2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
17 tcSimplifyInfer, tcSimplifyInferCheck,
18 tcSimplifyCheck, tcSimplifyRestricted,
19 tcSimplifyRuleLhs, tcSimplifyIPs,
20 tcSimplifySuperClasses,
21 tcSimplifyTop, tcSimplifyInteractive,
22 tcSimplifyBracket, tcSimplifyCheckPat,
24 tcSimplifyDeriv, tcSimplifyDefault,
30 #include "HsVersions.h"
32 import {-# SOURCE #-} TcUnify( unifyType )
73 %************************************************************************
77 %************************************************************************
79 --------------------------------------
80 Notes on functional dependencies (a bug)
81 --------------------------------------
88 instance D a b => C a b -- Undecidable
89 -- (Not sure if it's crucial to this eg)
90 f :: C a b => a -> Bool
93 g :: C a b => a -> Bool
96 Here f typechecks, but g does not!! Reason: before doing improvement,
97 we reduce the (C a b1) constraint from the call of f to (D a b1).
99 Here is a more complicated example:
101 | > class Foo a b | a->b
103 | > class Bar a b | a->b
107 | > instance Bar Obj Obj
109 | > instance (Bar a b) => Foo a b
111 | > foo:: (Foo a b) => a -> String
114 | > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
120 | Could not deduce (Bar a b) from the context (Foo a b)
121 | arising from use of `foo' at <interactive>:1
123 | Add (Bar a b) to the expected type of an expression
124 | In the first argument of `runFoo', namely `foo'
125 | In the definition of `it': it = runFoo foo
127 | Why all of the sudden does GHC need the constraint Bar a b? The
128 | function foo didn't ask for that...
130 The trouble is that to type (runFoo foo), GHC has to solve the problem:
132 Given constraint Foo a b
133 Solve constraint Foo a b'
135 Notice that b and b' aren't the same. To solve this, just do
136 improvement and then they are the same. But GHC currently does
141 That is usually fine, but it isn't here, because it sees that Foo a b is
142 not the same as Foo a b', and so instead applies the instance decl for
143 instance Bar a b => Foo a b. And that's where the Bar constraint comes
146 The Right Thing is to improve whenever the constraint set changes at
147 all. Not hard in principle, but it'll take a bit of fiddling to do.
149 Note [Choosing which variables to quantify]
150 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
151 Suppose we are about to do a generalisation step. We have in our hand
154 T the type of the RHS
155 C the constraints from that RHS
157 The game is to figure out
159 Q the set of type variables over which to quantify
160 Ct the constraints we will *not* quantify over
161 Cq the constraints we will quantify over
163 So we're going to infer the type
167 and float the constraints Ct further outwards.
169 Here are the things that *must* be true:
171 (A) Q intersect fv(G) = EMPTY limits how big Q can be
172 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
174 (A) says we can't quantify over a variable that's free in the environment.
175 (B) says we must quantify over all the truly free variables in T, else
176 we won't get a sufficiently general type.
178 We do not *need* to quantify over any variable that is fixed by the
179 free vars of the environment G.
181 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
183 Example: class H x y | x->y where ...
185 fv(G) = {a} C = {H a b, H c d}
188 (A) Q intersect {a} is empty
189 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
191 So Q can be {c,d}, {b,c,d}
193 In particular, it's perfectly OK to quantify over more type variables
194 than strictly necessary; there is no need to quantify over 'b', since
195 it is determined by 'a' which is free in the envt, but it's perfectly
196 OK to do so. However we must not quantify over 'a' itself.
198 Other things being equal, however, we'd like to quantify over as few
199 variables as possible: smaller types, fewer type applications, more
200 constraints can get into Ct instead of Cq. Here's a good way to
203 Q = grow( fv(T), C ) \ oclose( fv(G), C )
205 That is, quantify over all variable that that MIGHT be fixed by the
206 call site (which influences T), but which aren't DEFINITELY fixed by
207 G. This choice definitely quantifies over enough type variables,
208 albeit perhaps too many.
210 Why grow( fv(T), C ) rather than fv(T)? Consider
212 class H x y | x->y where ...
217 If we used fv(T) = {c} we'd get the type
219 forall c. H c d => c -> b
221 And then if the fn was called at several different c's, each of
222 which fixed d differently, we'd get a unification error, because
223 d isn't quantified. Solution: quantify d. So we must quantify
224 everything that might be influenced by c.
226 Why not oclose( fv(T), C )? Because we might not be able to see
227 all the functional dependencies yet:
229 class H x y | x->y where ...
230 instance H x y => Eq (T x y) where ...
235 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
236 apparent yet, and that's wrong. We must really quantify over d too.
238 There really isn't any point in quantifying over any more than
239 grow( fv(T), C ), because the call sites can't possibly influence
240 any other type variables.
244 -------------------------------------
246 -------------------------------------
248 It's very hard to be certain when a type is ambiguous. Consider
252 instance H x y => K (x,y)
254 Is this type ambiguous?
255 forall a b. (K (a,b), Eq b) => a -> a
257 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
258 now we see that a fixes b. So we can't tell about ambiguity for sure
259 without doing a full simplification. And even that isn't possible if
260 the context has some free vars that may get unified. Urgle!
262 Here's another example: is this ambiguous?
263 forall a b. Eq (T b) => a -> a
264 Not if there's an insance decl (with no context)
265 instance Eq (T b) where ...
267 You may say of this example that we should use the instance decl right
268 away, but you can't always do that:
270 class J a b where ...
271 instance J Int b where ...
273 f :: forall a b. J a b => a -> a
275 (Notice: no functional dependency in J's class decl.)
276 Here f's type is perfectly fine, provided f is only called at Int.
277 It's premature to complain when meeting f's signature, or even
278 when inferring a type for f.
282 However, we don't *need* to report ambiguity right away. It'll always
283 show up at the call site.... and eventually at main, which needs special
284 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
286 So here's the plan. We WARN about probable ambiguity if
288 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
290 (all tested before quantification).
291 That is, all the type variables in Cq must be fixed by the the variables
292 in the environment, or by the variables in the type.
294 Notice that we union before calling oclose. Here's an example:
296 class J a b c | a b -> c
300 forall b c. (J a b c) => b -> b
302 Only if we union {a} from G with {b} from T before using oclose,
303 do we see that c is fixed.
305 It's a bit vague exactly which C we should use for this oclose call. If we
306 don't fix enough variables we might complain when we shouldn't (see
307 the above nasty example). Nothing will be perfect. That's why we can
308 only issue a warning.
311 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
313 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
315 then c is a "bubble"; there's no way it can ever improve, and it's
316 certainly ambiguous. UNLESS it is a constant (sigh). And what about
321 instance H x y => K (x,y)
323 Is this type ambiguous?
324 forall a b. (K (a,b), Eq b) => a -> a
326 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
327 is a "bubble" that's a set of constraints
329 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
331 Hence another idea. To decide Q start with fv(T) and grow it
332 by transitive closure in Cq (no functional dependencies involved).
333 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
334 The definitely-ambiguous can then float out, and get smashed at top level
335 (which squashes out the constants, like Eq (T a) above)
338 --------------------------------------
339 Notes on principal types
340 --------------------------------------
345 f x = let g y = op (y::Int) in True
347 Here the principal type of f is (forall a. a->a)
348 but we'll produce the non-principal type
349 f :: forall a. C Int => a -> a
352 --------------------------------------
353 The need for forall's in constraints
354 --------------------------------------
356 [Exchange on Haskell Cafe 5/6 Dec 2000]
358 class C t where op :: t -> Bool
359 instance C [t] where op x = True
361 p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
362 q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
364 The definitions of p and q differ only in the order of the components in
365 the pair on their right-hand sides. And yet:
367 ghc and "Typing Haskell in Haskell" reject p, but accept q;
368 Hugs rejects q, but accepts p;
369 hbc rejects both p and q;
370 nhc98 ... (Malcolm, can you fill in the blank for us!).
372 The type signature for f forces context reduction to take place, and
373 the results of this depend on whether or not the type of y is known,
374 which in turn depends on which component of the pair the type checker
377 Solution: if y::m a, float out the constraints
378 Monad m, forall c. C (m c)
379 When m is later unified with [], we can solve both constraints.
382 --------------------------------------
383 Notes on implicit parameters
384 --------------------------------------
386 Note [Inheriting implicit parameters]
387 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
392 where f is *not* a top-level binding.
393 From the RHS of f we'll get the constraint (?y::Int).
394 There are two types we might infer for f:
398 (so we get ?y from the context of f's definition), or
400 f :: (?y::Int) => Int -> Int
402 At first you might think the first was better, becuase then
403 ?y behaves like a free variable of the definition, rather than
404 having to be passed at each call site. But of course, the WHOLE
405 IDEA is that ?y should be passed at each call site (that's what
406 dynamic binding means) so we'd better infer the second.
408 BOTTOM LINE: when *inferring types* you *must* quantify
409 over implicit parameters. See the predicate isFreeWhenInferring.
412 Note [Implicit parameters and ambiguity]
413 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
414 Only a *class* predicate can give rise to ambiguity
415 An *implicit parameter* cannot. For example:
416 foo :: (?x :: [a]) => Int
418 is fine. The call site will suppply a particular 'x'
420 Furthermore, the type variables fixed by an implicit parameter
421 propagate to the others. E.g.
422 foo :: (Show a, ?x::[a]) => Int
424 The type of foo looks ambiguous. But it isn't, because at a call site
426 let ?x = 5::Int in foo
427 and all is well. In effect, implicit parameters are, well, parameters,
428 so we can take their type variables into account as part of the
429 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
432 Question 2: type signatures
433 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
434 BUT WATCH OUT: When you supply a type signature, we can't force you
435 to quantify over implicit parameters. For example:
439 This is perfectly reasonable. We do not want to insist on
441 (?x + 1) :: (?x::Int => Int)
443 That would be silly. Here, the definition site *is* the occurrence site,
444 so the above strictures don't apply. Hence the difference between
445 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
446 and tcSimplifyCheckBind (which does not).
448 What about when you supply a type signature for a binding?
449 Is it legal to give the following explicit, user type
450 signature to f, thus:
455 At first sight this seems reasonable, but it has the nasty property
456 that adding a type signature changes the dynamic semantics.
459 (let f x = (x::Int) + ?y
460 in (f 3, f 3 with ?y=5)) with ?y = 6
466 in (f 3, f 3 with ?y=5)) with ?y = 6
470 Indeed, simply inlining f (at the Haskell source level) would change the
473 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
474 semantics for a Haskell program without knowing its typing, so if you
475 change the typing you may change the semantics.
477 To make things consistent in all cases where we are *checking* against
478 a supplied signature (as opposed to inferring a type), we adopt the
481 a signature does not need to quantify over implicit params.
483 [This represents a (rather marginal) change of policy since GHC 5.02,
484 which *required* an explicit signature to quantify over all implicit
485 params for the reasons mentioned above.]
487 But that raises a new question. Consider
489 Given (signature) ?x::Int
490 Wanted (inferred) ?x::Int, ?y::Bool
492 Clearly we want to discharge the ?x and float the ?y out. But
493 what is the criterion that distinguishes them? Clearly it isn't
494 what free type variables they have. The Right Thing seems to be
495 to float a constraint that
496 neither mentions any of the quantified type variables
497 nor any of the quantified implicit parameters
499 See the predicate isFreeWhenChecking.
502 Question 3: monomorphism
503 ~~~~~~~~~~~~~~~~~~~~~~~~
504 There's a nasty corner case when the monomorphism restriction bites:
508 The argument above suggests that we *must* generalise
509 over the ?y parameter, to get
510 z :: (?y::Int) => Int,
511 but the monomorphism restriction says that we *must not*, giving
513 Why does the momomorphism restriction say this? Because if you have
515 let z = x + ?y in z+z
517 you might not expect the addition to be done twice --- but it will if
518 we follow the argument of Question 2 and generalise over ?y.
521 Question 4: top level
522 ~~~~~~~~~~~~~~~~~~~~~
523 At the top level, monomorhism makes no sense at all.
526 main = let ?x = 5 in print foo
530 woggle :: (?x :: Int) => Int -> Int
533 We definitely don't want (foo :: Int) with a top-level implicit parameter
534 (?x::Int) becuase there is no way to bind it.
539 (A) Always generalise over implicit parameters
540 Bindings that fall under the monomorphism restriction can't
544 * Inlining remains valid
545 * No unexpected loss of sharing
546 * But simple bindings like
548 will be rejected, unless you add an explicit type signature
549 (to avoid the monomorphism restriction)
550 z :: (?y::Int) => Int
552 This seems unacceptable
554 (B) Monomorphism restriction "wins"
555 Bindings that fall under the monomorphism restriction can't
557 Always generalise over implicit parameters *except* for bindings
558 that fall under the monomorphism restriction
561 * Inlining isn't valid in general
562 * No unexpected loss of sharing
563 * Simple bindings like
565 accepted (get value of ?y from binding site)
567 (C) Always generalise over implicit parameters
568 Bindings that fall under the monomorphism restriction can't
569 be generalised, EXCEPT for implicit parameters
571 * Inlining remains valid
572 * Unexpected loss of sharing (from the extra generalisation)
573 * Simple bindings like
575 accepted (get value of ?y from occurrence sites)
580 None of these choices seems very satisfactory. But at least we should
581 decide which we want to do.
583 It's really not clear what is the Right Thing To Do. If you see
587 would you expect the value of ?y to be got from the *occurrence sites*
588 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
589 case of function definitions, the answer is clearly the former, but
590 less so in the case of non-fucntion definitions. On the other hand,
591 if we say that we get the value of ?y from the definition site of 'z',
592 then inlining 'z' might change the semantics of the program.
594 Choice (C) really says "the monomorphism restriction doesn't apply
595 to implicit parameters". Which is fine, but remember that every
596 innocent binding 'x = ...' that mentions an implicit parameter in
597 the RHS becomes a *function* of that parameter, called at each
598 use of 'x'. Now, the chances are that there are no intervening 'with'
599 clauses that bind ?y, so a decent compiler should common up all
600 those function calls. So I think I strongly favour (C). Indeed,
601 one could make a similar argument for abolishing the monomorphism
602 restriction altogether.
604 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
608 %************************************************************************
610 \subsection{tcSimplifyInfer}
612 %************************************************************************
614 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
616 1. Compute Q = grow( fvs(T), C )
618 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
619 predicates will end up in Ct; we deal with them at the top level
621 3. Try improvement, using functional dependencies
623 4. If Step 3 did any unification, repeat from step 1
624 (Unification can change the result of 'grow'.)
626 Note: we don't reduce dictionaries in step 2. For example, if we have
627 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
628 after step 2. However note that we may therefore quantify over more
629 type variables than we absolutely have to.
631 For the guts, we need a loop, that alternates context reduction and
632 improvement with unification. E.g. Suppose we have
634 class C x y | x->y where ...
636 and tcSimplify is called with:
638 Then improvement unifies a with b, giving
641 If we need to unify anything, we rattle round the whole thing all over
648 -> TcTyVarSet -- fv(T); type vars
650 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked and quantified)
651 [Inst], -- Dict Ids that must be bound here (zonked)
652 TcDictBinds) -- Bindings
653 -- Any free (escaping) Insts are tossed into the environment
658 tcSimplifyInfer doc tau_tvs wanted
659 = do { tau_tvs1 <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
660 ; wanted' <- mapM zonkInst wanted -- Zonk before deciding quantified tyvars
661 ; gbl_tvs <- tcGetGlobalTyVars
662 ; let preds1 = fdPredsOfInsts wanted'
663 gbl_tvs1 = oclose preds1 gbl_tvs
664 qtvs = grow preds1 tau_tvs1 `minusVarSet` gbl_tvs1
665 -- See Note [Choosing which variables to quantify]
667 -- To maximise sharing, remove from consideration any
668 -- constraints that don't mention qtvs at all
669 ; let (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
672 -- To make types simple, reduce as much as possible
673 ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (grow preds1 tau_tvs1) $$ ppr gbl_tvs $$
674 ppr gbl_tvs1 $$ ppr free $$ ppr bound))
675 ; (irreds1, binds1) <- tryHardCheckLoop doc bound
677 -- Note [Inference and implication constraints]
678 ; let want_dict d = tyVarsOfInst d `intersectsVarSet` qtvs
679 ; (irreds2, binds2) <- approximateImplications doc want_dict irreds1
681 -- Now work out all over again which type variables to quantify,
682 -- exactly in the same way as before, but starting from irreds2. Why?
683 -- a) By now improvment may have taken place, and we must *not*
684 -- quantify over any variable free in the environment
685 -- tc137 (function h inside g) is an example
687 -- b) Do not quantify over constraints that *now* do not
688 -- mention quantified type variables, because they are
689 -- simply ambiguous (or might be bound further out). Example:
690 -- f :: Eq b => a -> (a, b)
692 -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
693 -- We decide to quantify over 'alpha' alone, but free1 does not include f77
694 -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
695 -- constraint (Eq beta), which we dump back into the free set
696 -- See test tcfail181
698 -- c) irreds may contain type variables not previously mentioned,
699 -- e.g. instance D a x => Foo [a]
701 -- Then after simplifying we'll get (D a x), and x is fresh
702 -- We must quantify over x else it'll be totally unbound
703 ; tau_tvs2 <- zonkTcTyVarsAndFV (varSetElems tau_tvs1)
704 ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
705 -- Note that we start from gbl_tvs1
706 -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
707 -- we've already put some of the original preds1 into frees
708 -- E.g. wanteds = C a b (where a->b)
711 -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
712 -- irreds2 will be empty. But we don't want to generalise over b!
713 ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
714 qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
715 ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
718 -- Turn the quantified meta-type variables into real type variables
719 ; qtvs2 <- zonkQuantifiedTyVars (varSetElems qtvs)
721 -- We can't abstract over any remaining unsolved
722 -- implications so instead just float them outwards. Ugh.
723 ; let (q_dicts0, implics) = partition isAbstractableInst irreds3
724 ; loc <- getInstLoc (ImplicOrigin doc)
725 ; implic_bind <- bindIrreds loc qtvs2 q_dicts0 implics
727 -- Prepare equality instances for quantification
728 ; let (q_eqs0,q_dicts) = partition isEqInst q_dicts0
729 ; q_eqs <- mapM finalizeEqInst q_eqs0
731 ; return (qtvs2, q_eqs ++ q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
732 -- NB: when we are done, we might have some bindings, but
733 -- the final qtvs might be empty. See Note [NO TYVARS] below.
735 approximateImplications :: SDoc -> (Inst -> Bool) -> [Inst] -> TcM ([Inst], TcDictBinds)
736 -- Note [Inference and implication constraints]
737 -- Given a bunch of Dict and ImplicInsts, try to approximate the implications by
738 -- - fetching any dicts inside them that are free
739 -- - using those dicts as cruder constraints, to solve the implications
740 -- - returning the extra ones too
742 approximateImplications doc want_dict irreds
744 = return (irreds, emptyBag)
746 = do { extra_dicts' <- mapM cloneDict extra_dicts
747 ; tryHardCheckLoop doc (extra_dicts' ++ irreds) }
748 -- By adding extra_dicts', we make them
749 -- available to solve the implication constraints
751 extra_dicts = get_dicts (filter isImplicInst irreds)
753 get_dicts :: [Inst] -> [Inst] -- Returns only Dicts
754 -- Find the wanted constraints in implication constraints that satisfy
755 -- want_dict, and are not bound by forall's in the constraint itself
756 get_dicts ds = concatMap get_dict ds
758 get_dict d@(Dict {}) | want_dict d = [d]
760 get_dict (ImplicInst {tci_tyvars = tvs, tci_wanted = wanteds})
761 = [ d | let tv_set = mkVarSet tvs
762 , d <- get_dicts wanteds
763 , not (tyVarsOfInst d `intersectsVarSet` tv_set)]
764 get_dict i@(EqInst {}) | want_dict i = [i]
766 get_dict other = pprPanic "approximateImplications" (ppr other)
769 Note [Inference and implication constraints]
770 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
771 Suppose we have a wanted implication constraint (perhaps arising from
772 a nested pattern match) like
774 and we are now trying to quantify over 'a' when inferring the type for
775 a function. In principle it's possible that there might be an instance
776 instance (C a, E a) => D [a]
777 so the context (E a) would suffice. The Right Thing is to abstract over
778 the implication constraint, but we don't do that (a) because it'll be
779 surprising to programmers and (b) because we don't have the machinery to deal
780 with 'given' implications.
782 So our best approximation is to make (D [a]) part of the inferred
783 context, so we can use that to discharge the implication. Hence
784 the strange function get_dicts in approximateImplications.
786 The common cases are more clear-cut, when we have things like
788 Here, abstracting over (C b) is not an approximation at all -- but see
789 Note [Freeness and implications].
791 See Trac #1430 and test tc228.
795 -----------------------------------------------------------
796 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
797 -- against, but we don't know the type variables over which we are going to quantify.
798 -- This happens when we have a type signature for a mutually recursive group
801 -> TcTyVarSet -- fv(T)
804 -> TcM ([TyVar], -- Fully zonked, and quantified
805 TcDictBinds) -- Bindings
807 tcSimplifyInferCheck loc tau_tvs givens wanteds
808 = do { traceTc (text "tcSimplifyInferCheck <-" <+> ppr wanteds)
809 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
811 -- Figure out which type variables to quantify over
812 -- You might think it should just be the signature tyvars,
813 -- but in bizarre cases you can get extra ones
814 -- f :: forall a. Num a => a -> a
815 -- f x = fst (g (x, head [])) + 1
817 -- Here we infer g :: forall a b. a -> b -> (b,a)
818 -- We don't want g to be monomorphic in b just because
819 -- f isn't quantified over b.
820 ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
821 ; all_tvs <- zonkTcTyVarsAndFV all_tvs
822 ; gbl_tvs <- tcGetGlobalTyVars
823 ; let qtvs = varSetElems (all_tvs `minusVarSet` gbl_tvs)
824 -- We could close gbl_tvs, but its not necessary for
825 -- soundness, and it'll only affect which tyvars, not which
826 -- dictionaries, we quantify over
828 ; qtvs' <- zonkQuantifiedTyVars qtvs
830 -- Now we are back to normal (c.f. tcSimplCheck)
831 ; implic_bind <- bindIrreds loc qtvs' givens irreds
833 ; traceTc (text "tcSimplifyInferCheck ->" <+> ppr (implic_bind))
834 ; return (qtvs', binds `unionBags` implic_bind) }
837 Note [Squashing methods]
838 ~~~~~~~~~~~~~~~~~~~~~~~~~
839 Be careful if you want to float methods more:
840 truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
841 From an application (truncate f i) we get
844 If we have also have a second occurrence of truncate, we get
847 When simplifying with i,f free, we might still notice that
848 t1=t3; but alas, the binding for t2 (which mentions t1)
849 may continue to float out!
854 class Y a b | a -> b where
857 instance Y [[a]] a where
860 k :: X a -> X a -> X a
862 g :: Num a => [X a] -> [X a]
865 h ys = ys ++ map (k (y [[0]])) xs
867 The excitement comes when simplifying the bindings for h. Initially
868 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
869 From this we get t1:=:t2, but also various bindings. We can't forget
870 the bindings (because of [LOOP]), but in fact t1 is what g is
873 The net effect of [NO TYVARS]
876 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
877 isFreeWhenInferring qtvs inst
878 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
879 && isInheritableInst inst -- and no implicit parameter involved
880 -- see Note [Inheriting implicit parameters]
882 {- No longer used (with implication constraints)
883 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
884 -> NameSet -- Quantified implicit parameters
886 isFreeWhenChecking qtvs ips inst
887 = isFreeWrtTyVars qtvs inst
888 && isFreeWrtIPs ips inst
891 isFreeWrtTyVars qtvs inst = tyVarsOfInst inst `disjointVarSet` qtvs
892 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
896 %************************************************************************
898 \subsection{tcSimplifyCheck}
900 %************************************************************************
902 @tcSimplifyCheck@ is used when we know exactly the set of variables
903 we are going to quantify over. For example, a class or instance declaration.
906 -----------------------------------------------------------
907 -- tcSimplifyCheck is used when checking expression type signatures,
908 -- class decls, instance decls etc.
909 tcSimplifyCheck :: InstLoc
910 -> [TcTyVar] -- Quantify over these
913 -> TcM TcDictBinds -- Bindings
914 tcSimplifyCheck loc qtvs givens wanteds
915 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
916 do { traceTc (text "tcSimplifyCheck")
917 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
918 ; implic_bind <- bindIrreds loc qtvs givens irreds
919 ; return (binds `unionBags` implic_bind) }
921 -----------------------------------------------------------
922 -- tcSimplifyCheckPat is used for existential pattern match
923 tcSimplifyCheckPat :: InstLoc
925 -> [TcTyVar] -- Quantify over these
928 -> TcM TcDictBinds -- Bindings
929 tcSimplifyCheckPat loc co_vars qtvs givens wanteds
930 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
931 do { traceTc (text "tcSimplifyCheckPat")
932 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
933 ; implic_bind <- bindIrredsR loc qtvs co_vars givens irreds
934 ; return (binds `unionBags` implic_bind) }
936 -----------------------------------------------------------
937 bindIrreds :: InstLoc -> [TcTyVar]
940 bindIrreds loc qtvs givens irreds
941 = bindIrredsR loc qtvs [] givens irreds
943 bindIrredsR :: InstLoc -> [TcTyVar] -> [CoVar] -> [Inst] -> [Inst]
945 -- Make a binding that binds 'irreds', by generating an implication
946 -- constraint for them, *and* throwing the constraint into the LIE
947 bindIrredsR loc qtvs co_vars givens irreds
951 = do { let givens' = filter isAbstractableInst givens
952 -- The givens can (redundantly) include methods
953 -- We want to retain both EqInsts and Dicts
954 -- There should be no implicadtion constraints
955 -- See Note [Pruning the givens in an implication constraint]
957 -- If there are no 'givens', then it's safe to
958 -- partition the 'wanteds' by their qtvs, thereby trimming irreds
959 -- See Note [Freeness and implications]
960 ; irreds' <- if null givens'
962 { let qtv_set = mkVarSet qtvs
963 (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
965 ; return real_irreds }
968 ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
969 ; (implics, bind) <- makeImplicationBind loc all_tvs givens' irreds'
970 -- This call does the real work
971 -- If irreds' is empty, it does something sensible
976 makeImplicationBind :: InstLoc -> [TcTyVar]
978 -> TcM ([Inst], TcDictBinds)
979 -- Make a binding that binds 'irreds', by generating an implication
980 -- constraint for them, *and* throwing the constraint into the LIE
981 -- The binding looks like
982 -- (ir1, .., irn) = f qtvs givens
983 -- where f is (evidence for) the new implication constraint
984 -- f :: forall qtvs. {reft} givens => (ir1, .., irn)
985 -- qtvs includes coercion variables
987 -- This binding must line up the 'rhs' in reduceImplication
988 makeImplicationBind loc all_tvs
989 givens -- Guaranteed all Dicts
992 | null irreds -- If there are no irreds, we are done
993 = return ([], emptyBag)
994 | otherwise -- Otherwise we must generate a binding
995 = do { uniq <- newUnique
996 ; span <- getSrcSpanM
997 ; let (eq_givens, dict_givens) = partition isEqInst givens
998 eq_tyvar_cos = mkTyVarTys (varSetElems $ tyVarsOfTypes $ map eqInstType eq_givens)
999 -- Urgh! See line 2187 or thereabouts. I believe that all these
1000 -- 'givens' must be a simple CoVar. This MUST be cleaned up.
1002 ; let name = mkInternalName uniq (mkVarOcc "ic") span
1003 implic_inst = ImplicInst { tci_name = name, tci_reft = emptyRefinement,
1004 tci_tyvars = all_tvs,
1005 tci_given = (eq_givens ++ dict_givens),
1006 tci_wanted = irreds, tci_loc = loc }
1007 ; let -- only create binder for dict_irreds
1008 (eq_irreds, dict_irreds) = partition isEqInst irreds
1009 n_dict_irreds = length dict_irreds
1010 dict_irred_ids = map instToId dict_irreds
1011 tup_ty = mkTupleTy Boxed n_dict_irreds (map idType dict_irred_ids)
1012 pat = TuplePat (map nlVarPat dict_irred_ids) Boxed tup_ty
1013 rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
1014 co = mkWpApps (map instToId dict_givens)
1015 <.> mkWpTyApps eq_tyvar_cos
1016 <.> mkWpTyApps (mkTyVarTys all_tvs)
1017 bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs
1018 | otherwise = PatBind { pat_lhs = L span pat,
1019 pat_rhs = unguardedGRHSs rhs,
1020 pat_rhs_ty = tup_ty,
1021 bind_fvs = placeHolderNames }
1022 ; traceTc $ text "makeImplicationBind" <+> ppr implic_inst
1023 ; return ([implic_inst], unitBag (L span bind))
1026 -----------------------------------------------------------
1027 tryHardCheckLoop :: SDoc
1029 -> TcM ([Inst], TcDictBinds)
1031 tryHardCheckLoop doc wanteds
1032 = do { (irreds,binds) <- checkLoop (mkRedEnv doc try_me []) wanteds
1033 ; return (irreds,binds)
1036 try_me inst = ReduceMe AddSCs
1037 -- Here's the try-hard bit
1039 -----------------------------------------------------------
1040 gentleCheckLoop :: InstLoc
1043 -> TcM ([Inst], TcDictBinds)
1045 gentleCheckLoop inst_loc givens wanteds
1046 = do { (irreds,binds) <- checkLoop env wanteds
1047 ; return (irreds,binds)
1050 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
1052 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1054 -- When checking against a given signature
1055 -- we MUST be very gentle: Note [Check gently]
1057 gentleInferLoop :: SDoc -> [Inst]
1058 -> TcM ([Inst], TcDictBinds)
1059 gentleInferLoop doc wanteds
1060 = do { (irreds, binds) <- checkLoop env wanteds
1061 ; return (irreds, binds) }
1063 env = mkRedEnv doc try_me []
1064 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1069 ~~~~~~~~~~~~~~~~~~~~
1070 We have to very careful about not simplifying too vigorously
1075 f :: Show b => T b -> b
1076 f (MkT x) = show [x]
1078 Inside the pattern match, which binds (a:*, x:a), we know that
1080 Hence we have a dictionary for Show [a] available; and indeed we
1081 need it. We are going to build an implication contraint
1082 forall a. (b~[a]) => Show [a]
1083 Later, we will solve this constraint using the knowledge (Show b)
1085 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
1086 thing becomes insoluble. So we simplify gently (get rid of literals
1087 and methods only, plus common up equal things), deferring the real
1088 work until top level, when we solve the implication constraint
1089 with tryHardCheckLooop.
1093 -----------------------------------------------------------
1096 -> TcM ([Inst], TcDictBinds)
1097 -- Precondition: givens are completely rigid
1098 -- Postcondition: returned Insts are zonked
1100 checkLoop env wanteds
1101 = go env wanteds (return ())
1102 where go env wanteds elim_skolems
1103 = do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
1104 ; env' <- zonkRedEnv env
1105 ; wanteds' <- zonkInsts wanteds
1107 ; (improved, binds, irreds, elim_more_skolems)
1108 <- reduceContext env' wanteds'
1109 ; let elim_skolems' = elim_skolems >> elim_more_skolems
1111 ; if not improved then
1112 elim_skolems' >> return (irreds, binds)
1115 -- If improvement did some unification, we go round again.
1116 -- We start again with irreds, not wanteds
1117 -- Using an instance decl might have introduced a fresh type
1118 -- variable which might have been unified, so we'd get an
1119 -- infinite loop if we started again with wanteds!
1121 { (irreds1, binds1) <- go env' irreds elim_skolems'
1122 ; return (irreds1, binds `unionBags` binds1) } }
1125 Note [Zonking RedEnv]
1126 ~~~~~~~~~~~~~~~~~~~~~
1127 It might appear as if the givens in RedEnv are always rigid, but that is not
1128 necessarily the case for programs involving higher-rank types that have class
1129 contexts constraining the higher-rank variables. An example from tc237 in the
1132 class Modular s a | s -> a
1134 wim :: forall a w. Integral a
1135 => a -> (forall s. Modular s a => M s w) -> w
1136 wim i k = error "urk"
1138 test5 :: (Modular s a, Integral a) => M s a
1141 test4 = wim 4 test4'
1143 Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
1144 quantified further outside. When type checking test4, we have to check
1145 whether the signature of test5 is an instance of
1147 (forall s. Modular s a => M s w)
1149 Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
1152 Given the FD of Modular in this example, class improvement will instantiate
1153 t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
1154 Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
1155 the givens, we will get into a loop as improveOne uses the unification engine
1156 TcGadt.tcUnifyTys, which doesn't know about mutable type variables.
1161 class If b t e r | b t e -> r
1164 class Lte a b c | a b -> c where lte :: a -> b -> c
1166 instance (Lte a b l,If l b a c) => Max a b c
1168 Wanted: Max Z (S x) y
1170 Then we'll reduce using the Max instance to:
1171 (Lte Z (S x) l, If l (S x) Z y)
1172 and improve by binding l->T, after which we can do some reduction
1173 on both the Lte and If constraints. What we *can't* do is start again
1174 with (Max Z (S x) y)!
1178 %************************************************************************
1180 tcSimplifySuperClasses
1182 %************************************************************************
1184 Note [SUPERCLASS-LOOP 1]
1185 ~~~~~~~~~~~~~~~~~~~~~~~~
1186 We have to be very, very careful when generating superclasses, lest we
1187 accidentally build a loop. Here's an example:
1191 class S a => C a where { opc :: a -> a }
1192 class S b => D b where { opd :: b -> b }
1194 instance C Int where
1197 instance D Int where
1200 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1201 Simplifying, we may well get:
1202 $dfCInt = :C ds1 (opd dd)
1205 Notice that we spot that we can extract ds1 from dd.
1207 Alas! Alack! We can do the same for (instance D Int):
1209 $dfDInt = :D ds2 (opc dc)
1213 And now we've defined the superclass in terms of itself.
1215 Solution: never generate a superclass selectors at all when
1216 satisfying the superclass context of an instance declaration.
1218 Two more nasty cases are in
1223 tcSimplifySuperClasses
1228 tcSimplifySuperClasses loc givens sc_wanteds
1229 = do { traceTc (text "tcSimplifySuperClasses")
1230 ; (irreds,binds1) <- checkLoop env sc_wanteds
1231 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1232 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1235 env = mkRedEnv (pprInstLoc loc) try_me givens
1236 try_me inst = ReduceMe NoSCs
1237 -- Like tryHardCheckLoop, but with NoSCs
1241 %************************************************************************
1243 \subsection{tcSimplifyRestricted}
1245 %************************************************************************
1247 tcSimplifyRestricted infers which type variables to quantify for a
1248 group of restricted bindings. This isn't trivial.
1251 We want to quantify over a to get id :: forall a. a->a
1254 We do not want to quantify over a, because there's an Eq a
1255 constraint, so we get eq :: a->a->Bool (notice no forall)
1258 RHS has type 'tau', whose free tyvars are tau_tvs
1259 RHS has constraints 'wanteds'
1262 Quantify over (tau_tvs \ ftvs(wanteds))
1263 This is bad. The constraints may contain (Monad (ST s))
1264 where we have instance Monad (ST s) where...
1265 so there's no need to be monomorphic in s!
1267 Also the constraint might be a method constraint,
1268 whose type mentions a perfectly innocent tyvar:
1269 op :: Num a => a -> b -> a
1270 Here, b is unconstrained. A good example would be
1272 We want to infer the polymorphic type
1273 foo :: forall b. b -> b
1276 Plan B (cunning, used for a long time up to and including GHC 6.2)
1277 Step 1: Simplify the constraints as much as possible (to deal
1278 with Plan A's problem). Then set
1279 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1281 Step 2: Now simplify again, treating the constraint as 'free' if
1282 it does not mention qtvs, and trying to reduce it otherwise.
1283 The reasons for this is to maximise sharing.
1285 This fails for a very subtle reason. Suppose that in the Step 2
1286 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1287 In the Step 1 this constraint might have been simplified, perhaps to
1288 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1289 This won't happen in Step 2... but that in turn might prevent some other
1290 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1291 and that in turn breaks the invariant that no constraints are quantified over.
1293 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1298 Step 1: Simplify the constraints as much as possible (to deal
1299 with Plan A's problem). Then set
1300 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1301 Return the bindings from Step 1.
1304 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1307 instance (HasBinary ty IO) => HasCodedValue ty
1309 foo :: HasCodedValue a => String -> IO a
1311 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1312 doDecodeIO codedValue view
1313 = let { act = foo "foo" } in act
1315 You might think this should work becuase the call to foo gives rise to a constraint
1316 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1317 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1318 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1320 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1324 Plan D (a variant of plan B)
1325 Step 1: Simplify the constraints as much as possible (to deal
1326 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1327 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1329 Step 2: Now simplify again, treating the constraint as 'free' if
1330 it does not mention qtvs, and trying to reduce it otherwise.
1332 The point here is that it's generally OK to have too few qtvs; that is,
1333 to make the thing more monomorphic than it could be. We don't want to
1334 do that in the common cases, but in wierd cases it's ok: the programmer
1335 can always add a signature.
1337 Too few qtvs => too many wanteds, which is what happens if you do less
1342 tcSimplifyRestricted -- Used for restricted binding groups
1343 -- i.e. ones subject to the monomorphism restriction
1346 -> [Name] -- Things bound in this group
1347 -> TcTyVarSet -- Free in the type of the RHSs
1348 -> [Inst] -- Free in the RHSs
1349 -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
1350 TcDictBinds) -- Bindings
1351 -- tcSimpifyRestricted returns no constraints to
1352 -- quantify over; by definition there are none.
1353 -- They are all thrown back in the LIE
1355 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1356 -- Zonk everything in sight
1357 = do { traceTc (text "tcSimplifyRestricted")
1358 ; wanteds' <- zonkInsts wanteds
1360 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1361 -- dicts; the idea is to get rid of as many type
1362 -- variables as possible, and we don't want to stop
1363 -- at (say) Monad (ST s), because that reduces
1364 -- immediately, with no constraint on s.
1366 -- BUT do no improvement! See Plan D above
1367 -- HOWEVER, some unification may take place, if we instantiate
1368 -- a method Inst with an equality constraint
1369 ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
1370 ; (_imp, _binds, constrained_dicts, elim_skolems)
1371 <- reduceContext env wanteds'
1374 -- Next, figure out the tyvars we will quantify over
1375 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1376 ; gbl_tvs' <- tcGetGlobalTyVars
1377 ; constrained_dicts' <- zonkInsts constrained_dicts
1379 ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
1380 -- As in tcSimplifyInfer
1382 -- Do not quantify over constrained type variables:
1383 -- this is the monomorphism restriction
1384 constrained_tvs' = tyVarsOfInsts constrained_dicts'
1385 qtvs = qtvs1 `minusVarSet` constrained_tvs'
1386 pp_bndrs = pprWithCommas (quotes . ppr) bndrs
1389 ; warn_mono <- doptM Opt_WarnMonomorphism
1390 ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
1391 (vcat[ ptext SLIT("the Monomorphism Restriction applies to the binding")
1392 <> plural bndrs <+> ptext SLIT("for") <+> pp_bndrs,
1393 ptext SLIT("Consider giving a type signature for") <+> pp_bndrs])
1395 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1396 pprInsts wanteds, pprInsts constrained_dicts',
1398 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1400 -- The first step may have squashed more methods than
1401 -- necessary, so try again, this time more gently, knowing the exact
1402 -- set of type variables to quantify over.
1404 -- We quantify only over constraints that are captured by qtvs;
1405 -- these will just be a subset of non-dicts. This in contrast
1406 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1407 -- all *non-inheritable* constraints too. This implements choice
1408 -- (B) under "implicit parameter and monomorphism" above.
1410 -- Remember that we may need to do *some* simplification, to
1411 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1412 -- just to float all constraints
1414 -- At top level, we *do* squash methods becuase we want to
1415 -- expose implicit parameters to the test that follows
1416 ; let is_nested_group = isNotTopLevel top_lvl
1417 try_me inst | isFreeWrtTyVars qtvs inst,
1418 (is_nested_group || isDict inst) = Stop
1419 | otherwise = ReduceMe AddSCs
1420 env = mkNoImproveRedEnv doc try_me
1421 ; (_imp, binds, irreds, elim_skolems) <- reduceContext env wanteds'
1424 -- See "Notes on implicit parameters, Question 4: top level"
1425 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1426 if is_nested_group then
1428 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1429 ; addTopIPErrs bndrs bad_ips
1430 ; extendLIEs non_ips }
1432 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1433 ; return (qtvs', binds) }
1437 %************************************************************************
1441 %************************************************************************
1443 On the LHS of transformation rules we only simplify methods and constants,
1444 getting dictionaries. We want to keep all of them unsimplified, to serve
1445 as the available stuff for the RHS of the rule.
1447 Example. Consider the following left-hand side of a rule
1449 f (x == y) (y > z) = ...
1451 If we typecheck this expression we get constraints
1453 d1 :: Ord a, d2 :: Eq a
1455 We do NOT want to "simplify" to the LHS
1457 forall x::a, y::a, z::a, d1::Ord a.
1458 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1462 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1463 f ((==) d2 x y) ((>) d1 y z) = ...
1465 Here is another example:
1467 fromIntegral :: (Integral a, Num b) => a -> b
1468 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1470 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1471 we *dont* want to get
1473 forall dIntegralInt.
1474 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1476 because the scsel will mess up RULE matching. Instead we want
1478 forall dIntegralInt, dNumInt.
1479 fromIntegral Int Int dIntegralInt dNumInt = id Int
1483 g (x == y) (y == z) = ..
1485 where the two dictionaries are *identical*, we do NOT WANT
1487 forall x::a, y::a, z::a, d1::Eq a
1488 f ((==) d1 x y) ((>) d1 y z) = ...
1490 because that will only match if the dict args are (visibly) equal.
1491 Instead we want to quantify over the dictionaries separately.
1493 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1494 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1495 from scratch, rather than further parameterise simpleReduceLoop etc
1498 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1499 tcSimplifyRuleLhs wanteds
1500 = go [] emptyBag wanteds
1503 = return (dicts, binds)
1504 go dicts binds (w:ws)
1506 = go (w:dicts) binds ws
1508 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1509 -- to fromInteger; this looks fragile to me
1510 ; lookup_result <- lookupSimpleInst w'
1511 ; case lookup_result of
1513 go dicts (addInstToDictBind binds w rhs) (ws' ++ ws)
1514 NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w)
1518 tcSimplifyBracket is used when simplifying the constraints arising from
1519 a Template Haskell bracket [| ... |]. We want to check that there aren't
1520 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1521 Show instance), but we aren't otherwise interested in the results.
1522 Nor do we care about ambiguous dictionaries etc. We will type check
1523 this bracket again at its usage site.
1526 tcSimplifyBracket :: [Inst] -> TcM ()
1527 tcSimplifyBracket wanteds
1528 = do { tryHardCheckLoop doc wanteds
1531 doc = text "tcSimplifyBracket"
1535 %************************************************************************
1537 \subsection{Filtering at a dynamic binding}
1539 %************************************************************************
1544 we must discharge all the ?x constraints from B. We also do an improvement
1545 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1547 Actually, the constraints from B might improve the types in ?x. For example
1549 f :: (?x::Int) => Char -> Char
1552 then the constraint (?x::Int) arising from the call to f will
1553 force the binding for ?x to be of type Int.
1556 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1559 -- We need a loop so that we do improvement, and then
1560 -- (next time round) generate a binding to connect the two
1562 -- Here the two ?x's have different types, and improvement
1563 -- makes them the same.
1565 tcSimplifyIPs given_ips wanteds
1566 = do { wanteds' <- zonkInsts wanteds
1567 ; given_ips' <- zonkInsts given_ips
1568 -- Unusually for checking, we *must* zonk the given_ips
1570 ; let env = mkRedEnv doc try_me given_ips'
1571 ; (improved, binds, irreds, elim_skolems) <- reduceContext env wanteds'
1574 ; if not improved then
1575 ASSERT( all is_free irreds )
1576 do { extendLIEs irreds
1579 tcSimplifyIPs given_ips wanteds }
1581 doc = text "tcSimplifyIPs" <+> ppr given_ips
1582 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1583 is_free inst = isFreeWrtIPs ip_set inst
1585 -- Simplify any methods that mention the implicit parameter
1586 try_me inst | is_free inst = Stop
1587 | otherwise = ReduceMe NoSCs
1591 %************************************************************************
1593 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1595 %************************************************************************
1597 When doing a binding group, we may have @Insts@ of local functions.
1598 For example, we might have...
1600 let f x = x + 1 -- orig local function (overloaded)
1601 f.1 = f Int -- two instances of f
1606 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1607 where @f@ is in scope; those @Insts@ must certainly not be passed
1608 upwards towards the top-level. If the @Insts@ were binding-ified up
1609 there, they would have unresolvable references to @f@.
1611 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1612 For each method @Inst@ in the @init_lie@ that mentions one of the
1613 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1614 @LIE@), as well as the @HsBinds@ generated.
1617 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1618 -- Simlifies only MethodInsts, and generate only bindings of form
1620 -- We're careful not to even generate bindings of the form
1622 -- You'd think that'd be fine, but it interacts with what is
1623 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1625 bindInstsOfLocalFuns wanteds local_ids
1626 | null overloaded_ids = do
1629 return emptyLHsBinds
1632 = do { (irreds, binds) <- gentleInferLoop doc for_me
1633 ; extendLIEs not_for_me
1637 doc = text "bindInsts" <+> ppr local_ids
1638 overloaded_ids = filter is_overloaded local_ids
1639 is_overloaded id = isOverloadedTy (idType id)
1640 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1642 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1643 -- so it's worth building a set, so that
1644 -- lookup (in isMethodFor) is faster
1648 %************************************************************************
1650 \subsection{Data types for the reduction mechanism}
1652 %************************************************************************
1654 The main control over context reduction is here
1658 = RedEnv { red_doc :: SDoc -- The context
1659 , red_try_me :: Inst -> WhatToDo
1660 , red_improve :: Bool -- True <=> do improvement
1661 , red_givens :: [Inst] -- All guaranteed rigid
1663 -- but see Note [Rigidity]
1664 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1665 -- See Note [RedStack]
1669 -- The red_givens are rigid so far as cmpInst is concerned.
1670 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1671 -- let ?x = e in ...
1672 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1673 -- But that doesn't affect the comparison, which is based only on mame.
1676 -- The red_stack pair (n,insts) pair is just used for error reporting.
1677 -- 'n' is always the depth of the stack.
1678 -- The 'insts' is the stack of Insts being reduced: to produce X
1679 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1682 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1683 mkRedEnv doc try_me givens
1684 = RedEnv { red_doc = doc, red_try_me = try_me,
1685 red_givens = givens,
1687 red_improve = True }
1689 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1690 -- Do not do improvement; no givens
1691 mkNoImproveRedEnv doc try_me
1692 = RedEnv { red_doc = doc, red_try_me = try_me,
1695 red_improve = True }
1698 = ReduceMe WantSCs -- Try to reduce this
1699 -- If there's no instance, add the inst to the
1700 -- irreductible ones, but don't produce an error
1701 -- message of any kind.
1702 -- It might be quite legitimate such as (Eq a)!
1704 | Stop -- Return as irreducible unless it can
1705 -- be reduced to a constant in one step
1706 -- Do not add superclasses; see
1708 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1709 -- of a predicate when adding it to the avails
1710 -- The reason for this flag is entirely the super-class loop problem
1711 -- Note [SUPER-CLASS LOOP 1]
1713 zonkRedEnv :: RedEnv -> TcM RedEnv
1715 = do { givens' <- mapM zonkInst (red_givens env)
1716 ; return $ env {red_givens = givens'}
1721 %************************************************************************
1723 \subsection[reduce]{@reduce@}
1725 %************************************************************************
1727 Note [Ancestor Equalities]
1728 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1729 During context reduction, we add to the wanted equalities also those
1730 equalities that (transitively) occur in superclass contexts of wanted
1731 class constraints. Consider the following code
1733 class a ~ Int => C a
1736 If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
1737 substituting Int for a. Hence, we ultimately want (C Int), which we
1738 discharge with the explicit instance.
1741 reduceContext :: RedEnv
1743 -> TcM (ImprovementDone,
1744 TcDictBinds, -- Dictionary bindings
1745 [Inst], -- Irreducible
1746 TcM ()) -- Undo skolems from SkolemOccurs
1748 reduceContext env wanteds
1749 = do { traceTc (text "reduceContext" <+> (vcat [
1750 text "----------------------",
1752 text "given" <+> ppr (red_givens env),
1753 text "wanted" <+> ppr wanteds,
1754 text "----------------------"
1758 ; let givens = red_givens env
1759 (given_eqs0, given_dicts0) = partition isEqInst givens
1760 (wanted_eqs0, wanted_non_eqs) = partition isEqInst wanteds
1761 (wanted_implics0, wanted_dicts0) = partition isImplicInst wanted_non_eqs
1763 -- We want to add as wanted equalities those that (transitively)
1764 -- occur in superclass contexts of wanted class constraints.
1765 -- See Note [Ancestor Equalities]
1766 ; ancestor_eqs <- ancestorEqualities wanted_dicts0
1767 ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
1768 ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
1770 -- 1. Normalise the *given* *equality* constraints
1771 ; (given_eqs, eliminate_skolems) <- normaliseGivenEqs given_eqs0
1773 -- 2. Normalise the *given* *dictionary* constraints
1774 -- wrt. the toplevel and given equations
1775 ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
1778 -- 5. Build the Avail mapping from "given_dicts"
1779 ; (init_state, extra_givens) <- getLIE $ do
1780 { init_state <- foldlM addGiven emptyAvails given_dicts
1784 -- *** ToDo: what to do with the "extra_givens"? For the
1785 -- moment I'm simply discarding them, which is probably wrong
1787 -- 7. Normalise the *wanted* *dictionary* constraints
1788 -- wrt. the toplevel and given equations
1789 -- NB: normalisation includes zonking as part of what it does
1790 -- so it's important to do it after any unifications
1791 -- that happened as a result of the addGivens
1792 ; (wanted_dicts,normalise_binds1) <- normaliseWantedDicts given_eqs wanted_dicts0
1794 -- 6. Solve the *wanted* *dictionary* constraints (not implications)
1795 -- This may expose some further equational constraints...
1796 ; (avails, extra_eqs) <- getLIE (reduceList env wanted_dicts init_state)
1797 ; (dict_binds, bound_dicts, dict_irreds) <- extractResults avails wanted_dicts
1798 ; traceTc $ text "reduceContext extractresults" <+> vcat
1799 [ppr avails,ppr wanted_dicts,ppr dict_binds]
1801 -- *** ToDo: what to do with the "extra_eqs"? For the
1802 -- moment I'm simply discarding them, which is probably wrong
1804 -- Solve the wanted *implications*. In doing so, we can provide
1805 -- as "given" all the dicts that were originally given,
1806 -- *or* for which we now have bindings,
1807 -- *or* which are now irreds
1808 ; let implic_env = env { red_givens = givens ++ bound_dicts ++ dict_irreds }
1809 ; (implic_binds_s, implic_irreds_s) <- mapAndUnzipM (reduceImplication implic_env) wanted_implics0
1810 ; let implic_binds = unionManyBags implic_binds_s
1811 implic_irreds = concat implic_irreds_s
1813 -- 3. Solve the *wanted* *equation* constraints
1814 ; eq_irreds0 <- solveWantedEqs given_eqs wanted_eqs
1816 -- 4. Normalise the *wanted* equality constraints with respect to
1818 ; eq_irreds <- normaliseWantedEqs eq_irreds0
1820 -- 8. Substitute the wanted *equations* in the wanted *dictionaries*
1821 ; let irreds = dict_irreds ++ implic_irreds
1822 ; (norm_irreds, normalise_binds2) <- substEqInDictInsts True {-wanted-}
1825 -- 9. eliminate the artificial skolem constants introduced in 1.
1826 -- ; eliminate_skolems
1828 -- Figure out whether we should go round again
1829 -- My current plan is to see if any of the mutable tyvars in
1830 -- givens or irreds has been filled in by improvement.
1831 -- If so, there is merit in going around again, because
1832 -- we may make further progress
1834 -- ToDo: is it only mutable stuff? We may have exposed new
1835 -- equality constraints and should probably go round again
1836 -- then as well. But currently we are dropping them on the
1839 ; let all_irreds = norm_irreds ++ eq_irreds
1840 ; improved <- anyM isFilledMetaTyVar $ varSetElems $
1841 tyVarsOfInsts (givens ++ all_irreds)
1843 -- The old plan (fragile)
1844 -- improveed = availsImproved avails
1845 -- || (not $ isEmptyBag normalise_binds1)
1846 -- || (not $ isEmptyBag normalise_binds2)
1847 -- || (any isEqInst irreds)
1849 ; traceTc (text "reduceContext end" <+> (vcat [
1850 text "----------------------",
1852 text "given" <+> ppr givens,
1853 text "given_eqs" <+> ppr given_eqs,
1854 text "wanted" <+> ppr wanteds,
1855 text "wanted_dicts" <+> ppr wanted_dicts,
1857 text "avails" <+> pprAvails avails,
1858 text "improved =" <+> ppr improved,
1859 text "(all) irreds = " <+> ppr all_irreds,
1860 text "dict-binds = " <+> ppr dict_binds,
1861 text "implic-binds = " <+> ppr implic_binds,
1862 text "----------------------"
1866 given_binds `unionBags` normalise_binds1
1867 `unionBags` normalise_binds2
1868 `unionBags` dict_binds
1869 `unionBags` implic_binds,
1874 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1875 tcImproveOne avails inst
1876 | not (isDict inst) = return False
1878 = do { inst_envs <- tcGetInstEnvs
1879 ; let eqns = improveOne (classInstances inst_envs)
1880 (dictPred inst, pprInstArising inst)
1881 [ (dictPred p, pprInstArising p)
1882 | p <- availsInsts avails, isDict p ]
1883 -- Avails has all the superclasses etc (good)
1884 -- It also has all the intermediates of the deduction (good)
1885 -- It does not have duplicates (good)
1886 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1887 -- so that improve will see them separate
1888 ; traceTc (text "improveOne" <+> ppr inst)
1891 unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))]
1892 -> TcM ImprovementDone
1893 unifyEqns [] = return False
1895 = do { traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns))
1899 unify ((qtvs, pairs), what1, what2)
1900 = addErrCtxtM (mkEqnMsg what1 what2) $ do
1901 (_, _, tenv) <- tcInstTyVars (varSetElems qtvs)
1902 mapM_ (unif_pr tenv) pairs
1903 unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1905 pprEquationDoc (eqn, (p1,w1), (p2,w2)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
1907 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
1908 = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
1909 ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
1910 ; let msg = vcat [ptext SLIT("When using functional dependencies to combine"),
1911 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
1912 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
1913 ; return (tidy_env, msg) }
1916 The main context-reduction function is @reduce@. Here's its game plan.
1919 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
1920 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
1921 = do { traceTc (text "reduceList " <+> (ppr wanteds $$ ppr state))
1925 dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n)
1926 2 (ifPprDebug (nest 2 (pprStack stk))))
1929 ; if n >= ctxtStkDepth dopts then
1930 failWithTc (reduceDepthErr n stk)
1934 go [] state = return state
1935 go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
1938 -- Base case: we're done!
1939 reduce env wanted avails
1940 -- It's the same as an existing inst, or a superclass thereof
1941 | Just avail <- findAvail avails wanted
1942 = do { traceTc (text "reduce: found " <+> ppr wanted)
1947 = do { traceTc (text "reduce" <+> ppr wanted $$ ppr avails)
1948 ; case red_try_me env wanted of {
1949 Stop -> try_simple (addIrred NoSCs);
1950 -- See Note [No superclasses for Stop]
1952 ReduceMe want_scs -> do -- It should be reduced
1953 { (avails, lookup_result) <- reduceInst env avails wanted
1954 ; case lookup_result of
1955 NoInstance -> addIrred want_scs avails wanted
1956 -- Add it and its superclasses
1958 GenInst [] rhs -> addWanted want_scs avails wanted rhs []
1960 GenInst wanteds' rhs
1961 -> do { avails1 <- addIrred NoSCs avails wanted
1962 ; avails2 <- reduceList env wanteds' avails1
1963 ; addWanted want_scs avails2 wanted rhs wanteds' } }
1964 -- Temporarily do addIrred *before* the reduceList,
1965 -- which has the effect of adding the thing we are trying
1966 -- to prove to the database before trying to prove the things it
1967 -- needs. See note [RECURSIVE DICTIONARIES]
1968 -- NB: we must not do an addWanted before, because that adds the
1969 -- superclasses too, and that can lead to a spurious loop; see
1970 -- the examples in [SUPERCLASS-LOOP]
1971 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1974 -- First, see if the inst can be reduced to a constant in one step
1975 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
1976 -- Don't bother for implication constraints, which take real work
1977 try_simple do_this_otherwise
1978 = do { res <- lookupSimpleInst wanted
1980 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
1981 other -> do_this_otherwise avails wanted }
1985 Note [SUPERCLASS-LOOP 2]
1986 ~~~~~~~~~~~~~~~~~~~~~~~~
1987 But the above isn't enough. Suppose we are *given* d1:Ord a,
1988 and want to deduce (d2:C [a]) where
1990 class Ord a => C a where
1991 instance Ord [a] => C [a] where ...
1993 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
1994 superclasses of C [a] to avails. But we must not overwrite the binding
1995 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
1998 Here's another variant, immortalised in tcrun020
1999 class Monad m => C1 m
2000 class C1 m => C2 m x
2001 instance C2 Maybe Bool
2002 For the instance decl we need to build (C1 Maybe), and it's no good if
2003 we run around and add (C2 Maybe Bool) and its superclasses to the avails
2004 before we search for C1 Maybe.
2006 Here's another example
2007 class Eq b => Foo a b
2008 instance Eq a => Foo [a] a
2012 we'll first deduce that it holds (via the instance decl). We must not
2013 then overwrite the Eq t constraint with a superclass selection!
2015 At first I had a gross hack, whereby I simply did not add superclass constraints
2016 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
2017 becuase it lost legitimate superclass sharing, and it still didn't do the job:
2018 I found a very obscure program (now tcrun021) in which improvement meant the
2019 simplifier got two bites a the cherry... so something seemed to be an Stop
2020 first time, but reducible next time.
2022 Now we implement the Right Solution, which is to check for loops directly
2023 when adding superclasses. It's a bit like the occurs check in unification.
2026 Note [RECURSIVE DICTIONARIES]
2027 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2029 data D r = ZeroD | SuccD (r (D r));
2031 instance (Eq (r (D r))) => Eq (D r) where
2032 ZeroD == ZeroD = True
2033 (SuccD a) == (SuccD b) = a == b
2036 equalDC :: D [] -> D [] -> Bool;
2039 We need to prove (Eq (D [])). Here's how we go:
2043 by instance decl, holds if
2047 by instance decl of Eq, holds if
2049 where d2 = dfEqList d3
2052 But now we can "tie the knot" to give
2058 and it'll even run! The trick is to put the thing we are trying to prove
2059 (in this case Eq (D []) into the database before trying to prove its
2060 contributing clauses.
2063 %************************************************************************
2065 Reducing a single constraint
2067 %************************************************************************
2070 ---------------------------------------------
2071 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
2072 reduceInst env avails other_inst
2073 = do { result <- lookupSimpleInst other_inst
2074 ; return (avails, result) }
2077 Note [Equational Constraints in Implication Constraints]
2078 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2080 An implication constraint is of the form
2082 where Given and Wanted may contain both equational and dictionary
2083 constraints. The delay and reduction of these two kinds of constraints
2086 -) In the generated code, wanted Dictionary constraints are wrapped up in an
2087 implication constraint that is created at the code site where the wanted
2088 dictionaries can be reduced via a let-binding. This let-bound implication
2089 constraint is deconstructed at the use-site of the wanted dictionaries.
2091 -) While the reduction of equational constraints is also delayed, the delay
2092 is not manifest in the generated code. The required evidence is generated
2093 in the code directly at the use-site. There is no let-binding and deconstruction
2094 necessary. The main disadvantage is that we cannot exploit sharing as the
2095 same evidence may be generated at multiple use-sites. However, this disadvantage
2096 is limited because it only concerns coercions which are erased.
2098 The different treatment is motivated by the different in representation. Dictionary
2099 constraints require manifest runtime dictionaries, while equations require coercions
2103 ---------------------------------------------
2104 reduceImplication :: RedEnv
2106 -> TcM (TcDictBinds, [Inst])
2109 Suppose we are simplifying the constraint
2110 forall bs. extras => wanted
2111 in the context of an overall simplification problem with givens 'givens'.
2114 * The 'givens' need not mention any of the quantified type variables
2115 e.g. forall {}. Eq a => Eq [a]
2116 forall {}. C Int => D (Tree Int)
2118 This happens when you have something like
2120 T1 :: Eq a => a -> T a
2123 f x = ...(case x of { T1 v -> v==v })...
2126 -- ToDo: should we instantiate tvs? I think it's not necessary
2128 -- Note on coercion variables:
2130 -- The extra given coercion variables are bound at two different sites:
2131 -- -) in the creation context of the implication constraint
2132 -- the solved equational constraints use these binders
2134 -- -) at the solving site of the implication constraint
2135 -- the solved dictionaries use these binders
2136 -- these binders are generated by reduceImplication
2138 reduceImplication env
2139 orig_implic@(ImplicInst { tci_name = name, tci_loc = inst_loc,
2140 tci_tyvars = tvs, tci_reft = emptyRefinement,
2141 tci_given = extra_givens, tci_wanted = wanteds })
2142 = do { -- Solve the sub-problem
2143 ; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
2144 env' = env { red_givens = extra_givens ++ red_givens env
2145 , red_doc = sep [ptext SLIT("reduceImplication for")
2147 nest 2 (parens $ ptext SLIT("within")
2149 , red_try_me = try_me }
2151 ; traceTc (text "reduceImplication" <+> vcat
2152 [ ppr (red_givens env), ppr extra_givens,
2154 ; (irreds, binds) <- checkLoop env' wanteds
2155 ; let (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
2156 -- SLPJ Sept 07: I think this is bogus; currently
2157 -- there are no Eqinsts in extra_givens
2158 dict_ids = map instToId extra_dict_givens
2160 -- Note [Reducing implication constraints]
2161 -- Tom -- update note, put somewhere!
2163 ; traceTc (text "reduceImplication result" <+> vcat
2164 [ppr irreds, ppr binds])
2166 ; -- extract superclass binds
2167 -- (sc_binds,_) <- extractResults avails []
2168 -- ; traceTc (text "reduceImplication sc_binds" <+> vcat
2169 -- [ppr sc_binds, ppr avails])
2172 -- SLPJ Sept 07: what if improvement happened inside the checkLoop?
2173 -- Then we must iterate the outer loop too!
2175 ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
2177 -- Progress is no longer measered by the number of bindings
2178 ; if (isEmptyLHsBinds binds) && (not $ null irreds) then -- No progress
2179 -- If there are any irreds, we back off and do nothing
2180 return (emptyBag, [orig_implic])
2182 { (simpler_implic_insts, bind)
2183 <- makeImplicationBind inst_loc tvs extra_givens irreds
2184 -- This binding is useless if the recursive simplification
2185 -- made no progress; but currently we don't try to optimise that
2186 -- case. After all, we only try hard to reduce at top level, or
2187 -- when inferring types.
2189 ; let dict_wanteds = filter (not . isEqInst) wanteds
2190 -- TOMDO: given equational constraints bug!
2191 -- we need a different evidence for given
2192 -- equations depending on whether we solve
2193 -- dictionary constraints or equational constraints
2195 eq_tyvars = varSetElems $ tyVarsOfTypes $ map eqInstType extra_eq_givens
2196 -- SLPJ Sept07: this looks Utterly Wrong to me, but I think
2197 -- that current extra_givens has no EqInsts, so
2198 -- it makes no difference
2199 co = wrap_inline -- Note [Always inline implication constraints]
2201 <.> mkWpLams eq_tyvars
2202 <.> mkWpLams dict_ids
2203 <.> WpLet (binds `unionBags` bind)
2204 wrap_inline | null dict_ids = idHsWrapper
2205 | otherwise = WpInline
2206 rhs = mkHsWrap co payload
2207 loc = instLocSpan inst_loc
2208 payload | [dict_wanted] <- dict_wanteds = HsVar (instToId dict_wanted)
2209 | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) dict_wanteds) Boxed
2212 ; traceTc (vcat [text "reduceImplication" <+> ppr name,
2213 ppr simpler_implic_insts,
2214 text "->" <+> ppr rhs])
2215 ; return (unitBag (L loc (VarBind (instToId orig_implic) (L loc rhs))),
2216 simpler_implic_insts)
2221 Note [Always inline implication constraints]
2222 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2223 Suppose an implication constraint floats out of an INLINE function.
2224 Then although the implication has a single call site, it won't be
2225 inlined. And that is bad because it means that even if there is really
2226 *no* overloading (type signatures specify the exact types) there will
2227 still be dictionary passing in the resulting code. To avert this,
2228 we mark the implication constraints themselves as INLINE, at least when
2229 there is no loss of sharing as a result.
2231 Note [Freeness and implications]
2232 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2233 It's hard to say when an implication constraint can be floated out. Consider
2234 forall {} Eq a => Foo [a]
2235 The (Foo [a]) doesn't mention any of the quantified variables, but it
2236 still might be partially satisfied by the (Eq a).
2238 There is a useful special case when it *is* easy to partition the
2239 constraints, namely when there are no 'givens'. Consider
2240 forall {a}. () => Bar b
2241 There are no 'givens', and so there is no reason to capture (Bar b).
2242 We can let it float out. But if there is even one constraint we
2243 must be much more careful:
2244 forall {a}. C a b => Bar (m b)
2245 because (C a b) might have a superclass (D b), from which we might
2246 deduce (Bar [b]) when m later gets instantiated to []. Ha!
2248 Here is an even more exotic example
2250 Now consider the constraint
2251 forall b. D Int b => C Int
2252 We can satisfy the (C Int) from the superclass of D, so we don't want
2253 to float the (C Int) out, even though it mentions no type variable in
2256 Note [Pruning the givens in an implication constraint]
2257 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2258 Suppose we are about to form the implication constraint
2259 forall tvs. Eq a => Ord b
2260 The (Eq a) cannot contribute to the (Ord b), because it has no access to
2261 the type variable 'b'. So we could filter out the (Eq a) from the givens.
2263 Doing so would be a bit tidier, but all the implication constraints get
2264 simplified away by the optimiser, so it's no great win. So I don't take
2265 advantage of that at the moment.
2267 If you do, BE CAREFUL of wobbly type variables.
2270 %************************************************************************
2272 Avails and AvailHow: the pool of evidence
2274 %************************************************************************
2278 data Avails = Avails !ImprovementDone !AvailEnv
2280 type ImprovementDone = Bool -- True <=> some unification has happened
2281 -- so some Irreds might now be reducible
2282 -- keys that are now
2284 type AvailEnv = FiniteMap Inst AvailHow
2286 = IsIrred -- Used for irreducible dictionaries,
2287 -- which are going to be lambda bound
2289 | Given Inst -- Used for dictionaries for which we have a binding
2290 -- e.g. those "given" in a signature
2292 | Rhs -- Used when there is a RHS
2293 (LHsExpr TcId) -- The RHS
2294 [Inst] -- Insts free in the RHS; we need these too
2296 instance Outputable Avails where
2299 pprAvails (Avails imp avails)
2300 = vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty)
2302 vcat [ sep [ppr inst, nest 2 (equals <+> ppr avail)]
2303 | (inst,avail) <- fmToList avails ]]
2305 instance Outputable AvailHow where
2308 -------------------------
2309 pprAvail :: AvailHow -> SDoc
2310 pprAvail IsIrred = text "Irred"
2311 pprAvail (Given x) = text "Given" <+> ppr x
2312 pprAvail (Rhs rhs bs) = sep [text "Rhs" <+> ppr bs,
2315 -------------------------
2316 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
2317 extendAvailEnv env inst avail = addToFM env inst avail
2319 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
2320 findAvailEnv env wanted = lookupFM env wanted
2321 -- NB 1: the Ord instance of Inst compares by the class/type info
2322 -- *not* by unique. So
2323 -- d1::C Int == d2::C Int
2325 emptyAvails :: Avails
2326 emptyAvails = Avails False emptyFM
2328 findAvail :: Avails -> Inst -> Maybe AvailHow
2329 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
2331 elemAvails :: Inst -> Avails -> Bool
2332 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
2334 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
2336 extendAvails avails@(Avails imp env) inst avail
2337 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
2338 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
2340 availsInsts :: Avails -> [Inst]
2341 availsInsts (Avails _ avails) = keysFM avails
2343 availsImproved (Avails imp _) = imp
2345 updateImprovement :: Avails -> Avails -> Avails
2346 -- (updateImprovement a1 a2) sets a1's improvement flag from a2
2347 updateImprovement (Avails _ avails1) (Avails imp2 _) = Avails imp2 avails1
2350 Extracting the bindings from a bunch of Avails.
2351 The bindings do *not* come back sorted in dependency order.
2352 We assume that they'll be wrapped in a big Rec, so that the
2353 dependency analyser can sort them out later
2356 type DoneEnv = FiniteMap Inst [Id]
2357 -- Tracks which things we have evidence for
2359 extractResults :: Avails
2361 -> TcM (TcDictBinds, -- Bindings
2362 [Inst], -- The insts bound by the bindings
2363 [Inst]) -- Irreducible ones
2364 -- Note [Reducing implication constraints]
2366 extractResults (Avails _ avails) wanteds
2367 = go emptyBag [] [] emptyFM wanteds
2369 go :: TcDictBinds -- Bindings for dicts
2370 -> [Inst] -- Bound by the bindings
2372 -> DoneEnv -- Has an entry for each inst in the above three sets
2374 -> TcM (TcDictBinds, [Inst], [Inst])
2375 go binds bound_dicts irreds done []
2376 = return (binds, bound_dicts, irreds)
2378 go binds bound_dicts irreds done (w:ws)
2379 | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w
2380 = if w_id `elem` done_ids then
2381 go binds bound_dicts irreds done ws
2383 go (add_bind (nlHsVar done_id)) bound_dicts irreds
2384 (addToFM done w (done_id : w_id : rest_done_ids)) ws
2386 | otherwise -- Not yet done
2387 = case findAvailEnv avails w of
2388 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
2389 go binds bound_dicts irreds done ws
2391 Just IsIrred -> go binds bound_dicts (w:irreds) done' ws
2393 Just (Rhs rhs ws') -> go (add_bind rhs) (w:bound_dicts) irreds done' (ws' ++ ws)
2395 Just (Given g) -> go binds' bound_dicts irreds (addToFM done w [g_id]) ws
2398 binds' | w_id == g_id = binds
2399 | otherwise = add_bind (nlHsVar g_id)
2402 done' = addToFM done w [w_id]
2403 add_bind rhs = addInstToDictBind binds w rhs
2407 Note [No superclasses for Stop]
2408 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2409 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2410 add it to avails, so that any other equal Insts will be commoned up
2411 right here. However, we do *not* add superclasses. If we have
2414 but a is not bound here, then we *don't* want to derive dn from df
2415 here lest we lose sharing.
2418 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2419 addWanted want_scs avails wanted rhs_expr wanteds
2420 = addAvailAndSCs want_scs avails wanted avail
2422 avail = Rhs rhs_expr wanteds
2424 addGiven :: Avails -> Inst -> TcM Avails
2425 addGiven avails given = addAvailAndSCs AddSCs avails given (Given given)
2426 -- Always add superclasses for 'givens'
2428 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2429 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2430 -- so the assert isn't true
2434 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2435 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2436 addAvailAndSCs want_scs avails irred IsIrred
2438 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2439 addAvailAndSCs want_scs avails inst avail
2440 | not (isClassDict inst) = extendAvails avails inst avail
2441 | NoSCs <- want_scs = extendAvails avails inst avail
2442 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2443 ; avails' <- extendAvails avails inst avail
2444 ; addSCs is_loop avails' inst }
2446 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2447 -- Note: this compares by *type*, not by Unique
2448 deps = findAllDeps (unitVarSet (instToVar inst)) avail
2449 dep_tys = map idType (varSetElems deps)
2451 findAllDeps :: IdSet -> AvailHow -> IdSet
2452 -- Find all the Insts that this one depends on
2453 -- See Note [SUPERCLASS-LOOP 2]
2454 -- Watch out, though. Since the avails may contain loops
2455 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2456 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2457 findAllDeps so_far other = so_far
2459 find_all :: IdSet -> Inst -> IdSet
2461 | isEqInst kid = so_far
2462 | kid_id `elemVarSet` so_far = so_far
2463 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2464 | otherwise = so_far'
2466 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2467 kid_id = instToId kid
2469 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2470 -- Add all the superclasses of the Inst to Avails
2471 -- The first param says "don't do this because the original thing
2472 -- depends on this one, so you'd build a loop"
2473 -- Invariant: the Inst is already in Avails.
2475 addSCs is_loop avails dict
2476 = ASSERT( isDict dict )
2477 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2478 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2480 (clas, tys) = getDictClassTys dict
2481 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2482 sc_theta' = filter (not . isEqPred) $
2483 substTheta (zipTopTvSubst tyvars tys) sc_theta
2485 add_sc avails (sc_dict, sc_sel)
2486 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2487 | is_given sc_dict = return avails
2488 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2489 ; addSCs is_loop avails' sc_dict }
2491 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2492 co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
2494 is_given :: Inst -> Bool
2495 is_given sc_dict = case findAvail avails sc_dict of
2496 Just (Given _) -> True -- Given is cheaper than superclass selection
2499 -- From the a set of insts obtain all equalities that (transitively) occur in
2500 -- superclass contexts of class constraints (aka the ancestor equalities).
2502 ancestorEqualities :: [Inst] -> TcM [Inst]
2504 = mapM mkWantedEqInst -- turn only equality predicates..
2505 . filter isEqPred -- ..into wanted equality insts
2507 . addAEsToBag emptyBag -- collect the superclass constraints..
2508 . map dictPred -- ..of all predicates in a bag
2509 . filter isClassDict
2511 addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
2512 addAEsToBag bag [] = bag
2513 addAEsToBag bag (pred:preds)
2514 | pred `elemBag` bag = addAEsToBag bag preds
2515 | isEqPred pred = addAEsToBag bagWithPred preds
2516 | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
2517 | otherwise = addAEsToBag bag preds
2519 bagWithPred = bag `snocBag` pred
2520 predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
2522 (tyvars, sc_theta, _, _) = classBigSig clas
2523 (clas, tys) = getClassPredTys pred
2527 %************************************************************************
2529 \section{tcSimplifyTop: defaulting}
2531 %************************************************************************
2534 @tcSimplifyTop@ is called once per module to simplify all the constant
2535 and ambiguous Insts.
2537 We need to be careful of one case. Suppose we have
2539 instance Num a => Num (Foo a b) where ...
2541 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2542 to (Num x), and default x to Int. But what about y??
2544 It's OK: the final zonking stage should zap y to (), which is fine.
2548 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2549 tcSimplifyTop wanteds
2550 = tc_simplify_top doc False wanteds
2552 doc = text "tcSimplifyTop"
2554 tcSimplifyInteractive wanteds
2555 = tc_simplify_top doc True wanteds
2557 doc = text "tcSimplifyInteractive"
2559 -- The TcLclEnv should be valid here, solely to improve
2560 -- error message generation for the monomorphism restriction
2561 tc_simplify_top doc interactive wanteds
2562 = do { dflags <- getDOpts
2563 ; wanteds <- zonkInsts wanteds
2564 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2566 ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
2567 ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
2568 -- ; (irreds1, binds1) <- gentleInferLoop doc1 wanteds
2569 ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
2570 ; (irreds2, binds2) <- approximateImplications doc2 (\d -> True) irreds1
2571 ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
2573 -- Use the defaulting rules to do extra unification
2574 -- NB: irreds2 are already zonked
2575 ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
2577 -- Deal with implicit parameters
2578 ; let (bad_ips, non_ips) = partition isIPDict irreds3
2579 (ambigs, others) = partition isTyVarDict non_ips
2581 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2583 ; addNoInstanceErrs others
2584 ; addTopAmbigErrs ambigs
2586 ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
2588 doc1 = doc <+> ptext SLIT("(first round)")
2589 doc2 = doc <+> ptext SLIT("(approximate)")
2590 doc3 = doc <+> ptext SLIT("(disambiguate)")
2593 If a dictionary constrains a type variable which is
2594 * not mentioned in the environment
2595 * and not mentioned in the type of the expression
2596 then it is ambiguous. No further information will arise to instantiate
2597 the type variable; nor will it be generalised and turned into an extra
2598 parameter to a function.
2600 It is an error for this to occur, except that Haskell provided for
2601 certain rules to be applied in the special case of numeric types.
2603 * at least one of its classes is a numeric class, and
2604 * all of its classes are numeric or standard
2605 then the type variable can be defaulted to the first type in the
2606 default-type list which is an instance of all the offending classes.
2608 So here is the function which does the work. It takes the ambiguous
2609 dictionaries and either resolves them (producing bindings) or
2610 complains. It works by splitting the dictionary list by type
2611 variable, and using @disambigOne@ to do the real business.
2613 @disambigOne@ assumes that its arguments dictionaries constrain all
2614 the same type variable.
2616 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2617 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2618 the most common use of defaulting is code like:
2620 _ccall_ foo `seqPrimIO` bar
2622 Since we're not using the result of @foo@, the result if (presumably)
2626 disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
2627 -- Just does unification to fix the default types
2628 -- The Insts are assumed to be pre-zonked
2629 disambiguate doc interactive dflags insts
2631 = return (insts, emptyBag)
2633 | null defaultable_groups
2634 = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
2635 ; return (insts, emptyBag) }
2638 = do { -- Figure out what default types to use
2639 default_tys <- getDefaultTys extended_defaulting ovl_strings
2641 ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2642 ; mapM_ (disambigGroup default_tys) defaultable_groups
2644 -- disambigGroup does unification, hence try again
2645 ; tryHardCheckLoop doc insts }
2648 extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
2649 ovl_strings = dopt Opt_OverloadedStrings dflags
2651 unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
2652 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2653 (unaries, bad_tvs_s) = partitionWith find_unary insts
2654 bad_tvs = unionVarSets bad_tvs_s
2656 -- Finds unary type-class constraints
2657 find_unary d@(Dict {tci_pred = ClassP cls [ty]})
2658 | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
2659 find_unary inst = Right (tyVarsOfInst inst)
2661 -- Group by type variable
2662 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2663 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2664 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2666 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2667 defaultable_group ds@((_,_,tv):_)
2668 = isTyConableTyVar tv -- Note [Avoiding spurious errors]
2669 && not (tv `elemVarSet` bad_tvs)
2670 && defaultable_classes [c | (_,c,_) <- ds]
2671 defaultable_group [] = panic "defaultable_group"
2673 defaultable_classes clss
2674 | extended_defaulting = any isInteractiveClass clss
2675 | otherwise = all is_std_class clss && (any is_num_class clss)
2677 -- In interactive mode, or with -fextended-default-rules,
2678 -- we default Show a to Show () to avoid graututious errors on "show []"
2679 isInteractiveClass cls
2680 = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2682 is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2683 -- is_num_class adds IsString to the standard numeric classes,
2684 -- when -foverloaded-strings is enabled
2686 is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2687 -- Similarly is_std_class
2689 -----------------------
2690 disambigGroup :: [Type] -- The default types
2691 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2692 -> TcM () -- Just does unification, to fix the default types
2694 disambigGroup default_tys dicts
2695 = try_default default_tys
2697 (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
2698 classes = [c | (_,c,_) <- dicts]
2700 try_default [] = return ()
2701 try_default (default_ty : default_tys)
2702 = tryTcLIE_ (try_default default_tys) $
2703 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2704 -- This may fail; then the tryTcLIE_ kicks in
2705 -- Failure here is caused by there being no type in the
2706 -- default list which can satisfy all the ambiguous classes.
2707 -- For example, if Real a is reqd, but the only type in the
2708 -- default list is Int.
2710 -- After this we can't fail
2711 ; warnDefault dicts default_ty
2712 ; unifyType default_ty (mkTyVarTy tyvar)
2713 ; return () -- TOMDO: do something with the coercion
2717 -----------------------
2718 getDefaultTys :: Bool -> Bool -> TcM [Type]
2719 getDefaultTys extended_deflts ovl_strings
2720 = do { mb_defaults <- getDeclaredDefaultTys
2721 ; case mb_defaults of {
2722 Just tys -> return tys ; -- User-supplied defaults
2725 -- No use-supplied default
2726 -- Use [Integer, Double], plus modifications
2727 { integer_ty <- tcMetaTy integerTyConName
2728 ; checkWiredInTyCon doubleTyCon
2729 ; string_ty <- tcMetaTy stringTyConName
2730 ; return (opt_deflt extended_deflts unitTy
2731 -- Note [Default unitTy]
2733 [integer_ty,doubleTy]
2735 opt_deflt ovl_strings string_ty) } } }
2737 opt_deflt True ty = [ty]
2738 opt_deflt False ty = []
2741 Note [Default unitTy]
2742 ~~~~~~~~~~~~~~~~~~~~~
2743 In interative mode (or with -fextended-default-rules) we add () as the first type we
2744 try when defaulting. This has very little real impact, except in the following case.
2746 Text.Printf.printf "hello"
2747 This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
2748 want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
2749 default the 'a' to (), rather than to Integer (which is what would otherwise happen;
2750 and then GHCi doesn't attempt to print the (). So in interactive mode, we add
2751 () to the list of defaulting types. See Trac #1200.
2753 Note [Avoiding spurious errors]
2754 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2755 When doing the unification for defaulting, we check for skolem
2756 type variables, and simply don't default them. For example:
2757 f = (*) -- Monomorphic
2758 g :: Num a => a -> a
2760 Here, we get a complaint when checking the type signature for g,
2761 that g isn't polymorphic enough; but then we get another one when
2762 dealing with the (Num a) context arising from f's definition;
2763 we try to unify a with Int (to default it), but find that it's
2764 already been unified with the rigid variable from g's type sig
2767 %************************************************************************
2769 \subsection[simple]{@Simple@ versions}
2771 %************************************************************************
2773 Much simpler versions when there are no bindings to make!
2775 @tcSimplifyThetas@ simplifies class-type constraints formed by
2776 @deriving@ declarations and when specialising instances. We are
2777 only interested in the simplified bunch of class/type constraints.
2779 It simplifies to constraints of the form (C a b c) where
2780 a,b,c are type variables. This is required for the context of
2781 instance declarations.
2784 tcSimplifyDeriv :: InstOrigin
2786 -> ThetaType -- Wanted
2787 -> TcM ThetaType -- Needed
2788 -- Given instance (wanted) => C inst_ty
2789 -- Simplify 'wanted' as much as possible
2791 tcSimplifyDeriv orig tyvars theta
2792 = do { (tvs, _, tenv) <- tcInstTyVars tyvars
2793 -- The main loop may do unification, and that may crash if
2794 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2795 -- ToDo: what if two of them do get unified?
2796 ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
2797 ; (irreds, _) <- tryHardCheckLoop doc wanteds
2799 ; let (tv_dicts, others) = partition ok irreds
2800 ; addNoInstanceErrs others
2801 -- See Note [Exotic derived instance contexts] in TcMType
2803 ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2804 simpl_theta = substTheta rev_env (map dictPred tv_dicts)
2805 -- This reverse-mapping is a pain, but the result
2806 -- should mention the original TyVars not TcTyVars
2808 ; return simpl_theta }
2810 doc = ptext SLIT("deriving classes for a data type")
2812 ok dict | isDict dict = validDerivPred (dictPred dict)
2817 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2818 used with \tr{default} declarations. We are only interested in
2819 whether it worked or not.
2822 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2825 tcSimplifyDefault theta = do
2826 wanteds <- newDictBndrsO DefaultOrigin theta
2827 (irreds, _) <- tryHardCheckLoop doc wanteds
2828 addNoInstanceErrs irreds
2832 traceTc (ptext SLIT("tcSimplifyDefault failing")) >> failM
2834 doc = ptext SLIT("default declaration")
2838 %************************************************************************
2840 \section{Errors and contexts}
2842 %************************************************************************
2844 ToDo: for these error messages, should we note the location as coming
2845 from the insts, or just whatever seems to be around in the monad just
2849 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2850 -> [Inst] -- The offending Insts
2852 -- Group together insts with the same origin
2853 -- We want to report them together in error messages
2855 groupErrs report_err []
2857 groupErrs report_err (inst:insts)
2858 = do { do_one (inst:friends)
2859 ; groupErrs report_err others }
2861 -- (It may seem a bit crude to compare the error messages,
2862 -- but it makes sure that we combine just what the user sees,
2863 -- and it avoids need equality on InstLocs.)
2864 (friends, others) = partition is_friend insts
2865 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2866 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2867 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2868 -- Add location and context information derived from the Insts
2870 -- Add the "arising from..." part to a message about bunch of dicts
2871 addInstLoc :: [Inst] -> Message -> Message
2872 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
2874 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
2875 addTopIPErrs bndrs []
2877 addTopIPErrs bndrs ips
2878 = do { dflags <- getDOpts
2879 ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
2881 (tidy_env, tidy_ips) = tidyInsts ips
2883 = vcat [sep [ptext SLIT("Implicit parameters escape from"),
2884 nest 2 (ptext SLIT("the monomorphic top-level binding")
2885 <> plural bndrs <+> ptext SLIT("of")
2886 <+> pprBinders bndrs <> colon)],
2887 nest 2 (vcat (map ppr_ip ips)),
2888 monomorphism_fix dflags]
2889 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
2891 topIPErrs :: [Inst] -> TcM ()
2893 = groupErrs report tidy_dicts
2895 (tidy_env, tidy_dicts) = tidyInsts dicts
2896 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2897 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2898 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2900 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
2902 addNoInstanceErrs insts
2903 = do { let (tidy_env, tidy_insts) = tidyInsts insts
2904 ; reportNoInstances tidy_env Nothing tidy_insts }
2908 -> Maybe (InstLoc, [Inst]) -- Context
2909 -- Nothing => top level
2910 -- Just (d,g) => d describes the construct
2912 -> [Inst] -- What is wanted (can include implications)
2915 reportNoInstances tidy_env mb_what insts
2916 = groupErrs (report_no_instances tidy_env mb_what) insts
2918 report_no_instances tidy_env mb_what insts
2919 = do { inst_envs <- tcGetInstEnvs
2920 ; let (implics, insts1) = partition isImplicInst insts
2921 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
2922 (eqInsts, insts3) = partition isEqInst insts2
2923 ; traceTc (text "reportNoInstances" <+> vcat
2924 [ppr insts, ppr implics, ppr insts1, ppr insts2])
2925 ; mapM_ complain_implic implics
2926 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
2927 ; groupErrs complain_no_inst insts3
2928 ; mapM_ (addErrTcM . mk_eq_err) eqInsts
2931 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
2933 complain_implic inst -- Recurse!
2934 = reportNoInstances tidy_env
2935 (Just (tci_loc inst, tci_given inst))
2938 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
2939 -- Right msg => overlap message
2940 -- Left inst => no instance
2941 check_overlap inst_envs wanted
2942 | not (isClassDict wanted) = Left wanted
2944 = case lookupInstEnv inst_envs clas tys of
2945 -- The case of exactly one match and no unifiers means a
2946 -- successful lookup. That can't happen here, because dicts
2947 -- only end up here if they didn't match in Inst.lookupInst
2949 ([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
2951 ([], _) -> Left wanted -- No match
2952 res -> Right (mk_overlap_msg wanted res)
2954 (clas,tys) = getDictClassTys wanted
2956 mk_overlap_msg dict (matches, unifiers)
2957 = ASSERT( not (null matches) )
2958 vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
2959 <+> pprPred (dictPred dict))),
2960 sep [ptext SLIT("Matching instances") <> colon,
2961 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
2962 if not (isSingleton matches)
2963 then -- Two or more matches
2965 else -- One match, plus some unifiers
2966 ASSERT( not (null unifiers) )
2967 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
2968 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
2969 ptext SLIT("To pick the first instance above, use -fallow-incoherent-instances"),
2970 ptext SLIT("when compiling the other instance declarations")])]
2972 ispecs = [ispec | (ispec, _) <- matches]
2974 mk_eq_err :: Inst -> (TidyEnv, SDoc)
2975 mk_eq_err inst = misMatchMsg tidy_env (eqInstTys inst)
2977 mk_no_inst_err insts
2978 | null insts = empty
2980 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
2981 not (isEmptyVarSet (tyVarsOfInsts insts))
2982 = vcat [ addInstLoc insts $
2983 sep [ ptext SLIT("Could not deduce") <+> pprDictsTheta insts
2984 , nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta givens]
2985 , show_fixes (fix1 loc : fixes2) ]
2987 | otherwise -- Top level
2988 = vcat [ addInstLoc insts $
2989 ptext SLIT("No instance") <> plural insts
2990 <+> ptext SLIT("for") <+> pprDictsTheta insts
2991 , show_fixes fixes2 ]
2994 fix1 loc = sep [ ptext SLIT("add") <+> pprDictsTheta insts
2995 <+> ptext SLIT("to the context of"),
2996 nest 2 (ppr (instLocOrigin loc)) ]
2997 -- I'm not sure it helps to add the location
2998 -- nest 2 (ptext SLIT("at") <+> ppr (instLocSpan loc)) ]
3000 fixes2 | null instance_dicts = []
3001 | otherwise = [sep [ptext SLIT("add an instance declaration for"),
3002 pprDictsTheta instance_dicts]]
3003 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
3004 -- Insts for which it is worth suggesting an adding an instance declaration
3005 -- Exclude implicit parameters, and tyvar dicts
3007 show_fixes :: [SDoc] -> SDoc
3008 show_fixes [] = empty
3009 show_fixes (f:fs) = sep [ptext SLIT("Possible fix:"),
3010 nest 2 (vcat (f : map (ptext SLIT("or") <+>) fs))]
3012 addTopAmbigErrs dicts
3013 -- Divide into groups that share a common set of ambiguous tyvars
3014 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
3015 -- See Note [Avoiding spurious errors]
3016 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
3018 (tidy_env, tidy_dicts) = tidyInsts dicts
3020 tvs_of :: Inst -> [TcTyVar]
3021 tvs_of d = varSetElems (tyVarsOfInst d)
3022 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
3024 report :: [(Inst,[TcTyVar])] -> TcM ()
3025 report pairs@((inst,tvs) : _) = do -- The pairs share a common set of ambiguous tyvars
3026 (tidy_env, mono_msg) <- mkMonomorphismMsg tidy_env tvs
3027 setSrcSpan (instSpan inst) $
3028 -- the location of the first one will do for the err message
3029 addErrTcM (tidy_env, msg $$ mono_msg)
3031 dicts = map fst pairs
3032 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
3033 pprQuotedList tvs <+> in_msg,
3034 nest 2 (pprDictsInFull dicts)]
3035 in_msg = text "in the constraint" <> plural dicts <> colon
3036 report [] = panic "addTopAmbigErrs"
3039 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
3040 -- There's an error with these Insts; if they have free type variables
3041 -- it's probably caused by the monomorphism restriction.
3042 -- Try to identify the offending variable
3043 -- ASSUMPTION: the Insts are fully zonked
3044 mkMonomorphismMsg tidy_env inst_tvs
3045 = do { dflags <- getDOpts
3046 ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
3047 ; return (tidy_env, mk_msg dflags docs) }
3049 mk_msg _ _ | any isRuntimeUnk inst_tvs
3050 = vcat [ptext SLIT("Cannot resolve unknown runtime types:") <+>
3051 (pprWithCommas ppr inst_tvs),
3052 ptext SLIT("Use :print or :force to determine these types")]
3053 mk_msg _ [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
3054 -- This happens in things like
3055 -- f x = show (read "foo")
3056 -- where monomorphism doesn't play any role
3058 = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
3060 monomorphism_fix dflags]
3062 monomorphism_fix :: DynFlags -> SDoc
3063 monomorphism_fix dflags
3064 = ptext SLIT("Probable fix:") <+> vcat
3065 [ptext SLIT("give these definition(s) an explicit type signature"),
3066 if dopt Opt_MonomorphismRestriction dflags
3067 then ptext SLIT("or use -fno-monomorphism-restriction")
3068 else empty] -- Only suggest adding "-fno-monomorphism-restriction"
3069 -- if it is not already set!
3071 warnDefault ups default_ty = do
3072 warn_flag <- doptM Opt_WarnTypeDefaults
3073 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
3075 dicts = [d | (d,_,_) <- ups]
3078 (_, tidy_dicts) = tidyInsts dicts
3079 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
3080 quotes (ppr default_ty),
3081 pprDictsInFull tidy_dicts]
3083 reduceDepthErr n stack
3084 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
3085 ptext SLIT("Use -fcontext-stack=N to increase stack size to N"),
3086 nest 4 (pprStack stack)]
3088 pprStack stack = vcat (map pprInstInFull stack)