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
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 = KeepDictWithoutSCs -- See notes above re "WithoutSCs"
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 | KeepDictWithoutSCs -- Return as irreducible; don't add its superclasses
1218 -- Rather specialised: see notes with tcSimplifyToDicts
1220 | DontReduceUnlessConstant -- Return as irreducible unless it can
1221 -- be reduced to a constant in one step
1223 | Free -- Return as free
1225 reduceMe :: Inst -> WhatToDo
1226 reduceMe inst = ReduceMe
1228 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1229 -- of a predicate when adding it to the avails
1235 type Avails = FiniteMap Inst Avail
1236 emptyAvails = emptyFM
1239 = IsFree -- Used for free Insts
1240 | Irred -- Used for irreducible dictionaries,
1241 -- which are going to be lambda bound
1243 | Given TcId -- Used for dictionaries for which we have a binding
1244 -- e.g. those "given" in a signature
1245 Bool -- True <=> actually consumed (splittable IPs only)
1247 | NoRhs -- Used for Insts like (CCallable f)
1248 -- where no witness is required.
1251 | Rhs -- Used when there is a RHS
1252 (LHsExpr TcId) -- The RHS
1253 [Inst] -- Insts free in the RHS; we need these too
1255 | Linear -- Splittable Insts only.
1256 Int -- The Int is always 2 or more; indicates how
1257 -- many copies are required
1258 Inst -- The splitter
1259 Avail -- Where the "master copy" is
1261 | LinRhss -- Splittable Insts only; this is used only internally
1262 -- by extractResults, where a Linear
1263 -- is turned into an LinRhss
1264 [LHsExpr TcId] -- A supply of suitable RHSs
1266 pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1267 | (inst,avail) <- fmToList avails ]
1269 instance Outputable Avail where
1272 pprAvail NoRhs = text "<no rhs>"
1273 pprAvail IsFree = text "Free"
1274 pprAvail Irred = text "Irred"
1275 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1276 if b then text "(used)" else empty
1277 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1278 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1279 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1282 Extracting the bindings from a bunch of Avails.
1283 The bindings do *not* come back sorted in dependency order.
1284 We assume that they'll be wrapped in a big Rec, so that the
1285 dependency analyser can sort them out later
1289 extractResults :: Avails
1291 -> TcM (TcDictBinds, -- Bindings
1292 [Inst], -- Irreducible ones
1293 [Inst]) -- Free ones
1295 extractResults avails wanteds
1296 = go avails emptyBag [] [] wanteds
1298 go avails binds irreds frees []
1299 = returnM (binds, irreds, frees)
1301 go avails binds irreds frees (w:ws)
1302 = case lookupFM avails w of
1303 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1304 go avails binds irreds frees ws
1306 Just NoRhs -> go avails binds irreds frees ws
1307 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1308 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1310 Just (Given id _) -> go avails new_binds irreds frees ws
1312 new_binds | id == instToId w = binds
1313 | otherwise = addBind binds w (L (instSpan w) (HsVar id))
1314 -- The sought Id can be one of the givens, via a superclass chain
1315 -- and then we definitely don't want to generate an x=x binding!
1317 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1319 new_binds = addBind binds w rhs
1321 Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
1322 -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) ->
1323 split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) ->
1324 go (addToFM avails w (LinRhss rhss))
1325 (binds `unionBags` binds')
1326 irreds' frees' (split_inst : w : ws)
1328 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1329 -> go new_avails new_binds irreds frees ws
1331 new_binds = addBind binds w rhs
1332 new_avails = addToFM avails w (LinRhss rhss)
1334 get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
1335 get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
1336 returnM (w':irreds, frees, instToId w')
1337 get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
1338 returnM (irreds, w':frees, instToId w')
1341 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1342 | otherwise = addToFM avails w NoRhs
1343 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1344 -- than Given, else we end up with bogus bindings.
1346 add_free avails w | isMethod w = avails
1347 | otherwise = add_given avails w
1349 -- Do *not* replace Free by Given if it's a method.
1350 -- The following situation shows why this is bad:
1351 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1352 -- From an application (truncate f i) we get
1353 -- t1 = truncate at f
1355 -- If we have also have a second occurrence of truncate, we get
1356 -- t3 = truncate at f
1358 -- When simplifying with i,f free, we might still notice that
1359 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1360 -- will continue to float out!
1361 -- (split n i a) returns: n rhss
1362 -- auxiliary bindings
1363 -- 1 or 0 insts to add to irreds
1366 split :: Int -> TcId -> TcId -> Inst
1367 -> TcM (TcDictBinds, [LHsExpr TcId])
1368 -- (split n split_id root_id wanted) returns
1369 -- * a list of 'n' expressions, all of which witness 'avail'
1370 -- * a bunch of auxiliary bindings to support these expressions
1371 -- * one or zero insts needed to witness the whole lot
1372 -- (maybe be zero if the initial Inst is a Given)
1374 -- NB: 'wanted' is just a template
1376 split n split_id root_id wanted
1379 ty = linearInstType wanted
1380 pair_ty = mkTyConApp pairTyCon [ty,ty]
1381 id = instToId wanted
1384 span = instSpan wanted
1386 go 1 = returnM (emptyBag, [L span $ HsVar root_id])
1388 go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
1389 expand n rhss `thenM` \ (binds2, rhss') ->
1390 returnM (binds1 `unionBags` binds2, rhss')
1393 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1394 -- e.g. expand 3 [rhs1, rhs2]
1395 -- = ( { x = split rhs1 },
1396 -- [fst x, snd x, rhs2] )
1398 | n `rem` 2 == 0 = go rhss -- n is even
1399 | otherwise = go (tail rhss) `thenM` \ (binds', rhss') ->
1400 returnM (binds', head rhss : rhss')
1402 go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') ->
1403 returnM (listToBag binds', concat rhss')
1405 do_one rhs = newUnique `thenM` \ uniq ->
1406 tcLookupId fstName `thenM` \ fst_id ->
1407 tcLookupId sndName `thenM` \ snd_id ->
1409 x = mkUserLocal occ uniq pair_ty loc
1411 returnM (L span (VarBind x (mk_app span split_id rhs)),
1412 [mk_fs_app span fst_id ty x, mk_fs_app span snd_id ty x])
1414 mk_fs_app span id ty var = L span (HsVar id) `mkHsTyApp` [ty,ty] `mkHsApp` (L span (HsVar var))
1416 mk_app span id rhs = L span (HsApp (L span (HsVar id)) rhs)
1418 addBind binds inst rhs = binds `unionBags` unitBag (L (instLocSrcSpan (instLoc inst))
1419 (VarBind (instToId inst) rhs))
1420 instSpan wanted = instLocSrcSpan (instLoc wanted)
1424 %************************************************************************
1426 \subsection[reduce]{@reduce@}
1428 %************************************************************************
1430 When the "what to do" predicate doesn't depend on the quantified type variables,
1431 matters are easier. We don't need to do any zonking, unless the improvement step
1432 does something, in which case we zonk before iterating.
1434 The "given" set is always empty.
1437 simpleReduceLoop :: SDoc
1438 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1440 -> TcM ([Inst], -- Free
1442 [Inst]) -- Irreducible
1444 simpleReduceLoop doc try_me wanteds
1445 = mappM zonkInst wanteds `thenM` \ wanteds' ->
1446 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1447 if no_improvement then
1448 returnM (frees, binds, irreds)
1450 simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
1451 returnM (frees1, binds `unionBags` binds1, irreds1)
1457 reduceContext :: SDoc
1458 -> (Inst -> WhatToDo)
1461 -> TcM (Bool, -- True <=> improve step did no unification
1463 TcDictBinds, -- Dictionary bindings
1464 [Inst]) -- Irreducible
1466 reduceContext doc try_me givens wanteds
1468 traceTc (text "reduceContext" <+> (vcat [
1469 text "----------------------",
1471 text "given" <+> ppr givens,
1472 text "wanted" <+> ppr wanteds,
1473 text "----------------------"
1476 -- Build the Avail mapping from "givens"
1477 foldlM addGiven emptyAvails givens `thenM` \ init_state ->
1480 reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
1482 -- Do improvement, using everything in avails
1483 -- In particular, avails includes all superclasses of everything
1484 tcImprove avails `thenM` \ no_improvement ->
1486 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1488 traceTc (text "reduceContext end" <+> (vcat [
1489 text "----------------------",
1491 text "given" <+> ppr givens,
1492 text "wanted" <+> ppr wanteds,
1494 text "avails" <+> pprAvails avails,
1495 text "frees" <+> ppr frees,
1496 text "no_improvement =" <+> ppr no_improvement,
1497 text "----------------------"
1500 returnM (no_improvement, frees, binds, irreds)
1502 -- reduceContextWithoutImprovement differs from reduceContext
1503 -- (a) no improvement
1504 -- (b) 'givens' is assumed empty
1505 reduceContextWithoutImprovement doc try_me wanteds
1507 traceTc (text "reduceContextWithoutImprovement" <+> (vcat [
1508 text "----------------------",
1510 text "wanted" <+> ppr wanteds,
1511 text "----------------------"
1515 reduceList (0,[]) try_me wanteds emptyAvails `thenM` \ avails ->
1516 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1518 traceTc (text "reduceContextWithoutImprovement end" <+> (vcat [
1519 text "----------------------",
1521 text "wanted" <+> ppr wanteds,
1523 text "avails" <+> pprAvails avails,
1524 text "frees" <+> ppr frees,
1525 text "----------------------"
1528 returnM (frees, binds, irreds)
1530 tcImprove :: Avails -> TcM Bool -- False <=> no change
1531 -- Perform improvement using all the predicates in Avails
1533 = tcGetInstEnvs `thenM` \ (home_ie, pkg_ie) ->
1535 preds = [ (pred, pp_loc)
1536 | inst <- keysFM avails,
1537 let pp_loc = pprInstLoc (instLoc inst),
1538 pred <- fdPredsOfInst inst
1540 -- Avails has all the superclasses etc (good)
1541 -- It also has all the intermediates of the deduction (good)
1542 -- It does not have duplicates (good)
1543 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1544 -- so that improve will see them separate
1545 eqns = improve get_insts preds
1546 get_insts clas = classInstances home_ie clas ++ classInstances pkg_ie clas
1551 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
1552 mappM_ unify eqns `thenM_`
1555 unify ((qtvs, t1, t2), doc)
1557 tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1558 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1561 The main context-reduction function is @reduce@. Here's its game plan.
1564 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1565 -- along with its depth
1566 -> (Inst -> WhatToDo)
1573 try_me: given an inst, this function returns
1575 DontReduce return this in "irreds"
1576 Free return this in "frees"
1578 wanteds: The list of insts to reduce
1579 state: An accumulating parameter of type Avails
1580 that contains the state of the algorithm
1582 It returns a Avails.
1584 The (n,stack) pair is just used for error reporting.
1585 n is always the depth of the stack.
1586 The stack is the stack of Insts being reduced: to produce X
1587 I had to produce Y, to produce Y I had to produce Z, and so on.
1590 reduceList (n,stack) try_me wanteds state
1591 | n > opt_MaxContextReductionDepth
1592 = failWithTc (reduceDepthErr n stack)
1598 pprTrace "Interesting! Context reduction stack deeper than 8:"
1599 (nest 2 (pprStack stack))
1604 go [] state = returnM state
1605 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
1608 -- Base case: we're done!
1609 reduce stack try_me wanted avails
1610 -- It's the same as an existing inst, or a superclass thereof
1611 | Just avail <- isAvailable avails wanted
1612 = if isLinearInst wanted then
1613 addLinearAvailable avails avail wanted `thenM` \ (avails', wanteds') ->
1614 reduceList stack try_me wanteds' avails'
1616 returnM avails -- No op for non-linear things
1619 = case try_me wanted of {
1621 KeepDictWithoutSCs -> addIrred NoSCs avails wanted
1623 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1624 -- First, see if the inst can be reduced to a constant in one step
1625 try_simple (addIrred AddSCs) -- Assume want superclasses
1627 ; Free -> -- It's free so just chuck it upstairs
1628 -- First, see if the inst can be reduced to a constant in one step
1631 ; ReduceMe -> -- It should be reduced
1632 lookupInst wanted `thenM` \ lookup_result ->
1633 case lookup_result of
1634 GenInst wanteds' rhs -> addIrred NoSCs avails wanted `thenM` \ avails1 ->
1635 reduceList stack try_me wanteds' avails1 `thenM` \ avails2 ->
1636 addWanted avails2 wanted rhs wanteds'
1637 -- Experiment with temporarily doing addIrred *before* the reduceList,
1638 -- which has the effect of adding the thing we are trying
1639 -- to prove to the database before trying to prove the things it
1640 -- needs. See note [RECURSIVE DICTIONARIES]
1641 -- NB: we must not do an addWanted before, because that adds the
1642 -- superclasses too, and thaat can lead to a spurious loop; see
1643 -- the examples in [SUPERCLASS-LOOP]
1644 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1646 SimpleInst rhs -> addWanted avails wanted rhs []
1648 NoInstance -> -- No such instance!
1649 -- Add it and its superclasses
1650 addIrred AddSCs avails wanted
1653 try_simple do_this_otherwise
1654 = lookupInst wanted `thenM` \ lookup_result ->
1655 case lookup_result of
1656 SimpleInst rhs -> addWanted avails wanted rhs []
1657 other -> do_this_otherwise avails wanted
1662 -------------------------
1663 isAvailable :: Avails -> Inst -> Maybe Avail
1664 isAvailable avails wanted = lookupFM avails wanted
1665 -- NB 1: the Ord instance of Inst compares by the class/type info
1666 -- *not* by unique. So
1667 -- d1::C Int == d2::C Int
1669 addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
1670 addLinearAvailable avails avail wanted
1671 -- avails currently maps [wanted -> avail]
1672 -- Extend avails to reflect a neeed for an extra copy of avail
1674 | Just avail' <- split_avail avail
1675 = returnM (addToFM avails wanted avail', [])
1678 = tcLookupId splitName `thenM` \ split_id ->
1679 tcInstClassOp (instLoc wanted) split_id
1680 [linearInstType wanted] `thenM` \ split_inst ->
1681 returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1684 split_avail :: Avail -> Maybe Avail
1685 -- (Just av) if there's a modified version of avail that
1686 -- we can use to replace avail in avails
1687 -- Nothing if there isn't, so we need to create a Linear
1688 split_avail (Linear n i a) = Just (Linear (n+1) i a)
1689 split_avail (Given id used) | not used = Just (Given id True)
1690 | otherwise = Nothing
1691 split_avail Irred = Nothing
1692 split_avail IsFree = Nothing
1693 split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
1695 -------------------------
1696 addFree :: Avails -> Inst -> TcM Avails
1697 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1698 -- to avails, so that any other equal Insts will be commoned up right
1699 -- here rather than also being tossed upstairs. This is really just
1700 -- an optimisation, and perhaps it is more trouble that it is worth,
1701 -- as the following comments show!
1703 -- NB: do *not* add superclasses. If we have
1706 -- but a is not bound here, then we *don't* want to derive
1707 -- dn from df here lest we lose sharing.
1709 addFree avails free = returnM (addToFM avails free IsFree)
1711 addWanted :: Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
1712 addWanted avails wanted rhs_expr wanteds
1713 = addAvailAndSCs avails wanted avail
1715 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1716 | otherwise = ASSERT( null wanteds ) NoRhs
1718 addGiven :: Avails -> Inst -> TcM Avails
1719 addGiven avails given = addAvailAndSCs avails given (Given (instToId given) False)
1720 -- No ASSERT( not (given `elemFM` avails) ) because in an instance
1721 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
1722 -- so the assert isn't true
1724 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
1725 addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
1726 addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
1727 addAvailAndSCs avails irred Irred
1729 addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
1730 addAvailAndSCs avails inst avail
1731 | not (isClassDict inst) = returnM avails1
1732 | otherwise = traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) `thenM_`
1733 addSCs is_loop avails1 inst
1735 avails1 = addToFM avails inst avail
1736 is_loop inst = any (`tcEqType` idType (instToId inst)) dep_tys
1737 -- Note: this compares by *type*, not by Unique
1738 deps = findAllDeps emptyVarSet avail
1739 dep_tys = map idType (varSetElems deps)
1741 findAllDeps :: IdSet -> Avail -> IdSet
1742 -- Find all the Insts that this one depends on
1743 -- See Note [SUPERCLASS-LOOP]
1744 -- Watch out, though. Since the avails may contain loops
1745 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
1746 findAllDeps so_far (Rhs _ kids)
1748 (extendVarSetList so_far (map instToId kids)) -- Add the kids to so_far
1749 [a | Just a <- map (lookupFM avails) kids] -- Find the kids' Avail
1750 findAllDeps so_far other = so_far
1753 addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
1754 -- Add all the superclasses of the Inst to Avails
1755 -- The first param says "dont do this because the original thing
1756 -- depends on this one, so you'd build a loop"
1757 -- Invariant: the Inst is already in Avails.
1759 addSCs is_loop avails dict
1760 = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
1761 foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1763 (clas, tys) = getDictClassTys dict
1764 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1765 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1767 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1768 | add_me sc_dict = addSCs is_loop avails' sc_dict
1769 | otherwise = returnM avails
1771 sc_sel_rhs = mkHsDictApp (mkHsTyApp (L (instSpan dict) (HsVar sc_sel)) tys) [instToId dict]
1772 avails' = addToFM avails sc_dict (Rhs sc_sel_rhs [dict])
1774 add_me :: Inst -> Bool
1776 | is_loop sc_dict = False -- See Note [SUPERCLASS-LOOP]
1777 | otherwise = case lookupFM avails sc_dict of
1778 Just (Given _ _) -> False -- Given is cheaper than superclass selection
1782 Note [SUPERCLASS-LOOP]: Checking for loops
1783 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1784 We have to be careful here. If we are *given* d1:Ord a,
1785 and want to deduce (d2:C [a]) where
1787 class Ord a => C a where
1788 instance Ord a => C [a] where ...
1790 Then we'll use the instance decl to deduce C [a] and then add the
1791 superclasses of C [a] to avails. But we must not overwrite the binding
1792 for d1:Ord a (which is given) with a superclass selection or we'll just
1795 Here's another variant, immortalised in tcrun020
1796 class Monad m => C1 m
1797 class C1 m => C2 m x
1798 instance C2 Maybe Bool
1799 For the instance decl we need to build (C1 Maybe), and it's no good if
1800 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1801 before we search for C1 Maybe.
1803 Here's another example
1804 class Eq b => Foo a b
1805 instance Eq a => Foo [a] a
1809 we'll first deduce that it holds (via the instance decl). We must not
1810 then overwrite the Eq t constraint with a superclass selection!
1812 At first I had a gross hack, whereby I simply did not add superclass constraints
1813 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1814 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1815 I found a very obscure program (now tcrun021) in which improvement meant the
1816 simplifier got two bites a the cherry... so something seemed to be an Irred
1817 first time, but reducible next time.
1819 Now we implement the Right Solution, which is to check for loops directly
1820 when adding superclasses. It's a bit like the occurs check in unification.
1823 Note [RECURSIVE DICTIONARIES]
1824 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1826 data D r = ZeroD | SuccD (r (D r));
1828 instance (Eq (r (D r))) => Eq (D r) where
1829 ZeroD == ZeroD = True
1830 (SuccD a) == (SuccD b) = a == b
1833 equalDC :: D [] -> D [] -> Bool;
1836 We need to prove (Eq (D [])). Here's how we go:
1840 by instance decl, holds if
1844 by instance decl of Eq, holds if
1846 where d2 = dfEqList d3
1849 But now we can "tie the knot" to give
1855 and it'll even run! The trick is to put the thing we are trying to prove
1856 (in this case Eq (D []) into the database before trying to prove its
1857 contributing clauses.
1860 %************************************************************************
1862 \section{tcSimplifyTop: defaulting}
1864 %************************************************************************
1867 @tcSimplifyTop@ is called once per module to simplify all the constant
1868 and ambiguous Insts.
1870 We need to be careful of one case. Suppose we have
1872 instance Num a => Num (Foo a b) where ...
1874 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1875 to (Num x), and default x to Int. But what about y??
1877 It's OK: the final zonking stage should zap y to (), which is fine.
1881 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
1882 tcSimplifyTop wanteds = tc_simplify_top False {- Not interactive loop -} wanteds
1883 tcSimplifyInteractive wanteds = tc_simplify_top True {- Interactive loop -} wanteds
1886 -- The TcLclEnv should be valid here, solely to improve
1887 -- error message generation for the monomorphism restriction
1888 tc_simplify_top is_interactive wanteds
1889 = getLclEnv `thenM` \ lcl_env ->
1890 traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
1891 simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
1892 ASSERT( null frees )
1895 -- All the non-std ones are definite errors
1896 (stds, non_stds) = partition isStdClassTyVarDict irreds
1898 -- Group by type variable
1899 std_groups = equivClasses cmp_by_tyvar stds
1901 -- Pick the ones which its worth trying to disambiguate
1902 -- namely, the onese whose type variable isn't bound
1903 -- up with one of the non-standard classes
1904 (std_oks, std_bads) = partition worth_a_try std_groups
1905 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1906 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1908 -- Collect together all the bad guys
1909 bad_guys = non_stds ++ concat std_bads
1910 (bad_ips, non_ips) = partition isIPDict bad_guys
1911 (no_insts, ambigs) = partition no_inst non_ips
1912 no_inst d = not (isTyVarDict d)
1913 -- Previously, there was a more elaborate no_inst definition:
1914 -- no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1915 -- fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
1916 -- But that seems over-elaborate to me; it only bites for class decls with
1917 -- fundeps like this: class C a b | -> b where ...
1920 -- Report definite errors
1921 groupErrs (addNoInstanceErrs Nothing []) no_insts `thenM_`
1922 addTopIPErrs bad_ips `thenM_`
1924 -- Deal with ambiguity errors, but only if
1925 -- if there has not been an error so far; errors often
1926 -- give rise to spurious ambiguous Insts
1927 ifErrsM (returnM []) (
1929 -- Complain about the ones that don't fall under
1930 -- the Haskell rules for disambiguation
1931 -- This group includes both non-existent instances
1932 -- e.g. Num (IO a) and Eq (Int -> Int)
1933 -- and ambiguous dictionaries
1935 addTopAmbigErrs ambigs `thenM_`
1937 -- Disambiguate the ones that look feasible
1938 mappM (disambigGroup is_interactive) std_oks
1939 ) `thenM` \ binds_ambig ->
1941 returnM (binds `unionBags` unionManyBags binds_ambig)
1943 ----------------------------------
1944 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1946 get_tv d = case getDictClassTys d of
1947 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1948 get_clas d = case getDictClassTys d of
1949 (clas, [ty]) -> clas
1952 If a dictionary constrains a type variable which is
1953 * not mentioned in the environment
1954 * and not mentioned in the type of the expression
1955 then it is ambiguous. No further information will arise to instantiate
1956 the type variable; nor will it be generalised and turned into an extra
1957 parameter to a function.
1959 It is an error for this to occur, except that Haskell provided for
1960 certain rules to be applied in the special case of numeric types.
1962 * at least one of its classes is a numeric class, and
1963 * all of its classes are numeric or standard
1964 then the type variable can be defaulted to the first type in the
1965 default-type list which is an instance of all the offending classes.
1967 So here is the function which does the work. It takes the ambiguous
1968 dictionaries and either resolves them (producing bindings) or
1969 complains. It works by splitting the dictionary list by type
1970 variable, and using @disambigOne@ to do the real business.
1972 @disambigOne@ assumes that its arguments dictionaries constrain all
1973 the same type variable.
1975 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1976 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1977 the most common use of defaulting is code like:
1979 _ccall_ foo `seqPrimIO` bar
1981 Since we're not using the result of @foo@, the result if (presumably)
1985 disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
1986 -> [Inst] -- All standard classes of form (C a)
1989 disambigGroup is_interactive dicts
1990 | any std_default_class classes -- Guaranteed all standard classes
1991 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1992 -- SO, TRY DEFAULT TYPES IN ORDER
1994 -- Failure here is caused by there being no type in the
1995 -- default list which can satisfy all the ambiguous classes.
1996 -- For example, if Real a is reqd, but the only type in the
1997 -- default list is Int.
1998 get_default_tys `thenM` \ default_tys ->
2000 try_default [] -- No defaults work, so fail
2003 try_default (default_ty : default_tys)
2004 = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
2005 -- default_tys instead
2006 tcSimplifyDefault theta `thenM` \ _ ->
2009 theta = [mkClassPred clas [default_ty] | clas <- classes]
2011 -- See if any default works
2012 tryM (try_default default_tys) `thenM` \ mb_ty ->
2015 Right chosen_default_ty -> choose_default chosen_default_ty
2017 | otherwise -- No defaults
2021 tyvar = get_tv (head dicts) -- Should be non-empty
2022 classes = map get_clas dicts
2024 std_default_class cls
2025 = isNumericClass cls
2026 || (is_interactive &&
2027 classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2028 -- In interactive mode, we default Show a to Show ()
2029 -- to avoid graututious errors on "show []"
2031 choose_default default_ty -- Commit to tyvar = default_ty
2032 = -- Bind the type variable
2033 unifyTauTy default_ty (mkTyVarTy tyvar) `thenM_`
2034 -- and reduce the context, for real this time
2035 simpleReduceLoop (text "disambig" <+> ppr dicts)
2036 reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
2037 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
2038 warnDefault dicts default_ty `thenM_`
2041 bomb_out = addTopAmbigErrs dicts `thenM_`
2045 = do { mb_defaults <- getDefaultTys
2046 ; case mb_defaults of
2047 Just tys -> return tys
2048 Nothing -> -- No use-supplied default;
2049 -- use [Integer, Double]
2050 do { integer_ty <- tcMetaTy integerTyConName
2051 ; return [integer_ty, doubleTy] } }
2054 [Aside - why the defaulting mechanism is turned off when
2055 dealing with arguments and results to ccalls.
2057 When typechecking _ccall_s, TcExpr ensures that the external
2058 function is only passed arguments (and in the other direction,
2059 results) of a restricted set of 'native' types. This is
2060 implemented via the help of the pseudo-type classes,
2061 @CReturnable@ (CR) and @CCallable@ (CC.)
2063 The interaction between the defaulting mechanism for numeric
2064 values and CC & CR can be a bit puzzling to the user at times.
2073 What type has 'x' got here? That depends on the default list
2074 in operation, if it is equal to Haskell 98's default-default
2075 of (Integer, Double), 'x' has type Double, since Integer
2076 is not an instance of CR. If the default list is equal to
2077 Haskell 1.4's default-default of (Int, Double), 'x' has type
2080 To try to minimise the potential for surprises here, the
2081 defaulting mechanism is turned off in the presence of
2082 CCallable and CReturnable.
2087 %************************************************************************
2089 \subsection[simple]{@Simple@ versions}
2091 %************************************************************************
2093 Much simpler versions when there are no bindings to make!
2095 @tcSimplifyThetas@ simplifies class-type constraints formed by
2096 @deriving@ declarations and when specialising instances. We are
2097 only interested in the simplified bunch of class/type constraints.
2099 It simplifies to constraints of the form (C a b c) where
2100 a,b,c are type variables. This is required for the context of
2101 instance declarations.
2104 tcSimplifyDeriv :: [TyVar]
2105 -> ThetaType -- Wanted
2106 -> TcM ThetaType -- Needed
2108 tcSimplifyDeriv tyvars theta
2109 = tcInstTyVars VanillaTv tyvars `thenM` \ (tvs, _, tenv) ->
2110 -- The main loop may do unification, and that may crash if
2111 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2112 -- ToDo: what if two of them do get unified?
2113 newDicts DataDeclOrigin (substTheta tenv theta) `thenM` \ wanteds ->
2114 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
2115 ASSERT( null frees ) -- reduceMe never returns Free
2117 doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
2119 tv_set = mkVarSet tvs
2121 (bad_insts, ok_insts) = partition is_bad_inst irreds
2123 = let pred = dictPred dict -- reduceMe squashes all non-dicts
2124 in isEmptyVarSet (tyVarsOfPred pred)
2125 -- Things like (Eq T) are bad
2126 || (not undecidable_ok && not (isTyVarClassPred pred))
2127 -- The returned dictionaries should be of form (C a b)
2128 -- (where a, b are type variables).
2129 -- We allow non-tyvar dicts if we had -fallow-undecidable-instances,
2130 -- but note that risks non-termination in the 'deriving' context-inference
2131 -- fixpoint loop. It is useful for situations like
2132 -- data Min h a = E | M a (h a)
2133 -- which gives the instance decl
2134 -- instance (Eq a, Eq (h a)) => Eq (Min h a)
2136 simpl_theta = map dictPred ok_insts
2137 weird_preds = [pred | pred <- simpl_theta
2138 , not (tyVarsOfPred pred `subVarSet` tv_set)]
2139 -- Check for a bizarre corner case, when the derived instance decl should
2140 -- have form instance C a b => D (T a) where ...
2141 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
2142 -- of problems; in particular, it's hard to compare solutions for
2143 -- equality when finding the fixpoint. So I just rule it out for now.
2145 rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
2146 -- This reverse-mapping is a Royal Pain,
2147 -- but the result should mention TyVars not TcTyVars
2150 addNoInstanceErrs Nothing [] bad_insts `thenM_`
2151 mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_`
2152 checkAmbiguity tvs simpl_theta tv_set `thenM_`
2153 returnM (substTheta rev_env simpl_theta)
2155 doc = ptext SLIT("deriving classes for a data type")
2158 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2159 used with \tr{default} declarations. We are only interested in
2160 whether it worked or not.
2163 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2166 tcSimplifyDefault theta
2167 = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
2168 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
2169 ASSERT( null frees ) -- try_me never returns Free
2170 addNoInstanceErrs Nothing [] irreds `thenM_`
2176 doc = ptext SLIT("default declaration")
2180 %************************************************************************
2182 \section{Errors and contexts}
2184 %************************************************************************
2186 ToDo: for these error messages, should we note the location as coming
2187 from the insts, or just whatever seems to be around in the monad just
2191 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2192 -> [Inst] -- The offending Insts
2194 -- Group together insts with the same origin
2195 -- We want to report them together in error messages
2197 groupErrs report_err []
2199 groupErrs report_err (inst:insts)
2200 = do_one (inst:friends) `thenM_`
2201 groupErrs report_err others
2204 -- (It may seem a bit crude to compare the error messages,
2205 -- but it makes sure that we combine just what the user sees,
2206 -- and it avoids need equality on InstLocs.)
2207 (friends, others) = partition is_friend insts
2208 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2209 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2210 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2211 -- Add location and context information derived from the Insts
2213 -- Add the "arising from..." part to a message about bunch of dicts
2214 addInstLoc :: [Inst] -> Message -> Message
2215 addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
2218 plural xs = char 's'
2221 = groupErrs report tidy_dicts
2223 (tidy_env, tidy_dicts) = tidyInsts dicts
2224 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2225 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2226 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2228 addNoInstanceErrs :: Maybe SDoc -- Nothing => top level
2229 -- Just d => d describes the construct
2230 -> [Inst] -- What is given by the context or type sig
2231 -> [Inst] -- What is wanted
2233 addNoInstanceErrs mb_what givens []
2235 addNoInstanceErrs mb_what givens dicts
2236 = -- Some of the dicts are here because there is no instances
2237 -- and some because there are too many instances (overlap)
2238 -- The first thing we do is separate them
2239 getDOpts `thenM` \ dflags ->
2240 tcGetInstEnvs `thenM` \ inst_envs ->
2242 (tidy_env1, tidy_givens) = tidyInsts givens
2243 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
2245 -- Run through the dicts, generating a message for each
2246 -- overlapping one, but simply accumulating all the
2247 -- no-instance ones so they can be reported as a group
2248 (overlap_doc, no_inst_dicts) = foldl check_overlap (empty, []) tidy_dicts
2249 check_overlap (overlap_doc, no_inst_dicts) dict
2250 | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts)
2252 = case lookupInstEnv dflags inst_envs clas tys of
2253 -- The case of exactly one match and no unifiers means
2254 -- a successful lookup. That can't happen here.
2256 ([m],[]) -> pprPanic "addNoInstanceErrs" (ppr dict)
2258 ([], _) -> (overlap_doc, dict : no_inst_dicts) -- No match
2259 res -> (mk_overlap_msg dict res $$ overlap_doc, no_inst_dicts)
2261 (clas,tys) = getDictClassTys dict
2264 -- Now generate a good message for the no-instance bunch
2265 mk_probable_fix tidy_env2 mb_what no_inst_dicts `thenM` \ (tidy_env3, probable_fix) ->
2267 no_inst_doc | null no_inst_dicts = empty
2268 | otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix]
2269 heading | null givens = ptext SLIT("No instance") <> plural no_inst_dicts <+>
2270 ptext SLIT("for") <+> pprDictsTheta no_inst_dicts
2271 | otherwise = sep [ptext SLIT("Could not deduce") <+> pprDictsTheta no_inst_dicts,
2272 nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta tidy_givens]
2274 -- And emit both the non-instance and overlap messages
2275 addErrTcM (tidy_env3, no_inst_doc $$ overlap_doc)
2277 mk_overlap_msg dict (matches, unifiers)
2278 = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
2279 <+> pprPred (dictPred dict))),
2280 sep [ptext SLIT("Matching instances") <> colon,
2281 nest 2 (pprDFuns (dfuns ++ unifiers))],
2282 ASSERT( not (null matches) )
2283 if not (isSingleton matches)
2284 then -- Two or more matches
2286 else -- One match, plus some unifiers
2287 ASSERT( not (null unifiers) )
2288 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
2289 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
2290 ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])]
2292 dfuns = [df | (_, (_,_,df)) <- matches]
2294 mk_probable_fix tidy_env Nothing dicts -- Top level
2295 = mkMonomorphismMsg tidy_env dicts
2296 mk_probable_fix tidy_env (Just what) dicts -- Nested (type signatures, instance decls)
2297 = returnM (tidy_env, sep [ptext SLIT("Probable fix:"), nest 2 fix1, nest 2 fix2])
2299 fix1 = sep [ptext SLIT("Add") <+> pprDictsTheta dicts,
2300 ptext SLIT("to the") <+> what]
2302 fix2 | null instance_dicts = empty
2303 | otherwise = ptext SLIT("Or add an instance declaration for")
2304 <+> pprDictsTheta instance_dicts
2305 instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
2306 -- Insts for which it is worth suggesting an adding an instance declaration
2307 -- Exclude implicit parameters, and tyvar dicts
2310 addTopAmbigErrs dicts
2311 -- Divide into groups that share a common set of ambiguous tyvars
2312 = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
2314 (tidy_env, tidy_dicts) = tidyInsts dicts
2316 tvs_of :: Inst -> [TcTyVar]
2317 tvs_of d = varSetElems (tyVarsOfInst d)
2318 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
2320 report :: [(Inst,[TcTyVar])] -> TcM ()
2321 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
2322 = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
2323 addSrcSpan (instLocSrcSpan (instLoc inst)) $
2324 -- the location of the first one will do for the err message
2325 addErrTcM (tidy_env, msg $$ mono_msg)
2327 dicts = map fst pairs
2328 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
2329 pprQuotedList tvs <+> in_msg,
2330 nest 2 (pprDictsInFull dicts)]
2331 in_msg | isSingleton dicts = text "in the top-level constraint:"
2332 | otherwise = text "in these top-level constraints:"
2335 mkMonomorphismMsg :: TidyEnv -> [Inst] -> TcM (TidyEnv, Message)
2336 -- There's an error with these Insts; if they have free type variables
2337 -- it's probably caused by the monomorphism restriction.
2338 -- Try to identify the offending variable
2339 -- ASSUMPTION: the Insts are fully zonked
2340 mkMonomorphismMsg tidy_env insts
2341 | isEmptyVarSet inst_tvs
2342 = returnM (tidy_env, empty)
2344 = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
2345 returnM (tidy_env, mk_msg docs)
2348 inst_tvs = tyVarsOfInsts insts
2350 mk_msg [] = empty -- This happens in things like
2351 -- f x = show (read "foo")
2352 -- whre monomorphism doesn't play any role
2353 mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
2355 ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
2357 warnDefault dicts default_ty
2358 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
2359 addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
2362 (_, tidy_dicts) = tidyInsts dicts
2363 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
2364 quotes (ppr default_ty),
2365 pprDictsInFull tidy_dicts]
2367 -- Used for the ...Thetas variants; all top level
2369 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2370 ptext SLIT("type variables that are not data type parameters"),
2371 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2373 reduceDepthErr n stack
2374 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2375 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
2376 nest 4 (pprStack stack)]
2378 pprStack stack = vcat (map pprInstInFull stack)