2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcSimplify]{TcSimplify}
10 tcSimplifyInfer, tcSimplifyInferCheck,
11 tcSimplifyCheck, tcSimplifyRestricted,
12 tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
15 tcSimplifyDeriv, tcSimplifyDefault,
19 #include "HsVersions.h"
21 import {-# SOURCE #-} TcUnify( unifyTauTy )
23 import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
24 import TcHsSyn ( TcExpr, TcId,
25 TcMonoBinds, TcDictBinds
29 import Inst ( lookupInst, LookupInstResult(..),
30 tyVarsOfInst, fdPredsOfInsts, fdPredsOfInst, newDicts,
31 isDict, isClassDict, isLinearInst, linearInstType,
32 isStdClassTyVarDict, isMethodFor, isMethod,
33 instToId, tyVarsOfInsts, cloneDict,
34 ipNamesOfInsts, ipNamesOfInst, dictPred,
35 instBindingRequired, instCanBeGeneralised,
36 newDictsFromOld, tcInstClassOp,
37 getDictClassTys, isTyVarDict,
38 instLoc, zonkInst, tidyInsts, tidyMoreInsts,
39 Inst, pprInsts, pprInstsInFull,
40 isIPDict, isInheritableInst
42 import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv, tcLookupId, findGlobals )
43 import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
44 import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
45 import TcType ( TcTyVar, TcTyVarSet, ThetaType, TyVarDetails(VanillaTv),
46 mkClassPred, isOverloadedTy, mkTyConApp,
47 mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
49 import Id ( idType, mkUserLocal )
51 import Name ( getOccName, getSrcLoc )
52 import NameSet ( NameSet, mkNameSet, elemNameSet )
53 import Class ( classBigSig )
54 import FunDeps ( oclose, grow, improve, pprEquationDoc )
55 import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
56 import PrelNames ( splitName, fstName, sndName )
58 import Subst ( mkTopTyVarSubst, substTheta, substTy )
59 import TysWiredIn ( unitTy, pairTyCon )
60 import ErrUtils ( Message )
62 import VarEnv ( TidyEnv )
65 import ListSetOps ( equivClasses )
66 import Util ( zipEqual, isSingleton )
67 import List ( partition )
72 %************************************************************************
76 %************************************************************************
78 --------------------------------------
79 Notes on quantification
80 --------------------------------------
82 Suppose we are about to do a generalisation step.
87 C the constraints from that RHS
89 The game is to figure out
91 Q the set of type variables over which to quantify
92 Ct the constraints we will *not* quantify over
93 Cq the constraints we will quantify over
95 So we're going to infer the type
99 and float the constraints Ct further outwards.
101 Here are the things that *must* be true:
103 (A) Q intersect fv(G) = EMPTY limits how big Q can be
104 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
106 (A) says we can't quantify over a variable that's free in the
107 environment. (B) says we must quantify over all the truly free
108 variables in T, else we won't get a sufficiently general type. We do
109 not *need* to quantify over any variable that is fixed by the free
110 vars of the environment G.
112 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
114 Example: class H x y | x->y where ...
116 fv(G) = {a} C = {H a b, H c d}
119 (A) Q intersect {a} is empty
120 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
122 So Q can be {c,d}, {b,c,d}
124 Other things being equal, however, we'd like to quantify over as few
125 variables as possible: smaller types, fewer type applications, more
126 constraints can get into Ct instead of Cq.
129 -----------------------------------------
132 fv(T) the free type vars of T
134 oclose(vs,C) The result of extending the set of tyvars vs
135 using the functional dependencies from C
137 grow(vs,C) The result of extend the set of tyvars vs
138 using all conceivable links from C.
140 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
141 Then grow(vs,C) = {a,b,c}
143 Note that grow(vs,C) `superset` grow(vs,simplify(C))
144 That is, simplfication can only shrink the result of grow.
147 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
148 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
151 -----------------------------------------
155 Here's a good way to choose Q:
157 Q = grow( fv(T), C ) \ oclose( fv(G), C )
159 That is, quantify over all variable that that MIGHT be fixed by the
160 call site (which influences T), but which aren't DEFINITELY fixed by
161 G. This choice definitely quantifies over enough type variables,
162 albeit perhaps too many.
164 Why grow( fv(T), C ) rather than fv(T)? Consider
166 class H x y | x->y where ...
171 If we used fv(T) = {c} we'd get the type
173 forall c. H c d => c -> b
175 And then if the fn was called at several different c's, each of
176 which fixed d differently, we'd get a unification error, because
177 d isn't quantified. Solution: quantify d. So we must quantify
178 everything that might be influenced by c.
180 Why not oclose( fv(T), C )? Because we might not be able to see
181 all the functional dependencies yet:
183 class H x y | x->y where ...
184 instance H x y => Eq (T x y) where ...
189 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
190 apparent yet, and that's wrong. We must really quantify over d too.
193 There really isn't any point in quantifying over any more than
194 grow( fv(T), C ), because the call sites can't possibly influence
195 any other type variables.
199 --------------------------------------
201 --------------------------------------
203 It's very hard to be certain when a type is ambiguous. Consider
207 instance H x y => K (x,y)
209 Is this type ambiguous?
210 forall a b. (K (a,b), Eq b) => a -> a
212 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
213 now we see that a fixes b. So we can't tell about ambiguity for sure
214 without doing a full simplification. And even that isn't possible if
215 the context has some free vars that may get unified. Urgle!
217 Here's another example: is this ambiguous?
218 forall a b. Eq (T b) => a -> a
219 Not if there's an insance decl (with no context)
220 instance Eq (T b) where ...
222 You may say of this example that we should use the instance decl right
223 away, but you can't always do that:
225 class J a b where ...
226 instance J Int b where ...
228 f :: forall a b. J a b => a -> a
230 (Notice: no functional dependency in J's class decl.)
231 Here f's type is perfectly fine, provided f is only called at Int.
232 It's premature to complain when meeting f's signature, or even
233 when inferring a type for f.
237 However, we don't *need* to report ambiguity right away. It'll always
238 show up at the call site.... and eventually at main, which needs special
239 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
241 So here's the plan. We WARN about probable ambiguity if
243 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
245 (all tested before quantification).
246 That is, all the type variables in Cq must be fixed by the the variables
247 in the environment, or by the variables in the type.
249 Notice that we union before calling oclose. Here's an example:
251 class J a b c | a b -> c
255 forall b c. (J a b c) => b -> b
257 Only if we union {a} from G with {b} from T before using oclose,
258 do we see that c is fixed.
260 It's a bit vague exactly which C we should use for this oclose call. If we
261 don't fix enough variables we might complain when we shouldn't (see
262 the above nasty example). Nothing will be perfect. That's why we can
263 only issue a warning.
266 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
268 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
270 then c is a "bubble"; there's no way it can ever improve, and it's
271 certainly ambiguous. UNLESS it is a constant (sigh). And what about
276 instance H x y => K (x,y)
278 Is this type ambiguous?
279 forall a b. (K (a,b), Eq b) => a -> a
281 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
282 is a "bubble" that's a set of constraints
284 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
286 Hence another idea. To decide Q start with fv(T) and grow it
287 by transitive closure in Cq (no functional dependencies involved).
288 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
289 The definitely-ambiguous can then float out, and get smashed at top level
290 (which squashes out the constants, like Eq (T a) above)
293 --------------------------------------
294 Notes on principal types
295 --------------------------------------
300 f x = let g y = op (y::Int) in True
302 Here the principal type of f is (forall a. a->a)
303 but we'll produce the non-principal type
304 f :: forall a. C Int => a -> a
307 --------------------------------------
308 Notes on implicit parameters
309 --------------------------------------
311 Question 1: can we "inherit" implicit parameters
312 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
317 where f is *not* a top-level binding.
318 From the RHS of f we'll get the constraint (?y::Int).
319 There are two types we might infer for f:
323 (so we get ?y from the context of f's definition), or
325 f :: (?y::Int) => Int -> Int
327 At first you might think the first was better, becuase then
328 ?y behaves like a free variable of the definition, rather than
329 having to be passed at each call site. But of course, the WHOLE
330 IDEA is that ?y should be passed at each call site (that's what
331 dynamic binding means) so we'd better infer the second.
333 BOTTOM LINE: when *inferring types* you *must* quantify
334 over implicit parameters. See the predicate isFreeWhenInferring.
337 Question 2: type signatures
338 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
339 BUT WATCH OUT: When you supply a type signature, we can't force you
340 to quantify over implicit parameters. For example:
344 This is perfectly reasonable. We do not want to insist on
346 (?x + 1) :: (?x::Int => Int)
348 That would be silly. Here, the definition site *is* the occurrence site,
349 so the above strictures don't apply. Hence the difference between
350 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
351 and tcSimplifyCheckBind (which does not).
353 What about when you supply a type signature for a binding?
354 Is it legal to give the following explicit, user type
355 signature to f, thus:
360 At first sight this seems reasonable, but it has the nasty property
361 that adding a type signature changes the dynamic semantics.
364 (let f x = (x::Int) + ?y
365 in (f 3, f 3 with ?y=5)) with ?y = 6
371 in (f 3, f 3 with ?y=5)) with ?y = 6
375 Indeed, simply inlining f (at the Haskell source level) would change the
378 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
379 semantics for a Haskell program without knowing its typing, so if you
380 change the typing you may change the semantics.
382 To make things consistent in all cases where we are *checking* against
383 a supplied signature (as opposed to inferring a type), we adopt the
386 a signature does not need to quantify over implicit params.
388 [This represents a (rather marginal) change of policy since GHC 5.02,
389 which *required* an explicit signature to quantify over all implicit
390 params for the reasons mentioned above.]
392 But that raises a new question. Consider
394 Given (signature) ?x::Int
395 Wanted (inferred) ?x::Int, ?y::Bool
397 Clearly we want to discharge the ?x and float the ?y out. But
398 what is the criterion that distinguishes them? Clearly it isn't
399 what free type variables they have. The Right Thing seems to be
400 to float a constraint that
401 neither mentions any of the quantified type variables
402 nor any of the quantified implicit parameters
404 See the predicate isFreeWhenChecking.
407 Question 3: monomorphism
408 ~~~~~~~~~~~~~~~~~~~~~~~~
409 There's a nasty corner case when the monomorphism restriction bites:
413 The argument above suggests that we *must* generalise
414 over the ?y parameter, to get
415 z :: (?y::Int) => Int,
416 but the monomorphism restriction says that we *must not*, giving
418 Why does the momomorphism restriction say this? Because if you have
420 let z = x + ?y in z+z
422 you might not expect the addition to be done twice --- but it will if
423 we follow the argument of Question 2 and generalise over ?y.
429 (A) Always generalise over implicit parameters
430 Bindings that fall under the monomorphism restriction can't
434 * Inlining remains valid
435 * No unexpected loss of sharing
436 * But simple bindings like
438 will be rejected, unless you add an explicit type signature
439 (to avoid the monomorphism restriction)
440 z :: (?y::Int) => Int
442 This seems unacceptable
444 (B) Monomorphism restriction "wins"
445 Bindings that fall under the monomorphism restriction can't
447 Always generalise over implicit parameters *except* for bindings
448 that fall under the monomorphism restriction
451 * Inlining isn't valid in general
452 * No unexpected loss of sharing
453 * Simple bindings like
455 accepted (get value of ?y from binding site)
457 (C) Always generalise over implicit parameters
458 Bindings that fall under the monomorphism restriction can't
459 be generalised, EXCEPT for implicit parameters
461 * Inlining remains valid
462 * Unexpected loss of sharing (from the extra generalisation)
463 * Simple bindings like
465 accepted (get value of ?y from occurrence sites)
470 None of these choices seems very satisfactory. But at least we should
471 decide which we want to do.
473 It's really not clear what is the Right Thing To Do. If you see
477 would you expect the value of ?y to be got from the *occurrence sites*
478 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
479 case of function definitions, the answer is clearly the former, but
480 less so in the case of non-fucntion definitions. On the other hand,
481 if we say that we get the value of ?y from the definition site of 'z',
482 then inlining 'z' might change the semantics of the program.
484 Choice (C) really says "the monomorphism restriction doesn't apply
485 to implicit parameters". Which is fine, but remember that every
486 innocent binding 'x = ...' that mentions an implicit parameter in
487 the RHS becomes a *function* of that parameter, called at each
488 use of 'x'. Now, the chances are that there are no intervening 'with'
489 clauses that bind ?y, so a decent compiler should common up all
490 those function calls. So I think I strongly favour (C). Indeed,
491 one could make a similar argument for abolishing the monomorphism
492 restriction altogether.
494 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
498 %************************************************************************
500 \subsection{tcSimplifyInfer}
502 %************************************************************************
504 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
506 1. Compute Q = grow( fvs(T), C )
508 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
509 predicates will end up in Ct; we deal with them at the top level
511 3. Try improvement, using functional dependencies
513 4. If Step 3 did any unification, repeat from step 1
514 (Unification can change the result of 'grow'.)
516 Note: we don't reduce dictionaries in step 2. For example, if we have
517 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
518 after step 2. However note that we may therefore quantify over more
519 type variables than we absolutely have to.
521 For the guts, we need a loop, that alternates context reduction and
522 improvement with unification. E.g. Suppose we have
524 class C x y | x->y where ...
526 and tcSimplify is called with:
528 Then improvement unifies a with b, giving
531 If we need to unify anything, we rattle round the whole thing all over
538 -> TcTyVarSet -- fv(T); type vars
540 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
541 TcDictBinds, -- Bindings
542 [TcId]) -- Dict Ids that must be bound here (zonked)
543 -- Any free (escaping) Insts are tossed into the environment
548 tcSimplifyInfer doc tau_tvs wanted_lie
549 = inferLoop doc (varSetElems tau_tvs)
550 wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
552 -- Check for non-generalisable insts
553 mappM_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenM_`
555 extendLIEs frees `thenM_`
556 returnM (qtvs, binds, map instToId irreds)
558 inferLoop doc tau_tvs wanteds
560 zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' ->
561 mappM zonkInst wanteds `thenM` \ wanteds' ->
562 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
564 preds = fdPredsOfInsts wanteds'
565 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
568 | isFreeWhenInferring qtvs inst = Free
569 | isClassDict inst = DontReduceUnlessConstant -- Dicts
570 | otherwise = ReduceMe -- Lits and Methods
573 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
576 if no_improvement then
577 returnM (varSetElems qtvs, frees, binds, irreds)
579 -- If improvement did some unification, we go round again. There
580 -- are two subtleties:
581 -- a) We start again with irreds, not wanteds
582 -- Using an instance decl might have introduced a fresh type variable
583 -- which might have been unified, so we'd get an infinite loop
584 -- if we started again with wanteds! See example [LOOP]
586 -- b) It's also essential to re-process frees, because unification
587 -- might mean that a type variable that looked free isn't now.
589 -- Hence the (irreds ++ frees)
591 -- However, NOTICE that when we are done, we might have some bindings, but
592 -- the final qtvs might be empty. See [NO TYVARS] below.
594 inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) ->
595 returnM (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
600 class If b t e r | b t e -> r
603 class Lte a b c | a b -> c where lte :: a -> b -> c
605 instance (Lte a b l,If l b a c) => Max a b c
607 Wanted: Max Z (S x) y
609 Then we'll reduce using the Max instance to:
610 (Lte Z (S x) l, If l (S x) Z y)
611 and improve by binding l->T, after which we can do some reduction
612 on both the Lte and If constraints. What we *can't* do is start again
613 with (Max Z (S x) y)!
617 class Y a b | a -> b where
620 instance Y [[a]] a where
623 k :: X a -> X a -> X a
625 g :: Num a => [X a] -> [X a]
628 h ys = ys ++ map (k (y [[0]])) xs
630 The excitement comes when simplifying the bindings for h. Initially
631 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
632 From this we get t1:=:t2, but also various bindings. We can't forget
633 the bindings (because of [LOOP]), but in fact t1 is what g is
636 The net effect of [NO TYVARS]
639 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
640 isFreeWhenInferring qtvs inst
641 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
642 && isInheritableInst inst -- And no implicit parameter involved
643 -- (see "Notes on implicit parameters")
645 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
646 -> NameSet -- Quantified implicit parameters
648 isFreeWhenChecking qtvs ips inst
649 = isFreeWrtTyVars qtvs inst
650 && isFreeWrtIPs ips inst
652 isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
653 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
657 %************************************************************************
659 \subsection{tcSimplifyCheck}
661 %************************************************************************
663 @tcSimplifyCheck@ is used when we know exactly the set of variables
664 we are going to quantify over. For example, a class or instance declaration.
669 -> [TcTyVar] -- Quantify over these
672 -> TcM TcDictBinds -- Bindings
674 -- tcSimplifyCheck is used when checking expression type signatures,
675 -- class decls, instance decls etc.
677 -- NB: tcSimplifyCheck does not consult the
678 -- global type variables in the environment; so you don't
679 -- need to worry about setting them before calling tcSimplifyCheck
680 tcSimplifyCheck doc qtvs givens wanted_lie
681 = tcSimplCheck doc get_qtvs
682 givens wanted_lie `thenM` \ (qtvs', binds) ->
685 get_qtvs = zonkTcTyVarsAndFV qtvs
688 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
689 -- against, but we don't know the type variables over which we are going to quantify.
690 -- This happens when we have a type signature for a mutually recursive group
693 -> TcTyVarSet -- fv(T)
696 -> TcM ([TcTyVar], -- Variables over which to quantify
697 TcDictBinds) -- Bindings
699 tcSimplifyInferCheck doc tau_tvs givens wanted_lie
700 = tcSimplCheck doc get_qtvs givens wanted_lie
702 -- Figure out which type variables to quantify over
703 -- You might think it should just be the signature tyvars,
704 -- but in bizarre cases you can get extra ones
705 -- f :: forall a. Num a => a -> a
706 -- f x = fst (g (x, head [])) + 1
708 -- Here we infer g :: forall a b. a -> b -> (b,a)
709 -- We don't want g to be monomorphic in b just because
710 -- f isn't quantified over b.
711 all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
713 get_qtvs = zonkTcTyVarsAndFV all_tvs `thenM` \ all_tvs' ->
714 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
716 qtvs = all_tvs' `minusVarSet` gbl_tvs
717 -- We could close gbl_tvs, but its not necessary for
718 -- soundness, and it'll only affect which tyvars, not which
719 -- dictionaries, we quantify over
724 Here is the workhorse function for all three wrappers.
727 tcSimplCheck doc get_qtvs givens wanted_lie
728 = check_loop givens wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
730 -- Complain about any irreducible ones
731 complainCheck doc givens irreds `thenM_`
734 extendLIEs frees `thenM_`
735 returnM (qtvs, binds)
738 ip_set = mkNameSet (ipNamesOfInsts givens)
740 check_loop givens wanteds
742 mappM zonkInst givens `thenM` \ givens' ->
743 mappM zonkInst wanteds `thenM` \ wanteds' ->
744 get_qtvs `thenM` \ qtvs' ->
748 -- When checking against a given signature we always reduce
749 -- until we find a match against something given, or can't reduce
750 try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
751 | otherwise = ReduceMe
753 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
756 if no_improvement then
757 returnM (varSetElems qtvs', frees, binds, irreds)
759 check_loop givens' (irreds ++ frees) `thenM` \ (qtvs', frees1, binds1, irreds1) ->
760 returnM (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1)
764 %************************************************************************
766 \subsection{tcSimplifyRestricted}
768 %************************************************************************
771 tcSimplifyRestricted -- Used for restricted binding groups
772 -- i.e. ones subject to the monomorphism restriction
774 -> TcTyVarSet -- Free in the type of the RHSs
775 -> [Inst] -- Free in the RHSs
776 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
777 TcDictBinds) -- Bindings
779 tcSimplifyRestricted doc tau_tvs wanteds
780 = -- First squash out all methods, to find the constrained tyvars
781 -- We can't just take the free vars of wanted_lie because that'll
782 -- have methods that may incidentally mention entirely unconstrained variables
783 -- e.g. a call to f :: Eq a => a -> b -> b
784 -- Here, b is unconstrained. A good example would be
786 -- We want to infer the polymorphic type
787 -- foo :: forall b. b -> b
789 try_me inst = ReduceMe -- Reduce as far as we can. Don't stop at
790 -- dicts; the idea is to get rid of as many type
791 -- variables as possible, and we don't want to stop
792 -- at (say) Monad (ST s), because that reduces
793 -- immediately, with no constraint on s.
795 simpleReduceLoop doc try_me wanteds `thenM` \ (_, _, constrained_dicts) ->
797 -- Next, figure out the tyvars we will quantify over
798 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
799 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
801 constrained_tvs = tyVarsOfInsts constrained_dicts
802 qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs)
803 `minusVarSet` constrained_tvs
806 -- The first step may have squashed more methods than
807 -- necessary, so try again, this time knowing the exact
808 -- set of type variables to quantify over.
810 -- We quantify only over constraints that are captured by qtvs;
811 -- these will just be a subset of non-dicts. This in contrast
812 -- to normal inference (using isFreeWhenInferring) in which we quantify over
813 -- all *non-inheritable* constraints too. This implements choice
814 -- (B) under "implicit parameter and monomorphism" above.
816 -- Remember that we may need to do *some* simplification, to
817 -- (for example) squash {Monad (ST s)} into {}. It's not enough
818 -- just to float all constraints
819 mappM zonkInst wanteds `thenM` \ wanteds' ->
821 try_me inst | isFreeWrtTyVars qtvs inst = Free
822 | otherwise = ReduceMe
824 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
825 ASSERT( no_improvement )
826 ASSERT( null irreds )
827 -- No need to loop because simpleReduceLoop will have
828 -- already done any improvement necessary
830 extendLIEs frees `thenM_`
831 returnM (varSetElems qtvs, binds)
835 %************************************************************************
837 \subsection{tcSimplifyToDicts}
839 %************************************************************************
841 On the LHS of transformation rules we only simplify methods and constants,
842 getting dictionaries. We want to keep all of them unsimplified, to serve
843 as the available stuff for the RHS of the rule.
845 The same thing is used for specialise pragmas. Consider
848 {-# SPECIALISE f :: Int -> Int #-}
851 The type checker generates a binding like:
853 f_spec = (f :: Int -> Int)
855 and we want to end up with
857 f_spec = _inline_me_ (f Int dNumInt)
859 But that means that we must simplify the Method for f to (f Int dNumInt)!
860 So tcSimplifyToDicts squeezes out all Methods.
862 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
864 fromIntegral :: (Integral a, Num b) => a -> b
865 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
867 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
871 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
873 because the scsel will mess up matching. Instead we want
875 forall dIntegralInt, dNumInt.
876 fromIntegral Int Int dIntegralInt dNumInt = id Int
878 Hence "DontReduce NoSCs"
881 tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds)
882 tcSimplifyToDicts wanteds
883 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
884 -- Since try_me doesn't look at types, we don't need to
885 -- do any zonking, so it's safe to call reduceContext directly
887 extendLIEs irreds `thenM_`
891 doc = text "tcSimplifyToDicts"
893 -- Reduce methods and lits only; stop as soon as we get a dictionary
894 try_me inst | isDict inst = DontReduce NoSCs
895 | otherwise = ReduceMe
900 tcSimplifyBracket is used when simplifying the constraints arising from
901 a Template Haskell bracket [| ... |]. We want to check that there aren't
902 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
903 Show instance), but we aren't otherwise interested in the results.
904 Nor do we care about ambiguous dictionaries etc. We will type check
905 this bracket again at its usage site.
908 tcSimplifyBracket :: [Inst] -> TcM ()
909 tcSimplifyBracket wanteds
910 = simpleReduceLoop doc try_me wanteds `thenM_`
914 doc = text "tcSimplifyBracket"
915 try_me inst = ReduceMe
919 %************************************************************************
921 \subsection{Filtering at a dynamic binding}
923 %************************************************************************
928 we must discharge all the ?x constraints from B. We also do an improvement
929 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
931 Actually, the constraints from B might improve the types in ?x. For example
933 f :: (?x::Int) => Char -> Char
936 then the constraint (?x::Int) arising from the call to f will
937 force the binding for ?x to be of type Int.
940 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
943 tcSimplifyIPs given_ips wanteds
944 = simpl_loop given_ips wanteds `thenM` \ (frees, binds) ->
945 extendLIEs frees `thenM_`
948 doc = text "tcSimplifyIPs" <+> ppr given_ips
949 ip_set = mkNameSet (ipNamesOfInsts given_ips)
951 -- Simplify any methods that mention the implicit parameter
952 try_me inst | isFreeWrtIPs ip_set inst = Free
953 | otherwise = ReduceMe
955 simpl_loop givens wanteds
956 = mappM zonkInst givens `thenM` \ givens' ->
957 mappM zonkInst wanteds `thenM` \ wanteds' ->
959 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
961 if no_improvement then
962 ASSERT( null irreds )
963 returnM (frees, binds)
965 simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) ->
966 returnM (frees1, binds `AndMonoBinds` binds1)
970 %************************************************************************
972 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
974 %************************************************************************
976 When doing a binding group, we may have @Insts@ of local functions.
977 For example, we might have...
979 let f x = x + 1 -- orig local function (overloaded)
980 f.1 = f Int -- two instances of f
985 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
986 where @f@ is in scope; those @Insts@ must certainly not be passed
987 upwards towards the top-level. If the @Insts@ were binding-ified up
988 there, they would have unresolvable references to @f@.
990 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
991 For each method @Inst@ in the @init_lie@ that mentions one of the
992 @Ids@, we create a binding. We return the remaining @Insts@ (in an
993 @LIE@), as well as the @HsBinds@ generated.
996 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcMonoBinds
998 bindInstsOfLocalFuns wanteds local_ids
999 | null overloaded_ids
1001 = extendLIEs wanteds `thenM_`
1002 returnM EmptyMonoBinds
1005 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
1006 ASSERT( null irreds )
1007 extendLIEs frees `thenM_`
1010 doc = text "bindInsts" <+> ppr local_ids
1011 overloaded_ids = filter is_overloaded local_ids
1012 is_overloaded id = isOverloadedTy (idType id)
1014 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1015 -- so it's worth building a set, so that
1016 -- lookup (in isMethodFor) is faster
1018 try_me inst | isMethodFor overloaded_set inst = ReduceMe
1023 %************************************************************************
1025 \subsection{Data types for the reduction mechanism}
1027 %************************************************************************
1029 The main control over context reduction is here
1033 = ReduceMe -- Try to reduce this
1034 -- If there's no instance, behave exactly like
1035 -- DontReduce: add the inst to
1036 -- the irreductible ones, but don't
1037 -- produce an error message of any kind.
1038 -- It might be quite legitimate such as (Eq a)!
1040 | DontReduce WantSCs -- Return as irreducible
1042 | DontReduceUnlessConstant -- Return as irreducible unless it can
1043 -- be reduced to a constant in one step
1045 | Free -- Return as free
1047 reduceMe :: Inst -> WhatToDo
1048 reduceMe inst = ReduceMe
1050 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1051 -- of a predicate when adding it to the avails
1057 type Avails = FiniteMap Inst Avail
1060 = IsFree -- Used for free Insts
1061 | Irred -- Used for irreducible dictionaries,
1062 -- which are going to be lambda bound
1064 | Given TcId -- Used for dictionaries for which we have a binding
1065 -- e.g. those "given" in a signature
1066 Bool -- True <=> actually consumed (splittable IPs only)
1068 | NoRhs -- Used for Insts like (CCallable f)
1069 -- where no witness is required.
1071 | Rhs -- Used when there is a RHS
1073 [Inst] -- Insts free in the RHS; we need these too
1075 | Linear -- Splittable Insts only.
1076 Int -- The Int is always 2 or more; indicates how
1077 -- many copies are required
1078 Inst -- The splitter
1079 Avail -- Where the "master copy" is
1081 | LinRhss -- Splittable Insts only; this is used only internally
1082 -- by extractResults, where a Linear
1083 -- is turned into an LinRhss
1084 [TcExpr] -- A supply of suitable RHSs
1086 pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1087 | (inst,avail) <- fmToList avails ]
1089 instance Outputable Avail where
1092 pprAvail NoRhs = text "<no rhs>"
1093 pprAvail IsFree = text "Free"
1094 pprAvail Irred = text "Irred"
1095 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1096 if b then text "(used)" else empty
1097 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1098 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1099 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1102 Extracting the bindings from a bunch of Avails.
1103 The bindings do *not* come back sorted in dependency order.
1104 We assume that they'll be wrapped in a big Rec, so that the
1105 dependency analyser can sort them out later
1109 extractResults :: Avails
1111 -> TcM (TcDictBinds, -- Bindings
1112 [Inst], -- Irreducible ones
1113 [Inst]) -- Free ones
1115 extractResults avails wanteds
1116 = go avails EmptyMonoBinds [] [] wanteds
1118 go avails binds irreds frees []
1119 = returnM (binds, irreds, frees)
1121 go avails binds irreds frees (w:ws)
1122 = case lookupFM avails w of
1123 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1124 go avails binds irreds frees ws
1126 Just NoRhs -> go avails binds irreds frees ws
1127 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1128 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1130 Just (Given id _) -> go avails new_binds irreds frees ws
1132 new_binds | id == instToId w = binds
1133 | otherwise = addBind binds w (HsVar id)
1134 -- The sought Id can be one of the givens, via a superclass chain
1135 -- and then we definitely don't want to generate an x=x binding!
1137 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1139 new_binds = addBind binds w rhs
1141 Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
1142 -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) ->
1143 split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) ->
1144 go (addToFM avails w (LinRhss rhss))
1145 (binds `AndMonoBinds` binds')
1146 irreds' frees' (split_inst : w : ws)
1148 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1149 -> go new_avails new_binds irreds frees ws
1151 new_binds = addBind binds w rhs
1152 new_avails = addToFM avails w (LinRhss rhss)
1154 get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
1155 get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
1156 returnM (w':irreds, frees, instToId w')
1157 get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
1158 returnM (irreds, w':frees, instToId w')
1161 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1162 | otherwise = addToFM avails w NoRhs
1163 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1164 -- than Given, else we end up with bogus bindings.
1166 add_free avails w | isMethod w = avails
1167 | otherwise = add_given avails w
1169 -- Do *not* replace Free by Given if it's a method.
1170 -- The following situation shows why this is bad:
1171 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1172 -- From an application (truncate f i) we get
1173 -- t1 = truncate at f
1175 -- If we have also have a second occurrence of truncate, we get
1176 -- t3 = truncate at f
1178 -- When simplifying with i,f free, we might still notice that
1179 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1180 -- will continue to float out!
1181 -- (split n i a) returns: n rhss
1182 -- auxiliary bindings
1183 -- 1 or 0 insts to add to irreds
1186 split :: Int -> TcId -> TcId -> Inst
1187 -> TcM (TcDictBinds, [TcExpr])
1188 -- (split n split_id root_id wanted) returns
1189 -- * a list of 'n' expressions, all of which witness 'avail'
1190 -- * a bunch of auxiliary bindings to support these expressions
1191 -- * one or zero insts needed to witness the whole lot
1192 -- (maybe be zero if the initial Inst is a Given)
1194 -- NB: 'wanted' is just a template
1196 split n split_id root_id wanted
1199 ty = linearInstType wanted
1200 pair_ty = mkTyConApp pairTyCon [ty,ty]
1201 id = instToId wanted
1205 go 1 = returnM (EmptyMonoBinds, [HsVar root_id])
1207 go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
1208 expand n rhss `thenM` \ (binds2, rhss') ->
1209 returnM (binds1 `AndMonoBinds` binds2, rhss')
1212 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1213 -- e.g. expand 3 [rhs1, rhs2]
1214 -- = ( { x = split rhs1 },
1215 -- [fst x, snd x, rhs2] )
1217 | n `rem` 2 == 0 = go rhss -- n is even
1218 | otherwise = go (tail rhss) `thenM` \ (binds', rhss') ->
1219 returnM (binds', head rhss : rhss')
1221 go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') ->
1222 returnM (andMonoBindList binds', concat rhss')
1224 do_one rhs = newUnique `thenM` \ uniq ->
1225 tcLookupId fstName `thenM` \ fst_id ->
1226 tcLookupId sndName `thenM` \ snd_id ->
1228 x = mkUserLocal occ uniq pair_ty loc
1230 returnM (VarMonoBind x (mk_app split_id rhs),
1231 [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])
1233 mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var
1235 mk_app id rhs = HsApp (HsVar id) rhs
1237 addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
1241 %************************************************************************
1243 \subsection[reduce]{@reduce@}
1245 %************************************************************************
1247 When the "what to do" predicate doesn't depend on the quantified type variables,
1248 matters are easier. We don't need to do any zonking, unless the improvement step
1249 does something, in which case we zonk before iterating.
1251 The "given" set is always empty.
1254 simpleReduceLoop :: SDoc
1255 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1257 -> TcM ([Inst], -- Free
1259 [Inst]) -- Irreducible
1261 simpleReduceLoop doc try_me wanteds
1262 = mappM zonkInst wanteds `thenM` \ wanteds' ->
1263 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1264 if no_improvement then
1265 returnM (frees, binds, irreds)
1267 simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
1268 returnM (frees1, binds `AndMonoBinds` binds1, irreds1)
1274 reduceContext :: SDoc
1275 -> (Inst -> WhatToDo)
1278 -> TcM (Bool, -- True <=> improve step did no unification
1280 TcDictBinds, -- Dictionary bindings
1281 [Inst]) -- Irreducible
1283 reduceContext doc try_me givens wanteds
1285 traceTc (text "reduceContext" <+> (vcat [
1286 text "----------------------",
1288 text "given" <+> ppr givens,
1289 text "wanted" <+> ppr wanteds,
1290 text "----------------------"
1293 -- Build the Avail mapping from "givens"
1294 foldlM addGiven emptyFM givens `thenM` \ init_state ->
1297 reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
1299 -- Do improvement, using everything in avails
1300 -- In particular, avails includes all superclasses of everything
1301 tcImprove avails `thenM` \ no_improvement ->
1303 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1305 traceTc (text "reduceContext end" <+> (vcat [
1306 text "----------------------",
1308 text "given" <+> ppr givens,
1309 text "wanted" <+> ppr wanteds,
1311 text "avails" <+> pprAvails avails,
1312 text "frees" <+> ppr frees,
1313 text "no_improvement =" <+> ppr no_improvement,
1314 text "----------------------"
1317 returnM (no_improvement, frees, binds, irreds)
1320 = tcGetInstEnv `thenM` \ inst_env ->
1322 preds = [ (pred, pp_loc)
1323 | inst <- keysFM avails,
1324 let pp_loc = pprInstLoc (instLoc inst),
1325 pred <- fdPredsOfInst inst
1327 -- Avails has all the superclasses etc (good)
1328 -- It also has all the intermediates of the deduction (good)
1329 -- It does not have duplicates (good)
1330 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1331 -- so that improve will see them separate
1332 eqns = improve (classInstEnv inst_env) preds
1337 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
1338 mappM_ unify eqns `thenM_`
1341 unify ((qtvs, t1, t2), doc)
1343 tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1344 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1347 The main context-reduction function is @reduce@. Here's its game plan.
1350 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1351 -- along with its depth
1352 -> (Inst -> WhatToDo)
1359 try_me: given an inst, this function returns
1361 DontReduce return this in "irreds"
1362 Free return this in "frees"
1364 wanteds: The list of insts to reduce
1365 state: An accumulating parameter of type Avails
1366 that contains the state of the algorithm
1368 It returns a Avails.
1370 The (n,stack) pair is just used for error reporting.
1371 n is always the depth of the stack.
1372 The stack is the stack of Insts being reduced: to produce X
1373 I had to produce Y, to produce Y I had to produce Z, and so on.
1376 reduceList (n,stack) try_me wanteds state
1377 | n > opt_MaxContextReductionDepth
1378 = failWithTc (reduceDepthErr n stack)
1384 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1389 go [] state = returnM state
1390 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
1393 -- Base case: we're done!
1394 reduce stack try_me wanted state
1395 -- It's the same as an existing inst, or a superclass thereof
1396 | Just avail <- isAvailable state wanted
1397 = if isLinearInst wanted then
1398 addLinearAvailable state avail wanted `thenM` \ (state', wanteds') ->
1399 reduceList stack try_me wanteds' state'
1401 returnM state -- No op for non-linear things
1404 = case try_me wanted of {
1406 DontReduce want_scs -> addIrred want_scs state wanted
1408 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1409 -- First, see if the inst can be reduced to a constant in one step
1410 try_simple (addIrred AddSCs) -- Assume want superclasses
1412 ; Free -> -- It's free so just chuck it upstairs
1413 -- First, see if the inst can be reduced to a constant in one step
1416 ; ReduceMe -> -- It should be reduced
1417 lookupInst wanted `thenM` \ lookup_result ->
1418 case lookup_result of
1419 GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenM` \ state' ->
1420 addWanted state' wanted rhs wanteds'
1421 SimpleInst rhs -> addWanted state wanted rhs []
1423 NoInstance -> -- No such instance!
1424 -- Add it and its superclasses
1425 addIrred AddSCs state wanted
1429 try_simple do_this_otherwise
1430 = lookupInst wanted `thenM` \ lookup_result ->
1431 case lookup_result of
1432 SimpleInst rhs -> addWanted state wanted rhs []
1433 other -> do_this_otherwise state wanted
1438 -------------------------
1439 isAvailable :: Avails -> Inst -> Maybe Avail
1440 isAvailable avails wanted = lookupFM avails wanted
1441 -- NB 1: the Ord instance of Inst compares by the class/type info
1442 -- *not* by unique. So
1443 -- d1::C Int == d2::C Int
1445 addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
1446 addLinearAvailable avails avail wanted
1447 -- avails currently maps [wanted -> avail]
1448 -- Extend avails to reflect a neeed for an extra copy of avail
1450 | Just avail' <- split_avail avail
1451 = returnM (addToFM avails wanted avail', [])
1454 = tcLookupId splitName `thenM` \ split_id ->
1455 tcInstClassOp (instLoc wanted) split_id
1456 [linearInstType wanted] `thenM` \ split_inst ->
1457 returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1460 split_avail :: Avail -> Maybe Avail
1461 -- (Just av) if there's a modified version of avail that
1462 -- we can use to replace avail in avails
1463 -- Nothing if there isn't, so we need to create a Linear
1464 split_avail (Linear n i a) = Just (Linear (n+1) i a)
1465 split_avail (Given id used) | not used = Just (Given id True)
1466 | otherwise = Nothing
1467 split_avail Irred = Nothing
1468 split_avail IsFree = Nothing
1469 split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
1471 -------------------------
1472 addFree :: Avails -> Inst -> TcM Avails
1473 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1474 -- to avails, so that any other equal Insts will be commoned up right
1475 -- here rather than also being tossed upstairs. This is really just
1476 -- an optimisation, and perhaps it is more trouble that it is worth,
1477 -- as the following comments show!
1479 -- NB: do *not* add superclasses. If we have
1482 -- but a is not bound here, then we *don't* want to derive
1483 -- dn from df here lest we lose sharing.
1485 addFree avails free = returnM (addToFM avails free IsFree)
1487 addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> TcM Avails
1488 addWanted avails wanted rhs_expr wanteds
1489 = ASSERT2( not (wanted `elemFM` avails), ppr wanted $$ ppr avails )
1490 addAvailAndSCs avails wanted avail
1492 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1493 | otherwise = ASSERT( null wanteds ) NoRhs
1495 addGiven :: Avails -> Inst -> TcM Avails
1496 addGiven state given = addAvailAndSCs state given (Given (instToId given) False)
1497 -- No ASSERT( not (given `elemFM` avails) ) because in an instance
1498 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
1499 -- so the assert isn't true
1501 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
1502 addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
1503 addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
1504 addAvailAndSCs avails irred Irred
1506 addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
1507 addAvailAndSCs avails inst avail
1508 | not (isClassDict inst) = returnM avails1
1509 | otherwise = addSCs is_loop avails1 inst
1511 avails1 = addToFM avails inst avail
1512 is_loop inst = inst `elem` deps -- Note: this compares by *type*, not by Unique
1513 deps = findAllDeps avails avail
1515 findAllDeps :: Avails -> Avail -> [Inst]
1516 -- Find all the Insts that this one depends on
1517 -- See Note [SUPERCLASS-LOOP]
1518 findAllDeps avails (Rhs _ kids) = kids ++ concat (map (find_all_deps_help avails) kids)
1519 findAllDeps avails other = []
1521 find_all_deps_help :: Avails -> Inst -> [Inst]
1522 find_all_deps_help avails inst
1523 = case lookupFM avails inst of
1524 Just avail -> findAllDeps avails avail
1527 addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
1528 -- Add all the superclasses of the Inst to Avails
1529 -- The first param says "dont do this because the original thing
1530 -- depends on this one, so you'd build a loop"
1531 -- Invariant: the Inst is already in Avails.
1533 addSCs is_loop avails dict
1534 = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
1535 foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1537 (clas, tys) = getDictClassTys dict
1538 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1539 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1541 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1542 = case lookupFM avails sc_dict of
1543 Just (Given _ _) -> returnM avails -- Given is cheaper than
1544 -- a superclass selection
1545 Just other | is_loop sc_dict -> returnM avails -- See Note [SUPERCLASS-LOOP]
1546 | otherwise -> returnM avails' -- SCs already added
1548 Nothing -> addSCs is_loop avails' sc_dict
1550 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1551 avail = Rhs sc_sel_rhs [dict]
1552 avails' = addToFM avails sc_dict avail
1555 Note [SUPERCLASS-LOOP]: Checking for loops
1556 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1557 We have to be careful here. If we are *given* d1:Ord a,
1558 and want to deduce (d2:C [a]) where
1560 class Ord a => C a where
1561 instance Ord a => C [a] where ...
1563 Then we'll use the instance decl to deduce C [a] and then add the
1564 superclasses of C [a] to avails. But we must not overwrite the binding
1565 for d1:Ord a (which is given) with a superclass selection or we'll just
1568 Here's another example
1569 class Eq b => Foo a b
1570 instance Eq a => Foo [a] a
1574 we'll first deduce that it holds (via the instance decl). We must not
1575 then overwrite the Eq t constraint with a superclass selection!
1577 At first I had a gross hack, whereby I simply did not add superclass constraints
1578 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1579 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1580 I found a very obscure program (now tcrun021) in which improvement meant the
1581 simplifier got two bites a the cherry... so something seemed to be an Irred
1582 first time, but reducible next time.
1584 Now we implement the Right Solution, which is to check for loops directly
1585 when adding superclasses. It's a bit like the occurs check in unification.
1589 %************************************************************************
1591 \section{tcSimplifyTop: defaulting}
1593 %************************************************************************
1596 @tcSimplifyTop@ is called once per module to simplify all the constant
1597 and ambiguous Insts.
1599 We need to be careful of one case. Suppose we have
1601 instance Num a => Num (Foo a b) where ...
1603 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1604 to (Num x), and default x to Int. But what about y??
1606 It's OK: the final zonking stage should zap y to (), which is fine.
1610 tcSimplifyTop :: [Inst] -> TcM TcDictBinds
1611 -- The TcLclEnv should be valid here, solely to improve
1612 -- error message generation for the monomorphism restriction
1613 tcSimplifyTop wanteds
1614 = getLclEnv `thenM` \ lcl_env ->
1615 traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
1616 simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
1617 ASSERT( null frees )
1620 -- All the non-std ones are definite errors
1621 (stds, non_stds) = partition isStdClassTyVarDict irreds
1623 -- Group by type variable
1624 std_groups = equivClasses cmp_by_tyvar stds
1626 -- Pick the ones which its worth trying to disambiguate
1627 -- namely, the onese whose type variable isn't bound
1628 -- up with one of the non-standard classes
1629 (std_oks, std_bads) = partition worth_a_try std_groups
1630 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1631 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1633 -- Collect together all the bad guys
1634 bad_guys = non_stds ++ concat std_bads
1635 (tidy_env, tidy_dicts) = tidyInsts bad_guys
1636 (bad_ips, non_ips) = partition isIPDict tidy_dicts
1637 (no_insts, ambigs) = partition no_inst non_ips
1638 no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1639 fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
1642 -- Report definite errors
1643 addTopInstanceErrs tidy_env no_insts `thenM_`
1644 addTopIPErrs tidy_env bad_ips `thenM_`
1646 -- Deal with ambiguity errors, but only if
1647 -- if there has not been an error so far; errors often
1648 -- give rise to spurious ambiguous Insts
1649 ifErrsM (returnM []) (
1651 -- Complain about the ones that don't fall under
1652 -- the Haskell rules for disambiguation
1653 -- This group includes both non-existent instances
1654 -- e.g. Num (IO a) and Eq (Int -> Int)
1655 -- and ambiguous dictionaries
1657 addTopAmbigErrs (tidy_env, ambigs) `thenM_`
1659 -- Disambiguate the ones that look feasible
1660 mappM disambigGroup std_oks
1661 ) `thenM` \ binds_ambig ->
1663 returnM (binds `andMonoBinds` andMonoBindList binds_ambig)
1665 ----------------------------------
1666 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1668 get_tv d = case getDictClassTys d of
1669 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1670 get_clas d = case getDictClassTys d of
1671 (clas, [ty]) -> clas
1674 If a dictionary constrains a type variable which is
1675 * not mentioned in the environment
1676 * and not mentioned in the type of the expression
1677 then it is ambiguous. No further information will arise to instantiate
1678 the type variable; nor will it be generalised and turned into an extra
1679 parameter to a function.
1681 It is an error for this to occur, except that Haskell provided for
1682 certain rules to be applied in the special case of numeric types.
1684 * at least one of its classes is a numeric class, and
1685 * all of its classes are numeric or standard
1686 then the type variable can be defaulted to the first type in the
1687 default-type list which is an instance of all the offending classes.
1689 So here is the function which does the work. It takes the ambiguous
1690 dictionaries and either resolves them (producing bindings) or
1691 complains. It works by splitting the dictionary list by type
1692 variable, and using @disambigOne@ to do the real business.
1694 @disambigOne@ assumes that its arguments dictionaries constrain all
1695 the same type variable.
1697 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1698 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1699 the most common use of defaulting is code like:
1701 _ccall_ foo `seqPrimIO` bar
1703 Since we're not using the result of @foo@, the result if (presumably)
1707 disambigGroup :: [Inst] -- All standard classes of form (C a)
1711 | any isNumericClass classes -- Guaranteed all standard classes
1712 -- see comment at the end of function for reasons as to
1713 -- why the defaulting mechanism doesn't apply to groups that
1714 -- include CCallable or CReturnable dicts.
1715 && not (any isCcallishClass classes)
1716 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1717 -- SO, TRY DEFAULT TYPES IN ORDER
1719 -- Failure here is caused by there being no type in the
1720 -- default list which can satisfy all the ambiguous classes.
1721 -- For example, if Real a is reqd, but the only type in the
1722 -- default list is Int.
1723 getDefaultTys `thenM` \ default_tys ->
1725 try_default [] -- No defaults work, so fail
1728 try_default (default_ty : default_tys)
1729 = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
1730 -- default_tys instead
1731 tcSimplifyDefault theta `thenM` \ _ ->
1734 theta = [mkClassPred clas [default_ty] | clas <- classes]
1736 -- See if any default works
1737 tryM (try_default default_tys) `thenM` \ mb_ty ->
1739 Left _ -> -- If not, add an AmbigErr
1740 addTopAmbigErrs (tidyInsts dicts) `thenM_`
1741 returnM EmptyMonoBinds ;
1743 Right chosen_default_ty ->
1745 -- If so, bind the type variable
1746 -- and reduce the context, for real this time
1747 unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenM_`
1748 simpleReduceLoop (text "disambig" <+> ppr dicts)
1749 reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
1750 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1751 warnDefault dicts chosen_default_ty `thenM_`
1754 | all isCreturnableClass classes
1755 = -- Default CCall stuff to (); we don't even both to check that () is an
1756 -- instance of CReturnable, because we know it is.
1757 unifyTauTy (mkTyVarTy tyvar) unitTy `thenM_`
1758 returnM EmptyMonoBinds
1760 | otherwise -- No defaults
1761 = addTopAmbigErrs (tidyInsts dicts) `thenM_`
1762 returnM EmptyMonoBinds
1765 tyvar = get_tv (head dicts) -- Should be non-empty
1766 classes = map get_clas dicts
1769 [Aside - why the defaulting mechanism is turned off when
1770 dealing with arguments and results to ccalls.
1772 When typechecking _ccall_s, TcExpr ensures that the external
1773 function is only passed arguments (and in the other direction,
1774 results) of a restricted set of 'native' types. This is
1775 implemented via the help of the pseudo-type classes,
1776 @CReturnable@ (CR) and @CCallable@ (CC.)
1778 The interaction between the defaulting mechanism for numeric
1779 values and CC & CR can be a bit puzzling to the user at times.
1788 What type has 'x' got here? That depends on the default list
1789 in operation, if it is equal to Haskell 98's default-default
1790 of (Integer, Double), 'x' has type Double, since Integer
1791 is not an instance of CR. If the default list is equal to
1792 Haskell 1.4's default-default of (Int, Double), 'x' has type
1795 To try to minimise the potential for surprises here, the
1796 defaulting mechanism is turned off in the presence of
1797 CCallable and CReturnable.
1802 %************************************************************************
1804 \subsection[simple]{@Simple@ versions}
1806 %************************************************************************
1808 Much simpler versions when there are no bindings to make!
1810 @tcSimplifyThetas@ simplifies class-type constraints formed by
1811 @deriving@ declarations and when specialising instances. We are
1812 only interested in the simplified bunch of class/type constraints.
1814 It simplifies to constraints of the form (C a b c) where
1815 a,b,c are type variables. This is required for the context of
1816 instance declarations.
1819 tcSimplifyDeriv :: [TyVar]
1820 -> ThetaType -- Wanted
1821 -> TcM ThetaType -- Needed
1823 tcSimplifyDeriv tyvars theta
1824 = tcInstTyVars VanillaTv tyvars `thenM` \ (tvs, _, tenv) ->
1825 -- The main loop may do unification, and that may crash if
1826 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
1827 -- ToDo: what if two of them do get unified?
1828 newDicts DataDeclOrigin (substTheta tenv theta) `thenM` \ wanteds ->
1829 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
1830 ASSERT( null frees ) -- reduceMe never returns Free
1832 doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
1834 tv_set = mkVarSet tvs
1835 simpl_theta = map dictPred irreds -- reduceMe squashes all non-dicts
1838 | isEmptyVarSet pred_tyvars -- Things like (Eq T) should be rejected
1839 = addErrTc (noInstErr pred)
1841 | not undecidable_ok && not (isTyVarClassPred pred)
1842 -- Check that the returned dictionaries are all of form (C a b)
1843 -- (where a, b are type variables).
1844 -- We allow this if we had -fallow-undecidable-instances,
1845 -- but note that risks non-termination in the 'deriving' context-inference
1846 -- fixpoint loop. It is useful for situations like
1847 -- data Min h a = E | M a (h a)
1848 -- which gives the instance decl
1849 -- instance (Eq a, Eq (h a)) => Eq (Min h a)
1850 = addErrTc (noInstErr pred)
1852 | not (pred_tyvars `subVarSet` tv_set)
1853 -- Check for a bizarre corner case, when the derived instance decl should
1854 -- have form instance C a b => D (T a) where ...
1855 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1856 -- of problems; in particular, it's hard to compare solutions for
1857 -- equality when finding the fixpoint. So I just rule it out for now.
1858 = addErrTc (badDerivedPred pred)
1863 pred_tyvars = tyVarsOfPred pred
1865 rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
1866 -- This reverse-mapping is a Royal Pain,
1867 -- but the result should mention TyVars not TcTyVars
1870 mappM check_pred simpl_theta `thenM_`
1871 checkAmbiguity tvs simpl_theta tv_set `thenM_`
1872 returnM (substTheta rev_env simpl_theta)
1874 doc = ptext SLIT("deriving classes for a data type")
1877 @tcSimplifyDefault@ just checks class-type constraints, essentially;
1878 used with \tr{default} declarations. We are only interested in
1879 whether it worked or not.
1882 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
1885 tcSimplifyDefault theta
1886 = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
1887 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
1888 ASSERT( null frees ) -- try_me never returns Free
1889 mappM (addErrTc . noInstErr) irreds `thenM_`
1895 doc = ptext SLIT("default declaration")
1899 %************************************************************************
1901 \section{Errors and contexts}
1903 %************************************************************************
1905 ToDo: for these error messages, should we note the location as coming
1906 from the insts, or just whatever seems to be around in the monad just
1910 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
1911 -> [Inst] -- The offending Insts
1913 -- Group together insts with the same origin
1914 -- We want to report them together in error messages
1916 groupErrs report_err []
1918 groupErrs report_err (inst:insts)
1919 = do_one (inst:friends) `thenM_`
1920 groupErrs report_err others
1923 -- (It may seem a bit crude to compare the error messages,
1924 -- but it makes sure that we combine just what the user sees,
1925 -- and it avoids need equality on InstLocs.)
1926 (friends, others) = partition is_friend insts
1927 loc_msg = showSDoc (pprInstLoc (instLoc inst))
1928 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
1929 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
1930 -- Add location and context information derived from the Insts
1932 -- Add the "arising from..." part to a message about bunch of dicts
1933 addInstLoc :: [Inst] -> Message -> Message
1934 addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
1937 plural xs = char 's'
1940 addTopIPErrs tidy_env tidy_dicts
1941 = groupErrs report tidy_dicts
1943 report dicts = addErrTcM (tidy_env, mk_msg dicts)
1944 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
1945 plural tidy_dicts <+> pprInsts tidy_dicts)
1947 -- Used for top-level irreducibles
1948 addTopInstanceErrs tidy_env tidy_dicts
1949 = groupErrs report tidy_dicts
1951 report dicts = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
1952 addErrTcM (tidy_env, mk_msg dicts $$ mono_msg)
1953 mk_msg dicts = addInstLoc dicts (ptext SLIT("No instance") <> plural tidy_dicts <+>
1954 ptext SLIT("for") <+> pprInsts tidy_dicts)
1957 addTopAmbigErrs (tidy_env, tidy_dicts)
1958 -- Divide into groups that share a common set of ambiguous tyvars
1959 = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
1961 tvs_of :: Inst -> [TcTyVar]
1962 tvs_of d = varSetElems (tyVarsOfInst d)
1963 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
1965 report :: [(Inst,[TcTyVar])] -> TcM ()
1966 report pairs@((_,tvs) : _) -- The pairs share a common set of ambiguous tyvars
1967 = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
1968 addErrTcM (tidy_env, msg $$ mono_msg)
1970 dicts = map fst pairs
1971 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
1972 pprQuotedList tvs <+> in_msg,
1973 nest 2 (pprInstsInFull dicts)]
1974 in_msg | isSingleton dicts = text "in the top-level constraint:"
1975 | otherwise = text "in these top-level constraints:"
1978 mkMonomorphismMsg :: TidyEnv -> [Inst] -> TcM (TidyEnv, Message)
1979 -- There's an error with these Insts; if they have free type variables
1980 -- it's probably caused by the monomorphism restriction.
1981 -- Try to identify the offending variable
1982 -- ASSUMPTION: the Insts are fully zonked
1983 mkMonomorphismMsg tidy_env insts
1984 | isEmptyVarSet inst_tvs
1985 = returnM (tidy_env, empty)
1987 = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
1988 returnM (tidy_env, mk_msg docs)
1991 inst_tvs = tyVarsOfInsts insts
1993 mk_msg [] = empty -- This happens in things like
1994 -- f x = show (read "foo")
1995 -- whre monomorphism doesn't play any role
1996 mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
1998 ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
2000 warnDefault dicts default_ty
2001 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
2002 addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
2005 (_, tidy_dicts) = tidyInsts dicts
2006 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
2007 quotes (ppr default_ty),
2008 pprInstsInFull tidy_dicts]
2010 complainCheck doc givens irreds
2011 = mappM zonkInst given_dicts_and_ips `thenM` \ givens' ->
2012 groupErrs (addNoInstanceErrs doc givens') irreds `thenM_`
2015 given_dicts_and_ips = filter (not . isMethod) givens
2016 -- Filter out methods, which are only added to
2017 -- the given set as an optimisation
2019 addNoInstanceErrs what_doc givens dicts
2020 = getDOpts `thenM` \ dflags ->
2021 tcGetInstEnv `thenM` \ inst_env ->
2023 (tidy_env1, tidy_givens) = tidyInsts givens
2024 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
2026 doc = vcat [addInstLoc dicts $
2027 sep [herald <+> pprInsts tidy_dicts,
2028 nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
2030 ptext SLIT("Probable fix:"),
2034 herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
2035 unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
2038 -- The error message when we don't find a suitable instance
2039 -- is complicated by the fact that sometimes this is because
2040 -- there is no instance, and sometimes it's because there are
2041 -- too many instances (overlap). See the comments in TcEnv.lhs
2042 -- with the InstEnv stuff.
2045 | not ambig_overlap = empty
2047 = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
2048 nest 4 (ptext SLIT("depends on the instantiation of") <+>
2049 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInsts tidy_dicts))))]
2051 fix1 = sep [ptext SLIT("Add") <+> pprInsts tidy_dicts,
2052 ptext SLIT("to the") <+> what_doc]
2054 fix2 | null instance_dicts
2057 = ptext SLIT("Or add an instance declaration for") <+> pprInsts instance_dicts
2059 instance_dicts = [d | d <- tidy_dicts, isClassDict d, not (isTyVarDict d)]
2060 -- Insts for which it is worth suggesting an adding an instance declaration
2061 -- Exclude implicit parameters, and tyvar dicts
2063 -- Checks for the ambiguous case when we have overlapping instances
2064 ambig_overlap = any ambig_overlap1 dicts
2067 = case lookupInstEnv dflags inst_env clas tys of
2068 NoMatch ambig -> ambig
2072 (clas,tys) = getDictClassTys dict
2074 addErrTcM (tidy_env2, doc)
2076 -- Used for the ...Thetas variants; all top level
2077 noInstErr pred = ptext SLIT("No instance for") <+> quotes (ppr pred)
2080 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2081 ptext SLIT("type variables that are not data type parameters"),
2082 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2084 reduceDepthErr n stack
2085 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2086 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
2087 nest 4 (pprInstsInFull stack)]
2089 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)
2091 -----------------------------------------------
2093 = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
2094 nest 4 (ppr inst <+> pprInstLoc (instLoc inst))])