%
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1995
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[TcSimplify]{TcSimplify}
-\begin{code}
-#include "HsVersions.h"
+
+\begin{code}
module TcSimplify (
- tcSimplify, tcSimplifyAndCheck,
- tcSimplifyTop, tcSimplifyThetas, tcSimplifyCheckThetas, tcSimplifyRank2,
+ tcSimplifyInfer, tcSimplifyInferCheck,
+ tcSimplifyCheck, tcSimplifyRestricted,
+ tcSimplifyToDicts, tcSimplifyIPs,
+ tcSimplifyTop, tcSimplifyInteractive,
+ tcSimplifyBracket,
+
+ tcSimplifyDeriv, tcSimplifyDefault,
bindInstsOfLocalFuns
) where
-IMPORT_Trace -- ToDo: rm (debugging)
-import Outputable
-import Pretty
-
-import TcMonad -- typechecking monadic machinery
-import TcMonadFns ( newDicts, applyTcSubstAndExpectTyVars )
-import AbsSyn -- the stuff being typechecked
+#include "HsVersions.h"
-import AbsUniType ( isSuperClassOf, getTyVar, eqTyVar, ltTyVar,
- instantiateThetaTy, isFunType, getUniDataTyCon,
- getSuperDictSelId, InstTyEnv(..)
- IF_ATTACK_PRAGMAS(COMMA isTyVarTy COMMA pprUniType)
- IF_ATTACK_PRAGMAS(COMMA assocMaybe)
- )
-import UniType ( UniType(..) ) -- ******* CHEATING ************
-import Disambig ( disambiguateDicts )
-import Errors ( reduceErr, genCantGenErr, Error(..) )
-import Id ( mkInstId )
-import Inst ( extractTyVarsFromInst, isTyVarDict, matchesInst,
- instBindingRequired, instCanBeGeneralised,
- Inst(..), -- We import the CONCRETE type, because
- -- TcSimplify is allowed to see the rep
- -- of Insts
- InstOrigin, OverloadedLit, InstTemplate
+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,
+ newDictsFromOld, tcInstClassOp,
+ getDictClassTys, isTyVarDict,
+ instLoc, zonkInst, tidyInsts, tidyMoreInsts,
+ Inst, pprInsts, pprDictsInFull, pprInstInFull, tcGetInstEnvs,
+ isInheritableInst, pprDFuns, pprDictsTheta
)
-import InstEnv
-import LIE
-import ListSetOps ( minusList )
-import Maybes ( catMaybes, maybeToBool, Maybe(..) )
-import Util
+import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals, pprBinders )
+import InstEnv ( lookupInstEnv, classInstances )
+import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
+import TcType ( TcTyVar, TcTyVarSet, ThetaType,
+ mkClassPred, isOverloadedTy, mkTyConApp,
+ mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
+ tyVarsOfPred, tcEqType, pprPred )
+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[tcSimplify-main]{Main entry function}
+\subsection{NOTES}
%* *
%************************************************************************
-* May modify the substitution to bind ambiguous type variables.
+ --------------------------------------
+ Notes on functional dependencies (a bug)
+ --------------------------------------
-Specification
-~~~~~~~~~~~~~
-(1) If an inst constrains only ``global'' type variables, (or none),
- return it as a ``global'' inst.
+| > 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...
-OTHERWISE
+The trouble is that to type (runFoo foo), GHC has to solve the problem:
-(2) Simplify it repeatedly (checking for (1) of course) until it is a dict
- constraining only a type variable.
+ Given constraint Foo a b
+ Solve constraint Foo a b'
-(3) If it constrains a ``local'' type variable, return it as a ``local'' inst.
- Otherwise it must be ambiguous, so try to resolve the ambiguity.
+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.
-\begin{code}
-tcSimpl :: Bool -- True <=> Don't simplify const insts
- -> [TyVar] -- ``Global'' type variables
- -> [TyVar] -- ``Local'' type variables
- -> [Inst] -- Given; these constrain only local tyvars
- -> [Inst] -- Wanted
- -> TcM ([Inst], -- Free
- [(Inst,TypecheckedExpr)],-- Bindings
- [Inst]) -- Remaining wanteds; no dups
-
-tcSimpl dont_squash_consts global_tvs local_tvs givens wanteds
- =
- -- Make sure the insts and type variables are fixed points of the substitution
- applyTcSubstAndExpectTyVars global_tvs `thenNF_Tc` \ global_tvs ->
- applyTcSubstAndExpectTyVars local_tvs `thenNF_Tc` \ local_tvs ->
- applyTcSubstToInsts givens `thenNF_Tc` \ givens ->
- applyTcSubstToInsts wanteds `thenNF_Tc` \ wanteds ->
- let
- is_elem1 = isIn "tcSimpl1"
- is_elem2 = isIn "tcSimpl2"
- in
- -- Deal with duplicates and type constructors
- elimTyCons
- dont_squash_consts (\tv -> tv `is_elem1` global_tvs)
- givens wanteds `thenTc` \ (globals, tycon_binds, locals_and_ambigs) ->
+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.
- -- Now disambiguate if necessary
- let
- (ambigs, unambigs) = partition (is_ambiguous local_tvs) locals_and_ambigs
- (locals, cant_generalise) = partition instCanBeGeneralised unambigs
- in
- checkTc (not (null cant_generalise)) (genCantGenErr cant_generalise) `thenTc_`
- (if (null ambigs) then
- -- No ambiguous dictionaries. Just bash on with the results
- -- of the elimTyCons
- returnTc (globals, tycon_binds, locals_and_ambigs)
+ --------------------------------------
+ Notes on quantification
+ --------------------------------------
- else
+Suppose we are about to do a generalisation step.
+We have in our hand
- -- Some ambiguous dictionaries. We now disambiguate them,
- -- which binds the offending type variables to suitable types in the
- -- substitution, and then we retry the whole process. This
- -- time there won't be any ambiguous ones.
- -- There's no need to back-substitute on global and local tvs,
- -- because the ambiguous type variables can't be in either.
+ G the environment
+ T the type of the RHS
+ C the constraints from that RHS
- -- Why do we retry the whole process? Because binding a type variable
- -- to a particular type might enable a short-cut simplification which
- -- elimTyCons will have missed the first time.
+The game is to figure out
- disambiguateDicts ambigs `thenTc_`
- applyTcSubstToInsts givens `thenNF_Tc` \ givens ->
- applyTcSubstToInsts wanteds `thenNF_Tc` \ wanteds ->
- elimTyCons
- dont_squash_consts (\tv -> tv `is_elem2` global_tvs)
- givens wanteds
+ 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
- ) {- End of the "if" -} `thenTc` \ (globals, tycon_binds, locals) ->
+So we're going to infer the type
- -- Deal with superclass relationships
- elimSCs givens locals `thenNF_Tc` \ (sc_binds, locals2) ->
+ forall Q. Cq => T
- -- Finished
- returnTc (globals, sc_binds ++ tycon_binds, locals2)
- where
- is_ambiguous local_tvs (Dict _ _ ty _)
- = getTyVar "is_ambiguous" ty `not_elem` local_tvs
- where
- not_elem = isn'tIn "is_ambiguous"
-\end{code}
+and float the constraints Ct further outwards.
-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.
+Here are the things that *must* be true:
-\begin{code}
-tcSimplify
- :: Bool -- True <=> top level
- -> [TyVar] -- ``Global'' type variables
- -> [TyVar] -- ``Local'' type variables
- -> [Inst] -- Wanted
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)],-- Bindings
- [Inst]) -- Remaining wanteds; no dups
-
-tcSimplify top_level global_tvs local_tvs wanteds
- = tcSimpl (not top_level) global_tvs local_tvs [] wanteds
-\end{code}
+ (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
-@tcSimplifyAndCheck@ is similar to the above, except that it checks
-that there is an empty wanted-set at the end.
+(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.
-It may still return some of constant insts, which have
-to be resolved finally at the end.
+ BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
-\begin{code}
-tcSimplifyAndCheck
- :: Bool -- True <=> top level
- -> [TyVar] -- ``Global'' type variables
- -> [TyVar] -- ``Local'' type variables
- -> [Inst] -- Given
- -> [Inst] -- Wanted
- -> UnifyErrContext -- Context info for error
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)]) -- Bindings
-
-tcSimplifyAndCheck top_level global_tvs local_tvs givens wanteds err_ctxt
- = tcSimpl (not top_level) global_tvs local_tvs givens wanteds
- `thenTc` \ (free_insts, binds, wanteds') ->
- checkTc (not (null wanteds')) (reduceErr wanteds' err_ctxt)
- `thenTc_`
- returnTc (free_insts, binds)
-\end{code}
+Example: class H x y | x->y where ...
-@tcSimplifyRank2@ checks that the argument of a rank-2 polymorphic function
-is not overloaded.
+ fv(G) = {a} C = {H a b, H c d}
+ T = c -> b
-\begin{code}
-tcSimplifyRank2 :: [TyVar] -- ``Local'' type variables; guaranteed fixpoint of subst
- -> [Inst] -- Given
- -> UnifyErrContext
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)]) -- Bindings
-
-tcSimplifyRank2 local_tvs givens err_ctxt
- = applyTcSubstToInsts givens `thenNF_Tc` \ givens' ->
- elimTyCons False
- (\tv -> not (tv `is_elem` local_tvs))
- -- This predicate claims that all
- -- any non-local tyvars are global,
- -- thereby postponing dealing with
- -- ambiguity until the enclosing Gen
- [] givens' `thenTc` \ (free, dict_binds, wanteds) ->
-
- checkTc (not (null wanteds)) (reduceErr wanteds err_ctxt) `thenTc_`
-
- returnTc (free, dict_binds)
- where
- is_elem = isIn "tcSimplifyRank2"
-\end{code}
+ (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.
+
+
+
+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.
+
+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)
+
+(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.
+
+Notice that we union before calling oclose. Here's an example:
+
+ class J a b c | a b -> c
+ fv(G) = {a}
+
+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.
+
+
+Can we ever be *certain* about ambiguity? Yes: if there's a constraint
+
+ c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
+
+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?
+
+ 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
+
+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
+
+
+ --------------------------------------
+ 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:
+
+ f x = (x::Int) + ?y
+
+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:
+
+ f :: Int -> Int
+
+(so we get ?y from the context of f's definition), or
+
+ f :: (?y::Int) => Int -> Int
+
+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.
+
+BOTTOM LINE: when *inferring types* you *must* quantify
+over implicit parameters. See the predicate isFreeWhenInferring.
+
+
+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:
+
+ (?x + 1) :: Int
+
+This is perfectly reasonable. We do not want to insist on
+
+ (?x + 1) :: (?x::Int => Int)
+
+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:
+
+ f :: Int -> Int
+ f x = (x::Int) + ?y
+
+At first sight this seems reasonable, but it has the nasty property
+that adding a type signature changes the dynamic semantics.
+Consider this:
+
+ (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
+
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyInfer}
+%* *
+%************************************************************************
+
+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.
-@tcSimplifyTop@ deals with constant @Insts@, using the standard simplification
-mechansim with the extra flag to say ``beat out constant insts''.
\begin{code}
-tcSimplifyTop :: [Inst] -> TcM [(Inst, TypecheckedExpr)]
-tcSimplifyTop dicts
- = tcSimpl False [] [] [] dicts `thenTc` \ (_, binds, _) ->
- returnTc binds
+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}
-@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.
\begin{code}
-tcSimplifyThetas :: (Class -> TauType -> InstOrigin) -- Creates an origin for the dummy dicts
- -> [(Class, TauType)] -- Simplify this
- -> TcM [(Class, TauType)] -- Result
+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
+ preds = fdPredsOfInsts wanteds'
+ qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
-tcSimplifyThetas mk_inst_origin theta
- = let
- dicts = map mk_dummy_dict theta
+ try_me inst
+ | isFreeWhenInferring qtvs inst = Free
+ | isClassDict inst = DontReduceUnlessConstant -- Dicts
+ | otherwise = ReduceMe -- Lits and Methods
in
- -- Do the business (this is just the heart of "tcSimpl")
- elimTyCons False (\tv -> False) [] dicts `thenTc` \ (_, _, dicts2) ->
+ 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}
- -- Deal with superclass relationships
- elimSCs [] dicts2 `thenNF_Tc` \ (_, dicts3) ->
+Example [LOOP]
- returnTc (map unmk_dummy_dict dicts3)
- where
- mk_dummy_dict (clas, ty)
- = Dict uniq clas ty (mk_inst_origin clas ty)
+ 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
- uniq = panic "tcSimplifyThetas:uniq"
+Wanted: Max Z (S x) y
- unmk_dummy_dict (Dict _ clas ty _) = (clas, ty)
-\end{code}
+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)!
-@tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
-used with \tr{default} declarations. We are only interested in
-whether it worked or not.
+[NO TYVARS]
-\begin{code}
-tcSimplifyCheckThetas :: InstOrigin -- context; for error msg
- -> [(Class, TauType)] -- Simplify this
- -> TcM ()
+ 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
-tcSimplifyCheckThetas origin theta
- = let
- dicts = map mk_dummy_dict theta
- in
- -- Do the business (this is just the heart of "tcSimpl")
- elimTyCons False (\tv -> False) [] dicts `thenTc` \ _ ->
+ g :: Num a => [X a] -> [X a]
+ g xs = h xs
+ where
+ h ys = ys ++ map (k (y [[0]])) xs
- returnTc ()
- where
- mk_dummy_dict (clas, ty)
- = Dict uniq clas ty origin
+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.
+
+The net effect of [NO TYVARS]
- uniq = panic "tcSimplifyCheckThetas:uniq"
+\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[elimTyCons]{@elimTyCons@}
+\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}
-elimTyCons :: Bool -- True <=> Don't simplify const insts
- -> (TyVar -> Bool) -- Free tyvar predicate
- -> [Inst] -- Given
- -> [Inst] -- Wanted
- -> TcM ([Inst], -- Free
- [(Inst, TypecheckedExpr)], -- Bindings
- [Inst] -- Remaining wanteds; no dups;
- -- dicts only (no Methods)
- )
+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
+ = tcSimplCheck doc get_qtvs
+ givens wanted_lie `thenM` \ (qtvs', binds) ->
+ returnM binds
+ where
+-- get_qtvs = zonkTcTyVarsAndFV qtvs
+ get_qtvs = return (mkVarSet qtvs)
+
+
+-- 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
+ = tcSimplCheck doc get_qtvs givens wanted_lie
+ where
+ -- 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}
+
+Here is the workhorse function for all three wrappers.
+
+\begin{code}
+tcSimplCheck doc get_qtvs givens wanted_lie
+ = check_loop givens wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
+
+ -- Complain about any irreducible ones
+ mappM zonkInst given_dicts_and_ips `thenM` \ givens' ->
+ groupErrs (addNoInstanceErrs (Just doc) givens') irreds `thenM_`
+
+ -- Done
+ extendLIEs frees `thenM_`
+ returnM (qtvs, 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
+ 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}
-The bindings returned may mention any or all of ``givens'', so the
-order in which the generated binds are put together is {\em tricky}.
-Case~4 of @try@ is the general case to see.
-When we do @eTC givens (wanted:wanteds)@ [some details omitted], we...
+%************************************************************************
+%* *
+\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.
- (1) first look up @wanted@; this gives us one binding to heave in:
- wanted = rhs
+ 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.
- (2) step (1) also gave us some @simpler_wanteds@; we simplify
- these and get some (simpler-wanted-)bindings {\em that must be
- in scope} for the @wanted=rhs@ binding above!
+ Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
+ the problem.
- (3) we simplify the remaining @wanteds@ (recursive call), giving
- us yet more bindings.
-The final arrangement of the {\em non-recursive} bindings is
+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.
- let <simpler-wanted-binds> in
- let wanted = rhs in
- let <yet-more-bindings> ...
\begin{code}
-elimTyCons dont_squash_consts is_free_tv givens wanteds
- = eTC givens wanteds
- where
- eTC :: [Inst] -> [Inst]
- -> TcM ([Inst], [(Inst, TypecheckedExpr)], [Inst])
-
- eTC _ [] = returnTc ([], [], [])
-
- eTC givens (wanted:wanteds) = try givens wanted wanteds
- (extractTyVarsFromInst wanted)
- (find_equiv givens wanted)
- -- find_equiv looks in "givens" for an inst equivalent to "wanted"
- -- This is used only in Case 2 below; it's like a guard which also
- -- returns a result.
-
- try :: [Inst] -> Inst -> [Inst] -> [TyVar] -> (Maybe Inst)
- -> TcM ([Inst], [(Inst, TypecheckedExpr)], [Inst])
-
- -- Case 0: same as existing dict, so build a simple binding
- try givens wanted wanteds tvs_of_wanted (Just this)
- = eTC givens wanteds `thenTc` \ (frees, binds, wanteds') ->
- let
- -- Create a new binding iff it's needed
- new_binds | instBindingRequired wanted = (wanted, Var (mkInstId this)):binds
- | otherwise = binds
- in
- returnTc (frees, new_binds, wanteds')
-
- -- Case 1: constrains no type variables at all
- -- In this case we have a quick go to see if it has an
- -- instance which requires no inputs (ie a constant); if so we use
- -- it; if not, we give up on the instance and just heave it out the
- -- top in the free result
- try givens wanted wanteds tvs_of_wanted _ | null tvs_of_wanted
- = simplify_it dont_squash_consts {- If dont_squash_consts is true,
- simplify only if trival -}
- givens wanted wanteds
-
- -- Case 2: constrains free vars only, so fling it out the top in free_ids
- try givens wanted wanteds tvs_of_wanted _
- | all is_free_tv tvs_of_wanted
- = eTC (wanted:givens) wanteds `thenTc` \ (frees, binds, wanteds') ->
- returnTc (wanted:frees, binds, wanteds')
-
- -- Case 3: is a dict constraining only a tyvar,
- -- so return it as part of the "wanteds" result
- try givens wanted wanteds tvs_of_wanted _
- | isTyVarDict wanted
- = eTC (wanted:givens) wanteds `thenTc` \ (frees, binds, wanteds') ->
- returnTc (frees, binds, wanted:wanteds')
-
- -- Case 4: is not a simple dict, so look up in instance environment
- try givens wanted wanteds tvs_of_wanted _
- = simplify_it False {- Simplify even if not trivial -}
- givens wanted wanteds
-
- simplify_it only_if_trivial givens wanted wanteds
- = if not (instBindingRequired wanted) then
- -- No binding required for this chap, so squash right away
- lookupNoBindInst_Tc wanted `thenTc` \ simpler_wanteds ->
-
- eTC givens simpler_wanteds `thenTc` \ (frees1, binds1, wanteds1) ->
- let
- new_givens = [new_given | (new_given,rhs) <- binds1]
- -- Typically binds1 is empty
- in
- eTC givens wanteds `thenTc` \ (frees2, binds2, wanteds2) ->
-
- returnTc (frees1 ++ frees2,
- binds1 ++ binds2,
- wanteds1 ++ wanteds2)
-
- else -- An binding is required for this inst
- lookupInst_Tc wanted `thenTc` \ (rhs, simpler_wanteds) ->
-
- if (only_if_trivial && not_var rhs) then
- -- Ho ho! It isn't trivial to simplify "wanted",
- -- because the rhs isn't a simple variable. The flag
- -- dont_squash_consts tells us to give up now and
- -- just fling it out the top.
- eTC (wanted:givens) wanteds `thenTc` \ (frees, binds, wanteds') ->
- returnTc (wanted:frees, binds, wanteds')
- else
- -- Aha! Either it's easy, or dont_squash_consts is
- -- False, so we must do it right here.
-
- eTC givens simpler_wanteds `thenTc` \ (frees1, binds1, wanteds1) ->
- let
- new_givens = [new_given | (new_given,rhs) <- binds1]
- in
- eTC (new_givens ++ [wanted] ++ wanteds1 ++ givens) wanteds
- `thenTc` \ (frees2, binds2, wanteds2) ->
- returnTc (frees1 ++ frees2,
- binds1 ++ [(wanted, rhs)] ++ binds2,
- wanteds1 ++ wanteds2)
- where
- not_var :: TypecheckedExpr -> Bool
- not_var (Var _) = False
- not_var other = True
-
- find_equiv :: [Inst] -> Inst -> Maybe Inst
- -- Look through the argument list for an inst which is
- -- equivalent to the second arg.
-
- find_equiv [] wanted = Nothing
- find_equiv (given:givens) wanted
- | wanted `matchesInst` given = Just given
- | otherwise = find_equiv givens wanted
+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
+ 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[elimSCs]{@elimSCs@}
+\subsection{tcSimplifyToDicts}
%* *
%************************************************************************
-\begin{code}
-elimSCs :: [Inst] -- Given; no dups
- -> [Inst] -- Wanted; no dups; all dictionaries, all
- -- constraining just a type variable
- -> NF_TcM ([(Inst,TypecheckedExpr)], -- Bindings
- [Inst]) -- Minimal wanted set
+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.
-elimSCs givens wanteds
- = -- Sort the wanteds so that subclasses occur before superclasses
- elimSCs_help
- [dict | dict@(Dict _ _ _ _) <- givens] -- Filter out non-dictionaries
- (sortSC wanteds)
+The same thing is used for specialise pragmas. Consider
-elimSCs_help :: [Inst] -- Given; no dups
- -> [Inst] -- Wanted; no dups;
- -> NF_TcM ([(Inst,TypecheckedExpr)],-- Bindings
- [Inst]) -- Minimal wanted set
+ f :: Num a => a -> a
+ {-# SPECIALISE f :: Int -> Int #-}
+ f = ...
-elimSCs_help given [] = returnNF_Tc ([], [])
+The type checker generates a binding like:
-elimSCs_help givens (wanted@(Dict _ wanted_class wanted_ty wanted_orig):wanteds)
- = case (trySC givens wanted_class wanted_ty) of
+ f_spec = (f :: Int -> Int)
- Nothing -> -- No superclass relnship found
- elimSCs_help (wanted:givens) wanteds `thenNF_Tc` \ (binds, wanteds') ->
- returnNF_Tc (binds, wanted:wanteds')
+and we want to end up with
- Just (given, classes) -> -- Aha! There's a superclass relnship
+ f_spec = _inline_me_ (f Int dNumInt)
- -- Build intermediate dictionaries
- let
- theta = [ (clas, wanted_ty) | clas <- classes ]
- in
- newDicts wanted_orig theta `thenNF_Tc` \ intermediates ->
+But that means that we must simplify the Method for f to (f Int dNumInt)!
+So tcSimplifyToDicts squeezes out all Methods.
- -- Deal with the recursive call
- elimSCs_help (wanted : (intermediates ++ givens)) wanteds
- `thenNF_Tc` \ (binds, wanteds') ->
+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 matching. Instead we want
+
+ forall dIntegralInt, dNumInt.
+ fromIntegral Int Int dIntegralInt dNumInt = id Int
+
+Hence "WithoutSCs"
+
+\begin{code}
+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 )
+ extendLIEs irreds `thenM_`
+ returnM binds
- -- Create bindings for the wanted dictionary and the intermediates.
- -- Later binds may depend on earlier ones, so each new binding is pushed
- -- on the front of the accumulating parameter list of bindings
- let
- new_binds = mk_binds wanted wanted_class (intermediates ++ [given]) []
- in
- returnNF_Tc (new_binds ++ binds, wanteds')
where
- mk_binds :: Inst -- Define this
- -> Class -- ...whose class is this
- -> [Inst] -- In terms of this sub-class chain
- -> [(Inst, TypecheckedExpr)] -- Push the binding on front of these
- -> [(Inst, TypecheckedExpr)]
-
- mk_binds dict clas [] binds_so_far = binds_so_far
- mk_binds dict clas (dict_sub@(Dict _ dict_sub_class ty _):dicts_sub) binds_so_far
- = mk_binds dict_sub dict_sub_class dicts_sub (new_bind:binds_so_far)
- where
- new_bind = (dict, DictApp (TyApp (Var (getSuperDictSelId dict_sub_class clas))
- [ty])
- [mkInstId dict_sub])
-
-
-trySC :: [Inst] -- Givens
- -> Class -> UniType -- Wanted
- -> Maybe (Inst, [Class]) -- Nothing if no link; Just (given, classes)
- -- if wanted can be given in terms of given, with
- -- intermediate classes specified
-trySC givens wanted_class wanted_ty
- = case subclass_relns of
- [] -> Nothing
- ((given, classes, _): _) -> Just (given, classes)
+ doc = text "tcSimplifyToDicts"
+
+ -- Reduce methods and lits only; stop as soon as we get a dictionary
+ try_me inst | isDict inst = KeepDictWithoutSCs -- See notes above re "WithoutSCs"
+ | otherwise = ReduceMe
+\end{code}
+
+
+
+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}
+tcSimplifyBracket :: [Inst] -> TcM ()
+tcSimplifyBracket wanteds
+ = simpleReduceLoop doc reduceMe wanteds `thenM_`
+ returnM ()
where
- subclass_relns :: [(Inst, [Class], Int)] -- Subclass of wanted,
- -- intervening classes,
- -- and number of intervening classes
- -- Sorted with shortest link first
- subclass_relns = sortLt reln_lt (catMaybes (map find_subclass_reln givens))
+ doc = text "tcSimplifyBracket"
+\end{code}
- reln_lt :: (Inst, [Class], Int) -> (Inst, [Class], Int) -> Bool
- (_,_,n1) `reln_lt` (_,_,n2) = n1 < n2
- find_subclass_reln given@(Dict _ given_class given_ty _)
- | wanted_ty == given_ty
- = case (wanted_class `isSuperClassOf` given_class) of
+%************************************************************************
+%* *
+\subsection{Filtering at a dynamic binding}
+%* *
+%************************************************************************
- Just classes -> Just (given,
- classes,
- length classes)
+When we have
+ let ?x = R in B
- Nothing -> Nothing
+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.
- | otherwise = Nothing
+Actually, the constraints from B might improve the types in ?x. For example
+ f :: (?x::Int) => Char -> Char
+ let ?x = 3 in f 'c'
-sortSC :: [Inst] -- Expected to be all dicts (no MethodIds), all of
- -- which constrain type variables
- -> [Inst] -- Sorted with subclasses before superclasses
+then the constraint (?x::Int) arising from the call to f will
+force the binding for ?x to be of type Int.
-sortSC dicts = sortLt lt dicts
+\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
- (Dict _ c1 ty1 _) `lt` (Dict _ c2 ty2 _)
- = tv1 `ltTyVar` tv2 ||
- (tv1 `eqTyVar` tv2 && maybeToBool (c2 `isSuperClassOf` c1))
- where
- tv1 = getTyVar "sortSC" ty1
- tv2 = getTyVar "sortSC" ty2
+ 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
+
+ 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}
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 @Binds@ generated.
+@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
+
+ | 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
+ | otherwise = Free
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Data types for the reduction mechanism}
+%* *
+%************************************************************************
+
+The main control over context reduction is here
\begin{code}
-bindInstsOfLocalFuns :: LIE -> [Id] -> NF_TcM (LIE, TypecheckedMonoBinds)
+data WhatToDo
+ = ReduceMe -- 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)!
+
+ | 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
+
+reduceMe :: Inst -> WhatToDo
+reduceMe inst = ReduceMe
+
+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 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
+
+ | 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
+
+ | 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
+
+pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
+ | (inst,avail) <- fmToList avails ]
+
+instance Outputable Avail where
+ ppr = pprAvail
+
+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}
+
+
+%************************************************************************
+%* *
+\subsection[reduce]{@reduce@}
+%* *
+%************************************************************************
+
+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 (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 "----------------------",
+ doc,
+ text "given" <+> ppr givens,
+ text "wanted" <+> ppr wanteds,
+ text "----------------------"
+ ])) `thenM_`
+
+ -- Build the Avail mapping from "givens"
+ foldlM addGiven emptyAvails givens `thenM` \ init_state ->
+
+ -- Do the real work
+ reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
+
+ -- 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 "----------------------",
+ doc,
+ text "given" <+> ppr givens,
+ text "wanted" <+> ppr wanteds,
+ text "----",
+ text "avails" <+> pprAvails avails,
+ text "frees" <+> ppr frees,
+ text "no_improvement =" <+> ppr no_improvement,
+ text "----------------------"
+ ])) `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.
+
+\begin{code}
+reduceList :: (Int,[Inst]) -- Stack (for err msgs)
+ -- along with its depth
+ -> (Inst -> WhatToDo)
+ -> [Inst]
+ -> Avails
+ -> TcM Avails
+\end{code}
+
+@reduce@ is passed
+ try_me: given an inst, this function returns
+ Reduce reduce this
+ DontReduce return this in "irreds"
+ Free return this in "frees"
+
+ wanteds: The list of insts to reduce
+ state: An accumulating parameter of type Avails
+ that contains the state of the algorithm
+
+ It returns a Avails.
-bindInstsOfLocalFuns init_lie local_ids
- = let
- insts = unMkLIE init_lie
+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.
+
+\begin{code}
+reduceList (n,stack) try_me wanteds state
+ | n > opt_MaxContextReductionDepth
+ = failWithTc (reduceDepthErr n stack)
+
+ | otherwise
+ =
+#ifdef DEBUG
+ (if n > 8 then
+ pprTrace "Interesting! Context reduction stack deeper than 8:"
+ (nest 2 (pprStack stack))
+ else (\x->x))
+#endif
+ go wanteds state
+ where
+ 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 avails
+ -- It's the same as an existing inst, or a superclass thereof
+ | 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 {
+
+ 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
+
+ ; 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
+
+ ; ReduceMe -> -- It should be reduced
+ lookupInst wanted `thenM` \ lookup_result ->
+ case lookup_result of
+ GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 ->
+ reduceList stack try_me wanteds' avails1 `thenM` \ avails2 ->
+ addWanted 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 avails wanted rhs []
+
+ NoInstance -> -- No such instance!
+ -- Add it and its superclasses
+ addIrred AddSCs avails wanted
+ }
+ where
+ try_simple do_this_otherwise
+ = lookupInst wanted `thenM` \ lookup_result ->
+ case lookup_result of
+ SimpleInst rhs -> addWanted avails wanted rhs []
+ other -> do_this_otherwise avails wanted
+\end{code}
+
+
+\begin{code}
+-------------------------
+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
+
+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' <- split_avail avail
+ = returnM (addToFM avails wanted avail', [])
+
+ | 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])
+
+ where
+ 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!
+ --
+ -- 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
+ -- dn from df here lest we lose sharing.
+ --
+addFree avails free = returnM (addToFM avails free IsFree)
+
+addWanted :: Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
+addWanted avails wanted rhs_expr wanteds
+ = addAvailAndSCs avails wanted avail
+ where
+ avail | instBindingRequired wanted = Rhs rhs_expr wanteds
+ | otherwise = ASSERT( null wanteds ) NoRhs
+
+addGiven :: Avails -> Inst -> TcM Avails
+addGiven avails given = addAvailAndSCs avails given (Given (instToId given) False)
+ -- 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 NoSCs avails irred = returnM (addToFM avails irred Irred)
+addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
+ addAvailAndSCs avails irred Irred
+
+addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
+addAvailAndSCs avails inst avail
+ | not (isClassDict inst) = returnM avails1
+ | otherwise = traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) `thenM_`
+ addSCs is_loop avails1 inst
+ where
+ avails1 = addToFM avails inst avail
+ is_loop inst = any (`tcEqType` idType (instToId inst)) 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 :: (Inst -> 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
+ = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
+ 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' = substTheta (zipTopTvSubst tyvars tys) sc_theta
+
+ add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
+ | add_me sc_dict = addSCs is_loop avails' sc_dict
+ | otherwise = returnM avails
+ 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])
+
+ add_me :: Inst -> Bool
+ add_me sc_dict
+ | is_loop sc_dict = False -- See Note [SUPERCLASS-LOOP]
+ | otherwise = case lookupFM avails sc_dict of
+ Just (Given _ _) -> False -- Given is cheaper than superclass selection
+ other -> True
+\end{code}
+
+Note [SUPERCLASS-LOOP]: Checking for loops
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We have to be careful here. If we are *given* d1:Ord a,
+and want to deduce (d2:C [a]) where
+
+ 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!
+
+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.
+
+Here's another example
+ class Eq b => Foo a b
+ instance Eq a => Foo [a] a
+If we are reducing
+ (Foo [t] t)
+
+we'll first deduce that it holds (via the instance decl). We must not
+then overwrite the Eq t constraint with a superclass selection!
+
+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.
+
+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.
+
+
+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 = (==);
+
+We need to prove (Eq (D [])). Here's how we go:
+
+ d1 : Eq (D [])
+
+by instance decl, holds if
+ d2 : Eq [D []]
+ where d1 = dfEqD d2
+
+by instance decl of Eq, holds if
+ d3 : D []
+ where d2 = dfEqList d3
+ d1 = dfEqD d2
+
+But now we can "tie the knot" to give
+
+ d3 = d1
+ d2 = dfEqList d3
+ d1 = dfEqD d2
+
+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.
+
+
+%************************************************************************
+%* *
+\section{tcSimplifyTop: defaulting}
+%* *
+%************************************************************************
+
+
+@tcSimplifyTop@ is called once per module to simplify all the constant
+and ambiguous Insts.
+
+We need to be careful of one case. Suppose we have
+
+ instance Num a => Num (Foo a b) where ...
+
+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??
+
+It's OK: the final zonking stage should zap y to (), which is fine.
+
+
+\begin{code}
+tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
+tcSimplifyTop wanteds = tc_simplify_top False {- Not interactive loop -} wanteds
+tcSimplifyInteractive wanteds = tc_simplify_top True {- Interactive loop -} wanteds
+
+
+-- The TcLclEnv should be valid here, solely to improve
+-- error message generation for the monomorphism restriction
+tc_simplify_top is_interactive wanteds
+ = getLclEnv `thenM` \ lcl_env ->
+ traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
+ simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, 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
+ -- 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 is_ambig non_ips
+ is_ambig d = not (tyVarsOfInst d `subVarSet` fixed_tvs)
+ fixed_tvs = oclose (fdPredsOfInsts irreds) emptyVarSet
+ -- If the dict has free type variables, it's almost certainly ambiguous,
+ -- and that's the first thing to fix.
+ -- Otherwise, addNoInstanceErrs does the right thing
+ -- I say "almost certain" because we might have
+ -- class C a b | a -> B where ...
+ -- plus an Inst (C Int x). Then the 'x' isn't ambiguous; it's just that
+ -- there's no instance decl for (C Int ...). Hence the oclose.
+ in
+
+ -- Report definite errors
+ groupErrs (addNoInstanceErrs Nothing []) no_insts `thenM_`
+ strangeTopIPErrs bad_ips `thenM_`
+
+ -- Deal with ambiguity errors, but only if
+ -- if there has not been an error so far; errors often
+ -- give rise to spurious ambiguous Insts
+ ifErrsM (returnM []) (
+
+ -- 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 `thenM_`
+
+ -- Disambiguate the ones that look feasible
+ mappM (disambigGroup is_interactive) std_oks
+ ) `thenM` \ binds_ambig ->
+
+ returnM (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
+ * 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
+ * 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.
+
+So here is the function which does the work. It takes the ambiguous
+dictionaries and either resolves them (producing bindings) or
+complains. It works by splitting the dictionary list by type
+variable, and using @disambigOne@ to do the real business.
+
+@disambigOne@ assumes that its arguments dictionaries constrain all
+the same type variable.
+
+ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
+@()@ instead of @Int@. I reckon this is the Right Thing to do since
+the most common use of defaulting is code like:
+\begin{verbatim}
+ _ccall_ foo `seqPrimIO` bar
+\end{verbatim}
+Since we're not using the result of @foo@, the result if (presumably)
+@void@.
+
+\begin{code}
+disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
+ -> [Inst] -- All standard classes of form (C a)
+ -> TcM TcDictBinds
+
+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
+
+ -- Failure here is caused by there being no type in the
+ -- 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.
+ get_default_tys `thenM` \ default_tys ->
+ let
+ try_default [] -- No defaults work, so fail
+ = failM
+
+ try_default (default_ty : default_tys)
+ = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
+ -- default_tys instead
+ tcSimplifyDefault theta `thenM` \ _ ->
+ returnM default_ty
+ where
+ theta = [mkClassPred clas [default_ty] | clas <- classes]
in
- bind_insts insts [] 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
- bind_insts :: [Inst] -- Insts to mangle
- -> [Inst] -- accum. Insts to return
- -> TypecheckedMonoBinds -- accum. Binds to return
- -> NF_TcM (LIE, TypecheckedMonoBinds)
+ 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}
- bind_insts [] acc_insts acc_binds
- = returnNF_Tc (mkLIE acc_insts, acc_binds)
+[Aside - why the defaulting mechanism is turned off when
+ dealing with arguments and results to ccalls.
- bind_insts (inst@(Method uniq id tys orig):insts) acc_insts acc_binds
- | id `is_elem` local_ids
- = noFailTc (lookupInst_Tc inst) `thenNF_Tc` \ (expr, dict_insts) ->
- let
- bind = VarMonoBind (mkInstId inst) expr
- in
- bind_insts insts (dict_insts ++ acc_insts) (bind `AndMonoBinds` acc_binds)
+When typechecking _ccall_s, TcExpr ensures that the external
+function is only passed arguments (and in the other direction,
+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,
- bind_insts (some_other_inst:insts) acc_insts acc_binds
- -- Either not a method, or a method instance for an id not in local_ids
- = bind_insts insts (some_other_inst:acc_insts) acc_binds
+ x <- _ccall_ f
+ if (x /= 0) then
+ _ccall_ g x
+ else
+ return ()
- is_elem = isIn "bindInstsOfLocalFuns"
+What type has 'x' got here? That depends on the default list
+in operation, if it is equal to Haskell 98's default-default
+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.
+
+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}
+%* *
+%************************************************************************
+
+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}
+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
+ -- (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_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_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
+ (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]
+ 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
+
+ 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"))
+
+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
+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 (pprStack stack)]
+
+pprStack stack = vcat (map pprInstInFull stack)
\end{code}
projects / ghc-hetmet.git / blobdiff