2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
17 tcSimplifyInfer, tcSimplifyInferCheck,
18 tcSimplifyCheck, tcSimplifyRestricted,
19 tcSimplifyRuleLhs, tcSimplifyIPs,
20 tcSimplifySuperClasses,
21 tcSimplifyTop, tcSimplifyInteractive,
22 tcSimplifyBracket, tcSimplifyCheckPat,
24 tcSimplifyDeriv, tcSimplifyDefault,
25 bindInstsOfLocalFuns, bindIrreds,
30 #include "HsVersions.h"
32 import {-# SOURCE #-} TcUnify( unifyType )
73 %************************************************************************
77 %************************************************************************
79 --------------------------------------
80 Notes on functional dependencies (a bug)
81 --------------------------------------
88 instance D a b => C a b -- Undecidable
89 -- (Not sure if it's crucial to this eg)
90 f :: C a b => a -> Bool
93 g :: C a b => a -> Bool
96 Here f typechecks, but g does not!! Reason: before doing improvement,
97 we reduce the (C a b1) constraint from the call of f to (D a b1).
99 Here is a more complicated example:
101 | > class Foo a b | a->b
103 | > class Bar a b | a->b
107 | > instance Bar Obj Obj
109 | > instance (Bar a b) => Foo a b
111 | > foo:: (Foo a b) => a -> String
114 | > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
120 | Could not deduce (Bar a b) from the context (Foo a b)
121 | arising from use of `foo' at <interactive>:1
123 | Add (Bar a b) to the expected type of an expression
124 | In the first argument of `runFoo', namely `foo'
125 | In the definition of `it': it = runFoo foo
127 | Why all of the sudden does GHC need the constraint Bar a b? The
128 | function foo didn't ask for that...
130 The trouble is that to type (runFoo foo), GHC has to solve the problem:
132 Given constraint Foo a b
133 Solve constraint Foo a b'
135 Notice that b and b' aren't the same. To solve this, just do
136 improvement and then they are the same. But GHC currently does
141 That is usually fine, but it isn't here, because it sees that Foo a b is
142 not the same as Foo a b', and so instead applies the instance decl for
143 instance Bar a b => Foo a b. And that's where the Bar constraint comes
146 The Right Thing is to improve whenever the constraint set changes at
147 all. Not hard in principle, but it'll take a bit of fiddling to do.
149 Note [Choosing which variables to quantify]
150 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
151 Suppose we are about to do a generalisation step. We have in our hand
154 T the type of the RHS
155 C the constraints from that RHS
157 The game is to figure out
159 Q the set of type variables over which to quantify
160 Ct the constraints we will *not* quantify over
161 Cq the constraints we will quantify over
163 So we're going to infer the type
167 and float the constraints Ct further outwards.
169 Here are the things that *must* be true:
171 (A) Q intersect fv(G) = EMPTY limits how big Q can be
172 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
174 (A) says we can't quantify over a variable that's free in the environment.
175 (B) says we must quantify over all the truly free variables in T, else
176 we won't get a sufficiently general type.
178 We do not *need* to quantify over any variable that is fixed by the
179 free vars of the environment G.
181 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
183 Example: class H x y | x->y where ...
185 fv(G) = {a} C = {H a b, H c d}
188 (A) Q intersect {a} is empty
189 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
191 So Q can be {c,d}, {b,c,d}
193 In particular, it's perfectly OK to quantify over more type variables
194 than strictly necessary; there is no need to quantify over 'b', since
195 it is determined by 'a' which is free in the envt, but it's perfectly
196 OK to do so. However we must not quantify over 'a' itself.
198 Other things being equal, however, we'd like to quantify over as few
199 variables as possible: smaller types, fewer type applications, more
200 constraints can get into Ct instead of Cq. Here's a good way to
203 Q = grow( fv(T), C ) \ oclose( fv(G), C )
205 That is, quantify over all variable that that MIGHT be fixed by the
206 call site (which influences T), but which aren't DEFINITELY fixed by
207 G. This choice definitely quantifies over enough type variables,
208 albeit perhaps too many.
210 Why grow( fv(T), C ) rather than fv(T)? Consider
212 class H x y | x->y where ...
217 If we used fv(T) = {c} we'd get the type
219 forall c. H c d => c -> b
221 And then if the fn was called at several different c's, each of
222 which fixed d differently, we'd get a unification error, because
223 d isn't quantified. Solution: quantify d. So we must quantify
224 everything that might be influenced by c.
226 Why not oclose( fv(T), C )? Because we might not be able to see
227 all the functional dependencies yet:
229 class H x y | x->y where ...
230 instance H x y => Eq (T x y) where ...
235 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
236 apparent yet, and that's wrong. We must really quantify over d too.
238 There really isn't any point in quantifying over any more than
239 grow( fv(T), C ), because the call sites can't possibly influence
240 any other type variables.
244 -------------------------------------
246 -------------------------------------
248 It's very hard to be certain when a type is ambiguous. Consider
252 instance H x y => K (x,y)
254 Is this type ambiguous?
255 forall a b. (K (a,b), Eq b) => a -> a
257 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
258 now we see that a fixes b. So we can't tell about ambiguity for sure
259 without doing a full simplification. And even that isn't possible if
260 the context has some free vars that may get unified. Urgle!
262 Here's another example: is this ambiguous?
263 forall a b. Eq (T b) => a -> a
264 Not if there's an insance decl (with no context)
265 instance Eq (T b) where ...
267 You may say of this example that we should use the instance decl right
268 away, but you can't always do that:
270 class J a b where ...
271 instance J Int b where ...
273 f :: forall a b. J a b => a -> a
275 (Notice: no functional dependency in J's class decl.)
276 Here f's type is perfectly fine, provided f is only called at Int.
277 It's premature to complain when meeting f's signature, or even
278 when inferring a type for f.
282 However, we don't *need* to report ambiguity right away. It'll always
283 show up at the call site.... and eventually at main, which needs special
284 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
286 So here's the plan. We WARN about probable ambiguity if
288 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
290 (all tested before quantification).
291 That is, all the type variables in Cq must be fixed by the the variables
292 in the environment, or by the variables in the type.
294 Notice that we union before calling oclose. Here's an example:
296 class J a b c | a b -> c
300 forall b c. (J a b c) => b -> b
302 Only if we union {a} from G with {b} from T before using oclose,
303 do we see that c is fixed.
305 It's a bit vague exactly which C we should use for this oclose call. If we
306 don't fix enough variables we might complain when we shouldn't (see
307 the above nasty example). Nothing will be perfect. That's why we can
308 only issue a warning.
311 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
313 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
315 then c is a "bubble"; there's no way it can ever improve, and it's
316 certainly ambiguous. UNLESS it is a constant (sigh). And what about
321 instance H x y => K (x,y)
323 Is this type ambiguous?
324 forall a b. (K (a,b), Eq b) => a -> a
326 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
327 is a "bubble" that's a set of constraints
329 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
331 Hence another idea. To decide Q start with fv(T) and grow it
332 by transitive closure in Cq (no functional dependencies involved).
333 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
334 The definitely-ambiguous can then float out, and get smashed at top level
335 (which squashes out the constants, like Eq (T a) above)
338 --------------------------------------
339 Notes on principal types
340 --------------------------------------
345 f x = let g y = op (y::Int) in True
347 Here the principal type of f is (forall a. a->a)
348 but we'll produce the non-principal type
349 f :: forall a. C Int => a -> a
352 --------------------------------------
353 The need for forall's in constraints
354 --------------------------------------
356 [Exchange on Haskell Cafe 5/6 Dec 2000]
358 class C t where op :: t -> Bool
359 instance C [t] where op x = True
361 p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
362 q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
364 The definitions of p and q differ only in the order of the components in
365 the pair on their right-hand sides. And yet:
367 ghc and "Typing Haskell in Haskell" reject p, but accept q;
368 Hugs rejects q, but accepts p;
369 hbc rejects both p and q;
370 nhc98 ... (Malcolm, can you fill in the blank for us!).
372 The type signature for f forces context reduction to take place, and
373 the results of this depend on whether or not the type of y is known,
374 which in turn depends on which component of the pair the type checker
377 Solution: if y::m a, float out the constraints
378 Monad m, forall c. C (m c)
379 When m is later unified with [], we can solve both constraints.
382 --------------------------------------
383 Notes on implicit parameters
384 --------------------------------------
386 Note [Inheriting implicit parameters]
387 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
392 where f is *not* a top-level binding.
393 From the RHS of f we'll get the constraint (?y::Int).
394 There are two types we might infer for f:
398 (so we get ?y from the context of f's definition), or
400 f :: (?y::Int) => Int -> Int
402 At first you might think the first was better, becuase then
403 ?y behaves like a free variable of the definition, rather than
404 having to be passed at each call site. But of course, the WHOLE
405 IDEA is that ?y should be passed at each call site (that's what
406 dynamic binding means) so we'd better infer the second.
408 BOTTOM LINE: when *inferring types* you *must* quantify
409 over implicit parameters. See the predicate isFreeWhenInferring.
412 Note [Implicit parameters and ambiguity]
413 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
414 What type should we infer for this?
415 f x = (show ?y, x::Int)
416 Since we must quantify over the ?y, the most plausible type is
417 f :: (Show a, ?y::a) => Int -> (String, Int)
418 But notice that the type of the RHS is (String,Int), with no type
419 varibables mentioned at all! The type of f looks ambiguous. But
420 it isn't, because at a call site we might have
421 let ?y = 5::Int in f 7
422 and all is well. In effect, implicit parameters are, well, parameters,
423 so we can take their type variables into account as part of the
424 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
427 Question 2: type signatures
428 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
429 BUT WATCH OUT: When you supply a type signature, we can't force you
430 to quantify over implicit parameters. For example:
434 This is perfectly reasonable. We do not want to insist on
436 (?x + 1) :: (?x::Int => Int)
438 That would be silly. Here, the definition site *is* the occurrence site,
439 so the above strictures don't apply. Hence the difference between
440 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
441 and tcSimplifyCheckBind (which does not).
443 What about when you supply a type signature for a binding?
444 Is it legal to give the following explicit, user type
445 signature to f, thus:
450 At first sight this seems reasonable, but it has the nasty property
451 that adding a type signature changes the dynamic semantics.
454 (let f x = (x::Int) + ?y
455 in (f 3, f 3 with ?y=5)) with ?y = 6
461 in (f 3, f 3 with ?y=5)) with ?y = 6
465 Indeed, simply inlining f (at the Haskell source level) would change the
468 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
469 semantics for a Haskell program without knowing its typing, so if you
470 change the typing you may change the semantics.
472 To make things consistent in all cases where we are *checking* against
473 a supplied signature (as opposed to inferring a type), we adopt the
476 a signature does not need to quantify over implicit params.
478 [This represents a (rather marginal) change of policy since GHC 5.02,
479 which *required* an explicit signature to quantify over all implicit
480 params for the reasons mentioned above.]
482 But that raises a new question. Consider
484 Given (signature) ?x::Int
485 Wanted (inferred) ?x::Int, ?y::Bool
487 Clearly we want to discharge the ?x and float the ?y out. But
488 what is the criterion that distinguishes them? Clearly it isn't
489 what free type variables they have. The Right Thing seems to be
490 to float a constraint that
491 neither mentions any of the quantified type variables
492 nor any of the quantified implicit parameters
494 See the predicate isFreeWhenChecking.
497 Question 3: monomorphism
498 ~~~~~~~~~~~~~~~~~~~~~~~~
499 There's a nasty corner case when the monomorphism restriction bites:
503 The argument above suggests that we *must* generalise
504 over the ?y parameter, to get
505 z :: (?y::Int) => Int,
506 but the monomorphism restriction says that we *must not*, giving
508 Why does the momomorphism restriction say this? Because if you have
510 let z = x + ?y in z+z
512 you might not expect the addition to be done twice --- but it will if
513 we follow the argument of Question 2 and generalise over ?y.
516 Question 4: top level
517 ~~~~~~~~~~~~~~~~~~~~~
518 At the top level, monomorhism makes no sense at all.
521 main = let ?x = 5 in print foo
525 woggle :: (?x :: Int) => Int -> Int
528 We definitely don't want (foo :: Int) with a top-level implicit parameter
529 (?x::Int) becuase there is no way to bind it.
534 (A) Always generalise over implicit parameters
535 Bindings that fall under the monomorphism restriction can't
539 * Inlining remains valid
540 * No unexpected loss of sharing
541 * But simple bindings like
543 will be rejected, unless you add an explicit type signature
544 (to avoid the monomorphism restriction)
545 z :: (?y::Int) => Int
547 This seems unacceptable
549 (B) Monomorphism restriction "wins"
550 Bindings that fall under the monomorphism restriction can't
552 Always generalise over implicit parameters *except* for bindings
553 that fall under the monomorphism restriction
556 * Inlining isn't valid in general
557 * No unexpected loss of sharing
558 * Simple bindings like
560 accepted (get value of ?y from binding site)
562 (C) Always generalise over implicit parameters
563 Bindings that fall under the monomorphism restriction can't
564 be generalised, EXCEPT for implicit parameters
566 * Inlining remains valid
567 * Unexpected loss of sharing (from the extra generalisation)
568 * Simple bindings like
570 accepted (get value of ?y from occurrence sites)
575 None of these choices seems very satisfactory. But at least we should
576 decide which we want to do.
578 It's really not clear what is the Right Thing To Do. If you see
582 would you expect the value of ?y to be got from the *occurrence sites*
583 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
584 case of function definitions, the answer is clearly the former, but
585 less so in the case of non-fucntion definitions. On the other hand,
586 if we say that we get the value of ?y from the definition site of 'z',
587 then inlining 'z' might change the semantics of the program.
589 Choice (C) really says "the monomorphism restriction doesn't apply
590 to implicit parameters". Which is fine, but remember that every
591 innocent binding 'x = ...' that mentions an implicit parameter in
592 the RHS becomes a *function* of that parameter, called at each
593 use of 'x'. Now, the chances are that there are no intervening 'with'
594 clauses that bind ?y, so a decent compiler should common up all
595 those function calls. So I think I strongly favour (C). Indeed,
596 one could make a similar argument for abolishing the monomorphism
597 restriction altogether.
599 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
603 %************************************************************************
605 \subsection{tcSimplifyInfer}
607 %************************************************************************
609 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
611 1. Compute Q = grow( fvs(T), C )
613 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
614 predicates will end up in Ct; we deal with them at the top level
616 3. Try improvement, using functional dependencies
618 4. If Step 3 did any unification, repeat from step 1
619 (Unification can change the result of 'grow'.)
621 Note: we don't reduce dictionaries in step 2. For example, if we have
622 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
623 after step 2. However note that we may therefore quantify over more
624 type variables than we absolutely have to.
626 For the guts, we need a loop, that alternates context reduction and
627 improvement with unification. E.g. Suppose we have
629 class C x y | x->y where ...
631 and tcSimplify is called with:
633 Then improvement unifies a with b, giving
636 If we need to unify anything, we rattle round the whole thing all over
643 -> TcTyVarSet -- fv(T); type vars
645 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked and quantified)
646 [Inst], -- Dict Ids that must be bound here (zonked)
647 TcDictBinds) -- Bindings
648 -- Any free (escaping) Insts are tossed into the environment
653 tcSimplifyInfer doc tau_tvs wanted
654 = do { tau_tvs1 <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
655 ; wanted' <- mappM zonkInst wanted -- Zonk before deciding quantified tyvars
656 ; gbl_tvs <- tcGetGlobalTyVars
657 ; let preds1 = fdPredsOfInsts wanted'
658 gbl_tvs1 = oclose preds1 gbl_tvs
659 qtvs = grow preds1 tau_tvs1 `minusVarSet` gbl_tvs1
660 -- See Note [Choosing which variables to quantify]
662 -- To maximise sharing, remove from consideration any
663 -- constraints that don't mention qtvs at all
664 ; let (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
667 -- To make types simple, reduce as much as possible
668 ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (grow preds1 tau_tvs1) $$ ppr gbl_tvs $$
669 ppr gbl_tvs1 $$ ppr free $$ ppr bound))
670 ; (irreds1, binds1) <- tryHardCheckLoop doc bound
672 -- Note [Inference and implication constraints]
673 ; let want_dict d = tyVarsOfInst d `intersectsVarSet` qtvs
674 ; (irreds2, binds2) <- approximateImplications doc want_dict irreds1
676 -- Now work out all over again which type variables to quantify,
677 -- exactly in the same way as before, but starting from irreds2. Why?
678 -- a) By now improvment may have taken place, and we must *not*
679 -- quantify over any variable free in the environment
680 -- tc137 (function h inside g) is an example
682 -- b) Do not quantify over constraints that *now* do not
683 -- mention quantified type variables, because they are
684 -- simply ambiguous (or might be bound further out). Example:
685 -- f :: Eq b => a -> (a, b)
687 -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
688 -- We decide to quantify over 'alpha' alone, but free1 does not include f77
689 -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
690 -- constraint (Eq beta), which we dump back into the free set
691 -- See test tcfail181
693 -- c) irreds may contain type variables not previously mentioned,
694 -- e.g. instance D a x => Foo [a]
696 -- Then after simplifying we'll get (D a x), and x is fresh
697 -- We must quantify over x else it'll be totally unbound
698 ; tau_tvs2 <- zonkTcTyVarsAndFV (varSetElems tau_tvs1)
699 ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
700 -- Note that we start from gbl_tvs1
701 -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
702 -- we've already put some of the original preds1 into frees
703 -- E.g. wanteds = C a b (where a->b)
706 -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
707 -- irreds2 will be empty. But we don't want to generalise over b!
708 ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
709 qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
710 ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
713 -- Turn the quantified meta-type variables into real type variables
714 ; qtvs2 <- zonkQuantifiedTyVars (varSetElems qtvs)
716 -- We can't abstract over any remaining unsolved
717 -- implications so instead just float them outwards. Ugh.
718 ; let (q_dicts0, implics) = partition isAbstractableInst irreds3
719 ; loc <- getInstLoc (ImplicOrigin doc)
720 ; implic_bind <- bindIrreds loc qtvs2 q_dicts0 implics
722 -- Prepare equality instances for quantification
723 ; let (q_eqs0,q_dicts) = partition isEqInst q_dicts0
724 ; q_eqs <- mappM finalizeEqInst q_eqs0
726 ; return (qtvs2, q_eqs ++ q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
727 -- NB: when we are done, we might have some bindings, but
728 -- the final qtvs might be empty. See Note [NO TYVARS] below.
730 approximateImplications :: SDoc -> (Inst -> Bool) -> [Inst] -> TcM ([Inst], TcDictBinds)
731 -- Note [Inference and implication constraints]
732 -- Given a bunch of Dict and ImplicInsts, try to approximate the implications by
733 -- - fetching any dicts inside them that are free
734 -- - using those dicts as cruder constraints, to solve the implications
735 -- - returning the extra ones too
737 approximateImplications doc want_dict irreds
739 = return (irreds, emptyBag)
741 = do { extra_dicts' <- mapM cloneDict extra_dicts
742 ; tryHardCheckLoop doc (extra_dicts' ++ irreds) }
743 -- By adding extra_dicts', we make them
744 -- available to solve the implication constraints
746 extra_dicts = get_dicts (filter isImplicInst irreds)
748 get_dicts :: [Inst] -> [Inst] -- Returns only Dicts
749 -- Find the wanted constraints in implication constraints that satisfy
750 -- want_dict, and are not bound by forall's in the constraint itself
751 get_dicts ds = concatMap get_dict ds
753 get_dict d@(Dict {}) | want_dict d = [d]
755 get_dict (ImplicInst {tci_tyvars = tvs, tci_wanted = wanteds})
756 = [ d | let tv_set = mkVarSet tvs
757 , d <- get_dicts wanteds
758 , not (tyVarsOfInst d `intersectsVarSet` tv_set)]
759 get_dict i@(EqInst {}) | want_dict i = [i]
761 get_dict other = pprPanic "approximateImplications" (ppr other)
764 Note [Inference and implication constraints]
765 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
766 Suppose we have a wanted implication constraint (perhaps arising from
767 a nested pattern match) like
769 and we are now trying to quantify over 'a' when inferring the type for
770 a function. In principle it's possible that there might be an instance
771 instance (C a, E a) => D [a]
772 so the context (E a) would suffice. The Right Thing is to abstract over
773 the implication constraint, but we don't do that (a) because it'll be
774 surprising to programmers and (b) because we don't have the machinery to deal
775 with 'given' implications.
777 So our best approximation is to make (D [a]) part of the inferred
778 context, so we can use that to discharge the implication. Hence
779 the strange function get_dictsin approximateImplications.
781 The common cases are more clear-cut, when we have things like
783 Here, abstracting over (C b) is not an approximation at all -- but see
784 Note [Freeness and implications].
786 See Trac #1430 and test tc228.
790 -----------------------------------------------------------
791 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
792 -- against, but we don't know the type variables over which we are going to quantify.
793 -- This happens when we have a type signature for a mutually recursive group
796 -> TcTyVarSet -- fv(T)
799 -> TcM ([TyVar], -- Fully zonked, and quantified
800 TcDictBinds) -- Bindings
802 tcSimplifyInferCheck loc tau_tvs givens wanteds
803 = do { traceTc (text "tcSimplifyInferCheck <-" <+> ppr wanteds)
804 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
806 -- Figure out which type variables to quantify over
807 -- You might think it should just be the signature tyvars,
808 -- but in bizarre cases you can get extra ones
809 -- f :: forall a. Num a => a -> a
810 -- f x = fst (g (x, head [])) + 1
812 -- Here we infer g :: forall a b. a -> b -> (b,a)
813 -- We don't want g to be monomorphic in b just because
814 -- f isn't quantified over b.
815 ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
816 ; all_tvs <- zonkTcTyVarsAndFV all_tvs
817 ; gbl_tvs <- tcGetGlobalTyVars
818 ; let qtvs = varSetElems (all_tvs `minusVarSet` gbl_tvs)
819 -- We could close gbl_tvs, but its not necessary for
820 -- soundness, and it'll only affect which tyvars, not which
821 -- dictionaries, we quantify over
823 ; qtvs' <- zonkQuantifiedTyVars qtvs
825 -- Now we are back to normal (c.f. tcSimplCheck)
826 ; implic_bind <- bindIrreds loc qtvs' givens irreds
828 ; traceTc (text "tcSimplifyInferCheck ->" <+> ppr (implic_bind))
829 ; return (qtvs', binds `unionBags` implic_bind) }
832 Note [Squashing methods]
833 ~~~~~~~~~~~~~~~~~~~~~~~~~
834 Be careful if you want to float methods more:
835 truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
836 From an application (truncate f i) we get
839 If we have also have a second occurrence of truncate, we get
842 When simplifying with i,f free, we might still notice that
843 t1=t3; but alas, the binding for t2 (which mentions t1)
844 may continue to float out!
849 class Y a b | a -> b where
852 instance Y [[a]] a where
855 k :: X a -> X a -> X a
857 g :: Num a => [X a] -> [X a]
860 h ys = ys ++ map (k (y [[0]])) xs
862 The excitement comes when simplifying the bindings for h. Initially
863 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
864 From this we get t1:=:t2, but also various bindings. We can't forget
865 the bindings (because of [LOOP]), but in fact t1 is what g is
868 The net effect of [NO TYVARS]
871 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
872 isFreeWhenInferring qtvs inst
873 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
874 && isInheritableInst inst -- and no implicit parameter involved
875 -- see Note [Inheriting implicit parameters]
877 {- No longer used (with implication constraints)
878 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
879 -> NameSet -- Quantified implicit parameters
881 isFreeWhenChecking qtvs ips inst
882 = isFreeWrtTyVars qtvs inst
883 && isFreeWrtIPs ips inst
886 isFreeWrtTyVars qtvs inst = tyVarsOfInst inst `disjointVarSet` qtvs
887 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
891 %************************************************************************
893 \subsection{tcSimplifyCheck}
895 %************************************************************************
897 @tcSimplifyCheck@ is used when we know exactly the set of variables
898 we are going to quantify over. For example, a class or instance declaration.
901 -----------------------------------------------------------
902 -- tcSimplifyCheck is used when checking expression type signatures,
903 -- class decls, instance decls etc.
904 tcSimplifyCheck :: InstLoc
905 -> [TcTyVar] -- Quantify over these
908 -> TcM TcDictBinds -- Bindings
909 tcSimplifyCheck loc qtvs givens wanteds
910 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
911 do { traceTc (text "tcSimplifyCheck")
912 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
913 ; implic_bind <- bindIrreds loc qtvs givens irreds
914 ; return (binds `unionBags` implic_bind) }
916 -----------------------------------------------------------
917 -- tcSimplifyCheckPat is used for existential pattern match
918 tcSimplifyCheckPat :: InstLoc
919 -> [CoVar] -> Refinement
920 -> [TcTyVar] -- Quantify over these
923 -> TcM TcDictBinds -- Bindings
924 tcSimplifyCheckPat loc co_vars reft qtvs givens wanteds
925 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
926 do { traceTc (text "tcSimplifyCheckPat")
927 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
928 ; implic_bind <- bindIrredsR loc qtvs co_vars reft
930 ; return (binds `unionBags` implic_bind) }
932 -----------------------------------------------------------
933 bindIrreds :: InstLoc -> [TcTyVar]
936 bindIrreds loc qtvs givens irreds
937 = bindIrredsR loc qtvs [] emptyRefinement givens irreds
939 bindIrredsR :: InstLoc -> [TcTyVar] -> [CoVar]
940 -> Refinement -> [Inst] -> [Inst]
942 -- Make a binding that binds 'irreds', by generating an implication
943 -- constraint for them, *and* throwing the constraint into the LIE
944 bindIrredsR loc qtvs co_vars reft givens irreds
948 = do { let givens' = filter isDict givens
949 -- The givens can include methods
950 -- See Note [Pruning the givens in an implication constraint]
952 -- If there are no 'givens' *and* the refinement is empty
953 -- (the refinement is like more givens), then it's safe to
954 -- partition the 'wanteds' by their qtvs, thereby trimming irreds
955 -- See Note [Freeness and implications]
956 ; irreds' <- if null givens' && isEmptyRefinement reft
958 { let qtv_set = mkVarSet qtvs
959 (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
961 ; return real_irreds }
964 ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
965 ; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds'
966 -- This call does the real work
967 -- If irreds' is empty, it does something sensible
972 makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement
974 -> TcM ([Inst], TcDictBinds)
975 -- Make a binding that binds 'irreds', by generating an implication
976 -- constraint for them, *and* throwing the constraint into the LIE
977 -- The binding looks like
978 -- (ir1, .., irn) = f qtvs givens
979 -- where f is (evidence for) the new implication constraint
980 -- f :: forall qtvs. {reft} givens => (ir1, .., irn)
981 -- qtvs includes coercion variables
983 -- This binding must line up the 'rhs' in reduceImplication
984 makeImplicationBind loc all_tvs reft
985 givens -- Guaranteed all Dicts (TOMDO: true?)
987 | null irreds -- If there are no irreds, we are done
988 = return ([], emptyBag)
989 | otherwise -- Otherwise we must generate a binding
990 = do { uniq <- newUnique
991 ; span <- getSrcSpanM
992 ; let (eq_givens, dict_givens) = partition isEqInst givens
993 eq_tyvar_cos = map TyVarTy $ uniqSetToList $ tyVarsOfTypes $ map eqInstType eq_givens
994 ; let name = mkInternalName uniq (mkVarOcc "ic") span
995 implic_inst = ImplicInst { tci_name = name, tci_reft = reft,
996 tci_tyvars = all_tvs,
997 tci_given = (eq_givens ++ dict_givens),
998 tci_wanted = irreds, tci_loc = loc }
999 ; let -- only create binder for dict_irreds
1000 (eq_irreds, dict_irreds) = partition isEqInst irreds
1001 n_dict_irreds = length dict_irreds
1002 dict_irred_ids = map instToId dict_irreds
1003 tup_ty = mkTupleTy Boxed n_dict_irreds (map idType dict_irred_ids)
1004 pat = TuplePat (map nlVarPat dict_irred_ids) Boxed tup_ty
1005 rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
1006 co = mkWpApps (map instToId dict_givens) <.> mkWpTyApps eq_tyvar_cos <.> mkWpTyApps (mkTyVarTys all_tvs)
1007 bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs
1008 | otherwise = PatBind { pat_lhs = L span pat,
1009 pat_rhs = unguardedGRHSs rhs,
1010 pat_rhs_ty = tup_ty,
1011 bind_fvs = placeHolderNames }
1012 ; -- pprTrace "Make implic inst" (ppr (implic_inst,irreds,dict_irreds,tup_ty)) $
1013 return ([implic_inst], unitBag (L span bind)) }
1015 -----------------------------------------------------------
1016 tryHardCheckLoop :: SDoc
1018 -> TcM ([Inst], TcDictBinds)
1020 tryHardCheckLoop doc wanteds
1021 = do { (irreds,binds,_) <- checkLoop (mkRedEnv doc try_me []) wanteds
1022 ; return (irreds,binds)
1025 try_me inst = ReduceMe AddSCs
1026 -- Here's the try-hard bit
1028 -----------------------------------------------------------
1029 gentleCheckLoop :: InstLoc
1032 -> TcM ([Inst], TcDictBinds)
1034 gentleCheckLoop inst_loc givens wanteds
1035 = do { (irreds,binds,_) <- checkLoop env wanteds
1036 ; return (irreds,binds)
1039 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
1041 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1043 -- When checking against a given signature
1044 -- we MUST be very gentle: Note [Check gently]
1048 ~~~~~~~~~~~~~~~~~~~~
1049 We have to very careful about not simplifying too vigorously
1054 f :: Show b => T b -> b
1055 f (MkT x) = show [x]
1057 Inside the pattern match, which binds (a:*, x:a), we know that
1059 Hence we have a dictionary for Show [a] available; and indeed we
1060 need it. We are going to build an implication contraint
1061 forall a. (b~[a]) => Show [a]
1062 Later, we will solve this constraint using the knowledg e(Show b)
1064 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
1065 thing becomes insoluble. So we simplify gently (get rid of literals
1066 and methods only, plus common up equal things), deferring the real
1067 work until top level, when we solve the implication constraint
1068 with tryHardCheckLooop.
1072 -----------------------------------------------------------
1075 -> TcM ([Inst], TcDictBinds,
1076 [Inst]) -- needed givens
1077 -- Precondition: givens are completely rigid
1078 -- Postcondition: returned Insts are zonked
1080 checkLoop env wanteds
1082 where go env wanteds needed_givens
1083 = do { -- Givens are skolems, so no need to zonk them
1084 wanteds' <- zonkInsts wanteds
1086 ; (improved, binds, irreds, more_needed_givens) <- reduceContext env wanteds'
1088 ; let all_needed_givens = needed_givens ++ more_needed_givens
1090 ; if not improved then
1091 return (irreds, binds, all_needed_givens)
1094 -- If improvement did some unification, we go round again.
1095 -- We start again with irreds, not wanteds
1096 -- Using an instance decl might have introduced a fresh type variable
1097 -- which might have been unified, so we'd get an infinite loop
1098 -- if we started again with wanteds! See Note [LOOP]
1099 { (irreds1, binds1, all_needed_givens1) <- go env irreds all_needed_givens
1100 ; return (irreds1, binds `unionBags` binds1, all_needed_givens1) } }
1105 class If b t e r | b t e -> r
1108 class Lte a b c | a b -> c where lte :: a -> b -> c
1110 instance (Lte a b l,If l b a c) => Max a b c
1112 Wanted: Max Z (S x) y
1114 Then we'll reduce using the Max instance to:
1115 (Lte Z (S x) l, If l (S x) Z y)
1116 and improve by binding l->T, after which we can do some reduction
1117 on both the Lte and If constraints. What we *can't* do is start again
1118 with (Max Z (S x) y)!
1122 %************************************************************************
1124 tcSimplifySuperClasses
1126 %************************************************************************
1128 Note [SUPERCLASS-LOOP 1]
1129 ~~~~~~~~~~~~~~~~~~~~~~~~
1130 We have to be very, very careful when generating superclasses, lest we
1131 accidentally build a loop. Here's an example:
1135 class S a => C a where { opc :: a -> a }
1136 class S b => D b where { opd :: b -> b }
1138 instance C Int where
1141 instance D Int where
1144 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1145 Simplifying, we may well get:
1146 $dfCInt = :C ds1 (opd dd)
1149 Notice that we spot that we can extract ds1 from dd.
1151 Alas! Alack! We can do the same for (instance D Int):
1153 $dfDInt = :D ds2 (opc dc)
1157 And now we've defined the superclass in terms of itself.
1159 Solution: never generate a superclass selectors at all when
1160 satisfying the superclass context of an instance declaration.
1162 Two more nasty cases are in
1167 tcSimplifySuperClasses
1172 tcSimplifySuperClasses loc givens sc_wanteds
1173 = do { traceTc (text "tcSimplifySuperClasses")
1174 ; (irreds,binds1,_) <- checkLoop env sc_wanteds
1175 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1176 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1179 env = mkRedEnv (pprInstLoc loc) try_me givens
1180 try_me inst = ReduceMe NoSCs
1181 -- Like tryHardCheckLoop, but with NoSCs
1185 %************************************************************************
1187 \subsection{tcSimplifyRestricted}
1189 %************************************************************************
1191 tcSimplifyRestricted infers which type variables to quantify for a
1192 group of restricted bindings. This isn't trivial.
1195 We want to quantify over a to get id :: forall a. a->a
1198 We do not want to quantify over a, because there's an Eq a
1199 constraint, so we get eq :: a->a->Bool (notice no forall)
1202 RHS has type 'tau', whose free tyvars are tau_tvs
1203 RHS has constraints 'wanteds'
1206 Quantify over (tau_tvs \ ftvs(wanteds))
1207 This is bad. The constraints may contain (Monad (ST s))
1208 where we have instance Monad (ST s) where...
1209 so there's no need to be monomorphic in s!
1211 Also the constraint might be a method constraint,
1212 whose type mentions a perfectly innocent tyvar:
1213 op :: Num a => a -> b -> a
1214 Here, b is unconstrained. A good example would be
1216 We want to infer the polymorphic type
1217 foo :: forall b. b -> b
1220 Plan B (cunning, used for a long time up to and including GHC 6.2)
1221 Step 1: Simplify the constraints as much as possible (to deal
1222 with Plan A's problem). Then set
1223 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1225 Step 2: Now simplify again, treating the constraint as 'free' if
1226 it does not mention qtvs, and trying to reduce it otherwise.
1227 The reasons for this is to maximise sharing.
1229 This fails for a very subtle reason. Suppose that in the Step 2
1230 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1231 In the Step 1 this constraint might have been simplified, perhaps to
1232 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1233 This won't happen in Step 2... but that in turn might prevent some other
1234 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1235 and that in turn breaks the invariant that no constraints are quantified over.
1237 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1242 Step 1: Simplify the constraints as much as possible (to deal
1243 with Plan A's problem). Then set
1244 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1245 Return the bindings from Step 1.
1248 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1251 instance (HasBinary ty IO) => HasCodedValue ty
1253 foo :: HasCodedValue a => String -> IO a
1255 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1256 doDecodeIO codedValue view
1257 = let { act = foo "foo" } in act
1259 You might think this should work becuase the call to foo gives rise to a constraint
1260 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1261 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1262 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1264 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1268 Plan D (a variant of plan B)
1269 Step 1: Simplify the constraints as much as possible (to deal
1270 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1271 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1273 Step 2: Now simplify again, treating the constraint as 'free' if
1274 it does not mention qtvs, and trying to reduce it otherwise.
1276 The point here is that it's generally OK to have too few qtvs; that is,
1277 to make the thing more monomorphic than it could be. We don't want to
1278 do that in the common cases, but in wierd cases it's ok: the programmer
1279 can always add a signature.
1281 Too few qtvs => too many wanteds, which is what happens if you do less
1286 tcSimplifyRestricted -- Used for restricted binding groups
1287 -- i.e. ones subject to the monomorphism restriction
1290 -> [Name] -- Things bound in this group
1291 -> TcTyVarSet -- Free in the type of the RHSs
1292 -> [Inst] -- Free in the RHSs
1293 -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
1294 TcDictBinds) -- Bindings
1295 -- tcSimpifyRestricted returns no constraints to
1296 -- quantify over; by definition there are none.
1297 -- They are all thrown back in the LIE
1299 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1300 -- Zonk everything in sight
1301 = do { traceTc (text "tcSimplifyRestricted")
1302 ; wanteds' <- zonkInsts wanteds
1304 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1305 -- dicts; the idea is to get rid of as many type
1306 -- variables as possible, and we don't want to stop
1307 -- at (say) Monad (ST s), because that reduces
1308 -- immediately, with no constraint on s.
1310 -- BUT do no improvement! See Plan D above
1311 -- HOWEVER, some unification may take place, if we instantiate
1312 -- a method Inst with an equality constraint
1313 ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
1314 ; (_imp, _binds, constrained_dicts, _) <- reduceContext env wanteds'
1316 -- Next, figure out the tyvars we will quantify over
1317 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1318 ; gbl_tvs' <- tcGetGlobalTyVars
1319 ; constrained_dicts' <- zonkInsts constrained_dicts
1321 ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
1322 -- As in tcSimplifyInfer
1324 -- Do not quantify over constrained type variables:
1325 -- this is the monomorphism restriction
1326 constrained_tvs' = tyVarsOfInsts constrained_dicts'
1327 qtvs = qtvs1 `minusVarSet` constrained_tvs'
1328 pp_bndrs = pprWithCommas (quotes . ppr) bndrs
1331 ; warn_mono <- doptM Opt_WarnMonomorphism
1332 ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
1333 (vcat[ ptext SLIT("the Monomorphism Restriction applies to the binding")
1334 <> plural bndrs <+> ptext SLIT("for") <+> pp_bndrs,
1335 ptext SLIT("Consider giving a type signature for") <+> pp_bndrs])
1337 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1338 pprInsts wanteds, pprInsts constrained_dicts',
1340 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1342 -- The first step may have squashed more methods than
1343 -- necessary, so try again, this time more gently, knowing the exact
1344 -- set of type variables to quantify over.
1346 -- We quantify only over constraints that are captured by qtvs;
1347 -- these will just be a subset of non-dicts. This in contrast
1348 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1349 -- all *non-inheritable* constraints too. This implements choice
1350 -- (B) under "implicit parameter and monomorphism" above.
1352 -- Remember that we may need to do *some* simplification, to
1353 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1354 -- just to float all constraints
1356 -- At top level, we *do* squash methods becuase we want to
1357 -- expose implicit parameters to the test that follows
1358 ; let is_nested_group = isNotTopLevel top_lvl
1359 try_me inst | isFreeWrtTyVars qtvs inst,
1360 (is_nested_group || isDict inst) = Stop
1361 | otherwise = ReduceMe AddSCs
1362 env = mkNoImproveRedEnv doc try_me
1363 ; (_imp, binds, irreds, _) <- reduceContext env wanteds'
1365 -- See "Notes on implicit parameters, Question 4: top level"
1366 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1367 if is_nested_group then
1369 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1370 ; addTopIPErrs bndrs bad_ips
1371 ; extendLIEs non_ips }
1373 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1374 ; return (qtvs', binds) }
1378 %************************************************************************
1382 %************************************************************************
1384 On the LHS of transformation rules we only simplify methods and constants,
1385 getting dictionaries. We want to keep all of them unsimplified, to serve
1386 as the available stuff for the RHS of the rule.
1388 Example. Consider the following left-hand side of a rule
1390 f (x == y) (y > z) = ...
1392 If we typecheck this expression we get constraints
1394 d1 :: Ord a, d2 :: Eq a
1396 We do NOT want to "simplify" to the LHS
1398 forall x::a, y::a, z::a, d1::Ord a.
1399 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1403 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1404 f ((==) d2 x y) ((>) d1 y z) = ...
1406 Here is another example:
1408 fromIntegral :: (Integral a, Num b) => a -> b
1409 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1411 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1412 we *dont* want to get
1414 forall dIntegralInt.
1415 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1417 because the scsel will mess up RULE matching. Instead we want
1419 forall dIntegralInt, dNumInt.
1420 fromIntegral Int Int dIntegralInt dNumInt = id Int
1424 g (x == y) (y == z) = ..
1426 where the two dictionaries are *identical*, we do NOT WANT
1428 forall x::a, y::a, z::a, d1::Eq a
1429 f ((==) d1 x y) ((>) d1 y z) = ...
1431 because that will only match if the dict args are (visibly) equal.
1432 Instead we want to quantify over the dictionaries separately.
1434 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1435 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1436 from scratch, rather than further parameterise simpleReduceLoop etc
1439 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1440 tcSimplifyRuleLhs wanteds
1441 = go [] emptyBag wanteds
1444 = return (dicts, binds)
1445 go dicts binds (w:ws)
1447 = go (w:dicts) binds ws
1449 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1450 -- to fromInteger; this looks fragile to me
1451 ; lookup_result <- lookupSimpleInst w'
1452 ; case lookup_result of
1453 GenInst ws' rhs -> go dicts (addBind binds w rhs) (ws' ++ ws)
1454 NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w)
1458 tcSimplifyBracket is used when simplifying the constraints arising from
1459 a Template Haskell bracket [| ... |]. We want to check that there aren't
1460 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1461 Show instance), but we aren't otherwise interested in the results.
1462 Nor do we care about ambiguous dictionaries etc. We will type check
1463 this bracket again at its usage site.
1466 tcSimplifyBracket :: [Inst] -> TcM ()
1467 tcSimplifyBracket wanteds
1468 = do { tryHardCheckLoop doc wanteds
1471 doc = text "tcSimplifyBracket"
1475 %************************************************************************
1477 \subsection{Filtering at a dynamic binding}
1479 %************************************************************************
1484 we must discharge all the ?x constraints from B. We also do an improvement
1485 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1487 Actually, the constraints from B might improve the types in ?x. For example
1489 f :: (?x::Int) => Char -> Char
1492 then the constraint (?x::Int) arising from the call to f will
1493 force the binding for ?x to be of type Int.
1496 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1499 -- We need a loop so that we do improvement, and then
1500 -- (next time round) generate a binding to connect the two
1502 -- Here the two ?x's have different types, and improvement
1503 -- makes them the same.
1505 tcSimplifyIPs given_ips wanteds
1506 = do { wanteds' <- zonkInsts wanteds
1507 ; given_ips' <- zonkInsts given_ips
1508 -- Unusually for checking, we *must* zonk the given_ips
1510 ; let env = mkRedEnv doc try_me given_ips'
1511 ; (improved, binds, irreds, _) <- reduceContext env wanteds'
1513 ; if not improved then
1514 ASSERT( all is_free irreds )
1515 do { extendLIEs irreds
1518 tcSimplifyIPs given_ips wanteds }
1520 doc = text "tcSimplifyIPs" <+> ppr given_ips
1521 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1522 is_free inst = isFreeWrtIPs ip_set inst
1524 -- Simplify any methods that mention the implicit parameter
1525 try_me inst | is_free inst = Stop
1526 | otherwise = ReduceMe NoSCs
1530 %************************************************************************
1532 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1534 %************************************************************************
1536 When doing a binding group, we may have @Insts@ of local functions.
1537 For example, we might have...
1539 let f x = x + 1 -- orig local function (overloaded)
1540 f.1 = f Int -- two instances of f
1545 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1546 where @f@ is in scope; those @Insts@ must certainly not be passed
1547 upwards towards the top-level. If the @Insts@ were binding-ified up
1548 there, they would have unresolvable references to @f@.
1550 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1551 For each method @Inst@ in the @init_lie@ that mentions one of the
1552 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1553 @LIE@), as well as the @HsBinds@ generated.
1556 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1557 -- Simlifies only MethodInsts, and generate only bindings of form
1559 -- We're careful not to even generate bindings of the form
1561 -- You'd think that'd be fine, but it interacts with what is
1562 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1564 bindInstsOfLocalFuns wanteds local_ids
1565 | null overloaded_ids
1567 = extendLIEs wanteds `thenM_`
1568 returnM emptyLHsBinds
1571 = do { (irreds, binds,_) <- checkLoop env for_me
1572 ; extendLIEs not_for_me
1576 env = mkRedEnv doc try_me []
1577 doc = text "bindInsts" <+> ppr local_ids
1578 overloaded_ids = filter is_overloaded local_ids
1579 is_overloaded id = isOverloadedTy (idType id)
1580 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1582 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1583 -- so it's worth building a set, so that
1584 -- lookup (in isMethodFor) is faster
1585 try_me inst | isMethod inst = ReduceMe NoSCs
1590 %************************************************************************
1592 \subsection{Data types for the reduction mechanism}
1594 %************************************************************************
1596 The main control over context reduction is here
1600 = RedEnv { red_doc :: SDoc -- The context
1601 , red_try_me :: Inst -> WhatToDo
1602 , red_improve :: Bool -- True <=> do improvement
1603 , red_givens :: [Inst] -- All guaranteed rigid
1605 -- but see Note [Rigidity]
1606 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1607 -- See Note [RedStack]
1611 -- The red_givens are rigid so far as cmpInst is concerned.
1612 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1613 -- let ?x = e in ...
1614 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1615 -- But that doesn't affect the comparison, which is based only on mame.
1618 -- The red_stack pair (n,insts) pair is just used for error reporting.
1619 -- 'n' is always the depth of the stack.
1620 -- The 'insts' is the stack of Insts being reduced: to produce X
1621 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1624 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1625 mkRedEnv doc try_me givens
1626 = RedEnv { red_doc = doc, red_try_me = try_me,
1627 red_givens = givens, red_stack = (0,[]),
1628 red_improve = True }
1630 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1631 -- Do not do improvement; no givens
1632 mkNoImproveRedEnv doc try_me
1633 = RedEnv { red_doc = doc, red_try_me = try_me,
1634 red_givens = [], red_stack = (0,[]),
1635 red_improve = True }
1638 = ReduceMe WantSCs -- Try to reduce this
1639 -- If there's no instance, add the inst to the
1640 -- irreductible ones, but don't produce an error
1641 -- message of any kind.
1642 -- It might be quite legitimate such as (Eq a)!
1644 | Stop -- Return as irreducible unless it can
1645 -- be reduced to a constant in one step
1646 -- Do not add superclasses; see
1648 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1649 -- of a predicate when adding it to the avails
1650 -- The reason for this flag is entirely the super-class loop problem
1651 -- Note [SUPER-CLASS LOOP 1]
1655 %************************************************************************
1657 \subsection[reduce]{@reduce@}
1659 %************************************************************************
1661 Note [Ancestor Equalities]
1662 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1663 During context reduction, we add to the wanted equalities also those
1664 equalities that (transitively) occur in superclass contexts of wanted
1665 class constraints. Consider the following code
1667 class a ~ Int => C a
1670 If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
1671 substituting Int for a. Hence, we ultimately want (C Int), which we
1672 discharge with the explicit instance.
1675 reduceContext :: RedEnv
1677 -> TcM (ImprovementDone,
1678 TcDictBinds, -- Dictionary bindings
1679 [Inst], -- Irreducible
1680 [Inst]) -- Needed givens
1682 reduceContext env wanteds
1683 = do { traceTc (text "reduceContext" <+> (vcat [
1684 text "----------------------",
1686 text "given" <+> ppr (red_givens env),
1687 text "wanted" <+> ppr wanteds,
1688 text "----------------------"
1691 ; let givens = red_givens env
1692 (given_eqs0, given_dicts0) = partition isEqInst givens
1693 (wanted_eqs0, wanted_dicts) = partition isEqInst wanteds
1695 -- We want to add as wanted equalities those that (transitively)
1696 -- occur in superclass contexts of wanted class constraints.
1697 -- See Note [Ancestor Equalities]
1698 ; ancestor_eqs <- ancestorEqualities wanted_dicts
1699 ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
1700 ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
1702 -- 1. Normalise the *given* *equality* constraints
1703 ; (given_eqs, eliminate_skolems) <- normaliseGivens given_eqs0
1705 -- 2. Normalise the *given* *dictionary* constraints
1706 -- wrt. the toplevel and given equations
1707 ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
1710 -- 3. Solve the *wanted* *equation* constraints
1711 ; eq_irreds0 <- solveWanteds given_eqs wanted_eqs
1713 -- 4. Normalise the *wanted* equality constraints with respect to
1715 ; eq_irreds <- normaliseWanteds eq_irreds0
1717 -- 5. Build the Avail mapping from "given_dicts"
1718 ; init_state <- foldlM addGiven emptyAvails given_dicts
1720 -- 6. Solve the *wanted* *dictionary* constraints
1721 -- This may expose some further equational constraints...
1722 ; wanted_dicts' <- zonkInsts wanted_dicts
1723 ; avails <- reduceList env wanted_dicts' init_state
1724 ; (binds, irreds0, needed_givens) <- extractResults avails wanted_dicts'
1725 ; traceTc $ text "reduceContext extractresults" <+> vcat
1726 [ppr avails,ppr wanted_dicts',ppr binds,ppr needed_givens]
1728 -- 7. Normalise the *wanted* *dictionary* constraints
1729 -- wrt. the toplevel and given equations
1730 ; (irreds1,normalise_binds1) <- normaliseWantedDicts given_eqs irreds0
1732 -- 8. Substitute the wanted *equations* in the wanted *dictionaries*
1733 ; (irreds,normalise_binds2) <- substEqInDictInsts eq_irreds irreds1
1735 -- 9. eliminate the artificial skolem constants introduced in 1.
1738 -- If there was some FD improvement,
1739 -- or new wanted equations have been exposed,
1740 -- we should have another go at solving.
1741 ; let improved = availsImproved avails
1742 || (not $ isEmptyBag normalise_binds1)
1743 || (not $ isEmptyBag normalise_binds2)
1744 || (any isEqInst irreds)
1746 ; traceTc (text "reduceContext end" <+> (vcat [
1747 text "----------------------",
1749 text "given" <+> ppr (red_givens env),
1750 text "wanted" <+> ppr wanteds,
1752 text "avails" <+> pprAvails avails,
1753 text "improved =" <+> ppr improved,
1754 text "irreds = " <+> ppr irreds,
1755 text "binds = " <+> ppr binds,
1756 text "needed givens = " <+> ppr needed_givens,
1757 text "----------------------"
1761 given_binds `unionBags` normalise_binds1
1762 `unionBags` normalise_binds2
1764 irreds ++ eq_irreds,
1768 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1769 tcImproveOne avails inst
1770 | not (isDict inst) = return False
1772 = do { inst_envs <- tcGetInstEnvs
1773 ; let eqns = improveOne (classInstances inst_envs)
1774 (dictPred inst, pprInstArising inst)
1775 [ (dictPred p, pprInstArising p)
1776 | p <- availsInsts avails, isDict p ]
1777 -- Avails has all the superclasses etc (good)
1778 -- It also has all the intermediates of the deduction (good)
1779 -- It does not have duplicates (good)
1780 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1781 -- so that improve will see them separate
1782 ; traceTc (text "improveOne" <+> ppr inst)
1785 unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))]
1786 -> TcM ImprovementDone
1787 unifyEqns [] = return False
1789 = do { traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns))
1793 unify ((qtvs, pairs), what1, what2)
1794 = addErrCtxtM (mkEqnMsg what1 what2) $
1795 tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1796 mapM_ (unif_pr tenv) pairs
1797 unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1799 pprEquationDoc (eqn, (p1,w1), (p2,w2)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
1801 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
1802 = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
1803 ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
1804 ; let msg = vcat [ptext SLIT("When using functional dependencies to combine"),
1805 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
1806 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
1807 ; return (tidy_env, msg) }
1810 The main context-reduction function is @reduce@. Here's its game plan.
1813 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
1814 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
1815 = do { dopts <- getDOpts
1818 dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n)
1819 2 (ifPprDebug (nest 2 (pprStack stk))))
1822 ; if n >= ctxtStkDepth dopts then
1823 failWithTc (reduceDepthErr n stk)
1827 go [] state = return state
1828 go (w:ws) state = do { traceTc (text "reduceList " <+> ppr (w:ws) <+> ppr state)
1829 ; state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
1832 -- Base case: we're done!
1833 reduce env wanted avails
1834 -- It's the same as an existing inst, or a superclass thereof
1835 | Just avail <- findAvail avails wanted
1836 = do { traceTc (text "reduce: found " <+> ppr wanted)
1841 = do { traceTc (text "reduce" <+> ppr avails <+> ppr wanted)
1842 ; case red_try_me env wanted of {
1843 Stop -> try_simple (addIrred NoSCs);
1844 -- See Note [No superclasses for Stop]
1846 ReduceMe want_scs -> do -- It should be reduced
1847 { (avails, lookup_result) <- reduceInst env avails wanted
1848 ; case lookup_result of
1849 NoInstance -> addIrred want_scs avails wanted
1850 -- Add it and its superclasses
1852 GenInst [] rhs -> addWanted want_scs avails wanted rhs []
1854 GenInst wanteds' rhs
1855 -> do { avails1 <- addIrred NoSCs avails wanted
1856 ; avails2 <- reduceList env wanteds' avails1
1857 ; addWanted want_scs avails2 wanted rhs wanteds' } }
1858 -- Temporarily do addIrred *before* the reduceList,
1859 -- which has the effect of adding the thing we are trying
1860 -- to prove to the database before trying to prove the things it
1861 -- needs. See note [RECURSIVE DICTIONARIES]
1862 -- NB: we must not do an addWanted before, because that adds the
1863 -- superclasses too, and that can lead to a spurious loop; see
1864 -- the examples in [SUPERCLASS-LOOP]
1865 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1868 -- First, see if the inst can be reduced to a constant in one step
1869 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
1870 -- Don't bother for implication constraints, which take real work
1871 try_simple do_this_otherwise
1872 = do { res <- lookupSimpleInst wanted
1874 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
1875 other -> do_this_otherwise avails wanted }
1879 Note [SUPERCLASS-LOOP 2]
1880 ~~~~~~~~~~~~~~~~~~~~~~~~
1881 But the above isn't enough. Suppose we are *given* d1:Ord a,
1882 and want to deduce (d2:C [a]) where
1884 class Ord a => C a where
1885 instance Ord [a] => C [a] where ...
1887 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
1888 superclasses of C [a] to avails. But we must not overwrite the binding
1889 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
1892 Here's another variant, immortalised in tcrun020
1893 class Monad m => C1 m
1894 class C1 m => C2 m x
1895 instance C2 Maybe Bool
1896 For the instance decl we need to build (C1 Maybe), and it's no good if
1897 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1898 before we search for C1 Maybe.
1900 Here's another example
1901 class Eq b => Foo a b
1902 instance Eq a => Foo [a] a
1906 we'll first deduce that it holds (via the instance decl). We must not
1907 then overwrite the Eq t constraint with a superclass selection!
1909 At first I had a gross hack, whereby I simply did not add superclass constraints
1910 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1911 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1912 I found a very obscure program (now tcrun021) in which improvement meant the
1913 simplifier got two bites a the cherry... so something seemed to be an Stop
1914 first time, but reducible next time.
1916 Now we implement the Right Solution, which is to check for loops directly
1917 when adding superclasses. It's a bit like the occurs check in unification.
1920 Note [RECURSIVE DICTIONARIES]
1921 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1923 data D r = ZeroD | SuccD (r (D r));
1925 instance (Eq (r (D r))) => Eq (D r) where
1926 ZeroD == ZeroD = True
1927 (SuccD a) == (SuccD b) = a == b
1930 equalDC :: D [] -> D [] -> Bool;
1933 We need to prove (Eq (D [])). Here's how we go:
1937 by instance decl, holds if
1941 by instance decl of Eq, holds if
1943 where d2 = dfEqList d3
1946 But now we can "tie the knot" to give
1952 and it'll even run! The trick is to put the thing we are trying to prove
1953 (in this case Eq (D []) into the database before trying to prove its
1954 contributing clauses.
1957 %************************************************************************
1959 Reducing a single constraint
1961 %************************************************************************
1964 ---------------------------------------------
1965 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
1966 reduceInst env avails (ImplicInst { tci_tyvars = tvs, tci_reft = reft, tci_loc = loc,
1967 tci_given = extra_givens, tci_wanted = wanteds })
1968 = reduceImplication env avails reft tvs extra_givens wanteds loc
1970 reduceInst env avails other_inst
1971 = do { result <- lookupSimpleInst other_inst
1972 ; return (avails, result) }
1975 Note [Equational Constraints in Implication Constraints]
1976 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1978 An equational constraint is of the form
1980 where Given and Wanted may contain both equational and dictionary
1981 constraints. The delay and reduction of these two kinds of constraints
1984 -) In the generated code, wanted Dictionary constraints are wrapped up in an
1985 implication constraint that is created at the code site where the wanted
1986 dictionaries can be reduced via a let-binding. This let-bound implication
1987 constraint is deconstructed at the use-site of the wanted dictionaries.
1989 -) While the reduction of equational constraints is also delayed, the delay
1990 is not manifest in the generated code. The required evidence is generated
1991 in the code directly at the use-site. There is no let-binding and deconstruction
1992 necessary. The main disadvantage is that we cannot exploit sharing as the
1993 same evidence may be generated at multiple use-sites. However, this disadvantage
1994 is limited because it only concerns coercions which are erased.
1996 The different treatment is motivated by the different in representation. Dictionary
1997 constraints require manifest runtime dictionaries, while equations require coercions
2001 ---------------------------------------------
2002 reduceImplication :: RedEnv
2004 -> Refinement -- May refine the givens; often empty
2005 -> [TcTyVar] -- Quantified type variables; all skolems
2006 -> [Inst] -- Extra givens; all rigid
2009 -> TcM (Avails, LookupInstResult)
2012 Suppose we are simplifying the constraint
2013 forall bs. extras => wanted
2014 in the context of an overall simplification problem with givens 'givens',
2015 and refinment 'reft'.
2018 * The refinement is often empty
2020 * The 'extra givens' need not mention any of the quantified type variables
2021 e.g. forall {}. Eq a => Eq [a]
2022 forall {}. C Int => D (Tree Int)
2024 This happens when you have something like
2026 T1 :: Eq a => a -> T a
2029 f x = ...(case x of { T1 v -> v==v })...
2032 -- ToDo: should we instantiate tvs? I think it's not necessary
2034 -- Note on coercion variables:
2036 -- The extra given coercion variables are bound at two different sites:
2037 -- -) in the creation context of the implication constraint
2038 -- the solved equational constraints use these binders
2040 -- -) at the solving site of the implication constraint
2041 -- the solved dictionaries use these binders
2042 -- these binders are generated by reduceImplication
2044 reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc
2045 = do { -- Add refined givens, and the extra givens
2047 (refined_red_givens,refined_avails)
2048 <- if isEmptyRefinement reft then return (red_givens env,orig_avails)
2049 else foldlM (addRefinedGiven reft) ([],orig_avails) (red_givens env)
2051 -- Solve the sub-problem
2052 ; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
2053 env' = env { red_givens = refined_red_givens ++ extra_givens ++ availsInsts orig_avails
2054 , red_try_me = try_me }
2056 ; traceTc (text "reduceImplication" <+> vcat
2058 ppr (red_givens env), ppr extra_givens,
2059 ppr reft, ppr wanteds])
2060 ; (irreds,binds,needed_givens0) <- checkLoop env' wanteds
2061 ; let needed_givens1 = [ng | ng <- needed_givens0, notElem ng extra_givens]
2063 -- Note [Reducing implication constraints]
2064 -- Tom -- update note, put somewhere!
2066 ; traceTc (text "reduceImplication result" <+> vcat
2067 [ppr irreds, ppr binds, ppr needed_givens1])
2068 -- ; avails <- reduceList env' wanteds avails
2070 -- -- Extract the binding
2071 -- ; (binds, irreds) <- extractResults avails wanteds
2072 ; (refinement_binds,needed_givens) <- extractLocalResults refined_avails needed_givens1
2073 ; traceTc (text "reduceImplication local results" <+> vcat
2074 [ppr refinement_binds, ppr needed_givens])
2076 ; -- extract superclass binds
2077 -- (sc_binds,_) <- extractResults avails []
2078 -- ; traceTc (text "reduceImplication sc_binds" <+> vcat
2079 -- [ppr sc_binds, ppr avails])
2082 -- We always discard the extra avails we've generated;
2083 -- but we remember if we have done any (global) improvement
2084 -- ; let ret_avails = avails
2085 ; let ret_avails = orig_avails
2086 -- ; let ret_avails = updateImprovement orig_avails avails
2088 ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
2090 -- Porgress is no longer measered by the number of bindings
2091 -- ; if isEmptyLHsBinds binds then -- No progress
2092 ; if (isEmptyLHsBinds binds) && (not $ null irreds) then
2093 return (ret_avails, NoInstance)
2096 ; (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds
2097 -- This binding is useless if the recursive simplification
2098 -- made no progress; but currently we don't try to optimise that
2099 -- case. After all, we only try hard to reduce at top level, or
2100 -- when inferring types.
2102 ; let dict_wanteds = filter (not . isEqInst) wanteds
2103 (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
2104 dict_ids = map instToId extra_dict_givens
2105 -- TOMDO: given equational constraints bug!
2106 -- we need a different evidence for given
2107 -- equations depending on whether we solve
2108 -- dictionary constraints or equational constraints
2109 eq_tyvars = uniqSetToList $ tyVarsOfTypes $ map eqInstType extra_eq_givens
2110 -- dict_ids = map instToId extra_givens
2111 co = mkWpTyLams tvs <.> mkWpTyLams eq_tyvars <.> mkWpLams dict_ids <.> WpLet (binds `unionBags` refinement_binds `unionBags` bind)
2112 rhs = mkHsWrap co payload
2113 loc = instLocSpan inst_loc
2114 payload | [dict_wanted] <- dict_wanteds = HsVar (instToId dict_wanted)
2115 | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) dict_wanteds) Boxed
2118 ; traceTc (text "reduceImplication ->" <+> vcat
2121 -- If there are any irreds, we back off and return NoInstance
2122 ; return (ret_avails, GenInst (implic_insts ++ needed_givens) (L loc rhs))
2127 Note [Reducing implication constraints]
2128 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2129 Suppose we are trying to simplify
2130 (Ord a, forall b. C a b => (W [a] b, D c b))
2132 instance (C a b, Ord a) => W [a] b
2133 When solving the implication constraint, we'll start with
2135 in the Avails. Then we add (C a b -> Given) and solve. Extracting
2136 the results gives us a binding for the (W [a] b), with an Irred of
2137 (Ord a, D c b). Now, the (Ord a) comes from "outside" the implication,
2138 but the (D d b) is from "inside". So we want to generate a Rhs binding
2141 ic = /\b \dc:C a b). (df a b dc do, ic' b dc)
2144 ic' :: forall b. C a b => D c b
2146 The 'depending on' part of the Rhs is important, because it drives
2147 the extractResults code.
2149 The "inside" and "outside" distinction is what's going on with 'inner' and
2150 'outer' in reduceImplication
2153 Note [Freeness and implications]
2154 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2155 It's hard to say when an implication constraint can be floated out. Consider
2156 forall {} Eq a => Foo [a]
2157 The (Foo [a]) doesn't mention any of the quantified variables, but it
2158 still might be partially satisfied by the (Eq a).
2160 There is a useful special case when it *is* easy to partition the
2161 constraints, namely when there are no 'givens'. Consider
2162 forall {a}. () => Bar b
2163 There are no 'givens', and so there is no reason to capture (Bar b).
2164 We can let it float out. But if there is even one constraint we
2165 must be much more careful:
2166 forall {a}. C a b => Bar (m b)
2167 because (C a b) might have a superclass (D b), from which we might
2168 deduce (Bar [b]) when m later gets instantiated to []. Ha!
2170 Here is an even more exotic example
2172 Now consider the constraint
2173 forall b. D Int b => C Int
2174 We can satisfy the (C Int) from the superclass of D, so we don't want
2175 to float the (C Int) out, even though it mentions no type variable in
2178 Note [Pruning the givens in an implication constraint]
2179 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2180 Suppose we are about to form the implication constraint
2181 forall tvs. Eq a => Ord b
2182 The (Eq a) cannot contribute to the (Ord b), because it has no access to
2183 the type variable 'b'. So we could filter out the (Eq a) from the givens.
2185 Doing so would be a bit tidier, but all the implication constraints get
2186 simplified away by the optimiser, so it's no great win. So I don't take
2187 advantage of that at the moment.
2189 If you do, BE CAREFUL of wobbly type variables.
2192 %************************************************************************
2194 Avails and AvailHow: the pool of evidence
2196 %************************************************************************
2200 data Avails = Avails !ImprovementDone !AvailEnv
2202 type ImprovementDone = Bool -- True <=> some unification has happened
2203 -- so some Irreds might now be reducible
2204 -- keys that are now
2206 type AvailEnv = FiniteMap Inst AvailHow
2208 = IsIrred -- Used for irreducible dictionaries,
2209 -- which are going to be lambda bound
2211 | Given TcId -- Used for dictionaries for which we have a binding
2212 -- e.g. those "given" in a signature
2214 | Rhs -- Used when there is a RHS
2215 (LHsExpr TcId) -- The RHS
2216 [Inst] -- Insts free in the RHS; we need these too
2218 instance Outputable Avails where
2221 pprAvails (Avails imp avails)
2222 = vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty)
2223 , nest 2 (vcat [sep [ppr inst, nest 2 (equals <+> ppr avail)]
2224 | (inst,avail) <- fmToList avails ])]
2226 instance Outputable AvailHow where
2229 -------------------------
2230 pprAvail :: AvailHow -> SDoc
2231 pprAvail IsIrred = text "Irred"
2232 pprAvail (Given x) = text "Given" <+> ppr x
2233 pprAvail (Rhs rhs bs) = text "Rhs" <+> sep [ppr rhs, braces (ppr bs)]
2235 -------------------------
2236 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
2237 extendAvailEnv env inst avail = addToFM env inst avail
2239 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
2240 findAvailEnv env wanted = lookupFM env wanted
2241 -- NB 1: the Ord instance of Inst compares by the class/type info
2242 -- *not* by unique. So
2243 -- d1::C Int == d2::C Int
2245 emptyAvails :: Avails
2246 emptyAvails = Avails False emptyFM
2248 findAvail :: Avails -> Inst -> Maybe AvailHow
2249 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
2251 elemAvails :: Inst -> Avails -> Bool
2252 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
2254 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
2256 extendAvails avails@(Avails imp env) inst avail
2257 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
2258 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
2260 availsInsts :: Avails -> [Inst]
2261 availsInsts (Avails _ avails) = keysFM avails
2263 availsImproved (Avails imp _) = imp
2265 updateImprovement :: Avails -> Avails -> Avails
2266 -- (updateImprovement a1 a2) sets a1's improvement flag from a2
2267 updateImprovement (Avails _ avails1) (Avails imp2 _) = Avails imp2 avails1
2270 Extracting the bindings from a bunch of Avails.
2271 The bindings do *not* come back sorted in dependency order.
2272 We assume that they'll be wrapped in a big Rec, so that the
2273 dependency analyser can sort them out later
2276 extractResults :: Avails
2278 -> TcM ( TcDictBinds, -- Bindings
2279 [Inst], -- Irreducible ones
2280 [Inst]) -- Needed givens, i.e. ones used in the bindings
2282 extractResults (Avails _ avails) wanteds
2283 = go avails emptyBag [] [] wanteds
2285 go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst] -> [Inst]
2286 -> TcM (TcDictBinds, [Inst], [Inst])
2287 go avails binds irreds givens []
2288 = returnM (binds, irreds, givens)
2290 go avails binds irreds givens (w:ws)
2291 = case findAvailEnv avails w of
2292 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
2293 go avails binds irreds givens ws
2296 | id == w_id -> go avails binds irreds (w:givens) ws
2297 | otherwise -> go avails (addBind binds w (nlHsVar id)) irreds (update_id w id:givens) ws
2298 -- The sought Id can be one of the givens, via a superclass chain
2299 -- and then we definitely don't want to generate an x=x binding!
2301 Just IsIrred -> go (add_given avails w) binds (w:irreds) givens ws
2302 -- The add_given handles the case where we want (Ord a, Eq a), and we
2303 -- don't want to emit *two* Irreds for Ord a, one via the superclass chain
2304 -- This showed up in a dupliated Ord constraint in the error message for
2307 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds givens (ws' ++ ws)
2309 new_binds = addBind binds w rhs
2312 update_id m@(Method{}) id = m {tci_id = id}
2313 update_id w id = w {tci_name = idName id}
2315 add_given avails w = extendAvailEnv avails w (Given (instToId w))
2317 extractLocalResults :: Avails
2319 -> TcM ( TcDictBinds, -- Bindings
2320 [Inst]) -- Needed givens, i.e. ones used in the bindings
2322 extractLocalResults (Avails _ avails) wanteds
2323 = go avails emptyBag [] wanteds
2325 go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst]
2326 -> TcM (TcDictBinds, [Inst])
2327 go avails binds givens []
2328 = returnM (binds, givens)
2330 go avails binds givens (w:ws)
2331 = case findAvailEnv avails w of
2332 Nothing -> -- pprTrace "Urk: extractLocalResults" (ppr w) $
2333 go avails binds givens ws
2336 go avails binds givens ws
2339 | id == w_id -> go avails binds (w:givens) ws
2340 | otherwise -> go avails binds (w{tci_name=idName id}:givens) ws
2341 -- The sought Id can be one of the givens, via a superclass chain
2342 -- and then we definitely don't want to generate an x=x binding!
2344 Just (Rhs rhs ws') -> go (add_given avails w) new_binds givens (ws' ++ ws)
2346 new_binds = addBind binds w rhs
2350 add_given avails w = extendAvailEnv avails w (Given (instToId w))
2354 Note [No superclasses for Stop]
2355 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2356 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2357 add it to avails, so that any other equal Insts will be commoned up
2358 right here. However, we do *not* add superclasses. If we have
2361 but a is not bound here, then we *don't* want to derive dn from df
2362 here lest we lose sharing.
2365 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2366 addWanted want_scs avails wanted rhs_expr wanteds
2367 = addAvailAndSCs want_scs avails wanted avail
2369 avail = Rhs rhs_expr wanteds
2371 addGiven :: Avails -> Inst -> TcM Avails
2372 addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given))
2373 -- Always add superclasses for 'givens'
2375 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2376 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2377 -- so the assert isn't true
2379 addRefinedGiven :: Refinement -> ([Inst], Avails) -> Inst -> TcM ([Inst], Avails)
2380 addRefinedGiven reft (refined_givens, avails) given
2381 | isDict given -- We sometimes have 'given' methods, but they
2382 -- are always optional, so we can drop them
2383 , let pred = dictPred given
2384 , isRefineablePred pred -- See Note [ImplicInst rigidity]
2385 , Just (co, pred) <- refinePred reft pred
2386 = do { new_given <- newDictBndr (instLoc given) pred
2387 ; let rhs = L (instSpan given) $
2388 HsWrap (WpCo co) (HsVar (instToId given))
2389 ; avails <- addAvailAndSCs AddSCs avails new_given (Rhs rhs [given])
2390 ; return (new_given:refined_givens, avails) }
2391 -- ToDo: the superclasses of the original given all exist in Avails
2392 -- so we could really just cast them, but it's more awkward to do,
2393 -- and hopefully the optimiser will spot the duplicated work
2395 = return (refined_givens, avails)
2398 Note [ImplicInst rigidity]
2399 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2401 C :: forall ab. (Eq a, Ord b) => b -> T a
2403 ...(case x of C v -> <body>)...
2405 From the case (where x::T ty) we'll get an implication constraint
2406 forall b. (Eq ty, Ord b) => <body-constraints>
2407 Now suppose <body-constraints> itself has an implication constraint
2409 forall c. <reft> => <payload>
2410 Then, we can certainly apply the refinement <reft> to the Ord b, becuase it is
2411 existential, but we probably should not apply it to the (Eq ty) because it may
2412 be wobbly. Hence the isRigidInst
2414 @Insts@ are ordered by their class/type info, rather than by their
2415 unique. This allows the context-reduction mechanism to use standard finite
2416 maps to do their stuff. It's horrible that this code is here, rather
2417 than with the Avails handling stuff in TcSimplify
2420 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2421 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2422 addAvailAndSCs want_scs avails irred IsIrred
2424 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2425 addAvailAndSCs want_scs avails inst avail
2426 | not (isClassDict inst) = extendAvails avails inst avail
2427 | NoSCs <- want_scs = extendAvails avails inst avail
2428 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2429 ; avails' <- extendAvails avails inst avail
2430 ; addSCs is_loop avails' inst }
2432 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2433 -- Note: this compares by *type*, not by Unique
2434 deps = findAllDeps (unitVarSet (instToVar inst)) avail
2435 dep_tys = map idType (varSetElems deps)
2437 findAllDeps :: IdSet -> AvailHow -> IdSet
2438 -- Find all the Insts that this one depends on
2439 -- See Note [SUPERCLASS-LOOP 2]
2440 -- Watch out, though. Since the avails may contain loops
2441 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2442 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2443 findAllDeps so_far other = so_far
2445 find_all :: IdSet -> Inst -> IdSet
2447 | kid_id `elemVarSet` so_far = so_far
2448 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2449 | otherwise = so_far'
2451 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2452 kid_id = instToId kid
2454 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2455 -- Add all the superclasses of the Inst to Avails
2456 -- The first param says "dont do this because the original thing
2457 -- depends on this one, so you'd build a loop"
2458 -- Invariant: the Inst is already in Avails.
2460 addSCs is_loop avails dict
2461 = ASSERT( isDict dict )
2462 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2463 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2465 (clas, tys) = getDictClassTys dict
2466 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2467 sc_theta' = filter (not . isEqPred) $
2468 substTheta (zipTopTvSubst tyvars tys) sc_theta
2470 add_sc avails (sc_dict, sc_sel)
2471 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2472 | is_given sc_dict = return avails
2473 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2474 ; addSCs is_loop avails' sc_dict }
2476 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2477 co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
2479 is_given :: Inst -> Bool
2480 is_given sc_dict = case findAvail avails sc_dict of
2481 Just (Given _) -> True -- Given is cheaper than superclass selection
2484 -- From the a set of insts obtain all equalities that (transitively) occur in
2485 -- superclass contexts of class constraints (aka the ancestor equalities).
2487 ancestorEqualities :: [Inst] -> TcM [Inst]
2489 = mapM mkWantedEqInst -- turn only equality predicates..
2490 . filter isEqPred -- ..into wanted equality insts
2492 . addAEsToBag emptyBag -- collect the superclass constraints..
2493 . map dictPred -- ..of all predicates in a bag
2494 . filter isClassDict
2496 addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
2497 addAEsToBag bag [] = bag
2498 addAEsToBag bag (pred:preds)
2499 | pred `elemBag` bag = addAEsToBag bag preds
2500 | isEqPred pred = addAEsToBag bagWithPred preds
2501 | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
2502 | otherwise = addAEsToBag bag preds
2504 bagWithPred = bag `snocBag` pred
2505 predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
2507 (tyvars, sc_theta, _, _) = classBigSig clas
2508 (clas, tys) = getClassPredTys pred
2512 %************************************************************************
2514 \section{tcSimplifyTop: defaulting}
2516 %************************************************************************
2519 @tcSimplifyTop@ is called once per module to simplify all the constant
2520 and ambiguous Insts.
2522 We need to be careful of one case. Suppose we have
2524 instance Num a => Num (Foo a b) where ...
2526 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2527 to (Num x), and default x to Int. But what about y??
2529 It's OK: the final zonking stage should zap y to (), which is fine.
2533 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2534 tcSimplifyTop wanteds
2535 = tc_simplify_top doc False wanteds
2537 doc = text "tcSimplifyTop"
2539 tcSimplifyInteractive wanteds
2540 = tc_simplify_top doc True wanteds
2542 doc = text "tcSimplifyInteractive"
2544 -- The TcLclEnv should be valid here, solely to improve
2545 -- error message generation for the monomorphism restriction
2546 tc_simplify_top doc interactive wanteds
2547 = do { dflags <- getDOpts
2548 ; wanteds <- zonkInsts wanteds
2549 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2551 ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
2552 ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
2553 ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
2554 ; (irreds2, binds2) <- approximateImplications doc2 (\d -> True) irreds1
2555 ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
2557 -- Use the defaulting rules to do extra unification
2558 -- NB: irreds2 are already zonked
2559 ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
2561 -- Deal with implicit parameters
2562 ; let (bad_ips, non_ips) = partition isIPDict irreds3
2563 (ambigs, others) = partition isTyVarDict non_ips
2565 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2567 ; addNoInstanceErrs others
2568 ; addTopAmbigErrs ambigs
2570 ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
2572 doc1 = doc <+> ptext SLIT("(first round)")
2573 doc2 = doc <+> ptext SLIT("(approximate)")
2574 doc3 = doc <+> ptext SLIT("(disambiguate)")
2577 If a dictionary constrains a type variable which is
2578 * not mentioned in the environment
2579 * and not mentioned in the type of the expression
2580 then it is ambiguous. No further information will arise to instantiate
2581 the type variable; nor will it be generalised and turned into an extra
2582 parameter to a function.
2584 It is an error for this to occur, except that Haskell provided for
2585 certain rules to be applied in the special case of numeric types.
2587 * at least one of its classes is a numeric class, and
2588 * all of its classes are numeric or standard
2589 then the type variable can be defaulted to the first type in the
2590 default-type list which is an instance of all the offending classes.
2592 So here is the function which does the work. It takes the ambiguous
2593 dictionaries and either resolves them (producing bindings) or
2594 complains. It works by splitting the dictionary list by type
2595 variable, and using @disambigOne@ to do the real business.
2597 @disambigOne@ assumes that its arguments dictionaries constrain all
2598 the same type variable.
2600 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2601 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2602 the most common use of defaulting is code like:
2604 _ccall_ foo `seqPrimIO` bar
2606 Since we're not using the result of @foo@, the result if (presumably)
2610 disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
2611 -- Just does unification to fix the default types
2612 -- The Insts are assumed to be pre-zonked
2613 disambiguate doc interactive dflags insts
2615 = return (insts, emptyBag)
2617 | null defaultable_groups
2618 = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
2619 ; return (insts, emptyBag) }
2622 = do { -- Figure out what default types to use
2623 default_tys <- getDefaultTys extended_defaulting ovl_strings
2625 ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2626 ; mapM_ (disambigGroup default_tys) defaultable_groups
2628 -- disambigGroup does unification, hence try again
2629 ; tryHardCheckLoop doc insts }
2632 extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
2633 ovl_strings = dopt Opt_OverloadedStrings dflags
2635 unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
2636 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2637 (unaries, bad_tvs_s) = partitionWith find_unary insts
2638 bad_tvs = unionVarSets bad_tvs_s
2640 -- Finds unary type-class constraints
2641 find_unary d@(Dict {tci_pred = ClassP cls [ty]})
2642 | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
2643 find_unary inst = Right (tyVarsOfInst inst)
2645 -- Group by type variable
2646 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2647 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2648 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2650 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2651 defaultable_group ds@((_,_,tv):_)
2652 = isTyConableTyVar tv -- Note [Avoiding spurious errors]
2653 && not (tv `elemVarSet` bad_tvs)
2654 && defaultable_classes [c | (_,c,_) <- ds]
2655 defaultable_group [] = panic "defaultable_group"
2657 defaultable_classes clss
2658 | extended_defaulting = any isInteractiveClass clss
2659 | otherwise = all is_std_class clss && (any is_num_class clss)
2661 -- In interactive mode, or with -fextended-default-rules,
2662 -- we default Show a to Show () to avoid graututious errors on "show []"
2663 isInteractiveClass cls
2664 = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2666 is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2667 -- is_num_class adds IsString to the standard numeric classes,
2668 -- when -foverloaded-strings is enabled
2670 is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2671 -- Similarly is_std_class
2673 -----------------------
2674 disambigGroup :: [Type] -- The default types
2675 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2676 -> TcM () -- Just does unification, to fix the default types
2678 disambigGroup default_tys dicts
2679 = try_default default_tys
2681 (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
2682 classes = [c | (_,c,_) <- dicts]
2684 try_default [] = return ()
2685 try_default (default_ty : default_tys)
2686 = tryTcLIE_ (try_default default_tys) $
2687 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2688 -- This may fail; then the tryTcLIE_ kicks in
2689 -- Failure here is caused by there being no type in the
2690 -- default list which can satisfy all the ambiguous classes.
2691 -- For example, if Real a is reqd, but the only type in the
2692 -- default list is Int.
2694 -- After this we can't fail
2695 ; warnDefault dicts default_ty
2696 ; unifyType default_ty (mkTyVarTy tyvar)
2697 ; return () -- TOMDO: do something with the coercion
2701 -----------------------
2702 getDefaultTys :: Bool -> Bool -> TcM [Type]
2703 getDefaultTys extended_deflts ovl_strings
2704 = do { mb_defaults <- getDeclaredDefaultTys
2705 ; case mb_defaults of {
2706 Just tys -> return tys ; -- User-supplied defaults
2709 -- No use-supplied default
2710 -- Use [Integer, Double], plus modifications
2711 { integer_ty <- tcMetaTy integerTyConName
2712 ; checkWiredInTyCon doubleTyCon
2713 ; string_ty <- tcMetaTy stringTyConName
2714 ; return (opt_deflt extended_deflts unitTy
2715 -- Note [Default unitTy]
2717 [integer_ty,doubleTy]
2719 opt_deflt ovl_strings string_ty) } } }
2721 opt_deflt True ty = [ty]
2722 opt_deflt False ty = []
2725 Note [Default unitTy]
2726 ~~~~~~~~~~~~~~~~~~~~~
2727 In interative mode (or with -fextended-default-rules) we add () as the first type we
2728 try when defaulting. This has very little real impact, except in the following case.
2730 Text.Printf.printf "hello"
2731 This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
2732 want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
2733 default the 'a' to (), rather than to Integer (which is what would otherwise happen;
2734 and then GHCi doesn't attempt to print the (). So in interactive mode, we add
2735 () to the list of defaulting types. See Trac #1200.
2737 Note [Avoiding spurious errors]
2738 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2739 When doing the unification for defaulting, we check for skolem
2740 type variables, and simply don't default them. For example:
2741 f = (*) -- Monomorphic
2742 g :: Num a => a -> a
2744 Here, we get a complaint when checking the type signature for g,
2745 that g isn't polymorphic enough; but then we get another one when
2746 dealing with the (Num a) context arising from f's definition;
2747 we try to unify a with Int (to default it), but find that it's
2748 already been unified with the rigid variable from g's type sig
2751 %************************************************************************
2753 \subsection[simple]{@Simple@ versions}
2755 %************************************************************************
2757 Much simpler versions when there are no bindings to make!
2759 @tcSimplifyThetas@ simplifies class-type constraints formed by
2760 @deriving@ declarations and when specialising instances. We are
2761 only interested in the simplified bunch of class/type constraints.
2763 It simplifies to constraints of the form (C a b c) where
2764 a,b,c are type variables. This is required for the context of
2765 instance declarations.
2768 tcSimplifyDeriv :: InstOrigin
2770 -> ThetaType -- Wanted
2771 -> TcM ThetaType -- Needed
2772 -- Given instance (wanted) => C inst_ty
2773 -- Simplify 'wanted' as much as possible
2775 tcSimplifyDeriv orig tyvars theta
2776 = do { (tvs, _, tenv) <- tcInstTyVars tyvars
2777 -- The main loop may do unification, and that may crash if
2778 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2779 -- ToDo: what if two of them do get unified?
2780 ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
2781 ; (irreds, _) <- tryHardCheckLoop doc wanteds
2783 ; let (tv_dicts, others) = partition ok irreds
2784 ; addNoInstanceErrs others
2785 -- See Note [Exotic derived instance contexts] in TcMType
2787 ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2788 simpl_theta = substTheta rev_env (map dictPred tv_dicts)
2789 -- This reverse-mapping is a pain, but the result
2790 -- should mention the original TyVars not TcTyVars
2792 ; return simpl_theta }
2794 doc = ptext SLIT("deriving classes for a data type")
2796 ok dict | isDict dict = validDerivPred (dictPred dict)
2801 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2802 used with \tr{default} declarations. We are only interested in
2803 whether it worked or not.
2806 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2809 tcSimplifyDefault theta
2810 = newDictBndrsO DefaultOrigin theta `thenM` \ wanteds ->
2811 tryHardCheckLoop doc wanteds `thenM` \ (irreds, _) ->
2812 addNoInstanceErrs irreds `thenM_`
2818 doc = ptext SLIT("default declaration")
2822 %************************************************************************
2824 \section{Errors and contexts}
2826 %************************************************************************
2828 ToDo: for these error messages, should we note the location as coming
2829 from the insts, or just whatever seems to be around in the monad just
2833 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2834 -> [Inst] -- The offending Insts
2836 -- Group together insts with the same origin
2837 -- We want to report them together in error messages
2839 groupErrs report_err []
2841 groupErrs report_err (inst:insts)
2842 = do_one (inst:friends) `thenM_`
2843 groupErrs report_err others
2846 -- (It may seem a bit crude to compare the error messages,
2847 -- but it makes sure that we combine just what the user sees,
2848 -- and it avoids need equality on InstLocs.)
2849 (friends, others) = partition is_friend insts
2850 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2851 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2852 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2853 -- Add location and context information derived from the Insts
2855 -- Add the "arising from..." part to a message about bunch of dicts
2856 addInstLoc :: [Inst] -> Message -> Message
2857 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
2859 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
2860 addTopIPErrs bndrs []
2862 addTopIPErrs bndrs ips
2863 = do { dflags <- getDOpts
2864 ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
2866 (tidy_env, tidy_ips) = tidyInsts ips
2868 = vcat [sep [ptext SLIT("Implicit parameters escape from"),
2869 nest 2 (ptext SLIT("the monomorphic top-level binding")
2870 <> plural bndrs <+> ptext SLIT("of")
2871 <+> pprBinders bndrs <> colon)],
2872 nest 2 (vcat (map ppr_ip ips)),
2873 monomorphism_fix dflags]
2874 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
2876 topIPErrs :: [Inst] -> TcM ()
2878 = groupErrs report tidy_dicts
2880 (tidy_env, tidy_dicts) = tidyInsts dicts
2881 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2882 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2883 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2885 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
2887 addNoInstanceErrs insts
2888 = do { let (tidy_env, tidy_insts) = tidyInsts insts
2889 ; reportNoInstances tidy_env Nothing tidy_insts }
2893 -> Maybe (InstLoc, [Inst]) -- Context
2894 -- Nothing => top level
2895 -- Just (d,g) => d describes the construct
2897 -> [Inst] -- What is wanted (can include implications)
2900 reportNoInstances tidy_env mb_what insts
2901 = groupErrs (report_no_instances tidy_env mb_what) insts
2903 report_no_instances tidy_env mb_what insts
2904 = do { inst_envs <- tcGetInstEnvs
2905 ; let (implics, insts1) = partition isImplicInst insts
2906 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
2907 (eqInsts, insts3) = partition isEqInst insts2
2908 ; traceTc (text "reportNoInstances" <+> vcat
2909 [ppr implics, ppr insts1, ppr insts2])
2910 ; mapM_ complain_implic implics
2911 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
2912 ; groupErrs complain_no_inst insts3
2913 ; mapM_ complain_eq eqInsts
2916 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
2918 complain_implic inst -- Recurse!
2919 = reportNoInstances tidy_env
2920 (Just (tci_loc inst, tci_given inst))
2923 complain_eq EqInst {tci_left = lty, tci_right = rty,
2924 tci_loc = InstLoc _ _ ctxt}
2925 = do { (env, msg) <- misMatchMsg lty rty
2927 failWithTcM (env, msg)
2930 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
2931 -- Right msg => overlap message
2932 -- Left inst => no instance
2933 check_overlap inst_envs wanted
2934 | not (isClassDict wanted) = Left wanted
2936 = case lookupInstEnv inst_envs clas tys of
2937 -- The case of exactly one match and no unifiers means a
2938 -- successful lookup. That can't happen here, because dicts
2939 -- only end up here if they didn't match in Inst.lookupInst
2941 ([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
2943 ([], _) -> Left wanted -- No match
2944 res -> Right (mk_overlap_msg wanted res)
2946 (clas,tys) = getDictClassTys wanted
2948 mk_overlap_msg dict (matches, unifiers)
2949 = ASSERT( not (null matches) )
2950 vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
2951 <+> pprPred (dictPred dict))),
2952 sep [ptext SLIT("Matching instances") <> colon,
2953 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
2954 if not (isSingleton matches)
2955 then -- Two or more matches
2957 else -- One match, plus some unifiers
2958 ASSERT( not (null unifiers) )
2959 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
2960 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
2961 ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
2962 ptext SLIT("when compiling the other instance declarations")])]
2964 ispecs = [ispec | (ispec, _) <- matches]
2966 mk_no_inst_err insts
2967 | null insts = empty
2969 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
2970 not (isEmptyVarSet (tyVarsOfInsts insts))
2971 = vcat [ addInstLoc insts $
2972 sep [ ptext SLIT("Could not deduce") <+> pprDictsTheta insts
2973 , nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta givens]
2974 , show_fixes (fix1 loc : fixes2) ]
2976 | otherwise -- Top level
2977 = vcat [ addInstLoc insts $
2978 ptext SLIT("No instance") <> plural insts
2979 <+> ptext SLIT("for") <+> pprDictsTheta insts
2980 , show_fixes fixes2 ]
2983 fix1 loc = sep [ ptext SLIT("add") <+> pprDictsTheta insts
2984 <+> ptext SLIT("to the context of"),
2985 nest 2 (ppr (instLocOrigin loc)) ]
2986 -- I'm not sure it helps to add the location
2987 -- nest 2 (ptext SLIT("at") <+> ppr (instLocSpan loc)) ]
2989 fixes2 | null instance_dicts = []
2990 | otherwise = [sep [ptext SLIT("add an instance declaration for"),
2991 pprDictsTheta instance_dicts]]
2992 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
2993 -- Insts for which it is worth suggesting an adding an instance declaration
2994 -- Exclude implicit parameters, and tyvar dicts
2996 show_fixes :: [SDoc] -> SDoc
2997 show_fixes [] = empty
2998 show_fixes (f:fs) = sep [ptext SLIT("Possible fix:"),
2999 nest 2 (vcat (f : map (ptext SLIT("or") <+>) fs))]
3001 addTopAmbigErrs dicts
3002 -- Divide into groups that share a common set of ambiguous tyvars
3003 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
3004 -- See Note [Avoiding spurious errors]
3005 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
3007 (tidy_env, tidy_dicts) = tidyInsts dicts
3009 tvs_of :: Inst -> [TcTyVar]
3010 tvs_of d = varSetElems (tyVarsOfInst d)
3011 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
3013 report :: [(Inst,[TcTyVar])] -> TcM ()
3014 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
3015 = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) ->
3016 setSrcSpan (instSpan inst) $
3017 -- the location of the first one will do for the err message
3018 addErrTcM (tidy_env, msg $$ mono_msg)
3020 dicts = map fst pairs
3021 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
3022 pprQuotedList tvs <+> in_msg,
3023 nest 2 (pprDictsInFull dicts)]
3024 in_msg = text "in the constraint" <> plural dicts <> colon
3025 report [] = panic "addTopAmbigErrs"
3028 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
3029 -- There's an error with these Insts; if they have free type variables
3030 -- it's probably caused by the monomorphism restriction.
3031 -- Try to identify the offending variable
3032 -- ASSUMPTION: the Insts are fully zonked
3033 mkMonomorphismMsg tidy_env inst_tvs
3034 = do { dflags <- getDOpts
3035 ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
3036 ; return (tidy_env, mk_msg dflags docs) }
3038 mk_msg _ _ | any isRuntimeUnk inst_tvs
3039 = vcat [ptext SLIT("Cannot resolve unknown runtime types:") <+>
3040 (pprWithCommas ppr inst_tvs),
3041 ptext SLIT("Use :print or :force to determine these types")]
3042 mk_msg _ [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
3043 -- This happens in things like
3044 -- f x = show (read "foo")
3045 -- where monomorphism doesn't play any role
3047 = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
3049 monomorphism_fix dflags]
3051 isRuntimeUnk :: TcTyVar -> Bool
3052 isRuntimeUnk x | SkolemTv RuntimeUnkSkol <- tcTyVarDetails x = True
3055 monomorphism_fix :: DynFlags -> SDoc
3056 monomorphism_fix dflags
3057 = ptext SLIT("Probable fix:") <+> vcat
3058 [ptext SLIT("give these definition(s) an explicit type signature"),
3059 if dopt Opt_MonomorphismRestriction dflags
3060 then ptext SLIT("or use -fno-monomorphism-restriction")
3061 else empty] -- Only suggest adding "-fno-monomorphism-restriction"
3062 -- if it is not already set!
3064 warnDefault ups default_ty
3065 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
3066 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
3068 dicts = [d | (d,_,_) <- ups]
3071 (_, tidy_dicts) = tidyInsts dicts
3072 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
3073 quotes (ppr default_ty),
3074 pprDictsInFull tidy_dicts]
3076 reduceDepthErr n stack
3077 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
3078 ptext SLIT("Use -fcontext-stack=N to increase stack size to N"),
3079 nest 4 (pprStack stack)]
3081 pprStack stack = vcat (map pprInstInFull stack)
3083 -----------------------
3084 misMatchMsg :: TcType -> TcType -> TcM (TidyEnv, SDoc)
3085 -- Generate the message when two types fail to match,
3086 -- going to some trouble to make it helpful.
3087 -- The argument order is: actual type, expected type
3088 misMatchMsg ty_act ty_exp
3089 = do { env0 <- tcInitTidyEnv
3090 ; ty_exp <- zonkTcType ty_exp
3091 ; ty_act <- zonkTcType ty_act
3092 ; (env1, pp_exp, extra_exp) <- ppr_ty env0 ty_exp
3093 ; (env2, pp_act, extra_act) <- ppr_ty env1 ty_act
3095 sep [sep [ptext SLIT("Couldn't match expected type") <+> pp_exp,
3097 ptext SLIT("against inferred type") <+> pp_act],
3098 nest 2 (extra_exp $$ extra_act)]) }
3100 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
3102 = do { let (env1, tidy_ty) = tidyOpenType env ty
3103 ; (env2, extra) <- ppr_extra env1 tidy_ty
3104 ; return (env2, quotes (ppr tidy_ty), extra) }
3106 -- (ppr_extra env ty) shows extra info about 'ty'
3107 ppr_extra env (TyVarTy tv)
3108 | isSkolemTyVar tv || isSigTyVar tv
3109 = return (env1, pprSkolTvBinding tv1)
3111 (env1, tv1) = tidySkolemTyVar env tv
3113 ppr_extra env ty = return (env, empty) -- Normal case