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,
18 bindInstsOfLocalFuns, bindIrreds,
21 #include "HsVersions.h"
23 import {-# SOURCE #-} TcUnify( unifyType )
60 %************************************************************************
64 %************************************************************************
66 --------------------------------------
67 Notes on functional dependencies (a bug)
68 --------------------------------------
75 instance D a b => C a b -- Undecidable
76 -- (Not sure if it's crucial to this eg)
77 f :: C a b => a -> Bool
80 g :: C a b => a -> Bool
83 Here f typechecks, but g does not!! Reason: before doing improvement,
84 we reduce the (C a b1) constraint from the call of f to (D a b1).
86 Here is a more complicated example:
88 | > class Foo a b | a->b
90 | > class Bar a b | a->b
94 | > instance Bar Obj Obj
96 | > instance (Bar a b) => Foo a b
98 | > foo:: (Foo a b) => a -> String
101 | > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
107 | Could not deduce (Bar a b) from the context (Foo a b)
108 | arising from use of `foo' at <interactive>:1
110 | Add (Bar a b) to the expected type of an expression
111 | In the first argument of `runFoo', namely `foo'
112 | In the definition of `it': it = runFoo foo
114 | Why all of the sudden does GHC need the constraint Bar a b? The
115 | function foo didn't ask for that...
117 The trouble is that to type (runFoo foo), GHC has to solve the problem:
119 Given constraint Foo a b
120 Solve constraint Foo a b'
122 Notice that b and b' aren't the same. To solve this, just do
123 improvement and then they are the same. But GHC currently does
128 That is usually fine, but it isn't here, because it sees that Foo a b is
129 not the same as Foo a b', and so instead applies the instance decl for
130 instance Bar a b => Foo a b. And that's where the Bar constraint comes
133 The Right Thing is to improve whenever the constraint set changes at
134 all. Not hard in principle, but it'll take a bit of fiddling to do.
138 --------------------------------------
139 Notes on quantification
140 --------------------------------------
142 Suppose we are about to do a generalisation step.
146 T the type of the RHS
147 C the constraints from that RHS
149 The game is to figure out
151 Q the set of type variables over which to quantify
152 Ct the constraints we will *not* quantify over
153 Cq the constraints we will quantify over
155 So we're going to infer the type
159 and float the constraints Ct further outwards.
161 Here are the things that *must* be true:
163 (A) Q intersect fv(G) = EMPTY limits how big Q can be
164 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
166 (A) says we can't quantify over a variable that's free in the
167 environment. (B) says we must quantify over all the truly free
168 variables in T, else we won't get a sufficiently general type. We do
169 not *need* to quantify over any variable that is fixed by the free
170 vars of the environment G.
172 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
174 Example: class H x y | x->y where ...
176 fv(G) = {a} C = {H a b, H c d}
179 (A) Q intersect {a} is empty
180 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
182 So Q can be {c,d}, {b,c,d}
184 Other things being equal, however, we'd like to quantify over as few
185 variables as possible: smaller types, fewer type applications, more
186 constraints can get into Ct instead of Cq.
189 -----------------------------------------
192 fv(T) the free type vars of T
194 oclose(vs,C) The result of extending the set of tyvars vs
195 using the functional dependencies from C
197 grow(vs,C) The result of extend the set of tyvars vs
198 using all conceivable links from C.
200 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
201 Then grow(vs,C) = {a,b,c}
203 Note that grow(vs,C) `superset` grow(vs,simplify(C))
204 That is, simplfication can only shrink the result of grow.
207 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
208 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
211 -----------------------------------------
215 Here's a good way to choose Q:
217 Q = grow( fv(T), C ) \ oclose( fv(G), C )
219 That is, quantify over all variable that that MIGHT be fixed by the
220 call site (which influences T), but which aren't DEFINITELY fixed by
221 G. This choice definitely quantifies over enough type variables,
222 albeit perhaps too many.
224 Why grow( fv(T), C ) rather than fv(T)? Consider
226 class H x y | x->y where ...
231 If we used fv(T) = {c} we'd get the type
233 forall c. H c d => c -> b
235 And then if the fn was called at several different c's, each of
236 which fixed d differently, we'd get a unification error, because
237 d isn't quantified. Solution: quantify d. So we must quantify
238 everything that might be influenced by c.
240 Why not oclose( fv(T), C )? Because we might not be able to see
241 all the functional dependencies yet:
243 class H x y | x->y where ...
244 instance H x y => Eq (T x y) where ...
249 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
250 apparent yet, and that's wrong. We must really quantify over d too.
253 There really isn't any point in quantifying over any more than
254 grow( fv(T), C ), because the call sites can't possibly influence
255 any other type variables.
259 -------------------------------------
261 -------------------------------------
263 It's very hard to be certain when a type is ambiguous. Consider
267 instance H x y => K (x,y)
269 Is this type ambiguous?
270 forall a b. (K (a,b), Eq b) => a -> a
272 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
273 now we see that a fixes b. So we can't tell about ambiguity for sure
274 without doing a full simplification. And even that isn't possible if
275 the context has some free vars that may get unified. Urgle!
277 Here's another example: is this ambiguous?
278 forall a b. Eq (T b) => a -> a
279 Not if there's an insance decl (with no context)
280 instance Eq (T b) where ...
282 You may say of this example that we should use the instance decl right
283 away, but you can't always do that:
285 class J a b where ...
286 instance J Int b where ...
288 f :: forall a b. J a b => a -> a
290 (Notice: no functional dependency in J's class decl.)
291 Here f's type is perfectly fine, provided f is only called at Int.
292 It's premature to complain when meeting f's signature, or even
293 when inferring a type for f.
297 However, we don't *need* to report ambiguity right away. It'll always
298 show up at the call site.... and eventually at main, which needs special
299 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
301 So here's the plan. We WARN about probable ambiguity if
303 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
305 (all tested before quantification).
306 That is, all the type variables in Cq must be fixed by the the variables
307 in the environment, or by the variables in the type.
309 Notice that we union before calling oclose. Here's an example:
311 class J a b c | a b -> c
315 forall b c. (J a b c) => b -> b
317 Only if we union {a} from G with {b} from T before using oclose,
318 do we see that c is fixed.
320 It's a bit vague exactly which C we should use for this oclose call. If we
321 don't fix enough variables we might complain when we shouldn't (see
322 the above nasty example). Nothing will be perfect. That's why we can
323 only issue a warning.
326 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
328 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
330 then c is a "bubble"; there's no way it can ever improve, and it's
331 certainly ambiguous. UNLESS it is a constant (sigh). And what about
336 instance H x y => K (x,y)
338 Is this type ambiguous?
339 forall a b. (K (a,b), Eq b) => a -> a
341 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
342 is a "bubble" that's a set of constraints
344 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
346 Hence another idea. To decide Q start with fv(T) and grow it
347 by transitive closure in Cq (no functional dependencies involved).
348 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
349 The definitely-ambiguous can then float out, and get smashed at top level
350 (which squashes out the constants, like Eq (T a) above)
353 --------------------------------------
354 Notes on principal types
355 --------------------------------------
360 f x = let g y = op (y::Int) in True
362 Here the principal type of f is (forall a. a->a)
363 but we'll produce the non-principal type
364 f :: forall a. C Int => a -> a
367 --------------------------------------
368 The need for forall's in constraints
369 --------------------------------------
371 [Exchange on Haskell Cafe 5/6 Dec 2000]
373 class C t where op :: t -> Bool
374 instance C [t] where op x = True
376 p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
377 q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
379 The definitions of p and q differ only in the order of the components in
380 the pair on their right-hand sides. And yet:
382 ghc and "Typing Haskell in Haskell" reject p, but accept q;
383 Hugs rejects q, but accepts p;
384 hbc rejects both p and q;
385 nhc98 ... (Malcolm, can you fill in the blank for us!).
387 The type signature for f forces context reduction to take place, and
388 the results of this depend on whether or not the type of y is known,
389 which in turn depends on which component of the pair the type checker
392 Solution: if y::m a, float out the constraints
393 Monad m, forall c. C (m c)
394 When m is later unified with [], we can solve both constraints.
397 --------------------------------------
398 Notes on implicit parameters
399 --------------------------------------
401 Note [Inheriting implicit parameters]
402 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
407 where f is *not* a top-level binding.
408 From the RHS of f we'll get the constraint (?y::Int).
409 There are two types we might infer for f:
413 (so we get ?y from the context of f's definition), or
415 f :: (?y::Int) => Int -> Int
417 At first you might think the first was better, becuase then
418 ?y behaves like a free variable of the definition, rather than
419 having to be passed at each call site. But of course, the WHOLE
420 IDEA is that ?y should be passed at each call site (that's what
421 dynamic binding means) so we'd better infer the second.
423 BOTTOM LINE: when *inferring types* you *must* quantify
424 over implicit parameters. See the predicate isFreeWhenInferring.
427 Note [Implicit parameters and ambiguity]
428 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
429 What type should we infer for this?
430 f x = (show ?y, x::Int)
431 Since we must quantify over the ?y, the most plausible type is
432 f :: (Show a, ?y::a) => Int -> (String, Int)
433 But notice that the type of the RHS is (String,Int), with no type
434 varibables mentioned at all! The type of f looks ambiguous. But
435 it isn't, because at a call site we might have
436 let ?y = 5::Int in f 7
437 and all is well. In effect, implicit parameters are, well, parameters,
438 so we can take their type variables into account as part of the
439 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
442 Question 2: type signatures
443 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
444 BUT WATCH OUT: When you supply a type signature, we can't force you
445 to quantify over implicit parameters. For example:
449 This is perfectly reasonable. We do not want to insist on
451 (?x + 1) :: (?x::Int => Int)
453 That would be silly. Here, the definition site *is* the occurrence site,
454 so the above strictures don't apply. Hence the difference between
455 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
456 and tcSimplifyCheckBind (which does not).
458 What about when you supply a type signature for a binding?
459 Is it legal to give the following explicit, user type
460 signature to f, thus:
465 At first sight this seems reasonable, but it has the nasty property
466 that adding a type signature changes the dynamic semantics.
469 (let f x = (x::Int) + ?y
470 in (f 3, f 3 with ?y=5)) with ?y = 6
476 in (f 3, f 3 with ?y=5)) with ?y = 6
480 Indeed, simply inlining f (at the Haskell source level) would change the
483 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
484 semantics for a Haskell program without knowing its typing, so if you
485 change the typing you may change the semantics.
487 To make things consistent in all cases where we are *checking* against
488 a supplied signature (as opposed to inferring a type), we adopt the
491 a signature does not need to quantify over implicit params.
493 [This represents a (rather marginal) change of policy since GHC 5.02,
494 which *required* an explicit signature to quantify over all implicit
495 params for the reasons mentioned above.]
497 But that raises a new question. Consider
499 Given (signature) ?x::Int
500 Wanted (inferred) ?x::Int, ?y::Bool
502 Clearly we want to discharge the ?x and float the ?y out. But
503 what is the criterion that distinguishes them? Clearly it isn't
504 what free type variables they have. The Right Thing seems to be
505 to float a constraint that
506 neither mentions any of the quantified type variables
507 nor any of the quantified implicit parameters
509 See the predicate isFreeWhenChecking.
512 Question 3: monomorphism
513 ~~~~~~~~~~~~~~~~~~~~~~~~
514 There's a nasty corner case when the monomorphism restriction bites:
518 The argument above suggests that we *must* generalise
519 over the ?y parameter, to get
520 z :: (?y::Int) => Int,
521 but the monomorphism restriction says that we *must not*, giving
523 Why does the momomorphism restriction say this? Because if you have
525 let z = x + ?y in z+z
527 you might not expect the addition to be done twice --- but it will if
528 we follow the argument of Question 2 and generalise over ?y.
531 Question 4: top level
532 ~~~~~~~~~~~~~~~~~~~~~
533 At the top level, monomorhism makes no sense at all.
536 main = let ?x = 5 in print foo
540 woggle :: (?x :: Int) => Int -> Int
543 We definitely don't want (foo :: Int) with a top-level implicit parameter
544 (?x::Int) becuase there is no way to bind it.
549 (A) Always generalise over implicit parameters
550 Bindings that fall under the monomorphism restriction can't
554 * Inlining remains valid
555 * No unexpected loss of sharing
556 * But simple bindings like
558 will be rejected, unless you add an explicit type signature
559 (to avoid the monomorphism restriction)
560 z :: (?y::Int) => Int
562 This seems unacceptable
564 (B) Monomorphism restriction "wins"
565 Bindings that fall under the monomorphism restriction can't
567 Always generalise over implicit parameters *except* for bindings
568 that fall under the monomorphism restriction
571 * Inlining isn't valid in general
572 * No unexpected loss of sharing
573 * Simple bindings like
575 accepted (get value of ?y from binding site)
577 (C) Always generalise over implicit parameters
578 Bindings that fall under the monomorphism restriction can't
579 be generalised, EXCEPT for implicit parameters
581 * Inlining remains valid
582 * Unexpected loss of sharing (from the extra generalisation)
583 * Simple bindings like
585 accepted (get value of ?y from occurrence sites)
590 None of these choices seems very satisfactory. But at least we should
591 decide which we want to do.
593 It's really not clear what is the Right Thing To Do. If you see
597 would you expect the value of ?y to be got from the *occurrence sites*
598 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
599 case of function definitions, the answer is clearly the former, but
600 less so in the case of non-fucntion definitions. On the other hand,
601 if we say that we get the value of ?y from the definition site of 'z',
602 then inlining 'z' might change the semantics of the program.
604 Choice (C) really says "the monomorphism restriction doesn't apply
605 to implicit parameters". Which is fine, but remember that every
606 innocent binding 'x = ...' that mentions an implicit parameter in
607 the RHS becomes a *function* of that parameter, called at each
608 use of 'x'. Now, the chances are that there are no intervening 'with'
609 clauses that bind ?y, so a decent compiler should common up all
610 those function calls. So I think I strongly favour (C). Indeed,
611 one could make a similar argument for abolishing the monomorphism
612 restriction altogether.
614 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
618 %************************************************************************
620 \subsection{tcSimplifyInfer}
622 %************************************************************************
624 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
626 1. Compute Q = grow( fvs(T), C )
628 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
629 predicates will end up in Ct; we deal with them at the top level
631 3. Try improvement, using functional dependencies
633 4. If Step 3 did any unification, repeat from step 1
634 (Unification can change the result of 'grow'.)
636 Note: we don't reduce dictionaries in step 2. For example, if we have
637 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
638 after step 2. However note that we may therefore quantify over more
639 type variables than we absolutely have to.
641 For the guts, we need a loop, that alternates context reduction and
642 improvement with unification. E.g. Suppose we have
644 class C x y | x->y where ...
646 and tcSimplify is called with:
648 Then improvement unifies a with b, giving
651 If we need to unify anything, we rattle round the whole thing all over
658 -> TcTyVarSet -- fv(T); type vars
660 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
661 [Inst], -- Dict Ids that must be bound here (zonked)
662 TcDictBinds) -- Bindings
663 -- Any free (escaping) Insts are tossed into the environment
668 tcSimplifyInfer doc tau_tvs wanted
669 = do { tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
670 ; wanted' <- mappM zonkInst wanted -- Zonk before deciding quantified tyvars
671 ; gbl_tvs <- tcGetGlobalTyVars
672 ; let preds = fdPredsOfInsts wanted'
673 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
674 (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
675 ; traceTc (text "infer" <+> (ppr preds $$ ppr (grow preds tau_tvs') $$ ppr gbl_tvs $$ ppr (oclose preds gbl_tvs) $$ ppr free $$ ppr bound))
678 -- To make types simple, reduce as much as possible
679 ; let try_me inst = ReduceMe AddSCs
680 ; (irreds, binds) <- checkLoop (mkRedEnv doc try_me []) bound
682 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
684 -- We can't abstract over implications
685 ; let (dicts, implics) = partition isDict irreds
686 ; loc <- getInstLoc (ImplicOrigin doc)
687 ; implic_bind <- bindIrreds loc qtvs' dicts implics
689 ; return (qtvs', dicts, binds `unionBags` implic_bind) }
690 -- NB: when we are done, we might have some bindings, but
691 -- the final qtvs might be empty. See Note [NO TYVARS] below.
695 -----------------------------------------------------------
696 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
697 -- against, but we don't know the type variables over which we are going to quantify.
698 -- This happens when we have a type signature for a mutually recursive group
701 -> TcTyVarSet -- fv(T)
704 -> TcM ([TyVar], -- Fully zonked, and quantified
705 TcDictBinds) -- Bindings
707 tcSimplifyInferCheck loc tau_tvs givens wanteds
708 = do { (irreds, binds) <- innerCheckLoop loc givens wanteds
710 -- Figure out which type variables to quantify over
711 -- You might think it should just be the signature tyvars,
712 -- but in bizarre cases you can get extra ones
713 -- f :: forall a. Num a => a -> a
714 -- f x = fst (g (x, head [])) + 1
716 -- Here we infer g :: forall a b. a -> b -> (b,a)
717 -- We don't want g to be monomorphic in b just because
718 -- f isn't quantified over b.
719 ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
720 ; all_tvs <- zonkTcTyVarsAndFV all_tvs
721 ; gbl_tvs <- tcGetGlobalTyVars
722 ; let qtvs = varSetElems (all_tvs `minusVarSet` gbl_tvs)
723 -- We could close gbl_tvs, but its not necessary for
724 -- soundness, and it'll only affect which tyvars, not which
725 -- dictionaries, we quantify over
727 ; qtvs' <- zonkQuantifiedTyVars qtvs
729 -- Now we are back to normal (c.f. tcSimplCheck)
730 ; implic_bind <- bindIrreds loc qtvs' givens irreds
732 ; return (qtvs', binds `unionBags` implic_bind) }
735 Note [Squashing methods]
736 ~~~~~~~~~~~~~~~~~~~~~~~~~
737 Be careful if you want to float methods more:
738 truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
739 From an application (truncate f i) we get
742 If we have also have a second occurrence of truncate, we get
745 When simplifying with i,f free, we might still notice that
746 t1=t3; but alas, the binding for t2 (which mentions t1)
747 may continue to float out!
752 class Y a b | a -> b where
755 instance Y [[a]] a where
758 k :: X a -> X a -> X a
760 g :: Num a => [X a] -> [X a]
763 h ys = ys ++ map (k (y [[0]])) xs
765 The excitement comes when simplifying the bindings for h. Initially
766 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
767 From this we get t1:=:t2, but also various bindings. We can't forget
768 the bindings (because of [LOOP]), but in fact t1 is what g is
771 The net effect of [NO TYVARS]
774 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
775 isFreeWhenInferring qtvs inst
776 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
777 && isInheritableInst inst -- and no implicit parameter involved
778 -- see Note [Inheriting implicit parameters]
780 {- No longer used (with implication constraints)
781 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
782 -> NameSet -- Quantified implicit parameters
784 isFreeWhenChecking qtvs ips inst
785 = isFreeWrtTyVars qtvs inst
786 && isFreeWrtIPs ips inst
789 isFreeWrtTyVars qtvs inst = tyVarsOfInst inst `disjointVarSet` qtvs
790 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
794 %************************************************************************
796 \subsection{tcSimplifyCheck}
798 %************************************************************************
800 @tcSimplifyCheck@ is used when we know exactly the set of variables
801 we are going to quantify over. For example, a class or instance declaration.
804 -----------------------------------------------------------
805 -- tcSimplifyCheck is used when checking expression type signatures,
806 -- class decls, instance decls etc.
807 tcSimplifyCheck :: InstLoc
808 -> [TcTyVar] -- Quantify over these
811 -> TcM TcDictBinds -- Bindings
812 tcSimplifyCheck loc qtvs givens wanteds
813 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
814 do { (irreds, binds) <- innerCheckLoop loc givens wanteds
815 ; implic_bind <- bindIrreds loc qtvs givens irreds
816 ; return (binds `unionBags` implic_bind) }
818 -----------------------------------------------------------
819 -- tcSimplifyCheckPat is used for existential pattern match
820 tcSimplifyCheckPat :: InstLoc
821 -> [CoVar] -> Refinement
822 -> [TcTyVar] -- Quantify over these
825 -> TcM TcDictBinds -- Bindings
826 tcSimplifyCheckPat loc co_vars reft qtvs givens wanteds
827 = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
828 do { (irreds, binds) <- innerCheckLoop loc givens wanteds
829 ; implic_bind <- bindIrredsR loc qtvs co_vars reft
831 ; return (binds `unionBags` implic_bind) }
833 -----------------------------------------------------------
834 bindIrreds :: InstLoc -> [TcTyVar]
837 bindIrreds loc qtvs givens irreds
838 = bindIrredsR loc qtvs [] emptyRefinement givens irreds
840 bindIrredsR :: InstLoc -> [TcTyVar] -> [CoVar]
841 -> Refinement -> [Inst] -> [Inst]
843 -- Make a binding that binds 'irreds', by generating an implication
844 -- constraint for them, *and* throwing the constraint into the LIE
845 bindIrredsR loc qtvs co_vars reft givens irreds
849 = do { let givens' = filter isDict givens
850 -- The givens can include methods
852 -- If there are no 'givens', then it's safe to
853 -- partition the 'wanteds' by their qtvs, thereby trimming irreds
854 -- See Note [Freeness and implications]
855 ; irreds' <- if null givens'
857 { let qtv_set = mkVarSet qtvs
858 (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
860 ; return real_irreds }
863 ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
864 ; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds'
865 -- This call does the real work
870 makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement
872 -> TcM ([Inst], TcDictBinds)
873 -- Make a binding that binds 'irreds', by generating an implication
874 -- constraint for them, *and* throwing the constraint into the LIE
875 -- The binding looks like
876 -- (ir1, .., irn) = f qtvs givens
877 -- where f is (evidence for) the new implication constraint
879 -- This binding must line up the 'rhs' in reduceImplication
880 makeImplicationBind loc all_tvs reft
881 givens -- Guaranteed all Dicts
883 | null irreds -- If there are no irreds, we are done
884 = return ([], emptyBag)
885 | otherwise -- Otherwise we must generate a binding
886 = do { uniq <- newUnique
887 ; span <- getSrcSpanM
888 ; let name = mkInternalName uniq (mkVarOcc "ic") (srcSpanStart span)
889 implic_inst = ImplicInst { tci_name = name, tci_reft = reft,
890 tci_tyvars = all_tvs,
892 tci_wanted = irreds, tci_loc = loc }
894 ; let n_irreds = length irreds
895 irred_ids = map instToId irreds
896 tup_ty = mkTupleTy Boxed n_irreds (map idType irred_ids)
897 pat = TuplePat (map nlVarPat irred_ids) Boxed tup_ty
898 rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
899 co = mkWpApps (map instToId givens) <.> mkWpTyApps (mkTyVarTys all_tvs)
900 bind | n_irreds==1 = VarBind (head irred_ids) rhs
901 | otherwise = PatBind { pat_lhs = L span pat,
902 pat_rhs = unguardedGRHSs rhs,
904 bind_fvs = placeHolderNames }
905 ; -- pprTrace "Make implic inst" (ppr implic_inst) $
906 return ([implic_inst], unitBag (L span bind)) }
908 -----------------------------------------------------------
911 -> TcM ([Inst], TcDictBinds)
913 topCheckLoop doc wanteds
914 = checkLoop (mkRedEnv doc try_me []) wanteds
916 try_me inst = ReduceMe AddSCs
918 -----------------------------------------------------------
919 innerCheckLoop :: InstLoc
922 -> TcM ([Inst], TcDictBinds)
924 innerCheckLoop inst_loc givens wanteds
925 = checkLoop env wanteds
927 env = mkRedEnv (pprInstLoc inst_loc) try_me givens
929 try_me inst | isMethodOrLit inst = ReduceMe AddSCs
931 -- When checking against a given signature
932 -- we MUST be very gentle: Note [Check gently]
937 We have to very careful about not simplifying too vigorously
942 f :: Show b => T b -> b
945 Inside the pattern match, which binds (a:*, x:a), we know that
947 Hence we have a dictionary for Show [a] available; and indeed we
948 need it. We are going to build an implication contraint
949 forall a. (b~[a]) => Show [a]
950 Later, we will solve this constraint using the knowledge (Show b)
952 But we MUST NOT reduce (Show [a]) to (Show a), else the whole
953 thing becomes insoluble. So we simplify gently (get rid of literals
954 and methods only, plus common up equal things), deferring the real
955 work until top level, when we solve the implication constraint
960 -----------------------------------------------------------
963 -> TcM ([Inst], TcDictBinds)
964 -- Precondition: givens are completely rigid
966 checkLoop env wanteds
967 = do { -- Givens are skolems, so no need to zonk them
968 wanteds' <- mappM zonkInst wanteds
970 ; (improved, binds, irreds) <- reduceContext env wanteds'
972 ; if not improved then
973 return (irreds, binds)
976 -- If improvement did some unification, we go round again.
977 -- We start again with irreds, not wanteds
978 -- Using an instance decl might have introduced a fresh type variable
979 -- which might have been unified, so we'd get an infinite loop
980 -- if we started again with wanteds! See Note [LOOP]
981 { (irreds1, binds1) <- checkLoop env irreds
982 ; return (irreds1, binds `unionBags` binds1) } }
987 class If b t e r | b t e -> r
990 class Lte a b c | a b -> c where lte :: a -> b -> c
992 instance (Lte a b l,If l b a c) => Max a b c
994 Wanted: Max Z (S x) y
996 Then we'll reduce using the Max instance to:
997 (Lte Z (S x) l, If l (S x) Z y)
998 and improve by binding l->T, after which we can do some reduction
999 on both the Lte and If constraints. What we *can't* do is start again
1000 with (Max Z (S x) y)!
1004 %************************************************************************
1006 tcSimplifySuperClasses
1008 %************************************************************************
1010 Note [SUPERCLASS-LOOP 1]
1011 ~~~~~~~~~~~~~~~~~~~~~~~~
1012 We have to be very, very careful when generating superclasses, lest we
1013 accidentally build a loop. Here's an example:
1017 class S a => C a where { opc :: a -> a }
1018 class S b => D b where { opd :: b -> b }
1020 instance C Int where
1023 instance D Int where
1026 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
1027 Simplifying, we may well get:
1028 $dfCInt = :C ds1 (opd dd)
1031 Notice that we spot that we can extract ds1 from dd.
1033 Alas! Alack! We can do the same for (instance D Int):
1035 $dfDInt = :D ds2 (opc dc)
1039 And now we've defined the superclass in terms of itself.
1041 Solution: never generate a superclass selectors at all when
1042 satisfying the superclass context of an instance declaration.
1044 Two more nasty cases are in
1049 tcSimplifySuperClasses
1054 tcSimplifySuperClasses loc givens sc_wanteds
1055 = do { (irreds, binds1) <- checkLoop env sc_wanteds
1056 ; let (tidy_env, tidy_irreds) = tidyInsts irreds
1057 ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds
1060 env = mkRedEnv (pprInstLoc loc) try_me givens
1061 try_me inst = ReduceMe NoSCs
1062 -- Like topCheckLoop, but with NoSCs
1066 %************************************************************************
1068 \subsection{tcSimplifyRestricted}
1070 %************************************************************************
1072 tcSimplifyRestricted infers which type variables to quantify for a
1073 group of restricted bindings. This isn't trivial.
1076 We want to quantify over a to get id :: forall a. a->a
1079 We do not want to quantify over a, because there's an Eq a
1080 constraint, so we get eq :: a->a->Bool (notice no forall)
1083 RHS has type 'tau', whose free tyvars are tau_tvs
1084 RHS has constraints 'wanteds'
1087 Quantify over (tau_tvs \ ftvs(wanteds))
1088 This is bad. The constraints may contain (Monad (ST s))
1089 where we have instance Monad (ST s) where...
1090 so there's no need to be monomorphic in s!
1092 Also the constraint might be a method constraint,
1093 whose type mentions a perfectly innocent tyvar:
1094 op :: Num a => a -> b -> a
1095 Here, b is unconstrained. A good example would be
1097 We want to infer the polymorphic type
1098 foo :: forall b. b -> b
1101 Plan B (cunning, used for a long time up to and including GHC 6.2)
1102 Step 1: Simplify the constraints as much as possible (to deal
1103 with Plan A's problem). Then set
1104 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1106 Step 2: Now simplify again, treating the constraint as 'free' if
1107 it does not mention qtvs, and trying to reduce it otherwise.
1108 The reasons for this is to maximise sharing.
1110 This fails for a very subtle reason. Suppose that in the Step 2
1111 a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
1112 In the Step 1 this constraint might have been simplified, perhaps to
1113 (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
1114 This won't happen in Step 2... but that in turn might prevent some other
1115 constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
1116 and that in turn breaks the invariant that no constraints are quantified over.
1118 Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
1123 Step 1: Simplify the constraints as much as possible (to deal
1124 with Plan A's problem). Then set
1125 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1126 Return the bindings from Step 1.
1129 A note about Plan C (arising from "bug" reported by George Russel March 2004)
1132 instance (HasBinary ty IO) => HasCodedValue ty
1134 foo :: HasCodedValue a => String -> IO a
1136 doDecodeIO :: HasCodedValue a => () -> () -> IO a
1137 doDecodeIO codedValue view
1138 = let { act = foo "foo" } in act
1140 You might think this should work becuase the call to foo gives rise to a constraint
1141 (HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
1142 restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
1143 constraint using the (rather bogus) instance declaration, and now we are stuffed.
1145 I claim this is not really a bug -- but it bit Sergey as well as George. So here's
1149 Plan D (a variant of plan B)
1150 Step 1: Simplify the constraints as much as possible (to deal
1151 with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
1152 qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
1154 Step 2: Now simplify again, treating the constraint as 'free' if
1155 it does not mention qtvs, and trying to reduce it otherwise.
1157 The point here is that it's generally OK to have too few qtvs; that is,
1158 to make the thing more monomorphic than it could be. We don't want to
1159 do that in the common cases, but in wierd cases it's ok: the programmer
1160 can always add a signature.
1162 Too few qtvs => too many wanteds, which is what happens if you do less
1167 tcSimplifyRestricted -- Used for restricted binding groups
1168 -- i.e. ones subject to the monomorphism restriction
1171 -> [Name] -- Things bound in this group
1172 -> TcTyVarSet -- Free in the type of the RHSs
1173 -> [Inst] -- Free in the RHSs
1174 -> TcM ([TyVar], -- Tyvars to quantify (zonked)
1175 TcDictBinds) -- Bindings
1176 -- tcSimpifyRestricted returns no constraints to
1177 -- quantify over; by definition there are none.
1178 -- They are all thrown back in the LIE
1180 tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
1181 -- Zonk everything in sight
1182 = do { wanteds' <- mappM zonkInst wanteds
1184 -- 'ReduceMe': Reduce as far as we can. Don't stop at
1185 -- dicts; the idea is to get rid of as many type
1186 -- variables as possible, and we don't want to stop
1187 -- at (say) Monad (ST s), because that reduces
1188 -- immediately, with no constraint on s.
1190 -- BUT do no improvement! See Plan D above
1191 -- HOWEVER, some unification may take place, if we instantiate
1192 -- a method Inst with an equality constraint
1193 ; let env = mkNoImproveRedEnv doc (\i -> ReduceMe AddSCs)
1194 ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds'
1196 -- Next, figure out the tyvars we will quantify over
1197 ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
1198 ; gbl_tvs' <- tcGetGlobalTyVars
1199 ; constrained_dicts' <- mappM zonkInst constrained_dicts
1201 ; let constrained_tvs' = tyVarsOfInsts constrained_dicts'
1202 qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
1203 `minusVarSet` constrained_tvs'
1204 ; traceTc (text "tcSimplifyRestricted" <+> vcat [
1205 pprInsts wanteds, pprInsts constrained_dicts',
1207 ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
1209 -- The first step may have squashed more methods than
1210 -- necessary, so try again, this time more gently, knowing the exact
1211 -- set of type variables to quantify over.
1213 -- We quantify only over constraints that are captured by qtvs;
1214 -- these will just be a subset of non-dicts. This in contrast
1215 -- to normal inference (using isFreeWhenInferring) in which we quantify over
1216 -- all *non-inheritable* constraints too. This implements choice
1217 -- (B) under "implicit parameter and monomorphism" above.
1219 -- Remember that we may need to do *some* simplification, to
1220 -- (for example) squash {Monad (ST s)} into {}. It's not enough
1221 -- just to float all constraints
1223 -- At top level, we *do* squash methods becuase we want to
1224 -- expose implicit parameters to the test that follows
1225 ; let is_nested_group = isNotTopLevel top_lvl
1226 try_me inst | isFreeWrtTyVars qtvs inst,
1227 (is_nested_group || isDict inst) = Stop
1228 | otherwise = ReduceMe AddSCs
1229 env = mkNoImproveRedEnv doc try_me
1230 ; (_imp, binds, irreds) <- reduceContext env wanteds'
1232 -- See "Notes on implicit parameters, Question 4: top level"
1233 ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
1234 if is_nested_group then
1236 else do { let (bad_ips, non_ips) = partition isIPDict irreds
1237 ; addTopIPErrs bndrs bad_ips
1238 ; extendLIEs non_ips }
1240 ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
1241 ; return (qtvs', binds) }
1245 %************************************************************************
1249 %************************************************************************
1251 On the LHS of transformation rules we only simplify methods and constants,
1252 getting dictionaries. We want to keep all of them unsimplified, to serve
1253 as the available stuff for the RHS of the rule.
1255 Example. Consider the following left-hand side of a rule
1257 f (x == y) (y > z) = ...
1259 If we typecheck this expression we get constraints
1261 d1 :: Ord a, d2 :: Eq a
1263 We do NOT want to "simplify" to the LHS
1265 forall x::a, y::a, z::a, d1::Ord a.
1266 f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
1270 forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
1271 f ((==) d2 x y) ((>) d1 y z) = ...
1273 Here is another example:
1275 fromIntegral :: (Integral a, Num b) => a -> b
1276 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
1278 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
1279 we *dont* want to get
1281 forall dIntegralInt.
1282 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
1284 because the scsel will mess up RULE matching. Instead we want
1286 forall dIntegralInt, dNumInt.
1287 fromIntegral Int Int dIntegralInt dNumInt = id Int
1291 g (x == y) (y == z) = ..
1293 where the two dictionaries are *identical*, we do NOT WANT
1295 forall x::a, y::a, z::a, d1::Eq a
1296 f ((==) d1 x y) ((>) d1 y z) = ...
1298 because that will only match if the dict args are (visibly) equal.
1299 Instead we want to quantify over the dictionaries separately.
1301 In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
1302 all dicts unchanged, with absolutely no sharing. It's simpler to do this
1303 from scratch, rather than further parameterise simpleReduceLoop etc
1306 tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
1307 tcSimplifyRuleLhs wanteds
1308 = go [] emptyBag wanteds
1311 = return (dicts, binds)
1312 go dicts binds (w:ws)
1314 = go (w:dicts) binds ws
1316 = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
1317 -- to fromInteger; this looks fragile to me
1318 ; lookup_result <- lookupSimpleInst w'
1319 ; case lookup_result of
1320 GenInst ws' rhs -> go dicts (addBind binds w rhs) (ws' ++ ws)
1321 NoInstance -> pprPanic "tcSimplifyRuleLhs" (ppr w)
1325 tcSimplifyBracket is used when simplifying the constraints arising from
1326 a Template Haskell bracket [| ... |]. We want to check that there aren't
1327 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
1328 Show instance), but we aren't otherwise interested in the results.
1329 Nor do we care about ambiguous dictionaries etc. We will type check
1330 this bracket again at its usage site.
1333 tcSimplifyBracket :: [Inst] -> TcM ()
1334 tcSimplifyBracket wanteds
1335 = do { topCheckLoop doc wanteds
1338 doc = text "tcSimplifyBracket"
1342 %************************************************************************
1344 \subsection{Filtering at a dynamic binding}
1346 %************************************************************************
1351 we must discharge all the ?x constraints from B. We also do an improvement
1352 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
1354 Actually, the constraints from B might improve the types in ?x. For example
1356 f :: (?x::Int) => Char -> Char
1359 then the constraint (?x::Int) arising from the call to f will
1360 force the binding for ?x to be of type Int.
1363 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
1366 -- We need a loop so that we do improvement, and then
1367 -- (next time round) generate a binding to connect the two
1369 -- Here the two ?x's have different types, and improvement
1370 -- makes them the same.
1372 tcSimplifyIPs given_ips wanteds
1373 = do { wanteds' <- mappM zonkInst wanteds
1374 ; given_ips' <- mappM zonkInst given_ips
1375 -- Unusually for checking, we *must* zonk the given_ips
1377 ; let env = mkRedEnv doc try_me given_ips'
1378 ; (improved, binds, irreds) <- reduceContext env wanteds'
1380 ; if not improved then
1381 ASSERT( all is_free irreds )
1382 do { extendLIEs irreds
1385 tcSimplifyIPs given_ips wanteds }
1387 doc = text "tcSimplifyIPs" <+> ppr given_ips
1388 ip_set = mkNameSet (ipNamesOfInsts given_ips)
1389 is_free inst = isFreeWrtIPs ip_set inst
1391 -- Simplify any methods that mention the implicit parameter
1392 try_me inst | is_free inst = Stop
1393 | otherwise = ReduceMe NoSCs
1397 %************************************************************************
1399 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
1401 %************************************************************************
1403 When doing a binding group, we may have @Insts@ of local functions.
1404 For example, we might have...
1406 let f x = x + 1 -- orig local function (overloaded)
1407 f.1 = f Int -- two instances of f
1412 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1413 where @f@ is in scope; those @Insts@ must certainly not be passed
1414 upwards towards the top-level. If the @Insts@ were binding-ified up
1415 there, they would have unresolvable references to @f@.
1417 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1418 For each method @Inst@ in the @init_lie@ that mentions one of the
1419 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1420 @LIE@), as well as the @HsBinds@ generated.
1423 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
1424 -- Simlifies only MethodInsts, and generate only bindings of form
1426 -- We're careful not to even generate bindings of the form
1428 -- You'd think that'd be fine, but it interacts with what is
1429 -- arguably a bug in Match.tidyEqnInfo (see notes there)
1431 bindInstsOfLocalFuns wanteds local_ids
1432 | null overloaded_ids
1434 = extendLIEs wanteds `thenM_`
1435 returnM emptyLHsBinds
1438 = do { (irreds, binds) <- checkLoop env for_me
1439 ; extendLIEs not_for_me
1443 env = mkRedEnv doc try_me []
1444 doc = text "bindInsts" <+> ppr local_ids
1445 overloaded_ids = filter is_overloaded local_ids
1446 is_overloaded id = isOverloadedTy (idType id)
1447 (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
1449 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1450 -- so it's worth building a set, so that
1451 -- lookup (in isMethodFor) is faster
1452 try_me inst | isMethod inst = ReduceMe NoSCs
1457 %************************************************************************
1459 \subsection{Data types for the reduction mechanism}
1461 %************************************************************************
1463 The main control over context reduction is here
1467 = RedEnv { red_doc :: SDoc -- The context
1468 , red_try_me :: Inst -> WhatToDo
1469 , red_improve :: Bool -- True <=> do improvement
1470 , red_givens :: [Inst] -- All guaranteed rigid
1472 -- but see Note [Rigidity]
1473 , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
1474 -- See Note [RedStack]
1478 -- The red_givens are rigid so far as cmpInst is concerned.
1479 -- There is one case where they are not totally rigid, namely in tcSimplifyIPs
1480 -- let ?x = e in ...
1481 -- Here, the given is (?x::a), where 'a' is not necy a rigid type
1482 -- But that doesn't affect the comparison, which is based only on mame.
1485 -- The red_stack pair (n,insts) pair is just used for error reporting.
1486 -- 'n' is always the depth of the stack.
1487 -- The 'insts' is the stack of Insts being reduced: to produce X
1488 -- I had to produce Y, to produce Y I had to produce Z, and so on.
1491 mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
1492 mkRedEnv doc try_me givens
1493 = RedEnv { red_doc = doc, red_try_me = try_me,
1494 red_givens = givens, red_stack = (0,[]),
1495 red_improve = True }
1497 mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
1498 -- Do not do improvement; no givens
1499 mkNoImproveRedEnv doc try_me
1500 = RedEnv { red_doc = doc, red_try_me = try_me,
1501 red_givens = [], red_stack = (0,[]),
1502 red_improve = True }
1505 = ReduceMe WantSCs -- Try to reduce this
1506 -- If there's no instance, add the inst to the
1507 -- irreductible ones, but don't produce an error
1508 -- message of any kind.
1509 -- It might be quite legitimate such as (Eq a)!
1511 | Stop -- Return as irreducible unless it can
1512 -- be reduced to a constant in one step
1513 -- Do not add superclasses; see
1515 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1516 -- of a predicate when adding it to the avails
1517 -- The reason for this flag is entirely the super-class loop problem
1518 -- Note [SUPER-CLASS LOOP 1]
1521 %************************************************************************
1523 \subsection[reduce]{@reduce@}
1525 %************************************************************************
1529 reduceContext :: RedEnv
1531 -> TcM (ImprovementDone,
1532 TcDictBinds, -- Dictionary bindings
1533 [Inst]) -- Irreducible
1535 reduceContext env wanteds
1536 = do { traceTc (text "reduceContext" <+> (vcat [
1537 text "----------------------",
1539 text "given" <+> ppr (red_givens env),
1540 text "wanted" <+> ppr wanteds,
1541 text "----------------------"
1544 -- Build the Avail mapping from "givens"
1545 ; init_state <- foldlM addGiven emptyAvails (red_givens env)
1548 ; avails <- reduceList env wanteds init_state
1550 ; let improved = availsImproved avails
1551 ; (binds, irreds) <- extractResults avails wanteds
1553 ; traceTc (text "reduceContext end" <+> (vcat [
1554 text "----------------------",
1556 text "given" <+> ppr (red_givens env),
1557 text "wanted" <+> ppr wanteds,
1559 text "avails" <+> pprAvails avails,
1560 text "improved =" <+> ppr improved,
1561 text "----------------------"
1564 ; return (improved, binds, irreds) }
1566 tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
1567 tcImproveOne avails inst
1568 | not (isDict inst) = return False
1570 = do { inst_envs <- tcGetInstEnvs
1571 ; let eqns = improveOne (classInstances inst_envs)
1572 (dictPred inst, pprInstArising inst)
1573 [ (dictPred p, pprInstArising p)
1574 | p <- availsInsts avails, isDict p ]
1575 -- Avails has all the superclasses etc (good)
1576 -- It also has all the intermediates of the deduction (good)
1577 -- It does not have duplicates (good)
1578 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1579 -- so that improve will see them separate
1580 ; traceTc (text "improveOne" <+> ppr inst)
1583 unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))]
1584 -> TcM ImprovementDone
1585 unifyEqns [] = return False
1587 = do { traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns))
1591 unify ((qtvs, pairs), what1, what2)
1592 = addErrCtxtM (mkEqnMsg what1 what2) $
1593 tcInstTyVars (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1594 mapM_ (unif_pr tenv) pairs
1595 unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
1597 pprEquationDoc (eqn, (p1,w1), (p2,w2)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
1599 mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
1600 = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2
1601 ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' }
1602 ; let msg = vcat [ptext SLIT("When using functional dependencies to combine"),
1603 nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
1604 nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
1605 ; return (tidy_env, msg) }
1608 The main context-reduction function is @reduce@. Here's its game plan.
1611 reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
1612 reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
1613 = do { dopts <- getDOpts
1616 dumpTcRn (hang (ptext SLIT("Interesting! Context reduction stack depth") <+> int n)
1617 2 (ifPprDebug (nest 2 (pprStack stk))))
1620 ; if n >= ctxtStkDepth dopts then
1621 failWithTc (reduceDepthErr n stk)
1625 go [] state = return state
1626 go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
1629 -- Base case: we're done!
1630 reduce env wanted avails
1631 -- It's the same as an existing inst, or a superclass thereof
1632 | Just avail <- findAvail avails wanted
1636 = case red_try_me env wanted of {
1637 ; Stop -> try_simple (addIrred NoSCs) -- See Note [No superclasses for Stop]
1639 ; ReduceMe want_scs -> -- It should be reduced
1640 reduceInst env avails wanted `thenM` \ (avails, lookup_result) ->
1641 case lookup_result of
1642 NoInstance -> -- No such instance!
1643 -- Add it and its superclasses
1644 addIrred want_scs avails wanted
1646 GenInst [] rhs -> addWanted want_scs avails wanted rhs []
1648 GenInst wanteds' rhs -> do { avails1 <- addIrred NoSCs avails wanted
1649 ; avails2 <- reduceList env wanteds' avails1
1650 ; addWanted want_scs avails2 wanted rhs wanteds' }
1651 -- Temporarily do addIrred *before* the reduceList,
1652 -- which has the effect of adding the thing we are trying
1653 -- to prove to the database before trying to prove the things it
1654 -- needs. See note [RECURSIVE DICTIONARIES]
1655 -- NB: we must not do an addWanted before, because that adds the
1656 -- superclasses too, and thaat can lead to a spurious loop; see
1657 -- the examples in [SUPERCLASS-LOOP]
1658 -- So we do an addIrred before, and then overwrite it afterwards with addWanted
1662 -- First, see if the inst can be reduced to a constant in one step
1663 -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
1664 -- Don't bother for implication constraints, which take real work
1665 try_simple do_this_otherwise
1666 = do { res <- lookupSimpleInst wanted
1668 GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
1669 other -> do_this_otherwise avails wanted }
1673 Note [SUPERCLASS-LOOP 2]
1674 ~~~~~~~~~~~~~~~~~~~~~~~~
1675 But the above isn't enough. Suppose we are *given* d1:Ord a,
1676 and want to deduce (d2:C [a]) where
1678 class Ord a => C a where
1679 instance Ord [a] => C [a] where ...
1681 Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
1682 superclasses of C [a] to avails. But we must not overwrite the binding
1683 for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
1686 Here's another variant, immortalised in tcrun020
1687 class Monad m => C1 m
1688 class C1 m => C2 m x
1689 instance C2 Maybe Bool
1690 For the instance decl we need to build (C1 Maybe), and it's no good if
1691 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1692 before we search for C1 Maybe.
1694 Here's another example
1695 class Eq b => Foo a b
1696 instance Eq a => Foo [a] a
1700 we'll first deduce that it holds (via the instance decl). We must not
1701 then overwrite the Eq t constraint with a superclass selection!
1703 At first I had a gross hack, whereby I simply did not add superclass constraints
1704 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1705 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1706 I found a very obscure program (now tcrun021) in which improvement meant the
1707 simplifier got two bites a the cherry... so something seemed to be an Stop
1708 first time, but reducible next time.
1710 Now we implement the Right Solution, which is to check for loops directly
1711 when adding superclasses. It's a bit like the occurs check in unification.
1714 Note [RECURSIVE DICTIONARIES]
1715 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1717 data D r = ZeroD | SuccD (r (D r));
1719 instance (Eq (r (D r))) => Eq (D r) where
1720 ZeroD == ZeroD = True
1721 (SuccD a) == (SuccD b) = a == b
1724 equalDC :: D [] -> D [] -> Bool;
1727 We need to prove (Eq (D [])). Here's how we go:
1731 by instance decl, holds if
1735 by instance decl of Eq, holds if
1737 where d2 = dfEqList d3
1740 But now we can "tie the knot" to give
1746 and it'll even run! The trick is to put the thing we are trying to prove
1747 (in this case Eq (D []) into the database before trying to prove its
1748 contributing clauses.
1751 %************************************************************************
1753 Reducing a single constraint
1755 %************************************************************************
1758 ---------------------------------------------
1759 reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
1760 reduceInst env avails (ImplicInst { tci_tyvars = tvs, tci_reft = reft, tci_loc = loc,
1761 tci_given = extra_givens, tci_wanted = wanteds })
1762 = reduceImplication env avails reft tvs extra_givens wanteds loc
1764 reduceInst env avails other_inst
1765 = do { result <- lookupSimpleInst other_inst
1766 ; return (avails, result) }
1770 ---------------------------------------------
1771 reduceImplication :: RedEnv
1773 -> Refinement -- May refine the givens; often empty
1774 -> [TcTyVar] -- Quantified type variables; all skolems
1775 -> [Inst] -- Extra givens; all rigid
1778 -> TcM (Avails, LookupInstResult)
1781 Suppose we are simplifying the constraint
1782 forall bs. extras => wanted
1783 in the context of an overall simplification problem with givens 'givens',
1784 and refinment 'reft'.
1787 * The refinement is often empty
1789 * The 'extra givens' need not mention any of the quantified type variables
1790 e.g. forall {}. Eq a => Eq [a]
1791 forall {}. C Int => D (Tree Int)
1793 This happens when you have something like
1795 T1 :: Eq a => a -> T a
1798 f x = ...(case x of { T1 v -> v==v })...
1801 -- ToDo: should we instantiate tvs? I think it's not necessary
1803 -- ToDo: what about improvement? There may be some improvement
1804 -- exposed as a result of the simplifications done by reduceList
1805 -- which are discarded if we back off.
1806 -- This is almost certainly Wrong, but we'll fix it when dealing
1807 -- better with equality constraints
1808 reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc
1809 = do { -- Add refined givens, and the extra givens
1810 (refined_red_givens, avails)
1811 <- if isEmptyRefinement reft then return (red_givens env, orig_avails)
1812 else foldlM (addRefinedGiven reft) ([], orig_avails) (red_givens env)
1813 ; avails <- foldlM addGiven avails extra_givens
1815 -- Solve the sub-problem
1816 ; let try_me inst = ReduceMe AddSCs -- Note [Freeness and implications]
1817 env' = env { red_givens = refined_red_givens ++ extra_givens
1818 , red_try_me = try_me }
1820 ; traceTc (text "reduceImplication" <+> vcat
1822 ppr (red_givens env), ppr extra_givens,
1823 ppr reft, ppr wanteds, ppr avails ])
1824 ; avails <- reduceList env' wanteds avails
1826 -- Extract the binding
1827 ; (binds, irreds) <- extractResults avails wanteds
1829 -- We always discard the extra avails we've generated;
1830 -- but we remember if we have done any (global) improvement
1831 ; let ret_avails = updateImprovement orig_avails avails
1833 ; if isEmptyLHsBinds binds then -- No progress
1834 return (ret_avails, NoInstance)
1836 { (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds
1837 -- This binding is useless if the recursive simplification
1838 -- made no progress; but currently we don't try to optimise that
1839 -- case. After all, we only try hard to reduce at top level, or
1840 -- when inferring types.
1842 ; let dict_ids = map instToId extra_givens
1843 co = mkWpTyLams tvs <.> mkWpLams dict_ids <.> WpLet (binds `unionBags` bind)
1844 rhs = mkHsWrap co payload
1845 loc = instLocSpan inst_loc
1846 payload | isSingleton wanteds = HsVar (instToId (head wanteds))
1847 | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) wanteds) Boxed
1849 -- If there are any irreds, we back off and return NoInstance
1850 ; return (ret_avails, GenInst implic_insts (L loc rhs))
1854 Note [Freeness and implications]
1855 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1856 It's hard to say when an implication constraint can be floated out. Consider
1857 forall {} Eq a => Foo [a]
1858 The (Foo [a]) doesn't mention any of the quantified variables, but it
1859 still might be partially satisfied by the (Eq a).
1861 There is a useful special case when it *is* easy to partition the
1862 constraints, namely when there are no 'givens'. Consider
1863 forall {a}. () => Bar b
1864 There are no 'givens', and so there is no reason to capture (Bar b).
1865 We can let it float out. But if there is even one constraint we
1866 must be much more careful:
1867 forall {a}. C a b => Bar (m b)
1868 because (C a b) might have a superclass (D b), from which we might
1869 deduce (Bar [b]) when m later gets instantiated to []. Ha!
1871 Here is an even more exotic example
1873 Now consider the constraint
1874 forall b. D Int b => C Int
1875 We can satisfy the (C Int) from the superclass of D, so we don't want
1876 to float the (C Int) out, even though it mentions no type variable in
1879 %************************************************************************
1881 Avails and AvailHow: the pool of evidence
1883 %************************************************************************
1887 data Avails = Avails !ImprovementDone !AvailEnv
1889 type ImprovementDone = Bool -- True <=> some unification has happened
1890 -- so some Irreds might now be reducible
1891 -- keys that are now
1893 type AvailEnv = FiniteMap Inst AvailHow
1895 = IsIrred -- Used for irreducible dictionaries,
1896 -- which are going to be lambda bound
1898 | Given TcId -- Used for dictionaries for which we have a binding
1899 -- e.g. those "given" in a signature
1901 | Rhs -- Used when there is a RHS
1902 (LHsExpr TcId) -- The RHS
1903 [Inst] -- Insts free in the RHS; we need these too
1905 instance Outputable Avails where
1908 pprAvails (Avails imp avails)
1909 = vcat [ ptext SLIT("Avails") <> (if imp then ptext SLIT("[improved]") else empty)
1910 , nest 2 (vcat [sep [ppr inst, nest 2 (equals <+> ppr avail)]
1911 | (inst,avail) <- fmToList avails ])]
1913 instance Outputable AvailHow where
1916 -------------------------
1917 pprAvail :: AvailHow -> SDoc
1918 pprAvail IsIrred = text "Irred"
1919 pprAvail (Given x) = text "Given" <+> ppr x
1920 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1922 -------------------------
1923 extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
1924 extendAvailEnv env inst avail = addToFM env inst avail
1926 findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
1927 findAvailEnv env wanted = lookupFM env wanted
1928 -- NB 1: the Ord instance of Inst compares by the class/type info
1929 -- *not* by unique. So
1930 -- d1::C Int == d2::C Int
1932 emptyAvails :: Avails
1933 emptyAvails = Avails False emptyFM
1935 findAvail :: Avails -> Inst -> Maybe AvailHow
1936 findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
1938 elemAvails :: Inst -> Avails -> Bool
1939 elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
1941 extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
1943 extendAvails avails@(Avails imp env) inst avail
1944 = do { imp1 <- tcImproveOne avails inst -- Do any improvement
1945 ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
1947 availsInsts :: Avails -> [Inst]
1948 availsInsts (Avails _ avails) = keysFM avails
1950 availsImproved (Avails imp _) = imp
1952 updateImprovement :: Avails -> Avails -> Avails
1953 -- (updateImprovement a1 a2) sets a1's improvement flag from a2
1954 updateImprovement (Avails _ avails1) (Avails imp2 _) = Avails imp2 avails1
1957 Extracting the bindings from a bunch of Avails.
1958 The bindings do *not* come back sorted in dependency order.
1959 We assume that they'll be wrapped in a big Rec, so that the
1960 dependency analyser can sort them out later
1963 extractResults :: Avails
1965 -> TcM ( TcDictBinds, -- Bindings
1966 [Inst]) -- Irreducible ones
1968 extractResults (Avails _ avails) wanteds
1969 = go avails emptyBag [] wanteds
1971 go :: AvailEnv -> TcDictBinds -> [Inst] -> [Inst]
1972 -> TcM (TcDictBinds, [Inst])
1973 go avails binds irreds []
1974 = returnM (binds, irreds)
1976 go avails binds irreds (w:ws)
1977 = case findAvailEnv avails w of
1978 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1979 go avails binds irreds ws
1981 Just IsIrred -> go (add_given avails w) binds (w:irreds) ws
1985 -> go avails binds irreds ws
1986 -- The sought Id can be one of the givens, via a superclass chain
1987 -- and then we definitely don't want to generate an x=x binding!
1990 -> go avails (addBind binds w (nlHsVar id)) irreds ws
1992 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds (ws' ++ ws)
1994 new_binds = addBind binds w rhs
1996 add_given avails w = extendAvailEnv avails w (Given (instToId w))
1998 addBind binds inst rhs = binds `unionBags` unitBag (L (instSpan inst)
1999 (VarBind (instToId inst) rhs))
2003 Note [No superclasses for Stop]
2004 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2005 When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
2006 add it to avails, so that any other equal Insts will be commoned up
2007 right here. However, we do *not* add superclasses. If we have
2010 but a is not bound here, then we *don't* want to derive dn from df
2011 here lest we lose sharing.
2014 addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
2015 addWanted want_scs avails wanted rhs_expr wanteds
2016 = addAvailAndSCs want_scs avails wanted avail
2018 avail = Rhs rhs_expr wanteds
2020 addGiven :: Avails -> Inst -> TcM Avails
2021 addGiven avails given = addAvailAndSCs AddSCs avails given (Given (instToId given))
2022 -- Always add superclasses for 'givens'
2024 -- No ASSERT( not (given `elemAvails` avails) ) because in an instance
2025 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
2026 -- so the assert isn't true
2028 addRefinedGiven :: Refinement -> ([Inst], Avails) -> Inst -> TcM ([Inst], Avails)
2029 addRefinedGiven reft (refined_givens, avails) given
2030 | isDict given -- We sometimes have 'given' methods, but they
2031 -- are always optional, so we can drop them
2032 , let pred = dictPred given
2033 , isRefineablePred pred -- See Note [ImplicInst rigidity]
2034 , Just (co, pred) <- refinePred reft pred
2035 = do { new_given <- newDictBndr (instLoc given) pred
2036 ; let rhs = L (instSpan given) $
2037 HsWrap (WpCo co) (HsVar (instToId given))
2038 ; avails <- addAvailAndSCs AddSCs avails new_given (Rhs rhs [given])
2039 ; return (new_given:refined_givens, avails) }
2040 -- ToDo: the superclasses of the original given all exist in Avails
2041 -- so we could really just cast them, but it's more awkward to do,
2042 -- and hopefully the optimiser will spot the duplicated work
2044 = return (refined_givens, avails)
2047 Note [ImplicInst rigidity]
2048 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2050 C :: forall ab. (Eq a, Ord b) => b -> T a
2052 ...(case x of C v -> <body>)...
2054 From the case (where x::T ty) we'll get an implication constraint
2055 forall b. (Eq ty, Ord b) => <body-constraints>
2056 Now suppose <body-constraints> itself has an implication constraint
2058 forall c. <reft> => <payload>
2059 Then, we can certainly apply the refinement <reft> to the Ord b, becuase it is
2060 existential, but we probably should not apply it to the (Eq ty) because it may
2061 be wobbly. Hence the isRigidInst
2063 @Insts@ are ordered by their class/type info, rather than by their
2064 unique. This allows the context-reduction mechanism to use standard finite
2065 maps to do their stuff. It's horrible that this code is here, rather
2066 than with the Avails handling stuff in TcSimplify
2069 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
2070 addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
2071 addAvailAndSCs want_scs avails irred IsIrred
2073 addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
2074 addAvailAndSCs want_scs avails inst avail
2075 | not (isClassDict inst) = extendAvails avails inst avail
2076 | NoSCs <- want_scs = extendAvails avails inst avail
2077 | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
2078 ; avails' <- extendAvails avails inst avail
2079 ; addSCs is_loop avails' inst }
2081 is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
2082 -- Note: this compares by *type*, not by Unique
2083 deps = findAllDeps (unitVarSet (instToId inst)) avail
2084 dep_tys = map idType (varSetElems deps)
2086 findAllDeps :: IdSet -> AvailHow -> IdSet
2087 -- Find all the Insts that this one depends on
2088 -- See Note [SUPERCLASS-LOOP 2]
2089 -- Watch out, though. Since the avails may contain loops
2090 -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
2091 findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
2092 findAllDeps so_far other = so_far
2094 find_all :: IdSet -> Inst -> IdSet
2096 | kid_id `elemVarSet` so_far = so_far
2097 | Just avail <- findAvail avails kid = findAllDeps so_far' avail
2098 | otherwise = so_far'
2100 so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
2101 kid_id = instToId kid
2103 addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
2104 -- Add all the superclasses of the Inst to Avails
2105 -- The first param says "dont do this because the original thing
2106 -- depends on this one, so you'd build a loop"
2107 -- Invariant: the Inst is already in Avails.
2109 addSCs is_loop avails dict
2110 = ASSERT( isDict dict )
2111 do { sc_dicts <- newDictBndrs (instLoc dict) sc_theta'
2112 ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
2114 (clas, tys) = getDictClassTys dict
2115 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
2116 sc_theta' = substTheta (zipTopTvSubst tyvars tys) sc_theta
2118 add_sc avails (sc_dict, sc_sel)
2119 | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
2120 | is_given sc_dict = return avails
2121 | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
2122 ; addSCs is_loop avails' sc_dict }
2124 sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
2125 co_fn = WpApp (instToId dict) <.> mkWpTyApps tys
2127 is_given :: Inst -> Bool
2128 is_given sc_dict = case findAvail avails sc_dict of
2129 Just (Given _) -> True -- Given is cheaper than superclass selection
2133 %************************************************************************
2135 \section{tcSimplifyTop: defaulting}
2137 %************************************************************************
2140 @tcSimplifyTop@ is called once per module to simplify all the constant
2141 and ambiguous Insts.
2143 We need to be careful of one case. Suppose we have
2145 instance Num a => Num (Foo a b) where ...
2147 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
2148 to (Num x), and default x to Int. But what about y??
2150 It's OK: the final zonking stage should zap y to (), which is fine.
2154 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
2155 tcSimplifyTop wanteds
2156 = tc_simplify_top doc False wanteds
2158 doc = text "tcSimplifyTop"
2160 tcSimplifyInteractive wanteds
2161 = tc_simplify_top doc True wanteds
2163 doc = text "tcSimplifyInteractive"
2165 -- The TcLclEnv should be valid here, solely to improve
2166 -- error message generation for the monomorphism restriction
2167 tc_simplify_top doc interactive wanteds
2168 = do { wanteds <- mapM zonkInst wanteds
2169 ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
2171 ; (irreds1, binds1) <- topCheckLoop doc wanteds
2173 ; if null irreds1 then
2176 -- OK, so there are some errors
2177 { -- Use the defaulting rules to do extra unification
2178 -- NB: irreds are already zonked
2179 ; extended_default <- if interactive then return True
2180 else doptM Opt_ExtendedDefaultRules
2181 ; disambiguate extended_default irreds1 -- Does unification
2182 ; (irreds2, binds2) <- topCheckLoop doc irreds1
2184 -- Deal with implicit parameter
2185 ; let (bad_ips, non_ips) = partition isIPDict irreds2
2186 (ambigs, others) = partition isTyVarDict non_ips
2188 ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
2190 ; addNoInstanceErrs others
2191 ; addTopAmbigErrs ambigs
2193 ; return (binds1 `unionBags` binds2) }}
2196 If a dictionary constrains a type variable which is
2197 * not mentioned in the environment
2198 * and not mentioned in the type of the expression
2199 then it is ambiguous. No further information will arise to instantiate
2200 the type variable; nor will it be generalised and turned into an extra
2201 parameter to a function.
2203 It is an error for this to occur, except that Haskell provided for
2204 certain rules to be applied in the special case of numeric types.
2206 * at least one of its classes is a numeric class, and
2207 * all of its classes are numeric or standard
2208 then the type variable can be defaulted to the first type in the
2209 default-type list which is an instance of all the offending classes.
2211 So here is the function which does the work. It takes the ambiguous
2212 dictionaries and either resolves them (producing bindings) or
2213 complains. It works by splitting the dictionary list by type
2214 variable, and using @disambigOne@ to do the real business.
2216 @disambigOne@ assumes that its arguments dictionaries constrain all
2217 the same type variable.
2219 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
2220 @()@ instead of @Int@. I reckon this is the Right Thing to do since
2221 the most common use of defaulting is code like:
2223 _ccall_ foo `seqPrimIO` bar
2225 Since we're not using the result of @foo@, the result if (presumably)
2229 disambiguate :: Bool -> [Inst] -> TcM ()
2230 -- Just does unification to fix the default types
2231 -- The Insts are assumed to be pre-zonked
2232 disambiguate extended_defaulting insts
2233 | null defaultable_groups
2234 = do { traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2237 = do { -- Figure out what default types to use
2238 mb_defaults <- getDefaultTys
2239 ; default_tys <- case mb_defaults of
2240 Just tys -> return tys
2241 Nothing -> -- No use-supplied default;
2242 -- use [Integer, Double]
2243 do { integer_ty <- tcMetaTy integerTyConName
2244 ; checkWiredInTyCon doubleTyCon
2245 ; return [integer_ty, doubleTy] }
2246 ; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
2247 ; mapM_ (disambigGroup default_tys) defaultable_groups }
2249 unaries :: [(Inst,Class, TcTyVar)] -- (C tv) constraints
2250 bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
2251 (unaries, bad_tvs) = getDefaultableDicts insts
2253 -- Group by type variable
2254 defaultable_groups :: [[(Inst,Class,TcTyVar)]]
2255 defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
2256 cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
2258 defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
2259 defaultable_group ds@((_,_,tv):_)
2260 = not (isImmutableTyVar tv) -- Note [Avoiding spurious errors]
2261 && not (tv `elemVarSet` bad_tvs)
2262 && defaultable_classes [c | (_,c,_) <- ds]
2263 defaultable_group [] = panic "defaultable_group"
2265 defaultable_classes clss
2266 | extended_defaulting = any isInteractiveClass clss
2267 | otherwise = all isStandardClass clss && any isNumericClass clss
2269 -- In interactive mode, or with -fextended-default-rules,
2270 -- we default Show a to Show () to avoid graututious errors on "show []"
2271 isInteractiveClass cls
2272 = isNumericClass cls
2273 || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
2276 disambigGroup :: [Type] -- The default types
2277 -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
2278 -> TcM () -- Just does unification, to fix the default types
2280 disambigGroup default_tys dicts
2281 = try_default default_tys
2283 (_,_,tyvar) = head dicts -- Should be non-empty
2284 classes = [c | (_,c,_) <- dicts]
2286 try_default [] = return ()
2287 try_default (default_ty : default_tys)
2288 = tryTcLIE_ (try_default default_tys) $
2289 do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
2290 -- This may fail; then the tryTcLIE_ kicks in
2291 -- Failure here is caused by there being no type in the
2292 -- default list which can satisfy all the ambiguous classes.
2293 -- For example, if Real a is reqd, but the only type in the
2294 -- default list is Int.
2296 -- After this we can't fail
2297 ; warnDefault dicts default_ty
2298 ; unifyType default_ty (mkTyVarTy tyvar) }
2301 Note [Avoiding spurious errors]
2302 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2303 When doing the unification for defaulting, we check for skolem
2304 type variables, and simply don't default them. For example:
2305 f = (*) -- Monomorphic
2306 g :: Num a => a -> a
2308 Here, we get a complaint when checking the type signature for g,
2309 that g isn't polymorphic enough; but then we get another one when
2310 dealing with the (Num a) context arising from f's definition;
2311 we try to unify a with Int (to default it), but find that it's
2312 already been unified with the rigid variable from g's type sig
2315 %************************************************************************
2317 \subsection[simple]{@Simple@ versions}
2319 %************************************************************************
2321 Much simpler versions when there are no bindings to make!
2323 @tcSimplifyThetas@ simplifies class-type constraints formed by
2324 @deriving@ declarations and when specialising instances. We are
2325 only interested in the simplified bunch of class/type constraints.
2327 It simplifies to constraints of the form (C a b c) where
2328 a,b,c are type variables. This is required for the context of
2329 instance declarations.
2332 tcSimplifyDeriv :: InstOrigin
2335 -> ThetaType -- Wanted
2336 -> TcM ThetaType -- Needed
2338 tcSimplifyDeriv orig tc tyvars theta
2339 = tcInstTyVars tyvars `thenM` \ (tvs, _, tenv) ->
2340 -- The main loop may do unification, and that may crash if
2341 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
2342 -- ToDo: what if two of them do get unified?
2343 newDictBndrsO orig (substTheta tenv theta) `thenM` \ wanteds ->
2344 topCheckLoop doc wanteds `thenM` \ (irreds, _) ->
2346 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
2347 doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
2349 inst_ty = mkTyConApp tc (mkTyVarTys tvs)
2350 (ok_insts, bad_insts) = partition is_ok_inst irreds
2352 = isDict inst -- Exclude implication consraints
2353 && (isTyVarClassPred pred || (gla_exts && ok_gla_pred pred))
2355 pred = dictPred inst
2357 ok_gla_pred pred = null (checkInstTermination [inst_ty] [pred])
2358 -- See Note [Deriving context]
2360 tv_set = mkVarSet tvs
2361 simpl_theta = map dictPred ok_insts
2362 weird_preds = [pred | pred <- simpl_theta
2363 , not (tyVarsOfPred pred `subVarSet` tv_set)]
2365 -- Check for a bizarre corner case, when the derived instance decl should
2366 -- have form instance C a b => D (T a) where ...
2367 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
2368 -- of problems; in particular, it's hard to compare solutions for
2369 -- equality when finding the fixpoint. So I just rule it out for now.
2371 rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
2372 -- This reverse-mapping is a Royal Pain,
2373 -- but the result should mention TyVars not TcTyVars
2375 -- In effect, the bad and wierd insts cover all of the cases that
2376 -- would make checkValidInstance fail; if it were called right after tcSimplifyDeriv
2377 -- * wierd_preds ensures unambiguous instances (checkAmbiguity in checkValidInstance)
2378 -- * ok_gla_pred ensures termination (checkInstTermination in checkValidInstance)
2379 addNoInstanceErrs bad_insts `thenM_`
2380 mapM_ (addErrTc . badDerivedPred) weird_preds `thenM_`
2381 returnM (substTheta rev_env simpl_theta)
2383 doc = ptext SLIT("deriving classes for a data type")
2386 Note [Deriving context]
2387 ~~~~~~~~~~~~~~~~~~~~~~~
2388 With -fglasgow-exts, we allow things like (C Int a) in the simplified
2389 context for a derived instance declaration, because at a use of this
2390 instance, we might know that a=Bool, and have an instance for (C Int
2393 We nevertheless insist that each predicate meets the termination
2394 conditions. If not, the deriving mechanism generates larger and larger
2395 constraints. Example:
2397 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
2399 Note the lack of a Show instance for Succ. First we'll generate
2400 instance (Show (Succ a), Show a) => Show (Seq a)
2402 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
2403 and so on. Instead we want to complain of no instance for (Show (Succ a)).
2407 @tcSimplifyDefault@ just checks class-type constraints, essentially;
2408 used with \tr{default} declarations. We are only interested in
2409 whether it worked or not.
2412 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
2415 tcSimplifyDefault theta
2416 = newDictBndrsO DefaultOrigin theta `thenM` \ wanteds ->
2417 topCheckLoop doc wanteds `thenM` \ (irreds, _) ->
2418 addNoInstanceErrs irreds `thenM_`
2424 doc = ptext SLIT("default declaration")
2428 %************************************************************************
2430 \section{Errors and contexts}
2432 %************************************************************************
2434 ToDo: for these error messages, should we note the location as coming
2435 from the insts, or just whatever seems to be around in the monad just
2439 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
2440 -> [Inst] -- The offending Insts
2442 -- Group together insts with the same origin
2443 -- We want to report them together in error messages
2445 groupErrs report_err []
2447 groupErrs report_err (inst:insts)
2448 = do_one (inst:friends) `thenM_`
2449 groupErrs report_err others
2452 -- (It may seem a bit crude to compare the error messages,
2453 -- but it makes sure that we combine just what the user sees,
2454 -- and it avoids need equality on InstLocs.)
2455 (friends, others) = partition is_friend insts
2456 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2457 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2458 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2459 -- Add location and context information derived from the Insts
2461 -- Add the "arising from..." part to a message about bunch of dicts
2462 addInstLoc :: [Inst] -> Message -> Message
2463 addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
2465 addTopIPErrs :: [Name] -> [Inst] -> TcM ()
2466 addTopIPErrs bndrs []
2468 addTopIPErrs bndrs ips
2469 = addErrTcM (tidy_env, mk_msg tidy_ips)
2471 (tidy_env, tidy_ips) = tidyInsts ips
2472 mk_msg ips = vcat [sep [ptext SLIT("Implicit parameters escape from"),
2473 nest 2 (ptext SLIT("the monomorphic top-level binding")
2474 <> plural bndrs <+> ptext SLIT("of")
2475 <+> pprBinders bndrs <> colon)],
2476 nest 2 (vcat (map ppr_ip ips)),
2478 ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
2480 topIPErrs :: [Inst] -> TcM ()
2482 = groupErrs report tidy_dicts
2484 (tidy_env, tidy_dicts) = tidyInsts dicts
2485 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2486 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2487 plural tidy_dicts <+> pprDictsTheta tidy_dicts)
2489 addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
2491 addNoInstanceErrs insts
2492 = do { let (tidy_env, tidy_insts) = tidyInsts insts
2493 ; reportNoInstances tidy_env Nothing tidy_insts }
2497 -> Maybe (InstLoc, [Inst]) -- Context
2498 -- Nothing => top level
2499 -- Just (d,g) => d describes the construct
2501 -> [Inst] -- What is wanted (can include implications)
2504 reportNoInstances tidy_env mb_what insts
2505 = groupErrs (report_no_instances tidy_env mb_what) insts
2507 report_no_instances tidy_env mb_what insts
2508 = do { inst_envs <- tcGetInstEnvs
2509 ; let (implics, insts1) = partition isImplicInst insts
2510 (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
2511 ; traceTc (text "reportNoInstnces" <+> vcat
2512 [ppr implics, ppr insts1, ppr insts2])
2513 ; mapM_ complain_implic implics
2514 ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
2515 ; groupErrs complain_no_inst insts2 }
2517 complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
2519 complain_implic inst -- Recurse!
2520 = reportNoInstances tidy_env
2521 (Just (tci_loc inst, tci_given inst))
2524 check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
2525 -- Right msg => overlap message
2526 -- Left inst => no instance
2527 check_overlap inst_envs wanted
2528 | not (isClassDict wanted) = Left wanted
2530 = case lookupInstEnv inst_envs clas tys of
2531 -- The case of exactly one match and no unifiers means
2532 -- a successful lookup. That can't happen here, becuase
2533 -- dicts only end up here if they didn't match in Inst.lookupInst
2535 ([m],[]) -> pprPanic "reportNoInstance" (ppr wanted)
2537 ([], _) -> Left wanted -- No match
2538 res -> Right (mk_overlap_msg wanted res)
2540 (clas,tys) = getDictClassTys wanted
2542 mk_overlap_msg dict (matches, unifiers)
2543 = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for")
2544 <+> pprPred (dictPred dict))),
2545 sep [ptext SLIT("Matching instances") <> colon,
2546 nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
2547 ASSERT( not (null matches) )
2548 if not (isSingleton matches)
2549 then -- Two or more matches
2551 else -- One match, plus some unifiers
2552 ASSERT( not (null unifiers) )
2553 parens (vcat [ptext SLIT("The choice depends on the instantiation of") <+>
2554 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
2555 ptext SLIT("Use -fallow-incoherent-instances to use the first choice above")])]
2557 ispecs = [ispec | (_, ispec) <- matches]
2559 mk_no_inst_err insts
2560 | null insts = empty
2562 | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
2563 not (isEmptyVarSet (tyVarsOfInsts insts))
2564 = vcat [ addInstLoc insts $
2565 sep [ ptext SLIT("Could not deduce") <+> pprDictsTheta insts
2566 , nest 2 $ ptext SLIT("from the context") <+> pprDictsTheta givens]
2567 , show_fixes (fix1 loc : fixes2) ]
2569 | otherwise -- Top level
2570 = vcat [ addInstLoc insts $
2571 ptext SLIT("No instance") <> plural insts
2572 <+> ptext SLIT("for") <+> pprDictsTheta insts
2573 , show_fixes fixes2 ]
2576 fix1 loc = sep [ ptext SLIT("add") <+> pprDictsTheta insts
2577 <+> ptext SLIT("to the context of"),
2578 nest 2 (ppr (instLocOrigin loc)) ]
2579 -- I'm not sure it helps to add the location
2580 -- nest 2 (ptext SLIT("at") <+> ppr (instLocSpan loc)) ]
2582 fixes2 | null instance_dicts = []
2583 | otherwise = [sep [ptext SLIT("add an instance declaration for"),
2584 pprDictsTheta instance_dicts]]
2585 instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
2586 -- Insts for which it is worth suggesting an adding an instance declaration
2587 -- Exclude implicit parameters, and tyvar dicts
2589 show_fixes :: [SDoc] -> SDoc
2590 show_fixes [] = empty
2591 show_fixes (f:fs) = sep [ptext SLIT("Possible fix:"),
2592 nest 2 (vcat (f : map (ptext SLIT("or") <+>) fs))]
2594 addTopAmbigErrs dicts
2595 -- Divide into groups that share a common set of ambiguous tyvars
2596 = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
2597 -- See Note [Avoiding spurious errors]
2598 mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
2600 (tidy_env, tidy_dicts) = tidyInsts dicts
2602 tvs_of :: Inst -> [TcTyVar]
2603 tvs_of d = varSetElems (tyVarsOfInst d)
2604 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
2606 report :: [(Inst,[TcTyVar])] -> TcM ()
2607 report pairs@((inst,tvs) : _) -- The pairs share a common set of ambiguous tyvars
2608 = mkMonomorphismMsg tidy_env tvs `thenM` \ (tidy_env, mono_msg) ->
2609 setSrcSpan (instSpan inst) $
2610 -- the location of the first one will do for the err message
2611 addErrTcM (tidy_env, msg $$ mono_msg)
2613 dicts = map fst pairs
2614 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
2615 pprQuotedList tvs <+> in_msg,
2616 nest 2 (pprDictsInFull dicts)]
2617 in_msg = text "in the constraint" <> plural dicts <> colon
2618 report [] = panic "addTopAmbigErrs"
2621 mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
2622 -- There's an error with these Insts; if they have free type variables
2623 -- it's probably caused by the monomorphism restriction.
2624 -- Try to identify the offending variable
2625 -- ASSUMPTION: the Insts are fully zonked
2626 mkMonomorphismMsg tidy_env inst_tvs
2627 = findGlobals (mkVarSet inst_tvs) tidy_env `thenM` \ (tidy_env, docs) ->
2628 returnM (tidy_env, mk_msg docs)
2630 mk_msg [] = ptext SLIT("Probable fix: add a type signature that fixes these type variable(s)")
2631 -- This happens in things like
2632 -- f x = show (read "foo")
2633 -- where monomorphism doesn't play any role
2634 mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
2638 monomorphism_fix :: SDoc
2639 monomorphism_fix = ptext SLIT("Probable fix:") <+>
2640 (ptext SLIT("give these definition(s) an explicit type signature")
2641 $$ ptext SLIT("or use -fno-monomorphism-restriction"))
2643 warnDefault ups default_ty
2644 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
2645 addInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
2647 dicts = [d | (d,_,_) <- ups]
2650 (_, tidy_dicts) = tidyInsts dicts
2651 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
2652 quotes (ppr default_ty),
2653 pprDictsInFull tidy_dicts]
2655 -- Used for the ...Thetas variants; all top level
2657 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2658 ptext SLIT("type variables that are not data type parameters"),
2659 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2661 reduceDepthErr n stack
2662 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2663 ptext SLIT("Use -fcontext-stack=N to increase stack size to N"),
2664 nest 4 (pprStack stack)]
2666 pprStack stack = vcat (map pprInstInFull stack)