2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
10 tcSimplifyInfer, tcSimplifyInferCheck,
11 tcSimplifyCheck, tcSimplifyRestricted,
12 tcSimplifyRuleLhs, tcSimplifyIPs,
13 tcSimplifySuperClasses,
14 tcSimplifyTop, tcSimplifyInteractive,
15 tcSimplifyBracket, tcSimplifyCheckPat,
17 tcSimplifyDeriv, tcSimplifyDefault,
23 #include "HsVersions.h"
25 import {-# SOURCE #-} TcUnify( unifyType )
29 import TcHsSyn ( hsLPatType )
37 import DsUtils -- Big-tuple functions
66 %************************************************************************
70 %************************************************************************
72 --------------------------------------
73 Notes on functional dependencies (a bug)
74 --------------------------------------
81 instance D a b => C a b -- Undecidable
82 -- (Not sure if it's crucial to this eg)
83 f :: C a b => a -> Bool
86 g :: C a b => a -> Bool
89 Here f typechecks, but g does not!! Reason: before doing improvement,
90 we reduce the (C a b1) constraint from the call of f to (D a b1).
92 Here is a more complicated example:
95 > class Foo a b | a->b
97 > class Bar a b | a->b
101 > instance Bar Obj Obj
103 > instance (Bar a b) => Foo a b
105 > foo:: (Foo a b) => a -> String
108 > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
114 Could not deduce (Bar a b) from the context (Foo a b)
115 arising from use of `foo' at <interactive>:1
117 Add (Bar a b) to the expected type of an expression
118 In the first argument of `runFoo', namely `foo'
119 In the definition of `it': it = runFoo foo
121 Why all of the sudden does GHC need the constraint Bar a b? The
122 function foo didn't ask for that...
125 The trouble is that to type (runFoo foo), GHC has to solve the problem:
127 Given constraint Foo a b
128 Solve constraint Foo a b'
130 Notice that b and b' aren't the same. To solve this, just do
131 improvement and then they are the same. But GHC currently does
136 That is usually fine, but it isn't here, because it sees that Foo a b is
137 not the same as Foo a b', and so instead applies the instance decl for
138 instance Bar a b => Foo a b. And that's where the Bar constraint comes
141 The Right Thing is to improve whenever the constraint set changes at
142 all. Not hard in principle, but it'll take a bit of fiddling to do.
144 Note [Choosing which variables to quantify]
145 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
146 Suppose we are about to do a generalisation step. We have in our hand
149 T the type of the RHS
150 C the constraints from that RHS
152 The game is to figure out
154 Q the set of type variables over which to quantify
155 Ct the constraints we will *not* quantify over
156 Cq the constraints we will quantify over
158 So we're going to infer the type
162 and float the constraints Ct further outwards.
164 Here are the things that *must* be true:
166 (A) Q intersect fv(G) = EMPTY limits how big Q can be
167 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
169 (A) says we can't quantify over a variable that's free in the environment.
170 (B) says we must quantify over all the truly free variables in T, else
171 we won't get a sufficiently general type.
173 We do not *need* to quantify over any variable that is fixed by the
174 free vars of the environment G.
176 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
178 Example: class H x y | x->y where ...
180 fv(G) = {a} C = {H a b, H c d}
183 (A) Q intersect {a} is empty
184 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
186 So Q can be {c,d}, {b,c,d}
188 In particular, it's perfectly OK to quantify over more type variables
189 than strictly necessary; there is no need to quantify over 'b', since
190 it is determined by 'a' which is free in the envt, but it's perfectly
191 OK to do so. However we must not quantify over 'a' itself.
193 Other things being equal, however, we'd like to quantify over as few
194 variables as possible: smaller types, fewer type applications, more
195 constraints can get into Ct instead of Cq. Here's a good way to
198 Q = grow( fv(T), C ) \ oclose( fv(G), C )
200 That is, quantify over all variable that that MIGHT be fixed by the
201 call site (which influences T), but which aren't DEFINITELY fixed by
202 G. This choice definitely quantifies over enough type variables,
203 albeit perhaps too many.
205 Why grow( fv(T), C ) rather than fv(T)? Consider
207 class H x y | x->y where ...
212 If we used fv(T) = {c} we'd get the type
214 forall c. H c d => c -> b
216 And then if the fn was called at several different c's, each of
217 which fixed d differently, we'd get a unification error, because
218 d isn't quantified. Solution: quantify d. So we must quantify
219 everything that might be influenced by c.
221 Why not oclose( fv(T), C )? Because we might not be able to see
222 all the functional dependencies yet:
224 class H x y | x->y where ...
225 instance H x y => Eq (T x y) where ...
230 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
231 apparent yet, and that's wrong. We must really quantify over d too.
233 There really isn't any point in quantifying over any more than
234 grow( fv(T), C ), because the call sites can't possibly influence
235 any other type variables.
239 -------------------------------------
241 -------------------------------------
243 It's very hard to be certain when a type is ambiguous. Consider
247 instance H x y => K (x,y)
249 Is this type ambiguous?
250 forall a b. (K (a,b), Eq b) => a -> a
252 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
253 now we see that a fixes b. So we can't tell about ambiguity for sure
254 without doing a full simplification. And even that isn't possible if
255 the context has some free vars that may get unified. Urgle!
257 Here's another example: is this ambiguous?
258 forall a b. Eq (T b) => a -> a
259 Not if there's an insance decl (with no context)
260 instance Eq (T b) where ...
262 You may say of this example that we should use the instance decl right
263 away, but you can't always do that:
265 class J a b where ...
266 instance J Int b where ...
268 f :: forall a b. J a b => a -> a
270 (Notice: no functional dependency in J's class decl.)
271 Here f's type is perfectly fine, provided f is only called at Int.
272 It's premature to complain when meeting f's signature, or even
273 when inferring a type for f.
277 However, we don't *need* to report ambiguity right away. It'll always
278 show up at the call site.... and eventually at main, which needs special
279 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
281 So here's the plan. We WARN about probable ambiguity if
283 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
285 (all tested before quantification).
286 That is, all the type variables in Cq must be fixed by the the variables
287 in the environment, or by the variables in the type.
289 Notice that we union before calling oclose. Here's an example:
291 class J a b c | a b -> c
295 forall b c. (J a b c) => b -> b
297 Only if we union {a} from G with {b} from T before using oclose,
298 do we see that c is fixed.
300 It's a bit vague exactly which C we should use for this oclose call. If we
301 don't fix enough variables we might complain when we shouldn't (see
302 the above nasty example). Nothing will be perfect. That's why we can
303 only issue a warning.
306 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
308 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
310 then c is a "bubble"; there's no way it can ever improve, and it's
311 certainly ambiguous. UNLESS it is a constant (sigh). And what about
316 instance H x y => K (x,y)
318 Is this type ambiguous?
319 forall a b. (K (a,b), Eq b) => a -> a
321 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
322 is a "bubble" that's a set of constraints
324 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
326 Hence another idea. To decide Q start with fv(T) and grow it
327 by transitive closure in Cq (no functional dependencies involved).
328 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
329 The definitely-ambiguous can then float out, and get smashed at top level
330 (which squashes out the constants, like Eq (T a) above)
333 --------------------------------------
334 Notes on principal types
335 --------------------------------------
340 f x = let g y = op (y::Int) in True
342 Here the principal type of f is (forall a. a->a)
343 but we'll produce the non-principal type
344 f :: forall a. C Int => a -> a
347 --------------------------------------
348 The need for forall's in constraints
349 --------------------------------------
351 [Exchange on Haskell Cafe 5/6 Dec 2000]
353 class C t where op :: t -> Bool
354 instance C [t] where op x = True
356 p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
357 q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
359 The definitions of p and q differ only in the order of the components in
360 the pair on their right-hand sides. And yet:
362 ghc and "Typing Haskell in Haskell" reject p, but accept q;
363 Hugs rejects q, but accepts p;
364 hbc rejects both p and q;
365 nhc98 ... (Malcolm, can you fill in the blank for us!).
367 The type signature for f forces context reduction to take place, and
368 the results of this depend on whether or not the type of y is known,
369 which in turn depends on which component of the pair the type checker
372 Solution: if y::m a, float out the constraints
373 Monad m, forall c. C (m c)
374 When m is later unified with [], we can solve both constraints.
377 --------------------------------------
378 Notes on implicit parameters
379 --------------------------------------
381 Note [Inheriting implicit parameters]
382 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
387 where f is *not* a top-level binding.
388 From the RHS of f we'll get the constraint (?y::Int).
389 There are two types we might infer for f:
393 (so we get ?y from the context of f's definition), or
395 f :: (?y::Int) => Int -> Int
397 At first you might think the first was better, becuase then
398 ?y behaves like a free variable of the definition, rather than
399 having to be passed at each call site. But of course, the WHOLE
400 IDEA is that ?y should be passed at each call site (that's what
401 dynamic binding means) so we'd better infer the second.
403 BOTTOM LINE: when *inferring types* you *must* quantify
404 over implicit parameters. See the predicate isFreeWhenInferring.
407 Note [Implicit parameters and ambiguity]
408 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
409 Only a *class* predicate can give rise to ambiguity
410 An *implicit parameter* cannot. For example:
411 foo :: (?x :: [a]) => Int
413 is fine. The call site will suppply a particular 'x'
415 Furthermore, the type variables fixed by an implicit parameter
416 propagate to the others. E.g.
417 foo :: (Show a, ?x::[a]) => Int
419 The type of foo looks ambiguous. But it isn't, because at a call site
421 let ?x = 5::Int in foo
422 and all is well. In effect, implicit parameters are, well, parameters,
423 so we can take their type variables into account as part of the
424 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
427 Question 2: type signatures
428 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
429 BUT WATCH OUT: When you supply a type signature, we can't force you
430 to quantify over implicit parameters. For example:
434 This is perfectly reasonable. We do not want to insist on
436 (?x + 1) :: (?x::Int => Int)
438 That would be silly. Here, the definition site *is* the occurrence site,
439 so the above strictures don't apply. Hence the difference between
440 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
441 and tcSimplifyCheckBind (which does not).
443 What about when you supply a type signature for a binding?
444 Is it legal to give the following explicit, user type
445 signature to f, thus:
450 At first sight this seems reasonable, but it has the nasty property
451 that adding a type signature changes the dynamic semantics.
454 (let f x = (x::Int) + ?y
455 in (f 3, f 3 with ?y=5)) with ?y = 6
461 in (f 3, f 3 with ?y=5)) with ?y = 6
465 Indeed, simply inlining f (at the Haskell source level) would change the
468 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
469 semantics for a Haskell program without knowing its typing, so if you
470 change the typing you may change the semantics.
472 To make things consistent in all cases where we are *checking* against
473 a supplied signature (as opposed to inferring a type), we adopt the
476 a signature does not need to quantify over implicit params.
478 [This represents a (rather marginal) change of policy since GHC 5.02,
479 which *required* an explicit signature to quantify over all implicit
480 params for the reasons mentioned above.]
482 But that raises a new question. Consider
484 Given (signature) ?x::Int
485 Wanted (inferred) ?x::Int, ?y::Bool
487 Clearly we want to discharge the ?x and float the ?y out. But
488 what is the criterion that distinguishes them? Clearly it isn't
489 what free type variables they have. The Right Thing seems to be
490 to float a constraint that
491 neither mentions any of the quantified type variables
492 nor any of the quantified implicit parameters
494 See the predicate isFreeWhenChecking.
497 Question 3: monomorphism
498 ~~~~~~~~~~~~~~~~~~~~~~~~
499 There's a nasty corner case when the monomorphism restriction bites:
503 The argument above suggests that we *must* generalise
504 over the ?y parameter, to get
505 z :: (?y::Int) => Int,
506 but the monomorphism restriction says that we *must not*, giving
508 Why does the momomorphism restriction say this? Because if you have
510 let z = x + ?y in z+z
512 you might not expect the addition to be done twice --- but it will if
513 we follow the argument of Question 2 and generalise over ?y.
516 Question 4: top level
517 ~~~~~~~~~~~~~~~~~~~~~
518 At the top level, monomorhism makes no sense at all.
521 main = let ?x = 5 in print foo
525 woggle :: (?x :: Int) => Int -> Int
528 We definitely don't want (foo :: Int) with a top-level implicit parameter
529 (?x::Int) becuase there is no way to bind it.
534 (A) Always generalise over implicit parameters
535 Bindings that fall under the monomorphism restriction can't
539 * Inlining remains valid
540 * No unexpected loss of sharing
541 * But simple bindings like
543 will be rejected, unless you add an explicit type signature
544 (to avoid the monomorphism restriction)
545 z :: (?y::Int) => Int
547 This seems unacceptable
549 (B) Monomorphism restriction "wins"
550 Bindings that fall under the monomorphism restriction can't
552 Always generalise over implicit parameters *except* for bindings
553 that fall under the monomorphism restriction
556 * Inlining isn't valid in general
557 * No unexpected loss of sharing
558 * Simple bindings like
560 accepted (get value of ?y from binding site)
562 (C) Always generalise over implicit parameters
563 Bindings that fall under the monomorphism restriction can't
564 be generalised, EXCEPT for implicit parameters
566 * Inlining remains valid
567 * Unexpected loss of sharing (from the extra generalisation)
568 * Simple bindings like
570 accepted (get value of ?y from occurrence sites)
575 None of these choices seems very satisfactory. But at least we should
576 decide which we want to do.
578 It's really not clear what is the Right Thing To Do. If you see
582 would you expect the value of ?y to be got from the *occurrence sites*
583 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
584 case of function definitions, the answer is clearly the former, but
585 less so in the case of non-fucntion definitions. On the other hand,
586 if we say that we get the value of ?y from the definition site of 'z',
587 then inlining 'z' might change the semantics of the program.
589 Choice (C) really says "the monomorphism restriction doesn't apply
590 to implicit parameters". Which is fine, but remember that every
591 innocent binding 'x = ...' that mentions an implicit parameter in
592 the RHS becomes a *function* of that parameter, called at each
593 use of 'x'. Now, the chances are that there are no intervening 'with'
594 clauses that bind ?y, so a decent compiler should common up all
595 those function calls. So I think I strongly favour (C). Indeed,
596 one could make a similar argument for abolishing the monomorphism
597 restriction altogether.
599 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
603 %************************************************************************
605 \subsection{tcSimplifyInfer}
607 %************************************************************************
609 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
611 1. Compute Q = grow( fvs(T), C )
613 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
614 predicates will end up in Ct; we deal with them at the top level
616 3. Try improvement, using functional dependencies
618 4. If Step 3 did any unification, repeat from step 1
619 (Unification can change the result of 'grow'.)
621 Note: we don't reduce dictionaries in step 2. For example, if we have
622 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
623 after step 2. However note that we may therefore quantify over more
624 type variables than we absolutely have to.
626 For the guts, we need a loop, that alternates context reduction and
627 improvement with unification. E.g. Suppose we have
629 class C x y | x->y where ...
631 and tcSimplify is called with:
633 Then improvement unifies a with b, giving
636 If we need to unify anything, we rattle round the whole thing all over
643 -> TcTyVarSet -- fv(T); type vars
645 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked and quantified)
646 [Inst], -- Dict Ids that must be bound here (zonked)
647 TcDictBinds) -- Bindings
648 -- Any free (escaping) Insts are tossed into the environment
653 tcSimplifyInfer doc tau_tvs wanted
654 = do { tau_tvs1 <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
655 ; wanted' <- mapM zonkInst wanted -- Zonk before deciding quantified tyvars
656 ; gbl_tvs <- tcGetGlobalTyVars
657 ; let preds1 = fdPredsOfInsts wanted'
658 gbl_tvs1 = oclose preds1 gbl_tvs
659 qtvs = grow preds1 tau_tvs1 `minusVarSet` gbl_tvs1
660 -- See Note [Choosing which variables to quantify]
662 -- To maximise sharing, remove from consideration any
663 -- constraints that don't mention qtvs at all
664 ; let (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
667 -- To make types simple, reduce as much as possible
668 ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (grow preds1 tau_tvs1) $$ ppr gbl_tvs $$
669 ppr gbl_tvs1 $$ ppr free $$ ppr bound))
670 ; (irreds1, binds1) <- tryHardCheckLoop doc bound
672 -- Note [Inference and implication constraints]
673 ; let want_dict d = tyVarsOfInst d `intersectsVarSet` qtvs
674 ; (irreds2, binds2) <- approximateImplications doc want_dict irreds1
676 -- Now work out all over again which type variables to quantify,
677 -- exactly in the same way as before, but starting from irreds2. Why?
678 -- a) By now improvment may have taken place, and we must *not*
679 -- quantify over any variable free in the environment
680 -- tc137 (function h inside g) is an example
682 -- b) Do not quantify over constraints that *now* do not
683 -- mention quantified type variables, because they are
684 -- simply ambiguous (or might be bound further out). Example:
685 -- f :: Eq b => a -> (a, b)
687 -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
688 -- We decide to quantify over 'alpha' alone, but free1 does not include f77
689 -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
690 -- constraint (Eq beta), which we dump back into the free set
691 -- See test tcfail181
693 -- c) irreds may contain type variables not previously mentioned,
694 -- e.g. instance D a x => Foo [a]
696 -- Then after simplifying we'll get (D a x), and x is fresh
697 -- We must quantify over x else it'll be totally unbound
698 ; tau_tvs2 <- zonkTcTyVarsAndFV (varSetElems tau_tvs1)
699 ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
700 -- Note that we start from gbl_tvs1
701 -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
702 -- we've already put some of the original preds1 into frees
703 -- E.g. wanteds = C a b (where a->b)
706 -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
707 -- irreds2 will be empty. But we don't want to generalise over b!
708 ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
709 qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
710 ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
713 -- Turn the quantified meta-type variables into real type variables
714 ; qtvs2 <- zonkQuantifiedTyVars (varSetElems qtvs)
716 -- We can't abstract over any remaining unsolved
717 -- implications so instead just float them outwards. Ugh.
718 ; let (q_dicts0, implics) = partition isAbstractableInst irreds3
719 ; loc <- getInstLoc (ImplicOrigin doc)
720 ; implic_bind <- bindIrreds loc qtvs2 q_dicts0 implics
722 -- Prepare equality instances for quantification
723 ; let (q_eqs0,q_dicts) = partition isEqInst q_dicts0
724 ; q_eqs <- mapM finalizeEqInst q_eqs0
726 ; return (qtvs2, q_eqs ++ q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
727 -- NB: when we are done, we might have some bindings, but
728 -- the final qtvs might be empty. See Note [NO TYVARS] below.
730 approximateImplications :: SDoc -> (Inst -> Bool) -> [Inst] -> TcM ([Inst], TcDictBinds)
731 -- Note [Inference and implication constraints]
732 -- Given a bunch of Dict and ImplicInsts, try to approximate the implications by
733 -- - fetching any dicts inside them that are free
734 -- - using those dicts as cruder constraints, to solve the implications
735 -- - returning the extra ones too
737 approximateImplications doc want_dict irreds
739 = return (irreds, emptyBag)
741 = do { extra_dicts' <- mapM cloneDict extra_dicts
742 ; tryHardCheckLoop doc (extra_dicts' ++ irreds) }
743 -- By adding extra_dicts', we make them
744 -- available to solve the implication constraints
746 extra_dicts = get_dicts (filter isImplicInst irreds)
748 get_dicts :: [Inst] -> [Inst] -- Returns only Dicts
749 -- Find the wanted constraints in implication constraints that satisfy
750 -- want_dict, and are not bound by forall's in the constraint itself
751 get_dicts ds = concatMap get_dict ds
753 get_dict d@(Dict {}) | want_dict d = [d]
755 get_dict (ImplicInst {tci_tyvars = tvs, tci_wanted = wanteds})
756 = [ d | let tv_set = mkVarSet tvs
757 , d <- get_dicts wanteds
758 , not (tyVarsOfInst d `intersectsVarSet` tv_set)]
759 get_dict i@(EqInst {}) | want_dict i = [i]
761 get_dict other = pprPanic "approximateImplications" (ppr other)
764 Note [Inference and implication constraints]
765 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
766 Suppose we have a wanted implication constraint (perhaps arising from
767 a nested pattern match) like
769 and we are now trying to quantify over 'a' when inferring the type for
770 a function. In principle it's possible that there might be an instance
771 instance (C a, E a) => D [a]
772 so the context (E a) would suffice. The Right Thing is to abstract over
773 the implication constraint, but we don't do that (a) because it'll be
774 surprising to programmers and (b) because we don't have the machinery to deal
775 with 'given' implications.
777 So our best approximation is to make (D [a]) part of the inferred
778 context, so we can use that to discharge the implication. Hence
779 the strange function get_dicts in approximateImplications.
781 The common cases are more clear-cut, when we have things like
783 Here, abstracting over (C b) is not an approximation at all -- but see
784 Note [Freeness and implications].
786 See Trac #1430 and test tc228.
790 -----------------------------------------------------------
791 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
792 -- against, but we don't know the type variables over which we are going to quantify.
793 -- This happens when we have a type signature for a mutually recursive group
796 -> TcTyVarSet -- fv(T)
799 -> TcM ([TyVar], -- Fully zonked, and quantified
800 TcDictBinds) -- Bindings
802 tcSimplifyInferCheck loc tau_tvs givens wanteds
803 = do { traceTc (text "tcSimplifyInferCheck <-" <+> ppr wanteds)
804 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
806 -- Figure out which type variables to quantify over
807 -- You might think it should just be the signature tyvars,
808 -- but in bizarre cases you can get extra ones
809 -- f :: forall a. Num a => a -> a
810 -- f x = fst (g (x, head [])) + 1
812 -- Here we infer g :: forall a b. a -> b -> (b,a)
813 -- We don't want g to be monomorphic in b just because
814 -- f isn't quantified over b.
815 ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
816 ; all_tvs <- zonkTcTyVarsAndFV all_tvs
817 ; gbl_tvs <- tcGetGlobalTyVars
818 ; let qtvs = varSetElems (all_tvs `minusVarSet` gbl_tvs)
819 -- We could close gbl_tvs, but its not necessary for
820 -- soundness, and it'll only affect which tyvars, not which
821 -- dictionaries, we quantify over
823 ; qtvs' <- zonkQuantifiedTyVars qtvs
825 -- Now we are back to normal (c.f. tcSimplCheck)
826 ; implic_bind <- bindIrreds loc qtvs' givens irreds
828 ; traceTc (text "tcSimplifyInferCheck ->" <+> ppr (implic_bind))
829 ; return (qtvs', binds `unionBags` implic_bind) }
832 Note [Squashing methods]
833 ~~~~~~~~~~~~~~~~~~~~~~~~~
834 Be careful if you want to float methods more:
835 truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
836 From an application (truncate f i) we get
839 If we have also have a second occurrence of truncate, we get
842 When simplifying with i,f free, we might still notice that
843 t1=t3; but alas, the binding for t2 (which mentions t1)
844 may continue to float out!
849 class Y a b | a -> b where
852 instance Y [[a]] a where
855 k :: X a -> X a -> X a
857 g :: Num a => [X a] -> [X a]
860 h ys = ys ++ map (k (y [[0]])) xs
862 The excitement comes when simplifying the bindings for h. Initially
863 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
864 From this we get t1:=:t2, but also various bindings. We can't forget
865 the bindings (because of [LOOP]), but in fact t1 is what g is
868 The net effect of [NO TYVARS]
871 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
872 isFreeWhenInferring qtvs inst
873 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
874 && isInheritableInst inst -- and no implicit parameter involved
875 -- see Note [Inheriting implicit parameters]
877 {- No longer used (with implication constraints)
878 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
879 -> NameSet -- Quantified implicit parameters
881 isFreeWhenChecking qtvs ips inst
882 = isFreeWrtTyVars qtvs inst
883 && isFreeWrtIPs ips inst
886 isFreeWrtTyVars :: VarSet -> Inst -> Bool
887 isFreeWrtTyVars qtvs inst = tyVarsOfInst inst `disjointVarSet` qtvs
888 isFreeWrtIPs :: NameSet -> Inst -> Bool
889 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
893 %************************************************************************
895 \subsection{tcSimplifyCheck}
897 %************************************************************************
899 @tcSimplifyCheck@ is used when we know exactly the set of variables
900 we are going to quantify over. For example, a class or instance declaration.
903 -----------------------------------------------------------
904 -- tcSimplifyCheck is used when checking expression type signatures,
905 -- class decls, instance decls etc.
906 tcSimplifyCheck :: InstLoc
907 -> [TcTyVar] -- Quantify over these
910 -> TcM TcDictBinds -- Bindings
911 tcSimplifyCheck loc qtvs givens wanteds
912 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
913 do { traceTc (text "tcSimplifyCheck")
914 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
915 ; implic_bind <- bindIrreds loc qtvs givens irreds
916 ; return (binds `unionBags` implic_bind) }
918 -----------------------------------------------------------
919 -- tcSimplifyCheckPat is used for existential pattern match
920 tcSimplifyCheckPat :: InstLoc
921 -> [TcTyVar] -- Quantify over these
924 -> TcM TcDictBinds -- Bindings
925 tcSimplifyCheckPat loc qtvs givens wanteds
926 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
927 do { traceTc (text "tcSimplifyCheckPat")
928 ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
929 ; implic_bind <- bindIrredsR loc qtvs givens irreds
930 ; return (binds `unionBags` implic_bind) }
932 -----------------------------------------------------------
933 bindIrreds :: InstLoc -> [TcTyVar]
936 bindIrreds loc qtvs givens irreds
937 = bindIrredsR loc qtvs givens irreds
939 bindIrredsR :: InstLoc -> [TcTyVar] -> [Inst] -> [Inst] -> TcM TcDictBinds
940 -- Make a binding that binds 'irreds', by generating an implication
941 -- constraint for them, *and* throwing the constraint into the LIE
942 bindIrredsR loc qtvs givens irreds
946 = do { let givens' = filter isAbstractableInst givens
947 -- The givens can (redundantly) include methods
948 -- We want to retain both EqInsts and Dicts
949 -- There should be no implicadtion constraints
950 -- See Note [Pruning the givens in an implication constraint]
952 -- If there are no 'givens', then it's safe to
953 -- partition the 'wanteds' by their qtvs, thereby trimming irreds
954 -- See Note [Freeness and implications]
955 ; irreds' <- if null givens'
957 { let qtv_set = mkVarSet qtvs
958 (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
960 ; return real_irreds }
963 ; (implics, bind) <- makeImplicationBind loc qtvs givens' irreds'
964 -- This call does the real work
965 -- If irreds' is empty, it does something sensible
970 makeImplicationBind :: InstLoc -> [TcTyVar]
972 -> TcM ([Inst], TcDictBinds)
973 -- Make a binding that binds 'irreds', by generating an implication
974 -- constraint for them, *and* throwing the constraint into the LIE
975 -- The binding looks like
976 -- (ir1, .., irn) = f qtvs givens
977 -- where f is (evidence for) the new implication constraint
978 -- f :: forall qtvs. {reft} givens => (ir1, .., irn)
979 -- qtvs includes coercion variables
981 -- This binding must line up the 'rhs' in reduceImplication
982 makeImplicationBind loc all_tvs
983 givens -- Guaranteed all Dicts
986 | null irreds -- If there are no irreds, we are done
987 = return ([], emptyBag)
988 | otherwise -- Otherwise we must generate a binding
989 = do { uniq <- newUnique
990 ; span <- getSrcSpanM
991 ; let (eq_givens, dict_givens) = partition isEqInst givens
992 eq_tyvar_cos = mkTyVarTys (varSetElems $ tyVarsOfTypes $ map eqInstType eq_givens)
993 -- Urgh! See line 2187 or thereabouts. I believe that all these
994 -- 'givens' must be a simple CoVar. This MUST be cleaned up.
996 ; let name = mkInternalName uniq (mkVarOcc "ic") span
997 implic_inst = ImplicInst { tci_name = name,
998 tci_tyvars = all_tvs,
999 tci_given = (eq_givens ++ dict_givens),
1000 tci_wanted = irreds, tci_loc = loc }
1001 ; let -- only create binder for dict_irreds
1002 (_, dict_irreds) = partition isEqInst irreds
1003 dict_irred_ids = map instToId dict_irreds
1004 lpat = mkBigLHsPatTup (map (L span . VarPat) dict_irred_ids)
1005 rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
1006 co = mkWpApps (map instToId dict_givens)
1007 <.> mkWpTyApps eq_tyvar_cos
1008 <.> mkWpTyApps (mkTyVarTys all_tvs)
1009 bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs
1010 | otherwise = PatBind { pat_lhs = lpat,
1011 pat_rhs = unguardedGRHSs rhs,
1012 pat_rhs_ty = hsLPatType lpat,
1013 bind_fvs = placeHolderNames }
1014 ; traceTc $ text "makeImplicationBind" <+> ppr implic_inst
1015 ; return ([implic_inst], unitBag (L span bind))
1018 -----------------------------------------------------------
1019 tryHardCheckLoop :: SDoc
1021 -> TcM ([Inst], TcDictBinds)
1023 tryHardCheckLoop doc wanteds
1024 = do { (irreds,binds) <- checkLoop (mkRedEnv doc try_me []) wanteds
1025 ; return (irreds,binds)
1028 try_me _ = ReduceMe AddSCs
1029 -- Here's the try-hard bit
1031 -----------------------------------------------------------
1032 gentleCheckLoop :: InstLoc
1035 -> TcM ([Inst], TcDictBinds)
1037 gentleCheckLoop inst_loc givens wanteds
1038 = do { (irreds,binds) <- checkLoop env wanteds
1039 ; return (irreds,binds)
1042 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
1044 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1046 -- When checking against a given signature
1047 -- we MUST be very gentle: Note [Check gently]
1049 gentleInferLoop :: SDoc -> [Inst]
1050 -> TcM ([Inst], TcDictBinds)
1051 gentleInferLoop doc wanteds
1052 = do { (irreds, binds) <- checkLoop env wanteds
1053 ; return (irreds, binds) }
1055 env = mkRedEnv doc try_me []
1056 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1061 ~~~~~~~~~~~~~~~~~~~~
1062 We have to very careful about not simplifying too vigorously
1067 f :: Show b => T b -> b
1068 f (MkT x) = show [x]
1070 Inside the pattern match, which binds (a:*, x:a), we know that
1072 Hence we have a dictionary for Show [a] available; and indeed we
1073 need it. We are going to build an implication contraint
1074 forall a. (b~[a]) => Show [a]
1075 Later, we will solve this constraint using the knowledge (Show b)
1077 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
1078 thing becomes insoluble. So we simplify gently (get rid of literals
1079 and methods only, plus common up equal things), deferring the real
1080 work until top level, when we solve the implication constraint
1081 with tryHardCheckLooop.
1085 -----------------------------------------------------------
1088 -> TcM ([Inst], TcDictBinds)
1089 -- Precondition: givens are completely rigid
1090 -- Postcondition: returned Insts are zonked
1092 checkLoop env wanteds
1093 = go env wanteds (return ())
1094 where go env wanteds elim_skolems
1095 = do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
1096 ; env' <- zonkRedEnv env
1097 ; wanteds' <- zonkInsts wanteds
1099 ; (improved, binds, irreds, elim_more_skolems)
1100 <- reduceContext env' wanteds'
1101 ; let elim_skolems' = elim_skolems >> elim_more_skolems
1103 ; if not improved then
1104 elim_skolems' >> return (irreds, binds)
1107 -- If improvement did some unification, we go round again.
1108 -- We start again with irreds, not wanteds
1109 -- Using an instance decl might have introduced a fresh type
1110 -- variable which might have been unified, so we'd get an
1111 -- infinite loop if we started again with wanteds!
1113 { (irreds1, binds1) <- go env' irreds elim_skolems'
1114 ; return (irreds1, binds `unionBags` binds1) } }
1117 Note [Zonking RedEnv]
1118 ~~~~~~~~~~~~~~~~~~~~~
1119 It might appear as if the givens in RedEnv are always rigid, but that is not
1120 necessarily the case for programs involving higher-rank types that have class
1121 contexts constraining the higher-rank variables. An example from tc237 in the
1124 class Modular s a | s -> a
1126 wim :: forall a w. Integral a
1127 => a -> (forall s. Modular s a => M s w) -> w
1128 wim i k = error "urk"
1130 test5 :: (Modular s a, Integral a) => M s a
1133 test4 = wim 4 test4'
1135 Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
1136 quantified further outside. When type checking test4, we have to check
1137 whether the signature of test5 is an instance of
1139 (forall s. Modular s a => M s w)
1141 Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
1144 Given the FD of Modular in this example, class improvement will instantiate
1145 t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
1146 Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
1147 the givens, we will get into a loop as improveOne uses the unification engine
1148 Unify.tcUnifyTys, which doesn't know about mutable type variables.
1153 class If b t e r | b t e -> r
1156 class Lte a b c | a b -> c where lte :: a -> b -> c
1158 instance (Lte a b l,If l b a c) => Max a b c
1160 Wanted: Max Z (S x) y
1162 Then we'll reduce using the Max instance to:
1163 (Lte Z (S x) l, If l (S x) Z y)
1164 and improve by binding l->T, after which we can do some reduction
1165 on both the Lte and If constraints. What we *can't* do is start again
1166 with (Max Z (S x) y)!
1170 %************************************************************************
1172 tcSimplifySuperClasses
1174 %************************************************************************
1176 Note [SUPERCLASS-LOOP 1]
1177 ~~~~~~~~~~~~~~~~~~~~~~~~
1178 We have to be very, very careful when generating superclasses, lest we
1179 accidentally build a loop. Here's an example:
1183 class S a => C a where { opc :: a -> a }
1184 class S b => D b where { opd :: b -> b }
1186 instance C Int where
1189 instance D Int where
1192 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1193 Simplifying, we may well get:
1194 $dfCInt = :C ds1 (opd dd)
1197 Notice that we spot that we can extract ds1 from dd.
1199 Alas! Alack! We can do the same for (instance D Int):
1201 $dfDInt = :D ds2 (opc dc)
1205 And now we've defined the superclass in terms of itself.
1207 Solution: never generate a superclass selectors at all when
1208 satisfying the superclass context of an instance declaration.
1210 Two more nasty cases are in
1215 tcSimplifySuperClasses
1220 tcSimplifySuperClasses loc givens sc_wanteds
1221 = do { traceTc (text "tcSimplifySuperClasses")
1222 ; (irreds,binds1) <- checkLoop env sc_wanteds
1223 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1224 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1227 env = mkRedEnv (pprInstLoc loc) try_me givens
1228 try_me _ = ReduceMe NoSCs
1229 -- Like tryHardCheckLoop, but with NoSCs
1233 %************************************************************************
1235 \subsection{tcSimplifyRestricted}
1237 %************************************************************************
1239 tcSimplifyRestricted infers which type variables to quantify for a
1240 group of restricted bindings. This isn't trivial.
1243 We want to quantify over a to get id :: forall a. a->a
1246 We do not want to quantify over a, because there's an Eq a
1247 constraint, so we get eq :: a->a->Bool (notice no forall)
1250 RHS has type 'tau', whose free tyvars are tau_tvs
1251 RHS has constraints 'wanteds'
1254 Quantify over (tau_tvs \ ftvs(wanteds))
1255 This is bad. The constraints may contain (Monad (ST s))
1256 where we have instance Monad (ST s) where...
1257 so there's no need to be monomorphic in s!
1259 Also the constraint might be a method constraint,
1260 whose type mentions a perfectly innocent tyvar:
1261 op :: Num a => a -> b -> a
1262 Here, b is unconstrained. A good example would be
1264 We want to infer the polymorphic type
1265 foo :: forall b. b -> b
1268 Plan B (cunning, used for a long time up to and including GHC 6.2)
1269 Step 1: Simplify the constraints as much as possible (to deal
1270 with Plan A's problem). Then set
1271 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1273 Step 2: Now simplify again, treating the constraint as 'free' if
1274 it does not mention qtvs, and trying to reduce it otherwise.
1275 The reasons for this is to maximise sharing.
1277 This fails for a very subtle reason. Suppose that in the Step 2
1278 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1279 In the Step 1 this constraint might have been simplified, perhaps to
1280 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1281 This won't happen in Step 2... but that in turn might prevent some other
1282 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1283 and that in turn breaks the invariant that no constraints are quantified over.
1285 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1290 Step 1: Simplify the constraints as much as possible (to deal
1291 with Plan A's problem). Then set
1292 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1293 Return the bindings from Step 1.
1296 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1299 instance (HasBinary ty IO) => HasCodedValue ty
1301 foo :: HasCodedValue a => String -> IO a
1303 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1304 doDecodeIO codedValue view
1305 = let { act = foo "foo" } in act
1307 You might think this should work becuase the call to foo gives rise to a constraint
1308 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1309 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1310 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1312 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1316 Plan D (a variant of plan B)
1317 Step 1: Simplify the constraints as much as possible (to deal
1318 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1319 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1321 Step 2: Now simplify again, treating the constraint as 'free' if
1322 it does not mention qtvs, and trying to reduce it otherwise.
1324 The point here is that it's generally OK to have too few qtvs; that is,
1325 to make the thing more monomorphic than it could be. We don't want to
1326 do that in the common cases, but in wierd cases it's ok: the programmer
1327 can always add a signature.
1329 Too few qtvs => too many wanteds, which is what happens if you do less
1334 tcSimplifyRestricted -- Used for restricted binding groups
1335 -- i.e. ones subject to the monomorphism restriction
1338 -> [Name] -- Things bound in this group
1339 -> TcTyVarSet -- Free in the type of the RHSs
1340 -> [Inst] -- Free in the RHSs
1341 -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
1342 TcDictBinds) -- Bindings
1343 -- tcSimpifyRestricted returns no constraints to
1344 -- quantify over; by definition there are none.
1345 -- They are all thrown back in the LIE
1347 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1348 -- Zonk everything in sight
1349 = do { traceTc (text "tcSimplifyRestricted")
1350 ; wanteds' <- zonkInsts wanteds
1352 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1353 -- dicts; the idea is to get rid of as many type
1354 -- variables as possible, and we don't want to stop
1355 -- at (say) Monad (ST s), because that reduces
1356 -- immediately, with no constraint on s.
1358 -- BUT do no improvement! See Plan D above
1359 -- HOWEVER, some unification may take place, if we instantiate
1360 -- a method Inst with an equality constraint
1361 ; let env = mkNoImproveRedEnv doc (\_ -> ReduceMe AddSCs)
1362 ; (_imp, _binds, constrained_dicts, elim_skolems)
1363 <- reduceContext env wanteds'
1366 -- Next, figure out the tyvars we will quantify over
1367 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1368 ; gbl_tvs' <- tcGetGlobalTyVars
1369 ; constrained_dicts' <- zonkInsts constrained_dicts
1371 ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
1372 -- As in tcSimplifyInfer
1374 -- Do not quantify over constrained type variables:
1375 -- this is the monomorphism restriction
1376 constrained_tvs' = tyVarsOfInsts constrained_dicts'
1377 qtvs = qtvs1 `minusVarSet` constrained_tvs'
1378 pp_bndrs = pprWithCommas (quotes . ppr) bndrs
1381 ; warn_mono <- doptM Opt_WarnMonomorphism
1382 ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
1383 (vcat[ ptext (sLit "the Monomorphism Restriction applies to the binding")
1384 <> plural bndrs <+> ptext (sLit "for") <+> pp_bndrs,
1385 ptext (sLit "Consider giving a type signature for") <+> pp_bndrs])
1387 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1388 pprInsts wanteds, pprInsts constrained_dicts',
1390 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1392 -- The first step may have squashed more methods than
1393 -- necessary, so try again, this time more gently, knowing the exact
1394 -- set of type variables to quantify over.
1396 -- We quantify only over constraints that are captured by qtvs;
1397 -- these will just be a subset of non-dicts. This in contrast
1398 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1399 -- all *non-inheritable* constraints too. This implements choice
1400 -- (B) under "implicit parameter and monomorphism" above.
1402 -- Remember that we may need to do *some* simplification, to
1403 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1404 -- just to float all constraints
1406 -- At top level, we *do* squash methods becuase we want to
1407 -- expose implicit parameters to the test that follows
1408 ; let is_nested_group = isNotTopLevel top_lvl
1409 try_me inst | isFreeWrtTyVars qtvs inst,
1410 (is_nested_group || isDict inst) = Stop
1411 | otherwise = ReduceMe AddSCs
1412 env = mkNoImproveRedEnv doc try_me
1413 ; (_imp, binds, irreds, elim_skolems) <- reduceContext env wanteds'
1416 -- See "Notes on implicit parameters, Question 4: top level"
1417 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1418 if is_nested_group then
1420 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1421 ; addTopIPErrs bndrs bad_ips
1422 ; extendLIEs non_ips }
1424 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1425 ; return (qtvs', binds) }
1429 %************************************************************************
1433 %************************************************************************
1435 On the LHS of transformation rules we only simplify methods and constants,
1436 getting dictionaries. We want to keep all of them unsimplified, to serve
1437 as the available stuff for the RHS of the rule.
1439 Example. Consider the following left-hand side of a rule
1441 f (x == y) (y > z) = ...
1443 If we typecheck this expression we get constraints
1445 d1 :: Ord a, d2 :: Eq a
1447 We do NOT want to "simplify" to the LHS
1449 forall x::a, y::a, z::a, d1::Ord a.
1450 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1454 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1455 f ((==) d2 x y) ((>) d1 y z) = ...
1457 Here is another example:
1459 fromIntegral :: (Integral a, Num b) => a -> b
1460 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1462 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1463 we *dont* want to get
1465 forall dIntegralInt.
1466 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1468 because the scsel will mess up RULE matching. Instead we want
1470 forall dIntegralInt, dNumInt.
1471 fromIntegral Int Int dIntegralInt dNumInt = id Int
1475 g (x == y) (y == z) = ..
1477 where the two dictionaries are *identical*, we do NOT WANT
1479 forall x::a, y::a, z::a, d1::Eq a
1480 f ((==) d1 x y) ((>) d1 y z) = ...
1482 because that will only match if the dict args are (visibly) equal.
1483 Instead we want to quantify over the dictionaries separately.
1485 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1486 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1487 from scratch, rather than further parameterise simpleReduceLoop etc.
1488 Simpler, maybe, but alas not simple (see Trac #2494)
1490 * Type errors may give rise to an (unsatisfiable) equality constraint
1492 * Applications of a higher-rank function on the LHS may give
1493 rise to an implication constraint, esp if there are unsatisfiable
1494 equality constraints inside.
1497 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1498 tcSimplifyRuleLhs wanteds
1499 = do { wanteds' <- zonkInsts wanteds
1500 ; (irreds, binds) <- go [] emptyBag wanteds'
1501 ; let (dicts, bad_irreds) = partition isDict irreds
1502 ; traceTc (text "tcSimplifyrulelhs" <+> pprInsts bad_irreds)
1503 ; addNoInstanceErrs (nub bad_irreds)
1504 -- The nub removes duplicates, which has
1505 -- not happened otherwise (see notes above)
1506 ; return (dicts, binds) }
1508 go :: [Inst] -> TcDictBinds -> [Inst] -> TcM ([Inst], TcDictBinds)
1510 = return (irreds, binds)
1511 go irreds binds (w:ws)
1513 = go (w:irreds) binds ws
1514 | isImplicInst w -- Have a go at reducing the implication
1515 = do { (binds1, irreds1) <- reduceImplication red_env w
1516 ; let (bad_irreds, ok_irreds) = partition isImplicInst irreds1
1517 ; go (bad_irreds ++ irreds)
1518 (binds `unionBags` binds1)
1521 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1522 -- to fromInteger; this looks fragile to me
1523 ; lookup_result <- lookupSimpleInst w'
1524 ; case lookup_result of
1525 NoInstance -> go (w:irreds) binds ws
1526 GenInst ws' rhs -> go irreds binds' (ws' ++ ws)
1528 binds' = addInstToDictBind binds w rhs
1531 -- Sigh: we need to reduce inside implications
1532 red_env = mkRedEnv doc try_me []
1533 doc = ptext (sLit "Implication constraint in RULE lhs")
1534 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
1535 | otherwise = Stop -- Be gentle
1538 tcSimplifyBracket is used when simplifying the constraints arising from
1539 a Template Haskell bracket [| ... |]. We want to check that there aren't
1540 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1541 Show instance), but we aren't otherwise interested in the results.
1542 Nor do we care about ambiguous dictionaries etc. We will type check
1543 this bracket again at its usage site.
1546 tcSimplifyBracket :: [Inst] -> TcM ()
1547 tcSimplifyBracket wanteds
1548 = do { tryHardCheckLoop doc wanteds
1551 doc = text "tcSimplifyBracket"
1555 %************************************************************************
1557 \subsection{Filtering at a dynamic binding}
1559 %************************************************************************
1564 we must discharge all the ?x constraints from B. We also do an improvement
1565 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1567 Actually, the constraints from B might improve the types in ?x. For example
1569 f :: (?x::Int) => Char -> Char
1572 then the constraint (?x::Int) arising from the call to f will
1573 force the binding for ?x to be of type Int.
1576 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1579 -- We need a loop so that we do improvement, and then
1580 -- (next time round) generate a binding to connect the two
1582 -- Here the two ?x's have different types, and improvement
1583 -- makes them the same.
1585 tcSimplifyIPs given_ips wanteds
1586 = do { wanteds' <- zonkInsts wanteds
1587 ; given_ips' <- zonkInsts given_ips
1588 -- Unusually for checking, we *must* zonk the given_ips
1590 ; let env = mkRedEnv doc try_me given_ips'
1591 ; (improved, binds, irreds, elim_skolems) <- reduceContext env wanteds'
1594 ; if not improved then
1595 ASSERT( all is_free irreds )
1596 do { extendLIEs irreds
1599 tcSimplifyIPs given_ips wanteds }
1601 doc = text "tcSimplifyIPs" <+> ppr given_ips
1602 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1603 is_free inst = isFreeWrtIPs ip_set inst
1605 -- Simplify any methods that mention the implicit parameter
1606 try_me inst | is_free inst = Stop
1607 | otherwise = ReduceMe NoSCs
1611 %************************************************************************
1613 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1615 %************************************************************************
1617 When doing a binding group, we may have @Insts@ of local functions.
1618 For example, we might have...
1620 let f x = x + 1 -- orig local function (overloaded)
1621 f.1 = f Int -- two instances of f
1626 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1627 where @f@ is in scope; those @Insts@ must certainly not be passed
1628 upwards towards the top-level. If the @Insts@ were binding-ified up
1629 there, they would have unresolvable references to @f@.
1631 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1632 For each method @Inst@ in the @init_lie@ that mentions one of the
1633 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1634 @LIE@), as well as the @HsBinds@ generated.
1637 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1638 -- Simlifies only MethodInsts, and generate only bindings of form
1640 -- We're careful not to even generate bindings of the form
1642 -- You'd think that'd be fine, but it interacts with what is
1643 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1645 bindInstsOfLocalFuns wanteds local_ids
1646 | null overloaded_ids = do
1649 return emptyLHsBinds
1652 = do { (irreds, binds) <- gentleInferLoop doc for_me
1653 ; extendLIEs not_for_me
1657 doc = text "bindInsts" <+> ppr local_ids
1658 overloaded_ids = filter is_overloaded local_ids
1659 is_overloaded id = isOverloadedTy (idType id)
1660 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1662 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1663 -- so it's worth building a set, so that
1664 -- lookup (in isMethodFor) is faster
1668 %************************************************************************
1670 \subsection{Data types for the reduction mechanism}
1672 %************************************************************************
1674 The main control over context reduction is here
1678 = RedEnv { red_doc :: SDoc -- The context
1679 , red_try_me :: Inst -> WhatToDo
1680 , red_improve :: Bool -- True <=> do improvement
1681 , red_givens :: [Inst] -- All guaranteed rigid
1683 -- but see Note [Rigidity]
1684 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1685 -- See Note [RedStack]
1689 -- The red_givens are rigid so far as cmpInst is concerned.
1690 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1691 -- let ?x = e in ...
1692 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1693 -- But that doesn't affect the comparison, which is based only on mame.
1696 -- The red_stack pair (n,insts) pair is just used for error reporting.
1697 -- 'n' is always the depth of the stack.
1698 -- The 'insts' is the stack of Insts being reduced: to produce X
1699 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1702 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1703 mkRedEnv doc try_me givens
1704 = RedEnv { red_doc = doc, red_try_me = try_me,
1705 red_givens = givens,
1707 red_improve = True }
1709 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1710 -- Do not do improvement; no givens
1711 mkNoImproveRedEnv doc try_me
1712 = RedEnv { red_doc = doc, red_try_me = try_me,
1715 red_improve = True }
1718 = ReduceMe WantSCs -- Try to reduce this
1719 -- If there's no instance, add the inst to the
1720 -- irreductible ones, but don't produce an error
1721 -- message of any kind.
1722 -- It might be quite legitimate such as (Eq a)!
1724 | Stop -- Return as irreducible unless it can
1725 -- be reduced to a constant in one step
1726 -- Do not add superclasses; see
1728 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1729 -- of a predicate when adding it to the avails
1730 -- The reason for this flag is entirely the super-class loop problem
1731 -- Note [SUPER-CLASS LOOP 1]
1733 zonkRedEnv :: RedEnv -> TcM RedEnv
1735 = do { givens' <- mapM zonkInst (red_givens env)
1736 ; return $ env {red_givens = givens'}
1741 %************************************************************************
1743 \subsection[reduce]{@reduce@}
1745 %************************************************************************
1747 Note [Ancestor Equalities]
1748 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1749 During context reduction, we add to the wanted equalities also those
1750 equalities that (transitively) occur in superclass contexts of wanted
1751 class constraints. Consider the following code
1753 class a ~ Int => C a
1756 If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
1757 substituting Int for a. Hence, we ultimately want (C Int), which we
1758 discharge with the explicit instance.
1761 reduceContext :: RedEnv
1763 -> TcM (ImprovementDone,
1764 TcDictBinds, -- Dictionary bindings
1765 [Inst], -- Irreducible
1766 TcM ()) -- Undo skolems from SkolemOccurs
1768 reduceContext env wanteds
1769 = do { traceTc (text "reduceContext" <+> (vcat [
1770 text "----------------------",
1772 text "given" <+> ppr (red_givens env),
1773 text "wanted" <+> ppr wanteds,
1774 text "----------------------"
1778 ; let givens = red_givens env
1779 (given_eqs0, given_dicts0) = partition isEqInst givens
1780 (wanted_eqs0, wanted_non_eqs) = partition isEqInst wanteds
1781 (wanted_implics0, wanted_dicts) = partition isImplicInst wanted_non_eqs
1783 -- We want to add as wanted equalities those that (transitively)
1784 -- occur in superclass contexts of wanted class constraints.
1785 -- See Note [Ancestor Equalities]
1786 ; ancestor_eqs <- ancestorEqualities wanted_dicts
1787 ; let wanted_eqs = wanted_eqs0 ++ ancestor_eqs
1788 ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
1790 -- 1. Normalise the *given* *equality* constraints
1791 ; (given_eqs, eliminate_skolems) <- normaliseGivenEqs given_eqs0
1793 -- 2. Normalise the *given* *dictionary* constraints
1794 -- wrt. the toplevel and given equations
1795 ; (given_dicts, given_binds) <- normaliseGivenDicts given_eqs
1798 -- 5. Build the Avail mapping from "given_dicts"
1799 ; (init_state, _) <- getLIE $ do
1800 { init_state <- foldlM addGiven emptyAvails given_dicts
1804 -- !!! ToDo: what to do with the "extra_givens"? For the
1805 -- moment I'm simply discarding them, which is probably wrong
1807 -- 6. Solve the *wanted* *dictionary* constraints (not implications)
1808 -- This may expose some further equational constraints...
1809 ; (avails, extra_eqs) <- getLIE (reduceList env wanted_dicts init_state)
1810 ; (dict_binds, bound_dicts, dict_irreds)
1811 <- extractResults avails wanted_dicts
1812 ; traceTc $ text "reduceContext extractresults" <+> vcat
1813 [ppr avails, ppr wanted_dicts, ppr dict_binds]
1815 -- Solve the wanted *implications*. In doing so, we can provide
1816 -- as "given" all the dicts that were originally given,
1817 -- *or* for which we now have bindings,
1818 -- *or* which are now irreds
1819 ; let implic_env = env { red_givens = givens ++ bound_dicts
1821 ; (implic_binds_s, implic_irreds_s)
1822 <- mapAndUnzipM (reduceImplication implic_env) wanted_implics0
1823 ; let implic_binds = unionManyBags implic_binds_s
1824 implic_irreds = concat implic_irreds_s
1826 -- Normalise the wanted equality constraints
1827 ; eq_irreds <- normaliseWantedEqs given_eqs (wanted_eqs ++ extra_eqs)
1829 -- Normalise the wanted dictionaries
1830 ; let irreds = dict_irreds ++ implic_irreds
1831 eqs = eq_irreds ++ given_eqs
1832 ; (norm_irreds, normalise_binds) <- normaliseWantedDicts eqs irreds
1834 -- Figure out whether we should go round again. We do so in either
1836 -- (1) If any of the mutable tyvars in givens or irreds has been
1837 -- filled in by improvement, there is merit in going around
1838 -- again, because we may make further progress.
1839 -- (2) If we managed to normalise any dicts, there is merit in going
1840 -- around gain, because reduceList may be able to get further.
1842 -- ToDo: We may have exposed new
1843 -- equality constraints and should probably go round again
1844 -- then as well. But currently we are dropping them on the
1847 ; let all_irreds = norm_irreds ++ eq_irreds
1848 ; improvedMetaTy <- anyM isFilledMetaTyVar $ varSetElems $
1849 tyVarsOfInsts (givens ++ all_irreds)
1850 ; let improvedDicts = not $ isEmptyBag normalise_binds
1851 improved = improvedMetaTy || improvedDicts
1853 -- The old plan (fragile)
1854 -- improveed = availsImproved avails
1855 -- || (not $ isEmptyBag normalise_binds1)
1856 -- || (not $ isEmptyBag normalise_binds2)
1857 -- || (any isEqInst irreds)
1859 ; traceTc (text "reduceContext end" <+> (vcat [
1860 text "----------------------",
1862 text "given" <+> ppr givens,
1863 text "given_eqs" <+> ppr given_eqs,
1864 text "wanted" <+> ppr wanteds,
1865 text "wanted_dicts" <+> ppr wanted_dicts,
1867 text "avails" <+> pprAvails avails,
1868 text "improved =" <+> ppr improved,
1869 text "(all) irreds = " <+> ppr all_irreds,
1870 text "dict-binds = " <+> ppr dict_binds,
1871 text "implic-binds = " <+> ppr implic_binds,
1872 text "----------------------"
1876 given_binds `unionBags` normalise_binds
1877 `unionBags` dict_binds
1878 `unionBags` implic_binds,
1883 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1884 tcImproveOne avails inst
1885 | not (isDict inst) = return False
1887 = do { inst_envs <- tcGetInstEnvs
1888 ; let eqns = improveOne (classInstances inst_envs)
1889 (dictPred inst, pprInstArising inst)
1890 [ (dictPred p, pprInstArising p)
1891 | p <- availsInsts avails, isDict p ]
1892 -- Avails has all the superclasses etc (good)
1893 -- It also has all the intermediates of the deduction (good)
1894 -- It does not have duplicates (good)
1895 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1896 -- so that improve will see them separate
1897 ; traceTc (text "improveOne" <+> ppr inst)
1900 unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))]
1901 -> TcM ImprovementDone
1902 unifyEqns [] = return False
1904 = do { traceTc (ptext (sLit "Improve:") <+> vcat (map pprEquationDoc eqns))
1908 unify ((qtvs, pairs), what1, what2)
1909 = addErrCtxtM (mkEqnMsg what1 what2) $ do
1910 (_, _, tenv) <- tcInstTyVars (varSetElems qtvs)
1911 mapM_ (unif_pr tenv) pairs
1912 unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1914 pprEquationDoc :: (Equation, (PredType, SDoc), (PredType, SDoc)) -> SDoc
1915 pprEquationDoc (eqn, (p1, _), (p2, _)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
1917 mkEqnMsg :: (TcPredType, SDoc) -> (TcPredType, SDoc) -> TidyEnv
1918 -> TcM (TidyEnv, SDoc)
1919 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
1920 = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
1921 ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
1922 ; let msg = vcat [ptext (sLit "When using functional dependencies to combine"),
1923 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
1924 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
1925 ; return (tidy_env, msg) }
1928 The main context-reduction function is @reduce@. Here's its game plan.
1931 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
1932 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
1933 = do { traceTc (text "reduceList " <+> (ppr wanteds $$ ppr state))
1935 ; when (debugIsOn && (n > 8)) $ do
1936 debugDumpTcRn (hang (ptext (sLit "Interesting! Context reduction stack depth") <+> int n)
1937 2 (ifPprDebug (nest 2 (pprStack stk))))
1938 ; if n >= ctxtStkDepth dopts then
1939 failWithTc (reduceDepthErr n stk)
1943 go [] state = return state
1944 go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
1947 -- Base case: we're done!
1948 reduce :: RedEnv -> Inst -> Avails -> TcM Avails
1949 reduce env wanted avails
1950 -- It's the same as an existing inst, or a superclass thereof
1951 | Just _ <- findAvail avails wanted
1952 = do { traceTc (text "reduce: found " <+> ppr wanted)
1957 = do { traceTc (text "reduce" <+> ppr wanted $$ ppr avails)
1958 ; case red_try_me env wanted of {
1959 Stop -> try_simple (addIrred NoSCs);
1960 -- See Note [No superclasses for Stop]
1962 ReduceMe want_scs -> do -- It should be reduced
1963 { (avails, lookup_result) <- reduceInst env avails wanted
1964 ; case lookup_result of
1965 NoInstance -> addIrred want_scs avails wanted
1966 -- Add it and its superclasses
1968 GenInst [] rhs -> addWanted want_scs avails wanted rhs []
1970 GenInst wanteds' rhs
1971 -> do { avails1 <- addIrred NoSCs avails wanted
1972 ; avails2 <- reduceList env wanteds' avails1
1973 ; addWanted want_scs avails2 wanted rhs wanteds' } }
1974 -- Temporarily do addIrred *before* the reduceList,
1975 -- which has the effect of adding the thing we are trying
1976 -- to prove to the database before trying to prove the things it
1977 -- needs. See note [RECURSIVE DICTIONARIES]
1978 -- NB: we must not do an addWanted before, because that adds the
1979 -- superclasses too, and that can lead to a spurious loop; see
1980 -- the examples in [SUPERCLASS-LOOP]
1981 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1984 -- First, see if the inst can be reduced to a constant in one step
1985 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
1986 -- Don't bother for implication constraints, which take real work
1987 try_simple do_this_otherwise
1988 = do { res <- lookupSimpleInst wanted
1990 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
1991 _ -> do_this_otherwise avails wanted }
1995 Note [SUPERCLASS-LOOP 2]
1996 ~~~~~~~~~~~~~~~~~~~~~~~~
1997 But the above isn't enough. Suppose we are *given* d1:Ord a,
1998 and want to deduce (d2:C [a]) where
2000 class Ord a => C a where
2001 instance Ord [a] => C [a] where ...
2003 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
2004 superclasses of C [a] to avails. But we must not overwrite the binding
2005 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
2008 Here's another variant, immortalised in tcrun020
2009 class Monad m => C1 m
2010 class C1 m => C2 m x
2011 instance C2 Maybe Bool
2012 For the instance decl we need to build (C1 Maybe), and it's no good if
2013 we run around and add (C2 Maybe Bool) and its superclasses to the avails
2014 before we search for C1 Maybe.
2016 Here's another example
2017 class Eq b => Foo a b
2018 instance Eq a => Foo [a] a
2022 we'll first deduce that it holds (via the instance decl). We must not
2023 then overwrite the Eq t constraint with a superclass selection!
2025 At first I had a gross hack, whereby I simply did not add superclass constraints
2026 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
2027 becuase it lost legitimate superclass sharing, and it still didn't do the job:
2028 I found a very obscure program (now tcrun021) in which improvement meant the
2029 simplifier got two bites a the cherry... so something seemed to be an Stop
2030 first time, but reducible next time.
2032 Now we implement the Right Solution, which is to check for loops directly
2033 when adding superclasses. It's a bit like the occurs check in unification.
2036 Note [RECURSIVE DICTIONARIES]
2037 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2039 data D r = ZeroD | SuccD (r (D r));
2041 instance (Eq (r (D r))) => Eq (D r) where
2042 ZeroD == ZeroD = True
2043 (SuccD a) == (SuccD b) = a == b
2046 equalDC :: D [] -> D [] -> Bool;
2049 We need to prove (Eq (D [])). Here's how we go:
2053 by instance decl, holds if
2057 by instance decl of Eq, holds if
2059 where d2 = dfEqList d3
2062 But now we can "tie the knot" to give
2068 and it'll even run! The trick is to put the thing we are trying to prove
2069 (in this case Eq (D []) into the database before trying to prove its
2070 contributing clauses.
2073 %************************************************************************
2075 Reducing a single constraint
2077 %************************************************************************
2080 ---------------------------------------------
2081 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
2082 reduceInst _ avails other_inst
2083 = do { result <- lookupSimpleInst other_inst
2084 ; return (avails, result) }
2087 Note [Equational Constraints in Implication Constraints]
2088 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2090 An implication constraint is of the form
2092 where Given and Wanted may contain both equational and dictionary
2093 constraints. The delay and reduction of these two kinds of constraints
2096 -) In the generated code, wanted Dictionary constraints are wrapped up in an
2097 implication constraint that is created at the code site where the wanted
2098 dictionaries can be reduced via a let-binding. This let-bound implication
2099 constraint is deconstructed at the use-site of the wanted dictionaries.
2101 -) While the reduction of equational constraints is also delayed, the delay
2102 is not manifest in the generated code. The required evidence is generated
2103 in the code directly at the use-site. There is no let-binding and deconstruction
2104 necessary. The main disadvantage is that we cannot exploit sharing as the
2105 same evidence may be generated at multiple use-sites. However, this disadvantage
2106 is limited because it only concerns coercions which are erased.
2108 The different treatment is motivated by the different in representation. Dictionary
2109 constraints require manifest runtime dictionaries, while equations require coercions
2113 ---------------------------------------------
2114 reduceImplication :: RedEnv
2116 -> TcM (TcDictBinds, [Inst])
2119 Suppose we are simplifying the constraint
2120 forall bs. extras => wanted
2121 in the context of an overall simplification problem with givens 'givens'.
2124 * The 'givens' need not mention any of the quantified type variables
2125 e.g. forall {}. Eq a => Eq [a]
2126 forall {}. C Int => D (Tree Int)
2128 This happens when you have something like
2130 T1 :: Eq a => a -> T a
2133 f x = ...(case x of { T1 v -> v==v })...
2136 -- ToDo: should we instantiate tvs? I think it's not necessary
2138 -- Note on coercion variables:
2140 -- The extra given coercion variables are bound at two different sites:
2141 -- -) in the creation context of the implication constraint
2142 -- the solved equational constraints use these binders
2144 -- -) at the solving site of the implication constraint
2145 -- the solved dictionaries use these binders
2146 -- these binders are generated by reduceImplication
2148 reduceImplication env
2149 orig_implic@(ImplicInst { tci_name = name, tci_loc = inst_loc,
2151 tci_given = extra_givens, tci_wanted = wanteds })
2152 = do { -- Solve the sub-problem
2153 ; let try_me _ = ReduceMe AddSCs -- Note [Freeness and implications]
2154 env' = env { red_givens = extra_givens ++ red_givens env
2155 , red_doc = sep [ptext (sLit "reduceImplication for")
2157 nest 2 (parens $ ptext (sLit "within")
2159 , red_try_me = try_me }
2161 ; traceTc (text "reduceImplication" <+> vcat
2162 [ ppr (red_givens env), ppr extra_givens,
2164 ; (irreds, binds) <- checkLoop env' wanteds
2165 ; let (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens
2166 -- SLPJ Sept 07: I think this is bogus; currently
2167 -- there are no Eqinsts in extra_givens
2168 dict_ids = map instToId extra_dict_givens
2170 -- Note [Reducing implication constraints]
2171 -- Tom -- update note, put somewhere!
2173 ; traceTc (text "reduceImplication result" <+> vcat
2174 [ppr irreds, ppr binds])
2176 ; -- extract superclass binds
2177 -- (sc_binds,_) <- extractResults avails []
2178 -- ; traceTc (text "reduceImplication sc_binds" <+> vcat
2179 -- [ppr sc_binds, ppr avails])
2182 -- SLPJ Sept 07: what if improvement happened inside the checkLoop?
2183 -- Then we must iterate the outer loop too!
2185 ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds)))
2187 -- Progress is no longer measered by the number of bindings
2188 ; if (isEmptyLHsBinds binds) && (not $ null irreds) then -- No progress
2189 -- If there are any irreds, we back off and do nothing
2190 return (emptyBag, [orig_implic])
2192 { (simpler_implic_insts, bind)
2193 <- makeImplicationBind inst_loc tvs extra_givens irreds
2194 -- This binding is useless if the recursive simplification
2195 -- made no progress; but currently we don't try to optimise that
2196 -- case. After all, we only try hard to reduce at top level, or
2197 -- when inferring types.
2199 ; let dict_wanteds = filter (not . isEqInst) wanteds
2200 -- TOMDO: given equational constraints bug!
2201 -- we need a different evidence for given
2202 -- equations depending on whether we solve
2203 -- dictionary constraints or equational constraints
2205 eq_tyvars = varSetElems $ tyVarsOfTypes $ map eqInstType extra_eq_givens
2206 -- SLPJ Sept07: this looks Utterly Wrong to me, but I think
2207 -- that current extra_givens has no EqInsts, so
2208 -- it makes no difference
2209 co = wrap_inline -- Note [Always inline implication constraints]
2211 <.> mkWpLams eq_tyvars
2212 <.> mkWpLams dict_ids
2213 <.> WpLet (binds `unionBags` bind)
2214 wrap_inline | null dict_ids = idHsWrapper
2215 | otherwise = WpInline
2216 rhs = mkLHsWrap co payload
2217 loc = instLocSpan inst_loc
2218 payload = mkBigLHsTup (map (L loc . HsVar . instToId) dict_wanteds)
2221 ; traceTc (vcat [text "reduceImplication" <+> ppr name,
2222 ppr simpler_implic_insts,
2223 text "->" <+> ppr rhs])
2224 ; return (unitBag (L loc (VarBind (instToId orig_implic) rhs)),
2225 simpler_implic_insts)
2228 reduceImplication _ i = pprPanic "reduceImplication" (ppr i)
2231 Note [Always inline implication constraints]
2232 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2233 Suppose an implication constraint floats out of an INLINE function.
2234 Then although the implication has a single call site, it won't be
2235 inlined. And that is bad because it means that even if there is really
2236 *no* overloading (type signatures specify the exact types) there will
2237 still be dictionary passing in the resulting code. To avert this,
2238 we mark the implication constraints themselves as INLINE, at least when
2239 there is no loss of sharing as a result.
2241 Note [Freeness and implications]
2242 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2243 It's hard to say when an implication constraint can be floated out. Consider
2244 forall {} Eq a => Foo [a]
2245 The (Foo [a]) doesn't mention any of the quantified variables, but it
2246 still might be partially satisfied by the (Eq a).
2248 There is a useful special case when it *is* easy to partition the
2249 constraints, namely when there are no 'givens'. Consider
2250 forall {a}. () => Bar b
2251 There are no 'givens', and so there is no reason to capture (Bar b).
2252 We can let it float out. But if there is even one constraint we
2253 must be much more careful:
2254 forall {a}. C a b => Bar (m b)
2255 because (C a b) might have a superclass (D b), from which we might
2256 deduce (Bar [b]) when m later gets instantiated to []. Ha!
2258 Here is an even more exotic example
2260 Now consider the constraint
2261 forall b. D Int b => C Int
2262 We can satisfy the (C Int) from the superclass of D, so we don't want
2263 to float the (C Int) out, even though it mentions no type variable in
2266 One more example: the constraint
2268 instance (C a, E c) => E (a,c)
2270 constraint: forall b. D Int b => E (Int,c)
2272 You might think that the (D Int b) can't possibly contribute
2273 to solving (E (Int,c)), since the latter mentions 'c'. But
2274 in fact it can, because solving the (E (Int,c)) constraint needs
2277 and the (C Int) can be satisfied from the superclass of (D Int b).
2278 So we must still not float (E (Int,c)) out.
2280 To think about: special cases for unary type classes?
2282 Note [Pruning the givens in an implication constraint]
2283 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2284 Suppose we are about to form the implication constraint
2285 forall tvs. Eq a => Ord b
2286 The (Eq a) cannot contribute to the (Ord b), because it has no access to
2287 the type variable 'b'. So we could filter out the (Eq a) from the givens.
2288 But BE CAREFUL of the examples above in [Freeness and implications].
2290 Doing so would be a bit tidier, but all the implication constraints get
2291 simplified away by the optimiser, so it's no great win. So I don't take
2292 advantage of that at the moment.
2294 If you do, BE CAREFUL of wobbly type variables.
2297 %************************************************************************
2299 Avails and AvailHow: the pool of evidence
2301 %************************************************************************
2305 data Avails = Avails !ImprovementDone !AvailEnv
2307 type ImprovementDone = Bool -- True <=> some unification has happened
2308 -- so some Irreds might now be reducible
2309 -- keys that are now
2311 type AvailEnv = FiniteMap Inst AvailHow
2313 = IsIrred -- Used for irreducible dictionaries,
2314 -- which are going to be lambda bound
2316 | Given Inst -- Used for dictionaries for which we have a binding
2317 -- e.g. those "given" in a signature
2319 | Rhs -- Used when there is a RHS
2320 (LHsExpr TcId) -- The RHS
2321 [Inst] -- Insts free in the RHS; we need these too
2323 instance Outputable Avails where
2326 pprAvails :: Avails -> SDoc
2327 pprAvails (Avails imp avails)
2328 = vcat [ ptext (sLit "Avails") <> (if imp then ptext (sLit "[improved]") else empty)
2330 vcat [ sep [ppr inst, nest 2 (equals <+> ppr avail)]
2331 | (inst,avail) <- fmToList avails ]]
2333 instance Outputable AvailHow where
2336 -------------------------
2337 pprAvail :: AvailHow -> SDoc
2338 pprAvail IsIrred = text "Irred"
2339 pprAvail (Given x) = text "Given" <+> ppr x
2340 pprAvail (Rhs rhs bs) = sep [text "Rhs" <+> ppr bs,
2343 -------------------------
2344 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
2345 extendAvailEnv env inst avail = addToFM env inst avail
2347 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
2348 findAvailEnv env wanted = lookupFM env wanted
2349 -- NB 1: the Ord instance of Inst compares by the class/type info
2350 -- *not* by unique. So
2351 -- d1::C Int == d2::C Int
2353 emptyAvails :: Avails
2354 emptyAvails = Avails False emptyFM
2356 findAvail :: Avails -> Inst -> Maybe AvailHow
2357 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
2359 elemAvails :: Inst -> Avails -> Bool
2360 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
2362 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
2364 extendAvails avails@(Avails imp env) inst avail
2365 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
2366 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
2368 availsInsts :: Avails -> [Inst]
2369 availsInsts (Avails _ avails) = keysFM avails
2371 _availsImproved :: Avails -> ImprovementDone
2372 _availsImproved (Avails imp _) = imp
2375 Extracting the bindings from a bunch of Avails.
2376 The bindings do *not* come back sorted in dependency order.
2377 We assume that they'll be wrapped in a big Rec, so that the
2378 dependency analyser can sort them out later
2381 type DoneEnv = FiniteMap Inst [Id]
2382 -- Tracks which things we have evidence for
2384 extractResults :: Avails
2386 -> TcM (TcDictBinds, -- Bindings
2387 [Inst], -- The insts bound by the bindings
2388 [Inst]) -- Irreducible ones
2389 -- Note [Reducing implication constraints]
2391 extractResults (Avails _ avails) wanteds
2392 = go emptyBag [] [] emptyFM wanteds
2394 go :: TcDictBinds -- Bindings for dicts
2395 -> [Inst] -- Bound by the bindings
2397 -> DoneEnv -- Has an entry for each inst in the above three sets
2399 -> TcM (TcDictBinds, [Inst], [Inst])
2400 go binds bound_dicts irreds _ []
2401 = return (binds, bound_dicts, irreds)
2403 go binds bound_dicts irreds done (w:ws)
2404 | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w
2405 = if w_id `elem` done_ids then
2406 go binds bound_dicts irreds done ws
2408 go (add_bind (nlHsVar done_id)) bound_dicts irreds
2409 (addToFM done w (done_id : w_id : rest_done_ids)) ws
2411 | otherwise -- Not yet done
2412 = case findAvailEnv avails w of
2413 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
2414 go binds bound_dicts irreds done ws
2416 Just IsIrred -> go binds bound_dicts (w:irreds) done' ws
2418 Just (Rhs rhs ws') -> go (add_bind rhs) (w:bound_dicts) irreds done' (ws' ++ ws)
2420 Just (Given g) -> go binds' bound_dicts irreds (addToFM done w [g_id]) ws
2423 binds' | w_id == g_id = binds
2424 | otherwise = add_bind (nlHsVar g_id)
2427 done' = addToFM done w [w_id]
2428 add_bind rhs = addInstToDictBind binds w rhs
2432 Note [No superclasses for Stop]
2433 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2434 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2435 add it to avails, so that any other equal Insts will be commoned up
2436 right here. However, we do *not* add superclasses. If we have
2439 but a is not bound here, then we *don't* want to derive dn from df
2440 here lest we lose sharing.
2443 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2444 addWanted want_scs avails wanted rhs_expr wanteds
2445 = addAvailAndSCs want_scs avails wanted avail
2447 avail = Rhs rhs_expr wanteds
2449 addGiven :: Avails -> Inst -> TcM Avails
2450 addGiven avails given = addAvailAndSCs AddSCs avails given (Given given)
2451 -- Always add superclasses for 'givens'
2453 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2454 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2455 -- so the assert isn't true
2459 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2460 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2461 addAvailAndSCs want_scs avails irred IsIrred
2463 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2464 addAvailAndSCs want_scs avails inst avail
2465 | not (isClassDict inst) = extendAvails avails inst avail
2466 | NoSCs <- want_scs = extendAvails avails inst avail
2467 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2468 ; avails' <- extendAvails avails inst avail
2469 ; addSCs is_loop avails' inst }
2471 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2472 -- Note: this compares by *type*, not by Unique
2473 deps = findAllDeps (unitVarSet (instToVar inst)) avail
2474 dep_tys = map idType (varSetElems deps)
2476 findAllDeps :: IdSet -> AvailHow -> IdSet
2477 -- Find all the Insts that this one depends on
2478 -- See Note [SUPERCLASS-LOOP 2]
2479 -- Watch out, though. Since the avails may contain loops
2480 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2481 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2482 findAllDeps so_far _ = so_far
2484 find_all :: IdSet -> Inst -> IdSet
2486 | isEqInst kid = so_far
2487 | kid_id `elemVarSet` so_far = so_far
2488 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2489 | otherwise = so_far'
2491 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2492 kid_id = instToId kid
2494 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2495 -- Add all the superclasses of the Inst to Avails
2496 -- The first param says "don't do this because the original thing
2497 -- depends on this one, so you'd build a loop"
2498 -- Invariant: the Inst is already in Avails.
2500 addSCs is_loop avails dict
2501 = ASSERT( isDict dict )
2502 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2503 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2505 (clas, tys) = getDictClassTys dict
2506 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2507 sc_theta' = filter (not . isEqPred) $
2508 substTheta (zipTopTvSubst tyvars tys) sc_theta
2510 add_sc avails (sc_dict, sc_sel)
2511 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2512 | is_given sc_dict = return avails
2513 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2514 ; addSCs is_loop avails' sc_dict }
2516 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2517 co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
2519 is_given :: Inst -> Bool
2520 is_given sc_dict = case findAvail avails sc_dict of
2521 Just (Given _) -> True -- Given is cheaper than superclass selection
2524 -- From the a set of insts obtain all equalities that (transitively) occur in
2525 -- superclass contexts of class constraints (aka the ancestor equalities).
2527 ancestorEqualities :: [Inst] -> TcM [Inst]
2529 = mapM mkWantedEqInst -- turn only equality predicates..
2530 . filter isEqPred -- ..into wanted equality insts
2532 . addAEsToBag emptyBag -- collect the superclass constraints..
2533 . map dictPred -- ..of all predicates in a bag
2534 . filter isClassDict
2536 addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
2537 addAEsToBag bag [] = bag
2538 addAEsToBag bag (pred:preds)
2539 | pred `elemBag` bag = addAEsToBag bag preds
2540 | isEqPred pred = addAEsToBag bagWithPred preds
2541 | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
2542 | otherwise = addAEsToBag bag preds
2544 bagWithPred = bag `snocBag` pred
2545 predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
2547 (tyvars, sc_theta, _, _) = classBigSig clas
2548 (clas, tys) = getClassPredTys pred
2552 %************************************************************************
2554 \section{tcSimplifyTop: defaulting}
2556 %************************************************************************
2559 @tcSimplifyTop@ is called once per module to simplify all the constant
2560 and ambiguous Insts.
2562 We need to be careful of one case. Suppose we have
2564 instance Num a => Num (Foo a b) where ...
2566 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2567 to (Num x), and default x to Int. But what about y??
2569 It's OK: the final zonking stage should zap y to (), which is fine.
2573 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2574 tcSimplifyTop wanteds
2575 = tc_simplify_top doc False wanteds
2577 doc = text "tcSimplifyTop"
2579 tcSimplifyInteractive wanteds
2580 = tc_simplify_top doc True wanteds
2582 doc = text "tcSimplifyInteractive"
2584 -- The TcLclEnv should be valid here, solely to improve
2585 -- error message generation for the monomorphism restriction
2586 tc_simplify_top :: SDoc -> Bool -> [Inst] -> TcM (Bag (LHsBind TcId))
2587 tc_simplify_top doc interactive wanteds
2588 = do { dflags <- getDOpts
2589 ; wanteds <- zonkInsts wanteds
2590 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2592 ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
2593 ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
2594 -- ; (irreds1, binds1) <- gentleInferLoop doc1 wanteds
2595 ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
2596 ; (irreds2, binds2) <- approximateImplications doc2 (\_ -> True) irreds1
2597 ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
2599 -- Use the defaulting rules to do extra unification
2600 -- NB: irreds2 are already zonked
2601 ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
2603 -- Deal with implicit parameters
2604 ; let (bad_ips, non_ips) = partition isIPDict irreds3
2605 (ambigs, others) = partition isTyVarDict non_ips
2607 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2609 ; addNoInstanceErrs others
2610 ; addTopAmbigErrs ambigs
2612 ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
2614 doc1 = doc <+> ptext (sLit "(first round)")
2615 doc2 = doc <+> ptext (sLit "(approximate)")
2616 doc3 = doc <+> ptext (sLit "(disambiguate)")
2619 If a dictionary constrains a type variable which is
2620 * not mentioned in the environment
2621 * and not mentioned in the type of the expression
2622 then it is ambiguous. No further information will arise to instantiate
2623 the type variable; nor will it be generalised and turned into an extra
2624 parameter to a function.
2626 It is an error for this to occur, except that Haskell provided for
2627 certain rules to be applied in the special case of numeric types.
2629 * at least one of its classes is a numeric class, and
2630 * all of its classes are numeric or standard
2631 then the type variable can be defaulted to the first type in the
2632 default-type list which is an instance of all the offending classes.
2634 So here is the function which does the work. It takes the ambiguous
2635 dictionaries and either resolves them (producing bindings) or
2636 complains. It works by splitting the dictionary list by type
2637 variable, and using @disambigOne@ to do the real business.
2639 @disambigOne@ assumes that its arguments dictionaries constrain all
2640 the same type variable.
2642 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2643 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2644 the most common use of defaulting is code like:
2646 _ccall_ foo `seqPrimIO` bar
2648 Since we're not using the result of @foo@, the result if (presumably)
2652 disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
2653 -- Just does unification to fix the default types
2654 -- The Insts are assumed to be pre-zonked
2655 disambiguate doc interactive dflags insts
2657 = return (insts, emptyBag)
2659 | null defaultable_groups
2660 = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
2661 ; return (insts, emptyBag) }
2664 = do { -- Figure out what default types to use
2665 default_tys <- getDefaultTys extended_defaulting ovl_strings
2667 ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2668 ; mapM_ (disambigGroup default_tys) defaultable_groups
2670 -- disambigGroup does unification, hence try again
2671 ; tryHardCheckLoop doc insts }
2674 extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
2675 ovl_strings = dopt Opt_OverloadedStrings dflags
2677 unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
2678 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2679 (unaries, bad_tvs_s) = partitionWith find_unary insts
2680 bad_tvs = unionVarSets bad_tvs_s
2682 -- Finds unary type-class constraints
2683 find_unary d@(Dict {tci_pred = ClassP cls [ty]})
2684 | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
2685 find_unary inst = Right (tyVarsOfInst inst)
2687 -- Group by type variable
2688 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2689 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2690 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2692 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2693 defaultable_group ds@((_,_,tv):_)
2694 = isTyConableTyVar tv -- Note [Avoiding spurious errors]
2695 && not (tv `elemVarSet` bad_tvs)
2696 && defaultable_classes [c | (_,c,_) <- ds]
2697 defaultable_group [] = panic "defaultable_group"
2699 defaultable_classes clss
2700 | extended_defaulting = any isInteractiveClass clss
2701 | otherwise = all is_std_class clss && (any is_num_class clss)
2703 -- In interactive mode, or with -XExtendedDefaultRules,
2704 -- we default Show a to Show () to avoid graututious errors on "show []"
2705 isInteractiveClass cls
2706 = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2708 is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2709 -- is_num_class adds IsString to the standard numeric classes,
2710 -- when -foverloaded-strings is enabled
2712 is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2713 -- Similarly is_std_class
2715 -----------------------
2716 disambigGroup :: [Type] -- The default types
2717 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2718 -> TcM () -- Just does unification, to fix the default types
2720 disambigGroup default_tys dicts
2721 = try_default default_tys
2723 (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
2724 classes = [c | (_,c,_) <- dicts]
2726 try_default [] = return ()
2727 try_default (default_ty : default_tys)
2728 = tryTcLIE_ (try_default default_tys) $
2729 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2730 -- This may fail; then the tryTcLIE_ kicks in
2731 -- Failure here is caused by there being no type in the
2732 -- default list which can satisfy all the ambiguous classes.
2733 -- For example, if Real a is reqd, but the only type in the
2734 -- default list is Int.
2736 -- After this we can't fail
2737 ; warnDefault dicts default_ty
2738 ; unifyType default_ty (mkTyVarTy tyvar)
2739 ; return () -- TOMDO: do something with the coercion
2743 -----------------------
2744 getDefaultTys :: Bool -> Bool -> TcM [Type]
2745 getDefaultTys extended_deflts ovl_strings
2746 = do { mb_defaults <- getDeclaredDefaultTys
2747 ; case mb_defaults of {
2748 Just tys -> return tys ; -- User-supplied defaults
2751 -- No use-supplied default
2752 -- Use [Integer, Double], plus modifications
2753 { integer_ty <- tcMetaTy integerTyConName
2754 ; checkWiredInTyCon doubleTyCon
2755 ; string_ty <- tcMetaTy stringTyConName
2756 ; return (opt_deflt extended_deflts unitTy
2757 -- Note [Default unitTy]
2759 [integer_ty,doubleTy]
2761 opt_deflt ovl_strings string_ty) } } }
2763 opt_deflt True ty = [ty]
2764 opt_deflt False _ = []
2767 Note [Default unitTy]
2768 ~~~~~~~~~~~~~~~~~~~~~
2769 In interative mode (or with -XExtendedDefaultRules) we add () as the first type we
2770 try when defaulting. This has very little real impact, except in the following case.
2772 Text.Printf.printf "hello"
2773 This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
2774 want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
2775 default the 'a' to (), rather than to Integer (which is what would otherwise happen;
2776 and then GHCi doesn't attempt to print the (). So in interactive mode, we add
2777 () to the list of defaulting types. See Trac #1200.
2779 Note [Avoiding spurious errors]
2780 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2781 When doing the unification for defaulting, we check for skolem
2782 type variables, and simply don't default them. For example:
2783 f = (*) -- Monomorphic
2784 g :: Num a => a -> a
2786 Here, we get a complaint when checking the type signature for g,
2787 that g isn't polymorphic enough; but then we get another one when
2788 dealing with the (Num a) context arising from f's definition;
2789 we try to unify a with Int (to default it), but find that it's
2790 already been unified with the rigid variable from g's type sig
2793 %************************************************************************
2795 \subsection[simple]{@Simple@ versions}
2797 %************************************************************************
2799 Much simpler versions when there are no bindings to make!
2801 @tcSimplifyThetas@ simplifies class-type constraints formed by
2802 @deriving@ declarations and when specialising instances. We are
2803 only interested in the simplified bunch of class/type constraints.
2805 It simplifies to constraints of the form (C a b c) where
2806 a,b,c are type variables. This is required for the context of
2807 instance declarations.
2810 tcSimplifyDeriv :: InstOrigin
2812 -> ThetaType -- Wanted
2813 -> TcM ThetaType -- Needed
2814 -- Given instance (wanted) => C inst_ty
2815 -- Simplify 'wanted' as much as possible
2817 tcSimplifyDeriv orig tyvars theta
2818 = do { (tvs, _, tenv) <- tcInstTyVars tyvars
2819 -- The main loop may do unification, and that may crash if
2820 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2821 -- ToDo: what if two of them do get unified?
2822 ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
2823 ; (irreds, _) <- tryHardCheckLoop doc wanteds
2825 ; let (tv_dicts, others) = partition ok irreds
2826 ; addNoInstanceErrs others
2827 -- See Note [Exotic derived instance contexts] in TcMType
2829 ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2830 simpl_theta = substTheta rev_env (map dictPred tv_dicts)
2831 -- This reverse-mapping is a pain, but the result
2832 -- should mention the original TyVars not TcTyVars
2834 ; return simpl_theta }
2836 doc = ptext (sLit "deriving classes for a data type")
2838 ok dict | isDict dict = validDerivPred (dictPred dict)
2843 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2844 used with \tr{default} declarations. We are only interested in
2845 whether it worked or not.
2848 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2851 tcSimplifyDefault theta = do
2852 wanteds <- newDictBndrsO DefaultOrigin theta
2853 (irreds, _) <- tryHardCheckLoop doc wanteds
2854 addNoInstanceErrs irreds
2858 traceTc (ptext (sLit "tcSimplifyDefault failing")) >> failM
2860 doc = ptext (sLit "default declaration")
2864 %************************************************************************
2866 \section{Errors and contexts}
2868 %************************************************************************
2870 ToDo: for these error messages, should we note the location as coming
2871 from the insts, or just whatever seems to be around in the monad just
2875 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2876 -> [Inst] -- The offending Insts
2878 -- Group together insts with the same origin
2879 -- We want to report them together in error messages
2883 groupErrs report_err (inst:insts)
2884 = do { do_one (inst:friends)
2885 ; groupErrs report_err others }
2887 -- (It may seem a bit crude to compare the error messages,
2888 -- but it makes sure that we combine just what the user sees,
2889 -- and it avoids need equality on InstLocs.)
2890 (friends, others) = partition is_friend insts
2891 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2892 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2893 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2894 -- Add location and context information derived from the Insts
2896 -- Add the "arising from..." part to a message about bunch of dicts
2897 addInstLoc :: [Inst] -> Message -> Message
2898 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
2900 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
2903 addTopIPErrs bndrs ips
2904 = do { dflags <- getDOpts
2905 ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
2907 (tidy_env, tidy_ips) = tidyInsts ips
2909 = vcat [sep [ptext (sLit "Implicit parameters escape from"),
2910 nest 2 (ptext (sLit "the monomorphic top-level binding")
2911 <> plural bndrs <+> ptext (sLit "of")
2912 <+> pprBinders bndrs <> colon)],
2913 nest 2 (vcat (map ppr_ip ips)),
2914 monomorphism_fix dflags]
2915 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
2917 topIPErrs :: [Inst] -> TcM ()
2919 = groupErrs report tidy_dicts
2921 (tidy_env, tidy_dicts) = tidyInsts dicts
2922 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2923 mk_msg dicts = addInstLoc dicts (ptext (sLit "Unbound implicit parameter") <>
2924 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2926 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
2928 addNoInstanceErrs insts
2929 = do { let (tidy_env, tidy_insts) = tidyInsts insts
2930 ; reportNoInstances tidy_env Nothing tidy_insts }
2934 -> Maybe (InstLoc, [Inst]) -- Context
2935 -- Nothing => top level
2936 -- Just (d,g) => d describes the construct
2938 -> [Inst] -- What is wanted (can include implications)
2941 reportNoInstances tidy_env mb_what insts
2942 = groupErrs (report_no_instances tidy_env mb_what) insts
2944 report_no_instances :: TidyEnv -> Maybe (InstLoc, [Inst]) -> [Inst] -> TcM ()
2945 report_no_instances tidy_env mb_what insts
2946 = do { inst_envs <- tcGetInstEnvs
2947 ; let (implics, insts1) = partition isImplicInst insts
2948 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
2949 (eqInsts, insts3) = partition isEqInst insts2
2950 ; traceTc (text "reportNoInstances" <+> vcat
2951 [ppr insts, ppr implics, ppr insts1, ppr insts2])
2952 ; mapM_ complain_implic implics
2953 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
2954 ; groupErrs complain_no_inst insts3
2955 ; mapM_ (addErrTcM . mk_eq_err) eqInsts
2958 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
2960 complain_implic inst -- Recurse!
2961 = reportNoInstances tidy_env
2962 (Just (tci_loc inst, tci_given inst))
2965 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
2966 -- Right msg => overlap message
2967 -- Left inst => no instance
2968 check_overlap inst_envs wanted
2969 | not (isClassDict wanted) = Left wanted
2971 = case lookupInstEnv inst_envs clas tys of
2972 ([], _) -> Left wanted -- No match
2973 -- The case of exactly one match and no unifiers means a
2974 -- successful lookup. That can't happen here, because dicts
2975 -- only end up here if they didn't match in Inst.lookupInst
2977 | debugIsOn -> pprPanic "reportNoInstance" (ppr wanted)
2978 res -> Right (mk_overlap_msg wanted res)
2980 (clas,tys) = getDictClassTys wanted
2982 mk_overlap_msg dict (matches, unifiers)
2983 = ASSERT( not (null matches) )
2984 vcat [ addInstLoc [dict] ((ptext (sLit "Overlapping instances for")
2985 <+> pprPred (dictPred dict))),
2986 sep [ptext (sLit "Matching instances") <> colon,
2987 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
2988 if not (isSingleton matches)
2989 then -- Two or more matches
2991 else -- One match, plus some unifiers
2992 ASSERT( not (null unifiers) )
2993 parens (vcat [ptext (sLit "The choice depends on the instantiation of") <+>
2994 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
2995 ptext (sLit "To pick the first instance above, use -XIncoherentInstances"),
2996 ptext (sLit "when compiling the other instance declarations")])]
2998 ispecs = [ispec | (ispec, _) <- matches]
3000 mk_eq_err :: Inst -> (TidyEnv, SDoc)
3001 mk_eq_err inst = misMatchMsg tidy_env (eqInstTys inst)
3003 mk_no_inst_err insts
3004 | null insts = empty
3006 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
3007 not (isEmptyVarSet (tyVarsOfInsts insts))
3008 = vcat [ addInstLoc insts $
3009 sep [ ptext (sLit "Could not deduce") <+> pprDictsTheta insts
3010 , nest 2 $ ptext (sLit "from the context") <+> pprDictsTheta givens]
3011 , show_fixes (fix1 loc : fixes2) ]
3013 | otherwise -- Top level
3014 = vcat [ addInstLoc insts $
3015 ptext (sLit "No instance") <> plural insts
3016 <+> ptext (sLit "for") <+> pprDictsTheta insts
3017 , show_fixes fixes2 ]
3020 fix1 loc = sep [ ptext (sLit "add") <+> pprDictsTheta insts
3021 <+> ptext (sLit "to the context of"),
3022 nest 2 (ppr (instLocOrigin loc)) ]
3023 -- I'm not sure it helps to add the location
3024 -- nest 2 (ptext (sLit "at") <+> ppr (instLocSpan loc)) ]
3026 fixes2 | null instance_dicts = []
3027 | otherwise = [sep [ptext (sLit "add an instance declaration for"),
3028 pprDictsTheta instance_dicts]]
3029 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
3030 -- Insts for which it is worth suggesting an adding an instance declaration
3031 -- Exclude implicit parameters, and tyvar dicts
3033 show_fixes :: [SDoc] -> SDoc
3034 show_fixes [] = empty
3035 show_fixes (f:fs) = sep [ptext (sLit "Possible fix:"),
3036 nest 2 (vcat (f : map (ptext (sLit "or") <+>) fs))]
3038 addTopAmbigErrs :: [Inst] -> TcRn ()
3039 addTopAmbigErrs dicts
3040 -- Divide into groups that share a common set of ambiguous tyvars
3041 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
3042 -- See Note [Avoiding spurious errors]
3043 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
3045 (tidy_env, tidy_dicts) = tidyInsts dicts
3047 tvs_of :: Inst -> [TcTyVar]
3048 tvs_of d = varSetElems (tyVarsOfInst d)
3049 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
3051 report :: [(Inst,[TcTyVar])] -> TcM ()
3052 report pairs@((inst,tvs) : _) = do -- The pairs share a common set of ambiguous tyvars
3053 (tidy_env, mono_msg) <- mkMonomorphismMsg tidy_env tvs
3054 setSrcSpan (instSpan inst) $
3055 -- the location of the first one will do for the err message
3056 addErrTcM (tidy_env, msg $$ mono_msg)
3058 dicts = map fst pairs
3059 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
3060 pprQuotedList tvs <+> in_msg,
3061 nest 2 (pprDictsInFull dicts)]
3062 in_msg = text "in the constraint" <> plural dicts <> colon
3063 report [] = panic "addTopAmbigErrs"
3066 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
3067 -- There's an error with these Insts; if they have free type variables
3068 -- it's probably caused by the monomorphism restriction.
3069 -- Try to identify the offending variable
3070 -- ASSUMPTION: the Insts are fully zonked
3071 mkMonomorphismMsg tidy_env inst_tvs
3072 = do { dflags <- getDOpts
3073 ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
3074 ; return (tidy_env, mk_msg dflags docs) }
3076 mk_msg _ _ | any isRuntimeUnk inst_tvs
3077 = vcat [ptext (sLit "Cannot resolve unknown runtime types:") <+>
3078 (pprWithCommas ppr inst_tvs),
3079 ptext (sLit "Use :print or :force to determine these types")]
3080 mk_msg _ [] = ptext (sLit "Probable fix: add a type signature that fixes these type variable(s)")
3081 -- This happens in things like
3082 -- f x = show (read "foo")
3083 -- where monomorphism doesn't play any role
3085 = vcat [ptext (sLit "Possible cause: the monomorphism restriction applied to the following:"),
3087 monomorphism_fix dflags]
3089 monomorphism_fix :: DynFlags -> SDoc
3090 monomorphism_fix dflags
3091 = ptext (sLit "Probable fix:") <+> vcat
3092 [ptext (sLit "give these definition(s) an explicit type signature"),
3093 if dopt Opt_MonomorphismRestriction dflags
3094 then ptext (sLit "or use -XNoMonomorphismRestriction")
3095 else empty] -- Only suggest adding "-XNoMonomorphismRestriction"
3096 -- if it is not already set!
3098 warnDefault :: [(Inst, Class, Var)] -> Type -> TcM ()
3099 warnDefault ups default_ty = do
3100 warn_flag <- doptM Opt_WarnTypeDefaults
3101 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
3103 dicts = [d | (d,_,_) <- ups]
3106 (_, tidy_dicts) = tidyInsts dicts
3107 warn_msg = vcat [ptext (sLit "Defaulting the following constraint(s) to type") <+>
3108 quotes (ppr default_ty),
3109 pprDictsInFull tidy_dicts]
3111 reduceDepthErr :: Int -> [Inst] -> SDoc
3112 reduceDepthErr n stack
3113 = vcat [ptext (sLit "Context reduction stack overflow; size =") <+> int n,
3114 ptext (sLit "Use -fcontext-stack=N to increase stack size to N"),
3115 nest 4 (pprStack stack)]
3117 pprStack :: [Inst] -> SDoc
3118 pprStack stack = vcat (map pprInstInFull stack)