2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcSimplify]{TcSimplify}
10 tcSimplifyInfer, tcSimplifyInferCheck, tcSimplifyCheck,
12 tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
14 tcSimplifyThetas, tcSimplifyCheckThetas,
18 #include "HsVersions.h"
20 import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
21 import TcHsSyn ( TcExpr, TcId,
22 TcMonoBinds, TcDictBinds
26 import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
27 tyVarsOfInst, predsOfInsts, predsOfInst,
29 isStdClassTyVarDict, isMethodFor,
30 instToId, tyVarsOfInsts,
31 instBindingRequired, instCanBeGeneralised,
32 newDictsFromOld, instMentionsIPs,
33 getDictClassTys, isTyVarDict,
34 instLoc, pprInst, zonkInst, tidyInsts,
35 Inst, LIE, pprInsts, pprInstsInFull,
38 import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv )
39 import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
41 import TcType ( zonkTcTyVarsAndFV, tcInstTyVars )
42 import TcUnify ( unifyTauTy )
45 import NameSet ( mkNameSet )
46 import Class ( classBigSig )
47 import FunDeps ( oclose, grow, improve )
48 import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
50 import Type ( Type, ThetaType, PredType, mkClassPred,
51 mkTyVarTy, getTyVar, isTyVarClassPred,
52 splitSigmaTy, tyVarsOfPred,
53 getClassPredTys_maybe, isClassPred, isIPPred,
56 import Subst ( mkTopTyVarSubst, substTheta, substTy )
57 import TysWiredIn ( unitTy )
61 import ListSetOps ( equivClasses )
62 import Util ( zipEqual )
63 import List ( partition )
68 %************************************************************************
72 %************************************************************************
74 --------------------------------------
75 Notes on quantification
76 --------------------------------------
78 Suppose we are about to do a generalisation step.
83 C the constraints from that RHS
85 The game is to figure out
87 Q the set of type variables over which to quantify
88 Ct the constraints we will *not* quantify over
89 Cq the constraints we will quantify over
91 So we're going to infer the type
95 and float the constraints Ct further outwards.
97 Here are the things that *must* be true:
99 (A) Q intersect fv(G) = EMPTY limits how big Q can be
100 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
102 (A) says we can't quantify over a variable that's free in the
103 environment. (B) says we must quantify over all the truly free
104 variables in T, else we won't get a sufficiently general type. We do
105 not *need* to quantify over any variable that is fixed by the free
106 vars of the environment G.
108 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
110 Example: class H x y | x->y where ...
112 fv(G) = {a} C = {H a b, H c d}
115 (A) Q intersect {a} is empty
116 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
118 So Q can be {c,d}, {b,c,d}
120 Other things being equal, however, we'd like to quantify over as few
121 variables as possible: smaller types, fewer type applications, more
122 constraints can get into Ct instead of Cq.
125 -----------------------------------------
128 fv(T) the free type vars of T
130 oclose(vs,C) The result of extending the set of tyvars vs
131 using the functional dependencies from C
133 grow(vs,C) The result of extend the set of tyvars vs
134 using all conceivable links from C.
136 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
137 Then grow(vs,C) = {a,b,c}
139 Note that grow(vs,C) `superset` grow(vs,simplify(C))
140 That is, simplfication can only shrink the result of grow.
143 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
144 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
147 -----------------------------------------
151 Here's a good way to choose Q:
153 Q = grow( fv(T), C ) \ oclose( fv(G), C )
155 That is, quantify over all variable that that MIGHT be fixed by the
156 call site (which influences T), but which aren't DEFINITELY fixed by
157 G. This choice definitely quantifies over enough type variables,
158 albeit perhaps too many.
160 Why grow( fv(T), C ) rather than fv(T)? Consider
162 class H x y | x->y where ...
167 If we used fv(T) = {c} we'd get the type
169 forall c. H c d => c -> b
171 And then if the fn was called at several different c's, each of
172 which fixed d differently, we'd get a unification error, because
173 d isn't quantified. Solution: quantify d. So we must quantify
174 everything that might be influenced by c.
176 Why not oclose( fv(T), C )? Because we might not be able to see
177 all the functional dependencies yet:
179 class H x y | x->y where ...
180 instance H x y => Eq (T x y) where ...
185 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
186 apparent yet, and that's wrong. We must really quantify over d too.
189 There really isn't any point in quantifying over any more than
190 grow( fv(T), C ), because the call sites can't possibly influence
191 any other type variables.
195 --------------------------------------
197 --------------------------------------
199 It's very hard to be certain when a type is ambiguous. Consider
203 instance H x y => K (x,y)
205 Is this type ambiguous?
206 forall a b. (K (a,b), Eq b) => a -> a
208 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
209 now we see that a fixes b. So we can't tell about ambiguity for sure
210 without doing a full simplification. And even that isn't possible if
211 the context has some free vars that may get unified. Urgle!
213 Here's another example: is this ambiguous?
214 forall a b. Eq (T b) => a -> a
215 Not if there's an insance decl (with no context)
216 instance Eq (T b) where ...
218 You may say of this example that we should use the instance decl right
219 away, but you can't always do that:
221 class J a b where ...
222 instance J Int b where ...
224 f :: forall a b. J a b => a -> a
226 (Notice: no functional dependency in J's class decl.)
227 Here f's type is perfectly fine, provided f is only called at Int.
228 It's premature to complain when meeting f's signature, or even
229 when inferring a type for f.
233 However, we don't *need* to report ambiguity right away. It'll always
234 show up at the call site.... and eventually at main, which needs special
235 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
237 So heres the plan. We WARN about probable ambiguity if
239 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
241 (all tested before quantification).
242 That is, all the type variables in Cq must be fixed by the the variables
243 in the environment, or by the variables in the type.
245 Notice that we union before calling oclose. Here's an example:
247 class J a b c | a b -> c
251 forall b c. (J a b c) => b -> b
253 Only if we union {a} from G with {b} from T before using oclose,
254 do we see that c is fixed.
256 It's a bit vague exactly which C we should use for this oclose call. If we
257 don't fix enough variables we might complain when we shouldn't (see
258 the above nasty example). Nothing will be perfect. That's why we can
259 only issue a warning.
262 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
264 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
266 then c is a "bubble"; there's no way it can ever improve, and it's
267 certainly ambiguous. UNLESS it is a constant (sigh). And what about
272 instance H x y => K (x,y)
274 Is this type ambiguous?
275 forall a b. (K (a,b), Eq b) => a -> a
277 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
278 is a "bubble" that's a set of constraints
280 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
282 Hence another idea. To decide Q start with fv(T) and grow it
283 by transitive closure in Cq (no functional dependencies involved).
284 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
285 The definitely-ambigous can then float out, and get smashed at top level
286 (which squashes out the constants, like Eq (T a) above)
289 --------------------------------------
290 Notes on implicit parameters
291 --------------------------------------
297 Then we get an LIE like (?y::Int). Doesn't constrain a type variable,
298 but we must nevertheless infer a type like
300 f :: (?y::Int) => Int -> Int
302 so that f is passed the value of y at the call site. Is this legal?
307 Should f be overloaded on "?y" ? Or does the type signature say that it
308 shouldn't be? Our position is that it should be illegal. Otherwise
309 you can change the *dynamic* semantics by adding a type signature:
311 (let f x = x + ?y -- f :: (?y::Int) => Int -> Int
312 in (f 3, f 3 with ?y=5)) with ?y = 6
318 in (f 3, f 3 with ?y=5)) with ?y = 6
322 URK! Let's not do this. So this is illegal:
327 There's a nasty corner case when the monomorphism restriction bites:
331 The argument above suggests that we must generalise over the ?y parameter,
332 but the monomorphism restriction says that we can't. The current
333 implementation chooses to let the monomorphism restriction 'win' in this
334 case, but it's not clear what the Right Thing is.
336 BOTTOM LINE: you *must* quantify over implicit parameters.
339 --------------------------------------
340 Notes on principal types
341 --------------------------------------
346 f x = let g y = op (y::Int) in True
348 Here the principal type of f is (forall a. a->a)
349 but we'll produce the non-principal type
350 f :: forall a. C Int => a -> a
353 %************************************************************************
355 \subsection{tcSimplifyInfer}
357 %************************************************************************
359 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
361 1. Compute Q = grow( fvs(T), C )
363 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
364 predicates will end up in Ct; we deal with them at the top level
366 3. Try improvement, using functional dependencies
368 4. If Step 3 did any unification, repeat from step 1
369 (Unification can change the result of 'grow'.)
371 Note: we don't reduce dictionaries in step 2. For example, if we have
372 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
373 after step 2. However note that we may therefore quantify over more
374 type variables than we absolutely have to.
376 For the guts, we need a loop, that alternates context reduction and
377 improvement with unification. E.g. Suppose we have
379 class C x y | x->y where ...
381 and tcSimplify is called with:
383 Then improvement unifies a with b, giving
386 If we need to unify anything, we rattle round the whole thing all over
393 -> [TcTyVar] -- fv(T); type vars
395 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
397 TcDictBinds, -- Bindings
398 [TcId]) -- Dict Ids that must be bound here (zonked)
403 tcSimplifyInfer doc tau_tvs wanted_lie
404 = inferLoop doc tau_tvs (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
406 -- Check for non-generalisable insts
407 mapTc_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenTc_`
409 returnTc (qtvs, mkLIE frees, binds, map instToId irreds)
411 inferLoop doc tau_tvs wanteds
413 zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
414 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
415 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
417 preds = predsOfInsts wanteds'
418 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
421 | isFree qtvs inst = Free
422 | isClassDict inst = DontReduceUnlessConstant -- Dicts
423 | otherwise = ReduceMe -- Lits and Methods
426 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
429 if no_improvement then
430 returnTc (varSetElems qtvs, frees, binds, irreds)
432 -- If improvement did some unification, we go round again. There
433 -- are two subtleties:
434 -- a) We start again with irreds, not wanteds
435 -- Using an instance decl might have introduced a fresh type variable
436 -- which might have been unified, so we'd get an infinite loop
437 -- if we started again with wanteds! See example [LOOP]
439 -- b) It's also essential to re-process frees, because unification
440 -- might mean that a type variable that looked free isn't now.
442 -- Hence the (irreds ++ frees)
444 inferLoop doc tau_tvs (irreds ++ frees) `thenTc` \ (qtvs1, frees1, binds1, irreds1) ->
445 returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
450 class If b t e r | b t e -> r
453 class Lte a b c | a b -> c where lte :: a -> b -> c
455 instance (Lte a b l,If l b a c) => Max a b c
457 Wanted: Max Z (S x) y
459 Then we'll reduce using the Max instance to:
460 (Lte Z (S x) l, If l (S x) Z y)
461 and improve by binding l->T, after which we can do some reduction
462 on both the Lte and If constraints. What we *can't* do is start again
463 with (Max Z (S x) y)!
467 = not (tyVarsOfInst inst `intersectsVarSet` qtvs) -- Constrains no quantified vars
468 && all inheritablePred (predsOfInst inst) -- And no implicit parameter involved
469 -- (see "Notes on implicit parameters")
473 %************************************************************************
475 \subsection{tcSimplifyCheck}
477 %************************************************************************
479 @tcSimplifyCheck@ is used when we know exactly the set of variables
480 we are going to quantify over. For example, a class or instance declaration.
485 -> [TcTyVar] -- Quantify over these
489 TcDictBinds) -- Bindings
491 tcSimplifyCheck doc qtvs givens wanted_lie
492 = checkLoop doc qtvs givens (lieToList wanted_lie) try `thenTc` \ (frees, binds, irreds) ->
494 -- Complain about any irreducible ones
495 complainCheck doc givens irreds `thenNF_Tc_`
498 returnTc (mkLIE frees, binds)
500 -- When checking against a given signature we always reduce
501 -- until we find a match against something given, or can't reduce
502 try qtvs inst | isFree qtvs inst = Free
503 | otherwise = ReduceMe
505 tcSimplifyRestricted doc qtvs givens wanted_lie
506 = checkLoop doc qtvs givens (lieToList wanted_lie) try `thenTc` \ (frees, binds, irreds) ->
508 -- Complain about any irreducible ones
509 complainCheck doc givens irreds `thenNF_Tc_`
512 returnTc (mkLIE frees, binds)
514 try qtvs inst | not (tyVarsOfInst inst `intersectsVarSet` qtvs) = Free
515 | otherwise = ReduceMe
517 checkLoop doc qtvs givens wanteds try_me
519 zonkTcTyVarsAndFV qtvs `thenNF_Tc` \ qtvs' ->
520 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
521 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
524 reduceContext doc (try_me qtvs') givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
527 if no_improvement then
528 returnTc (frees, binds, irreds)
530 checkLoop doc qtvs givens' (irreds ++ frees) try_me `thenTc` \ (frees1, binds1, irreds1) ->
531 returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
533 complainCheck doc givens irreds
534 = mapNF_Tc zonkInst given_dicts `thenNF_Tc` \ givens' ->
535 mapNF_Tc (addNoInstanceErr doc given_dicts) irreds `thenNF_Tc_`
538 given_dicts = filter isDict givens
539 -- Filter out methods, which are only added to
540 -- the given set as an optimisation
545 %************************************************************************
547 \subsection{tcSimplifyAndCheck}
549 %************************************************************************
551 @tcSimplifyInferCheck@ is used when we know the consraints we are to simplify
552 against, but we don't know the type variables over which we are going to quantify.
553 This happens when we have a type signature for a mutually recursive
559 -> [TcTyVar] -- fv(T)
562 -> TcM ([TcTyVar], -- Variables over which to quantify
564 TcDictBinds) -- Bindings
566 tcSimplifyInferCheck doc tau_tvs givens wanted
567 = inferCheckLoop doc tau_tvs givens (lieToList wanted) `thenTc` \ (qtvs, frees, binds, irreds) ->
569 -- Complain about any irreducible ones
570 complainCheck doc givens irreds `thenNF_Tc_`
573 returnTc (qtvs, mkLIE frees, binds)
575 inferCheckLoop doc tau_tvs givens wanteds
577 zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
578 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
579 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
580 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
583 -- Figure out what we are going to generalise over
584 -- You might think it should just be the signature tyvars,
585 -- but in bizarre cases you can get extra ones
586 -- f :: forall a. Num a => a -> a
587 -- f x = fst (g (x, head [])) + 1
589 -- Here we infer g :: forall a b. a -> b -> (b,a)
590 -- We don't want g to be monomorphic in b just because
591 -- f isn't quantified over b.
592 qtvs = (tau_tvs' `unionVarSet` tyVarsOfInsts givens') `minusVarSet` gbl_tvs
593 -- We could close gbl_tvs, but its not necessary for
594 -- soundness, and it'll only affect which tyvars, not which
595 -- dictionaries, we quantify over
597 -- When checking against a given signature we always reduce
598 -- until we find a match against something given, or can't reduce
599 try_me inst | isFree qtvs inst = Free
600 | otherwise = ReduceMe
603 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
606 if no_improvement then
607 returnTc (varSetElems qtvs, frees, binds, irreds)
609 inferCheckLoop doc tau_tvs givens' (irreds ++ frees) `thenTc` \ (qtvs1, frees1, binds1, irreds1) ->
610 returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
614 %************************************************************************
616 \subsection{tcSimplifyToDicts}
618 %************************************************************************
620 On the LHS of transformation rules we only simplify methods and constants,
621 getting dictionaries. We want to keep all of them unsimplified, to serve
622 as the available stuff for the RHS of the rule.
624 The same thing is used for specialise pragmas. Consider
627 {-# SPECIALISE f :: Int -> Int #-}
630 The type checker generates a binding like:
632 f_spec = (f :: Int -> Int)
634 and we want to end up with
636 f_spec = _inline_me_ (f Int dNumInt)
638 But that means that we must simplify the Method for f to (f Int dNumInt)!
639 So tcSimplifyToDicts squeezes out all Methods.
641 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
643 fromIntegral :: (Integral a, Num b) => a -> b
644 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
646 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
650 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
652 because the scsel will mess up matching. Instead we want
654 forall dIntegralInt, dNumInt.
655 fromIntegral Int Int dIntegralInt dNumInt = id Int
657 Hence "DontReduce NoSCs"
660 tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
661 tcSimplifyToDicts wanted_lie
662 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
663 -- Since try_me doesn't look at types, we don't need to
664 -- do any zonking, so it's safe to call reduceContext directly
666 returnTc (irreds, binds)
669 doc = text "tcSimplifyToDicts"
670 wanteds = lieToList wanted_lie
672 -- Reduce methods and lits only; stop as soon as we get a dictionary
673 try_me inst | isDict inst = DontReduce NoSCs
674 | otherwise = ReduceMe
678 %************************************************************************
680 \subsection{Filtering at a dynamic binding}
682 %************************************************************************
687 we must discharge all the ?x constraints from B. We also do an improvement
688 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2. No need to iterate, though.
691 tcSimplifyIPs :: [Name] -- The implicit parameters bound here
693 -> TcM (LIE, TcDictBinds)
694 tcSimplifyIPs ip_names wanted_lie
695 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
696 -- The irreducible ones should be a subset of the implicit
697 -- parameters we provided
698 ASSERT( all here_ip irreds )
699 returnTc (mkLIE frees, binds)
702 doc = text "tcSimplifyIPs" <+> ppr ip_names
703 wanteds = lieToList wanted_lie
704 ip_set = mkNameSet ip_names
705 here_ip ip = isDict ip && ip `instMentionsIPs` ip_set
707 -- Simplify any methods that mention the implicit parameter
708 try_me inst | inst `instMentionsIPs` ip_set = ReduceMe
713 %************************************************************************
715 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
717 %************************************************************************
719 When doing a binding group, we may have @Insts@ of local functions.
720 For example, we might have...
722 let f x = x + 1 -- orig local function (overloaded)
723 f.1 = f Int -- two instances of f
728 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
729 where @f@ is in scope; those @Insts@ must certainly not be passed
730 upwards towards the top-level. If the @Insts@ were binding-ified up
731 there, they would have unresolvable references to @f@.
733 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
734 For each method @Inst@ in the @init_lie@ that mentions one of the
735 @Ids@, we create a binding. We return the remaining @Insts@ (in an
736 @LIE@), as well as the @HsBinds@ generated.
739 bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
741 bindInstsOfLocalFuns init_lie local_ids
742 | null overloaded_ids
744 = returnTc (init_lie, EmptyMonoBinds)
747 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
748 ASSERT( null irreds )
749 returnTc (mkLIE frees, binds)
751 doc = text "bindInsts" <+> ppr local_ids
752 wanteds = lieToList init_lie
753 overloaded_ids = filter is_overloaded local_ids
754 is_overloaded id = case splitSigmaTy (idType id) of
755 (_, theta, _) -> not (null theta)
757 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
758 -- so it's worth building a set, so that
759 -- lookup (in isMethodFor) is faster
761 try_me inst | isMethodFor overloaded_set inst = ReduceMe
766 %************************************************************************
768 \subsection{Data types for the reduction mechanism}
770 %************************************************************************
772 The main control over context reduction is here
776 = ReduceMe -- Try to reduce this
777 -- If there's no instance, behave exactly like
778 -- DontReduce: add the inst to
779 -- the irreductible ones, but don't
780 -- produce an error message of any kind.
781 -- It might be quite legitimate such as (Eq a)!
783 | DontReduce WantSCs -- Return as irreducible
785 | DontReduceUnlessConstant -- Return as irreducible unless it can
786 -- be reduced to a constant in one step
788 | Free -- Return as free
790 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
791 -- of a predicate when adding it to the avails
797 type RedState = (Avails, -- What's available
798 [Inst]) -- Insts for which try_me returned Free
800 type Avails = FiniteMap Inst Avail
803 = Irred -- Used for irreducible dictionaries,
804 -- which are going to be lambda bound
806 | BoundTo TcId -- Used for dictionaries for which we have a binding
807 -- e.g. those "given" in a signature
809 | NoRhs -- Used for Insts like (CCallable f)
810 -- where no witness is required.
812 | Rhs -- Used when there is a RHS
814 [Inst] -- Insts free in the RHS; we need these too
816 pprAvails avails = vcat [ppr inst <+> equals <+> pprAvail avail
817 | (inst,avail) <- fmToList avails ]
819 instance Outputable Avail where
822 pprAvail NoRhs = text "<no rhs>"
823 pprAvail Irred = text "Irred"
824 pprAvail (BoundTo x) = text "Bound to" <+> ppr x
825 pprAvail (Rhs rhs bs) = ppr rhs <+> braces (ppr bs)
828 Extracting the bindings from a bunch of Avails.
829 The bindings do *not* come back sorted in dependency order.
830 We assume that they'll be wrapped in a big Rec, so that the
831 dependency analyser can sort them out later
835 bindsAndIrreds :: Avails
837 -> (TcDictBinds, -- Bindings
838 [Inst]) -- Irreducible ones
840 bindsAndIrreds avails wanteds
841 = go avails EmptyMonoBinds [] wanteds
843 go avails binds irreds [] = (binds, irreds)
845 go avails binds irreds (w:ws)
846 = case lookupFM avails w of
847 Nothing -> -- Free guys come out here
848 -- (If we didn't do addFree we could use this as the
849 -- criterion for free-ness, and pick up the free ones here too)
850 go avails binds irreds ws
852 Just NoRhs -> go avails binds irreds ws
854 Just Irred -> go (addToFM avails w (BoundTo (instToId w))) binds (w:irreds) ws
856 Just (BoundTo id) -> go avails new_binds irreds ws
858 -- For implicit parameters, all occurrences share the same
859 -- Id, so there is no need for synonym bindings
860 new_binds | new_id == id = binds
861 | otherwise = addBind binds new_id (HsVar id)
864 Just (Rhs rhs ws') -> go avails' (addBind binds id rhs) irreds (ws' ++ ws)
867 avails' = addToFM avails w (BoundTo id)
869 addBind binds id rhs = binds `AndMonoBinds` VarMonoBind id rhs
873 %************************************************************************
875 \subsection[reduce]{@reduce@}
877 %************************************************************************
879 When the "what to do" predicate doesn't depend on the quantified type variables,
880 matters are easier. We don't need to do any zonking, unless the improvement step
881 does something, in which case we zonk before iterating.
883 The "given" set is always empty.
886 simpleReduceLoop :: SDoc
887 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
889 -> TcM ([Inst], -- Free
891 [Inst]) -- Irreducible
893 simpleReduceLoop doc try_me wanteds
894 = mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
895 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
896 if no_improvement then
897 returnTc (frees, binds, irreds)
899 simpleReduceLoop doc try_me (irreds ++ frees) `thenTc` \ (frees1, binds1, irreds1) ->
900 returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
906 reduceContext :: SDoc
907 -> (Inst -> WhatToDo)
910 -> NF_TcM (Bool, -- True <=> improve step did no unification
912 TcDictBinds, -- Dictionary bindings
913 [Inst]) -- Irreducible
915 reduceContext doc try_me givens wanteds
917 traceTc (text "reduceContext" <+> (vcat [
918 text "----------------------",
920 text "given" <+> ppr givens,
921 text "wanted" <+> ppr wanteds,
922 text "----------------------"
925 -- Build the Avail mapping from "givens"
926 foldlNF_Tc addGiven (emptyFM, []) givens `thenNF_Tc` \ init_state ->
929 reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ state@(avails, frees) ->
931 -- Do improvement, using everything in avails
932 -- In particular, avails includes all superclasses of everything
933 tcImprove avails `thenTc` \ no_improvement ->
935 traceTc (text "reduceContext end" <+> (vcat [
936 text "----------------------",
938 text "given" <+> ppr givens,
939 text "wanted" <+> ppr wanteds,
941 text "avails" <+> pprAvails avails,
942 text "frees" <+> ppr frees,
943 text "no_improvement =" <+> ppr no_improvement,
944 text "----------------------"
947 (binds, irreds) = bindsAndIrreds avails wanteds
949 returnTc (no_improvement, frees, binds, irreds)
952 = tcGetInstEnv `thenTc` \ inst_env ->
954 preds = predsOfInsts (keysFM avails)
955 -- Avails has all the superclasses etc (good)
956 -- It also has all the intermediates of the deduction (good)
957 -- It does not have duplicates (good)
958 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
959 -- so that improve will see them separate
960 eqns = improve (classInstEnv inst_env) preds
965 traceTc (ptext SLIT("Improve:") <+> vcat (map ppr_eqn eqns)) `thenNF_Tc_`
966 mapTc_ unify eqns `thenTc_`
969 unify (qtvs, t1, t2) = tcInstTyVars (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) ->
970 unifyTauTy (substTy tenv t1) (substTy tenv t2)
971 ppr_eqn (qtvs, t1, t2) = ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)) <+>
972 ppr t1 <+> equals <+> ppr t2
975 The main context-reduction function is @reduce@. Here's its game plan.
978 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
979 -- along with its depth
980 -> (Inst -> WhatToDo)
987 try_me: given an inst, this function returns
989 DontReduce return this in "irreds"
990 Free return this in "frees"
992 wanteds: The list of insts to reduce
993 state: An accumulating parameter of type RedState
994 that contains the state of the algorithm
996 It returns a RedState.
998 The (n,stack) pair is just used for error reporting.
999 n is always the depth of the stack.
1000 The stack is the stack of Insts being reduced: to produce X
1001 I had to produce Y, to produce Y I had to produce Z, and so on.
1004 reduceList (n,stack) try_me wanteds state
1005 | n > opt_MaxContextReductionDepth
1006 = failWithTc (reduceDepthErr n stack)
1012 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1017 go [] state = returnTc state
1018 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
1021 -- Base case: we're done!
1022 reduce stack try_me wanted state
1023 -- It's the same as an existing inst, or a superclass thereof
1024 | isAvailable state wanted
1028 = case try_me wanted of {
1030 DontReduce want_scs -> addIrred want_scs state wanted
1032 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1033 -- First, see if the inst can be reduced to a constant in one step
1034 try_simple (addIrred AddSCs) -- Assume want superclasses
1036 ; Free -> -- It's free so just chuck it upstairs
1037 -- First, see if the inst can be reduced to a constant in one step
1040 ; ReduceMe -> -- It should be reduced
1041 lookupInst wanted `thenNF_Tc` \ lookup_result ->
1042 case lookup_result of
1043 GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenTc` \ state' ->
1044 addWanted state' wanted rhs wanteds'
1045 SimpleInst rhs -> addWanted state wanted rhs []
1047 NoInstance -> -- No such instance!
1048 -- Add it and its superclasses
1049 addIrred AddSCs state wanted
1053 try_simple do_this_otherwise
1054 = lookupInst wanted `thenNF_Tc` \ lookup_result ->
1055 case lookup_result of
1056 SimpleInst rhs -> addWanted state wanted rhs []
1057 other -> do_this_otherwise state wanted
1062 isAvailable :: RedState -> Inst -> Bool
1063 isAvailable (avails, _) wanted = wanted `elemFM` avails
1064 -- NB: the Ord instance of Inst compares by the class/type info
1065 -- *not* by unique. So
1066 -- d1::C Int == d2::C Int
1068 -------------------------
1069 addFree :: RedState -> Inst -> NF_TcM RedState
1070 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1071 -- to avails, so that any other equal Insts will be commoned up right
1072 -- here rather than also being tossed upstairs. This is really just
1073 -- an optimisation, and perhaps it is more trouble that it is worth,
1074 -- as the following comments show!
1076 -- NB1: do *not* add superclasses. If we have
1079 -- but a is not bound here, then we *don't* want to derive
1080 -- dn from df here lest we lose sharing.
1082 -- NB2: do *not* add the Inst to avails at all if it's a method.
1083 -- The following situation shows why this is bad:
1084 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1085 -- From an application (truncate f i) we get
1086 -- t1 = truncate at f
1088 -- If we have also have a second occurrence of truncate, we get
1089 -- t3 = truncate at f
1091 -- When simplifying with i,f free, we might still notice that
1092 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1093 -- will continue to float out!
1094 -- Solution: never put methods in avail till they are captured
1095 -- in which case addFree isn't used
1097 -- NB3: make sure that CCallable/CReturnable use NoRhs rather
1098 -- than BoundTo, else we end up with bogus bindings.
1099 -- c.f. instBindingRequired in addWanted
1100 addFree (avails, frees) free
1101 | isDict free = returnNF_Tc (addToFM avails free avail, free:frees)
1102 | otherwise = returnNF_Tc (avails, free:frees)
1104 avail | instBindingRequired free = BoundTo (instToId free)
1107 addWanted :: RedState -> Inst -> TcExpr -> [Inst] -> NF_TcM RedState
1108 addWanted state@(avails, frees) wanted rhs_expr wanteds
1109 -- Do *not* add superclasses as well. Here's an example of why not
1110 -- class Eq a => Foo a b
1111 -- instance Eq a => Foo [a] a
1112 -- If we are reducing
1114 -- we'll first deduce that it holds (via the instance decl). We
1115 -- must not then overwrite the Eq t constraint with a superclass selection!
1116 -- ToDo: this isn't entirely unsatisfactory, because
1117 -- we may also lose some entirely-legitimate sharing this way
1119 = ASSERT( not (isAvailable state wanted) )
1120 returnNF_Tc (addToFM avails wanted avail, frees)
1122 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1123 | otherwise = ASSERT( null wanteds ) NoRhs
1125 addGiven :: RedState -> Inst -> NF_TcM RedState
1126 addGiven state given = addAvailAndSCs state given (BoundTo (instToId given))
1128 addIrred :: WantSCs -> RedState -> Inst -> NF_TcM RedState
1129 addIrred NoSCs (avails,frees) irred = returnNF_Tc (addToFM avails irred Irred, frees)
1130 addIrred AddSCs state irred = addAvailAndSCs state irred Irred
1132 addAvailAndSCs :: RedState -> Inst -> Avail -> NF_TcM RedState
1133 addAvailAndSCs (avails, frees) wanted avail
1134 = add_avail_and_scs avails wanted avail `thenNF_Tc` \ avails' ->
1135 returnNF_Tc (avails', frees)
1137 ---------------------
1138 add_avail_and_scs :: Avails -> Inst -> Avail -> NF_TcM Avails
1139 add_avail_and_scs avails wanted avail
1140 = add_scs (addToFM avails wanted avail) wanted
1142 add_scs :: Avails -> Inst -> NF_TcM Avails
1143 -- Add all the superclasses of the Inst to Avails
1144 -- Invariant: the Inst is already in Avails.
1147 | not (isClassDict dict)
1148 = returnNF_Tc avails
1150 | otherwise -- It is a dictionary
1151 = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
1152 foldlNF_Tc add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1154 (clas, tys) = getDictClassTys dict
1155 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1156 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1158 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1159 = case lookupFM avails sc_dict of
1160 Just (BoundTo _) -> returnNF_Tc avails -- See Note [SUPER] below
1161 other -> add_avail_and_scs avails sc_dict avail
1163 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1164 avail = Rhs sc_sel_rhs [dict]
1167 Note [SUPER]. We have to be careful here. If we are *given* d1:Ord a,
1168 and want to deduce (d2:C [a]) where
1170 class Ord a => C a where
1171 instance Ord a => C [a] where ...
1173 Then we'll use the instance decl to deduce C [a] and then add the
1174 superclasses of C [a] to avails. But we must not overwrite the binding
1175 for d1:Ord a (which is given) with a superclass selection or we'll just
1176 build a loop! Hence looking for BoundTo. Crudely, BoundTo is cheaper
1180 %************************************************************************
1182 \section{tcSimplifyTop: defaulting}
1184 %************************************************************************
1187 If a dictionary constrains a type variable which is
1188 * not mentioned in the environment
1189 * and not mentioned in the type of the expression
1190 then it is ambiguous. No further information will arise to instantiate
1191 the type variable; nor will it be generalised and turned into an extra
1192 parameter to a function.
1194 It is an error for this to occur, except that Haskell provided for
1195 certain rules to be applied in the special case of numeric types.
1197 * at least one of its classes is a numeric class, and
1198 * all of its classes are numeric or standard
1199 then the type variable can be defaulted to the first type in the
1200 default-type list which is an instance of all the offending classes.
1202 So here is the function which does the work. It takes the ambiguous
1203 dictionaries and either resolves them (producing bindings) or
1204 complains. It works by splitting the dictionary list by type
1205 variable, and using @disambigOne@ to do the real business.
1207 @tcSimplifyTop@ is called once per module to simplify all the constant
1208 and ambiguous Insts.
1210 We need to be careful of one case. Suppose we have
1212 instance Num a => Num (Foo a b) where ...
1214 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1215 to (Num x), and default x to Int. But what about y??
1217 It's OK: the final zonking stage should zap y to (), which is fine.
1221 tcSimplifyTop :: LIE -> TcM TcDictBinds
1222 tcSimplifyTop wanted_lie
1223 = simpleReduceLoop (text "tcSimplTop") try_me wanteds `thenTc` \ (frees, binds, irreds) ->
1224 ASSERT( null frees )
1227 -- All the non-std ones are definite errors
1228 (stds, non_stds) = partition isStdClassTyVarDict irreds
1230 -- Group by type variable
1231 std_groups = equivClasses cmp_by_tyvar stds
1233 -- Pick the ones which its worth trying to disambiguate
1234 (std_oks, std_bads) = partition worth_a_try std_groups
1236 -- Have a try at disambiguation
1237 -- if the type variable isn't bound
1238 -- up with one of the non-standard classes
1239 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1240 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1242 -- Collect together all the bad guys
1243 bad_guys = non_stds ++ concat std_bads
1245 -- Disambiguate the ones that look feasible
1246 mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
1248 -- And complain about the ones that don't
1249 -- This group includes both non-existent instances
1250 -- e.g. Num (IO a) and Eq (Int -> Int)
1251 -- and ambiguous dictionaries
1253 addTopAmbigErrs bad_guys `thenNF_Tc_`
1255 returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
1257 wanteds = lieToList wanted_lie
1258 try_me inst = ReduceMe
1260 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1262 get_tv d = case getDictClassTys d of
1263 (clas, [ty]) -> getTyVar "tcSimplifyTop" ty
1264 get_clas d = case getDictClassTys d of
1265 (clas, [ty]) -> clas
1268 @disambigOne@ assumes that its arguments dictionaries constrain all
1269 the same type variable.
1271 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1272 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1273 the most common use of defaulting is code like:
1275 _ccall_ foo `seqPrimIO` bar
1277 Since we're not using the result of @foo@, the result if (presumably)
1281 disambigGroup :: [Inst] -- All standard classes of form (C a)
1285 | any isNumericClass classes -- Guaranteed all standard classes
1286 -- see comment at the end of function for reasons as to
1287 -- why the defaulting mechanism doesn't apply to groups that
1288 -- include CCallable or CReturnable dicts.
1289 && not (any isCcallishClass classes)
1290 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1291 -- SO, TRY DEFAULT TYPES IN ORDER
1293 -- Failure here is caused by there being no type in the
1294 -- default list which can satisfy all the ambiguous classes.
1295 -- For example, if Real a is reqd, but the only type in the
1296 -- default list is Int.
1297 tcGetDefaultTys `thenNF_Tc` \ default_tys ->
1299 try_default [] -- No defaults work, so fail
1302 try_default (default_ty : default_tys)
1303 = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
1304 -- default_tys instead
1305 tcSimplifyCheckThetas [] theta `thenTc` \ _ ->
1308 theta = [mkClassPred clas [default_ty] | clas <- classes]
1310 -- See if any default works, and if so bind the type variable to it
1311 -- If not, add an AmbigErr
1312 recoverTc (addAmbigErrs dicts `thenNF_Tc_`
1313 returnTc EmptyMonoBinds) $
1315 try_default default_tys `thenTc` \ chosen_default_ty ->
1317 -- Bind the type variable and reduce the context, for real this time
1318 unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
1319 simpleReduceLoop (text "disambig" <+> ppr dicts)
1320 try_me dicts `thenTc` \ (frees, binds, ambigs) ->
1321 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1322 warnDefault dicts chosen_default_ty `thenTc_`
1325 | all isCreturnableClass classes
1326 = -- Default CCall stuff to (); we don't even both to check that () is an
1327 -- instance of CReturnable, because we know it is.
1328 unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
1329 returnTc EmptyMonoBinds
1331 | otherwise -- No defaults
1332 = addAmbigErrs dicts `thenNF_Tc_`
1333 returnTc EmptyMonoBinds
1336 try_me inst = ReduceMe -- This reduce should not fail
1337 tyvar = get_tv (head dicts) -- Should be non-empty
1338 classes = map get_clas dicts
1341 [Aside - why the defaulting mechanism is turned off when
1342 dealing with arguments and results to ccalls.
1344 When typechecking _ccall_s, TcExpr ensures that the external
1345 function is only passed arguments (and in the other direction,
1346 results) of a restricted set of 'native' types. This is
1347 implemented via the help of the pseudo-type classes,
1348 @CReturnable@ (CR) and @CCallable@ (CC.)
1350 The interaction between the defaulting mechanism for numeric
1351 values and CC & CR can be a bit puzzling to the user at times.
1360 What type has 'x' got here? That depends on the default list
1361 in operation, if it is equal to Haskell 98's default-default
1362 of (Integer, Double), 'x' has type Double, since Integer
1363 is not an instance of CR. If the default list is equal to
1364 Haskell 1.4's default-default of (Int, Double), 'x' has type
1367 To try to minimise the potential for surprises here, the
1368 defaulting mechanism is turned off in the presence of
1369 CCallable and CReturnable.
1374 %************************************************************************
1376 \subsection[simple]{@Simple@ versions}
1378 %************************************************************************
1380 Much simpler versions when there are no bindings to make!
1382 @tcSimplifyThetas@ simplifies class-type constraints formed by
1383 @deriving@ declarations and when specialising instances. We are
1384 only interested in the simplified bunch of class/type constraints.
1386 It simplifies to constraints of the form (C a b c) where
1387 a,b,c are type variables. This is required for the context of
1388 instance declarations.
1391 tcSimplifyThetas :: ThetaType -- Wanted
1392 -> TcM ThetaType -- Needed
1394 tcSimplifyThetas wanteds
1395 = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
1396 reduceSimple [] wanteds `thenNF_Tc` \ irreds ->
1398 -- For multi-param Haskell, check that the returned dictionaries
1399 -- don't have any of the form (C Int Bool) for which
1400 -- we expect an instance here
1401 -- For Haskell 98, check that all the constraints are of the form C a,
1402 -- where a is a type variable
1403 bad_guys | glaExts = [pred | pred <- irreds,
1404 isEmptyVarSet (tyVarsOfPred pred)]
1405 | otherwise = [pred | pred <- irreds,
1406 not (isTyVarClassPred pred)]
1408 if null bad_guys then
1411 mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
1415 @tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
1416 used with \tr{default} declarations. We are only interested in
1417 whether it worked or not.
1420 tcSimplifyCheckThetas :: ThetaType -- Given
1421 -> ThetaType -- Wanted
1424 tcSimplifyCheckThetas givens wanteds
1425 = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
1429 mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
1435 type AvailsSimple = FiniteMap PredType Bool
1436 -- True => irreducible
1437 -- False => given, or can be derived from a given or from an irreducible
1439 reduceSimple :: ThetaType -- Given
1440 -> ThetaType -- Wanted
1441 -> NF_TcM ThetaType -- Irreducible
1443 reduceSimple givens wanteds
1444 = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
1445 returnNF_Tc [pred | (pred,True) <- fmToList givens_fm']
1447 givens_fm = foldl addNonIrred emptyFM givens
1449 reduce_simple :: (Int,ThetaType) -- Stack
1452 -> NF_TcM AvailsSimple
1454 reduce_simple (n,stack) avails wanteds
1457 go avails [] = returnNF_Tc avails
1458 go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
1461 reduce_simple_help stack givens wanted
1462 | wanted `elemFM` givens
1463 = returnNF_Tc givens
1465 | Just (clas, tys) <- getClassPredTys_maybe wanted
1466 = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
1468 Nothing -> returnNF_Tc (addSimpleIrred givens wanted)
1469 Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
1472 = returnNF_Tc (addSimpleIrred givens wanted)
1474 addSimpleIrred :: AvailsSimple -> PredType -> AvailsSimple
1475 addSimpleIrred givens pred
1476 = addSCs (addToFM givens pred True) pred
1478 addNonIrred :: AvailsSimple -> PredType -> AvailsSimple
1479 addNonIrred givens pred
1480 = addSCs (addToFM givens pred False) pred
1483 | not (isClassPred pred) = givens
1484 | otherwise = foldl add givens sc_theta
1486 Just (clas,tys) = getClassPredTys_maybe pred
1487 (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
1488 sc_theta = substTheta (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
1491 = case lookupFM givens ct of
1492 Nothing -> -- Add it and its superclasses
1493 addSCs (addToFM givens ct False) ct
1495 Just True -> -- Set its flag to False; superclasses already done
1496 addToFM givens ct False
1498 Just False -> -- Already done
1504 %************************************************************************
1506 \section{Errors and contexts}
1508 %************************************************************************
1510 ToDo: for these error messages, should we note the location as coming
1511 from the insts, or just whatever seems to be around in the monad just
1515 addTopAmbigErrs dicts
1516 = mapNF_Tc complain tidy_dicts
1518 fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet
1519 (tidy_env, tidy_dicts) = tidyInsts dicts
1520 complain d | any isIPPred (predsOfInst d) = addTopIPErr tidy_env d
1521 | not (isTyVarDict d) ||
1522 tyVarsOfInst d `subVarSet` fixed_tvs = addTopInstanceErr tidy_env d
1523 | otherwise = addAmbigErr tidy_env d
1525 addTopIPErr tidy_env tidy_dict
1526 = addInstErrTcM (instLoc tidy_dict)
1528 ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
1530 -- Used for top-level irreducibles
1531 addTopInstanceErr tidy_env tidy_dict
1532 = addInstErrTcM (instLoc tidy_dict)
1534 ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
1537 = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts
1539 (tidy_env, tidy_dicts) = tidyInsts dicts
1541 addAmbigErr tidy_env tidy_dict
1542 = addInstErrTcM (instLoc tidy_dict)
1544 sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs,
1545 nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
1547 ambig_tvs = varSetElems (tyVarsOfInst tidy_dict)
1549 warnDefault dicts default_ty
1550 = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag ->
1551 tcAddSrcLoc (get_loc (head dicts)) (warnTc warn_flag warn_msg)
1554 (_, tidy_dicts) = tidyInsts dicts
1555 get_loc i = case instLoc i of { (_,loc,_) -> loc }
1556 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
1557 quotes (ppr default_ty),
1558 pprInstsInFull tidy_dicts]
1560 -- The error message when we don't find a suitable instance
1561 -- is complicated by the fact that sometimes this is because
1562 -- there is no instance, and sometimes it's because there are
1563 -- too many instances (overlap). See the comments in TcEnv.lhs
1564 -- with the InstEnv stuff.
1565 addNoInstanceErr what_doc givens dict
1566 = tcGetInstEnv `thenNF_Tc` \ inst_env ->
1568 doc = vcat [sep [herald <+> quotes (pprInst tidy_dict),
1569 nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
1571 ptext SLIT("Probable fix:"),
1575 herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
1576 unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
1580 | not ambig_overlap = empty
1582 = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
1583 nest 4 (ptext SLIT("depends on the instantiation of") <+>
1584 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst tidy_dict))))]
1586 fix1 = sep [ptext SLIT("Add") <+> quotes (pprInst tidy_dict),
1587 ptext SLIT("to the") <+> what_doc]
1589 fix2 | isTyVarDict dict || ambig_overlap
1592 = ptext SLIT("Or add an instance declaration for") <+> quotes (pprInst tidy_dict)
1594 (tidy_env, tidy_dict:tidy_givens) = tidyInsts (dict:givens)
1596 -- Checks for the ambiguous case when we have overlapping instances
1597 ambig_overlap | isClassDict dict
1598 = case lookupInstEnv inst_env clas tys of
1599 NoMatch ambig -> ambig
1603 (clas,tys) = getDictClassTys dict
1605 addInstErrTcM (instLoc dict) (tidy_env, doc)
1607 -- Used for the ...Thetas variants; all top level
1609 = addErrTc (ptext SLIT("No instance for") <+> quotes (ppr pred))
1611 reduceDepthErr n stack
1612 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
1613 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
1614 nest 4 (pprInstsInFull stack)]
1616 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)
1618 -----------------------------------------------
1620 = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
1621 nest 4 (ppr inst <+> pprInstLoc (instLoc inst))])