2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcSimplify]{TcSimplify}
10 tcSimplifyInfer, tcSimplifyInferCheck, tcSimplifyCheck,
12 tcSimplifyToDicts, tcSimplifyIPs, tcSimplifyTop,
14 tcSimplifyThetas, tcSimplifyCheckThetas,
18 #include "HsVersions.h"
20 import HsSyn ( MonoBinds(..), HsExpr(..), andMonoBinds, andMonoBindList )
21 import TcHsSyn ( TcExpr, TcId,
22 TcMonoBinds, TcDictBinds
26 import Inst ( lookupInst, lookupSimpleInst, LookupInstResult(..),
27 tyVarsOfInst, predsOfInsts, predsOfInst,
29 isStdClassTyVarDict, isMethodFor,
30 instToId, tyVarsOfInsts,
31 instBindingRequired, instCanBeGeneralised,
32 newDictsFromOld, instMentionsIPs,
33 getDictClassTys, isTyVarDict,
34 instLoc, pprInst, zonkInst, tidyInsts,
35 Inst, LIE, pprInsts, pprInstsInFull,
38 import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv )
39 import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
41 import TcType ( zonkTcTyVarsAndFV, tcInstTyVars )
42 import TcUnify ( unifyTauTy )
45 import NameSet ( mkNameSet )
46 import Class ( classBigSig )
47 import FunDeps ( oclose, grow, improve )
48 import PrelInfo ( isNumericClass, isCreturnableClass, isCcallishClass )
50 import Type ( Type, ThetaType, PredType, mkClassPred,
51 mkTyVarTy, getTyVar, isTyVarClassPred,
52 splitSigmaTy, tyVarsOfPred,
53 getClassPredTys_maybe, isClassPred, isIPPred,
56 import Subst ( mkTopTyVarSubst, substTheta, substTy )
57 import TysWiredIn ( unitTy )
61 import ListSetOps ( equivClasses )
62 import Util ( zipEqual )
63 import List ( partition )
68 %************************************************************************
72 %************************************************************************
74 --------------------------------------
75 Notes on quantification
76 --------------------------------------
78 Suppose we are about to do a generalisation step.
83 C the constraints from that RHS
85 The game is to figure out
87 Q the set of type variables over which to quantify
88 Ct the constraints we will *not* quantify over
89 Cq the constraints we will quantify over
91 So we're going to infer the type
95 and float the constraints Ct further outwards.
97 Here are the things that *must* be true:
99 (A) Q intersect fv(G) = EMPTY limits how big Q can be
100 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
102 (A) says we can't quantify over a variable that's free in the
103 environment. (B) says we must quantify over all the truly free
104 variables in T, else we won't get a sufficiently general type. We do
105 not *need* to quantify over any variable that is fixed by the free
106 vars of the environment G.
108 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
110 Example: class H x y | x->y where ...
112 fv(G) = {a} C = {H a b, H c d}
115 (A) Q intersect {a} is empty
116 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
118 So Q can be {c,d}, {b,c,d}
120 Other things being equal, however, we'd like to quantify over as few
121 variables as possible: smaller types, fewer type applications, more
122 constraints can get into Ct instead of Cq.
125 -----------------------------------------
128 fv(T) the free type vars of T
130 oclose(vs,C) The result of extending the set of tyvars vs
131 using the functional dependencies from C
133 grow(vs,C) The result of extend the set of tyvars vs
134 using all conceivable links from C.
136 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
137 Then grow(vs,C) = {a,b,c}
139 Note that grow(vs,C) `superset` grow(vs,simplify(C))
140 That is, simplfication can only shrink the result of grow.
143 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
144 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
147 -----------------------------------------
151 Here's a good way to choose Q:
153 Q = grow( fv(T), C ) \ oclose( fv(G), C )
155 That is, quantify over all variable that that MIGHT be fixed by the
156 call site (which influences T), but which aren't DEFINITELY fixed by
157 G. This choice definitely quantifies over enough type variables,
158 albeit perhaps too many.
160 Why grow( fv(T), C ) rather than fv(T)? Consider
162 class H x y | x->y where ...
167 If we used fv(T) = {c} we'd get the type
169 forall c. H c d => c -> b
171 And then if the fn was called at several different c's, each of
172 which fixed d differently, we'd get a unification error, because
173 d isn't quantified. Solution: quantify d. So we must quantify
174 everything that might be influenced by c.
176 Why not oclose( fv(T), C )? Because we might not be able to see
177 all the functional dependencies yet:
179 class H x y | x->y where ...
180 instance H x y => Eq (T x y) where ...
185 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
186 apparent yet, and that's wrong. We must really quantify over d too.
189 There really isn't any point in quantifying over any more than
190 grow( fv(T), C ), because the call sites can't possibly influence
191 any other type variables.
195 --------------------------------------
197 --------------------------------------
199 It's very hard to be certain when a type is ambiguous. Consider
203 instance H x y => K (x,y)
205 Is this type ambiguous?
206 forall a b. (K (a,b), Eq b) => a -> a
208 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
209 now we see that a fixes b. So we can't tell about ambiguity for sure
210 without doing a full simplification. And even that isn't possible if
211 the context has some free vars that may get unified. Urgle!
213 Here's another example: is this ambiguous?
214 forall a b. Eq (T b) => a -> a
215 Not if there's an insance decl (with no context)
216 instance Eq (T b) where ...
218 You may say of this example that we should use the instance decl right
219 away, but you can't always do that:
221 class J a b where ...
222 instance J Int b where ...
224 f :: forall a b. J a b => a -> a
226 (Notice: no functional dependency in J's class decl.)
227 Here f's type is perfectly fine, provided f is only called at Int.
228 It's premature to complain when meeting f's signature, or even
229 when inferring a type for f.
233 However, we don't *need* to report ambiguity right away. It'll always
234 show up at the call site.... and eventually at main, which needs special
235 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
237 So heres the plan. We WARN about probable ambiguity if
239 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
241 (all tested before quantification).
242 That is, all the type variables in Cq must be fixed by the the variables
243 in the environment, or by the variables in the type.
245 Notice that we union before calling oclose. Here's an example:
247 class J a b c | a b -> c
251 forall b c. (J a b c) => b -> b
253 Only if we union {a} from G with {b} from T before using oclose,
254 do we see that c is fixed.
256 It's a bit vague exactly which C we should use for this oclose call. If we
257 don't fix enough variables we might complain when we shouldn't (see
258 the above nasty example). Nothing will be perfect. That's why we can
259 only issue a warning.
262 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
264 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
266 then c is a "bubble"; there's no way it can ever improve, and it's
267 certainly ambiguous. UNLESS it is a constant (sigh). And what about
272 instance H x y => K (x,y)
274 Is this type ambiguous?
275 forall a b. (K (a,b), Eq b) => a -> a
277 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
278 is a "bubble" that's a set of constraints
280 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
282 Hence another idea. To decide Q start with fv(T) and grow it
283 by transitive closure in Cq (no functional dependencies involved).
284 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
285 The definitely-ambigous can then float out, and get smashed at top level
286 (which squashes out the constants, like Eq (T a) above)
289 --------------------------------------
290 Notes on implicit parameters
291 --------------------------------------
297 Then we get an LIE like (?y::Int). Doesn't constrain a type variable,
298 but we must nevertheless infer a type like
300 f :: (?y::Int) => Int -> Int
302 so that f is passed the value of y at the call site. Is this legal?
307 Should f be overloaded on "?y" ? Or does the type signature say that it
308 shouldn't be? Our position is that it should be illegal. Otherwise
309 you can change the *dynamic* semantics by adding a type signature:
311 (let f x = x + ?y -- f :: (?y::Int) => Int -> Int
312 in (f 3, f 3 with ?y=5)) with ?y = 6
318 in (f 3, f 3 with ?y=5)) with ?y = 6
322 URK! Let's not do this. So this is illegal:
327 BOTTOM LINE: you *must* quantify over implicit parameters.
330 --------------------------------------
331 Notes on principal types
332 --------------------------------------
337 f x = let g y = op (y::Int) in True
339 Here the principal type of f is (forall a. a->a)
340 but we'll produce the non-principal type
341 f :: forall a. C Int => a -> a
344 %************************************************************************
346 \subsection{tcSimplifyInfer}
348 %************************************************************************
350 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
352 1. Compute Q = grow( fvs(T), C )
354 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
355 predicates will end up in Ct; we deal with them at the top level
357 3. Try improvement, using functional dependencies
359 4. If Step 3 did any unification, repeat from step 1
360 (Unification can change the result of 'grow'.)
362 Note: we don't reduce dictionaries in step 2. For example, if we have
363 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
364 after step 2. However note that we may therefore quantify over more
365 type variables than we absolutely have to.
367 For the guts, we need a loop, that alternates context reduction and
368 improvement with unification. E.g. Suppose we have
370 class C x y | x->y where ...
372 and tcSimplify is called with:
374 Then improvement unifies a with b, giving
377 If we need to unify anything, we rattle round the whole thing all over
384 -> [TcTyVar] -- fv(T); type vars
386 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
388 TcDictBinds, -- Bindings
389 [TcId]) -- Dict Ids that must be bound here (zonked)
394 tcSimplifyInfer doc tau_tvs wanted_lie
395 = inferLoop doc tau_tvs (lieToList wanted_lie) `thenTc` \ (qtvs, frees, binds, irreds) ->
397 -- Check for non-generalisable insts
398 mapTc_ addCantGenErr (filter (not . instCanBeGeneralised) irreds) `thenTc_`
400 returnTc (qtvs, mkLIE frees, binds, map instToId irreds)
402 inferLoop doc tau_tvs wanteds
404 zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
405 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
406 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
408 preds = predsOfInsts wanteds'
409 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
412 | isFree qtvs inst = Free
413 | isClassDict inst = DontReduceUnlessConstant -- Dicts
414 | otherwise = ReduceMe -- Lits and Methods
417 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
420 if no_improvement then
421 returnTc (varSetElems qtvs, frees, binds, irreds)
423 -- If improvement did some unification, we go round again. There
424 -- are two subtleties:
425 -- a) We start again with irreds, not wanteds
426 -- Using an instance decl might have introduced a fresh type variable
427 -- which might have been unified, so we'd get an infinite loop
428 -- if we started again with wanteds! See example [LOOP]
430 -- b) It's also essential to re-process frees, because unification
431 -- might mean that a type variable that looked free isn't now.
433 -- Hence the (irreds ++ frees)
435 inferLoop doc tau_tvs (irreds ++ frees) `thenTc` \ (qtvs1, frees1, binds1, irreds1) ->
436 returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
441 class If b t e r | b t e -> r
444 class Lte a b c | a b -> c where lte :: a -> b -> c
446 instance (Lte a b l,If l b a c) => Max a b c
448 Wanted: Max Z (S x) y
450 Then we'll reduce using the Max instance to:
451 (Lte Z (S x) l, If l (S x) Z y)
452 and improve by binding l->T, after which we can do some reduction
453 on both the Lte and If constraints. What we *can't* do is start again
454 with (Max Z (S x) y)!
458 = not (tyVarsOfInst inst `intersectsVarSet` qtvs) -- Constrains no quantified vars
459 && all inheritablePred (predsOfInst inst) -- And no implicit parameter involved
460 -- (see "Notes on implicit parameters")
464 %************************************************************************
466 \subsection{tcSimplifyCheck}
468 %************************************************************************
470 @tcSimplifyCheck@ is used when we know exactly the set of variables
471 we are going to quantify over. For example, a class or instance declaration.
476 -> [TcTyVar] -- Quantify over these
480 TcDictBinds) -- Bindings
482 tcSimplifyCheck doc qtvs givens wanted_lie
483 = checkLoop doc qtvs givens (lieToList wanted_lie) try `thenTc` \ (frees, binds, irreds) ->
485 -- Complain about any irreducible ones
486 complainCheck doc givens irreds `thenNF_Tc_`
489 returnTc (mkLIE frees, binds)
491 -- When checking against a given signature we always reduce
492 -- until we find a match against something given, or can't reduce
493 try qtvs inst | isFree qtvs inst = Free
494 | otherwise = ReduceMe
496 tcSimplifyRestricted doc qtvs givens wanted_lie
497 = checkLoop doc qtvs givens (lieToList wanted_lie) try `thenTc` \ (frees, binds, irreds) ->
499 -- Complain about any irreducible ones
500 complainCheck doc givens irreds `thenNF_Tc_`
503 returnTc (mkLIE frees, binds)
505 try qtvs inst | not (tyVarsOfInst inst `intersectsVarSet` qtvs) = Free
506 | otherwise = ReduceMe
508 checkLoop doc qtvs givens wanteds try_me
510 zonkTcTyVarsAndFV qtvs `thenNF_Tc` \ qtvs' ->
511 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
512 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
515 reduceContext doc (try_me qtvs') givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
518 if no_improvement then
519 returnTc (frees, binds, irreds)
521 checkLoop doc qtvs givens' (irreds ++ frees) try_me `thenTc` \ (frees1, binds1, irreds1) ->
522 returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
524 complainCheck doc givens irreds
525 = mapNF_Tc zonkInst given_dicts `thenNF_Tc` \ givens' ->
526 mapNF_Tc (addNoInstanceErr doc given_dicts) irreds `thenNF_Tc_`
529 given_dicts = filter isDict givens
530 -- Filter out methods, which are only added to
531 -- the given set as an optimisation
536 %************************************************************************
538 \subsection{tcSimplifyAndCheck}
540 %************************************************************************
542 @tcSimplifyInferCheck@ is used when we know the consraints we are to simplify
543 against, but we don't know the type variables over which we are going to quantify.
544 This happens when we have a type signature for a mutually recursive
550 -> [TcTyVar] -- fv(T)
553 -> TcM ([TcTyVar], -- Variables over which to quantify
555 TcDictBinds) -- Bindings
557 tcSimplifyInferCheck doc tau_tvs givens wanted
558 = inferCheckLoop doc tau_tvs givens (lieToList wanted) `thenTc` \ (qtvs, frees, binds, irreds) ->
560 -- Complain about any irreducible ones
561 complainCheck doc givens irreds `thenNF_Tc_`
564 returnTc (qtvs, mkLIE frees, binds)
566 inferCheckLoop doc tau_tvs givens wanteds
568 zonkTcTyVarsAndFV tau_tvs `thenNF_Tc` \ tau_tvs' ->
569 mapNF_Tc zonkInst givens `thenNF_Tc` \ givens' ->
570 mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
571 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
574 -- Figure out what we are going to generalise over
575 -- You might think it should just be the signature tyvars,
576 -- but in bizarre cases you can get extra ones
577 -- f :: forall a. Num a => a -> a
578 -- f x = fst (g (x, head [])) + 1
580 -- Here we infer g :: forall a b. a -> b -> (b,a)
581 -- We don't want g to be monomorphic in b just because
582 -- f isn't quantified over b.
583 qtvs = (tau_tvs' `unionVarSet` tyVarsOfInsts givens') `minusVarSet` gbl_tvs
584 -- We could close gbl_tvs, but its not necessary for
585 -- soundness, and it'll only affect which tyvars, not which
586 -- dictionaries, we quantify over
588 -- When checking against a given signature we always reduce
589 -- until we find a match against something given, or can't reduce
590 try_me inst | isFree qtvs inst = Free
591 | otherwise = ReduceMe
594 reduceContext doc try_me givens' wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
597 if no_improvement then
598 returnTc (varSetElems qtvs, frees, binds, irreds)
600 inferCheckLoop doc tau_tvs givens' (irreds ++ frees) `thenTc` \ (qtvs1, frees1, binds1, irreds1) ->
601 returnTc (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
605 %************************************************************************
607 \subsection{tcSimplifyToDicts}
609 %************************************************************************
611 On the LHS of transformation rules we only simplify methods and constants,
612 getting dictionaries. We want to keep all of them unsimplified, to serve
613 as the available stuff for the RHS of the rule.
615 The same thing is used for specialise pragmas. Consider
618 {-# SPECIALISE f :: Int -> Int #-}
621 The type checker generates a binding like:
623 f_spec = (f :: Int -> Int)
625 and we want to end up with
627 f_spec = _inline_me_ (f Int dNumInt)
629 But that means that we must simplify the Method for f to (f Int dNumInt)!
630 So tcSimplifyToDicts squeezes out all Methods.
632 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
634 fromIntegral :: (Integral a, Num b) => a -> b
635 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
637 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
641 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
643 because the scsel will mess up matching. Instead we want
645 forall dIntegralInt, dNumInt.
646 fromIntegral Int Int dIntegralInt dNumInt = id Int
648 Hence "DontReduce NoSCs"
651 tcSimplifyToDicts :: LIE -> TcM ([Inst], TcDictBinds)
652 tcSimplifyToDicts wanted_lie
653 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
654 -- Since try_me doesn't look at types, we don't need to
655 -- do any zonking, so it's safe to call reduceContext directly
657 returnTc (irreds, binds)
660 doc = text "tcSimplifyToDicts"
661 wanteds = lieToList wanted_lie
663 -- Reduce methods and lits only; stop as soon as we get a dictionary
664 try_me inst | isDict inst = DontReduce NoSCs
665 | otherwise = ReduceMe
669 %************************************************************************
671 \subsection{Filtering at a dynamic binding}
673 %************************************************************************
678 we must discharge all the ?x constraints from B. We also do an improvement
679 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2. No need to iterate, though.
682 tcSimplifyIPs :: [Name] -- The implicit parameters bound here
684 -> TcM (LIE, TcDictBinds)
685 tcSimplifyIPs ip_names wanted_lie
686 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
687 -- The irreducible ones should be a subset of the implicit
688 -- parameters we provided
689 ASSERT( all here_ip irreds )
690 returnTc (mkLIE frees, binds)
693 doc = text "tcSimplifyIPs" <+> ppr ip_names
694 wanteds = lieToList wanted_lie
695 ip_set = mkNameSet ip_names
696 here_ip ip = isDict ip && ip `instMentionsIPs` ip_set
698 -- Simplify any methods that mention the implicit parameter
699 try_me inst | inst `instMentionsIPs` ip_set = ReduceMe
704 %************************************************************************
706 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
708 %************************************************************************
710 When doing a binding group, we may have @Insts@ of local functions.
711 For example, we might have...
713 let f x = x + 1 -- orig local function (overloaded)
714 f.1 = f Int -- two instances of f
719 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
720 where @f@ is in scope; those @Insts@ must certainly not be passed
721 upwards towards the top-level. If the @Insts@ were binding-ified up
722 there, they would have unresolvable references to @f@.
724 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
725 For each method @Inst@ in the @init_lie@ that mentions one of the
726 @Ids@, we create a binding. We return the remaining @Insts@ (in an
727 @LIE@), as well as the @HsBinds@ generated.
730 bindInstsOfLocalFuns :: LIE -> [TcId] -> TcM (LIE, TcMonoBinds)
732 bindInstsOfLocalFuns init_lie local_ids
733 | null overloaded_ids
735 = returnTc (init_lie, EmptyMonoBinds)
738 = simpleReduceLoop doc try_me wanteds `thenTc` \ (frees, binds, irreds) ->
739 ASSERT( null irreds )
740 returnTc (mkLIE frees, binds)
742 doc = text "bindInsts" <+> ppr local_ids
743 wanteds = lieToList init_lie
744 overloaded_ids = filter is_overloaded local_ids
745 is_overloaded id = case splitSigmaTy (idType id) of
746 (_, theta, _) -> not (null theta)
748 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
749 -- so it's worth building a set, so that
750 -- lookup (in isMethodFor) is faster
752 try_me inst | isMethodFor overloaded_set inst = ReduceMe
757 %************************************************************************
759 \subsection{Data types for the reduction mechanism}
761 %************************************************************************
763 The main control over context reduction is here
767 = ReduceMe -- Try to reduce this
768 -- If there's no instance, behave exactly like
769 -- DontReduce: add the inst to
770 -- the irreductible ones, but don't
771 -- produce an error message of any kind.
772 -- It might be quite legitimate such as (Eq a)!
774 | DontReduce WantSCs -- Return as irreducible
776 | DontReduceUnlessConstant -- Return as irreducible unless it can
777 -- be reduced to a constant in one step
779 | Free -- Return as free
781 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
782 -- of a predicate when adding it to the avails
788 type RedState = (Avails, -- What's available
789 [Inst]) -- Insts for which try_me returned Free
791 type Avails = FiniteMap Inst Avail
794 = Irred -- Used for irreducible dictionaries,
795 -- which are going to be lambda bound
797 | BoundTo TcId -- Used for dictionaries for which we have a binding
798 -- e.g. those "given" in a signature
800 | NoRhs -- Used for Insts like (CCallable f)
801 -- where no witness is required.
803 | Rhs -- Used when there is a RHS
805 [Inst] -- Insts free in the RHS; we need these too
807 pprAvails avails = vcat [ppr inst <+> equals <+> pprAvail avail
808 | (inst,avail) <- fmToList avails ]
810 instance Outputable Avail where
813 pprAvail NoRhs = text "<no rhs>"
814 pprAvail Irred = text "Irred"
815 pprAvail (BoundTo x) = text "Bound to" <+> ppr x
816 pprAvail (Rhs rhs bs) = ppr rhs <+> braces (ppr bs)
819 Extracting the bindings from a bunch of Avails.
820 The bindings do *not* come back sorted in dependency order.
821 We assume that they'll be wrapped in a big Rec, so that the
822 dependency analyser can sort them out later
826 bindsAndIrreds :: Avails
828 -> (TcDictBinds, -- Bindings
829 [Inst]) -- Irreducible ones
831 bindsAndIrreds avails wanteds
832 = go avails EmptyMonoBinds [] wanteds
834 go avails binds irreds [] = (binds, irreds)
836 go avails binds irreds (w:ws)
837 = case lookupFM avails w of
838 Nothing -> -- Free guys come out here
839 -- (If we didn't do addFree we could use this as the
840 -- criterion for free-ness, and pick up the free ones here too)
841 go avails binds irreds ws
843 Just NoRhs -> go avails binds irreds ws
845 Just Irred -> go (addToFM avails w (BoundTo (instToId w))) binds (w:irreds) ws
847 Just (BoundTo id) -> go avails new_binds irreds ws
849 -- For implicit parameters, all occurrences share the same
850 -- Id, so there is no need for synonym bindings
851 new_binds | new_id == id = binds
852 | otherwise = addBind binds new_id (HsVar id)
855 Just (Rhs rhs ws') -> go avails' (addBind binds id rhs) irreds (ws' ++ ws)
858 avails' = addToFM avails w (BoundTo id)
860 addBind binds id rhs = binds `AndMonoBinds` VarMonoBind id rhs
864 %************************************************************************
866 \subsection[reduce]{@reduce@}
868 %************************************************************************
870 When the "what to do" predicate doesn't depend on the quantified type variables,
871 matters are easier. We don't need to do any zonking, unless the improvement step
872 does something, in which case we zonk before iterating.
874 The "given" set is always empty.
877 simpleReduceLoop :: SDoc
878 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
880 -> TcM ([Inst], -- Free
882 [Inst]) -- Irreducible
884 simpleReduceLoop doc try_me wanteds
885 = mapNF_Tc zonkInst wanteds `thenNF_Tc` \ wanteds' ->
886 reduceContext doc try_me [] wanteds' `thenTc` \ (no_improvement, frees, binds, irreds) ->
887 if no_improvement then
888 returnTc (frees, binds, irreds)
890 simpleReduceLoop doc try_me (irreds ++ frees) `thenTc` \ (frees1, binds1, irreds1) ->
891 returnTc (frees1, binds `AndMonoBinds` binds1, irreds1)
897 reduceContext :: SDoc
898 -> (Inst -> WhatToDo)
901 -> NF_TcM (Bool, -- True <=> improve step did no unification
903 TcDictBinds, -- Dictionary bindings
904 [Inst]) -- Irreducible
906 reduceContext doc try_me givens wanteds
908 traceTc (text "reduceContext" <+> (vcat [
909 text "----------------------",
911 text "given" <+> ppr givens,
912 text "wanted" <+> ppr wanteds,
913 text "----------------------"
916 -- Build the Avail mapping from "givens"
917 foldlNF_Tc addGiven (emptyFM, []) givens `thenNF_Tc` \ init_state ->
920 reduceList (0,[]) try_me wanteds init_state `thenNF_Tc` \ state@(avails, frees) ->
922 -- Do improvement, using everything in avails
923 -- In particular, avails includes all superclasses of everything
924 tcImprove avails `thenTc` \ no_improvement ->
926 traceTc (text "reduceContext end" <+> (vcat [
927 text "----------------------",
929 text "given" <+> ppr givens,
930 text "wanted" <+> ppr wanteds,
932 text "avails" <+> pprAvails avails,
933 text "frees" <+> ppr frees,
934 text "no_improvement =" <+> ppr no_improvement,
935 text "----------------------"
938 (binds, irreds) = bindsAndIrreds avails wanteds
940 returnTc (no_improvement, frees, binds, irreds)
943 = tcGetInstEnv `thenTc` \ inst_env ->
945 preds = predsOfInsts (keysFM avails)
946 -- Avails has all the superclasses etc (good)
947 -- It also has all the intermediates of the deduction (good)
948 -- It does not have duplicates (good)
949 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
950 -- so that improve will see them separate
951 eqns = improve (classInstEnv inst_env) preds
956 traceTc (ptext SLIT("Improve:") <+> vcat (map ppr_eqn eqns)) `thenNF_Tc_`
957 mapTc_ unify eqns `thenTc_`
960 unify (qtvs, t1, t2) = tcInstTyVars (varSetElems qtvs) `thenNF_Tc` \ (_, _, tenv) ->
961 unifyTauTy (substTy tenv t1) (substTy tenv t2)
962 ppr_eqn (qtvs, t1, t2) = ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)) <+>
963 ppr t1 <+> equals <+> ppr t2
966 The main context-reduction function is @reduce@. Here's its game plan.
969 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
970 -- along with its depth
971 -> (Inst -> WhatToDo)
978 try_me: given an inst, this function returns
980 DontReduce return this in "irreds"
981 Free return this in "frees"
983 wanteds: The list of insts to reduce
984 state: An accumulating parameter of type RedState
985 that contains the state of the algorithm
987 It returns a RedState.
989 The (n,stack) pair is just used for error reporting.
990 n is always the depth of the stack.
991 The stack is the stack of Insts being reduced: to produce X
992 I had to produce Y, to produce Y I had to produce Z, and so on.
995 reduceList (n,stack) try_me wanteds state
996 | n > opt_MaxContextReductionDepth
997 = failWithTc (reduceDepthErr n stack)
1003 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1008 go [] state = returnTc state
1009 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenTc` \ state' ->
1012 -- Base case: we're done!
1013 reduce stack try_me wanted state
1014 -- It's the same as an existing inst, or a superclass thereof
1015 | isAvailable state wanted
1019 = case try_me wanted of {
1021 DontReduce want_scs -> addIrred want_scs state wanted
1023 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1024 -- First, see if the inst can be reduced to a constant in one step
1025 try_simple (addIrred AddSCs) -- Assume want superclasses
1027 ; Free -> -- It's free so just chuck it upstairs
1028 -- First, see if the inst can be reduced to a constant in one step
1031 ; ReduceMe -> -- It should be reduced
1032 lookupInst wanted `thenNF_Tc` \ lookup_result ->
1033 case lookup_result of
1034 GenInst wanteds' rhs -> reduceList stack try_me wanteds' state `thenTc` \ state' ->
1035 addWanted state' wanted rhs wanteds'
1036 SimpleInst rhs -> addWanted state wanted rhs []
1038 NoInstance -> -- No such instance!
1039 -- Add it and its superclasses
1040 addIrred AddSCs state wanted
1044 try_simple do_this_otherwise
1045 = lookupInst wanted `thenNF_Tc` \ lookup_result ->
1046 case lookup_result of
1047 SimpleInst rhs -> addWanted state wanted rhs []
1048 other -> do_this_otherwise state wanted
1053 isAvailable :: RedState -> Inst -> Bool
1054 isAvailable (avails, _) wanted = wanted `elemFM` avails
1055 -- NB: the Ord instance of Inst compares by the class/type info
1056 -- *not* by unique. So
1057 -- d1::C Int == d2::C Int
1059 -------------------------
1060 addFree :: RedState -> Inst -> NF_TcM RedState
1061 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1062 -- to avails, so that any other equal Insts will be commoned up right
1063 -- here rather than also being tossed upstairs. This is really just
1064 -- an optimisation, and perhaps it is more trouble that it is worth,
1065 -- as the following comments show!
1067 -- NB1: do *not* add superclasses. If we have
1070 -- but a is not bound here, then we *don't* want to derive
1071 -- dn from df here lest we lose sharing.
1073 -- NB2: do *not* add the Inst to avails at all if it's a method.
1074 -- The following situation shows why this is bad:
1075 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1076 -- From an application (truncate f i) we get
1077 -- t1 = truncate at f
1079 -- If we have also have a second occurrence of truncate, we get
1080 -- t3 = truncate at f
1082 -- When simplifying with i,f free, we might still notice that
1083 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1084 -- will continue to float out!
1085 -- Solution: never put methods in avail till they are captured
1086 -- in which case addFree isn't used
1088 -- NB3: make sure that CCallable/CReturnable use NoRhs rather
1089 -- than BoundTo, else we end up with bogus bindings.
1090 -- c.f. instBindingRequired in addWanted
1091 addFree (avails, frees) free
1092 | isDict free = returnNF_Tc (addToFM avails free avail, free:frees)
1093 | otherwise = returnNF_Tc (avails, free:frees)
1095 avail | instBindingRequired free = BoundTo (instToId free)
1098 addWanted :: RedState -> Inst -> TcExpr -> [Inst] -> NF_TcM RedState
1099 addWanted state@(avails, frees) wanted rhs_expr wanteds
1100 -- Do *not* add superclasses as well. Here's an example of why not
1101 -- class Eq a => Foo a b
1102 -- instance Eq a => Foo [a] a
1103 -- If we are reducing
1105 -- we'll first deduce that it holds (via the instance decl). We
1106 -- must not then overwrite the Eq t constraint with a superclass selection!
1107 -- ToDo: this isn't entirely unsatisfactory, because
1108 -- we may also lose some entirely-legitimate sharing this way
1110 = ASSERT( not (isAvailable state wanted) )
1111 returnNF_Tc (addToFM avails wanted avail, frees)
1113 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1114 | otherwise = ASSERT( null wanteds ) NoRhs
1116 addGiven :: RedState -> Inst -> NF_TcM RedState
1117 addGiven state given = addAvailAndSCs state given (BoundTo (instToId given))
1119 addIrred :: WantSCs -> RedState -> Inst -> NF_TcM RedState
1120 addIrred NoSCs (avails,frees) irred = returnNF_Tc (addToFM avails irred Irred, frees)
1121 addIrred AddSCs state irred = addAvailAndSCs state irred Irred
1123 addAvailAndSCs :: RedState -> Inst -> Avail -> NF_TcM RedState
1124 addAvailAndSCs (avails, frees) wanted avail
1125 = add_avail_and_scs avails wanted avail `thenNF_Tc` \ avails' ->
1126 returnNF_Tc (avails', frees)
1128 ---------------------
1129 add_avail_and_scs :: Avails -> Inst -> Avail -> NF_TcM Avails
1130 add_avail_and_scs avails wanted avail
1131 = add_scs (addToFM avails wanted avail) wanted
1133 add_scs :: Avails -> Inst -> NF_TcM Avails
1134 -- Add all the superclasses of the Inst to Avails
1135 -- Invariant: the Inst is already in Avails.
1138 | not (isClassDict dict)
1139 = returnNF_Tc avails
1141 | otherwise -- It is a dictionary
1142 = newDictsFromOld dict sc_theta' `thenNF_Tc` \ sc_dicts ->
1143 foldlNF_Tc add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1145 (clas, tys) = getDictClassTys dict
1146 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1147 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1149 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1150 = case lookupFM avails sc_dict of
1151 Just (BoundTo _) -> returnNF_Tc avails -- See Note [SUPER] below
1152 other -> add_avail_and_scs avails sc_dict avail
1154 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1155 avail = Rhs sc_sel_rhs [dict]
1158 Note [SUPER]. We have to be careful here. If we are *given* d1:Ord a,
1159 and want to deduce (d2:C [a]) where
1161 class Ord a => C a where
1162 instance Ord a => C [a] where ...
1164 Then we'll use the instance decl to deduce C [a] and then add the
1165 superclasses of C [a] to avails. But we must not overwrite the binding
1166 for d1:Ord a (which is given) with a superclass selection or we'll just
1167 build a loop! Hence looking for BoundTo. Crudely, BoundTo is cheaper
1171 %************************************************************************
1173 \section{tcSimplifyTop: defaulting}
1175 %************************************************************************
1178 If a dictionary constrains a type variable which is
1179 * not mentioned in the environment
1180 * and not mentioned in the type of the expression
1181 then it is ambiguous. No further information will arise to instantiate
1182 the type variable; nor will it be generalised and turned into an extra
1183 parameter to a function.
1185 It is an error for this to occur, except that Haskell provided for
1186 certain rules to be applied in the special case of numeric types.
1188 * at least one of its classes is a numeric class, and
1189 * all of its classes are numeric or standard
1190 then the type variable can be defaulted to the first type in the
1191 default-type list which is an instance of all the offending classes.
1193 So here is the function which does the work. It takes the ambiguous
1194 dictionaries and either resolves them (producing bindings) or
1195 complains. It works by splitting the dictionary list by type
1196 variable, and using @disambigOne@ to do the real business.
1198 @tcSimplifyTop@ is called once per module to simplify all the constant
1199 and ambiguous Insts.
1201 We need to be careful of one case. Suppose we have
1203 instance Num a => Num (Foo a b) where ...
1205 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1206 to (Num x), and default x to Int. But what about y??
1208 It's OK: the final zonking stage should zap y to (), which is fine.
1212 tcSimplifyTop :: LIE -> TcM TcDictBinds
1213 tcSimplifyTop wanted_lie
1214 = simpleReduceLoop (text "tcSimplTop") try_me wanteds `thenTc` \ (frees, binds, irreds) ->
1215 ASSERT( null frees )
1218 -- All the non-std ones are definite errors
1219 (stds, non_stds) = partition isStdClassTyVarDict irreds
1221 -- Group by type variable
1222 std_groups = equivClasses cmp_by_tyvar stds
1224 -- Pick the ones which its worth trying to disambiguate
1225 (std_oks, std_bads) = partition worth_a_try std_groups
1227 -- Have a try at disambiguation
1228 -- if the type variable isn't bound
1229 -- up with one of the non-standard classes
1230 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1231 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1233 -- Collect together all the bad guys
1234 bad_guys = non_stds ++ concat std_bads
1236 -- Disambiguate the ones that look feasible
1237 mapTc disambigGroup std_oks `thenTc` \ binds_ambig ->
1239 -- And complain about the ones that don't
1240 -- This group includes both non-existent instances
1241 -- e.g. Num (IO a) and Eq (Int -> Int)
1242 -- and ambiguous dictionaries
1244 addTopAmbigErrs bad_guys `thenNF_Tc_`
1246 returnTc (binds `andMonoBinds` andMonoBindList binds_ambig)
1248 wanteds = lieToList wanted_lie
1249 try_me inst = ReduceMe
1251 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1253 get_tv d = case getDictClassTys d of
1254 (clas, [ty]) -> getTyVar "tcSimplifyTop" ty
1255 get_clas d = case getDictClassTys d of
1256 (clas, [ty]) -> clas
1259 @disambigOne@ assumes that its arguments dictionaries constrain all
1260 the same type variable.
1262 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1263 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1264 the most common use of defaulting is code like:
1266 _ccall_ foo `seqPrimIO` bar
1268 Since we're not using the result of @foo@, the result if (presumably)
1272 disambigGroup :: [Inst] -- All standard classes of form (C a)
1276 | any isNumericClass classes -- Guaranteed all standard classes
1277 -- see comment at the end of function for reasons as to
1278 -- why the defaulting mechanism doesn't apply to groups that
1279 -- include CCallable or CReturnable dicts.
1280 && not (any isCcallishClass classes)
1281 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1282 -- SO, TRY DEFAULT TYPES IN ORDER
1284 -- Failure here is caused by there being no type in the
1285 -- default list which can satisfy all the ambiguous classes.
1286 -- For example, if Real a is reqd, but the only type in the
1287 -- default list is Int.
1288 tcGetDefaultTys `thenNF_Tc` \ default_tys ->
1290 try_default [] -- No defaults work, so fail
1293 try_default (default_ty : default_tys)
1294 = tryTc_ (try_default default_tys) $ -- If default_ty fails, we try
1295 -- default_tys instead
1296 tcSimplifyCheckThetas [] theta `thenTc` \ _ ->
1299 theta = [mkClassPred clas [default_ty] | clas <- classes]
1301 -- See if any default works, and if so bind the type variable to it
1302 -- If not, add an AmbigErr
1303 recoverTc (addAmbigErrs dicts `thenNF_Tc_`
1304 returnTc EmptyMonoBinds) $
1306 try_default default_tys `thenTc` \ chosen_default_ty ->
1308 -- Bind the type variable and reduce the context, for real this time
1309 unifyTauTy chosen_default_ty (mkTyVarTy tyvar) `thenTc_`
1310 simpleReduceLoop (text "disambig" <+> ppr dicts)
1311 try_me dicts `thenTc` \ (frees, binds, ambigs) ->
1312 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1313 warnDefault dicts chosen_default_ty `thenTc_`
1316 | all isCreturnableClass classes
1317 = -- Default CCall stuff to (); we don't even both to check that () is an
1318 -- instance of CReturnable, because we know it is.
1319 unifyTauTy (mkTyVarTy tyvar) unitTy `thenTc_`
1320 returnTc EmptyMonoBinds
1322 | otherwise -- No defaults
1323 = addAmbigErrs dicts `thenNF_Tc_`
1324 returnTc EmptyMonoBinds
1327 try_me inst = ReduceMe -- This reduce should not fail
1328 tyvar = get_tv (head dicts) -- Should be non-empty
1329 classes = map get_clas dicts
1332 [Aside - why the defaulting mechanism is turned off when
1333 dealing with arguments and results to ccalls.
1335 When typechecking _ccall_s, TcExpr ensures that the external
1336 function is only passed arguments (and in the other direction,
1337 results) of a restricted set of 'native' types. This is
1338 implemented via the help of the pseudo-type classes,
1339 @CReturnable@ (CR) and @CCallable@ (CC.)
1341 The interaction between the defaulting mechanism for numeric
1342 values and CC & CR can be a bit puzzling to the user at times.
1351 What type has 'x' got here? That depends on the default list
1352 in operation, if it is equal to Haskell 98's default-default
1353 of (Integer, Double), 'x' has type Double, since Integer
1354 is not an instance of CR. If the default list is equal to
1355 Haskell 1.4's default-default of (Int, Double), 'x' has type
1358 To try to minimise the potential for surprises here, the
1359 defaulting mechanism is turned off in the presence of
1360 CCallable and CReturnable.
1365 %************************************************************************
1367 \subsection[simple]{@Simple@ versions}
1369 %************************************************************************
1371 Much simpler versions when there are no bindings to make!
1373 @tcSimplifyThetas@ simplifies class-type constraints formed by
1374 @deriving@ declarations and when specialising instances. We are
1375 only interested in the simplified bunch of class/type constraints.
1377 It simplifies to constraints of the form (C a b c) where
1378 a,b,c are type variables. This is required for the context of
1379 instance declarations.
1382 tcSimplifyThetas :: ThetaType -- Wanted
1383 -> TcM ThetaType -- Needed
1385 tcSimplifyThetas wanteds
1386 = doptsTc Opt_GlasgowExts `thenNF_Tc` \ glaExts ->
1387 reduceSimple [] wanteds `thenNF_Tc` \ irreds ->
1389 -- For multi-param Haskell, check that the returned dictionaries
1390 -- don't have any of the form (C Int Bool) for which
1391 -- we expect an instance here
1392 -- For Haskell 98, check that all the constraints are of the form C a,
1393 -- where a is a type variable
1394 bad_guys | glaExts = [pred | pred <- irreds,
1395 isEmptyVarSet (tyVarsOfPred pred)]
1396 | otherwise = [pred | pred <- irreds,
1397 not (isTyVarClassPred pred)]
1399 if null bad_guys then
1402 mapNF_Tc addNoInstErr bad_guys `thenNF_Tc_`
1406 @tcSimplifyCheckThetas@ just checks class-type constraints, essentially;
1407 used with \tr{default} declarations. We are only interested in
1408 whether it worked or not.
1411 tcSimplifyCheckThetas :: ThetaType -- Given
1412 -> ThetaType -- Wanted
1415 tcSimplifyCheckThetas givens wanteds
1416 = reduceSimple givens wanteds `thenNF_Tc` \ irreds ->
1420 mapNF_Tc addNoInstErr irreds `thenNF_Tc_`
1426 type AvailsSimple = FiniteMap PredType Bool
1427 -- True => irreducible
1428 -- False => given, or can be derived from a given or from an irreducible
1430 reduceSimple :: ThetaType -- Given
1431 -> ThetaType -- Wanted
1432 -> NF_TcM ThetaType -- Irreducible
1434 reduceSimple givens wanteds
1435 = reduce_simple (0,[]) givens_fm wanteds `thenNF_Tc` \ givens_fm' ->
1436 returnNF_Tc [pred | (pred,True) <- fmToList givens_fm']
1438 givens_fm = foldl addNonIrred emptyFM givens
1440 reduce_simple :: (Int,ThetaType) -- Stack
1443 -> NF_TcM AvailsSimple
1445 reduce_simple (n,stack) avails wanteds
1448 go avails [] = returnNF_Tc avails
1449 go avails (w:ws) = reduce_simple_help (n+1,w:stack) avails w `thenNF_Tc` \ avails' ->
1452 reduce_simple_help stack givens wanted
1453 | wanted `elemFM` givens
1454 = returnNF_Tc givens
1456 | Just (clas, tys) <- getClassPredTys_maybe wanted
1457 = lookupSimpleInst clas tys `thenNF_Tc` \ maybe_theta ->
1459 Nothing -> returnNF_Tc (addSimpleIrred givens wanted)
1460 Just theta -> reduce_simple stack (addNonIrred givens wanted) theta
1463 = returnNF_Tc (addSimpleIrred givens wanted)
1465 addSimpleIrred :: AvailsSimple -> PredType -> AvailsSimple
1466 addSimpleIrred givens pred
1467 = addSCs (addToFM givens pred True) pred
1469 addNonIrred :: AvailsSimple -> PredType -> AvailsSimple
1470 addNonIrred givens pred
1471 = addSCs (addToFM givens pred False) pred
1474 | not (isClassPred pred) = givens
1475 | otherwise = foldl add givens sc_theta
1477 Just (clas,tys) = getClassPredTys_maybe pred
1478 (tyvars, sc_theta_tmpl, _, _) = classBigSig clas
1479 sc_theta = substTheta (mkTopTyVarSubst tyvars tys) sc_theta_tmpl
1482 = case lookupFM givens ct of
1483 Nothing -> -- Add it and its superclasses
1484 addSCs (addToFM givens ct False) ct
1486 Just True -> -- Set its flag to False; superclasses already done
1487 addToFM givens ct False
1489 Just False -> -- Already done
1495 %************************************************************************
1497 \section{Errors and contexts}
1499 %************************************************************************
1501 ToDo: for these error messages, should we note the location as coming
1502 from the insts, or just whatever seems to be around in the monad just
1506 addTopAmbigErrs dicts
1507 = mapNF_Tc complain tidy_dicts
1509 fixed_tvs = oclose (predsOfInsts tidy_dicts) emptyVarSet
1510 (tidy_env, tidy_dicts) = tidyInsts dicts
1511 complain d | any isIPPred (predsOfInst d) = addTopIPErr tidy_env d
1512 | not (isTyVarDict d) ||
1513 tyVarsOfInst d `subVarSet` fixed_tvs = addTopInstanceErr tidy_env d
1514 | otherwise = addAmbigErr tidy_env d
1516 addTopIPErr tidy_env tidy_dict
1517 = addInstErrTcM (instLoc tidy_dict)
1519 ptext SLIT("Unbound implicit parameter") <+> quotes (pprInst tidy_dict))
1521 -- Used for top-level irreducibles
1522 addTopInstanceErr tidy_env tidy_dict
1523 = addInstErrTcM (instLoc tidy_dict)
1525 ptext SLIT("No instance for") <+> quotes (pprInst tidy_dict))
1528 = mapNF_Tc (addAmbigErr tidy_env) tidy_dicts
1530 (tidy_env, tidy_dicts) = tidyInsts dicts
1532 addAmbigErr tidy_env tidy_dict
1533 = addInstErrTcM (instLoc tidy_dict)
1535 sep [text "Ambiguous type variable(s)" <+> pprQuotedList ambig_tvs,
1536 nest 4 (text "in the constraint" <+> quotes (pprInst tidy_dict))])
1538 ambig_tvs = varSetElems (tyVarsOfInst tidy_dict)
1540 warnDefault dicts default_ty
1541 = doptsTc Opt_WarnTypeDefaults `thenTc` \ warn_flag ->
1542 tcAddSrcLoc (get_loc (head dicts)) (warnTc warn_flag warn_msg)
1545 (_, tidy_dicts) = tidyInsts dicts
1546 get_loc i = case instLoc i of { (_,loc,_) -> loc }
1547 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
1548 quotes (ppr default_ty),
1549 pprInstsInFull tidy_dicts]
1551 -- The error message when we don't find a suitable instance
1552 -- is complicated by the fact that sometimes this is because
1553 -- there is no instance, and sometimes it's because there are
1554 -- too many instances (overlap). See the comments in TcEnv.lhs
1555 -- with the InstEnv stuff.
1556 addNoInstanceErr what_doc givens dict
1557 = tcGetInstEnv `thenNF_Tc` \ inst_env ->
1559 doc = vcat [sep [herald <+> quotes (pprInst tidy_dict),
1560 nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
1562 ptext SLIT("Probable fix:"),
1566 herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
1567 unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
1571 | not ambig_overlap = empty
1573 = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
1574 nest 4 (ptext SLIT("depends on the instantiation of") <+>
1575 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst tidy_dict))))]
1577 fix1 = sep [ptext SLIT("Add") <+> quotes (pprInst tidy_dict),
1578 ptext SLIT("to the") <+> what_doc]
1580 fix2 | isTyVarDict dict || ambig_overlap
1583 = ptext SLIT("Or add an instance declaration for") <+> quotes (pprInst tidy_dict)
1585 (tidy_env, tidy_dict:tidy_givens) = tidyInsts (dict:givens)
1587 -- Checks for the ambiguous case when we have overlapping instances
1588 ambig_overlap | isClassDict dict
1589 = case lookupInstEnv inst_env clas tys of
1590 NoMatch ambig -> ambig
1594 (clas,tys) = getDictClassTys dict
1596 addInstErrTcM (instLoc dict) (tidy_env, doc)
1598 -- Used for the ...Thetas variants; all top level
1600 = addErrTc (ptext SLIT("No instance for") <+> quotes (ppr pred))
1602 reduceDepthErr n stack
1603 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
1604 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
1605 nest 4 (pprInstsInFull stack)]
1607 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)
1609 -----------------------------------------------
1611 = addErrTc (sep [ptext SLIT("Cannot generalise these overloadings (in a _ccall_):"),
1612 nest 4 (ppr inst <+> pprInstLoc (instLoc inst))])