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,
41 isIPDict, isInheritableInst
43 import TcEnv ( tcGetGlobalTyVars, tcGetInstEnv, tcLookupId, findGlobals )
44 import InstEnv ( lookupInstEnv, classInstEnv, InstLookupResult(..) )
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, showClassKey, eqClassKey, ordClassKey)
58 import HscTypes ( GhciMode(Interactive) )
60 import Subst ( mkTopTyVarSubst, substTheta, substTy )
61 import TysWiredIn ( unitTy, pairTyCon )
62 import ErrUtils ( Message )
64 import VarEnv ( TidyEnv )
67 import ListSetOps ( equivClasses )
68 import Unique ( hasKey )
69 import Util ( zipEqual, isSingleton )
70 import List ( partition )
75 %************************************************************************
79 %************************************************************************
81 --------------------------------------
82 Notes on quantification
83 --------------------------------------
85 Suppose we are about to do a generalisation step.
90 C the constraints from that RHS
92 The game is to figure out
94 Q the set of type variables over which to quantify
95 Ct the constraints we will *not* quantify over
96 Cq the constraints we will quantify over
98 So we're going to infer the type
102 and float the constraints Ct further outwards.
104 Here are the things that *must* be true:
106 (A) Q intersect fv(G) = EMPTY limits how big Q can be
107 (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
109 (A) says we can't quantify over a variable that's free in the
110 environment. (B) says we must quantify over all the truly free
111 variables in T, else we won't get a sufficiently general type. We do
112 not *need* to quantify over any variable that is fixed by the free
113 vars of the environment G.
115 BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
117 Example: class H x y | x->y where ...
119 fv(G) = {a} C = {H a b, H c d}
122 (A) Q intersect {a} is empty
123 (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
125 So Q can be {c,d}, {b,c,d}
127 Other things being equal, however, we'd like to quantify over as few
128 variables as possible: smaller types, fewer type applications, more
129 constraints can get into Ct instead of Cq.
132 -----------------------------------------
135 fv(T) the free type vars of T
137 oclose(vs,C) The result of extending the set of tyvars vs
138 using the functional dependencies from C
140 grow(vs,C) The result of extend the set of tyvars vs
141 using all conceivable links from C.
143 E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e}
144 Then grow(vs,C) = {a,b,c}
146 Note that grow(vs,C) `superset` grow(vs,simplify(C))
147 That is, simplfication can only shrink the result of grow.
150 oclose is conservative one way: v `elem` oclose(vs,C) => v is definitely fixed by vs
151 grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C)
154 -----------------------------------------
158 Here's a good way to choose Q:
160 Q = grow( fv(T), C ) \ oclose( fv(G), C )
162 That is, quantify over all variable that that MIGHT be fixed by the
163 call site (which influences T), but which aren't DEFINITELY fixed by
164 G. This choice definitely quantifies over enough type variables,
165 albeit perhaps too many.
167 Why grow( fv(T), C ) rather than fv(T)? Consider
169 class H x y | x->y where ...
174 If we used fv(T) = {c} we'd get the type
176 forall c. H c d => c -> b
178 And then if the fn was called at several different c's, each of
179 which fixed d differently, we'd get a unification error, because
180 d isn't quantified. Solution: quantify d. So we must quantify
181 everything that might be influenced by c.
183 Why not oclose( fv(T), C )? Because we might not be able to see
184 all the functional dependencies yet:
186 class H x y | x->y where ...
187 instance H x y => Eq (T x y) where ...
192 Now oclose(fv(T),C) = {c}, because the functional dependency isn't
193 apparent yet, and that's wrong. We must really quantify over d too.
196 There really isn't any point in quantifying over any more than
197 grow( fv(T), C ), because the call sites can't possibly influence
198 any other type variables.
202 --------------------------------------
204 --------------------------------------
206 It's very hard to be certain when a type is ambiguous. Consider
210 instance H x y => K (x,y)
212 Is this type ambiguous?
213 forall a b. (K (a,b), Eq b) => a -> a
215 Looks like it! But if we simplify (K (a,b)) we get (H a b) and
216 now we see that a fixes b. So we can't tell about ambiguity for sure
217 without doing a full simplification. And even that isn't possible if
218 the context has some free vars that may get unified. Urgle!
220 Here's another example: is this ambiguous?
221 forall a b. Eq (T b) => a -> a
222 Not if there's an insance decl (with no context)
223 instance Eq (T b) where ...
225 You may say of this example that we should use the instance decl right
226 away, but you can't always do that:
228 class J a b where ...
229 instance J Int b where ...
231 f :: forall a b. J a b => a -> a
233 (Notice: no functional dependency in J's class decl.)
234 Here f's type is perfectly fine, provided f is only called at Int.
235 It's premature to complain when meeting f's signature, or even
236 when inferring a type for f.
240 However, we don't *need* to report ambiguity right away. It'll always
241 show up at the call site.... and eventually at main, which needs special
242 treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
244 So here's the plan. We WARN about probable ambiguity if
246 fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
248 (all tested before quantification).
249 That is, all the type variables in Cq must be fixed by the the variables
250 in the environment, or by the variables in the type.
252 Notice that we union before calling oclose. Here's an example:
254 class J a b c | a b -> c
258 forall b c. (J a b c) => b -> b
260 Only if we union {a} from G with {b} from T before using oclose,
261 do we see that c is fixed.
263 It's a bit vague exactly which C we should use for this oclose call. If we
264 don't fix enough variables we might complain when we shouldn't (see
265 the above nasty example). Nothing will be perfect. That's why we can
266 only issue a warning.
269 Can we ever be *certain* about ambiguity? Yes: if there's a constraint
271 c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
273 then c is a "bubble"; there's no way it can ever improve, and it's
274 certainly ambiguous. UNLESS it is a constant (sigh). And what about
279 instance H x y => K (x,y)
281 Is this type ambiguous?
282 forall a b. (K (a,b), Eq b) => a -> a
284 Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
285 is a "bubble" that's a set of constraints
287 Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
289 Hence another idea. To decide Q start with fv(T) and grow it
290 by transitive closure in Cq (no functional dependencies involved).
291 Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
292 The definitely-ambiguous can then float out, and get smashed at top level
293 (which squashes out the constants, like Eq (T a) above)
296 --------------------------------------
297 Notes on principal types
298 --------------------------------------
303 f x = let g y = op (y::Int) in True
305 Here the principal type of f is (forall a. a->a)
306 but we'll produce the non-principal type
307 f :: forall a. C Int => a -> a
310 --------------------------------------
311 Notes on implicit parameters
312 --------------------------------------
314 Question 1: can we "inherit" implicit parameters
315 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
320 where f is *not* a top-level binding.
321 From the RHS of f we'll get the constraint (?y::Int).
322 There are two types we might infer for f:
326 (so we get ?y from the context of f's definition), or
328 f :: (?y::Int) => Int -> Int
330 At first you might think the first was better, becuase then
331 ?y behaves like a free variable of the definition, rather than
332 having to be passed at each call site. But of course, the WHOLE
333 IDEA is that ?y should be passed at each call site (that's what
334 dynamic binding means) so we'd better infer the second.
336 BOTTOM LINE: when *inferring types* you *must* quantify
337 over implicit parameters. See the predicate isFreeWhenInferring.
340 Question 2: type signatures
341 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
342 BUT WATCH OUT: When you supply a type signature, we can't force you
343 to quantify over implicit parameters. For example:
347 This is perfectly reasonable. We do not want to insist on
349 (?x + 1) :: (?x::Int => Int)
351 That would be silly. Here, the definition site *is* the occurrence site,
352 so the above strictures don't apply. Hence the difference between
353 tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
354 and tcSimplifyCheckBind (which does not).
356 What about when you supply a type signature for a binding?
357 Is it legal to give the following explicit, user type
358 signature to f, thus:
363 At first sight this seems reasonable, but it has the nasty property
364 that adding a type signature changes the dynamic semantics.
367 (let f x = (x::Int) + ?y
368 in (f 3, f 3 with ?y=5)) with ?y = 6
374 in (f 3, f 3 with ?y=5)) with ?y = 6
378 Indeed, simply inlining f (at the Haskell source level) would change the
381 Nevertheless, as Launchbury says (email Oct 01) we can't really give the
382 semantics for a Haskell program without knowing its typing, so if you
383 change the typing you may change the semantics.
385 To make things consistent in all cases where we are *checking* against
386 a supplied signature (as opposed to inferring a type), we adopt the
389 a signature does not need to quantify over implicit params.
391 [This represents a (rather marginal) change of policy since GHC 5.02,
392 which *required* an explicit signature to quantify over all implicit
393 params for the reasons mentioned above.]
395 But that raises a new question. Consider
397 Given (signature) ?x::Int
398 Wanted (inferred) ?x::Int, ?y::Bool
400 Clearly we want to discharge the ?x and float the ?y out. But
401 what is the criterion that distinguishes them? Clearly it isn't
402 what free type variables they have. The Right Thing seems to be
403 to float a constraint that
404 neither mentions any of the quantified type variables
405 nor any of the quantified implicit parameters
407 See the predicate isFreeWhenChecking.
410 Question 3: monomorphism
411 ~~~~~~~~~~~~~~~~~~~~~~~~
412 There's a nasty corner case when the monomorphism restriction bites:
416 The argument above suggests that we *must* generalise
417 over the ?y parameter, to get
418 z :: (?y::Int) => Int,
419 but the monomorphism restriction says that we *must not*, giving
421 Why does the momomorphism restriction say this? Because if you have
423 let z = x + ?y in z+z
425 you might not expect the addition to be done twice --- but it will if
426 we follow the argument of Question 2 and generalise over ?y.
432 (A) Always generalise over implicit parameters
433 Bindings that fall under the monomorphism restriction can't
437 * Inlining remains valid
438 * No unexpected loss of sharing
439 * But simple bindings like
441 will be rejected, unless you add an explicit type signature
442 (to avoid the monomorphism restriction)
443 z :: (?y::Int) => Int
445 This seems unacceptable
447 (B) Monomorphism restriction "wins"
448 Bindings that fall under the monomorphism restriction can't
450 Always generalise over implicit parameters *except* for bindings
451 that fall under the monomorphism restriction
454 * Inlining isn't valid in general
455 * No unexpected loss of sharing
456 * Simple bindings like
458 accepted (get value of ?y from binding site)
460 (C) Always generalise over implicit parameters
461 Bindings that fall under the monomorphism restriction can't
462 be generalised, EXCEPT for implicit parameters
464 * Inlining remains valid
465 * Unexpected loss of sharing (from the extra generalisation)
466 * Simple bindings like
468 accepted (get value of ?y from occurrence sites)
473 None of these choices seems very satisfactory. But at least we should
474 decide which we want to do.
476 It's really not clear what is the Right Thing To Do. If you see
480 would you expect the value of ?y to be got from the *occurrence sites*
481 of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
482 case of function definitions, the answer is clearly the former, but
483 less so in the case of non-fucntion definitions. On the other hand,
484 if we say that we get the value of ?y from the definition site of 'z',
485 then inlining 'z' might change the semantics of the program.
487 Choice (C) really says "the monomorphism restriction doesn't apply
488 to implicit parameters". Which is fine, but remember that every
489 innocent binding 'x = ...' that mentions an implicit parameter in
490 the RHS becomes a *function* of that parameter, called at each
491 use of 'x'. Now, the chances are that there are no intervening 'with'
492 clauses that bind ?y, so a decent compiler should common up all
493 those function calls. So I think I strongly favour (C). Indeed,
494 one could make a similar argument for abolishing the monomorphism
495 restriction altogether.
497 BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
501 %************************************************************************
503 \subsection{tcSimplifyInfer}
505 %************************************************************************
507 tcSimplify is called when we *inferring* a type. Here's the overall game plan:
509 1. Compute Q = grow( fvs(T), C )
511 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
512 predicates will end up in Ct; we deal with them at the top level
514 3. Try improvement, using functional dependencies
516 4. If Step 3 did any unification, repeat from step 1
517 (Unification can change the result of 'grow'.)
519 Note: we don't reduce dictionaries in step 2. For example, if we have
520 Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
521 after step 2. However note that we may therefore quantify over more
522 type variables than we absolutely have to.
524 For the guts, we need a loop, that alternates context reduction and
525 improvement with unification. E.g. Suppose we have
527 class C x y | x->y where ...
529 and tcSimplify is called with:
531 Then improvement unifies a with b, giving
534 If we need to unify anything, we rattle round the whole thing all over
541 -> TcTyVarSet -- fv(T); type vars
543 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
544 TcDictBinds, -- Bindings
545 [TcId]) -- Dict Ids that must be bound here (zonked)
546 -- Any free (escaping) Insts are tossed into the environment
551 tcSimplifyInfer doc tau_tvs wanted_lie
552 = inferLoop doc (varSetElems tau_tvs)
553 wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
555 extendLIEs frees `thenM_`
556 returnM (qtvs, binds, map instToId irreds)
558 inferLoop doc tau_tvs wanteds
560 zonkTcTyVarsAndFV tau_tvs `thenM` \ tau_tvs' ->
561 mappM zonkInst wanteds `thenM` \ wanteds' ->
562 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
564 preds = fdPredsOfInsts wanteds'
565 qtvs = grow preds tau_tvs' `minusVarSet` oclose preds gbl_tvs
568 | isFreeWhenInferring qtvs inst = Free
569 | isClassDict inst = DontReduceUnlessConstant -- Dicts
570 | otherwise = ReduceMe -- Lits and Methods
572 traceTc (text "infloop" <+> vcat [ppr tau_tvs', ppr wanteds', ppr preds, ppr (grow preds tau_tvs'), ppr qtvs]) `thenM_`
574 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
577 if no_improvement then
578 returnM (varSetElems qtvs, frees, binds, irreds)
580 -- If improvement did some unification, we go round again. There
581 -- are two subtleties:
582 -- a) We start again with irreds, not wanteds
583 -- Using an instance decl might have introduced a fresh type variable
584 -- which might have been unified, so we'd get an infinite loop
585 -- if we started again with wanteds! See example [LOOP]
587 -- b) It's also essential to re-process frees, because unification
588 -- might mean that a type variable that looked free isn't now.
590 -- Hence the (irreds ++ frees)
592 -- However, NOTICE that when we are done, we might have some bindings, but
593 -- the final qtvs might be empty. See [NO TYVARS] below.
595 inferLoop doc tau_tvs (irreds ++ frees) `thenM` \ (qtvs1, frees1, binds1, irreds1) ->
596 returnM (qtvs1, frees1, binds `AndMonoBinds` binds1, irreds1)
601 class If b t e r | b t e -> r
604 class Lte a b c | a b -> c where lte :: a -> b -> c
606 instance (Lte a b l,If l b a c) => Max a b c
608 Wanted: Max Z (S x) y
610 Then we'll reduce using the Max instance to:
611 (Lte Z (S x) l, If l (S x) Z y)
612 and improve by binding l->T, after which we can do some reduction
613 on both the Lte and If constraints. What we *can't* do is start again
614 with (Max Z (S x) y)!
618 class Y a b | a -> b where
621 instance Y [[a]] a where
624 k :: X a -> X a -> X a
626 g :: Num a => [X a] -> [X a]
629 h ys = ys ++ map (k (y [[0]])) xs
631 The excitement comes when simplifying the bindings for h. Initially
632 try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
633 From this we get t1:=:t2, but also various bindings. We can't forget
634 the bindings (because of [LOOP]), but in fact t1 is what g is
637 The net effect of [NO TYVARS]
640 isFreeWhenInferring :: TyVarSet -> Inst -> Bool
641 isFreeWhenInferring qtvs inst
642 = isFreeWrtTyVars qtvs inst -- Constrains no quantified vars
643 && isInheritableInst inst -- And no implicit parameter involved
644 -- (see "Notes on implicit parameters")
646 isFreeWhenChecking :: TyVarSet -- Quantified tyvars
647 -> NameSet -- Quantified implicit parameters
649 isFreeWhenChecking qtvs ips inst
650 = isFreeWrtTyVars qtvs inst
651 && isFreeWrtIPs ips inst
653 isFreeWrtTyVars qtvs inst = not (tyVarsOfInst inst `intersectsVarSet` qtvs)
654 isFreeWrtIPs ips inst = not (any (`elemNameSet` ips) (ipNamesOfInst inst))
658 %************************************************************************
660 \subsection{tcSimplifyCheck}
662 %************************************************************************
664 @tcSimplifyCheck@ is used when we know exactly the set of variables
665 we are going to quantify over. For example, a class or instance declaration.
670 -> [TcTyVar] -- Quantify over these
673 -> TcM TcDictBinds -- Bindings
675 -- tcSimplifyCheck is used when checking expression type signatures,
676 -- class decls, instance decls etc.
678 -- NB: tcSimplifyCheck does not consult the
679 -- global type variables in the environment; so you don't
680 -- need to worry about setting them before calling tcSimplifyCheck
681 tcSimplifyCheck doc qtvs givens wanted_lie
682 = tcSimplCheck doc get_qtvs
683 givens wanted_lie `thenM` \ (qtvs', binds) ->
686 get_qtvs = zonkTcTyVarsAndFV qtvs
689 -- tcSimplifyInferCheck is used when we know the constraints we are to simplify
690 -- against, but we don't know the type variables over which we are going to quantify.
691 -- This happens when we have a type signature for a mutually recursive group
694 -> TcTyVarSet -- fv(T)
697 -> TcM ([TcTyVar], -- Variables over which to quantify
698 TcDictBinds) -- Bindings
700 tcSimplifyInferCheck doc tau_tvs givens wanted_lie
701 = tcSimplCheck doc get_qtvs givens wanted_lie
703 -- Figure out which type variables to quantify over
704 -- You might think it should just be the signature tyvars,
705 -- but in bizarre cases you can get extra ones
706 -- f :: forall a. Num a => a -> a
707 -- f x = fst (g (x, head [])) + 1
709 -- Here we infer g :: forall a b. a -> b -> (b,a)
710 -- We don't want g to be monomorphic in b just because
711 -- f isn't quantified over b.
712 all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
714 get_qtvs = zonkTcTyVarsAndFV all_tvs `thenM` \ all_tvs' ->
715 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
717 qtvs = all_tvs' `minusVarSet` gbl_tvs
718 -- We could close gbl_tvs, but its not necessary for
719 -- soundness, and it'll only affect which tyvars, not which
720 -- dictionaries, we quantify over
725 Here is the workhorse function for all three wrappers.
728 tcSimplCheck doc get_qtvs givens wanted_lie
729 = check_loop givens wanted_lie `thenM` \ (qtvs, frees, binds, irreds) ->
731 -- Complain about any irreducible ones
732 complainCheck doc givens irreds `thenM_`
735 extendLIEs frees `thenM_`
736 returnM (qtvs, binds)
739 ip_set = mkNameSet (ipNamesOfInsts givens)
741 check_loop givens wanteds
743 mappM zonkInst givens `thenM` \ givens' ->
744 mappM zonkInst wanteds `thenM` \ wanteds' ->
745 get_qtvs `thenM` \ qtvs' ->
749 -- When checking against a given signature we always reduce
750 -- until we find a match against something given, or can't reduce
751 try_me inst | isFreeWhenChecking qtvs' ip_set inst = Free
752 | otherwise = ReduceMe
754 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
757 if no_improvement then
758 returnM (varSetElems qtvs', frees, binds, irreds)
760 check_loop givens' (irreds ++ frees) `thenM` \ (qtvs', frees1, binds1, irreds1) ->
761 returnM (qtvs', frees1, binds `AndMonoBinds` binds1, irreds1)
765 %************************************************************************
767 \subsection{tcSimplifyRestricted}
769 %************************************************************************
772 tcSimplifyRestricted -- Used for restricted binding groups
773 -- i.e. ones subject to the monomorphism restriction
775 -> TcTyVarSet -- Free in the type of the RHSs
776 -> [Inst] -- Free in the RHSs
777 -> TcM ([TcTyVar], -- Tyvars to quantify (zonked)
778 TcDictBinds) -- Bindings
780 tcSimplifyRestricted doc tau_tvs wanteds
781 = -- First squash out all methods, to find the constrained tyvars
782 -- We can't just take the free vars of wanted_lie because that'll
783 -- have methods that may incidentally mention entirely unconstrained variables
784 -- e.g. a call to f :: Eq a => a -> b -> b
785 -- Here, b is unconstrained. A good example would be
787 -- We want to infer the polymorphic type
788 -- foo :: forall b. b -> b
790 -- 'reduceMe': Reduce as far as we can. Don't stop at
791 -- dicts; the idea is to get rid of as many type
792 -- variables as possible, and we don't want to stop
793 -- at (say) Monad (ST s), because that reduces
794 -- immediately, with no constraint on s.
795 simpleReduceLoop doc reduceMe wanteds `thenM` \ (foo_frees, foo_binds, constrained_dicts) ->
797 -- Next, figure out the tyvars we will quantify over
798 zonkTcTyVarsAndFV (varSetElems tau_tvs) `thenM` \ tau_tvs' ->
799 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
801 constrained_tvs = tyVarsOfInsts constrained_dicts
802 qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs)
803 `minusVarSet` constrained_tvs
805 traceTc (text "tcSimplifyRestricted" <+> vcat [
806 pprInsts wanteds, pprInsts foo_frees, pprInsts constrained_dicts,
808 ppr constrained_tvs, ppr tau_tvs', ppr qtvs ]) `thenM_`
810 -- The first step may have squashed more methods than
811 -- necessary, so try again, this time knowing the exact
812 -- set of type variables to quantify over.
814 -- We quantify only over constraints that are captured by qtvs;
815 -- these will just be a subset of non-dicts. This in contrast
816 -- to normal inference (using isFreeWhenInferring) in which we quantify over
817 -- all *non-inheritable* constraints too. This implements choice
818 -- (B) under "implicit parameter and monomorphism" above.
820 -- Remember that we may need to do *some* simplification, to
821 -- (for example) squash {Monad (ST s)} into {}. It's not enough
822 -- just to float all constraints
823 restrict_loop doc qtvs wanteds
824 -- We still need a loop because improvement can take place
825 -- E.g. if we have (C (T a)) and the instance decl
826 -- instance D Int b => C (T a) where ...
827 -- and there's a functional dependency for D. Then we may improve
828 -- the tyep variable 'b'.
830 restrict_loop doc qtvs wanteds
831 = mappM zonkInst wanteds `thenM` \ wanteds' ->
832 zonkTcTyVarsAndFV (varSetElems qtvs) `thenM` \ qtvs' ->
834 try_me inst | isFreeWrtTyVars qtvs' inst = Free
835 | otherwise = ReduceMe
837 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
838 if no_improvement then
839 ASSERT( null irreds )
840 extendLIEs frees `thenM_`
841 returnM (varSetElems qtvs', binds)
843 restrict_loop doc qtvs' (irreds ++ frees) `thenM` \ (qtvs1, binds1) ->
844 returnM (qtvs1, binds `AndMonoBinds` binds1)
848 %************************************************************************
850 \subsection{tcSimplifyToDicts}
852 %************************************************************************
854 On the LHS of transformation rules we only simplify methods and constants,
855 getting dictionaries. We want to keep all of them unsimplified, to serve
856 as the available stuff for the RHS of the rule.
858 The same thing is used for specialise pragmas. Consider
861 {-# SPECIALISE f :: Int -> Int #-}
864 The type checker generates a binding like:
866 f_spec = (f :: Int -> Int)
868 and we want to end up with
870 f_spec = _inline_me_ (f Int dNumInt)
872 But that means that we must simplify the Method for f to (f Int dNumInt)!
873 So tcSimplifyToDicts squeezes out all Methods.
875 IMPORTANT NOTE: we *don't* want to do superclass commoning up. Consider
877 fromIntegral :: (Integral a, Num b) => a -> b
878 {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
880 Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
884 fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
886 because the scsel will mess up matching. Instead we want
888 forall dIntegralInt, dNumInt.
889 fromIntegral Int Int dIntegralInt dNumInt = id Int
891 Hence "DontReduce NoSCs"
894 tcSimplifyToDicts :: [Inst] -> TcM (TcDictBinds)
895 tcSimplifyToDicts wanteds
896 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
897 -- Since try_me doesn't look at types, we don't need to
898 -- do any zonking, so it's safe to call reduceContext directly
900 extendLIEs irreds `thenM_`
904 doc = text "tcSimplifyToDicts"
906 -- Reduce methods and lits only; stop as soon as we get a dictionary
907 try_me inst | isDict inst = DontReduce NoSCs
908 | otherwise = ReduceMe
913 tcSimplifyBracket is used when simplifying the constraints arising from
914 a Template Haskell bracket [| ... |]. We want to check that there aren't
915 any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
916 Show instance), but we aren't otherwise interested in the results.
917 Nor do we care about ambiguous dictionaries etc. We will type check
918 this bracket again at its usage site.
921 tcSimplifyBracket :: [Inst] -> TcM ()
922 tcSimplifyBracket wanteds
923 = simpleReduceLoop doc reduceMe wanteds `thenM_`
926 doc = text "tcSimplifyBracket"
930 %************************************************************************
932 \subsection{Filtering at a dynamic binding}
934 %************************************************************************
939 we must discharge all the ?x constraints from B. We also do an improvement
940 step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
942 Actually, the constraints from B might improve the types in ?x. For example
944 f :: (?x::Int) => Char -> Char
947 then the constraint (?x::Int) arising from the call to f will
948 force the binding for ?x to be of type Int.
951 tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
954 tcSimplifyIPs given_ips wanteds
955 = simpl_loop given_ips wanteds `thenM` \ (frees, binds) ->
956 extendLIEs frees `thenM_`
959 doc = text "tcSimplifyIPs" <+> ppr given_ips
960 ip_set = mkNameSet (ipNamesOfInsts given_ips)
962 -- Simplify any methods that mention the implicit parameter
963 try_me inst | isFreeWrtIPs ip_set inst = Free
964 | otherwise = ReduceMe
966 simpl_loop givens wanteds
967 = mappM zonkInst givens `thenM` \ givens' ->
968 mappM zonkInst wanteds `thenM` \ wanteds' ->
970 reduceContext doc try_me givens' wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
972 if no_improvement then
973 ASSERT( null irreds )
974 returnM (frees, binds)
976 simpl_loop givens' (irreds ++ frees) `thenM` \ (frees1, binds1) ->
977 returnM (frees1, binds `AndMonoBinds` binds1)
981 %************************************************************************
983 \subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
985 %************************************************************************
987 When doing a binding group, we may have @Insts@ of local functions.
988 For example, we might have...
990 let f x = x + 1 -- orig local function (overloaded)
991 f.1 = f Int -- two instances of f
996 The point is: we must drop the bindings for @f.1@ and @f.2@ here,
997 where @f@ is in scope; those @Insts@ must certainly not be passed
998 upwards towards the top-level. If the @Insts@ were binding-ified up
999 there, they would have unresolvable references to @f@.
1001 We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
1002 For each method @Inst@ in the @init_lie@ that mentions one of the
1003 @Ids@, we create a binding. We return the remaining @Insts@ (in an
1004 @LIE@), as well as the @HsBinds@ generated.
1007 bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcMonoBinds
1009 bindInstsOfLocalFuns wanteds local_ids
1010 | null overloaded_ids
1012 = extendLIEs wanteds `thenM_`
1013 returnM EmptyMonoBinds
1016 = simpleReduceLoop doc try_me wanteds `thenM` \ (frees, binds, irreds) ->
1017 ASSERT( null irreds )
1018 extendLIEs frees `thenM_`
1021 doc = text "bindInsts" <+> ppr local_ids
1022 overloaded_ids = filter is_overloaded local_ids
1023 is_overloaded id = isOverloadedTy (idType id)
1025 overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
1026 -- so it's worth building a set, so that
1027 -- lookup (in isMethodFor) is faster
1029 try_me inst | isMethodFor overloaded_set inst = ReduceMe
1034 %************************************************************************
1036 \subsection{Data types for the reduction mechanism}
1038 %************************************************************************
1040 The main control over context reduction is here
1044 = ReduceMe -- Try to reduce this
1045 -- If there's no instance, behave exactly like
1046 -- DontReduce: add the inst to
1047 -- the irreductible ones, but don't
1048 -- produce an error message of any kind.
1049 -- It might be quite legitimate such as (Eq a)!
1051 | DontReduce WantSCs -- Return as irreducible
1053 | DontReduceUnlessConstant -- Return as irreducible unless it can
1054 -- be reduced to a constant in one step
1056 | Free -- Return as free
1058 reduceMe :: Inst -> WhatToDo
1059 reduceMe inst = ReduceMe
1061 data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
1062 -- of a predicate when adding it to the avails
1068 type Avails = FiniteMap Inst Avail
1071 = IsFree -- Used for free Insts
1072 | Irred -- Used for irreducible dictionaries,
1073 -- which are going to be lambda bound
1075 | Given TcId -- Used for dictionaries for which we have a binding
1076 -- e.g. those "given" in a signature
1077 Bool -- True <=> actually consumed (splittable IPs only)
1079 | NoRhs -- Used for Insts like (CCallable f)
1080 -- where no witness is required.
1083 | Rhs -- Used when there is a RHS
1085 [Inst] -- Insts free in the RHS; we need these too
1087 | Linear -- Splittable Insts only.
1088 Int -- The Int is always 2 or more; indicates how
1089 -- many copies are required
1090 Inst -- The splitter
1091 Avail -- Where the "master copy" is
1093 | LinRhss -- Splittable Insts only; this is used only internally
1094 -- by extractResults, where a Linear
1095 -- is turned into an LinRhss
1096 [TcExpr] -- A supply of suitable RHSs
1098 pprAvails avails = vcat [sep [ppr inst, nest 2 (equals <+> pprAvail avail)]
1099 | (inst,avail) <- fmToList avails ]
1101 instance Outputable Avail where
1104 pprAvail NoRhs = text "<no rhs>"
1105 pprAvail IsFree = text "Free"
1106 pprAvail Irred = text "Irred"
1107 pprAvail (Given x b) = text "Given" <+> ppr x <+>
1108 if b then text "(used)" else empty
1109 pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs)
1110 pprAvail (Linear n i a) = text "Linear" <+> ppr n <+> braces (ppr i) <+> ppr a
1111 pprAvail (LinRhss rhss) = text "LinRhss" <+> ppr rhss
1114 Extracting the bindings from a bunch of Avails.
1115 The bindings do *not* come back sorted in dependency order.
1116 We assume that they'll be wrapped in a big Rec, so that the
1117 dependency analyser can sort them out later
1121 extractResults :: Avails
1123 -> TcM (TcDictBinds, -- Bindings
1124 [Inst], -- Irreducible ones
1125 [Inst]) -- Free ones
1127 extractResults avails wanteds
1128 = go avails EmptyMonoBinds [] [] wanteds
1130 go avails binds irreds frees []
1131 = returnM (binds, irreds, frees)
1133 go avails binds irreds frees (w:ws)
1134 = case lookupFM avails w of
1135 Nothing -> pprTrace "Urk: extractResults" (ppr w) $
1136 go avails binds irreds frees ws
1138 Just NoRhs -> go avails binds irreds frees ws
1139 Just IsFree -> go (add_free avails w) binds irreds (w:frees) ws
1140 Just Irred -> go (add_given avails w) binds (w:irreds) frees ws
1142 Just (Given id _) -> go avails new_binds irreds frees ws
1144 new_binds | id == instToId w = binds
1145 | otherwise = addBind binds w (HsVar id)
1146 -- The sought Id can be one of the givens, via a superclass chain
1147 -- and then we definitely don't want to generate an x=x binding!
1149 Just (Rhs rhs ws') -> go (add_given avails w) new_binds irreds frees (ws' ++ ws)
1151 new_binds = addBind binds w rhs
1153 Just (Linear n split_inst avail) -- Transform Linear --> LinRhss
1154 -> get_root irreds frees avail w `thenM` \ (irreds', frees', root_id) ->
1155 split n (instToId split_inst) root_id w `thenM` \ (binds', rhss) ->
1156 go (addToFM avails w (LinRhss rhss))
1157 (binds `AndMonoBinds` binds')
1158 irreds' frees' (split_inst : w : ws)
1160 Just (LinRhss (rhs:rhss)) -- Consume one of the Rhss
1161 -> go new_avails new_binds irreds frees ws
1163 new_binds = addBind binds w rhs
1164 new_avails = addToFM avails w (LinRhss rhss)
1166 get_root irreds frees (Given id _) w = returnM (irreds, frees, id)
1167 get_root irreds frees Irred w = cloneDict w `thenM` \ w' ->
1168 returnM (w':irreds, frees, instToId w')
1169 get_root irreds frees IsFree w = cloneDict w `thenM` \ w' ->
1170 returnM (irreds, w':frees, instToId w')
1173 | instBindingRequired w = addToFM avails w (Given (instToId w) True)
1174 | otherwise = addToFM avails w NoRhs
1175 -- NB: make sure that CCallable/CReturnable use NoRhs rather
1176 -- than Given, else we end up with bogus bindings.
1178 add_free avails w | isMethod w = avails
1179 | otherwise = add_given avails w
1181 -- Do *not* replace Free by Given if it's a method.
1182 -- The following situation shows why this is bad:
1183 -- truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
1184 -- From an application (truncate f i) we get
1185 -- t1 = truncate at f
1187 -- If we have also have a second occurrence of truncate, we get
1188 -- t3 = truncate at f
1190 -- When simplifying with i,f free, we might still notice that
1191 -- t1=t3; but alas, the binding for t2 (which mentions t1)
1192 -- will continue to float out!
1193 -- (split n i a) returns: n rhss
1194 -- auxiliary bindings
1195 -- 1 or 0 insts to add to irreds
1198 split :: Int -> TcId -> TcId -> Inst
1199 -> TcM (TcDictBinds, [TcExpr])
1200 -- (split n split_id root_id wanted) returns
1201 -- * a list of 'n' expressions, all of which witness 'avail'
1202 -- * a bunch of auxiliary bindings to support these expressions
1203 -- * one or zero insts needed to witness the whole lot
1204 -- (maybe be zero if the initial Inst is a Given)
1206 -- NB: 'wanted' is just a template
1208 split n split_id root_id wanted
1211 ty = linearInstType wanted
1212 pair_ty = mkTyConApp pairTyCon [ty,ty]
1213 id = instToId wanted
1217 go 1 = returnM (EmptyMonoBinds, [HsVar root_id])
1219 go n = go ((n+1) `div` 2) `thenM` \ (binds1, rhss) ->
1220 expand n rhss `thenM` \ (binds2, rhss') ->
1221 returnM (binds1 `AndMonoBinds` binds2, rhss')
1224 -- Given ((n+1)/2) rhss, make n rhss, using auxiliary bindings
1225 -- e.g. expand 3 [rhs1, rhs2]
1226 -- = ( { x = split rhs1 },
1227 -- [fst x, snd x, rhs2] )
1229 | n `rem` 2 == 0 = go rhss -- n is even
1230 | otherwise = go (tail rhss) `thenM` \ (binds', rhss') ->
1231 returnM (binds', head rhss : rhss')
1233 go rhss = mapAndUnzipM do_one rhss `thenM` \ (binds', rhss') ->
1234 returnM (andMonoBindList binds', concat rhss')
1236 do_one rhs = newUnique `thenM` \ uniq ->
1237 tcLookupId fstName `thenM` \ fst_id ->
1238 tcLookupId sndName `thenM` \ snd_id ->
1240 x = mkUserLocal occ uniq pair_ty loc
1242 returnM (VarMonoBind x (mk_app split_id rhs),
1243 [mk_fs_app fst_id ty x, mk_fs_app snd_id ty x])
1245 mk_fs_app id ty var = HsVar id `TyApp` [ty,ty] `HsApp` HsVar var
1247 mk_app id rhs = HsApp (HsVar id) rhs
1249 addBind binds inst rhs = binds `AndMonoBinds` VarMonoBind (instToId inst) rhs
1253 %************************************************************************
1255 \subsection[reduce]{@reduce@}
1257 %************************************************************************
1259 When the "what to do" predicate doesn't depend on the quantified type variables,
1260 matters are easier. We don't need to do any zonking, unless the improvement step
1261 does something, in which case we zonk before iterating.
1263 The "given" set is always empty.
1266 simpleReduceLoop :: SDoc
1267 -> (Inst -> WhatToDo) -- What to do, *not* based on the quantified type variables
1269 -> TcM ([Inst], -- Free
1271 [Inst]) -- Irreducible
1273 simpleReduceLoop doc try_me wanteds
1274 = mappM zonkInst wanteds `thenM` \ wanteds' ->
1275 reduceContext doc try_me [] wanteds' `thenM` \ (no_improvement, frees, binds, irreds) ->
1276 if no_improvement then
1277 returnM (frees, binds, irreds)
1279 simpleReduceLoop doc try_me (irreds ++ frees) `thenM` \ (frees1, binds1, irreds1) ->
1280 returnM (frees1, binds `AndMonoBinds` binds1, irreds1)
1286 reduceContext :: SDoc
1287 -> (Inst -> WhatToDo)
1290 -> TcM (Bool, -- True <=> improve step did no unification
1292 TcDictBinds, -- Dictionary bindings
1293 [Inst]) -- Irreducible
1295 reduceContext doc try_me givens wanteds
1297 traceTc (text "reduceContext" <+> (vcat [
1298 text "----------------------",
1300 text "given" <+> ppr givens,
1301 text "wanted" <+> ppr wanteds,
1302 text "----------------------"
1305 -- Build the Avail mapping from "givens"
1306 foldlM addGiven emptyFM givens `thenM` \ init_state ->
1309 reduceList (0,[]) try_me wanteds init_state `thenM` \ avails ->
1311 -- Do improvement, using everything in avails
1312 -- In particular, avails includes all superclasses of everything
1313 tcImprove avails `thenM` \ no_improvement ->
1315 extractResults avails wanteds `thenM` \ (binds, irreds, frees) ->
1317 traceTc (text "reduceContext end" <+> (vcat [
1318 text "----------------------",
1320 text "given" <+> ppr givens,
1321 text "wanted" <+> ppr wanteds,
1323 text "avails" <+> pprAvails avails,
1324 text "frees" <+> ppr frees,
1325 text "no_improvement =" <+> ppr no_improvement,
1326 text "----------------------"
1329 returnM (no_improvement, frees, binds, irreds)
1332 = tcGetInstEnv `thenM` \ inst_env ->
1334 preds = [ (pred, pp_loc)
1335 | inst <- keysFM avails,
1336 let pp_loc = pprInstLoc (instLoc inst),
1337 pred <- fdPredsOfInst inst
1339 -- Avails has all the superclasses etc (good)
1340 -- It also has all the intermediates of the deduction (good)
1341 -- It does not have duplicates (good)
1342 -- NB that (?x::t1) and (?x::t2) will be held separately in avails
1343 -- so that improve will see them separate
1344 eqns = improve (classInstEnv inst_env) preds
1349 traceTc (ptext SLIT("Improve:") <+> vcat (map pprEquationDoc eqns)) `thenM_`
1350 mappM_ unify eqns `thenM_`
1353 unify ((qtvs, t1, t2), doc)
1355 tcInstTyVars VanillaTv (varSetElems qtvs) `thenM` \ (_, _, tenv) ->
1356 unifyTauTy (substTy tenv t1) (substTy tenv t2)
1359 The main context-reduction function is @reduce@. Here's its game plan.
1362 reduceList :: (Int,[Inst]) -- Stack (for err msgs)
1363 -- along with its depth
1364 -> (Inst -> WhatToDo)
1371 try_me: given an inst, this function returns
1373 DontReduce return this in "irreds"
1374 Free return this in "frees"
1376 wanteds: The list of insts to reduce
1377 state: An accumulating parameter of type Avails
1378 that contains the state of the algorithm
1380 It returns a Avails.
1382 The (n,stack) pair is just used for error reporting.
1383 n is always the depth of the stack.
1384 The stack is the stack of Insts being reduced: to produce X
1385 I had to produce Y, to produce Y I had to produce Z, and so on.
1388 reduceList (n,stack) try_me wanteds state
1389 | n > opt_MaxContextReductionDepth
1390 = failWithTc (reduceDepthErr n stack)
1396 pprTrace "Jeepers! ReduceContext:" (reduceDepthMsg n stack)
1401 go [] state = returnM state
1402 go (w:ws) state = reduce (n+1, w:stack) try_me w state `thenM` \ state' ->
1405 -- Base case: we're done!
1406 reduce stack try_me wanted state
1407 -- It's the same as an existing inst, or a superclass thereof
1408 | Just avail <- isAvailable state wanted
1409 = if isLinearInst wanted then
1410 addLinearAvailable state avail wanted `thenM` \ (state', wanteds') ->
1411 reduceList stack try_me wanteds' state'
1413 returnM state -- No op for non-linear things
1416 = case try_me wanted of {
1418 DontReduce want_scs -> addIrred want_scs state wanted
1420 ; DontReduceUnlessConstant -> -- It's irreducible (or at least should not be reduced)
1421 -- First, see if the inst can be reduced to a constant in one step
1422 try_simple (addIrred AddSCs) -- Assume want superclasses
1424 ; Free -> -- It's free so just chuck it upstairs
1425 -- First, see if the inst can be reduced to a constant in one step
1428 ; ReduceMe -> -- It should be reduced
1429 lookupInst wanted `thenM` \ lookup_result ->
1430 case lookup_result of
1431 GenInst wanteds' rhs -> addWanted state wanted rhs wanteds' `thenM` \ state' ->
1432 reduceList stack try_me wanteds' state'
1433 -- Experiment with doing addWanted *before* the reduceList,
1434 -- which has the effect of adding the thing we are trying
1435 -- to prove to the database before trying to prove the things it
1436 -- needs. See note [RECURSIVE DICTIONARIES]
1438 SimpleInst rhs -> addWanted state wanted rhs []
1440 NoInstance -> -- No such instance!
1441 -- Add it and its superclasses
1442 addIrred AddSCs state wanted
1446 try_simple do_this_otherwise
1447 = lookupInst wanted `thenM` \ lookup_result ->
1448 case lookup_result of
1449 SimpleInst rhs -> addWanted state wanted rhs []
1450 other -> do_this_otherwise state wanted
1455 -------------------------
1456 isAvailable :: Avails -> Inst -> Maybe Avail
1457 isAvailable avails wanted = lookupFM avails wanted
1458 -- NB 1: the Ord instance of Inst compares by the class/type info
1459 -- *not* by unique. So
1460 -- d1::C Int == d2::C Int
1462 addLinearAvailable :: Avails -> Avail -> Inst -> TcM (Avails, [Inst])
1463 addLinearAvailable avails avail wanted
1464 -- avails currently maps [wanted -> avail]
1465 -- Extend avails to reflect a neeed for an extra copy of avail
1467 | Just avail' <- split_avail avail
1468 = returnM (addToFM avails wanted avail', [])
1471 = tcLookupId splitName `thenM` \ split_id ->
1472 tcInstClassOp (instLoc wanted) split_id
1473 [linearInstType wanted] `thenM` \ split_inst ->
1474 returnM (addToFM avails wanted (Linear 2 split_inst avail), [split_inst])
1477 split_avail :: Avail -> Maybe Avail
1478 -- (Just av) if there's a modified version of avail that
1479 -- we can use to replace avail in avails
1480 -- Nothing if there isn't, so we need to create a Linear
1481 split_avail (Linear n i a) = Just (Linear (n+1) i a)
1482 split_avail (Given id used) | not used = Just (Given id True)
1483 | otherwise = Nothing
1484 split_avail Irred = Nothing
1485 split_avail IsFree = Nothing
1486 split_avail other = pprPanic "addLinearAvailable" (ppr avail $$ ppr wanted $$ ppr avails)
1488 -------------------------
1489 addFree :: Avails -> Inst -> TcM Avails
1490 -- When an Inst is tossed upstairs as 'free' we nevertheless add it
1491 -- to avails, so that any other equal Insts will be commoned up right
1492 -- here rather than also being tossed upstairs. This is really just
1493 -- an optimisation, and perhaps it is more trouble that it is worth,
1494 -- as the following comments show!
1496 -- NB: do *not* add superclasses. If we have
1499 -- but a is not bound here, then we *don't* want to derive
1500 -- dn from df here lest we lose sharing.
1502 addFree avails free = returnM (addToFM avails free IsFree)
1504 addWanted :: Avails -> Inst -> TcExpr -> [Inst] -> TcM Avails
1505 addWanted avails wanted rhs_expr wanteds
1506 = ASSERT2( not (wanted `elemFM` avails), ppr wanted $$ ppr avails )
1507 addAvailAndSCs avails wanted avail
1509 avail | instBindingRequired wanted = Rhs rhs_expr wanteds
1510 | otherwise = ASSERT( null wanteds ) NoRhs
1512 addGiven :: Avails -> Inst -> TcM Avails
1513 addGiven state given = addAvailAndSCs state given (Given (instToId given) False)
1514 -- No ASSERT( not (given `elemFM` avails) ) because in an instance
1515 -- decl for Ord t we can add both Ord t and Eq t as 'givens',
1516 -- so the assert isn't true
1518 addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
1519 addIrred NoSCs avails irred = returnM (addToFM avails irred Irred)
1520 addIrred AddSCs avails irred = ASSERT2( not (irred `elemFM` avails), ppr irred $$ ppr avails )
1521 addAvailAndSCs avails irred Irred
1523 addAvailAndSCs :: Avails -> Inst -> Avail -> TcM Avails
1524 addAvailAndSCs avails inst avail
1525 | not (isClassDict inst) = returnM avails1
1526 | otherwise = addSCs is_loop avails1 inst
1528 avails1 = addToFM avails inst avail
1529 is_loop inst = inst `elem` deps -- Note: this compares by *type*, not by Unique
1530 deps = findAllDeps avails avail
1532 findAllDeps :: Avails -> Avail -> [Inst]
1533 -- Find all the Insts that this one depends on
1534 -- See Note [SUPERCLASS-LOOP]
1535 findAllDeps avails (Rhs _ kids) = kids ++ concat (map (find_all_deps_help avails) kids)
1536 findAllDeps avails other = []
1538 find_all_deps_help :: Avails -> Inst -> [Inst]
1539 find_all_deps_help avails inst
1540 = case lookupFM avails inst of
1541 Just avail -> findAllDeps avails avail
1544 addSCs :: (Inst -> Bool) -> Avails -> Inst -> TcM Avails
1545 -- Add all the superclasses of the Inst to Avails
1546 -- The first param says "dont do this because the original thing
1547 -- depends on this one, so you'd build a loop"
1548 -- Invariant: the Inst is already in Avails.
1550 addSCs is_loop avails dict
1551 = newDictsFromOld dict sc_theta' `thenM` \ sc_dicts ->
1552 foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels)
1554 (clas, tys) = getDictClassTys dict
1555 (tyvars, sc_theta, sc_sels, _) = classBigSig clas
1556 sc_theta' = substTheta (mkTopTyVarSubst tyvars tys) sc_theta
1558 add_sc avails (sc_dict, sc_sel) -- Add it, and its superclasses
1559 = case lookupFM avails sc_dict of
1560 Just (Given _ _) -> returnM avails -- Given is cheaper than
1561 -- a superclass selection
1562 Just other | is_loop sc_dict -> returnM avails -- See Note [SUPERCLASS-LOOP]
1563 | otherwise -> returnM avails' -- SCs already added
1565 Nothing -> addSCs is_loop avails' sc_dict
1567 sc_sel_rhs = DictApp (TyApp (HsVar sc_sel) tys) [instToId dict]
1568 avail = Rhs sc_sel_rhs [dict]
1569 avails' = addToFM avails sc_dict avail
1572 Note [SUPERCLASS-LOOP]: Checking for loops
1573 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1574 We have to be careful here. If we are *given* d1:Ord a,
1575 and want to deduce (d2:C [a]) where
1577 class Ord a => C a where
1578 instance Ord a => C [a] where ...
1580 Then we'll use the instance decl to deduce C [a] and then add the
1581 superclasses of C [a] to avails. But we must not overwrite the binding
1582 for d1:Ord a (which is given) with a superclass selection or we'll just
1585 Here's another example
1586 class Eq b => Foo a b
1587 instance Eq a => Foo [a] a
1591 we'll first deduce that it holds (via the instance decl). We must not
1592 then overwrite the Eq t constraint with a superclass selection!
1594 At first I had a gross hack, whereby I simply did not add superclass constraints
1595 in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
1596 becuase it lost legitimate superclass sharing, and it still didn't do the job:
1597 I found a very obscure program (now tcrun021) in which improvement meant the
1598 simplifier got two bites a the cherry... so something seemed to be an Irred
1599 first time, but reducible next time.
1601 Now we implement the Right Solution, which is to check for loops directly
1602 when adding superclasses. It's a bit like the occurs check in unification.
1605 Note [RECURSIVE DICTIONARIES]
1606 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1608 data D r = ZeroD | SuccD (r (D r));
1610 instance (Eq (r (D r))) => Eq (D r) where
1611 ZeroD == ZeroD = True
1612 (SuccD a) == (SuccD b) = a == b
1615 equalDC :: D [] -> D [] -> Bool;
1618 We need to prove (Eq (D [])). Here's how we go:
1622 by instance decl, holds if
1626 by instance decl of Eq, holds if
1628 where d2 = dfEqList d2
1631 But now we can "tie the knot" to give
1637 and it'll even run! The trick is to put the thing we are trying to prove
1638 (in this case Eq (D []) into the database before trying to prove its
1639 contributing clauses.
1642 %************************************************************************
1644 \section{tcSimplifyTop: defaulting}
1646 %************************************************************************
1649 @tcSimplifyTop@ is called once per module to simplify all the constant
1650 and ambiguous Insts.
1652 We need to be careful of one case. Suppose we have
1654 instance Num a => Num (Foo a b) where ...
1656 and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
1657 to (Num x), and default x to Int. But what about y??
1659 It's OK: the final zonking stage should zap y to (), which is fine.
1663 tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
1664 tcSimplifyTop wanteds = tc_simplify_top False {- Not interactive loop -} wanteds
1665 tcSimplifyInteractive wanteds = tc_simplify_top True {- Interactive loop -} wanteds
1668 -- The TcLclEnv should be valid here, solely to improve
1669 -- error message generation for the monomorphism restriction
1670 tc_simplify_top is_interactive wanteds
1671 = getLclEnv `thenM` \ lcl_env ->
1672 traceTc (text "tcSimplifyTop" <+> ppr (lclEnvElts lcl_env)) `thenM_`
1673 simpleReduceLoop (text "tcSimplTop") reduceMe wanteds `thenM` \ (frees, binds, irreds) ->
1674 ASSERT( null frees )
1677 -- All the non-std ones are definite errors
1678 (stds, non_stds) = partition isStdClassTyVarDict irreds
1680 -- Group by type variable
1681 std_groups = equivClasses cmp_by_tyvar stds
1683 -- Pick the ones which its worth trying to disambiguate
1684 -- namely, the onese whose type variable isn't bound
1685 -- up with one of the non-standard classes
1686 (std_oks, std_bads) = partition worth_a_try std_groups
1687 worth_a_try group@(d:_) = not (non_std_tyvars `intersectsVarSet` tyVarsOfInst d)
1688 non_std_tyvars = unionVarSets (map tyVarsOfInst non_stds)
1690 -- Collect together all the bad guys
1691 bad_guys = non_stds ++ concat std_bads
1692 (tidy_env, tidy_dicts) = tidyInsts bad_guys
1693 (bad_ips, non_ips) = partition isIPDict tidy_dicts
1694 (no_insts, ambigs) = partition no_inst non_ips
1695 no_inst d = not (isTyVarDict d)
1696 -- Previously, there was a more elaborate no_inst definition:
1697 -- no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
1698 -- fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
1699 -- But that seems over-elaborate to me; it only bites for class decls with
1700 -- fundeps like this: class C a b | -> b where ...
1703 -- Report definite errors
1704 addTopInstanceErrs tidy_env no_insts `thenM_`
1705 addTopIPErrs tidy_env bad_ips `thenM_`
1707 -- Deal with ambiguity errors, but only if
1708 -- if there has not been an error so far; errors often
1709 -- give rise to spurious ambiguous Insts
1710 ifErrsM (returnM []) (
1712 -- Complain about the ones that don't fall under
1713 -- the Haskell rules for disambiguation
1714 -- This group includes both non-existent instances
1715 -- e.g. Num (IO a) and Eq (Int -> Int)
1716 -- and ambiguous dictionaries
1718 addTopAmbigErrs (tidy_env, ambigs) `thenM_`
1720 -- Disambiguate the ones that look feasible
1721 mappM (disambigGroup is_interactive) std_oks
1722 ) `thenM` \ binds_ambig ->
1724 returnM (binds `andMonoBinds` andMonoBindList binds_ambig)
1726 ----------------------------------
1727 d1 `cmp_by_tyvar` d2 = get_tv d1 `compare` get_tv d2
1729 get_tv d = case getDictClassTys d of
1730 (clas, [ty]) -> tcGetTyVar "tcSimplify" ty
1731 get_clas d = case getDictClassTys d of
1732 (clas, [ty]) -> clas
1735 If a dictionary constrains a type variable which is
1736 * not mentioned in the environment
1737 * and not mentioned in the type of the expression
1738 then it is ambiguous. No further information will arise to instantiate
1739 the type variable; nor will it be generalised and turned into an extra
1740 parameter to a function.
1742 It is an error for this to occur, except that Haskell provided for
1743 certain rules to be applied in the special case of numeric types.
1745 * at least one of its classes is a numeric class, and
1746 * all of its classes are numeric or standard
1747 then the type variable can be defaulted to the first type in the
1748 default-type list which is an instance of all the offending classes.
1750 So here is the function which does the work. It takes the ambiguous
1751 dictionaries and either resolves them (producing bindings) or
1752 complains. It works by splitting the dictionary list by type
1753 variable, and using @disambigOne@ to do the real business.
1755 @disambigOne@ assumes that its arguments dictionaries constrain all
1756 the same type variable.
1758 ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
1759 @()@ instead of @Int@. I reckon this is the Right Thing to do since
1760 the most common use of defaulting is code like:
1762 _ccall_ foo `seqPrimIO` bar
1764 Since we're not using the result of @foo@, the result if (presumably)
1768 disambigGroup :: Bool -- True <=> simplifying at top-level interactive loop
1769 -> [Inst] -- All standard classes of form (C a)
1772 disambigGroup is_interactive dicts
1773 | any std_default_class classes -- Guaranteed all standard classes
1774 = -- THE DICTS OBEY THE DEFAULTABLE CONSTRAINT
1775 -- SO, TRY DEFAULT TYPES IN ORDER
1777 -- Failure here is caused by there being no type in the
1778 -- default list which can satisfy all the ambiguous classes.
1779 -- For example, if Real a is reqd, but the only type in the
1780 -- default list is Int.
1781 getDefaultTys `thenM` \ default_tys ->
1783 try_default [] -- No defaults work, so fail
1786 try_default (default_ty : default_tys)
1787 = tryTcLIE_ (try_default default_tys) $ -- If default_ty fails, we try
1788 -- default_tys instead
1789 tcSimplifyDefault theta `thenM` \ _ ->
1792 theta = [mkClassPred clas [default_ty] | clas <- classes]
1794 -- See if any default works
1795 tryM (try_default default_tys) `thenM` \ mb_ty ->
1798 Right chosen_default_ty -> choose_default chosen_default_ty
1800 | otherwise -- No defaults
1804 tyvar = get_tv (head dicts) -- Should be non-empty
1805 classes = map get_clas dicts
1807 std_default_class cls
1808 = isNumericClass cls
1809 || (is_interactive &&
1810 classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
1811 -- In interactive mode, we default Show a to Show ()
1812 -- to avoid graututious errors on "show []"
1814 choose_default default_ty -- Commit to tyvar = default_ty
1815 = -- Bind the type variable
1816 unifyTauTy default_ty (mkTyVarTy tyvar) `thenM_`
1817 -- and reduce the context, for real this time
1818 simpleReduceLoop (text "disambig" <+> ppr dicts)
1819 reduceMe dicts `thenM` \ (frees, binds, ambigs) ->
1820 WARN( not (null frees && null ambigs), ppr frees $$ ppr ambigs )
1821 warnDefault dicts default_ty `thenM_`
1824 bomb_out = addTopAmbigErrs (tidyInsts dicts) `thenM_`
1825 returnM EmptyMonoBinds
1828 [Aside - why the defaulting mechanism is turned off when
1829 dealing with arguments and results to ccalls.
1831 When typechecking _ccall_s, TcExpr ensures that the external
1832 function is only passed arguments (and in the other direction,
1833 results) of a restricted set of 'native' types. This is
1834 implemented via the help of the pseudo-type classes,
1835 @CReturnable@ (CR) and @CCallable@ (CC.)
1837 The interaction between the defaulting mechanism for numeric
1838 values and CC & CR can be a bit puzzling to the user at times.
1847 What type has 'x' got here? That depends on the default list
1848 in operation, if it is equal to Haskell 98's default-default
1849 of (Integer, Double), 'x' has type Double, since Integer
1850 is not an instance of CR. If the default list is equal to
1851 Haskell 1.4's default-default of (Int, Double), 'x' has type
1854 To try to minimise the potential for surprises here, the
1855 defaulting mechanism is turned off in the presence of
1856 CCallable and CReturnable.
1861 %************************************************************************
1863 \subsection[simple]{@Simple@ versions}
1865 %************************************************************************
1867 Much simpler versions when there are no bindings to make!
1869 @tcSimplifyThetas@ simplifies class-type constraints formed by
1870 @deriving@ declarations and when specialising instances. We are
1871 only interested in the simplified bunch of class/type constraints.
1873 It simplifies to constraints of the form (C a b c) where
1874 a,b,c are type variables. This is required for the context of
1875 instance declarations.
1878 tcSimplifyDeriv :: [TyVar]
1879 -> ThetaType -- Wanted
1880 -> TcM ThetaType -- Needed
1882 tcSimplifyDeriv tyvars theta
1883 = tcInstTyVars VanillaTv tyvars `thenM` \ (tvs, _, tenv) ->
1884 -- The main loop may do unification, and that may crash if
1885 -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
1886 -- ToDo: what if two of them do get unified?
1887 newDicts DataDeclOrigin (substTheta tenv theta) `thenM` \ wanteds ->
1888 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
1889 ASSERT( null frees ) -- reduceMe never returns Free
1891 doptM Opt_AllowUndecidableInstances `thenM` \ undecidable_ok ->
1893 tv_set = mkVarSet tvs
1894 simpl_theta = map dictPred irreds -- reduceMe squashes all non-dicts
1897 | isEmptyVarSet pred_tyvars -- Things like (Eq T) should be rejected
1898 = addErrTc (noInstErr pred)
1900 | not undecidable_ok && not (isTyVarClassPred pred)
1901 -- Check that the returned dictionaries are all of form (C a b)
1902 -- (where a, b are type variables).
1903 -- We allow this if we had -fallow-undecidable-instances,
1904 -- but note that risks non-termination in the 'deriving' context-inference
1905 -- fixpoint loop. It is useful for situations like
1906 -- data Min h a = E | M a (h a)
1907 -- which gives the instance decl
1908 -- instance (Eq a, Eq (h a)) => Eq (Min h a)
1909 = addErrTc (noInstErr pred)
1911 | not (pred_tyvars `subVarSet` tv_set)
1912 -- Check for a bizarre corner case, when the derived instance decl should
1913 -- have form instance C a b => D (T a) where ...
1914 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1915 -- of problems; in particular, it's hard to compare solutions for
1916 -- equality when finding the fixpoint. So I just rule it out for now.
1917 = addErrTc (badDerivedPred pred)
1922 pred_tyvars = tyVarsOfPred pred
1924 rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
1925 -- This reverse-mapping is a Royal Pain,
1926 -- but the result should mention TyVars not TcTyVars
1929 mappM check_pred simpl_theta `thenM_`
1930 checkAmbiguity tvs simpl_theta tv_set `thenM_`
1931 returnM (substTheta rev_env simpl_theta)
1933 doc = ptext SLIT("deriving classes for a data type")
1936 @tcSimplifyDefault@ just checks class-type constraints, essentially;
1937 used with \tr{default} declarations. We are only interested in
1938 whether it worked or not.
1941 tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
1944 tcSimplifyDefault theta
1945 = newDicts DataDeclOrigin theta `thenM` \ wanteds ->
1946 simpleReduceLoop doc reduceMe wanteds `thenM` \ (frees, _, irreds) ->
1947 ASSERT( null frees ) -- try_me never returns Free
1948 mappM (addErrTc . noInstErr) irreds `thenM_`
1954 doc = ptext SLIT("default declaration")
1958 %************************************************************************
1960 \section{Errors and contexts}
1962 %************************************************************************
1964 ToDo: for these error messages, should we note the location as coming
1965 from the insts, or just whatever seems to be around in the monad just
1969 groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
1970 -> [Inst] -- The offending Insts
1972 -- Group together insts with the same origin
1973 -- We want to report them together in error messages
1975 groupErrs report_err []
1977 groupErrs report_err (inst:insts)
1978 = do_one (inst:friends) `thenM_`
1979 groupErrs report_err others
1982 -- (It may seem a bit crude to compare the error messages,
1983 -- but it makes sure that we combine just what the user sees,
1984 -- and it avoids need equality on InstLocs.)
1985 (friends, others) = partition is_friend insts
1986 loc_msg = showSDoc (pprInstLoc (instLoc inst))
1987 is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
1988 do_one insts = addInstCtxt (instLoc (head insts)) (report_err insts)
1989 -- Add location and context information derived from the Insts
1991 -- Add the "arising from..." part to a message about bunch of dicts
1992 addInstLoc :: [Inst] -> Message -> Message
1993 addInstLoc insts msg = msg $$ nest 2 (pprInstLoc (instLoc (head insts)))
1996 plural xs = char 's'
1999 addTopIPErrs tidy_env tidy_dicts
2000 = groupErrs report tidy_dicts
2002 report dicts = addErrTcM (tidy_env, mk_msg dicts)
2003 mk_msg dicts = addInstLoc dicts (ptext SLIT("Unbound implicit parameter") <>
2004 plural tidy_dicts <+> pprInsts tidy_dicts)
2006 -- Used for top-level irreducibles
2007 addTopInstanceErrs tidy_env tidy_dicts
2008 = groupErrs report tidy_dicts
2010 report dicts = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
2011 addErrTcM (tidy_env, mk_msg dicts $$ mono_msg)
2012 mk_msg dicts = addInstLoc dicts (ptext SLIT("No instance") <> plural tidy_dicts <+>
2013 ptext SLIT("for") <+> pprInsts tidy_dicts)
2016 addTopAmbigErrs (tidy_env, tidy_dicts)
2017 -- Divide into groups that share a common set of ambiguous tyvars
2018 = mapM report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
2020 tvs_of :: Inst -> [TcTyVar]
2021 tvs_of d = varSetElems (tyVarsOfInst d)
2022 cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
2024 report :: [(Inst,[TcTyVar])] -> TcM ()
2025 report pairs@((_,tvs) : _) -- The pairs share a common set of ambiguous tyvars
2026 = mkMonomorphismMsg tidy_env dicts `thenM` \ (tidy_env, mono_msg) ->
2027 addErrTcM (tidy_env, msg $$ mono_msg)
2029 dicts = map fst pairs
2030 msg = sep [text "Ambiguous type variable" <> plural tvs <+>
2031 pprQuotedList tvs <+> in_msg,
2032 nest 2 (pprInstsInFull dicts)]
2033 in_msg | isSingleton dicts = text "in the top-level constraint:"
2034 | otherwise = text "in these top-level constraints:"
2037 mkMonomorphismMsg :: TidyEnv -> [Inst] -> TcM (TidyEnv, Message)
2038 -- There's an error with these Insts; if they have free type variables
2039 -- it's probably caused by the monomorphism restriction.
2040 -- Try to identify the offending variable
2041 -- ASSUMPTION: the Insts are fully zonked
2042 mkMonomorphismMsg tidy_env insts
2043 | isEmptyVarSet inst_tvs
2044 = returnM (tidy_env, empty)
2046 = findGlobals inst_tvs tidy_env `thenM` \ (tidy_env, docs) ->
2047 returnM (tidy_env, mk_msg docs)
2050 inst_tvs = tyVarsOfInsts insts
2052 mk_msg [] = empty -- This happens in things like
2053 -- f x = show (read "foo")
2054 -- whre monomorphism doesn't play any role
2055 mk_msg docs = vcat [ptext SLIT("Possible cause: the monomorphism restriction applied to the following:"),
2057 ptext SLIT("Probable fix: give these definition(s) an explicit type signature")]
2059 warnDefault dicts default_ty
2060 = doptM Opt_WarnTypeDefaults `thenM` \ warn_flag ->
2061 addInstCtxt (instLoc (head dicts)) (warnTc warn_flag warn_msg)
2064 (_, tidy_dicts) = tidyInsts dicts
2065 warn_msg = vcat [ptext SLIT("Defaulting the following constraint(s) to type") <+>
2066 quotes (ppr default_ty),
2067 pprInstsInFull tidy_dicts]
2069 complainCheck doc givens irreds
2070 = mappM zonkInst given_dicts_and_ips `thenM` \ givens' ->
2071 groupErrs (addNoInstanceErrs doc givens') irreds `thenM_`
2074 given_dicts_and_ips = filter (not . isMethod) givens
2075 -- Filter out methods, which are only added to
2076 -- the given set as an optimisation
2078 addNoInstanceErrs what_doc givens dicts
2079 = getDOpts `thenM` \ dflags ->
2080 tcGetInstEnv `thenM` \ inst_env ->
2082 (tidy_env1, tidy_givens) = tidyInsts givens
2083 (tidy_env2, tidy_dicts) = tidyMoreInsts tidy_env1 dicts
2085 doc = vcat [addInstLoc dicts $
2086 sep [herald <+> pprInsts tidy_dicts,
2087 nest 4 $ ptext SLIT("from the context") <+> pprInsts tidy_givens],
2089 ptext SLIT("Probable fix:"),
2093 herald = ptext SLIT("Could not") <+> unambig_doc <+> ptext SLIT("deduce")
2094 unambig_doc | ambig_overlap = ptext SLIT("unambiguously")
2097 -- The error message when we don't find a suitable instance
2098 -- is complicated by the fact that sometimes this is because
2099 -- there is no instance, and sometimes it's because there are
2100 -- too many instances (overlap). See the comments in TcEnv.lhs
2101 -- with the InstEnv stuff.
2104 | not ambig_overlap = empty
2106 = vcat [ptext SLIT("The choice of (overlapping) instance declaration"),
2107 nest 4 (ptext SLIT("depends on the instantiation of") <+>
2108 quotes (pprWithCommas ppr (varSetElems (tyVarsOfInsts tidy_dicts))))]
2110 fix1 = sep [ptext SLIT("Add") <+> pprInsts tidy_dicts,
2111 ptext SLIT("to the") <+> what_doc]
2113 fix2 | null instance_dicts
2116 = ptext SLIT("Or add an instance declaration for") <+> pprInsts instance_dicts
2118 instance_dicts = [d | d <- tidy_dicts, isClassDict d, not (isTyVarDict d)]
2119 -- Insts for which it is worth suggesting an adding an instance declaration
2120 -- Exclude implicit parameters, and tyvar dicts
2122 -- Checks for the ambiguous case when we have overlapping instances
2123 ambig_overlap = any ambig_overlap1 dicts
2126 = case lookupInstEnv dflags inst_env clas tys of
2127 NoMatch ambig -> ambig
2131 (clas,tys) = getDictClassTys dict
2133 addErrTcM (tidy_env2, doc)
2135 -- Used for the ...Thetas variants; all top level
2136 noInstErr pred = ptext SLIT("No instance for") <+> quotes (ppr pred)
2139 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
2140 ptext SLIT("type variables that are not data type parameters"),
2141 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
2143 reduceDepthErr n stack
2144 = vcat [ptext SLIT("Context reduction stack overflow; size =") <+> int n,
2145 ptext SLIT("Use -fcontext-stack20 to increase stack size to (e.g.) 20"),
2146 nest 4 (pprInstsInFull stack)]
2148 reduceDepthMsg n stack = nest 4 (pprInstsInFull stack)