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.
976 -- The binding looks like
977 -- (ir1, .., irn) = f qtvs givens
978 -- where f is (evidence for) the new implication constraint
979 -- f :: forall qtvs. givens => (ir1, .., irn)
980 -- qtvs includes coercion variables.
982 -- This binding must line up the 'rhs' in reduceImplication
983 makeImplicationBind loc all_tvs
984 givens -- Guaranteed all Dicts or EqInsts
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
993 -- extract equality binders
994 eq_cotvs = map eqInstType eq_givens
996 -- make the implication constraint instance
997 name = mkInternalName uniq (mkVarOcc "ic") span
998 implic_inst = ImplicInst { tci_name = name,
999 tci_tyvars = all_tvs,
1000 tci_given = (eq_givens ++ dict_givens),
1001 -- same order as binders
1002 tci_wanted = irreds,
1005 -- create binders for the irreducible dictionaries
1006 dict_irreds = filter (not . isEqInst) irreds
1007 dict_irred_ids = map instToId dict_irreds
1008 lpat = mkBigLHsPatTup (map (L span . VarPat) dict_irred_ids)
1010 -- create the binding
1011 rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
1012 co = mkWpApps (map instToId dict_givens)
1013 <.> mkWpTyApps eq_cotvs
1014 <.> mkWpTyApps (mkTyVarTys all_tvs)
1015 bind | [dict_irred_id] <- dict_irred_ids
1016 = VarBind dict_irred_id rhs
1018 = PatBind { pat_lhs = lpat
1019 , pat_rhs = unguardedGRHSs rhs
1020 , pat_rhs_ty = hsLPatType lpat
1021 , bind_fvs = placeHolderNames
1024 ; traceTc $ text "makeImplicationBind" <+> ppr implic_inst
1025 ; return ([implic_inst], unitBag (L span bind))
1028 -----------------------------------------------------------
1029 tryHardCheckLoop :: SDoc
1031 -> TcM ([Inst], TcDictBinds)
1033 tryHardCheckLoop doc wanteds
1034 = do { (irreds,binds) <- checkLoop (mkInferRedEnv doc try_me) wanteds
1035 ; return (irreds,binds)
1039 -- Here's the try-hard bit
1041 -----------------------------------------------------------
1042 gentleCheckLoop :: InstLoc
1045 -> TcM ([Inst], TcDictBinds)
1047 gentleCheckLoop inst_loc givens wanteds
1048 = do { (irreds,binds) <- checkLoop env wanteds
1049 ; return (irreds,binds)
1052 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
1054 try_me inst | isMethodOrLit inst = ReduceMe
1056 -- When checking against a given signature
1057 -- we MUST be very gentle: Note [Check gently]
1059 gentleInferLoop :: SDoc -> [Inst]
1060 -> TcM ([Inst], TcDictBinds)
1061 gentleInferLoop doc wanteds
1062 = do { (irreds, binds) <- checkLoop env wanteds
1063 ; return (irreds, binds) }
1065 env = mkInferRedEnv doc try_me
1066 try_me inst | isMethodOrLit inst = ReduceMe
1071 ~~~~~~~~~~~~~~~~~~~~
1072 We have to very careful about not simplifying too vigorously
1077 f :: Show b => T b -> b
1078 f (MkT x) = show [x]
1080 Inside the pattern match, which binds (a:*, x:a), we know that
1082 Hence we have a dictionary for Show [a] available; and indeed we
1083 need it. We are going to build an implication contraint
1084 forall a. (b~[a]) => Show [a]
1085 Later, we will solve this constraint using the knowledge (Show b)
1087 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
1088 thing becomes insoluble. So we simplify gently (get rid of literals
1089 and methods only, plus common up equal things), deferring the real
1090 work until top level, when we solve the implication constraint
1091 with tryHardCheckLooop.
1095 -----------------------------------------------------------
1098 -> TcM ([Inst], TcDictBinds)
1099 -- Precondition: givens are completely rigid
1100 -- Postcondition: returned Insts are zonked
1102 checkLoop env wanteds
1104 where go env wanteds
1105 = do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
1106 ; env' <- zonkRedEnv env
1107 ; wanteds' <- zonkInsts wanteds
1109 ; (improved, binds, irreds) <- reduceContext env' wanteds'
1111 ; if null irreds || not improved then
1112 return (irreds, binds)
1115 -- If improvement did some unification, we go round again.
1116 -- We start again with irreds, not wanteds
1117 -- Using an instance decl might have introduced a fresh type
1118 -- variable which might have been unified, so we'd get an
1119 -- infinite loop if we started again with wanteds!
1121 { (irreds1, binds1) <- go env' irreds
1122 ; return (irreds1, binds `unionBags` binds1) } }
1125 Note [Zonking RedEnv]
1126 ~~~~~~~~~~~~~~~~~~~~~
1127 It might appear as if the givens in RedEnv are always rigid, but that is not
1128 necessarily the case for programs involving higher-rank types that have class
1129 contexts constraining the higher-rank variables. An example from tc237 in the
1132 class Modular s a | s -> a
1134 wim :: forall a w. Integral a
1135 => a -> (forall s. Modular s a => M s w) -> w
1136 wim i k = error "urk"
1138 test5 :: (Modular s a, Integral a) => M s a
1141 test4 = wim 4 test4'
1143 Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
1144 quantified further outside. When type checking test4, we have to check
1145 whether the signature of test5 is an instance of
1147 (forall s. Modular s a => M s w)
1149 Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
1152 Given the FD of Modular in this example, class improvement will instantiate
1153 t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
1154 Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
1155 the givens, we will get into a loop as improveOne uses the unification engine
1156 Unify.tcUnifyTys, which doesn't know about mutable type variables.
1161 class If b t e r | b t e -> r
1164 class Lte a b c | a b -> c where lte :: a -> b -> c
1166 instance (Lte a b l,If l b a c) => Max a b c
1168 Wanted: Max Z (S x) y
1170 Then we'll reduce using the Max instance to:
1171 (Lte Z (S x) l, If l (S x) Z y)
1172 and improve by binding l->T, after which we can do some reduction
1173 on both the Lte and If constraints. What we *can't* do is start again
1174 with (Max Z (S x) y)!
1178 %************************************************************************
1180 tcSimplifySuperClasses
1182 %************************************************************************
1184 Note [SUPERCLASS-LOOP 1]
1185 ~~~~~~~~~~~~~~~~~~~~~~~~
1186 We have to be very, very careful when generating superclasses, lest we
1187 accidentally build a loop. Here's an example:
1191 class S a => C a where { opc :: a -> a }
1192 class S b => D b where { opd :: b -> b }
1194 instance C Int where
1197 instance D Int where
1200 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1201 Simplifying, we may well get:
1202 $dfCInt = :C ds1 (opd dd)
1205 Notice that we spot that we can extract ds1 from dd.
1207 Alas! Alack! We can do the same for (instance D Int):
1209 $dfDInt = :D ds2 (opc dc)
1213 And now we've defined the superclass in terms of itself.
1214 Two more nasty cases are in
1219 - Satisfy the superclass context *all by itself*
1220 (tcSimplifySuperClasses)
1221 - And do so completely; i.e. no left-over constraints
1222 to mix with the constraints arising from method declarations
1225 Note [Recursive instances and superclases]
1226 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1227 Consider this code, which arises in the context of "Scrap Your
1228 Boilerplate with Class".
1232 instance Sat (ctx Char) => Data ctx Char
1233 instance (Sat (ctx [a]), Data ctx a) => Data ctx [a]
1235 class Data Maybe a => Foo a
1237 instance Foo t => Sat (Maybe t)
1239 instance Data Maybe a => Foo a
1240 instance Foo a => Foo [a]
1243 In the instance for Foo [a], when generating evidence for the superclasses
1244 (ie in tcSimplifySuperClasses) we need a superclass (Data Maybe [a]).
1245 Using the instance for Data, we therefore need
1246 (Sat (Maybe [a], Data Maybe a)
1247 But we are given (Foo a), and hence its superclass (Data Maybe a).
1248 So that leaves (Sat (Maybe [a])). Using the instance for Sat means
1249 we need (Foo [a]). And that is the very dictionary we are bulding
1250 an instance for! So we must put that in the "givens". So in this
1252 Given: Foo a, Foo [a]
1253 Watend: Data Maybe [a]
1255 BUT we must *not not not* put the *superclasses* of (Foo [a]) in
1256 the givens, which is what 'addGiven' would normally do. Why? Because
1257 (Data Maybe [a]) is the superclass, so we'd "satisfy" the wanted
1258 by selecting a superclass from Foo [a], which simply makes a loop.
1260 On the other hand we *must* put the superclasses of (Foo a) in
1261 the givens, as you can see from the derivation described above.
1263 Conclusion: in the very special case of tcSimplifySuperClasses
1264 we have one 'given' (namely the "this" dictionary) whose superclasses
1265 must not be added to 'givens' by addGiven. That is the *whole* reason
1266 for the red_given_scs field in RedEnv, and the function argument to
1270 tcSimplifySuperClasses
1272 -> Inst -- The dict whose superclasses
1273 -- are being figured out
1277 tcSimplifySuperClasses loc this givens sc_wanteds
1278 = do { traceTc (text "tcSimplifySuperClasses")
1279 ; (irreds,binds1) <- checkLoop env sc_wanteds
1280 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1281 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1284 env = RedEnv { red_doc = pprInstLoc loc,
1285 red_try_me = try_me,
1286 red_givens = this:givens,
1287 red_given_scs = add_scs,
1289 red_improve = False } -- No unification vars
1290 add_scs g | g==this = NoSCs
1291 | otherwise = AddSCs
1293 try_me _ = ReduceMe -- Try hard, so we completely solve the superclass
1294 -- constraints right here. See Note [SUPERCLASS-LOOP 1]
1298 %************************************************************************
1300 \subsection{tcSimplifyRestricted}
1302 %************************************************************************
1304 tcSimplifyRestricted infers which type variables to quantify for a
1305 group of restricted bindings. This isn't trivial.
1308 We want to quantify over a to get id :: forall a. a->a
1311 We do not want to quantify over a, because there's an Eq a
1312 constraint, so we get eq :: a->a->Bool (notice no forall)
1315 RHS has type 'tau', whose free tyvars are tau_tvs
1316 RHS has constraints 'wanteds'
1319 Quantify over (tau_tvs \ ftvs(wanteds))
1320 This is bad. The constraints may contain (Monad (ST s))
1321 where we have instance Monad (ST s) where...
1322 so there's no need to be monomorphic in s!
1324 Also the constraint might be a method constraint,
1325 whose type mentions a perfectly innocent tyvar:
1326 op :: Num a => a -> b -> a
1327 Here, b is unconstrained. A good example would be
1329 We want to infer the polymorphic type
1330 foo :: forall b. b -> b
1333 Plan B (cunning, used for a long time up to and including GHC 6.2)
1334 Step 1: Simplify the constraints as much as possible (to deal
1335 with Plan A's problem). Then set
1336 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1338 Step 2: Now simplify again, treating the constraint as 'free' if
1339 it does not mention qtvs, and trying to reduce it otherwise.
1340 The reasons for this is to maximise sharing.
1342 This fails for a very subtle reason. Suppose that in the Step 2
1343 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1344 In the Step 1 this constraint might have been simplified, perhaps to
1345 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1346 This won't happen in Step 2... but that in turn might prevent some other
1347 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1348 and that in turn breaks the invariant that no constraints are quantified over.
1350 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1355 Step 1: Simplify the constraints as much as possible (to deal
1356 with Plan A's problem). Then set
1357 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1358 Return the bindings from Step 1.
1361 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1364 instance (HasBinary ty IO) => HasCodedValue ty
1366 foo :: HasCodedValue a => String -> IO a
1368 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1369 doDecodeIO codedValue view
1370 = let { act = foo "foo" } in act
1372 You might think this should work becuase the call to foo gives rise to a constraint
1373 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1374 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1375 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1377 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1381 Plan D (a variant of plan B)
1382 Step 1: Simplify the constraints as much as possible (to deal
1383 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1384 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1386 Step 2: Now simplify again, treating the constraint as 'free' if
1387 it does not mention qtvs, and trying to reduce it otherwise.
1389 The point here is that it's generally OK to have too few qtvs; that is,
1390 to make the thing more monomorphic than it could be. We don't want to
1391 do that in the common cases, but in wierd cases it's ok: the programmer
1392 can always add a signature.
1394 Too few qtvs => too many wanteds, which is what happens if you do less
1399 tcSimplifyRestricted -- Used for restricted binding groups
1400 -- i.e. ones subject to the monomorphism restriction
1403 -> [Name] -- Things bound in this group
1404 -> TcTyVarSet -- Free in the type of the RHSs
1405 -> [Inst] -- Free in the RHSs
1406 -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
1407 TcDictBinds) -- Bindings
1408 -- tcSimpifyRestricted returns no constraints to
1409 -- quantify over; by definition there are none.
1410 -- They are all thrown back in the LIE
1412 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1413 -- Zonk everything in sight
1414 = do { traceTc (text "tcSimplifyRestricted")
1415 ; wanteds' <- zonkInsts wanteds
1417 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1418 -- dicts; the idea is to get rid of as many type
1419 -- variables as possible, and we don't want to stop
1420 -- at (say) Monad (ST s), because that reduces
1421 -- immediately, with no constraint on s.
1423 -- BUT do no improvement! See Plan D above
1424 -- HOWEVER, some unification may take place, if we instantiate
1425 -- a method Inst with an equality constraint
1426 ; let env = mkNoImproveRedEnv doc (\_ -> ReduceMe)
1427 ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds'
1429 -- Next, figure out the tyvars we will quantify over
1430 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1431 ; gbl_tvs' <- tcGetGlobalTyVars
1432 ; constrained_dicts' <- zonkInsts constrained_dicts
1434 ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
1435 -- As in tcSimplifyInfer
1437 -- Do not quantify over constrained type variables:
1438 -- this is the monomorphism restriction
1439 constrained_tvs' = tyVarsOfInsts constrained_dicts'
1440 qtvs = qtvs1 `minusVarSet` constrained_tvs'
1441 pp_bndrs = pprWithCommas (quotes . ppr) bndrs
1444 ; warn_mono <- doptM Opt_WarnMonomorphism
1445 ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
1446 (vcat[ ptext (sLit "the Monomorphism Restriction applies to the binding")
1447 <> plural bndrs <+> ptext (sLit "for") <+> pp_bndrs,
1448 ptext (sLit "Consider giving a type signature for") <+> pp_bndrs])
1450 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1451 pprInsts wanteds, pprInsts constrained_dicts',
1453 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1455 -- The first step may have squashed more methods than
1456 -- necessary, so try again, this time more gently, knowing the exact
1457 -- set of type variables to quantify over.
1459 -- We quantify only over constraints that are captured by qtvs;
1460 -- these will just be a subset of non-dicts. This in contrast
1461 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1462 -- all *non-inheritable* constraints too. This implements choice
1463 -- (B) under "implicit parameter and monomorphism" above.
1465 -- Remember that we may need to do *some* simplification, to
1466 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1467 -- just to float all constraints
1469 -- At top level, we *do* squash methods becuase we want to
1470 -- expose implicit parameters to the test that follows
1471 ; let is_nested_group = isNotTopLevel top_lvl
1472 try_me inst | isFreeWrtTyVars qtvs inst,
1473 (is_nested_group || isDict inst) = Stop
1474 | otherwise = ReduceMe
1475 env = mkNoImproveRedEnv doc try_me
1476 ; (_imp, binds, irreds) <- reduceContext env wanteds'
1478 -- See "Notes on implicit parameters, Question 4: top level"
1479 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1480 if is_nested_group then
1482 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1483 ; addTopIPErrs bndrs bad_ips
1484 ; extendLIEs non_ips }
1486 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1487 ; return (qtvs', binds) }
1491 %************************************************************************
1495 %************************************************************************
1497 On the LHS of transformation rules we only simplify methods and constants,
1498 getting dictionaries. We want to keep all of them unsimplified, to serve
1499 as the available stuff for the RHS of the rule.
1501 Example. Consider the following left-hand side of a rule
1503 f (x == y) (y > z) = ...
1505 If we typecheck this expression we get constraints
1507 d1 :: Ord a, d2 :: Eq a
1509 We do NOT want to "simplify" to the LHS
1511 forall x::a, y::a, z::a, d1::Ord a.
1512 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1516 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1517 f ((==) d2 x y) ((>) d1 y z) = ...
1519 Here is another example:
1521 fromIntegral :: (Integral a, Num b) => a -> b
1522 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1524 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1525 we *dont* want to get
1527 forall dIntegralInt.
1528 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1530 because the scsel will mess up RULE matching. Instead we want
1532 forall dIntegralInt, dNumInt.
1533 fromIntegral Int Int dIntegralInt dNumInt = id Int
1537 g (x == y) (y == z) = ..
1539 where the two dictionaries are *identical*, we do NOT WANT
1541 forall x::a, y::a, z::a, d1::Eq a
1542 f ((==) d1 x y) ((>) d1 y z) = ...
1544 because that will only match if the dict args are (visibly) equal.
1545 Instead we want to quantify over the dictionaries separately.
1547 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1548 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1549 from scratch, rather than further parameterise simpleReduceLoop etc.
1550 Simpler, maybe, but alas not simple (see Trac #2494)
1552 * Type errors may give rise to an (unsatisfiable) equality constraint
1554 * Applications of a higher-rank function on the LHS may give
1555 rise to an implication constraint, esp if there are unsatisfiable
1556 equality constraints inside.
1559 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1560 tcSimplifyRuleLhs wanteds
1561 = do { wanteds' <- zonkInsts wanteds
1562 ; (irreds, binds) <- go [] emptyBag wanteds'
1563 ; let (dicts, bad_irreds) = partition isDict irreds
1564 ; traceTc (text "tcSimplifyrulelhs" <+> pprInsts bad_irreds)
1565 ; addNoInstanceErrs (nub bad_irreds)
1566 -- The nub removes duplicates, which has
1567 -- not happened otherwise (see notes above)
1568 ; return (dicts, binds) }
1570 go :: [Inst] -> TcDictBinds -> [Inst] -> TcM ([Inst], TcDictBinds)
1572 = return (irreds, binds)
1573 go irreds binds (w:ws)
1575 = go (w:irreds) binds ws
1576 | isImplicInst w -- Have a go at reducing the implication
1577 = do { (binds1, irreds1) <- reduceImplication red_env w
1578 ; let (bad_irreds, ok_irreds) = partition isImplicInst irreds1
1579 ; go (bad_irreds ++ irreds)
1580 (binds `unionBags` binds1)
1583 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1584 -- to fromInteger; this looks fragile to me
1585 ; lookup_result <- lookupSimpleInst w'
1586 ; case lookup_result of
1587 NoInstance -> go (w:irreds) binds ws
1588 GenInst ws' rhs -> go irreds binds' (ws' ++ ws)
1590 binds' = addInstToDictBind binds w rhs
1593 -- Sigh: we need to reduce inside implications
1594 red_env = mkInferRedEnv doc try_me
1595 doc = ptext (sLit "Implication constraint in RULE lhs")
1596 try_me inst | isMethodOrLit inst = ReduceMe
1597 | otherwise = Stop -- Be gentle
1600 tcSimplifyBracket is used when simplifying the constraints arising from
1601 a Template Haskell bracket [| ... |]. We want to check that there aren't
1602 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1603 Show instance), but we aren't otherwise interested in the results.
1604 Nor do we care about ambiguous dictionaries etc. We will type check
1605 this bracket again at its usage site.
1608 tcSimplifyBracket :: [Inst] -> TcM ()
1609 tcSimplifyBracket wanteds
1610 = do { tryHardCheckLoop doc wanteds
1613 doc = text "tcSimplifyBracket"
1617 %************************************************************************
1619 \subsection{Filtering at a dynamic binding}
1621 %************************************************************************
1626 we must discharge all the ?x constraints from B. We also do an improvement
1627 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1629 Actually, the constraints from B might improve the types in ?x. For example
1631 f :: (?x::Int) => Char -> Char
1634 then the constraint (?x::Int) arising from the call to f will
1635 force the binding for ?x to be of type Int.
1638 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1641 -- We need a loop so that we do improvement, and then
1642 -- (next time round) generate a binding to connect the two
1644 -- Here the two ?x's have different types, and improvement
1645 -- makes them the same.
1647 tcSimplifyIPs given_ips wanteds
1648 = do { wanteds' <- zonkInsts wanteds
1649 ; given_ips' <- zonkInsts given_ips
1650 -- Unusually for checking, we *must* zonk the given_ips
1652 ; let env = mkRedEnv doc try_me given_ips'
1653 ; (improved, binds, irreds) <- reduceContext env wanteds'
1655 ; if null irreds || not improved then
1656 ASSERT( all is_free irreds )
1657 do { extendLIEs irreds
1660 -- If improvement did some unification, we go round again.
1661 -- We start again with irreds, not wanteds
1662 -- Using an instance decl might have introduced a fresh type
1663 -- variable which might have been unified, so we'd get an
1664 -- infinite loop if we started again with wanteds!
1666 { binds1 <- tcSimplifyIPs given_ips' irreds
1667 ; return $ binds `unionBags` binds1
1670 doc = text "tcSimplifyIPs" <+> ppr given_ips
1671 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1672 is_free inst = isFreeWrtIPs ip_set inst
1674 -- Simplify any methods that mention the implicit parameter
1675 try_me inst | is_free inst = Stop
1676 | otherwise = ReduceMe
1680 %************************************************************************
1682 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1684 %************************************************************************
1686 When doing a binding group, we may have @Insts@ of local functions.
1687 For example, we might have...
1689 let f x = x + 1 -- orig local function (overloaded)
1690 f.1 = f Int -- two instances of f
1695 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1696 where @f@ is in scope; those @Insts@ must certainly not be passed
1697 upwards towards the top-level. If the @Insts@ were binding-ified up
1698 there, they would have unresolvable references to @f@.
1700 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1701 For each method @Inst@ in the @init_lie@ that mentions one of the
1702 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1703 @LIE@), as well as the @HsBinds@ generated.
1706 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1707 -- Simlifies only MethodInsts, and generate only bindings of form
1709 -- We're careful not to even generate bindings of the form
1711 -- You'd think that'd be fine, but it interacts with what is
1712 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1714 bindInstsOfLocalFuns wanteds local_ids
1715 | null overloaded_ids = do
1718 return emptyLHsBinds
1721 = do { (irreds, binds) <- gentleInferLoop doc for_me
1722 ; extendLIEs not_for_me
1726 doc = text "bindInsts" <+> ppr local_ids
1727 overloaded_ids = filter is_overloaded local_ids
1728 is_overloaded id = isOverloadedTy (idType id)
1729 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1731 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1732 -- so it's worth building a set, so that
1733 -- lookup (in isMethodFor) is faster
1737 %************************************************************************
1739 \subsection{Data types for the reduction mechanism}
1741 %************************************************************************
1743 The main control over context reduction is here
1747 = RedEnv { red_doc :: SDoc -- The context
1748 , red_try_me :: Inst -> WhatToDo
1749 , red_improve :: Bool -- True <=> do improvement
1750 , red_givens :: [Inst] -- All guaranteed rigid
1751 -- Always dicts & equalities
1752 -- but see Note [Rigidity]
1754 , red_given_scs :: Inst -> WantSCs -- See Note [Recursive instances and superclases]
1756 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1757 -- See Note [RedStack]
1761 -- The red_givens are rigid so far as cmpInst is concerned.
1762 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1763 -- let ?x = e in ...
1764 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1765 -- But that doesn't affect the comparison, which is based only on mame.
1768 -- The red_stack pair (n,insts) pair is just used for error reporting.
1769 -- 'n' is always the depth of the stack.
1770 -- The 'insts' is the stack of Insts being reduced: to produce X
1771 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1774 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1775 mkRedEnv doc try_me givens
1776 = RedEnv { red_doc = doc, red_try_me = try_me,
1777 red_givens = givens,
1778 red_given_scs = const AddSCs,
1780 red_improve = True }
1782 mkInferRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1784 mkInferRedEnv doc try_me
1785 = RedEnv { red_doc = doc, red_try_me = try_me,
1787 red_given_scs = const AddSCs,
1789 red_improve = True }
1791 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1792 -- Do not do improvement; no givens
1793 mkNoImproveRedEnv doc try_me
1794 = RedEnv { red_doc = doc, red_try_me = try_me,
1796 red_given_scs = const AddSCs,
1798 red_improve = True }
1801 = ReduceMe -- Try to reduce this
1802 -- If there's no instance, add the inst to the
1803 -- irreductible ones, but don't produce an error
1804 -- message of any kind.
1805 -- It might be quite legitimate such as (Eq a)!
1807 | Stop -- Return as irreducible unless it can
1808 -- be reduced to a constant in one step
1809 -- Do not add superclasses; see
1811 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1812 -- of a predicate when adding it to the avails
1813 -- The reason for this flag is entirely the super-class loop problem
1814 -- Note [SUPER-CLASS LOOP 1]
1816 zonkRedEnv :: RedEnv -> TcM RedEnv
1818 = do { givens' <- mapM zonkInst (red_givens env)
1819 ; return $ env {red_givens = givens'}
1824 %************************************************************************
1826 \subsection[reduce]{@reduce@}
1828 %************************************************************************
1830 Note [Ancestor Equalities]
1831 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1832 During context reduction, we add to the wanted equalities also those
1833 equalities that (transitively) occur in superclass contexts of wanted
1834 class constraints. Consider the following code
1836 class a ~ Int => C a
1839 If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
1840 substituting Int for a. Hence, we ultimately want (C Int), which we
1841 discharge with the explicit instance.
1844 reduceContext :: RedEnv
1846 -> TcM (ImprovementDone,
1847 TcDictBinds, -- Dictionary bindings
1848 [Inst]) -- Irreducible
1850 reduceContext env wanteds0
1851 = do { traceTc (text "reduceContext" <+> (vcat [
1852 text "----------------------",
1854 text "given" <+> ppr (red_givens env),
1855 text "wanted" <+> ppr wanteds0,
1856 text "----------------------"
1859 -- We want to add as wanted equalities those that (transitively)
1860 -- occur in superclass contexts of wanted class constraints.
1861 -- See Note [Ancestor Equalities]
1862 ; ancestor_eqs <- ancestorEqualities wanteds0
1863 ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
1865 -- Normalise and solve all equality constraints as far as possible
1866 -- and normalise all dictionary constraints wrt to the reduced
1867 -- equalities. The returned wanted constraints include the
1868 -- irreducible wanted equalities.
1869 ; let wanteds = wanteds0 ++ ancestor_eqs
1870 givens = red_givens env
1874 eq_improved) <- tcReduceEqs givens wanteds
1875 ; traceTc $ text "reduceContext: tcReduceEqs result" <+> vcat
1876 [ppr givens', ppr wanteds', ppr normalise_binds]
1878 -- Build the Avail mapping from "given_dicts"
1879 ; (init_state, _) <- getLIE $ do
1880 { init_state <- foldlM (addGiven (red_given_scs env))
1885 -- Solve the *wanted* *dictionary* constraints (not implications)
1886 -- This may expose some further equational constraints in the course
1887 -- of improvement due to functional dependencies if any of the
1888 -- involved unifications gets deferred.
1889 ; let (wanted_implics, wanted_dicts) = partition isImplicInst wanteds'
1890 ; (avails, extra_eqs) <- getLIE (reduceList env wanted_dicts init_state)
1891 -- The getLIE is reqd because reduceList does improvement
1892 -- (via extendAvails) which may in turn do unification
1895 dict_irreds) <- extractResults avails wanted_dicts
1896 ; traceTc $ text "reduceContext: extractResults" <+> vcat
1897 [ppr avails, ppr wanted_dicts, ppr dict_binds]
1899 -- Solve the wanted *implications*. In doing so, we can provide
1900 -- as "given" all the dicts that were originally given,
1901 -- *or* for which we now have bindings,
1902 -- *or* which are now irreds
1903 -- NB: Equality irreds need to be converted, as the recursive
1904 -- invocation of the solver will still treat them as wanteds
1906 ; let implic_env = env { red_givens
1907 = givens ++ bound_dicts ++
1908 map wantedToLocalEqInst dict_irreds }
1909 ; (implic_binds_s, implic_irreds_s)
1910 <- mapAndUnzipM (reduceImplication implic_env) wanted_implics
1911 ; let implic_binds = unionManyBags implic_binds_s
1912 implic_irreds = concat implic_irreds_s
1914 -- Collect all irreducible instances, and determine whether we should
1915 -- go round again. We do so in either of two cases:
1916 -- (1) If dictionary reduction or equality solving led to
1917 -- improvement (i.e., instantiated type variables).
1918 -- (2) If we reduced dictionaries (i.e., got dictionary bindings),
1919 -- they may have exposed further opportunities to normalise
1920 -- family applications. See Note [Dictionary Improvement]
1922 -- NB: We do *not* go around for new extra_eqs. Morally, we should,
1923 -- but we can't without risking non-termination (see #2688). By
1924 -- not going around, we miss some legal programs mixing FDs and
1925 -- TFs, but we never claimed to support such programs in the
1926 -- current implementation anyway.
1928 ; let all_irreds = dict_irreds ++ implic_irreds ++ extra_eqs
1929 avails_improved = availsImproved avails
1930 improvedFlexible = avails_improved || eq_improved
1931 reduced_dicts = not (isEmptyBag dict_binds)
1932 improved = improvedFlexible || reduced_dicts
1934 improvedHint = (if avails_improved then " [AVAILS]" else "") ++
1935 (if eq_improved then " [EQ]" else "")
1937 ; traceTc (text "reduceContext end" <+> (vcat [
1938 text "----------------------",
1940 text "given" <+> ppr givens,
1941 text "wanted" <+> ppr wanteds0,
1943 text "avails" <+> pprAvails avails,
1944 text "improved =" <+> ppr improved <+> text improvedHint,
1945 text "(all) irreds = " <+> ppr all_irreds,
1946 text "dict-binds = " <+> ppr dict_binds,
1947 text "implic-binds = " <+> ppr implic_binds,
1948 text "----------------------"
1952 normalise_binds `unionBags` dict_binds
1953 `unionBags` implic_binds,
1957 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1958 tcImproveOne avails inst
1959 | not (isDict inst) = return False
1961 = do { inst_envs <- tcGetInstEnvs
1962 ; let eqns = improveOne (classInstances inst_envs)
1963 (dictPred inst, pprInstArising inst)
1964 [ (dictPred p, pprInstArising p)
1965 | p <- availsInsts avails, isDict p ]
1966 -- Avails has all the superclasses etc (good)
1967 -- It also has all the intermediates of the deduction (good)
1968 -- It does not have duplicates (good)
1969 -- NB that (?x::t1) and (?x::t2) will be held separately in
1970 -- avails so that improve will see them separate
1971 ; traceTc (text "improveOne" <+> ppr inst)
1974 unifyEqns :: [(Equation, (PredType, SDoc), (PredType, SDoc))]
1975 -> TcM ImprovementDone
1976 unifyEqns [] = return False
1978 = do { traceTc (ptext (sLit "Improve:") <+> vcat (map pprEquationDoc eqns))
1979 ; improved <- mapM unify eqns
1980 ; return $ or improved
1983 unify ((qtvs, pairs), what1, what2)
1984 = addErrCtxtM (mkEqnMsg what1 what2) $
1985 do { let freeTyVars = unionVarSets (map tvs_pr pairs)
1987 ; (_, _, tenv) <- tcInstTyVars (varSetElems qtvs)
1988 ; mapM_ (unif_pr tenv) pairs
1989 ; anyM isFilledMetaTyVar $ varSetElems freeTyVars
1992 unif_pr tenv (ty1, ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1994 tvs_pr (ty1, ty2) = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
1996 pprEquationDoc :: (Equation, (PredType, SDoc), (PredType, SDoc)) -> SDoc
1997 pprEquationDoc (eqn, (p1, _), (p2, _))
1998 = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
2000 mkEqnMsg :: (TcPredType, SDoc) -> (TcPredType, SDoc) -> TidyEnv
2001 -> TcM (TidyEnv, SDoc)
2002 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
2003 = do { pred1' <- zonkTcPredType pred1
2004 ; pred2' <- zonkTcPredType pred2
2005 ; let { pred1'' = tidyPred tidy_env pred1'
2006 ; pred2'' = tidyPred tidy_env pred2' }
2007 ; let msg = vcat [ptext (sLit "When using functional dependencies to combine"),
2008 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
2009 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
2010 ; return (tidy_env, msg) }
2013 Note [Dictionary Improvement]
2014 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2015 In reduceContext, we first reduce equalities and then class constraints.
2016 However, the letter may expose further opportunities for the former. Hence,
2017 we need to go around again if dictionary reduction produced any dictionary
2018 bindings. The following example demonstrated the point:
2020 data EX _x _y (p :: * -> *)
2025 class Base (Def p) => Prop p where
2029 instance Prop () where
2032 instance (Base (Def (p ANY))) => Base (EX _x _y p)
2033 instance (Prop (p ANY)) => Prop (EX _x _y p) where
2034 type Def (EX _x _y p) = EX _x _y p
2037 instance Prop (FOO x) where
2038 type Def (FOO x) = ()
2041 instance Prop BAR where
2042 type Def BAR = EX () () FOO
2044 During checking the last instance declaration, we need to check the superclass
2045 cosntraint Base (Def BAR), which family normalisation reduced to
2046 Base (EX () () FOO). Chasing the instance for Base (EX _x _y p), gives us
2047 Base (Def (FOO ANY)), which again requires family normalisation of Def to
2048 Base () before we can finish.
2051 The main context-reduction function is @reduce@. Here's its game plan.
2054 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
2055 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
2056 = do { traceTc (text "reduceList " <+> (ppr wanteds $$ ppr state))
2058 ; when (debugIsOn && (n > 8)) $ do
2059 debugDumpTcRn (hang (ptext (sLit "Interesting! Context reduction stack depth") <+> int n)
2060 2 (ifPprDebug (nest 2 (pprStack stk))))
2061 ; if n >= ctxtStkDepth dopts then
2062 failWithTc (reduceDepthErr n stk)
2066 go [] state = return state
2067 go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
2070 -- Base case: we're done!
2071 reduce :: RedEnv -> Inst -> Avails -> TcM Avails
2072 reduce env wanted avails
2074 -- We don't reduce equalities here (and they must not end up as irreds
2079 -- It's the same as an existing inst, or a superclass thereof
2080 | Just _ <- findAvail avails wanted
2081 = do { traceTc (text "reduce: found " <+> ppr wanted)
2086 = do { traceTc (text "reduce" <+> ppr wanted $$ ppr avails)
2087 ; case red_try_me env wanted of {
2088 Stop -> try_simple (addIrred NoSCs);
2089 -- See Note [No superclasses for Stop]
2091 ReduceMe -> do -- It should be reduced
2092 { (avails, lookup_result) <- reduceInst env avails wanted
2093 ; case lookup_result of
2094 NoInstance -> addIrred AddSCs avails wanted
2095 -- Add it and its superclasses
2097 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
2099 GenInst wanteds' rhs
2100 -> do { avails1 <- addIrred NoSCs avails wanted
2101 ; avails2 <- reduceList env wanteds' avails1
2102 ; addWanted AddSCs avails2 wanted rhs wanteds' } }
2103 -- Temporarily do addIrred *before* the reduceList,
2104 -- which has the effect of adding the thing we are trying
2105 -- to prove to the database before trying to prove the things it
2106 -- needs. See note [RECURSIVE DICTIONARIES]
2107 -- NB: we must not do an addWanted before, because that adds the
2108 -- superclasses too, and that can lead to a spurious loop; see
2109 -- the examples in [SUPERCLASS-LOOP]
2110 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
2113 -- First, see if the inst can be reduced to a constant in one step
2114 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
2115 -- Don't bother for implication constraints, which take real work
2116 try_simple do_this_otherwise
2117 = do { res <- lookupSimpleInst wanted
2119 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
2120 _ -> do_this_otherwise avails wanted }
2124 Note [RECURSIVE DICTIONARIES]
2125 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2127 data D r = ZeroD | SuccD (r (D r));
2129 instance (Eq (r (D r))) => Eq (D r) where
2130 ZeroD == ZeroD = True
2131 (SuccD a) == (SuccD b) = a == b
2134 equalDC :: D [] -> D [] -> Bool;
2137 We need to prove (Eq (D [])). Here's how we go:
2141 by instance decl, holds if
2145 by instance decl of Eq, holds if
2147 where d2 = dfEqList d3
2150 But now we can "tie the knot" to give
2156 and it'll even run! The trick is to put the thing we are trying to prove
2157 (in this case Eq (D []) into the database before trying to prove its
2158 contributing clauses.
2160 Note [SUPERCLASS-LOOP 2]
2161 ~~~~~~~~~~~~~~~~~~~~~~~~
2162 We need to be careful when adding "the constaint we are trying to prove".
2163 Suppose we are *given* d1:Ord a, and want to deduce (d2:C [a]) where
2165 class Ord a => C a where
2166 instance Ord [a] => C [a] where ...
2168 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
2169 superclasses of C [a] to avails. But we must not overwrite the binding
2170 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
2173 Here's another variant, immortalised in tcrun020
2174 class Monad m => C1 m
2175 class C1 m => C2 m x
2176 instance C2 Maybe Bool
2177 For the instance decl we need to build (C1 Maybe), and it's no good if
2178 we run around and add (C2 Maybe Bool) and its superclasses to the avails
2179 before we search for C1 Maybe.
2181 Here's another example
2182 class Eq b => Foo a b
2183 instance Eq a => Foo [a] a
2187 we'll first deduce that it holds (via the instance decl). We must not
2188 then overwrite the Eq t constraint with a superclass selection!
2190 At first I had a gross hack, whereby I simply did not add superclass constraints
2191 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
2192 becuase it lost legitimate superclass sharing, and it still didn't do the job:
2193 I found a very obscure program (now tcrun021) in which improvement meant the
2194 simplifier got two bites a the cherry... so something seemed to be an Stop
2195 first time, but reducible next time.
2197 Now we implement the Right Solution, which is to check for loops directly
2198 when adding superclasses. It's a bit like the occurs check in unification.
2202 %************************************************************************
2204 Reducing a single constraint
2206 %************************************************************************
2209 ---------------------------------------------
2210 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
2211 reduceInst _ avails other_inst
2212 = do { result <- lookupSimpleInst other_inst
2213 ; return (avails, result) }
2216 Note [Equational Constraints in Implication Constraints]
2217 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2219 An implication constraint is of the form
2221 where Given and Wanted may contain both equational and dictionary
2222 constraints. The delay and reduction of these two kinds of constraints
2225 -) In the generated code, wanted Dictionary constraints are wrapped up in an
2226 implication constraint that is created at the code site where the wanted
2227 dictionaries can be reduced via a let-binding. This let-bound implication
2228 constraint is deconstructed at the use-site of the wanted dictionaries.
2230 -) While the reduction of equational constraints is also delayed, the delay
2231 is not manifest in the generated code. The required evidence is generated
2232 in the code directly at the use-site. There is no let-binding and deconstruction
2233 necessary. The main disadvantage is that we cannot exploit sharing as the
2234 same evidence may be generated at multiple use-sites. However, this disadvantage
2235 is limited because it only concerns coercions which are erased.
2237 The different treatment is motivated by the different in representation. Dictionary
2238 constraints require manifest runtime dictionaries, while equations require coercions
2242 ---------------------------------------------
2243 reduceImplication :: RedEnv
2245 -> TcM (TcDictBinds, [Inst])
2248 Suppose we are simplifying the constraint
2249 forall bs. extras => wanted
2250 in the context of an overall simplification problem with givens 'givens'.
2253 * The 'givens' need not mention any of the quantified type variables
2254 e.g. forall {}. Eq a => Eq [a]
2255 forall {}. C Int => D (Tree Int)
2257 This happens when you have something like
2259 T1 :: Eq a => a -> T a
2262 f x = ...(case x of { T1 v -> v==v })...
2265 -- ToDo: should we instantiate tvs? I think it's not necessary
2267 -- Note on coercion variables:
2269 -- The extra given coercion variables are bound at two different
2272 -- -) in the creation context of the implication constraint
2273 -- the solved equational constraints use these binders
2275 -- -) at the solving site of the implication constraint
2276 -- the solved dictionaries use these binders;
2277 -- these binders are generated by reduceImplication
2279 -- Note [Binders for equalities]
2280 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2281 -- To reuse the binders of local/given equalities in the binders of
2282 -- implication constraints, it is crucial that these given equalities
2283 -- always have the form
2285 -- where cotv is a simple coercion type variable (and not a more
2286 -- complex coercion term). We require that the extra_givens always
2287 -- have this form and exploit the special form when generating binders.
2288 reduceImplication env
2289 orig_implic@(ImplicInst { tci_name = name, tci_loc = inst_loc,
2291 tci_given = extra_givens, tci_wanted = wanteds
2293 = do { -- Solve the sub-problem
2294 ; let try_me _ = ReduceMe -- Note [Freeness and implications]
2295 env' = env { red_givens = extra_givens ++ red_givens env
2296 , red_doc = sep [ptext (sLit "reduceImplication for")
2298 nest 2 (parens $ ptext (sLit "within")
2300 , red_try_me = try_me }
2302 ; traceTc (text "reduceImplication" <+> vcat
2303 [ ppr (red_givens env), ppr extra_givens,
2305 ; (irreds, binds) <- checkLoop env' wanteds
2307 ; traceTc (text "reduceImplication result" <+> vcat
2308 [ppr irreds, ppr binds])
2310 ; -- extract superclass binds
2311 -- (sc_binds,_) <- extractResults avails []
2312 -- ; traceTc (text "reduceImplication sc_binds" <+> vcat
2313 -- [ppr sc_binds, ppr avails])
2316 -- SLPJ Sept 07: what if improvement happened inside the checkLoop?
2317 -- Then we must iterate the outer loop too!
2319 ; didntSolveWantedEqs <- allM wantedEqInstIsUnsolved wanteds
2320 -- we solve wanted eqs by side effect!
2322 -- Progress is no longer measered by the number of bindings
2323 -- If there are any irreds, but no bindings and no solved
2324 -- equalities, we back off and do nothing
2325 ; let backOff = isEmptyLHsBinds binds && -- no new bindings
2326 (not $ null irreds) && -- but still some irreds
2327 didntSolveWantedEqs -- no instantiated cotv
2329 ; if backOff then -- No progress
2330 return (emptyBag, [orig_implic])
2332 { (simpler_implic_insts, bind)
2333 <- makeImplicationBind inst_loc tvs extra_givens irreds
2334 -- This binding is useless if the recursive simplification
2335 -- made no progress; but currently we don't try to optimise that
2336 -- case. After all, we only try hard to reduce at top level, or
2337 -- when inferring types.
2339 ; let -- extract Id binders for dicts and CoTyVar binders for eqs;
2340 -- see Note [Binders for equalities]
2341 (extra_eq_givens, extra_dict_givens) = partition isEqInst
2343 eq_cotvs = map instToVar extra_eq_givens
2344 dict_ids = map instToId extra_dict_givens
2346 -- Note [Always inline implication constraints]
2347 wrap_inline | null dict_ids = idHsWrapper
2348 | otherwise = WpInline
2351 <.> mkWpTyLams eq_cotvs
2352 <.> mkWpLams dict_ids
2353 <.> WpLet (binds `unionBags` bind)
2354 rhs = mkLHsWrap co payload
2355 loc = instLocSpan inst_loc
2356 -- wanted equalities are solved by updating their
2357 -- cotv; we don't generate bindings for them
2358 dict_bndrs = map (L loc . HsVar . instToId)
2359 . filter (not . isEqInst)
2361 payload = mkBigLHsTup dict_bndrs
2364 ; traceTc (vcat [text "reduceImplication" <+> ppr name,
2365 ppr simpler_implic_insts,
2366 text "->" <+> ppr rhs])
2367 ; return (unitBag (L loc (VarBind (instToId orig_implic) rhs)),
2368 simpler_implic_insts)
2371 reduceImplication _ i = pprPanic "reduceImplication" (ppr i)
2374 Note [Always inline implication constraints]
2375 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2376 Suppose an implication constraint floats out of an INLINE function.
2377 Then although the implication has a single call site, it won't be
2378 inlined. And that is bad because it means that even if there is really
2379 *no* overloading (type signatures specify the exact types) there will
2380 still be dictionary passing in the resulting code. To avert this,
2381 we mark the implication constraints themselves as INLINE, at least when
2382 there is no loss of sharing as a result.
2384 Note [Freeness and implications]
2385 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2386 It's hard to say when an implication constraint can be floated out. Consider
2387 forall {} Eq a => Foo [a]
2388 The (Foo [a]) doesn't mention any of the quantified variables, but it
2389 still might be partially satisfied by the (Eq a).
2391 There is a useful special case when it *is* easy to partition the
2392 constraints, namely when there are no 'givens'. Consider
2393 forall {a}. () => Bar b
2394 There are no 'givens', and so there is no reason to capture (Bar b).
2395 We can let it float out. But if there is even one constraint we
2396 must be much more careful:
2397 forall {a}. C a b => Bar (m b)
2398 because (C a b) might have a superclass (D b), from which we might
2399 deduce (Bar [b]) when m later gets instantiated to []. Ha!
2401 Here is an even more exotic example
2403 Now consider the constraint
2404 forall b. D Int b => C Int
2405 We can satisfy the (C Int) from the superclass of D, so we don't want
2406 to float the (C Int) out, even though it mentions no type variable in
2409 One more example: the constraint
2411 instance (C a, E c) => E (a,c)
2413 constraint: forall b. D Int b => E (Int,c)
2415 You might think that the (D Int b) can't possibly contribute
2416 to solving (E (Int,c)), since the latter mentions 'c'. But
2417 in fact it can, because solving the (E (Int,c)) constraint needs
2420 and the (C Int) can be satisfied from the superclass of (D Int b).
2421 So we must still not float (E (Int,c)) out.
2423 To think about: special cases for unary type classes?
2425 Note [Pruning the givens in an implication constraint]
2426 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2427 Suppose we are about to form the implication constraint
2428 forall tvs. Eq a => Ord b
2429 The (Eq a) cannot contribute to the (Ord b), because it has no access to
2430 the type variable 'b'. So we could filter out the (Eq a) from the givens.
2431 But BE CAREFUL of the examples above in [Freeness and implications].
2433 Doing so would be a bit tidier, but all the implication constraints get
2434 simplified away by the optimiser, so it's no great win. So I don't take
2435 advantage of that at the moment.
2437 If you do, BE CAREFUL of wobbly type variables.
2440 %************************************************************************
2442 Avails and AvailHow: the pool of evidence
2444 %************************************************************************
2448 data Avails = Avails !ImprovementDone !AvailEnv
2450 type ImprovementDone = Bool -- True <=> some unification has happened
2451 -- so some Irreds might now be reducible
2452 -- keys that are now
2454 type AvailEnv = FiniteMap Inst AvailHow
2456 = IsIrred -- Used for irreducible dictionaries,
2457 -- which are going to be lambda bound
2459 | Given Inst -- Used for dictionaries for which we have a binding
2460 -- e.g. those "given" in a signature
2462 | Rhs -- Used when there is a RHS
2463 (LHsExpr TcId) -- The RHS
2464 [Inst] -- Insts free in the RHS; we need these too
2466 instance Outputable Avails where
2469 pprAvails :: Avails -> SDoc
2470 pprAvails (Avails imp avails)
2471 = vcat [ ptext (sLit "Avails") <> (if imp then ptext (sLit "[improved]") else empty)
2473 vcat [ sep [ppr inst, nest 2 (equals <+> ppr avail)]
2474 | (inst,avail) <- fmToList avails ]]
2476 instance Outputable AvailHow where
2479 -------------------------
2480 pprAvail :: AvailHow -> SDoc
2481 pprAvail IsIrred = text "Irred"
2482 pprAvail (Given x) = text "Given" <+> ppr x
2483 pprAvail (Rhs rhs bs) = sep [text "Rhs" <+> ppr bs,
2486 -------------------------
2487 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
2488 extendAvailEnv env inst avail = addToFM env inst avail
2490 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
2491 findAvailEnv env wanted = lookupFM env wanted
2492 -- NB 1: the Ord instance of Inst compares by the class/type info
2493 -- *not* by unique. So
2494 -- d1::C Int == d2::C Int
2496 emptyAvails :: Avails
2497 emptyAvails = Avails False emptyFM
2499 findAvail :: Avails -> Inst -> Maybe AvailHow
2500 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
2502 elemAvails :: Inst -> Avails -> Bool
2503 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
2505 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
2507 extendAvails avails@(Avails imp env) inst avail
2508 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
2509 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
2511 availsInsts :: Avails -> [Inst]
2512 availsInsts (Avails _ avails) = keysFM avails
2514 availsImproved :: Avails -> ImprovementDone
2515 availsImproved (Avails imp _) = imp
2518 Extracting the bindings from a bunch of Avails.
2519 The bindings do *not* come back sorted in dependency order.
2520 We assume that they'll be wrapped in a big Rec, so that the
2521 dependency analyser can sort them out later
2524 type DoneEnv = FiniteMap Inst [Id]
2525 -- Tracks which things we have evidence for
2527 extractResults :: Avails
2529 -> TcM (TcDictBinds, -- Bindings
2530 [Inst], -- The insts bound by the bindings
2531 [Inst]) -- Irreducible ones
2532 -- Note [Reducing implication constraints]
2534 extractResults (Avails _ avails) wanteds
2535 = go emptyBag [] [] emptyFM wanteds
2537 go :: TcDictBinds -- Bindings for dicts
2538 -> [Inst] -- Bound by the bindings
2540 -> DoneEnv -- Has an entry for each inst in the above three sets
2542 -> TcM (TcDictBinds, [Inst], [Inst])
2543 go binds bound_dicts irreds _ []
2544 = return (binds, bound_dicts, irreds)
2546 go binds bound_dicts irreds done (w:ws)
2548 = go binds bound_dicts (w:irreds) done' ws
2550 | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w
2551 = if w_id `elem` done_ids then
2552 go binds bound_dicts irreds done ws
2554 go (add_bind (nlHsVar done_id)) bound_dicts irreds
2555 (addToFM done w (done_id : w_id : rest_done_ids)) ws
2557 | otherwise -- Not yet done
2558 = case findAvailEnv avails w of
2559 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
2560 go binds bound_dicts irreds done ws
2562 Just IsIrred -> go binds bound_dicts (w:irreds) done' ws
2564 Just (Rhs rhs ws') -> go (add_bind rhs) (w:bound_dicts) irreds done' (ws' ++ ws)
2566 Just (Given g) -> go binds' bound_dicts irreds (addToFM done w [g_id]) ws
2569 binds' | w_id == g_id = binds
2570 | otherwise = add_bind (nlHsVar g_id)
2573 done' = addToFM done w [w_id]
2574 add_bind rhs = addInstToDictBind binds w rhs
2578 Note [No superclasses for Stop]
2579 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2580 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2581 add it to avails, so that any other equal Insts will be commoned up
2582 right here. However, we do *not* add superclasses. If we have
2585 but a is not bound here, then we *don't* want to derive dn from df
2586 here lest we lose sharing.
2589 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2590 addWanted want_scs avails wanted rhs_expr wanteds
2591 = addAvailAndSCs want_scs avails wanted avail
2593 avail = Rhs rhs_expr wanteds
2595 addGiven :: (Inst -> WantSCs) -> Avails -> Inst -> TcM Avails
2596 addGiven want_scs avails given = addAvailAndSCs (want_scs given) avails given (Given given)
2597 -- Conditionally add superclasses for 'givens'
2598 -- See Note [Recursive instances and superclases]
2600 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2601 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2602 -- so the assert isn't true
2606 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2607 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2608 addAvailAndSCs want_scs avails irred IsIrred
2610 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2611 addAvailAndSCs want_scs avails inst avail
2612 | not (isClassDict inst) = extendAvails avails inst avail
2613 | NoSCs <- want_scs = extendAvails avails inst avail
2614 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2615 ; avails' <- extendAvails avails inst avail
2616 ; addSCs is_loop avails' inst }
2618 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2619 -- Note: this compares by *type*, not by Unique
2620 deps = findAllDeps (unitVarSet (instToVar inst)) avail
2621 dep_tys = map idType (varSetElems deps)
2623 findAllDeps :: IdSet -> AvailHow -> IdSet
2624 -- Find all the Insts that this one depends on
2625 -- See Note [SUPERCLASS-LOOP 2]
2626 -- Watch out, though. Since the avails may contain loops
2627 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2628 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2629 findAllDeps so_far _ = so_far
2631 find_all :: IdSet -> Inst -> IdSet
2633 | isEqInst kid = so_far
2634 | kid_id `elemVarSet` so_far = so_far
2635 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2636 | otherwise = so_far'
2638 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2639 kid_id = instToId kid
2641 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2642 -- Add all the superclasses of the Inst to Avails
2643 -- The first param says "don't do this because the original thing
2644 -- depends on this one, so you'd build a loop"
2645 -- Invariant: the Inst is already in Avails.
2647 addSCs is_loop avails dict
2648 = ASSERT( isDict dict )
2649 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2650 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2652 (clas, tys) = getDictClassTys dict
2653 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2654 sc_theta' = filter (not . isEqPred) $
2655 substTheta (zipTopTvSubst tyvars tys) sc_theta
2657 add_sc avails (sc_dict, sc_sel)
2658 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2659 | is_given sc_dict = return avails
2660 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2661 ; addSCs is_loop avails' sc_dict }
2663 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2664 co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
2666 is_given :: Inst -> Bool
2667 is_given sc_dict = case findAvail avails sc_dict of
2668 Just (Given _) -> True -- Given is cheaper than superclass selection
2671 -- From the a set of insts obtain all equalities that (transitively) occur in
2672 -- superclass contexts of class constraints (aka the ancestor equalities).
2674 ancestorEqualities :: [Inst] -> TcM [Inst]
2676 = mapM mkWantedEqInst -- turn only equality predicates..
2677 . filter isEqPred -- ..into wanted equality insts
2679 . addAEsToBag emptyBag -- collect the superclass constraints..
2680 . map dictPred -- ..of all predicates in a bag
2681 . filter isClassDict
2683 addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
2684 addAEsToBag bag [] = bag
2685 addAEsToBag bag (pred:preds)
2686 | pred `elemBag` bag = addAEsToBag bag preds
2687 | isEqPred pred = addAEsToBag bagWithPred preds
2688 | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
2689 | otherwise = addAEsToBag bag preds
2691 bagWithPred = bag `snocBag` pred
2692 predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
2694 (tyvars, sc_theta, _, _) = classBigSig clas
2695 (clas, tys) = getClassPredTys pred
2699 %************************************************************************
2701 \section{tcSimplifyTop: defaulting}
2703 %************************************************************************
2706 @tcSimplifyTop@ is called once per module to simplify all the constant
2707 and ambiguous Insts.
2709 We need to be careful of one case. Suppose we have
2711 instance Num a => Num (Foo a b) where ...
2713 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2714 to (Num x), and default x to Int. But what about y??
2716 It's OK: the final zonking stage should zap y to (), which is fine.
2720 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2721 tcSimplifyTop wanteds
2722 = tc_simplify_top doc False wanteds
2724 doc = text "tcSimplifyTop"
2726 tcSimplifyInteractive wanteds
2727 = tc_simplify_top doc True wanteds
2729 doc = text "tcSimplifyInteractive"
2731 -- The TcLclEnv should be valid here, solely to improve
2732 -- error message generation for the monomorphism restriction
2733 tc_simplify_top :: SDoc -> Bool -> [Inst] -> TcM (Bag (LHsBind TcId))
2734 tc_simplify_top doc interactive wanteds
2735 = do { dflags <- getDOpts
2736 ; wanteds <- zonkInsts wanteds
2737 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2739 ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
2740 ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
2741 -- ; (irreds1, binds1) <- gentleInferLoop doc1 wanteds
2742 ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
2743 ; (irreds2, binds2) <- approximateImplications doc2 (\_ -> True) irreds1
2744 ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
2746 -- Use the defaulting rules to do extra unification
2747 -- NB: irreds2 are already zonked
2748 ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
2750 -- Deal with implicit parameters
2751 ; let (bad_ips, non_ips) = partition isIPDict irreds3
2752 (ambigs, others) = partition isTyVarDict non_ips
2754 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2756 ; addNoInstanceErrs others
2757 ; addTopAmbigErrs ambigs
2759 ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
2761 doc1 = doc <+> ptext (sLit "(first round)")
2762 doc2 = doc <+> ptext (sLit "(approximate)")
2763 doc3 = doc <+> ptext (sLit "(disambiguate)")
2766 If a dictionary constrains a type variable which is
2767 * not mentioned in the environment
2768 * and not mentioned in the type of the expression
2769 then it is ambiguous. No further information will arise to instantiate
2770 the type variable; nor will it be generalised and turned into an extra
2771 parameter to a function.
2773 It is an error for this to occur, except that Haskell provided for
2774 certain rules to be applied in the special case of numeric types.
2776 * at least one of its classes is a numeric class, and
2777 * all of its classes are numeric or standard
2778 then the type variable can be defaulted to the first type in the
2779 default-type list which is an instance of all the offending classes.
2781 So here is the function which does the work. It takes the ambiguous
2782 dictionaries and either resolves them (producing bindings) or
2783 complains. It works by splitting the dictionary list by type
2784 variable, and using @disambigOne@ to do the real business.
2786 @disambigOne@ assumes that its arguments dictionaries constrain all
2787 the same type variable.
2789 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2790 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2791 the most common use of defaulting is code like:
2793 _ccall_ foo `seqPrimIO` bar
2795 Since we're not using the result of @foo@, the result if (presumably)
2799 disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
2800 -- Just does unification to fix the default types
2801 -- The Insts are assumed to be pre-zonked
2802 disambiguate doc interactive dflags insts
2804 = return (insts, emptyBag)
2806 | null defaultable_groups
2807 = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
2808 ; return (insts, emptyBag) }
2811 = do { -- Figure out what default types to use
2812 default_tys <- getDefaultTys extended_defaulting ovl_strings
2814 ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2815 ; mapM_ (disambigGroup default_tys) defaultable_groups
2817 -- disambigGroup does unification, hence try again
2818 ; tryHardCheckLoop doc insts }
2821 extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
2822 ovl_strings = dopt Opt_OverloadedStrings dflags
2824 unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
2825 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2826 (unaries, bad_tvs_s) = partitionWith find_unary insts
2827 bad_tvs = unionVarSets bad_tvs_s
2829 -- Finds unary type-class constraints
2830 find_unary d@(Dict {tci_pred = ClassP cls [ty]})
2831 | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
2832 find_unary inst = Right (tyVarsOfInst inst)
2834 -- Group by type variable
2835 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2836 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2837 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2839 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2840 defaultable_group ds@((_,_,tv):_)
2841 = isTyConableTyVar tv -- Note [Avoiding spurious errors]
2842 && not (tv `elemVarSet` bad_tvs)
2843 && defaultable_classes [c | (_,c,_) <- ds]
2844 defaultable_group [] = panic "defaultable_group"
2846 defaultable_classes clss
2847 | extended_defaulting = any isInteractiveClass clss
2848 | otherwise = all is_std_class clss && (any is_num_class clss)
2850 -- In interactive mode, or with -XExtendedDefaultRules,
2851 -- we default Show a to Show () to avoid graututious errors on "show []"
2852 isInteractiveClass cls
2853 = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2855 is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2856 -- is_num_class adds IsString to the standard numeric classes,
2857 -- when -foverloaded-strings is enabled
2859 is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
2860 -- Similarly is_std_class
2862 -----------------------
2863 disambigGroup :: [Type] -- The default types
2864 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2865 -> TcM () -- Just does unification, to fix the default types
2867 disambigGroup default_tys dicts
2868 = try_default default_tys
2870 (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
2871 classes = [c | (_,c,_) <- dicts]
2873 try_default [] = return ()
2874 try_default (default_ty : default_tys)
2875 = tryTcLIE_ (try_default default_tys) $
2876 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2877 -- This may fail; then the tryTcLIE_ kicks in
2878 -- Failure here is caused by there being no type in the
2879 -- default list which can satisfy all the ambiguous classes.
2880 -- For example, if Real a is reqd, but the only type in the
2881 -- default list is Int.
2883 -- After this we can't fail
2884 ; warnDefault dicts default_ty
2885 ; unifyType default_ty (mkTyVarTy tyvar)
2886 ; return () -- TOMDO: do something with the coercion
2890 -----------------------
2891 getDefaultTys :: Bool -> Bool -> TcM [Type]
2892 getDefaultTys extended_deflts ovl_strings
2893 = do { mb_defaults <- getDeclaredDefaultTys
2894 ; case mb_defaults of {
2895 Just tys -> return tys ; -- User-supplied defaults
2898 -- No use-supplied default
2899 -- Use [Integer, Double], plus modifications
2900 { integer_ty <- tcMetaTy integerTyConName
2901 ; checkWiredInTyCon doubleTyCon
2902 ; string_ty <- tcMetaTy stringTyConName
2903 ; return (opt_deflt extended_deflts unitTy
2904 -- Note [Default unitTy]
2906 [integer_ty,doubleTy]
2908 opt_deflt ovl_strings string_ty) } } }
2910 opt_deflt True ty = [ty]
2911 opt_deflt False _ = []
2914 Note [Default unitTy]
2915 ~~~~~~~~~~~~~~~~~~~~~
2916 In interative mode (or with -XExtendedDefaultRules) we add () as the first type we
2917 try when defaulting. This has very little real impact, except in the following case.
2919 Text.Printf.printf "hello"
2920 This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
2921 want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
2922 default the 'a' to (), rather than to Integer (which is what would otherwise happen;
2923 and then GHCi doesn't attempt to print the (). So in interactive mode, we add
2924 () to the list of defaulting types. See Trac #1200.
2926 Note [Avoiding spurious errors]
2927 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2928 When doing the unification for defaulting, we check for skolem
2929 type variables, and simply don't default them. For example:
2930 f = (*) -- Monomorphic
2931 g :: Num a => a -> a
2933 Here, we get a complaint when checking the type signature for g,
2934 that g isn't polymorphic enough; but then we get another one when
2935 dealing with the (Num a) context arising from f's definition;
2936 we try to unify a with Int (to default it), but find that it's
2937 already been unified with the rigid variable from g's type sig
2940 %************************************************************************
2942 \subsection[simple]{@Simple@ versions}
2944 %************************************************************************
2946 Much simpler versions when there are no bindings to make!
2948 @tcSimplifyThetas@ simplifies class-type constraints formed by
2949 @deriving@ declarations and when specialising instances. We are
2950 only interested in the simplified bunch of class/type constraints.
2952 It simplifies to constraints of the form (C a b c) where
2953 a,b,c are type variables. This is required for the context of
2954 instance declarations.
2957 tcSimplifyDeriv :: InstOrigin
2959 -> ThetaType -- Wanted
2960 -> TcM ThetaType -- Needed
2961 -- Given instance (wanted) => C inst_ty
2962 -- Simplify 'wanted' as much as possible
2964 tcSimplifyDeriv orig tyvars theta
2965 = do { (tvs, _, tenv) <- tcInstTyVars tyvars
2966 -- The main loop may do unification, and that may crash if
2967 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2968 -- ToDo: what if two of them do get unified?
2969 ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
2970 ; (irreds, _) <- tryHardCheckLoop doc wanteds
2972 ; let (tv_dicts, others) = partition ok irreds
2973 ; addNoInstanceErrs others
2974 -- See Note [Exotic derived instance contexts] in TcMType
2976 ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2977 simpl_theta = substTheta rev_env (map dictPred tv_dicts)
2978 -- This reverse-mapping is a pain, but the result
2979 -- should mention the original TyVars not TcTyVars
2981 ; return simpl_theta }
2983 doc = ptext (sLit "deriving classes for a data type")
2985 ok dict | isDict dict = validDerivPred (dictPred dict)
2990 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2991 used with \tr{default} declarations. We are only interested in
2992 whether it worked or not.
2995 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2998 tcSimplifyDefault theta = do
2999 wanteds <- newDictBndrsO DefaultOrigin theta
3000 (irreds, _) <- tryHardCheckLoop doc wanteds
3001 addNoInstanceErrs irreds
3005 traceTc (ptext (sLit "tcSimplifyDefault failing")) >> failM
3007 doc = ptext (sLit "default declaration")
3011 %************************************************************************
3013 \section{Errors and contexts}
3015 %************************************************************************
3017 ToDo: for these error messages, should we note the location as coming
3018 from the insts, or just whatever seems to be around in the monad just
3022 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
3023 -> [Inst] -- The offending Insts
3025 -- Group together insts with the same origin
3026 -- We want to report them together in error messages
3030 groupErrs report_err (inst:insts)
3031 = do { do_one (inst:friends)
3032 ; groupErrs report_err others }
3034 -- (It may seem a bit crude to compare the error messages,
3035 -- but it makes sure that we combine just what the user sees,
3036 -- and it avoids need equality on InstLocs.)
3037 (friends, others) = partition is_friend insts
3038 loc_msg = showSDoc (pprInstLoc (instLoc inst))
3039 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
3040 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
3041 -- Add location and context information derived from the Insts
3043 -- Add the "arising from..." part to a message about bunch of dicts
3044 addInstLoc :: [Inst] -> Message -> Message
3045 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
3047 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
3050 addTopIPErrs bndrs ips
3051 = do { dflags <- getDOpts
3052 ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
3054 (tidy_env, tidy_ips) = tidyInsts ips
3056 = vcat [sep [ptext (sLit "Implicit parameters escape from"),
3057 nest 2 (ptext (sLit "the monomorphic top-level binding")
3058 <> plural bndrs <+> ptext (sLit "of")
3059 <+> pprBinders bndrs <> colon)],
3060 nest 2 (vcat (map ppr_ip ips)),
3061 monomorphism_fix dflags]
3062 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
3064 topIPErrs :: [Inst] -> TcM ()
3066 = groupErrs report tidy_dicts
3068 (tidy_env, tidy_dicts) = tidyInsts dicts
3069 report dicts = addErrTcM (tidy_env, mk_msg dicts)
3070 mk_msg dicts = addInstLoc dicts (ptext (sLit "Unbound implicit parameter") <>
3071 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
3073 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
3075 addNoInstanceErrs insts
3076 = do { let (tidy_env, tidy_insts) = tidyInsts insts
3077 ; reportNoInstances tidy_env Nothing tidy_insts }
3081 -> Maybe (InstLoc, [Inst]) -- Context
3082 -- Nothing => top level
3083 -- Just (d,g) => d describes the construct
3085 -> [Inst] -- What is wanted (can include implications)
3088 reportNoInstances tidy_env mb_what insts
3089 = groupErrs (report_no_instances tidy_env mb_what) insts
3091 report_no_instances :: TidyEnv -> Maybe (InstLoc, [Inst]) -> [Inst] -> TcM ()
3092 report_no_instances tidy_env mb_what insts
3093 = do { inst_envs <- tcGetInstEnvs
3094 ; let (implics, insts1) = partition isImplicInst insts
3095 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
3096 (eqInsts, insts3) = partition isEqInst insts2
3097 ; traceTc (text "reportNoInstances" <+> vcat
3098 [ppr insts, ppr implics, ppr insts1, ppr insts2])
3099 ; mapM_ complain_implic implics
3100 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
3101 ; groupErrs complain_no_inst insts3
3102 ; mapM_ (addErrTcM . mk_eq_err) eqInsts
3105 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
3107 complain_implic inst -- Recurse!
3108 = reportNoInstances tidy_env
3109 (Just (tci_loc inst, tci_given inst))
3112 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
3113 -- Right msg => overlap message
3114 -- Left inst => no instance
3115 check_overlap inst_envs wanted
3116 | not (isClassDict wanted) = Left wanted
3118 = case lookupInstEnv inst_envs clas tys of
3119 ([], _) -> Left wanted -- No match
3120 -- The case of exactly one match and no unifiers means a
3121 -- successful lookup. That can't happen here, because dicts
3122 -- only end up here if they didn't match in Inst.lookupInst
3124 | debugIsOn -> pprPanic "reportNoInstance" (ppr wanted)
3125 res -> Right (mk_overlap_msg wanted res)
3127 (clas,tys) = getDictClassTys wanted
3129 mk_overlap_msg dict (matches, unifiers)
3130 = ASSERT( not (null matches) )
3131 vcat [ addInstLoc [dict] ((ptext (sLit "Overlapping instances for")
3132 <+> pprPred (dictPred dict))),
3133 sep [ptext (sLit "Matching instances") <> colon,
3134 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
3135 if not (isSingleton matches)
3136 then -- Two or more matches
3138 else -- One match, plus some unifiers
3139 ASSERT( not (null unifiers) )
3140 parens (vcat [ptext (sLit "The choice depends on the instantiation of") <+>
3141 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
3142 ptext (sLit "To pick the first instance above, use -XIncoherentInstances"),
3143 ptext (sLit "when compiling the other instance declarations")])]
3145 ispecs = [ispec | (ispec, _) <- matches]
3147 mk_eq_err :: Inst -> (TidyEnv, SDoc)
3148 mk_eq_err inst = misMatchMsg tidy_env (eqInstTys inst)
3150 mk_no_inst_err insts
3151 | null insts = empty
3153 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
3154 not (isEmptyVarSet (tyVarsOfInsts insts))
3155 = vcat [ addInstLoc insts $
3156 sep [ ptext (sLit "Could not deduce") <+> pprDictsTheta insts
3157 , nest 2 $ ptext (sLit "from the context") <+> pprDictsTheta givens]
3158 , show_fixes (fix1 loc : fixes2) ]
3160 | otherwise -- Top level
3161 = vcat [ addInstLoc insts $
3162 ptext (sLit "No instance") <> plural insts
3163 <+> ptext (sLit "for") <+> pprDictsTheta insts
3164 , show_fixes fixes2 ]
3167 fix1 loc = sep [ ptext (sLit "add") <+> pprDictsTheta insts
3168 <+> ptext (sLit "to the context of"),
3169 nest 2 (ppr (instLocOrigin loc)) ]
3170 -- I'm not sure it helps to add the location
3171 -- nest 2 (ptext (sLit "at") <+> ppr (instLocSpan loc)) ]
3173 fixes2 | null instance_dicts = []
3174 | otherwise = [sep [ptext (sLit "add an instance declaration for"),
3175 pprDictsTheta instance_dicts]]
3176 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
3177 -- Insts for which it is worth suggesting an adding an instance declaration
3178 -- Exclude implicit parameters, and tyvar dicts
3180 show_fixes :: [SDoc] -> SDoc
3181 show_fixes [] = empty
3182 show_fixes (f:fs) = sep [ptext (sLit "Possible fix:"),
3183 nest 2 (vcat (f : map (ptext (sLit "or") <+>) fs))]
3185 addTopAmbigErrs :: [Inst] -> TcRn ()
3186 addTopAmbigErrs dicts
3187 -- Divide into groups that share a common set of ambiguous tyvars
3188 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
3189 -- See Note [Avoiding spurious errors]
3190 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
3192 (tidy_env, tidy_dicts) = tidyInsts dicts
3194 tvs_of :: Inst -> [TcTyVar]
3195 tvs_of d = varSetElems (tyVarsOfInst d)
3196 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
3198 report :: [(Inst,[TcTyVar])] -> TcM ()
3199 report pairs@((inst,tvs) : _) = do -- The pairs share a common set of ambiguous tyvars
3200 (tidy_env, mono_msg) <- mkMonomorphismMsg tidy_env tvs
3201 setSrcSpan (instSpan inst) $
3202 -- the location of the first one will do for the err message
3203 addErrTcM (tidy_env, msg $$ mono_msg)
3205 dicts = map fst pairs
3206 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
3207 pprQuotedList tvs <+> in_msg,
3208 nest 2 (pprDictsInFull dicts)]
3209 in_msg = text "in the constraint" <> plural dicts <> colon
3210 report [] = panic "addTopAmbigErrs"
3213 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
3214 -- There's an error with these Insts; if they have free type variables
3215 -- it's probably caused by the monomorphism restriction.
3216 -- Try to identify the offending variable
3217 -- ASSUMPTION: the Insts are fully zonked
3218 mkMonomorphismMsg tidy_env inst_tvs
3219 = do { dflags <- getDOpts
3220 ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
3221 ; return (tidy_env, mk_msg dflags docs) }
3223 mk_msg _ _ | any isRuntimeUnk inst_tvs
3224 = vcat [ptext (sLit "Cannot resolve unknown runtime types:") <+>
3225 (pprWithCommas ppr inst_tvs),
3226 ptext (sLit "Use :print or :force to determine these types")]
3227 mk_msg _ [] = ptext (sLit "Probable fix: add a type signature that fixes these type variable(s)")
3228 -- This happens in things like
3229 -- f x = show (read "foo")
3230 -- where monomorphism doesn't play any role
3232 = vcat [ptext (sLit "Possible cause: the monomorphism restriction applied to the following:"),
3234 monomorphism_fix dflags]
3236 monomorphism_fix :: DynFlags -> SDoc
3237 monomorphism_fix dflags
3238 = ptext (sLit "Probable fix:") <+> vcat
3239 [ptext (sLit "give these definition(s) an explicit type signature"),
3240 if dopt Opt_MonomorphismRestriction dflags
3241 then ptext (sLit "or use -XNoMonomorphismRestriction")
3242 else empty] -- Only suggest adding "-XNoMonomorphismRestriction"
3243 -- if it is not already set!
3245 warnDefault :: [(Inst, Class, Var)] -> Type -> TcM ()
3246 warnDefault ups default_ty = do
3247 warn_flag <- doptM Opt_WarnTypeDefaults
3248 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
3250 dicts = [d | (d,_,_) <- ups]
3253 (_, tidy_dicts) = tidyInsts dicts
3254 warn_msg = vcat [ptext (sLit "Defaulting the following constraint(s) to type") <+>
3255 quotes (ppr default_ty),
3256 pprDictsInFull tidy_dicts]
3258 reduceDepthErr :: Int -> [Inst] -> SDoc
3259 reduceDepthErr n stack
3260 = vcat [ptext (sLit "Context reduction stack overflow; size =") <+> int n,
3261 ptext (sLit "Use -fcontext-stack=N to increase stack size to N"),
3262 nest 4 (pprStack stack)]
3264 pprStack :: [Inst] -> SDoc
3265 pprStack stack = vcat (map pprInstInFull stack)