2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcSimplify]{TcSimplify}
10 tcSimplifyInfer, tcSimplifyInferCheck,
11 tcSimplifyCheck, tcSimplifyRestricted,
12 tcSimplifyToDicts, tcSimplifyIPs,
13 tcSimplifyTop, tcSimplifyInteractive,
16 tcSimplifyDeriv, tcSimplifyDefault,
20 #include "HsVersions.h"
22 import {-# SOURCE #-} TcUnify( unifyTauTy )
24 import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
25 import TcHsSyn ( TcExpr, TcId,
26 TcMonoBinds, TcDictBinds
30 import Inst ( lookupInst, LookupInstResult(..),
31 tyVarsOfInst, fdPredsOfInsts, fdPredsOfInst, newDicts,
32 isDict, isClassDict, isLinearInst, linearInstType,
33 isStdClassTyVarDict, isMethodFor, isMethod,
34 instToId, tyVarsOfInsts, cloneDict,
35 ipNamesOfInsts, ipNamesOfInst, dictPred,
37 newDictsFromOld, tcInstClassOp,
38 getDictClassTys, isTyVarDict,
39 instLoc, zonkInst, tidyInsts, tidyMoreInsts,
40 Inst, pprInsts, pprInstsInFull, tcGetInstEnvs,
41 isIPDict, isInheritableInst, pprDFuns
43 import TcEnv ( tcGetGlobalTyVars, tcLookupId, findGlobals )
44 import InstEnv ( lookupInstEnv, classInstEnv )
45 import TcMType ( zonkTcTyVarsAndFV, tcInstTyVars, checkAmbiguity )
46 import TcType ( TcTyVar, TcTyVarSet, ThetaType, TyVarDetails(VanillaTv),
47 mkClassPred, isOverloadedTy, mkTyConApp,
48 mkTyVarTy, tcGetTyVar, isTyVarClassPred, mkTyVarTys,
50 import Id ( idType, mkUserLocal )
52 import Name ( getOccName, getSrcLoc )
53 import NameSet ( NameSet, mkNameSet, elemNameSet )
54 import Class ( classBigSig, classKey )
55 import FunDeps ( oclose, grow, improve, pprEquationDoc )
56 import PrelInfo ( isNumericClass )
57 import PrelNames ( splitName, fstName, sndName, integerTyConName,
58 showClassKey, eqClassKey, ordClassKey )
59 import Subst ( mkTopTyVarSubst, substTheta, substTy )
60 import TysWiredIn ( pairTyCon, doubleTy )
61 import ErrUtils ( Message )
63 import VarEnv ( TidyEnv )
66 import ListSetOps ( equivClasses )
67 import Util ( zipEqual, isSingleton )
68 import List ( partition )
73 %************************************************************************
77 %************************************************************************
79 --------------------------------------
80 Notes on quantification
81 --------------------------------------
83 Suppose we are about to do a generalisation step.
88 C the constraints from that RHS
90 The game is to figure out
92 Q the set of type variables over which to quantify
93 Ct the constraints we will *not* quantify over
94 Cq the constraints we will quantify over
96 So we're going to infer the type
100 and float the constraints Ct further outwards.
102 Here are the things that *must* be true:
104 (A) Q intersect fv(G) = EMPTY limits how big Q can be
105 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
107 (A) says we can't quantify over a variable that's free in the
108 environment. (B) says we must quantify over all the truly free
109 variables in T, else we won't get a sufficiently general type. We do
110 not *need* to quantify over any variable that is fixed by the free
111 vars of the environment G.
113 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
115 Example: class H x y | x->y where ...
117 fv(G) = {a} C = {H a b, H c d}
120 (A) Q intersect {a} is empty
121 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
123 So Q can be {c,d}, {b,c,d}
125 Other things being equal, however, we'd like to quantify over as few
126 variables as possible: smaller types, fewer type applications, more
127 constraints can get into Ct instead of Cq.
130 -----------------------------------------
133 fv(T) the free type vars of T
135 oclose(vs,C) The result of extending the set of tyvars vs
136 using the functional dependencies from C
138 grow(vs,C) The result of extend the set of tyvars vs
139 using all conceivable links from C.
141 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
142 Then grow(vs,C) = {a,b,c}
144 Note that grow(vs,C) `superset` grow(vs,simplify(C))
145 That is, simplfication can only shrink the result of grow.
148 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
149 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
152 -----------------------------------------
156 Here's a good way to choose Q:
158 Q = grow( fv(T), C ) \ oclose( fv(G), C )
160 That is, quantify over all variable that that MIGHT be fixed by the
161 call site (which influences T), but which aren't DEFINITELY fixed by
162 G. This choice definitely quantifies over enough type variables,
163 albeit perhaps too many.
165 Why grow( fv(T), C ) rather than fv(T)? Consider
167 class H x y | x->y where ...
172 If we used fv(T) = {c} we'd get the type
174 forall c. H c d => c -> b
176 And then if the fn was called at several different c's, each of
177 which fixed d differently, we'd get a unification error, because
178 d isn't quantified. Solution: quantify d. So we must quantify
179 everything that might be influenced by c.
181 Why not oclose( fv(T), C )? Because we might not be able to see
182 all the functional dependencies yet:
184 class H x y | x->y where ...
185 instance H x y => Eq (T x y) where ...
190 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
191 apparent yet, and that's wrong. We must really quantify over d too.
194 There really isn't any point in quantifying over any more than
195 grow( fv(T), C ), because the call sites can't possibly influence
196 any other type variables.
200 --------------------------------------
202 --------------------------------------
204 It's very hard to be certain when a type is ambiguous. Consider
208 instance H x y => K (x,y)
210 Is this type ambiguous?
211 forall a b. (K (a,b), Eq b) => a -> a
213 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
214 now we see that a fixes b. So we can't tell about ambiguity for sure
215 without doing a full simplification. And even that isn't possible if
216 the context has some free vars that may get unified. Urgle!
218 Here's another example: is this ambiguous?
219 forall a b. Eq (T b) => a -> a
220 Not if there's an insance decl (with no context)
221 instance Eq (T b) where ...
223 You may say of this example that we should use the instance decl right
224 away, but you can't always do that:
226 class J a b where ...
227 instance J Int b where ...
229 f :: forall a b. J a b => a -> a
231 (Notice: no functional dependency in J's class decl.)
232 Here f's type is perfectly fine, provided f is only called at Int.
233 It's premature to complain when meeting f's signature, or even
234 when inferring a type for f.
238 However, we don't *need* to report ambiguity right away. It'll always
239 show up at the call site.... and eventually at main, which needs special
240 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
242 So here's the plan. We WARN about probable ambiguity if
244 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
246 (all tested before quantification).
247 That is, all the type variables in Cq must be fixed by the the variables
248 in the environment, or by the variables in the type.
250 Notice that we union before calling oclose. Here's an example:
252 class J a b c | a b -> c
256 forall b c. (J a b c) => b -> b
258 Only if we union {a} from G with {b} from T before using oclose,
259 do we see that c is fixed.
261 It's a bit vague exactly which C we should use for this oclose call. If we
262 don't fix enough variables we might complain when we shouldn't (see
263 the above nasty example). Nothing will be perfect. That's why we can
264 only issue a warning.
267 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
269 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
271 then c is a "bubble"; there's no way it can ever improve, and it's
272 certainly ambiguous. UNLESS it is a constant (sigh). And what about
277 instance H x y => K (x,y)
279 Is this type ambiguous?
280 forall a b. (K (a,b), Eq b) => a -> a
282 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
283 is a "bubble" that's a set of constraints
285 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
287 Hence another idea. To decide Q start with fv(T) and grow it
288 by transitive closure in Cq (no functional dependencies involved).
289 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
290 The definitely-ambiguous can then float out, and get smashed at top level
291 (which squashes out the constants, like Eq (T a) above)
294 --------------------------------------
295 Notes on principal types
296 --------------------------------------
301 f x = let g y = op (y::Int) in True
303 Here the principal type of f is (forall a. a->a)
304 but we'll produce the non-principal type
305 f :: forall a. C Int => a -> a
308 --------------------------------------
309 Notes on implicit parameters
310 --------------------------------------
312 Question 1: can we "inherit" implicit parameters
313 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
318 where f is *not* a top-level binding.
319 From the RHS of f we'll get the constraint (?y::Int).
320 There are two types we might infer for f:
324 (so we get ?y from the context of f's definition), or
326 f :: (?y::Int) => Int -> Int
328 At first you might think the first was better, becuase then
329 ?y behaves like a free variable of the definition, rather than
330 having to be passed at each call site. But of course, the WHOLE
331 IDEA is that ?y should be passed at each call site (that's what
332 dynamic binding means) so we'd better infer the second.
334 BOTTOM LINE: when *inferring types* you *must* quantify
335 over implicit parameters. See the predicate isFreeWhenInferring.
338 Question 2: type signatures
339 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
340 BUT WATCH OUT: When you supply a type signature, we can't force you
341 to quantify over implicit parameters. For example:
345 This is perfectly reasonable. We do not want to insist on
347 (?x + 1) :: (?x::Int => Int)
349 That would be silly. Here, the definition site *is* the occurrence site,
350 so the above strictures don't apply. Hence the difference between
351 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
352 and tcSimplifyCheckBind (which does not).
354 What about when you supply a type signature for a binding?
355 Is it legal to give the following explicit, user type
356 signature to f, thus:
361 At first sight this seems reasonable, but it has the nasty property
362 that adding a type signature changes the dynamic semantics.
365 (let f x = (x::Int) + ?y
366 in (f 3, f 3 with ?y=5)) with ?y = 6
372 in (f 3, f 3 with ?y=5)) with ?y = 6
376 Indeed, simply inlining f (at the Haskell source level) would change the
379 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
380 semantics for a Haskell program without knowing its typing, so if you
381 change the typing you may change the semantics.
383 To make things consistent in all cases where we are *checking* against
384 a supplied signature (as opposed to inferring a type), we adopt the
387 a signature does not need to quantify over implicit params.
389 [This represents a (rather marginal) change of policy since GHC 5.02,
390 which *required* an explicit signature to quantify over all implicit
391 params for the reasons mentioned above.]
393 But that raises a new question. Consider
395 Given (signature) ?x::Int
396 Wanted (inferred) ?x::Int, ?y::Bool
398 Clearly we want to discharge the ?x and float the ?y out. But
399 what is the criterion that distinguishes them? Clearly it isn't
400 what free type variables they have. The Right Thing seems to be
401 to float a constraint that
402 neither mentions any of the quantified type variables
403 nor any of the quantified implicit parameters
405 See the predicate isFreeWhenChecking.
408 Question 3: monomorphism
409 ~~~~~~~~~~~~~~~~~~~~~~~~
410 There's a nasty corner case when the monomorphism restriction bites:
414 The argument above suggests that we *must* generalise
415 over the ?y parameter, to get
416 z :: (?y::Int) => Int,
417 but the monomorphism restriction says that we *must not*, giving
419 Why does the momomorphism restriction say this? Because if you have
421 let z = x + ?y in z+z
423 you might not expect the addition to be done twice --- but it will if
424 we follow the argument of Question 2 and generalise over ?y.
430 (A) Always generalise over implicit parameters
431 Bindings that fall under the monomorphism restriction can't
435 * Inlining remains valid
436 * No unexpected loss of sharing
437 * But simple bindings like
439 will be rejected, unless you add an explicit type signature
440 (to avoid the monomorphism restriction)
441 z :: (?y::Int) => Int
443 This seems unacceptable
445 (B) Monomorphism restriction "wins"
446 Bindings that fall under the monomorphism restriction can't
448 Always generalise over implicit parameters *except* for bindings
449 that fall under the monomorphism restriction
452 * Inlining isn't valid in general
453 * No unexpected loss of sharing
454 * Simple bindings like
456 accepted (get value of ?y from binding site)
458 (C) Always generalise over implicit parameters
459 Bindings that fall under the monomorphism restriction can't
460 be generalised, EXCEPT for implicit parameters
462 * Inlining remains valid
463 * Unexpected loss of sharing (from the extra generalisation)
464 * Simple bindings like
466 accepted (get value of ?y from occurrence sites)
471 None of these choices seems very satisfactory. But at least we should
472 decide which we want to do.
474 It's really not clear what is the Right Thing To Do. If you see
478 would you expect the value of ?y to be got from the *occurrence sites*
479 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
480 case of function definitions, the answer is clearly the former, but
481 less so in the case of non-fucntion definitions. On the other hand,
482 if we say that we get the value of ?y from the definition site of 'z',
483 then inlining 'z' might change the semantics of the program.
485 Choice (C) really says "the monomorphism restriction doesn't apply
486 to implicit parameters". Which is fine, but remember that every
487 innocent binding 'x = ...' that mentions an implicit parameter in
488 the RHS becomes a *function* of that parameter, called at each
489 use of 'x'. Now, the chances are that there are no intervening 'with'
490 clauses that bind ?y, so a decent compiler should common up all
491 those function calls. So I think I strongly favour (C). Indeed,
492 one could make a similar argument for abolishing the monomorphism
493 restriction altogether.
495 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
499 %************************************************************************
501 \subsection{tcSimplifyInfer}
503 %************************************************************************
505 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
507 1. Compute Q = grow( fvs(T), C )
509 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
510 predicates will end up in Ct; we deal with them at the top level
512 3. Try improvement, using functional dependencies
514 4. If Step 3 did any unification, repeat from step 1
515 (Unification can change the result of 'grow'.)
517 Note: we don't reduce dictionaries in step 2. For example, if we have
518 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
519 after step 2. However note that we may therefore quantify over more
520 type variables than we absolutely have to.
522 For the guts, we need a loop, that alternates context reduction and
523 improvement with unification. E.g. Suppose we have
525 class C x y | x->y where ...
527 and tcSimplify is called with:
529 Then improvement unifies a with b, giving
532 If we need to unify anything, we rattle round the whole thing all over
539 -> TcTyVarSet -- fv(T); type vars
541 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
542 TcDictBinds, -- Bindings
543 [TcId]) -- Dict Ids that must be bound here (zonked)
544 -- Any free (escaping) Insts are tossed into the environment
549 tcSimplifyInfer doc tau_tvs wanted_lie
550 = inferLoop doc (varSetElems tau_tvs)
551 wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
553 extendLIEs frees `thenM_`
554 returnM (qtvs, binds, map instToId irreds)
556 inferLoop doc tau_tvs wanteds
558 zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' ->
559 mappM zonkInst wanteds `thenM` \ wanteds' ->
560 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
562 preds = fdPredsOfInsts wanteds'
563 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
566 | isFreeWhenInferring qtvs inst = Free
567 | isClassDict inst = DontReduceUnlessConstant -- Dicts
568 | otherwise = ReduceMe -- Lits and Methods
570 traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds, ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_`
572 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
575 if no_improvement then
576 returnM (varSetElems qtvs, frees, binds, irreds)
578 -- If improvement did some unification, we go round again. There
579 -- are two subtleties:
580 -- a) We start again with irreds, not wanteds
581 -- Using an instance decl might have introduced a fresh type variable
582 -- which might have been unified, so we'd get an infinite loop
583 -- if we started again with wanteds! See example [LOOP]
585 -- b) It's also essential to re-process frees, because unification
586 -- might mean that a type variable that looked free isn't now.
588 -- Hence the (irreds ++ frees)
590 -- However, NOTICE that when we are done, we might have some bindings, but
591 -- the final qtvs might be empty. See [NO TYVARS] below.
593 inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) ->
594 returnM (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
599 class If b t e r | b t e -> r
602 class Lte a b c | a b -> c where lte :: a -> b -> c
604 instance (Lte a b l,If l b a c) => Max a b c
606 Wanted: Max Z (S x) y
608 Then we'll reduce using the Max instance to:
609 (Lte Z (S x) l, If l (S x) Z y)
610 and improve by binding l->T, after which we can do some reduction
611 on both the Lte and If constraints. What we *can't* do is start again
612 with (Max Z (S x) y)!
616 class Y a b | a -> b where
619 instance Y [[a]] a where
622 k :: X a -> X a -> X a
624 g :: Num a => [X a] -> [X a]
627 h ys = ys ++ map (k (y [[0]])) xs
629 The excitement comes when simplifying the bindings for h. Initially
630 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
631 From this we get t1:=:t2, but also various bindings. We can't forget
632 the bindings (because of [LOOP]), but in fact t1 is what g is
635 The net effect of [NO TYVARS]
638 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
639 isFreeWhenInferring qtvs inst
640 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
641 && isInheritableInst inst -- And no implicit parameter involved
642 -- (see "Notes on implicit parameters")
644 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
645 -> NameSet -- Quantified implicit parameters
647 isFreeWhenChecking qtvs ips inst
648 = isFreeWrtTyVars qtvs inst
649 && isFreeWrtIPs ips inst
651 isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
652 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
656 %************************************************************************
658 \subsection{tcSimplifyCheck}
660 %************************************************************************
662 @tcSimplifyCheck@ is used when we know exactly the set of variables
663 we are going to quantify over. For example, a class or instance declaration.
668 -> [TcTyVar] -- Quantify over these
671 -> TcM TcDictBinds -- Bindings
673 -- tcSimplifyCheck is used when checking expression type signatures,
674 -- class decls, instance decls etc.
676 -- NB: tcSimplifyCheck does not consult the
677 -- global type variables in the environment; so you don't
678 -- need to worry about setting them before calling tcSimplifyCheck
679 tcSimplifyCheck doc qtvs givens wanted_lie
680 = tcSimplCheck doc get_qtvs
681 givens wanted_lie `thenM` \ (qtvs', binds) ->
684 get_qtvs = zonkTcTyVarsAndFV qtvs
687 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
688 -- against, but we don't know the type variables over which we are going to quantify.
689 -- This happens when we have a type signature for a mutually recursive group
692 -> TcTyVarSet -- fv(T)
695 -> TcM ([TcTyVar], -- Variables over which to quantify
696 TcDictBinds) -- Bindings
698 tcSimplifyInferCheck doc tau_tvs givens wanted_lie
699 = tcSimplCheck doc get_qtvs givens wanted_lie
701 -- Figure out which type variables to quantify over
702 -- You might think it should just be the signature tyvars,
703 -- but in bizarre cases you can get extra ones
704 -- f :: forall a. Num a => a -> a
705 -- f x = fst (g (x, head [])) + 1
707 -- Here we infer g :: forall a b. a -> b -> (b,a)
708 -- We don't want g to be monomorphic in b just because
709 -- f isn't quantified over b.
710 all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
712 get_qtvs = zonkTcTyVarsAndFV all_tvs `thenM` \ all_tvs' ->
713 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
715 qtvs = all_tvs' `minusVarSet` gbl_tvs
716 -- We could close gbl_tvs, but its not necessary for
717 -- soundness, and it'll only affect which tyvars, not which
718 -- dictionaries, we quantify over
723 Here is the workhorse function for all three wrappers.
726 tcSimplCheck doc get_qtvs givens wanted_lie
727 = check_loop givens wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
729 -- Complain about any irreducible ones
730 mappM zonkInst given_dicts_and_ips `thenM` \ givens' ->
731 groupErrs (addNoInstanceErrs (Just doc) givens') irreds `thenM_`
734 extendLIEs frees `thenM_`
735 returnM (qtvs, binds)
738 given_dicts_and_ips = filter (not . isMethod) givens
739 -- For error reporting, filter out methods, which are
740 -- only added to the given set as an optimisation
742 ip_set = mkNameSet (ipNamesOfInsts givens)
744 check_loop givens wanteds
746 mappM zonkInst givens `thenM` \ givens' ->
747 mappM zonkInst wanteds `thenM` \ wanteds' ->
748 get_qtvs `thenM` \ qtvs' ->
752 -- When checking against a given signature we always reduce
753 -- until we find a match against something given, or can't reduce
754 try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
755 | otherwise = ReduceMe
757 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
760 if no_improvement then
761 returnM (varSetElems qtvs', frees, binds, irreds)
763 check_loop givens' (irreds ++ frees) `thenM` \ (qtvs', frees1, binds1, irreds1) ->
764 returnM (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1)
768 %************************************************************************
770 \subsection{tcSimplifyRestricted}
772 %************************************************************************
775 tcSimplifyRestricted -- Used for restricted binding groups
776 -- i.e. ones subject to the monomorphism restriction
778 -> TcTyVarSet -- Free in the type of the RHSs
779 -> [Inst] -- Free in the RHSs
780 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
781 TcDictBinds) -- Bindings
783 tcSimplifyRestricted doc tau_tvs wanteds
784 = -- First squash out all methods, to find the constrained tyvars
785 -- We can't just take the free vars of wanted_lie because that'll
786 -- have methods that may incidentally mention entirely unconstrained variables
787 -- e.g. a call to f :: Eq a => a -> b -> b
788 -- Here, b is unconstrained. A good example would be
790 -- We want to infer the polymorphic type
791 -- foo :: forall b. b -> b
793 -- 'reduceMe': Reduce as far as we can. Don't stop at
794 -- dicts; the idea is to get rid of as many type
795 -- variables as possible, and we don't want to stop
796 -- at (say) Monad (ST s), because that reduces
797 -- immediately, with no constraint on s.
798 simpleReduceLoop doc reduceMe wanteds `thenM` \ (foo_frees, foo_binds, constrained_dicts) ->
800 -- Next, figure out the tyvars we will quantify over
801 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
802 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
804 constrained_tvs = tyVarsOfInsts constrained_dicts
805 qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs)
806 `minusVarSet` constrained_tvs
808 traceTc (text "tcSimplifyRestricted" <+> vcat [
809 pprInsts wanteds, pprInsts foo_frees, pprInsts constrained_dicts,
811 ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_`
813 -- The first step may have squashed more methods than
814 -- necessary, so try again, this time knowing the exact
815 -- set of type variables to quantify over.
817 -- We quantify only over constraints that are captured by qtvs;
818 -- these will just be a subset of non-dicts. This in contrast
819 -- to normal inference (using isFreeWhenInferring) in which we quantify over
820 -- all *non-inheritable* constraints too. This implements choice
821 -- (B) under "implicit parameter and monomorphism" above.
823 -- Remember that we may need to do *some* simplification, to
824 -- (for example) squash {Monad (ST s)} into {}. It's not enough
825 -- just to float all constraints
826 restrict_loop doc qtvs wanteds
827 -- We still need a loop because improvement can take place
828 -- E.g. if we have (C (T a)) and the instance decl
829 -- instance D Int b => C (T a) where ...
830 -- and there's a functional dependency for D. Then we may improve
831 -- the tyep variable 'b'.
833 restrict_loop doc qtvs wanteds
834 = mappM zonkInst wanteds `thenM` \ wanteds' ->
835 zonkTcTyVarsAndFV (varSetElems qtvs) `thenM` \ qtvs' ->
837 try_me inst | isFreeWrtTyVars qtvs' inst = Free
838 | otherwise = ReduceMe
840 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
841 if no_improvement then
842 ASSERT( null irreds )
843 extendLIEs frees `thenM_`
844 returnM (varSetElems qtvs', binds)
846 restrict_loop doc qtvs' (irreds ++ frees) `thenM` \ (qtvs1, binds1) ->
847 returnM (qtvs1, binds `AndMonoBinds` binds1)
851 %************************************************************************
853 \subsection{tcSimplifyToDicts}
855 %************************************************************************
857 On the LHS of transformation rules we only simplify methods and constants,
858 getting dictionaries. We want to keep all of them unsimplified, to serve
859 as the available stuff for the RHS of the rule.
861 The same thing is used for specialise pragmas. Consider
864 {-# SPECIALISE f :: Int -> Int #-}
867 The type checker generates a binding like:
869 f_spec = (f :: Int -> Int)
871 and we want to end up with
873 f_spec = _inline_me_ (f Int dNumInt)
875 But that means that we must simplify the Method for f to (f Int dNumInt)!
876 So tcSimplifyToDicts squeezes out all Methods.
878 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
880 fromIntegral :: (Integral a, Num b) => a -> b
881 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
883 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
887 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
889 because the scsel will mess up matching. Instead we want
891 forall dIntegralInt, dNumInt.
892 fromIntegral Int Int dIntegralInt dNumInt = id Int
894 Hence "DontReduce NoSCs"
897 tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds)
898 tcSimplifyToDicts wanteds
899 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
900 -- Since try_me doesn't look at types, we don't need to
901 -- do any zonking, so it's safe to call reduceContext directly
903 extendLIEs irreds `thenM_`
907 doc = text "tcSimplifyToDicts"
909 -- Reduce methods and lits only; stop as soon as we get a dictionary
910 try_me inst | isDict inst = DontReduce NoSCs
911 | otherwise = ReduceMe
916 tcSimplifyBracket is used when simplifying the constraints arising from
917 a Template Haskell bracket [| ... |]. We want to check that there aren't
918 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
919 Show instance), but we aren't otherwise interested in the results.
920 Nor do we care about ambiguous dictionaries etc. We will type check
921 this bracket again at its usage site.
924 tcSimplifyBracket :: [Inst] -> TcM ()
925 tcSimplifyBracket wanteds
926 = simpleReduceLoop doc reduceMe wanteds `thenM_`
929 doc = text "tcSimplifyBracket"
933 %************************************************************************
935 \subsection{Filtering at a dynamic binding}
937 %************************************************************************
942 we must discharge all the ?x constraints from B. We also do an improvement
943 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
945 Actually, the constraints from B might improve the types in ?x. For example
947 f :: (?x::Int) => Char -> Char
950 then the constraint (?x::Int) arising from the call to f will
951 force the binding for ?x to be of type Int.
954 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
957 tcSimplifyIPs given_ips wanteds
958 = simpl_loop given_ips wanteds `thenM` \ (frees, binds) ->
959 extendLIEs frees `thenM_`
962 doc = text "tcSimplifyIPs" <+> ppr given_ips
963 ip_set = mkNameSet (ipNamesOfInsts given_ips)
965 -- Simplify any methods that mention the implicit parameter
966 try_me inst | isFreeWrtIPs ip_set inst = Free
967 | otherwise = ReduceMe
969 simpl_loop givens wanteds
970 = mappM zonkInst givens `thenM` \ givens' ->
971 mappM zonkInst wanteds `thenM` \ wanteds' ->
973 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
975 if no_improvement then
976 ASSERT( null irreds )
977 returnM (frees, binds)
979 simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) ->
980 returnM (frees1, binds `AndMonoBinds` binds1)
984 %************************************************************************
986 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
988 %************************************************************************
990 When doing a binding group, we may have @Insts@ of local functions.
991 For example, we might have...
993 let f x = x + 1 -- orig local function (overloaded)
994 f.1 = f Int -- two instances of f
999 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
1000 where @f@ is in scope; those @Insts@ must certainly not be passed
1001 upwards towards the top-level. If the @Insts@ were binding-ified up
1002 there, they would have unresolvable references to @f@.
1004 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1005 For each method @Inst@ in the @init_lie@ that mentions one of the
1006 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1007 @LIE@), as well as the @HsBinds@ generated.
1010 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcMonoBinds
1012 bindInstsOfLocalFuns wanteds local_ids
1013 | null overloaded_ids
1015 = extendLIEs wanteds `thenM_`
1016 returnM EmptyMonoBinds
1019 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
1020 ASSERT( null irreds )
1021 extendLIEs frees `thenM_`
1024 doc = text "bindInsts" <+> ppr local_ids
1025 overloaded_ids = filter is_overloaded local_ids
1026 is_overloaded id = isOverloadedTy (idType id)
1028 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1029 -- so it's worth building a set, so that
1030 -- lookup (in isMethodFor) is faster
1032 try_me inst | isMethodFor overloaded_set inst = ReduceMe
1037 %************************************************************************
1039 \subsection{Data types for the reduction mechanism}
1041 %************************************************************************
1043 The main control over context reduction is here
1047 = ReduceMe -- Try to reduce this
1048 -- If there's no instance, behave exactly like
1049 -- DontReduce: add the inst to
1050 -- the irreductible ones, but don't
1051 -- produce an error message of any kind.
1052 -- It might be quite legitimate such as (Eq a)!
1054 | DontReduce WantSCs -- Return as irreducible
1056 | DontReduceUnlessConstant -- Return as irreducible unless it can
1057 -- be reduced to a constant in one step
1059 | Free -- Return as free
1061 reduceMe :: Inst -> WhatToDo
1062 reduceMe inst = ReduceMe
1064 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1065 -- of a predicate when adding it to the avails
1071 type Avails = FiniteMap Inst Avail
1074 = IsFree -- Used for free Insts
1075 | Irred -- Used for irreducible dictionaries,
1076 -- which are going to be lambda bound
1078 | Given TcId -- Used for dictionaries for which we have a binding
1079 -- e.g. those "given" in a signature
1080 Bool -- True <=> actually consumed (splittable IPs only)
1082 | NoRhs -- Used for Insts like (CCallable f)
1083 -- where no witness is required.
1086 | Rhs -- Used when there is a RHS
1088 [Inst] -- Insts free in the RHS; we need these too
1090 | Linear -- Splittable Insts only.
1091 Int -- The Int is always 2 or more; indicates how
1092 -- many copies are required
1093 Inst -- The splitter
1094 Avail -- Where the "master copy" is
1096 | LinRhss -- Splittable Insts only; this is used only internally
1097 -- by extractResults, where a Linear
1098 -- is turned into an LinRhss
1099 [TcExpr] -- A supply of suitable RHSs
1101 pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1102 | (inst,avail) <- fmToList avails ]
1104 instance Outputable Avail where
1107 pprAvail NoRhs = text "<no rhs>"
1108 pprAvail IsFree = text "Free"
1109 pprAvail Irred = text "Irred"
1110 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1111 if b then text "(used)" else empty
1112 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1113 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1114 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1117 Extracting the bindings from a bunch of Avails.
1118 The bindings do *not* come back sorted in dependency order.
1119 We assume that they'll be wrapped in a big Rec, so that the
1120 dependency analyser can sort them out later
1124 extractResults :: Avails
1126 -> TcM (TcDictBinds, -- Bindings
1127 [Inst], -- Irreducible ones
1128 [Inst]) -- Free ones
1130 extractResults avails wanteds
1131 = go avails EmptyMonoBinds [] [] wanteds
1133 go avails binds irreds frees []
1134 = returnM (binds, irreds, frees)
1136 go avails binds irreds frees (w:ws)
1137 = case lookupFM avails w of
1138 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1139 go avails binds irreds frees ws
1141 Just NoRhs -> go avails binds irreds frees ws
1142 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1143 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1145 Just (Given id _) -> go avails new_binds irreds frees ws
1147 new_binds | id == instToId w = binds
1148 | otherwise = addBind binds w (HsVar id)
1149 -- The sought Id can be one of the givens, via a superclass chain
1150 -- and then we definitely don't want to generate an x=x binding!
1152 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1154 new_binds = addBind binds w rhs
1156 Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
1157 -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) ->
1158 split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) ->
1159 go (addToFM avails w (LinRhss rhss))
1160 (binds `AndMonoBinds` binds')
1161 irreds' frees' (split_inst : w : ws)
1163 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1164 -> go new_avails new_binds irreds frees ws
1166 new_binds = addBind binds w rhs
1167 new_avails = addToFM avails w (LinRhss rhss)
1169 get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
1170 get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
1171 returnM (w':irreds, frees, instToId w')
1172 get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
1173 returnM (irreds, w':frees, instToId w')
1176 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1177 | otherwise = addToFM avails w NoRhs
1178 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1179 -- than Given, else we end up with bogus bindings.
1181 add_free avails w | isMethod w = avails
1182 | otherwise = add_given avails w
1184 -- Do *not* replace Free by Given if it's a method.
1185 -- The following situation shows why this is bad:
1186 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1187 -- From an application (truncate f i) we get
1188 -- t1 = truncate at f
1190 -- If we have also have a second occurrence of truncate, we get
1191 -- t3 = truncate at f
1193 -- When simplifying with i,f free, we might still notice that
1194 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1195 -- will continue to float out!
1196 -- (split n i a) returns: n rhss
1197 -- auxiliary bindings
1198 -- 1 or 0 insts to add to irreds
1201 split :: Int -> TcId -> TcId -> Inst
1202 -> TcM (TcDictBinds, [TcExpr])
1203 -- (split n split_id root_id wanted) returns
1204 -- * a list of 'n' expressions, all of which witness 'avail'
1205 -- * a bunch of auxiliary bindings to support these expressions
1206 -- * one or zero insts needed to witness the whole lot
1207 -- (maybe be zero if the initial Inst is a Given)
1209 -- NB: 'wanted' is just a template
1211 split n split_id root_id wanted
1214 ty = linearInstType wanted
1215 pair_ty = mkTyConApp pairTyCon [ty,ty]
1216 id = instToId wanted
1220 go 1 = returnM (EmptyMonoBinds, [HsVar root_id])
1222 go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
1223 expand n rhss `thenM` \ (binds2, rhss') ->
1224 returnM (binds1 `AndMonoBinds` binds2, rhss')
1227 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1228 -- e.g. expand 3 [rhs1, rhs2]
1229 -- = ( { x = split rhs1 },
1230 -- [fst x, snd x, rhs2] )
1232 | n `rem` 2 == 0 = go rhss -- n is even
1233 | otherwise = go (tail rhss) `thenM` \ (binds', rhss') ->
1234 returnM (binds', head rhss : rhss')
1236 go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') ->
1237 returnM (andMonoBindList binds', concat rhss')
1239 do_one rhs = newUnique `thenM` \ uniq ->
1240 tcLookupId fstName `thenM` \ fst_id ->
1241 tcLookupId sndName `thenM` \ snd_id ->
1243 x = mkUserLocal occ uniq pair_ty loc
1245 returnM (VarMonoBind x (mk_app split_id rhs),
1246 [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])
1248 mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var
1250 mk_app id rhs = HsApp (HsVar id) rhs
1252 addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
1256 %************************************************************************
1258 \subsection[reduce]{@reduce@}
1260 %************************************************************************
1262 When the "what to do" predicate doesn't depend on the quantified type variables,
1263 matters are easier. We don't need to do any zonking, unless the improvement step
1264 does something, in which case we zonk before iterating.
1266 The "given" set is always empty.
1269 simpleReduceLoop :: SDoc
1270 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1272 -> TcM ([Inst], -- Free
1274 [Inst]) -- Irreducible
1276 simpleReduceLoop doc try_me wanteds
1277 = mappM zonkInst wanteds `thenM` \ wanteds' ->
1278 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1279 if no_improvement then
1280 returnM (frees, binds, irreds)
1282 simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
1283 returnM (frees1, binds `AndMonoBinds` binds1, irreds1)
1289 reduceContext :: SDoc
1290 -> (Inst -> WhatToDo)
1293 -> TcM (Bool, -- True <=> improve step did no unification
1295 TcDictBinds, -- Dictionary bindings
1296 [Inst]) -- Irreducible
1298 reduceContext doc try_me givens wanteds
1300 traceTc (text "reduceContext" <+> (vcat [
1301 text "----------------------",
1303 text "given" <+> ppr givens,
1304 text "wanted" <+> ppr wanteds,
1305 text "----------------------"
1308 -- Build the Avail mapping from "givens"
1309 foldlM addGiven emptyFM givens `thenM` \ init_state ->
1312 reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
1314 -- Do improvement, using everything in avails
1315 -- In particular, avails includes all superclasses of everything
1316 tcImprove avails `thenM` \ no_improvement ->
1318 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1320 traceTc (text "reduceContext end" <+> (vcat [
1321 text "----------------------",
1323 text "given" <+> ppr givens,
1324 text "wanted" <+> ppr wanteds,
1326 text "avails" <+> pprAvails avails,
1327 text "frees" <+> ppr frees,
1328 text "no_improvement =" <+> ppr no_improvement,
1329 text "----------------------"
1332 returnM (no_improvement, frees, binds, irreds)
1334 tcImprove :: Avails -> TcM Bool -- False <=> no change
1335 -- Perform improvement using all the predicates in Avails
1337 = tcGetInstEnvs `thenM` \ (home_ie, pkg_ie) ->
1339 preds = [ (pred, pp_loc)
1340 | inst <- keysFM avails,
1341 let pp_loc = pprInstLoc (instLoc inst),
1342 pred <- fdPredsOfInst inst
1344 -- Avails has all the superclasses etc (good)
1345 -- It also has all the intermediates of the deduction (good)
1346 -- It does not have duplicates (good)
1347 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1348 -- so that improve will see them separate
1349 eqns = improve get_insts preds
1350 get_insts clas = classInstEnv home_ie clas ++ classInstEnv pkg_ie clas
1355 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
1356 mappM_ unify eqns `thenM_`
1359 unify ((qtvs, t1, t2), doc)
1361 tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1362 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1365 The main context-reduction function is @reduce@. Here's its game plan.
1368 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1369 -- along with its depth
1370 -> (Inst -> WhatToDo)
1377 try_me: given an inst, this function returns
1379 DontReduce return this in "irreds"
1380 Free return this in "frees"
1382 wanteds: The list of insts to reduce
1383 state: An accumulating parameter of type Avails
1384 that contains the state of the algorithm
1386 It returns a Avails.
1388 The (n,stack) pair is just used for error reporting.
1389 n is always the depth of the stack.
1390 The stack is the stack of Insts being reduced: to produce X
1391 I had to produce Y, to produce Y I had to produce Z, and so on.
1394 reduceList (n,stack) try_me wanteds state
1395 | n > opt_MaxContextReductionDepth
1396 = failWithTc (reduceDepthErr n stack)
1402 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1407 go [] state = returnM state
1408 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
1411 -- Base case: we're done!
1412 reduce stack try_me wanted state
1413 -- It's the same as an existing inst, or a superclass thereof
1414 | Just avail <- isAvailable state wanted
1415 = if isLinearInst wanted then
1416 addLinearAvailable state avail wanted `thenM` \ (state', wanteds') ->
1417 reduceList stack try_me wanteds' state'
1419 returnM state -- No op for non-linear things
1422 = case try_me wanted of {
1424 DontReduce want_scs -> addIrred want_scs state wanted
1426 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1427 -- First, see if the inst can be reduced to a constant in one step
1428 try_simple (addIrred AddSCs) -- Assume want superclasses
1430 ; Free -> -- It's free so just chuck it upstairs
1431 -- First, see if the inst can be reduced to a constant in one step
1434 ; ReduceMe -> -- It should be reduced
1435 lookupInst wanted `thenM` \ lookup_result ->
1436 case lookup_result of
1437 GenInst wanteds' rhs -> addWanted state wanted rhs wanteds' `thenM` \ state' ->
1438 reduceList stack try_me wanteds' state'
1439 -- Experiment with doing addWanted *before* the reduceList,
1440 -- which has the effect of adding the thing we are trying
1441 -- to prove to the database before trying to prove the things it
1442 -- needs. See note [RECURSIVE DICTIONARIES]
1444 SimpleInst rhs -> addWanted state wanted rhs []
1446 NoInstance -> -- No such instance!
1447 -- Add it and its superclasses
1448 addIrred AddSCs state wanted
1452 try_simple do_this_otherwise
1453 = lookupInst wanted `thenM` \ lookup_result ->
1454 case lookup_result of
1455 SimpleInst rhs -> addWanted state wanted rhs []
1456 other -> do_this_otherwise state wanted
1461 -------------------------
1462 isAvailable :: Avails -> Inst -> Maybe Avail
1463 isAvailable avails wanted = lookupFM avails wanted
1464 -- NB 1: the Ord instance of Inst compares by the class/type info
1465 -- *not* by unique. So
1466 -- d1::C Int == d2::C Int
1468 addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
1469 addLinearAvailable avails avail wanted
1470 -- avails currently maps [wanted -> avail]
1471 -- Extend avails to reflect a neeed for an extra copy of avail
1473 | Just avail' <- split_avail avail
1474 = returnM (addToFM avails wanted avail', [])
1477 = tcLookupId splitName `thenM` \ split_id ->
1478 tcInstClassOp (instLoc wanted) split_id
1479 [linearInstType wanted] `thenM` \ split_inst ->
1480 returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1483 split_avail :: Avail -> Maybe Avail
1484 -- (Just av) if there's a modified version of avail that
1485 -- we can use to replace avail in avails
1486 -- Nothing if there isn't, so we need to create a Linear
1487 split_avail (Linear n i a) = Just (Linear (n+1) i a)
1488 split_avail (Given id used) | not used = Just (Given id True)
1489 | otherwise = Nothing
1490 split_avail Irred = Nothing
1491 split_avail IsFree = Nothing
1492 split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
1494 -------------------------
1495 addFree :: Avails -> Inst -> TcM Avails
1496 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1497 -- to avails, so that any other equal Insts will be commoned up right
1498 -- here rather than also being tossed upstairs. This is really just
1499 -- an optimisation, and perhaps it is more trouble that it is worth,
1500 -- as the following comments show!
1502 -- NB: do *not* add superclasses. If we have
1505 -- but a is not bound here, then we *don't* want to derive
1506 -- dn from df here lest we lose sharing.
1508 addFree avails free = returnM (addToFM avails free IsFree)
1510 addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> TcM Avails
1511 addWanted avails wanted rhs_expr wanteds
1512 = ASSERT2( not (wanted `elemFM` avails), ppr wanted $$ ppr avails )
1513 addAvailAndSCs avails wanted avail
1515 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1516 | otherwise = ASSERT( null wanteds ) NoRhs
1518 addGiven :: Avails -> Inst -> TcM Avails
1519 addGiven state given = addAvailAndSCs state given (Given (instToId given) False)
1520 -- No ASSERT( not (given `elemFM` avails) ) because in an instance
1521 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
1522 -- so the assert isn't true
1524 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
1525 addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
1526 addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
1527 addAvailAndSCs avails irred Irred
1529 addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
1530 addAvailAndSCs avails inst avail
1531 | not (isClassDict inst) = returnM avails1
1532 | otherwise = traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps]) `thenM_`
1533 addSCs is_loop avails1 inst
1535 avails1 = addToFM avails inst avail
1536 is_loop inst = inst `elem` deps -- Note: this compares by *type*, not by Unique
1537 deps = findAllDeps avails avail
1539 findAllDeps :: Avails -> Avail -> [Inst]
1540 -- Find all the Insts that this one depends on
1541 -- See Note [SUPERCLASS-LOOP]
1542 findAllDeps avails (Rhs _ kids) = kids ++ concat (map (find_all_deps_help avails) kids)
1543 findAllDeps avails other = []
1545 find_all_deps_help :: Avails -> Inst -> [Inst]
1546 find_all_deps_help avails inst
1547 = case lookupFM avails inst of
1548 Just avail -> findAllDeps avails avail
1551 addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
1552 -- Add all the superclasses of the Inst to Avails
1553 -- The first param says "dont do this because the original thing
1554 -- depends on this one, so you'd build a loop"
1555 -- Invariant: the Inst is already in Avails.
1557 addSCs is_loop avails dict
1558 = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
1559 foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1561 (clas, tys) = getDictClassTys dict
1562 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1563 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1565 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1567 = returnM avails -- See Note [SUPERCLASS-LOOP]
1569 = case lookupFM avails sc_dict of
1570 Just (Given _ _) -> returnM avails -- Given is cheaper than superclass selection
1571 Just other -> returnM avails' -- SCs already added
1572 Nothing -> addSCs is_loop avails' sc_dict
1574 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1575 avail = Rhs sc_sel_rhs [dict]
1576 avails' = addToFM avails sc_dict avail
1579 Note [SUPERCLASS-LOOP]: Checking for loops
1580 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1581 We have to be careful here. If we are *given* d1:Ord a,
1582 and want to deduce (d2:C [a]) where
1584 class Ord a => C a where
1585 instance Ord a => C [a] where ...
1587 Then we'll use the instance decl to deduce C [a] and then add the
1588 superclasses of C [a] to avails. But we must not overwrite the binding
1589 for d1:Ord a (which is given) with a superclass selection or we'll just
1592 Here's another variant, immortalised in tcrun020
1593 class Monad m => C1 m
1594 class C1 m => C2 m x
1595 instance C2 Maybe Bool
1596 For the instance decl we need to build (C1 Maybe), and it's no good if
1597 we run around and add (C2 Maybe Bool) and its superclasses to the avails
1598 before we search for C1 Maybe.
1600 Here's another example
1601 class Eq b => Foo a b
1602 instance Eq a => Foo [a] a
1606 we'll first deduce that it holds (via the instance decl). We must not
1607 then overwrite the Eq t constraint with a superclass selection!
1609 At first I had a gross hack, whereby I simply did not add superclass constraints
1610 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1611 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1612 I found a very obscure program (now tcrun021) in which improvement meant the
1613 simplifier got two bites a the cherry... so something seemed to be an Irred
1614 first time, but reducible next time.
1616 Now we implement the Right Solution, which is to check for loops directly
1617 when adding superclasses. It's a bit like the occurs check in unification.
1620 Note [RECURSIVE DICTIONARIES]
1621 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1623 data D r = ZeroD | SuccD (r (D r));
1625 instance (Eq (r (D r))) => Eq (D r) where
1626 ZeroD == ZeroD = True
1627 (SuccD a) == (SuccD b) = a == b
1630 equalDC :: D [] -> D [] -> Bool;
1633 We need to prove (Eq (D [])). Here's how we go:
1637 by instance decl, holds if
1641 by instance decl of Eq, holds if
1643 where d2 = dfEqList d2
1646 But now we can "tie the knot" to give
1652 and it'll even run! The trick is to put the thing we are trying to prove
1653 (in this case Eq (D []) into the database before trying to prove its
1654 contributing clauses.
1657 %************************************************************************
1659 \section{tcSimplifyTop: defaulting}
1661 %************************************************************************
1664 @tcSimplifyTop@ is called once per module to simplify all the constant
1665 and ambiguous Insts.
1667 We need to be careful of one case. Suppose we have
1669 instance Num a => Num (Foo a b) where ...
1671 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1672 to (Num x), and default x to Int. But what about y??
1674 It's OK: the final zonking stage should zap y to (), which is fine.
1678 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
1679 tcSimplifyTop wanteds = tc_simplify_top False {- Not interactive loop -} wanteds
1680 tcSimplifyInteractive wanteds = tc_simplify_top True {- Interactive loop -} wanteds
1683 -- The TcLclEnv should be valid here, solely to improve
1684 -- error message generation for the monomorphism restriction
1685 tc_simplify_top is_interactive wanteds
1686 = getLclEnv `thenM` \ lcl_env ->
1687 traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
1688 simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
1689 ASSERT( null frees )
1692 -- All the non-std ones are definite errors
1693 (stds, non_stds) = partition isStdClassTyVarDict irreds
1695 -- Group by type variable
1696 std_groups = equivClasses cmp_by_tyvar stds
1698 -- Pick the ones which its worth trying to disambiguate
1699 -- namely, the onese whose type variable isn't bound
1700 -- up with one of the non-standard classes
1701 (std_oks, std_bads) = partition worth_a_try std_groups
1702 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1703 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1705 -- Collect together all the bad guys
1706 bad_guys = non_stds ++ concat std_bads
1707 (bad_ips, non_ips) = partition isIPDict bad_guys
1708 (no_insts, ambigs) = partition no_inst non_ips
1709 no_inst d = not (isTyVarDict d)
1710 -- Previously, there was a more elaborate no_inst definition:
1711 -- no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1712 -- fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
1713 -- But that seems over-elaborate to me; it only bites for class decls with
1714 -- fundeps like this: class C a b | -> b where ...
1717 -- Report definite errors
1718 groupErrs (addNoInstanceErrs Nothing []) no_insts `thenM_`
1719 addTopIPErrs bad_ips `thenM_`
1721 -- Deal with ambiguity errors, but only if
1722 -- if there has not been an error so far; errors often
1723 -- give rise to spurious ambiguous Insts
1724 ifErrsM (returnM []) (
1726 -- Complain about the ones that don't fall under
1727 -- the Haskell rules for disambiguation
1728 -- This group includes both non-existent instances
1729 -- e.g. Num (IO a) and Eq (Int -> Int)
1730 -- and ambiguous dictionaries
1732 addTopAmbigErrs ambigs `thenM_`
1734 -- Disambiguate the ones that look feasible
1735 mappM (disambigGroup is_interactive) std_oks
1736 ) `thenM` \ binds_ambig ->
1738 returnM (binds `andMonoBinds` andMonoBindList binds_ambig)
1740 ----------------------------------
1741 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1743 get_tv d = case getDictClassTys d of
1744 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1745 get_clas d = case getDictClassTys d of
1746 (clas, [ty]) -> clas
1749 If a dictionary constrains a type variable which is
1750 * not mentioned in the environment
1751 * and not mentioned in the type of the expression
1752 then it is ambiguous. No further information will arise to instantiate
1753 the type variable; nor will it be generalised and turned into an extra
1754 parameter to a function.
1756 It is an error for this to occur, except that Haskell provided for
1757 certain rules to be applied in the special case of numeric types.
1759 * at least one of its classes is a numeric class, and
1760 * all of its classes are numeric or standard
1761 then the type variable can be defaulted to the first type in the
1762 default-type list which is an instance of all the offending classes.
1764 So here is the function which does the work. It takes the ambiguous
1765 dictionaries and either resolves them (producing bindings) or
1766 complains. It works by splitting the dictionary list by type
1767 variable, and using @disambigOne@ to do the real business.
1769 @disambigOne@ assumes that its arguments dictionaries constrain all
1770 the same type variable.
1772 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1773 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1774 the most common use of defaulting is code like:
1776 _ccall_ foo `seqPrimIO` bar
1778 Since we're not using the result of @foo@, the result if (presumably)
1782 disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
1783 -> [Inst] -- All standard classes of form (C a)
1786 disambigGroup is_interactive dicts
1787 | any std_default_class classes -- Guaranteed all standard classes
1788 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1789 -- SO, TRY DEFAULT TYPES IN ORDER
1791 -- Failure here is caused by there being no type in the
1792 -- default list which can satisfy all the ambiguous classes.
1793 -- For example, if Real a is reqd, but the only type in the
1794 -- default list is Int.
1795 get_default_tys `thenM` \ default_tys ->
1797 try_default [] -- No defaults work, so fail
1800 try_default (default_ty : default_tys)
1801 = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
1802 -- default_tys instead
1803 tcSimplifyDefault theta `thenM` \ _ ->
1806 theta = [mkClassPred clas [default_ty] | clas <- classes]
1808 -- See if any default works
1809 tryM (try_default default_tys) `thenM` \ mb_ty ->
1812 Right chosen_default_ty -> choose_default chosen_default_ty
1814 | otherwise -- No defaults
1818 tyvar = get_tv (head dicts) -- Should be non-empty
1819 classes = map get_clas dicts
1821 std_default_class cls
1822 = isNumericClass cls
1823 || (is_interactive &&
1824 classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
1825 -- In interactive mode, we default Show a to Show ()
1826 -- to avoid graututious errors on "show []"
1828 choose_default default_ty -- Commit to tyvar = default_ty
1829 = -- Bind the type variable
1830 unifyTauTy default_ty (mkTyVarTy tyvar) `thenM_`
1831 -- and reduce the context, for real this time
1832 simpleReduceLoop (text "disambig" <+> ppr dicts)
1833 reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
1834 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1835 warnDefault dicts default_ty `thenM_`
1838 bomb_out = addTopAmbigErrs dicts `thenM_`
1839 returnM EmptyMonoBinds
1842 = do { mb_defaults <- getDefaultTys
1843 ; case mb_defaults of
1844 Just tys -> return tys
1845 Nothing -> -- No use-supplied default;
1846 -- use [Integer, Double]
1847 do { integer_ty <- tcMetaTy integerTyConName
1848 ; return [integer_ty, doubleTy] } }
1851 [Aside - why the defaulting mechanism is turned off when
1852 dealing with arguments and results to ccalls.
1854 When typechecking _ccall_s, TcExpr ensures that the external
1855 function is only passed arguments (and in the other direction,
1856 results) of a restricted set of 'native' types. This is
1857 implemented via the help of the pseudo-type classes,
1858 @CReturnable@ (CR) and @CCallable@ (CC.)
1860 The interaction between the defaulting mechanism for numeric
1861 values and CC & CR can be a bit puzzling to the user at times.
1870 What type has 'x' got here? That depends on the default list
1871 in operation, if it is equal to Haskell 98's default-default
1872 of (Integer, Double), 'x' has type Double, since Integer
1873 is not an instance of CR. If the default list is equal to
1874 Haskell 1.4's default-default of (Int, Double), 'x' has type
1877 To try to minimise the potential for surprises here, the
1878 defaulting mechanism is turned off in the presence of
1879 CCallable and CReturnable.
1884 %************************************************************************
1886 \subsection[simple]{@Simple@ versions}
1888 %************************************************************************
1890 Much simpler versions when there are no bindings to make!
1892 @tcSimplifyThetas@ simplifies class-type constraints formed by
1893 @deriving@ declarations and when specialising instances. We are
1894 only interested in the simplified bunch of class/type constraints.
1896 It simplifies to constraints of the form (C a b c) where
1897 a,b,c are type variables. This is required for the context of
1898 instance declarations.
1901 tcSimplifyDeriv :: [TyVar]
1902 -> ThetaType -- Wanted
1903 -> TcM ThetaType -- Needed
1905 tcSimplifyDeriv tyvars theta
1906 = tcInstTyVars VanillaTv tyvars `thenM` \ (tvs, _, tenv) ->
1907 -- The main loop may do unification, and that may crash if
1908 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
1909 -- ToDo: what if two of them do get unified?
1910 newDicts DataDeclOrigin (substTheta tenv theta) `thenM` \ wanteds ->
1911 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
1912 ASSERT( null frees ) -- reduceMe never returns Free
1914 doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
1916 tv_set = mkVarSet tvs
1917 simpl_theta = map dictPred irreds -- reduceMe squashes all non-dicts
1920 | isEmptyVarSet pred_tyvars -- Things like (Eq T) should be rejected
1921 = addErrTc (noInstErr pred)
1923 | not undecidable_ok && not (isTyVarClassPred pred)
1924 -- Check that the returned dictionaries are all of form (C a b)
1925 -- (where a, b are type variables).
1926 -- We allow this if we had -fallow-undecidable-instances,
1927 -- but note that risks non-termination in the 'deriving' context-inference
1928 -- fixpoint loop. It is useful for situations like
1929 -- data Min h a = E | M a (h a)
1930 -- which gives the instance decl
1931 -- instance (Eq a, Eq (h a)) => Eq (Min h a)
1932 = addErrTc (noInstErr pred)
1934 | not (pred_tyvars `subVarSet` tv_set)
1935 -- Check for a bizarre corner case, when the derived instance decl should
1936 -- have form instance C a b => D (T a) where ...
1937 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1938 -- of problems; in particular, it's hard to compare solutions for
1939 -- equality when finding the fixpoint. So I just rule it out for now.
1940 = addErrTc (badDerivedPred pred)
1945 pred_tyvars = tyVarsOfPred pred
1947 rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
1948 -- This reverse-mapping is a Royal Pain,
1949 -- but the result should mention TyVars not TcTyVars
1952 mappM check_pred simpl_theta `thenM_`
1953 checkAmbiguity tvs simpl_theta tv_set `thenM_`
1954 returnM (substTheta rev_env simpl_theta)
1956 doc = ptext SLIT("deriving classes for a data type")
1959 @tcSimplifyDefault@ just checks class-type constraints, essentially;
1960 used with \tr{default} declarations. We are only interested in
1961 whether it worked or not.
1964 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
1967 tcSimplifyDefault theta
1968 = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
1969 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
1970 ASSERT( null frees ) -- try_me never returns Free
1971 mappM (addErrTc . noInstErr) irreds `thenM_`
1977 doc = ptext SLIT("default declaration")
1981 %************************************************************************
1983 \section{Errors and contexts}
1985 %************************************************************************
1987 ToDo: for these error messages, should we note the location as coming
1988 from the insts, or just whatever seems to be around in the monad just
1992 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
1993 -> [Inst] -- The offending Insts
1995 -- Group together insts with the same origin
1996 -- We want to report them together in error messages
1998 groupErrs report_err []
2000 groupErrs report_err (inst:insts)
2001 = do_one (inst:friends) `thenM_`
2002 groupErrs report_err others
2005 -- (It may seem a bit crude to compare the error messages,
2006 -- but it makes sure that we combine just what the user sees,
2007 -- and it avoids need equality on InstLocs.)
2008 (friends, others) = partition is_friend insts
2009 loc_msg = showSDoc (pprInstLoc (instLoc inst))
2010 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
2011 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
2012 -- Add location and context information derived from the Insts
2014 -- Add the "arising from..." part to a message about bunch of dicts
2015 addInstLoc :: [Inst] -> Message -> Message
2016 addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
2019 plural xs = char 's'
2022 = groupErrs report tidy_dicts
2024 (tidy_env, tidy_dicts) = tidyInsts dicts
2025 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2026 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2027 plural tidy_dicts <+> pprInsts tidy_dicts)
2029 addNoInstanceErrs :: Maybe SDoc -- Nothing => top level
2030 -- Just d => d describes the construct
2031 -> [Inst] -- What is given by the context or type sig
2032 -> [Inst] -- What is wanted
2034 addNoInstanceErrs mb_what givens []
2036 addNoInstanceErrs mb_what givens dicts
2037 = -- Some of the dicts are here because there is no instances
2038 -- and some because there are too many instances (overlap)
2039 -- The first thing we do is separate them
2040 getDOpts `thenM` \ dflags ->
2041 tcGetInstEnvs `thenM` \ inst_envs ->
2043 (tidy_env1, tidy_givens) = tidyInsts givens
2044 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
2046 -- Run through the dicts, generating a message for each
2047 -- overlapping one, but simply accumulating all the
2048 -- no-instance ones so they can be reported as a group
2049 (overlap_doc, no_inst_dicts) = foldl check_overlap (empty, []) tidy_dicts
2050 check_overlap (overlap_doc, no_inst_dicts) dict
2051 | not (isClassDict dict) = (overlap_doc, dict : no_inst_dicts)
2053 = case lookupInstEnv dflags inst_envs clas tys of
2054 ([], _) -> (overlap_doc, dict : no_inst_dicts) -- No matches
2055 inst_res -> (mk_overlap_msg dict inst_res $$ overlap_doc, no_inst_dicts)
2057 (clas,tys) = getDictClassTys dict
2059 mk_probable_fix tidy_env2 mb_what no_inst_dicts `thenM` \ (tidy_env3, probable_fix) ->
2061 no_inst_doc | null no_inst_dicts = empty
2062 | otherwise = vcat [addInstLoc no_inst_dicts heading, probable_fix]
2063 heading | null givens = ptext SLIT("No instance") <> plural no_inst_dicts <+>
2064 ptext SLIT("for") <+> pprInsts no_inst_dicts
2065 | otherwise = sep [ptext SLIT("Could not deduce") <+> pprInsts no_inst_dicts,
2066 nest 2 $ ptext SLIT("from the context") <+> pprInsts tidy_givens]
2068 addErrTcM (tidy_env3, no_inst_doc $$ overlap_doc)
2071 mk_overlap_msg dict (matches, unifiers)
2072 = vcat [ addInstLoc [dict] ((ptext SLIT("Overlapping instances for") <+> ppr dict)),
2073 sep [ptext SLIT("Matching instances") <> colon,
2074 nest 2 (pprDFuns (dfuns ++ unifiers))],
2077 else parens (ptext SLIT("The choice depends on the instantiation of") <+>
2078 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))))]
2080 dfuns = [df | (_, (_,_,df)) <- matches]
2082 mk_probable_fix tidy_env Nothing dicts -- Top level
2083 = mkMonomorphismMsg tidy_env dicts
2084 mk_probable_fix tidy_env (Just what) dicts -- Nested (type signatures, instance decls)
2085 = returnM (tidy_env, sep [ptext SLIT("Probable fix:"), nest 2 fix1, nest 2 fix2])
2087 fix1 = sep [ptext SLIT("Add") <+> pprInsts dicts,
2088 ptext SLIT("to the") <+> what]
2090 fix2 | null instance_dicts = empty
2091 | otherwise = ptext SLIT("Or add an instance declaration for")
2092 <+> pprInsts instance_dicts
2093 instance_dicts = [d | d <- dicts, isClassDict d, not (isTyVarDict d)]
2094 -- Insts for which it is worth suggesting an adding an instance declaration
2095 -- Exclude implicit parameters, and tyvar dicts
2098 addTopAmbigErrs dicts
2099 -- Divide into groups that share a common set of ambiguous tyvars
2100 = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
2102 (tidy_env, tidy_dicts) = tidyInsts dicts
2104 tvs_of :: Inst -> [TcTyVar]
2105 tvs_of d = varSetElems (tyVarsOfInst d)
2106 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
2108 report :: [(Inst,[TcTyVar])] -> TcM ()
2109 report pairs@((_,tvs) : _) -- The pairs share a common set of ambiguous tyvars
2110 = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
2111 addErrTcM (tidy_env, msg $$ mono_msg)
2113 dicts = map fst pairs
2114 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
2115 pprQuotedList tvs <+> in_msg,
2116 nest 2 (pprInstsInFull dicts)]
2117 in_msg | isSingleton dicts = text "in the top-level constraint:"
2118 | otherwise = text "in these top-level constraints:"
2121 mkMonomorphismMsg :: TidyEnv -> [Inst] -> TcM (TidyEnv, Message)
2122 -- There's an error with these Insts; if they have free type variables
2123 -- it's probably caused by the monomorphism restriction.
2124 -- Try to identify the offending variable
2125 -- ASSUMPTION: the Insts are fully zonked
2126 mkMonomorphismMsg tidy_env insts
2127 | isEmptyVarSet inst_tvs
2128 = returnM (tidy_env, empty)
2130 = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
2131 returnM (tidy_env, mk_msg docs)
2134 inst_tvs = tyVarsOfInsts insts
2136 mk_msg [] = empty -- This happens in things like
2137 -- f x = show (read "foo")
2138 -- whre monomorphism doesn't play any role
2139 mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
2141 ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
2143 warnDefault dicts default_ty
2144 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
2145 addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
2148 (_, tidy_dicts) = tidyInsts dicts
2149 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
2150 quotes (ppr default_ty),
2151 pprInstsInFull tidy_dicts]
2153 -- Used for the ...Thetas variants; all top level
2154 noInstErr pred = ptext SLIT("No instance for") <+> quotes (ppr pred)
2157 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2158 ptext SLIT("type variables that are not data type parameters"),
2159 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2161 reduceDepthErr n stack
2162 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2163 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
2164 nest 4 (pprInstsInFull stack)]
2166 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)