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, pprInstInFull, tcGetInstEnvs,
39 isIPDict, isInheritableInst, pprDFuns, pprDictsTheta
41 import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals )
42 import InstEnv ( lookupInstEnv, classInstances )
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 (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
902 and that in turn 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.
915 A note about Plan C (arising from "bug" reported by George Russel March 2004)
918 instance (HasBinary ty IO) => HasCodedValue ty
920 foo :: HasCodedValue a => String -> IO a
922 doDecodeIO :: HasCodedValue a => () -> () -> IO a
923 doDecodeIO codedValue view
924 = let { act = foo "foo" } in act
926 You might think this should work becuase the call to foo gives rise to a constraint
927 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
928 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
929 constraint using the (rather bogus) instance declaration, and now we are stuffed.
931 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
935 Plan D (a variant of plan B)
936 Step 1: Simplify the constraints as much as possible (to deal
937 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
938 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
940 Step 2: Now simplify again, treating the constraint as 'free' if
941 it does not mention qtvs, and trying to reduce it otherwise.
943 The point here is that it's generally OK to have too few qtvs; that is,
944 to make the thing more monomorphic than it could be. We don't want to
945 do that in the common cases, but in wierd cases it's ok: the programmer
946 can always add a signature.
948 Too few qtvs => too many wanteds, which is what happens if you do less
953 tcSimplifyRestricted -- Used for restricted binding groups
954 -- i.e. ones subject to the monomorphism restriction
956 -> TcTyVarSet -- Free in the type of the RHSs
957 -> [Inst] -- Free in the RHSs
958 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
959 TcDictBinds) -- Bindings
960 -- tcSimpifyRestricted returns no constraints to
961 -- quantify over; by definition there are none.
962 -- They are all thrown back in the LIE
964 tcSimplifyRestricted doc tau_tvs wanteds
965 -- Zonk everything in sight
966 = mappM zonkInst wanteds `thenM` \ wanteds' ->
967 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
968 tcGetGlobalTyVars `thenM` \ gbl_tvs' ->
970 -- 'reduceMe': Reduce as far as we can. Don't stop at
971 -- dicts; the idea is to get rid of as many type
972 -- variables as possible, and we don't want to stop
973 -- at (say) Monad (ST s), because that reduces
974 -- immediately, with no constraint on s.
976 -- BUT do no improvement! See Plan D above
977 reduceContextWithoutImprovement
978 doc reduceMe wanteds' `thenM` \ (_frees, _binds, constrained_dicts) ->
980 -- Next, figure out the tyvars we will quantify over
982 constrained_tvs = tyVarsOfInsts constrained_dicts
983 qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
984 `minusVarSet` constrained_tvs
985 try_me inst | isFreeWrtTyVars qtvs inst = Free
986 | otherwise = ReduceMe
988 traceTc (text "tcSimplifyRestricted" <+> vcat [
989 pprInsts wanteds, pprInsts _frees, pprInsts constrained_dicts,
991 ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_`
993 -- The first step may have squashed more methods than
994 -- necessary, so try again, this time more gently, knowing the exact
995 -- set of type variables to quantify over.
997 -- We quantify only over constraints that are captured by qtvs;
998 -- these will just be a subset of non-dicts. This in contrast
999 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1000 -- all *non-inheritable* constraints too. This implements choice
1001 -- (B) under "implicit parameter and monomorphism" above.
1003 -- Remember that we may need to do *some* simplification, to
1004 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1005 -- just to float all constraints
1006 reduceContextWithoutImprovement
1007 doc try_me wanteds' `thenM` \ (frees, binds, irreds) ->
1008 ASSERT( null irreds )
1009 extendLIEs frees `thenM_`
1010 returnM (varSetElems qtvs, binds)
1014 %************************************************************************
1016 \subsection{tcSimplifyToDicts}
1018 %************************************************************************
1020 On the LHS of transformation rules we only simplify methods and constants,
1021 getting dictionaries. We want to keep all of them unsimplified, to serve
1022 as the available stuff for the RHS of the rule.
1024 The same thing is used for specialise pragmas. Consider
1026 f :: Num a => a -> a
1027 {-# SPECIALISE f :: Int -> Int #-}
1030 The type checker generates a binding like:
1032 f_spec = (f :: Int -> Int)
1034 and we want to end up with
1036 f_spec = _inline_me_ (f Int dNumInt)
1038 But that means that we must simplify the Method for f to (f Int dNumInt)!
1039 So tcSimplifyToDicts squeezes out all Methods.
1041 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
1043 fromIntegral :: (Integral a, Num b) => a -> b
1044 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1046 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
1049 forall dIntegralInt.
1050 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1052 because the scsel will mess up matching. Instead we want
1054 forall dIntegralInt, dNumInt.
1055 fromIntegral Int Int dIntegralInt dNumInt = id Int
1057 Hence "DontReduce NoSCs"
1060 tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds)
1061 tcSimplifyToDicts wanteds
1062 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
1063 -- Since try_me doesn't look at types, we don't need to
1064 -- do any zonking, so it's safe to call reduceContext directly
1065 ASSERT( null frees )
1066 extendLIEs irreds `thenM_`
1070 doc = text "tcSimplifyToDicts"
1072 -- Reduce methods and lits only; stop as soon as we get a dictionary
1073 try_me inst | isDict inst = DontReduce NoSCs -- See notes above for why NoSCs
1074 | otherwise = ReduceMe
1079 tcSimplifyBracket is used when simplifying the constraints arising from
1080 a Template Haskell bracket [| ... |]. We want to check that there aren't
1081 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1082 Show instance), but we aren't otherwise interested in the results.
1083 Nor do we care about ambiguous dictionaries etc. We will type check
1084 this bracket again at its usage site.
1087 tcSimplifyBracket :: [Inst] -> TcM ()
1088 tcSimplifyBracket wanteds
1089 = simpleReduceLoop doc reduceMe wanteds `thenM_`
1092 doc = text "tcSimplifyBracket"
1096 %************************************************************************
1098 \subsection{Filtering at a dynamic binding}
1100 %************************************************************************
1105 we must discharge all the ?x constraints from B. We also do an improvement
1106 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1108 Actually, the constraints from B might improve the types in ?x. For example
1110 f :: (?x::Int) => Char -> Char
1113 then the constraint (?x::Int) arising from the call to f will
1114 force the binding for ?x to be of type Int.
1117 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1120 tcSimplifyIPs given_ips wanteds
1121 = simpl_loop given_ips wanteds `thenM` \ (frees, binds) ->
1122 extendLIEs frees `thenM_`
1125 doc = text "tcSimplifyIPs" <+> ppr given_ips
1126 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1128 -- Simplify any methods that mention the implicit parameter
1129 try_me inst | isFreeWrtIPs ip_set inst = Free
1130 | otherwise = ReduceMe
1132 simpl_loop givens wanteds
1133 = mappM zonkInst givens `thenM` \ givens' ->
1134 mappM zonkInst wanteds `thenM` \ wanteds' ->
1136 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1138 if no_improvement then
1139 ASSERT( null irreds )
1140 returnM (frees, binds)
1142 simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) ->
1143 returnM (frees1, binds `unionBags` binds1)
1147 %************************************************************************
1149 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1151 %************************************************************************
1153 When doing a binding group, we may have @Insts@ of local functions.
1154 For example, we might have...
1156 let f x = x + 1 -- orig local function (overloaded)
1157 f.1 = f Int -- two instances of f
1162 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1163 where @f@ is in scope; those @Insts@ must certainly not be passed
1164 upwards towards the top-level. If the @Insts@ were binding-ified up
1165 there, they would have unresolvable references to @f@.
1167 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1168 For each method @Inst@ in the @init_lie@ that mentions one of the
1169 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1170 @LIE@), as well as the @HsBinds@ generated.
1173 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM (LHsBinds TcId)
1175 bindInstsOfLocalFuns wanteds local_ids
1176 | null overloaded_ids
1178 = extendLIEs wanteds `thenM_`
1182 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
1183 ASSERT( null irreds )
1184 extendLIEs frees `thenM_`
1187 doc = text "bindInsts" <+> ppr local_ids
1188 overloaded_ids = filter is_overloaded local_ids
1189 is_overloaded id = isOverloadedTy (idType id)
1191 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1192 -- so it's worth building a set, so that
1193 -- lookup (in isMethodFor) is faster
1195 try_me inst | isMethodFor overloaded_set inst = ReduceMe
1200 %************************************************************************
1202 \subsection{Data types for the reduction mechanism}
1204 %************************************************************************
1206 The main control over context reduction is here
1210 = ReduceMe -- Try to reduce this
1211 -- If there's no instance, behave exactly like
1212 -- DontReduce: add the inst to
1213 -- the irreductible ones, but don't
1214 -- produce an error message of any kind.
1215 -- It might be quite legitimate such as (Eq a)!
1217 | DontReduce WantSCs -- Return as irreducible
1219 | DontReduceUnlessConstant -- Return as irreducible unless it can
1220 -- be reduced to a constant in one step
1222 | Free -- Return as free
1224 reduceMe :: Inst -> WhatToDo
1225 reduceMe inst = ReduceMe
1227 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1228 -- of a predicate when adding it to the avails
1234 type Avails = FiniteMap Inst Avail
1235 emptyAvails = emptyFM
1238 = IsFree -- Used for free Insts
1239 | Irred -- Used for irreducible dictionaries,
1240 -- which are going to be lambda bound
1242 | Given TcId -- Used for dictionaries for which we have a binding
1243 -- e.g. those "given" in a signature
1244 Bool -- True <=> actually consumed (splittable IPs only)
1246 | NoRhs -- Used for Insts like (CCallable f)
1247 -- where no witness is required.
1250 | Rhs -- Used when there is a RHS
1251 (LHsExpr TcId) -- The RHS
1252 [Inst] -- Insts free in the RHS; we need these too
1254 | Linear -- Splittable Insts only.
1255 Int -- The Int is always 2 or more; indicates how
1256 -- many copies are required
1257 Inst -- The splitter
1258 Avail -- Where the "master copy" is
1260 | LinRhss -- Splittable Insts only; this is used only internally
1261 -- by extractResults, where a Linear
1262 -- is turned into an LinRhss
1263 [LHsExpr TcId] -- A supply of suitable RHSs
1265 pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1266 | (inst,avail) <- fmToList avails ]
1268 instance Outputable Avail where
1271 pprAvail NoRhs = text "<no rhs>"
1272 pprAvail IsFree = text "Free"
1273 pprAvail Irred = text "Irred"
1274 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1275 if b then text "(used)" else empty
1276 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1277 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1278 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1281 Extracting the bindings from a bunch of Avails.
1282 The bindings do *not* come back sorted in dependency order.
1283 We assume that they'll be wrapped in a big Rec, so that the
1284 dependency analyser can sort them out later
1288 extractResults :: Avails
1290 -> TcM (TcDictBinds, -- Bindings
1291 [Inst], -- Irreducible ones
1292 [Inst]) -- Free ones
1294 extractResults avails wanteds
1295 = go avails emptyBag [] [] wanteds
1297 go avails binds irreds frees []
1298 = returnM (binds, irreds, frees)
1300 go avails binds irreds frees (w:ws)
1301 = case lookupFM avails w of
1302 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1303 go avails binds irreds frees ws
1305 Just NoRhs -> go avails binds irreds frees ws
1306 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1307 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1309 Just (Given id _) -> go avails new_binds irreds frees ws
1311 new_binds | id == instToId w = binds
1312 | otherwise = addBind binds w (L (instSpan w) (HsVar id))
1313 -- The sought Id can be one of the givens, via a superclass chain
1314 -- and then we definitely don't want to generate an x=x binding!
1316 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1318 new_binds = addBind binds w rhs
1320 Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
1321 -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) ->
1322 split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) ->
1323 go (addToFM avails w (LinRhss rhss))
1324 (binds `unionBags` binds')
1325 irreds' frees' (split_inst : w : ws)
1327 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1328 -> go new_avails new_binds irreds frees ws
1330 new_binds = addBind binds w rhs
1331 new_avails = addToFM avails w (LinRhss rhss)
1333 get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
1334 get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
1335 returnM (w':irreds, frees, instToId w')
1336 get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
1337 returnM (irreds, w':frees, instToId w')
1340 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1341 | otherwise = addToFM avails w NoRhs
1342 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1343 -- than Given, else we end up with bogus bindings.
1345 add_free avails w | isMethod w = avails
1346 | otherwise = add_given avails w
1348 -- Do *not* replace Free by Given if it's a method.
1349 -- The following situation shows why this is bad:
1350 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1351 -- From an application (truncate f i) we get
1352 -- t1 = truncate at f
1354 -- If we have also have a second occurrence of truncate, we get
1355 -- t3 = truncate at f
1357 -- When simplifying with i,f free, we might still notice that
1358 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1359 -- will continue to float out!
1360 -- (split n i a) returns: n rhss
1361 -- auxiliary bindings
1362 -- 1 or 0 insts to add to irreds
1365 split :: Int -> TcId -> TcId -> Inst
1366 -> TcM (TcDictBinds, [LHsExpr TcId])
1367 -- (split n split_id root_id wanted) returns
1368 -- * a list of 'n' expressions, all of which witness 'avail'
1369 -- * a bunch of auxiliary bindings to support these expressions
1370 -- * one or zero insts needed to witness the whole lot
1371 -- (maybe be zero if the initial Inst is a Given)
1373 -- NB: 'wanted' is just a template
1375 split n split_id root_id wanted
1378 ty = linearInstType wanted
1379 pair_ty = mkTyConApp pairTyCon [ty,ty]
1380 id = instToId wanted
1383 span = instSpan wanted
1385 go 1 = returnM (emptyBag, [L span $ HsVar root_id])
1387 go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
1388 expand n rhss `thenM` \ (binds2, rhss') ->
1389 returnM (binds1 `unionBags` binds2, rhss')
1392 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1393 -- e.g. expand 3 [rhs1, rhs2]
1394 -- = ( { x = split rhs1 },
1395 -- [fst x, snd x, rhs2] )
1397 | n `rem` 2 == 0 = go rhss -- n is even
1398 | otherwise = go (tail rhss) `thenM` \ (binds', rhss') ->
1399 returnM (binds', head rhss : rhss')
1401 go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') ->
1402 returnM (listToBag binds', concat rhss')
1404 do_one rhs = newUnique `thenM` \ uniq ->
1405 tcLookupId fstName `thenM` \ fst_id ->
1406 tcLookupId sndName `thenM` \ snd_id ->
1408 x = mkUserLocal occ uniq pair_ty loc
1410 returnM (L span (VarBind x (mk_app span split_id rhs)),
1411 [mk_fs_app span fst_id ty x, mk_fs_app span snd_id ty x])
1413 mk_fs_app span id ty var = L span (HsVar id) `mkHsTyApp` [ty,ty] `mkHsApp` (L span (HsVar var))
1415 mk_app span id rhs = L span (HsApp (L span (HsVar id)) rhs)
1417 addBind binds inst rhs = binds `unionBags` unitBag (L (instLocSrcSpan (instLoc inst))
1418 (VarBind (instToId inst) rhs))
1419 instSpan wanted = instLocSrcSpan (instLoc wanted)
1423 %************************************************************************
1425 \subsection[reduce]{@reduce@}
1427 %************************************************************************
1429 When the "what to do" predicate doesn't depend on the quantified type variables,
1430 matters are easier. We don't need to do any zonking, unless the improvement step
1431 does something, in which case we zonk before iterating.
1433 The "given" set is always empty.
1436 simpleReduceLoop :: SDoc
1437 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1439 -> TcM ([Inst], -- Free
1441 [Inst]) -- Irreducible
1443 simpleReduceLoop doc try_me wanteds
1444 = mappM zonkInst wanteds `thenM` \ wanteds' ->
1445 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1446 if no_improvement then
1447 returnM (frees, binds, irreds)
1449 simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
1450 returnM (frees1, binds `unionBags` binds1, irreds1)
1456 reduceContext :: SDoc
1457 -> (Inst -> WhatToDo)
1460 -> TcM (Bool, -- True <=> improve step did no unification
1462 TcDictBinds, -- Dictionary bindings
1463 [Inst]) -- Irreducible
1465 reduceContext doc try_me givens wanteds
1467 traceTc (text "reduceContext" <+> (vcat [
1468 text "----------------------",
1470 text "given" <+> ppr givens,
1471 text "wanted" <+> ppr wanteds,
1472 text "----------------------"
1475 -- Build the Avail mapping from "givens"
1476 foldlM addGiven emptyAvails givens `thenM` \ init_state ->
1479 reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
1481 -- Do improvement, using everything in avails
1482 -- In particular, avails includes all superclasses of everything
1483 tcImprove avails `thenM` \ no_improvement ->
1485 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1487 traceTc (text "reduceContext end" <+> (vcat [
1488 text "----------------------",
1490 text "given" <+> ppr givens,
1491 text "wanted" <+> ppr wanteds,
1493 text "avails" <+> pprAvails avails,
1494 text "frees" <+> ppr frees,
1495 text "no_improvement =" <+> ppr no_improvement,
1496 text "----------------------"
1499 returnM (no_improvement, frees, binds, irreds)
1501 -- reduceContextWithoutImprovement differs from reduceContext
1502 -- (a) no improvement
1503 -- (b) 'givens' is assumed empty
1504 reduceContextWithoutImprovement doc try_me wanteds
1506 traceTc (text "reduceContextWithoutImprovement" <+> (vcat [
1507 text "----------------------",
1509 text "wanted" <+> ppr wanteds,
1510 text "----------------------"
1514 reduceList (0,[]) try_me wanteds emptyAvails `thenM` \ avails ->
1515 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1517 traceTc (text "reduceContextWithoutImprovement end" <+> (vcat [
1518 text "----------------------",
1520 text "wanted" <+> ppr wanteds,
1522 text "avails" <+> pprAvails avails,
1523 text "frees" <+> ppr frees,
1524 text "----------------------"
1527 returnM (frees, binds, irreds)
1529 tcImprove :: Avails -> TcM Bool -- False <=> no change
1530 -- Perform improvement using all the predicates in Avails
1532 = tcGetInstEnvs `thenM` \ (home_ie, pkg_ie) ->
1534 preds = [ (pred, pp_loc)
1535 | inst <- keysFM avails,
1536 let pp_loc = pprInstLoc (instLoc inst),
1537 pred <- fdPredsOfInst inst
1539 -- Avails has all the superclasses etc (good)
1540 -- It also has all the intermediates of the deduction (good)
1541 -- It does not have duplicates (good)
1542 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1543 -- so that improve will see them separate
1544 eqns = improve get_insts preds
1545 get_insts clas = classInstances home_ie clas ++ classInstances pkg_ie clas
1550 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
1551 mappM_ unify eqns `thenM_`
1554 unify ((qtvs, t1, t2), doc)
1556 tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1557 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1560 The main context-reduction function is @reduce@. Here's its game plan.
1563 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1564 -- along with its depth
1565 -> (Inst -> WhatToDo)
1572 try_me: given an inst, this function returns
1574 DontReduce return this in "irreds"
1575 Free return this in "frees"
1577 wanteds: The list of insts to reduce
1578 state: An accumulating parameter of type Avails
1579 that contains the state of the algorithm
1581 It returns a Avails.
1583 The (n,stack) pair is just used for error reporting.
1584 n is always the depth of the stack.
1585 The stack is the stack of Insts being reduced: to produce X
1586 I had to produce Y, to produce Y I had to produce Z, and so on.
1589 reduceList (n,stack) try_me wanteds state
1590 | n > opt_MaxContextReductionDepth
1591 = failWithTc (reduceDepthErr n stack)
1597 pprTrace "Interesting! Context reduction stack deeper than 8:"
1598 (nest 2 (pprStack stack))
1603 go [] state = returnM state
1604 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
1607 -- Base case: we're done!
1608 reduce stack try_me wanted avails
1609 -- It's the same as an existing inst, or a superclass thereof
1610 | Just avail <- isAvailable avails wanted
1611 = if isLinearInst wanted then
1612 addLinearAvailable avails avail wanted `thenM` \ (avails', wanteds') ->
1613 reduceList stack try_me wanteds' avails'
1615 returnM avails -- No op for non-linear things
1618 = case try_me wanted of {
1620 DontReduce want_scs -> addIrred want_scs avails wanted
1622 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1623 -- First, see if the inst can be reduced to a constant in one step
1624 try_simple (addIrred AddSCs) -- Assume want superclasses
1626 ; Free -> -- It's free so just chuck it upstairs
1627 -- First, see if the inst can be reduced to a constant in one step
1630 ; ReduceMe -> -- It should be reduced
1631 lookupInst wanted `thenM` \ lookup_result ->
1632 case lookup_result of
1633 GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 ->
1634 reduceList stack try_me wanteds' avails1 `thenM` \ avails2 ->
1635 addWanted avails2 wanted rhs wanteds'
1636 -- Experiment with temporarily doing addIrred *before* the reduceList,
1637 -- which has the effect of adding the thing we are trying
1638 -- to prove to the database before trying to prove the things it
1639 -- needs. See note [RECURSIVE DICTIONARIES]
1640 -- NB: we must not do an addWanted before, because that adds the
1641 -- superclasses too, and thaat can lead to a spurious loop; see
1642 -- the examples in [SUPERCLASS-LOOP]
1643 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1645 SimpleInst rhs -> addWanted avails wanted rhs []
1647 NoInstance -> -- No such instance!
1648 -- Add it and its superclasses
1649 addIrred AddSCs avails wanted
1652 try_simple do_this_otherwise
1653 = lookupInst wanted `thenM` \ lookup_result ->
1654 case lookup_result of
1655 SimpleInst rhs -> addWanted avails wanted rhs []
1656 other -> do_this_otherwise avails wanted
1661 -------------------------
1662 isAvailable :: Avails -> Inst -> Maybe Avail
1663 isAvailable avails wanted = lookupFM avails wanted
1664 -- NB 1: the Ord instance of Inst compares by the class/type info
1665 -- *not* by unique. So
1666 -- d1::C Int == d2::C Int
1668 addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
1669 addLinearAvailable avails avail wanted
1670 -- avails currently maps [wanted -> avail]
1671 -- Extend avails to reflect a neeed for an extra copy of avail
1673 | Just avail' <- split_avail avail
1674 = returnM (addToFM avails wanted avail', [])
1677 = tcLookupId splitName `thenM` \ split_id ->
1678 tcInstClassOp (instLoc wanted) split_id
1679 [linearInstType wanted] `thenM` \ split_inst ->
1680 returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1683 split_avail :: Avail -> Maybe Avail
1684 -- (Just av) if there's a modified version of avail that
1685 -- we can use to replace avail in avails
1686 -- Nothing if there isn't, so we need to create a Linear
1687 split_avail (Linear n i a) = Just (Linear (n+1) i a)
1688 split_avail (Given id used) | not used = Just (Given id True)
1689 | otherwise = Nothing
1690 split_avail Irred = Nothing
1691 split_avail IsFree = Nothing
1692 split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
1694 -------------------------
1695 addFree :: Avails -> Inst -> TcM Avails
1696 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1697 -- to avails, so that any other equal Insts will be commoned up right
1698 -- here rather than also being tossed upstairs. This is really just
1699 -- an optimisation, and perhaps it is more trouble that it is worth,
1700 -- as the following comments show!
1702 -- NB: do *not* add superclasses. If we have
1705 -- but a is not bound here, then we *don't* want to derive
1706 -- dn from df here lest we lose sharing.
1708 addFree avails free = returnM (addToFM avails free IsFree)
1710 addWanted :: Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
1711 addWanted avails wanted rhs_expr wanteds
1712 = addAvailAndSCs avails wanted avail
1714 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1715 | otherwise = ASSERT( null wanteds ) NoRhs
1717 addGiven :: Avails -> Inst -> TcM Avails
1718 addGiven avails given = addAvailAndSCs avails given (Given (instToId given) False)
1719 -- No ASSERT( not (given `elemFM` avails) ) because in an instance
1720 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
1721 -- so the assert isn't true
1723 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
1724 addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
1725 addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
1726 addAvailAndSCs avails irred Irred
1728 addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
1729 addAvailAndSCs avails inst avail
1730 | not (isClassDict inst) = returnM avails1
1731 | otherwise = traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) `thenM_`
1732 addSCs is_loop avails1 inst
1734 avails1 = addToFM avails inst avail
1735 is_loop inst = any (`tcEqType` idType (instToId inst)) dep_tys
1736 -- Note: this compares by *type*, not by Unique
1737 deps = findAllDeps emptyVarSet avail
1738 dep_tys = map idType (varSetElems deps)
1740 findAllDeps :: IdSet -> Avail -> IdSet
1741 -- Find all the Insts that this one depends on
1742 -- See Note [SUPERCLASS-LOOP]
1743 -- Watch out, though. Since the avails may contain loops
1744 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
1745 findAllDeps so_far (Rhs _ kids)
1747 (extendVarSetList so_far (map instToId kids)) -- Add the kids to so_far
1748 [a | Just a <- map (lookupFM avails) kids] -- Find the kids' Avail
1749 findAllDeps so_far other = so_far
1752 addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
1753 -- Add all the superclasses of the Inst to Avails
1754 -- The first param says "dont do this because the original thing
1755 -- depends on this one, so you'd build a loop"
1756 -- Invariant: the Inst is already in Avails.
1758 addSCs is_loop avails dict
1759 = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
1760 foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1762 (clas, tys) = getDictClassTys dict
1763 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1764 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1766 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1767 | add_me sc_dict = addSCs is_loop avails' sc_dict
1768 | otherwise = returnM avails
1770 sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict]
1771 avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict])
1773 add_me :: Inst -> Bool
1775 | is_loop sc_dict = False -- See Note [SUPERCLASS-LOOP]
1776 | otherwise = case lookupFM avails sc_dict of
1777 Just (Given _ _) -> False -- Given is cheaper than superclass selection
1781 Note [SUPERCLASS-LOOP]: Checking for loops
1782 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1783 We have to be careful here. If we are *given* d1:Ord a,
1784 and want to deduce (d2:C [a]) where
1786 class Ord a => C a where
1787 instance Ord a => C [a] where ...
1789 Then we'll use the instance decl to deduce C [a] and then add the
1790 superclasses of C [a] to avails. But we must not overwrite the binding
1791 for d1:Ord a (which is given) with a superclass selection or we'll just
1794 Here's another variant, immortalised in tcrun020
1795 class Monad m => C1 m
1796 class C1 m => C2 m x
1797 instance C2 Maybe Bool
1798 For the instance decl we need to build (C1 Maybe), and it's no good if
1799 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1800 before we search for C1 Maybe.
1802 Here's another example
1803 class Eq b => Foo a b
1804 instance Eq a => Foo [a] a
1808 we'll first deduce that it holds (via the instance decl). We must not
1809 then overwrite the Eq t constraint with a superclass selection!
1811 At first I had a gross hack, whereby I simply did not add superclass constraints
1812 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1813 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1814 I found a very obscure program (now tcrun021) in which improvement meant the
1815 simplifier got two bites a the cherry... so something seemed to be an Irred
1816 first time, but reducible next time.
1818 Now we implement the Right Solution, which is to check for loops directly
1819 when adding superclasses. It's a bit like the occurs check in unification.
1822 Note [RECURSIVE DICTIONARIES]
1823 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1825 data D r = ZeroD | SuccD (r (D r));
1827 instance (Eq (r (D r))) => Eq (D r) where
1828 ZeroD == ZeroD = True
1829 (SuccD a) == (SuccD b) = a == b
1832 equalDC :: D [] -> D [] -> Bool;
1835 We need to prove (Eq (D [])). Here's how we go:
1839 by instance decl, holds if
1843 by instance decl of Eq, holds if
1845 where d2 = dfEqList d3
1848 But now we can "tie the knot" to give
1854 and it'll even run! The trick is to put the thing we are trying to prove
1855 (in this case Eq (D []) into the database before trying to prove its
1856 contributing clauses.
1859 %************************************************************************
1861 \section{tcSimplifyTop: defaulting}
1863 %************************************************************************
1866 @tcSimplifyTop@ is called once per module to simplify all the constant
1867 and ambiguous Insts.
1869 We need to be careful of one case. Suppose we have
1871 instance Num a => Num (Foo a b) where ...
1873 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1874 to (Num x), and default x to Int. But what about y??
1876 It's OK: the final zonking stage should zap y to (), which is fine.
1880 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
1881 tcSimplifyTop wanteds = tc_simplify_top False {- Not interactive loop -} wanteds
1882 tcSimplifyInteractive wanteds = tc_simplify_top True {- Interactive loop -} wanteds
1885 -- The TcLclEnv should be valid here, solely to improve
1886 -- error message generation for the monomorphism restriction
1887 tc_simplify_top is_interactive wanteds
1888 = getLclEnv `thenM` \ lcl_env ->
1889 traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
1890 simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
1891 ASSERT( null frees )
1894 -- All the non-std ones are definite errors
1895 (stds, non_stds) = partition isStdClassTyVarDict irreds
1897 -- Group by type variable
1898 std_groups = equivClasses cmp_by_tyvar stds
1900 -- Pick the ones which its worth trying to disambiguate
1901 -- namely, the onese whose type variable isn't bound
1902 -- up with one of the non-standard classes
1903 (std_oks, std_bads) = partition worth_a_try std_groups
1904 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1905 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1907 -- Collect together all the bad guys
1908 bad_guys = non_stds ++ concat std_bads
1909 (bad_ips, non_ips) = partition isIPDict bad_guys
1910 (no_insts, ambigs) = partition no_inst non_ips
1911 no_inst d = not (isTyVarDict d)
1912 -- Previously, there was a more elaborate no_inst definition:
1913 -- no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1914 -- fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
1915 -- But that seems over-elaborate to me; it only bites for class decls with
1916 -- fundeps like this: class C a b | -> b where ...
1919 -- Report definite errors
1920 groupErrs (addNoInstanceErrs Nothing []) no_insts `thenM_`
1921 addTopIPErrs bad_ips `thenM_`
1923 -- Deal with ambiguity errors, but only if
1924 -- if there has not been an error so far; errors often
1925 -- give rise to spurious ambiguous Insts
1926 ifErrsM (returnM []) (
1928 -- Complain about the ones that don't fall under
1929 -- the Haskell rules for disambiguation
1930 -- This group includes both non-existent instances
1931 -- e.g. Num (IO a) and Eq (Int -> Int)
1932 -- and ambiguous dictionaries
1934 addTopAmbigErrs ambigs `thenM_`
1936 -- Disambiguate the ones that look feasible
1937 mappM (disambigGroup is_interactive) std_oks
1938 ) `thenM` \ binds_ambig ->
1940 returnM (binds `unionBags` unionManyBags binds_ambig)
1942 ----------------------------------
1943 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1945 get_tv d = case getDictClassTys d of
1946 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1947 get_clas d = case getDictClassTys d of
1948 (clas, [ty]) -> clas
1951 If a dictionary constrains a type variable which is
1952 * not mentioned in the environment
1953 * and not mentioned in the type of the expression
1954 then it is ambiguous. No further information will arise to instantiate
1955 the type variable; nor will it be generalised and turned into an extra
1956 parameter to a function.
1958 It is an error for this to occur, except that Haskell provided for
1959 certain rules to be applied in the special case of numeric types.
1961 * at least one of its classes is a numeric class, and
1962 * all of its classes are numeric or standard
1963 then the type variable can be defaulted to the first type in the
1964 default-type list which is an instance of all the offending classes.
1966 So here is the function which does the work. It takes the ambiguous
1967 dictionaries and either resolves them (producing bindings) or
1968 complains. It works by splitting the dictionary list by type
1969 variable, and using @disambigOne@ to do the real business.
1971 @disambigOne@ assumes that its arguments dictionaries constrain all
1972 the same type variable.
1974 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1975 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1976 the most common use of defaulting is code like:
1978 _ccall_ foo `seqPrimIO` bar
1980 Since we're not using the result of @foo@, the result if (presumably)
1984 disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
1985 -> [Inst] -- All standard classes of form (C a)
1988 disambigGroup is_interactive dicts
1989 | any std_default_class classes -- Guaranteed all standard classes
1990 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1991 -- SO, TRY DEFAULT TYPES IN ORDER
1993 -- Failure here is caused by there being no type in the
1994 -- default list which can satisfy all the ambiguous classes.
1995 -- For example, if Real a is reqd, but the only type in the
1996 -- default list is Int.
1997 get_default_tys `thenM` \ default_tys ->
1999 try_default [] -- No defaults work, so fail
2002 try_default (default_ty : default_tys)
2003 = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
2004 -- default_tys instead
2005 tcSimplifyDefault theta `thenM` \ _ ->
2008 theta = [mkClassPred clas [default_ty] | clas <- classes]
2010 -- See if any default works
2011 tryM (try_default default_tys) `thenM` \ mb_ty ->
2014 Right chosen_default_ty -> choose_default chosen_default_ty
2016 | otherwise -- No defaults
2020 tyvar = get_tv (head dicts) -- Should be non-empty
2021 classes = map get_clas dicts
2023 std_default_class cls
2024 = isNumericClass cls
2025 || (is_interactive &&
2026 classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2027 -- In interactive mode, we default Show a to Show ()
2028 -- to avoid graututious errors on "show []"
2030 choose_default default_ty -- Commit to tyvar = default_ty
2031 = -- Bind the type variable
2032 unifyTauTy default_ty (mkTyVarTy tyvar) `thenM_`
2033 -- and reduce the context, for real this time
2034 simpleReduceLoop (text "disambig" <+> ppr dicts)
2035 reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
2036 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
2037 warnDefault dicts default_ty `thenM_`
2040 bomb_out = addTopAmbigErrs dicts `thenM_`
2044 = do { mb_defaults <- getDefaultTys
2045 ; case mb_defaults of
2046 Just tys -> return tys
2047 Nothing -> -- No use-supplied default;
2048 -- use [Integer, Double]
2049 do { integer_ty <- tcMetaTy integerTyConName
2050 ; return [integer_ty, doubleTy] } }
2053 [Aside - why the defaulting mechanism is turned off when
2054 dealing with arguments and results to ccalls.
2056 When typechecking _ccall_s, TcExpr ensures that the external
2057 function is only passed arguments (and in the other direction,
2058 results) of a restricted set of 'native' types. This is
2059 implemented via the help of the pseudo-type classes,
2060 @CReturnable@ (CR) and @CCallable@ (CC.)
2062 The interaction between the defaulting mechanism for numeric
2063 values and CC & CR can be a bit puzzling to the user at times.
2072 What type has 'x' got here? That depends on the default list
2073 in operation, if it is equal to Haskell 98's default-default
2074 of (Integer, Double), 'x' has type Double, since Integer
2075 is not an instance of CR. If the default list is equal to
2076 Haskell 1.4's default-default of (Int, Double), 'x' has type
2079 To try to minimise the potential for surprises here, the
2080 defaulting mechanism is turned off in the presence of
2081 CCallable and CReturnable.
2086 %************************************************************************
2088 \subsection[simple]{@Simple@ versions}
2090 %************************************************************************
2092 Much simpler versions when there are no bindings to make!
2094 @tcSimplifyThetas@ simplifies class-type constraints formed by
2095 @deriving@ declarations and when specialising instances. We are
2096 only interested in the simplified bunch of class/type constraints.
2098 It simplifies to constraints of the form (C a b c) where
2099 a,b,c are type variables. This is required for the context of
2100 instance declarations.
2103 tcSimplifyDeriv :: [TyVar]
2104 -> ThetaType -- Wanted
2105 -> TcM ThetaType -- Needed
2107 tcSimplifyDeriv tyvars theta
2108 = tcInstTyVars VanillaTv tyvars `thenM` \ (tvs, _, tenv) ->
2109 -- The main loop may do unification, and that may crash if
2110 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2111 -- ToDo: what if two of them do get unified?
2112 newDicts DataDeclOrigin (substTheta tenv theta) `thenM` \ wanteds ->
2113 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
2114 ASSERT( null frees ) -- reduceMe never returns Free
2116 doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
2118 tv_set = mkVarSet tvs
2120 (bad_insts, ok_insts) = partition is_bad_inst irreds
2122 = let pred = dictPred dict -- reduceMe squashes all non-dicts
2123 in isEmptyVarSet (tyVarsOfPred pred)
2124 -- Things like (Eq T) are bad
2125 || (not undecidable_ok && not (isTyVarClassPred pred))
2126 -- The returned dictionaries should be of form (C a b)
2127 -- (where a, b are type variables).
2128 -- We allow non-tyvar dicts if we had -fallow-undecidable-instances,
2129 -- but note that risks non-termination in the 'deriving' context-inference
2130 -- fixpoint loop. It is useful for situations like
2131 -- data Min h a = E | M a (h a)
2132 -- which gives the instance decl
2133 -- instance (Eq a, Eq (h a)) => Eq (Min h a)
2135 simpl_theta = map dictPred ok_insts
2136 weird_preds = [pred | pred <- simpl_theta
2137 , not (tyVarsOfPred pred `subVarSet` tv_set)]
2138 -- Check for a bizarre corner case, when the derived instance decl should
2139 -- have form instance C a b => D (T a) where ...
2140 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
2141 -- of problems; in particular, it's hard to compare solutions for
2142 -- equality when finding the fixpoint. So I just rule it out for now.
2144 rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
2145 -- This reverse-mapping is a Royal Pain,
2146 -- but the result should mention TyVars not TcTyVars
2149 addNoInstanceErrs Nothing [] bad_insts `thenM_`
2150 mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_`
2151 checkAmbiguity tvs simpl_theta tv_set `thenM_`
2152 returnM (substTheta rev_env simpl_theta)
2154 doc = ptext SLIT("deriving classes for a data type")
2157 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2158 used with \tr{default} declarations. We are only interested in
2159 whether it worked or not.
2162 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2165 tcSimplifyDefault theta
2166 = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
2167 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
2168 ASSERT( null frees ) -- try_me never returns Free
2169 addNoInstanceErrs Nothing [] irreds `thenM_`
2175 doc = ptext SLIT("default declaration")
2179 %************************************************************************
2181 \section{Errors and contexts}
2183 %************************************************************************
2185 ToDo: for these error messages, should we note the location as coming
2186 from the insts, or just whatever seems to be around in the monad just
2190 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2191 -> [Inst] -- The offending Insts
2193 -- Group together insts with the same origin
2194 -- We want to report them together in error messages
2196 groupErrs report_err []
2198 groupErrs report_err (inst:insts)
2199 = do_one (inst:friends) `thenM_`
2200 groupErrs report_err others
2203 -- (It may seem a bit crude to compare the error messages,
2204 -- but it makes sure that we combine just what the user sees,
2205 -- and it avoids need equality on InstLocs.)
2206 (friends, others) = partition is_friend insts
2207 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2208 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2209 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2210 -- Add location and context information derived from the Insts
2212 -- Add the "arising from..." part to a message about bunch of dicts
2213 addInstLoc :: [Inst] -> Message -> Message
2214 addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
2217 plural xs = char 's'
2220 = groupErrs report tidy_dicts
2222 (tidy_env, tidy_dicts) = tidyInsts dicts
2223 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2224 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2225 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2227 addNoInstanceErrs :: Maybe SDoc -- Nothing => top level
2228 -- Just d => d describes the construct
2229 -> [Inst] -- What is given by the context or type sig
2230 -> [Inst] -- What is wanted
2232 addNoInstanceErrs mb_what givens []
2234 addNoInstanceErrs mb_what givens dicts
2235 = -- Some of the dicts are here because there is no instances
2236 -- and some because there are too many instances (overlap)
2237 -- The first thing we do is separate them
2238 getDOpts `thenM` \ dflags ->
2239 tcGetInstEnvs `thenM` \ inst_envs ->
2241 (tidy_env1, tidy_givens) = tidyInsts givens
2242 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
2244 -- Run through the dicts, generating a message for each
2245 -- overlapping one, but simply accumulating all the
2246 -- no-instance ones so they can be reported as a group
2247 (overlap_doc, no_inst_dicts) = foldl check_overlap (empty, []) tidy_dicts
2248 check_overlap (overlap_doc, no_inst_dicts) dict
2249 | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts)
2251 = case lookupInstEnv dflags inst_envs clas tys of
2252 -- The case of exactly one match and no unifiers means
2253 -- a successful lookup. That can't happen here.
2255 ([m],[]) -> pprPanic "addNoInstanceErrs" (ppr dict)
2257 ([], _) -> (overlap_doc, dict : no_inst_dicts) -- No match
2258 res -> (mk_overlap_msg dict res $$ overlap_doc, no_inst_dicts)
2260 (clas,tys) = getDictClassTys dict
2263 -- Now generate a good message for the no-instance bunch
2264 mk_probable_fix tidy_env2 mb_what no_inst_dicts `thenM` \ (tidy_env3, probable_fix) ->
2266 no_inst_doc | null no_inst_dicts = empty
2267 | otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix]
2268 heading | null givens = ptext SLIT("No instance") <> plural no_inst_dicts <+>
2269 ptext SLIT("for") <+> pprDictsTheta no_inst_dicts
2270 | otherwise = sep [ptext SLIT("Could not deduce") <+> pprDictsTheta no_inst_dicts,
2271 nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta tidy_givens]
2273 -- And emit both the non-instance and overlap messages
2274 addErrTcM (tidy_env3, no_inst_doc $$ overlap_doc)
2276 mk_overlap_msg dict (matches, unifiers)
2277 = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
2278 <+> pprPred (dictPred dict))),
2279 sep [ptext SLIT("Matching instances") <> colon,
2280 nest 2 (pprDFuns (dfuns ++ unifiers))],
2281 ASSERT( not (null matches) )
2282 if not (isSingleton matches)
2283 then -- Two or more matches
2285 else -- One match, plus some unifiers
2286 ASSERT( not (null unifiers) )
2287 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
2288 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
2289 ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])]
2291 dfuns = [df | (_, (_,_,df)) <- matches]
2293 mk_probable_fix tidy_env Nothing dicts -- Top level
2294 = mkMonomorphismMsg tidy_env dicts
2295 mk_probable_fix tidy_env (Just what) dicts -- Nested (type signatures, instance decls)
2296 = returnM (tidy_env, sep [ptext SLIT("Probable fix:"), nest 2 fix1, nest 2 fix2])
2298 fix1 = sep [ptext SLIT("Add") <+> pprDictsTheta dicts,
2299 ptext SLIT("to the") <+> what]
2301 fix2 | null instance_dicts = empty
2302 | otherwise = ptext SLIT("Or add an instance declaration for")
2303 <+> pprDictsTheta instance_dicts
2304 instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
2305 -- Insts for which it is worth suggesting an adding an instance declaration
2306 -- Exclude implicit parameters, and tyvar dicts
2309 addTopAmbigErrs dicts
2310 -- Divide into groups that share a common set of ambiguous tyvars
2311 = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
2313 (tidy_env, tidy_dicts) = tidyInsts dicts
2315 tvs_of :: Inst -> [TcTyVar]
2316 tvs_of d = varSetElems (tyVarsOfInst d)
2317 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
2319 report :: [(Inst,[TcTyVar])] -> TcM ()
2320 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
2321 = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
2322 addSrcSpan (instLocSrcSpan (instLoc inst)) $
2323 -- the location of the first one will do for the err message
2324 addErrTcM (tidy_env, msg $$ mono_msg)
2326 dicts = map fst pairs
2327 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
2328 pprQuotedList tvs <+> in_msg,
2329 nest 2 (pprDictsInFull dicts)]
2330 in_msg | isSingleton dicts = text "in the top-level constraint:"
2331 | otherwise = text "in these top-level constraints:"
2334 mkMonomorphismMsg :: TidyEnv -> [Inst] -> TcM (TidyEnv, Message)
2335 -- There's an error with these Insts; if they have free type variables
2336 -- it's probably caused by the monomorphism restriction.
2337 -- Try to identify the offending variable
2338 -- ASSUMPTION: the Insts are fully zonked
2339 mkMonomorphismMsg tidy_env insts
2340 | isEmptyVarSet inst_tvs
2341 = returnM (tidy_env, empty)
2343 = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
2344 returnM (tidy_env, mk_msg docs)
2347 inst_tvs = tyVarsOfInsts insts
2349 mk_msg [] = empty -- This happens in things like
2350 -- f x = show (read "foo")
2351 -- whre monomorphism doesn't play any role
2352 mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
2354 ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
2356 warnDefault dicts default_ty
2357 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
2358 addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
2361 (_, tidy_dicts) = tidyInsts dicts
2362 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
2363 quotes (ppr default_ty),
2364 pprDictsInFull tidy_dicts]
2366 -- Used for the ...Thetas variants; all top level
2368 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2369 ptext SLIT("type variables that are not data type parameters"),
2370 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2372 reduceDepthErr n stack
2373 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2374 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
2375 nest 4 (pprStack stack)]
2377 pprStack stack = vcat (map pprInstInFull stack)