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 ; -- pprTrace "Make implic inst" (ppr (implic_inst,irreds,dict_irreds,tup_ty)) $
1026 return ([implic_inst], unitBag (L span bind)) }
1028 -----------------------------------------------------------
1029 tryHardCheckLoop :: SDoc
1031 -> TcM ([Inst], TcDictBinds)
1033 tryHardCheckLoop doc wanteds
1034 = do { (irreds,binds,_) <- checkLoop (mkRedEnv doc try_me []) wanteds
1035 ; return (irreds,binds)
1038 try_me inst = ReduceMe AddSCs
1039 -- Here's the try-hard bit
1041 -----------------------------------------------------------
1042 gentleCheckLoop :: InstLoc
1045 -> TcM ([Inst], TcDictBinds)
1047 gentleCheckLoop inst_loc givens wanteds
1048 = do { (irreds,binds,_) <- checkLoop env wanteds
1049 ; return (irreds,binds)
1052 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
1054 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1056 -- When checking against a given signature
1057 -- we MUST be very gentle: Note [Check gently]
1059 gentleInferLoop :: SDoc -> [Inst]
1060 -> TcM ([Inst], TcDictBinds)
1061 gentleInferLoop doc wanteds
1062 = do { (irreds, binds, _) <- checkLoop env wanteds
1063 ; return (irreds, binds) }
1065 env = mkRedEnv doc try_me []
1066 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1071 ~~~~~~~~~~~~~~~~~~~~
1072 We have to very careful about not simplifying too vigorously
1077 f :: Show b => T b -> b
1078 f (MkT x) = show [x]
1080 Inside the pattern match, which binds (a:*, x:a), we know that
1082 Hence we have a dictionary for Show [a] available; and indeed we
1083 need it. We are going to build an implication contraint
1084 forall a. (b~[a]) => Show [a]
1085 Later, we will solve this constraint using the knowledge (Show b)
1087 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
1088 thing becomes insoluble. So we simplify gently (get rid of literals
1089 and methods only, plus common up equal things), deferring the real
1090 work until top level, when we solve the implication constraint
1091 with tryHardCheckLooop.
1095 -----------------------------------------------------------
1098 -> TcM ([Inst], TcDictBinds,
1099 [Inst]) -- needed givens
1100 -- Precondition: givens are completely rigid
1101 -- Postcondition: returned Insts are zonked
1103 checkLoop env wanteds
1105 where go env wanteds needed_givens
1106 = do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
1107 ; env' <- zonkRedEnv env
1108 ; wanteds' <- zonkInsts wanteds
1110 ; (improved, binds, irreds, more_needed_givens) <- reduceContext env' wanteds'
1112 ; let all_needed_givens = needed_givens ++ more_needed_givens
1114 ; if not improved then
1115 return (irreds, binds, all_needed_givens)
1118 -- If improvement did some unification, we go round again.
1119 -- We start again with irreds, not wanteds
1120 -- Using an instance decl might have introduced a fresh type variable
1121 -- which might have been unified, so we'd get an infinite loop
1122 -- if we started again with wanteds! See Note [LOOP]
1123 { (irreds1, binds1, all_needed_givens1) <- go env' irreds all_needed_givens
1124 ; return (irreds1, binds `unionBags` binds1, all_needed_givens1) } }
1127 Note [Zonking RedEnv]
1128 ~~~~~~~~~~~~~~~~~~~~~
1129 It might appear as if the givens in RedEnv are always rigid, but that is not
1130 necessarily the case for programs involving higher-rank types that have class
1131 contexts constraining the higher-rank variables. An example from tc237 in the
1134 class Modular s a | s -> a
1136 wim :: forall a w. Integral a
1137 => a -> (forall s. Modular s a => M s w) -> w
1138 wim i k = error "urk"
1140 test5 :: (Modular s a, Integral a) => M s a
1143 test4 = wim 4 test4'
1145 Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
1146 quantified further outside. When type checking test4, we have to check
1147 whether the signature of test5 is an instance of
1149 (forall s. Modular s a => M s w)
1151 Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
1154 Given the FD of Modular in this example, class improvement will instantiate
1155 t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
1156 Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
1157 the givens, we will get into a loop as improveOne uses the unification engine
1158 TcGadt.tcUnifyTys, which doesn't know about mutable type variables.
1163 class If b t e r | b t e -> r
1166 class Lte a b c | a b -> c where lte :: a -> b -> c
1168 instance (Lte a b l,If l b a c) => Max a b c
1170 Wanted: Max Z (S x) y
1172 Then we'll reduce using the Max instance to:
1173 (Lte Z (S x) l, If l (S x) Z y)
1174 and improve by binding l->T, after which we can do some reduction
1175 on both the Lte and If constraints. What we *can't* do is start again
1176 with (Max Z (S x) y)!
1180 %************************************************************************
1182 tcSimplifySuperClasses
1184 %************************************************************************
1186 Note [SUPERCLASS-LOOP 1]
1187 ~~~~~~~~~~~~~~~~~~~~~~~~
1188 We have to be very, very careful when generating superclasses, lest we
1189 accidentally build a loop. Here's an example:
1193 class S a => C a where { opc :: a -> a }
1194 class S b => D b where { opd :: b -> b }
1196 instance C Int where
1199 instance D Int where
1202 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1203 Simplifying, we may well get:
1204 $dfCInt = :C ds1 (opd dd)
1207 Notice that we spot that we can extract ds1 from dd.
1209 Alas! Alack! We can do the same for (instance D Int):
1211 $dfDInt = :D ds2 (opc dc)
1215 And now we've defined the superclass in terms of itself.
1217 Solution: never generate a superclass selectors at all when
1218 satisfying the superclass context of an instance declaration.
1220 Two more nasty cases are in
1225 tcSimplifySuperClasses
1230 tcSimplifySuperClasses loc givens sc_wanteds
1231 = do { traceTc (text "tcSimplifySuperClasses")
1232 ; (irreds,binds1,_) <- checkLoop env sc_wanteds
1233 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1234 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1237 env = mkRedEnv (pprInstLoc loc) try_me givens
1238 try_me inst = ReduceMe NoSCs
1239 -- Like tryHardCheckLoop, but with NoSCs
1243 %************************************************************************
1245 \subsection{tcSimplifyRestricted}
1247 %************************************************************************
1249 tcSimplifyRestricted infers which type variables to quantify for a
1250 group of restricted bindings. This isn't trivial.
1253 We want to quantify over a to get id :: forall a. a->a
1256 We do not want to quantify over a, because there's an Eq a
1257 constraint, so we get eq :: a->a->Bool (notice no forall)
1260 RHS has type 'tau', whose free tyvars are tau_tvs
1261 RHS has constraints 'wanteds'
1264 Quantify over (tau_tvs \ ftvs(wanteds))
1265 This is bad. The constraints may contain (Monad (ST s))
1266 where we have instance Monad (ST s) where...
1267 so there's no need to be monomorphic in s!
1269 Also the constraint might be a method constraint,
1270 whose type mentions a perfectly innocent tyvar:
1271 op :: Num a => a -> b -> a
1272 Here, b is unconstrained. A good example would be
1274 We want to infer the polymorphic type
1275 foo :: forall b. b -> b
1278 Plan B (cunning, used for a long time up to and including GHC 6.2)
1279 Step 1: Simplify the constraints as much as possible (to deal
1280 with Plan A's problem). Then set
1281 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1283 Step 2: Now simplify again, treating the constraint as 'free' if
1284 it does not mention qtvs, and trying to reduce it otherwise.
1285 The reasons for this is to maximise sharing.
1287 This fails for a very subtle reason. Suppose that in the Step 2
1288 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1289 In the Step 1 this constraint might have been simplified, perhaps to
1290 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1291 This won't happen in Step 2... but that in turn might prevent some other
1292 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1293 and that in turn breaks the invariant that no constraints are quantified over.
1295 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1300 Step 1: Simplify the constraints as much as possible (to deal
1301 with Plan A's problem). Then set
1302 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1303 Return the bindings from Step 1.
1306 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1309 instance (HasBinary ty IO) => HasCodedValue ty
1311 foo :: HasCodedValue a => String -> IO a
1313 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1314 doDecodeIO codedValue view
1315 = let { act = foo "foo" } in act
1317 You might think this should work becuase the call to foo gives rise to a constraint
1318 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1319 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1320 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1322 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1326 Plan D (a variant of plan B)
1327 Step 1: Simplify the constraints as much as possible (to deal
1328 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1329 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1331 Step 2: Now simplify again, treating the constraint as 'free' if
1332 it does not mention qtvs, and trying to reduce it otherwise.
1334 The point here is that it's generally OK to have too few qtvs; that is,
1335 to make the thing more monomorphic than it could be. We don't want to
1336 do that in the common cases, but in wierd cases it's ok: the programmer
1337 can always add a signature.
1339 Too few qtvs => too many wanteds, which is what happens if you do less
1344 tcSimplifyRestricted -- Used for restricted binding groups
1345 -- i.e. ones subject to the monomorphism restriction
1348 -> [Name] -- Things bound in this group
1349 -> TcTyVarSet -- Free in the type of the RHSs
1350 -> [Inst] -- Free in the RHSs
1351 -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
1352 TcDictBinds) -- Bindings
1353 -- tcSimpifyRestricted returns no constraints to
1354 -- quantify over; by definition there are none.
1355 -- They are all thrown back in the LIE
1357 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1358 -- Zonk everything in sight
1359 = do { traceTc (text "tcSimplifyRestricted")
1360 ; wanteds' <- zonkInsts wanteds
1362 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1363 -- dicts; the idea is to get rid of as many type
1364 -- variables as possible, and we don't want to stop
1365 -- at (say) Monad (ST s), because that reduces
1366 -- immediately, with no constraint on s.
1368 -- BUT do no improvement! See Plan D above
1369 -- HOWEVER, some unification may take place, if we instantiate
1370 -- a method Inst with an equality constraint
1371 ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
1372 ; (_imp, _binds, constrained_dicts, _) <- reduceContext env wanteds'
1374 -- Next, figure out the tyvars we will quantify over
1375 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1376 ; gbl_tvs' <- tcGetGlobalTyVars
1377 ; constrained_dicts' <- zonkInsts constrained_dicts
1379 ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
1380 -- As in tcSimplifyInfer
1382 -- Do not quantify over constrained type variables:
1383 -- this is the monomorphism restriction
1384 constrained_tvs' = tyVarsOfInsts constrained_dicts'
1385 qtvs = qtvs1 `minusVarSet` constrained_tvs'
1386 pp_bndrs = pprWithCommas (quotes . ppr) bndrs
1389 ; warn_mono <- doptM Opt_WarnMonomorphism
1390 ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
1391 (vcat[ ptext SLIT("the Monomorphism Restriction applies to the binding")
1392 <> plural bndrs <+> ptext SLIT("for") <+> pp_bndrs,
1393 ptext SLIT("Consider giving a type signature for") <+> pp_bndrs])
1395 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1396 pprInsts wanteds, pprInsts constrained_dicts',
1398 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1400 -- The first step may have squashed more methods than
1401 -- necessary, so try again, this time more gently, knowing the exact
1402 -- set of type variables to quantify over.
1404 -- We quantify only over constraints that are captured by qtvs;
1405 -- these will just be a subset of non-dicts. This in contrast
1406 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1407 -- all *non-inheritable* constraints too. This implements choice
1408 -- (B) under "implicit parameter and monomorphism" above.
1410 -- Remember that we may need to do *some* simplification, to
1411 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1412 -- just to float all constraints
1414 -- At top level, we *do* squash methods becuase we want to
1415 -- expose implicit parameters to the test that follows
1416 ; let is_nested_group = isNotTopLevel top_lvl
1417 try_me inst | isFreeWrtTyVars qtvs inst,
1418 (is_nested_group || isDict inst) = Stop
1419 | otherwise = ReduceMe AddSCs
1420 env = mkNoImproveRedEnv doc try_me
1421 ; (_imp, binds, irreds, _) <- reduceContext env wanteds'
1423 -- See "Notes on implicit parameters, Question 4: top level"
1424 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1425 if is_nested_group then
1427 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1428 ; addTopIPErrs bndrs bad_ips
1429 ; extendLIEs non_ips }
1431 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1432 ; return (qtvs', binds) }
1436 %************************************************************************
1440 %************************************************************************
1442 On the LHS of transformation rules we only simplify methods and constants,
1443 getting dictionaries. We want to keep all of them unsimplified, to serve
1444 as the available stuff for the RHS of the rule.
1446 Example. Consider the following left-hand side of a rule
1448 f (x == y) (y > z) = ...
1450 If we typecheck this expression we get constraints
1452 d1 :: Ord a, d2 :: Eq a
1454 We do NOT want to "simplify" to the LHS
1456 forall x::a, y::a, z::a, d1::Ord a.
1457 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1461 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1462 f ((==) d2 x y) ((>) d1 y z) = ...
1464 Here is another example:
1466 fromIntegral :: (Integral a, Num b) => a -> b
1467 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1469 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1470 we *dont* want to get
1472 forall dIntegralInt.
1473 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1475 because the scsel will mess up RULE matching. Instead we want
1477 forall dIntegralInt, dNumInt.
1478 fromIntegral Int Int dIntegralInt dNumInt = id Int
1482 g (x == y) (y == z) = ..
1484 where the two dictionaries are *identical*, we do NOT WANT
1486 forall x::a, y::a, z::a, d1::Eq a
1487 f ((==) d1 x y) ((>) d1 y z) = ...
1489 because that will only match if the dict args are (visibly) equal.
1490 Instead we want to quantify over the dictionaries separately.
1492 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1493 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1494 from scratch, rather than further parameterise simpleReduceLoop etc
1497 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1498 tcSimplifyRuleLhs wanteds
1499 = go [] emptyBag wanteds
1502 = return (dicts, binds)
1503 go dicts binds (w:ws)
1505 = go (w:dicts) binds ws
1507 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1508 -- to fromInteger; this looks fragile to me
1509 ; lookup_result <- lookupSimpleInst w'
1510 ; case lookup_result of
1512 go dicts (addInstToDictBind binds w rhs) (ws' ++ ws)
1513 NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w)
1517 tcSimplifyBracket is used when simplifying the constraints arising from
1518 a Template Haskell bracket [| ... |]. We want to check that there aren't
1519 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1520 Show instance), but we aren't otherwise interested in the results.
1521 Nor do we care about ambiguous dictionaries etc. We will type check
1522 this bracket again at its usage site.
1525 tcSimplifyBracket :: [Inst] -> TcM ()
1526 tcSimplifyBracket wanteds
1527 = do { tryHardCheckLoop doc wanteds
1530 doc = text "tcSimplifyBracket"
1534 %************************************************************************
1536 \subsection{Filtering at a dynamic binding}
1538 %************************************************************************
1543 we must discharge all the ?x constraints from B. We also do an improvement
1544 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1546 Actually, the constraints from B might improve the types in ?x. For example
1548 f :: (?x::Int) => Char -> Char
1551 then the constraint (?x::Int) arising from the call to f will
1552 force the binding for ?x to be of type Int.
1555 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1558 -- We need a loop so that we do improvement, and then
1559 -- (next time round) generate a binding to connect the two
1561 -- Here the two ?x's have different types, and improvement
1562 -- makes them the same.
1564 tcSimplifyIPs given_ips wanteds
1565 = do { wanteds' <- zonkInsts wanteds
1566 ; given_ips' <- zonkInsts given_ips
1567 -- Unusually for checking, we *must* zonk the given_ips
1569 ; let env = mkRedEnv doc try_me given_ips'
1570 ; (improved, binds, irreds, _) <- reduceContext env wanteds'
1572 ; if not improved then
1573 ASSERT( all is_free irreds )
1574 do { extendLIEs irreds
1577 tcSimplifyIPs given_ips wanteds }
1579 doc = text "tcSimplifyIPs" <+> ppr given_ips
1580 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1581 is_free inst = isFreeWrtIPs ip_set inst
1583 -- Simplify any methods that mention the implicit parameter
1584 try_me inst | is_free inst = Stop
1585 | otherwise = ReduceMe NoSCs
1589 %************************************************************************
1591 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1593 %************************************************************************
1595 When doing a binding group, we may have @Insts@ of local functions.
1596 For example, we might have...
1598 let f x = x + 1 -- orig local function (overloaded)
1599 f.1 = f Int -- two instances of f
1604 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1605 where @f@ is in scope; those @Insts@ must certainly not be passed
1606 upwards towards the top-level. If the @Insts@ were binding-ified up
1607 there, they would have unresolvable references to @f@.
1609 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1610 For each method @Inst@ in the @init_lie@ that mentions one of the
1611 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1612 @LIE@), as well as the @HsBinds@ generated.
1615 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1616 -- Simlifies only MethodInsts, and generate only bindings of form
1618 -- We're careful not to even generate bindings of the form
1620 -- You'd think that'd be fine, but it interacts with what is
1621 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1623 bindInstsOfLocalFuns wanteds local_ids
1624 | null overloaded_ids
1626 = extendLIEs wanteds `thenM_`
1627 returnM emptyLHsBinds
1630 = do { (irreds, binds) <- gentleInferLoop doc for_me
1631 ; extendLIEs not_for_me
1635 doc = text "bindInsts" <+> ppr local_ids
1636 overloaded_ids = filter is_overloaded local_ids
1637 is_overloaded id = isOverloadedTy (idType id)
1638 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1640 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1641 -- so it's worth building a set, so that
1642 -- lookup (in isMethodFor) is faster
1646 %************************************************************************
1648 \subsection{Data types for the reduction mechanism}
1650 %************************************************************************
1652 The main control over context reduction is here
1656 = RedEnv { red_doc :: SDoc -- The context
1657 , red_try_me :: Inst -> WhatToDo
1658 , red_improve :: Bool -- True <=> do improvement
1659 , red_givens :: [Inst] -- All guaranteed rigid
1661 -- but see Note [Rigidity]
1662 , red_reft :: Refinement -- The refinement to apply to the 'givens'
1663 -- You should think of it as 'given equalities'
1664 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1665 -- See Note [RedStack]
1669 -- The red_givens are rigid so far as cmpInst is concerned.
1670 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1671 -- let ?x = e in ...
1672 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1673 -- But that doesn't affect the comparison, which is based only on mame.
1676 -- The red_stack pair (n,insts) pair is just used for error reporting.
1677 -- 'n' is always the depth of the stack.
1678 -- The 'insts' is the stack of Insts being reduced: to produce X
1679 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1682 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1683 mkRedEnv doc try_me givens
1684 = RedEnv { red_doc = doc, red_try_me = try_me,
1685 red_givens = givens,
1686 red_reft = emptyRefinement,
1688 red_improve = True }
1690 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1691 -- Do not do improvement; no givens
1692 mkNoImproveRedEnv doc try_me
1693 = RedEnv { red_doc = doc, red_try_me = try_me,
1694 red_givens = [], red_reft = emptyRefinement,
1696 red_improve = True }
1699 = ReduceMe WantSCs -- Try to reduce this
1700 -- If there's no instance, add the inst to the
1701 -- irreductible ones, but don't produce an error
1702 -- message of any kind.
1703 -- It might be quite legitimate such as (Eq a)!
1705 | Stop -- Return as irreducible unless it can
1706 -- be reduced to a constant in one step
1707 -- Do not add superclasses; see
1709 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1710 -- of a predicate when adding it to the avails
1711 -- The reason for this flag is entirely the super-class loop problem
1712 -- Note [SUPER-CLASS LOOP 1]
1714 zonkRedEnv :: RedEnv -> TcM RedEnv
1716 = do { givens' <- mappM zonkInst (red_givens env)
1717 ; return $ env {red_givens = givens'}
1722 %************************************************************************
1724 \subsection[reduce]{@reduce@}
1726 %************************************************************************
1728 Note [Ancestor Equalities]
1729 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1730 During context reduction, we add to the wanted equalities also those
1731 equalities that (transitively) occur in superclass contexts of wanted
1732 class constraints. Consider the following code
1734 class a ~ Int => C a
1737 If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
1738 substituting Int for a. Hence, we ultimately want (C Int), which we
1739 discharge with the explicit instance.
1742 reduceContext :: RedEnv
1744 -> TcM (ImprovementDone,
1745 TcDictBinds, -- Dictionary bindings
1746 [Inst], -- Irreducible
1747 [Inst]) -- Needed givens
1749 reduceContext env wanteds
1750 = do { traceTc (text "reduceContext" <+> (vcat [
1751 text "----------------------",
1753 text "given" <+> ppr (red_givens env),
1754 text "wanted" <+> ppr wanteds,
1755 text "----------------------"
1759 ; let givens = red_givens env
1760 (given_eqs0, given_dicts0) = partition isEqInst givens
1761 (wanted_eqs0, wanted_dicts0) = partition isEqInst wanteds
1763 -- We want to add as wanted equalities those that (transitively)
1764 -- occur in superclass contexts of wanted class constraints.
1765 -- See Note [Ancestor Equalities]
1766 ; ancestor_eqs <- ancestorEqualities wanted_dicts0
1767 ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
1768 ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
1770 -- 1. Normalise the *given* *equality* constraints
1771 ; (given_eqs, eliminate_skolems) <- normaliseGivenEqs given_eqs0
1773 -- 2. Normalise the *given* *dictionary* constraints
1774 -- wrt. the toplevel and given equations
1775 ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
1778 -- 5. Build the Avail mapping from "given_dicts"
1779 -- Add dicts refined by the current type refinement
1780 ; (init_state, extra_givens) <- getLIE $ do
1781 { init_state <- foldlM addGiven emptyAvails given_dicts
1782 ; let reft = red_reft env
1783 ; if isEmptyRefinement reft then return init_state
1784 else foldlM (addRefinedGiven reft)
1785 init_state given_dicts }
1787 -- *** ToDo: what to do with the "extra_givens"? For the
1788 -- moment I'm simply discarding them, which is probably wrong
1790 -- 7. Normalise the *wanted* *dictionary* constraints
1791 -- wrt. the toplevel and given equations
1792 -- NB: normalisation includes zonking as part of what it does
1793 -- so it's important to do it after any unifications
1794 -- that happened as a result of the addGivens
1795 ; (wanted_dicts,normalise_binds1) <- normaliseWantedDicts given_eqs wanted_dicts0
1797 -- 6. Solve the *wanted* *dictionary* constraints
1798 -- This may expose some further equational constraints...
1799 ; (avails, extra_eqs) <- getLIE (reduceList env wanted_dicts init_state)
1800 ; let (binds, irreds1, needed_givens) = extractResults avails wanted_dicts
1801 ; traceTc $ text "reduceContext extractresults" <+> vcat
1802 [ppr avails,ppr wanted_dicts,ppr binds,ppr needed_givens]
1804 -- *** ToDo: what to do with the "extra_eqs"? For the
1805 -- moment I'm simply discarding them, which is probably wrong
1807 -- 3. Solve the *wanted* *equation* constraints
1808 ; eq_irreds0 <- solveWantedEqs given_eqs wanted_eqs
1810 -- 4. Normalise the *wanted* equality constraints with respect to
1812 ; eq_irreds <- normaliseWantedEqs eq_irreds0
1814 -- 8. Substitute the wanted *equations* in the wanted *dictionaries*
1815 ; (irreds,normalise_binds2) <- substEqInDictInsts eq_irreds irreds1
1817 -- 9. eliminate the artificial skolem constants introduced in 1.
1820 -- Figure out whether we should go round again
1821 -- My current plan is to see if any of the mutable tyvars in
1822 -- givens or irreds has been filled in by improvement.
1823 -- If so, there is merit in going around again, because
1824 -- we may make further progress
1826 -- ToDo: is it only mutable stuff? We may have exposed new
1827 -- equality constraints and should probably go round again
1828 -- then as well. But currently we are dropping them on the
1831 ; let all_irreds = irreds ++ eq_irreds
1832 ; improved <- anyM isFilledMetaTyVar $ varSetElems $
1833 tyVarsOfInsts (givens ++ all_irreds)
1835 -- The old plan (fragile)
1836 -- improveed = availsImproved avails
1837 -- || (not $ isEmptyBag normalise_binds1)
1838 -- || (not $ isEmptyBag normalise_binds2)
1839 -- || (any isEqInst irreds)
1841 ; traceTc (text "reduceContext end" <+> (vcat [
1842 text "----------------------",
1844 text "given" <+> ppr givens,
1845 text "given_eqs" <+> ppr given_eqs,
1846 text "wanted" <+> ppr wanteds,
1847 text "wanted_dicts" <+> ppr wanted_dicts,
1849 text "avails" <+> pprAvails avails,
1850 text "improved =" <+> ppr improved,
1851 text "irreds = " <+> ppr irreds,
1852 text "binds = " <+> ppr binds,
1853 text "needed givens = " <+> ppr needed_givens,
1854 text "----------------------"
1858 given_binds `unionBags` normalise_binds1
1859 `unionBags` normalise_binds2
1865 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1866 tcImproveOne avails inst
1867 | not (isDict inst) = return False
1869 = do { inst_envs <- tcGetInstEnvs
1870 ; let eqns = improveOne (classInstances inst_envs)
1871 (dictPred inst, pprInstArising inst)
1872 [ (dictPred p, pprInstArising p)
1873 | p <- availsInsts avails, isDict p ]
1874 -- Avails has all the superclasses etc (good)
1875 -- It also has all the intermediates of the deduction (good)
1876 -- It does not have duplicates (good)
1877 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1878 -- so that improve will see them separate
1879 ; traceTc (text "improveOne" <+> ppr inst)
1882 unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))]
1883 -> TcM ImprovementDone
1884 unifyEqns [] = return False
1886 = do { traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns))
1890 unify ((qtvs, pairs), what1, what2)
1891 = addErrCtxtM (mkEqnMsg what1 what2) $
1892 tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1893 mapM_ (unif_pr tenv) pairs
1894 unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1896 pprEquationDoc (eqn, (p1,w1), (p2,w2)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
1898 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
1899 = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
1900 ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
1901 ; let msg = vcat [ptext SLIT("When using functional dependencies to combine"),
1902 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
1903 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
1904 ; return (tidy_env, msg) }
1907 The main context-reduction function is @reduce@. Here's its game plan.
1910 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
1911 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
1912 = do { traceTc (text "reduceList " <+> (ppr wanteds $$ ppr state))
1916 dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n)
1917 2 (ifPprDebug (nest 2 (pprStack stk))))
1920 ; if n >= ctxtStkDepth dopts then
1921 failWithTc (reduceDepthErr n stk)
1925 go [] state = return state
1926 go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
1929 -- Base case: we're done!
1930 reduce env wanted avails
1931 -- It's the same as an existing inst, or a superclass thereof
1932 | Just avail <- findAvail avails wanted
1933 = do { traceTc (text "reduce: found " <+> ppr wanted)
1938 = do { traceTc (text "reduce" <+> ppr wanted $$ ppr avails)
1939 ; case red_try_me env wanted of {
1940 Stop -> try_simple (addIrred NoSCs);
1941 -- See Note [No superclasses for Stop]
1943 ReduceMe want_scs -> do -- It should be reduced
1944 { (avails, lookup_result) <- reduceInst env avails wanted
1945 ; case lookup_result of
1946 NoInstance -> addIrred want_scs avails wanted
1947 -- Add it and its superclasses
1949 GenInst [] rhs -> addWanted want_scs avails wanted rhs []
1951 GenInst wanteds' rhs
1952 -> do { avails1 <- addIrred NoSCs avails wanted
1953 ; avails2 <- reduceList env wanteds' avails1
1954 ; addWanted want_scs avails2 wanted rhs wanteds' } }
1955 -- Temporarily do addIrred *before* the reduceList,
1956 -- which has the effect of adding the thing we are trying
1957 -- to prove to the database before trying to prove the things it
1958 -- needs. See note [RECURSIVE DICTIONARIES]
1959 -- NB: we must not do an addWanted before, because that adds the
1960 -- superclasses too, and that can lead to a spurious loop; see
1961 -- the examples in [SUPERCLASS-LOOP]
1962 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1965 -- First, see if the inst can be reduced to a constant in one step
1966 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
1967 -- Don't bother for implication constraints, which take real work
1968 try_simple do_this_otherwise
1969 = do { res <- lookupSimpleInst wanted
1971 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
1972 other -> do_this_otherwise avails wanted }
1976 Note [SUPERCLASS-LOOP 2]
1977 ~~~~~~~~~~~~~~~~~~~~~~~~
1978 But the above isn't enough. Suppose we are *given* d1:Ord a,
1979 and want to deduce (d2:C [a]) where
1981 class Ord a => C a where
1982 instance Ord [a] => C [a] where ...
1984 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
1985 superclasses of C [a] to avails. But we must not overwrite the binding
1986 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
1989 Here's another variant, immortalised in tcrun020
1990 class Monad m => C1 m
1991 class C1 m => C2 m x
1992 instance C2 Maybe Bool
1993 For the instance decl we need to build (C1 Maybe), and it's no good if
1994 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1995 before we search for C1 Maybe.
1997 Here's another example
1998 class Eq b => Foo a b
1999 instance Eq a => Foo [a] a
2003 we'll first deduce that it holds (via the instance decl). We must not
2004 then overwrite the Eq t constraint with a superclass selection!
2006 At first I had a gross hack, whereby I simply did not add superclass constraints
2007 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
2008 becuase it lost legitimate superclass sharing, and it still didn't do the job:
2009 I found a very obscure program (now tcrun021) in which improvement meant the
2010 simplifier got two bites a the cherry... so something seemed to be an Stop
2011 first time, but reducible next time.
2013 Now we implement the Right Solution, which is to check for loops directly
2014 when adding superclasses. It's a bit like the occurs check in unification.
2017 Note [RECURSIVE DICTIONARIES]
2018 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2020 data D r = ZeroD | SuccD (r (D r));
2022 instance (Eq (r (D r))) => Eq (D r) where
2023 ZeroD == ZeroD = True
2024 (SuccD a) == (SuccD b) = a == b
2027 equalDC :: D [] -> D [] -> Bool;
2030 We need to prove (Eq (D [])). Here's how we go:
2034 by instance decl, holds if
2038 by instance decl of Eq, holds if
2040 where d2 = dfEqList d3
2043 But now we can "tie the knot" to give
2049 and it'll even run! The trick is to put the thing we are trying to prove
2050 (in this case Eq (D []) into the database before trying to prove its
2051 contributing clauses.
2054 %************************************************************************
2056 Reducing a single constraint
2058 %************************************************************************
2061 ---------------------------------------------
2062 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
2063 reduceInst env avails (ImplicInst { tci_name = name,
2064 tci_tyvars = tvs, tci_reft = reft, tci_loc = loc,
2065 tci_given = extra_givens, tci_wanted = wanteds })
2066 = reduceImplication env avails name reft tvs extra_givens wanteds loc
2068 reduceInst env avails other_inst
2069 = do { result <- lookupSimpleInst other_inst
2070 ; return (avails, result) }
2073 Note [Equational Constraints in Implication Constraints]
2074 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2076 An implication constraint is of the form
2078 where Given and Wanted may contain both equational and dictionary
2079 constraints. The delay and reduction of these two kinds of constraints
2082 -) In the generated code, wanted Dictionary constraints are wrapped up in an
2083 implication constraint that is created at the code site where the wanted
2084 dictionaries can be reduced via a let-binding. This let-bound implication
2085 constraint is deconstructed at the use-site of the wanted dictionaries.
2087 -) While the reduction of equational constraints is also delayed, the delay
2088 is not manifest in the generated code. The required evidence is generated
2089 in the code directly at the use-site. There is no let-binding and deconstruction
2090 necessary. The main disadvantage is that we cannot exploit sharing as the
2091 same evidence may be generated at multiple use-sites. However, this disadvantage
2092 is limited because it only concerns coercions which are erased.
2094 The different treatment is motivated by the different in representation. Dictionary
2095 constraints require manifest runtime dictionaries, while equations require coercions
2099 ---------------------------------------------
2100 reduceImplication :: RedEnv
2103 -> Refinement -- May refine the givens; often empty
2104 -> [TcTyVar] -- Quantified type variables; all skolems
2105 -> [Inst] -- Extra givens; all rigid
2108 -> TcM (Avails, LookupInstResult)
2111 Suppose we are simplifying the constraint
2112 forall bs. extras => wanted
2113 in the context of an overall simplification problem with givens 'givens',
2114 and refinment 'reft'.
2117 * The refinement is often empty
2119 * The 'extra givens' need not mention any of the quantified type variables
2120 e.g. forall {}. Eq a => Eq [a]
2121 forall {}. C Int => D (Tree Int)
2123 This happens when you have something like
2125 T1 :: Eq a => a -> T a
2128 f x = ...(case x of { T1 v -> v==v })...
2131 -- ToDo: should we instantiate tvs? I think it's not necessary
2133 -- Note on coercion variables:
2135 -- The extra given coercion variables are bound at two different sites:
2136 -- -) in the creation context of the implication constraint
2137 -- the solved equational constraints use these binders
2139 -- -) at the solving site of the implication constraint
2140 -- the solved dictionaries use these binders
2141 -- these binders are generated by reduceImplication
2143 reduceImplication env orig_avails name reft tvs extra_givens wanteds inst_loc
2144 = do { -- Add refined givens, and the extra givens
2146 -- (refined_red_givens,refined_avails)
2147 -- <- if isEmptyRefinement reft then return (red_givens env,orig_avails)
2148 -- else foldlM (addRefinedGiven reft) ([],orig_avails) (red_givens env)
2149 -- Commented out SLPJ Sept 07; see comment with extractLocalResults below
2150 let refined_red_givens = []
2152 -- Solve the sub-problem
2153 ; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
2154 env' = env { red_givens = extra_givens ++ availsInsts orig_avails
2156 , red_doc = sep [ptext SLIT("reduceImplication for") <+> ppr name,
2157 nest 2 (parens $ ptext SLIT("within") <+> red_doc env)]
2158 , red_try_me = try_me }
2160 ; traceTc (text "reduceImplication" <+> vcat
2162 ppr (red_givens env), ppr extra_givens,
2163 ppr reft, ppr wanteds])
2164 ; (irreds,binds,needed_givens0) <- checkLoop env' wanteds
2165 ; let (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
2166 -- SLPJ Sept 07: I think this is bogus; currently
2167 -- there are no Eqinsts in extra_givens
2168 dict_ids = map instToId extra_dict_givens
2170 -- needed_givens0 is the free vars of the bindings
2171 -- Remove the ones we are going to lambda-bind
2172 -- Use the actual dictionary identity *not* equality on Insts
2173 -- (Mind you, it should make no difference here.)
2174 ; let needed_givens = [ng | ng <- needed_givens0
2175 , instToVar ng `notElem` dict_ids]
2177 -- Note [Reducing implication constraints]
2178 -- Tom -- update note, put somewhere!
2180 ; traceTc (text "reduceImplication result" <+> vcat
2181 [ppr irreds, ppr binds, ppr needed_givens])
2183 ; -- extract superclass binds
2184 -- (sc_binds,_) <- extractResults avails []
2185 -- ; traceTc (text "reduceImplication sc_binds" <+> vcat
2186 -- [ppr sc_binds, ppr avails])
2189 -- We always discard the extra avails we've generated;
2190 -- but we remember if we have done any (global) improvement
2191 -- ; let ret_avails = avails
2192 ; let ret_avails = orig_avails
2193 -- ; let ret_avails = updateImprovement orig_avails avails
2195 -- SLPJ Sept 07: what if improvement happened inside the checkLoop?
2196 -- Then we must iterate the outer loop too!
2198 ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
2200 -- Progress is no longer measered by the number of bindings
2201 ; if (isEmptyLHsBinds binds) && (not $ null irreds) then -- No progress
2202 -- If there are any irreds, we back off and return NoInstance
2203 return (ret_avails, NoInstance)
2205 { (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds
2206 -- This binding is useless if the recursive simplification
2207 -- made no progress; but currently we don't try to optimise that
2208 -- case. After all, we only try hard to reduce at top level, or
2209 -- when inferring types.
2211 ; let dict_wanteds = filter (not . isEqInst) wanteds
2212 -- TOMDO: given equational constraints bug!
2213 -- we need a different evidence for given
2214 -- equations depending on whether we solve
2215 -- dictionary constraints or equational constraints
2217 eq_tyvars = varSetElems $ tyVarsOfTypes $ map eqInstType extra_eq_givens
2218 -- SLPJ Sept07: this looks Utterly Wrong to me, but I think
2219 -- that current extra_givens has no EqInsts, so
2220 -- it makes no difference
2221 co = wrap_inline -- Note [Always inline implication constraints]
2223 <.> mkWpTyLams eq_tyvars
2224 <.> mkWpLams dict_ids
2225 <.> WpLet (binds `unionBags` bind)
2226 wrap_inline | null dict_ids = idHsWrapper
2227 | otherwise = WpInline
2228 rhs = mkHsWrap co payload
2229 loc = instLocSpan inst_loc
2230 payload | [dict_wanted] <- dict_wanteds = HsVar (instToId dict_wanted)
2231 | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) dict_wanteds) Boxed
2234 ; traceTc (vcat [text "reduceImplication" <+> ppr name,
2236 text "->" <+> sep [ppr needed_givens, ppr rhs]])
2237 ; return (ret_avails, GenInst (implic_insts ++ needed_givens) (L loc rhs))
2242 Note [Always inline implication constraints]
2243 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2244 Suppose an implication constraint floats out of an INLINE function.
2245 Then although the implication has a single call site, it won't be
2246 inlined. And that is bad because it means that even if there is really
2247 *no* overloading (type signatures specify the exact types) there will
2248 still be dictionary passing in the resulting code. To avert this,
2249 we mark the implication constraints themselves as INLINE, at least when
2250 there is no loss of sharing as a result.
2252 Note [Reducing implication constraints]
2253 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2254 Suppose we are trying to simplify
2256 ic: (forall b. C a b => (W [a] b, D c b)) )
2258 instance (C a b, Ord a) => W [a] b
2259 When solving the implication constraint, we'll start with
2261 in the Avails. Then we add (C a b -> Given) and solve. Extracting
2262 the results gives us a binding for the (W [a] b), with an Irred of
2263 (Ord a, D c b). Now, the (Ord a) comes from "outside" the implication,
2264 but the (D d b) is from "inside". So we want to generate a GenInst
2269 ic' :: forall b. C a b => D c b]
2270 (/\b \(dc:C a b). (df a b dc do, ic' b dc))
2272 The first arg of GenInst gives the free dictionary variables of the
2273 second argument -- the "needed givens". And that list in turn is
2274 vital because it's used to determine what other dicts must be solved.
2275 This very list ends up in the second field of the Rhs, and drives
2278 The need for this field is why we have to return "needed givens"
2279 from extractResults, reduceContext, checkLoop, and so on.
2281 NB: the "needed givens" in a GenInst or Rhs, may contain two dicts
2282 with the same type but different Ids, e.g. [d12 :: Eq a, d81 :: Eq a]
2283 That says we must generate a binding for both d12 and d81.
2285 The "inside" and "outside" distinction is what's going on with 'inner' and
2286 'outer' in reduceImplication
2289 Note [Freeness and implications]
2290 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2291 It's hard to say when an implication constraint can be floated out. Consider
2292 forall {} Eq a => Foo [a]
2293 The (Foo [a]) doesn't mention any of the quantified variables, but it
2294 still might be partially satisfied by the (Eq a).
2296 There is a useful special case when it *is* easy to partition the
2297 constraints, namely when there are no 'givens'. Consider
2298 forall {a}. () => Bar b
2299 There are no 'givens', and so there is no reason to capture (Bar b).
2300 We can let it float out. But if there is even one constraint we
2301 must be much more careful:
2302 forall {a}. C a b => Bar (m b)
2303 because (C a b) might have a superclass (D b), from which we might
2304 deduce (Bar [b]) when m later gets instantiated to []. Ha!
2306 Here is an even more exotic example
2308 Now consider the constraint
2309 forall b. D Int b => C Int
2310 We can satisfy the (C Int) from the superclass of D, so we don't want
2311 to float the (C Int) out, even though it mentions no type variable in
2314 Note [Pruning the givens in an implication constraint]
2315 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2316 Suppose we are about to form the implication constraint
2317 forall tvs. Eq a => Ord b
2318 The (Eq a) cannot contribute to the (Ord b), because it has no access to
2319 the type variable 'b'. So we could filter out the (Eq a) from the givens.
2321 Doing so would be a bit tidier, but all the implication constraints get
2322 simplified away by the optimiser, so it's no great win. So I don't take
2323 advantage of that at the moment.
2325 If you do, BE CAREFUL of wobbly type variables.
2328 %************************************************************************
2330 Avails and AvailHow: the pool of evidence
2332 %************************************************************************
2336 data Avails = Avails !ImprovementDone !AvailEnv
2338 type ImprovementDone = Bool -- True <=> some unification has happened
2339 -- so some Irreds might now be reducible
2340 -- keys that are now
2342 type AvailEnv = FiniteMap Inst AvailHow
2344 = IsIrred -- Used for irreducible dictionaries,
2345 -- which are going to be lambda bound
2347 | Given Inst -- Used for dictionaries for which we have a binding
2348 -- e.g. those "given" in a signature
2350 | Rhs -- Used when there is a RHS
2351 (LHsExpr TcId) -- The RHS
2352 [Inst] -- Insts free in the RHS; we need these too
2354 instance Outputable Avails where
2357 pprAvails (Avails imp avails)
2358 = vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty)
2360 vcat [ sep [ppr inst, nest 2 (equals <+> ppr avail)]
2361 | (inst,avail) <- fmToList avails ]]
2363 instance Outputable AvailHow where
2366 -------------------------
2367 pprAvail :: AvailHow -> SDoc
2368 pprAvail IsIrred = text "Irred"
2369 pprAvail (Given x) = text "Given" <+> ppr x
2370 pprAvail (Rhs rhs bs) = sep [text "Rhs" <+> ppr bs,
2373 -------------------------
2374 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
2375 extendAvailEnv env inst avail = addToFM env inst avail
2377 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
2378 findAvailEnv env wanted = lookupFM env wanted
2379 -- NB 1: the Ord instance of Inst compares by the class/type info
2380 -- *not* by unique. So
2381 -- d1::C Int == d2::C Int
2383 emptyAvails :: Avails
2384 emptyAvails = Avails False emptyFM
2386 findAvail :: Avails -> Inst -> Maybe AvailHow
2387 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
2389 elemAvails :: Inst -> Avails -> Bool
2390 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
2392 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
2394 extendAvails avails@(Avails imp env) inst avail
2395 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
2396 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
2398 availsInsts :: Avails -> [Inst]
2399 availsInsts (Avails _ avails) = keysFM avails
2401 availsImproved (Avails imp _) = imp
2403 updateImprovement :: Avails -> Avails -> Avails
2404 -- (updateImprovement a1 a2) sets a1's improvement flag from a2
2405 updateImprovement (Avails _ avails1) (Avails imp2 _) = Avails imp2 avails1
2408 Extracting the bindings from a bunch of Avails.
2409 The bindings do *not* come back sorted in dependency order.
2410 We assume that they'll be wrapped in a big Rec, so that the
2411 dependency analyser can sort them out later
2414 type DoneEnv = FiniteMap Inst [Id]
2415 -- Tracks which things we have evidence for
2417 extractResults :: Avails
2419 -> (TcDictBinds, -- Bindings
2420 [Inst], -- Irreducible ones
2421 [Inst]) -- Needed givens, i.e. ones used in the bindings
2422 -- Postcondition: needed-givens = free vars( binds ) \ irreds
2423 -- needed-gives is subset of Givens in incoming Avails
2424 -- Note [Reducing implication constraints]
2426 extractResults (Avails _ avails) wanteds
2427 = go emptyBag [] [] emptyFM wanteds
2429 go :: TcDictBinds -- Bindings for dicts
2431 -> [Inst] -- Needed givens
2432 -> DoneEnv -- Has an entry for each inst in the above three sets
2434 -> (TcDictBinds, [Inst], [Inst])
2435 go binds irreds givens done []
2436 = (binds, irreds, givens)
2438 go binds irreds givens done (w:ws)
2439 | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w
2440 = if w_id `elem` done_ids then
2441 go binds irreds givens done ws
2443 go (add_bind (nlHsVar done_id)) irreds givens
2444 (addToFM done w (done_id : w_id : rest_done_ids)) ws
2446 | otherwise -- Not yet done
2447 = case findAvailEnv avails w of
2448 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
2449 go binds irreds givens done ws
2451 Just IsIrred -> go binds (w:irreds) givens done' ws
2453 Just (Rhs rhs ws') -> go (add_bind rhs) irreds givens done' (ws' ++ ws)
2455 Just (Given g) -> go binds' irreds (g:givens) (addToFM done w [g_id]) ws
2458 binds' | w_id == g_id = binds
2459 | otherwise = add_bind (nlHsVar g_id)
2462 done' = addToFM done w [w_id]
2463 add_bind rhs = addInstToDictBind binds w rhs
2467 Note [No superclasses for Stop]
2468 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2469 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2470 add it to avails, so that any other equal Insts will be commoned up
2471 right here. However, we do *not* add superclasses. If we have
2474 but a is not bound here, then we *don't* want to derive dn from df
2475 here lest we lose sharing.
2478 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2479 addWanted want_scs avails wanted rhs_expr wanteds
2480 = addAvailAndSCs want_scs avails wanted avail
2482 avail = Rhs rhs_expr wanteds
2484 addGiven :: Avails -> Inst -> TcM Avails
2485 addGiven avails given = addAvailAndSCs AddSCs avails given (Given given)
2486 -- Always add superclasses for 'givens'
2488 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2489 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2490 -- so the assert isn't true
2492 addRefinedGiven :: Refinement -> Avails -> Inst -> TcM Avails
2493 addRefinedGiven reft avails given
2494 | isDict given -- We sometimes have 'given' methods, but they
2495 -- are always optional, so we can drop them
2496 , let pred = dictPred given
2497 , isRefineablePred pred -- See Note [ImplicInst rigidity]
2498 , Just (co, pred) <- refinePred reft pred
2499 = do { new_given <- newDictBndr (instLoc given) pred
2500 ; let rhs = L (instSpan given) $
2501 HsWrap (WpCo co) (HsVar (instToId given))
2502 ; addAvailAndSCs AddSCs avails new_given (Rhs rhs [given]) }
2503 -- ToDo: the superclasses of the original given all exist in Avails
2504 -- so we could really just cast them, but it's more awkward to do,
2505 -- and hopefully the optimiser will spot the duplicated work
2510 Note [ImplicInst rigidity]
2511 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2513 C :: forall ab. (Eq a, Ord b) => b -> T a
2515 ...(case x of C v -> <body>)...
2517 From the case (where x::T ty) we'll get an implication constraint
2518 forall b. (Eq ty, Ord b) => <body-constraints>
2519 Now suppose <body-constraints> itself has an implication constraint
2521 forall c. <reft> => <payload>
2522 Then, we can certainly apply the refinement <reft> to the Ord b, becuase it is
2523 existential, but we probably should not apply it to the (Eq ty) because it may
2524 be wobbly. Hence the isRigidInst
2526 @Insts@ are ordered by their class/type info, rather than by their
2527 unique. This allows the context-reduction mechanism to use standard finite
2528 maps to do their stuff. It's horrible that this code is here, rather
2529 than with the Avails handling stuff in TcSimplify
2532 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2533 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2534 addAvailAndSCs want_scs avails irred IsIrred
2536 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2537 addAvailAndSCs want_scs avails inst avail
2538 | not (isClassDict inst) = extendAvails avails inst avail
2539 | NoSCs <- want_scs = extendAvails avails inst avail
2540 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2541 ; avails' <- extendAvails avails inst avail
2542 ; addSCs is_loop avails' inst }
2544 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2545 -- Note: this compares by *type*, not by Unique
2546 deps = findAllDeps (unitVarSet (instToVar inst)) avail
2547 dep_tys = map idType (varSetElems deps)
2549 findAllDeps :: IdSet -> AvailHow -> IdSet
2550 -- Find all the Insts that this one depends on
2551 -- See Note [SUPERCLASS-LOOP 2]
2552 -- Watch out, though. Since the avails may contain loops
2553 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2554 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2555 findAllDeps so_far other = so_far
2557 find_all :: IdSet -> Inst -> IdSet
2559 | isEqInst kid = so_far
2560 | kid_id `elemVarSet` so_far = so_far
2561 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2562 | otherwise = so_far'
2564 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2565 kid_id = instToId kid
2567 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2568 -- Add all the superclasses of the Inst to Avails
2569 -- The first param says "don't do this because the original thing
2570 -- depends on this one, so you'd build a loop"
2571 -- Invariant: the Inst is already in Avails.
2573 addSCs is_loop avails dict
2574 = ASSERT( isDict dict )
2575 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2576 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2578 (clas, tys) = getDictClassTys dict
2579 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2580 sc_theta' = filter (not . isEqPred) $
2581 substTheta (zipTopTvSubst tyvars tys) sc_theta
2583 add_sc avails (sc_dict, sc_sel)
2584 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2585 | is_given sc_dict = return avails
2586 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2587 ; addSCs is_loop avails' sc_dict }
2589 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2590 co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
2592 is_given :: Inst -> Bool
2593 is_given sc_dict = case findAvail avails sc_dict of
2594 Just (Given _) -> True -- Given is cheaper than superclass selection
2597 -- From the a set of insts obtain all equalities that (transitively) occur in
2598 -- superclass contexts of class constraints (aka the ancestor equalities).
2600 ancestorEqualities :: [Inst] -> TcM [Inst]
2602 = mapM mkWantedEqInst -- turn only equality predicates..
2603 . filter isEqPred -- ..into wanted equality insts
2605 . addAEsToBag emptyBag -- collect the superclass constraints..
2606 . map dictPred -- ..of all predicates in a bag
2607 . filter isClassDict
2609 addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
2610 addAEsToBag bag [] = bag
2611 addAEsToBag bag (pred:preds)
2612 | pred `elemBag` bag = addAEsToBag bag preds
2613 | isEqPred pred = addAEsToBag bagWithPred preds
2614 | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
2615 | otherwise = addAEsToBag bag preds
2617 bagWithPred = bag `snocBag` pred
2618 predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
2620 (tyvars, sc_theta, _, _) = classBigSig clas
2621 (clas, tys) = getClassPredTys pred
2625 %************************************************************************
2627 \section{tcSimplifyTop: defaulting}
2629 %************************************************************************
2632 @tcSimplifyTop@ is called once per module to simplify all the constant
2633 and ambiguous Insts.
2635 We need to be careful of one case. Suppose we have
2637 instance Num a => Num (Foo a b) where ...
2639 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2640 to (Num x), and default x to Int. But what about y??
2642 It's OK: the final zonking stage should zap y to (), which is fine.
2646 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2647 tcSimplifyTop wanteds
2648 = tc_simplify_top doc False wanteds
2650 doc = text "tcSimplifyTop"
2652 tcSimplifyInteractive wanteds
2653 = tc_simplify_top doc True wanteds
2655 doc = text "tcSimplifyInteractive"
2657 -- The TcLclEnv should be valid here, solely to improve
2658 -- error message generation for the monomorphism restriction
2659 tc_simplify_top doc interactive wanteds
2660 = do { dflags <- getDOpts
2661 ; wanteds <- zonkInsts wanteds
2662 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2664 ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
2665 ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
2666 -- ; (irreds1, binds1) <- gentleInferLoop doc1 wanteds
2667 ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
2668 ; (irreds2, binds2) <- approximateImplications doc2 (\d -> True) irreds1
2669 ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
2671 -- Use the defaulting rules to do extra unification
2672 -- NB: irreds2 are already zonked
2673 ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
2675 -- Deal with implicit parameters
2676 ; let (bad_ips, non_ips) = partition isIPDict irreds3
2677 (ambigs, others) = partition isTyVarDict non_ips
2679 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2681 ; addNoInstanceErrs others
2682 ; addTopAmbigErrs ambigs
2684 ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
2686 doc1 = doc <+> ptext SLIT("(first round)")
2687 doc2 = doc <+> ptext SLIT("(approximate)")
2688 doc3 = doc <+> ptext SLIT("(disambiguate)")
2691 If a dictionary constrains a type variable which is
2692 * not mentioned in the environment
2693 * and not mentioned in the type of the expression
2694 then it is ambiguous. No further information will arise to instantiate
2695 the type variable; nor will it be generalised and turned into an extra
2696 parameter to a function.
2698 It is an error for this to occur, except that Haskell provided for
2699 certain rules to be applied in the special case of numeric types.
2701 * at least one of its classes is a numeric class, and
2702 * all of its classes are numeric or standard
2703 then the type variable can be defaulted to the first type in the
2704 default-type list which is an instance of all the offending classes.
2706 So here is the function which does the work. It takes the ambiguous
2707 dictionaries and either resolves them (producing bindings) or
2708 complains. It works by splitting the dictionary list by type
2709 variable, and using @disambigOne@ to do the real business.
2711 @disambigOne@ assumes that its arguments dictionaries constrain all
2712 the same type variable.
2714 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2715 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2716 the most common use of defaulting is code like:
2718 _ccall_ foo `seqPrimIO` bar
2720 Since we're not using the result of @foo@, the result if (presumably)
2724 disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
2725 -- Just does unification to fix the default types
2726 -- The Insts are assumed to be pre-zonked
2727 disambiguate doc interactive dflags insts
2729 = return (insts, emptyBag)
2731 | null defaultable_groups
2732 = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
2733 ; return (insts, emptyBag) }
2736 = do { -- Figure out what default types to use
2737 default_tys <- getDefaultTys extended_defaulting ovl_strings
2739 ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2740 ; mapM_ (disambigGroup default_tys) defaultable_groups
2742 -- disambigGroup does unification, hence try again
2743 ; tryHardCheckLoop doc insts }
2746 extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
2747 ovl_strings = dopt Opt_OverloadedStrings dflags
2749 unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
2750 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2751 (unaries, bad_tvs_s) = partitionWith find_unary insts
2752 bad_tvs = unionVarSets bad_tvs_s
2754 -- Finds unary type-class constraints
2755 find_unary d@(Dict {tci_pred = ClassP cls [ty]})
2756 | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
2757 find_unary inst = Right (tyVarsOfInst inst)
2759 -- Group by type variable
2760 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2761 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2762 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2764 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2765 defaultable_group ds@((_,_,tv):_)
2766 = isTyConableTyVar tv -- Note [Avoiding spurious errors]
2767 && not (tv `elemVarSet` bad_tvs)
2768 && defaultable_classes [c | (_,c,_) <- ds]
2769 defaultable_group [] = panic "defaultable_group"
2771 defaultable_classes clss
2772 | extended_defaulting = any isInteractiveClass clss
2773 | otherwise = all is_std_class clss && (any is_num_class clss)
2775 -- In interactive mode, or with -fextended-default-rules,
2776 -- we default Show a to Show () to avoid graututious errors on "show []"
2777 isInteractiveClass cls
2778 = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2780 is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2781 -- is_num_class adds IsString to the standard numeric classes,
2782 -- when -foverloaded-strings is enabled
2784 is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2785 -- Similarly is_std_class
2787 -----------------------
2788 disambigGroup :: [Type] -- The default types
2789 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2790 -> TcM () -- Just does unification, to fix the default types
2792 disambigGroup default_tys dicts
2793 = try_default default_tys
2795 (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
2796 classes = [c | (_,c,_) <- dicts]
2798 try_default [] = return ()
2799 try_default (default_ty : default_tys)
2800 = tryTcLIE_ (try_default default_tys) $
2801 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2802 -- This may fail; then the tryTcLIE_ kicks in
2803 -- Failure here is caused by there being no type in the
2804 -- default list which can satisfy all the ambiguous classes.
2805 -- For example, if Real a is reqd, but the only type in the
2806 -- default list is Int.
2808 -- After this we can't fail
2809 ; warnDefault dicts default_ty
2810 ; unifyType default_ty (mkTyVarTy tyvar)
2811 ; return () -- TOMDO: do something with the coercion
2815 -----------------------
2816 getDefaultTys :: Bool -> Bool -> TcM [Type]
2817 getDefaultTys extended_deflts ovl_strings
2818 = do { mb_defaults <- getDeclaredDefaultTys
2819 ; case mb_defaults of {
2820 Just tys -> return tys ; -- User-supplied defaults
2823 -- No use-supplied default
2824 -- Use [Integer, Double], plus modifications
2825 { integer_ty <- tcMetaTy integerTyConName
2826 ; checkWiredInTyCon doubleTyCon
2827 ; string_ty <- tcMetaTy stringTyConName
2828 ; return (opt_deflt extended_deflts unitTy
2829 -- Note [Default unitTy]
2831 [integer_ty,doubleTy]
2833 opt_deflt ovl_strings string_ty) } } }
2835 opt_deflt True ty = [ty]
2836 opt_deflt False ty = []
2839 Note [Default unitTy]
2840 ~~~~~~~~~~~~~~~~~~~~~
2841 In interative mode (or with -fextended-default-rules) we add () as the first type we
2842 try when defaulting. This has very little real impact, except in the following case.
2844 Text.Printf.printf "hello"
2845 This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
2846 want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
2847 default the 'a' to (), rather than to Integer (which is what would otherwise happen;
2848 and then GHCi doesn't attempt to print the (). So in interactive mode, we add
2849 () to the list of defaulting types. See Trac #1200.
2851 Note [Avoiding spurious errors]
2852 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2853 When doing the unification for defaulting, we check for skolem
2854 type variables, and simply don't default them. For example:
2855 f = (*) -- Monomorphic
2856 g :: Num a => a -> a
2858 Here, we get a complaint when checking the type signature for g,
2859 that g isn't polymorphic enough; but then we get another one when
2860 dealing with the (Num a) context arising from f's definition;
2861 we try to unify a with Int (to default it), but find that it's
2862 already been unified with the rigid variable from g's type sig
2865 %************************************************************************
2867 \subsection[simple]{@Simple@ versions}
2869 %************************************************************************
2871 Much simpler versions when there are no bindings to make!
2873 @tcSimplifyThetas@ simplifies class-type constraints formed by
2874 @deriving@ declarations and when specialising instances. We are
2875 only interested in the simplified bunch of class/type constraints.
2877 It simplifies to constraints of the form (C a b c) where
2878 a,b,c are type variables. This is required for the context of
2879 instance declarations.
2882 tcSimplifyDeriv :: InstOrigin
2884 -> ThetaType -- Wanted
2885 -> TcM ThetaType -- Needed
2886 -- Given instance (wanted) => C inst_ty
2887 -- Simplify 'wanted' as much as possible
2889 tcSimplifyDeriv orig tyvars theta
2890 = do { (tvs, _, tenv) <- tcInstTyVars tyvars
2891 -- The main loop may do unification, and that may crash if
2892 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2893 -- ToDo: what if two of them do get unified?
2894 ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
2895 ; (irreds, _) <- tryHardCheckLoop doc wanteds
2897 ; let (tv_dicts, others) = partition ok irreds
2898 ; addNoInstanceErrs others
2899 -- See Note [Exotic derived instance contexts] in TcMType
2901 ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2902 simpl_theta = substTheta rev_env (map dictPred tv_dicts)
2903 -- This reverse-mapping is a pain, but the result
2904 -- should mention the original TyVars not TcTyVars
2906 ; return simpl_theta }
2908 doc = ptext SLIT("deriving classes for a data type")
2910 ok dict | isDict dict = validDerivPred (dictPred dict)
2915 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2916 used with \tr{default} declarations. We are only interested in
2917 whether it worked or not.
2920 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2923 tcSimplifyDefault theta
2924 = newDictBndrsO DefaultOrigin theta `thenM` \ wanteds ->
2925 tryHardCheckLoop doc wanteds `thenM` \ (irreds, _) ->
2926 addNoInstanceErrs irreds `thenM_`
2930 traceTc (ptext SLIT("tcSimplifyDefault failing")) >> failM
2932 doc = ptext SLIT("default declaration")
2936 %************************************************************************
2938 \section{Errors and contexts}
2940 %************************************************************************
2942 ToDo: for these error messages, should we note the location as coming
2943 from the insts, or just whatever seems to be around in the monad just
2947 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2948 -> [Inst] -- The offending Insts
2950 -- Group together insts with the same origin
2951 -- We want to report them together in error messages
2953 groupErrs report_err []
2955 groupErrs report_err (inst:insts)
2956 = do { do_one (inst:friends)
2957 ; groupErrs report_err others }
2959 -- (It may seem a bit crude to compare the error messages,
2960 -- but it makes sure that we combine just what the user sees,
2961 -- and it avoids need equality on InstLocs.)
2962 (friends, others) = partition is_friend insts
2963 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2964 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2965 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2966 -- Add location and context information derived from the Insts
2968 -- Add the "arising from..." part to a message about bunch of dicts
2969 addInstLoc :: [Inst] -> Message -> Message
2970 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
2972 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
2973 addTopIPErrs bndrs []
2975 addTopIPErrs bndrs ips
2976 = do { dflags <- getDOpts
2977 ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
2979 (tidy_env, tidy_ips) = tidyInsts ips
2981 = vcat [sep [ptext SLIT("Implicit parameters escape from"),
2982 nest 2 (ptext SLIT("the monomorphic top-level binding")
2983 <> plural bndrs <+> ptext SLIT("of")
2984 <+> pprBinders bndrs <> colon)],
2985 nest 2 (vcat (map ppr_ip ips)),
2986 monomorphism_fix dflags]
2987 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
2989 topIPErrs :: [Inst] -> TcM ()
2991 = groupErrs report tidy_dicts
2993 (tidy_env, tidy_dicts) = tidyInsts dicts
2994 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2995 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2996 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2998 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
3000 addNoInstanceErrs insts
3001 = do { let (tidy_env, tidy_insts) = tidyInsts insts
3002 ; reportNoInstances tidy_env Nothing tidy_insts }
3006 -> Maybe (InstLoc, [Inst]) -- Context
3007 -- Nothing => top level
3008 -- Just (d,g) => d describes the construct
3010 -> [Inst] -- What is wanted (can include implications)
3013 reportNoInstances tidy_env mb_what insts
3014 = groupErrs (report_no_instances tidy_env mb_what) insts
3016 report_no_instances tidy_env mb_what insts
3017 = do { inst_envs <- tcGetInstEnvs
3018 ; let (implics, insts1) = partition isImplicInst insts
3019 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
3020 (eqInsts, insts3) = partition isEqInst insts2
3021 ; traceTc (text "reportNoInstances" <+> vcat
3022 [ppr insts, ppr implics, ppr insts1, ppr insts2])
3023 ; mapM_ complain_implic implics
3024 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
3025 ; groupErrs complain_no_inst insts3
3026 ; mapM_ (addErrTcM . mk_eq_err) eqInsts
3029 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
3031 complain_implic inst -- Recurse!
3032 = reportNoInstances tidy_env
3033 (Just (tci_loc inst, tci_given inst))
3036 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
3037 -- Right msg => overlap message
3038 -- Left inst => no instance
3039 check_overlap inst_envs wanted
3040 | not (isClassDict wanted) = Left wanted
3042 = case lookupInstEnv inst_envs clas tys of
3043 -- The case of exactly one match and no unifiers means a
3044 -- successful lookup. That can't happen here, because dicts
3045 -- only end up here if they didn't match in Inst.lookupInst
3047 ([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
3049 ([], _) -> Left wanted -- No match
3050 res -> Right (mk_overlap_msg wanted res)
3052 (clas,tys) = getDictClassTys wanted
3054 mk_overlap_msg dict (matches, unifiers)
3055 = ASSERT( not (null matches) )
3056 vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
3057 <+> pprPred (dictPred dict))),
3058 sep [ptext SLIT("Matching instances") <> colon,
3059 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
3060 if not (isSingleton matches)
3061 then -- Two or more matches
3063 else -- One match, plus some unifiers
3064 ASSERT( not (null unifiers) )
3065 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
3066 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
3067 ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
3068 ptext SLIT("when compiling the other instance declarations")])]
3070 ispecs = [ispec | (ispec, _) <- matches]
3072 mk_eq_err :: Inst -> (TidyEnv, SDoc)
3073 mk_eq_err inst = misMatchMsg tidy_env (eqInstTys inst)
3075 mk_no_inst_err insts
3076 | null insts = empty
3078 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
3079 not (isEmptyVarSet (tyVarsOfInsts insts))
3080 = vcat [ addInstLoc insts $
3081 sep [ ptext SLIT("Could not deduce") <+> pprDictsTheta insts
3082 , nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta givens]
3083 , show_fixes (fix1 loc : fixes2) ]
3085 | otherwise -- Top level
3086 = vcat [ addInstLoc insts $
3087 ptext SLIT("No instance") <> plural insts
3088 <+> ptext SLIT("for") <+> pprDictsTheta insts
3089 , show_fixes fixes2 ]
3092 fix1 loc = sep [ ptext SLIT("add") <+> pprDictsTheta insts
3093 <+> ptext SLIT("to the context of"),
3094 nest 2 (ppr (instLocOrigin loc)) ]
3095 -- I'm not sure it helps to add the location
3096 -- nest 2 (ptext SLIT("at") <+> ppr (instLocSpan loc)) ]
3098 fixes2 | null instance_dicts = []
3099 | otherwise = [sep [ptext SLIT("add an instance declaration for"),
3100 pprDictsTheta instance_dicts]]
3101 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
3102 -- Insts for which it is worth suggesting an adding an instance declaration
3103 -- Exclude implicit parameters, and tyvar dicts
3105 show_fixes :: [SDoc] -> SDoc
3106 show_fixes [] = empty
3107 show_fixes (f:fs) = sep [ptext SLIT("Possible fix:"),
3108 nest 2 (vcat (f : map (ptext SLIT("or") <+>) fs))]
3110 addTopAmbigErrs dicts
3111 -- Divide into groups that share a common set of ambiguous tyvars
3112 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
3113 -- See Note [Avoiding spurious errors]
3114 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
3116 (tidy_env, tidy_dicts) = tidyInsts dicts
3118 tvs_of :: Inst -> [TcTyVar]
3119 tvs_of d = varSetElems (tyVarsOfInst d)
3120 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
3122 report :: [(Inst,[TcTyVar])] -> TcM ()
3123 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
3124 = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) ->
3125 setSrcSpan (instSpan inst) $
3126 -- the location of the first one will do for the err message
3127 addErrTcM (tidy_env, msg $$ mono_msg)
3129 dicts = map fst pairs
3130 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
3131 pprQuotedList tvs <+> in_msg,
3132 nest 2 (pprDictsInFull dicts)]
3133 in_msg = text "in the constraint" <> plural dicts <> colon
3134 report [] = panic "addTopAmbigErrs"
3137 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
3138 -- There's an error with these Insts; if they have free type variables
3139 -- it's probably caused by the monomorphism restriction.
3140 -- Try to identify the offending variable
3141 -- ASSUMPTION: the Insts are fully zonked
3142 mkMonomorphismMsg tidy_env inst_tvs
3143 = do { dflags <- getDOpts
3144 ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
3145 ; return (tidy_env, mk_msg dflags docs) }
3147 mk_msg _ _ | any isRuntimeUnk inst_tvs
3148 = vcat [ptext SLIT("Cannot resolve unknown runtime types:") <+>
3149 (pprWithCommas ppr inst_tvs),
3150 ptext SLIT("Use :print or :force to determine these types")]
3151 mk_msg _ [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
3152 -- This happens in things like
3153 -- f x = show (read "foo")
3154 -- where monomorphism doesn't play any role
3156 = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
3158 monomorphism_fix dflags]
3160 monomorphism_fix :: DynFlags -> SDoc
3161 monomorphism_fix dflags
3162 = ptext SLIT("Probable fix:") <+> vcat
3163 [ptext SLIT("give these definition(s) an explicit type signature"),
3164 if dopt Opt_MonomorphismRestriction dflags
3165 then ptext SLIT("or use -fno-monomorphism-restriction")
3166 else empty] -- Only suggest adding "-fno-monomorphism-restriction"
3167 -- if it is not already set!
3169 warnDefault ups default_ty
3170 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
3171 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
3173 dicts = [d | (d,_,_) <- ups]
3176 (_, tidy_dicts) = tidyInsts dicts
3177 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
3178 quotes (ppr default_ty),
3179 pprDictsInFull tidy_dicts]
3181 reduceDepthErr n stack
3182 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
3183 ptext SLIT("Use -fcontext-stack=N to increase stack size to N"),
3184 nest 4 (pprStack stack)]
3186 pprStack stack = vcat (map pprInstInFull stack)