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,
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_dicts in 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 isAbstractableInst givens
954 -- The givens can (redundantly) include methods
955 -- We want to retain both EqInsts and Dicts
956 -- There should be no implicadtion constraints
957 -- See Note [Pruning the givens in an implication constraint]
959 -- If there are no 'givens' *and* the refinement is empty
960 -- (the refinement is like more givens), then it's safe to
961 -- partition the 'wanteds' by their qtvs, thereby trimming irreds
962 -- See Note [Freeness and implications]
963 ; irreds' <- if null givens' && isEmptyRefinement reft
965 { let qtv_set = mkVarSet qtvs
966 (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
968 ; return real_irreds }
971 ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
972 ; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds'
973 -- This call does the real work
974 -- If irreds' is empty, it does something sensible
979 makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement
981 -> TcM ([Inst], TcDictBinds)
982 -- Make a binding that binds 'irreds', by generating an implication
983 -- constraint for them, *and* throwing the constraint into the LIE
984 -- The binding looks like
985 -- (ir1, .., irn) = f qtvs givens
986 -- where f is (evidence for) the new implication constraint
987 -- f :: forall qtvs. {reft} givens => (ir1, .., irn)
988 -- qtvs includes coercion variables
990 -- This binding must line up the 'rhs' in reduceImplication
991 makeImplicationBind loc all_tvs reft
992 givens -- Guaranteed all Dicts
995 | null irreds -- If there are no irreds, we are done
996 = return ([], emptyBag)
997 | otherwise -- Otherwise we must generate a binding
998 = do { uniq <- newUnique
999 ; span <- getSrcSpanM
1000 ; let (eq_givens, dict_givens) = partition isEqInst givens
1001 eq_tyvar_cos = mkTyVarTys (varSetElems $ tyVarsOfTypes $ map eqInstType eq_givens)
1002 -- Urgh! See line 2187 or thereabouts. I believe that all these
1003 -- 'givens' must be a simple CoVar. This MUST be cleaned up.
1005 ; let name = mkInternalName uniq (mkVarOcc "ic") span
1006 implic_inst = ImplicInst { tci_name = name, tci_reft = reft,
1007 tci_tyvars = all_tvs,
1008 tci_given = (eq_givens ++ dict_givens),
1009 tci_wanted = irreds, tci_loc = loc }
1010 ; let -- only create binder for dict_irreds
1011 (eq_irreds, dict_irreds) = partition isEqInst irreds
1012 n_dict_irreds = length dict_irreds
1013 dict_irred_ids = map instToId dict_irreds
1014 tup_ty = mkTupleTy Boxed n_dict_irreds (map idType dict_irred_ids)
1015 pat = TuplePat (map nlVarPat dict_irred_ids) Boxed tup_ty
1016 rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
1017 co = mkWpApps (map instToId dict_givens)
1018 <.> mkWpTyApps eq_tyvar_cos
1019 <.> mkWpTyApps (mkTyVarTys all_tvs)
1020 bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs
1021 | otherwise = PatBind { pat_lhs = L span pat,
1022 pat_rhs = unguardedGRHSs rhs,
1023 pat_rhs_ty = tup_ty,
1024 bind_fvs = placeHolderNames }
1025 ; traceTc $ text "makeImplicationBind" <+> ppr implic_inst
1026 ; return ([implic_inst], unitBag (L span bind))
1029 -----------------------------------------------------------
1030 tryHardCheckLoop :: SDoc
1032 -> TcM ([Inst], TcDictBinds)
1034 tryHardCheckLoop doc wanteds
1035 = do { (irreds,binds,_) <- checkLoop (mkRedEnv doc try_me []) wanteds
1036 ; return (irreds,binds)
1039 try_me inst = ReduceMe AddSCs
1040 -- Here's the try-hard bit
1042 -----------------------------------------------------------
1043 gentleCheckLoop :: InstLoc
1046 -> TcM ([Inst], TcDictBinds)
1048 gentleCheckLoop inst_loc givens wanteds
1049 = do { (irreds,binds,_) <- checkLoop env wanteds
1050 ; return (irreds,binds)
1053 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
1055 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1057 -- When checking against a given signature
1058 -- we MUST be very gentle: Note [Check gently]
1060 gentleInferLoop :: SDoc -> [Inst]
1061 -> TcM ([Inst], TcDictBinds)
1062 gentleInferLoop doc wanteds
1063 = do { (irreds, binds, _) <- checkLoop env wanteds
1064 ; return (irreds, binds) }
1066 env = mkRedEnv doc try_me []
1067 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1072 ~~~~~~~~~~~~~~~~~~~~
1073 We have to very careful about not simplifying too vigorously
1078 f :: Show b => T b -> b
1079 f (MkT x) = show [x]
1081 Inside the pattern match, which binds (a:*, x:a), we know that
1083 Hence we have a dictionary for Show [a] available; and indeed we
1084 need it. We are going to build an implication contraint
1085 forall a. (b~[a]) => Show [a]
1086 Later, we will solve this constraint using the knowledge (Show b)
1088 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
1089 thing becomes insoluble. So we simplify gently (get rid of literals
1090 and methods only, plus common up equal things), deferring the real
1091 work until top level, when we solve the implication constraint
1092 with tryHardCheckLooop.
1096 -----------------------------------------------------------
1099 -> TcM ([Inst], TcDictBinds,
1100 [Inst]) -- needed givens
1101 -- Precondition: givens are completely rigid
1102 -- Postcondition: returned Insts are zonked
1104 checkLoop env wanteds
1106 where go env wanteds needed_givens
1107 = do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
1108 ; env' <- zonkRedEnv env
1109 ; wanteds' <- zonkInsts wanteds
1111 ; (improved, binds, irreds, more_needed_givens) <- reduceContext env' wanteds'
1113 ; let all_needed_givens = needed_givens ++ more_needed_givens
1115 ; if not improved then
1116 return (irreds, binds, all_needed_givens)
1119 -- If improvement did some unification, we go round again.
1120 -- We start again with irreds, not wanteds
1121 -- Using an instance decl might have introduced a fresh type variable
1122 -- which might have been unified, so we'd get an infinite loop
1123 -- if we started again with wanteds! See Note [LOOP]
1124 { (irreds1, binds1, all_needed_givens1) <- go env' irreds all_needed_givens
1125 ; return (irreds1, binds `unionBags` binds1, all_needed_givens1) } }
1128 Note [Zonking RedEnv]
1129 ~~~~~~~~~~~~~~~~~~~~~
1130 It might appear as if the givens in RedEnv are always rigid, but that is not
1131 necessarily the case for programs involving higher-rank types that have class
1132 contexts constraining the higher-rank variables. An example from tc237 in the
1135 class Modular s a | s -> a
1137 wim :: forall a w. Integral a
1138 => a -> (forall s. Modular s a => M s w) -> w
1139 wim i k = error "urk"
1141 test5 :: (Modular s a, Integral a) => M s a
1144 test4 = wim 4 test4'
1146 Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
1147 quantified further outside. When type checking test4, we have to check
1148 whether the signature of test5 is an instance of
1150 (forall s. Modular s a => M s w)
1152 Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
1155 Given the FD of Modular in this example, class improvement will instantiate
1156 t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
1157 Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
1158 the givens, we will get into a loop as improveOne uses the unification engine
1159 TcGadt.tcUnifyTys, which doesn't know about mutable type variables.
1164 class If b t e r | b t e -> r
1167 class Lte a b c | a b -> c where lte :: a -> b -> c
1169 instance (Lte a b l,If l b a c) => Max a b c
1171 Wanted: Max Z (S x) y
1173 Then we'll reduce using the Max instance to:
1174 (Lte Z (S x) l, If l (S x) Z y)
1175 and improve by binding l->T, after which we can do some reduction
1176 on both the Lte and If constraints. What we *can't* do is start again
1177 with (Max Z (S x) y)!
1181 %************************************************************************
1183 tcSimplifySuperClasses
1185 %************************************************************************
1187 Note [SUPERCLASS-LOOP 1]
1188 ~~~~~~~~~~~~~~~~~~~~~~~~
1189 We have to be very, very careful when generating superclasses, lest we
1190 accidentally build a loop. Here's an example:
1194 class S a => C a where { opc :: a -> a }
1195 class S b => D b where { opd :: b -> b }
1197 instance C Int where
1200 instance D Int where
1203 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1204 Simplifying, we may well get:
1205 $dfCInt = :C ds1 (opd dd)
1208 Notice that we spot that we can extract ds1 from dd.
1210 Alas! Alack! We can do the same for (instance D Int):
1212 $dfDInt = :D ds2 (opc dc)
1216 And now we've defined the superclass in terms of itself.
1218 Solution: never generate a superclass selectors at all when
1219 satisfying the superclass context of an instance declaration.
1221 Two more nasty cases are in
1226 tcSimplifySuperClasses
1231 tcSimplifySuperClasses loc givens sc_wanteds
1232 = do { traceTc (text "tcSimplifySuperClasses")
1233 ; (irreds,binds1,_) <- checkLoop env sc_wanteds
1234 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1235 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1238 env = mkRedEnv (pprInstLoc loc) try_me givens
1239 try_me inst = ReduceMe NoSCs
1240 -- Like tryHardCheckLoop, but with NoSCs
1244 %************************************************************************
1246 \subsection{tcSimplifyRestricted}
1248 %************************************************************************
1250 tcSimplifyRestricted infers which type variables to quantify for a
1251 group of restricted bindings. This isn't trivial.
1254 We want to quantify over a to get id :: forall a. a->a
1257 We do not want to quantify over a, because there's an Eq a
1258 constraint, so we get eq :: a->a->Bool (notice no forall)
1261 RHS has type 'tau', whose free tyvars are tau_tvs
1262 RHS has constraints 'wanteds'
1265 Quantify over (tau_tvs \ ftvs(wanteds))
1266 This is bad. The constraints may contain (Monad (ST s))
1267 where we have instance Monad (ST s) where...
1268 so there's no need to be monomorphic in s!
1270 Also the constraint might be a method constraint,
1271 whose type mentions a perfectly innocent tyvar:
1272 op :: Num a => a -> b -> a
1273 Here, b is unconstrained. A good example would be
1275 We want to infer the polymorphic type
1276 foo :: forall b. b -> b
1279 Plan B (cunning, used for a long time up to and including GHC 6.2)
1280 Step 1: Simplify the constraints as much as possible (to deal
1281 with Plan A's problem). Then set
1282 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1284 Step 2: Now simplify again, treating the constraint as 'free' if
1285 it does not mention qtvs, and trying to reduce it otherwise.
1286 The reasons for this is to maximise sharing.
1288 This fails for a very subtle reason. Suppose that in the Step 2
1289 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1290 In the Step 1 this constraint might have been simplified, perhaps to
1291 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1292 This won't happen in Step 2... but that in turn might prevent some other
1293 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1294 and that in turn breaks the invariant that no constraints are quantified over.
1296 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1301 Step 1: Simplify the constraints as much as possible (to deal
1302 with Plan A's problem). Then set
1303 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1304 Return the bindings from Step 1.
1307 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1310 instance (HasBinary ty IO) => HasCodedValue ty
1312 foo :: HasCodedValue a => String -> IO a
1314 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1315 doDecodeIO codedValue view
1316 = let { act = foo "foo" } in act
1318 You might think this should work becuase the call to foo gives rise to a constraint
1319 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1320 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1321 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1323 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1327 Plan D (a variant of plan B)
1328 Step 1: Simplify the constraints as much as possible (to deal
1329 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1330 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1332 Step 2: Now simplify again, treating the constraint as 'free' if
1333 it does not mention qtvs, and trying to reduce it otherwise.
1335 The point here is that it's generally OK to have too few qtvs; that is,
1336 to make the thing more monomorphic than it could be. We don't want to
1337 do that in the common cases, but in wierd cases it's ok: the programmer
1338 can always add a signature.
1340 Too few qtvs => too many wanteds, which is what happens if you do less
1345 tcSimplifyRestricted -- Used for restricted binding groups
1346 -- i.e. ones subject to the monomorphism restriction
1349 -> [Name] -- Things bound in this group
1350 -> TcTyVarSet -- Free in the type of the RHSs
1351 -> [Inst] -- Free in the RHSs
1352 -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
1353 TcDictBinds) -- Bindings
1354 -- tcSimpifyRestricted returns no constraints to
1355 -- quantify over; by definition there are none.
1356 -- They are all thrown back in the LIE
1358 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1359 -- Zonk everything in sight
1360 = do { traceTc (text "tcSimplifyRestricted")
1361 ; wanteds' <- zonkInsts wanteds
1363 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1364 -- dicts; the idea is to get rid of as many type
1365 -- variables as possible, and we don't want to stop
1366 -- at (say) Monad (ST s), because that reduces
1367 -- immediately, with no constraint on s.
1369 -- BUT do no improvement! See Plan D above
1370 -- HOWEVER, some unification may take place, if we instantiate
1371 -- a method Inst with an equality constraint
1372 ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
1373 ; (_imp, _binds, constrained_dicts, _) <- reduceContext env wanteds'
1375 -- Next, figure out the tyvars we will quantify over
1376 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1377 ; gbl_tvs' <- tcGetGlobalTyVars
1378 ; constrained_dicts' <- zonkInsts constrained_dicts
1380 ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
1381 -- As in tcSimplifyInfer
1383 -- Do not quantify over constrained type variables:
1384 -- this is the monomorphism restriction
1385 constrained_tvs' = tyVarsOfInsts constrained_dicts'
1386 qtvs = qtvs1 `minusVarSet` constrained_tvs'
1387 pp_bndrs = pprWithCommas (quotes . ppr) bndrs
1390 ; warn_mono <- doptM Opt_WarnMonomorphism
1391 ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
1392 (vcat[ ptext SLIT("the Monomorphism Restriction applies to the binding")
1393 <> plural bndrs <+> ptext SLIT("for") <+> pp_bndrs,
1394 ptext SLIT("Consider giving a type signature for") <+> pp_bndrs])
1396 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1397 pprInsts wanteds, pprInsts constrained_dicts',
1399 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1401 -- The first step may have squashed more methods than
1402 -- necessary, so try again, this time more gently, knowing the exact
1403 -- set of type variables to quantify over.
1405 -- We quantify only over constraints that are captured by qtvs;
1406 -- these will just be a subset of non-dicts. This in contrast
1407 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1408 -- all *non-inheritable* constraints too. This implements choice
1409 -- (B) under "implicit parameter and monomorphism" above.
1411 -- Remember that we may need to do *some* simplification, to
1412 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1413 -- just to float all constraints
1415 -- At top level, we *do* squash methods becuase we want to
1416 -- expose implicit parameters to the test that follows
1417 ; let is_nested_group = isNotTopLevel top_lvl
1418 try_me inst | isFreeWrtTyVars qtvs inst,
1419 (is_nested_group || isDict inst) = Stop
1420 | otherwise = ReduceMe AddSCs
1421 env = mkNoImproveRedEnv doc try_me
1422 ; (_imp, binds, irreds, _) <- reduceContext env wanteds'
1424 -- See "Notes on implicit parameters, Question 4: top level"
1425 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1426 if is_nested_group then
1428 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1429 ; addTopIPErrs bndrs bad_ips
1430 ; extendLIEs non_ips }
1432 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1433 ; return (qtvs', binds) }
1437 %************************************************************************
1441 %************************************************************************
1443 On the LHS of transformation rules we only simplify methods and constants,
1444 getting dictionaries. We want to keep all of them unsimplified, to serve
1445 as the available stuff for the RHS of the rule.
1447 Example. Consider the following left-hand side of a rule
1449 f (x == y) (y > z) = ...
1451 If we typecheck this expression we get constraints
1453 d1 :: Ord a, d2 :: Eq a
1455 We do NOT want to "simplify" to the LHS
1457 forall x::a, y::a, z::a, d1::Ord a.
1458 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1462 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1463 f ((==) d2 x y) ((>) d1 y z) = ...
1465 Here is another example:
1467 fromIntegral :: (Integral a, Num b) => a -> b
1468 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1470 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1471 we *dont* want to get
1473 forall dIntegralInt.
1474 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1476 because the scsel will mess up RULE matching. Instead we want
1478 forall dIntegralInt, dNumInt.
1479 fromIntegral Int Int dIntegralInt dNumInt = id Int
1483 g (x == y) (y == z) = ..
1485 where the two dictionaries are *identical*, we do NOT WANT
1487 forall x::a, y::a, z::a, d1::Eq a
1488 f ((==) d1 x y) ((>) d1 y z) = ...
1490 because that will only match if the dict args are (visibly) equal.
1491 Instead we want to quantify over the dictionaries separately.
1493 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1494 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1495 from scratch, rather than further parameterise simpleReduceLoop etc
1498 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1499 tcSimplifyRuleLhs wanteds
1500 = go [] emptyBag wanteds
1503 = return (dicts, binds)
1504 go dicts binds (w:ws)
1506 = go (w:dicts) binds ws
1508 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1509 -- to fromInteger; this looks fragile to me
1510 ; lookup_result <- lookupSimpleInst w'
1511 ; case lookup_result of
1513 go dicts (addInstToDictBind binds w rhs) (ws' ++ ws)
1514 NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w)
1518 tcSimplifyBracket is used when simplifying the constraints arising from
1519 a Template Haskell bracket [| ... |]. We want to check that there aren't
1520 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1521 Show instance), but we aren't otherwise interested in the results.
1522 Nor do we care about ambiguous dictionaries etc. We will type check
1523 this bracket again at its usage site.
1526 tcSimplifyBracket :: [Inst] -> TcM ()
1527 tcSimplifyBracket wanteds
1528 = do { tryHardCheckLoop doc wanteds
1531 doc = text "tcSimplifyBracket"
1535 %************************************************************************
1537 \subsection{Filtering at a dynamic binding}
1539 %************************************************************************
1544 we must discharge all the ?x constraints from B. We also do an improvement
1545 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1547 Actually, the constraints from B might improve the types in ?x. For example
1549 f :: (?x::Int) => Char -> Char
1552 then the constraint (?x::Int) arising from the call to f will
1553 force the binding for ?x to be of type Int.
1556 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1559 -- We need a loop so that we do improvement, and then
1560 -- (next time round) generate a binding to connect the two
1562 -- Here the two ?x's have different types, and improvement
1563 -- makes them the same.
1565 tcSimplifyIPs given_ips wanteds
1566 = do { wanteds' <- zonkInsts wanteds
1567 ; given_ips' <- zonkInsts given_ips
1568 -- Unusually for checking, we *must* zonk the given_ips
1570 ; let env = mkRedEnv doc try_me given_ips'
1571 ; (improved, binds, irreds, _) <- reduceContext env wanteds'
1573 ; if not improved then
1574 ASSERT( all is_free irreds )
1575 do { extendLIEs irreds
1578 tcSimplifyIPs given_ips wanteds }
1580 doc = text "tcSimplifyIPs" <+> ppr given_ips
1581 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1582 is_free inst = isFreeWrtIPs ip_set inst
1584 -- Simplify any methods that mention the implicit parameter
1585 try_me inst | is_free inst = Stop
1586 | otherwise = ReduceMe NoSCs
1590 %************************************************************************
1592 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1594 %************************************************************************
1596 When doing a binding group, we may have @Insts@ of local functions.
1597 For example, we might have...
1599 let f x = x + 1 -- orig local function (overloaded)
1600 f.1 = f Int -- two instances of f
1605 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1606 where @f@ is in scope; those @Insts@ must certainly not be passed
1607 upwards towards the top-level. If the @Insts@ were binding-ified up
1608 there, they would have unresolvable references to @f@.
1610 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1611 For each method @Inst@ in the @init_lie@ that mentions one of the
1612 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1613 @LIE@), as well as the @HsBinds@ generated.
1616 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1617 -- Simlifies only MethodInsts, and generate only bindings of form
1619 -- We're careful not to even generate bindings of the form
1621 -- You'd think that'd be fine, but it interacts with what is
1622 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1624 bindInstsOfLocalFuns wanteds local_ids
1625 | null overloaded_ids
1627 = extendLIEs wanteds `thenM_`
1628 returnM emptyLHsBinds
1631 = do { (irreds, binds) <- gentleInferLoop doc for_me
1632 ; extendLIEs not_for_me
1636 doc = text "bindInsts" <+> ppr local_ids
1637 overloaded_ids = filter is_overloaded local_ids
1638 is_overloaded id = isOverloadedTy (idType id)
1639 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1641 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1642 -- so it's worth building a set, so that
1643 -- lookup (in isMethodFor) is faster
1647 %************************************************************************
1649 \subsection{Data types for the reduction mechanism}
1651 %************************************************************************
1653 The main control over context reduction is here
1657 = RedEnv { red_doc :: SDoc -- The context
1658 , red_try_me :: Inst -> WhatToDo
1659 , red_improve :: Bool -- True <=> do improvement
1660 , red_givens :: [Inst] -- All guaranteed rigid
1662 -- but see Note [Rigidity]
1663 , red_reft :: Refinement -- The refinement to apply to the 'givens'
1664 -- You should think of it as 'given equalities'
1665 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1666 -- See Note [RedStack]
1670 -- The red_givens are rigid so far as cmpInst is concerned.
1671 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1672 -- let ?x = e in ...
1673 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1674 -- But that doesn't affect the comparison, which is based only on mame.
1677 -- The red_stack pair (n,insts) pair is just used for error reporting.
1678 -- 'n' is always the depth of the stack.
1679 -- The 'insts' is the stack of Insts being reduced: to produce X
1680 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1683 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1684 mkRedEnv doc try_me givens
1685 = RedEnv { red_doc = doc, red_try_me = try_me,
1686 red_givens = givens,
1687 red_reft = emptyRefinement,
1689 red_improve = True }
1691 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1692 -- Do not do improvement; no givens
1693 mkNoImproveRedEnv doc try_me
1694 = RedEnv { red_doc = doc, red_try_me = try_me,
1695 red_givens = [], red_reft = emptyRefinement,
1697 red_improve = True }
1700 = ReduceMe WantSCs -- Try to reduce this
1701 -- If there's no instance, add the inst to the
1702 -- irreductible ones, but don't produce an error
1703 -- message of any kind.
1704 -- It might be quite legitimate such as (Eq a)!
1706 | Stop -- Return as irreducible unless it can
1707 -- be reduced to a constant in one step
1708 -- Do not add superclasses; see
1710 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1711 -- of a predicate when adding it to the avails
1712 -- The reason for this flag is entirely the super-class loop problem
1713 -- Note [SUPER-CLASS LOOP 1]
1715 zonkRedEnv :: RedEnv -> TcM RedEnv
1717 = do { givens' <- mappM zonkInst (red_givens env)
1718 ; return $ env {red_givens = givens'}
1723 %************************************************************************
1725 \subsection[reduce]{@reduce@}
1727 %************************************************************************
1729 Note [Ancestor Equalities]
1730 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1731 During context reduction, we add to the wanted equalities also those
1732 equalities that (transitively) occur in superclass contexts of wanted
1733 class constraints. Consider the following code
1735 class a ~ Int => C a
1738 If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
1739 substituting Int for a. Hence, we ultimately want (C Int), which we
1740 discharge with the explicit instance.
1743 reduceContext :: RedEnv
1745 -> TcM (ImprovementDone,
1746 TcDictBinds, -- Dictionary bindings
1747 [Inst], -- Irreducible
1748 [Inst]) -- Needed givens
1750 reduceContext env wanteds
1751 = do { traceTc (text "reduceContext" <+> (vcat [
1752 text "----------------------",
1754 text "given" <+> ppr (red_givens env),
1755 text "wanted" <+> ppr wanteds,
1756 text "----------------------"
1760 ; let givens = red_givens env
1761 (given_eqs0, given_dicts0) = partition isEqInst givens
1762 (wanted_eqs0, wanted_dicts0) = partition isEqInst wanteds
1764 -- We want to add as wanted equalities those that (transitively)
1765 -- occur in superclass contexts of wanted class constraints.
1766 -- See Note [Ancestor Equalities]
1767 ; ancestor_eqs <- ancestorEqualities wanted_dicts0
1768 ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
1769 ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
1771 -- 1. Normalise the *given* *equality* constraints
1772 ; (given_eqs, eliminate_skolems) <- normaliseGivenEqs given_eqs0
1774 -- 2. Normalise the *given* *dictionary* constraints
1775 -- wrt. the toplevel and given equations
1776 ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
1779 -- 5. Build the Avail mapping from "given_dicts"
1780 -- Add dicts refined by the current type refinement
1781 ; (init_state, extra_givens) <- getLIE $ do
1782 { init_state <- foldlM addGiven emptyAvails given_dicts
1783 ; let reft = red_reft env
1784 ; if isEmptyRefinement reft then return init_state
1785 else foldlM (addRefinedGiven reft)
1786 init_state given_dicts }
1788 -- *** ToDo: what to do with the "extra_givens"? For the
1789 -- moment I'm simply discarding them, which is probably wrong
1791 -- 7. Normalise the *wanted* *dictionary* constraints
1792 -- wrt. the toplevel and given equations
1793 -- NB: normalisation includes zonking as part of what it does
1794 -- so it's important to do it after any unifications
1795 -- that happened as a result of the addGivens
1796 ; (wanted_dicts,normalise_binds1) <- normaliseWantedDicts given_eqs wanted_dicts0
1798 -- 6. Solve the *wanted* *dictionary* constraints
1799 -- This may expose some further equational constraints...
1800 ; (avails, extra_eqs) <- getLIE (reduceList env wanted_dicts init_state)
1801 ; let (binds, irreds1, needed_givens) = extractResults avails wanted_dicts
1802 ; traceTc $ text "reduceContext extractresults" <+> vcat
1803 [ppr avails,ppr wanted_dicts,ppr binds,ppr needed_givens]
1805 -- *** ToDo: what to do with the "extra_eqs"? For the
1806 -- moment I'm simply discarding them, which is probably wrong
1808 -- 3. Solve the *wanted* *equation* constraints
1809 ; eq_irreds0 <- solveWantedEqs given_eqs wanted_eqs
1811 -- 4. Normalise the *wanted* equality constraints with respect to
1813 ; eq_irreds <- normaliseWantedEqs eq_irreds0
1815 -- 8. Substitute the wanted *equations* in the wanted *dictionaries*
1816 ; (irreds,normalise_binds2) <- substEqInDictInsts eq_irreds irreds1
1818 -- 9. eliminate the artificial skolem constants introduced in 1.
1821 -- Figure out whether we should go round again
1822 -- My current plan is to see if any of the mutable tyvars in
1823 -- givens or irreds has been filled in by improvement.
1824 -- If so, there is merit in going around again, because
1825 -- we may make further progress
1827 -- ToDo: is it only mutable stuff? We may have exposed new
1828 -- equality constraints and should probably go round again
1829 -- then as well. But currently we are dropping them on the
1832 ; let all_irreds = irreds ++ eq_irreds
1833 ; improved <- anyM isFilledMetaTyVar $ varSetElems $
1834 tyVarsOfInsts (givens ++ all_irreds)
1836 -- The old plan (fragile)
1837 -- improveed = availsImproved avails
1838 -- || (not $ isEmptyBag normalise_binds1)
1839 -- || (not $ isEmptyBag normalise_binds2)
1840 -- || (any isEqInst irreds)
1842 ; traceTc (text "reduceContext end" <+> (vcat [
1843 text "----------------------",
1845 text "given" <+> ppr givens,
1846 text "given_eqs" <+> ppr given_eqs,
1847 text "wanted" <+> ppr wanteds,
1848 text "wanted_dicts" <+> ppr wanted_dicts,
1850 text "avails" <+> pprAvails avails,
1851 text "improved =" <+> ppr improved,
1852 text "(all) irreds = " <+> ppr all_irreds,
1853 text "binds = " <+> ppr binds,
1854 text "needed givens = " <+> ppr needed_givens,
1855 text "----------------------"
1859 given_binds `unionBags` normalise_binds1
1860 `unionBags` normalise_binds2
1866 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1867 tcImproveOne avails inst
1868 | not (isDict inst) = return False
1870 = do { inst_envs <- tcGetInstEnvs
1871 ; let eqns = improveOne (classInstances inst_envs)
1872 (dictPred inst, pprInstArising inst)
1873 [ (dictPred p, pprInstArising p)
1874 | p <- availsInsts avails, isDict p ]
1875 -- Avails has all the superclasses etc (good)
1876 -- It also has all the intermediates of the deduction (good)
1877 -- It does not have duplicates (good)
1878 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1879 -- so that improve will see them separate
1880 ; traceTc (text "improveOne" <+> ppr inst)
1883 unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))]
1884 -> TcM ImprovementDone
1885 unifyEqns [] = return False
1887 = do { traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns))
1891 unify ((qtvs, pairs), what1, what2)
1892 = addErrCtxtM (mkEqnMsg what1 what2) $
1893 tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1894 mapM_ (unif_pr tenv) pairs
1895 unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1897 pprEquationDoc (eqn, (p1,w1), (p2,w2)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
1899 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
1900 = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
1901 ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
1902 ; let msg = vcat [ptext SLIT("When using functional dependencies to combine"),
1903 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
1904 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
1905 ; return (tidy_env, msg) }
1908 The main context-reduction function is @reduce@. Here's its game plan.
1911 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
1912 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
1913 = do { traceTc (text "reduceList " <+> (ppr wanteds $$ ppr state))
1917 dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n)
1918 2 (ifPprDebug (nest 2 (pprStack stk))))
1921 ; if n >= ctxtStkDepth dopts then
1922 failWithTc (reduceDepthErr n stk)
1926 go [] state = return state
1927 go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
1930 -- Base case: we're done!
1931 reduce env wanted avails
1932 -- It's the same as an existing inst, or a superclass thereof
1933 | Just avail <- findAvail avails wanted
1934 = do { traceTc (text "reduce: found " <+> ppr wanted)
1939 = do { traceTc (text "reduce" <+> ppr wanted $$ ppr avails)
1940 ; case red_try_me env wanted of {
1941 Stop -> try_simple (addIrred NoSCs);
1942 -- See Note [No superclasses for Stop]
1944 ReduceMe want_scs -> do -- It should be reduced
1945 { (avails, lookup_result) <- reduceInst env avails wanted
1946 ; case lookup_result of
1947 NoInstance -> addIrred want_scs avails wanted
1948 -- Add it and its superclasses
1950 GenInst [] rhs -> addWanted want_scs avails wanted rhs []
1952 GenInst wanteds' rhs
1953 -> do { avails1 <- addIrred NoSCs avails wanted
1954 ; avails2 <- reduceList env wanteds' avails1
1955 ; addWanted want_scs avails2 wanted rhs wanteds' } }
1956 -- Temporarily do addIrred *before* the reduceList,
1957 -- which has the effect of adding the thing we are trying
1958 -- to prove to the database before trying to prove the things it
1959 -- needs. See note [RECURSIVE DICTIONARIES]
1960 -- NB: we must not do an addWanted before, because that adds the
1961 -- superclasses too, and that can lead to a spurious loop; see
1962 -- the examples in [SUPERCLASS-LOOP]
1963 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1966 -- First, see if the inst can be reduced to a constant in one step
1967 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
1968 -- Don't bother for implication constraints, which take real work
1969 try_simple do_this_otherwise
1970 = do { res <- lookupSimpleInst wanted
1972 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
1973 other -> do_this_otherwise avails wanted }
1977 Note [SUPERCLASS-LOOP 2]
1978 ~~~~~~~~~~~~~~~~~~~~~~~~
1979 But the above isn't enough. Suppose we are *given* d1:Ord a,
1980 and want to deduce (d2:C [a]) where
1982 class Ord a => C a where
1983 instance Ord [a] => C [a] where ...
1985 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
1986 superclasses of C [a] to avails. But we must not overwrite the binding
1987 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
1990 Here's another variant, immortalised in tcrun020
1991 class Monad m => C1 m
1992 class C1 m => C2 m x
1993 instance C2 Maybe Bool
1994 For the instance decl we need to build (C1 Maybe), and it's no good if
1995 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1996 before we search for C1 Maybe.
1998 Here's another example
1999 class Eq b => Foo a b
2000 instance Eq a => Foo [a] a
2004 we'll first deduce that it holds (via the instance decl). We must not
2005 then overwrite the Eq t constraint with a superclass selection!
2007 At first I had a gross hack, whereby I simply did not add superclass constraints
2008 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
2009 becuase it lost legitimate superclass sharing, and it still didn't do the job:
2010 I found a very obscure program (now tcrun021) in which improvement meant the
2011 simplifier got two bites a the cherry... so something seemed to be an Stop
2012 first time, but reducible next time.
2014 Now we implement the Right Solution, which is to check for loops directly
2015 when adding superclasses. It's a bit like the occurs check in unification.
2018 Note [RECURSIVE DICTIONARIES]
2019 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2021 data D r = ZeroD | SuccD (r (D r));
2023 instance (Eq (r (D r))) => Eq (D r) where
2024 ZeroD == ZeroD = True
2025 (SuccD a) == (SuccD b) = a == b
2028 equalDC :: D [] -> D [] -> Bool;
2031 We need to prove (Eq (D [])). Here's how we go:
2035 by instance decl, holds if
2039 by instance decl of Eq, holds if
2041 where d2 = dfEqList d3
2044 But now we can "tie the knot" to give
2050 and it'll even run! The trick is to put the thing we are trying to prove
2051 (in this case Eq (D []) into the database before trying to prove its
2052 contributing clauses.
2055 %************************************************************************
2057 Reducing a single constraint
2059 %************************************************************************
2062 ---------------------------------------------
2063 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
2064 reduceInst env avails (ImplicInst { tci_name = name,
2065 tci_tyvars = tvs, tci_reft = reft, tci_loc = loc,
2066 tci_given = extra_givens, tci_wanted = wanteds })
2067 = reduceImplication env avails name reft tvs extra_givens wanteds loc
2069 reduceInst env avails other_inst
2070 = do { result <- lookupSimpleInst other_inst
2071 ; return (avails, result) }
2074 Note [Equational Constraints in Implication Constraints]
2075 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2077 An implication constraint is of the form
2079 where Given and Wanted may contain both equational and dictionary
2080 constraints. The delay and reduction of these two kinds of constraints
2083 -) In the generated code, wanted Dictionary constraints are wrapped up in an
2084 implication constraint that is created at the code site where the wanted
2085 dictionaries can be reduced via a let-binding. This let-bound implication
2086 constraint is deconstructed at the use-site of the wanted dictionaries.
2088 -) While the reduction of equational constraints is also delayed, the delay
2089 is not manifest in the generated code. The required evidence is generated
2090 in the code directly at the use-site. There is no let-binding and deconstruction
2091 necessary. The main disadvantage is that we cannot exploit sharing as the
2092 same evidence may be generated at multiple use-sites. However, this disadvantage
2093 is limited because it only concerns coercions which are erased.
2095 The different treatment is motivated by the different in representation. Dictionary
2096 constraints require manifest runtime dictionaries, while equations require coercions
2100 ---------------------------------------------
2101 reduceImplication :: RedEnv
2104 -> Refinement -- May refine the givens; often empty
2105 -> [TcTyVar] -- Quantified type variables; all skolems
2106 -> [Inst] -- Extra givens; all rigid
2109 -> TcM (Avails, LookupInstResult)
2112 Suppose we are simplifying the constraint
2113 forall bs. extras => wanted
2114 in the context of an overall simplification problem with givens 'givens',
2115 and refinment 'reft'.
2118 * The refinement is often empty
2120 * The 'extra givens' need not mention any of the quantified type variables
2121 e.g. forall {}. Eq a => Eq [a]
2122 forall {}. C Int => D (Tree Int)
2124 This happens when you have something like
2126 T1 :: Eq a => a -> T a
2129 f x = ...(case x of { T1 v -> v==v })...
2132 -- ToDo: should we instantiate tvs? I think it's not necessary
2134 -- Note on coercion variables:
2136 -- The extra given coercion variables are bound at two different sites:
2137 -- -) in the creation context of the implication constraint
2138 -- the solved equational constraints use these binders
2140 -- -) at the solving site of the implication constraint
2141 -- the solved dictionaries use these binders
2142 -- these binders are generated by reduceImplication
2144 reduceImplication env orig_avails name reft tvs extra_givens wanteds inst_loc
2145 = do { -- Add refined givens, and the extra givens
2147 -- (refined_red_givens,refined_avails)
2148 -- <- if isEmptyRefinement reft then return (red_givens env,orig_avails)
2149 -- else foldlM (addRefinedGiven reft) ([],orig_avails) (red_givens env)
2150 -- Commented out SLPJ Sept 07; see comment with extractLocalResults below
2151 let refined_red_givens = []
2153 -- Solve the sub-problem
2154 ; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
2155 env' = env { red_givens = extra_givens ++ availsInsts orig_avails
2157 , red_doc = sep [ptext SLIT("reduceImplication for") <+> ppr name,
2158 nest 2 (parens $ ptext SLIT("within") <+> red_doc env)]
2159 , red_try_me = try_me }
2161 ; traceTc (text "reduceImplication" <+> vcat
2163 ppr (red_givens env), ppr extra_givens,
2164 ppr reft, ppr wanteds])
2165 ; (irreds,binds,needed_givens0) <- checkLoop env' wanteds
2166 ; let (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
2167 -- SLPJ Sept 07: I think this is bogus; currently
2168 -- there are no Eqinsts in extra_givens
2169 dict_ids = map instToId extra_dict_givens
2171 -- needed_givens0 is the free vars of the bindings
2172 -- Remove the ones we are going to lambda-bind
2173 -- Use the actual dictionary identity *not* equality on Insts
2174 -- (Mind you, it should make no difference here.)
2175 ; let needed_givens = [ng | ng <- needed_givens0
2176 , instToVar ng `notElem` dict_ids]
2178 -- Note [Reducing implication constraints]
2179 -- Tom -- update note, put somewhere!
2181 ; traceTc (text "reduceImplication result" <+> vcat
2182 [ppr irreds, ppr binds, ppr needed_givens])
2184 ; -- extract superclass binds
2185 -- (sc_binds,_) <- extractResults avails []
2186 -- ; traceTc (text "reduceImplication sc_binds" <+> vcat
2187 -- [ppr sc_binds, ppr avails])
2190 -- We always discard the extra avails we've generated;
2191 -- but we remember if we have done any (global) improvement
2192 -- ; let ret_avails = avails
2193 ; let ret_avails = orig_avails
2194 -- ; let ret_avails = updateImprovement orig_avails avails
2196 -- SLPJ Sept 07: what if improvement happened inside the checkLoop?
2197 -- Then we must iterate the outer loop too!
2199 ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
2201 -- Progress is no longer measered by the number of bindings
2202 ; if (isEmptyLHsBinds binds) && (not $ null irreds) then -- No progress
2203 -- If there are any irreds, we back off and return NoInstance
2204 return (ret_avails, NoInstance)
2206 { (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds
2207 -- This binding is useless if the recursive simplification
2208 -- made no progress; but currently we don't try to optimise that
2209 -- case. After all, we only try hard to reduce at top level, or
2210 -- when inferring types.
2212 ; let dict_wanteds = filter (not . isEqInst) wanteds
2213 -- TOMDO: given equational constraints bug!
2214 -- we need a different evidence for given
2215 -- equations depending on whether we solve
2216 -- dictionary constraints or equational constraints
2218 eq_tyvars = varSetElems $ tyVarsOfTypes $ map eqInstType extra_eq_givens
2219 -- SLPJ Sept07: this looks Utterly Wrong to me, but I think
2220 -- that current extra_givens has no EqInsts, so
2221 -- it makes no difference
2222 co = wrap_inline -- Note [Always inline implication constraints]
2224 <.> mkWpTyLams eq_tyvars
2225 <.> mkWpLams dict_ids
2226 <.> WpLet (binds `unionBags` bind)
2227 wrap_inline | null dict_ids = idHsWrapper
2228 | otherwise = WpInline
2229 rhs = mkHsWrap co payload
2230 loc = instLocSpan inst_loc
2231 payload | [dict_wanted] <- dict_wanteds = HsVar (instToId dict_wanted)
2232 | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) dict_wanteds) Boxed
2235 ; traceTc (vcat [text "reduceImplication" <+> ppr name,
2237 text "->" <+> sep [ppr needed_givens, ppr rhs]])
2238 ; return (ret_avails, GenInst (implic_insts ++ needed_givens) (L loc rhs))
2243 Note [Always inline implication constraints]
2244 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2245 Suppose an implication constraint floats out of an INLINE function.
2246 Then although the implication has a single call site, it won't be
2247 inlined. And that is bad because it means that even if there is really
2248 *no* overloading (type signatures specify the exact types) there will
2249 still be dictionary passing in the resulting code. To avert this,
2250 we mark the implication constraints themselves as INLINE, at least when
2251 there is no loss of sharing as a result.
2253 Note [Reducing implication constraints]
2254 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2255 Suppose we are trying to simplify
2257 ic: (forall b. C a b => (W [a] b, D c b)) )
2259 instance (C a b, Ord a) => W [a] b
2260 When solving the implication constraint, we'll start with
2262 in the Avails. Then we add (C a b -> Given) and solve. Extracting
2263 the results gives us a binding for the (W [a] b), with an Irred of
2264 (Ord a, D c b). Now, the (Ord a) comes from "outside" the implication,
2265 but the (D d b) is from "inside". So we want to generate a GenInst
2270 ic' :: forall b. C a b => D c b]
2271 (/\b \(dc:C a b). (df a b dc do, ic' b dc))
2273 The first arg of GenInst gives the free dictionary variables of the
2274 second argument -- the "needed givens". And that list in turn is
2275 vital because it's used to determine what other dicts must be solved.
2276 This very list ends up in the second field of the Rhs, and drives
2279 The need for this field is why we have to return "needed givens"
2280 from extractResults, reduceContext, checkLoop, and so on.
2282 NB: the "needed givens" in a GenInst or Rhs, may contain two dicts
2283 with the same type but different Ids, e.g. [d12 :: Eq a, d81 :: Eq a]
2284 That says we must generate a binding for both d12 and d81.
2286 The "inside" and "outside" distinction is what's going on with 'inner' and
2287 'outer' in reduceImplication
2290 Note [Freeness and implications]
2291 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2292 It's hard to say when an implication constraint can be floated out. Consider
2293 forall {} Eq a => Foo [a]
2294 The (Foo [a]) doesn't mention any of the quantified variables, but it
2295 still might be partially satisfied by the (Eq a).
2297 There is a useful special case when it *is* easy to partition the
2298 constraints, namely when there are no 'givens'. Consider
2299 forall {a}. () => Bar b
2300 There are no 'givens', and so there is no reason to capture (Bar b).
2301 We can let it float out. But if there is even one constraint we
2302 must be much more careful:
2303 forall {a}. C a b => Bar (m b)
2304 because (C a b) might have a superclass (D b), from which we might
2305 deduce (Bar [b]) when m later gets instantiated to []. Ha!
2307 Here is an even more exotic example
2309 Now consider the constraint
2310 forall b. D Int b => C Int
2311 We can satisfy the (C Int) from the superclass of D, so we don't want
2312 to float the (C Int) out, even though it mentions no type variable in
2315 Note [Pruning the givens in an implication constraint]
2316 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2317 Suppose we are about to form the implication constraint
2318 forall tvs. Eq a => Ord b
2319 The (Eq a) cannot contribute to the (Ord b), because it has no access to
2320 the type variable 'b'. So we could filter out the (Eq a) from the givens.
2322 Doing so would be a bit tidier, but all the implication constraints get
2323 simplified away by the optimiser, so it's no great win. So I don't take
2324 advantage of that at the moment.
2326 If you do, BE CAREFUL of wobbly type variables.
2329 %************************************************************************
2331 Avails and AvailHow: the pool of evidence
2333 %************************************************************************
2337 data Avails = Avails !ImprovementDone !AvailEnv
2339 type ImprovementDone = Bool -- True <=> some unification has happened
2340 -- so some Irreds might now be reducible
2341 -- keys that are now
2343 type AvailEnv = FiniteMap Inst AvailHow
2345 = IsIrred -- Used for irreducible dictionaries,
2346 -- which are going to be lambda bound
2348 | Given Inst -- Used for dictionaries for which we have a binding
2349 -- e.g. those "given" in a signature
2351 | Rhs -- Used when there is a RHS
2352 (LHsExpr TcId) -- The RHS
2353 [Inst] -- Insts free in the RHS; we need these too
2355 instance Outputable Avails where
2358 pprAvails (Avails imp avails)
2359 = vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty)
2361 vcat [ sep [ppr inst, nest 2 (equals <+> ppr avail)]
2362 | (inst,avail) <- fmToList avails ]]
2364 instance Outputable AvailHow where
2367 -------------------------
2368 pprAvail :: AvailHow -> SDoc
2369 pprAvail IsIrred = text "Irred"
2370 pprAvail (Given x) = text "Given" <+> ppr x
2371 pprAvail (Rhs rhs bs) = sep [text "Rhs" <+> ppr bs,
2374 -------------------------
2375 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
2376 extendAvailEnv env inst avail = addToFM env inst avail
2378 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
2379 findAvailEnv env wanted = lookupFM env wanted
2380 -- NB 1: the Ord instance of Inst compares by the class/type info
2381 -- *not* by unique. So
2382 -- d1::C Int == d2::C Int
2384 emptyAvails :: Avails
2385 emptyAvails = Avails False emptyFM
2387 findAvail :: Avails -> Inst -> Maybe AvailHow
2388 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
2390 elemAvails :: Inst -> Avails -> Bool
2391 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
2393 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
2395 extendAvails avails@(Avails imp env) inst avail
2396 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
2397 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
2399 availsInsts :: Avails -> [Inst]
2400 availsInsts (Avails _ avails) = keysFM avails
2402 availsImproved (Avails imp _) = imp
2404 updateImprovement :: Avails -> Avails -> Avails
2405 -- (updateImprovement a1 a2) sets a1's improvement flag from a2
2406 updateImprovement (Avails _ avails1) (Avails imp2 _) = Avails imp2 avails1
2409 Extracting the bindings from a bunch of Avails.
2410 The bindings do *not* come back sorted in dependency order.
2411 We assume that they'll be wrapped in a big Rec, so that the
2412 dependency analyser can sort them out later
2415 type DoneEnv = FiniteMap Inst [Id]
2416 -- Tracks which things we have evidence for
2418 extractResults :: Avails
2420 -> (TcDictBinds, -- Bindings
2421 [Inst], -- Irreducible ones
2422 [Inst]) -- Needed givens, i.e. ones used in the bindings
2423 -- Postcondition: needed-givens = free vars( binds ) \ irreds
2424 -- needed-gives is subset of Givens in incoming Avails
2425 -- Note [Reducing implication constraints]
2427 extractResults (Avails _ avails) wanteds
2428 = go emptyBag [] [] emptyFM wanteds
2430 go :: TcDictBinds -- Bindings for dicts
2432 -> [Inst] -- Needed givens
2433 -> DoneEnv -- Has an entry for each inst in the above three sets
2435 -> (TcDictBinds, [Inst], [Inst])
2436 go binds irreds givens done []
2437 = (binds, irreds, givens)
2439 go binds irreds givens done (w:ws)
2440 | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w
2441 = if w_id `elem` done_ids then
2442 go binds irreds givens done ws
2444 go (add_bind (nlHsVar done_id)) irreds givens
2445 (addToFM done w (done_id : w_id : rest_done_ids)) ws
2447 | otherwise -- Not yet done
2448 = case findAvailEnv avails w of
2449 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
2450 go binds irreds givens done ws
2452 Just IsIrred -> go binds (w:irreds) givens done' ws
2454 Just (Rhs rhs ws') -> go (add_bind rhs) irreds givens done' (ws' ++ ws)
2456 Just (Given g) -> go binds' irreds (g:givens) (addToFM done w [g_id]) ws
2459 binds' | w_id == g_id = binds
2460 | otherwise = add_bind (nlHsVar g_id)
2463 done' = addToFM done w [w_id]
2464 add_bind rhs = addInstToDictBind binds w rhs
2468 Note [No superclasses for Stop]
2469 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2470 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2471 add it to avails, so that any other equal Insts will be commoned up
2472 right here. However, we do *not* add superclasses. If we have
2475 but a is not bound here, then we *don't* want to derive dn from df
2476 here lest we lose sharing.
2479 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2480 addWanted want_scs avails wanted rhs_expr wanteds
2481 = addAvailAndSCs want_scs avails wanted avail
2483 avail = Rhs rhs_expr wanteds
2485 addGiven :: Avails -> Inst -> TcM Avails
2486 addGiven avails given = addAvailAndSCs AddSCs avails given (Given given)
2487 -- Always add superclasses for 'givens'
2489 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2490 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2491 -- so the assert isn't true
2493 addRefinedGiven :: Refinement -> Avails -> Inst -> TcM Avails
2494 addRefinedGiven reft avails given
2495 | isDict given -- We sometimes have 'given' methods, but they
2496 -- are always optional, so we can drop them
2497 , let pred = dictPred given
2498 , isRefineablePred pred -- See Note [ImplicInst rigidity]
2499 , Just (co, pred) <- refinePred reft pred
2500 = do { new_given <- newDictBndr (instLoc given) pred
2501 ; let rhs = L (instSpan given) $
2502 HsWrap (WpCo co) (HsVar (instToId given))
2503 ; addAvailAndSCs AddSCs avails new_given (Rhs rhs [given]) }
2504 -- ToDo: the superclasses of the original given all exist in Avails
2505 -- so we could really just cast them, but it's more awkward to do,
2506 -- and hopefully the optimiser will spot the duplicated work
2511 Note [ImplicInst rigidity]
2512 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2514 C :: forall ab. (Eq a, Ord b) => b -> T a
2516 ...(case x of C v -> <body>)...
2518 From the case (where x::T ty) we'll get an implication constraint
2519 forall b. (Eq ty, Ord b) => <body-constraints>
2520 Now suppose <body-constraints> itself has an implication constraint
2522 forall c. <reft> => <payload>
2523 Then, we can certainly apply the refinement <reft> to the Ord b, becuase it is
2524 existential, but we probably should not apply it to the (Eq ty) because it may
2525 be wobbly. Hence the isRigidInst
2527 @Insts@ are ordered by their class/type info, rather than by their
2528 unique. This allows the context-reduction mechanism to use standard finite
2529 maps to do their stuff. It's horrible that this code is here, rather
2530 than with the Avails handling stuff in TcSimplify
2533 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2534 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2535 addAvailAndSCs want_scs avails irred IsIrred
2537 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2538 addAvailAndSCs want_scs avails inst avail
2539 | not (isClassDict inst) = extendAvails avails inst avail
2540 | NoSCs <- want_scs = extendAvails avails inst avail
2541 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2542 ; avails' <- extendAvails avails inst avail
2543 ; addSCs is_loop avails' inst }
2545 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2546 -- Note: this compares by *type*, not by Unique
2547 deps = findAllDeps (unitVarSet (instToVar inst)) avail
2548 dep_tys = map idType (varSetElems deps)
2550 findAllDeps :: IdSet -> AvailHow -> IdSet
2551 -- Find all the Insts that this one depends on
2552 -- See Note [SUPERCLASS-LOOP 2]
2553 -- Watch out, though. Since the avails may contain loops
2554 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2555 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2556 findAllDeps so_far other = so_far
2558 find_all :: IdSet -> Inst -> IdSet
2560 | isEqInst kid = so_far
2561 | kid_id `elemVarSet` so_far = so_far
2562 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2563 | otherwise = so_far'
2565 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2566 kid_id = instToId kid
2568 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2569 -- Add all the superclasses of the Inst to Avails
2570 -- The first param says "don't do this because the original thing
2571 -- depends on this one, so you'd build a loop"
2572 -- Invariant: the Inst is already in Avails.
2574 addSCs is_loop avails dict
2575 = ASSERT( isDict dict )
2576 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2577 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2579 (clas, tys) = getDictClassTys dict
2580 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2581 sc_theta' = filter (not . isEqPred) $
2582 substTheta (zipTopTvSubst tyvars tys) sc_theta
2584 add_sc avails (sc_dict, sc_sel)
2585 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2586 | is_given sc_dict = return avails
2587 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2588 ; addSCs is_loop avails' sc_dict }
2590 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2591 co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
2593 is_given :: Inst -> Bool
2594 is_given sc_dict = case findAvail avails sc_dict of
2595 Just (Given _) -> True -- Given is cheaper than superclass selection
2598 -- From the a set of insts obtain all equalities that (transitively) occur in
2599 -- superclass contexts of class constraints (aka the ancestor equalities).
2601 ancestorEqualities :: [Inst] -> TcM [Inst]
2603 = mapM mkWantedEqInst -- turn only equality predicates..
2604 . filter isEqPred -- ..into wanted equality insts
2606 . addAEsToBag emptyBag -- collect the superclass constraints..
2607 . map dictPred -- ..of all predicates in a bag
2608 . filter isClassDict
2610 addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
2611 addAEsToBag bag [] = bag
2612 addAEsToBag bag (pred:preds)
2613 | pred `elemBag` bag = addAEsToBag bag preds
2614 | isEqPred pred = addAEsToBag bagWithPred preds
2615 | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
2616 | otherwise = addAEsToBag bag preds
2618 bagWithPred = bag `snocBag` pred
2619 predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
2621 (tyvars, sc_theta, _, _) = classBigSig clas
2622 (clas, tys) = getClassPredTys pred
2626 %************************************************************************
2628 \section{tcSimplifyTop: defaulting}
2630 %************************************************************************
2633 @tcSimplifyTop@ is called once per module to simplify all the constant
2634 and ambiguous Insts.
2636 We need to be careful of one case. Suppose we have
2638 instance Num a => Num (Foo a b) where ...
2640 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2641 to (Num x), and default x to Int. But what about y??
2643 It's OK: the final zonking stage should zap y to (), which is fine.
2647 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2648 tcSimplifyTop wanteds
2649 = tc_simplify_top doc False wanteds
2651 doc = text "tcSimplifyTop"
2653 tcSimplifyInteractive wanteds
2654 = tc_simplify_top doc True wanteds
2656 doc = text "tcSimplifyInteractive"
2658 -- The TcLclEnv should be valid here, solely to improve
2659 -- error message generation for the monomorphism restriction
2660 tc_simplify_top doc interactive wanteds
2661 = do { dflags <- getDOpts
2662 ; wanteds <- zonkInsts wanteds
2663 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2665 ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
2666 ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
2667 -- ; (irreds1, binds1) <- gentleInferLoop doc1 wanteds
2668 ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
2669 ; (irreds2, binds2) <- approximateImplications doc2 (\d -> True) irreds1
2670 ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
2672 -- Use the defaulting rules to do extra unification
2673 -- NB: irreds2 are already zonked
2674 ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
2676 -- Deal with implicit parameters
2677 ; let (bad_ips, non_ips) = partition isIPDict irreds3
2678 (ambigs, others) = partition isTyVarDict non_ips
2680 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2682 ; addNoInstanceErrs others
2683 ; addTopAmbigErrs ambigs
2685 ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
2687 doc1 = doc <+> ptext SLIT("(first round)")
2688 doc2 = doc <+> ptext SLIT("(approximate)")
2689 doc3 = doc <+> ptext SLIT("(disambiguate)")
2692 If a dictionary constrains a type variable which is
2693 * not mentioned in the environment
2694 * and not mentioned in the type of the expression
2695 then it is ambiguous. No further information will arise to instantiate
2696 the type variable; nor will it be generalised and turned into an extra
2697 parameter to a function.
2699 It is an error for this to occur, except that Haskell provided for
2700 certain rules to be applied in the special case of numeric types.
2702 * at least one of its classes is a numeric class, and
2703 * all of its classes are numeric or standard
2704 then the type variable can be defaulted to the first type in the
2705 default-type list which is an instance of all the offending classes.
2707 So here is the function which does the work. It takes the ambiguous
2708 dictionaries and either resolves them (producing bindings) or
2709 complains. It works by splitting the dictionary list by type
2710 variable, and using @disambigOne@ to do the real business.
2712 @disambigOne@ assumes that its arguments dictionaries constrain all
2713 the same type variable.
2715 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2716 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2717 the most common use of defaulting is code like:
2719 _ccall_ foo `seqPrimIO` bar
2721 Since we're not using the result of @foo@, the result if (presumably)
2725 disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
2726 -- Just does unification to fix the default types
2727 -- The Insts are assumed to be pre-zonked
2728 disambiguate doc interactive dflags insts
2730 = return (insts, emptyBag)
2732 | null defaultable_groups
2733 = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
2734 ; return (insts, emptyBag) }
2737 = do { -- Figure out what default types to use
2738 default_tys <- getDefaultTys extended_defaulting ovl_strings
2740 ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2741 ; mapM_ (disambigGroup default_tys) defaultable_groups
2743 -- disambigGroup does unification, hence try again
2744 ; tryHardCheckLoop doc insts }
2747 extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
2748 ovl_strings = dopt Opt_OverloadedStrings dflags
2750 unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
2751 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2752 (unaries, bad_tvs_s) = partitionWith find_unary insts
2753 bad_tvs = unionVarSets bad_tvs_s
2755 -- Finds unary type-class constraints
2756 find_unary d@(Dict {tci_pred = ClassP cls [ty]})
2757 | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
2758 find_unary inst = Right (tyVarsOfInst inst)
2760 -- Group by type variable
2761 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2762 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2763 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2765 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2766 defaultable_group ds@((_,_,tv):_)
2767 = isTyConableTyVar tv -- Note [Avoiding spurious errors]
2768 && not (tv `elemVarSet` bad_tvs)
2769 && defaultable_classes [c | (_,c,_) <- ds]
2770 defaultable_group [] = panic "defaultable_group"
2772 defaultable_classes clss
2773 | extended_defaulting = any isInteractiveClass clss
2774 | otherwise = all is_std_class clss && (any is_num_class clss)
2776 -- In interactive mode, or with -fextended-default-rules,
2777 -- we default Show a to Show () to avoid graututious errors on "show []"
2778 isInteractiveClass cls
2779 = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2781 is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2782 -- is_num_class adds IsString to the standard numeric classes,
2783 -- when -foverloaded-strings is enabled
2785 is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2786 -- Similarly is_std_class
2788 -----------------------
2789 disambigGroup :: [Type] -- The default types
2790 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2791 -> TcM () -- Just does unification, to fix the default types
2793 disambigGroup default_tys dicts
2794 = try_default default_tys
2796 (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
2797 classes = [c | (_,c,_) <- dicts]
2799 try_default [] = return ()
2800 try_default (default_ty : default_tys)
2801 = tryTcLIE_ (try_default default_tys) $
2802 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2803 -- This may fail; then the tryTcLIE_ kicks in
2804 -- Failure here is caused by there being no type in the
2805 -- default list which can satisfy all the ambiguous classes.
2806 -- For example, if Real a is reqd, but the only type in the
2807 -- default list is Int.
2809 -- After this we can't fail
2810 ; warnDefault dicts default_ty
2811 ; unifyType default_ty (mkTyVarTy tyvar)
2812 ; return () -- TOMDO: do something with the coercion
2816 -----------------------
2817 getDefaultTys :: Bool -> Bool -> TcM [Type]
2818 getDefaultTys extended_deflts ovl_strings
2819 = do { mb_defaults <- getDeclaredDefaultTys
2820 ; case mb_defaults of {
2821 Just tys -> return tys ; -- User-supplied defaults
2824 -- No use-supplied default
2825 -- Use [Integer, Double], plus modifications
2826 { integer_ty <- tcMetaTy integerTyConName
2827 ; checkWiredInTyCon doubleTyCon
2828 ; string_ty <- tcMetaTy stringTyConName
2829 ; return (opt_deflt extended_deflts unitTy
2830 -- Note [Default unitTy]
2832 [integer_ty,doubleTy]
2834 opt_deflt ovl_strings string_ty) } } }
2836 opt_deflt True ty = [ty]
2837 opt_deflt False ty = []
2840 Note [Default unitTy]
2841 ~~~~~~~~~~~~~~~~~~~~~
2842 In interative mode (or with -fextended-default-rules) we add () as the first type we
2843 try when defaulting. This has very little real impact, except in the following case.
2845 Text.Printf.printf "hello"
2846 This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
2847 want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
2848 default the 'a' to (), rather than to Integer (which is what would otherwise happen;
2849 and then GHCi doesn't attempt to print the (). So in interactive mode, we add
2850 () to the list of defaulting types. See Trac #1200.
2852 Note [Avoiding spurious errors]
2853 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2854 When doing the unification for defaulting, we check for skolem
2855 type variables, and simply don't default them. For example:
2856 f = (*) -- Monomorphic
2857 g :: Num a => a -> a
2859 Here, we get a complaint when checking the type signature for g,
2860 that g isn't polymorphic enough; but then we get another one when
2861 dealing with the (Num a) context arising from f's definition;
2862 we try to unify a with Int (to default it), but find that it's
2863 already been unified with the rigid variable from g's type sig
2866 %************************************************************************
2868 \subsection[simple]{@Simple@ versions}
2870 %************************************************************************
2872 Much simpler versions when there are no bindings to make!
2874 @tcSimplifyThetas@ simplifies class-type constraints formed by
2875 @deriving@ declarations and when specialising instances. We are
2876 only interested in the simplified bunch of class/type constraints.
2878 It simplifies to constraints of the form (C a b c) where
2879 a,b,c are type variables. This is required for the context of
2880 instance declarations.
2883 tcSimplifyDeriv :: InstOrigin
2885 -> ThetaType -- Wanted
2886 -> TcM ThetaType -- Needed
2887 -- Given instance (wanted) => C inst_ty
2888 -- Simplify 'wanted' as much as possible
2890 tcSimplifyDeriv orig tyvars theta
2891 = do { (tvs, _, tenv) <- tcInstTyVars tyvars
2892 -- The main loop may do unification, and that may crash if
2893 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2894 -- ToDo: what if two of them do get unified?
2895 ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
2896 ; (irreds, _) <- tryHardCheckLoop doc wanteds
2898 ; let (tv_dicts, others) = partition ok irreds
2899 ; addNoInstanceErrs others
2900 -- See Note [Exotic derived instance contexts] in TcMType
2902 ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2903 simpl_theta = substTheta rev_env (map dictPred tv_dicts)
2904 -- This reverse-mapping is a pain, but the result
2905 -- should mention the original TyVars not TcTyVars
2907 ; return simpl_theta }
2909 doc = ptext SLIT("deriving classes for a data type")
2911 ok dict | isDict dict = validDerivPred (dictPred dict)
2916 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2917 used with \tr{default} declarations. We are only interested in
2918 whether it worked or not.
2921 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2924 tcSimplifyDefault theta
2925 = newDictBndrsO DefaultOrigin theta `thenM` \ wanteds ->
2926 tryHardCheckLoop doc wanteds `thenM` \ (irreds, _) ->
2927 addNoInstanceErrs irreds `thenM_`
2931 traceTc (ptext SLIT("tcSimplifyDefault failing")) >> failM
2933 doc = ptext SLIT("default declaration")
2937 %************************************************************************
2939 \section{Errors and contexts}
2941 %************************************************************************
2943 ToDo: for these error messages, should we note the location as coming
2944 from the insts, or just whatever seems to be around in the monad just
2948 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2949 -> [Inst] -- The offending Insts
2951 -- Group together insts with the same origin
2952 -- We want to report them together in error messages
2954 groupErrs report_err []
2956 groupErrs report_err (inst:insts)
2957 = do { do_one (inst:friends)
2958 ; groupErrs report_err others }
2960 -- (It may seem a bit crude to compare the error messages,
2961 -- but it makes sure that we combine just what the user sees,
2962 -- and it avoids need equality on InstLocs.)
2963 (friends, others) = partition is_friend insts
2964 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2965 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2966 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2967 -- Add location and context information derived from the Insts
2969 -- Add the "arising from..." part to a message about bunch of dicts
2970 addInstLoc :: [Inst] -> Message -> Message
2971 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
2973 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
2974 addTopIPErrs bndrs []
2976 addTopIPErrs bndrs ips
2977 = do { dflags <- getDOpts
2978 ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
2980 (tidy_env, tidy_ips) = tidyInsts ips
2982 = vcat [sep [ptext SLIT("Implicit parameters escape from"),
2983 nest 2 (ptext SLIT("the monomorphic top-level binding")
2984 <> plural bndrs <+> ptext SLIT("of")
2985 <+> pprBinders bndrs <> colon)],
2986 nest 2 (vcat (map ppr_ip ips)),
2987 monomorphism_fix dflags]
2988 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
2990 topIPErrs :: [Inst] -> TcM ()
2992 = groupErrs report tidy_dicts
2994 (tidy_env, tidy_dicts) = tidyInsts dicts
2995 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2996 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2997 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2999 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
3001 addNoInstanceErrs insts
3002 = do { let (tidy_env, tidy_insts) = tidyInsts insts
3003 ; reportNoInstances tidy_env Nothing tidy_insts }
3007 -> Maybe (InstLoc, [Inst]) -- Context
3008 -- Nothing => top level
3009 -- Just (d,g) => d describes the construct
3011 -> [Inst] -- What is wanted (can include implications)
3014 reportNoInstances tidy_env mb_what insts
3015 = groupErrs (report_no_instances tidy_env mb_what) insts
3017 report_no_instances tidy_env mb_what insts
3018 = do { inst_envs <- tcGetInstEnvs
3019 ; let (implics, insts1) = partition isImplicInst insts
3020 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
3021 (eqInsts, insts3) = partition isEqInst insts2
3022 ; traceTc (text "reportNoInstances" <+> vcat
3023 [ppr insts, ppr implics, ppr insts1, ppr insts2])
3024 ; mapM_ complain_implic implics
3025 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
3026 ; groupErrs complain_no_inst insts3
3027 ; mapM_ (addErrTcM . mk_eq_err) eqInsts
3030 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
3032 complain_implic inst -- Recurse!
3033 = reportNoInstances tidy_env
3034 (Just (tci_loc inst, tci_given inst))
3037 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
3038 -- Right msg => overlap message
3039 -- Left inst => no instance
3040 check_overlap inst_envs wanted
3041 | not (isClassDict wanted) = Left wanted
3043 = case lookupInstEnv inst_envs clas tys of
3044 -- The case of exactly one match and no unifiers means a
3045 -- successful lookup. That can't happen here, because dicts
3046 -- only end up here if they didn't match in Inst.lookupInst
3048 ([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
3050 ([], _) -> Left wanted -- No match
3051 res -> Right (mk_overlap_msg wanted res)
3053 (clas,tys) = getDictClassTys wanted
3055 mk_overlap_msg dict (matches, unifiers)
3056 = ASSERT( not (null matches) )
3057 vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
3058 <+> pprPred (dictPred dict))),
3059 sep [ptext SLIT("Matching instances") <> colon,
3060 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
3061 if not (isSingleton matches)
3062 then -- Two or more matches
3064 else -- One match, plus some unifiers
3065 ASSERT( not (null unifiers) )
3066 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
3067 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
3068 ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
3069 ptext SLIT("when compiling the other instance declarations")])]
3071 ispecs = [ispec | (ispec, _) <- matches]
3073 mk_eq_err :: Inst -> (TidyEnv, SDoc)
3074 mk_eq_err inst = misMatchMsg tidy_env (eqInstTys inst)
3076 mk_no_inst_err insts
3077 | null insts = empty
3079 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
3080 not (isEmptyVarSet (tyVarsOfInsts insts))
3081 = vcat [ addInstLoc insts $
3082 sep [ ptext SLIT("Could not deduce") <+> pprDictsTheta insts
3083 , nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta givens]
3084 , show_fixes (fix1 loc : fixes2) ]
3086 | otherwise -- Top level
3087 = vcat [ addInstLoc insts $
3088 ptext SLIT("No instance") <> plural insts
3089 <+> ptext SLIT("for") <+> pprDictsTheta insts
3090 , show_fixes fixes2 ]
3093 fix1 loc = sep [ ptext SLIT("add") <+> pprDictsTheta insts
3094 <+> ptext SLIT("to the context of"),
3095 nest 2 (ppr (instLocOrigin loc)) ]
3096 -- I'm not sure it helps to add the location
3097 -- nest 2 (ptext SLIT("at") <+> ppr (instLocSpan loc)) ]
3099 fixes2 | null instance_dicts = []
3100 | otherwise = [sep [ptext SLIT("add an instance declaration for"),
3101 pprDictsTheta instance_dicts]]
3102 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
3103 -- Insts for which it is worth suggesting an adding an instance declaration
3104 -- Exclude implicit parameters, and tyvar dicts
3106 show_fixes :: [SDoc] -> SDoc
3107 show_fixes [] = empty
3108 show_fixes (f:fs) = sep [ptext SLIT("Possible fix:"),
3109 nest 2 (vcat (f : map (ptext SLIT("or") <+>) fs))]
3111 addTopAmbigErrs dicts
3112 -- Divide into groups that share a common set of ambiguous tyvars
3113 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
3114 -- See Note [Avoiding spurious errors]
3115 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
3117 (tidy_env, tidy_dicts) = tidyInsts dicts
3119 tvs_of :: Inst -> [TcTyVar]
3120 tvs_of d = varSetElems (tyVarsOfInst d)
3121 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
3123 report :: [(Inst,[TcTyVar])] -> TcM ()
3124 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
3125 = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) ->
3126 setSrcSpan (instSpan inst) $
3127 -- the location of the first one will do for the err message
3128 addErrTcM (tidy_env, msg $$ mono_msg)
3130 dicts = map fst pairs
3131 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
3132 pprQuotedList tvs <+> in_msg,
3133 nest 2 (pprDictsInFull dicts)]
3134 in_msg = text "in the constraint" <> plural dicts <> colon
3135 report [] = panic "addTopAmbigErrs"
3138 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
3139 -- There's an error with these Insts; if they have free type variables
3140 -- it's probably caused by the monomorphism restriction.
3141 -- Try to identify the offending variable
3142 -- ASSUMPTION: the Insts are fully zonked
3143 mkMonomorphismMsg tidy_env inst_tvs
3144 = do { dflags <- getDOpts
3145 ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
3146 ; return (tidy_env, mk_msg dflags docs) }
3148 mk_msg _ _ | any isRuntimeUnk inst_tvs
3149 = vcat [ptext SLIT("Cannot resolve unknown runtime types:") <+>
3150 (pprWithCommas ppr inst_tvs),
3151 ptext SLIT("Use :print or :force to determine these types")]
3152 mk_msg _ [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
3153 -- This happens in things like
3154 -- f x = show (read "foo")
3155 -- where monomorphism doesn't play any role
3157 = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
3159 monomorphism_fix dflags]
3161 monomorphism_fix :: DynFlags -> SDoc
3162 monomorphism_fix dflags
3163 = ptext SLIT("Probable fix:") <+> vcat
3164 [ptext SLIT("give these definition(s) an explicit type signature"),
3165 if dopt Opt_MonomorphismRestriction dflags
3166 then ptext SLIT("or use -fno-monomorphism-restriction")
3167 else empty] -- Only suggest adding "-fno-monomorphism-restriction"
3168 -- if it is not already set!
3170 warnDefault ups default_ty
3171 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
3172 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
3174 dicts = [d | (d,_,_) <- ups]
3177 (_, tidy_dicts) = tidyInsts dicts
3178 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
3179 quotes (ppr default_ty),
3180 pprDictsInFull tidy_dicts]
3182 reduceDepthErr n stack
3183 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
3184 ptext SLIT("Use -fcontext-stack=N to increase stack size to N"),
3185 nest 4 (pprStack stack)]
3187 pprStack stack = vcat (map pprInstInFull stack)