2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcSimplify]{TcSimplify}
10 tcSimplifyInfer, tcSimplifyInferCheck,
11 tcSimplifyCheck, tcSimplifyRestricted,
12 tcSimplifyToDicts, tcSimplifyIPs,
13 tcSimplifyTop, tcSimplifyInteractive,
16 tcSimplifyDeriv, tcSimplifyDefault,
20 #include "HsVersions.h"
22 import {-# SOURCE #-} TcUnify( unifyTauTy )
24 import HsSyn ( HsBind(..), LHsBinds, HsExpr(..), LHsExpr, pprLHsBinds )
25 import TcHsSyn ( TcId, TcDictBinds, mkHsApp, mkHsTyApp, mkHsDictApp )
28 import Inst ( lookupInst, LookupInstResult(..),
29 tyVarsOfInst, fdPredsOfInsts, fdPredsOfInst, newDicts,
30 isDict, isClassDict, isLinearInst, linearInstType,
31 isStdClassTyVarDict, isMethodFor, isMethod,
32 instToId, tyVarsOfInsts, cloneDict,
33 ipNamesOfInsts, ipNamesOfInst, dictPred,
35 newDictsFromOld, tcInstClassOp,
36 getDictClassTys, isTyVarDict,
37 instLoc, zonkInst, tidyInsts, tidyMoreInsts,
38 Inst, pprInsts, pprDictsInFull, tcGetInstEnvs,
39 isIPDict, isInheritableInst, pprDFuns, pprDictsTheta
41 import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals )
42 import InstEnv ( lookupInstEnv, classInstEnv )
43 import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
44 import TcType ( TcTyVar, TcTyVarSet, ThetaType, TyVarDetails(VanillaTv),
45 mkClassPred, isOverloadedTy, mkTyConApp,
46 mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
47 tyVarsOfPred, tcEqType, pprPred )
48 import Id ( idType, mkUserLocal )
50 import Name ( getOccName, getSrcLoc )
51 import NameSet ( NameSet, mkNameSet, elemNameSet )
52 import Class ( classBigSig, classKey )
53 import FunDeps ( oclose, grow, improve, pprEquationDoc )
54 import PrelInfo ( isNumericClass )
55 import PrelNames ( splitName, fstName, sndName, integerTyConName,
56 showClassKey, eqClassKey, ordClassKey )
57 import Subst ( mkTopTyVarSubst, substTheta, substTy )
58 import TysWiredIn ( pairTyCon, doubleTy )
59 import ErrUtils ( Message )
61 import VarEnv ( TidyEnv )
65 import ListSetOps ( equivClasses )
66 import Util ( zipEqual, isSingleton )
67 import List ( partition )
68 import SrcLoc ( Located(..) )
73 %************************************************************************
77 %************************************************************************
79 --------------------------------------
80 Notes on functional dependencies (a bug)
81 --------------------------------------
83 | > class Foo a b | a->b
85 | > class Bar a b | a->b
89 | > instance Bar Obj Obj
91 | > instance (Bar a b) => Foo a b
93 | > foo:: (Foo a b) => a -> String
96 | > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
102 | Could not deduce (Bar a b) from the context (Foo a b)
103 | arising from use of `foo' at <interactive>:1
105 | Add (Bar a b) to the expected type of an expression
106 | In the first argument of `runFoo', namely `foo'
107 | In the definition of `it': it = runFoo foo
109 | Why all of the sudden does GHC need the constraint Bar a b? The
110 | function foo didn't ask for that...
112 The trouble is that to type (runFoo foo), GHC has to solve the problem:
114 Given constraint Foo a b
115 Solve constraint Foo a b'
117 Notice that b and b' aren't the same. To solve this, just do
118 improvement and then they are the same. But GHC currently does
123 That is usually fine, but it isn't here, because it sees that Foo a b is
124 not the same as Foo a b', and so instead applies the instance decl for
125 instance Bar a b => Foo a b. And that's where the Bar constraint comes
128 The Right Thing is to improve whenever the constraint set changes at
129 all. Not hard in principle, but it'll take a bit of fiddling to do.
133 --------------------------------------
134 Notes on quantification
135 --------------------------------------
137 Suppose we are about to do a generalisation step.
141 T the type of the RHS
142 C the constraints from that RHS
144 The game is to figure out
146 Q the set of type variables over which to quantify
147 Ct the constraints we will *not* quantify over
148 Cq the constraints we will quantify over
150 So we're going to infer the type
154 and float the constraints Ct further outwards.
156 Here are the things that *must* be true:
158 (A) Q intersect fv(G) = EMPTY limits how big Q can be
159 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
161 (A) says we can't quantify over a variable that's free in the
162 environment. (B) says we must quantify over all the truly free
163 variables in T, else we won't get a sufficiently general type. We do
164 not *need* to quantify over any variable that is fixed by the free
165 vars of the environment G.
167 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
169 Example: class H x y | x->y where ...
171 fv(G) = {a} C = {H a b, H c d}
174 (A) Q intersect {a} is empty
175 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
177 So Q can be {c,d}, {b,c,d}
179 Other things being equal, however, we'd like to quantify over as few
180 variables as possible: smaller types, fewer type applications, more
181 constraints can get into Ct instead of Cq.
184 -----------------------------------------
187 fv(T) the free type vars of T
189 oclose(vs,C) The result of extending the set of tyvars vs
190 using the functional dependencies from C
192 grow(vs,C) The result of extend the set of tyvars vs
193 using all conceivable links from C.
195 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
196 Then grow(vs,C) = {a,b,c}
198 Note that grow(vs,C) `superset` grow(vs,simplify(C))
199 That is, simplfication can only shrink the result of grow.
202 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
203 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
206 -----------------------------------------
210 Here's a good way to choose Q:
212 Q = grow( fv(T), C ) \ oclose( fv(G), C )
214 That is, quantify over all variable that that MIGHT be fixed by the
215 call site (which influences T), but which aren't DEFINITELY fixed by
216 G. This choice definitely quantifies over enough type variables,
217 albeit perhaps too many.
219 Why grow( fv(T), C ) rather than fv(T)? Consider
221 class H x y | x->y where ...
226 If we used fv(T) = {c} we'd get the type
228 forall c. H c d => c -> b
230 And then if the fn was called at several different c's, each of
231 which fixed d differently, we'd get a unification error, because
232 d isn't quantified. Solution: quantify d. So we must quantify
233 everything that might be influenced by c.
235 Why not oclose( fv(T), C )? Because we might not be able to see
236 all the functional dependencies yet:
238 class H x y | x->y where ...
239 instance H x y => Eq (T x y) where ...
244 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
245 apparent yet, and that's wrong. We must really quantify over d too.
248 There really isn't any point in quantifying over any more than
249 grow( fv(T), C ), because the call sites can't possibly influence
250 any other type variables.
254 --------------------------------------
256 --------------------------------------
258 It's very hard to be certain when a type is ambiguous. Consider
262 instance H x y => K (x,y)
264 Is this type ambiguous?
265 forall a b. (K (a,b), Eq b) => a -> a
267 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
268 now we see that a fixes b. So we can't tell about ambiguity for sure
269 without doing a full simplification. And even that isn't possible if
270 the context has some free vars that may get unified. Urgle!
272 Here's another example: is this ambiguous?
273 forall a b. Eq (T b) => a -> a
274 Not if there's an insance decl (with no context)
275 instance Eq (T b) where ...
277 You may say of this example that we should use the instance decl right
278 away, but you can't always do that:
280 class J a b where ...
281 instance J Int b where ...
283 f :: forall a b. J a b => a -> a
285 (Notice: no functional dependency in J's class decl.)
286 Here f's type is perfectly fine, provided f is only called at Int.
287 It's premature to complain when meeting f's signature, or even
288 when inferring a type for f.
292 However, we don't *need* to report ambiguity right away. It'll always
293 show up at the call site.... and eventually at main, which needs special
294 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
296 So here's the plan. We WARN about probable ambiguity if
298 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
300 (all tested before quantification).
301 That is, all the type variables in Cq must be fixed by the the variables
302 in the environment, or by the variables in the type.
304 Notice that we union before calling oclose. Here's an example:
306 class J a b c | a b -> c
310 forall b c. (J a b c) => b -> b
312 Only if we union {a} from G with {b} from T before using oclose,
313 do we see that c is fixed.
315 It's a bit vague exactly which C we should use for this oclose call. If we
316 don't fix enough variables we might complain when we shouldn't (see
317 the above nasty example). Nothing will be perfect. That's why we can
318 only issue a warning.
321 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
323 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
325 then c is a "bubble"; there's no way it can ever improve, and it's
326 certainly ambiguous. UNLESS it is a constant (sigh). And what about
331 instance H x y => K (x,y)
333 Is this type ambiguous?
334 forall a b. (K (a,b), Eq b) => a -> a
336 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
337 is a "bubble" that's a set of constraints
339 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
341 Hence another idea. To decide Q start with fv(T) and grow it
342 by transitive closure in Cq (no functional dependencies involved).
343 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
344 The definitely-ambiguous can then float out, and get smashed at top level
345 (which squashes out the constants, like Eq (T a) above)
348 --------------------------------------
349 Notes on principal types
350 --------------------------------------
355 f x = let g y = op (y::Int) in True
357 Here the principal type of f is (forall a. a->a)
358 but we'll produce the non-principal type
359 f :: forall a. C Int => a -> a
362 --------------------------------------
363 The need for forall's in constraints
364 --------------------------------------
366 [Exchange on Haskell Cafe 5/6 Dec 2000]
368 class C t where op :: t -> Bool
369 instance C [t] where op x = True
371 p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
372 q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
374 The definitions of p and q differ only in the order of the components in
375 the pair on their right-hand sides. And yet:
377 ghc and "Typing Haskell in Haskell" reject p, but accept q;
378 Hugs rejects q, but accepts p;
379 hbc rejects both p and q;
380 nhc98 ... (Malcolm, can you fill in the blank for us!).
382 The type signature for f forces context reduction to take place, and
383 the results of this depend on whether or not the type of y is known,
384 which in turn depends on which component of the pair the type checker
387 Solution: if y::m a, float out the constraints
388 Monad m, forall c. C (m c)
389 When m is later unified with [], we can solve both constraints.
392 --------------------------------------
393 Notes on implicit parameters
394 --------------------------------------
396 Question 1: can we "inherit" implicit parameters
397 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
402 where f is *not* a top-level binding.
403 From the RHS of f we'll get the constraint (?y::Int).
404 There are two types we might infer for f:
408 (so we get ?y from the context of f's definition), or
410 f :: (?y::Int) => Int -> Int
412 At first you might think the first was better, becuase then
413 ?y behaves like a free variable of the definition, rather than
414 having to be passed at each call site. But of course, the WHOLE
415 IDEA is that ?y should be passed at each call site (that's what
416 dynamic binding means) so we'd better infer the second.
418 BOTTOM LINE: when *inferring types* you *must* quantify
419 over implicit parameters. See the predicate isFreeWhenInferring.
422 Question 2: type signatures
423 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
424 BUT WATCH OUT: When you supply a type signature, we can't force you
425 to quantify over implicit parameters. For example:
429 This is perfectly reasonable. We do not want to insist on
431 (?x + 1) :: (?x::Int => Int)
433 That would be silly. Here, the definition site *is* the occurrence site,
434 so the above strictures don't apply. Hence the difference between
435 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
436 and tcSimplifyCheckBind (which does not).
438 What about when you supply a type signature for a binding?
439 Is it legal to give the following explicit, user type
440 signature to f, thus:
445 At first sight this seems reasonable, but it has the nasty property
446 that adding a type signature changes the dynamic semantics.
449 (let f x = (x::Int) + ?y
450 in (f 3, f 3 with ?y=5)) with ?y = 6
456 in (f 3, f 3 with ?y=5)) with ?y = 6
460 Indeed, simply inlining f (at the Haskell source level) would change the
463 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
464 semantics for a Haskell program without knowing its typing, so if you
465 change the typing you may change the semantics.
467 To make things consistent in all cases where we are *checking* against
468 a supplied signature (as opposed to inferring a type), we adopt the
471 a signature does not need to quantify over implicit params.
473 [This represents a (rather marginal) change of policy since GHC 5.02,
474 which *required* an explicit signature to quantify over all implicit
475 params for the reasons mentioned above.]
477 But that raises a new question. Consider
479 Given (signature) ?x::Int
480 Wanted (inferred) ?x::Int, ?y::Bool
482 Clearly we want to discharge the ?x and float the ?y out. But
483 what is the criterion that distinguishes them? Clearly it isn't
484 what free type variables they have. The Right Thing seems to be
485 to float a constraint that
486 neither mentions any of the quantified type variables
487 nor any of the quantified implicit parameters
489 See the predicate isFreeWhenChecking.
492 Question 3: monomorphism
493 ~~~~~~~~~~~~~~~~~~~~~~~~
494 There's a nasty corner case when the monomorphism restriction bites:
498 The argument above suggests that we *must* generalise
499 over the ?y parameter, to get
500 z :: (?y::Int) => Int,
501 but the monomorphism restriction says that we *must not*, giving
503 Why does the momomorphism restriction say this? Because if you have
505 let z = x + ?y in z+z
507 you might not expect the addition to be done twice --- but it will if
508 we follow the argument of Question 2 and generalise over ?y.
514 (A) Always generalise over implicit parameters
515 Bindings that fall under the monomorphism restriction can't
519 * Inlining remains valid
520 * No unexpected loss of sharing
521 * But simple bindings like
523 will be rejected, unless you add an explicit type signature
524 (to avoid the monomorphism restriction)
525 z :: (?y::Int) => Int
527 This seems unacceptable
529 (B) Monomorphism restriction "wins"
530 Bindings that fall under the monomorphism restriction can't
532 Always generalise over implicit parameters *except* for bindings
533 that fall under the monomorphism restriction
536 * Inlining isn't valid in general
537 * No unexpected loss of sharing
538 * Simple bindings like
540 accepted (get value of ?y from binding site)
542 (C) Always generalise over implicit parameters
543 Bindings that fall under the monomorphism restriction can't
544 be generalised, EXCEPT for implicit parameters
546 * Inlining remains valid
547 * Unexpected loss of sharing (from the extra generalisation)
548 * Simple bindings like
550 accepted (get value of ?y from occurrence sites)
555 None of these choices seems very satisfactory. But at least we should
556 decide which we want to do.
558 It's really not clear what is the Right Thing To Do. If you see
562 would you expect the value of ?y to be got from the *occurrence sites*
563 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
564 case of function definitions, the answer is clearly the former, but
565 less so in the case of non-fucntion definitions. On the other hand,
566 if we say that we get the value of ?y from the definition site of 'z',
567 then inlining 'z' might change the semantics of the program.
569 Choice (C) really says "the monomorphism restriction doesn't apply
570 to implicit parameters". Which is fine, but remember that every
571 innocent binding 'x = ...' that mentions an implicit parameter in
572 the RHS becomes a *function* of that parameter, called at each
573 use of 'x'. Now, the chances are that there are no intervening 'with'
574 clauses that bind ?y, so a decent compiler should common up all
575 those function calls. So I think I strongly favour (C). Indeed,
576 one could make a similar argument for abolishing the monomorphism
577 restriction altogether.
579 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
583 %************************************************************************
585 \subsection{tcSimplifyInfer}
587 %************************************************************************
589 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
591 1. Compute Q = grow( fvs(T), C )
593 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
594 predicates will end up in Ct; we deal with them at the top level
596 3. Try improvement, using functional dependencies
598 4. If Step 3 did any unification, repeat from step 1
599 (Unification can change the result of 'grow'.)
601 Note: we don't reduce dictionaries in step 2. For example, if we have
602 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
603 after step 2. However note that we may therefore quantify over more
604 type variables than we absolutely have to.
606 For the guts, we need a loop, that alternates context reduction and
607 improvement with unification. E.g. Suppose we have
609 class C x y | x->y where ...
611 and tcSimplify is called with:
613 Then improvement unifies a with b, giving
616 If we need to unify anything, we rattle round the whole thing all over
623 -> TcTyVarSet -- fv(T); type vars
625 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
626 TcDictBinds, -- Bindings
627 [TcId]) -- Dict Ids that must be bound here (zonked)
628 -- Any free (escaping) Insts are tossed into the environment
633 tcSimplifyInfer doc tau_tvs wanted_lie
634 = inferLoop doc (varSetElems tau_tvs)
635 wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
637 extendLIEs frees `thenM_`
638 returnM (qtvs, binds, map instToId irreds)
640 inferLoop doc tau_tvs wanteds
642 zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' ->
643 mappM zonkInst wanteds `thenM` \ wanteds' ->
644 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
646 preds = fdPredsOfInsts wanteds'
647 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
650 | isFreeWhenInferring qtvs inst = Free
651 | isClassDict inst = DontReduceUnlessConstant -- Dicts
652 | otherwise = ReduceMe -- Lits and Methods
654 traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds, ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_`
656 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
659 if no_improvement then
660 returnM (varSetElems qtvs, frees, binds, irreds)
662 -- If improvement did some unification, we go round again. There
663 -- are two subtleties:
664 -- a) We start again with irreds, not wanteds
665 -- Using an instance decl might have introduced a fresh type variable
666 -- which might have been unified, so we'd get an infinite loop
667 -- if we started again with wanteds! See example [LOOP]
669 -- b) It's also essential to re-process frees, because unification
670 -- might mean that a type variable that looked free isn't now.
672 -- Hence the (irreds ++ frees)
674 -- However, NOTICE that when we are done, we might have some bindings, but
675 -- the final qtvs might be empty. See [NO TYVARS] below.
677 inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) ->
678 returnM (qtvs1, frees1, binds `unionBags` binds1, irreds1)
683 class If b t e r | b t e -> r
686 class Lte a b c | a b -> c where lte :: a -> b -> c
688 instance (Lte a b l,If l b a c) => Max a b c
690 Wanted: Max Z (S x) y
692 Then we'll reduce using the Max instance to:
693 (Lte Z (S x) l, If l (S x) Z y)
694 and improve by binding l->T, after which we can do some reduction
695 on both the Lte and If constraints. What we *can't* do is start again
696 with (Max Z (S x) y)!
700 class Y a b | a -> b where
703 instance Y [[a]] a where
706 k :: X a -> X a -> X a
708 g :: Num a => [X a] -> [X a]
711 h ys = ys ++ map (k (y [[0]])) xs
713 The excitement comes when simplifying the bindings for h. Initially
714 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
715 From this we get t1:=:t2, but also various bindings. We can't forget
716 the bindings (because of [LOOP]), but in fact t1 is what g is
719 The net effect of [NO TYVARS]
722 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
723 isFreeWhenInferring qtvs inst
724 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
725 && isInheritableInst inst -- And no implicit parameter involved
726 -- (see "Notes on implicit parameters")
728 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
729 -> NameSet -- Quantified implicit parameters
731 isFreeWhenChecking qtvs ips inst
732 = isFreeWrtTyVars qtvs inst
733 && isFreeWrtIPs ips inst
735 isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
736 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
740 %************************************************************************
742 \subsection{tcSimplifyCheck}
744 %************************************************************************
746 @tcSimplifyCheck@ is used when we know exactly the set of variables
747 we are going to quantify over. For example, a class or instance declaration.
752 -> [TcTyVar] -- Quantify over these
755 -> TcM TcDictBinds -- Bindings
757 -- tcSimplifyCheck is used when checking expression type signatures,
758 -- class decls, instance decls etc.
760 -- NB: tcSimplifyCheck does not consult the
761 -- global type variables in the environment; so you don't
762 -- need to worry about setting them before calling tcSimplifyCheck
763 tcSimplifyCheck doc qtvs givens wanted_lie
764 = tcSimplCheck doc get_qtvs
765 givens wanted_lie `thenM` \ (qtvs', binds) ->
768 get_qtvs = zonkTcTyVarsAndFV qtvs
771 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
772 -- against, but we don't know the type variables over which we are going to quantify.
773 -- This happens when we have a type signature for a mutually recursive group
776 -> TcTyVarSet -- fv(T)
779 -> TcM ([TcTyVar], -- Variables over which to quantify
780 TcDictBinds) -- Bindings
782 tcSimplifyInferCheck doc tau_tvs givens wanted_lie
783 = tcSimplCheck doc get_qtvs givens wanted_lie
785 -- Figure out which type variables to quantify over
786 -- You might think it should just be the signature tyvars,
787 -- but in bizarre cases you can get extra ones
788 -- f :: forall a. Num a => a -> a
789 -- f x = fst (g (x, head [])) + 1
791 -- Here we infer g :: forall a b. a -> b -> (b,a)
792 -- We don't want g to be monomorphic in b just because
793 -- f isn't quantified over b.
794 all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
796 get_qtvs = zonkTcTyVarsAndFV all_tvs `thenM` \ all_tvs' ->
797 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
799 qtvs = all_tvs' `minusVarSet` gbl_tvs
800 -- We could close gbl_tvs, but its not necessary for
801 -- soundness, and it'll only affect which tyvars, not which
802 -- dictionaries, we quantify over
807 Here is the workhorse function for all three wrappers.
810 tcSimplCheck doc get_qtvs givens wanted_lie
811 = check_loop givens wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
813 -- Complain about any irreducible ones
814 mappM zonkInst given_dicts_and_ips `thenM` \ givens' ->
815 groupErrs (addNoInstanceErrs (Just doc) givens') irreds `thenM_`
818 extendLIEs frees `thenM_`
819 returnM (qtvs, binds)
822 given_dicts_and_ips = filter (not . isMethod) givens
823 -- For error reporting, filter out methods, which are
824 -- only added to the given set as an optimisation
826 ip_set = mkNameSet (ipNamesOfInsts givens)
828 check_loop givens wanteds
830 mappM zonkInst givens `thenM` \ givens' ->
831 mappM zonkInst wanteds `thenM` \ wanteds' ->
832 get_qtvs `thenM` \ qtvs' ->
836 -- When checking against a given signature we always reduce
837 -- until we find a match against something given, or can't reduce
838 try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
839 | otherwise = ReduceMe
841 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
844 if no_improvement then
845 returnM (varSetElems qtvs', frees, binds, irreds)
847 check_loop givens' (irreds ++ frees) `thenM` \ (qtvs', frees1, binds1, irreds1) ->
848 returnM (qtvs', frees1, binds `unionBags` binds1, irreds1)
852 %************************************************************************
854 \subsection{tcSimplifyRestricted}
856 %************************************************************************
858 tcSimplifyRestricted infers which type variables to quantify for a
859 group of restricted bindings. This isn't trivial.
862 We want to quantify over a to get id :: forall a. a->a
865 We do not want to quantify over a, because there's an Eq a
866 constraint, so we get eq :: a->a->Bool (notice no forall)
869 RHS has type 'tau', whose free tyvars are tau_tvs
870 RHS has constraints 'wanteds'
873 Quantify over (tau_tvs \ ftvs(wanteds))
874 This is bad. The constraints may contain (Monad (ST s))
875 where we have instance Monad (ST s) where...
876 so there's no need to be monomorphic in s!
878 Also the constraint might be a method constraint,
879 whose type mentions a perfectly innocent tyvar:
880 op :: Num a => a -> b -> a
881 Here, b is unconstrained. A good example would be
883 We want to infer the polymorphic type
884 foo :: forall b. b -> b
887 Plan B (cunning, used for a long time up to and including GHC 6.2)
888 Step 1: Simplify the constraints as much as possible (to deal
889 with Plan A's problem). Then set
890 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
892 Step 2: Now simplify again, treating the constraint as 'free' if
893 it does not mention qtvs, and trying to reduce it otherwise.
894 The reasons for this is to maximise sharing.
896 This fails for a very subtle reason. Suppose that in the Step 2
897 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
898 In the Step 1 this constraint might have been simplified, perhaps to
899 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
900 This won't happen in Step 2... but that in turn might prevent some other
901 constraint mentioning 'b' from being simplified... and that in turn
902 breaks the invariant that no constraints are quantified over.
904 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
909 Step 1: Simplify the constraints as much as possible (to deal
910 with Plan A's problem). Then set
911 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
912 Return the bindings from Step 1.
917 tcSimplifyRestricted -- Used for restricted binding groups
918 -- i.e. ones subject to the monomorphism restriction
920 -> TcTyVarSet -- Free in the type of the RHSs
921 -> [Inst] -- Free in the RHSs
922 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
923 TcDictBinds) -- Bindings
924 -- tcSimpifyRestricted returns no constraints to
925 -- quantify over; by definition there are none.
926 -- They are all thrown back in the LIE
928 tcSimplifyRestricted doc tau_tvs wanteds
929 -- 'reduceMe': Reduce as far as we can. Don't stop at
930 -- dicts; the idea is to get rid of as many type
931 -- variables as possible, and we don't want to stop
932 -- at (say) Monad (ST s), because that reduces
933 -- immediately, with no constraint on s.
934 = simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
937 -- Next, figure out the tyvars we will quantify over
938 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
939 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
941 constrained_tvs = tyVarsOfInsts irreds
942 qtvs = (tau_tvs' `minusVarSet` constrained_tvs)
943 `minusVarSet` oclose (fdPredsOfInsts irreds) gbl_tvs
944 -- The second minusVarSet arranges not to quantify over
945 -- any tyvars that are functionally determined by ones in
948 traceTc (text "tcSimplifyRestricted" <+> vcat [
949 pprInsts wanteds, pprInsts frees, pprInsts irreds,
951 ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_`
953 extendLIEs irreds `thenM_`
954 returnM (varSetElems qtvs, binds)
958 %************************************************************************
960 \subsection{tcSimplifyToDicts}
962 %************************************************************************
964 On the LHS of transformation rules we only simplify methods and constants,
965 getting dictionaries. We want to keep all of them unsimplified, to serve
966 as the available stuff for the RHS of the rule.
968 The same thing is used for specialise pragmas. Consider
971 {-# SPECIALISE f :: Int -> Int #-}
974 The type checker generates a binding like:
976 f_spec = (f :: Int -> Int)
978 and we want to end up with
980 f_spec = _inline_me_ (f Int dNumInt)
982 But that means that we must simplify the Method for f to (f Int dNumInt)!
983 So tcSimplifyToDicts squeezes out all Methods.
985 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
987 fromIntegral :: (Integral a, Num b) => a -> b
988 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
990 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
994 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
996 because the scsel will mess up matching. Instead we want
998 forall dIntegralInt, dNumInt.
999 fromIntegral Int Int dIntegralInt dNumInt = id Int
1001 Hence "DontReduce NoSCs"
1004 tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds)
1005 tcSimplifyToDicts wanteds
1006 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
1007 -- Since try_me doesn't look at types, we don't need to
1008 -- do any zonking, so it's safe to call reduceContext directly
1009 ASSERT( null frees )
1010 extendLIEs irreds `thenM_`
1014 doc = text "tcSimplifyToDicts"
1016 -- Reduce methods and lits only; stop as soon as we get a dictionary
1017 try_me inst | isDict inst = DontReduce NoSCs -- See notes above for why NoSCs
1018 | otherwise = ReduceMe
1023 tcSimplifyBracket is used when simplifying the constraints arising from
1024 a Template Haskell bracket [| ... |]. We want to check that there aren't
1025 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1026 Show instance), but we aren't otherwise interested in the results.
1027 Nor do we care about ambiguous dictionaries etc. We will type check
1028 this bracket again at its usage site.
1031 tcSimplifyBracket :: [Inst] -> TcM ()
1032 tcSimplifyBracket wanteds
1033 = simpleReduceLoop doc reduceMe wanteds `thenM_`
1036 doc = text "tcSimplifyBracket"
1040 %************************************************************************
1042 \subsection{Filtering at a dynamic binding}
1044 %************************************************************************
1049 we must discharge all the ?x constraints from B. We also do an improvement
1050 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1052 Actually, the constraints from B might improve the types in ?x. For example
1054 f :: (?x::Int) => Char -> Char
1057 then the constraint (?x::Int) arising from the call to f will
1058 force the binding for ?x to be of type Int.
1061 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1064 tcSimplifyIPs given_ips wanteds
1065 = simpl_loop given_ips wanteds `thenM` \ (frees, binds) ->
1066 extendLIEs frees `thenM_`
1069 doc = text "tcSimplifyIPs" <+> ppr given_ips
1070 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1072 -- Simplify any methods that mention the implicit parameter
1073 try_me inst | isFreeWrtIPs ip_set inst = Free
1074 | otherwise = ReduceMe
1076 simpl_loop givens wanteds
1077 = mappM zonkInst givens `thenM` \ givens' ->
1078 mappM zonkInst wanteds `thenM` \ wanteds' ->
1080 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1082 if no_improvement then
1083 ASSERT( null irreds )
1084 returnM (frees, binds)
1086 simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) ->
1087 returnM (frees1, binds `unionBags` binds1)
1091 %************************************************************************
1093 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1095 %************************************************************************
1097 When doing a binding group, we may have @Insts@ of local functions.
1098 For example, we might have...
1100 let f x = x + 1 -- orig local function (overloaded)
1101 f.1 = f Int -- two instances of f
1106 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1107 where @f@ is in scope; those @Insts@ must certainly not be passed
1108 upwards towards the top-level. If the @Insts@ were binding-ified up
1109 there, they would have unresolvable references to @f@.
1111 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1112 For each method @Inst@ in the @init_lie@ that mentions one of the
1113 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1114 @LIE@), as well as the @HsBinds@ generated.
1117 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM (LHsBinds TcId)
1119 bindInstsOfLocalFuns wanteds local_ids
1120 | null overloaded_ids
1122 = extendLIEs wanteds `thenM_`
1126 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
1127 ASSERT( null irreds )
1128 extendLIEs frees `thenM_`
1131 doc = text "bindInsts" <+> ppr local_ids
1132 overloaded_ids = filter is_overloaded local_ids
1133 is_overloaded id = isOverloadedTy (idType id)
1135 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1136 -- so it's worth building a set, so that
1137 -- lookup (in isMethodFor) is faster
1139 try_me inst | isMethodFor overloaded_set inst = ReduceMe
1144 %************************************************************************
1146 \subsection{Data types for the reduction mechanism}
1148 %************************************************************************
1150 The main control over context reduction is here
1154 = ReduceMe -- Try to reduce this
1155 -- If there's no instance, behave exactly like
1156 -- DontReduce: add the inst to
1157 -- the irreductible ones, but don't
1158 -- produce an error message of any kind.
1159 -- It might be quite legitimate such as (Eq a)!
1161 | DontReduce WantSCs -- Return as irreducible
1163 | DontReduceUnlessConstant -- Return as irreducible unless it can
1164 -- be reduced to a constant in one step
1166 | Free -- Return as free
1168 reduceMe :: Inst -> WhatToDo
1169 reduceMe inst = ReduceMe
1171 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1172 -- of a predicate when adding it to the avails
1178 type Avails = FiniteMap Inst Avail
1181 = IsFree -- Used for free Insts
1182 | Irred -- Used for irreducible dictionaries,
1183 -- which are going to be lambda bound
1185 | Given TcId -- Used for dictionaries for which we have a binding
1186 -- e.g. those "given" in a signature
1187 Bool -- True <=> actually consumed (splittable IPs only)
1189 | NoRhs -- Used for Insts like (CCallable f)
1190 -- where no witness is required.
1193 | Rhs -- Used when there is a RHS
1194 (LHsExpr TcId) -- The RHS
1195 [Inst] -- Insts free in the RHS; we need these too
1197 | Linear -- Splittable Insts only.
1198 Int -- The Int is always 2 or more; indicates how
1199 -- many copies are required
1200 Inst -- The splitter
1201 Avail -- Where the "master copy" is
1203 | LinRhss -- Splittable Insts only; this is used only internally
1204 -- by extractResults, where a Linear
1205 -- is turned into an LinRhss
1206 [LHsExpr TcId] -- A supply of suitable RHSs
1208 pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1209 | (inst,avail) <- fmToList avails ]
1211 instance Outputable Avail where
1214 pprAvail NoRhs = text "<no rhs>"
1215 pprAvail IsFree = text "Free"
1216 pprAvail Irred = text "Irred"
1217 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1218 if b then text "(used)" else empty
1219 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1220 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1221 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1224 Extracting the bindings from a bunch of Avails.
1225 The bindings do *not* come back sorted in dependency order.
1226 We assume that they'll be wrapped in a big Rec, so that the
1227 dependency analyser can sort them out later
1231 extractResults :: Avails
1233 -> TcM (TcDictBinds, -- Bindings
1234 [Inst], -- Irreducible ones
1235 [Inst]) -- Free ones
1237 extractResults avails wanteds
1238 = go avails emptyBag [] [] wanteds
1240 go avails binds irreds frees []
1241 = returnM (binds, irreds, frees)
1243 go avails binds irreds frees (w:ws)
1244 = case lookupFM avails w of
1245 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1246 go avails binds irreds frees ws
1248 Just NoRhs -> go avails binds irreds frees ws
1249 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1250 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1252 Just (Given id _) -> go avails new_binds irreds frees ws
1254 new_binds | id == instToId w = binds
1255 | otherwise = addBind binds w (L (instSpan w) (HsVar id))
1256 -- The sought Id can be one of the givens, via a superclass chain
1257 -- and then we definitely don't want to generate an x=x binding!
1259 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1261 new_binds = addBind binds w rhs
1263 Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
1264 -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) ->
1265 split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) ->
1266 go (addToFM avails w (LinRhss rhss))
1267 (binds `unionBags` binds')
1268 irreds' frees' (split_inst : w : ws)
1270 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1271 -> go new_avails new_binds irreds frees ws
1273 new_binds = addBind binds w rhs
1274 new_avails = addToFM avails w (LinRhss rhss)
1276 get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
1277 get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
1278 returnM (w':irreds, frees, instToId w')
1279 get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
1280 returnM (irreds, w':frees, instToId w')
1283 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1284 | otherwise = addToFM avails w NoRhs
1285 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1286 -- than Given, else we end up with bogus bindings.
1288 add_free avails w | isMethod w = avails
1289 | otherwise = add_given avails w
1291 -- Do *not* replace Free by Given if it's a method.
1292 -- The following situation shows why this is bad:
1293 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1294 -- From an application (truncate f i) we get
1295 -- t1 = truncate at f
1297 -- If we have also have a second occurrence of truncate, we get
1298 -- t3 = truncate at f
1300 -- When simplifying with i,f free, we might still notice that
1301 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1302 -- will continue to float out!
1303 -- (split n i a) returns: n rhss
1304 -- auxiliary bindings
1305 -- 1 or 0 insts to add to irreds
1308 split :: Int -> TcId -> TcId -> Inst
1309 -> TcM (TcDictBinds, [LHsExpr TcId])
1310 -- (split n split_id root_id wanted) returns
1311 -- * a list of 'n' expressions, all of which witness 'avail'
1312 -- * a bunch of auxiliary bindings to support these expressions
1313 -- * one or zero insts needed to witness the whole lot
1314 -- (maybe be zero if the initial Inst is a Given)
1316 -- NB: 'wanted' is just a template
1318 split n split_id root_id wanted
1321 ty = linearInstType wanted
1322 pair_ty = mkTyConApp pairTyCon [ty,ty]
1323 id = instToId wanted
1326 span = instSpan wanted
1328 go 1 = returnM (emptyBag, [L span $ HsVar root_id])
1330 go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
1331 expand n rhss `thenM` \ (binds2, rhss') ->
1332 returnM (binds1 `unionBags` binds2, rhss')
1335 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1336 -- e.g. expand 3 [rhs1, rhs2]
1337 -- = ( { x = split rhs1 },
1338 -- [fst x, snd x, rhs2] )
1340 | n `rem` 2 == 0 = go rhss -- n is even
1341 | otherwise = go (tail rhss) `thenM` \ (binds', rhss') ->
1342 returnM (binds', head rhss : rhss')
1344 go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') ->
1345 returnM (listToBag binds', concat rhss')
1347 do_one rhs = newUnique `thenM` \ uniq ->
1348 tcLookupId fstName `thenM` \ fst_id ->
1349 tcLookupId sndName `thenM` \ snd_id ->
1351 x = mkUserLocal occ uniq pair_ty loc
1353 returnM (L span (VarBind x (mk_app span split_id rhs)),
1354 [mk_fs_app span fst_id ty x, mk_fs_app span snd_id ty x])
1356 mk_fs_app span id ty var = L span (HsVar id) `mkHsTyApp` [ty,ty] `mkHsApp` (L span (HsVar var))
1358 mk_app span id rhs = L span (HsApp (L span (HsVar id)) rhs)
1360 addBind binds inst rhs = binds `unionBags` unitBag (L (instLocSrcSpan (instLoc inst))
1361 (VarBind (instToId inst) rhs))
1362 instSpan wanted = instLocSrcSpan (instLoc wanted)
1366 %************************************************************************
1368 \subsection[reduce]{@reduce@}
1370 %************************************************************************
1372 When the "what to do" predicate doesn't depend on the quantified type variables,
1373 matters are easier. We don't need to do any zonking, unless the improvement step
1374 does something, in which case we zonk before iterating.
1376 The "given" set is always empty.
1379 simpleReduceLoop :: SDoc
1380 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1382 -> TcM ([Inst], -- Free
1384 [Inst]) -- Irreducible
1386 simpleReduceLoop doc try_me wanteds
1387 = mappM zonkInst wanteds `thenM` \ wanteds' ->
1388 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1389 if no_improvement then
1390 returnM (frees, binds, irreds)
1392 simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
1393 returnM (frees1, binds `unionBags` binds1, irreds1)
1399 reduceContext :: SDoc
1400 -> (Inst -> WhatToDo)
1403 -> TcM (Bool, -- True <=> improve step did no unification
1405 TcDictBinds, -- Dictionary bindings
1406 [Inst]) -- Irreducible
1408 reduceContext doc try_me givens wanteds
1410 traceTc (text "reduceContext" <+> (vcat [
1411 text "----------------------",
1413 text "given" <+> ppr givens,
1414 text "wanted" <+> ppr wanteds,
1415 text "----------------------"
1418 -- Build the Avail mapping from "givens"
1419 foldlM addGiven emptyFM givens `thenM` \ init_state ->
1422 reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
1424 -- Do improvement, using everything in avails
1425 -- In particular, avails includes all superclasses of everything
1426 tcImprove avails `thenM` \ no_improvement ->
1428 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1430 traceTc (text "reduceContext end" <+> (vcat [
1431 text "----------------------",
1433 text "given" <+> ppr givens,
1434 text "wanted" <+> ppr wanteds,
1436 text "avails" <+> pprAvails avails,
1437 text "frees" <+> ppr frees,
1438 text "no_improvement =" <+> ppr no_improvement,
1439 text "----------------------"
1442 returnM (no_improvement, frees, binds, irreds)
1444 tcImprove :: Avails -> TcM Bool -- False <=> no change
1445 -- Perform improvement using all the predicates in Avails
1447 = tcGetInstEnvs `thenM` \ (home_ie, pkg_ie) ->
1449 preds = [ (pred, pp_loc)
1450 | inst <- keysFM avails,
1451 let pp_loc = pprInstLoc (instLoc inst),
1452 pred <- fdPredsOfInst inst
1454 -- Avails has all the superclasses etc (good)
1455 -- It also has all the intermediates of the deduction (good)
1456 -- It does not have duplicates (good)
1457 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1458 -- so that improve will see them separate
1459 eqns = improve get_insts preds
1460 get_insts clas = classInstEnv home_ie clas ++ classInstEnv pkg_ie clas
1465 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
1466 mappM_ unify eqns `thenM_`
1469 unify ((qtvs, t1, t2), doc)
1471 tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1472 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1475 The main context-reduction function is @reduce@. Here's its game plan.
1478 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1479 -- along with its depth
1480 -> (Inst -> WhatToDo)
1487 try_me: given an inst, this function returns
1489 DontReduce return this in "irreds"
1490 Free return this in "frees"
1492 wanteds: The list of insts to reduce
1493 state: An accumulating parameter of type Avails
1494 that contains the state of the algorithm
1496 It returns a Avails.
1498 The (n,stack) pair is just used for error reporting.
1499 n is always the depth of the stack.
1500 The stack is the stack of Insts being reduced: to produce X
1501 I had to produce Y, to produce Y I had to produce Z, and so on.
1504 reduceList (n,stack) try_me wanteds state
1505 | n > opt_MaxContextReductionDepth
1506 = failWithTc (reduceDepthErr n stack)
1512 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1517 go [] state = returnM state
1518 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
1521 -- Base case: we're done!
1522 reduce stack try_me wanted avails
1523 -- It's the same as an existing inst, or a superclass thereof
1524 | Just avail <- isAvailable avails wanted
1525 = if isLinearInst wanted then
1526 addLinearAvailable avails avail wanted `thenM` \ (avails', wanteds') ->
1527 reduceList stack try_me wanteds' avails'
1529 returnM avails -- No op for non-linear things
1532 = case try_me wanted of {
1534 DontReduce want_scs -> addIrred want_scs avails wanted
1536 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1537 -- First, see if the inst can be reduced to a constant in one step
1538 try_simple (addIrred AddSCs) -- Assume want superclasses
1540 ; Free -> -- It's free so just chuck it upstairs
1541 -- First, see if the inst can be reduced to a constant in one step
1544 ; ReduceMe -> -- It should be reduced
1545 lookupInst wanted `thenM` \ lookup_result ->
1546 case lookup_result of
1547 GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 ->
1548 reduceList stack try_me wanteds' avails1 `thenM` \ avails2 ->
1549 addWanted avails2 wanted rhs wanteds'
1550 -- Experiment with temporarily doing addIrred *before* the reduceList,
1551 -- which has the effect of adding the thing we are trying
1552 -- to prove to the database before trying to prove the things it
1553 -- needs. See note [RECURSIVE DICTIONARIES]
1554 -- NB: we must not do an addWanted before, because that adds the
1555 -- superclasses too, and thaat can lead to a spurious loop; see
1556 -- the examples in [SUPERCLASS-LOOP]
1557 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1559 SimpleInst rhs -> addWanted avails wanted rhs []
1561 NoInstance -> -- No such instance!
1562 -- Add it and its superclasses
1563 addIrred AddSCs avails wanted
1566 try_simple do_this_otherwise
1567 = lookupInst wanted `thenM` \ lookup_result ->
1568 case lookup_result of
1569 SimpleInst rhs -> addWanted avails wanted rhs []
1570 other -> do_this_otherwise avails wanted
1575 -------------------------
1576 isAvailable :: Avails -> Inst -> Maybe Avail
1577 isAvailable avails wanted = lookupFM avails wanted
1578 -- NB 1: the Ord instance of Inst compares by the class/type info
1579 -- *not* by unique. So
1580 -- d1::C Int == d2::C Int
1582 addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
1583 addLinearAvailable avails avail wanted
1584 -- avails currently maps [wanted -> avail]
1585 -- Extend avails to reflect a neeed for an extra copy of avail
1587 | Just avail' <- split_avail avail
1588 = returnM (addToFM avails wanted avail', [])
1591 = tcLookupId splitName `thenM` \ split_id ->
1592 tcInstClassOp (instLoc wanted) split_id
1593 [linearInstType wanted] `thenM` \ split_inst ->
1594 returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1597 split_avail :: Avail -> Maybe Avail
1598 -- (Just av) if there's a modified version of avail that
1599 -- we can use to replace avail in avails
1600 -- Nothing if there isn't, so we need to create a Linear
1601 split_avail (Linear n i a) = Just (Linear (n+1) i a)
1602 split_avail (Given id used) | not used = Just (Given id True)
1603 | otherwise = Nothing
1604 split_avail Irred = Nothing
1605 split_avail IsFree = Nothing
1606 split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
1608 -------------------------
1609 addFree :: Avails -> Inst -> TcM Avails
1610 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1611 -- to avails, so that any other equal Insts will be commoned up right
1612 -- here rather than also being tossed upstairs. This is really just
1613 -- an optimisation, and perhaps it is more trouble that it is worth,
1614 -- as the following comments show!
1616 -- NB: do *not* add superclasses. If we have
1619 -- but a is not bound here, then we *don't* want to derive
1620 -- dn from df here lest we lose sharing.
1622 addFree avails free = returnM (addToFM avails free IsFree)
1624 addWanted :: Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
1625 addWanted avails wanted rhs_expr wanteds
1626 = addAvailAndSCs avails wanted avail
1628 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1629 | otherwise = ASSERT( null wanteds ) NoRhs
1631 addGiven :: Avails -> Inst -> TcM Avails
1632 addGiven avails given = addAvailAndSCs avails given (Given (instToId given) False)
1633 -- No ASSERT( not (given `elemFM` avails) ) because in an instance
1634 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
1635 -- so the assert isn't true
1637 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
1638 addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
1639 addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
1640 addAvailAndSCs avails irred Irred
1642 addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
1643 addAvailAndSCs avails inst avail
1644 | not (isClassDict inst) = returnM avails1
1645 | otherwise = traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) `thenM_`
1646 addSCs is_loop avails1 inst
1648 avails1 = addToFM avails inst avail
1649 is_loop inst = any (`tcEqType` idType (instToId inst)) dep_tys
1650 -- Note: this compares by *type*, not by Unique
1651 deps = findAllDeps emptyVarSet avail
1652 dep_tys = map idType (varSetElems deps)
1654 findAllDeps :: IdSet -> Avail -> IdSet
1655 -- Find all the Insts that this one depends on
1656 -- See Note [SUPERCLASS-LOOP]
1657 -- Watch out, though. Since the avails may contain loops
1658 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
1659 findAllDeps so_far (Rhs _ kids)
1661 (extendVarSetList so_far (map instToId kids)) -- Add the kids to so_far
1662 [a | Just a <- map (lookupFM avails) kids] -- Find the kids' Avail
1663 findAllDeps so_far other = so_far
1666 addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
1667 -- Add all the superclasses of the Inst to Avails
1668 -- The first param says "dont do this because the original thing
1669 -- depends on this one, so you'd build a loop"
1670 -- Invariant: the Inst is already in Avails.
1672 addSCs is_loop avails dict
1673 = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
1674 foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1676 (clas, tys) = getDictClassTys dict
1677 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1678 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1680 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1681 | add_me sc_dict = addSCs is_loop avails' sc_dict
1682 | otherwise = returnM avails
1684 sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict]
1685 avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict])
1687 add_me :: Inst -> Bool
1689 | is_loop sc_dict = False -- See Note [SUPERCLASS-LOOP]
1690 | otherwise = case lookupFM avails sc_dict of
1691 Just (Given _ _) -> False -- Given is cheaper than superclass selection
1695 Note [SUPERCLASS-LOOP]: Checking for loops
1696 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1697 We have to be careful here. If we are *given* d1:Ord a,
1698 and want to deduce (d2:C [a]) where
1700 class Ord a => C a where
1701 instance Ord a => C [a] where ...
1703 Then we'll use the instance decl to deduce C [a] and then add the
1704 superclasses of C [a] to avails. But we must not overwrite the binding
1705 for d1:Ord a (which is given) with a superclass selection or we'll just
1708 Here's another variant, immortalised in tcrun020
1709 class Monad m => C1 m
1710 class C1 m => C2 m x
1711 instance C2 Maybe Bool
1712 For the instance decl we need to build (C1 Maybe), and it's no good if
1713 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1714 before we search for C1 Maybe.
1716 Here's another example
1717 class Eq b => Foo a b
1718 instance Eq a => Foo [a] a
1722 we'll first deduce that it holds (via the instance decl). We must not
1723 then overwrite the Eq t constraint with a superclass selection!
1725 At first I had a gross hack, whereby I simply did not add superclass constraints
1726 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1727 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1728 I found a very obscure program (now tcrun021) in which improvement meant the
1729 simplifier got two bites a the cherry... so something seemed to be an Irred
1730 first time, but reducible next time.
1732 Now we implement the Right Solution, which is to check for loops directly
1733 when adding superclasses. It's a bit like the occurs check in unification.
1736 Note [RECURSIVE DICTIONARIES]
1737 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1739 data D r = ZeroD | SuccD (r (D r));
1741 instance (Eq (r (D r))) => Eq (D r) where
1742 ZeroD == ZeroD = True
1743 (SuccD a) == (SuccD b) = a == b
1746 equalDC :: D [] -> D [] -> Bool;
1749 We need to prove (Eq (D [])). Here's how we go:
1753 by instance decl, holds if
1757 by instance decl of Eq, holds if
1759 where d2 = dfEqList d3
1762 But now we can "tie the knot" to give
1768 and it'll even run! The trick is to put the thing we are trying to prove
1769 (in this case Eq (D []) into the database before trying to prove its
1770 contributing clauses.
1773 %************************************************************************
1775 \section{tcSimplifyTop: defaulting}
1777 %************************************************************************
1780 @tcSimplifyTop@ is called once per module to simplify all the constant
1781 and ambiguous Insts.
1783 We need to be careful of one case. Suppose we have
1785 instance Num a => Num (Foo a b) where ...
1787 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1788 to (Num x), and default x to Int. But what about y??
1790 It's OK: the final zonking stage should zap y to (), which is fine.
1794 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
1795 tcSimplifyTop wanteds = tc_simplify_top False {- Not interactive loop -} wanteds
1796 tcSimplifyInteractive wanteds = tc_simplify_top True {- Interactive loop -} wanteds
1799 -- The TcLclEnv should be valid here, solely to improve
1800 -- error message generation for the monomorphism restriction
1801 tc_simplify_top is_interactive wanteds
1802 = getLclEnv `thenM` \ lcl_env ->
1803 traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
1804 simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
1805 ASSERT( null frees )
1808 -- All the non-std ones are definite errors
1809 (stds, non_stds) = partition isStdClassTyVarDict irreds
1811 -- Group by type variable
1812 std_groups = equivClasses cmp_by_tyvar stds
1814 -- Pick the ones which its worth trying to disambiguate
1815 -- namely, the onese whose type variable isn't bound
1816 -- up with one of the non-standard classes
1817 (std_oks, std_bads) = partition worth_a_try std_groups
1818 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1819 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1821 -- Collect together all the bad guys
1822 bad_guys = non_stds ++ concat std_bads
1823 (bad_ips, non_ips) = partition isIPDict bad_guys
1824 (no_insts, ambigs) = partition no_inst non_ips
1825 no_inst d = not (isTyVarDict d)
1826 -- Previously, there was a more elaborate no_inst definition:
1827 -- no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1828 -- fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
1829 -- But that seems over-elaborate to me; it only bites for class decls with
1830 -- fundeps like this: class C a b | -> b where ...
1833 -- Report definite errors
1834 groupErrs (addNoInstanceErrs Nothing []) no_insts `thenM_`
1835 addTopIPErrs bad_ips `thenM_`
1837 -- Deal with ambiguity errors, but only if
1838 -- if there has not been an error so far; errors often
1839 -- give rise to spurious ambiguous Insts
1840 ifErrsM (returnM []) (
1842 -- Complain about the ones that don't fall under
1843 -- the Haskell rules for disambiguation
1844 -- This group includes both non-existent instances
1845 -- e.g. Num (IO a) and Eq (Int -> Int)
1846 -- and ambiguous dictionaries
1848 addTopAmbigErrs ambigs `thenM_`
1850 -- Disambiguate the ones that look feasible
1851 mappM (disambigGroup is_interactive) std_oks
1852 ) `thenM` \ binds_ambig ->
1854 returnM (binds `unionBags` unionManyBags binds_ambig)
1856 ----------------------------------
1857 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1859 get_tv d = case getDictClassTys d of
1860 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1861 get_clas d = case getDictClassTys d of
1862 (clas, [ty]) -> clas
1865 If a dictionary constrains a type variable which is
1866 * not mentioned in the environment
1867 * and not mentioned in the type of the expression
1868 then it is ambiguous. No further information will arise to instantiate
1869 the type variable; nor will it be generalised and turned into an extra
1870 parameter to a function.
1872 It is an error for this to occur, except that Haskell provided for
1873 certain rules to be applied in the special case of numeric types.
1875 * at least one of its classes is a numeric class, and
1876 * all of its classes are numeric or standard
1877 then the type variable can be defaulted to the first type in the
1878 default-type list which is an instance of all the offending classes.
1880 So here is the function which does the work. It takes the ambiguous
1881 dictionaries and either resolves them (producing bindings) or
1882 complains. It works by splitting the dictionary list by type
1883 variable, and using @disambigOne@ to do the real business.
1885 @disambigOne@ assumes that its arguments dictionaries constrain all
1886 the same type variable.
1888 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1889 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1890 the most common use of defaulting is code like:
1892 _ccall_ foo `seqPrimIO` bar
1894 Since we're not using the result of @foo@, the result if (presumably)
1898 disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
1899 -> [Inst] -- All standard classes of form (C a)
1902 disambigGroup is_interactive dicts
1903 | any std_default_class classes -- Guaranteed all standard classes
1904 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1905 -- SO, TRY DEFAULT TYPES IN ORDER
1907 -- Failure here is caused by there being no type in the
1908 -- default list which can satisfy all the ambiguous classes.
1909 -- For example, if Real a is reqd, but the only type in the
1910 -- default list is Int.
1911 get_default_tys `thenM` \ default_tys ->
1913 try_default [] -- No defaults work, so fail
1916 try_default (default_ty : default_tys)
1917 = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
1918 -- default_tys instead
1919 tcSimplifyDefault theta `thenM` \ _ ->
1922 theta = [mkClassPred clas [default_ty] | clas <- classes]
1924 -- See if any default works
1925 tryM (try_default default_tys) `thenM` \ mb_ty ->
1928 Right chosen_default_ty -> choose_default chosen_default_ty
1930 | otherwise -- No defaults
1934 tyvar = get_tv (head dicts) -- Should be non-empty
1935 classes = map get_clas dicts
1937 std_default_class cls
1938 = isNumericClass cls
1939 || (is_interactive &&
1940 classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
1941 -- In interactive mode, we default Show a to Show ()
1942 -- to avoid graututious errors on "show []"
1944 choose_default default_ty -- Commit to tyvar = default_ty
1945 = -- Bind the type variable
1946 unifyTauTy default_ty (mkTyVarTy tyvar) `thenM_`
1947 -- and reduce the context, for real this time
1948 simpleReduceLoop (text "disambig" <+> ppr dicts)
1949 reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
1950 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1951 warnDefault dicts default_ty `thenM_`
1954 bomb_out = addTopAmbigErrs dicts `thenM_`
1958 = do { mb_defaults <- getDefaultTys
1959 ; case mb_defaults of
1960 Just tys -> return tys
1961 Nothing -> -- No use-supplied default;
1962 -- use [Integer, Double]
1963 do { integer_ty <- tcMetaTy integerTyConName
1964 ; return [integer_ty, doubleTy] } }
1967 [Aside - why the defaulting mechanism is turned off when
1968 dealing with arguments and results to ccalls.
1970 When typechecking _ccall_s, TcExpr ensures that the external
1971 function is only passed arguments (and in the other direction,
1972 results) of a restricted set of 'native' types. This is
1973 implemented via the help of the pseudo-type classes,
1974 @CReturnable@ (CR) and @CCallable@ (CC.)
1976 The interaction between the defaulting mechanism for numeric
1977 values and CC & CR can be a bit puzzling to the user at times.
1986 What type has 'x' got here? That depends on the default list
1987 in operation, if it is equal to Haskell 98's default-default
1988 of (Integer, Double), 'x' has type Double, since Integer
1989 is not an instance of CR. If the default list is equal to
1990 Haskell 1.4's default-default of (Int, Double), 'x' has type
1993 To try to minimise the potential for surprises here, the
1994 defaulting mechanism is turned off in the presence of
1995 CCallable and CReturnable.
2000 %************************************************************************
2002 \subsection[simple]{@Simple@ versions}
2004 %************************************************************************
2006 Much simpler versions when there are no bindings to make!
2008 @tcSimplifyThetas@ simplifies class-type constraints formed by
2009 @deriving@ declarations and when specialising instances. We are
2010 only interested in the simplified bunch of class/type constraints.
2012 It simplifies to constraints of the form (C a b c) where
2013 a,b,c are type variables. This is required for the context of
2014 instance declarations.
2017 tcSimplifyDeriv :: [TyVar]
2018 -> ThetaType -- Wanted
2019 -> TcM ThetaType -- Needed
2021 tcSimplifyDeriv tyvars theta
2022 = tcInstTyVars VanillaTv tyvars `thenM` \ (tvs, _, tenv) ->
2023 -- The main loop may do unification, and that may crash if
2024 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2025 -- ToDo: what if two of them do get unified?
2026 newDicts DataDeclOrigin (substTheta tenv theta) `thenM` \ wanteds ->
2027 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
2028 ASSERT( null frees ) -- reduceMe never returns Free
2030 doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
2032 tv_set = mkVarSet tvs
2034 (bad_insts, ok_insts) = partition is_bad_inst irreds
2036 = let pred = dictPred dict -- reduceMe squashes all non-dicts
2037 in isEmptyVarSet (tyVarsOfPred pred)
2038 -- Things like (Eq T) are bad
2039 || (not undecidable_ok && not (isTyVarClassPred pred))
2040 -- The returned dictionaries should be of form (C a b)
2041 -- (where a, b are type variables).
2042 -- We allow non-tyvar dicts if we had -fallow-undecidable-instances,
2043 -- but note that risks non-termination in the 'deriving' context-inference
2044 -- fixpoint loop. It is useful for situations like
2045 -- data Min h a = E | M a (h a)
2046 -- which gives the instance decl
2047 -- instance (Eq a, Eq (h a)) => Eq (Min h a)
2049 simpl_theta = map dictPred ok_insts
2050 weird_preds = [pred | pred <- simpl_theta
2051 , not (tyVarsOfPred pred `subVarSet` tv_set)]
2052 -- Check for a bizarre corner case, when the derived instance decl should
2053 -- have form instance C a b => D (T a) where ...
2054 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
2055 -- of problems; in particular, it's hard to compare solutions for
2056 -- equality when finding the fixpoint. So I just rule it out for now.
2058 rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
2059 -- This reverse-mapping is a Royal Pain,
2060 -- but the result should mention TyVars not TcTyVars
2063 addNoInstanceErrs Nothing [] bad_insts `thenM_`
2064 mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_`
2065 checkAmbiguity tvs simpl_theta tv_set `thenM_`
2066 returnM (substTheta rev_env simpl_theta)
2068 doc = ptext SLIT("deriving classes for a data type")
2071 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2072 used with \tr{default} declarations. We are only interested in
2073 whether it worked or not.
2076 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2079 tcSimplifyDefault theta
2080 = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
2081 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
2082 ASSERT( null frees ) -- try_me never returns Free
2083 addNoInstanceErrs Nothing [] irreds `thenM_`
2089 doc = ptext SLIT("default declaration")
2093 %************************************************************************
2095 \section{Errors and contexts}
2097 %************************************************************************
2099 ToDo: for these error messages, should we note the location as coming
2100 from the insts, or just whatever seems to be around in the monad just
2104 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2105 -> [Inst] -- The offending Insts
2107 -- Group together insts with the same origin
2108 -- We want to report them together in error messages
2110 groupErrs report_err []
2112 groupErrs report_err (inst:insts)
2113 = do_one (inst:friends) `thenM_`
2114 groupErrs report_err others
2117 -- (It may seem a bit crude to compare the error messages,
2118 -- but it makes sure that we combine just what the user sees,
2119 -- and it avoids need equality on InstLocs.)
2120 (friends, others) = partition is_friend insts
2121 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2122 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2123 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2124 -- Add location and context information derived from the Insts
2126 -- Add the "arising from..." part to a message about bunch of dicts
2127 addInstLoc :: [Inst] -> Message -> Message
2128 addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
2131 plural xs = char 's'
2134 = groupErrs report tidy_dicts
2136 (tidy_env, tidy_dicts) = tidyInsts dicts
2137 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2138 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2139 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2141 addNoInstanceErrs :: Maybe SDoc -- Nothing => top level
2142 -- Just d => d describes the construct
2143 -> [Inst] -- What is given by the context or type sig
2144 -> [Inst] -- What is wanted
2146 addNoInstanceErrs mb_what givens []
2148 addNoInstanceErrs mb_what givens dicts
2149 = -- Some of the dicts are here because there is no instances
2150 -- and some because there are too many instances (overlap)
2151 -- The first thing we do is separate them
2152 getDOpts `thenM` \ dflags ->
2153 tcGetInstEnvs `thenM` \ inst_envs ->
2155 (tidy_env1, tidy_givens) = tidyInsts givens
2156 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
2158 -- Run through the dicts, generating a message for each
2159 -- overlapping one, but simply accumulating all the
2160 -- no-instance ones so they can be reported as a group
2161 (overlap_doc, no_inst_dicts) = foldl check_overlap (empty, []) tidy_dicts
2162 check_overlap (overlap_doc, no_inst_dicts) dict
2163 | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts)
2165 = case lookupInstEnv dflags inst_envs clas tys of
2167 | length ms > 1 -> (mk_overlap_msg dict res $$ overlap_doc, no_inst_dicts)
2168 | otherwise -> (overlap_doc, dict : no_inst_dicts) -- No match
2169 -- NB: there can be exactly one match, in the case where we have
2170 -- instance C a where ...
2171 -- (In this case, lookupInst doesn't bother to look up,
2172 -- unless -fallow-undecidable-instances is set.)
2173 -- So we report this as "no instance" rather than "overlap"; the fix is
2174 -- to specify -fallow-undecidable-instances, but we leave that to the programmer!
2176 (clas,tys) = getDictClassTys dict
2178 mk_probable_fix tidy_env2 mb_what no_inst_dicts `thenM` \ (tidy_env3, probable_fix) ->
2180 no_inst_doc | null no_inst_dicts = empty
2181 | otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix]
2182 heading | null givens = ptext SLIT("No instance") <> plural no_inst_dicts <+>
2183 ptext SLIT("for") <+> pprDictsTheta no_inst_dicts
2184 | otherwise = sep [ptext SLIT("Could not deduce") <+> pprDictsTheta no_inst_dicts,
2185 nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta tidy_givens]
2187 addErrTcM (tidy_env3, no_inst_doc $$ overlap_doc)
2190 mk_overlap_msg dict (matches, unifiers)
2191 = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
2192 <+> pprPred (dictPred dict))),
2193 sep [ptext SLIT("Matching instances") <> colon,
2194 nest 2 (pprDFuns (dfuns ++ unifiers))],
2197 else parens (ptext SLIT("The choice depends on the instantiation of") <+>
2198 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))))]
2200 dfuns = [df | (_, (_,_,df)) <- matches]
2202 mk_probable_fix tidy_env Nothing dicts -- Top level
2203 = mkMonomorphismMsg tidy_env dicts
2204 mk_probable_fix tidy_env (Just what) dicts -- Nested (type signatures, instance decls)
2205 = returnM (tidy_env, sep [ptext SLIT("Probable fix:"), nest 2 fix1, nest 2 fix2])
2207 fix1 = sep [ptext SLIT("Add") <+> pprDictsTheta dicts,
2208 ptext SLIT("to the") <+> what]
2210 fix2 | null instance_dicts = empty
2211 | otherwise = ptext SLIT("Or add an instance declaration for")
2212 <+> pprDictsTheta instance_dicts
2213 instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
2214 -- Insts for which it is worth suggesting an adding an instance declaration
2215 -- Exclude implicit parameters, and tyvar dicts
2218 addTopAmbigErrs dicts
2219 -- Divide into groups that share a common set of ambiguous tyvars
2220 = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
2222 (tidy_env, tidy_dicts) = tidyInsts dicts
2224 tvs_of :: Inst -> [TcTyVar]
2225 tvs_of d = varSetElems (tyVarsOfInst d)
2226 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
2228 report :: [(Inst,[TcTyVar])] -> TcM ()
2229 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
2230 = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
2231 addSrcSpan (instLocSrcSpan (instLoc inst)) $
2232 -- the location of the first one will do for the err message
2233 addErrTcM (tidy_env, msg $$ mono_msg)
2235 dicts = map fst pairs
2236 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
2237 pprQuotedList tvs <+> in_msg,
2238 nest 2 (pprDictsInFull dicts)]
2239 in_msg | isSingleton dicts = text "in the top-level constraint:"
2240 | otherwise = text "in these top-level constraints:"
2243 mkMonomorphismMsg :: TidyEnv -> [Inst] -> TcM (TidyEnv, Message)
2244 -- There's an error with these Insts; if they have free type variables
2245 -- it's probably caused by the monomorphism restriction.
2246 -- Try to identify the offending variable
2247 -- ASSUMPTION: the Insts are fully zonked
2248 mkMonomorphismMsg tidy_env insts
2249 | isEmptyVarSet inst_tvs
2250 = returnM (tidy_env, empty)
2252 = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
2253 returnM (tidy_env, mk_msg docs)
2256 inst_tvs = tyVarsOfInsts insts
2258 mk_msg [] = empty -- This happens in things like
2259 -- f x = show (read "foo")
2260 -- whre monomorphism doesn't play any role
2261 mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
2263 ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
2265 warnDefault dicts default_ty
2266 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
2267 addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
2270 (_, tidy_dicts) = tidyInsts dicts
2271 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
2272 quotes (ppr default_ty),
2273 pprDictsInFull tidy_dicts]
2275 -- Used for the ...Thetas variants; all top level
2277 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2278 ptext SLIT("type variables that are not data type parameters"),
2279 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2281 reduceDepthErr n stack
2282 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2283 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
2284 nest 4 (pprDictsInFull stack)]
2286 reduceDepthMsg n stack = nest 4 (pprDictsInFull stack)