2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcSimplify]{TcSimplify}
10 tcSimplifyInfer, tcSimplifyInferCheck,
11 tcSimplifyCheck, tcSimplifyRestricted,
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,
28 isDict, isClassDict, instName,
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 TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, unifyTauTy )
42 import TcType ( ThetaType, PredType, mkClassPred, isOverloadedTy,
43 mkTyVarTy, tcGetTyVar, isTyVarClassPred,
44 tyVarsOfPred, getClassPredTys_maybe, isClassPred, isIPPred,
45 inheritablePred, predHasFDs )
47 import NameSet ( mkNameSet )
48 import Class ( classBigSig )
49 import FunDeps ( oclose, grow, improve )
50 import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
52 import Subst ( mkTopTyVarSubst, substTheta, substTy )
53 import TysWiredIn ( unitTy )
57 import ListSetOps ( equivClasses )
58 import Util ( zipEqual )
59 import List ( partition )
64 %************************************************************************
68 %************************************************************************
70 --------------------------------------
71 Notes on quantification
72 --------------------------------------
74 Suppose we are about to do a generalisation step.
79 C the constraints from that RHS
81 The game is to figure out
83 Q the set of type variables over which to quantify
84 Ct the constraints we will *not* quantify over
85 Cq the constraints we will quantify over
87 So we're going to infer the type
91 and float the constraints Ct further outwards.
93 Here are the things that *must* be true:
95 (A) Q intersect fv(G) = EMPTY limits how big Q can be
96 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
98 (A) says we can't quantify over a variable that's free in the
99 environment. (B) says we must quantify over all the truly free
100 variables in T, else we won't get a sufficiently general type. We do
101 not *need* to quantify over any variable that is fixed by the free
102 vars of the environment G.
104 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
106 Example: class H x y | x->y where ...
108 fv(G) = {a} C = {H a b, H c d}
111 (A) Q intersect {a} is empty
112 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
114 So Q can be {c,d}, {b,c,d}
116 Other things being equal, however, we'd like to quantify over as few
117 variables as possible: smaller types, fewer type applications, more
118 constraints can get into Ct instead of Cq.
121 -----------------------------------------
124 fv(T) the free type vars of T
126 oclose(vs,C) The result of extending the set of tyvars vs
127 using the functional dependencies from C
129 grow(vs,C) The result of extend the set of tyvars vs
130 using all conceivable links from C.
132 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
133 Then grow(vs,C) = {a,b,c}
135 Note that grow(vs,C) `superset` grow(vs,simplify(C))
136 That is, simplfication can only shrink the result of grow.
139 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
140 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
143 -----------------------------------------
147 Here's a good way to choose Q:
149 Q = grow( fv(T), C ) \ oclose( fv(G), C )
151 That is, quantify over all variable that that MIGHT be fixed by the
152 call site (which influences T), but which aren't DEFINITELY fixed by
153 G. This choice definitely quantifies over enough type variables,
154 albeit perhaps too many.
156 Why grow( fv(T), C ) rather than fv(T)? Consider
158 class H x y | x->y where ...
163 If we used fv(T) = {c} we'd get the type
165 forall c. H c d => c -> b
167 And then if the fn was called at several different c's, each of
168 which fixed d differently, we'd get a unification error, because
169 d isn't quantified. Solution: quantify d. So we must quantify
170 everything that might be influenced by c.
172 Why not oclose( fv(T), C )? Because we might not be able to see
173 all the functional dependencies yet:
175 class H x y | x->y where ...
176 instance H x y => Eq (T x y) where ...
181 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
182 apparent yet, and that's wrong. We must really quantify over d too.
185 There really isn't any point in quantifying over any more than
186 grow( fv(T), C ), because the call sites can't possibly influence
187 any other type variables.
191 --------------------------------------
193 --------------------------------------
195 It's very hard to be certain when a type is ambiguous. Consider
199 instance H x y => K (x,y)
201 Is this type ambiguous?
202 forall a b. (K (a,b), Eq b) => a -> a
204 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
205 now we see that a fixes b. So we can't tell about ambiguity for sure
206 without doing a full simplification. And even that isn't possible if
207 the context has some free vars that may get unified. Urgle!
209 Here's another example: is this ambiguous?
210 forall a b. Eq (T b) => a -> a
211 Not if there's an insance decl (with no context)
212 instance Eq (T b) where ...
214 You may say of this example that we should use the instance decl right
215 away, but you can't always do that:
217 class J a b where ...
218 instance J Int b where ...
220 f :: forall a b. J a b => a -> a
222 (Notice: no functional dependency in J's class decl.)
223 Here f's type is perfectly fine, provided f is only called at Int.
224 It's premature to complain when meeting f's signature, or even
225 when inferring a type for f.
229 However, we don't *need* to report ambiguity right away. It'll always
230 show up at the call site.... and eventually at main, which needs special
231 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
233 So here's the plan. We WARN about probable ambiguity if
235 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
237 (all tested before quantification).
238 That is, all the type variables in Cq must be fixed by the the variables
239 in the environment, or by the variables in the type.
241 Notice that we union before calling oclose. Here's an example:
243 class J a b c | a b -> c
247 forall b c. (J a b c) => b -> b
249 Only if we union {a} from G with {b} from T before using oclose,
250 do we see that c is fixed.
252 It's a bit vague exactly which C we should use for this oclose call. If we
253 don't fix enough variables we might complain when we shouldn't (see
254 the above nasty example). Nothing will be perfect. That's why we can
255 only issue a warning.
258 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
260 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
262 then c is a "bubble"; there's no way it can ever improve, and it's
263 certainly ambiguous. UNLESS it is a constant (sigh). And what about
268 instance H x y => K (x,y)
270 Is this type ambiguous?
271 forall a b. (K (a,b), Eq b) => a -> a
273 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
274 is a "bubble" that's a set of constraints
276 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
278 Hence another idea. To decide Q start with fv(T) and grow it
279 by transitive closure in Cq (no functional dependencies involved).
280 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
281 The definitely-ambiguous can then float out, and get smashed at top level
282 (which squashes out the constants, like Eq (T a) above)
285 --------------------------------------
286 Notes on principal types
287 --------------------------------------
292 f x = let g y = op (y::Int) in True
294 Here the principal type of f is (forall a. a->a)
295 but we'll produce the non-principal type
296 f :: forall a. C Int => a -> a
299 --------------------------------------
300 Notes on implicit parameters
301 --------------------------------------
303 Question 1: can we "inherit" implicit parameters
304 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
309 where f is *not* a top-level binding.
310 From the RHS of f we'll get the constraint (?y::Int).
311 There are two types we might infer for f:
315 (so we get ?y from the context of f's definition), or
317 f :: (?y::Int) => Int -> Int
319 At first you might think the first was better, becuase then
320 ?y behaves like a free variable of the definition, rather than
321 having to be passed at each call site. But of course, the WHOLE
322 IDEA is that ?y should be passed at each call site (that's what
323 dynamic binding means) so we'd better infer the second.
325 BOTTOM LINE: you *must* quantify over implicit parameters. See
326 isFreeAndInheritable.
328 BUT WATCH OUT: for *expressions*, this isn't right. Consider:
332 This is perfectly reasonable. We do not want to insist on
334 (?x + 1) :: (?x::Int => Int)
336 That would be silly. Here, the definition site *is* the occurrence site,
337 so the above strictures don't apply. Hence the difference between
338 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
339 and tcSimplifyCheckBind (which does not).
342 Question 2: type signatures
343 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
344 OK, so is it legal to give an explicit, user type signature to f, thus:
349 At first sight this seems reasonable, but it has the nasty property
350 that adding a type signature changes the dynamic semantics.
353 (let f x = (x::Int) + ?y
354 in (f 3, f 3 with ?y=5)) with ?y = 6
360 in (f 3, f 3 with ?y=5)) with ?y = 6
364 Indeed, simply inlining f (at the Haskell source level) would change the
367 Conclusion: the above type signature is illegal. You'll get a message
368 of the form "could not deduce (?y::Int) from ()".
371 Question 3: monomorphism
372 ~~~~~~~~~~~~~~~~~~~~~~~~
373 There's a nasty corner case when the monomorphism restriction bites:
377 The argument above suggests that we *must* generalise
378 over the ?y parameter, to get
379 z :: (?y::Int) => Int,
380 but the monomorphism restriction says that we *must not*, giving
382 Why does the momomorphism restriction say this? Because if you have
384 let z = x + ?y in z+z
386 you might not expect the addition to be done twice --- but it will if
387 we follow the argument of Question 2 and generalise over ?y.
393 (A) Always generalise over implicit parameters
394 Bindings that fall under the monomorphism restriction can't
398 * Inlining remains valid
399 * No unexpected loss of sharing
400 * But simple bindings like
402 will be rejected, unless you add an explicit type signature
403 (to avoid the monomorphism restriction)
404 z :: (?y::Int) => Int
406 This seems unacceptable
408 (B) Monomorphism restriction "wins"
409 Bindings that fall under the monomorphism restriction can't
411 Always generalise over implicit parameters *except* for bindings
412 that fall under the monomorphism restriction
415 * Inlining isn't valid in general
416 * No unexpected loss of sharing
417 * Simple bindings like
419 accepted (get value of ?y from binding site)
421 (C) Always generalise over implicit parameters
422 Bindings that fall under the monomorphism restriction can't
423 be generalised, EXCEPT for implicit parameters
425 * Inlining remains valid
426 * Unexpected loss of sharing (from the extra generalisation)
427 * Simple bindings like
429 accepted (get value of ?y from occurrence sites)
434 None of these choices seems very satisfactory. But at least we should
435 decide which we want to do.
437 It's really not clear what is the Right Thing To Do. If you see
441 would you expect the value of ?y to be got from the *occurrence sites*
442 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
443 case of function definitions, the answer is clearly the former, but
444 less so in the case of non-fucntion definitions. On the other hand,
445 if we say that we get the value of ?y from the definition site of 'z',
446 then inlining 'z' might change the semantics of the program.
448 Choice (C) really says "the monomorphism restriction doesn't apply
449 to implicit parameters". Which is fine, but remember that every
450 innocent binding 'x = ...' that mentions an implicit parameter in
451 the RHS becomes a *function* of that parameter, called at each
452 use of 'x'. Now, the chances are that there are no intervening 'with'
453 clauses that bind ?y, so a decent compiler should common up all
454 those function calls. So I think I strongly favour (C). Indeed,
455 one could make a similar argument for abolishing the monomorphism
456 restriction altogether.
458 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
462 %************************************************************************
464 \subsection{tcSimplifyInfer}
466 %************************************************************************
468 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
470 1. Compute Q = grow( fvs(T), C )
472 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
473 predicates will end up in Ct; we deal with them at the top level
475 3. Try improvement, using functional dependencies
477 4. If Step 3 did any unification, repeat from step 1
478 (Unification can change the result of 'grow'.)
480 Note: we don't reduce dictionaries in step 2. For example, if we have
481 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
482 after step 2. However note that we may therefore quantify over more
483 type variables than we absolutely have to.
485 For the guts, we need a loop, that alternates context reduction and
486 improvement with unification. E.g. Suppose we have
488 class C x y | x->y where ...
490 and tcSimplify is called with:
492 Then improvement unifies a with b, giving
495 If we need to unify anything, we rattle round the whole thing all over
502 -> TcTyVarSet -- fv(T); type vars
504 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
506 TcDictBinds, -- Bindings
507 [TcId]) -- Dict Ids that must be bound here (zonked)
512 tcSimplifyInfer doc tau_tvs wanted_lie
513 = inferLoop doc (varSetElems tau_tvs)
514 (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
516 -- Check for non-generalisable insts
517 mapTc_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenTc_`
519 returnTc (qtvs, mkLIE frees, binds, map instToId irreds)
521 inferLoop doc tau_tvs wanteds
523 zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
524 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
525 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
527 preds = predsOfInsts wanteds'
528 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
531 | isFreeAndInheritable qtvs inst = Free
532 | isClassDict inst = DontReduceUnlessConstant -- Dicts
533 | otherwise = ReduceMe -- Lits and Methods
536 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
539 if no_improvement then
540 returnTc (varSetElems qtvs, frees, binds, irreds)
542 -- If improvement did some unification, we go round again. There
543 -- are two subtleties:
544 -- a) We start again with irreds, not wanteds
545 -- Using an instance decl might have introduced a fresh type variable
546 -- which might have been unified, so we'd get an infinite loop
547 -- if we started again with wanteds! See example [LOOP]
549 -- b) It's also essential to re-process frees, because unification
550 -- might mean that a type variable that looked free isn't now.
552 -- Hence the (irreds ++ frees)
554 inferLoop doc tau_tvs (irreds ++ frees) `thenTc` \ (qtvs1, frees1, binds1, irreds1) ->
555 returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
560 class If b t e r | b t e -> r
563 class Lte a b c | a b -> c where lte :: a -> b -> c
565 instance (Lte a b l,If l b a c) => Max a b c
567 Wanted: Max Z (S x) y
569 Then we'll reduce using the Max instance to:
570 (Lte Z (S x) l, If l (S x) Z y)
571 and improve by binding l->T, after which we can do some reduction
572 on both the Lte and If constraints. What we *can't* do is start again
573 with (Max Z (S x) y)!
576 isFreeAndInheritable qtvs inst
577 = isFree qtvs inst -- Constrains no quantified vars
578 && all inheritablePred (predsOfInst inst) -- And no implicit parameter involved
579 -- (see "Notes on implicit parameters")
582 = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
586 %************************************************************************
588 \subsection{tcSimplifyCheck}
590 %************************************************************************
592 @tcSimplifyCheck@ is used when we know exactly the set of variables
593 we are going to quantify over. For example, a class or instance declaration.
598 -> [TcTyVar] -- Quantify over these
602 TcDictBinds) -- Bindings
604 -- tcSimplifyCheck is used when checking exprssion type signatures,
605 -- class decls, instance decls etc.
606 -- Note that we psss isFree (not isFreeAndInheritable) to tcSimplCheck
607 -- It's important that we can float out non-inheritable predicates
608 -- Example: (?x :: Int) is ok!
609 tcSimplifyCheck doc qtvs givens wanted_lie
610 = tcSimplCheck doc isFree get_qtvs
611 givens wanted_lie `thenTc` \ (qtvs', frees, binds) ->
612 returnTc (frees, binds)
614 get_qtvs = zonkTcTyVarsAndFV qtvs
617 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
618 -- against, but we don't know the type variables over which we are going to quantify.
619 -- This happens when we have a type signature for a mutually recursive group
622 -> TcTyVarSet -- fv(T)
625 -> TcM ([TcTyVar], -- Variables over which to quantify
627 TcDictBinds) -- Bindings
629 tcSimplifyInferCheck doc tau_tvs givens wanted_lie
630 = tcSimplCheck doc isFreeAndInheritable get_qtvs givens wanted_lie
632 -- Figure out which type variables to quantify over
633 -- You might think it should just be the signature tyvars,
634 -- but in bizarre cases you can get extra ones
635 -- f :: forall a. Num a => a -> a
636 -- f x = fst (g (x, head [])) + 1
638 -- Here we infer g :: forall a b. a -> b -> (b,a)
639 -- We don't want g to be monomorphic in b just because
640 -- f isn't quantified over b.
641 all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
643 get_qtvs = zonkTcTyVarsAndFV all_tvs `thenNF_Tc` \ all_tvs' ->
644 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
646 qtvs = all_tvs' `minusVarSet` gbl_tvs
647 -- We could close gbl_tvs, but its not necessary for
648 -- soundness, and it'll only affect which tyvars, not which
649 -- dictionaries, we quantify over
654 Here is the workhorse function for all three wrappers.
657 tcSimplCheck doc is_free get_qtvs givens wanted_lie
658 = check_loop givens (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
660 -- Complain about any irreducible ones
661 complainCheck doc givens irreds `thenNF_Tc_`
664 returnTc (qtvs, mkLIE frees, binds)
667 check_loop givens wanteds
669 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
670 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
671 get_qtvs `thenNF_Tc` \ qtvs' ->
675 -- When checking against a given signature we always reduce
676 -- until we find a match against something given, or can't reduce
677 try_me inst | is_free qtvs' inst = Free
678 | otherwise = ReduceMe
680 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
683 if no_improvement then
684 returnTc (varSetElems qtvs', frees, binds, irreds)
686 check_loop givens' (irreds ++ frees) `thenTc` \ (qtvs', frees1, binds1, irreds1) ->
687 returnTc (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1)
689 complainCheck doc givens irreds
690 = mapNF_Tc zonkInst given_dicts `thenNF_Tc` \ givens' ->
691 mapNF_Tc (addNoInstanceErr doc given_dicts) irreds `thenNF_Tc_`
694 given_dicts = filter isDict givens
695 -- Filter out methods, which are only added to
696 -- the given set as an optimisation
700 %************************************************************************
702 \subsection{tcSimplifyRestricted}
704 %************************************************************************
707 tcSimplifyRestricted -- Used for restricted binding groups
708 -- i.e. ones subject to the monomorphism restriction
710 -> TcTyVarSet -- Free in the type of the RHSs
711 -> LIE -- Free in the RHSs
712 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
714 TcDictBinds) -- Bindings
716 tcSimplifyRestricted doc tau_tvs wanted_lie
717 = -- First squash out all methods, to find the constrained tyvars
718 -- We can't just take the free vars of wanted_lie because that'll
719 -- have methods that may incidentally mention entirely unconstrained variables
720 -- e.g. a call to f :: Eq a => a -> b -> b
721 -- Here, b is unconstrained. A good example would be
723 -- We want to infer the polymorphic type
724 -- foo :: forall b. b -> b
725 tcSimplifyToDicts wanted_lie `thenTc` \ (dicts, _) ->
727 constrained_tvs = tyVarsOfInsts dicts
730 -- Next, figure out the tyvars we will quantify over
731 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenNF_Tc` \ tau_tvs' ->
732 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
734 qtvs = (tau_tvs' `minusVarSet` oclose (predsOfInsts dicts) gbl_tvs)
735 `minusVarSet` constrained_tvs
738 -- The first step may have squashed more methods than
739 -- necessary, so try again, this time knowing the exact
740 -- set of type variables to quantify over.
742 -- We quantify only over constraints that are captured by qtvs;
743 -- these will just be a subset of non-dicts. This in contrast
744 -- to normal inference (using isFreeAndInheritable) in which we quantify over
745 -- all *non-inheritable* constraints too. This implements choice
746 -- (B) under "implicit parameter and monomorphism" above.
747 mapNF_Tc zonkInst (lieToList wanted_lie) `thenNF_Tc` \ wanteds' ->
749 try_me inst | isFree qtvs inst = Free
750 | otherwise = ReduceMe
752 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
753 ASSERT( no_improvement )
754 ASSERT( null irreds )
755 -- No need to loop because tcSimplifyToDicts will have
756 -- already done any improvement necessary
758 returnTc (varSetElems qtvs, mkLIE frees, binds)
762 %************************************************************************
764 \subsection{tcSimplifyToDicts}
766 %************************************************************************
768 On the LHS of transformation rules we only simplify methods and constants,
769 getting dictionaries. We want to keep all of them unsimplified, to serve
770 as the available stuff for the RHS of the rule.
772 The same thing is used for specialise pragmas. Consider
775 {-# SPECIALISE f :: Int -> Int #-}
778 The type checker generates a binding like:
780 f_spec = (f :: Int -> Int)
782 and we want to end up with
784 f_spec = _inline_me_ (f Int dNumInt)
786 But that means that we must simplify the Method for f to (f Int dNumInt)!
787 So tcSimplifyToDicts squeezes out all Methods.
789 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
791 fromIntegral :: (Integral a, Num b) => a -> b
792 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
794 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
798 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
800 because the scsel will mess up matching. Instead we want
802 forall dIntegralInt, dNumInt.
803 fromIntegral Int Int dIntegralInt dNumInt = id Int
805 Hence "DontReduce NoSCs"
808 tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
809 tcSimplifyToDicts wanted_lie
810 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
811 -- Since try_me doesn't look at types, we don't need to
812 -- do any zonking, so it's safe to call reduceContext directly
814 returnTc (irreds, binds)
817 doc = text "tcSimplifyToDicts"
818 wanteds = lieToList wanted_lie
820 -- Reduce methods and lits only; stop as soon as we get a dictionary
821 try_me inst | isDict inst = DontReduce NoSCs
822 | otherwise = ReduceMe
826 %************************************************************************
828 \subsection{Filtering at a dynamic binding}
830 %************************************************************************
835 we must discharge all the ?x constraints from B. We also do an improvement
836 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
838 Actually, the constraints from B might improve the types in ?x. For example
840 f :: (?x::Int) => Char -> Char
843 then the constraint (?x::Int) arising from the call to f will
844 force the binding for ?x to be of type Int.
847 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
849 -> TcM (LIE, TcDictBinds)
850 tcSimplifyIPs given_ips wanted_lie
851 = simpl_loop given_ips wanteds `thenTc` \ (frees, binds) ->
852 returnTc (mkLIE frees, binds)
854 doc = text "tcSimplifyIPs" <+> ppr ip_names
855 wanteds = lieToList wanted_lie
856 ip_names = map instName given_ips
857 ip_set = mkNameSet ip_names
859 -- Simplify any methods that mention the implicit parameter
860 try_me inst | inst `instMentionsIPs` ip_set = ReduceMe
863 simpl_loop givens wanteds
864 = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
865 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
867 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
869 if no_improvement then
870 ASSERT( null irreds )
871 returnTc (frees, binds)
873 simpl_loop givens' (irreds ++ frees) `thenTc` \ (frees1, binds1) ->
874 returnTc (frees1, binds `AndMonoBinds` binds1)
878 %************************************************************************
880 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
882 %************************************************************************
884 When doing a binding group, we may have @Insts@ of local functions.
885 For example, we might have...
887 let f x = x + 1 -- orig local function (overloaded)
888 f.1 = f Int -- two instances of f
893 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
894 where @f@ is in scope; those @Insts@ must certainly not be passed
895 upwards towards the top-level. If the @Insts@ were binding-ified up
896 there, they would have unresolvable references to @f@.
898 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
899 For each method @Inst@ in the @init_lie@ that mentions one of the
900 @Ids@, we create a binding. We return the remaining @Insts@ (in an
901 @LIE@), as well as the @HsBinds@ generated.
904 bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
906 bindInstsOfLocalFuns init_lie local_ids
907 | null overloaded_ids
909 = returnTc (init_lie, EmptyMonoBinds)
912 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
913 ASSERT( null irreds )
914 returnTc (mkLIE frees, binds)
916 doc = text "bindInsts" <+> ppr local_ids
917 wanteds = lieToList init_lie
918 overloaded_ids = filter is_overloaded local_ids
919 is_overloaded id = isOverloadedTy (idType id)
921 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
922 -- so it's worth building a set, so that
923 -- lookup (in isMethodFor) is faster
925 try_me inst | isMethodFor overloaded_set inst = ReduceMe
930 %************************************************************************
932 \subsection{Data types for the reduction mechanism}
934 %************************************************************************
936 The main control over context reduction is here
940 = ReduceMe -- Try to reduce this
941 -- If there's no instance, behave exactly like
942 -- DontReduce: add the inst to
943 -- the irreductible ones, but don't
944 -- produce an error message of any kind.
945 -- It might be quite legitimate such as (Eq a)!
947 | DontReduce WantSCs -- Return as irreducible
949 | DontReduceUnlessConstant -- Return as irreducible unless it can
950 -- be reduced to a constant in one step
952 | Free -- Return as free
954 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
955 -- of a predicate when adding it to the avails
961 type RedState = (Avails, -- What's available
962 [Inst]) -- Insts for which try_me returned Free
964 type Avails = FiniteMap Inst Avail
967 = Irred -- Used for irreducible dictionaries,
968 -- which are going to be lambda bound
970 | BoundTo TcId -- Used for dictionaries for which we have a binding
971 -- e.g. those "given" in a signature
973 | NoRhs -- Used for Insts like (CCallable f)
974 -- where no witness is required.
976 | Rhs -- Used when there is a RHS
978 [Inst] -- Insts free in the RHS; we need these too
980 pprAvails avails = vcat [ppr inst <+> equals <+> pprAvail avail
981 | (inst,avail) <- fmToList avails ]
983 instance Outputable Avail where
986 pprAvail NoRhs = text "<no rhs>"
987 pprAvail Irred = text "Irred"
988 pprAvail (BoundTo x) = text "Bound to" <+> ppr x
989 pprAvail (Rhs rhs bs) = ppr rhs <+> braces (ppr bs)
992 Extracting the bindings from a bunch of Avails.
993 The bindings do *not* come back sorted in dependency order.
994 We assume that they'll be wrapped in a big Rec, so that the
995 dependency analyser can sort them out later
999 bindsAndIrreds :: Avails
1001 -> (TcDictBinds, -- Bindings
1002 [Inst]) -- Irreducible ones
1004 bindsAndIrreds avails wanteds
1005 = go avails EmptyMonoBinds [] wanteds
1007 go avails binds irreds [] = (binds, irreds)
1009 go avails binds irreds (w:ws)
1010 = case lookupFM avails w of
1011 Nothing -> -- Free guys come out here
1012 -- (If we didn't do addFree we could use this as the
1013 -- criterion for free-ness, and pick up the free ones here too)
1014 go avails binds irreds ws
1016 Just NoRhs -> go avails binds irreds ws
1018 Just Irred -> go (addToFM avails w (BoundTo (instToId w))) binds (w:irreds) ws
1020 Just (BoundTo id) -> go avails new_binds irreds ws
1022 -- For implicit parameters, all occurrences share the same
1023 -- Id, so there is no need for synonym bindings
1024 new_binds | new_id == id = binds
1025 | otherwise = addBind binds new_id (HsVar id)
1028 Just (Rhs rhs ws') -> go avails' (addBind binds id rhs) irreds (ws' ++ ws)
1031 avails' = addToFM avails w (BoundTo id)
1033 addBind binds id rhs = binds `AndMonoBinds` VarMonoBind id rhs
1037 %************************************************************************
1039 \subsection[reduce]{@reduce@}
1041 %************************************************************************
1043 When the "what to do" predicate doesn't depend on the quantified type variables,
1044 matters are easier. We don't need to do any zonking, unless the improvement step
1045 does something, in which case we zonk before iterating.
1047 The "given" set is always empty.
1050 simpleReduceLoop :: SDoc
1051 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1053 -> TcM ([Inst], -- Free
1055 [Inst]) -- Irreducible
1057 simpleReduceLoop doc try_me wanteds
1058 = mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
1059 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
1060 if no_improvement then
1061 returnTc (frees, binds, irreds)
1063 simpleReduceLoop doc try_me (irreds ++ frees) `thenTc` \ (frees1, binds1, irreds1) ->
1064 returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
1070 reduceContext :: SDoc
1071 -> (Inst -> WhatToDo)
1074 -> NF_TcM (Bool, -- True <=> improve step did no unification
1076 TcDictBinds, -- Dictionary bindings
1077 [Inst]) -- Irreducible
1079 reduceContext doc try_me givens wanteds
1081 traceTc (text "reduceContext" <+> (vcat [
1082 text "----------------------",
1084 text "given" <+> ppr givens,
1085 text "wanted" <+> ppr wanteds,
1086 text "----------------------"
1089 -- Build the Avail mapping from "givens"
1090 foldlNF_Tc addGiven (emptyFM, []) givens `thenNF_Tc` \ init_state ->
1093 reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ state@(avails, frees) ->
1095 -- Do improvement, using everything in avails
1096 -- In particular, avails includes all superclasses of everything
1097 tcImprove avails `thenTc` \ no_improvement ->
1099 traceTc (text "reduceContext end" <+> (vcat [
1100 text "----------------------",
1102 text "given" <+> ppr givens,
1103 text "wanted" <+> ppr wanteds,
1105 text "avails" <+> pprAvails avails,
1106 text "frees" <+> ppr frees,
1107 text "no_improvement =" <+> ppr no_improvement,
1108 text "----------------------"
1111 (binds, irreds) = bindsAndIrreds avails wanteds
1113 returnTc (no_improvement, frees, binds, irreds)
1116 = tcGetInstEnv `thenTc` \ inst_env ->
1118 preds = [ (pred, pp_loc)
1119 | inst <- keysFM avails,
1120 let pp_loc = pprInstLoc (instLoc inst),
1121 pred <- predsOfInst inst,
1124 -- Avails has all the superclasses etc (good)
1125 -- It also has all the intermediates of the deduction (good)
1126 -- It does not have duplicates (good)
1127 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1128 -- so that improve will see them separate
1129 eqns = improve (classInstEnv inst_env) preds
1134 traceTc (ptext SLIT("Improve:") <+> vcat (map ppr_eqn eqns)) `thenNF_Tc_`
1135 mapTc_ unify eqns `thenTc_`
1138 unify ((qtvs, t1, t2), doc)
1139 = tcAddErrCtxt doc $
1140 tcInstTyVars (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) ->
1141 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1142 ppr_eqn ((qtvs, t1, t2), doc)
1143 = vcat [ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs))
1144 <+> ppr t1 <+> ptext SLIT(":=:") <+> ppr t2,
1148 The main context-reduction function is @reduce@. Here's its game plan.
1151 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1152 -- along with its depth
1153 -> (Inst -> WhatToDo)
1160 try_me: given an inst, this function returns
1162 DontReduce return this in "irreds"
1163 Free return this in "frees"
1165 wanteds: The list of insts to reduce
1166 state: An accumulating parameter of type RedState
1167 that contains the state of the algorithm
1169 It returns a RedState.
1171 The (n,stack) pair is just used for error reporting.
1172 n is always the depth of the stack.
1173 The stack is the stack of Insts being reduced: to produce X
1174 I had to produce Y, to produce Y I had to produce Z, and so on.
1177 reduceList (n,stack) try_me wanteds state
1178 | n > opt_MaxContextReductionDepth
1179 = failWithTc (reduceDepthErr n stack)
1185 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1190 go [] state = returnTc state
1191 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
1194 -- Base case: we're done!
1195 reduce stack try_me wanted state
1196 -- It's the same as an existing inst, or a superclass thereof
1197 | isAvailable state wanted
1201 = case try_me wanted of {
1203 DontReduce want_scs -> addIrred want_scs state wanted
1205 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1206 -- First, see if the inst can be reduced to a constant in one step
1207 try_simple (addIrred AddSCs) -- Assume want superclasses
1209 ; Free -> -- It's free so just chuck it upstairs
1210 -- First, see if the inst can be reduced to a constant in one step
1213 ; ReduceMe -> -- It should be reduced
1214 lookupInst wanted `thenNF_Tc` \ lookup_result ->
1215 case lookup_result of
1216 GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenTc` \ state' ->
1217 addWanted state' wanted rhs wanteds'
1218 SimpleInst rhs -> addWanted state wanted rhs []
1220 NoInstance -> -- No such instance!
1221 -- Add it and its superclasses
1222 addIrred AddSCs state wanted
1226 try_simple do_this_otherwise
1227 = lookupInst wanted `thenNF_Tc` \ lookup_result ->
1228 case lookup_result of
1229 SimpleInst rhs -> addWanted state wanted rhs []
1230 other -> do_this_otherwise state wanted
1235 isAvailable :: RedState -> Inst -> Bool
1236 isAvailable (avails, _) wanted = wanted `elemFM` avails
1237 -- NB: the Ord instance of Inst compares by the class/type info
1238 -- *not* by unique. So
1239 -- d1::C Int == d2::C Int
1241 -------------------------
1242 addFree :: RedState -> Inst -> NF_TcM RedState
1243 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1244 -- to avails, so that any other equal Insts will be commoned up right
1245 -- here rather than also being tossed upstairs. This is really just
1246 -- an optimisation, and perhaps it is more trouble that it is worth,
1247 -- as the following comments show!
1249 -- NB1: do *not* add superclasses. If we have
1252 -- but a is not bound here, then we *don't* want to derive
1253 -- dn from df here lest we lose sharing.
1255 -- NB2: do *not* add the Inst to avails at all if it's a method.
1256 -- The following situation shows why this is bad:
1257 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1258 -- From an application (truncate f i) we get
1259 -- t1 = truncate at f
1261 -- If we have also have a second occurrence of truncate, we get
1262 -- t3 = truncate at f
1264 -- When simplifying with i,f free, we might still notice that
1265 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1266 -- will continue to float out!
1267 -- Solution: never put methods in avail till they are captured
1268 -- in which case addFree isn't used
1270 -- NB3: make sure that CCallable/CReturnable use NoRhs rather
1271 -- than BoundTo, else we end up with bogus bindings.
1272 -- c.f. instBindingRequired in addWanted
1273 addFree (avails, frees) free
1274 | isDict free = returnNF_Tc (addToFM avails free avail, free:frees)
1275 | otherwise = returnNF_Tc (avails, free:frees)
1277 avail | instBindingRequired free = BoundTo (instToId free)
1280 addWanted :: RedState -> Inst -> TcExpr -> [Inst] -> NF_TcM RedState
1281 addWanted state@(avails, frees) wanted rhs_expr wanteds
1282 -- Do *not* add superclasses as well. Here's an example of why not
1283 -- class Eq a => Foo a b
1284 -- instance Eq a => Foo [a] a
1285 -- If we are reducing
1287 -- we'll first deduce that it holds (via the instance decl). We
1288 -- must not then overwrite the Eq t constraint with a superclass selection!
1289 -- ToDo: this isn't entirely unsatisfactory, because
1290 -- we may also lose some entirely-legitimate sharing this way
1292 = ASSERT( not (isAvailable state wanted) )
1293 returnNF_Tc (addToFM avails wanted avail, frees)
1295 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1296 | otherwise = ASSERT( null wanteds ) NoRhs
1298 addGiven :: RedState -> Inst -> NF_TcM RedState
1299 addGiven state given = addAvailAndSCs state given (BoundTo (instToId given))
1301 addIrred :: WantSCs -> RedState -> Inst -> NF_TcM RedState
1302 addIrred NoSCs (avails,frees) irred = returnNF_Tc (addToFM avails irred Irred, frees)
1303 addIrred AddSCs state irred = addAvailAndSCs state irred Irred
1305 addAvailAndSCs :: RedState -> Inst -> Avail -> NF_TcM RedState
1306 addAvailAndSCs (avails, frees) wanted avail
1307 = add_avail_and_scs avails wanted avail `thenNF_Tc` \ avails' ->
1308 returnNF_Tc (avails', frees)
1310 ---------------------
1311 add_avail_and_scs :: Avails -> Inst -> Avail -> NF_TcM Avails
1312 add_avail_and_scs avails wanted avail
1313 = add_scs (addToFM avails wanted avail) wanted
1315 add_scs :: Avails -> Inst -> NF_TcM Avails
1316 -- Add all the superclasses of the Inst to Avails
1317 -- Invariant: the Inst is already in Avails.
1320 | not (isClassDict dict)
1321 = returnNF_Tc avails
1323 | otherwise -- It is a dictionary
1324 = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
1325 foldlNF_Tc add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1327 (clas, tys) = getDictClassTys dict
1328 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1329 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1331 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1332 = case lookupFM avails sc_dict of
1333 Just (BoundTo _) -> returnNF_Tc avails -- See Note [SUPER] below
1334 other -> add_avail_and_scs avails sc_dict avail
1336 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1337 avail = Rhs sc_sel_rhs [dict]
1340 Note [SUPER]. We have to be careful here. If we are *given* d1:Ord a,
1341 and want to deduce (d2:C [a]) where
1343 class Ord a => C a where
1344 instance Ord a => C [a] where ...
1346 Then we'll use the instance decl to deduce C [a] and then add the
1347 superclasses of C [a] to avails. But we must not overwrite the binding
1348 for d1:Ord a (which is given) with a superclass selection or we'll just
1349 build a loop! Hence looking for BoundTo. Crudely, BoundTo is cheaper
1353 %************************************************************************
1355 \section{tcSimplifyTop: defaulting}
1357 %************************************************************************
1360 If a dictionary constrains a type variable which is
1361 * not mentioned in the environment
1362 * and not mentioned in the type of the expression
1363 then it is ambiguous. No further information will arise to instantiate
1364 the type variable; nor will it be generalised and turned into an extra
1365 parameter to a function.
1367 It is an error for this to occur, except that Haskell provided for
1368 certain rules to be applied in the special case of numeric types.
1370 * at least one of its classes is a numeric class, and
1371 * all of its classes are numeric or standard
1372 then the type variable can be defaulted to the first type in the
1373 default-type list which is an instance of all the offending classes.
1375 So here is the function which does the work. It takes the ambiguous
1376 dictionaries and either resolves them (producing bindings) or
1377 complains. It works by splitting the dictionary list by type
1378 variable, and using @disambigOne@ to do the real business.
1380 @tcSimplifyTop@ is called once per module to simplify all the constant
1381 and ambiguous Insts.
1383 We need to be careful of one case. Suppose we have
1385 instance Num a => Num (Foo a b) where ...
1387 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1388 to (Num x), and default x to Int. But what about y??
1390 It's OK: the final zonking stage should zap y to (), which is fine.
1394 tcSimplifyTop :: LIE -> TcM TcDictBinds
1395 tcSimplifyTop wanted_lie
1396 = simpleReduceLoop (text "tcSimplTop") try_me wanteds `thenTc` \ (frees, binds, irreds) ->
1397 ASSERT( null frees )
1400 -- All the non-std ones are definite errors
1401 (stds, non_stds) = partition isStdClassTyVarDict irreds
1403 -- Group by type variable
1404 std_groups = equivClasses cmp_by_tyvar stds
1406 -- Pick the ones which its worth trying to disambiguate
1407 (std_oks, std_bads) = partition worth_a_try std_groups
1409 -- Have a try at disambiguation
1410 -- if the type variable isn't bound
1411 -- up with one of the non-standard classes
1412 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1413 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1415 -- Collect together all the bad guys
1416 bad_guys = non_stds ++ concat std_bads
1418 -- Disambiguate the ones that look feasible
1419 mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
1421 -- And complain about the ones that don't
1422 -- This group includes both non-existent instances
1423 -- e.g. Num (IO a) and Eq (Int -> Int)
1424 -- and ambiguous dictionaries
1426 addTopAmbigErrs bad_guys `thenNF_Tc_`
1428 returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
1430 wanteds = lieToList wanted_lie
1431 try_me inst = ReduceMe
1433 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1435 get_tv d = case getDictClassTys d of
1436 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1437 get_clas d = case getDictClassTys d of
1438 (clas, [ty]) -> clas
1441 @disambigOne@ assumes that its arguments dictionaries constrain all
1442 the same type variable.
1444 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1445 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1446 the most common use of defaulting is code like:
1448 _ccall_ foo `seqPrimIO` bar
1450 Since we're not using the result of @foo@, the result if (presumably)
1454 disambigGroup :: [Inst] -- All standard classes of form (C a)
1458 | any isNumericClass classes -- Guaranteed all standard classes
1459 -- see comment at the end of function for reasons as to
1460 -- why the defaulting mechanism doesn't apply to groups that
1461 -- include CCallable or CReturnable dicts.
1462 && not (any isCcallishClass classes)
1463 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1464 -- SO, TRY DEFAULT TYPES IN ORDER
1466 -- Failure here is caused by there being no type in the
1467 -- default list which can satisfy all the ambiguous classes.
1468 -- For example, if Real a is reqd, but the only type in the
1469 -- default list is Int.
1470 tcGetDefaultTys `thenNF_Tc` \ default_tys ->
1472 try_default [] -- No defaults work, so fail
1475 try_default (default_ty : default_tys)
1476 = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
1477 -- default_tys instead
1478 tcSimplifyCheckThetas [] theta `thenTc` \ _ ->
1481 theta = [mkClassPred clas [default_ty] | clas <- classes]
1483 -- See if any default works, and if so bind the type variable to it
1484 -- If not, add an AmbigErr
1485 recoverTc (addAmbigErrs dicts `thenNF_Tc_`
1486 returnTc EmptyMonoBinds) $
1488 try_default default_tys `thenTc` \ chosen_default_ty ->
1490 -- Bind the type variable and reduce the context, for real this time
1491 unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
1492 simpleReduceLoop (text "disambig" <+> ppr dicts)
1493 try_me dicts `thenTc` \ (frees, binds, ambigs) ->
1494 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1495 warnDefault dicts chosen_default_ty `thenTc_`
1498 | all isCreturnableClass classes
1499 = -- Default CCall stuff to (); we don't even both to check that () is an
1500 -- instance of CReturnable, because we know it is.
1501 unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
1502 returnTc EmptyMonoBinds
1504 | otherwise -- No defaults
1505 = addAmbigErrs dicts `thenNF_Tc_`
1506 returnTc EmptyMonoBinds
1509 try_me inst = ReduceMe -- This reduce should not fail
1510 tyvar = get_tv (head dicts) -- Should be non-empty
1511 classes = map get_clas dicts
1514 [Aside - why the defaulting mechanism is turned off when
1515 dealing with arguments and results to ccalls.
1517 When typechecking _ccall_s, TcExpr ensures that the external
1518 function is only passed arguments (and in the other direction,
1519 results) of a restricted set of 'native' types. This is
1520 implemented via the help of the pseudo-type classes,
1521 @CReturnable@ (CR) and @CCallable@ (CC.)
1523 The interaction between the defaulting mechanism for numeric
1524 values and CC & CR can be a bit puzzling to the user at times.
1533 What type has 'x' got here? That depends on the default list
1534 in operation, if it is equal to Haskell 98's default-default
1535 of (Integer, Double), 'x' has type Double, since Integer
1536 is not an instance of CR. If the default list is equal to
1537 Haskell 1.4's default-default of (Int, Double), 'x' has type
1540 To try to minimise the potential for surprises here, the
1541 defaulting mechanism is turned off in the presence of
1542 CCallable and CReturnable.
1547 %************************************************************************
1549 \subsection[simple]{@Simple@ versions}
1551 %************************************************************************
1553 Much simpler versions when there are no bindings to make!
1555 @tcSimplifyThetas@ simplifies class-type constraints formed by
1556 @deriving@ declarations and when specialising instances. We are
1557 only interested in the simplified bunch of class/type constraints.
1559 It simplifies to constraints of the form (C a b c) where
1560 a,b,c are type variables. This is required for the context of
1561 instance declarations.
1564 tcSimplifyThetas :: ThetaType -- Wanted
1565 -> TcM ThetaType -- Needed
1567 tcSimplifyThetas wanteds
1568 = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
1569 reduceSimple [] wanteds `thenNF_Tc` \ irreds ->
1571 -- For multi-param Haskell, check that the returned dictionaries
1572 -- don't have any of the form (C Int Bool) for which
1573 -- we expect an instance here
1574 -- For Haskell 98, check that all the constraints are of the form C a,
1575 -- where a is a type variable
1576 bad_guys | glaExts = [pred | pred <- irreds,
1577 isEmptyVarSet (tyVarsOfPred pred)]
1578 | otherwise = [pred | pred <- irreds,
1579 not (isTyVarClassPred pred)]
1581 if null bad_guys then
1584 mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
1588 @tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
1589 used with \tr{default} declarations. We are only interested in
1590 whether it worked or not.
1593 tcSimplifyCheckThetas :: ThetaType -- Given
1594 -> ThetaType -- Wanted
1597 tcSimplifyCheckThetas givens wanteds
1598 = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
1602 mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
1608 type AvailsSimple = FiniteMap PredType Bool
1609 -- True => irreducible
1610 -- False => given, or can be derived from a given or from an irreducible
1612 reduceSimple :: ThetaType -- Given
1613 -> ThetaType -- Wanted
1614 -> NF_TcM ThetaType -- Irreducible
1616 reduceSimple givens wanteds
1617 = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
1618 returnNF_Tc [pred | (pred,True) <- fmToList givens_fm']
1620 givens_fm = foldl addNonIrred emptyFM givens
1622 reduce_simple :: (Int,ThetaType) -- Stack
1625 -> NF_TcM AvailsSimple
1627 reduce_simple (n,stack) avails wanteds
1630 go avails [] = returnNF_Tc avails
1631 go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
1634 reduce_simple_help stack givens wanted
1635 | wanted `elemFM` givens
1636 = returnNF_Tc givens
1638 | Just (clas, tys) <- getClassPredTys_maybe wanted
1639 = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
1641 Nothing -> returnNF_Tc (addSimpleIrred givens wanted)
1642 Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
1645 = returnNF_Tc (addSimpleIrred givens wanted)
1647 addSimpleIrred :: AvailsSimple -> PredType -> AvailsSimple
1648 addSimpleIrred givens pred
1649 = addSCs (addToFM givens pred True) pred
1651 addNonIrred :: AvailsSimple -> PredType -> AvailsSimple
1652 addNonIrred givens pred
1653 = addSCs (addToFM givens pred False) pred
1656 | not (isClassPred pred) = givens
1657 | otherwise = foldl add givens sc_theta
1659 Just (clas,tys) = getClassPredTys_maybe pred
1660 (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
1661 sc_theta = substTheta (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
1664 = case lookupFM givens ct of
1665 Nothing -> -- Add it and its superclasses
1666 addSCs (addToFM givens ct False) ct
1668 Just True -> -- Set its flag to False; superclasses already done
1669 addToFM givens ct False
1671 Just False -> -- Already done
1677 %************************************************************************
1679 \section{Errors and contexts}
1681 %************************************************************************
1683 ToDo: for these error messages, should we note the location as coming
1684 from the insts, or just whatever seems to be around in the monad just
1688 addTopAmbigErrs dicts
1689 = mapNF_Tc complain tidy_dicts
1691 fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet
1692 (tidy_env, tidy_dicts) = tidyInsts dicts
1693 complain d | any isIPPred (predsOfInst d) = addTopIPErr tidy_env d
1694 | not (isTyVarDict d) ||
1695 tyVarsOfInst d `subVarSet` fixed_tvs = addTopInstanceErr tidy_env d
1696 | otherwise = addAmbigErr tidy_env d
1698 addTopIPErr tidy_env tidy_dict
1699 = addInstErrTcM (instLoc tidy_dict)
1701 ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
1703 -- Used for top-level irreducibles
1704 addTopInstanceErr tidy_env tidy_dict
1705 = addInstErrTcM (instLoc tidy_dict)
1707 ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
1710 = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts
1712 (tidy_env, tidy_dicts) = tidyInsts dicts
1714 addAmbigErr tidy_env tidy_dict
1715 = addInstErrTcM (instLoc tidy_dict)
1717 sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs,
1718 nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
1720 ambig_tvs = varSetElems (tyVarsOfInst tidy_dict)
1722 warnDefault dicts default_ty
1723 = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag ->
1724 tcAddSrcLoc (get_loc (head dicts)) (warnTc warn_flag warn_msg)
1727 (_, tidy_dicts) = tidyInsts dicts
1728 get_loc i = case instLoc i of { (_,loc,_) -> loc }
1729 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
1730 quotes (ppr default_ty),
1731 pprInstsInFull tidy_dicts]
1733 -- The error message when we don't find a suitable instance
1734 -- is complicated by the fact that sometimes this is because
1735 -- there is no instance, and sometimes it's because there are
1736 -- too many instances (overlap). See the comments in TcEnv.lhs
1737 -- with the InstEnv stuff.
1738 addNoInstanceErr what_doc givens dict
1739 = tcGetInstEnv `thenNF_Tc` \ inst_env ->
1741 doc = vcat [sep [herald <+> quotes (pprInst tidy_dict),
1742 nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
1744 ptext SLIT("Probable fix:"),
1748 herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
1749 unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
1753 | not ambig_overlap = empty
1755 = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
1756 nest 4 (ptext SLIT("depends on the instantiation of") <+>
1757 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst tidy_dict))))]
1759 fix1 = sep [ptext SLIT("Add") <+> quotes (pprInst tidy_dict),
1760 ptext SLIT("to the") <+> what_doc]
1762 fix2 | isTyVarDict dict
1763 || not (isClassDict dict) -- Don't suggest adding instance declarations for implicit parameters
1767 = ptext SLIT("Or add an instance declaration for") <+> quotes (pprInst tidy_dict)
1769 (tidy_env, tidy_dict:tidy_givens) = tidyInsts (dict:givens)
1771 -- Checks for the ambiguous case when we have overlapping instances
1772 ambig_overlap | isClassDict dict
1773 = case lookupInstEnv inst_env clas tys of
1774 NoMatch ambig -> ambig
1778 (clas,tys) = getDictClassTys dict
1780 addInstErrTcM (instLoc dict) (tidy_env, doc)
1782 -- Used for the ...Thetas variants; all top level
1784 = addErrTc (ptext SLIT("No instance for") <+> quotes (ppr pred))
1786 reduceDepthErr n stack
1787 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
1788 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
1789 nest 4 (pprInstsInFull stack)]
1791 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)
1793 -----------------------------------------------
1795 = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
1796 nest 4 (ppr inst <+> pprInstLoc (instLoc inst))])