\begin{code}
module TcSimplify (
- tcSimplifyInfer, tcSimplifyInferCheck, tcSimplifyCheck,
- tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
- tcSimplifyThetas, tcSimplifyCheckThetas,
+ tcSimplifyInfer, tcSimplifyInferCheck,
+ tcSimplifyCheck, tcSimplifyRestricted,
+ tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
+ tcSimplifyBracket,
+
+ tcSimplifyDeriv, tcSimplifyDefault,
bindInstsOfLocalFuns
) where
#include "HsVersions.h"
+import {-# SOURCE #-} TcUnify( unifyTauTy )
+import TcEnv -- temp
import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
-import TcHsSyn ( TcExpr, TcId,
+import TcHsSyn ( TcExpr, TcId,
TcMonoBinds, TcDictBinds
)
-import TcMonad
-import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
- tyVarsOfInst, predsOfInsts,
- isDict, isClassDict,
- isStdClassTyVarDict, isMethodFor,
- instToId, tyVarsOfInsts,
+import TcRnMonad
+import Inst ( lookupInst, LookupInstResult(..),
+ tyVarsOfInst, fdPredsOfInsts, fdPredsOfInst, newDicts,
+ isDict, isClassDict, isLinearInst, linearInstType,
+ isStdClassTyVarDict, isMethodFor, isMethod,
+ instToId, tyVarsOfInsts, cloneDict,
+ ipNamesOfInsts, ipNamesOfInst, dictPred,
instBindingRequired, instCanBeGeneralised,
- newDictsFromOld, instMentionsIPs,
- getDictClassTys, getIPs, isTyVarDict,
- instLoc, pprInst, zonkInst, tidyInst, tidyInsts,
- Inst, LIE, pprInsts, pprInstsInFull,
- mkLIE,
- lieToList
+ newDictsFromOld, tcInstClassOp,
+ getDictClassTys, isTyVarDict,
+ instLoc, zonkInst, tidyInsts, tidyMoreInsts,
+ Inst, pprInsts, pprInstsInFull,
+ isIPDict, isInheritableInst
)
-import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv )
+import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv, tcLookupId, findGlobals )
import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
-
-import TcType ( zonkTcTyVarsAndFV )
-import TcUnify ( unifyTauTy )
-import Id ( idType )
-import Name ( Name )
-import NameSet ( mkNameSet )
-import Class ( Class, classBigSig )
-import FunDeps ( oclose, grow, improve )
-import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
-
-import Type ( Type, ClassContext,
- mkTyVarTy, getTyVar,
- isTyVarTy, splitSigmaTy, tyVarsOfTypes
- )
-import Subst ( mkTopTyVarSubst, substClasses )
-import PprType ( pprClassPred )
-import TysWiredIn ( unitTy )
+import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
+import TcType ( TcTyVar, TcTyVarSet, ThetaType, TyVarDetails(VanillaTv),
+ mkClassPred, isOverloadedTy, mkTyConApp,
+ mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
+ tyVarsOfPred )
+import Id ( idType, mkUserLocal )
+import Var ( TyVar )
+import Name ( getOccName, getSrcLoc )
+import NameSet ( NameSet, mkNameSet, elemNameSet )
+import Class ( classBigSig )
+import FunDeps ( oclose, grow, improve, pprEquationDoc )
+import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
+import PrelNames ( splitName, fstName, sndName )
+
+import Subst ( mkTopTyVarSubst, substTheta, substTy )
+import TysWiredIn ( unitTy, pairTyCon )
+import ErrUtils ( Message )
import VarSet
+import VarEnv ( TidyEnv )
import FiniteMap
import Outputable
import ListSetOps ( equivClasses )
-import Util ( zipEqual, mapAccumL )
+import Util ( zipEqual, isSingleton )
import List ( partition )
import CmdLineOpts
\end{code}
%* *
%************************************************************************
- --------------------------------------
+ --------------------------------------
Notes on quantification
- --------------------------------------
+ --------------------------------------
Suppose we are about to do a generalisation step.
We have in our hand
forall Q. Cq => T
-and float the constraints Ct further outwards.
+and float the constraints Ct further outwards.
Here are the things that *must* be true:
using the functional dependencies from C
grow(vs,C) The result of extend the set of tyvars vs
- using all conceivable links from C.
+ 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
+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)
Why grow( fv(T), C ) rather than fv(T)? Consider
class H x y | x->y where ...
-
+
T = c->c
C = (H c d)
forall c. H c d => c -> b
- And then if the fn was called at several different c's, each of
+ 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.
- --------------------------------------
- Notes on ambiguity
- --------------------------------------
+ --------------------------------------
+ Notes on ambiguity
+ --------------------------------------
It's very hard to be certain when a type is ambiguous. Consider
show up at the call site.... and eventually at main, which needs special
treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
-So heres the plan. We WARN about probable ambiguity if
+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.
+in the environment, or by the variables in the type.
Notice that we union before calling oclose. Here's an example:
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.
+do we see that c is fixed.
-It's a bit vague exactly which C we should use for this oclose call. If we
+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.
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
+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?
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-ambigous can then float out, and get smashed at top level
+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
+
+
+ --------------------------------------
Notes on implicit parameters
- --------------------------------------
+ --------------------------------------
-Consider
+Question 1: can we "inherit" implicit parameters
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this:
- f x = ...?y...
+ f x = (x::Int) + ?y
-Then we get an LIE like (?y::Int). Doesn't constrain a type variable,
-but we must nevertheless infer a type like
+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
-so that f is passed the value of y at the call site. Is this legal?
-
+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 + ?y
+ f x = (x::Int) + ?y
-Should f be overloaded on "?y" ? Or does the type signature say that it
-shouldn't be? Our position is that it should be illegal. Otherwise
-you can change the *dynamic* semantics by adding a type signature:
+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 + ?y -- f :: (?y::Int) => Int -> Int
+ (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
+ (let f :: Int -> Int
+ f x = x + ?y
in (f 3, f 3 with ?y=5)) with ?y = 6
returns (3+6, 3+6)
-URK! Let's not do this. So this is illegal:
+Indeed, simply inlining f (at the Haskell source level) would change the
+dynamic semantics.
- f :: Int -> Int
- f x = x + ?y
+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.
-BOTTOM LINE: you *must* quantify over implicit parameters.
+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.
- --------------------------------------
- Notes on principal types
- --------------------------------------
+[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.
+
+
+
+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
- 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
-
%************************************************************************
%* *
\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
+
+ 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
+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.
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.
+again.
\begin{code}
tcSimplifyInfer
- :: SDoc
- -> [TcTyVar] -- fv(T); type vars
- -> LIE -- Wanted
+ :: SDoc
+ -> TcTyVarSet -- fv(T); type vars
+ -> [Inst] -- Wanted
-> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
- LIE, -- Free
TcDictBinds, -- Bindings
[TcId]) -- Dict Ids that must be bound here (zonked)
+ -- Any free (escaping) Insts are tossed into the environment
\end{code}
\begin{code}
tcSimplifyInfer doc tau_tvs wanted_lie
- = inferLoop doc tau_tvs (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
+ = inferLoop doc (varSetElems tau_tvs)
+ wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
-- Check for non-generalisable insts
- mapTc_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenTc_`
+ mappM_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenM_`
- returnTc (qtvs, mkLIE frees, binds, map instToId irreds)
+ extendLIEs frees `thenM_`
+ returnM (qtvs, binds, map instToId irreds)
inferLoop doc tau_tvs wanteds
= -- Step 1
- zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
- mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
- tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
+ zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' ->
+ mappM zonkInst wanteds `thenM` \ wanteds' ->
+ tcGetGlobalTyVars `thenM` \ gbl_tvs ->
let
- preds = predsOfInsts wanteds'
+ preds = fdPredsOfInsts wanteds'
qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
-
- try_me inst
- | isFree qtvs inst = Free
- | isClassDict inst = DontReduceUnlessConstant -- Dicts
- | otherwise = ReduceMe AddToIrreds -- Lits and Methods
+
+ try_me inst
+ | isFreeWhenInferring qtvs inst = Free
+ | isClassDict inst = DontReduceUnlessConstant -- Dicts
+ | otherwise = ReduceMe -- Lits and Methods
in
+ 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' `thenTc` \ (no_improvement, frees, binds, irreds) ->
-
+ reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
+
-- Step 3
if no_improvement then
- returnTc (varSetElems qtvs, frees, binds, irreds)
+ returnM (varSetElems qtvs, frees, binds, irreds)
else
- inferLoop doc tau_tvs wanteds
-\end{code}
+ -- 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)
-\begin{code}
-isFree qtvs inst
- = not (tyVarsOfInst inst `intersectsVarSet` qtvs) -- Constrains no quantified vars
- && null (getIPs inst) -- And no implicit parameter involved
- -- (see "Notes on implicit parameters")
+ -- 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 `AndMonoBinds` binds1, irreds1)
\end{code}
+Example [LOOP]
-%************************************************************************
-%* *
-\subsection{tcSimplifyCheck}
-%* *
-%************************************************************************
+ 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
-@tcSimplifyCheck@ is used when we know exactly the set of variables
-we are going to quantify over.
+Wanted: Max Z (S x) y
-\begin{code}
-tcSimplifyCheck
- :: SDoc
- -> [TcTyVar] -- Quantify over these
- -> [Inst] -- Given
- -> LIE -- Wanted
- -> TcM (LIE, -- Free
- TcDictBinds) -- Bindings
+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)!
-tcSimplifyCheck doc qtvs givens wanted_lie
- = checkLoop doc qtvs givens (lieToList wanted_lie) `thenTc` \ (frees, binds, irreds) ->
+[NO TYVARS]
- -- Complain about any irreducible ones
- complainCheck doc givens irreds `thenNF_Tc_`
+ 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
- -- Done
- returnTc (mkLIE frees, binds)
+ g :: Num a => [X a] -> [X a]
+ g xs = h xs
+ where
+ h ys = ys ++ map (k (y [[0]])) xs
-checkLoop doc qtvs givens wanteds
- = -- Step 1
- zonkTcTyVarsAndFV qtvs `thenNF_Tc` \ qtvs' ->
- mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
- mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
- 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 | isFree qtvs' inst = Free
- | otherwise = ReduceMe AddToIrreds
- in
- -- Step 2
- reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
-
- -- Step 3
- if no_improvement then
- returnTc (frees, binds, irreds)
- else
- checkLoop doc qtvs givens wanteds
+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.
-complainCheck doc givens irreds
- = mapNF_Tc zonkInst given_dicts `thenNF_Tc` \ givens' ->
- mapNF_Tc (addNoInstanceErr doc given_dicts) irreds `thenNF_Tc_`
- returnTc ()
- where
- given_dicts = filter isDict givens
- -- Filter out methods, which are only added to
- -- the given set as an optimisation
-\end{code}
+The net effect of [NO TYVARS]
+\begin{code}
+isFreeWhenInferring :: TyVarSet -> Inst -> Bool
+isFreeWhenInferring qtvs inst
+ = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
+ && isInheritableInst inst -- And no implicit parameter involved
+ -- (see "Notes on implicit parameters")
+
+isFreeWhenChecking :: TyVarSet -- Quantified tyvars
+ -> NameSet -- Quantified implicit parameters
+ -> Inst -> Bool
+isFreeWhenChecking qtvs ips inst
+ = isFreeWrtTyVars qtvs inst
+ && isFreeWrtIPs ips inst
+
+isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
+isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
+\end{code}
%************************************************************************
%* *
-\subsection{tcSimplifyAndCheck}
+\subsection{tcSimplifyCheck}
%* *
%************************************************************************
-@tcSimplifyInferCheck@ is used when we know the consraints we are to simplify
-against, but we don't know the type variables over which we are going to quantify.
+@tcSimplifyCheck@ is used when we know exactly the set of variables
+we are going to quantify over. For example, a class or instance declaration.
\begin{code}
+tcSimplifyCheck
+ :: SDoc
+ -> [TcTyVar] -- Quantify over these
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds -- Bindings
+
+-- tcSimplifyCheck is used when checking expression type signatures,
+-- class decls, instance decls etc.
+--
+-- NB: tcSimplifyCheck does not consult the
+-- global type variables in the environment; so you don't
+-- need to worry about setting them before calling tcSimplifyCheck
+tcSimplifyCheck doc qtvs givens wanted_lie
+ = tcSimplCheck doc get_qtvs
+ givens wanted_lie `thenM` \ (qtvs', binds) ->
+ returnM binds
+ where
+ get_qtvs = zonkTcTyVarsAndFV 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
- -> [TcTyVar] -- fv(T)
+ :: SDoc
+ -> TcTyVarSet -- fv(T)
-> [Inst] -- Given
- -> LIE -- Wanted
+ -> [Inst] -- Wanted
-> TcM ([TcTyVar], -- Variables over which to quantify
- LIE, -- Free
TcDictBinds) -- Bindings
-tcSimplifyInferCheck doc tau_tvs givens wanted
- = inferCheckLoop doc tau_tvs givens (lieToList wanted) `thenTc` \ (qtvs, frees, binds, irreds) ->
-
- -- Complain about any irreducible ones
- complainCheck doc givens irreds `thenNF_Tc_`
-
- -- Done
- returnTc (qtvs, mkLIE frees, binds)
-
-inferCheckLoop doc tau_tvs givens wanteds
- = -- Step 1
- zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
- mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
- mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
- tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
-
- let
- -- Figure out what we are going to generalise over
+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
-- 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.
- qtvs = (tau_tvs' `unionVarSet` tyVarsOfInsts givens') `minusVarSet` gbl_tvs
+ 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
+ -- soundness, and it'll only affect which tyvars, not which
-- dictionaries, we quantify over
+ in
+ returnM qtvs
+\end{code}
- -- When checking against a given signature we always reduce
- -- until we find a match against something given, or can't reduce
- try_me inst | isFree qtvs inst = Free
- | otherwise = ReduceMe AddToIrreds
+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
+ complainCheck doc givens irreds `thenM_`
+
+ -- Done
+ extendLIEs frees `thenM_`
+ returnM (qtvs, binds)
+
+ where
+ 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 `AndMonoBinds` binds1, irreds1)
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyRestricted}
+%* *
+%************************************************************************
+
+\begin{code}
+tcSimplifyRestricted -- Used for restricted binding groups
+ -- i.e. ones subject to the monomorphism restriction
+ :: SDoc
+ -> TcTyVarSet -- Free in the type of the RHSs
+ -> [Inst] -- Free in the RHSs
+ -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
+ TcDictBinds) -- Bindings
+
+tcSimplifyRestricted doc tau_tvs wanteds
+ = -- First squash out all methods, to find the constrained tyvars
+ -- We can't just take the free vars of wanted_lie because that'll
+ -- have methods that may incidentally mention entirely unconstrained variables
+ -- e.g. a call to f :: Eq a => a -> b -> b
+ -- Here, b is unconstrained. A good example would be
+ -- foo = f (3::Int)
+ -- We want to infer the polymorphic type
+ -- foo :: forall b. b -> b
+
+ -- '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.
+ simpleReduceLoop doc reduceMe wanteds `thenM` \ (foo_frees, foo_binds, constrained_dicts) ->
+
+ -- Next, figure out the tyvars we will quantify over
+ zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
+ tcGetGlobalTyVars `thenM` \ gbl_tvs ->
+ let
+ constrained_tvs = tyVarsOfInsts constrained_dicts
+ qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs)
+ `minusVarSet` constrained_tvs
in
- -- Step 2
- reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
-
- -- Step 3
+ traceTc (text "tcSimplifyRestricted" <+> vcat [
+ pprInsts wanteds, pprInsts foo_frees, pprInsts constrained_dicts,
+ ppr foo_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 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
+ restrict_loop doc qtvs wanteds
+ -- We still need a loop because improvement can take place
+ -- E.g. if we have (C (T a)) and the instance decl
+ -- instance D Int b => C (T a) where ...
+ -- and there's a functional dependency for D. Then we may improve
+ -- the tyep variable 'b'.
+
+restrict_loop doc qtvs wanteds
+ = mappM zonkInst wanteds `thenM` \ wanteds' ->
+ zonkTcTyVarsAndFV (varSetElems qtvs) `thenM` \ qtvs' ->
+ let
+ try_me inst | isFreeWrtTyVars qtvs' inst = Free
+ | otherwise = ReduceMe
+ in
+ reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
if no_improvement then
- returnTc (varSetElems qtvs, frees, binds, irreds)
+ ASSERT( null irreds )
+ extendLIEs frees `thenM_`
+ returnM (varSetElems qtvs', binds)
else
- inferCheckLoop doc tau_tvs givens wanteds
+ restrict_loop doc qtvs' (irreds ++ frees) `thenM` \ (qtvs1, binds1) ->
+ returnM (qtvs1, binds `AndMonoBinds` binds1)
\end{code}
-
%************************************************************************
%* *
\subsection{tcSimplifyToDicts}
as the available stuff for the RHS of the rule.
The same thing is used for specialise pragmas. Consider
-
+
f :: Num a => a -> a
{-# SPECIALISE f :: Int -> Int #-}
f = ...
f_spec = _inline_me_ (f Int dNumInt)
-But that means that we must simplify the Method for f to (f Int dNumInt)!
+But that means that we must simplify the Method for f to (f Int dNumInt)!
So tcSimplifyToDicts squeezes out all Methods.
+IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
+
+ fromIntegral :: (Integral a, Num b) => a -> b
+ {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
+
+Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
+want to get
+
+ forall dIntegralInt.
+ fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
+
+because the scsel will mess up matching. Instead we want
+
+ forall dIntegralInt, dNumInt.
+ fromIntegral Int Int dIntegralInt dNumInt = id Int
+
+Hence "DontReduce NoSCs"
+
\begin{code}
-tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
-tcSimplifyToDicts wanted_lie
- = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
- -- Since try_me doesn't look at types, we don't need to
+tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds)
+tcSimplifyToDicts wanteds
+ = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
+ -- Since try_me doesn't look at types, we don't need to
-- do any zonking, so it's safe to call reduceContext directly
ASSERT( null frees )
- returnTc (irreds, binds)
+ extendLIEs irreds `thenM_`
+ returnM binds
where
doc = text "tcSimplifyToDicts"
- wanteds = lieToList wanted_lie
-- Reduce methods and lits only; stop as soon as we get a dictionary
- try_me inst | isDict inst = DontReduce
- | otherwise = ReduceMe AddToIrreds
+ try_me inst | isDict inst = DontReduce NoSCs
+ | 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
+ doc = text "tcSimplifyBracket"
\end{code}
let ?x = R in B
we must discharge all the ?x constraints from B. We also do an improvement
-step; if we have ?x::t1 and ?x::t2 we must unify t1, t2. No need to iterate, though.
+step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
+
+Actually, the constraints from B might improve the types in ?x. For example
+
+ f :: (?x::Int) => Char -> Char
+ let ?x = 3 in f 'c'
+
+then the constraint (?x::Int) arising from the call to f will
+force the binding for ?x to be of type Int.
\begin{code}
-tcSimplifyIPs :: [Name] -- The implicit parameters bound here
- -> LIE
- -> TcM (LIE, TcDictBinds)
-tcSimplifyIPs ip_names wanted_lie
- = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
- -- The irreducible ones should be a subset of the implicit
- -- parameters we provided
- ASSERT( all here_ip irreds )
- returnTc (mkLIE frees, binds)
-
+tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds
+tcSimplifyIPs given_ips wanteds
+ = simpl_loop given_ips wanteds `thenM` \ (frees, binds) ->
+ extendLIEs frees `thenM_`
+ returnM binds
where
- doc = text "tcSimplifyIPs" <+> ppr ip_names
- wanteds = lieToList wanted_lie
- ip_set = mkNameSet ip_names
- here_ip ip = isDict ip && ip `instMentionsIPs` ip_set
+ doc = text "tcSimplifyIPs" <+> ppr given_ips
+ ip_set = mkNameSet (ipNamesOfInsts given_ips)
-- Simplify any methods that mention the implicit parameter
- try_me inst | inst `instMentionsIPs` ip_set = ReduceMe AddToIrreds
- | otherwise = Free
+ 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 `AndMonoBinds` binds1)
\end{code}
@LIE@), as well as the @HsBinds@ generated.
\begin{code}
-bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
+bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcMonoBinds
-bindInstsOfLocalFuns init_lie local_ids
- | null overloaded_ids
+bindInstsOfLocalFuns wanteds local_ids
+ | null overloaded_ids
-- Common case
- = returnTc (init_lie, EmptyMonoBinds)
+ = extendLIEs wanteds `thenM_`
+ returnM EmptyMonoBinds
| otherwise
- = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
+ = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
ASSERT( null irreds )
- returnTc (mkLIE frees, binds)
+ extendLIEs frees `thenM_`
+ returnM binds
where
doc = text "bindInsts" <+> ppr local_ids
- wanteds = lieToList init_lie
overloaded_ids = filter is_overloaded local_ids
- is_overloaded id = case splitSigmaTy (idType id) of
- (_, theta, _) -> not (null theta)
+ is_overloaded id = isOverloadedTy (idType id)
overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
- -- so it's worth building a set, so that
+ -- so it's worth building a set, so that
-- lookup (in isMethodFor) is faster
- try_me inst | isMethodFor overloaded_set inst = ReduceMe AddToIrreds
+ try_me inst | isMethodFor overloaded_set inst = ReduceMe
| otherwise = Free
\end{code}
The main control over context reduction is here
\begin{code}
-data WhatToDo
- = ReduceMe -- Try to reduce this
- NoInstanceAction -- What to do if there's no such instance
+data WhatToDo
+ = ReduceMe -- Try to reduce this
+ -- If there's no instance, behave exactly like
+ -- DontReduce: add the inst to
+ -- the irreductible ones, but don't
+ -- produce an error message of any kind.
+ -- It might be quite legitimate such as (Eq a)!
- | DontReduce -- Return as irreducible
+ | DontReduce WantSCs -- Return as irreducible
| DontReduceUnlessConstant -- Return as irreducible unless it can
-- be reduced to a constant in one step
| Free -- Return as free
-data NoInstanceAction
- = Stop -- Fail; no error message
- -- (Only used when tautology checking.)
+reduceMe :: Inst -> WhatToDo
+reduceMe inst = ReduceMe
- | AddToIrreds -- Just add the inst to the irreductible ones; don't
- -- produce an error message of any kind.
- -- It might be quite legitimate such as (Eq a)!
+data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
+ -- of a predicate when adding it to the avails
\end{code}
\begin{code}
-type RedState = (Avails, -- What's available
- [Inst]) -- Insts for which try_me returned Free
-
type Avails = FiniteMap Inst Avail
data Avail
- = Irred -- Used for irreducible dictionaries,
+ = IsFree -- Used for free Insts
+ | Irred -- Used for irreducible dictionaries,
-- which are going to be lambda bound
- | BoundTo TcId -- Used for dictionaries for which we have a binding
+ | 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.
- | Rhs -- Used when there is a RHS
+ | Rhs -- Used when there is a RHS
TcExpr -- The RHS
[Inst] -- Insts free in the RHS; we need these too
-pprAvails avails = vcat (map pprAvail (eltsFM avails))
+ | 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
+ [TcExpr] -- 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 Irred = text "Irred"
-pprAvail (BoundTo x) = text "Bound to" <+> ppr x
-pprAvail (Rhs rhs bs) = ppr rhs <+> braces (ppr bs)
+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 loop startes
\begin{code}
-bindsAndIrreds :: Avails
+extractResults :: Avails
-> [Inst] -- Wanted
- -> (TcDictBinds, -- Bindings
- [Inst]) -- Irreducible ones
+ -> TcM (TcDictBinds, -- Bindings
+ [Inst], -- Irreducible ones
+ [Inst]) -- Free ones
-bindsAndIrreds avails wanteds
- = go avails EmptyMonoBinds [] wanteds
+extractResults avails wanteds
+ = go avails EmptyMonoBinds [] [] wanteds
where
- go avails binds irreds [] = (binds, irreds)
+ go avails binds irreds frees []
+ = returnM (binds, irreds, frees)
- go avails binds irreds (w:ws)
+ go avails binds irreds frees (w:ws)
= case lookupFM avails w of
- Nothing -> -- Free guys come out here
- -- (If we didn't do addFree we could use this as the
- -- criterion for free-ness, and pick up the free ones here too)
- go avails binds irreds ws
-
- Just NoRhs -> go avails binds irreds ws
+ Nothing -> pprTrace "Urk: extractResults" (ppr w) $
+ go avails binds irreds frees ws
- Just Irred -> go (addToFM avails w (BoundTo (instToId w))) binds (w:irreds) 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 (BoundTo id) -> go avails new_binds irreds ws
+ Just (Given id _) -> go avails new_binds irreds frees ws
where
- -- For implicit parameters, all occurrences share the same
- -- Id, so there is no need for synonym bindings
- new_binds | new_id == id = binds
- | otherwise = binds `AndMonoBinds` new_bind
- new_bind = VarMonoBind new_id (HsVar id)
- new_id = instToId w
-
- Just (Rhs rhs ws') -> go avails' (binds `AndMonoBinds` new_bind) irreds (ws' ++ ws)
+ new_binds | id == instToId w = binds
+ | otherwise = addBind binds 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
- id = instToId w
- avails' = addToFM avails w (BoundTo id)
- new_bind = VarMonoBind id rhs
+ 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 `AndMonoBinds` 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, [TcExpr])
+-- (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
+
+ go 1 = returnM (EmptyMonoBinds, [HsVar root_id])
+
+ go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
+ expand n rhss `thenM` \ (binds2, rhss') ->
+ returnM (binds1 `AndMonoBinds` 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 (andMonoBindList 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 (VarMonoBind x (mk_app split_id rhs),
+ [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])
+
+mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var
+
+mk_app id rhs = HsApp (HsVar id) rhs
+
+addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
\end{code}
[Inst]) -- Irreducible
simpleReduceLoop doc try_me wanteds
- = mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
- reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
+ = mappM zonkInst wanteds `thenM` \ wanteds' ->
+ reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
if no_improvement then
- returnTc (frees, binds, irreds)
+ returnM (frees, binds, irreds)
else
- simpleReduceLoop doc try_me wanteds
-\end{code}
+ simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
+ returnM (frees1, binds `AndMonoBinds` binds1, irreds1)
+\end{code}
-> (Inst -> WhatToDo)
-> [Inst] -- Given
-> [Inst] -- Wanted
- -> NF_TcM (Bool, -- True <=> improve step did no unification
+ -> 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 [
+ traceTc (text "reduceContext" <+> (vcat [
text "----------------------",
doc,
text "given" <+> ppr givens,
text "wanted" <+> ppr wanteds,
text "----------------------"
- ])) `thenNF_Tc_`
+ ])) `thenM_`
--}
-- Build the Avail mapping from "givens"
- foldlNF_Tc addGiven (emptyFM, []) givens `thenNF_Tc` \ init_state ->
+ foldlM addGiven emptyFM givens `thenM` \ init_state ->
-- Do the real work
- reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ state@(avails, frees) ->
+ 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 `thenTc` \ no_improvement ->
+ 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 "----",
text "avails" <+> pprAvails avails,
text "frees" <+> ppr frees,
text "no_improvement =" <+> ppr no_improvement,
text "----------------------"
- ])) `thenNF_Tc_`
--}
- let
- (binds, irreds) = bindsAndIrreds avails wanteds
- in
- returnTc (no_improvement, frees, binds, irreds)
+ ])) `thenM_`
+
+ returnM (no_improvement, frees, binds, irreds)
tcImprove avails
- = tcGetInstEnv `thenTc` \ inst_env ->
+ = tcGetInstEnv `thenM` \ inst_env ->
let
- preds = predsOfInsts (keysFM avails)
+ preds = [ (pred, pp_loc)
+ | inst <- keysFM avails,
+ let pp_loc = pprInstLoc (instLoc inst),
+ pred <- fdPredsOfInst inst
+ ]
-- Avails has all the superclasses etc (good)
-- It also has all the intermediates of the deduction (good)
-- It does not have duplicates (good)
eqns = improve (classInstEnv inst_env) preds
in
if null eqns then
- returnTc True
+ returnM True
else
- mapTc_ (\ (t1,t2) -> unifyTauTy t1 t2) eqns `thenTc_`
- returnTc False
+ traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
+ mappM_ unify eqns `thenM_`
+ returnM False
+ where
+ unify ((qtvs, t1, t2), doc)
+ = addErrCtxt doc $
+ tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
+ unifyTauTy (substTy tenv t1) (substTy tenv t2)
\end{code}
The main context-reduction function is @reduce@. Here's its game plan.
-- along with its depth
-> (Inst -> WhatToDo)
-> [Inst]
- -> RedState
- -> TcM RedState
+ -> Avails
+ -> TcM Avails
\end{code}
@reduce@ is passed
Free return this in "frees"
wanteds: The list of insts to reduce
- state: An accumulating parameter of type RedState
+ state: An accumulating parameter of type Avails
that contains the state of the algorithm
-
- It returns a RedState.
-The (n,stack) pair is just used for error reporting.
+ It returns a Avails.
+
+The (n,stack) pair is just used for error reporting.
n is always the depth of the stack.
The stack is the stack of Insts being reduced: to produce X
I had to produce Y, to produce Y I had to produce Z, and so on.
#endif
go wanteds state
where
- go [] state = returnTc state
- go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
+ go [] state = returnM state
+ go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
go ws state'
-- Base case: we're done!
reduce stack try_me wanted state
-- It's the same as an existing inst, or a superclass thereof
- | isAvailable state wanted
- = returnTc state
+ | Just avail <- isAvailable state wanted
+ = if isLinearInst wanted then
+ addLinearAvailable state avail wanted `thenM` \ (state', wanteds') ->
+ reduceList stack try_me wanteds' state'
+ else
+ returnM state -- No op for non-linear things
| otherwise
= case try_me wanted of {
- DontReduce -> addIrred state wanted
+ DontReduce want_scs -> addIrred want_scs state 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
+ 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 no_instance_action -> -- It should be reduced
- lookupInst wanted `thenNF_Tc` \ lookup_result ->
+ ; ReduceMe -> -- It should be reduced
+ lookupInst wanted `thenM` \ lookup_result ->
case lookup_result of
- GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenTc` \ state' ->
+ GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenM` \ state' ->
addWanted state' wanted rhs wanteds'
SimpleInst rhs -> addWanted state wanted rhs []
- NoInstance -> -- No such instance!
- case no_instance_action of
- Stop -> failTc
- AddToIrreds -> addIrred state wanted
+ NoInstance -> -- No such instance!
+ -- Add it and its superclasses
+ addIrred AddSCs state wanted
}
where
try_simple do_this_otherwise
- = lookupInst wanted `thenNF_Tc` \ lookup_result ->
+ = lookupInst wanted `thenM` \ lookup_result ->
case lookup_result of
SimpleInst rhs -> addWanted state wanted rhs []
other -> do_this_otherwise state wanted
\begin{code}
-isAvailable :: RedState -> Inst -> Bool
-isAvailable (avails, _) wanted = wanted `elemFM` avails
- -- NB: the Ord instance of Inst compares by the class/type info
- -- *not* by unique. So
+-------------------------
+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 :: RedState -> Inst -> NF_TcM RedState
+addFree :: Avails -> Inst -> TcM Avails
-- When an Inst is tossed upstairs as 'free' we nevertheless add it
-- to avails, so that any other equal Insts will be commoned up right
-- here rather than also being tossed upstairs. This is really just
-- an optimisation, and perhaps it is more trouble that it is worth,
-- as the following comments show!
--
- -- NB1: do *not* add superclasses. If we have
+ -- NB: do *not* add superclasses. If we have
-- df::Floating a
-- dn::Num a
- -- but a is not bound here, then we *don't* want to derive
+ -- but a is not bound here, then we *don't* want to derive
-- dn from df here lest we lose sharing.
--
- -- NB2: do *not* add the Inst to avails at all if it's a method.
- -- The following situation shows why this is bad:
- -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
- -- From an application (truncate f i) we get
- -- t1 = truncate at f
- -- t2 = t1 at i
- -- If we have also have a 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!
- -- Solution: never put methods in avail till they are captured
- -- in which case addFree isn't used
- --
- -- NB3: make sure that CCallable/CReturnable use NoRhs rather
- -- than BoundTo, else we end up with bogus bindings.
- -- c.f. instBindingRequired in addWanted
-addFree (avails, frees) free
- | isDict free = returnNF_Tc (addToFM avails free avail, free:frees)
- | otherwise = returnNF_Tc (avails, free:frees)
- where
- avail | instBindingRequired free = BoundTo (instToId free)
- | otherwise = NoRhs
-
-addGiven :: RedState -> Inst -> NF_TcM RedState
-addGiven state given = add_avail state given (BoundTo (instToId given))
-
-addIrred :: RedState -> Inst -> NF_TcM RedState
-addIrred state irred = add_avail state irred Irred
+addFree avails free = returnM (addToFM avails free IsFree)
-addWanted :: RedState -> Inst -> TcExpr -> [Inst] -> NF_TcM RedState
-addWanted state wanted rhs_expr wanteds
- = ASSERT( not (isAvailable state wanted) )
- add_avail state wanted avail
- where
+addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> TcM Avails
+addWanted avails wanted rhs_expr wanteds
+ = ASSERT2( not (wanted `elemFM` avails), ppr wanted $$ ppr avails )
+ addAvailAndSCs avails wanted avail
+ where
avail | instBindingRequired wanted = Rhs rhs_expr wanteds
| otherwise = ASSERT( null wanteds ) NoRhs
-add_avail :: RedState -> Inst -> Avail -> NF_TcM RedState
-add_avail (avails, frees) wanted avail
- = addAvail avails wanted avail `thenNF_Tc` \ avails' ->
- returnNF_Tc (avails', frees)
-
----------------------
-addAvail :: Avails -> Inst -> Avail -> NF_TcM Avails
-addAvail avails wanted avail
- = addSuperClasses (addToFM avails wanted avail) wanted
-
-addSuperClasses :: Avails -> Inst -> NF_TcM Avails
+addGiven :: Avails -> Inst -> TcM Avails
+addGiven state given = addAvailAndSCs state given (Given (instToId given) False)
+ -- No ASSERT( not (given `elemFM` avails) ) because in an instance
+ -- decl for Ord t we can add both Ord t and Eq t as 'givens',
+ -- so the assert isn't true
+
+addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
+addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
+addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
+ addAvailAndSCs avails irred Irred
+
+addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
+addAvailAndSCs avails inst avail
+ | not (isClassDict inst) = returnM avails1
+ | otherwise = addSCs is_loop avails1 inst
+ where
+ avails1 = addToFM avails inst avail
+ is_loop inst = inst `elem` deps -- Note: this compares by *type*, not by Unique
+ deps = findAllDeps avails avail
+
+findAllDeps :: Avails -> Avail -> [Inst]
+-- Find all the Insts that this one depends on
+-- See Note [SUPERCLASS-LOOP]
+findAllDeps avails (Rhs _ kids) = kids ++ concat (map (find_all_deps_help avails) kids)
+findAllDeps avails other = []
+
+find_all_deps_help :: Avails -> Inst -> [Inst]
+find_all_deps_help avails inst
+ = case lookupFM avails inst of
+ Just avail -> findAllDeps avails avail
+ Nothing -> []
+
+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.
-addSuperClasses avails dict
- | not (isClassDict dict)
- = returnNF_Tc avails
-
- | otherwise -- It is a dictionary
- = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
- foldlNF_Tc add_sc avails (zipEqual "addSuperClasses" sc_dicts sc_sels)
+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' = substClasses (mkTopTyVarSubst tyvars tys) sc_theta
+ sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
= case lookupFM avails sc_dict of
- Just (BoundTo _) -> returnNF_Tc avails -- See Note [SUPER] below
- other -> addAvail avails sc_dict avail
+ Just (Given _ _) -> returnM avails -- Given is cheaper than
+ -- a superclass selection
+ Just other | is_loop sc_dict -> returnM avails -- See Note [SUPERCLASS-LOOP]
+ | otherwise -> returnM avails' -- SCs already added
+
+ Nothing -> addSCs is_loop avails' sc_dict
where
sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
avail = Rhs sc_sel_rhs [dict]
+ avails' = addToFM avails sc_dict avail
\end{code}
-Note [SUPER]. We have to be careful here. If we are *given* d1:Ord a,
+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] and then add the
+Then we'll use the instance decl to deduce C [a] and then add the
superclasses of C [a] to avails. But we must not overwrite the binding
for d1:Ord a (which is given) with a superclass selection or we'll just
-build a loop! Hence looking for BoundTo. Crudely, BoundTo is cheaper
-than a selection.
+build a loop!
+
+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.
+
%************************************************************************
%************************************************************************
-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.
-
@tcSimplifyTop@ is called once per module to simplify all the constant
and ambiguous Insts.
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??
+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 :: LIE -> TcM TcDictBinds
-tcSimplifyTop wanted_lie
- = simpleReduceLoop (text "tcSimplTop") try_me wanteds `thenTc` \ (frees, binds, irreds) ->
+tcSimplifyTop :: [Inst] -> TcM TcDictBinds
+-- The TcLclEnv should be valid here, solely to improve
+-- error message generation for the monomorphism restriction
+tcSimplifyTop 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
- (std_oks, std_bads) = partition worth_a_try std_groups
-
- -- Have a try at disambiguation
- -- if the type variable isn't bound
+ -- 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
+ bad_guys = non_stds ++ concat std_bads
+ (tidy_env, tidy_dicts) = tidyInsts bad_guys
+ (bad_ips, non_ips) = partition isIPDict tidy_dicts
+ (no_insts, ambigs) = partition no_inst non_ips
+ no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
+ fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
in
- -- Disambiguate the ones that look feasible
- mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
- -- And complain about the ones that don't
- addTopAmbigErrs bad_guys `thenNF_Tc_`
+ -- Report definite errors
+ addTopInstanceErrs tidy_env no_insts `thenM_`
+ addTopIPErrs tidy_env bad_ips `thenM_`
- returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
- where
- wanteds = lieToList wanted_lie
- try_me inst = ReduceMe AddToIrreds
+ -- 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 (tidy_env, ambigs) `thenM_`
- d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
+ -- Disambiguate the ones that look feasible
+ mappM disambigGroup std_oks
+ ) `thenM` \ binds_ambig ->
+
+ returnM (binds `andMonoBinds` andMonoBindList binds_ambig)
+
+----------------------------------
+d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
get_tv d = case getDictClassTys d of
- (clas, [ty]) -> getTyVar "tcSimplifyTop" ty
+ (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.
disambigGroup dicts
| any isNumericClass classes -- Guaranteed all standard classes
- -- see comment at the end of function for reasons as to
+ -- see comment at the end of function for reasons as to
-- why the defaulting mechanism doesn't apply to groups that
-- include CCallable or CReturnable dicts.
&& not (any isCcallishClass classes)
-- default list which can satisfy all the ambiguous classes.
-- For example, if Real a is reqd, but the only type in the
-- default list is Int.
- tcGetDefaultTys `thenNF_Tc` \ default_tys ->
+ getDefaultTys `thenM` \ default_tys ->
let
try_default [] -- No defaults work, so fail
- = failTc
+ = failM
try_default (default_ty : default_tys)
- = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
+ = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
-- default_tys instead
- tcSimplifyCheckThetas [] thetas `thenTc` \ _ ->
- returnTc default_ty
+ tcSimplifyDefault theta `thenM` \ _ ->
+ returnM default_ty
where
- thetas = classes `zip` repeat [default_ty]
+ theta = [mkClassPred clas [default_ty] | clas <- classes]
in
- -- See if any default works, and if so bind the type variable to it
- -- If not, add an AmbigErr
- recoverTc (addAmbigErrs dicts `thenNF_Tc_` returnTc EmptyMonoBinds) $
-
- try_default default_tys `thenTc` \ chosen_default_ty ->
-
- -- Bind the type variable and reduce the context, for real this time
- unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
+ -- See if any default works
+ tryM (try_default default_tys) `thenM` \ mb_ty ->
+ case mb_ty of {
+ Left _ -> -- If not, add an AmbigErr
+ addTopAmbigErrs (tidyInsts dicts) `thenM_`
+ returnM EmptyMonoBinds ;
+
+ Right chosen_default_ty ->
+
+ -- If so, bind the type variable
+ -- and reduce the context, for real this time
+ unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenM_`
simpleReduceLoop (text "disambig" <+> ppr dicts)
- try_me dicts `thenTc` \ (frees, binds, ambigs) ->
+ reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
- warnDefault dicts chosen_default_ty `thenTc_`
- returnTc binds
+ warnDefault dicts chosen_default_ty `thenM_`
+ returnM binds }
| all isCreturnableClass classes
- = -- Default CCall stuff to (); we don't even both to check that () is an
+ = -- Default CCall stuff to (); we don't even both to check that () is an
-- instance of CReturnable, because we know it is.
- unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
- returnTc EmptyMonoBinds
-
+ unifyTauTy (mkTyVarTy tyvar) unitTy `thenM_`
+ returnM EmptyMonoBinds
+
| otherwise -- No defaults
- = addAmbigErrs dicts `thenNF_Tc_`
- returnTc EmptyMonoBinds
+ = addTopAmbigErrs (tidyInsts dicts) `thenM_`
+ returnM EmptyMonoBinds
where
- try_me inst = ReduceMe AddToIrreds -- This reduce should not fail
tyvar = get_tv (head dicts) -- Should be non-empty
classes = map get_clas dicts
\end{code}
results) of a restricted set of 'native' types. This is
implemented via the help of the pseudo-type classes,
@CReturnable@ (CR) and @CCallable@ (CC.)
-
+
The interaction between the defaulting mechanism for numeric
values and CC & CR can be a bit puzzling to the user at times.
For example,
of (Integer, Double), 'x' has type Double, since Integer
is not an instance of CR. If the default list is equal to
Haskell 1.4's default-default of (Int, Double), 'x' has type
-Int.
+Int.
To try to minimise the potential for surprises here, the
defaulting mechanism is turned off in the presence of
CCallable and CReturnable.
-]
+End of aside]
%************************************************************************
instance declarations.
\begin{code}
-tcSimplifyThetas :: ClassContext -- Wanted
- -> TcM ClassContext -- Needed
-
-tcSimplifyThetas wanteds
- = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
- reduceSimple [] wanteds `thenNF_Tc` \ irreds ->
+tcSimplifyDeriv :: [TyVar]
+ -> ThetaType -- Wanted
+ -> TcM ThetaType -- Needed
+
+tcSimplifyDeriv tyvars theta
+ = tcInstTyVars VanillaTv 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 DataDeclOrigin (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
- -- For multi-param Haskell, check that the returned dictionaries
- -- don't have any of the form (C Int Bool) for which
- -- we expect an instance here
- -- For Haskell 98, check that all the constraints are of the form C a,
- -- where a is a type variable
- bad_guys | glaExts = [ct | ct@(clas,tys) <- irreds,
- isEmptyVarSet (tyVarsOfTypes tys)]
- | otherwise = [ct | ct@(clas,tys) <- irreds,
- not (all isTyVarTy tys)]
+ tv_set = mkVarSet tvs
+ simpl_theta = map dictPred irreds -- reduceMe squashes all non-dicts
+
+ check_pred pred
+ | isEmptyVarSet pred_tyvars -- Things like (Eq T) should be rejected
+ = addErrTc (noInstErr pred)
+
+ | not undecidable_ok && not (isTyVarClassPred pred)
+ -- Check that the returned dictionaries are all of form (C a b)
+ -- (where a, b are type variables).
+ -- We allow this 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)
+ = addErrTc (noInstErr pred)
+
+ | not (pred_tyvars `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.
+ = addErrTc (badDerivedPred pred)
+
+ | otherwise
+ = returnM ()
+ where
+ pred_tyvars = tyVarsOfPred pred
+
+ rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
+ -- This reverse-mapping is a Royal Pain,
+ -- but the result should mention TyVars not TcTyVars
in
- if null bad_guys then
- returnTc irreds
- else
- mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
- failTc
+
+ mappM check_pred simpl_theta `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}
-@tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
+@tcSimplifyDefault@ just checks class-type constraints, essentially;
used with \tr{default} declarations. We are only interested in
whether it worked or not.
\begin{code}
-tcSimplifyCheckThetas :: ClassContext -- Given
- -> ClassContext -- Wanted
- -> TcM ()
-
-tcSimplifyCheckThetas givens wanteds
- = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
+tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
+ -> TcM ()
+
+tcSimplifyDefault theta
+ = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
+ simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
+ ASSERT( null frees ) -- try_me never returns Free
+ mappM (addErrTc . noInstErr) irreds `thenM_`
if null irreds then
- returnTc ()
+ returnM ()
else
- mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
- failTc
-\end{code}
-
-
-\begin{code}
-type AvailsSimple = FiniteMap (Class,[Type]) Bool
- -- True => irreducible
- -- False => given, or can be derived from a given or from an irreducible
-
-reduceSimple :: ClassContext -- Given
- -> ClassContext -- Wanted
- -> NF_TcM ClassContext -- Irreducible
-
-reduceSimple givens wanteds
- = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
- returnNF_Tc [ct | (ct,True) <- fmToList givens_fm']
+ failM
where
- givens_fm = foldl addNonIrred emptyFM givens
-
-reduce_simple :: (Int,ClassContext) -- Stack
- -> AvailsSimple
- -> ClassContext
- -> NF_TcM AvailsSimple
-
-reduce_simple (n,stack) avails wanteds
- = go avails wanteds
- where
- go avails [] = returnNF_Tc avails
- go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
- go avails' ws
-
-reduce_simple_help stack givens wanted@(clas,tys)
- | wanted `elemFM` givens
- = returnNF_Tc givens
-
- | otherwise
- = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
-
- case maybe_theta of
- Nothing -> returnNF_Tc (addSimpleIrred givens wanted)
- Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
-
-addSimpleIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
-addSimpleIrred givens ct@(clas,tys)
- = addSCs (addToFM givens ct True) ct
-
-addNonIrred :: AvailsSimple -> (Class,[Type]) -> AvailsSimple
-addNonIrred givens ct@(clas,tys)
- = addSCs (addToFM givens ct False) ct
-
-addSCs givens ct@(clas,tys)
- = foldl add givens sc_theta
- where
- (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
- sc_theta = substClasses (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
-
- add givens ct@(clas, tys)
- = case lookupFM givens ct of
- Nothing -> -- Add it and its superclasses
- addSCs (addToFM givens ct False) ct
-
- Just True -> -- Set its flag to False; superclasses already done
- addToFM givens ct False
-
- Just False -> -- Already done
- givens
-
+ doc = ptext SLIT("default declaration")
\end{code}
now?
\begin{code}
-addTopAmbigErrs dicts
- = mapNF_Tc complain tidy_dicts
+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
- fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet
- (tidy_env, tidy_dicts) = tidyInsts emptyTidyEnv dicts
- complain d | not (null (getIPs d)) = addTopIPErr tidy_env d
- | tyVarsOfInst d `subVarSet` fixed_tvs = addTopInstanceErr tidy_env d
- | otherwise = addAmbigErr tidy_env d
+ -- (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
-addTopIPErr tidy_env tidy_dict
- = addInstErrTcM (instLoc tidy_dict)
- (tidy_env,
- ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
+-- 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'
--- Used for top-level irreducibles
-addTopInstanceErr tidy_env tidy_dict
- = addInstErrTcM (instLoc tidy_dict)
- (tidy_env,
- ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
-addAmbigErrs dicts
- = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts
+addTopIPErrs tidy_env tidy_dicts
+ = groupErrs report tidy_dicts
where
- (tidy_env, tidy_dicts) = tidyInsts emptyTidyEnv dicts
+ report dicts = addErrTcM (tidy_env, mk_msg dicts)
+ mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
+ plural tidy_dicts <+> pprInsts tidy_dicts)
-addAmbigErr tidy_env tidy_dict
- = addInstErrTcM (instLoc tidy_dict)
- (tidy_env,
- sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs,
- nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
+-- Used for top-level irreducibles
+addTopInstanceErrs tidy_env tidy_dicts
+ = groupErrs report tidy_dicts
where
- ambig_tvs = varSetElems (tyVarsOfInst tidy_dict)
+ report dicts = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
+ addErrTcM (tidy_env, mk_msg dicts $$ mono_msg)
+ mk_msg dicts = addInstLoc dicts (ptext SLIT("No instance") <> plural tidy_dicts <+>
+ ptext SLIT("for") <+> pprInsts tidy_dicts)
+
+
+addTopAmbigErrs (tidy_env, tidy_dicts)
+-- Divide into groups that share a common set of ambiguous tyvars
+ = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
+ where
+ tvs_of :: Inst -> [TcTyVar]
+ tvs_of d = varSetElems (tyVarsOfInst d)
+ cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
+
+ report :: [(Inst,[TcTyVar])] -> TcM ()
+ report pairs@((_,tvs) : _) -- The pairs share a common set of ambiguous tyvars
+ = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
+ addErrTcM (tidy_env, msg $$ mono_msg)
+ where
+ dicts = map fst pairs
+ msg = sep [text "Ambiguous type variable" <> plural tvs <+>
+ pprQuotedList tvs <+> in_msg,
+ nest 2 (pprInstsInFull dicts)]
+ in_msg | isSingleton dicts = text "in the top-level constraint:"
+ | otherwise = text "in these top-level constraints:"
+
+
+mkMonomorphismMsg :: TidyEnv -> [Inst] -> 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 insts
+ | isEmptyVarSet inst_tvs
+ = returnM (tidy_env, empty)
+ | otherwise
+ = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
+ returnM (tidy_env, mk_msg docs)
+ where
+ inst_tvs = tyVarsOfInsts insts
+
+ mk_msg [] = empty -- 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),
+ ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
+
warnDefault dicts default_ty
- = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag ->
- if warn_flag
- then mapNF_Tc warn groups `thenNF_Tc_` returnNF_Tc ()
- else returnNF_Tc ()
-
+ = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
+ addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
where
-- Tidy them first
- (_, tidy_dicts) = mapAccumL tidyInst emptyTidyEnv dicts
-
- -- Group the dictionaries by source location
- groups = equivClasses cmp tidy_dicts
- i1 `cmp` i2 = get_loc i1 `compare` get_loc i2
- get_loc i = case instLoc i of { (_,loc,_) -> loc }
-
- warn [dict] = tcAddSrcLoc (get_loc dict) $
- warnTc True (ptext SLIT("Defaulting") <+> quotes (pprInst dict) <+>
- ptext SLIT("to type") <+> quotes (ppr default_ty))
-
- warn dicts = tcAddSrcLoc (get_loc (head dicts)) $
- warnTc True (vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+> quotes (ppr default_ty),
- pprInstsInFull dicts])
-
--- The error message when we don't find a suitable instance
--- is complicated by the fact that sometimes this is because
--- there is no instance, and sometimes it's because there are
--- too many instances (overlap). See the comments in TcEnv.lhs
--- with the InstEnv stuff.
-addNoInstanceErr what_doc givens dict
- = tcGetInstEnv `thenNF_Tc` \ inst_env ->
+ (_, tidy_dicts) = tidyInsts dicts
+ warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
+ quotes (ppr default_ty),
+ pprInstsInFull tidy_dicts]
+
+complainCheck doc givens irreds
+ = mappM zonkInst given_dicts_and_ips `thenM` \ givens' ->
+ groupErrs (addNoInstanceErrs doc givens') irreds `thenM_`
+ returnM ()
+ where
+ given_dicts_and_ips = filter (not . isMethod) givens
+ -- Filter out methods, which are only added to
+ -- the given set as an optimisation
+
+addNoInstanceErrs what_doc givens dicts
+ = getDOpts `thenM` \ dflags ->
+ tcGetInstEnv `thenM` \ inst_env ->
let
- doc = vcat [sep [herald <+> quotes (pprInst tidy_dict),
+ (tidy_env1, tidy_givens) = tidyInsts givens
+ (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
+
+ doc = vcat [addInstLoc dicts $
+ sep [herald <+> pprInsts tidy_dicts,
nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
ambig_doc,
ptext SLIT("Probable fix:"),
nest 4 fix1,
nest 4 fix2]
-
+
herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
- unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
+ unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
| otherwise = empty
-
- ambig_doc
+
+ -- The error message when we don't find a suitable instance
+ -- is complicated by the fact that sometimes this is because
+ -- there is no instance, and sometimes it's because there are
+ -- too many instances (overlap). See the comments in TcEnv.lhs
+ -- with the InstEnv stuff.
+
+ ambig_doc
| not ambig_overlap = empty
- | otherwise
+ | otherwise
= vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
- nest 4 (ptext SLIT("depends on the instantiation of") <+>
- quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst tidy_dict))))]
-
- fix1 = sep [ptext SLIT("Add") <+> quotes (pprInst tidy_dict),
+ nest 4 (ptext SLIT("depends on the instantiation of") <+>
+ quotes (pprWithCommas ppr (varSetElems (tyVarsOfInsts tidy_dicts))))]
+
+ fix1 = sep [ptext SLIT("Add") <+> pprInsts tidy_dicts,
ptext SLIT("to the") <+> what_doc]
-
- fix2 | isTyVarDict dict || ambig_overlap
+
+ fix2 | null instance_dicts
= empty
| otherwise
- = ptext SLIT("Or add an instance declaration for") <+> quotes (pprInst tidy_dict)
-
- (tidy_env, tidy_dict:tidy_givens) = tidyInsts emptyTidyEnv (dict:givens)
-
+ = ptext SLIT("Or add an instance declaration for") <+> pprInsts instance_dicts
+
+ instance_dicts = [d | d <- tidy_dicts, isClassDict d, not (isTyVarDict d)]
+ -- Insts for which it is worth suggesting an adding an instance declaration
+ -- Exclude implicit parameters, and tyvar dicts
+
-- Checks for the ambiguous case when we have overlapping instances
- ambig_overlap | isClassDict dict
- = case lookupInstEnv inst_env clas tys of
+ ambig_overlap = any ambig_overlap1 dicts
+ ambig_overlap1 dict
+ | isClassDict dict
+ = case lookupInstEnv dflags inst_env clas tys of
NoMatch ambig -> ambig
other -> False
- | otherwise = False
- where
- (clas,tys) = getDictClassTys dict
+ | otherwise = False
+ where
+ (clas,tys) = getDictClassTys dict
in
- addInstErrTcM (instLoc dict) (tidy_env, doc)
+ addErrTcM (tidy_env2, doc)
-- Used for the ...Thetas variants; all top level
-addNoInstErr (c,ts)
- = addErrTc (ptext SLIT("No instance for") <+> quotes (pprClassPred c ts))
+noInstErr pred = ptext SLIT("No instance for") <+> quotes (ppr pred)
+
+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,
-----------------------------------------------
addCantGenErr inst
- = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
+ = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
nest 4 (ppr inst <+> pprInstLoc (instLoc inst))])
\end{code}
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