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 {-# SOURCE #-} TcUnify( unifyTauTy )
22 import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
23 import TcHsSyn ( TcExpr, TcId,
24 TcMonoBinds, TcDictBinds
28 import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
29 tyVarsOfInst, predsOfInsts, predsOfInst,
30 isDict, isClassDict, isLinearInst, linearInstType,
31 isStdClassTyVarDict, isMethodFor, isMethod,
32 instToId, tyVarsOfInsts, cloneDict,
33 ipNamesOfInsts, ipNamesOfInst,
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 )
44 import TcType ( TcTyVar, TcTyVarSet, ThetaType, PredType,
45 mkClassPred, isOverloadedTy, mkTyConApp,
46 mkTyVarTy, tcGetTyVar, isTyVarClassPred,
47 tyVarsOfPred, getClassPredTys_maybe, isClassPred, isIPPred,
48 inheritablePred, predHasFDs )
49 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 splitIdName, fstIdName, sndIdName )
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 inheritablePred (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.
673 -- Note that we psss isFree (not isFreeAndInheritable) to tcSimplCheck
674 -- It's important that we can float out non-inheritable predicates
675 -- Example: (?x :: Int) is ok!
676 tcSimplifyCheck doc qtvs givens wanted_lie
677 = tcSimplCheck doc get_qtvs
678 givens wanted_lie `thenTc` \ (qtvs', frees, binds) ->
679 returnTc (frees, binds)
681 get_qtvs = zonkTcTyVarsAndFV qtvs
684 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
685 -- against, but we don't know the type variables over which we are going to quantify.
686 -- This happens when we have a type signature for a mutually recursive group
689 -> TcTyVarSet -- fv(T)
692 -> TcM ([TcTyVar], -- Variables over which to quantify
694 TcDictBinds) -- Bindings
696 tcSimplifyInferCheck doc tau_tvs givens wanted_lie
697 = tcSimplCheck doc get_qtvs givens wanted_lie
699 -- Figure out which type variables to quantify over
700 -- You might think it should just be the signature tyvars,
701 -- but in bizarre cases you can get extra ones
702 -- f :: forall a. Num a => a -> a
703 -- f x = fst (g (x, head [])) + 1
705 -- Here we infer g :: forall a b. a -> b -> (b,a)
706 -- We don't want g to be monomorphic in b just because
707 -- f isn't quantified over b.
708 all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
710 get_qtvs = zonkTcTyVarsAndFV all_tvs `thenNF_Tc` \ all_tvs' ->
711 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
713 qtvs = all_tvs' `minusVarSet` gbl_tvs
714 -- We could close gbl_tvs, but its not necessary for
715 -- soundness, and it'll only affect which tyvars, not which
716 -- dictionaries, we quantify over
721 Here is the workhorse function for all three wrappers.
724 tcSimplCheck doc get_qtvs givens wanted_lie
725 = check_loop givens (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
727 -- Complain about any irreducible ones
728 complainCheck doc givens irreds `thenNF_Tc_`
731 returnTc (qtvs, mkLIE frees, binds)
734 ip_set = mkNameSet (ipNamesOfInsts givens)
736 check_loop givens wanteds
738 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
739 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
740 get_qtvs `thenNF_Tc` \ qtvs' ->
744 -- When checking against a given signature we always reduce
745 -- until we find a match against something given, or can't reduce
746 try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
747 | otherwise = ReduceMe
749 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
752 if no_improvement then
753 returnTc (varSetElems qtvs', frees, binds, irreds)
755 check_loop givens' (irreds ++ frees) `thenTc` \ (qtvs', frees1, binds1, irreds1) ->
756 returnTc (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1)
760 %************************************************************************
762 \subsection{tcSimplifyRestricted}
764 %************************************************************************
767 tcSimplifyRestricted -- Used for restricted binding groups
768 -- i.e. ones subject to the monomorphism restriction
770 -> TcTyVarSet -- Free in the type of the RHSs
771 -> LIE -- Free in the RHSs
772 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
774 TcDictBinds) -- Bindings
776 tcSimplifyRestricted doc tau_tvs wanted_lie
777 = -- First squash out all methods, to find the constrained tyvars
778 -- We can't just take the free vars of wanted_lie because that'll
779 -- have methods that may incidentally mention entirely unconstrained variables
780 -- e.g. a call to f :: Eq a => a -> b -> b
781 -- Here, b is unconstrained. A good example would be
783 -- We want to infer the polymorphic type
784 -- foo :: forall b. b -> b
786 wanteds = lieToList wanted_lie
787 try_me inst = ReduceMe -- Reduce as far as we can. Don't stop at
788 -- dicts; the idea is to get rid of as many type
789 -- variables as possible, and we don't want to stop
790 -- at (say) Monad (ST s), because that reduces
791 -- immediately, with no constraint on s.
793 simpleReduceLoop doc try_me wanteds `thenTc` \ (_, _, constrained_dicts) ->
795 -- Next, figure out the tyvars we will quantify over
796 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenNF_Tc` \ tau_tvs' ->
797 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
799 constrained_tvs = tyVarsOfInsts constrained_dicts
800 qtvs = (tau_tvs' `minusVarSet` oclose (predsOfInsts constrained_dicts) gbl_tvs)
801 `minusVarSet` constrained_tvs
804 -- The first step may have squashed more methods than
805 -- necessary, so try again, this time knowing the exact
806 -- set of type variables to quantify over.
808 -- We quantify only over constraints that are captured by qtvs;
809 -- these will just be a subset of non-dicts. This in contrast
810 -- to normal inference (using isFreeWhenInferring) in which we quantify over
811 -- all *non-inheritable* constraints too. This implements choice
812 -- (B) under "implicit parameter and monomorphism" above.
814 -- Remember that we may need to do *some* simplification, to
815 -- (for example) squash {Monad (ST s)} into {}. It's not enough
816 -- just to float all constraints
817 mapNF_Tc zonkInst (lieToList wanted_lie) `thenNF_Tc` \ wanteds' ->
819 try_me inst | isFreeWrtTyVars qtvs inst = Free
820 | otherwise = ReduceMe
822 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
823 ASSERT( no_improvement )
824 ASSERT( null irreds )
825 -- No need to loop because simpleReduceLoop will have
826 -- already done any improvement necessary
828 returnTc (varSetElems qtvs, mkLIE frees, binds)
832 %************************************************************************
834 \subsection{tcSimplifyToDicts}
836 %************************************************************************
838 On the LHS of transformation rules we only simplify methods and constants,
839 getting dictionaries. We want to keep all of them unsimplified, to serve
840 as the available stuff for the RHS of the rule.
842 The same thing is used for specialise pragmas. Consider
845 {-# SPECIALISE f :: Int -> Int #-}
848 The type checker generates a binding like:
850 f_spec = (f :: Int -> Int)
852 and we want to end up with
854 f_spec = _inline_me_ (f Int dNumInt)
856 But that means that we must simplify the Method for f to (f Int dNumInt)!
857 So tcSimplifyToDicts squeezes out all Methods.
859 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
861 fromIntegral :: (Integral a, Num b) => a -> b
862 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
864 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
868 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
870 because the scsel will mess up matching. Instead we want
872 forall dIntegralInt, dNumInt.
873 fromIntegral Int Int dIntegralInt dNumInt = id Int
875 Hence "DontReduce NoSCs"
878 tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
879 tcSimplifyToDicts wanted_lie
880 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
881 -- Since try_me doesn't look at types, we don't need to
882 -- do any zonking, so it's safe to call reduceContext directly
884 returnTc (irreds, binds)
887 doc = text "tcSimplifyToDicts"
888 wanteds = lieToList wanted_lie
890 -- Reduce methods and lits only; stop as soon as we get a dictionary
891 try_me inst | isDict inst = DontReduce NoSCs
892 | otherwise = ReduceMe
896 %************************************************************************
898 \subsection{Filtering at a dynamic binding}
900 %************************************************************************
905 we must discharge all the ?x constraints from B. We also do an improvement
906 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
908 Actually, the constraints from B might improve the types in ?x. For example
910 f :: (?x::Int) => Char -> Char
913 then the constraint (?x::Int) arising from the call to f will
914 force the binding for ?x to be of type Int.
917 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
919 -> TcM (LIE, TcDictBinds)
920 tcSimplifyIPs given_ips wanted_lie
921 = simpl_loop given_ips wanteds `thenTc` \ (frees, binds) ->
922 returnTc (mkLIE frees, binds)
924 doc = text "tcSimplifyIPs" <+> ppr given_ips
925 wanteds = lieToList wanted_lie
926 ip_set = mkNameSet (ipNamesOfInsts given_ips)
928 -- Simplify any methods that mention the implicit parameter
929 try_me inst | isFreeWrtIPs ip_set inst = Free
930 | otherwise = ReduceMe
932 simpl_loop givens wanteds
933 = mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
934 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
936 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
938 if no_improvement then
939 ASSERT( null irreds )
940 returnTc (frees, binds)
942 simpl_loop givens' (irreds ++ frees) `thenTc` \ (frees1, binds1) ->
943 returnTc (frees1, binds `AndMonoBinds` binds1)
947 %************************************************************************
949 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
951 %************************************************************************
953 When doing a binding group, we may have @Insts@ of local functions.
954 For example, we might have...
956 let f x = x + 1 -- orig local function (overloaded)
957 f.1 = f Int -- two instances of f
962 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
963 where @f@ is in scope; those @Insts@ must certainly not be passed
964 upwards towards the top-level. If the @Insts@ were binding-ified up
965 there, they would have unresolvable references to @f@.
967 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
968 For each method @Inst@ in the @init_lie@ that mentions one of the
969 @Ids@, we create a binding. We return the remaining @Insts@ (in an
970 @LIE@), as well as the @HsBinds@ generated.
973 bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
975 bindInstsOfLocalFuns init_lie local_ids
976 | null overloaded_ids
978 = returnTc (init_lie, EmptyMonoBinds)
981 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
982 ASSERT( null irreds )
983 returnTc (mkLIE frees, binds)
985 doc = text "bindInsts" <+> ppr local_ids
986 wanteds = lieToList init_lie
987 overloaded_ids = filter is_overloaded local_ids
988 is_overloaded id = isOverloadedTy (idType id)
990 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
991 -- so it's worth building a set, so that
992 -- lookup (in isMethodFor) is faster
994 try_me inst | isMethodFor overloaded_set inst = ReduceMe
999 %************************************************************************
1001 \subsection{Data types for the reduction mechanism}
1003 %************************************************************************
1005 The main control over context reduction is here
1009 = ReduceMe -- Try to reduce this
1010 -- If there's no instance, behave exactly like
1011 -- DontReduce: add the inst to
1012 -- the irreductible ones, but don't
1013 -- produce an error message of any kind.
1014 -- It might be quite legitimate such as (Eq a)!
1016 | DontReduce WantSCs -- Return as irreducible
1018 | DontReduceUnlessConstant -- Return as irreducible unless it can
1019 -- be reduced to a constant in one step
1021 | Free -- Return as free
1023 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1024 -- of a predicate when adding it to the avails
1030 type Avails = FiniteMap Inst Avail
1033 = IsFree -- Used for free Insts
1034 | Irred -- Used for irreducible dictionaries,
1035 -- which are going to be lambda bound
1037 | Given TcId -- Used for dictionaries for which we have a binding
1038 -- e.g. those "given" in a signature
1039 Bool -- True <=> actually consumed (splittable IPs only)
1041 | NoRhs -- Used for Insts like (CCallable f)
1042 -- where no witness is required.
1044 | Rhs -- Used when there is a RHS
1046 [Inst] -- Insts free in the RHS; we need these too
1048 | Linear -- Splittable Insts only.
1049 Int -- The Int is always 2 or more; indicates how
1050 -- many copies are required
1051 Inst -- The splitter
1052 Avail -- Where the "master copy" is
1054 | LinRhss -- Splittable Insts only; this is used only internally
1055 -- by extractResults, where a Linear
1056 -- is turned into an LinRhss
1057 [TcExpr] -- A supply of suitable RHSs
1059 pprAvails avails = vcat [ppr inst <+> equals <+> pprAvail avail
1060 | (inst,avail) <- fmToList avails ]
1062 instance Outputable Avail where
1065 pprAvail NoRhs = text "<no rhs>"
1066 pprAvail IsFree = text "Free"
1067 pprAvail Irred = text "Irred"
1068 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1069 if b then text "(used)" else empty
1070 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1071 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1072 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1075 Extracting the bindings from a bunch of Avails.
1076 The bindings do *not* come back sorted in dependency order.
1077 We assume that they'll be wrapped in a big Rec, so that the
1078 dependency analyser can sort them out later
1082 extractResults :: Avails
1084 -> NF_TcM (TcDictBinds, -- Bindings
1085 [Inst], -- Irreducible ones
1086 [Inst]) -- Free ones
1088 extractResults avails wanteds
1089 = go avails EmptyMonoBinds [] [] wanteds
1091 go avails binds irreds frees []
1092 = returnNF_Tc (binds, irreds, frees)
1094 go avails binds irreds frees (w:ws)
1095 = case lookupFM avails w of
1096 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1097 go avails binds irreds frees ws
1099 Just NoRhs -> go avails binds irreds frees ws
1100 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1101 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1103 Just (Given id _) -> go avails new_binds irreds frees ws
1105 new_binds | id == instToId w = binds
1106 | otherwise = addBind binds w (HsVar id)
1107 -- The sought Id can be one of the givens, via a superclass chain
1108 -- and then we definitely don't want to generate an x=x binding!
1110 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1112 new_binds = addBind binds w rhs
1114 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1115 -> go new_avails new_binds irreds frees ws
1117 new_binds = addBind binds w rhs
1118 new_avails = addToFM avails w (LinRhss rhss)
1120 Just (Linear n split_inst avail)
1121 -> split n (instToId split_inst) avail w `thenNF_Tc` \ (binds', (rhs:rhss), irreds') ->
1122 go (addToFM avails w (LinRhss rhss))
1123 (binds `AndMonoBinds` addBind binds' w rhs)
1124 (irreds' ++ irreds) frees (split_inst:ws)
1128 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1129 | otherwise = addToFM avails w NoRhs
1130 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1131 -- than Given, else we end up with bogus bindings.
1133 add_free avails w | isMethod w = avails
1134 | otherwise = add_given avails w
1136 -- Do *not* replace Free by Given if it's a method.
1137 -- The following situation shows why this is bad:
1138 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1139 -- From an application (truncate f i) we get
1140 -- t1 = truncate at f
1142 -- If we have also have a second occurrence of truncate, we get
1143 -- t3 = truncate at f
1145 -- When simplifying with i,f free, we might still notice that
1146 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1147 -- will continue to float out!
1148 -- (split n i a) returns: n rhss
1149 -- auxiliary bindings
1150 -- 1 or 0 insts to add to irreds
1153 split :: Int -> TcId -> Avail -> Inst
1154 -> NF_TcM (TcDictBinds, [TcExpr], [Inst])
1155 -- (split n split_id avail wanted) returns
1156 -- * a list of 'n' expressions, all of which witness 'avail'
1157 -- * a bunch of auxiliary bindings to support these expressions
1158 -- * one or zero insts needed to witness the whole lot
1159 -- (maybe be zero if the initial Inst is a Given)
1160 split n split_id avail wanted
1163 ty = linearInstType wanted
1164 pair_ty = mkTyConApp pairTyCon [ty,ty]
1165 id = instToId wanted
1169 go 1 = case avail of
1170 Given id _ -> returnNF_Tc (EmptyMonoBinds, [HsVar id], [])
1171 Irred -> cloneDict wanted `thenNF_Tc` \ w' ->
1172 returnNF_Tc (EmptyMonoBinds, [HsVar (instToId w')], [w'])
1174 go n = go ((n+1) `div` 2) `thenNF_Tc` \ (binds1, rhss, irred) ->
1175 expand n rhss `thenNF_Tc` \ (binds2, rhss') ->
1176 returnNF_Tc (binds1 `AndMonoBinds` binds2, rhss', irred)
1179 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1180 -- e.g. expand 3 [rhs1, rhs2]
1181 -- = ( { x = split rhs1 },
1182 -- [fst x, snd x, rhs2] )
1184 | n `rem` 2 == 0 = go rhss -- n is even
1185 | otherwise = go (tail rhss) `thenNF_Tc` \ (binds', rhss') ->
1186 returnNF_Tc (binds', head rhss : rhss')
1188 go rhss = mapAndUnzipNF_Tc do_one rhss `thenNF_Tc` \ (binds', rhss') ->
1189 returnNF_Tc (andMonoBindList binds', concat rhss')
1191 do_one rhs = tcGetUnique `thenNF_Tc` \ uniq ->
1192 tcLookupGlobalId fstIdName `thenNF_Tc` \ fst_id ->
1193 tcLookupGlobalId sndIdName `thenNF_Tc` \ snd_id ->
1195 x = mkUserLocal occ uniq pair_ty loc
1197 returnNF_Tc (VarMonoBind x (mk_app split_id rhs),
1198 [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])
1200 mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var
1202 mk_app id rhs = HsApp (HsVar id) rhs
1204 addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
1208 %************************************************************************
1210 \subsection[reduce]{@reduce@}
1212 %************************************************************************
1214 When the "what to do" predicate doesn't depend on the quantified type variables,
1215 matters are easier. We don't need to do any zonking, unless the improvement step
1216 does something, in which case we zonk before iterating.
1218 The "given" set is always empty.
1221 simpleReduceLoop :: SDoc
1222 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1224 -> TcM ([Inst], -- Free
1226 [Inst]) -- Irreducible
1228 simpleReduceLoop doc try_me wanteds
1229 = mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
1230 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
1231 if no_improvement then
1232 returnTc (frees, binds, irreds)
1234 simpleReduceLoop doc try_me (irreds ++ frees) `thenTc` \ (frees1, binds1, irreds1) ->
1235 returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
1241 reduceContext :: SDoc
1242 -> (Inst -> WhatToDo)
1245 -> NF_TcM (Bool, -- True <=> improve step did no unification
1247 TcDictBinds, -- Dictionary bindings
1248 [Inst]) -- Irreducible
1250 reduceContext doc try_me givens wanteds
1252 traceTc (text "reduceContext" <+> (vcat [
1253 text "----------------------",
1255 text "given" <+> ppr givens,
1256 text "wanted" <+> ppr wanteds,
1257 text "----------------------"
1260 -- Build the Avail mapping from "givens"
1261 foldlNF_Tc addGiven emptyFM givens `thenNF_Tc` \ init_state ->
1264 reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ avails ->
1266 -- Do improvement, using everything in avails
1267 -- In particular, avails includes all superclasses of everything
1268 tcImprove avails `thenTc` \ no_improvement ->
1270 extractResults avails wanteds `thenNF_Tc` \ (binds, irreds, frees) ->
1272 traceTc (text "reduceContext end" <+> (vcat [
1273 text "----------------------",
1275 text "given" <+> ppr givens,
1276 text "wanted" <+> ppr wanteds,
1278 text "avails" <+> pprAvails avails,
1279 text "frees" <+> ppr frees,
1280 text "no_improvement =" <+> ppr no_improvement,
1281 text "----------------------"
1284 returnTc (no_improvement, frees, binds, irreds)
1287 = tcGetInstEnv `thenTc` \ inst_env ->
1289 preds = [ (pred, pp_loc)
1290 | inst <- keysFM avails,
1291 let pp_loc = pprInstLoc (instLoc inst),
1292 pred <- predsOfInst inst,
1295 -- Avails has all the superclasses etc (good)
1296 -- It also has all the intermediates of the deduction (good)
1297 -- It does not have duplicates (good)
1298 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1299 -- so that improve will see them separate
1300 eqns = improve (classInstEnv inst_env) preds
1305 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenNF_Tc_`
1306 mapTc_ unify eqns `thenTc_`
1309 unify ((qtvs, t1, t2), doc)
1310 = tcAddErrCtxt doc $
1311 tcInstTyVars (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) ->
1312 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1315 The main context-reduction function is @reduce@. Here's its game plan.
1318 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1319 -- along with its depth
1320 -> (Inst -> WhatToDo)
1327 try_me: given an inst, this function returns
1329 DontReduce return this in "irreds"
1330 Free return this in "frees"
1332 wanteds: The list of insts to reduce
1333 state: An accumulating parameter of type Avails
1334 that contains the state of the algorithm
1336 It returns a Avails.
1338 The (n,stack) pair is just used for error reporting.
1339 n is always the depth of the stack.
1340 The stack is the stack of Insts being reduced: to produce X
1341 I had to produce Y, to produce Y I had to produce Z, and so on.
1344 reduceList (n,stack) try_me wanteds state
1345 | n > opt_MaxContextReductionDepth
1346 = failWithTc (reduceDepthErr n stack)
1352 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1357 go [] state = returnTc state
1358 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
1361 -- Base case: we're done!
1362 reduce stack try_me wanted state
1363 -- It's the same as an existing inst, or a superclass thereof
1364 | Just avail <- isAvailable state wanted
1365 = if isLinearInst wanted then
1366 addLinearAvailable state avail wanted `thenNF_Tc` \ (state', wanteds') ->
1367 reduceList stack try_me wanteds' state'
1369 returnTc state -- No op for non-linear things
1372 = case try_me wanted of {
1374 DontReduce want_scs -> addIrred want_scs state wanted
1376 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1377 -- First, see if the inst can be reduced to a constant in one step
1378 try_simple (addIrred AddSCs) -- Assume want superclasses
1380 ; Free -> -- It's free so just chuck it upstairs
1381 -- First, see if the inst can be reduced to a constant in one step
1384 ; ReduceMe -> -- It should be reduced
1385 lookupInst wanted `thenNF_Tc` \ lookup_result ->
1386 case lookup_result of
1387 GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenTc` \ state' ->
1388 addWanted state' wanted rhs wanteds'
1389 SimpleInst rhs -> addWanted state wanted rhs []
1391 NoInstance -> -- No such instance!
1392 -- Add it and its superclasses
1393 addIrred AddSCs state wanted
1397 try_simple do_this_otherwise
1398 = lookupInst wanted `thenNF_Tc` \ lookup_result ->
1399 case lookup_result of
1400 SimpleInst rhs -> addWanted state wanted rhs []
1401 other -> do_this_otherwise state wanted
1406 -------------------------
1407 isAvailable :: Avails -> Inst -> Maybe Avail
1408 isAvailable avails wanted = lookupFM avails wanted
1409 -- NB 1: the Ord instance of Inst compares by the class/type info
1410 -- *not* by unique. So
1411 -- d1::C Int == d2::C Int
1413 addLinearAvailable :: Avails -> Avail -> Inst -> NF_TcM (Avails, [Inst])
1414 addLinearAvailable avails avail wanted
1416 = tcLookupGlobalId splitIdName `thenNF_Tc` \ split_id ->
1417 newMethodAtLoc (instLoc wanted) split_id
1418 [linearInstType wanted] `thenNF_Tc` \ (split_inst,_) ->
1419 returnNF_Tc (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1422 = returnNF_Tc (addToFM avails wanted avail', [])
1424 avail' = case avail of
1425 Given id _ -> Given id True
1426 Linear n i a -> Linear (n+1) i a
1428 need_split Irred = True
1429 need_split (Given _ used) = used
1430 need_split (Linear _ _ _) = False
1432 -------------------------
1433 addFree :: Avails -> Inst -> NF_TcM Avails
1434 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1435 -- to avails, so that any other equal Insts will be commoned up right
1436 -- here rather than also being tossed upstairs. This is really just
1437 -- an optimisation, and perhaps it is more trouble that it is worth,
1438 -- as the following comments show!
1440 -- NB1: do *not* add superclasses. If we have
1443 -- but a is not bound here, then we *don't* want to derive
1444 -- dn from df here lest we lose sharing.
1446 addFree avails free = returnNF_Tc (addToFM avails free IsFree)
1448 addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> NF_TcM Avails
1449 addWanted avails wanted rhs_expr wanteds
1450 -- Do *not* add superclasses as well. Here's an example of why not
1451 -- class Eq a => Foo a b
1452 -- instance Eq a => Foo [a] a
1453 -- If we are reducing
1455 -- we'll first deduce that it holds (via the instance decl). We
1456 -- must not then overwrite the Eq t constraint with a superclass selection!
1457 -- ToDo: this isn't entirely unsatisfactory, because
1458 -- we may also lose some entirely-legitimate sharing this way
1460 = ASSERT( not (wanted `elemFM` avails) )
1461 returnNF_Tc (addToFM avails wanted avail)
1463 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1464 | otherwise = ASSERT( null wanteds ) NoRhs
1466 addGiven :: Avails -> Inst -> NF_TcM Avails
1467 addGiven state given = addAvailAndSCs state given (Given (instToId given) False)
1469 addIrred :: WantSCs -> Avails -> Inst -> NF_TcM Avails
1470 addIrred NoSCs state irred = returnNF_Tc (addToFM state irred Irred)
1471 addIrred AddSCs state irred = addAvailAndSCs state irred Irred
1473 addAvailAndSCs :: Avails -> Inst -> Avail -> NF_TcM Avails
1474 addAvailAndSCs avails wanted avail
1475 = add_scs (addToFM avails wanted avail) wanted
1477 add_scs :: Avails -> Inst -> NF_TcM Avails
1478 -- Add all the superclasses of the Inst to Avails
1479 -- Invariant: the Inst is already in Avails.
1482 | not (isClassDict dict)
1483 = returnNF_Tc avails
1485 | otherwise -- It is a dictionary
1486 = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
1487 foldlNF_Tc add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1489 (clas, tys) = getDictClassTys dict
1490 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1491 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1493 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1494 = case lookupFM avails sc_dict of
1495 Just (Given _ _) -> returnNF_Tc avails -- See Note [SUPER] below
1496 other -> addAvailAndSCs avails sc_dict avail
1498 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1499 avail = Rhs sc_sel_rhs [dict]
1502 Note [SUPER]. We have to be careful here. If we are *given* d1:Ord a,
1503 and want to deduce (d2:C [a]) where
1505 class Ord a => C a where
1506 instance Ord a => C [a] where ...
1508 Then we'll use the instance decl to deduce C [a] and then add the
1509 superclasses of C [a] to avails. But we must not overwrite the binding
1510 for d1:Ord a (which is given) with a superclass selection or we'll just
1511 build a loop! Hence looking for Given. Crudely, Given is cheaper
1515 %************************************************************************
1517 \section{tcSimplifyTop: defaulting}
1519 %************************************************************************
1522 If a dictionary constrains a type variable which is
1523 * not mentioned in the environment
1524 * and not mentioned in the type of the expression
1525 then it is ambiguous. No further information will arise to instantiate
1526 the type variable; nor will it be generalised and turned into an extra
1527 parameter to a function.
1529 It is an error for this to occur, except that Haskell provided for
1530 certain rules to be applied in the special case of numeric types.
1532 * at least one of its classes is a numeric class, and
1533 * all of its classes are numeric or standard
1534 then the type variable can be defaulted to the first type in the
1535 default-type list which is an instance of all the offending classes.
1537 So here is the function which does the work. It takes the ambiguous
1538 dictionaries and either resolves them (producing bindings) or
1539 complains. It works by splitting the dictionary list by type
1540 variable, and using @disambigOne@ to do the real business.
1542 @tcSimplifyTop@ is called once per module to simplify all the constant
1543 and ambiguous Insts.
1545 We need to be careful of one case. Suppose we have
1547 instance Num a => Num (Foo a b) where ...
1549 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1550 to (Num x), and default x to Int. But what about y??
1552 It's OK: the final zonking stage should zap y to (), which is fine.
1556 tcSimplifyTop :: LIE -> TcM TcDictBinds
1557 tcSimplifyTop wanted_lie
1558 = simpleReduceLoop (text "tcSimplTop") try_me wanteds `thenTc` \ (frees, binds, irreds) ->
1559 ASSERT( null frees )
1562 -- All the non-std ones are definite errors
1563 (stds, non_stds) = partition isStdClassTyVarDict irreds
1565 -- Group by type variable
1566 std_groups = equivClasses cmp_by_tyvar stds
1568 -- Pick the ones which its worth trying to disambiguate
1569 (std_oks, std_bads) = partition worth_a_try std_groups
1571 -- Have a try at disambiguation
1572 -- if the type variable isn't bound
1573 -- up with one of the non-standard classes
1574 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1575 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1577 -- Collect together all the bad guys
1578 bad_guys = non_stds ++ concat std_bads
1580 -- Disambiguate the ones that look feasible
1581 mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
1583 -- And complain about the ones that don't
1584 -- This group includes both non-existent instances
1585 -- e.g. Num (IO a) and Eq (Int -> Int)
1586 -- and ambiguous dictionaries
1588 addTopAmbigErrs bad_guys `thenNF_Tc_`
1590 returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
1592 wanteds = lieToList wanted_lie
1593 try_me inst = ReduceMe
1595 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1597 get_tv d = case getDictClassTys d of
1598 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1599 get_clas d = case getDictClassTys d of
1600 (clas, [ty]) -> clas
1603 @disambigOne@ assumes that its arguments dictionaries constrain all
1604 the same type variable.
1606 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1607 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1608 the most common use of defaulting is code like:
1610 _ccall_ foo `seqPrimIO` bar
1612 Since we're not using the result of @foo@, the result if (presumably)
1616 disambigGroup :: [Inst] -- All standard classes of form (C a)
1620 | any isNumericClass classes -- Guaranteed all standard classes
1621 -- see comment at the end of function for reasons as to
1622 -- why the defaulting mechanism doesn't apply to groups that
1623 -- include CCallable or CReturnable dicts.
1624 && not (any isCcallishClass classes)
1625 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1626 -- SO, TRY DEFAULT TYPES IN ORDER
1628 -- Failure here is caused by there being no type in the
1629 -- default list which can satisfy all the ambiguous classes.
1630 -- For example, if Real a is reqd, but the only type in the
1631 -- default list is Int.
1632 tcGetDefaultTys `thenNF_Tc` \ default_tys ->
1634 try_default [] -- No defaults work, so fail
1637 try_default (default_ty : default_tys)
1638 = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
1639 -- default_tys instead
1640 tcSimplifyCheckThetas [] theta `thenTc` \ _ ->
1643 theta = [mkClassPred clas [default_ty] | clas <- classes]
1645 -- See if any default works, and if so bind the type variable to it
1646 -- If not, add an AmbigErr
1647 recoverTc (addAmbigErrs dicts `thenNF_Tc_`
1648 returnTc EmptyMonoBinds) $
1650 try_default default_tys `thenTc` \ chosen_default_ty ->
1652 -- Bind the type variable and reduce the context, for real this time
1653 unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
1654 simpleReduceLoop (text "disambig" <+> ppr dicts)
1655 try_me dicts `thenTc` \ (frees, binds, ambigs) ->
1656 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1657 warnDefault dicts chosen_default_ty `thenTc_`
1660 | all isCreturnableClass classes
1661 = -- Default CCall stuff to (); we don't even both to check that () is an
1662 -- instance of CReturnable, because we know it is.
1663 unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
1664 returnTc EmptyMonoBinds
1666 | otherwise -- No defaults
1667 = addAmbigErrs dicts `thenNF_Tc_`
1668 returnTc EmptyMonoBinds
1671 try_me inst = ReduceMe -- This reduce should not fail
1672 tyvar = get_tv (head dicts) -- Should be non-empty
1673 classes = map get_clas dicts
1676 [Aside - why the defaulting mechanism is turned off when
1677 dealing with arguments and results to ccalls.
1679 When typechecking _ccall_s, TcExpr ensures that the external
1680 function is only passed arguments (and in the other direction,
1681 results) of a restricted set of 'native' types. This is
1682 implemented via the help of the pseudo-type classes,
1683 @CReturnable@ (CR) and @CCallable@ (CC.)
1685 The interaction between the defaulting mechanism for numeric
1686 values and CC & CR can be a bit puzzling to the user at times.
1695 What type has 'x' got here? That depends on the default list
1696 in operation, if it is equal to Haskell 98's default-default
1697 of (Integer, Double), 'x' has type Double, since Integer
1698 is not an instance of CR. If the default list is equal to
1699 Haskell 1.4's default-default of (Int, Double), 'x' has type
1702 To try to minimise the potential for surprises here, the
1703 defaulting mechanism is turned off in the presence of
1704 CCallable and CReturnable.
1709 %************************************************************************
1711 \subsection[simple]{@Simple@ versions}
1713 %************************************************************************
1715 Much simpler versions when there are no bindings to make!
1717 @tcSimplifyThetas@ simplifies class-type constraints formed by
1718 @deriving@ declarations and when specialising instances. We are
1719 only interested in the simplified bunch of class/type constraints.
1721 It simplifies to constraints of the form (C a b c) where
1722 a,b,c are type variables. This is required for the context of
1723 instance declarations.
1726 tcSimplifyThetas :: ThetaType -- Wanted
1727 -> TcM ThetaType -- Needed
1729 tcSimplifyThetas wanteds
1730 = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
1731 reduceSimple [] wanteds `thenNF_Tc` \ irreds ->
1733 -- For multi-param Haskell, check that the returned dictionaries
1734 -- don't have any of the form (C Int Bool) for which
1735 -- we expect an instance here
1736 -- For Haskell 98, check that all the constraints are of the form C a,
1737 -- where a is a type variable
1738 bad_guys | glaExts = [pred | pred <- irreds,
1739 isEmptyVarSet (tyVarsOfPred pred)]
1740 | otherwise = [pred | pred <- irreds,
1741 not (isTyVarClassPred pred)]
1743 if null bad_guys then
1746 mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
1750 @tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
1751 used with \tr{default} declarations. We are only interested in
1752 whether it worked or not.
1755 tcSimplifyCheckThetas :: ThetaType -- Given
1756 -> ThetaType -- Wanted
1759 tcSimplifyCheckThetas givens wanteds
1760 = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
1764 mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
1770 type AvailsSimple = FiniteMap PredType Bool
1771 -- True => irreducible
1772 -- False => given, or can be derived from a given or from an irreducible
1774 reduceSimple :: ThetaType -- Given
1775 -> ThetaType -- Wanted
1776 -> NF_TcM ThetaType -- Irreducible
1778 reduceSimple givens wanteds
1779 = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
1780 returnNF_Tc [pred | (pred,True) <- fmToList givens_fm']
1782 givens_fm = foldl addNonIrred emptyFM givens
1784 reduce_simple :: (Int,ThetaType) -- Stack
1787 -> NF_TcM AvailsSimple
1789 reduce_simple (n,stack) avails wanteds
1792 go avails [] = returnNF_Tc avails
1793 go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
1796 reduce_simple_help stack givens wanted
1797 | wanted `elemFM` givens
1798 = returnNF_Tc givens
1800 | Just (clas, tys) <- getClassPredTys_maybe wanted
1801 = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
1803 Nothing -> returnNF_Tc (addSimpleIrred givens wanted)
1804 Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
1807 = returnNF_Tc (addSimpleIrred givens wanted)
1809 addSimpleIrred :: AvailsSimple -> PredType -> AvailsSimple
1810 addSimpleIrred givens pred
1811 = addSCs (addToFM givens pred True) pred
1813 addNonIrred :: AvailsSimple -> PredType -> AvailsSimple
1814 addNonIrred givens pred
1815 = addSCs (addToFM givens pred False) pred
1818 | not (isClassPred pred) = givens
1819 | otherwise = foldl add givens sc_theta
1821 Just (clas,tys) = getClassPredTys_maybe pred
1822 (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
1823 sc_theta = substTheta (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
1826 = case lookupFM givens ct of
1827 Nothing -> -- Add it and its superclasses
1828 addSCs (addToFM givens ct False) ct
1830 Just True -> -- Set its flag to False; superclasses already done
1831 addToFM givens ct False
1833 Just False -> -- Already done
1839 %************************************************************************
1841 \section{Errors and contexts}
1843 %************************************************************************
1845 ToDo: for these error messages, should we note the location as coming
1846 from the insts, or just whatever seems to be around in the monad just
1850 groupInsts :: [Inst] -> [[Inst]]
1851 -- Group together insts with the same origin
1852 -- We want to report them together in error messages
1854 groupInsts (inst:insts) = (inst:friends) : groupInsts others
1856 -- (It may seem a bit crude to compare the error messages,
1857 -- but it makes sure that we combine just what the user sees,
1858 -- and it avoids need equality on InstLocs.)
1859 (friends, others) = partition is_friend insts
1860 loc_msg = showSDoc (pprInstLoc (instLoc inst))
1861 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
1864 addTopAmbigErrs dicts
1865 = mapNF_Tc (addTopInstanceErrs tidy_env) (groupInsts no_insts) `thenNF_Tc_`
1866 mapNF_Tc (addTopIPErrs tidy_env) (groupInsts bad_ips) `thenNF_Tc_`
1867 mapNF_Tc (addAmbigErr tidy_env) ambigs `thenNF_Tc_`
1870 fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet
1871 (tidy_env, tidy_dicts) = tidyInsts dicts
1872 (bad_ips, non_ips) = partition is_ip tidy_dicts
1873 (no_insts, ambigs) = partition no_inst non_ips
1874 is_ip d = any isIPPred (predsOfInst d)
1875 no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1878 plural xs = char 's'
1880 addTopIPErrs tidy_env tidy_dicts
1881 = addInstErrTcM (instLoc (head tidy_dicts))
1883 ptext SLIT("Unbound implicit parameter") <> plural tidy_dicts <+> pprInsts tidy_dicts)
1885 -- Used for top-level irreducibles
1886 addTopInstanceErrs tidy_env tidy_dicts
1887 = addInstErrTcM (instLoc (head tidy_dicts))
1889 ptext SLIT("No instance") <> plural tidy_dicts <+>
1890 ptext SLIT("for") <+> pprInsts tidy_dicts)
1893 = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts
1895 (tidy_env, tidy_dicts) = tidyInsts dicts
1897 addAmbigErr tidy_env tidy_dict
1898 = addInstErrTcM (instLoc tidy_dict)
1900 sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs,
1901 nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
1903 ambig_tvs = varSetElems (tyVarsOfInst tidy_dict)
1905 warnDefault dicts default_ty
1906 = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag ->
1907 tcAddSrcLoc (get_loc (head dicts)) (warnTc warn_flag warn_msg)
1910 (_, tidy_dicts) = tidyInsts dicts
1911 get_loc i = case instLoc i of { (_,loc,_) -> loc }
1912 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
1913 quotes (ppr default_ty),
1914 pprInstsInFull tidy_dicts]
1916 complainCheck doc givens irreds
1917 = mapNF_Tc zonkInst given_dicts_and_ips `thenNF_Tc` \ givens' ->
1918 mapNF_Tc (addNoInstanceErrs doc givens') (groupInsts irreds) `thenNF_Tc_`
1921 given_dicts_and_ips = filter (not . isMethod) givens
1922 -- Filter out methods, which are only added to
1923 -- the given set as an optimisation
1925 addNoInstanceErrs what_doc givens dicts
1926 = tcGetInstEnv `thenNF_Tc` \ inst_env ->
1928 (tidy_env1, tidy_givens) = tidyInsts givens
1929 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
1931 doc = vcat [sep [herald <+> pprInsts tidy_dicts,
1932 nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
1934 ptext SLIT("Probable fix:"),
1938 herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
1939 unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
1942 -- The error message when we don't find a suitable instance
1943 -- is complicated by the fact that sometimes this is because
1944 -- there is no instance, and sometimes it's because there are
1945 -- too many instances (overlap). See the comments in TcEnv.lhs
1946 -- with the InstEnv stuff.
1949 | not ambig_overlap = empty
1951 = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
1952 nest 4 (ptext SLIT("depends on the instantiation of") <+>
1953 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInsts tidy_dicts))))]
1955 fix1 = sep [ptext SLIT("Add") <+> pprInsts tidy_dicts,
1956 ptext SLIT("to the") <+> what_doc]
1958 fix2 | null instance_dicts
1961 = ptext SLIT("Or add an instance declaration for") <+> pprInsts instance_dicts
1963 instance_dicts = [d | d <- tidy_dicts, isClassDict d, not (isTyVarDict d)]
1964 -- Insts for which it is worth suggesting an adding an instance declaration
1965 -- Exclude implicit parameters, and tyvar dicts
1967 -- Checks for the ambiguous case when we have overlapping instances
1968 ambig_overlap = any ambig_overlap1 dicts
1971 = case lookupInstEnv inst_env clas tys of
1972 NoMatch ambig -> ambig
1976 (clas,tys) = getDictClassTys dict
1978 addInstErrTcM (instLoc (head dicts)) (tidy_env2, doc)
1980 -- Used for the ...Thetas variants; all top level
1982 = addErrTc (ptext SLIT("No instance for") <+> quotes (ppr pred))
1984 reduceDepthErr n stack
1985 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
1986 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
1987 nest 4 (pprInstsInFull stack)]
1989 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)
1991 -----------------------------------------------
1993 = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
1994 nest 4 (ppr inst <+> pprInstLoc (instLoc inst))])