%
\section[TcSimplify]{TcSimplify}
-Notes:
-Inference (local definitions)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-If the inst constrains a local type variable, then
- [ReduceMe] if it's a literal or method inst, reduce it
- [DontReduce] otherwise see whether the inst is just a constant
- if succeed, use it
- if not, add original to context
- This check gets rid of constant dictionaries without
- losing sharing.
+\begin{code}
+module TcSimplify (
+ tcSimplifyInfer, tcSimplifyInferCheck,
+ tcSimplifyCheck, tcSimplifyRestricted,
+ tcSimplifyToDicts, tcSimplifyIPs,
+ tcSimplifySuperClasses,
+ tcSimplifyTop, tcSimplifyInteractive,
+ tcSimplifyBracket,
+
+ tcSimplifyDeriv, tcSimplifyDefault,
+ bindInstsOfLocalFuns
+ ) where
+
+#include "HsVersions.h"
+
+import {-# SOURCE #-} TcUnify( unifyTauTy )
+import TcEnv -- temp
+import HsSyn ( HsBind(..), HsExpr(..), LHsExpr, emptyLHsBinds )
+import TcHsSyn ( TcId, TcDictBinds, mkHsApp, mkHsTyApp, mkHsDictApp )
+
+import TcRnMonad
+import Inst ( lookupInst, LookupInstResult(..),
+ tyVarsOfInst, fdPredsOfInsts, newDicts,
+ isDict, isClassDict, isLinearInst, linearInstType,
+ isStdClassTyVarDict, isMethodFor, isMethod,
+ instToId, tyVarsOfInsts, cloneDict,
+ ipNamesOfInsts, ipNamesOfInst, dictPred,
+ instBindingRequired, fdPredsOfInst,
+ newDictsAtLoc, tcInstClassOp,
+ getDictClassTys, isTyVarDict, instLoc,
+ zonkInst, tidyInsts, tidyMoreInsts,
+ Inst, pprInsts, pprDictsInFull, pprInstInFull, tcGetInstEnvs,
+ isInheritableInst, pprDFuns, pprDictsTheta
+ )
+import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals, pprBinders )
+import InstEnv ( lookupInstEnv, classInstances )
+import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
+import TcType ( TcTyVar, TcTyVarSet, ThetaType, TcPredType,
+ mkClassPred, isOverloadedTy, mkTyConApp, isSkolemTyVar,
+ mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
+ tyVarsOfPred, tcEqType, pprPred, mkPredTy )
+import Id ( idType, mkUserLocal )
+import Var ( TyVar )
+import Name ( Name, getOccName, getSrcLoc )
+import NameSet ( NameSet, mkNameSet, elemNameSet )
+import Class ( classBigSig, classKey )
+import FunDeps ( oclose, grow, improve, pprEquationDoc )
+import PrelInfo ( isNumericClass )
+import PrelNames ( splitName, fstName, sndName, integerTyConName,
+ showClassKey, eqClassKey, ordClassKey )
+import Type ( zipTopTvSubst, substTheta, substTy )
+import TysWiredIn ( pairTyCon, doubleTy )
+import ErrUtils ( Message )
+import BasicTypes ( TopLevelFlag, isNotTopLevel )
+import VarSet
+import VarEnv ( TidyEnv )
+import FiniteMap
+import Bag
+import Outputable
+import ListSetOps ( equivClasses )
+import Util ( zipEqual, isSingleton )
+import List ( partition )
+import SrcLoc ( Located(..) )
+import CmdLineOpts
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{NOTES}
+%* *
+%************************************************************************
+
+ --------------------------------------
+ Notes on functional dependencies (a bug)
+ --------------------------------------
+
+| > class Foo a b | a->b
+| >
+| > class Bar a b | a->b
+| >
+| > data Obj = Obj
+| >
+| > instance Bar Obj Obj
+| >
+| > instance (Bar a b) => Foo a b
+| >
+| > foo:: (Foo a b) => a -> String
+| > foo _ = "works"
+| >
+| > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
+| > runFoo f = f Obj
+|
+| *Test> runFoo foo
+|
+| <interactive>:1:
+| Could not deduce (Bar a b) from the context (Foo a b)
+| arising from use of `foo' at <interactive>:1
+| Probable fix:
+| Add (Bar a b) to the expected type of an expression
+| In the first argument of `runFoo', namely `foo'
+| In the definition of `it': it = runFoo foo
+|
+| Why all of the sudden does GHC need the constraint Bar a b? The
+| function foo didn't ask for that...
+
+The trouble is that to type (runFoo foo), GHC has to solve the problem:
+
+ Given constraint Foo a b
+ Solve constraint Foo a b'
+
+Notice that b and b' aren't the same. To solve this, just do
+improvement and then they are the same. But GHC currently does
+ simplify constraints
+ apply improvement
+ and loop
+
+That is usually fine, but it isn't here, because it sees that Foo a b is
+not the same as Foo a b', and so instead applies the instance decl for
+instance Bar a b => Foo a b. And that's where the Bar constraint comes
+from.
+
+The Right Thing is to improve whenever the constraint set changes at
+all. Not hard in principle, but it'll take a bit of fiddling to do.
+
+
+
+ --------------------------------------
+ Notes on quantification
+ --------------------------------------
+
+Suppose we are about to do a generalisation step.
+We have in our hand
+
+ G the environment
+ T the type of the RHS
+ C the constraints from that RHS
+
+The game is to figure out
+
+ Q the set of type variables over which to quantify
+ Ct the constraints we will *not* quantify over
+ Cq the constraints we will quantify over
+
+So we're going to infer the type
+
+ forall Q. Cq => T
+
+and float the constraints Ct further outwards.
+
+Here are the things that *must* be true:
+
+ (A) Q intersect fv(G) = EMPTY limits how big Q can be
+ (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
+
+(A) says we can't quantify over a variable that's free in the
+environment. (B) says we must quantify over all the truly free
+variables in T, else we won't get a sufficiently general type. We do
+not *need* to quantify over any variable that is fixed by the free
+vars of the environment G.
+
+ BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
+
+Example: class H x y | x->y where ...
+
+ fv(G) = {a} C = {H a b, H c d}
+ T = c -> b
+
+ (A) Q intersect {a} is empty
+ (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
+
+ So Q can be {c,d}, {b,c,d}
+
+Other things being equal, however, we'd like to quantify over as few
+variables as possible: smaller types, fewer type applications, more
+constraints can get into Ct instead of Cq.
+
+
+-----------------------------------------
+We will make use of
+
+ fv(T) the free type vars of T
+
+ oclose(vs,C) The result of extending the set of tyvars vs
+ using the functional dependencies from C
+
+ grow(vs,C) The result of extend the set of tyvars vs
+ using all conceivable links from C.
+
+ E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
+ Then grow(vs,C) = {a,b,c}
+
+ Note that grow(vs,C) `superset` grow(vs,simplify(C))
+ That is, simplfication can only shrink the result of grow.
+
+Notice that
+ oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
+ grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
+
+
+-----------------------------------------
+
+Choosing Q
+~~~~~~~~~~
+Here's a good way to choose Q:
+
+ Q = grow( fv(T), C ) \ oclose( fv(G), C )
+
+That is, quantify over all variable that that MIGHT be fixed by the
+call site (which influences T), but which aren't DEFINITELY fixed by
+G. This choice definitely quantifies over enough type variables,
+albeit perhaps too many.
+
+Why grow( fv(T), C ) rather than fv(T)? Consider
+
+ class H x y | x->y where ...
+
+ T = c->c
+ C = (H c d)
+
+ If we used fv(T) = {c} we'd get the type
+
+ forall c. H c d => c -> b
+
+ And then if the fn was called at several different c's, each of
+ which fixed d differently, we'd get a unification error, because
+ d isn't quantified. Solution: quantify d. So we must quantify
+ everything that might be influenced by c.
+
+Why not oclose( fv(T), C )? Because we might not be able to see
+all the functional dependencies yet:
+
+ class H x y | x->y where ...
+ instance H x y => Eq (T x y) where ...
+
+ T = c->c
+ C = (Eq (T c d))
+
+ Now oclose(fv(T),C) = {c}, because the functional dependency isn't
+ apparent yet, and that's wrong. We must really quantify over d too.
+
+
+There really isn't any point in quantifying over any more than
+grow( fv(T), C ), because the call sites can't possibly influence
+any other type variables.
+
+
+
+ --------------------------------------
+ Notes on ambiguity
+ --------------------------------------
+
+It's very hard to be certain when a type is ambiguous. Consider
+
+ class K x
+ class H x y | x -> y
+ instance H x y => K (x,y)
+
+Is this type ambiguous?
+ forall a b. (K (a,b), Eq b) => a -> a
+
+Looks like it! But if we simplify (K (a,b)) we get (H a b) and
+now we see that a fixes b. So we can't tell about ambiguity for sure
+without doing a full simplification. And even that isn't possible if
+the context has some free vars that may get unified. Urgle!
+
+Here's another example: is this ambiguous?
+ forall a b. Eq (T b) => a -> a
+Not if there's an insance decl (with no context)
+ instance Eq (T b) where ...
+
+You may say of this example that we should use the instance decl right
+away, but you can't always do that:
+
+ class J a b where ...
+ instance J Int b where ...
+
+ f :: forall a b. J a b => a -> a
+
+(Notice: no functional dependency in J's class decl.)
+Here f's type is perfectly fine, provided f is only called at Int.
+It's premature to complain when meeting f's signature, or even
+when inferring a type for f.
+
-If the inst does not constrain a local type variable then
- [Free] then throw it out as free.
-Inference (top level definitions)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-If the inst does not constrain a local type variable, then
- [FreeIfTautological] try for tautology;
- if so, throw it out as free
- (discarding result of tautology check)
- if not, make original inst part of the context
- (eliminating superclasses as usual)
+However, we don't *need* to report ambiguity right away. It'll always
+show up at the call site.... and eventually at main, which needs special
+treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
-If the inst constrains a local type variable, then
- as for inference (local defns)
+So here's the plan. We WARN about probable ambiguity if
+ fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
-Checking (local defns)
-~~~~~~~~
-If the inst constrains a local type variable then
- [ReduceMe] reduce (signal error on failure)
+(all tested before quantification).
+That is, all the type variables in Cq must be fixed by the the variables
+in the environment, or by the variables in the type.
-If the inst does not constrain a local type variable then
- [Free] throw it out as free.
+Notice that we union before calling oclose. Here's an example:
-Checking (top level)
-~~~~~~~~~~~~~~~~~~~~
-If the inst constrains a local type variable then
- as for checking (local defns)
+ class J a b c | a b -> c
+ fv(G) = {a}
-If the inst does not constrain a local type variable then
- as for checking (local defns)
+Is this ambiguous?
+ forall b c. (J a b c) => b -> b
+Only if we union {a} from G with {b} from T before using oclose,
+do we see that c is fixed.
+It's a bit vague exactly which C we should use for this oclose call. If we
+don't fix enough variables we might complain when we shouldn't (see
+the above nasty example). Nothing will be perfect. That's why we can
+only issue a warning.
-Checking once per module
-~~~~~~~~~~~~~~~~~~~~~~~~~
-For dicts of the form (C a), where C is a std class
- and "a" is a type variable,
- [DontReduce] add to context
-otherwise [ReduceMe] always reduce
+Can we ever be *certain* about ambiguity? Yes: if there's a constraint
-[NB: we may generate one Tree [Int] dict per module, so
- sharing is not complete.]
+ c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
-Sort out ambiguity at the end.
+then c is a "bubble"; there's no way it can ever improve, and it's
+certainly ambiguous. UNLESS it is a constant (sigh). And what about
+the nasty example?
-Principal types
-~~~~~~~~~~~~~~~
-class C a where
- op :: a -> a
+ class K x
+ class H x y | x -> y
+ instance H x y => K (x,y)
-f x = let g y = op (y::Int) in True
+Is this type ambiguous?
+ forall a b. (K (a,b), Eq b) => a -> a
+
+Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
+is a "bubble" that's a set of constraints
+
+ Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
+
+Hence another idea. To decide Q start with fv(T) and grow it
+by transitive closure in Cq (no functional dependencies involved).
+Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
+The definitely-ambiguous can then float out, and get smashed at top level
+(which squashes out the constants, like Eq (T a) above)
+
+
+ --------------------------------------
+ Notes on principal types
+ --------------------------------------
+
+ class C a where
+ op :: a -> a
+
+ f x = let g y = op (y::Int) in True
Here the principal type of f is (forall a. a->a)
but we'll produce the non-principal type
f :: forall a. C Int => a -> a
-Ambiguity
-~~~~~~~~~
+ --------------------------------------
+ The need for forall's in constraints
+ --------------------------------------
+
+[Exchange on Haskell Cafe 5/6 Dec 2000]
+
+ class C t where op :: t -> Bool
+ instance C [t] where op x = True
+
+ p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
+ q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
+
+The definitions of p and q differ only in the order of the components in
+the pair on their right-hand sides. And yet:
+
+ ghc and "Typing Haskell in Haskell" reject p, but accept q;
+ Hugs rejects q, but accepts p;
+ hbc rejects both p and q;
+ nhc98 ... (Malcolm, can you fill in the blank for us!).
+
+The type signature for f forces context reduction to take place, and
+the results of this depend on whether or not the type of y is known,
+which in turn depends on which component of the pair the type checker
+analyzes first.
+
+Solution: if y::m a, float out the constraints
+ Monad m, forall c. C (m c)
+When m is later unified with [], we can solve both constraints.
+
+
+ --------------------------------------
+ Notes on implicit parameters
+ --------------------------------------
+
+Question 1: can we "inherit" implicit parameters
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this:
- instance C (T a) Int where ...
- instance C (T a) Bool where ...
+ f x = (x::Int) + ?y
-and suppose we infer a context
+where f is *not* a top-level binding.
+From the RHS of f we'll get the constraint (?y::Int).
+There are two types we might infer for f:
- C (T x) y
+ f :: Int -> Int
-from some expression, where x and y are type varibles,
-and x is ambiguous, and y is being quantified over.
-Should we complain, or should we generate the type
+(so we get ?y from the context of f's definition), or
- forall x y. C (T x) y => <type not involving x>
+ f :: (?y::Int) => Int -> Int
-The idea is that at the call of the function we might
-know that y is Int (say), so the "x" isn't really ambiguous.
-Notice that we have to add "x" to the type variables over
-which we generalise.
+At first you might think the first was better, becuase then
+?y behaves like a free variable of the definition, rather than
+having to be passed at each call site. But of course, the WHOLE
+IDEA is that ?y should be passed at each call site (that's what
+dynamic binding means) so we'd better infer the second.
-Something similar can happen even if C constrains only ambiguous
-variables. Suppose we infer the context
+BOTTOM LINE: when *inferring types* you *must* quantify
+over implicit parameters. See the predicate isFreeWhenInferring.
- C [x]
-where x is ambiguous. Then we could infer the type
+Question 2: type signatures
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BUT WATCH OUT: When you supply a type signature, we can't force you
+to quantify over implicit parameters. For example:
- forall x. C [x] => <type not involving x>
+ (?x + 1) :: Int
-in the hope that at the call site there was an instance
-decl such as
+This is perfectly reasonable. We do not want to insist on
- instance Num a => C [a] where ...
+ (?x + 1) :: (?x::Int => Int)
-and hence the default mechanism would resolve the "a".
+That would be silly. Here, the definition site *is* the occurrence site,
+so the above strictures don't apply. Hence the difference between
+tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
+and tcSimplifyCheckBind (which does not).
+What about when you supply a type signature for a binding?
+Is it legal to give the following explicit, user type
+signature to f, thus:
-\begin{code}
-module TcSimplify (
- tcSimplify, tcSimplifyAndCheck, tcSimplifyToDicts,
- tcSimplifyTop, tcSimplifyThetas, tcSimplifyCheckThetas,
- bindInstsOfLocalFuns, partitionPredsOfLIE
- ) where
+ f :: Int -> Int
+ f x = (x::Int) + ?y
-#include "HsVersions.h"
+At first sight this seems reasonable, but it has the nasty property
+that adding a type signature changes the dynamic semantics.
+Consider this:
-import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
-import TcHsSyn ( TcExpr, TcId,
- TcMonoBinds, TcDictBinds
- )
+ (let f x = (x::Int) + ?y
+ in (f 3, f 3 with ?y=5)) with ?y = 6
+
+ returns (3+6, 3+5)
+vs
+ (let f :: Int -> Int
+ f x = x + ?y
+ in (f 3, f 3 with ?y=5)) with ?y = 6
+
+ returns (3+6, 3+6)
+
+Indeed, simply inlining f (at the Haskell source level) would change the
+dynamic semantics.
+
+Nevertheless, as Launchbury says (email Oct 01) we can't really give the
+semantics for a Haskell program without knowing its typing, so if you
+change the typing you may change the semantics.
+
+To make things consistent in all cases where we are *checking* against
+a supplied signature (as opposed to inferring a type), we adopt the
+rule:
+
+ a signature does not need to quantify over implicit params.
+
+[This represents a (rather marginal) change of policy since GHC 5.02,
+which *required* an explicit signature to quantify over all implicit
+params for the reasons mentioned above.]
+
+But that raises a new question. Consider
+
+ Given (signature) ?x::Int
+ Wanted (inferred) ?x::Int, ?y::Bool
+
+Clearly we want to discharge the ?x and float the ?y out. But
+what is the criterion that distinguishes them? Clearly it isn't
+what free type variables they have. The Right Thing seems to be
+to float a constraint that
+ neither mentions any of the quantified type variables
+ nor any of the quantified implicit parameters
+
+See the predicate isFreeWhenChecking.
+
+
+Question 3: monomorphism
+~~~~~~~~~~~~~~~~~~~~~~~~
+There's a nasty corner case when the monomorphism restriction bites:
+
+ z = (x::Int) + ?y
+
+The argument above suggests that we *must* generalise
+over the ?y parameter, to get
+ z :: (?y::Int) => Int,
+but the monomorphism restriction says that we *must not*, giving
+ z :: Int.
+Why does the momomorphism restriction say this? Because if you have
+
+ let z = x + ?y in z+z
+
+you might not expect the addition to be done twice --- but it will if
+we follow the argument of Question 2 and generalise over ?y.
+
+
+Question 4: top level
+~~~~~~~~~~~~~~~~~~~~~
+At the top level, monomorhism makes no sense at all.
+
+ module Main where
+ main = let ?x = 5 in print foo
+
+ foo = woggle 3
+
+ woggle :: (?x :: Int) => Int -> Int
+ woggle y = ?x + y
+
+We definitely don't want (foo :: Int) with a top-level implicit parameter
+(?x::Int) becuase there is no way to bind it.
+
+
+Possible choices
+~~~~~~~~~~~~~~~~
+(A) Always generalise over implicit parameters
+ Bindings that fall under the monomorphism restriction can't
+ be generalised
+
+ Consequences:
+ * Inlining remains valid
+ * No unexpected loss of sharing
+ * But simple bindings like
+ z = ?y + 1
+ will be rejected, unless you add an explicit type signature
+ (to avoid the monomorphism restriction)
+ z :: (?y::Int) => Int
+ z = ?y + 1
+ This seems unacceptable
+
+(B) Monomorphism restriction "wins"
+ Bindings that fall under the monomorphism restriction can't
+ be generalised
+ Always generalise over implicit parameters *except* for bindings
+ that fall under the monomorphism restriction
+
+ Consequences
+ * Inlining isn't valid in general
+ * No unexpected loss of sharing
+ * Simple bindings like
+ z = ?y + 1
+ accepted (get value of ?y from binding site)
+
+(C) Always generalise over implicit parameters
+ Bindings that fall under the monomorphism restriction can't
+ be generalised, EXCEPT for implicit parameters
+ Consequences
+ * Inlining remains valid
+ * Unexpected loss of sharing (from the extra generalisation)
+ * Simple bindings like
+ z = ?y + 1
+ accepted (get value of ?y from occurrence sites)
+
+
+Discussion
+~~~~~~~~~~
+None of these choices seems very satisfactory. But at least we should
+decide which we want to do.
+
+It's really not clear what is the Right Thing To Do. If you see
+
+ z = (x::Int) + ?y
+
+would you expect the value of ?y to be got from the *occurrence sites*
+of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
+case of function definitions, the answer is clearly the former, but
+less so in the case of non-fucntion definitions. On the other hand,
+if we say that we get the value of ?y from the definition site of 'z',
+then inlining 'z' might change the semantics of the program.
+
+Choice (C) really says "the monomorphism restriction doesn't apply
+to implicit parameters". Which is fine, but remember that every
+innocent binding 'x = ...' that mentions an implicit parameter in
+the RHS becomes a *function* of that parameter, called at each
+use of 'x'. Now, the chances are that there are no intervening 'with'
+clauses that bind ?y, so a decent compiler should common up all
+those function calls. So I think I strongly favour (C). Indeed,
+one could make a similar argument for abolishing the monomorphism
+restriction altogether.
+
+BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
-import TcMonad
-import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
- tyVarsOfInst,
- isDict, isClassDict, isMethod, notFunDep,
- isStdClassTyVarDict, isMethodFor,
- instToId, instBindingRequired, instCanBeGeneralised,
- newDictFromOld, newFunDepFromDict,
- getDictClassTys, getIPs, isTyVarDict,
- getDictPred_maybe, getMethodTheta_maybe,
- instLoc, pprInst, zonkInst, tidyInst, tidyInsts,
- Inst, LIE, pprInsts, pprInstsInFull,
- mkLIE, emptyLIE, unitLIE, consLIE, plusLIE,
- lieToList
- )
-import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv )
-import InstEnv ( lookupInstEnv, InstLookupResult(..) )
-
-import TcType ( TcTyVarSet )
-import TcUnify ( unifyTauTy )
-import Id ( idType )
-import Class ( Class, classBigSig )
-import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
-
-import Type ( Type, ClassContext,
- mkTyVarTy, getTyVar,
- isTyVarTy, splitSigmaTy, tyVarsOfTypes
- )
-import Subst ( mkTopTyVarSubst, substClasses )
-import PprType ( pprConstraint )
-import TysWiredIn ( unitTy )
-import VarSet
-import FiniteMap
-import Outputable
-import ListSetOps ( equivClasses )
-import Util ( zipEqual, mapAccumL )
-import List ( partition )
-import Maybe ( fromJust )
-import Maybes ( maybeToBool )
-import CmdLineOpts
-\end{code}
%************************************************************************
%* *
-\subsection[tcSimplify-main]{Main entry function}
+\subsection{tcSimplifyInfer}
%* *
%************************************************************************
-The main wrapper is @tcSimplify@. It just calls @tcSimpl@, but with
-the ``don't-squash-consts'' flag set depending on top-level ness. For
-top level defns we *do* squash constants, so that they stay local to a
-single defn. This makes things which are inlined more likely to be
-exportable, because their constants are "inside". Later passes will
-float them out if poss, after inlinings are sorted out.
+tcSimplify is called when we *inferring* a type. Here's the overall game plan:
+
+ 1. Compute Q = grow( fvs(T), C )
+
+ 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
+ predicates will end up in Ct; we deal with them at the top level
+
+ 3. Try improvement, using functional dependencies
+
+ 4. If Step 3 did any unification, repeat from step 1
+ (Unification can change the result of 'grow'.)
+
+Note: we don't reduce dictionaries in step 2. For example, if we have
+Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
+after step 2. However note that we may therefore quantify over more
+type variables than we absolutely have to.
+
+For the guts, we need a loop, that alternates context reduction and
+improvement with unification. E.g. Suppose we have
+
+ class C x y | x->y where ...
+
+and tcSimplify is called with:
+ (C Int a, C Int b)
+Then improvement unifies a with b, giving
+ (C Int a, C Int a)
+
+If we need to unify anything, we rattle round the whole thing all over
+again.
+
\begin{code}
-tcSimplify
- :: SDoc
- -> TcTyVarSet -- ``Local'' type variables
- -- ASSERT: this tyvar set is already zonked
- -> LIE -- Wanted
- -> TcM (LIE, -- Free
- TcDictBinds, -- Bindings
- LIE) -- Remaining wanteds; no dups
-
-tcSimplify str local_tvs wanted_lie
-{- this is just an optimization, and interferes with implicit params,
- disable it for now. same goes for tcSimplifyAndCheck
- | isEmptyVarSet local_tvs
- = returnTc (wanted_lie, EmptyMonoBinds, emptyLIE)
+tcSimplifyInfer
+ :: SDoc
+ -> TcTyVarSet -- fv(T); type vars
+ -> [Inst] -- Wanted
+ -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
+ TcDictBinds, -- Bindings
+ [TcId]) -- Dict Ids that must be bound here (zonked)
+ -- Any free (escaping) Insts are tossed into the environment
+\end{code}
- | otherwise
--}
- = reduceContext str try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
- -- Check for non-generalisable insts
- let
- cant_generalise = filter (not . instCanBeGeneralised) irreds
- in
- checkTc (null cant_generalise)
- (genCantGenErr cant_generalise) `thenTc_`
-
- -- Check for ambiguous insts.
- -- You might think these can't happen (I did) because an ambiguous
- -- inst like (Eq a) will get tossed out with "frees", and eventually
- -- dealt with by tcSimplifyTop.
- -- But we can get stuck with
- -- C a b
- -- where "a" is one of the local_tvs, but "b" is unconstrained.
- -- Then we must yell about the ambiguous b
- -- But we must only do so if "b" really is unconstrained; so
- -- we must grab the global tyvars to answer that question
- tcGetGlobalTyVars `thenNF_Tc` \ global_tvs ->
+\begin{code}
+tcSimplifyInfer doc tau_tvs wanted_lie
+ = inferLoop doc (varSetElems tau_tvs)
+ wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
+
+ extendLIEs frees `thenM_`
+ returnM (qtvs, binds, map instToId irreds)
+
+inferLoop doc tau_tvs wanteds
+ = -- Step 1
+ zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' ->
+ mappM zonkInst wanteds `thenM` \ wanteds' ->
+ tcGetGlobalTyVars `thenM` \ gbl_tvs ->
let
- avail_tvs = local_tvs `unionVarSet` global_tvs
- (irreds', bad_guys) = partition (isEmptyVarSet . ambig_tv_fn) irreds
- ambig_tv_fn dict = tyVarsOfInst dict `minusVarSet` avail_tvs
+ preds = fdPredsOfInsts wanteds'
+ qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
+
+ try_me inst
+ | isFreeWhenInferring qtvs inst = Free
+ | isClassDict inst = DontReduceUnlessConstant -- Dicts
+ | otherwise = ReduceMe NoSCs -- Lits and Methods
in
- addAmbigErrs ambig_tv_fn bad_guys `thenNF_Tc_`
+ traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds,
+ ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_`
+ -- Step 2
+ reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
+
+ -- Step 3
+ if no_improvement then
+ returnM (varSetElems qtvs, frees, binds, irreds)
+ else
+ -- If improvement did some unification, we go round again. There
+ -- are two subtleties:
+ -- a) We start again with irreds, not wanteds
+ -- Using an instance decl might have introduced a fresh type variable
+ -- which might have been unified, so we'd get an infinite loop
+ -- if we started again with wanteds! See example [LOOP]
+ --
+ -- b) It's also essential to re-process frees, because unification
+ -- might mean that a type variable that looked free isn't now.
+ --
+ -- Hence the (irreds ++ frees)
+
+ -- However, NOTICE that when we are done, we might have some bindings, but
+ -- the final qtvs might be empty. See [NO TYVARS] below.
+
+ inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) ->
+ returnM (qtvs1, frees1, binds `unionBags` binds1, irreds1)
+\end{code}
+
+Example [LOOP]
+
+ class If b t e r | b t e -> r
+ instance If T t e t
+ instance If F t e e
+ class Lte a b c | a b -> c where lte :: a -> b -> c
+ instance Lte Z b T
+ instance (Lte a b l,If l b a c) => Max a b c
+
+Wanted: Max Z (S x) y
+
+Then we'll reduce using the Max instance to:
+ (Lte Z (S x) l, If l (S x) Z y)
+and improve by binding l->T, after which we can do some reduction
+on both the Lte and If constraints. What we *can't* do is start again
+with (Max Z (S x) y)!
+
+[NO TYVARS]
+
+ class Y a b | a -> b where
+ y :: a -> X b
+
+ instance Y [[a]] a where
+ y ((x:_):_) = X x
+
+ k :: X a -> X a -> X a
+
+ g :: Num a => [X a] -> [X a]
+ g xs = h xs
+ where
+ h ys = ys ++ map (k (y [[0]])) xs
+The excitement comes when simplifying the bindings for h. Initially
+try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
+From this we get t1:=:t2, but also various bindings. We can't forget
+the bindings (because of [LOOP]), but in fact t1 is what g is
+polymorphic in.
- -- Finished
- returnTc (mkLIE frees, binds, mkLIE irreds')
+The net effect of [NO TYVARS]
+
+\begin{code}
+isFreeWhenInferring :: TyVarSet -> Inst -> Bool
+isFreeWhenInferring qtvs inst
+ = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
+ && isInheritableInst inst -- And no implicit parameter involved
+ -- (see "Notes on implicit parameters")
+
+isFreeWhenChecking :: TyVarSet -- Quantified tyvars
+ -> NameSet -- Quantified implicit parameters
+ -> Inst -> Bool
+isFreeWhenChecking qtvs ips inst
+ = isFreeWrtTyVars qtvs inst
+ && isFreeWrtIPs ips inst
+
+isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
+isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyCheck}
+%* *
+%************************************************************************
+
+@tcSimplifyCheck@ is used when we know exactly the set of variables
+we are going to quantify over. For example, a class or instance declaration.
+
+\begin{code}
+tcSimplifyCheck
+ :: SDoc
+ -> [TcTyVar] -- Quantify over these
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds -- Bindings
+
+-- tcSimplifyCheck is used when checking expression type signatures,
+-- class decls, instance decls etc.
+--
+-- NB: tcSimplifyCheck does not consult the
+-- global type variables in the environment; so you don't
+-- need to worry about setting them before calling tcSimplifyCheck
+tcSimplifyCheck doc qtvs givens wanted_lie
+ = ASSERT( all isSkolemTyVar qtvs )
+ do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie
+ ; extendLIEs frees
+ ; return binds }
+ where
+-- get_qtvs = zonkTcTyVarsAndFV qtvs
+ get_qtvs = return (mkVarSet qtvs) -- All skolems
+
+
+-- tcSimplifyInferCheck is used when we know the constraints we are to simplify
+-- against, but we don't know the type variables over which we are going to quantify.
+-- This happens when we have a type signature for a mutually recursive group
+tcSimplifyInferCheck
+ :: SDoc
+ -> TcTyVarSet -- fv(T)
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM ([TcTyVar], -- Variables over which to quantify
+ TcDictBinds) -- Bindings
+
+tcSimplifyInferCheck doc tau_tvs givens wanted_lie
+ = do { (qtvs', frees, binds) <- tcSimplCheck doc get_qtvs AddSCs givens wanted_lie
+ ; extendLIEs frees
+ ; return (qtvs', binds) }
where
- wanteds = lieToList wanted_lie
-
- try_me inst
- -- Does not constrain a local tyvar
- | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
- && null (getIPs inst)
- = -- if is_top_level then
- -- FreeIfTautological -- Special case for inference on
- -- -- top-level defns
- -- else
- Free
-
- -- We're infering (not checking) the type, and
- -- the inst constrains a local type variable
- | isClassDict inst = DontReduceUnlessConstant -- Dicts
- | otherwise = ReduceMe AddToIrreds -- Lits and Methods
+ -- Figure out which type variables to quantify over
+ -- You might think it should just be the signature tyvars,
+ -- but in bizarre cases you can get extra ones
+ -- f :: forall a. Num a => a -> a
+ -- f x = fst (g (x, head [])) + 1
+ -- g a b = (b,a)
+ -- Here we infer g :: forall a b. a -> b -> (b,a)
+ -- We don't want g to be monomorphic in b just because
+ -- f isn't quantified over b.
+ all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
+
+ get_qtvs = zonkTcTyVarsAndFV all_tvs `thenM` \ all_tvs' ->
+ tcGetGlobalTyVars `thenM` \ gbl_tvs ->
+ let
+ qtvs = all_tvs' `minusVarSet` gbl_tvs
+ -- We could close gbl_tvs, but its not necessary for
+ -- soundness, and it'll only affect which tyvars, not which
+ -- dictionaries, we quantify over
+ in
+ returnM qtvs
\end{code}
-@tcSimplifyAndCheck@ is similar to the above, except that it checks
-that there is an empty wanted-set at the end. It may still return
-some of constant insts, which have to be resolved finally at the end.
+Here is the workhorse function for all three wrappers.
\begin{code}
-tcSimplifyAndCheck
- :: SDoc
- -> TcTyVarSet -- ``Local'' type variables
- -- ASSERT: this tyvar set is already zonked
- -> LIE -- Given; constrain only local tyvars
- -> LIE -- Wanted
- -> TcM (LIE, -- Free
- TcDictBinds) -- Bindings
-
-tcSimplifyAndCheck str local_tvs given_lie wanted_lie
-{-
- | isEmptyVarSet local_tvs
- -- This can happen quite legitimately; for example in
- -- instance Num Int where ...
- = returnTc (wanted_lie, EmptyMonoBinds)
+tcSimplCheck doc get_qtvs want_scs givens wanted_lie
+ = do { (qtvs, frees, binds, irreds) <- check_loop givens wanted_lie
- | otherwise
--}
- = reduceContext str try_me givens wanteds `thenTc` \ (binds, frees, irreds) ->
+ -- Complain about any irreducible ones
+ ; if not (null irreds)
+ then do { givens' <- mappM zonkInst given_dicts_and_ips
+ ; groupErrs (addNoInstanceErrs (Just doc) givens') irreds }
+ else return ()
+
+ ; returnM (qtvs, frees, binds) }
+ where
+ given_dicts_and_ips = filter (not . isMethod) givens
+ -- For error reporting, filter out methods, which are
+ -- only added to the given set as an optimisation
+
+ ip_set = mkNameSet (ipNamesOfInsts givens)
+
+ check_loop givens wanteds
+ = -- Step 1
+ mappM zonkInst givens `thenM` \ givens' ->
+ mappM zonkInst wanteds `thenM` \ wanteds' ->
+ get_qtvs `thenM` \ qtvs' ->
+
+ -- Step 2
+ let
+ -- When checking against a given signature we always reduce
+ -- until we find a match against something given, or can't reduce
+ try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
+ | otherwise = ReduceMe want_scs
+ in
+ reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
+
+ -- Step 3
+ if no_improvement then
+ returnM (varSetElems qtvs', frees, binds, irreds)
+ else
+ check_loop givens' (irreds ++ frees) `thenM` \ (qtvs', frees1, binds1, irreds1) ->
+ returnM (qtvs', frees1, binds `unionBags` binds1, irreds1)
+\end{code}
+
+
+%************************************************************************
+%* *
+ tcSimplifySuperClasses
+%* *
+%************************************************************************
+
+Note [SUPERCLASS-LOOP 1]
+~~~~~~~~~~~~~~~~~~~~~~~~
+We have to be very, very careful when generating superclasses, lest we
+accidentally build a loop. Here's an example:
+
+ class S a
+
+ class S a => C a where { opc :: a -> a }
+ class S b => D b where { opd :: b -> b }
+
+ instance C Int where
+ opc = opd
+
+ instance D Int where
+ opd = opc
+
+From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
+Simplifying, we may well get:
+ $dfCInt = :C ds1 (opd dd)
+ dd = $dfDInt
+ ds1 = $p1 dd
+Notice that we spot that we can extract ds1 from dd.
+
+Alas! Alack! We can do the same for (instance D Int):
+
+ $dfDInt = :D ds2 (opc dc)
+ dc = $dfCInt
+ ds2 = $p1 dc
- -- Complain about any irreducible ones
- mapNF_Tc complain irreds `thenNF_Tc_`
+And now we've defined the superclass in terms of itself.
- -- Done
- returnTc (mkLIE frees, binds)
+Solution: never generate a superclass selectors at all when
+satisfying the superclass context of an instance declaration.
+
+Two more nasty cases are in
+ tcrun021
+ tcrun033
+
+\begin{code}
+tcSimplifySuperClasses qtvs givens sc_wanteds
+ = ASSERT( all isSkolemTyVar qtvs )
+ do { (_, frees, binds1) <- tcSimplCheck doc get_qtvs NoSCs givens sc_wanteds
+ ; binds2 <- tc_simplify_top doc False NoSCs frees
+ ; return (binds1 `unionBags` binds2) }
where
- givens = lieToList given_lie
- wanteds = lieToList wanted_lie
- given_dicts = filter isClassDict givens
-
- try_me inst
- -- Does not constrain a local tyvar
- | isEmptyVarSet (tyVarsOfInst inst `intersectVarSet` local_tvs)
- && (not (isMethod inst) || null (getIPs inst))
- = Free
-
- -- When checking against a given signature we always reduce
- -- until we find a match against something given, or can't reduce
- | otherwise
- = ReduceMe AddToIrreds
-
- complain dict = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
- addNoInstanceErr str given_dicts dict
+ get_qtvs = return (mkVarSet qtvs)
+ doc = ptext SLIT("instance declaration superclass context")
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyRestricted}
+%* *
+%************************************************************************
+
+tcSimplifyRestricted infers which type variables to quantify for a
+group of restricted bindings. This isn't trivial.
+
+Eg1: id = \x -> x
+ We want to quantify over a to get id :: forall a. a->a
+
+Eg2: eq = (==)
+ We do not want to quantify over a, because there's an Eq a
+ constraint, so we get eq :: a->a->Bool (notice no forall)
+
+So, assume:
+ RHS has type 'tau', whose free tyvars are tau_tvs
+ RHS has constraints 'wanteds'
+
+Plan A (simple)
+ Quantify over (tau_tvs \ ftvs(wanteds))
+ This is bad. The constraints may contain (Monad (ST s))
+ where we have instance Monad (ST s) where...
+ so there's no need to be monomorphic in s!
+
+ Also the constraint might be a method constraint,
+ whose type mentions a perfectly innocent tyvar:
+ op :: Num a => a -> b -> a
+ Here, b is unconstrained. A good example would be
+ foo = op (3::Int)
+ We want to infer the polymorphic type
+ foo :: forall b. b -> b
+
+
+Plan B (cunning, used for a long time up to and including GHC 6.2)
+ Step 1: Simplify the constraints as much as possible (to deal
+ with Plan A's problem). Then set
+ qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
+
+ Step 2: Now simplify again, treating the constraint as 'free' if
+ it does not mention qtvs, and trying to reduce it otherwise.
+ The reasons for this is to maximise sharing.
+
+ This fails for a very subtle reason. Suppose that in the Step 2
+ a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
+ In the Step 1 this constraint might have been simplified, perhaps to
+ (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
+ This won't happen in Step 2... but that in turn might prevent some other
+ constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
+ and that in turn breaks the invariant that no constraints are quantified over.
+
+ Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
+ the problem.
+
+
+Plan C (brutal)
+ Step 1: Simplify the constraints as much as possible (to deal
+ with Plan A's problem). Then set
+ qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
+ Return the bindings from Step 1.
+
+
+A note about Plan C (arising from "bug" reported by George Russel March 2004)
+Consider this:
+
+ instance (HasBinary ty IO) => HasCodedValue ty
+
+ foo :: HasCodedValue a => String -> IO a
+
+ doDecodeIO :: HasCodedValue a => () -> () -> IO a
+ doDecodeIO codedValue view
+ = let { act = foo "foo" } in act
+
+You might think this should work becuase the call to foo gives rise to a constraint
+(HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
+restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
+constraint using the (rather bogus) instance declaration, and now we are stuffed.
+
+I claim this is not really a bug -- but it bit Sergey as well as George. So here's
+plan D
+
+
+Plan D (a variant of plan B)
+ Step 1: Simplify the constraints as much as possible (to deal
+ with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
+ qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
+
+ Step 2: Now simplify again, treating the constraint as 'free' if
+ it does not mention qtvs, and trying to reduce it otherwise.
+
+ The point here is that it's generally OK to have too few qtvs; that is,
+ to make the thing more monomorphic than it could be. We don't want to
+ do that in the common cases, but in wierd cases it's ok: the programmer
+ can always add a signature.
+
+ Too few qtvs => too many wanteds, which is what happens if you do less
+ improvement.
+
+
+\begin{code}
+tcSimplifyRestricted -- Used for restricted binding groups
+ -- i.e. ones subject to the monomorphism restriction
+ :: SDoc
+ -> TopLevelFlag
+ -> [Name] -- Things bound in this group
+ -> TcTyVarSet -- Free in the type of the RHSs
+ -> [Inst] -- Free in the RHSs
+ -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
+ TcDictBinds) -- Bindings
+ -- tcSimpifyRestricted returns no constraints to
+ -- quantify over; by definition there are none.
+ -- They are all thrown back in the LIE
+
+tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
+ -- Zonk everything in sight
+ = mappM zonkInst wanteds `thenM` \ wanteds' ->
+ zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
+ tcGetGlobalTyVars `thenM` \ gbl_tvs' ->
+
+ -- 'reduceMe': Reduce as far as we can. Don't stop at
+ -- dicts; the idea is to get rid of as many type
+ -- variables as possible, and we don't want to stop
+ -- at (say) Monad (ST s), because that reduces
+ -- immediately, with no constraint on s.
+ --
+ -- BUT do no improvement! See Plan D above
+ reduceContextWithoutImprovement
+ doc reduceMe wanteds' `thenM` \ (_frees, _binds, constrained_dicts) ->
+
+ -- Next, figure out the tyvars we will quantify over
+ let
+ constrained_tvs = tyVarsOfInsts constrained_dicts
+ qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
+ `minusVarSet` constrained_tvs
+ in
+ traceTc (text "tcSimplifyRestricted" <+> vcat [
+ pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts,
+ ppr _binds,
+ ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_`
+
+ -- The first step may have squashed more methods than
+ -- necessary, so try again, this time more gently, knowing the exact
+ -- set of type variables to quantify over.
+ --
+ -- We quantify only over constraints that are captured by qtvs;
+ -- these will just be a subset of non-dicts. This in contrast
+ -- to normal inference (using isFreeWhenInferring) in which we quantify over
+ -- all *non-inheritable* constraints too. This implements choice
+ -- (B) under "implicit parameter and monomorphism" above.
+ --
+ -- Remember that we may need to do *some* simplification, to
+ -- (for example) squash {Monad (ST s)} into {}. It's not enough
+ -- just to float all constraints
+ --
+ -- At top level, we *do* squash methods becuase we want to
+ -- expose implicit parameters to the test that follows
+ let
+ is_nested_group = isNotTopLevel top_lvl
+ try_me inst | isFreeWrtTyVars qtvs inst,
+ (is_nested_group || isDict inst) = Free
+ | otherwise = ReduceMe AddSCs
+ in
+ reduceContextWithoutImprovement
+ doc try_me wanteds' `thenM` \ (frees, binds, irreds) ->
+ ASSERT( null irreds )
+
+ -- See "Notes on implicit parameters, Question 4: top level"
+ if is_nested_group then
+ extendLIEs frees `thenM_`
+ returnM (varSetElems qtvs, binds)
+ else
+ let
+ (non_ips, bad_ips) = partition isClassDict frees
+ in
+ addTopIPErrs bndrs bad_ips `thenM_`
+ extendLIEs non_ips `thenM_`
+ returnM (varSetElems qtvs, binds)
\end{code}
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyToDicts}
+%* *
+%************************************************************************
+
On the LHS of transformation rules we only simplify methods and constants,
getting dictionaries. We want to keep all of them unsimplified, to serve
as the available stuff for the RHS of the rule.
The same thing is used for specialise pragmas. Consider
-
+
f :: Num a => a -> a
{-# SPECIALISE f :: Int -> Int #-}
f = ...
f_spec = _inline_me_ (f Int dNumInt)
-But that means that we must simplify the Method for f to (f Int dNumInt)!
+But that means that we must simplify the Method for f to (f Int dNumInt)!
So tcSimplifyToDicts squeezes out all Methods.
+IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
+
+ fromIntegral :: (Integral a, Num b) => a -> b
+ {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
+
+Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
+want to get
+
+ forall dIntegralInt.
+ fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
+
+because the scsel will mess up RULE matching. Instead we want
+
+ forall dIntegralInt, dNumInt.
+ fromIntegral Int Int dIntegralInt dNumInt = id Int
+
+Hence "WithoutSCs"
+
\begin{code}
-tcSimplifyToDicts :: LIE -> TcM (LIE, TcDictBinds)
-tcSimplifyToDicts wanted_lie
- = reduceContext (text "tcSimplifyToDicts") try_me [] wanteds `thenTc` \ (binds, frees, irreds) ->
+tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds)
+tcSimplifyToDicts wanteds
+ = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
+ -- Since try_me doesn't look at types, we don't need to
+ -- do any zonking, so it's safe to call reduceContext directly
ASSERT( null frees )
- returnTc (mkLIE irreds, binds)
+ extendLIEs irreds `thenM_`
+ returnM binds
+
where
- wanteds = lieToList wanted_lie
+ doc = text "tcSimplifyToDicts"
-- Reduce methods and lits only; stop as soon as we get a dictionary
- try_me inst | isDict inst = DontReduce
- | otherwise = ReduceMe AddToIrreds
+ try_me inst | isDict inst = KeepDictWithoutSCs -- See notes above re "WithoutSCs"
+ | otherwise = ReduceMe NoSCs
\end{code}
-The following function partitions a LIE by a predicate defined
-over `Pred'icates (an unfortunate overloading of terminology!).
-This means it sometimes has to split up `Methods', in which case
-a binding is generated.
-It is used in `with' bindings to extract from the LIE the implicit
-parameters being bound.
+
+tcSimplifyBracket is used when simplifying the constraints arising from
+a Template Haskell bracket [| ... |]. We want to check that there aren't
+any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
+Show instance), but we aren't otherwise interested in the results.
+Nor do we care about ambiguous dictionaries etc. We will type check
+this bracket again at its usage site.
\begin{code}
-partitionPredsOfLIE pred lie
- = foldlTc (partPreds pred) (emptyLIE, emptyLIE, EmptyMonoBinds) insts
- where insts = lieToList lie
-
--- warning: the term `pred' is overloaded here!
-partPreds pred (lie1, lie2, binds) inst
- | maybeToBool maybe_pred
- = if pred p then
- returnTc (consLIE inst lie1, lie2, binds)
- else
- returnTc (lie1, consLIE inst lie2, binds)
- where maybe_pred = getDictPred_maybe inst
- Just p = maybe_pred
-
--- the assumption is that those satisfying `pred' are being extracted,
--- so we leave the method untouched when nothing satisfies `pred'
-partPreds pred (lie1, lie2, binds1) inst
- | maybeToBool maybe_theta
- = if any pred theta then
- zonkInst inst `thenTc` \ inst' ->
- tcSimplifyToDicts (unitLIE inst') `thenTc` \ (lie3, binds2) ->
- partitionPredsOfLIE pred lie3 `thenTc` \ (lie1', lie2', EmptyMonoBinds) ->
- returnTc (lie1 `plusLIE` lie1',
- lie2 `plusLIE` lie2',
- binds1 `AndMonoBinds` binds2)
- else
- returnTc (lie1, consLIE inst lie2, binds1)
- where maybe_theta = getMethodTheta_maybe inst
- Just theta = maybe_theta
+tcSimplifyBracket :: [Inst] -> TcM ()
+tcSimplifyBracket wanteds
+ = simpleReduceLoop doc reduceMe wanteds `thenM_`
+ returnM ()
+ where
+ doc = text "tcSimplifyBracket"
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Filtering at a dynamic binding}
+%* *
+%************************************************************************
+
+When we have
+ let ?x = R in B
+
+we must discharge all the ?x constraints from B. We also do an improvement
+step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
+
+Actually, the constraints from B might improve the types in ?x. For example
+
+ f :: (?x::Int) => Char -> Char
+ let ?x = 3 in f 'c'
+
+then the constraint (?x::Int) arising from the call to f will
+force the binding for ?x to be of type Int.
+
+\begin{code}
+tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds
+tcSimplifyIPs given_ips wanteds
+ = simpl_loop given_ips wanteds `thenM` \ (frees, binds) ->
+ extendLIEs frees `thenM_`
+ returnM binds
+ where
+ doc = text "tcSimplifyIPs" <+> ppr given_ips
+ ip_set = mkNameSet (ipNamesOfInsts given_ips)
+
+ -- Simplify any methods that mention the implicit parameter
+ try_me inst | isFreeWrtIPs ip_set inst = Free
+ | otherwise = ReduceMe NoSCs
+
+ simpl_loop givens wanteds
+ = mappM zonkInst givens `thenM` \ givens' ->
+ mappM zonkInst wanteds `thenM` \ wanteds' ->
+
+ reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
+
+ if no_improvement then
+ ASSERT( null irreds )
+ returnM (frees, binds)
+ else
+ simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) ->
+ returnM (frees1, binds `unionBags` binds1)
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
+%* *
+%************************************************************************
+
+When doing a binding group, we may have @Insts@ of local functions.
+For example, we might have...
+\begin{verbatim}
+let f x = x + 1 -- orig local function (overloaded)
+ f.1 = f Int -- two instances of f
+ f.2 = f Float
+ in
+ (f.1 5, f.2 6.7)
+\end{verbatim}
+The point is: we must drop the bindings for @f.1@ and @f.2@ here,
+where @f@ is in scope; those @Insts@ must certainly not be passed
+upwards towards the top-level. If the @Insts@ were binding-ified up
+there, they would have unresolvable references to @f@.
+
+We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
+For each method @Inst@ in the @init_lie@ that mentions one of the
+@Ids@, we create a binding. We return the remaining @Insts@ (in an
+@LIE@), as well as the @HsBinds@ generated.
+
+\begin{code}
+bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
+-- Simlifies only MethodInsts, and generate only bindings of form
+-- fm = f tys dicts
+-- We're careful not to even generate bindings of the form
+-- d1 = d2
+-- You'd think that'd be fine, but it interacts with what is
+-- arguably a bug in Match.tidyEqnInfo (see notes there)
+
+bindInstsOfLocalFuns wanteds local_ids
+ | null overloaded_ids
+ -- Common case
+ = extendLIEs wanteds `thenM_`
+ returnM emptyLHsBinds
-partPreds pred (lie1, lie2, binds) inst
- = returnTc (lie1, consLIE inst lie2, binds)
+ | otherwise
+ = simpleReduceLoop doc try_me for_me `thenM` \ (frees, binds, irreds) ->
+ ASSERT( null irreds )
+ extendLIEs not_for_me `thenM_`
+ extendLIEs frees `thenM_`
+ returnM binds
+ where
+ doc = text "bindInsts" <+> ppr local_ids
+ overloaded_ids = filter is_overloaded local_ids
+ is_overloaded id = isOverloadedTy (idType id)
+ (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
+
+ overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
+ -- so it's worth building a set, so that
+ -- lookup (in isMethodFor) is faster
+ try_me inst | isMethod inst = ReduceMe NoSCs
+ | otherwise = Free
\end{code}
The main control over context reduction is here
\begin{code}
-data WhatToDo
- = ReduceMe -- Try to reduce this
- NoInstanceAction -- What to do if there's no such instance
+data WhatToDo
+ = ReduceMe WantSCs -- Try to reduce this
+ -- If there's no instance, behave exactly like
+ -- DontReduce: add the inst to
+ -- the irreductible ones, but don't
+ -- produce an error message of any kind.
+ -- It might be quite legitimate such as (Eq a)!
- | DontReduce -- Return as irreducible
+ | KeepDictWithoutSCs -- Return as irreducible; don't add its superclasses
+ -- Rather specialised: see notes with tcSimplifyToDicts
| DontReduceUnlessConstant -- Return as irreducible unless it can
-- be reduced to a constant in one step
| Free -- Return as free
- | FreeIfTautological -- Return as free iff it's tautological;
- -- if not, return as irreducible
- -- The FreeIfTautological case is to allow the possibility
- -- of generating functions with types like
- -- f :: C Int => Int -> Int
- -- Here, the C Int isn't a tautology presumably because Int
- -- isn't an instance of C in this module; but perhaps it will
- -- be at f's call site(s). Haskell doesn't allow this at
- -- present.
-
-data NoInstanceAction
- = Stop -- Fail; no error message
- -- (Only used when tautology checking.)
-
- | AddToIrreds -- Just add the inst to the irreductible ones; don't
- -- produce an error message of any kind.
- -- It might be quite legitimate such as (Eq a)!
+reduceMe :: Inst -> WhatToDo
+reduceMe inst = ReduceMe AddSCs
+
+data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
+ -- of a predicate when adding it to the avails
\end{code}
-\begin{code}
-type RedState s
- = (Avails s, -- What's available
- [Inst], -- Insts for which try_me returned Free
- [Inst] -- Insts for which try_me returned DontReduce
- )
+\begin{code}
+type Avails = FiniteMap Inst Avail
+emptyAvails = emptyFM
+
+data Avail
+ = IsFree -- Used for free Insts
+ | Irred -- Used for irreducible dictionaries,
+ -- which are going to be lambda bound
+
+ | Given TcId -- Used for dictionaries for which we have a binding
+ -- e.g. those "given" in a signature
+ Bool -- True <=> actually consumed (splittable IPs only)
+
+ | NoRhs -- Used for Insts like (CCallable f)
+ -- where no witness is required.
+ -- ToDo: remove?
+
+ | Rhs -- Used when there is a RHS
+ (LHsExpr TcId) -- The RHS
+ [Inst] -- Insts free in the RHS; we need these too
-type Avails s = FiniteMap Inst Avail
+ | Linear -- Splittable Insts only.
+ Int -- The Int is always 2 or more; indicates how
+ -- many copies are required
+ Inst -- The splitter
+ Avail -- Where the "master copy" is
-data Avail
- = Avail
- TcId -- The "main Id"; that is, the Id for the Inst that
- -- caused this avail to be put into the finite map in the first place
- -- It is this Id that is bound to the RHS.
-
- RHS -- The RHS: an expression whose value is that Inst.
- -- The main Id should be bound to this RHS
-
- [TcId] -- Extra Ids that must all be bound to the main Id.
- -- At the end we generate a list of bindings
- -- { i1 = main_id; i2 = main_id; i3 = main_id; ... }
-
-data RHS
- = NoRhs -- Used for irreducible dictionaries,
- -- which are going to be lambda bound, or for those that are
- -- suppplied as "given" when checking againgst a signature.
- --
- -- NoRhs is also used for Insts like (CCallable f)
- -- where no witness is required.
+ | LinRhss -- Splittable Insts only; this is used only internally
+ -- by extractResults, where a Linear
+ -- is turned into an LinRhss
+ [LHsExpr TcId] -- A supply of suitable RHSs
- | Rhs -- Used when there is a RHS
- TcExpr
- Bool -- True => the RHS simply selects a superclass dictionary
- -- from a subclass dictionary.
- -- False => not so.
- -- This is useful info, because superclass selection
- -- is cheaper than building the dictionary using its dfun,
- -- and we can sometimes replace the latter with the former
-
- | PassiveScSel -- Used for as-yet-unactivated RHSs. For example suppose we have
- -- an (Ord t) dictionary; then we put an (Eq t) entry in
- -- the finite map, with an PassiveScSel. Then if the
- -- the (Eq t) binding is ever *needed* we make it an Rhs
- TcExpr
- [Inst] -- List of Insts that are free in the RHS.
- -- If the main Id is subsequently needed, we toss this list into
- -- the needed-inst pool so that we make sure their bindings
- -- will actually be produced.
- --
- -- Invariant: these Insts are already in the finite mapping
-
-
-pprAvails avails = vcat (map pprAvail (eltsFM avails))
-
-pprAvail (Avail main_id rhs ids)
- = ppr main_id <> colon <+> brackets (ppr ids) <+> pprRhs rhs
+pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
+ | (inst,avail) <- fmToList avails ]
instance Outputable Avail where
ppr = pprAvail
-pprRhs NoRhs = text "<no rhs>"
-pprRhs (Rhs rhs b) = ppr rhs
-pprRhs (PassiveScSel rhs is) = text "passive" <+> ppr rhs
+pprAvail NoRhs = text "<no rhs>"
+pprAvail IsFree = text "Free"
+pprAvail Irred = text "Irred"
+pprAvail (Given x b) = text "Given" <+> ppr x <+>
+ if b then text "(used)" else empty
+pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
+pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
+pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
+\end{code}
+
+Extracting the bindings from a bunch of Avails.
+The bindings do *not* come back sorted in dependency order.
+We assume that they'll be wrapped in a big Rec, so that the
+dependency analyser can sort them out later
+
+The loop startes
+\begin{code}
+extractResults :: Avails
+ -> [Inst] -- Wanted
+ -> TcM (TcDictBinds, -- Bindings
+ [Inst], -- Irreducible ones
+ [Inst]) -- Free ones
+
+extractResults avails wanteds
+ = go avails emptyBag [] [] wanteds
+ where
+ go avails binds irreds frees []
+ = returnM (binds, irreds, frees)
+
+ go avails binds irreds frees (w:ws)
+ = case lookupFM avails w of
+ Nothing -> pprTrace "Urk: extractResults" (ppr w) $
+ go avails binds irreds frees ws
+
+ Just NoRhs -> go avails binds irreds frees ws
+ Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
+ Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
+
+ Just (Given id _) -> go avails new_binds irreds frees ws
+ where
+ new_binds | id == instToId w = binds
+ | otherwise = addBind binds w (L (instSpan w) (HsVar id))
+ -- The sought Id can be one of the givens, via a superclass chain
+ -- and then we definitely don't want to generate an x=x binding!
+
+ Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
+ where
+ new_binds = addBind binds w rhs
+
+ Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
+ -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) ->
+ split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) ->
+ go (addToFM avails w (LinRhss rhss))
+ (binds `unionBags` binds')
+ irreds' frees' (split_inst : w : ws)
+
+ Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
+ -> go new_avails new_binds irreds frees ws
+ where
+ new_binds = addBind binds w rhs
+ new_avails = addToFM avails w (LinRhss rhss)
+
+ get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
+ get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
+ returnM (w':irreds, frees, instToId w')
+ get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
+ returnM (irreds, w':frees, instToId w')
+
+ add_given avails w
+ | instBindingRequired w = addToFM avails w (Given (instToId w) True)
+ | otherwise = addToFM avails w NoRhs
+ -- NB: make sure that CCallable/CReturnable use NoRhs rather
+ -- than Given, else we end up with bogus bindings.
+
+ add_free avails w | isMethod w = avails
+ | otherwise = add_given avails w
+ -- NB: Hack alert!
+ -- Do *not* replace Free by Given if it's a method.
+ -- The following situation shows why this is bad:
+ -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
+ -- From an application (truncate f i) we get
+ -- t1 = truncate at f
+ -- t2 = t1 at i
+ -- If we have also have a second occurrence of truncate, we get
+ -- t3 = truncate at f
+ -- t4 = t3 at i
+ -- When simplifying with i,f free, we might still notice that
+ -- t1=t3; but alas, the binding for t2 (which mentions t1)
+ -- will continue to float out!
+ -- (split n i a) returns: n rhss
+ -- auxiliary bindings
+ -- 1 or 0 insts to add to irreds
+
+
+split :: Int -> TcId -> TcId -> Inst
+ -> TcM (TcDictBinds, [LHsExpr TcId])
+-- (split n split_id root_id wanted) returns
+-- * a list of 'n' expressions, all of which witness 'avail'
+-- * a bunch of auxiliary bindings to support these expressions
+-- * one or zero insts needed to witness the whole lot
+-- (maybe be zero if the initial Inst is a Given)
+--
+-- NB: 'wanted' is just a template
+
+split n split_id root_id wanted
+ = go n
+ where
+ ty = linearInstType wanted
+ pair_ty = mkTyConApp pairTyCon [ty,ty]
+ id = instToId wanted
+ occ = getOccName id
+ loc = getSrcLoc id
+ span = instSpan wanted
+
+ go 1 = returnM (emptyBag, [L span $ HsVar root_id])
+
+ go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
+ expand n rhss `thenM` \ (binds2, rhss') ->
+ returnM (binds1 `unionBags` binds2, rhss')
+
+ -- (expand n rhss)
+ -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
+ -- e.g. expand 3 [rhs1, rhs2]
+ -- = ( { x = split rhs1 },
+ -- [fst x, snd x, rhs2] )
+ expand n rhss
+ | n `rem` 2 == 0 = go rhss -- n is even
+ | otherwise = go (tail rhss) `thenM` \ (binds', rhss') ->
+ returnM (binds', head rhss : rhss')
+ where
+ go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') ->
+ returnM (listToBag binds', concat rhss')
+
+ do_one rhs = newUnique `thenM` \ uniq ->
+ tcLookupId fstName `thenM` \ fst_id ->
+ tcLookupId sndName `thenM` \ snd_id ->
+ let
+ x = mkUserLocal occ uniq pair_ty loc
+ in
+ returnM (L span (VarBind x (mk_app span split_id rhs)),
+ [mk_fs_app span fst_id ty x, mk_fs_app span snd_id ty x])
+
+mk_fs_app span id ty var = L span (HsVar id) `mkHsTyApp` [ty,ty] `mkHsApp` (L span (HsVar var))
+
+mk_app span id rhs = L span (HsApp (L span (HsVar id)) rhs)
+
+addBind binds inst rhs = binds `unionBags` unitBag (L (instLocSrcSpan (instLoc inst))
+ (VarBind (instToId inst) rhs))
+instSpan wanted = instLocSrcSpan (instLoc wanted)
\end{code}
%* *
%************************************************************************
-The main entry point for context reduction is @reduceContext@:
+When the "what to do" predicate doesn't depend on the quantified type variables,
+matters are easier. We don't need to do any zonking, unless the improvement step
+does something, in which case we zonk before iterating.
+
+The "given" set is always empty.
+
+\begin{code}
+simpleReduceLoop :: SDoc
+ -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
+ -> [Inst] -- Wanted
+ -> TcM ([Inst], -- Free
+ TcDictBinds,
+ [Inst]) -- Irreducible
+
+simpleReduceLoop doc try_me wanteds
+ = mappM zonkInst wanteds `thenM` \ wanteds' ->
+ reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
+ if no_improvement then
+ returnM (frees, binds, irreds)
+ else
+ simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
+ returnM (frees1, binds `unionBags` binds1, irreds1)
+\end{code}
+
+
\begin{code}
-reduceContext :: SDoc -> (Inst -> WhatToDo)
- -> [Inst] -- Given
- -> [Inst] -- Wanted
- -> TcM (TcDictBinds,
- [Inst], -- Free
- [Inst]) -- Irreducible
-
-reduceContext str try_me givens wanteds
- = -- Zonking first
- mapNF_Tc zonkInst givens `thenNF_Tc` \ givens ->
- mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds ->
- -- JRL - process fundeps last. We eliminate fundeps by seeing
- -- what available classes generate them, so we need to process the
- -- classes first. (would it be useful to make LIEs ordered in the first place?)
- let (wantedOther, wantedFds) = partition notFunDep wanteds
- wanteds' = wantedOther ++ wantedFds in
-
-{-
- pprTrace "reduceContext" (vcat [
+reduceContext :: SDoc
+ -> (Inst -> WhatToDo)
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM (Bool, -- True <=> improve step did no unification
+ [Inst], -- Free
+ TcDictBinds, -- Dictionary bindings
+ [Inst]) -- Irreducible
+
+reduceContext doc try_me givens wanteds
+ =
+ traceTc (text "reduceContext" <+> (vcat [
text "----------------------",
- str,
+ doc,
text "given" <+> ppr givens,
text "wanted" <+> ppr wanteds,
text "----------------------"
- ]) $
--}
+ ])) `thenM_`
+
-- Build the Avail mapping from "givens"
- foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ avails ->
+ foldlM addGiven emptyAvails givens `thenM` \ init_state ->
-- Do the real work
- reduceList (0,[]) try_me wanteds' (avails, [], []) `thenNF_Tc` \ (avails, frees, irreds) ->
+ reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
- -- Extract the bindings from avails
- let
- binds = foldFM add_bind EmptyMonoBinds avails
-
- add_bind _ (Avail main_id rhs ids) binds
- = foldr add_synonym (add_rhs_bind rhs binds) ids
- where
- add_rhs_bind (Rhs rhs _) binds = binds `AndMonoBinds` VarMonoBind main_id rhs
- add_rhs_bind other binds = binds
-
- -- Add the trivial {x = y} bindings
- -- The main Id can end up in the list when it's first added passively
- -- and then activated, so we have to filter it out. A bit of a hack.
- add_synonym id binds
- | id /= main_id = binds `AndMonoBinds` VarMonoBind id (HsVar main_id)
- | otherwise = binds
- in
-{-
- pprTrace ("reduceContext end") (vcat [
+ -- Do improvement, using everything in avails
+ -- In particular, avails includes all superclasses of everything
+ tcImprove avails `thenM` \ no_improvement ->
+
+ extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
+
+ traceTc (text "reduceContext end" <+> (vcat [
text "----------------------",
- str,
+ doc,
text "given" <+> ppr givens,
text "wanted" <+> ppr wanteds,
- text "----",
+ text "----",
text "avails" <+> pprAvails avails,
text "frees" <+> ppr frees,
- text "irreds" <+> ppr irreds,
+ text "no_improvement =" <+> ppr no_improvement,
text "----------------------"
- ]) $
--}
- returnNF_Tc (binds, frees, irreds)
+ ])) `thenM_`
+
+ returnM (no_improvement, frees, binds, irreds)
+
+-- reduceContextWithoutImprovement differs from reduceContext
+-- (a) no improvement
+-- (b) 'givens' is assumed empty
+reduceContextWithoutImprovement doc try_me wanteds
+ =
+ traceTc (text "reduceContextWithoutImprovement" <+> (vcat [
+ text "----------------------",
+ doc,
+ text "wanted" <+> ppr wanteds,
+ text "----------------------"
+ ])) `thenM_`
+
+ -- Do the real work
+ reduceList (0,[]) try_me wanteds emptyAvails `thenM` \ avails ->
+ extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
+
+ traceTc (text "reduceContextWithoutImprovement end" <+> (vcat [
+ text "----------------------",
+ doc,
+ text "wanted" <+> ppr wanteds,
+ text "----",
+ text "avails" <+> pprAvails avails,
+ text "frees" <+> ppr frees,
+ text "----------------------"
+ ])) `thenM_`
+
+ returnM (frees, binds, irreds)
+
+tcImprove :: Avails -> TcM Bool -- False <=> no change
+-- Perform improvement using all the predicates in Avails
+tcImprove avails
+ = tcGetInstEnvs `thenM` \ inst_envs ->
+ let
+ preds = [ (pred, pp_loc)
+ | (inst, avail) <- fmToList avails,
+ pred <- get_preds inst avail,
+ let pp_loc = pprInstLoc (instLoc inst)
+ ]
+ -- Avails has all the superclasses etc (good)
+ -- It also has all the intermediates of the deduction (good)
+ -- It does not have duplicates (good)
+ -- NB that (?x::t1) and (?x::t2) will be held separately in avails
+ -- so that improve will see them separate
+
+ -- For free Methods, we want to take predicates from their context,
+ -- but for Methods that have been squished their context will already
+ -- be in Avails, and we don't want duplicates. Hence this rather
+ -- horrid get_preds function
+ get_preds inst IsFree = fdPredsOfInst inst
+ get_preds inst other | isDict inst = [dictPred inst]
+ | otherwise = []
+
+ eqns = improve get_insts preds
+ get_insts clas = classInstances inst_envs clas
+ in
+ if null eqns then
+ returnM True
+ else
+ traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
+ mappM_ unify eqns `thenM_`
+ returnM False
+ where
+ unify ((qtvs, pairs), doc)
+ = addErrCtxt doc $
+ tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
+ mapM_ (unif_pr tenv) pairs
+ unif_pr tenv (ty1,ty2) = unifyTauTy (substTy tenv ty1) (substTy tenv ty2)
\end{code}
The main context-reduction function is @reduce@. Here's its game plan.
-- along with its depth
-> (Inst -> WhatToDo)
-> [Inst]
- -> RedState s
- -> TcM (RedState s)
+ -> Avails
+ -> TcM Avails
\end{code}
@reduce@ is passed
Free return this in "frees"
wanteds: The list of insts to reduce
- state: An accumulating parameter of type RedState
+ state: An accumulating parameter of type Avails
that contains the state of the algorithm
-
- It returns a RedState.
-The (n,stack) pair is just used for error reporting.
+ It returns a Avails.
+
+The (n,stack) pair is just used for error reporting.
n is always the depth of the stack.
The stack is the stack of Insts being reduced: to produce X
I had to produce Y, to produce Y I had to produce Z, and so on.
=
#ifdef DEBUG
(if n > 8 then
- pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
+ pprTrace "Interesting! Context reduction stack deeper than 8:"
+ (nest 2 (pprStack stack))
else (\x->x))
#endif
go wanteds state
where
- go [] state = returnTc state
- go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
+ go [] state = returnM state
+ go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
go ws state'
-- Base case: we're done!
-reduce stack try_me wanted state@(avails, frees, irreds)
+reduce stack try_me wanted avails
-- It's the same as an existing inst, or a superclass thereof
- | wanted `elemFM` avails
- = returnTc (activate avails wanted, frees, irreds)
+ | Just avail <- isAvailable avails wanted
+ = if isLinearInst wanted then
+ addLinearAvailable avails avail wanted `thenM` \ (avails', wanteds') ->
+ reduceList stack try_me wanteds' avails'
+ else
+ returnM avails -- No op for non-linear things
| otherwise
= case try_me wanted of {
- ReduceMe no_instance_action -> -- It should be reduced
- lookupInst wanted `thenNF_Tc` \ lookup_result ->
- case lookup_result of
- GenInst wanteds' rhs -> use_instance wanteds' rhs
- SimpleInst rhs -> use_instance [] rhs
-
- NoInstance -> -- No such instance!
- case no_instance_action of
- Stop -> failTc
- AddToIrreds -> add_to_irreds
- ;
- Free -> -- It's free and this isn't a top-level binding, so just chuck it upstairs
- -- First, see if the inst can be reduced to a constant in one step
- lookupInst wanted `thenNF_Tc` \ lookup_result ->
- case lookup_result of
- SimpleInst rhs -> use_instance [] rhs
- other -> add_to_frees
-
-
-
- ;
- FreeIfTautological -> -- It's free and this is a top level binding, so
- -- check whether it's a tautology or not
- tryTc_
- add_to_irreds -- If tautology trial fails, add to irreds
-
- -- If tautology succeeds, just add to frees
- (reduce stack try_me_taut wanted (avails, [], []) `thenTc_`
- returnTc (avails, wanted:frees, irreds))
+ KeepDictWithoutSCs -> addIrred NoSCs avails wanted
+ ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
+ -- First, see if the inst can be reduced to a constant in one step
+ try_simple (addIrred AddSCs) -- Assume want superclasses
- ;
-
- DontReduce -> add_to_irreds
- ;
+ ; Free -> -- It's free so just chuck it upstairs
+ -- First, see if the inst can be reduced to a constant in one step
+ try_simple addFree
- DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
- -- See if the inst can be reduced to a constant in one step
- lookupInst wanted `thenNF_Tc` \ lookup_result ->
+ ; ReduceMe want_scs -> -- It should be reduced
+ lookupInst wanted `thenM` \ lookup_result ->
case lookup_result of
- SimpleInst rhs -> use_instance [] rhs
- other -> add_to_irreds
+ GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 ->
+ reduceList stack try_me wanteds' avails1 `thenM` \ avails2 ->
+ addWanted want_scs avails2 wanted rhs wanteds'
+ -- Experiment with temporarily doing addIrred *before* the reduceList,
+ -- which has the effect of adding the thing we are trying
+ -- to prove to the database before trying to prove the things it
+ -- needs. See note [RECURSIVE DICTIONARIES]
+ -- NB: we must not do an addWanted before, because that adds the
+ -- superclasses too, and thaat can lead to a spurious loop; see
+ -- the examples in [SUPERCLASS-LOOP]
+ -- So we do an addIrred before, and then overwrite it afterwards with addWanted
+
+ SimpleInst rhs -> addWanted want_scs avails wanted rhs []
+
+ NoInstance -> -- No such instance!
+ -- Add it and its superclasses
+ addIrred want_scs avails wanted
}
where
- -- The three main actions
- add_to_frees = let
- avails' = addFree avails wanted
- -- Add the thing to the avails set so any identical Insts
- -- will be commoned up with it right here
- in
- returnTc (avails', wanted:frees, irreds)
-
- add_to_irreds = addGiven avails wanted `thenNF_Tc` \ avails' ->
- returnTc (avails', frees, wanted:irreds)
-
- use_instance wanteds' rhs = addWanted avails wanted rhs `thenNF_Tc` \ avails' ->
- reduceList stack try_me wanteds' (avails', frees, irreds)
-
-
- -- The try-me to use when trying to identify tautologies
- -- It blunders on reducing as much as possible
- try_me_taut inst = ReduceMe Stop -- No error recovery
+ try_simple do_this_otherwise
+ = lookupInst wanted `thenM` \ lookup_result ->
+ case lookup_result of
+ SimpleInst rhs -> addWanted AddSCs avails wanted rhs []
+ other -> do_this_otherwise avails wanted
\end{code}
\begin{code}
-activate :: Avails s -> Inst -> Avails s
- -- Activate the binding for Inst, ensuring that a binding for the
- -- wanted Inst will be generated.
- -- (Activate its parent if necessary, recursively).
- -- Precondition: the Inst is in Avails already
-
-activate avails wanted
- | not (instBindingRequired wanted)
- = avails
+-------------------------
+isAvailable :: Avails -> Inst -> Maybe Avail
+isAvailable avails wanted = lookupFM avails wanted
+ -- NB 1: the Ord instance of Inst compares by the class/type info
+ -- *not* by unique. So
+ -- d1::C Int == d2::C Int
- | otherwise
- = case lookupFM avails wanted of
+addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
+addLinearAvailable avails avail wanted
+ -- avails currently maps [wanted -> avail]
+ -- Extend avails to reflect a neeed for an extra copy of avail
- Just (Avail main_id (PassiveScSel rhs insts) ids) ->
- foldl activate avails' insts -- Activate anything it needs
- where
- avails' = addToFM avails wanted avail'
- avail' = Avail main_id (Rhs rhs True) (wanted_id : ids) -- Activate it
+ | Just avail' <- split_avail avail
+ = returnM (addToFM avails wanted avail', [])
- Just (Avail main_id other_rhs ids) -> -- Just add to the synonyms list
- addToFM avails wanted (Avail main_id other_rhs (wanted_id : ids))
+ | otherwise
+ = tcLookupId splitName `thenM` \ split_id ->
+ tcInstClassOp (instLoc wanted) split_id
+ [linearInstType wanted] `thenM` \ split_inst ->
+ returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
- Nothing -> panic "activate"
- where
- wanted_id = instToId wanted
-
-addWanted avails wanted rhs_expr
- = ASSERT( not (wanted `elemFM` avails) )
- addFunDeps (addToFM avails wanted avail) wanted
- -- NB: we don't add the thing's superclasses too!
- -- Why not? Because addWanted is used when we've successfully used an
- -- instance decl to reduce something; e.g.
- -- d:Ord [a] = dfunOrd (d1:Eq [a]) (d2:Ord a)
- -- Note that we pass the superclasses to the dfun, so they will be "wanted".
- -- If we put the superclasses of "d" in avails, then we might end up
- -- expressing "d1" in terms of "d", which would be a disaster.
where
- avail = Avail (instToId wanted) rhs []
-
- rhs | instBindingRequired wanted = Rhs rhs_expr False -- Not superclass selection
- | otherwise = NoRhs
-
-addFree :: Avails s -> Inst -> (Avails s)
+ split_avail :: Avail -> Maybe Avail
+ -- (Just av) if there's a modified version of avail that
+ -- we can use to replace avail in avails
+ -- Nothing if there isn't, so we need to create a Linear
+ split_avail (Linear n i a) = Just (Linear (n+1) i a)
+ split_avail (Given id used) | not used = Just (Given id True)
+ | otherwise = Nothing
+ split_avail Irred = Nothing
+ split_avail IsFree = Nothing
+ split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
+
+-------------------------
+addFree :: Avails -> Inst -> TcM Avails
-- When an Inst is tossed upstairs as 'free' we nevertheless add it
-- to avails, so that any other equal Insts will be commoned up right
-- here rather than also being tossed upstairs. This is really just
-- an optimisation, and perhaps it is more trouble that it is worth,
-- as the following comments show!
--
- -- NB1: do *not* add superclasses. If we have
+ -- NB: do *not* add superclasses. If we have
-- df::Floating a
-- dn::Num a
- -- but a is not bound here, then we *don't* want to derive
+ -- but a is not bound here, then we *don't* want to derive
-- dn from df here lest we lose sharing.
--
- -- NB2: do *not* add the Inst to avails at all if it's a method.
- -- The following situation shows why this is bad:
- -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
- -- From an application (truncate f i) we get
- -- t1 = truncate at f
- -- t2 = t1 at i
- -- If we have also have a secon occurrence of truncate, we get
- -- t3 = truncate at f
- -- t4 = t3 at i
- -- When simplifying with i,f free, we might still notice that
- -- t1=t3; but alas, the binding for t2 (which mentions t1)
- -- will continue to float out!
- -- Solution: never put methods in avail till they are captured
- -- in which case addFree isn't used
-addFree avails free
- | isDict free = addToFM avails free (Avail (instToId free) NoRhs [])
- | otherwise = avails
-
-addGiven :: Avails s -> Inst -> NF_TcM (Avails s)
-addGiven avails given
- = -- ASSERT( not (given `elemFM` avails) )
- -- This assertion isn't necessarily true. It's permitted
- -- to given a redundant context in a type signature (eg (Ord a, Eq a) => ...)
- -- and when typechecking instance decls we generate redundant "givens" too.
- -- addAvail avails given avail
- addAvail avails given avail `thenNF_Tc` \av ->
- zonkInst given `thenNF_Tc` \given' ->
- returnNF_Tc av
- where
- avail = Avail (instToId given) NoRhs []
+addFree avails free = returnM (addToFM avails free IsFree)
-addAvail avails wanted avail
- = addSuperClasses (addToFM avails wanted avail) wanted
-
-addSuperClasses :: Avails s -> Inst -> NF_TcM (Avails s)
- -- Add all the superclasses of the Inst to Avails
- -- Invariant: the Inst is already in Avails.
-
-addSuperClasses avails dict
- | not (isClassDict dict)
- = returnNF_Tc avails
+addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
+addWanted want_scs avails wanted rhs_expr wanteds
+ = addAvailAndSCs want_scs avails wanted avail
+ where
+ avail | instBindingRequired wanted = Rhs rhs_expr wanteds
+ | otherwise = ASSERT( null wanteds ) NoRhs
- | otherwise -- It is a dictionary
- = foldlNF_Tc add_sc avails (zipEqual "addSuperClasses" sc_theta' sc_sels) `thenNF_Tc` \ avails' ->
- addFunDeps avails' dict
+addGiven :: Avails -> Inst -> TcM Avails
+addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given) False)
+ -- Always add superclasses for 'givens'
+ --
+ -- No ASSERT( not (given `elemFM` avails) ) because in an instance
+ -- decl for Ord t we can add both Ord t and Eq t as 'givens',
+ -- so the assert isn't true
+
+addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
+addIrred want_scs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
+ addAvailAndSCs want_scs avails irred Irred
+
+addAvailAndSCs :: WantSCs -> Avails -> Inst -> Avail -> TcM Avails
+addAvailAndSCs want_scs avails inst avail
+ | not (isClassDict inst) = return avails_with_inst
+ | NoSCs <- want_scs = return avails_with_inst
+ | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
+ ; addSCs is_loop avails_with_inst inst }
+ where
+ avails_with_inst = addToFM avails inst avail
+
+ is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
+ -- Note: this compares by *type*, not by Unique
+ deps = findAllDeps (unitVarSet (instToId inst)) avail
+ dep_tys = map idType (varSetElems deps)
+
+ findAllDeps :: IdSet -> Avail -> IdSet
+ -- Find all the Insts that this one depends on
+ -- See Note [SUPERCLASS-LOOP]
+ -- Watch out, though. Since the avails may contain loops
+ -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
+ findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
+ findAllDeps so_far other = so_far
+
+ find_all :: IdSet -> Inst -> IdSet
+ find_all so_far kid
+ | kid_id `elemVarSet` so_far = so_far
+ | Just avail <- lookupFM avails kid = findAllDeps so_far' avail
+ | otherwise = so_far'
+ where
+ so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
+ kid_id = instToId kid
+
+addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
+ -- Add all the superclasses of the Inst to Avails
+ -- The first param says "dont do this because the original thing
+ -- depends on this one, so you'd build a loop"
+ -- Invariant: the Inst is already in Avails.
+
+addSCs is_loop avails dict
+ = do { sc_dicts <- newDictsAtLoc (instLoc dict) sc_theta'
+ ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
where
(clas, tys) = getDictClassTys dict
(tyvars, sc_theta, sc_sels, _) = classBigSig clas
- sc_theta' = substClasses (mkTopTyVarSubst tyvars tys) sc_theta
-
- add_sc avails ((super_clas, super_tys), sc_sel)
- = newDictFromOld dict super_clas super_tys `thenNF_Tc` \ super_dict ->
- let
- sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys)
- [instToId dict]
- in
- case lookupFM avails super_dict of
-
- Just (Avail main_id (Rhs rhs False {- not sc selection -}) ids) ->
- -- Already there, but not as a superclass selector
- -- No need to look at its superclasses; since it's there
- -- already they must be already in avails
- -- However, we must remember to activate the dictionary
- -- from which it is (now) generated
- returnNF_Tc (activate avails' dict)
- where
- avails' = addToFM avails super_dict avail
- avail = Avail main_id (Rhs sc_sel_rhs True) ids -- Superclass selection
-
- Just (Avail _ _ _) -> returnNF_Tc avails
- -- Already there; no need to do anything
-
- Nothing ->
- -- Not there at all, so add it, and its superclasses
- addAvail avails super_dict avail
- where
- avail = Avail (instToId super_dict)
- (PassiveScSel sc_sel_rhs [dict])
- []
-
-addFunDeps :: Avails s -> Inst -> NF_TcM (Avails s)
- -- Add in the functional dependencies generated by the inst
-addFunDeps avails inst
- = newFunDepFromDict inst `thenNF_Tc` \ fdInst_maybe ->
- case fdInst_maybe of
- Nothing -> returnNF_Tc avails
- Just fdInst ->
- let fdAvail = Avail (instToId (fromJust fdInst_maybe)) NoRhs [] in
- addAvail avails fdInst fdAvail
-\end{code}
-
-%************************************************************************
-%* *
-\subsection[simple]{@Simple@ versions}
-%* *
-%************************************************************************
-
-Much simpler versions when there are no bindings to make!
-
-@tcSimplifyThetas@ simplifies class-type constraints formed by
-@deriving@ declarations and when specialising instances. We are
-only interested in the simplified bunch of class/type constraints.
-
-It simplifies to constraints of the form (C a b c) where
-a,b,c are type variables. This is required for the context of
-instance declarations.
-
-\begin{code}
-tcSimplifyThetas :: ClassContext -- Wanted
- -> TcM ClassContext -- Needed
-
-tcSimplifyThetas wanteds
- = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
- reduceSimple [] wanteds `thenNF_Tc` \ irreds ->
- let
- -- For multi-param Haskell, check that the returned dictionaries
- -- don't have any of the form (C Int Bool) for which
- -- we expect an instance here
- -- For Haskell 98, check that all the constraints are of the form C a,
- -- where a is a type variable
- bad_guys | glaExts = [ct | ct@(clas,tys) <- irreds,
- isEmptyVarSet (tyVarsOfTypes tys)]
- | otherwise = [ct | ct@(clas,tys) <- irreds,
- not (all isTyVarTy tys)]
- in
- if null bad_guys then
- returnTc irreds
- else
- mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
- failTc
+ sc_theta' = substTheta (zipTopTvSubst tyvars tys) sc_theta
+
+ add_sc avails (sc_dict, sc_sel)
+ | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
+ | is_given sc_dict = return avails
+ | otherwise = addSCs is_loop avails' sc_dict
+ where
+ sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict]
+ avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict])
+
+ is_given :: Inst -> Bool
+ is_given sc_dict = case lookupFM avails sc_dict of
+ Just (Given _ _) -> True -- Given is cheaper than superclass selection
+ other -> False
\end{code}
-@tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
-used with \tr{default} declarations. We are only interested in
-whether it worked or not.
-
-\begin{code}
-tcSimplifyCheckThetas :: ClassContext -- Given
- -> ClassContext -- Wanted
- -> TcM ()
+Note [SUPERCLASS-LOOP 2]
+~~~~~~~~~~~~~~~~~~~~~~~~
+But the above isn't enough. Suppose we are *given* d1:Ord a,
+and want to deduce (d2:C [a]) where
-tcSimplifyCheckThetas givens wanteds
- = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
- if null irreds then
- returnTc ()
- else
- mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
- failTc
-\end{code}
+ class Ord a => C a where
+ instance Ord [a] => C [a] where ...
+Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
+superclasses of C [a] to avails. But we must not overwrite the binding
+for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
+build a loop!
-\begin{code}
-type AvailsSimple = FiniteMap (Class,[Type]) Bool
- -- True => irreducible
- -- False => given, or can be derived from a given or from an irreducible
+Here's another variant, immortalised in tcrun020
+ class Monad m => C1 m
+ class C1 m => C2 m x
+ instance C2 Maybe Bool
+For the instance decl we need to build (C1 Maybe), and it's no good if
+we run around and add (C2 Maybe Bool) and its superclasses to the avails
+before we search for C1 Maybe.
-reduceSimple :: ClassContext -- Given
- -> ClassContext -- Wanted
- -> NF_TcM ClassContext -- Irreducible
+Here's another example
+ class Eq b => Foo a b
+ instance Eq a => Foo [a] a
+If we are reducing
+ (Foo [t] t)
-reduceSimple givens wanteds
- = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
- returnNF_Tc [ct | (ct,True) <- fmToList givens_fm']
- where
- givens_fm = foldl addNonIrred emptyFM givens
+we'll first deduce that it holds (via the instance decl). We must not
+then overwrite the Eq t constraint with a superclass selection!
-reduce_simple :: (Int,ClassContext) -- Stack
- -> AvailsSimple
- -> ClassContext
- -> NF_TcM AvailsSimple
+At first I had a gross hack, whereby I simply did not add superclass constraints
+in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
+becuase it lost legitimate superclass sharing, and it still didn't do the job:
+I found a very obscure program (now tcrun021) in which improvement meant the
+simplifier got two bites a the cherry... so something seemed to be an Irred
+first time, but reducible next time.
-reduce_simple (n,stack) avails wanteds
- = go avails wanteds
- where
- go avails [] = returnNF_Tc avails
- go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
- go avails' ws
+Now we implement the Right Solution, which is to check for loops directly
+when adding superclasses. It's a bit like the occurs check in unification.
-reduce_simple_help stack givens wanted@(clas,tys)
- | wanted `elemFM` givens
- = returnNF_Tc givens
- | otherwise
- = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
+Note [RECURSIVE DICTIONARIES]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ data D r = ZeroD | SuccD (r (D r));
+
+ instance (Eq (r (D r))) => Eq (D r) where
+ ZeroD == ZeroD = True
+ (SuccD a) == (SuccD b) = a == b
+ _ == _ = False;
+
+ equalDC :: D [] -> D [] -> Bool;
+ equalDC = (==);
- case maybe_theta of
- Nothing -> returnNF_Tc (addIrred givens wanted)
- Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
+We need to prove (Eq (D [])). Here's how we go:
-addIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
-addIrred givens ct@(clas,tys)
- = addSCs (addToFM givens ct True) ct
+ d1 : Eq (D [])
-addNonIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
-addNonIrred givens ct@(clas,tys)
- = addSCs (addToFM givens ct False) ct
+by instance decl, holds if
+ d2 : Eq [D []]
+ where d1 = dfEqD d2
-addSCs givens ct@(clas,tys)
- = foldl add givens sc_theta
- where
- (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
- sc_theta = substClasses (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
+by instance decl of Eq, holds if
+ d3 : D []
+ where d2 = dfEqList d3
+ d1 = dfEqD d2
- add givens ct@(clas, tys)
- = case lookupFM givens ct of
- Nothing -> -- Add it and its superclasses
- addSCs (addToFM givens ct False) ct
+But now we can "tie the knot" to give
- Just True -> -- Set its flag to False; superclasses already done
- addToFM givens ct False
+ d3 = d1
+ d2 = dfEqList d3
+ d1 = dfEqD d2
- Just False -> -- Already done
- givens
-
-\end{code}
+and it'll even run! The trick is to put the thing we are trying to prove
+(in this case Eq (D []) into the database before trying to prove its
+contributing clauses.
+
%************************************************************************
%* *
-\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
+\section{tcSimplifyTop: defaulting}
%* *
%************************************************************************
-When doing a binding group, we may have @Insts@ of local functions.
-For example, we might have...
-\begin{verbatim}
-let f x = x + 1 -- orig local function (overloaded)
- f.1 = f Int -- two instances of f
- f.2 = f Float
- in
- (f.1 5, f.2 6.7)
-\end{verbatim}
-The point is: we must drop the bindings for @f.1@ and @f.2@ here,
-where @f@ is in scope; those @Insts@ must certainly not be passed
-upwards towards the top-level. If the @Insts@ were binding-ified up
-there, they would have unresolvable references to @f@.
-
-We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
-For each method @Inst@ in the @init_lie@ that mentions one of the
-@Ids@, we create a binding. We return the remaining @Insts@ (in an
-@LIE@), as well as the @HsBinds@ generated.
-\begin{code}
-bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
+@tcSimplifyTop@ is called once per module to simplify all the constant
+and ambiguous Insts.
-bindInstsOfLocalFuns init_lie local_ids
- | null overloaded_ids || null lie_for_here
- -- Common case
- = returnTc (init_lie, EmptyMonoBinds)
+We need to be careful of one case. Suppose we have
- | otherwise
- = reduceContext (text "bindInsts" <+> ppr local_ids)
- try_me [] lie_for_here `thenTc` \ (binds, frees, irreds) ->
- ASSERT( null irreds )
- returnTc (mkLIE frees `plusLIE` mkLIE lie_not_for_here, binds)
- where
- overloaded_ids = filter is_overloaded local_ids
- is_overloaded id = case splitSigmaTy (idType id) of
- (_, theta, _) -> not (null theta)
+ instance Num a => Num (Foo a b) where ...
- overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
- -- so it's worth building a set, so that
- -- lookup (in isMethodFor) is faster
+and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
+to (Num x), and default x to Int. But what about y??
- -- No sense in repeatedly zonking lots of
- -- constant constraints so filter them out here
- (lie_for_here, lie_not_for_here) = partition (isMethodFor overloaded_set)
- (lieToList init_lie)
- try_me inst | isMethodFor overloaded_set inst = ReduceMe AddToIrreds
- | otherwise = Free
-\end{code}
+It's OK: the final zonking stage should zap y to (), which is fine.
-%************************************************************************
-%* *
-\section[Disambig]{Disambiguation of overloading}
-%* *
-%************************************************************************
+\begin{code}
+tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
+tcSimplifyTop wanteds
+ = tc_simplify_top doc False {- Not interactive loop -} AddSCs wanteds
+ where
+ doc = text "tcSimplifyTop"
+
+tcSimplifyInteractive wanteds
+ = tc_simplify_top doc True {- Interactive loop -} AddSCs wanteds
+ where
+ doc = text "tcSimplifyTop"
+
+-- The TcLclEnv should be valid here, solely to improve
+-- error message generation for the monomorphism restriction
+tc_simplify_top doc is_interactive want_scs wanteds
+ = do { lcl_env <- getLclEnv
+ ; traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env))
+
+ ; let try_me inst = ReduceMe want_scs
+ ; (frees, binds, irreds) <- simpleReduceLoop doc try_me wanteds
+
+ ; let
+ -- All the non-std ones are definite errors
+ (stds, non_stds) = partition isStdClassTyVarDict irreds
+
+ -- Group by type variable
+ std_groups = equivClasses cmp_by_tyvar stds
+
+ -- Pick the ones which its worth trying to disambiguate
+ -- namely, the onese whose type variable isn't bound
+ -- up with one of the non-standard classes
+ (std_oks, std_bads) = partition worth_a_try std_groups
+ worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
+ non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
+
+ -- Collect together all the bad guys
+ bad_guys = non_stds ++ concat std_bads
+ (non_ips, bad_ips) = partition isClassDict bad_guys
+ (ambigs, no_insts) = partition isTyVarDict non_ips
+ -- If the dict has no type constructors involved, it must be ambiguous,
+ -- except I suppose that another error with fundeps maybe should have
+ -- constrained those type variables
+
+ -- Report definite errors
+ ; ASSERT( null frees )
+ groupErrs (addNoInstanceErrs Nothing []) no_insts
+ ; strangeTopIPErrs bad_ips
+
+ -- Deal with ambiguity errors, but only if
+ -- if there has not been an error so far:
+ -- errors often give rise to spurious ambiguous Insts.
+ -- For example:
+ -- f = (*) -- Monomorphic
+ -- g :: Num a => a -> a
+ -- g x = f x x
+ -- Here, we get a complaint when checking the type signature for g,
+ -- that g isn't polymorphic enough; but then we get another one when
+ -- dealing with the (Num a) context arising from f's definition;
+ -- we try to unify a with Int (to default it), but find that it's
+ -- already been unified with the rigid variable from g's type sig
+ ; binds_ambig <- ifErrsM (returnM []) $
+ do { -- Complain about the ones that don't fall under
+ -- the Haskell rules for disambiguation
+ -- This group includes both non-existent instances
+ -- e.g. Num (IO a) and Eq (Int -> Int)
+ -- and ambiguous dictionaries
+ -- e.g. Num a
+ addTopAmbigErrs ambigs
+
+ -- Disambiguate the ones that look feasible
+ ; mappM (disambigGroup is_interactive) std_oks }
+
+ ; return (binds `unionBags` unionManyBags binds_ambig) }
+
+----------------------------------
+d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
+get_tv d = case getDictClassTys d of
+ (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
+get_clas d = case getDictClassTys d of
+ (clas, [ty]) -> clas
+\end{code}
If a dictionary constrains a type variable which is
-\begin{itemize}
-\item
-not mentioned in the environment
-\item
-and not mentioned in the type of the expression
-\end{itemize}
+ * not mentioned in the environment
+ * and not mentioned in the type of the expression
then it is ambiguous. No further information will arise to instantiate
the type variable; nor will it be generalised and turned into an extra
parameter to a function.
It is an error for this to occur, except that Haskell provided for
certain rules to be applied in the special case of numeric types.
-
Specifically, if
-\begin{itemize}
-\item
-at least one of its classes is a numeric class, and
-\item
-all of its classes are numeric or standard
-\end{itemize}
+ * at least one of its classes is a numeric class, and
+ * all of its classes are numeric or standard
then the type variable can be defaulted to the first type in the
default-type list which is an instance of all the offending classes.
complains. It works by splitting the dictionary list by type
variable, and using @disambigOne@ to do the real business.
-
-@tcSimplifyTop@ is called once per module to simplify
-all the constant and ambiguous Insts.
-
-\begin{code}
-tcSimplifyTop :: LIE -> TcM TcDictBinds
-tcSimplifyTop wanted_lie
- = reduceContext (text "tcSimplTop") try_me [] wanteds `thenTc` \ (binds1, frees, irreds) ->
- ASSERT( null frees )
-
- let
- -- All the non-std ones are definite errors
- (stds, non_stds) = partition isStdClassTyVarDict irreds
-
-
- -- Group by type variable
- std_groups = equivClasses cmp_by_tyvar stds
-
- -- Pick the ones which its worth trying to disambiguate
- (std_oks, std_bads) = partition worth_a_try std_groups
- -- Have a try at disambiguation
- -- if the type variable isn't bound
- -- up with one of the non-standard classes
- worth_a_try group@(d:_) = isEmptyVarSet (tyVarsOfInst d `intersectVarSet` non_std_tyvars)
- non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
-
- -- Collect together all the bad guys
- bad_guys = non_stds ++ concat std_bads
- in
-
- -- Disambiguate the ones that look feasible
- mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
-
- -- And complain about the ones that don't
- mapNF_Tc complain bad_guys `thenNF_Tc_`
-
- returnTc (binds1 `andMonoBinds` andMonoBindList binds_ambig)
- where
- wanteds = lieToList wanted_lie
- try_me inst = ReduceMe AddToIrreds
-
- d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
-
- complain d | not (null (getIPs d)) = addTopIPErr d
- | isEmptyVarSet (tyVarsOfInst d) = addTopInstanceErr d
- | otherwise = addAmbigErr tyVarsOfInst d
-
-get_tv d = case getDictClassTys d of
- (clas, [ty]) -> getTyVar "tcSimplifyTop" ty
-get_clas d = case getDictClassTys d of
- (clas, [ty]) -> clas
-\end{code}
-
@disambigOne@ assumes that its arguments dictionaries constrain all
the same type variable.
@void@.
\begin{code}
-disambigGroup :: [Inst] -- All standard classes of form (C a)
+disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
+ -> [Inst] -- All standard classes of form (C a)
-> TcM TcDictBinds
-disambigGroup dicts
- | any isNumericClass classes -- Guaranteed all standard classes
- -- see comment at the end of function for reasons as to
- -- why the defaulting mechanism doesn't apply to groups that
- -- include CCallable or CReturnable dicts.
- && not (any isCcallishClass classes)
+disambigGroup is_interactive dicts
+ | any std_default_class classes -- Guaranteed all standard classes
= -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
-- SO, TRY DEFAULT TYPES IN ORDER
-- default list which can satisfy all the ambiguous classes.
-- For example, if Real a is reqd, but the only type in the
-- default list is Int.
- tcGetDefaultTys `thenNF_Tc` \ default_tys ->
+ get_default_tys `thenM` \ default_tys ->
let
try_default [] -- No defaults work, so fail
- = failTc
+ = failM
try_default (default_ty : default_tys)
- = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
+ = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
-- default_tys instead
- tcSimplifyCheckThetas [] thetas `thenTc` \ _ ->
- returnTc default_ty
+ tcSimplifyDefault theta `thenM` \ _ ->
+ returnM default_ty
where
- thetas = classes `zip` repeat [default_ty]
+ theta = [mkClassPred clas [default_ty] | clas <- classes]
in
- -- See if any default works, and if so bind the type variable to it
- -- If not, add an AmbigErr
- recoverTc (complain dicts `thenNF_Tc_` returnTc EmptyMonoBinds) $
-
- try_default default_tys `thenTc` \ chosen_default_ty ->
-
- -- Bind the type variable and reduce the context, for real this time
- unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
- reduceContext (text "disambig" <+> ppr dicts)
- try_me [] dicts `thenTc` \ (binds, frees, ambigs) ->
- ASSERT( null frees && null ambigs )
- warnDefault dicts chosen_default_ty `thenTc_`
- returnTc binds
-
- | all isCreturnableClass classes
- = -- Default CCall stuff to (); we don't even both to check that () is an
- -- instance of CReturnable, because we know it is.
- unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
- returnTc EmptyMonoBinds
-
- | otherwise -- No defaults
- = complain dicts `thenNF_Tc_`
- returnTc EmptyMonoBinds
+ -- See if any default works
+ tryM (try_default default_tys) `thenM` \ mb_ty ->
+ case mb_ty of
+ Left _ -> bomb_out
+ Right chosen_default_ty -> choose_default chosen_default_ty
+
+ | otherwise -- No defaults
+ = bomb_out
where
- complain = addAmbigErrs tyVarsOfInst
- try_me inst = ReduceMe AddToIrreds -- This reduce should not fail
- tyvar = get_tv (head dicts) -- Should be non-empty
- classes = map get_clas dicts
+ tyvar = get_tv (head dicts) -- Should be non-empty
+ classes = map get_clas dicts
+
+ std_default_class cls
+ = isNumericClass cls
+ || (is_interactive &&
+ classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+ -- In interactive mode, we default Show a to Show ()
+ -- to avoid graututious errors on "show []"
+
+ choose_default default_ty -- Commit to tyvar = default_ty
+ = -- Bind the type variable
+ unifyTauTy default_ty (mkTyVarTy tyvar) `thenM_`
+ -- and reduce the context, for real this time
+ simpleReduceLoop (text "disambig" <+> ppr dicts)
+ reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
+ WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
+ warnDefault dicts default_ty `thenM_`
+ returnM binds
+
+ bomb_out = addTopAmbigErrs dicts `thenM_`
+ returnM emptyBag
+
+get_default_tys
+ = do { mb_defaults <- getDefaultTys
+ ; case mb_defaults of
+ Just tys -> return tys
+ Nothing -> -- No use-supplied default;
+ -- use [Integer, Double]
+ do { integer_ty <- tcMetaTy integerTyConName
+ ; return [integer_ty, doubleTy] } }
\end{code}
[Aside - why the defaulting mechanism is turned off when
results) of a restricted set of 'native' types. This is
implemented via the help of the pseudo-type classes,
@CReturnable@ (CR) and @CCallable@ (CC.)
-
+
The interaction between the defaulting mechanism for numeric
values and CC & CR can be a bit puzzling to the user at times.
For example,
of (Integer, Double), 'x' has type Double, since Integer
is not an instance of CR. If the default list is equal to
Haskell 1.4's default-default of (Int, Double), 'x' has type
-Int.
+Int.
To try to minimise the potential for surprises here, the
defaulting mechanism is turned off in the presence of
CCallable and CReturnable.
-]
+End of aside]
+
+
+%************************************************************************
+%* *
+\subsection[simple]{@Simple@ versions}
+%* *
+%************************************************************************
+
+Much simpler versions when there are no bindings to make!
+
+@tcSimplifyThetas@ simplifies class-type constraints formed by
+@deriving@ declarations and when specialising instances. We are
+only interested in the simplified bunch of class/type constraints.
+
+It simplifies to constraints of the form (C a b c) where
+a,b,c are type variables. This is required for the context of
+instance declarations.
+
+\begin{code}
+tcSimplifyDeriv :: [TyVar]
+ -> ThetaType -- Wanted
+ -> TcM ThetaType -- Needed
+
+tcSimplifyDeriv tyvars theta
+ = tcInstTyVars tyvars `thenM` \ (tvs, _, tenv) ->
+ -- The main loop may do unification, and that may crash if
+ -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
+ -- ToDo: what if two of them do get unified?
+ newDicts DerivOrigin (substTheta tenv theta) `thenM` \ wanteds ->
+ simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
+ ASSERT( null frees ) -- reduceMe never returns Free
+
+ doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
+ let
+ tv_set = mkVarSet tvs
+
+ (bad_insts, ok_insts) = partition is_bad_inst irreds
+ is_bad_inst dict
+ = let pred = dictPred dict -- reduceMe squashes all non-dicts
+ in isEmptyVarSet (tyVarsOfPred pred)
+ -- Things like (Eq T) are bad
+ || (not undecidable_ok && not (isTyVarClassPred pred))
+ -- The returned dictionaries should be of form (C a b)
+ -- (where a, b are type variables).
+ -- We allow non-tyvar dicts if we had -fallow-undecidable-instances,
+ -- but note that risks non-termination in the 'deriving' context-inference
+ -- fixpoint loop. It is useful for situations like
+ -- data Min h a = E | M a (h a)
+ -- which gives the instance decl
+ -- instance (Eq a, Eq (h a)) => Eq (Min h a)
+
+ simpl_theta = map dictPred ok_insts
+ weird_preds = [pred | pred <- simpl_theta
+ , not (tyVarsOfPred pred `subVarSet` tv_set)]
+ -- Check for a bizarre corner case, when the derived instance decl should
+ -- have form instance C a b => D (T a) where ...
+ -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
+ -- of problems; in particular, it's hard to compare solutions for
+ -- equality when finding the fixpoint. So I just rule it out for now.
+
+ rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
+ -- This reverse-mapping is a Royal Pain,
+ -- but the result should mention TyVars not TcTyVars
+ in
+
+ addNoInstanceErrs Nothing [] bad_insts `thenM_`
+ mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_`
+ checkAmbiguity tvs simpl_theta tv_set `thenM_`
+ returnM (substTheta rev_env simpl_theta)
+ where
+ doc = ptext SLIT("deriving classes for a data type")
+\end{code}
+
+@tcSimplifyDefault@ just checks class-type constraints, essentially;
+used with \tr{default} declarations. We are only interested in
+whether it worked or not.
+
+\begin{code}
+tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
+ -> TcM ()
+
+tcSimplifyDefault theta
+ = newDicts DefaultOrigin theta `thenM` \ wanteds ->
+ simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
+ ASSERT( null frees ) -- try_me never returns Free
+ addNoInstanceErrs Nothing [] irreds `thenM_`
+ if null irreds then
+ returnM ()
+ else
+ failM
+ where
+ doc = ptext SLIT("default declaration")
+\end{code}
+
+
+%************************************************************************
+%* *
+\section{Errors and contexts}
+%* *
+%************************************************************************
-Errors and contexts
-~~~~~~~~~~~~~~~~~~~
ToDo: for these error messages, should we note the location as coming
from the insts, or just whatever seems to be around in the monad just
now?
\begin{code}
-genCantGenErr insts -- Can't generalise these Insts
- = sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
- nest 4 (pprInstsInFull insts)
- ]
-
-addAmbigErrs ambig_tv_fn dicts = mapNF_Tc (addAmbigErr ambig_tv_fn) dicts
-
-addAmbigErr ambig_tv_fn dict
- = addInstErrTcM (instLoc dict)
- (tidy_env,
- sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs,
- nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
- where
- ambig_tvs = varSetElems (ambig_tv_fn tidy_dict)
- (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
-
-warnDefault dicts default_ty
- = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn ->
- if warn then warnTc True msg else returnNF_Tc ()
+groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
+ -> [Inst] -- The offending Insts
+ -> TcM ()
+-- Group together insts with the same origin
+-- We want to report them together in error messages
+
+groupErrs report_err []
+ = returnM ()
+groupErrs report_err (inst:insts)
+ = do_one (inst:friends) `thenM_`
+ groupErrs report_err others
where
- msg | length dicts > 1
- = (ptext SLIT("Defaulting the following constraint(s) to type") <+> quotes (ppr default_ty))
- $$ pprInstsInFull tidy_dicts
- | otherwise
- = ptext SLIT("Defaulting") <+> quotes (pprInst (head tidy_dicts)) <+>
- ptext SLIT("to type") <+> quotes (ppr default_ty)
-
- (_, tidy_dicts) = mapAccumL tidyInst emptyTidyEnv dicts
-
-addTopIPErr dict
- = addInstErrTcM (instLoc dict)
- (tidy_env,
- ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
+ -- (It may seem a bit crude to compare the error messages,
+ -- but it makes sure that we combine just what the user sees,
+ -- and it avoids need equality on InstLocs.)
+ (friends, others) = partition is_friend insts
+ loc_msg = showSDoc (pprInstLoc (instLoc inst))
+ is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
+ do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
+ -- Add location and context information derived from the Insts
+
+-- Add the "arising from..." part to a message about bunch of dicts
+addInstLoc :: [Inst] -> Message -> Message
+addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
+
+plural [x] = empty
+plural xs = char 's'
+
+addTopIPErrs :: [Name] -> [Inst] -> TcM ()
+addTopIPErrs bndrs []
+ = return ()
+addTopIPErrs bndrs ips
+ = addErrTcM (tidy_env, mk_msg tidy_ips)
where
- (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
-
--- Used for top-level irreducibles
-addTopInstanceErr dict
- = addInstErrTcM (instLoc dict)
- (tidy_env,
- ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
+ (tidy_env, tidy_ips) = tidyInsts ips
+ mk_msg ips = vcat [sep [ptext SLIT("Implicit parameters escape from the monomorphic top-level binding(s) of"),
+ pprBinders bndrs <> colon],
+ nest 2 (vcat (map ppr_ip ips)),
+ monomorphism_fix]
+ ppr_ip ip = pprPred (dictPred ip) <+> pprInstLoc (instLoc ip)
+
+strangeTopIPErrs :: [Inst] -> TcM ()
+strangeTopIPErrs dicts -- Strange, becuase addTopIPErrs should have caught them all
+ = groupErrs report tidy_dicts
where
- (tidy_env, tidy_dict) = tidyInst emptyTidyEnv dict
-
--- The error message when we don't find a suitable instance
--- is complicated by the fact that sometimes this is because
--- there is no instance, and sometimes it's because there are
--- too many instances (overlap). See the comments in TcEnv.lhs
--- with the InstEnv stuff.
-addNoInstanceErr str givens dict
- = tcGetInstEnv `thenNF_Tc` \ inst_env ->
+ (tidy_env, tidy_dicts) = tidyInsts dicts
+ report dicts = addErrTcM (tidy_env, mk_msg dicts)
+ mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
+ plural tidy_dicts <+> pprDictsTheta tidy_dicts)
+
+addNoInstanceErrs :: Maybe SDoc -- Nothing => top level
+ -- Just d => d describes the construct
+ -> [Inst] -- What is given by the context or type sig
+ -> [Inst] -- What is wanted
+ -> TcM ()
+addNoInstanceErrs mb_what givens []
+ = returnM ()
+addNoInstanceErrs mb_what givens dicts
+ = -- Some of the dicts are here because there is no instances
+ -- and some because there are too many instances (overlap)
+ getDOpts `thenM` \ dflags ->
+ tcGetInstEnvs `thenM` \ inst_envs ->
let
- doc = vcat [sep [herald <+> quotes (pprInst tidy_dict),
- nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
- ambig_doc,
- ptext SLIT("Probable fix:"),
- nest 4 fix1,
- nest 4 fix2]
-
- herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
- unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
- | otherwise = empty
-
- ambig_doc
- | not ambig_overlap = empty
- | otherwise
- = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
- nest 4 (ptext SLIT("depends on the instantiation of") <+>
- quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst tidy_dict))))]
-
- fix1 = sep [ptext SLIT("Add") <+> quotes (pprInst tidy_dict),
- ptext SLIT("to the") <+> str]
-
- fix2 | isTyVarDict dict || ambig_overlap
- = empty
- | otherwise
- = ptext SLIT("Or add an instance declaration for") <+> quotes (pprInst tidy_dict)
+ (tidy_env1, tidy_givens) = tidyInsts givens
+ (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
+
+ -- Run through the dicts, generating a message for each
+ -- overlapping one, but simply accumulating all the
+ -- no-instance ones so they can be reported as a group
+ (overlap_doc, no_inst_dicts) = foldl check_overlap (empty, []) tidy_dicts
+ check_overlap (overlap_doc, no_inst_dicts) dict
+ | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts)
+ | otherwise
+ = case lookupInstEnv dflags inst_envs clas tys of
+ -- The case of exactly one match and no unifiers means
+ -- a successful lookup. That can't happen here, becuase
+ -- dicts only end up here if they didn't match in Inst.lookupInst
+#ifdef DEBUG
+ ([m],[]) -> pprPanic "addNoInstanceErrs" (ppr dict)
+#endif
+ ([], _) -> (overlap_doc, dict : no_inst_dicts) -- No match
+ res -> (mk_overlap_msg dict res $$ overlap_doc, no_inst_dicts)
+ where
+ (clas,tys) = getDictClassTys dict
+ in
+
+ -- Now generate a good message for the no-instance bunch
+ mk_probable_fix tidy_env2 no_inst_dicts `thenM` \ (tidy_env3, probable_fix) ->
+ let
+ no_inst_doc | null no_inst_dicts = empty
+ | otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix]
+ heading | null givens = ptext SLIT("No instance") <> plural no_inst_dicts <+>
+ ptext SLIT("for") <+> pprDictsTheta no_inst_dicts
+ | otherwise = sep [ptext SLIT("Could not deduce") <+> pprDictsTheta no_inst_dicts,
+ nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta tidy_givens]
+ in
+ -- And emit both the non-instance and overlap messages
+ addErrTcM (tidy_env3, no_inst_doc $$ overlap_doc)
+ where
+ mk_overlap_msg dict (matches, unifiers)
+ = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
+ <+> pprPred (dictPred dict))),
+ sep [ptext SLIT("Matching instances") <> colon,
+ nest 2 (vcat [pprDFuns dfuns, pprDFuns unifiers])],
+ ASSERT( not (null matches) )
+ if not (isSingleton matches)
+ then -- Two or more matches
+ empty
+ else -- One match, plus some unifiers
+ ASSERT( not (null unifiers) )
+ parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
+ quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
+ ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])]
+ where
+ dfuns = [df | (_, (_,_,df)) <- matches]
+
+ mk_probable_fix tidy_env dicts
+ = returnM (tidy_env, sep [ptext SLIT("Probable fix:"), nest 2 (vcat fixes)])
+ where
+ fixes = add_ors (fix1 ++ fix2)
+
+ fix1 = case mb_what of
+ Nothing -> [] -- Top level
+ Just what -> -- Nested (type signatures, instance decls)
+ [ sep [ ptext SLIT("add") <+> pprDictsTheta dicts,
+ ptext SLIT("to the") <+> what] ]
+
+ fix2 | null instance_dicts = []
+ | otherwise = [ ptext SLIT("add an instance declaration for")
+ <+> pprDictsTheta instance_dicts ]
+ instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
+ -- Insts for which it is worth suggesting an adding an instance declaration
+ -- Exclude implicit parameters, and tyvar dicts
+
+ add_ors :: [SDoc] -> [SDoc] -- The empty case should not happen
+ add_ors [] = [ptext SLIT("[No suggested fixes]")] -- Strange
+ add_ors (f1:fs) = f1 : map (ptext SLIT("or") <+>) fs
+
+addTopAmbigErrs dicts
+-- Divide into groups that share a common set of ambiguous tyvars
+ = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
+ where
+ (tidy_env, tidy_dicts) = tidyInsts dicts
+
+ tvs_of :: Inst -> [TcTyVar]
+ tvs_of d = varSetElems (tyVarsOfInst d)
+ cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
- (tidy_env, tidy_dict:tidy_givens) = tidyInsts emptyTidyEnv (dict:givens)
+ report :: [(Inst,[TcTyVar])] -> TcM ()
+ report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
+ = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) ->
+ setSrcSpan (instLocSrcSpan (instLoc inst)) $
+ -- the location of the first one will do for the err message
+ addErrTcM (tidy_env, msg $$ mono_msg)
+ where
+ dicts = map fst pairs
+ msg = sep [text "Ambiguous type variable" <> plural tvs <+>
+ pprQuotedList tvs <+> in_msg,
+ nest 2 (pprDictsInFull dicts)]
+ in_msg = text "in the constraint" <> plural dicts <> colon
+
+
+mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
+-- There's an error with these Insts; if they have free type variables
+-- it's probably caused by the monomorphism restriction.
+-- Try to identify the offending variable
+-- ASSUMPTION: the Insts are fully zonked
+mkMonomorphismMsg tidy_env inst_tvs
+ = findGlobals (mkVarSet inst_tvs) tidy_env `thenM` \ (tidy_env, docs) ->
+ returnM (tidy_env, mk_msg docs)
+ where
+ mk_msg [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
+ -- This happens in things like
+ -- f x = show (read "foo")
+ -- whre monomorphism doesn't play any role
+ mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
+ nest 2 (vcat docs),
+ monomorphism_fix
+ ]
+monomorphism_fix :: SDoc
+monomorphism_fix = ptext SLIT("Probable fix:") <+>
+ (ptext SLIT("give these definition(s) an explicit type signature")
+ $$ ptext SLIT("or use -fno-monomorphism-restriction"))
- -- Checks for the ambiguous case when we have overlapping instances
- ambig_overlap | isClassDict dict
- = case lookupInstEnv inst_env clas tys of
- NoMatch ambig -> ambig
- other -> False
- | otherwise = False
- where
- (clas,tys) = getDictClassTys dict
- in
- addInstErrTcM (instLoc dict) (tidy_env, doc)
+warnDefault dicts default_ty
+ = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
+ addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
+ where
+ -- Tidy them first
+ (_, tidy_dicts) = tidyInsts dicts
+ warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
+ quotes (ppr default_ty),
+ pprDictsInFull tidy_dicts]
-- Used for the ...Thetas variants; all top level
-addNoInstErr (c,ts)
- = addErrTc (ptext SLIT("No instance for") <+> quotes (pprConstraint c ts))
+badDerivedPred pred
+ = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
+ ptext SLIT("type variables that are not data type parameters"),
+ nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
reduceDepthErr n stack
= vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
- nest 4 (pprInstsInFull stack)]
+ nest 4 (pprStack stack)]
-reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)
+pprStack stack = vcat (map pprInstInFull stack)
\end{code}
projects / ghc-hetmet.git / blobdiff