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 tcSimplifyDeriv, tcSimplifyDefault,
18 #include "HsVersions.h"
20 import {-# SOURCE #-} TcUnify( unifyTauTy )
22 import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
23 import TcHsSyn ( TcExpr, TcId,
24 TcMonoBinds, TcDictBinds
28 import Inst ( lookupInst, LookupInstResult(..),
29 tyVarsOfInst, predsOfInsts, predsOfInst, newDicts,
30 isDict, isClassDict, isLinearInst, linearInstType,
31 isStdClassTyVarDict, isMethodFor, isMethod,
32 instToId, tyVarsOfInsts, cloneDict,
33 ipNamesOfInsts, ipNamesOfInst, dictPred,
34 instBindingRequired, instCanBeGeneralised,
35 newDictsFromOld, newMethodAtLoc,
36 getDictClassTys, isTyVarDict,
37 instLoc, pprInst, zonkInst, tidyInsts, tidyMoreInsts,
38 Inst, LIE, pprInsts, pprInstsInFull,
41 import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv, tcLookupGlobalId )
42 import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
43 import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
44 import TcType ( TcTyVar, TcTyVarSet, ThetaType, TyVarDetails(VanillaTv),
45 mkClassPred, isOverloadedTy, mkTyConApp,
46 mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
47 tyVarsOfPred, isIPPred, isInheritablePred, predHasFDs )
48 import Id ( idType, mkUserLocal )
50 import Name ( getOccName, getSrcLoc )
51 import NameSet ( NameSet, mkNameSet, elemNameSet )
52 import Class ( classBigSig )
53 import FunDeps ( oclose, grow, improve, pprEquationDoc )
54 import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass,
55 splitName, fstName, sndName )
57 import Subst ( mkTopTyVarSubst, substTheta, substTy )
58 import TysWiredIn ( unitTy, pairTyCon )
62 import ListSetOps ( equivClasses )
63 import Util ( zipEqual )
64 import List ( partition )
69 %************************************************************************
73 %************************************************************************
75 --------------------------------------
76 Notes on quantification
77 --------------------------------------
79 Suppose we are about to do a generalisation step.
84 C the constraints from that RHS
86 The game is to figure out
88 Q the set of type variables over which to quantify
89 Ct the constraints we will *not* quantify over
90 Cq the constraints we will quantify over
92 So we're going to infer the type
96 and float the constraints Ct further outwards.
98 Here are the things that *must* be true:
100 (A) Q intersect fv(G) = EMPTY limits how big Q can be
101 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
103 (A) says we can't quantify over a variable that's free in the
104 environment. (B) says we must quantify over all the truly free
105 variables in T, else we won't get a sufficiently general type. We do
106 not *need* to quantify over any variable that is fixed by the free
107 vars of the environment G.
109 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
111 Example: class H x y | x->y where ...
113 fv(G) = {a} C = {H a b, H c d}
116 (A) Q intersect {a} is empty
117 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
119 So Q can be {c,d}, {b,c,d}
121 Other things being equal, however, we'd like to quantify over as few
122 variables as possible: smaller types, fewer type applications, more
123 constraints can get into Ct instead of Cq.
126 -----------------------------------------
129 fv(T) the free type vars of T
131 oclose(vs,C) The result of extending the set of tyvars vs
132 using the functional dependencies from C
134 grow(vs,C) The result of extend the set of tyvars vs
135 using all conceivable links from C.
137 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
138 Then grow(vs,C) = {a,b,c}
140 Note that grow(vs,C) `superset` grow(vs,simplify(C))
141 That is, simplfication can only shrink the result of grow.
144 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
145 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
148 -----------------------------------------
152 Here's a good way to choose Q:
154 Q = grow( fv(T), C ) \ oclose( fv(G), C )
156 That is, quantify over all variable that that MIGHT be fixed by the
157 call site (which influences T), but which aren't DEFINITELY fixed by
158 G. This choice definitely quantifies over enough type variables,
159 albeit perhaps too many.
161 Why grow( fv(T), C ) rather than fv(T)? Consider
163 class H x y | x->y where ...
168 If we used fv(T) = {c} we'd get the type
170 forall c. H c d => c -> b
172 And then if the fn was called at several different c's, each of
173 which fixed d differently, we'd get a unification error, because
174 d isn't quantified. Solution: quantify d. So we must quantify
175 everything that might be influenced by c.
177 Why not oclose( fv(T), C )? Because we might not be able to see
178 all the functional dependencies yet:
180 class H x y | x->y where ...
181 instance H x y => Eq (T x y) where ...
186 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
187 apparent yet, and that's wrong. We must really quantify over d too.
190 There really isn't any point in quantifying over any more than
191 grow( fv(T), C ), because the call sites can't possibly influence
192 any other type variables.
196 --------------------------------------
198 --------------------------------------
200 It's very hard to be certain when a type is ambiguous. Consider
204 instance H x y => K (x,y)
206 Is this type ambiguous?
207 forall a b. (K (a,b), Eq b) => a -> a
209 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
210 now we see that a fixes b. So we can't tell about ambiguity for sure
211 without doing a full simplification. And even that isn't possible if
212 the context has some free vars that may get unified. Urgle!
214 Here's another example: is this ambiguous?
215 forall a b. Eq (T b) => a -> a
216 Not if there's an insance decl (with no context)
217 instance Eq (T b) where ...
219 You may say of this example that we should use the instance decl right
220 away, but you can't always do that:
222 class J a b where ...
223 instance J Int b where ...
225 f :: forall a b. J a b => a -> a
227 (Notice: no functional dependency in J's class decl.)
228 Here f's type is perfectly fine, provided f is only called at Int.
229 It's premature to complain when meeting f's signature, or even
230 when inferring a type for f.
234 However, we don't *need* to report ambiguity right away. It'll always
235 show up at the call site.... and eventually at main, which needs special
236 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
238 So here's the plan. We WARN about probable ambiguity if
240 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
242 (all tested before quantification).
243 That is, all the type variables in Cq must be fixed by the the variables
244 in the environment, or by the variables in the type.
246 Notice that we union before calling oclose. Here's an example:
248 class J a b c | a b -> c
252 forall b c. (J a b c) => b -> b
254 Only if we union {a} from G with {b} from T before using oclose,
255 do we see that c is fixed.
257 It's a bit vague exactly which C we should use for this oclose call. If we
258 don't fix enough variables we might complain when we shouldn't (see
259 the above nasty example). Nothing will be perfect. That's why we can
260 only issue a warning.
263 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
265 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
267 then c is a "bubble"; there's no way it can ever improve, and it's
268 certainly ambiguous. UNLESS it is a constant (sigh). And what about
273 instance H x y => K (x,y)
275 Is this type ambiguous?
276 forall a b. (K (a,b), Eq b) => a -> a
278 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
279 is a "bubble" that's a set of constraints
281 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
283 Hence another idea. To decide Q start with fv(T) and grow it
284 by transitive closure in Cq (no functional dependencies involved).
285 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
286 The definitely-ambiguous can then float out, and get smashed at top level
287 (which squashes out the constants, like Eq (T a) above)
290 --------------------------------------
291 Notes on principal types
292 --------------------------------------
297 f x = let g y = op (y::Int) in True
299 Here the principal type of f is (forall a. a->a)
300 but we'll produce the non-principal type
301 f :: forall a. C Int => a -> a
304 --------------------------------------
305 Notes on implicit parameters
306 --------------------------------------
308 Question 1: can we "inherit" implicit parameters
309 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
314 where f is *not* a top-level binding.
315 From the RHS of f we'll get the constraint (?y::Int).
316 There are two types we might infer for f:
320 (so we get ?y from the context of f's definition), or
322 f :: (?y::Int) => Int -> Int
324 At first you might think the first was better, becuase then
325 ?y behaves like a free variable of the definition, rather than
326 having to be passed at each call site. But of course, the WHOLE
327 IDEA is that ?y should be passed at each call site (that's what
328 dynamic binding means) so we'd better infer the second.
330 BOTTOM LINE: when *inferring types* you *must* quantify
331 over implicit parameters. See the predicate isFreeWhenInferring.
334 Question 2: type signatures
335 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
336 BUT WATCH OUT: When you supply a type signature, we can't force you
337 to quantify over implicit parameters. For example:
341 This is perfectly reasonable. We do not want to insist on
343 (?x + 1) :: (?x::Int => Int)
345 That would be silly. Here, the definition site *is* the occurrence site,
346 so the above strictures don't apply. Hence the difference between
347 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
348 and tcSimplifyCheckBind (which does not).
350 What about when you supply a type signature for a binding?
351 Is it legal to give the following explicit, user type
352 signature to f, thus:
357 At first sight this seems reasonable, but it has the nasty property
358 that adding a type signature changes the dynamic semantics.
361 (let f x = (x::Int) + ?y
362 in (f 3, f 3 with ?y=5)) with ?y = 6
368 in (f 3, f 3 with ?y=5)) with ?y = 6
372 Indeed, simply inlining f (at the Haskell source level) would change the
375 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
376 semantics for a Haskell program without knowing its typing, so if you
377 change the typing you may change the semantics.
379 To make things consistent in all cases where we are *checking* against
380 a supplied signature (as opposed to inferring a type), we adopt the
383 a signature does not need to quantify over implicit params.
385 [This represents a (rather marginal) change of policy since GHC 5.02,
386 which *required* an explicit signature to quantify over all implicit
387 params for the reasons mentioned above.]
389 But that raises a new question. Consider
391 Given (signature) ?x::Int
392 Wanted (inferred) ?x::Int, ?y::Bool
394 Clearly we want to discharge the ?x and float the ?y out. But
395 what is the criterion that distinguishes them? Clearly it isn't
396 what free type variables they have. The Right Thing seems to be
397 to float a constraint that
398 neither mentions any of the quantified type variables
399 nor any of the quantified implicit parameters
401 See the predicate isFreeWhenChecking.
404 Question 3: monomorphism
405 ~~~~~~~~~~~~~~~~~~~~~~~~
406 There's a nasty corner case when the monomorphism restriction bites:
410 The argument above suggests that we *must* generalise
411 over the ?y parameter, to get
412 z :: (?y::Int) => Int,
413 but the monomorphism restriction says that we *must not*, giving
415 Why does the momomorphism restriction say this? Because if you have
417 let z = x + ?y in z+z
419 you might not expect the addition to be done twice --- but it will if
420 we follow the argument of Question 2 and generalise over ?y.
426 (A) Always generalise over implicit parameters
427 Bindings that fall under the monomorphism restriction can't
431 * Inlining remains valid
432 * No unexpected loss of sharing
433 * But simple bindings like
435 will be rejected, unless you add an explicit type signature
436 (to avoid the monomorphism restriction)
437 z :: (?y::Int) => Int
439 This seems unacceptable
441 (B) Monomorphism restriction "wins"
442 Bindings that fall under the monomorphism restriction can't
444 Always generalise over implicit parameters *except* for bindings
445 that fall under the monomorphism restriction
448 * Inlining isn't valid in general
449 * No unexpected loss of sharing
450 * Simple bindings like
452 accepted (get value of ?y from binding site)
454 (C) Always generalise over implicit parameters
455 Bindings that fall under the monomorphism restriction can't
456 be generalised, EXCEPT for implicit parameters
458 * Inlining remains valid
459 * Unexpected loss of sharing (from the extra generalisation)
460 * Simple bindings like
462 accepted (get value of ?y from occurrence sites)
467 None of these choices seems very satisfactory. But at least we should
468 decide which we want to do.
470 It's really not clear what is the Right Thing To Do. If you see
474 would you expect the value of ?y to be got from the *occurrence sites*
475 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
476 case of function definitions, the answer is clearly the former, but
477 less so in the case of non-fucntion definitions. On the other hand,
478 if we say that we get the value of ?y from the definition site of 'z',
479 then inlining 'z' might change the semantics of the program.
481 Choice (C) really says "the monomorphism restriction doesn't apply
482 to implicit parameters". Which is fine, but remember that every
483 innocent binding 'x = ...' that mentions an implicit parameter in
484 the RHS becomes a *function* of that parameter, called at each
485 use of 'x'. Now, the chances are that there are no intervening 'with'
486 clauses that bind ?y, so a decent compiler should common up all
487 those function calls. So I think I strongly favour (C). Indeed,
488 one could make a similar argument for abolishing the monomorphism
489 restriction altogether.
491 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
495 %************************************************************************
497 \subsection{tcSimplifyInfer}
499 %************************************************************************
501 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
503 1. Compute Q = grow( fvs(T), C )
505 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
506 predicates will end up in Ct; we deal with them at the top level
508 3. Try improvement, using functional dependencies
510 4. If Step 3 did any unification, repeat from step 1
511 (Unification can change the result of 'grow'.)
513 Note: we don't reduce dictionaries in step 2. For example, if we have
514 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
515 after step 2. However note that we may therefore quantify over more
516 type variables than we absolutely have to.
518 For the guts, we need a loop, that alternates context reduction and
519 improvement with unification. E.g. Suppose we have
521 class C x y | x->y where ...
523 and tcSimplify is called with:
525 Then improvement unifies a with b, giving
528 If we need to unify anything, we rattle round the whole thing all over
535 -> TcTyVarSet -- fv(T); type vars
537 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
539 TcDictBinds, -- Bindings
540 [TcId]) -- Dict Ids that must be bound here (zonked)
545 tcSimplifyInfer doc tau_tvs wanted_lie
546 = inferLoop doc (varSetElems tau_tvs)
547 (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
549 -- Check for non-generalisable insts
550 mapTc_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenTc_`
552 returnTc (qtvs, mkLIE frees, binds, map instToId irreds)
554 inferLoop doc tau_tvs wanteds
556 zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
557 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
558 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
560 preds = predsOfInsts wanteds'
561 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
564 | isFreeWhenInferring qtvs inst = Free
565 | isClassDict inst = DontReduceUnlessConstant -- Dicts
566 | otherwise = ReduceMe -- Lits and Methods
569 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
572 if no_improvement then
573 returnTc (varSetElems qtvs, frees, binds, irreds)
575 -- If improvement did some unification, we go round again. There
576 -- are two subtleties:
577 -- a) We start again with irreds, not wanteds
578 -- Using an instance decl might have introduced a fresh type variable
579 -- which might have been unified, so we'd get an infinite loop
580 -- if we started again with wanteds! See example [LOOP]
582 -- b) It's also essential to re-process frees, because unification
583 -- might mean that a type variable that looked free isn't now.
585 -- Hence the (irreds ++ frees)
587 -- However, NOTICE that when we are done, we might have some bindings, but
588 -- the final qtvs might be empty. See [NO TYVARS] below.
590 inferLoop doc tau_tvs (irreds ++ frees) `thenTc` \ (qtvs1, frees1, binds1, irreds1) ->
591 returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
596 class If b t e r | b t e -> r
599 class Lte a b c | a b -> c where lte :: a -> b -> c
601 instance (Lte a b l,If l b a c) => Max a b c
603 Wanted: Max Z (S x) y
605 Then we'll reduce using the Max instance to:
606 (Lte Z (S x) l, If l (S x) Z y)
607 and improve by binding l->T, after which we can do some reduction
608 on both the Lte and If constraints. What we *can't* do is start again
609 with (Max Z (S x) y)!
613 class Y a b | a -> b where
616 instance Y [[a]] a where
619 k :: X a -> X a -> X a
621 g :: Num a => [X a] -> [X a]
624 h ys = ys ++ map (k (y [[0]])) xs
626 The excitement comes when simplifying the bindings for h. Initially
627 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
628 From this we get t1:=:t2, but also various bindings. We can't forget
629 the bindings (because of [LOOP]), but in fact t1 is what g is
632 The net effect of [NO TYVARS]
635 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
636 isFreeWhenInferring qtvs inst
637 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
638 && all isInheritablePred (predsOfInst inst) -- And no implicit parameter involved
639 -- (see "Notes on implicit parameters")
641 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
642 -> NameSet -- Quantified implicit parameters
644 isFreeWhenChecking qtvs ips inst
645 = isFreeWrtTyVars qtvs inst
646 && isFreeWrtIPs ips inst
648 isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
649 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
653 %************************************************************************
655 \subsection{tcSimplifyCheck}
657 %************************************************************************
659 @tcSimplifyCheck@ is used when we know exactly the set of variables
660 we are going to quantify over. For example, a class or instance declaration.
665 -> [TcTyVar] -- Quantify over these
669 TcDictBinds) -- Bindings
671 -- tcSimplifyCheck is used when checking expression type signatures,
672 -- class decls, instance decls etc.
674 -- NB: tcSimplifyCheck does not consult the
675 -- global type variables in the environment; so you don't
676 -- need to worry about setting them before calling tcSimplifyCheck
677 tcSimplifyCheck doc qtvs givens wanted_lie
678 = tcSimplCheck doc get_qtvs
679 givens wanted_lie `thenTc` \ (qtvs', frees, binds) ->
680 returnTc (frees, binds)
682 get_qtvs = zonkTcTyVarsAndFV qtvs
685 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
686 -- against, but we don't know the type variables over which we are going to quantify.
687 -- This happens when we have a type signature for a mutually recursive group
690 -> TcTyVarSet -- fv(T)
693 -> TcM ([TcTyVar], -- Variables over which to quantify
695 TcDictBinds) -- Bindings
697 tcSimplifyInferCheck doc tau_tvs givens wanted_lie
698 = tcSimplCheck doc get_qtvs givens wanted_lie
700 -- Figure out which type variables to quantify over
701 -- You might think it should just be the signature tyvars,
702 -- but in bizarre cases you can get extra ones
703 -- f :: forall a. Num a => a -> a
704 -- f x = fst (g (x, head [])) + 1
706 -- Here we infer g :: forall a b. a -> b -> (b,a)
707 -- We don't want g to be monomorphic in b just because
708 -- f isn't quantified over b.
709 all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
711 get_qtvs = zonkTcTyVarsAndFV all_tvs `thenNF_Tc` \ all_tvs' ->
712 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
714 qtvs = all_tvs' `minusVarSet` gbl_tvs
715 -- We could close gbl_tvs, but its not necessary for
716 -- soundness, and it'll only affect which tyvars, not which
717 -- dictionaries, we quantify over
722 Here is the workhorse function for all three wrappers.
725 tcSimplCheck doc get_qtvs givens wanted_lie
726 = check_loop givens (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
728 -- Complain about any irreducible ones
729 complainCheck doc givens irreds `thenNF_Tc_`
732 returnTc (qtvs, mkLIE frees, binds)
735 ip_set = mkNameSet (ipNamesOfInsts givens)
737 check_loop givens wanteds
739 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
740 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
741 get_qtvs `thenNF_Tc` \ qtvs' ->
745 -- When checking against a given signature we always reduce
746 -- until we find a match against something given, or can't reduce
747 try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
748 | otherwise = ReduceMe
750 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
753 if no_improvement then
754 returnTc (varSetElems qtvs', frees, binds, irreds)
756 check_loop givens' (irreds ++ frees) `thenTc` \ (qtvs', frees1, binds1, irreds1) ->
757 returnTc (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1)
761 %************************************************************************
763 \subsection{tcSimplifyRestricted}
765 %************************************************************************
768 tcSimplifyRestricted -- Used for restricted binding groups
769 -- i.e. ones subject to the monomorphism restriction
771 -> TcTyVarSet -- Free in the type of the RHSs
772 -> LIE -- Free in the RHSs
773 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
775 TcDictBinds) -- Bindings
777 tcSimplifyRestricted doc tau_tvs wanted_lie
778 = -- First squash out all methods, to find the constrained tyvars
779 -- We can't just take the free vars of wanted_lie because that'll
780 -- have methods that may incidentally mention entirely unconstrained variables
781 -- e.g. a call to f :: Eq a => a -> b -> b
782 -- Here, b is unconstrained. A good example would be
784 -- We want to infer the polymorphic type
785 -- foo :: forall b. b -> b
787 wanteds = lieToList wanted_lie
788 try_me inst = ReduceMe -- Reduce as far as we can. Don't stop at
789 -- dicts; the idea is to get rid of as many type
790 -- variables as possible, and we don't want to stop
791 -- at (say) Monad (ST s), because that reduces
792 -- immediately, with no constraint on s.
794 simpleReduceLoop doc try_me wanteds `thenTc` \ (_, _, constrained_dicts) ->
796 -- Next, figure out the tyvars we will quantify over
797 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenNF_Tc` \ tau_tvs' ->
798 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
800 constrained_tvs = tyVarsOfInsts constrained_dicts
801 qtvs = (tau_tvs' `minusVarSet` oclose (predsOfInsts constrained_dicts) gbl_tvs)
802 `minusVarSet` constrained_tvs
805 -- The first step may have squashed more methods than
806 -- necessary, so try again, this time knowing the exact
807 -- set of type variables to quantify over.
809 -- We quantify only over constraints that are captured by qtvs;
810 -- these will just be a subset of non-dicts. This in contrast
811 -- to normal inference (using isFreeWhenInferring) in which we quantify over
812 -- all *non-inheritable* constraints too. This implements choice
813 -- (B) under "implicit parameter and monomorphism" above.
815 -- Remember that we may need to do *some* simplification, to
816 -- (for example) squash {Monad (ST s)} into {}. It's not enough
817 -- just to float all constraints
818 mapNF_Tc zonkInst (lieToList wanted_lie) `thenNF_Tc` \ wanteds' ->
820 try_me inst | isFreeWrtTyVars qtvs inst = Free
821 | otherwise = ReduceMe
823 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
824 ASSERT( no_improvement )
825 ASSERT( null irreds )
826 -- No need to loop because simpleReduceLoop will have
827 -- already done any improvement necessary
829 returnTc (varSetElems qtvs, mkLIE frees, binds)
833 %************************************************************************
835 \subsection{tcSimplifyToDicts}
837 %************************************************************************
839 On the LHS of transformation rules we only simplify methods and constants,
840 getting dictionaries. We want to keep all of them unsimplified, to serve
841 as the available stuff for the RHS of the rule.
843 The same thing is used for specialise pragmas. Consider
846 {-# SPECIALISE f :: Int -> Int #-}
849 The type checker generates a binding like:
851 f_spec = (f :: Int -> Int)
853 and we want to end up with
855 f_spec = _inline_me_ (f Int dNumInt)
857 But that means that we must simplify the Method for f to (f Int dNumInt)!
858 So tcSimplifyToDicts squeezes out all Methods.
860 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
862 fromIntegral :: (Integral a, Num b) => a -> b
863 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
865 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
869 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
871 because the scsel will mess up matching. Instead we want
873 forall dIntegralInt, dNumInt.
874 fromIntegral Int Int dIntegralInt dNumInt = id Int
876 Hence "DontReduce NoSCs"
879 tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
880 tcSimplifyToDicts wanted_lie
881 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
882 -- Since try_me doesn't look at types, we don't need to
883 -- do any zonking, so it's safe to call reduceContext directly
885 returnTc (irreds, binds)
888 doc = text "tcSimplifyToDicts"
889 wanteds = lieToList wanted_lie
891 -- Reduce methods and lits only; stop as soon as we get a dictionary
892 try_me inst | isDict inst = DontReduce NoSCs
893 | otherwise = ReduceMe
897 %************************************************************************
899 \subsection{Filtering at a dynamic binding}
901 %************************************************************************
906 we must discharge all the ?x constraints from B. We also do an improvement
907 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
909 Actually, the constraints from B might improve the types in ?x. For example
911 f :: (?x::Int) => Char -> Char
914 then the constraint (?x::Int) arising from the call to f will
915 force the binding for ?x to be of type Int.
918 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
920 -> TcM (LIE, TcDictBinds)
921 tcSimplifyIPs given_ips wanted_lie
922 = simpl_loop given_ips wanteds `thenTc` \ (frees, binds) ->
923 returnTc (mkLIE frees, binds)
925 doc = text "tcSimplifyIPs" <+> ppr given_ips
926 wanteds = lieToList wanted_lie
927 ip_set = mkNameSet (ipNamesOfInsts given_ips)
929 -- Simplify any methods that mention the implicit parameter
930 try_me inst | isFreeWrtIPs ip_set inst = Free
931 | otherwise = ReduceMe
933 simpl_loop givens wanteds
934 = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
935 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
937 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
939 if no_improvement then
940 ASSERT( null irreds )
941 returnTc (frees, binds)
943 simpl_loop givens' (irreds ++ frees) `thenTc` \ (frees1, binds1) ->
944 returnTc (frees1, binds `AndMonoBinds` binds1)
948 %************************************************************************
950 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
952 %************************************************************************
954 When doing a binding group, we may have @Insts@ of local functions.
955 For example, we might have...
957 let f x = x + 1 -- orig local function (overloaded)
958 f.1 = f Int -- two instances of f
963 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
964 where @f@ is in scope; those @Insts@ must certainly not be passed
965 upwards towards the top-level. If the @Insts@ were binding-ified up
966 there, they would have unresolvable references to @f@.
968 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
969 For each method @Inst@ in the @init_lie@ that mentions one of the
970 @Ids@, we create a binding. We return the remaining @Insts@ (in an
971 @LIE@), as well as the @HsBinds@ generated.
974 bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
976 bindInstsOfLocalFuns init_lie local_ids
977 | null overloaded_ids
979 = returnTc (init_lie, EmptyMonoBinds)
982 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
983 ASSERT( null irreds )
984 returnTc (mkLIE frees, binds)
986 doc = text "bindInsts" <+> ppr local_ids
987 wanteds = lieToList init_lie
988 overloaded_ids = filter is_overloaded local_ids
989 is_overloaded id = isOverloadedTy (idType id)
991 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
992 -- so it's worth building a set, so that
993 -- lookup (in isMethodFor) is faster
995 try_me inst | isMethodFor overloaded_set inst = ReduceMe
1000 %************************************************************************
1002 \subsection{Data types for the reduction mechanism}
1004 %************************************************************************
1006 The main control over context reduction is here
1010 = ReduceMe -- Try to reduce this
1011 -- If there's no instance, behave exactly like
1012 -- DontReduce: add the inst to
1013 -- the irreductible ones, but don't
1014 -- produce an error message of any kind.
1015 -- It might be quite legitimate such as (Eq a)!
1017 | DontReduce WantSCs -- Return as irreducible
1019 | DontReduceUnlessConstant -- Return as irreducible unless it can
1020 -- be reduced to a constant in one step
1022 | Free -- Return as free
1024 reduceMe :: Inst -> WhatToDo
1025 reduceMe inst = ReduceMe
1027 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1028 -- of a predicate when adding it to the avails
1034 type Avails = FiniteMap Inst Avail
1037 = IsFree -- Used for free Insts
1038 | Irred -- Used for irreducible dictionaries,
1039 -- which are going to be lambda bound
1041 | Given TcId -- Used for dictionaries for which we have a binding
1042 -- e.g. those "given" in a signature
1043 Bool -- True <=> actually consumed (splittable IPs only)
1045 | NoRhs -- Used for Insts like (CCallable f)
1046 -- where no witness is required.
1048 | Rhs -- Used when there is a RHS
1050 [Inst] -- Insts free in the RHS; we need these too
1052 | Linear -- Splittable Insts only.
1053 Int -- The Int is always 2 or more; indicates how
1054 -- many copies are required
1055 Inst -- The splitter
1056 Avail -- Where the "master copy" is
1058 | LinRhss -- Splittable Insts only; this is used only internally
1059 -- by extractResults, where a Linear
1060 -- is turned into an LinRhss
1061 [TcExpr] -- A supply of suitable RHSs
1063 pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1064 | (inst,avail) <- fmToList avails ]
1066 instance Outputable Avail where
1069 pprAvail NoRhs = text "<no rhs>"
1070 pprAvail IsFree = text "Free"
1071 pprAvail Irred = text "Irred"
1072 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1073 if b then text "(used)" else empty
1074 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1075 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1076 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1079 Extracting the bindings from a bunch of Avails.
1080 The bindings do *not* come back sorted in dependency order.
1081 We assume that they'll be wrapped in a big Rec, so that the
1082 dependency analyser can sort them out later
1086 extractResults :: Avails
1088 -> NF_TcM (TcDictBinds, -- Bindings
1089 [Inst], -- Irreducible ones
1090 [Inst]) -- Free ones
1092 extractResults avails wanteds
1093 = go avails EmptyMonoBinds [] [] wanteds
1095 go avails binds irreds frees []
1096 = returnNF_Tc (binds, irreds, frees)
1098 go avails binds irreds frees (w:ws)
1099 = case lookupFM avails w of
1100 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1101 go avails binds irreds frees ws
1103 Just NoRhs -> go avails binds irreds frees ws
1104 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1105 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1107 Just (Given id _) -> go avails new_binds irreds frees ws
1109 new_binds | id == instToId w = binds
1110 | otherwise = addBind binds w (HsVar id)
1111 -- The sought Id can be one of the givens, via a superclass chain
1112 -- and then we definitely don't want to generate an x=x binding!
1114 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1116 new_binds = addBind binds w rhs
1118 Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
1119 -> get_root irreds frees avail w `thenNF_Tc` \ (irreds', frees', root_id) ->
1120 split n (instToId split_inst) root_id w `thenNF_Tc` \ (binds', rhss) ->
1121 go (addToFM avails w (LinRhss rhss))
1122 (binds `AndMonoBinds` binds')
1123 irreds' frees' (split_inst : w : ws)
1125 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1126 -> go new_avails new_binds irreds frees ws
1128 new_binds = addBind binds w rhs
1129 new_avails = addToFM avails w (LinRhss rhss)
1131 get_root irreds frees (Given id _) w = returnNF_Tc (irreds, frees, id)
1132 get_root irreds frees Irred w = cloneDict w `thenNF_Tc` \ w' ->
1133 returnNF_Tc (w':irreds, frees, instToId w')
1134 get_root irreds frees IsFree w = cloneDict w `thenNF_Tc` \ w' ->
1135 returnNF_Tc (irreds, w':frees, instToId w')
1138 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1139 | otherwise = addToFM avails w NoRhs
1140 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1141 -- than Given, else we end up with bogus bindings.
1143 add_free avails w | isMethod w = avails
1144 | otherwise = add_given avails w
1146 -- Do *not* replace Free by Given if it's a method.
1147 -- The following situation shows why this is bad:
1148 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1149 -- From an application (truncate f i) we get
1150 -- t1 = truncate at f
1152 -- If we have also have a second occurrence of truncate, we get
1153 -- t3 = truncate at f
1155 -- When simplifying with i,f free, we might still notice that
1156 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1157 -- will continue to float out!
1158 -- (split n i a) returns: n rhss
1159 -- auxiliary bindings
1160 -- 1 or 0 insts to add to irreds
1163 split :: Int -> TcId -> TcId -> Inst
1164 -> NF_TcM (TcDictBinds, [TcExpr])
1165 -- (split n split_id root_id wanted) returns
1166 -- * a list of 'n' expressions, all of which witness 'avail'
1167 -- * a bunch of auxiliary bindings to support these expressions
1168 -- * one or zero insts needed to witness the whole lot
1169 -- (maybe be zero if the initial Inst is a Given)
1171 -- NB: 'wanted' is just a template
1173 split n split_id root_id wanted
1176 ty = linearInstType wanted
1177 pair_ty = mkTyConApp pairTyCon [ty,ty]
1178 id = instToId wanted
1182 go 1 = returnNF_Tc (EmptyMonoBinds, [HsVar root_id])
1184 go n = go ((n+1) `div` 2) `thenNF_Tc` \ (binds1, rhss) ->
1185 expand n rhss `thenNF_Tc` \ (binds2, rhss') ->
1186 returnNF_Tc (binds1 `AndMonoBinds` binds2, rhss')
1189 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1190 -- e.g. expand 3 [rhs1, rhs2]
1191 -- = ( { x = split rhs1 },
1192 -- [fst x, snd x, rhs2] )
1194 | n `rem` 2 == 0 = go rhss -- n is even
1195 | otherwise = go (tail rhss) `thenNF_Tc` \ (binds', rhss') ->
1196 returnNF_Tc (binds', head rhss : rhss')
1198 go rhss = mapAndUnzipNF_Tc do_one rhss `thenNF_Tc` \ (binds', rhss') ->
1199 returnNF_Tc (andMonoBindList binds', concat rhss')
1201 do_one rhs = tcGetUnique `thenNF_Tc` \ uniq ->
1202 tcLookupGlobalId fstName `thenNF_Tc` \ fst_id ->
1203 tcLookupGlobalId sndName `thenNF_Tc` \ snd_id ->
1205 x = mkUserLocal occ uniq pair_ty loc
1207 returnNF_Tc (VarMonoBind x (mk_app split_id rhs),
1208 [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])
1210 mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var
1212 mk_app id rhs = HsApp (HsVar id) rhs
1214 addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
1218 %************************************************************************
1220 \subsection[reduce]{@reduce@}
1222 %************************************************************************
1224 When the "what to do" predicate doesn't depend on the quantified type variables,
1225 matters are easier. We don't need to do any zonking, unless the improvement step
1226 does something, in which case we zonk before iterating.
1228 The "given" set is always empty.
1231 simpleReduceLoop :: SDoc
1232 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1234 -> TcM ([Inst], -- Free
1236 [Inst]) -- Irreducible
1238 simpleReduceLoop doc try_me wanteds
1239 = mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
1240 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
1241 if no_improvement then
1242 returnTc (frees, binds, irreds)
1244 simpleReduceLoop doc try_me (irreds ++ frees) `thenTc` \ (frees1, binds1, irreds1) ->
1245 returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
1251 reduceContext :: SDoc
1252 -> (Inst -> WhatToDo)
1255 -> NF_TcM (Bool, -- True <=> improve step did no unification
1257 TcDictBinds, -- Dictionary bindings
1258 [Inst]) -- Irreducible
1260 reduceContext doc try_me givens wanteds
1262 traceTc (text "reduceContext" <+> (vcat [
1263 text "----------------------",
1265 text "given" <+> ppr givens,
1266 text "wanted" <+> ppr wanteds,
1267 text "----------------------"
1270 -- Build the Avail mapping from "givens"
1271 foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ init_state ->
1274 reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ avails ->
1276 -- Do improvement, using everything in avails
1277 -- In particular, avails includes all superclasses of everything
1278 tcImprove avails `thenTc` \ no_improvement ->
1280 extractResults avails wanteds `thenNF_Tc` \ (binds, irreds, frees) ->
1282 traceTc (text "reduceContext end" <+> (vcat [
1283 text "----------------------",
1285 text "given" <+> ppr givens,
1286 text "wanted" <+> ppr wanteds,
1288 text "avails" <+> pprAvails avails,
1289 text "frees" <+> ppr frees,
1290 text "no_improvement =" <+> ppr no_improvement,
1291 text "----------------------"
1294 returnTc (no_improvement, frees, binds, irreds)
1297 = tcGetInstEnv `thenTc` \ inst_env ->
1299 preds = [ (pred, pp_loc)
1300 | inst <- keysFM avails,
1301 let pp_loc = pprInstLoc (instLoc inst),
1302 pred <- predsOfInst inst,
1305 -- Avails has all the superclasses etc (good)
1306 -- It also has all the intermediates of the deduction (good)
1307 -- It does not have duplicates (good)
1308 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1309 -- so that improve will see them separate
1310 eqns = improve (classInstEnv inst_env) preds
1315 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenNF_Tc_`
1316 mapTc_ unify eqns `thenTc_`
1319 unify ((qtvs, t1, t2), doc)
1320 = tcAddErrCtxt doc $
1321 tcInstTyVars VanillaTv (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) ->
1322 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1325 The main context-reduction function is @reduce@. Here's its game plan.
1328 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1329 -- along with its depth
1330 -> (Inst -> WhatToDo)
1337 try_me: given an inst, this function returns
1339 DontReduce return this in "irreds"
1340 Free return this in "frees"
1342 wanteds: The list of insts to reduce
1343 state: An accumulating parameter of type Avails
1344 that contains the state of the algorithm
1346 It returns a Avails.
1348 The (n,stack) pair is just used for error reporting.
1349 n is always the depth of the stack.
1350 The stack is the stack of Insts being reduced: to produce X
1351 I had to produce Y, to produce Y I had to produce Z, and so on.
1354 reduceList (n,stack) try_me wanteds state
1355 | n > opt_MaxContextReductionDepth
1356 = failWithTc (reduceDepthErr n stack)
1362 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1367 go [] state = returnTc state
1368 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
1371 -- Base case: we're done!
1372 reduce stack try_me wanted state
1373 -- It's the same as an existing inst, or a superclass thereof
1374 | Just avail <- isAvailable state wanted
1375 = if isLinearInst wanted then
1376 addLinearAvailable state avail wanted `thenNF_Tc` \ (state', wanteds') ->
1377 reduceList stack try_me wanteds' state'
1379 returnTc state -- No op for non-linear things
1382 = case try_me wanted of {
1384 DontReduce want_scs -> addIrred want_scs state wanted
1386 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1387 -- First, see if the inst can be reduced to a constant in one step
1388 try_simple (addIrred AddSCs) -- Assume want superclasses
1390 ; Free -> -- It's free so just chuck it upstairs
1391 -- First, see if the inst can be reduced to a constant in one step
1394 ; ReduceMe -> -- It should be reduced
1395 lookupInst wanted `thenNF_Tc` \ lookup_result ->
1396 case lookup_result of
1397 GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenTc` \ state' ->
1398 addWanted state' wanted rhs wanteds'
1399 SimpleInst rhs -> addWanted state wanted rhs []
1401 NoInstance -> -- No such instance!
1402 -- Add it and its superclasses
1403 addIrred AddSCs state wanted
1407 try_simple do_this_otherwise
1408 = lookupInst wanted `thenNF_Tc` \ lookup_result ->
1409 case lookup_result of
1410 SimpleInst rhs -> addWanted state wanted rhs []
1411 other -> do_this_otherwise state wanted
1416 -------------------------
1417 isAvailable :: Avails -> Inst -> Maybe Avail
1418 isAvailable avails wanted = lookupFM avails wanted
1419 -- NB 1: the Ord instance of Inst compares by the class/type info
1420 -- *not* by unique. So
1421 -- d1::C Int == d2::C Int
1423 addLinearAvailable :: Avails -> Avail -> Inst -> NF_TcM (Avails, [Inst])
1424 addLinearAvailable avails avail wanted
1425 -- avails currently maps [wanted -> avail]
1426 -- Extend avails to reflect a neeed for an extra copy of avail
1428 | Just avail' <- split_avail avail
1429 = returnNF_Tc (addToFM avails wanted avail', [])
1432 = tcLookupGlobalId splitName `thenNF_Tc` \ split_id ->
1433 newMethodAtLoc (instLoc wanted) split_id
1434 [linearInstType wanted] `thenNF_Tc` \ (split_inst,_) ->
1435 returnNF_Tc (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1438 split_avail :: Avail -> Maybe Avail
1439 -- (Just av) if there's a modified version of avail that
1440 -- we can use to replace avail in avails
1441 -- Nothing if there isn't, so we need to create a Linear
1442 split_avail (Linear n i a) = Just (Linear (n+1) i a)
1443 split_avail (Given id used) | not used = Just (Given id True)
1444 | otherwise = Nothing
1445 split_avail Irred = Nothing
1446 split_avail IsFree = Nothing
1447 split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
1449 -------------------------
1450 addFree :: Avails -> Inst -> NF_TcM Avails
1451 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1452 -- to avails, so that any other equal Insts will be commoned up right
1453 -- here rather than also being tossed upstairs. This is really just
1454 -- an optimisation, and perhaps it is more trouble that it is worth,
1455 -- as the following comments show!
1457 -- NB: do *not* add superclasses. If we have
1460 -- but a is not bound here, then we *don't* want to derive
1461 -- dn from df here lest we lose sharing.
1463 addFree avails free = returnNF_Tc (addToFM avails free IsFree)
1465 addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> NF_TcM Avails
1466 addWanted avails wanted rhs_expr wanteds
1467 = ASSERT2( not (wanted `elemFM` avails), ppr wanted $$ ppr avails )
1468 addAvailAndSCs avails wanted avail
1470 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1471 | otherwise = ASSERT( null wanteds ) NoRhs
1473 addGiven :: Avails -> Inst -> NF_TcM Avails
1474 addGiven state given = addAvailAndSCs state given (Given (instToId given) False)
1475 -- No ASSERT( not (given `elemFM` avails) ) because in an instance
1476 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
1477 -- so the assert isn't true
1479 addIrred :: WantSCs -> Avails -> Inst -> NF_TcM Avails
1480 addIrred NoSCs avails irred = returnNF_Tc (addToFM avails irred Irred)
1481 addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
1482 addAvailAndSCs avails irred Irred
1484 addAvailAndSCs :: Avails -> Inst -> Avail -> NF_TcM Avails
1485 addAvailAndSCs avails inst avail
1486 | not (isClassDict inst) = returnNF_Tc avails1
1487 | otherwise = addSCs is_loop avails1 inst
1489 avails1 = addToFM avails inst avail
1490 is_loop inst = inst `elem` deps -- Note: this compares by *type*, not by Unique
1491 deps = findAllDeps avails avail
1493 findAllDeps :: Avails -> Avail -> [Inst]
1494 -- Find all the Insts that this one depends on
1495 -- See Note [SUPERCLASS-LOOP]
1496 findAllDeps avails (Rhs _ kids) = kids ++ concat (map (find_all_deps_help avails) kids)
1497 findAllDeps avails other = []
1499 find_all_deps_help :: Avails -> Inst -> [Inst]
1500 find_all_deps_help avails inst
1501 = case lookupFM avails inst of
1502 Just avail -> findAllDeps avails avail
1505 addSCs :: (Inst -> Bool) -> Avails -> Inst -> NF_TcM Avails
1506 -- Add all the superclasses of the Inst to Avails
1507 -- The first param says "dont do this because the original thing
1508 -- depends on this one, so you'd build a loop"
1509 -- Invariant: the Inst is already in Avails.
1511 addSCs is_loop avails dict
1512 = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
1513 foldlNF_Tc add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1515 (clas, tys) = getDictClassTys dict
1516 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1517 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1519 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1520 = case lookupFM avails sc_dict of
1521 Just (Given _ _) -> returnNF_Tc avails -- Given is cheaper than
1522 -- a superclass selection
1523 Just other | is_loop sc_dict -> returnNF_Tc avails -- See Note [SUPERCLASS-LOOP]
1524 | otherwise -> returnNF_Tc avails' -- SCs already added
1526 Nothing -> addSCs is_loop avails' sc_dict
1528 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1529 avail = Rhs sc_sel_rhs [dict]
1530 avails' = addToFM avails sc_dict avail
1533 Note [SUPERCLASS-LOOP]: Checking for loops
1534 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1535 We have to be careful here. If we are *given* d1:Ord a,
1536 and want to deduce (d2:C [a]) where
1538 class Ord a => C a where
1539 instance Ord a => C [a] where ...
1541 Then we'll use the instance decl to deduce C [a] and then add the
1542 superclasses of C [a] to avails. But we must not overwrite the binding
1543 for d1:Ord a (which is given) with a superclass selection or we'll just
1546 Here's another example
1547 class Eq b => Foo a b
1548 instance Eq a => Foo [a] a
1552 we'll first deduce that it holds (via the instance decl). We must not
1553 then overwrite the Eq t constraint with a superclass selection!
1555 At first I had a gross hack, whereby I simply did not add superclass constraints
1556 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1557 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1558 I found a very obscure program (now tcrun021) in which improvement meant the
1559 simplifier got two bites a the cherry... so something seemed to be an Irred
1560 first time, but reducible next time.
1562 Now we implement the Right Solution, which is to check for loops directly
1563 when adding superclasses. It's a bit like the occurs check in unification.
1567 %************************************************************************
1569 \section{tcSimplifyTop: defaulting}
1571 %************************************************************************
1574 @tcSimplifyTop@ is called once per module to simplify all the constant
1575 and ambiguous Insts.
1577 We need to be careful of one case. Suppose we have
1579 instance Num a => Num (Foo a b) where ...
1581 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1582 to (Num x), and default x to Int. But what about y??
1584 It's OK: the final zonking stage should zap y to (), which is fine.
1588 tcSimplifyTop :: LIE -> TcM TcDictBinds
1589 tcSimplifyTop wanted_lie
1590 = simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenTc` \ (frees, binds, irreds) ->
1591 ASSERT( null frees )
1594 -- All the non-std ones are definite errors
1595 (stds, non_stds) = partition isStdClassTyVarDict irreds
1597 -- Group by type variable
1598 std_groups = equivClasses cmp_by_tyvar stds
1600 -- Pick the ones which its worth trying to disambiguate
1601 -- namely, the onese whose type variable isn't bound
1602 -- up with one of the non-standard classes
1603 (std_oks, std_bads) = partition worth_a_try std_groups
1604 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1605 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1607 -- Collect together all the bad guys
1608 bad_guys = non_stds ++ concat std_bads
1609 (tidy_env, tidy_dicts) = tidyInsts bad_guys
1610 (bad_ips, non_ips) = partition is_ip tidy_dicts
1611 (no_insts, ambigs) = partition no_inst non_ips
1612 is_ip d = any isIPPred (predsOfInst d)
1613 no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1614 fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet
1617 -- Report definite errors
1618 mapNF_Tc (addTopInstanceErrs tidy_env) (groupInsts no_insts) `thenNF_Tc_`
1619 mapNF_Tc (addTopIPErrs tidy_env) (groupInsts bad_ips) `thenNF_Tc_`
1621 -- Deal with ambiguity errors, but only if
1622 -- if there has not been an error so far; errors often
1623 -- give rise to spurious ambiguous Insts
1624 ifErrsTc (returnTc []) (
1626 -- Complain about the ones that don't fall under
1627 -- the Haskell rules for disambiguation
1628 -- This group includes both non-existent instances
1629 -- e.g. Num (IO a) and Eq (Int -> Int)
1630 -- and ambiguous dictionaries
1632 mapNF_Tc (addAmbigErr tidy_env) ambigs `thenNF_Tc_`
1634 -- Disambiguate the ones that look feasible
1635 mapTc disambigGroup std_oks
1636 ) `thenTc` \ binds_ambig ->
1638 returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
1640 wanteds = lieToList wanted_lie
1642 ----------------------------------
1643 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1645 get_tv d = case getDictClassTys d of
1646 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1647 get_clas d = case getDictClassTys d of
1648 (clas, [ty]) -> clas
1651 If a dictionary constrains a type variable which is
1652 * not mentioned in the environment
1653 * and not mentioned in the type of the expression
1654 then it is ambiguous. No further information will arise to instantiate
1655 the type variable; nor will it be generalised and turned into an extra
1656 parameter to a function.
1658 It is an error for this to occur, except that Haskell provided for
1659 certain rules to be applied in the special case of numeric types.
1661 * at least one of its classes is a numeric class, and
1662 * all of its classes are numeric or standard
1663 then the type variable can be defaulted to the first type in the
1664 default-type list which is an instance of all the offending classes.
1666 So here is the function which does the work. It takes the ambiguous
1667 dictionaries and either resolves them (producing bindings) or
1668 complains. It works by splitting the dictionary list by type
1669 variable, and using @disambigOne@ to do the real business.
1671 @disambigOne@ assumes that its arguments dictionaries constrain all
1672 the same type variable.
1674 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1675 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1676 the most common use of defaulting is code like:
1678 _ccall_ foo `seqPrimIO` bar
1680 Since we're not using the result of @foo@, the result if (presumably)
1684 disambigGroup :: [Inst] -- All standard classes of form (C a)
1688 | any isNumericClass classes -- Guaranteed all standard classes
1689 -- see comment at the end of function for reasons as to
1690 -- why the defaulting mechanism doesn't apply to groups that
1691 -- include CCallable or CReturnable dicts.
1692 && not (any isCcallishClass classes)
1693 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1694 -- SO, TRY DEFAULT TYPES IN ORDER
1696 -- Failure here is caused by there being no type in the
1697 -- default list which can satisfy all the ambiguous classes.
1698 -- For example, if Real a is reqd, but the only type in the
1699 -- default list is Int.
1700 tcGetDefaultTys `thenNF_Tc` \ default_tys ->
1702 try_default [] -- No defaults work, so fail
1705 try_default (default_ty : default_tys)
1706 = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
1707 -- default_tys instead
1708 tcSimplifyDefault theta `thenTc` \ _ ->
1711 theta = [mkClassPred clas [default_ty] | clas <- classes]
1713 -- See if any default works, and if so bind the type variable to it
1714 -- If not, add an AmbigErr
1715 recoverTc (addAmbigErrs dicts `thenNF_Tc_`
1716 returnTc EmptyMonoBinds) $
1718 try_default default_tys `thenTc` \ chosen_default_ty ->
1720 -- Bind the type variable and reduce the context, for real this time
1721 unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
1722 simpleReduceLoop (text "disambig" <+> ppr dicts)
1723 reduceMe dicts `thenTc` \ (frees, binds, ambigs) ->
1724 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1725 warnDefault dicts chosen_default_ty `thenTc_`
1728 | all isCreturnableClass classes
1729 = -- Default CCall stuff to (); we don't even both to check that () is an
1730 -- instance of CReturnable, because we know it is.
1731 unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
1732 returnTc EmptyMonoBinds
1734 | otherwise -- No defaults
1735 = addAmbigErrs dicts `thenNF_Tc_`
1736 returnTc EmptyMonoBinds
1739 tyvar = get_tv (head dicts) -- Should be non-empty
1740 classes = map get_clas dicts
1743 [Aside - why the defaulting mechanism is turned off when
1744 dealing with arguments and results to ccalls.
1746 When typechecking _ccall_s, TcExpr ensures that the external
1747 function is only passed arguments (and in the other direction,
1748 results) of a restricted set of 'native' types. This is
1749 implemented via the help of the pseudo-type classes,
1750 @CReturnable@ (CR) and @CCallable@ (CC.)
1752 The interaction between the defaulting mechanism for numeric
1753 values and CC & CR can be a bit puzzling to the user at times.
1762 What type has 'x' got here? That depends on the default list
1763 in operation, if it is equal to Haskell 98's default-default
1764 of (Integer, Double), 'x' has type Double, since Integer
1765 is not an instance of CR. If the default list is equal to
1766 Haskell 1.4's default-default of (Int, Double), 'x' has type
1769 To try to minimise the potential for surprises here, the
1770 defaulting mechanism is turned off in the presence of
1771 CCallable and CReturnable.
1776 %************************************************************************
1778 \subsection[simple]{@Simple@ versions}
1780 %************************************************************************
1782 Much simpler versions when there are no bindings to make!
1784 @tcSimplifyThetas@ simplifies class-type constraints formed by
1785 @deriving@ declarations and when specialising instances. We are
1786 only interested in the simplified bunch of class/type constraints.
1788 It simplifies to constraints of the form (C a b c) where
1789 a,b,c are type variables. This is required for the context of
1790 instance declarations.
1793 tcSimplifyDeriv :: [TyVar]
1794 -> ThetaType -- Wanted
1795 -> TcM ThetaType -- Needed
1797 tcSimplifyDeriv tyvars theta
1798 = tcInstTyVars VanillaTv tyvars `thenNF_Tc` \ (tvs, _, tenv) ->
1799 -- The main loop may do unification, and that may crash if
1800 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
1801 -- ToDo: what if two of them do get unified?
1802 newDicts DataDeclOrigin (substTheta tenv theta) `thenNF_Tc` \ wanteds ->
1803 simpleReduceLoop doc reduceMe wanteds `thenTc` \ (frees, _, irreds) ->
1804 ASSERT( null frees ) -- reduceMe never returns Free
1806 doptsTc Opt_AllowUndecidableInstances `thenNF_Tc` \ undecidable_ok ->
1808 tv_set = mkVarSet tvs
1809 simpl_theta = map dictPred irreds -- reduceMe squashes all non-dicts
1812 | isEmptyVarSet pred_tyvars -- Things like (Eq T) should be rejected
1813 = addErrTc (noInstErr pred)
1815 | not undecidable_ok && not (isTyVarClassPred pred)
1816 -- Check that the returned dictionaries are all of form (C a b)
1817 -- (where a, b are type variables).
1818 -- We allow this if we had -fallow-undecidable-instances,
1819 -- but note that risks non-termination in the 'deriving' context-inference
1820 -- fixpoint loop. It is useful for situations like
1821 -- data Min h a = E | M a (h a)
1822 -- which gives the instance decl
1823 -- instance (Eq a, Eq (h a)) => Eq (Min h a)
1824 = addErrTc (noInstErr pred)
1826 | not (pred_tyvars `subVarSet` tv_set)
1827 -- Check for a bizarre corner case, when the derived instance decl should
1828 -- have form instance C a b => D (T a) where ...
1829 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1830 -- of problems; in particular, it's hard to compare solutions for
1831 -- equality when finding the fixpoint. So I just rule it out for now.
1832 = addErrTc (badDerivedPred pred)
1837 pred_tyvars = tyVarsOfPred pred
1839 rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
1840 -- This reverse-mapping is a Royal Pain,
1841 -- but the result should mention TyVars not TcTyVars
1844 mapNF_Tc check_pred simpl_theta `thenNF_Tc_`
1845 checkAmbiguity tvs simpl_theta tv_set `thenTc_`
1846 returnTc (substTheta rev_env simpl_theta)
1848 doc = ptext SLIT("deriving classes for a data type")
1851 @tcSimplifyDefault@ just checks class-type constraints, essentially;
1852 used with \tr{default} declarations. We are only interested in
1853 whether it worked or not.
1856 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
1859 tcSimplifyDefault theta
1860 = newDicts DataDeclOrigin theta `thenNF_Tc` \ wanteds ->
1861 simpleReduceLoop doc reduceMe wanteds `thenTc` \ (frees, _, irreds) ->
1862 ASSERT( null frees ) -- try_me never returns Free
1863 mapNF_Tc (addErrTc . noInstErr) irreds `thenNF_Tc_`
1869 doc = ptext SLIT("default declaration")
1873 %************************************************************************
1875 \section{Errors and contexts}
1877 %************************************************************************
1879 ToDo: for these error messages, should we note the location as coming
1880 from the insts, or just whatever seems to be around in the monad just
1884 groupInsts :: [Inst] -> [[Inst]]
1885 -- Group together insts with the same origin
1886 -- We want to report them together in error messages
1888 groupInsts (inst:insts) = (inst:friends) : groupInsts others
1890 -- (It may seem a bit crude to compare the error messages,
1891 -- but it makes sure that we combine just what the user sees,
1892 -- and it avoids need equality on InstLocs.)
1893 (friends, others) = partition is_friend insts
1894 loc_msg = showSDoc (pprInstLoc (instLoc inst))
1895 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
1898 plural xs = char 's'
1900 addTopIPErrs tidy_env tidy_dicts
1901 = addInstErrTcM (instLoc (head tidy_dicts))
1903 ptext SLIT("Unbound implicit parameter") <> plural tidy_dicts <+> pprInsts tidy_dicts)
1905 -- Used for top-level irreducibles
1906 addTopInstanceErrs tidy_env tidy_dicts
1907 = addInstErrTcM (instLoc (head tidy_dicts))
1909 ptext SLIT("No instance") <> plural tidy_dicts <+>
1910 ptext SLIT("for") <+> pprInsts tidy_dicts)
1913 = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts
1915 (tidy_env, tidy_dicts) = tidyInsts dicts
1917 addAmbigErr tidy_env tidy_dict
1918 = addInstErrTcM (instLoc tidy_dict)
1920 sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs,
1921 nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
1923 ambig_tvs = varSetElems (tyVarsOfInst tidy_dict)
1925 warnDefault dicts default_ty
1926 = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag ->
1927 tcAddSrcLoc (get_loc (head dicts)) (warnTc warn_flag warn_msg)
1930 (_, tidy_dicts) = tidyInsts dicts
1931 get_loc i = case instLoc i of { (_,loc,_) -> loc }
1932 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
1933 quotes (ppr default_ty),
1934 pprInstsInFull tidy_dicts]
1936 complainCheck doc givens irreds
1937 = mapNF_Tc zonkInst given_dicts_and_ips `thenNF_Tc` \ givens' ->
1938 mapNF_Tc (addNoInstanceErrs doc givens') (groupInsts irreds) `thenNF_Tc_`
1941 given_dicts_and_ips = filter (not . isMethod) givens
1942 -- Filter out methods, which are only added to
1943 -- the given set as an optimisation
1945 addNoInstanceErrs what_doc givens dicts
1946 = getDOptsTc `thenNF_Tc` \ dflags ->
1947 tcGetInstEnv `thenNF_Tc` \ inst_env ->
1949 (tidy_env1, tidy_givens) = tidyInsts givens
1950 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
1952 doc = vcat [sep [herald <+> pprInsts tidy_dicts,
1953 nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
1955 ptext SLIT("Probable fix:"),
1959 herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
1960 unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
1963 -- The error message when we don't find a suitable instance
1964 -- is complicated by the fact that sometimes this is because
1965 -- there is no instance, and sometimes it's because there are
1966 -- too many instances (overlap). See the comments in TcEnv.lhs
1967 -- with the InstEnv stuff.
1970 | not ambig_overlap = empty
1972 = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
1973 nest 4 (ptext SLIT("depends on the instantiation of") <+>
1974 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInsts tidy_dicts))))]
1976 fix1 = sep [ptext SLIT("Add") <+> pprInsts tidy_dicts,
1977 ptext SLIT("to the") <+> what_doc]
1979 fix2 | null instance_dicts
1982 = ptext SLIT("Or add an instance declaration for") <+> pprInsts instance_dicts
1984 instance_dicts = [d | d <- tidy_dicts, isClassDict d, not (isTyVarDict d)]
1985 -- Insts for which it is worth suggesting an adding an instance declaration
1986 -- Exclude implicit parameters, and tyvar dicts
1988 -- Checks for the ambiguous case when we have overlapping instances
1989 ambig_overlap = any ambig_overlap1 dicts
1992 = case lookupInstEnv dflags inst_env clas tys of
1993 NoMatch ambig -> ambig
1997 (clas,tys) = getDictClassTys dict
1999 addInstErrTcM (instLoc (head dicts)) (tidy_env2, doc)
2001 -- Used for the ...Thetas variants; all top level
2002 noInstErr pred = ptext SLIT("No instance for") <+> quotes (ppr pred)
2005 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2006 ptext SLIT("type variables that are not data type parameters"),
2007 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2009 reduceDepthErr n stack
2010 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2011 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
2012 nest 4 (pprInstsInFull stack)]
2014 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)
2016 -----------------------------------------------
2018 = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
2019 nest 4 (ppr inst <+> pprInstLoc (instLoc inst))])