2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
17 tcSimplifyInfer, tcSimplifyInferCheck,
18 tcSimplifyCheck, tcSimplifyRestricted,
19 tcSimplifyRuleLhs, tcSimplifyIPs,
20 tcSimplifySuperClasses,
21 tcSimplifyTop, tcSimplifyInteractive,
22 tcSimplifyBracket, tcSimplifyCheckPat,
24 tcSimplifyDeriv, tcSimplifyDefault,
25 bindInstsOfLocalFuns, bindIrreds,
30 #include "HsVersions.h"
32 import {-# SOURCE #-} TcUnify( unifyType )
73 %************************************************************************
77 %************************************************************************
79 --------------------------------------
80 Notes on functional dependencies (a bug)
81 --------------------------------------
88 instance D a b => C a b -- Undecidable
89 -- (Not sure if it's crucial to this eg)
90 f :: C a b => a -> Bool
93 g :: C a b => a -> Bool
96 Here f typechecks, but g does not!! Reason: before doing improvement,
97 we reduce the (C a b1) constraint from the call of f to (D a b1).
99 Here is a more complicated example:
101 | > class Foo a b | a->b
103 | > class Bar a b | a->b
107 | > instance Bar Obj Obj
109 | > instance (Bar a b) => Foo a b
111 | > foo:: (Foo a b) => a -> String
114 | > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
120 | Could not deduce (Bar a b) from the context (Foo a b)
121 | arising from use of `foo' at <interactive>:1
123 | Add (Bar a b) to the expected type of an expression
124 | In the first argument of `runFoo', namely `foo'
125 | In the definition of `it': it = runFoo foo
127 | Why all of the sudden does GHC need the constraint Bar a b? The
128 | function foo didn't ask for that...
130 The trouble is that to type (runFoo foo), GHC has to solve the problem:
132 Given constraint Foo a b
133 Solve constraint Foo a b'
135 Notice that b and b' aren't the same. To solve this, just do
136 improvement and then they are the same. But GHC currently does
141 That is usually fine, but it isn't here, because it sees that Foo a b is
142 not the same as Foo a b', and so instead applies the instance decl for
143 instance Bar a b => Foo a b. And that's where the Bar constraint comes
146 The Right Thing is to improve whenever the constraint set changes at
147 all. Not hard in principle, but it'll take a bit of fiddling to do.
149 Note [Choosing which variables to quantify]
150 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
151 Suppose we are about to do a generalisation step. We have in our hand
154 T the type of the RHS
155 C the constraints from that RHS
157 The game is to figure out
159 Q the set of type variables over which to quantify
160 Ct the constraints we will *not* quantify over
161 Cq the constraints we will quantify over
163 So we're going to infer the type
167 and float the constraints Ct further outwards.
169 Here are the things that *must* be true:
171 (A) Q intersect fv(G) = EMPTY limits how big Q can be
172 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
174 (A) says we can't quantify over a variable that's free in the environment.
175 (B) says we must quantify over all the truly free variables in T, else
176 we won't get a sufficiently general type.
178 We do not *need* to quantify over any variable that is fixed by the
179 free vars of the environment G.
181 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
183 Example: class H x y | x->y where ...
185 fv(G) = {a} C = {H a b, H c d}
188 (A) Q intersect {a} is empty
189 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
191 So Q can be {c,d}, {b,c,d}
193 In particular, it's perfectly OK to quantify over more type variables
194 than strictly necessary; there is no need to quantify over 'b', since
195 it is determined by 'a' which is free in the envt, but it's perfectly
196 OK to do so. However we must not quantify over 'a' itself.
198 Other things being equal, however, we'd like to quantify over as few
199 variables as possible: smaller types, fewer type applications, more
200 constraints can get into Ct instead of Cq. Here's a good way to
203 Q = grow( fv(T), C ) \ oclose( fv(G), C )
205 That is, quantify over all variable that that MIGHT be fixed by the
206 call site (which influences T), but which aren't DEFINITELY fixed by
207 G. This choice definitely quantifies over enough type variables,
208 albeit perhaps too many.
210 Why grow( fv(T), C ) rather than fv(T)? Consider
212 class H x y | x->y where ...
217 If we used fv(T) = {c} we'd get the type
219 forall c. H c d => c -> b
221 And then if the fn was called at several different c's, each of
222 which fixed d differently, we'd get a unification error, because
223 d isn't quantified. Solution: quantify d. So we must quantify
224 everything that might be influenced by c.
226 Why not oclose( fv(T), C )? Because we might not be able to see
227 all the functional dependencies yet:
229 class H x y | x->y where ...
230 instance H x y => Eq (T x y) where ...
235 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
236 apparent yet, and that's wrong. We must really quantify over d too.
238 There really isn't any point in quantifying over any more than
239 grow( fv(T), C ), because the call sites can't possibly influence
240 any other type variables.
244 -------------------------------------
246 -------------------------------------
248 It's very hard to be certain when a type is ambiguous. Consider
252 instance H x y => K (x,y)
254 Is this type ambiguous?
255 forall a b. (K (a,b), Eq b) => a -> a
257 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
258 now we see that a fixes b. So we can't tell about ambiguity for sure
259 without doing a full simplification. And even that isn't possible if
260 the context has some free vars that may get unified. Urgle!
262 Here's another example: is this ambiguous?
263 forall a b. Eq (T b) => a -> a
264 Not if there's an insance decl (with no context)
265 instance Eq (T b) where ...
267 You may say of this example that we should use the instance decl right
268 away, but you can't always do that:
270 class J a b where ...
271 instance J Int b where ...
273 f :: forall a b. J a b => a -> a
275 (Notice: no functional dependency in J's class decl.)
276 Here f's type is perfectly fine, provided f is only called at Int.
277 It's premature to complain when meeting f's signature, or even
278 when inferring a type for f.
282 However, we don't *need* to report ambiguity right away. It'll always
283 show up at the call site.... and eventually at main, which needs special
284 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
286 So here's the plan. We WARN about probable ambiguity if
288 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
290 (all tested before quantification).
291 That is, all the type variables in Cq must be fixed by the the variables
292 in the environment, or by the variables in the type.
294 Notice that we union before calling oclose. Here's an example:
296 class J a b c | a b -> c
300 forall b c. (J a b c) => b -> b
302 Only if we union {a} from G with {b} from T before using oclose,
303 do we see that c is fixed.
305 It's a bit vague exactly which C we should use for this oclose call. If we
306 don't fix enough variables we might complain when we shouldn't (see
307 the above nasty example). Nothing will be perfect. That's why we can
308 only issue a warning.
311 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
313 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
315 then c is a "bubble"; there's no way it can ever improve, and it's
316 certainly ambiguous. UNLESS it is a constant (sigh). And what about
321 instance H x y => K (x,y)
323 Is this type ambiguous?
324 forall a b. (K (a,b), Eq b) => a -> a
326 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
327 is a "bubble" that's a set of constraints
329 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
331 Hence another idea. To decide Q start with fv(T) and grow it
332 by transitive closure in Cq (no functional dependencies involved).
333 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
334 The definitely-ambiguous can then float out, and get smashed at top level
335 (which squashes out the constants, like Eq (T a) above)
338 --------------------------------------
339 Notes on principal types
340 --------------------------------------
345 f x = let g y = op (y::Int) in True
347 Here the principal type of f is (forall a. a->a)
348 but we'll produce the non-principal type
349 f :: forall a. C Int => a -> a
352 --------------------------------------
353 The need for forall's in constraints
354 --------------------------------------
356 [Exchange on Haskell Cafe 5/6 Dec 2000]
358 class C t where op :: t -> Bool
359 instance C [t] where op x = True
361 p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
362 q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
364 The definitions of p and q differ only in the order of the components in
365 the pair on their right-hand sides. And yet:
367 ghc and "Typing Haskell in Haskell" reject p, but accept q;
368 Hugs rejects q, but accepts p;
369 hbc rejects both p and q;
370 nhc98 ... (Malcolm, can you fill in the blank for us!).
372 The type signature for f forces context reduction to take place, and
373 the results of this depend on whether or not the type of y is known,
374 which in turn depends on which component of the pair the type checker
377 Solution: if y::m a, float out the constraints
378 Monad m, forall c. C (m c)
379 When m is later unified with [], we can solve both constraints.
382 --------------------------------------
383 Notes on implicit parameters
384 --------------------------------------
386 Note [Inheriting implicit parameters]
387 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
392 where f is *not* a top-level binding.
393 From the RHS of f we'll get the constraint (?y::Int).
394 There are two types we might infer for f:
398 (so we get ?y from the context of f's definition), or
400 f :: (?y::Int) => Int -> Int
402 At first you might think the first was better, becuase then
403 ?y behaves like a free variable of the definition, rather than
404 having to be passed at each call site. But of course, the WHOLE
405 IDEA is that ?y should be passed at each call site (that's what
406 dynamic binding means) so we'd better infer the second.
408 BOTTOM LINE: when *inferring types* you *must* quantify
409 over implicit parameters. See the predicate isFreeWhenInferring.
412 Note [Implicit parameters and ambiguity]
413 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
414 Only a *class* predicate can give rise to ambiguity
415 An *implicit parameter* cannot. For example:
416 foo :: (?x :: [a]) => Int
418 is fine. The call site will suppply a particular 'x'
420 Furthermore, the type variables fixed by an implicit parameter
421 propagate to the others. E.g.
422 foo :: (Show a, ?x::[a]) => Int
424 The type of foo looks ambiguous. But it isn't, because at a call site
426 let ?x = 5::Int in foo
427 and all is well. In effect, implicit parameters are, well, parameters,
428 so we can take their type variables into account as part of the
429 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
432 Question 2: type signatures
433 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
434 BUT WATCH OUT: When you supply a type signature, we can't force you
435 to quantify over implicit parameters. For example:
439 This is perfectly reasonable. We do not want to insist on
441 (?x + 1) :: (?x::Int => Int)
443 That would be silly. Here, the definition site *is* the occurrence site,
444 so the above strictures don't apply. Hence the difference between
445 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
446 and tcSimplifyCheckBind (which does not).
448 What about when you supply a type signature for a binding?
449 Is it legal to give the following explicit, user type
450 signature to f, thus:
455 At first sight this seems reasonable, but it has the nasty property
456 that adding a type signature changes the dynamic semantics.
459 (let f x = (x::Int) + ?y
460 in (f 3, f 3 with ?y=5)) with ?y = 6
466 in (f 3, f 3 with ?y=5)) with ?y = 6
470 Indeed, simply inlining f (at the Haskell source level) would change the
473 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
474 semantics for a Haskell program without knowing its typing, so if you
475 change the typing you may change the semantics.
477 To make things consistent in all cases where we are *checking* against
478 a supplied signature (as opposed to inferring a type), we adopt the
481 a signature does not need to quantify over implicit params.
483 [This represents a (rather marginal) change of policy since GHC 5.02,
484 which *required* an explicit signature to quantify over all implicit
485 params for the reasons mentioned above.]
487 But that raises a new question. Consider
489 Given (signature) ?x::Int
490 Wanted (inferred) ?x::Int, ?y::Bool
492 Clearly we want to discharge the ?x and float the ?y out. But
493 what is the criterion that distinguishes them? Clearly it isn't
494 what free type variables they have. The Right Thing seems to be
495 to float a constraint that
496 neither mentions any of the quantified type variables
497 nor any of the quantified implicit parameters
499 See the predicate isFreeWhenChecking.
502 Question 3: monomorphism
503 ~~~~~~~~~~~~~~~~~~~~~~~~
504 There's a nasty corner case when the monomorphism restriction bites:
508 The argument above suggests that we *must* generalise
509 over the ?y parameter, to get
510 z :: (?y::Int) => Int,
511 but the monomorphism restriction says that we *must not*, giving
513 Why does the momomorphism restriction say this? Because if you have
515 let z = x + ?y in z+z
517 you might not expect the addition to be done twice --- but it will if
518 we follow the argument of Question 2 and generalise over ?y.
521 Question 4: top level
522 ~~~~~~~~~~~~~~~~~~~~~
523 At the top level, monomorhism makes no sense at all.
526 main = let ?x = 5 in print foo
530 woggle :: (?x :: Int) => Int -> Int
533 We definitely don't want (foo :: Int) with a top-level implicit parameter
534 (?x::Int) becuase there is no way to bind it.
539 (A) Always generalise over implicit parameters
540 Bindings that fall under the monomorphism restriction can't
544 * Inlining remains valid
545 * No unexpected loss of sharing
546 * But simple bindings like
548 will be rejected, unless you add an explicit type signature
549 (to avoid the monomorphism restriction)
550 z :: (?y::Int) => Int
552 This seems unacceptable
554 (B) Monomorphism restriction "wins"
555 Bindings that fall under the monomorphism restriction can't
557 Always generalise over implicit parameters *except* for bindings
558 that fall under the monomorphism restriction
561 * Inlining isn't valid in general
562 * No unexpected loss of sharing
563 * Simple bindings like
565 accepted (get value of ?y from binding site)
567 (C) Always generalise over implicit parameters
568 Bindings that fall under the monomorphism restriction can't
569 be generalised, EXCEPT for implicit parameters
571 * Inlining remains valid
572 * Unexpected loss of sharing (from the extra generalisation)
573 * Simple bindings like
575 accepted (get value of ?y from occurrence sites)
580 None of these choices seems very satisfactory. But at least we should
581 decide which we want to do.
583 It's really not clear what is the Right Thing To Do. If you see
587 would you expect the value of ?y to be got from the *occurrence sites*
588 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
589 case of function definitions, the answer is clearly the former, but
590 less so in the case of non-fucntion definitions. On the other hand,
591 if we say that we get the value of ?y from the definition site of 'z',
592 then inlining 'z' might change the semantics of the program.
594 Choice (C) really says "the monomorphism restriction doesn't apply
595 to implicit parameters". Which is fine, but remember that every
596 innocent binding 'x = ...' that mentions an implicit parameter in
597 the RHS becomes a *function* of that parameter, called at each
598 use of 'x'. Now, the chances are that there are no intervening 'with'
599 clauses that bind ?y, so a decent compiler should common up all
600 those function calls. So I think I strongly favour (C). Indeed,
601 one could make a similar argument for abolishing the monomorphism
602 restriction altogether.
604 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
608 %************************************************************************
610 \subsection{tcSimplifyInfer}
612 %************************************************************************
614 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
616 1. Compute Q = grow( fvs(T), C )
618 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
619 predicates will end up in Ct; we deal with them at the top level
621 3. Try improvement, using functional dependencies
623 4. If Step 3 did any unification, repeat from step 1
624 (Unification can change the result of 'grow'.)
626 Note: we don't reduce dictionaries in step 2. For example, if we have
627 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
628 after step 2. However note that we may therefore quantify over more
629 type variables than we absolutely have to.
631 For the guts, we need a loop, that alternates context reduction and
632 improvement with unification. E.g. Suppose we have
634 class C x y | x->y where ...
636 and tcSimplify is called with:
638 Then improvement unifies a with b, giving
641 If we need to unify anything, we rattle round the whole thing all over
648 -> TcTyVarSet -- fv(T); type vars
650 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked and quantified)
651 [Inst], -- Dict Ids that must be bound here (zonked)
652 TcDictBinds) -- Bindings
653 -- Any free (escaping) Insts are tossed into the environment
658 tcSimplifyInfer doc tau_tvs wanted
659 = do { tau_tvs1 <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
660 ; wanted' <- mappM zonkInst wanted -- Zonk before deciding quantified tyvars
661 ; gbl_tvs <- tcGetGlobalTyVars
662 ; let preds1 = fdPredsOfInsts wanted'
663 gbl_tvs1 = oclose preds1 gbl_tvs
664 qtvs = grow preds1 tau_tvs1 `minusVarSet` gbl_tvs1
665 -- See Note [Choosing which variables to quantify]
667 -- To maximise sharing, remove from consideration any
668 -- constraints that don't mention qtvs at all
669 ; let (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
672 -- To make types simple, reduce as much as possible
673 ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (grow preds1 tau_tvs1) $$ ppr gbl_tvs $$
674 ppr gbl_tvs1 $$ ppr free $$ ppr bound))
675 ; (irreds1, binds1) <- tryHardCheckLoop doc bound
677 -- Note [Inference and implication constraints]
678 ; let want_dict d = tyVarsOfInst d `intersectsVarSet` qtvs
679 ; (irreds2, binds2) <- approximateImplications doc want_dict irreds1
681 -- Now work out all over again which type variables to quantify,
682 -- exactly in the same way as before, but starting from irreds2. Why?
683 -- a) By now improvment may have taken place, and we must *not*
684 -- quantify over any variable free in the environment
685 -- tc137 (function h inside g) is an example
687 -- b) Do not quantify over constraints that *now* do not
688 -- mention quantified type variables, because they are
689 -- simply ambiguous (or might be bound further out). Example:
690 -- f :: Eq b => a -> (a, b)
692 -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
693 -- We decide to quantify over 'alpha' alone, but free1 does not include f77
694 -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
695 -- constraint (Eq beta), which we dump back into the free set
696 -- See test tcfail181
698 -- c) irreds may contain type variables not previously mentioned,
699 -- e.g. instance D a x => Foo [a]
701 -- Then after simplifying we'll get (D a x), and x is fresh
702 -- We must quantify over x else it'll be totally unbound
703 ; tau_tvs2 <- zonkTcTyVarsAndFV (varSetElems tau_tvs1)
704 ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
705 -- Note that we start from gbl_tvs1
706 -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
707 -- we've already put some of the original preds1 into frees
708 -- E.g. wanteds = C a b (where a->b)
711 -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
712 -- irreds2 will be empty. But we don't want to generalise over b!
713 ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
714 qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
715 ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
718 -- Turn the quantified meta-type variables into real type variables
719 ; qtvs2 <- zonkQuantifiedTyVars (varSetElems qtvs)
721 -- We can't abstract over any remaining unsolved
722 -- implications so instead just float them outwards. Ugh.
723 ; let (q_dicts0, implics) = partition isAbstractableInst irreds3
724 ; loc <- getInstLoc (ImplicOrigin doc)
725 ; implic_bind <- bindIrreds loc qtvs2 q_dicts0 implics
727 -- Prepare equality instances for quantification
728 ; let (q_eqs0,q_dicts) = partition isEqInst q_dicts0
729 ; q_eqs <- mappM finalizeEqInst q_eqs0
731 ; return (qtvs2, q_eqs ++ q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
732 -- NB: when we are done, we might have some bindings, but
733 -- the final qtvs might be empty. See Note [NO TYVARS] below.
735 approximateImplications :: SDoc -> (Inst -> Bool) -> [Inst] -> TcM ([Inst], TcDictBinds)
736 -- Note [Inference and implication constraints]
737 -- Given a bunch of Dict and ImplicInsts, try to approximate the implications by
738 -- - fetching any dicts inside them that are free
739 -- - using those dicts as cruder constraints, to solve the implications
740 -- - returning the extra ones too
742 approximateImplications doc want_dict irreds
744 = return (irreds, emptyBag)
746 = do { extra_dicts' <- mapM cloneDict extra_dicts
747 ; tryHardCheckLoop doc (extra_dicts' ++ irreds) }
748 -- By adding extra_dicts', we make them
749 -- available to solve the implication constraints
751 extra_dicts = get_dicts (filter isImplicInst irreds)
753 get_dicts :: [Inst] -> [Inst] -- Returns only Dicts
754 -- Find the wanted constraints in implication constraints that satisfy
755 -- want_dict, and are not bound by forall's in the constraint itself
756 get_dicts ds = concatMap get_dict ds
758 get_dict d@(Dict {}) | want_dict d = [d]
760 get_dict (ImplicInst {tci_tyvars = tvs, tci_wanted = wanteds})
761 = [ d | let tv_set = mkVarSet tvs
762 , d <- get_dicts wanteds
763 , not (tyVarsOfInst d `intersectsVarSet` tv_set)]
764 get_dict i@(EqInst {}) | want_dict i = [i]
766 get_dict other = pprPanic "approximateImplications" (ppr other)
769 Note [Inference and implication constraints]
770 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
771 Suppose we have a wanted implication constraint (perhaps arising from
772 a nested pattern match) like
774 and we are now trying to quantify over 'a' when inferring the type for
775 a function. In principle it's possible that there might be an instance
776 instance (C a, E a) => D [a]
777 so the context (E a) would suffice. The Right Thing is to abstract over
778 the implication constraint, but we don't do that (a) because it'll be
779 surprising to programmers and (b) because we don't have the machinery to deal
780 with 'given' implications.
782 So our best approximation is to make (D [a]) part of the inferred
783 context, so we can use that to discharge the implication. Hence
784 the strange function get_dictsin approximateImplications.
786 The common cases are more clear-cut, when we have things like
788 Here, abstracting over (C b) is not an approximation at all -- but see
789 Note [Freeness and implications].
791 See Trac #1430 and test tc228.
795 -----------------------------------------------------------
796 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
797 -- against, but we don't know the type variables over which we are going to quantify.
798 -- This happens when we have a type signature for a mutually recursive group
801 -> TcTyVarSet -- fv(T)
804 -> TcM ([TyVar], -- Fully zonked, and quantified
805 TcDictBinds) -- Bindings
807 tcSimplifyInferCheck loc tau_tvs givens wanteds
808 = do { traceTc (text "tcSimplifyInferCheck <-" <+> ppr wanteds)
809 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
811 -- Figure out which type variables to quantify over
812 -- You might think it should just be the signature tyvars,
813 -- but in bizarre cases you can get extra ones
814 -- f :: forall a. Num a => a -> a
815 -- f x = fst (g (x, head [])) + 1
817 -- Here we infer g :: forall a b. a -> b -> (b,a)
818 -- We don't want g to be monomorphic in b just because
819 -- f isn't quantified over b.
820 ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
821 ; all_tvs <- zonkTcTyVarsAndFV all_tvs
822 ; gbl_tvs <- tcGetGlobalTyVars
823 ; let qtvs = varSetElems (all_tvs `minusVarSet` gbl_tvs)
824 -- We could close gbl_tvs, but its not necessary for
825 -- soundness, and it'll only affect which tyvars, not which
826 -- dictionaries, we quantify over
828 ; qtvs' <- zonkQuantifiedTyVars qtvs
830 -- Now we are back to normal (c.f. tcSimplCheck)
831 ; implic_bind <- bindIrreds loc qtvs' givens irreds
833 ; traceTc (text "tcSimplifyInferCheck ->" <+> ppr (implic_bind))
834 ; return (qtvs', binds `unionBags` implic_bind) }
837 Note [Squashing methods]
838 ~~~~~~~~~~~~~~~~~~~~~~~~~
839 Be careful if you want to float methods more:
840 truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
841 From an application (truncate f i) we get
844 If we have also have a second occurrence of truncate, we get
847 When simplifying with i,f free, we might still notice that
848 t1=t3; but alas, the binding for t2 (which mentions t1)
849 may continue to float out!
854 class Y a b | a -> b where
857 instance Y [[a]] a where
860 k :: X a -> X a -> X a
862 g :: Num a => [X a] -> [X a]
865 h ys = ys ++ map (k (y [[0]])) xs
867 The excitement comes when simplifying the bindings for h. Initially
868 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
869 From this we get t1:=:t2, but also various bindings. We can't forget
870 the bindings (because of [LOOP]), but in fact t1 is what g is
873 The net effect of [NO TYVARS]
876 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
877 isFreeWhenInferring qtvs inst
878 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
879 && isInheritableInst inst -- and no implicit parameter involved
880 -- see Note [Inheriting implicit parameters]
882 {- No longer used (with implication constraints)
883 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
884 -> NameSet -- Quantified implicit parameters
886 isFreeWhenChecking qtvs ips inst
887 = isFreeWrtTyVars qtvs inst
888 && isFreeWrtIPs ips inst
891 isFreeWrtTyVars qtvs inst = tyVarsOfInst inst `disjointVarSet` qtvs
892 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
896 %************************************************************************
898 \subsection{tcSimplifyCheck}
900 %************************************************************************
902 @tcSimplifyCheck@ is used when we know exactly the set of variables
903 we are going to quantify over. For example, a class or instance declaration.
906 -----------------------------------------------------------
907 -- tcSimplifyCheck is used when checking expression type signatures,
908 -- class decls, instance decls etc.
909 tcSimplifyCheck :: InstLoc
910 -> [TcTyVar] -- Quantify over these
913 -> TcM TcDictBinds -- Bindings
914 tcSimplifyCheck loc qtvs givens wanteds
915 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
916 do { traceTc (text "tcSimplifyCheck")
917 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
918 ; implic_bind <- bindIrreds loc qtvs givens irreds
919 ; return (binds `unionBags` implic_bind) }
921 -----------------------------------------------------------
922 -- tcSimplifyCheckPat is used for existential pattern match
923 tcSimplifyCheckPat :: InstLoc
924 -> [CoVar] -> Refinement
925 -> [TcTyVar] -- Quantify over these
928 -> TcM TcDictBinds -- Bindings
929 tcSimplifyCheckPat loc co_vars reft qtvs givens wanteds
930 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
931 do { traceTc (text "tcSimplifyCheckPat")
932 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
933 ; implic_bind <- bindIrredsR loc qtvs co_vars reft
935 ; return (binds `unionBags` implic_bind) }
937 -----------------------------------------------------------
938 bindIrreds :: InstLoc -> [TcTyVar]
941 bindIrreds loc qtvs givens irreds
942 = bindIrredsR loc qtvs [] emptyRefinement givens irreds
944 bindIrredsR :: InstLoc -> [TcTyVar] -> [CoVar]
945 -> Refinement -> [Inst] -> [Inst]
947 -- Make a binding that binds 'irreds', by generating an implication
948 -- constraint for them, *and* throwing the constraint into the LIE
949 bindIrredsR loc qtvs co_vars reft givens irreds
953 = do { let givens' = filter isDict givens
954 -- The givens can include methods
955 -- See Note [Pruning the givens in an implication constraint]
957 -- If there are no 'givens' *and* the refinement is empty
958 -- (the refinement is like more givens), then it's safe to
959 -- partition the 'wanteds' by their qtvs, thereby trimming irreds
960 -- See Note [Freeness and implications]
961 ; irreds' <- if null givens' && isEmptyRefinement reft
963 { let qtv_set = mkVarSet qtvs
964 (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
966 ; return real_irreds }
969 ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
970 ; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds'
971 -- This call does the real work
972 -- If irreds' is empty, it does something sensible
977 makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement
979 -> TcM ([Inst], TcDictBinds)
980 -- Make a binding that binds 'irreds', by generating an implication
981 -- constraint for them, *and* throwing the constraint into the LIE
982 -- The binding looks like
983 -- (ir1, .., irn) = f qtvs givens
984 -- where f is (evidence for) the new implication constraint
985 -- f :: forall qtvs. {reft} givens => (ir1, .., irn)
986 -- qtvs includes coercion variables
988 -- This binding must line up the 'rhs' in reduceImplication
989 makeImplicationBind loc all_tvs reft
990 givens -- Guaranteed all Dicts (TOMDO: true?)
992 | null irreds -- If there are no irreds, we are done
993 = return ([], emptyBag)
994 | otherwise -- Otherwise we must generate a binding
995 = do { uniq <- newUnique
996 ; span <- getSrcSpanM
997 ; let (eq_givens, dict_givens) = partition isEqInst givens
998 eq_tyvar_cos = map TyVarTy $ uniqSetToList $ tyVarsOfTypes $ map eqInstType eq_givens
999 ; let name = mkInternalName uniq (mkVarOcc "ic") span
1000 implic_inst = ImplicInst { tci_name = name, tci_reft = reft,
1001 tci_tyvars = all_tvs,
1002 tci_given = (eq_givens ++ dict_givens),
1003 tci_wanted = irreds, tci_loc = loc }
1004 ; let -- only create binder for dict_irreds
1005 (eq_irreds, dict_irreds) = partition isEqInst irreds
1006 n_dict_irreds = length dict_irreds
1007 dict_irred_ids = map instToId dict_irreds
1008 tup_ty = mkTupleTy Boxed n_dict_irreds (map idType dict_irred_ids)
1009 pat = TuplePat (map nlVarPat dict_irred_ids) Boxed tup_ty
1010 rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
1011 co = mkWpApps (map instToId dict_givens) <.> mkWpTyApps eq_tyvar_cos <.> mkWpTyApps (mkTyVarTys all_tvs)
1012 bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs
1013 | otherwise = PatBind { pat_lhs = L span pat,
1014 pat_rhs = unguardedGRHSs rhs,
1015 pat_rhs_ty = tup_ty,
1016 bind_fvs = placeHolderNames }
1017 ; -- pprTrace "Make implic inst" (ppr (implic_inst,irreds,dict_irreds,tup_ty)) $
1018 return ([implic_inst], unitBag (L span bind)) }
1020 -----------------------------------------------------------
1021 tryHardCheckLoop :: SDoc
1023 -> TcM ([Inst], TcDictBinds)
1025 tryHardCheckLoop doc wanteds
1026 = do { (irreds,binds,_) <- checkLoop (mkRedEnv doc try_me []) wanteds
1027 ; return (irreds,binds)
1030 try_me inst = ReduceMe AddSCs
1031 -- Here's the try-hard bit
1033 -----------------------------------------------------------
1034 gentleCheckLoop :: InstLoc
1037 -> TcM ([Inst], TcDictBinds)
1039 gentleCheckLoop inst_loc givens wanteds
1040 = do { (irreds,binds,_) <- checkLoop env wanteds
1041 ; return (irreds,binds)
1044 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
1046 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1048 -- When checking against a given signature
1049 -- we MUST be very gentle: Note [Check gently]
1053 ~~~~~~~~~~~~~~~~~~~~
1054 We have to very careful about not simplifying too vigorously
1059 f :: Show b => T b -> b
1060 f (MkT x) = show [x]
1062 Inside the pattern match, which binds (a:*, x:a), we know that
1064 Hence we have a dictionary for Show [a] available; and indeed we
1065 need it. We are going to build an implication contraint
1066 forall a. (b~[a]) => Show [a]
1067 Later, we will solve this constraint using the knowledg e(Show b)
1069 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
1070 thing becomes insoluble. So we simplify gently (get rid of literals
1071 and methods only, plus common up equal things), deferring the real
1072 work until top level, when we solve the implication constraint
1073 with tryHardCheckLooop.
1077 -----------------------------------------------------------
1080 -> TcM ([Inst], TcDictBinds,
1081 [Inst]) -- needed givens
1082 -- Precondition: givens are completely rigid
1083 -- Postcondition: returned Insts are zonked
1085 checkLoop env wanteds
1087 where go env wanteds needed_givens
1088 = do { -- Givens are skolems, so no need to zonk them
1089 wanteds' <- zonkInsts wanteds
1091 ; (improved, binds, irreds, more_needed_givens) <- reduceContext env wanteds'
1093 ; let all_needed_givens = needed_givens ++ more_needed_givens
1095 ; if not improved then
1096 return (irreds, binds, all_needed_givens)
1099 -- If improvement did some unification, we go round again.
1100 -- We start again with irreds, not wanteds
1101 -- Using an instance decl might have introduced a fresh type variable
1102 -- which might have been unified, so we'd get an infinite loop
1103 -- if we started again with wanteds! See Note [LOOP]
1104 { (irreds1, binds1, all_needed_givens1) <- go env irreds all_needed_givens
1105 ; return (irreds1, binds `unionBags` binds1, all_needed_givens1) } }
1110 class If b t e r | b t e -> r
1113 class Lte a b c | a b -> c where lte :: a -> b -> c
1115 instance (Lte a b l,If l b a c) => Max a b c
1117 Wanted: Max Z (S x) y
1119 Then we'll reduce using the Max instance to:
1120 (Lte Z (S x) l, If l (S x) Z y)
1121 and improve by binding l->T, after which we can do some reduction
1122 on both the Lte and If constraints. What we *can't* do is start again
1123 with (Max Z (S x) y)!
1127 %************************************************************************
1129 tcSimplifySuperClasses
1131 %************************************************************************
1133 Note [SUPERCLASS-LOOP 1]
1134 ~~~~~~~~~~~~~~~~~~~~~~~~
1135 We have to be very, very careful when generating superclasses, lest we
1136 accidentally build a loop. Here's an example:
1140 class S a => C a where { opc :: a -> a }
1141 class S b => D b where { opd :: b -> b }
1143 instance C Int where
1146 instance D Int where
1149 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1150 Simplifying, we may well get:
1151 $dfCInt = :C ds1 (opd dd)
1154 Notice that we spot that we can extract ds1 from dd.
1156 Alas! Alack! We can do the same for (instance D Int):
1158 $dfDInt = :D ds2 (opc dc)
1162 And now we've defined the superclass in terms of itself.
1164 Solution: never generate a superclass selectors at all when
1165 satisfying the superclass context of an instance declaration.
1167 Two more nasty cases are in
1172 tcSimplifySuperClasses
1177 tcSimplifySuperClasses loc givens sc_wanteds
1178 = do { traceTc (text "tcSimplifySuperClasses")
1179 ; (irreds,binds1,_) <- checkLoop env sc_wanteds
1180 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1181 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1184 env = mkRedEnv (pprInstLoc loc) try_me givens
1185 try_me inst = ReduceMe NoSCs
1186 -- Like tryHardCheckLoop, but with NoSCs
1190 %************************************************************************
1192 \subsection{tcSimplifyRestricted}
1194 %************************************************************************
1196 tcSimplifyRestricted infers which type variables to quantify for a
1197 group of restricted bindings. This isn't trivial.
1200 We want to quantify over a to get id :: forall a. a->a
1203 We do not want to quantify over a, because there's an Eq a
1204 constraint, so we get eq :: a->a->Bool (notice no forall)
1207 RHS has type 'tau', whose free tyvars are tau_tvs
1208 RHS has constraints 'wanteds'
1211 Quantify over (tau_tvs \ ftvs(wanteds))
1212 This is bad. The constraints may contain (Monad (ST s))
1213 where we have instance Monad (ST s) where...
1214 so there's no need to be monomorphic in s!
1216 Also the constraint might be a method constraint,
1217 whose type mentions a perfectly innocent tyvar:
1218 op :: Num a => a -> b -> a
1219 Here, b is unconstrained. A good example would be
1221 We want to infer the polymorphic type
1222 foo :: forall b. b -> b
1225 Plan B (cunning, used for a long time up to and including GHC 6.2)
1226 Step 1: Simplify the constraints as much as possible (to deal
1227 with Plan A's problem). Then set
1228 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1230 Step 2: Now simplify again, treating the constraint as 'free' if
1231 it does not mention qtvs, and trying to reduce it otherwise.
1232 The reasons for this is to maximise sharing.
1234 This fails for a very subtle reason. Suppose that in the Step 2
1235 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1236 In the Step 1 this constraint might have been simplified, perhaps to
1237 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1238 This won't happen in Step 2... but that in turn might prevent some other
1239 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1240 and that in turn breaks the invariant that no constraints are quantified over.
1242 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1247 Step 1: Simplify the constraints as much as possible (to deal
1248 with Plan A's problem). Then set
1249 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1250 Return the bindings from Step 1.
1253 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1256 instance (HasBinary ty IO) => HasCodedValue ty
1258 foo :: HasCodedValue a => String -> IO a
1260 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1261 doDecodeIO codedValue view
1262 = let { act = foo "foo" } in act
1264 You might think this should work becuase the call to foo gives rise to a constraint
1265 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1266 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1267 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1269 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1273 Plan D (a variant of plan B)
1274 Step 1: Simplify the constraints as much as possible (to deal
1275 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1276 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1278 Step 2: Now simplify again, treating the constraint as 'free' if
1279 it does not mention qtvs, and trying to reduce it otherwise.
1281 The point here is that it's generally OK to have too few qtvs; that is,
1282 to make the thing more monomorphic than it could be. We don't want to
1283 do that in the common cases, but in wierd cases it's ok: the programmer
1284 can always add a signature.
1286 Too few qtvs => too many wanteds, which is what happens if you do less
1291 tcSimplifyRestricted -- Used for restricted binding groups
1292 -- i.e. ones subject to the monomorphism restriction
1295 -> [Name] -- Things bound in this group
1296 -> TcTyVarSet -- Free in the type of the RHSs
1297 -> [Inst] -- Free in the RHSs
1298 -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
1299 TcDictBinds) -- Bindings
1300 -- tcSimpifyRestricted returns no constraints to
1301 -- quantify over; by definition there are none.
1302 -- They are all thrown back in the LIE
1304 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1305 -- Zonk everything in sight
1306 = do { traceTc (text "tcSimplifyRestricted")
1307 ; wanteds' <- zonkInsts wanteds
1309 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1310 -- dicts; the idea is to get rid of as many type
1311 -- variables as possible, and we don't want to stop
1312 -- at (say) Monad (ST s), because that reduces
1313 -- immediately, with no constraint on s.
1315 -- BUT do no improvement! See Plan D above
1316 -- HOWEVER, some unification may take place, if we instantiate
1317 -- a method Inst with an equality constraint
1318 ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
1319 ; (_imp, _binds, constrained_dicts, _) <- reduceContext env wanteds'
1321 -- Next, figure out the tyvars we will quantify over
1322 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1323 ; gbl_tvs' <- tcGetGlobalTyVars
1324 ; constrained_dicts' <- zonkInsts constrained_dicts
1326 ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
1327 -- As in tcSimplifyInfer
1329 -- Do not quantify over constrained type variables:
1330 -- this is the monomorphism restriction
1331 constrained_tvs' = tyVarsOfInsts constrained_dicts'
1332 qtvs = qtvs1 `minusVarSet` constrained_tvs'
1333 pp_bndrs = pprWithCommas (quotes . ppr) bndrs
1336 ; warn_mono <- doptM Opt_WarnMonomorphism
1337 ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
1338 (vcat[ ptext SLIT("the Monomorphism Restriction applies to the binding")
1339 <> plural bndrs <+> ptext SLIT("for") <+> pp_bndrs,
1340 ptext SLIT("Consider giving a type signature for") <+> pp_bndrs])
1342 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1343 pprInsts wanteds, pprInsts constrained_dicts',
1345 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1347 -- The first step may have squashed more methods than
1348 -- necessary, so try again, this time more gently, knowing the exact
1349 -- set of type variables to quantify over.
1351 -- We quantify only over constraints that are captured by qtvs;
1352 -- these will just be a subset of non-dicts. This in contrast
1353 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1354 -- all *non-inheritable* constraints too. This implements choice
1355 -- (B) under "implicit parameter and monomorphism" above.
1357 -- Remember that we may need to do *some* simplification, to
1358 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1359 -- just to float all constraints
1361 -- At top level, we *do* squash methods becuase we want to
1362 -- expose implicit parameters to the test that follows
1363 ; let is_nested_group = isNotTopLevel top_lvl
1364 try_me inst | isFreeWrtTyVars qtvs inst,
1365 (is_nested_group || isDict inst) = Stop
1366 | otherwise = ReduceMe AddSCs
1367 env = mkNoImproveRedEnv doc try_me
1368 ; (_imp, binds, irreds, _) <- reduceContext env wanteds'
1370 -- See "Notes on implicit parameters, Question 4: top level"
1371 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1372 if is_nested_group then
1374 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1375 ; addTopIPErrs bndrs bad_ips
1376 ; extendLIEs non_ips }
1378 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1379 ; return (qtvs', binds) }
1383 %************************************************************************
1387 %************************************************************************
1389 On the LHS of transformation rules we only simplify methods and constants,
1390 getting dictionaries. We want to keep all of them unsimplified, to serve
1391 as the available stuff for the RHS of the rule.
1393 Example. Consider the following left-hand side of a rule
1395 f (x == y) (y > z) = ...
1397 If we typecheck this expression we get constraints
1399 d1 :: Ord a, d2 :: Eq a
1401 We do NOT want to "simplify" to the LHS
1403 forall x::a, y::a, z::a, d1::Ord a.
1404 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1408 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1409 f ((==) d2 x y) ((>) d1 y z) = ...
1411 Here is another example:
1413 fromIntegral :: (Integral a, Num b) => a -> b
1414 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1416 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1417 we *dont* want to get
1419 forall dIntegralInt.
1420 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1422 because the scsel will mess up RULE matching. Instead we want
1424 forall dIntegralInt, dNumInt.
1425 fromIntegral Int Int dIntegralInt dNumInt = id Int
1429 g (x == y) (y == z) = ..
1431 where the two dictionaries are *identical*, we do NOT WANT
1433 forall x::a, y::a, z::a, d1::Eq a
1434 f ((==) d1 x y) ((>) d1 y z) = ...
1436 because that will only match if the dict args are (visibly) equal.
1437 Instead we want to quantify over the dictionaries separately.
1439 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1440 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1441 from scratch, rather than further parameterise simpleReduceLoop etc
1444 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1445 tcSimplifyRuleLhs wanteds
1446 = go [] emptyBag wanteds
1449 = return (dicts, binds)
1450 go dicts binds (w:ws)
1452 = go (w:dicts) binds ws
1454 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1455 -- to fromInteger; this looks fragile to me
1456 ; lookup_result <- lookupSimpleInst w'
1457 ; case lookup_result of
1458 GenInst ws' rhs -> go dicts (addBind binds w rhs) (ws' ++ ws)
1459 NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w)
1463 tcSimplifyBracket is used when simplifying the constraints arising from
1464 a Template Haskell bracket [| ... |]. We want to check that there aren't
1465 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1466 Show instance), but we aren't otherwise interested in the results.
1467 Nor do we care about ambiguous dictionaries etc. We will type check
1468 this bracket again at its usage site.
1471 tcSimplifyBracket :: [Inst] -> TcM ()
1472 tcSimplifyBracket wanteds
1473 = do { tryHardCheckLoop doc wanteds
1476 doc = text "tcSimplifyBracket"
1480 %************************************************************************
1482 \subsection{Filtering at a dynamic binding}
1484 %************************************************************************
1489 we must discharge all the ?x constraints from B. We also do an improvement
1490 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1492 Actually, the constraints from B might improve the types in ?x. For example
1494 f :: (?x::Int) => Char -> Char
1497 then the constraint (?x::Int) arising from the call to f will
1498 force the binding for ?x to be of type Int.
1501 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1504 -- We need a loop so that we do improvement, and then
1505 -- (next time round) generate a binding to connect the two
1507 -- Here the two ?x's have different types, and improvement
1508 -- makes them the same.
1510 tcSimplifyIPs given_ips wanteds
1511 = do { wanteds' <- zonkInsts wanteds
1512 ; given_ips' <- zonkInsts given_ips
1513 -- Unusually for checking, we *must* zonk the given_ips
1515 ; let env = mkRedEnv doc try_me given_ips'
1516 ; (improved, binds, irreds, _) <- reduceContext env wanteds'
1518 ; if not improved then
1519 ASSERT( all is_free irreds )
1520 do { extendLIEs irreds
1523 tcSimplifyIPs given_ips wanteds }
1525 doc = text "tcSimplifyIPs" <+> ppr given_ips
1526 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1527 is_free inst = isFreeWrtIPs ip_set inst
1529 -- Simplify any methods that mention the implicit parameter
1530 try_me inst | is_free inst = Stop
1531 | otherwise = ReduceMe NoSCs
1535 %************************************************************************
1537 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1539 %************************************************************************
1541 When doing a binding group, we may have @Insts@ of local functions.
1542 For example, we might have...
1544 let f x = x + 1 -- orig local function (overloaded)
1545 f.1 = f Int -- two instances of f
1550 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1551 where @f@ is in scope; those @Insts@ must certainly not be passed
1552 upwards towards the top-level. If the @Insts@ were binding-ified up
1553 there, they would have unresolvable references to @f@.
1555 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1556 For each method @Inst@ in the @init_lie@ that mentions one of the
1557 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1558 @LIE@), as well as the @HsBinds@ generated.
1561 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1562 -- Simlifies only MethodInsts, and generate only bindings of form
1564 -- We're careful not to even generate bindings of the form
1566 -- You'd think that'd be fine, but it interacts with what is
1567 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1569 bindInstsOfLocalFuns wanteds local_ids
1570 | null overloaded_ids
1572 = extendLIEs wanteds `thenM_`
1573 returnM emptyLHsBinds
1576 = do { (irreds, binds,_) <- checkLoop env for_me
1577 ; extendLIEs not_for_me
1581 env = mkRedEnv doc try_me []
1582 doc = text "bindInsts" <+> ppr local_ids
1583 overloaded_ids = filter is_overloaded local_ids
1584 is_overloaded id = isOverloadedTy (idType id)
1585 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1587 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1588 -- so it's worth building a set, so that
1589 -- lookup (in isMethodFor) is faster
1590 try_me inst | isMethod inst = ReduceMe NoSCs
1595 %************************************************************************
1597 \subsection{Data types for the reduction mechanism}
1599 %************************************************************************
1601 The main control over context reduction is here
1605 = RedEnv { red_doc :: SDoc -- The context
1606 , red_try_me :: Inst -> WhatToDo
1607 , red_improve :: Bool -- True <=> do improvement
1608 , red_givens :: [Inst] -- All guaranteed rigid
1610 -- but see Note [Rigidity]
1611 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1612 -- See Note [RedStack]
1616 -- The red_givens are rigid so far as cmpInst is concerned.
1617 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1618 -- let ?x = e in ...
1619 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1620 -- But that doesn't affect the comparison, which is based only on mame.
1623 -- The red_stack pair (n,insts) pair is just used for error reporting.
1624 -- 'n' is always the depth of the stack.
1625 -- The 'insts' is the stack of Insts being reduced: to produce X
1626 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1629 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1630 mkRedEnv doc try_me givens
1631 = RedEnv { red_doc = doc, red_try_me = try_me,
1632 red_givens = givens, red_stack = (0,[]),
1633 red_improve = True }
1635 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1636 -- Do not do improvement; no givens
1637 mkNoImproveRedEnv doc try_me
1638 = RedEnv { red_doc = doc, red_try_me = try_me,
1639 red_givens = [], red_stack = (0,[]),
1640 red_improve = True }
1643 = ReduceMe WantSCs -- Try to reduce this
1644 -- If there's no instance, add the inst to the
1645 -- irreductible ones, but don't produce an error
1646 -- message of any kind.
1647 -- It might be quite legitimate such as (Eq a)!
1649 | Stop -- Return as irreducible unless it can
1650 -- be reduced to a constant in one step
1651 -- Do not add superclasses; see
1653 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1654 -- of a predicate when adding it to the avails
1655 -- The reason for this flag is entirely the super-class loop problem
1656 -- Note [SUPER-CLASS LOOP 1]
1660 %************************************************************************
1662 \subsection[reduce]{@reduce@}
1664 %************************************************************************
1666 Note [Ancestor Equalities]
1667 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1668 During context reduction, we add to the wanted equalities also those
1669 equalities that (transitively) occur in superclass contexts of wanted
1670 class constraints. Consider the following code
1672 class a ~ Int => C a
1675 If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
1676 substituting Int for a. Hence, we ultimately want (C Int), which we
1677 discharge with the explicit instance.
1680 reduceContext :: RedEnv
1682 -> TcM (ImprovementDone,
1683 TcDictBinds, -- Dictionary bindings
1684 [Inst], -- Irreducible
1685 [Inst]) -- Needed givens
1687 reduceContext env wanteds
1688 = do { traceTc (text "reduceContext" <+> (vcat [
1689 text "----------------------",
1691 text "given" <+> ppr (red_givens env),
1692 text "wanted" <+> ppr wanteds,
1693 text "----------------------"
1696 ; let givens = red_givens env
1697 (given_eqs0, given_dicts0) = partition isEqInst givens
1698 (wanted_eqs0, wanted_dicts) = partition isEqInst wanteds
1700 -- We want to add as wanted equalities those that (transitively)
1701 -- occur in superclass contexts of wanted class constraints.
1702 -- See Note [Ancestor Equalities]
1703 ; ancestor_eqs <- ancestorEqualities wanted_dicts
1704 ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
1705 ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
1707 -- 1. Normalise the *given* *equality* constraints
1708 ; (given_eqs, eliminate_skolems) <- normaliseGivens given_eqs0
1710 -- 2. Normalise the *given* *dictionary* constraints
1711 -- wrt. the toplevel and given equations
1712 ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
1715 -- 3. Solve the *wanted* *equation* constraints
1716 ; eq_irreds0 <- solveWanteds given_eqs wanted_eqs
1718 -- 4. Normalise the *wanted* equality constraints with respect to
1720 ; eq_irreds <- normaliseWanteds eq_irreds0
1722 -- 5. Build the Avail mapping from "given_dicts"
1723 ; init_state <- foldlM addGiven emptyAvails given_dicts
1725 -- 6. Solve the *wanted* *dictionary* constraints
1726 -- This may expose some further equational constraints...
1727 ; wanted_dicts' <- zonkInsts wanted_dicts
1728 ; avails <- reduceList env wanted_dicts' init_state
1729 ; (binds, irreds0, needed_givens) <- extractResults avails wanted_dicts'
1730 ; traceTc $ text "reduceContext extractresults" <+> vcat
1731 [ppr avails,ppr wanted_dicts',ppr binds,ppr needed_givens]
1733 -- 7. Normalise the *wanted* *dictionary* constraints
1734 -- wrt. the toplevel and given equations
1735 ; (irreds1,normalise_binds1) <- normaliseWantedDicts given_eqs irreds0
1737 -- 8. Substitute the wanted *equations* in the wanted *dictionaries*
1738 ; (irreds,normalise_binds2) <- substEqInDictInsts eq_irreds irreds1
1740 -- 9. eliminate the artificial skolem constants introduced in 1.
1743 -- If there was some FD improvement,
1744 -- or new wanted equations have been exposed,
1745 -- we should have another go at solving.
1746 ; let improved = availsImproved avails
1747 || (not $ isEmptyBag normalise_binds1)
1748 || (not $ isEmptyBag normalise_binds2)
1749 || (any isEqInst irreds)
1751 ; traceTc (text "reduceContext end" <+> (vcat [
1752 text "----------------------",
1754 text "given" <+> ppr (red_givens env),
1755 text "wanted" <+> ppr wanteds,
1757 text "avails" <+> pprAvails avails,
1758 text "improved =" <+> ppr improved,
1759 text "irreds = " <+> ppr irreds,
1760 text "binds = " <+> ppr binds,
1761 text "needed givens = " <+> ppr needed_givens,
1762 text "----------------------"
1766 given_binds `unionBags` normalise_binds1
1767 `unionBags` normalise_binds2
1769 irreds ++ eq_irreds,
1773 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1774 tcImproveOne avails inst
1775 | not (isDict inst) = return False
1777 = do { inst_envs <- tcGetInstEnvs
1778 ; let eqns = improveOne (classInstances inst_envs)
1779 (dictPred inst, pprInstArising inst)
1780 [ (dictPred p, pprInstArising p)
1781 | p <- availsInsts avails, isDict p ]
1782 -- Avails has all the superclasses etc (good)
1783 -- It also has all the intermediates of the deduction (good)
1784 -- It does not have duplicates (good)
1785 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1786 -- so that improve will see them separate
1787 ; traceTc (text "improveOne" <+> ppr inst)
1790 unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))]
1791 -> TcM ImprovementDone
1792 unifyEqns [] = return False
1794 = do { traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns))
1798 unify ((qtvs, pairs), what1, what2)
1799 = addErrCtxtM (mkEqnMsg what1 what2) $
1800 tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1801 mapM_ (unif_pr tenv) pairs
1802 unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1804 pprEquationDoc (eqn, (p1,w1), (p2,w2)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
1806 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
1807 = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
1808 ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
1809 ; let msg = vcat [ptext SLIT("When using functional dependencies to combine"),
1810 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
1811 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
1812 ; return (tidy_env, msg) }
1815 The main context-reduction function is @reduce@. Here's its game plan.
1818 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
1819 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
1820 = do { dopts <- getDOpts
1823 dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n)
1824 2 (ifPprDebug (nest 2 (pprStack stk))))
1827 ; if n >= ctxtStkDepth dopts then
1828 failWithTc (reduceDepthErr n stk)
1832 go [] state = return state
1833 go (w:ws) state = do { traceTc (text "reduceList " <+> ppr (w:ws) <+> ppr state)
1834 ; state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
1837 -- Base case: we're done!
1838 reduce env wanted avails
1839 -- It's the same as an existing inst, or a superclass thereof
1840 | Just avail <- findAvail avails wanted
1841 = do { traceTc (text "reduce: found " <+> ppr wanted)
1846 = do { traceTc (text "reduce" <+> ppr avails <+> ppr wanted)
1847 ; case red_try_me env wanted of {
1848 Stop -> try_simple (addIrred NoSCs);
1849 -- See Note [No superclasses for Stop]
1851 ReduceMe want_scs -> do -- It should be reduced
1852 { (avails, lookup_result) <- reduceInst env avails wanted
1853 ; case lookup_result of
1854 NoInstance -> addIrred want_scs avails wanted
1855 -- Add it and its superclasses
1857 GenInst [] rhs -> addWanted want_scs avails wanted rhs []
1859 GenInst wanteds' rhs
1860 -> do { avails1 <- addIrred NoSCs avails wanted
1861 ; avails2 <- reduceList env wanteds' avails1
1862 ; addWanted want_scs avails2 wanted rhs wanteds' } }
1863 -- Temporarily do addIrred *before* the reduceList,
1864 -- which has the effect of adding the thing we are trying
1865 -- to prove to the database before trying to prove the things it
1866 -- needs. See note [RECURSIVE DICTIONARIES]
1867 -- NB: we must not do an addWanted before, because that adds the
1868 -- superclasses too, and that can lead to a spurious loop; see
1869 -- the examples in [SUPERCLASS-LOOP]
1870 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1873 -- First, see if the inst can be reduced to a constant in one step
1874 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
1875 -- Don't bother for implication constraints, which take real work
1876 try_simple do_this_otherwise
1877 = do { res <- lookupSimpleInst wanted
1879 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
1880 other -> do_this_otherwise avails wanted }
1884 Note [SUPERCLASS-LOOP 2]
1885 ~~~~~~~~~~~~~~~~~~~~~~~~
1886 But the above isn't enough. Suppose we are *given* d1:Ord a,
1887 and want to deduce (d2:C [a]) where
1889 class Ord a => C a where
1890 instance Ord [a] => C [a] where ...
1892 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
1893 superclasses of C [a] to avails. But we must not overwrite the binding
1894 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
1897 Here's another variant, immortalised in tcrun020
1898 class Monad m => C1 m
1899 class C1 m => C2 m x
1900 instance C2 Maybe Bool
1901 For the instance decl we need to build (C1 Maybe), and it's no good if
1902 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1903 before we search for C1 Maybe.
1905 Here's another example
1906 class Eq b => Foo a b
1907 instance Eq a => Foo [a] a
1911 we'll first deduce that it holds (via the instance decl). We must not
1912 then overwrite the Eq t constraint with a superclass selection!
1914 At first I had a gross hack, whereby I simply did not add superclass constraints
1915 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1916 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1917 I found a very obscure program (now tcrun021) in which improvement meant the
1918 simplifier got two bites a the cherry... so something seemed to be an Stop
1919 first time, but reducible next time.
1921 Now we implement the Right Solution, which is to check for loops directly
1922 when adding superclasses. It's a bit like the occurs check in unification.
1925 Note [RECURSIVE DICTIONARIES]
1926 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1928 data D r = ZeroD | SuccD (r (D r));
1930 instance (Eq (r (D r))) => Eq (D r) where
1931 ZeroD == ZeroD = True
1932 (SuccD a) == (SuccD b) = a == b
1935 equalDC :: D [] -> D [] -> Bool;
1938 We need to prove (Eq (D [])). Here's how we go:
1942 by instance decl, holds if
1946 by instance decl of Eq, holds if
1948 where d2 = dfEqList d3
1951 But now we can "tie the knot" to give
1957 and it'll even run! The trick is to put the thing we are trying to prove
1958 (in this case Eq (D []) into the database before trying to prove its
1959 contributing clauses.
1962 %************************************************************************
1964 Reducing a single constraint
1966 %************************************************************************
1969 ---------------------------------------------
1970 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
1971 reduceInst env avails (ImplicInst { tci_tyvars = tvs, tci_reft = reft, tci_loc = loc,
1972 tci_given = extra_givens, tci_wanted = wanteds })
1973 = reduceImplication env avails reft tvs extra_givens wanteds loc
1975 reduceInst env avails other_inst
1976 = do { result <- lookupSimpleInst other_inst
1977 ; return (avails, result) }
1980 Note [Equational Constraints in Implication Constraints]
1981 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1983 An equational constraint is of the form
1985 where Given and Wanted may contain both equational and dictionary
1986 constraints. The delay and reduction of these two kinds of constraints
1989 -) In the generated code, wanted Dictionary constraints are wrapped up in an
1990 implication constraint that is created at the code site where the wanted
1991 dictionaries can be reduced via a let-binding. This let-bound implication
1992 constraint is deconstructed at the use-site of the wanted dictionaries.
1994 -) While the reduction of equational constraints is also delayed, the delay
1995 is not manifest in the generated code. The required evidence is generated
1996 in the code directly at the use-site. There is no let-binding and deconstruction
1997 necessary. The main disadvantage is that we cannot exploit sharing as the
1998 same evidence may be generated at multiple use-sites. However, this disadvantage
1999 is limited because it only concerns coercions which are erased.
2001 The different treatment is motivated by the different in representation. Dictionary
2002 constraints require manifest runtime dictionaries, while equations require coercions
2006 ---------------------------------------------
2007 reduceImplication :: RedEnv
2009 -> Refinement -- May refine the givens; often empty
2010 -> [TcTyVar] -- Quantified type variables; all skolems
2011 -> [Inst] -- Extra givens; all rigid
2014 -> TcM (Avails, LookupInstResult)
2017 Suppose we are simplifying the constraint
2018 forall bs. extras => wanted
2019 in the context of an overall simplification problem with givens 'givens',
2020 and refinment 'reft'.
2023 * The refinement is often empty
2025 * The 'extra givens' need not mention any of the quantified type variables
2026 e.g. forall {}. Eq a => Eq [a]
2027 forall {}. C Int => D (Tree Int)
2029 This happens when you have something like
2031 T1 :: Eq a => a -> T a
2034 f x = ...(case x of { T1 v -> v==v })...
2037 -- ToDo: should we instantiate tvs? I think it's not necessary
2039 -- Note on coercion variables:
2041 -- The extra given coercion variables are bound at two different sites:
2042 -- -) in the creation context of the implication constraint
2043 -- the solved equational constraints use these binders
2045 -- -) at the solving site of the implication constraint
2046 -- the solved dictionaries use these binders
2047 -- these binders are generated by reduceImplication
2049 reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc
2050 = do { -- Add refined givens, and the extra givens
2052 (refined_red_givens,refined_avails)
2053 <- if isEmptyRefinement reft then return (red_givens env,orig_avails)
2054 else foldlM (addRefinedGiven reft) ([],orig_avails) (red_givens env)
2056 -- Solve the sub-problem
2057 ; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
2058 env' = env { red_givens = refined_red_givens ++ extra_givens ++ availsInsts orig_avails
2059 , red_try_me = try_me }
2061 ; traceTc (text "reduceImplication" <+> vcat
2063 ppr (red_givens env), ppr extra_givens,
2064 ppr reft, ppr wanteds])
2065 ; (irreds,binds,needed_givens0) <- checkLoop env' wanteds
2066 ; let needed_givens1 = [ng | ng <- needed_givens0, notElem ng extra_givens]
2068 -- Note [Reducing implication constraints]
2069 -- Tom -- update note, put somewhere!
2071 ; traceTc (text "reduceImplication result" <+> vcat
2072 [ppr irreds, ppr binds, ppr needed_givens1])
2073 -- ; avails <- reduceList env' wanteds avails
2075 -- -- Extract the binding
2076 -- ; (binds, irreds) <- extractResults avails wanteds
2077 ; (refinement_binds,needed_givens) <- extractLocalResults refined_avails needed_givens1
2078 ; traceTc (text "reduceImplication local results" <+> vcat
2079 [ppr refinement_binds, ppr needed_givens])
2081 ; -- extract superclass binds
2082 -- (sc_binds,_) <- extractResults avails []
2083 -- ; traceTc (text "reduceImplication sc_binds" <+> vcat
2084 -- [ppr sc_binds, ppr avails])
2087 -- We always discard the extra avails we've generated;
2088 -- but we remember if we have done any (global) improvement
2089 -- ; let ret_avails = avails
2090 ; let ret_avails = orig_avails
2091 -- ; let ret_avails = updateImprovement orig_avails avails
2093 ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
2095 -- Porgress is no longer measered by the number of bindings
2096 -- ; if isEmptyLHsBinds binds then -- No progress
2097 ; if (isEmptyLHsBinds binds) && (not $ null irreds) then
2098 return (ret_avails, NoInstance)
2101 ; (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds
2102 -- This binding is useless if the recursive simplification
2103 -- made no progress; but currently we don't try to optimise that
2104 -- case. After all, we only try hard to reduce at top level, or
2105 -- when inferring types.
2107 ; let dict_wanteds = filter (not . isEqInst) wanteds
2108 (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
2109 dict_ids = map instToId extra_dict_givens
2110 -- TOMDO: given equational constraints bug!
2111 -- we need a different evidence for given
2112 -- equations depending on whether we solve
2113 -- dictionary constraints or equational constraints
2114 eq_tyvars = uniqSetToList $ tyVarsOfTypes $ map eqInstType extra_eq_givens
2115 -- dict_ids = map instToId extra_givens
2116 co = mkWpTyLams tvs <.> mkWpTyLams eq_tyvars <.> mkWpLams dict_ids <.> WpLet (binds `unionBags` refinement_binds `unionBags` bind)
2117 rhs = mkHsWrap co payload
2118 loc = instLocSpan inst_loc
2119 payload | [dict_wanted] <- dict_wanteds = HsVar (instToId dict_wanted)
2120 | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) dict_wanteds) Boxed
2123 ; traceTc (text "reduceImplication ->" <+> vcat
2126 -- If there are any irreds, we back off and return NoInstance
2127 ; return (ret_avails, GenInst (implic_insts ++ needed_givens) (L loc rhs))
2132 Note [Reducing implication constraints]
2133 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2134 Suppose we are trying to simplify
2135 (Ord a, forall b. C a b => (W [a] b, D c b))
2137 instance (C a b, Ord a) => W [a] b
2138 When solving the implication constraint, we'll start with
2140 in the Avails. Then we add (C a b -> Given) and solve. Extracting
2141 the results gives us a binding for the (W [a] b), with an Irred of
2142 (Ord a, D c b). Now, the (Ord a) comes from "outside" the implication,
2143 but the (D d b) is from "inside". So we want to generate a Rhs binding
2146 ic = /\b \dc:C a b). (df a b dc do, ic' b dc)
2149 ic' :: forall b. C a b => D c b
2151 The 'depending on' part of the Rhs is important, because it drives
2152 the extractResults code.
2154 The "inside" and "outside" distinction is what's going on with 'inner' and
2155 'outer' in reduceImplication
2158 Note [Freeness and implications]
2159 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2160 It's hard to say when an implication constraint can be floated out. Consider
2161 forall {} Eq a => Foo [a]
2162 The (Foo [a]) doesn't mention any of the quantified variables, but it
2163 still might be partially satisfied by the (Eq a).
2165 There is a useful special case when it *is* easy to partition the
2166 constraints, namely when there are no 'givens'. Consider
2167 forall {a}. () => Bar b
2168 There are no 'givens', and so there is no reason to capture (Bar b).
2169 We can let it float out. But if there is even one constraint we
2170 must be much more careful:
2171 forall {a}. C a b => Bar (m b)
2172 because (C a b) might have a superclass (D b), from which we might
2173 deduce (Bar [b]) when m later gets instantiated to []. Ha!
2175 Here is an even more exotic example
2177 Now consider the constraint
2178 forall b. D Int b => C Int
2179 We can satisfy the (C Int) from the superclass of D, so we don't want
2180 to float the (C Int) out, even though it mentions no type variable in
2183 Note [Pruning the givens in an implication constraint]
2184 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2185 Suppose we are about to form the implication constraint
2186 forall tvs. Eq a => Ord b
2187 The (Eq a) cannot contribute to the (Ord b), because it has no access to
2188 the type variable 'b'. So we could filter out the (Eq a) from the givens.
2190 Doing so would be a bit tidier, but all the implication constraints get
2191 simplified away by the optimiser, so it's no great win. So I don't take
2192 advantage of that at the moment.
2194 If you do, BE CAREFUL of wobbly type variables.
2197 %************************************************************************
2199 Avails and AvailHow: the pool of evidence
2201 %************************************************************************
2205 data Avails = Avails !ImprovementDone !AvailEnv
2207 type ImprovementDone = Bool -- True <=> some unification has happened
2208 -- so some Irreds might now be reducible
2209 -- keys that are now
2211 type AvailEnv = FiniteMap Inst AvailHow
2213 = IsIrred -- Used for irreducible dictionaries,
2214 -- which are going to be lambda bound
2216 | Given TcId -- Used for dictionaries for which we have a binding
2217 -- e.g. those "given" in a signature
2219 | Rhs -- Used when there is a RHS
2220 (LHsExpr TcId) -- The RHS
2221 [Inst] -- Insts free in the RHS; we need these too
2223 instance Outputable Avails where
2226 pprAvails (Avails imp avails)
2227 = vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty)
2228 , nest 2 (vcat [sep [ppr inst, nest 2 (equals <+> ppr avail)]
2229 | (inst,avail) <- fmToList avails ])]
2231 instance Outputable AvailHow where
2234 -------------------------
2235 pprAvail :: AvailHow -> SDoc
2236 pprAvail IsIrred = text "Irred"
2237 pprAvail (Given x) = text "Given" <+> ppr x
2238 pprAvail (Rhs rhs bs) = text "Rhs" <+> sep [ppr rhs, braces (ppr bs)]
2240 -------------------------
2241 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
2242 extendAvailEnv env inst avail = addToFM env inst avail
2244 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
2245 findAvailEnv env wanted = lookupFM env wanted
2246 -- NB 1: the Ord instance of Inst compares by the class/type info
2247 -- *not* by unique. So
2248 -- d1::C Int == d2::C Int
2250 emptyAvails :: Avails
2251 emptyAvails = Avails False emptyFM
2253 findAvail :: Avails -> Inst -> Maybe AvailHow
2254 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
2256 elemAvails :: Inst -> Avails -> Bool
2257 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
2259 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
2261 extendAvails avails@(Avails imp env) inst avail
2262 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
2263 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
2265 availsInsts :: Avails -> [Inst]
2266 availsInsts (Avails _ avails) = keysFM avails
2268 availsImproved (Avails imp _) = imp
2270 updateImprovement :: Avails -> Avails -> Avails
2271 -- (updateImprovement a1 a2) sets a1's improvement flag from a2
2272 updateImprovement (Avails _ avails1) (Avails imp2 _) = Avails imp2 avails1
2275 Extracting the bindings from a bunch of Avails.
2276 The bindings do *not* come back sorted in dependency order.
2277 We assume that they'll be wrapped in a big Rec, so that the
2278 dependency analyser can sort them out later
2281 extractResults :: Avails
2283 -> TcM ( TcDictBinds, -- Bindings
2284 [Inst], -- Irreducible ones
2285 [Inst]) -- Needed givens, i.e. ones used in the bindings
2287 extractResults (Avails _ avails) wanteds
2288 = go avails emptyBag [] [] wanteds
2290 go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst] -> [Inst]
2291 -> TcM (TcDictBinds, [Inst], [Inst])
2292 go avails binds irreds givens []
2293 = returnM (binds, irreds, givens)
2295 go avails binds irreds givens (w:ws)
2296 = case findAvailEnv avails w of
2297 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
2298 go avails binds irreds givens ws
2301 | id == w_id -> go avails binds irreds (w:givens) ws
2302 | otherwise -> go avails (addBind binds w (nlHsVar id)) irreds (update_id w id:givens) ws
2303 -- The sought Id can be one of the givens, via a superclass chain
2304 -- and then we definitely don't want to generate an x=x binding!
2306 Just IsIrred -> go (add_given avails w) binds (w:irreds) givens ws
2307 -- The add_given handles the case where we want (Ord a, Eq a), and we
2308 -- don't want to emit *two* Irreds for Ord a, one via the superclass chain
2309 -- This showed up in a dupliated Ord constraint in the error message for
2312 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds givens (ws' ++ ws)
2314 new_binds = addBind binds w rhs
2317 update_id m@(Method{}) id = m {tci_id = id}
2318 update_id w id = w {tci_name = idName id}
2320 add_given avails w = extendAvailEnv avails w (Given (instToId w))
2322 extractLocalResults :: Avails
2324 -> TcM ( TcDictBinds, -- Bindings
2325 [Inst]) -- Needed givens, i.e. ones used in the bindings
2327 extractLocalResults (Avails _ avails) wanteds
2328 = go avails emptyBag [] wanteds
2330 go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst]
2331 -> TcM (TcDictBinds, [Inst])
2332 go avails binds givens []
2333 = returnM (binds, givens)
2335 go avails binds givens (w:ws)
2336 = case findAvailEnv avails w of
2337 Nothing -> -- pprTrace "Urk: extractLocalResults" (ppr w) $
2338 go avails binds givens ws
2341 go avails binds givens ws
2344 | id == w_id -> go avails binds (w:givens) ws
2345 | otherwise -> go avails binds (w{tci_name=idName id}:givens) ws
2346 -- The sought Id can be one of the givens, via a superclass chain
2347 -- and then we definitely don't want to generate an x=x binding!
2349 Just (Rhs rhs ws') -> go (add_given avails w) new_binds givens (ws' ++ ws)
2351 new_binds = addBind binds w rhs
2355 add_given avails w = extendAvailEnv avails w (Given (instToId w))
2359 Note [No superclasses for Stop]
2360 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2361 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2362 add it to avails, so that any other equal Insts will be commoned up
2363 right here. However, we do *not* add superclasses. If we have
2366 but a is not bound here, then we *don't* want to derive dn from df
2367 here lest we lose sharing.
2370 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2371 addWanted want_scs avails wanted rhs_expr wanteds
2372 = addAvailAndSCs want_scs avails wanted avail
2374 avail = Rhs rhs_expr wanteds
2376 addGiven :: Avails -> Inst -> TcM Avails
2377 addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given))
2378 -- Always add superclasses for 'givens'
2380 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2381 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2382 -- so the assert isn't true
2384 addRefinedGiven :: Refinement -> ([Inst], Avails) -> Inst -> TcM ([Inst], Avails)
2385 addRefinedGiven reft (refined_givens, avails) given
2386 | isDict given -- We sometimes have 'given' methods, but they
2387 -- are always optional, so we can drop them
2388 , let pred = dictPred given
2389 , isRefineablePred pred -- See Note [ImplicInst rigidity]
2390 , Just (co, pred) <- refinePred reft pred
2391 = do { new_given <- newDictBndr (instLoc given) pred
2392 ; let rhs = L (instSpan given) $
2393 HsWrap (WpCo co) (HsVar (instToId given))
2394 ; avails <- addAvailAndSCs AddSCs avails new_given (Rhs rhs [given])
2395 ; return (new_given:refined_givens, avails) }
2396 -- ToDo: the superclasses of the original given all exist in Avails
2397 -- so we could really just cast them, but it's more awkward to do,
2398 -- and hopefully the optimiser will spot the duplicated work
2400 = return (refined_givens, avails)
2403 Note [ImplicInst rigidity]
2404 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2406 C :: forall ab. (Eq a, Ord b) => b -> T a
2408 ...(case x of C v -> <body>)...
2410 From the case (where x::T ty) we'll get an implication constraint
2411 forall b. (Eq ty, Ord b) => <body-constraints>
2412 Now suppose <body-constraints> itself has an implication constraint
2414 forall c. <reft> => <payload>
2415 Then, we can certainly apply the refinement <reft> to the Ord b, becuase it is
2416 existential, but we probably should not apply it to the (Eq ty) because it may
2417 be wobbly. Hence the isRigidInst
2419 @Insts@ are ordered by their class/type info, rather than by their
2420 unique. This allows the context-reduction mechanism to use standard finite
2421 maps to do their stuff. It's horrible that this code is here, rather
2422 than with the Avails handling stuff in TcSimplify
2425 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2426 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2427 addAvailAndSCs want_scs avails irred IsIrred
2429 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2430 addAvailAndSCs want_scs avails inst avail
2431 | not (isClassDict inst) = extendAvails avails inst avail
2432 | NoSCs <- want_scs = extendAvails avails inst avail
2433 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2434 ; avails' <- extendAvails avails inst avail
2435 ; addSCs is_loop avails' inst }
2437 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2438 -- Note: this compares by *type*, not by Unique
2439 deps = findAllDeps (unitVarSet (instToVar inst)) avail
2440 dep_tys = map idType (varSetElems deps)
2442 findAllDeps :: IdSet -> AvailHow -> IdSet
2443 -- Find all the Insts that this one depends on
2444 -- See Note [SUPERCLASS-LOOP 2]
2445 -- Watch out, though. Since the avails may contain loops
2446 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2447 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2448 findAllDeps so_far other = so_far
2450 find_all :: IdSet -> Inst -> IdSet
2452 | kid_id `elemVarSet` so_far = so_far
2453 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2454 | otherwise = so_far'
2456 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2457 kid_id = instToId kid
2459 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2460 -- Add all the superclasses of the Inst to Avails
2461 -- The first param says "dont do this because the original thing
2462 -- depends on this one, so you'd build a loop"
2463 -- Invariant: the Inst is already in Avails.
2465 addSCs is_loop avails dict
2466 = ASSERT( isDict dict )
2467 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2468 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2470 (clas, tys) = getDictClassTys dict
2471 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2472 sc_theta' = filter (not . isEqPred) $
2473 substTheta (zipTopTvSubst tyvars tys) sc_theta
2475 add_sc avails (sc_dict, sc_sel)
2476 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2477 | is_given sc_dict = return avails
2478 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2479 ; addSCs is_loop avails' sc_dict }
2481 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2482 co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
2484 is_given :: Inst -> Bool
2485 is_given sc_dict = case findAvail avails sc_dict of
2486 Just (Given _) -> True -- Given is cheaper than superclass selection
2489 -- From the a set of insts obtain all equalities that (transitively) occur in
2490 -- superclass contexts of class constraints (aka the ancestor equalities).
2492 ancestorEqualities :: [Inst] -> TcM [Inst]
2494 = mapM mkWantedEqInst -- turn only equality predicates..
2495 . filter isEqPred -- ..into wanted equality insts
2497 . addAEsToBag emptyBag -- collect the superclass constraints..
2498 . map dictPred -- ..of all predicates in a bag
2499 . filter isClassDict
2501 addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
2502 addAEsToBag bag [] = bag
2503 addAEsToBag bag (pred:preds)
2504 | pred `elemBag` bag = addAEsToBag bag preds
2505 | isEqPred pred = addAEsToBag bagWithPred preds
2506 | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
2507 | otherwise = addAEsToBag bag preds
2509 bagWithPred = bag `snocBag` pred
2510 predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
2512 (tyvars, sc_theta, _, _) = classBigSig clas
2513 (clas, tys) = getClassPredTys pred
2517 %************************************************************************
2519 \section{tcSimplifyTop: defaulting}
2521 %************************************************************************
2524 @tcSimplifyTop@ is called once per module to simplify all the constant
2525 and ambiguous Insts.
2527 We need to be careful of one case. Suppose we have
2529 instance Num a => Num (Foo a b) where ...
2531 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2532 to (Num x), and default x to Int. But what about y??
2534 It's OK: the final zonking stage should zap y to (), which is fine.
2538 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2539 tcSimplifyTop wanteds
2540 = tc_simplify_top doc False wanteds
2542 doc = text "tcSimplifyTop"
2544 tcSimplifyInteractive wanteds
2545 = tc_simplify_top doc True wanteds
2547 doc = text "tcSimplifyInteractive"
2549 -- The TcLclEnv should be valid here, solely to improve
2550 -- error message generation for the monomorphism restriction
2551 tc_simplify_top doc interactive wanteds
2552 = do { dflags <- getDOpts
2553 ; wanteds <- zonkInsts wanteds
2554 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2556 ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
2557 ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
2558 ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
2559 ; (irreds2, binds2) <- approximateImplications doc2 (\d -> True) irreds1
2560 ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
2562 -- Use the defaulting rules to do extra unification
2563 -- NB: irreds2 are already zonked
2564 ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
2566 -- Deal with implicit parameters
2567 ; let (bad_ips, non_ips) = partition isIPDict irreds3
2568 (ambigs, others) = partition isTyVarDict non_ips
2570 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2572 ; addNoInstanceErrs others
2573 ; addTopAmbigErrs ambigs
2575 ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
2577 doc1 = doc <+> ptext SLIT("(first round)")
2578 doc2 = doc <+> ptext SLIT("(approximate)")
2579 doc3 = doc <+> ptext SLIT("(disambiguate)")
2582 If a dictionary constrains a type variable which is
2583 * not mentioned in the environment
2584 * and not mentioned in the type of the expression
2585 then it is ambiguous. No further information will arise to instantiate
2586 the type variable; nor will it be generalised and turned into an extra
2587 parameter to a function.
2589 It is an error for this to occur, except that Haskell provided for
2590 certain rules to be applied in the special case of numeric types.
2592 * at least one of its classes is a numeric class, and
2593 * all of its classes are numeric or standard
2594 then the type variable can be defaulted to the first type in the
2595 default-type list which is an instance of all the offending classes.
2597 So here is the function which does the work. It takes the ambiguous
2598 dictionaries and either resolves them (producing bindings) or
2599 complains. It works by splitting the dictionary list by type
2600 variable, and using @disambigOne@ to do the real business.
2602 @disambigOne@ assumes that its arguments dictionaries constrain all
2603 the same type variable.
2605 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2606 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2607 the most common use of defaulting is code like:
2609 _ccall_ foo `seqPrimIO` bar
2611 Since we're not using the result of @foo@, the result if (presumably)
2615 disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
2616 -- Just does unification to fix the default types
2617 -- The Insts are assumed to be pre-zonked
2618 disambiguate doc interactive dflags insts
2620 = return (insts, emptyBag)
2622 | null defaultable_groups
2623 = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
2624 ; return (insts, emptyBag) }
2627 = do { -- Figure out what default types to use
2628 default_tys <- getDefaultTys extended_defaulting ovl_strings
2630 ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2631 ; mapM_ (disambigGroup default_tys) defaultable_groups
2633 -- disambigGroup does unification, hence try again
2634 ; tryHardCheckLoop doc insts }
2637 extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
2638 ovl_strings = dopt Opt_OverloadedStrings dflags
2640 unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
2641 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2642 (unaries, bad_tvs_s) = partitionWith find_unary insts
2643 bad_tvs = unionVarSets bad_tvs_s
2645 -- Finds unary type-class constraints
2646 find_unary d@(Dict {tci_pred = ClassP cls [ty]})
2647 | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
2648 find_unary inst = Right (tyVarsOfInst inst)
2650 -- Group by type variable
2651 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2652 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2653 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2655 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2656 defaultable_group ds@((_,_,tv):_)
2657 = isTyConableTyVar tv -- Note [Avoiding spurious errors]
2658 && not (tv `elemVarSet` bad_tvs)
2659 && defaultable_classes [c | (_,c,_) <- ds]
2660 defaultable_group [] = panic "defaultable_group"
2662 defaultable_classes clss
2663 | extended_defaulting = any isInteractiveClass clss
2664 | otherwise = all is_std_class clss && (any is_num_class clss)
2666 -- In interactive mode, or with -fextended-default-rules,
2667 -- we default Show a to Show () to avoid graututious errors on "show []"
2668 isInteractiveClass cls
2669 = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2671 is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2672 -- is_num_class adds IsString to the standard numeric classes,
2673 -- when -foverloaded-strings is enabled
2675 is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2676 -- Similarly is_std_class
2678 -----------------------
2679 disambigGroup :: [Type] -- The default types
2680 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2681 -> TcM () -- Just does unification, to fix the default types
2683 disambigGroup default_tys dicts
2684 = try_default default_tys
2686 (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
2687 classes = [c | (_,c,_) <- dicts]
2689 try_default [] = return ()
2690 try_default (default_ty : default_tys)
2691 = tryTcLIE_ (try_default default_tys) $
2692 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2693 -- This may fail; then the tryTcLIE_ kicks in
2694 -- Failure here is caused by there being no type in the
2695 -- default list which can satisfy all the ambiguous classes.
2696 -- For example, if Real a is reqd, but the only type in the
2697 -- default list is Int.
2699 -- After this we can't fail
2700 ; warnDefault dicts default_ty
2701 ; unifyType default_ty (mkTyVarTy tyvar)
2702 ; return () -- TOMDO: do something with the coercion
2706 -----------------------
2707 getDefaultTys :: Bool -> Bool -> TcM [Type]
2708 getDefaultTys extended_deflts ovl_strings
2709 = do { mb_defaults <- getDeclaredDefaultTys
2710 ; case mb_defaults of {
2711 Just tys -> return tys ; -- User-supplied defaults
2714 -- No use-supplied default
2715 -- Use [Integer, Double], plus modifications
2716 { integer_ty <- tcMetaTy integerTyConName
2717 ; checkWiredInTyCon doubleTyCon
2718 ; string_ty <- tcMetaTy stringTyConName
2719 ; return (opt_deflt extended_deflts unitTy
2720 -- Note [Default unitTy]
2722 [integer_ty,doubleTy]
2724 opt_deflt ovl_strings string_ty) } } }
2726 opt_deflt True ty = [ty]
2727 opt_deflt False ty = []
2730 Note [Default unitTy]
2731 ~~~~~~~~~~~~~~~~~~~~~
2732 In interative mode (or with -fextended-default-rules) we add () as the first type we
2733 try when defaulting. This has very little real impact, except in the following case.
2735 Text.Printf.printf "hello"
2736 This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
2737 want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
2738 default the 'a' to (), rather than to Integer (which is what would otherwise happen;
2739 and then GHCi doesn't attempt to print the (). So in interactive mode, we add
2740 () to the list of defaulting types. See Trac #1200.
2742 Note [Avoiding spurious errors]
2743 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2744 When doing the unification for defaulting, we check for skolem
2745 type variables, and simply don't default them. For example:
2746 f = (*) -- Monomorphic
2747 g :: Num a => a -> a
2749 Here, we get a complaint when checking the type signature for g,
2750 that g isn't polymorphic enough; but then we get another one when
2751 dealing with the (Num a) context arising from f's definition;
2752 we try to unify a with Int (to default it), but find that it's
2753 already been unified with the rigid variable from g's type sig
2756 %************************************************************************
2758 \subsection[simple]{@Simple@ versions}
2760 %************************************************************************
2762 Much simpler versions when there are no bindings to make!
2764 @tcSimplifyThetas@ simplifies class-type constraints formed by
2765 @deriving@ declarations and when specialising instances. We are
2766 only interested in the simplified bunch of class/type constraints.
2768 It simplifies to constraints of the form (C a b c) where
2769 a,b,c are type variables. This is required for the context of
2770 instance declarations.
2773 tcSimplifyDeriv :: InstOrigin
2775 -> ThetaType -- Wanted
2776 -> TcM ThetaType -- Needed
2777 -- Given instance (wanted) => C inst_ty
2778 -- Simplify 'wanted' as much as possible
2780 tcSimplifyDeriv orig tyvars theta
2781 = do { (tvs, _, tenv) <- tcInstTyVars tyvars
2782 -- The main loop may do unification, and that may crash if
2783 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2784 -- ToDo: what if two of them do get unified?
2785 ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
2786 ; (irreds, _) <- tryHardCheckLoop doc wanteds
2788 ; let (tv_dicts, others) = partition ok irreds
2789 ; addNoInstanceErrs others
2790 -- See Note [Exotic derived instance contexts] in TcMType
2792 ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2793 simpl_theta = substTheta rev_env (map dictPred tv_dicts)
2794 -- This reverse-mapping is a pain, but the result
2795 -- should mention the original TyVars not TcTyVars
2797 ; return simpl_theta }
2799 doc = ptext SLIT("deriving classes for a data type")
2801 ok dict | isDict dict = validDerivPred (dictPred dict)
2806 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2807 used with \tr{default} declarations. We are only interested in
2808 whether it worked or not.
2811 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2814 tcSimplifyDefault theta
2815 = newDictBndrsO DefaultOrigin theta `thenM` \ wanteds ->
2816 tryHardCheckLoop doc wanteds `thenM` \ (irreds, _) ->
2817 addNoInstanceErrs irreds `thenM_`
2823 doc = ptext SLIT("default declaration")
2827 %************************************************************************
2829 \section{Errors and contexts}
2831 %************************************************************************
2833 ToDo: for these error messages, should we note the location as coming
2834 from the insts, or just whatever seems to be around in the monad just
2838 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2839 -> [Inst] -- The offending Insts
2841 -- Group together insts with the same origin
2842 -- We want to report them together in error messages
2844 groupErrs report_err []
2846 groupErrs report_err (inst:insts)
2847 = do_one (inst:friends) `thenM_`
2848 groupErrs report_err others
2851 -- (It may seem a bit crude to compare the error messages,
2852 -- but it makes sure that we combine just what the user sees,
2853 -- and it avoids need equality on InstLocs.)
2854 (friends, others) = partition is_friend insts
2855 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2856 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2857 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2858 -- Add location and context information derived from the Insts
2860 -- Add the "arising from..." part to a message about bunch of dicts
2861 addInstLoc :: [Inst] -> Message -> Message
2862 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
2864 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
2865 addTopIPErrs bndrs []
2867 addTopIPErrs bndrs ips
2868 = do { dflags <- getDOpts
2869 ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
2871 (tidy_env, tidy_ips) = tidyInsts ips
2873 = vcat [sep [ptext SLIT("Implicit parameters escape from"),
2874 nest 2 (ptext SLIT("the monomorphic top-level binding")
2875 <> plural bndrs <+> ptext SLIT("of")
2876 <+> pprBinders bndrs <> colon)],
2877 nest 2 (vcat (map ppr_ip ips)),
2878 monomorphism_fix dflags]
2879 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
2881 topIPErrs :: [Inst] -> TcM ()
2883 = groupErrs report tidy_dicts
2885 (tidy_env, tidy_dicts) = tidyInsts dicts
2886 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2887 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2888 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2890 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
2892 addNoInstanceErrs insts
2893 = do { let (tidy_env, tidy_insts) = tidyInsts insts
2894 ; reportNoInstances tidy_env Nothing tidy_insts }
2898 -> Maybe (InstLoc, [Inst]) -- Context
2899 -- Nothing => top level
2900 -- Just (d,g) => d describes the construct
2902 -> [Inst] -- What is wanted (can include implications)
2905 reportNoInstances tidy_env mb_what insts
2906 = groupErrs (report_no_instances tidy_env mb_what) insts
2908 report_no_instances tidy_env mb_what insts
2909 = do { inst_envs <- tcGetInstEnvs
2910 ; let (implics, insts1) = partition isImplicInst insts
2911 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
2912 (eqInsts, insts3) = partition isEqInst insts2
2913 ; traceTc (text "reportNoInstances" <+> vcat
2914 [ppr implics, ppr insts1, ppr insts2])
2915 ; mapM_ complain_implic implics
2916 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
2917 ; groupErrs complain_no_inst insts3
2918 ; mapM_ complain_eq eqInsts
2921 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
2923 complain_implic inst -- Recurse!
2924 = reportNoInstances tidy_env
2925 (Just (tci_loc inst, tci_given inst))
2928 complain_eq EqInst {tci_left = lty, tci_right = rty,
2929 tci_loc = InstLoc _ _ ctxt}
2930 = do { (env, msg) <- misMatchMsg lty rty
2932 failWithTcM (env, msg)
2935 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
2936 -- Right msg => overlap message
2937 -- Left inst => no instance
2938 check_overlap inst_envs wanted
2939 | not (isClassDict wanted) = Left wanted
2941 = case lookupInstEnv inst_envs clas tys of
2942 -- The case of exactly one match and no unifiers means a
2943 -- successful lookup. That can't happen here, because dicts
2944 -- only end up here if they didn't match in Inst.lookupInst
2946 ([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
2948 ([], _) -> Left wanted -- No match
2949 res -> Right (mk_overlap_msg wanted res)
2951 (clas,tys) = getDictClassTys wanted
2953 mk_overlap_msg dict (matches, unifiers)
2954 = ASSERT( not (null matches) )
2955 vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
2956 <+> pprPred (dictPred dict))),
2957 sep [ptext SLIT("Matching instances") <> colon,
2958 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
2959 if not (isSingleton matches)
2960 then -- Two or more matches
2962 else -- One match, plus some unifiers
2963 ASSERT( not (null unifiers) )
2964 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
2965 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
2966 ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
2967 ptext SLIT("when compiling the other instance declarations")])]
2969 ispecs = [ispec | (ispec, _) <- matches]
2971 mk_no_inst_err insts
2972 | null insts = empty
2974 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
2975 not (isEmptyVarSet (tyVarsOfInsts insts))
2976 = vcat [ addInstLoc insts $
2977 sep [ ptext SLIT("Could not deduce") <+> pprDictsTheta insts
2978 , nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta givens]
2979 , show_fixes (fix1 loc : fixes2) ]
2981 | otherwise -- Top level
2982 = vcat [ addInstLoc insts $
2983 ptext SLIT("No instance") <> plural insts
2984 <+> ptext SLIT("for") <+> pprDictsTheta insts
2985 , show_fixes fixes2 ]
2988 fix1 loc = sep [ ptext SLIT("add") <+> pprDictsTheta insts
2989 <+> ptext SLIT("to the context of"),
2990 nest 2 (ppr (instLocOrigin loc)) ]
2991 -- I'm not sure it helps to add the location
2992 -- nest 2 (ptext SLIT("at") <+> ppr (instLocSpan loc)) ]
2994 fixes2 | null instance_dicts = []
2995 | otherwise = [sep [ptext SLIT("add an instance declaration for"),
2996 pprDictsTheta instance_dicts]]
2997 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
2998 -- Insts for which it is worth suggesting an adding an instance declaration
2999 -- Exclude implicit parameters, and tyvar dicts
3001 show_fixes :: [SDoc] -> SDoc
3002 show_fixes [] = empty
3003 show_fixes (f:fs) = sep [ptext SLIT("Possible fix:"),
3004 nest 2 (vcat (f : map (ptext SLIT("or") <+>) fs))]
3006 addTopAmbigErrs dicts
3007 -- Divide into groups that share a common set of ambiguous tyvars
3008 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
3009 -- See Note [Avoiding spurious errors]
3010 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
3012 (tidy_env, tidy_dicts) = tidyInsts dicts
3014 tvs_of :: Inst -> [TcTyVar]
3015 tvs_of d = varSetElems (tyVarsOfInst d)
3016 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
3018 report :: [(Inst,[TcTyVar])] -> TcM ()
3019 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
3020 = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) ->
3021 setSrcSpan (instSpan inst) $
3022 -- the location of the first one will do for the err message
3023 addErrTcM (tidy_env, msg $$ mono_msg)
3025 dicts = map fst pairs
3026 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
3027 pprQuotedList tvs <+> in_msg,
3028 nest 2 (pprDictsInFull dicts)]
3029 in_msg = text "in the constraint" <> plural dicts <> colon
3030 report [] = panic "addTopAmbigErrs"
3033 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
3034 -- There's an error with these Insts; if they have free type variables
3035 -- it's probably caused by the monomorphism restriction.
3036 -- Try to identify the offending variable
3037 -- ASSUMPTION: the Insts are fully zonked
3038 mkMonomorphismMsg tidy_env inst_tvs
3039 = do { dflags <- getDOpts
3040 ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
3041 ; return (tidy_env, mk_msg dflags docs) }
3043 mk_msg _ _ | any isRuntimeUnk inst_tvs
3044 = vcat [ptext SLIT("Cannot resolve unknown runtime types:") <+>
3045 (pprWithCommas ppr inst_tvs),
3046 ptext SLIT("Use :print or :force to determine these types")]
3047 mk_msg _ [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
3048 -- This happens in things like
3049 -- f x = show (read "foo")
3050 -- where monomorphism doesn't play any role
3052 = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
3054 monomorphism_fix dflags]
3056 isRuntimeUnk :: TcTyVar -> Bool
3057 isRuntimeUnk x | SkolemTv RuntimeUnkSkol <- tcTyVarDetails x = True
3060 monomorphism_fix :: DynFlags -> SDoc
3061 monomorphism_fix dflags
3062 = ptext SLIT("Probable fix:") <+> vcat
3063 [ptext SLIT("give these definition(s) an explicit type signature"),
3064 if dopt Opt_MonomorphismRestriction dflags
3065 then ptext SLIT("or use -fno-monomorphism-restriction")
3066 else empty] -- Only suggest adding "-fno-monomorphism-restriction"
3067 -- if it is not already set!
3069 warnDefault ups default_ty
3070 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
3071 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
3073 dicts = [d | (d,_,_) <- ups]
3076 (_, tidy_dicts) = tidyInsts dicts
3077 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
3078 quotes (ppr default_ty),
3079 pprDictsInFull tidy_dicts]
3081 reduceDepthErr n stack
3082 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
3083 ptext SLIT("Use -fcontext-stack=N to increase stack size to N"),
3084 nest 4 (pprStack stack)]
3086 pprStack stack = vcat (map pprInstInFull stack)
3088 -----------------------
3089 misMatchMsg :: TcType -> TcType -> TcM (TidyEnv, SDoc)
3090 -- Generate the message when two types fail to match,
3091 -- going to some trouble to make it helpful.
3092 -- The argument order is: actual type, expected type
3093 misMatchMsg ty_act ty_exp
3094 = do { env0 <- tcInitTidyEnv
3095 ; ty_exp <- zonkTcType ty_exp
3096 ; ty_act <- zonkTcType ty_act
3097 ; (env1, pp_exp, extra_exp) <- ppr_ty env0 ty_exp
3098 ; (env2, pp_act, extra_act) <- ppr_ty env1 ty_act
3100 sep [sep [ptext SLIT("Couldn't match expected type") <+> pp_exp,
3102 ptext SLIT("against inferred type") <+> pp_act],
3103 nest 2 (extra_exp $$ extra_act)]) }
3105 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
3107 = do { let (env1, tidy_ty) = tidyOpenType env ty
3108 ; (env2, extra) <- ppr_extra env1 tidy_ty
3109 ; return (env2, quotes (ppr tidy_ty), extra) }
3111 -- (ppr_extra env ty) shows extra info about 'ty'
3112 ppr_extra env (TyVarTy tv)
3113 | isSkolemTyVar tv || isSigTyVar tv
3114 = return (env1, pprSkolTvBinding tv1)
3116 (env1, tv1) = tidySkolemTyVar env tv
3118 ppr_extra env ty = return (env, empty) -- Normal case