Print which warning-flag controls an emitted warning
[ghc.git] / compiler / typecheck / TcValidity.hs
1 {-
2 (c) The University of Glasgow 2006
3 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 -}
5
6 {-# LANGUAGE CPP, TupleSections, ViewPatterns #-}
7
8 module TcValidity (
9 Rank, UserTypeCtxt(..), checkValidType, checkValidMonoType,
10 ContextKind(..), expectedKindInCtxt,
11 checkValidTheta, checkValidFamPats,
12 checkValidInstance, validDerivPred,
13 checkInstTermination,
14 ClsInfo, checkValidCoAxiom, checkValidCoAxBranch,
15 checkValidTyFamEqn,
16 arityErr, badATErr,
17 checkValidTelescope, checkZonkValidTelescope, checkValidInferredKinds
18 ) where
19
20 #include "HsVersions.h"
21
22 -- friends:
23 import TcUnify ( tcSubType_NC )
24 import TcSimplify ( simplifyAmbiguityCheck )
25 import TyCoRep
26 import TcType hiding ( sizeType, sizeTypes )
27 import TcMType
28 import PrelNames
29 import Type
30 import Coercion
31 import Unify( tcMatchTyX )
32 import Kind
33 import CoAxiom
34 import Class
35 import TyCon
36
37 -- others:
38 import HsSyn -- HsType
39 import TcRnMonad -- TcType, amongst others
40 import FunDeps
41 import FamInstEnv ( isDominatedBy, injectiveBranches,
42 InjectivityCheckResult(..) )
43 import FamInst ( makeInjectivityErrors )
44 import Name
45 import VarEnv
46 import VarSet
47 import ErrUtils
48 import DynFlags
49 import Util
50 import ListSetOps
51 import SrcLoc
52 import Outputable
53 import BasicTypes
54 import Module
55 import qualified GHC.LanguageExtensions as LangExt
56
57 import Control.Monad
58 import Data.Maybe
59 import Data.List ( (\\) )
60
61 {-
62 ************************************************************************
63 * *
64 Checking for ambiguity
65 * *
66 ************************************************************************
67
68 Note [The ambiguity check for type signatures]
69 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
70 checkAmbiguity is a check on *user-supplied type signatures*. It is
71 *purely* there to report functions that cannot possibly be called. So for
72 example we want to reject:
73 f :: C a => Int
74 The idea is there can be no legal calls to 'f' because every call will
75 give rise to an ambiguous constraint. We could soundly omit the
76 ambiguity check on type signatures entirely, at the expense of
77 delaying ambiguity errors to call sites. Indeed, the flag
78 -XAllowAmbiguousTypes switches off the ambiguity check.
79
80 What about things like this:
81 class D a b | a -> b where ..
82 h :: D Int b => Int
83 The Int may well fix 'b' at the call site, so that signature should
84 not be rejected. Moreover, using *visible* fundeps is too
85 conservative. Consider
86 class X a b where ...
87 class D a b | a -> b where ...
88 instance D a b => X [a] b where...
89 h :: X a b => a -> a
90 Here h's type looks ambiguous in 'b', but here's a legal call:
91 ...(h [True])...
92 That gives rise to a (X [Bool] beta) constraint, and using the
93 instance means we need (D Bool beta) and that fixes 'beta' via D's
94 fundep!
95
96 Behind all these special cases there is a simple guiding principle.
97 Consider
98
99 f :: <type>
100 f = ...blah...
101
102 g :: <type>
103 g = f
104
105 You would think that the definition of g would surely typecheck!
106 After all f has exactly the same type, and g=f. But in fact f's type
107 is instantiated and the instantiated constraints are solved against
108 the originals, so in the case an ambiguous type it won't work.
109 Consider our earlier example f :: C a => Int. Then in g's definition,
110 we'll instantiate to (C alpha) and try to deduce (C alpha) from (C a),
111 and fail.
112
113 So in fact we use this as our *definition* of ambiguity. We use a
114 very similar test for *inferred* types, to ensure that they are
115 unambiguous. See Note [Impedence matching] in TcBinds.
116
117 This test is very conveniently implemented by calling
118 tcSubType <type> <type>
119 This neatly takes account of the functional dependecy stuff above,
120 and implicit parameter (see Note [Implicit parameters and ambiguity]).
121 And this is what checkAmbiguity does.
122
123 What about this, though?
124 g :: C [a] => Int
125 Is every call to 'g' ambiguous? After all, we might have
126 intance C [a] where ...
127 at the call site. So maybe that type is ok! Indeed even f's
128 quintessentially ambiguous type might, just possibly be callable:
129 with -XFlexibleInstances we could have
130 instance C a where ...
131 and now a call could be legal after all! Well, we'll reject this
132 unless the instance is available *here*.
133
134 Note [When to call checkAmbiguity]
135 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
136 We call checkAmbiguity
137 (a) on user-specified type signatures
138 (b) in checkValidType
139
140 Conncerning (b), you might wonder about nested foralls. What about
141 f :: forall b. (forall a. Eq a => b) -> b
142 The nested forall is ambiguous. Originally we called checkAmbiguity
143 in the forall case of check_type, but that had two bad consequences:
144 * We got two error messages about (Eq b) in a nested forall like this:
145 g :: forall a. Eq a => forall b. Eq b => a -> a
146 * If we try to check for ambiguity of an nested forall like
147 (forall a. Eq a => b), the implication constraint doesn't bind
148 all the skolems, which results in "No skolem info" in error
149 messages (see Trac #10432).
150
151 To avoid this, we call checkAmbiguity once, at the top, in checkValidType.
152 (I'm still a bit worried about unbound skolems when the type mentions
153 in-scope type variables.)
154
155 In fact, because of the co/contra-variance implemented in tcSubType,
156 this *does* catch function f above. too.
157
158 Concerning (a) the ambiguity check is only used for *user* types, not
159 for types coming from inteface files. The latter can legitimately
160 have ambiguous types. Example
161
162 class S a where s :: a -> (Int,Int)
163 instance S Char where s _ = (1,1)
164 f:: S a => [a] -> Int -> (Int,Int)
165 f (_::[a]) x = (a*x,b)
166 where (a,b) = s (undefined::a)
167
168 Here the worker for f gets the type
169 fw :: forall a. S a => Int -> (# Int, Int #)
170
171
172 Note [Implicit parameters and ambiguity]
173 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
174 Only a *class* predicate can give rise to ambiguity
175 An *implicit parameter* cannot. For example:
176 foo :: (?x :: [a]) => Int
177 foo = length ?x
178 is fine. The call site will supply a particular 'x'
179
180 Furthermore, the type variables fixed by an implicit parameter
181 propagate to the others. E.g.
182 foo :: (Show a, ?x::[a]) => Int
183 foo = show (?x++?x)
184 The type of foo looks ambiguous. But it isn't, because at a call site
185 we might have
186 let ?x = 5::Int in foo
187 and all is well. In effect, implicit parameters are, well, parameters,
188 so we can take their type variables into account as part of the
189 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
190 -}
191
192 checkAmbiguity :: UserTypeCtxt -> Type -> TcM ()
193 checkAmbiguity ctxt ty
194 | wantAmbiguityCheck ctxt
195 = do { traceTc "Ambiguity check for" (ppr ty)
196 -- Solve the constraints eagerly because an ambiguous type
197 -- can cause a cascade of further errors. Since the free
198 -- tyvars are skolemised, we can safely use tcSimplifyTop
199 ; allow_ambiguous <- xoptM LangExt.AllowAmbiguousTypes
200 ; (_wrap, wanted) <- addErrCtxt (mk_msg allow_ambiguous) $
201 captureConstraints $
202 tcSubType_NC ctxt ty (mkCheckExpType ty)
203 ; simplifyAmbiguityCheck ty wanted
204
205 ; traceTc "Done ambiguity check for" (ppr ty) }
206
207 | otherwise
208 = return ()
209 where
210 mk_msg allow_ambiguous
211 = vcat [ text "In the ambiguity check for" <+> what
212 , ppUnless allow_ambiguous ambig_msg ]
213 ambig_msg = text "To defer the ambiguity check to use sites, enable AllowAmbiguousTypes"
214 what | Just n <- isSigMaybe ctxt = quotes (ppr n)
215 | otherwise = pprUserTypeCtxt ctxt
216
217 wantAmbiguityCheck :: UserTypeCtxt -> Bool
218 wantAmbiguityCheck ctxt
219 = case ctxt of
220 GhciCtxt -> False -- Allow ambiguous types in GHCi's :kind command
221 -- E.g. type family T a :: * -- T :: forall k. k -> *
222 -- Then :k T should work in GHCi, not complain that
223 -- (T k) is ambiguous!
224 _ -> True
225
226
227 checkUserTypeError :: Type -> TcM ()
228 -- Check to see if the type signature mentions "TypeError blah"
229 -- anywhere in it, and fail if so.
230 --
231 -- Very unsatisfactorily (Trac #11144) we need to tidy the type
232 -- because it may have come from an /inferred/ signature, not a
233 -- user-supplied one. This is really only a half-baked fix;
234 -- the other errors in checkValidType don't do tidying, and so
235 -- may give bad error messages when given an inferred type.
236 checkUserTypeError = check
237 where
238 check ty
239 | Just msg <- userTypeError_maybe ty = fail_with msg
240 | Just (_,ts) <- splitTyConApp_maybe ty = mapM_ check ts
241 | Just (t1,t2) <- splitAppTy_maybe ty = check t1 >> check t2
242 | Just (_,t1) <- splitForAllTy_maybe ty = check t1
243 | otherwise = return ()
244
245 fail_with msg = do { env0 <- tcInitTidyEnv
246 ; let (env1, tidy_msg) = tidyOpenType env0 msg
247 ; failWithTcM (env1, pprUserTypeErrorTy tidy_msg) }
248
249
250 {-
251 ************************************************************************
252 * *
253 Checking validity of a user-defined type
254 * *
255 ************************************************************************
256
257 When dealing with a user-written type, we first translate it from an HsType
258 to a Type, performing kind checking, and then check various things that should
259 be true about it. We don't want to perform these checks at the same time
260 as the initial translation because (a) they are unnecessary for interface-file
261 types and (b) when checking a mutually recursive group of type and class decls,
262 we can't "look" at the tycons/classes yet. Also, the checks are rather
263 diverse, and used to really mess up the other code.
264
265 One thing we check for is 'rank'.
266
267 Rank 0: monotypes (no foralls)
268 Rank 1: foralls at the front only, Rank 0 inside
269 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
270
271 basic ::= tyvar | T basic ... basic
272
273 r2 ::= forall tvs. cxt => r2a
274 r2a ::= r1 -> r2a | basic
275 r1 ::= forall tvs. cxt => r0
276 r0 ::= r0 -> r0 | basic
277
278 Another thing is to check that type synonyms are saturated.
279 This might not necessarily show up in kind checking.
280 type A i = i
281 data T k = MkT (k Int)
282 f :: T A -- BAD!
283 -}
284
285 checkValidType :: UserTypeCtxt -> Type -> TcM ()
286 -- Checks that a user-written type is valid for the given context
287 -- Assumes arguemt is fully zonked
288 -- Not used for instance decls; checkValidInstance instead
289 checkValidType ctxt ty
290 = do { traceTc "checkValidType" (ppr ty <+> text "::" <+> ppr (typeKind ty))
291 ; rankn_flag <- xoptM LangExt.RankNTypes
292 ; impred_flag <- xoptM LangExt.ImpredicativeTypes
293 ; let gen_rank :: Rank -> Rank
294 gen_rank r | rankn_flag = ArbitraryRank
295 | otherwise = r
296
297 rank1 = gen_rank r1
298 rank0 = gen_rank r0
299
300 r0 = rankZeroMonoType
301 r1 = LimitedRank True r0
302
303 rank
304 = case ctxt of
305 DefaultDeclCtxt-> MustBeMonoType
306 ResSigCtxt -> MustBeMonoType
307 PatSigCtxt -> rank0
308 RuleSigCtxt _ -> rank1
309 TySynCtxt _ -> rank0
310
311 ExprSigCtxt -> rank1
312 TypeAppCtxt | impred_flag -> ArbitraryRank
313 | otherwise -> tyConArgMonoType
314 -- Normally, ImpredicativeTypes is handled in check_arg_type,
315 -- but visible type applications don't go through there.
316 -- So we do this check here.
317
318 FunSigCtxt {} -> rank1
319 InfSigCtxt _ -> ArbitraryRank -- Inferred type
320 ConArgCtxt _ -> rank1 -- We are given the type of the entire
321 -- constructor, hence rank 1
322
323 ForSigCtxt _ -> rank1
324 SpecInstCtxt -> rank1
325 ThBrackCtxt -> rank1
326 GhciCtxt -> ArbitraryRank
327 _ -> panic "checkValidType"
328 -- Can't happen; not used for *user* sigs
329
330 ; env <- tcInitOpenTidyEnv (tyCoVarsOfType ty)
331
332 -- Check the internal validity of the type itself
333 ; check_type env ctxt rank ty
334
335 -- Check that the thing has kind Type, and is lifted if necessary.
336 -- Do this *after* check_type, because we can't usefully take
337 -- the kind of an ill-formed type such as (a~Int)
338 ; check_kind env ctxt ty
339
340 ; checkUserTypeError ty
341
342 -- Check for ambiguous types. See Note [When to call checkAmbiguity]
343 -- NB: this will happen even for monotypes, but that should be cheap;
344 -- and there may be nested foralls for the subtype test to examine
345 ; checkAmbiguity ctxt ty
346
347 ; traceTc "checkValidType done" (ppr ty <+> text "::" <+> ppr (typeKind ty)) }
348
349 checkValidMonoType :: Type -> TcM ()
350 -- Assumes arguemt is fully zonked
351 checkValidMonoType ty
352 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfType ty)
353 ; check_type env SigmaCtxt MustBeMonoType ty }
354
355 check_kind :: TidyEnv -> UserTypeCtxt -> TcType -> TcM ()
356 -- Check that the type's kind is acceptable for the context
357 check_kind env ctxt ty
358 | TySynCtxt {} <- ctxt
359 , returnsConstraintKind actual_kind
360 = do { ck <- xoptM LangExt.ConstraintKinds
361 ; if ck
362 then when (isConstraintKind actual_kind)
363 (do { dflags <- getDynFlags
364 ; check_pred_ty env dflags ctxt ty })
365 else addErrTcM (constraintSynErr env actual_kind) }
366
367 | otherwise
368 = case expectedKindInCtxt ctxt of
369 TheKind k -> checkTcM (tcEqType actual_kind k) (kindErr env actual_kind)
370 OpenKind -> checkTcM (classifiesTypeWithValues actual_kind) (kindErr env actual_kind)
371 AnythingKind -> return ()
372 where
373 actual_kind = typeKind ty
374
375 -- | The kind expected in a certain context.
376 data ContextKind = TheKind Kind -- ^ a specific kind
377 | AnythingKind -- ^ any kind will do
378 | OpenKind -- ^ something of the form @TYPE _@
379
380 -- Depending on the context, we might accept any kind (for instance, in a TH
381 -- splice), or only certain kinds (like in type signatures).
382 expectedKindInCtxt :: UserTypeCtxt -> ContextKind
383 expectedKindInCtxt (TySynCtxt _) = AnythingKind
384 expectedKindInCtxt ThBrackCtxt = AnythingKind
385 expectedKindInCtxt GhciCtxt = AnythingKind
386 -- The types in a 'default' decl can have varying kinds
387 -- See Note [Extended defaults]" in TcEnv
388 expectedKindInCtxt DefaultDeclCtxt = AnythingKind
389 expectedKindInCtxt TypeAppCtxt = AnythingKind
390 expectedKindInCtxt (ForSigCtxt _) = TheKind liftedTypeKind
391 expectedKindInCtxt InstDeclCtxt = TheKind constraintKind
392 expectedKindInCtxt SpecInstCtxt = TheKind constraintKind
393 expectedKindInCtxt _ = OpenKind
394
395 {-
396 Note [Higher rank types]
397 ~~~~~~~~~~~~~~~~~~~~~~~~
398 Technically
399 Int -> forall a. a->a
400 is still a rank-1 type, but it's not Haskell 98 (Trac #5957). So the
401 validity checker allow a forall after an arrow only if we allow it
402 before -- that is, with Rank2Types or RankNTypes
403 -}
404
405 data Rank = ArbitraryRank -- Any rank ok
406
407 | LimitedRank -- Note [Higher rank types]
408 Bool -- Forall ok at top
409 Rank -- Use for function arguments
410
411 | MonoType SDoc -- Monotype, with a suggestion of how it could be a polytype
412
413 | MustBeMonoType -- Monotype regardless of flags
414
415
416 rankZeroMonoType, tyConArgMonoType, synArgMonoType, constraintMonoType :: Rank
417 rankZeroMonoType = MonoType (text "Perhaps you intended to use RankNTypes or Rank2Types")
418 tyConArgMonoType = MonoType (text "GHC doesn't yet support impredicative polymorphism")
419 synArgMonoType = MonoType (text "Perhaps you intended to use LiberalTypeSynonyms")
420 constraintMonoType = MonoType (text "A constraint must be a monotype")
421
422 funArgResRank :: Rank -> (Rank, Rank) -- Function argument and result
423 funArgResRank (LimitedRank _ arg_rank) = (arg_rank, LimitedRank (forAllAllowed arg_rank) arg_rank)
424 funArgResRank other_rank = (other_rank, other_rank)
425
426 forAllAllowed :: Rank -> Bool
427 forAllAllowed ArbitraryRank = True
428 forAllAllowed (LimitedRank forall_ok _) = forall_ok
429 forAllAllowed _ = False
430
431 ----------------------------------------
432 -- | Fail with error message if the type is unlifted
433 check_lifted :: Type -> TcM ()
434 check_lifted _ = return ()
435
436 {- ------ Legacy comment ---------
437 The check_unlifted function seems entirely redundant. The
438 kind system should check for uses of unlifted types. So I've
439 removed the check. See Trac #11120 comment:19.
440
441 check_lifted ty
442 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfType ty)
443 ; checkTcM (not (isUnliftedType ty)) (unliftedArgErr env ty) }
444
445 unliftedArgErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
446 unliftedArgErr env ty = (env, sep [text "Illegal unlifted type:", ppr_tidy env ty])
447 ------ End of legacy comment --------- -}
448
449
450 check_type :: TidyEnv -> UserTypeCtxt -> Rank -> Type -> TcM ()
451 -- The args say what the *type context* requires, independent
452 -- of *flag* settings. You test the flag settings at usage sites.
453 --
454 -- Rank is allowed rank for function args
455 -- Rank 0 means no for-alls anywhere
456
457 check_type env ctxt rank ty
458 | not (null tvs && null theta)
459 = do { traceTc "check_type" (ppr ty $$ ppr (forAllAllowed rank))
460 ; checkTcM (forAllAllowed rank) (forAllTyErr env rank ty)
461 -- Reject e.g. (Maybe (?x::Int => Int)),
462 -- with a decent error message
463
464 ; check_valid_theta env' SigmaCtxt theta
465 -- Allow type T = ?x::Int => Int -> Int
466 -- but not type T = ?x::Int
467
468 ; check_type env' ctxt rank tau -- Allow foralls to right of arrow
469 ; checkTcM (not (any (`elemVarSet` tyCoVarsOfType phi_kind) tvs))
470 (forAllEscapeErr env' ty tau_kind)
471 }
472 where
473 (tvs, theta, tau) = tcSplitSigmaTy ty
474 tau_kind = typeKind tau
475
476 phi_kind | null theta = tau_kind
477 | otherwise = liftedTypeKind
478 -- If there are any constraints, the kind is *. (#11405)
479
480 (env', _) = tidyTyCoVarBndrs env tvs
481
482 check_type _ _ _ (TyVarTy _) = return ()
483
484 check_type env ctxt rank (ForAllTy (Anon arg_ty) res_ty)
485 = do { check_type env ctxt arg_rank arg_ty
486 ; check_type env ctxt res_rank res_ty }
487 where
488 (arg_rank, res_rank) = funArgResRank rank
489
490 check_type env ctxt rank (AppTy ty1 ty2)
491 = do { check_arg_type env ctxt rank ty1
492 ; check_arg_type env ctxt rank ty2 }
493
494 check_type env ctxt rank ty@(TyConApp tc tys)
495 | isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
496 = check_syn_tc_app env ctxt rank ty tc tys
497 | isUnboxedTupleTyCon tc = check_ubx_tuple env ctxt ty tys
498 | otherwise = mapM_ (check_arg_type env ctxt rank) tys
499
500 check_type _ _ _ (LitTy {}) = return ()
501
502 check_type env ctxt rank (CastTy ty _) = check_type env ctxt rank ty
503
504 check_type _ _ _ ty = pprPanic "check_type" (ppr ty)
505
506 ----------------------------------------
507 check_syn_tc_app :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType
508 -> TyCon -> [KindOrType] -> TcM ()
509 -- Used for type synonyms and type synonym families,
510 -- which must be saturated,
511 -- but not data families, which need not be saturated
512 check_syn_tc_app env ctxt rank ty tc tys
513 | tc_arity <= length tys -- Saturated
514 -- Check that the synonym has enough args
515 -- This applies equally to open and closed synonyms
516 -- It's OK to have an *over-applied* type synonym
517 -- data Tree a b = ...
518 -- type Foo a = Tree [a]
519 -- f :: Foo a b -> ...
520 = do { -- See Note [Liberal type synonyms]
521 ; liberal <- xoptM LangExt.LiberalTypeSynonyms
522 ; if not liberal || isTypeFamilyTyCon tc then
523 -- For H98 and synonym families, do check the type args
524 mapM_ check_arg tys
525
526 else -- In the liberal case (only for closed syns), expand then check
527 case coreView ty of
528 Just ty' -> check_type env ctxt rank ty'
529 Nothing -> pprPanic "check_tau_type" (ppr ty) }
530
531 | GhciCtxt <- ctxt -- Accept under-saturated type synonyms in
532 -- GHCi :kind commands; see Trac #7586
533 = mapM_ check_arg tys
534
535 | otherwise
536 = failWithTc (tyConArityErr tc tys)
537 where
538 tc_arity = tyConArity tc
539 check_arg | isTypeFamilyTyCon tc = check_arg_type env ctxt rank
540 | otherwise = check_type env ctxt synArgMonoType
541
542 ----------------------------------------
543 check_ubx_tuple :: TidyEnv -> UserTypeCtxt -> KindOrType
544 -> [KindOrType] -> TcM ()
545 check_ubx_tuple env ctxt ty tys
546 = do { ub_tuples_allowed <- xoptM LangExt.UnboxedTuples
547 ; checkTcM ub_tuples_allowed (ubxArgTyErr env ty)
548
549 ; impred <- xoptM LangExt.ImpredicativeTypes
550 ; let rank' = if impred then ArbitraryRank else tyConArgMonoType
551 -- c.f. check_arg_type
552 -- However, args are allowed to be unlifted, or
553 -- more unboxed tuples, so can't use check_arg_ty
554 ; mapM_ (check_type env ctxt rank') tys }
555
556 ----------------------------------------
557 check_arg_type :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType -> TcM ()
558 -- The sort of type that can instantiate a type variable,
559 -- or be the argument of a type constructor.
560 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
561 -- Other unboxed types are very occasionally allowed as type
562 -- arguments depending on the kind of the type constructor
563 --
564 -- For example, we want to reject things like:
565 --
566 -- instance Ord a => Ord (forall s. T s a)
567 -- and
568 -- g :: T s (forall b.b)
569 --
570 -- NB: unboxed tuples can have polymorphic or unboxed args.
571 -- This happens in the workers for functions returning
572 -- product types with polymorphic components.
573 -- But not in user code.
574 -- Anyway, they are dealt with by a special case in check_tau_type
575
576 check_arg_type _ _ _ (CoercionTy {}) = return ()
577
578 check_arg_type env ctxt rank ty
579 = do { impred <- xoptM LangExt.ImpredicativeTypes
580 ; let rank' = case rank of -- Predictive => must be monotype
581 MustBeMonoType -> MustBeMonoType -- Monotype, regardless
582 _other | impred -> ArbitraryRank
583 | otherwise -> tyConArgMonoType
584 -- Make sure that MustBeMonoType is propagated,
585 -- so that we don't suggest -XImpredicativeTypes in
586 -- (Ord (forall a.a)) => a -> a
587 -- and so that if it Must be a monotype, we check that it is!
588
589 ; check_type env ctxt rank' ty
590 ; check_lifted ty }
591 -- NB the isUnliftedType test also checks for
592 -- T State#
593 -- where there is an illegal partial application of State# (which has
594 -- kind * -> #); see Note [The kind invariant] in TyCoRep
595
596 ----------------------------------------
597 forAllTyErr :: TidyEnv -> Rank -> Type -> (TidyEnv, SDoc)
598 forAllTyErr env rank ty
599 = ( env
600 , vcat [ hang herald 2 (ppr_tidy env ty)
601 , suggestion ] )
602 where
603 (tvs, _theta, _tau) = tcSplitSigmaTy ty
604 herald | null tvs = text "Illegal qualified type:"
605 | otherwise = text "Illegal polymorphic type:"
606 suggestion = case rank of
607 LimitedRank {} -> text "Perhaps you intended to use RankNTypes or Rank2Types"
608 MonoType d -> d
609 _ -> Outputable.empty -- Polytype is always illegal
610
611 forAllEscapeErr :: TidyEnv -> Type -> Kind -> (TidyEnv, SDoc)
612 forAllEscapeErr env ty tau_kind
613 = ( env
614 , hang (vcat [ text "Quantified type's kind mentions quantified type variable"
615 , text "(skolem escape)" ])
616 2 (vcat [ text " type:" <+> ppr_tidy env ty
617 , text "of kind:" <+> ppr_tidy env tau_kind ]) )
618
619 ubxArgTyErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
620 ubxArgTyErr env ty = (env, sep [text "Illegal unboxed tuple type as function argument:", ppr_tidy env ty])
621
622 kindErr :: TidyEnv -> Kind -> (TidyEnv, SDoc)
623 kindErr env kind = (env, sep [text "Expecting an ordinary type, but found a type of kind", ppr_tidy env kind])
624
625 {-
626 Note [Liberal type synonyms]
627 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
628 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
629 doing validity checking. This allows us to instantiate a synonym defn
630 with a for-all type, or with a partially-applied type synonym.
631 e.g. type T a b = a
632 type S m = m ()
633 f :: S (T Int)
634 Here, T is partially applied, so it's illegal in H98. But if you
635 expand S first, then T we get just
636 f :: Int
637 which is fine.
638
639 IMPORTANT: suppose T is a type synonym. Then we must do validity
640 checking on an appliation (T ty1 ty2)
641
642 *either* before expansion (i.e. check ty1, ty2)
643 *or* after expansion (i.e. expand T ty1 ty2, and then check)
644 BUT NOT BOTH
645
646 If we do both, we get exponential behaviour!!
647
648 data TIACons1 i r c = c i ::: r c
649 type TIACons2 t x = TIACons1 t (TIACons1 t x)
650 type TIACons3 t x = TIACons2 t (TIACons1 t x)
651 type TIACons4 t x = TIACons2 t (TIACons2 t x)
652 type TIACons7 t x = TIACons4 t (TIACons3 t x)
653
654
655 ************************************************************************
656 * *
657 \subsection{Checking a theta or source type}
658 * *
659 ************************************************************************
660
661 Note [Implicit parameters in instance decls]
662 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
663 Implicit parameters _only_ allowed in type signatures; not in instance
664 decls, superclasses etc. The reason for not allowing implicit params in
665 instances is a bit subtle. If we allowed
666 instance (?x::Int, Eq a) => Foo [a] where ...
667 then when we saw
668 (e :: (?x::Int) => t)
669 it would be unclear how to discharge all the potential uses of the ?x
670 in e. For example, a constraint Foo [Int] might come out of e, and
671 applying the instance decl would show up two uses of ?x. Trac #8912.
672 -}
673
674 checkValidTheta :: UserTypeCtxt -> ThetaType -> TcM ()
675 -- Assumes arguemt is fully zonked
676 checkValidTheta ctxt theta
677 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypes theta)
678 ; addErrCtxtM (checkThetaCtxt ctxt theta) $
679 check_valid_theta env ctxt theta }
680
681 -------------------------
682 check_valid_theta :: TidyEnv -> UserTypeCtxt -> [PredType] -> TcM ()
683 check_valid_theta _ _ []
684 = return ()
685 check_valid_theta env ctxt theta
686 = do { dflags <- getDynFlags
687 ; warnTcM (Reason Opt_WarnDuplicateConstraints)
688 (wopt Opt_WarnDuplicateConstraints dflags && notNull dups)
689 (dupPredWarn env dups)
690 ; traceTc "check_valid_theta" (ppr theta)
691 ; mapM_ (check_pred_ty env dflags ctxt) theta }
692 where
693 (_,dups) = removeDups cmpType theta
694
695 -------------------------
696 {- Note [Validity checking for constraints]
697 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
698 We look through constraint synonyms so that we can see the underlying
699 constraint(s). For example
700 type Foo = ?x::Int
701 instance Foo => C T
702 We should reject the instance because it has an implicit parameter in
703 the context.
704
705 But we record, in 'under_syn', whether we have looked under a synonym
706 to avoid requiring language extensions at the use site. Main example
707 (Trac #9838):
708
709 {-# LANGUAGE ConstraintKinds #-}
710 module A where
711 type EqShow a = (Eq a, Show a)
712
713 module B where
714 import A
715 foo :: EqShow a => a -> String
716
717 We don't want to require ConstraintKinds in module B.
718 -}
719
720 check_pred_ty :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
721 -- Check the validity of a predicate in a signature
722 -- See Note [Validity checking for constraints]
723 check_pred_ty env dflags ctxt pred
724 = do { check_type env SigmaCtxt constraintMonoType pred
725 ; check_pred_help False env dflags ctxt pred }
726
727 check_pred_help :: Bool -- True <=> under a type synonym
728 -> TidyEnv
729 -> DynFlags -> UserTypeCtxt
730 -> PredType -> TcM ()
731 check_pred_help under_syn env dflags ctxt pred
732 | Just pred' <- coreView pred -- Switch on under_syn when going under a
733 -- synonym (Trac #9838, yuk)
734 = check_pred_help True env dflags ctxt pred'
735 | otherwise
736 = case splitTyConApp_maybe pred of
737 Just (tc, tys)
738 | isTupleTyCon tc
739 -> check_tuple_pred under_syn env dflags ctxt pred tys
740 -- NB: this equality check must come first, because (~) is a class,
741 -- too.
742 | tc `hasKey` heqTyConKey ||
743 tc `hasKey` eqTyConKey ||
744 tc `hasKey` eqPrimTyConKey
745 -> check_eq_pred env dflags pred tc tys
746 | Just cls <- tyConClass_maybe tc
747 -> check_class_pred env dflags ctxt pred cls tys -- Includes Coercible
748 _ -> check_irred_pred under_syn env dflags ctxt pred
749
750 check_eq_pred :: TidyEnv -> DynFlags -> PredType -> TyCon -> [TcType] -> TcM ()
751 check_eq_pred env dflags pred tc tys
752 = -- Equational constraints are valid in all contexts if type
753 -- families are permitted
754 do { checkTc (length tys == tyConArity tc) (tyConArityErr tc tys)
755 ; checkTcM (xopt LangExt.TypeFamilies dflags
756 || xopt LangExt.GADTs dflags)
757 (eqPredTyErr env pred) }
758
759 check_tuple_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> [PredType] -> TcM ()
760 check_tuple_pred under_syn env dflags ctxt pred ts
761 = do { -- See Note [ConstraintKinds in predicates]
762 checkTcM (under_syn || xopt LangExt.ConstraintKinds dflags)
763 (predTupleErr env pred)
764 ; mapM_ (check_pred_help under_syn env dflags ctxt) ts }
765 -- This case will not normally be executed because without
766 -- -XConstraintKinds tuple types are only kind-checked as *
767
768 check_irred_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
769 check_irred_pred under_syn env dflags ctxt pred
770 -- The predicate looks like (X t1 t2) or (x t1 t2) :: Constraint
771 -- where X is a type function
772 = do { -- If it looks like (x t1 t2), require ConstraintKinds
773 -- see Note [ConstraintKinds in predicates]
774 -- But (X t1 t2) is always ok because we just require ConstraintKinds
775 -- at the definition site (Trac #9838)
776 failIfTcM (not under_syn && not (xopt LangExt.ConstraintKinds dflags)
777 && hasTyVarHead pred)
778 (predIrredErr env pred)
779
780 -- Make sure it is OK to have an irred pred in this context
781 -- See Note [Irreducible predicates in superclasses]
782 ; failIfTcM (is_superclass ctxt
783 && not (xopt LangExt.UndecidableInstances dflags)
784 && has_tyfun_head pred)
785 (predSuperClassErr env pred) }
786 where
787 is_superclass ctxt = case ctxt of { ClassSCCtxt _ -> True; _ -> False }
788 has_tyfun_head ty
789 = case tcSplitTyConApp_maybe ty of
790 Just (tc, _) -> isTypeFamilyTyCon tc
791 Nothing -> False
792
793 {- Note [ConstraintKinds in predicates]
794 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
795 Don't check for -XConstraintKinds under a type synonym, because that
796 was done at the type synonym definition site; see Trac #9838
797 e.g. module A where
798 type C a = (Eq a, Ix a) -- Needs -XConstraintKinds
799 module B where
800 import A
801 f :: C a => a -> a -- Does *not* need -XConstraintKinds
802
803 Note [Irreducible predicates in superclasses]
804 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
805 Allowing type-family calls in class superclasses is somewhat dangerous
806 because we can write:
807
808 type family Fooish x :: * -> Constraint
809 type instance Fooish () = Foo
810 class Fooish () a => Foo a where
811
812 This will cause the constraint simplifier to loop because every time we canonicalise a
813 (Foo a) class constraint we add a (Fooish () a) constraint which will be immediately
814 solved to add+canonicalise another (Foo a) constraint. -}
815
816 -------------------------
817 check_class_pred :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> Class -> [TcType] -> TcM ()
818 check_class_pred env dflags ctxt pred cls tys
819 | isIPClass cls
820 = do { check_arity
821 ; checkTcM (okIPCtxt ctxt) (badIPPred env pred) }
822
823 | otherwise
824 = do { check_arity
825 ; checkTcM arg_tys_ok (env, predTyVarErr (tidyType env pred)) }
826 where
827 check_arity = checkTc (classArity cls == length tys)
828 (tyConArityErr (classTyCon cls) tys)
829 flexible_contexts = xopt LangExt.FlexibleContexts dflags
830 undecidable_ok = xopt LangExt.UndecidableInstances dflags
831
832 arg_tys_ok = case ctxt of
833 SpecInstCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
834 InstDeclCtxt -> checkValidClsArgs (flexible_contexts || undecidable_ok) cls tys
835 -- Further checks on head and theta
836 -- in checkInstTermination
837 _ -> checkValidClsArgs flexible_contexts cls tys
838
839 -------------------------
840 okIPCtxt :: UserTypeCtxt -> Bool
841 -- See Note [Implicit parameters in instance decls]
842 okIPCtxt (FunSigCtxt {}) = True
843 okIPCtxt (InfSigCtxt {}) = True
844 okIPCtxt ExprSigCtxt = True
845 okIPCtxt TypeAppCtxt = True
846 okIPCtxt PatSigCtxt = True
847 okIPCtxt ResSigCtxt = True
848 okIPCtxt GenSigCtxt = True
849 okIPCtxt (ConArgCtxt {}) = True
850 okIPCtxt (ForSigCtxt {}) = True -- ??
851 okIPCtxt ThBrackCtxt = True
852 okIPCtxt GhciCtxt = True
853 okIPCtxt SigmaCtxt = True
854 okIPCtxt (DataTyCtxt {}) = True
855 okIPCtxt (PatSynCtxt {}) = True
856 okIPCtxt (TySynCtxt {}) = True -- e.g. type Blah = ?x::Int
857 -- Trac #11466
858
859 okIPCtxt (ClassSCCtxt {}) = False
860 okIPCtxt (InstDeclCtxt {}) = False
861 okIPCtxt (SpecInstCtxt {}) = False
862 okIPCtxt (RuleSigCtxt {}) = False
863 okIPCtxt DefaultDeclCtxt = False
864
865 badIPPred :: TidyEnv -> PredType -> (TidyEnv, SDoc)
866 badIPPred env pred
867 = ( env
868 , text "Illegal implicit parameter" <+> quotes (ppr_tidy env pred) )
869
870 {-
871 Note [Kind polymorphic type classes]
872 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
873 MultiParam check:
874
875 class C f where... -- C :: forall k. k -> Constraint
876 instance C Maybe where...
877
878 The dictionary gets type [C * Maybe] even if it's not a MultiParam
879 type class.
880
881 Flexibility check:
882
883 class C f where... -- C :: forall k. k -> Constraint
884 data D a = D a
885 instance C D where
886
887 The dictionary gets type [C * (D *)]. IA0_TODO it should be
888 generalized actually.
889 -}
890
891 checkThetaCtxt :: UserTypeCtxt -> ThetaType -> TidyEnv -> TcM (TidyEnv, SDoc)
892 checkThetaCtxt ctxt theta env
893 = return ( env
894 , vcat [ text "In the context:" <+> pprTheta (tidyTypes env theta)
895 , text "While checking" <+> pprUserTypeCtxt ctxt ] )
896
897 eqPredTyErr, predTupleErr, predIrredErr, predSuperClassErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
898 eqPredTyErr env pred
899 = ( env
900 , text "Illegal equational constraint" <+> ppr_tidy env pred $$
901 parens (text "Use GADTs or TypeFamilies to permit this") )
902 predTupleErr env pred
903 = ( env
904 , hang (text "Illegal tuple constraint:" <+> ppr_tidy env pred)
905 2 (parens constraintKindsMsg) )
906 predIrredErr env pred
907 = ( env
908 , hang (text "Illegal constraint:" <+> ppr_tidy env pred)
909 2 (parens constraintKindsMsg) )
910 predSuperClassErr env pred
911 = ( env
912 , hang (text "Illegal constraint" <+> quotes (ppr_tidy env pred)
913 <+> text "in a superclass context")
914 2 (parens undecidableMsg) )
915
916 predTyVarErr :: PredType -> SDoc -- type is already tidied!
917 predTyVarErr pred
918 = vcat [ hang (text "Non type-variable argument")
919 2 (text "in the constraint:" <+> ppr pred)
920 , parens (text "Use FlexibleContexts to permit this") ]
921
922 constraintSynErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
923 constraintSynErr env kind
924 = ( env
925 , hang (text "Illegal constraint synonym of kind:" <+> quotes (ppr_tidy env kind))
926 2 (parens constraintKindsMsg) )
927
928 dupPredWarn :: TidyEnv -> [[PredType]] -> (TidyEnv, SDoc)
929 dupPredWarn env dups
930 = ( env
931 , text "Duplicate constraint" <> plural primaryDups <> text ":"
932 <+> pprWithCommas (ppr_tidy env) primaryDups )
933 where
934 primaryDups = map head dups
935
936 tyConArityErr :: TyCon -> [TcType] -> SDoc
937 -- For type-constructor arity errors, be careful to report
938 -- the number of /visible/ arguments required and supplied,
939 -- ignoring the /invisible/ arguments, which the user does not see.
940 -- (e.g. Trac #10516)
941 tyConArityErr tc tks
942 = arityErr (tyConFlavour tc) (tyConName tc)
943 tc_type_arity tc_type_args
944 where
945 vis_tks = filterOutInvisibleTypes tc tks
946
947 -- tc_type_arity = number of *type* args expected
948 -- tc_type_args = number of *type* args encountered
949 tc_type_arity = count isVisibleBinder $ tyConBinders tc
950 tc_type_args = length vis_tks
951
952 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
953 arityErr what name n m
954 = hsep [ text "The" <+> text what, quotes (ppr name), text "should have",
955 n_arguments <> comma, text "but has been given",
956 if m==0 then text "none" else int m]
957 where
958 n_arguments | n == 0 = text "no arguments"
959 | n == 1 = text "1 argument"
960 | True = hsep [int n, text "arguments"]
961
962 {-
963 ************************************************************************
964 * *
965 \subsection{Checking for a decent instance head type}
966 * *
967 ************************************************************************
968
969 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
970 it must normally look like: @instance Foo (Tycon a b c ...) ...@
971
972 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
973 flag is on, or (2)~the instance is imported (they must have been
974 compiled elsewhere). In these cases, we let them go through anyway.
975
976 We can also have instances for functions: @instance Foo (a -> b) ...@.
977 -}
978
979 checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
980 checkValidInstHead ctxt clas cls_args
981 = do { dflags <- getDynFlags
982
983 ; mod <- getModule
984 ; checkTc (getUnique clas `notElem` abstractClassKeys ||
985 nameModule (getName clas) == mod)
986 (instTypeErr clas cls_args abstract_class_msg)
987
988 -- Check language restrictions;
989 -- but not for SPECIALISE instance pragmas
990 ; let ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
991 ; unless spec_inst_prag $
992 do { checkTc (xopt LangExt.TypeSynonymInstances dflags ||
993 all tcInstHeadTyNotSynonym ty_args)
994 (instTypeErr clas cls_args head_type_synonym_msg)
995 ; checkTc (xopt LangExt.FlexibleInstances dflags ||
996 all tcInstHeadTyAppAllTyVars ty_args)
997 (instTypeErr clas cls_args head_type_args_tyvars_msg)
998 ; checkTc (xopt LangExt.MultiParamTypeClasses dflags ||
999 length ty_args == 1 || -- Only count type arguments
1000 (xopt LangExt.NullaryTypeClasses dflags &&
1001 null ty_args))
1002 (instTypeErr clas cls_args head_one_type_msg) }
1003
1004 ; mapM_ checkValidTypePat ty_args }
1005 where
1006 spec_inst_prag = case ctxt of { SpecInstCtxt -> True; _ -> False }
1007
1008 head_type_synonym_msg = parens (
1009 text "All instance types must be of the form (T t1 ... tn)" $$
1010 text "where T is not a synonym." $$
1011 text "Use TypeSynonymInstances if you want to disable this.")
1012
1013 head_type_args_tyvars_msg = parens (vcat [
1014 text "All instance types must be of the form (T a1 ... an)",
1015 text "where a1 ... an are *distinct type variables*,",
1016 text "and each type variable appears at most once in the instance head.",
1017 text "Use FlexibleInstances if you want to disable this."])
1018
1019 head_one_type_msg = parens (
1020 text "Only one type can be given in an instance head." $$
1021 text "Use MultiParamTypeClasses if you want to allow more, or zero.")
1022
1023 abstract_class_msg =
1024 text "Manual instances of this class are not permitted."
1025
1026 tcInstHeadTyNotSynonym :: Type -> Bool
1027 -- Used in Haskell-98 mode, for the argument types of an instance head
1028 -- These must not be type synonyms, but everywhere else type synonyms
1029 -- are transparent, so we need a special function here
1030 tcInstHeadTyNotSynonym ty
1031 = case ty of -- Do not use splitTyConApp,
1032 -- because that expands synonyms!
1033 TyConApp tc _ -> not (isTypeSynonymTyCon tc)
1034 _ -> True
1035
1036 tcInstHeadTyAppAllTyVars :: Type -> Bool
1037 -- Used in Haskell-98 mode, for the argument types of an instance head
1038 -- These must be a constructor applied to type variable arguments.
1039 -- But we allow kind instantiations.
1040 tcInstHeadTyAppAllTyVars ty
1041 | Just (tc, tys) <- tcSplitTyConApp_maybe (dropCasts ty)
1042 = ok (filterOutInvisibleTypes tc tys) -- avoid kinds
1043
1044 | otherwise
1045 = False
1046 where
1047 -- Check that all the types are type variables,
1048 -- and that each is distinct
1049 ok tys = equalLength tvs tys && hasNoDups tvs
1050 where
1051 tvs = mapMaybe tcGetTyVar_maybe tys
1052
1053 dropCasts :: Type -> Type
1054 -- See Note [Casts during validity checking]
1055 -- This function can turn a well-kinded type into an ill-kinded
1056 -- one, so I've kept it local to this module
1057 -- To consider: drop only UnivCo(HoleProv) casts
1058 dropCasts (CastTy ty _) = dropCasts ty
1059 dropCasts (AppTy t1 t2) = mkAppTy (dropCasts t1) (dropCasts t2)
1060 dropCasts (TyConApp tc tys) = mkTyConApp tc (map dropCasts tys)
1061 dropCasts (ForAllTy b ty) = ForAllTy (dropCastsB b) (dropCasts ty)
1062 dropCasts ty = ty -- LitTy, TyVarTy, CoercionTy
1063
1064 dropCastsB :: TyBinder -> TyBinder
1065 dropCastsB (Anon ty) = Anon (dropCasts ty)
1066 dropCastsB b = b -- Don't bother in the kind of a forall
1067
1068 abstractClassKeys :: [Unique]
1069 abstractClassKeys = [ heqTyConKey
1070 , eqTyConKey
1071 , coercibleTyConKey
1072 ] -- See Note [Equality class instances]
1073
1074 instTypeErr :: Class -> [Type] -> SDoc -> SDoc
1075 instTypeErr cls tys msg
1076 = hang (hang (text "Illegal instance declaration for")
1077 2 (quotes (pprClassPred cls tys)))
1078 2 msg
1079
1080 {- Note [Casts during validity checking]
1081 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1082 Consider the (bogus)
1083 instance Eq Char#
1084 We elaborate to 'Eq (Char# |> UnivCo(hole))' where the hole is an
1085 insoluble equality constraint for * ~ #. We'll report the insoluble
1086 constraint separately, but we don't want to *also* complain that Eq is
1087 not applied to a type constructor. So we look gaily look through
1088 CastTys here.
1089
1090 Another example: Eq (Either a). Then we actually get a cast in
1091 the middle:
1092 Eq ((Either |> g) a)
1093
1094
1095 Note [Valid 'deriving' predicate]
1096 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1097 validDerivPred checks for OK 'deriving' context. See Note [Exotic
1098 derived instance contexts] in TcDeriv. However the predicate is
1099 here because it uses sizeTypes, fvTypes.
1100
1101 It checks for three things
1102
1103 * No repeated variables (hasNoDups fvs)
1104
1105 * No type constructors. This is done by comparing
1106 sizeTypes tys == length (fvTypes tys)
1107 sizeTypes counts variables and constructors; fvTypes returns variables.
1108 So if they are the same, there must be no constructors. But there
1109 might be applications thus (f (g x)).
1110
1111 * Also check for a bizarre corner case, when the derived instance decl
1112 would look like
1113 instance C a b => D (T a) where ...
1114 Note that 'b' isn't a parameter of T. This gives rise to all sorts of
1115 problems; in particular, it's hard to compare solutions for equality
1116 when finding the fixpoint, and that means the inferContext loop does
1117 not converge. See Trac #5287.
1118
1119 Note [Equality class instances]
1120 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1121 We can't have users writing instances for the equality classes. But we
1122 still need to be able to write instances for them ourselves. So we allow
1123 instances only in the defining module.
1124
1125 -}
1126
1127 validDerivPred :: TyVarSet -> PredType -> Bool
1128 -- See Note [Valid 'deriving' predicate]
1129 validDerivPred tv_set pred
1130 = case classifyPredType pred of
1131 ClassPred cls _ -> cls `hasKey` typeableClassKey
1132 -- Typeable constraints are bigger than they appear due
1133 -- to kind polymorphism, but that's OK
1134 || check_tys
1135 EqPred {} -> False -- reject equality constraints
1136 _ -> True -- Non-class predicates are ok
1137 where
1138 check_tys = hasNoDups fvs
1139 -- use sizePred to ignore implicit args
1140 && sizePred pred == fromIntegral (length fvs)
1141 && all (`elemVarSet` tv_set) fvs
1142
1143 fvs = fvType pred
1144
1145 {-
1146 ************************************************************************
1147 * *
1148 \subsection{Checking instance for termination}
1149 * *
1150 ************************************************************************
1151 -}
1152
1153 checkValidInstance :: UserTypeCtxt -> LHsSigType Name -> Type
1154 -> TcM ([TyVar], ThetaType, Class, [Type])
1155 checkValidInstance ctxt hs_type ty
1156 | Just (clas,inst_tys) <- getClassPredTys_maybe tau
1157 , inst_tys `lengthIs` classArity clas
1158 = do { setSrcSpan head_loc (checkValidInstHead ctxt clas inst_tys)
1159 ; traceTc "checkValidInstance {" (ppr ty)
1160 ; checkValidTheta ctxt theta
1161
1162 -- The Termination and Coverate Conditions
1163 -- Check that instance inference will terminate (if we care)
1164 -- For Haskell 98 this will already have been done by checkValidTheta,
1165 -- but as we may be using other extensions we need to check.
1166 --
1167 -- Note that the Termination Condition is *more conservative* than
1168 -- the checkAmbiguity test we do on other type signatures
1169 -- e.g. Bar a => Bar Int is ambiguous, but it also fails
1170 -- the termination condition, because 'a' appears more often
1171 -- in the constraint than in the head
1172 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1173 ; if undecidable_ok
1174 then checkAmbiguity ctxt ty
1175 else checkInstTermination inst_tys theta
1176
1177 ; traceTc "cvi 2" (ppr ty)
1178
1179 ; case (checkInstCoverage undecidable_ok clas theta inst_tys) of
1180 IsValid -> return () -- Check succeeded
1181 NotValid msg -> addErrTc (instTypeErr clas inst_tys msg)
1182
1183 ; traceTc "End checkValidInstance }" empty
1184
1185 ; return (tvs, theta, clas, inst_tys) }
1186
1187 | otherwise
1188 = failWithTc (text "Malformed instance head:" <+> ppr tau)
1189 where
1190 (tvs, theta, tau) = tcSplitSigmaTy ty
1191
1192 -- The location of the "head" of the instance
1193 head_loc = getLoc (getLHsInstDeclHead hs_type)
1194
1195 {-
1196 Note [Paterson conditions]
1197 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1198 Termination test: the so-called "Paterson conditions" (see Section 5 of
1199 "Understanding functional dependencies via Constraint Handling Rules,
1200 JFP Jan 2007).
1201
1202 We check that each assertion in the context satisfies:
1203 (1) no variable has more occurrences in the assertion than in the head, and
1204 (2) the assertion has fewer constructors and variables (taken together
1205 and counting repetitions) than the head.
1206 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1207 (which have already been checked) guarantee termination.
1208
1209 The underlying idea is that
1210
1211 for any ground substitution, each assertion in the
1212 context has fewer type constructors than the head.
1213 -}
1214
1215 checkInstTermination :: [TcType] -> ThetaType -> TcM ()
1216 -- See Note [Paterson conditions]
1217 checkInstTermination tys theta
1218 = check_preds theta
1219 where
1220 head_fvs = fvTypes tys
1221 head_size = sizeTypes tys
1222
1223 check_preds :: [PredType] -> TcM ()
1224 check_preds preds = mapM_ check preds
1225
1226 check :: PredType -> TcM ()
1227 check pred
1228 = case classifyPredType pred of
1229 EqPred {} -> return () -- See Trac #4200.
1230 IrredPred {} -> check2 pred (sizeType pred)
1231 ClassPred cls tys
1232 | isTerminatingClass cls
1233 -> return ()
1234
1235 | isCTupleClass cls -- Look inside tuple predicates; Trac #8359
1236 -> check_preds tys
1237
1238 | otherwise
1239 -> check2 pred (sizeTypes $ filterOutInvisibleTypes (classTyCon cls) tys)
1240 -- Other ClassPreds
1241
1242 check2 pred pred_size
1243 | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what)
1244 | pred_size >= head_size = addErrTc (smallerMsg what)
1245 | otherwise = return ()
1246 where
1247 what = text "constraint" <+> quotes (ppr pred)
1248 bad_tvs = fvType pred \\ head_fvs
1249
1250 smallerMsg :: SDoc -> SDoc
1251 smallerMsg what
1252 = vcat [ hang (text "The" <+> what)
1253 2 (text "is no smaller than the instance head")
1254 , parens undecidableMsg ]
1255
1256 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc
1257 noMoreMsg tvs what
1258 = vcat [ hang (text "Variable" <> plural tvs <+> quotes (pprWithCommas ppr tvs)
1259 <+> occurs <+> text "more often")
1260 2 (sep [ text "in the" <+> what
1261 , text "than in the instance head" ])
1262 , parens undecidableMsg ]
1263 where
1264 occurs = if isSingleton tvs then text "occurs"
1265 else text "occur"
1266
1267 undecidableMsg, constraintKindsMsg :: SDoc
1268 undecidableMsg = text "Use UndecidableInstances to permit this"
1269 constraintKindsMsg = text "Use ConstraintKinds to permit this"
1270
1271 {-
1272 Note [Associated type instances]
1273 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1274 We allow this:
1275 class C a where
1276 type T x a
1277 instance C Int where
1278 type T (S y) Int = y
1279 type T Z Int = Char
1280
1281 Note that
1282 a) The variable 'x' is not bound by the class decl
1283 b) 'x' is instantiated to a non-type-variable in the instance
1284 c) There are several type instance decls for T in the instance
1285
1286 All this is fine. Of course, you can't give any *more* instances
1287 for (T ty Int) elsewhere, because it's an *associated* type.
1288
1289 Note [Checking consistent instantiation]
1290 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1291 class C a b where
1292 type T a x b
1293
1294 instance C [p] Int
1295 type T [p] y Int = (p,y,y) -- Induces the family instance TyCon
1296 -- type TR p y = (p,y,y)
1297
1298 So we
1299 * Form the mini-envt from the class type variables a,b
1300 to the instance decl types [p],Int: [a->[p], b->Int]
1301
1302 * Look at the tyvars a,x,b of the type family constructor T
1303 (it shares tyvars with the class C)
1304
1305 * Apply the mini-evnt to them, and check that the result is
1306 consistent with the instance types [p] y Int
1307
1308 We do *not* assume (at this point) the the bound variables of
1309 the associated type instance decl are the same as for the parent
1310 instance decl. So, for example,
1311
1312 instance C [p] Int
1313 type T [q] y Int = ...
1314
1315 would work equally well. Reason: making the *kind* variables line
1316 up is much harder. Example (Trac #7282):
1317 class Foo (xs :: [k]) where
1318 type Bar xs :: *
1319
1320 instance Foo '[] where
1321 type Bar '[] = Int
1322 Here the instance decl really looks like
1323 instance Foo k ('[] k) where
1324 type Bar k ('[] k) = Int
1325 but the k's are not scoped, and hence won't match Uniques.
1326
1327 So instead we just match structure, with tcMatchTyX, and check
1328 that distinct type variables match 1-1 with distinct type variables.
1329
1330 HOWEVER, we *still* make the instance type variables scope over the
1331 type instances, to pick up non-obvious kinds. Eg
1332 class Foo (a :: k) where
1333 type F a
1334 instance Foo (b :: k -> k) where
1335 type F b = Int
1336 Here the instance is kind-indexed and really looks like
1337 type F (k->k) (b::k->k) = Int
1338 But if the 'b' didn't scope, we would make F's instance too
1339 poly-kinded.
1340 -}
1341
1342 -- | Extra information needed when type-checking associated types. The 'Class' is
1343 -- the enclosing class, and the @VarEnv Type@ maps class variables to their
1344 -- instance types.
1345 type ClsInfo = (Class, VarEnv Type)
1346
1347 checkConsistentFamInst
1348 :: Maybe ClsInfo
1349 -> TyCon -- ^ Family tycon
1350 -> [TyVar] -- ^ Type variables of the family instance
1351 -> [Type] -- ^ Type patterns from instance
1352 -> TcM ()
1353 -- See Note [Checking consistent instantiation]
1354
1355 checkConsistentFamInst Nothing _ _ _ = return ()
1356 checkConsistentFamInst (Just (clas, mini_env)) fam_tc at_tvs at_tys
1357 = do { -- Check that the associated type indeed comes from this class
1358 checkTc (Just clas == tyConAssoc_maybe fam_tc)
1359 (badATErr (className clas) (tyConName fam_tc))
1360
1361 -- See Note [Checking consistent instantiation]
1362 -- Check right to left, so that we spot type variable
1363 -- inconsistencies before (more confusing) kind variables
1364 ; checkTc (check_args emptyTCvSubst (fam_tc_tvs `zip` at_tys))
1365 (wrongATArgErr fam_tc expected_args at_tys) }
1366 where
1367 fam_tc_tvs = tyConTyVars fam_tc
1368 expected_args = zipWith pick fam_tc_tvs at_tys
1369 pick fam_tc_tv at_ty = case lookupVarEnv mini_env fam_tc_tv of
1370 Just inst_ty -> inst_ty
1371 Nothing -> at_ty
1372
1373 check_args :: TCvSubst -> [(TyVar,Type)] -> Bool
1374 check_args subst ((fam_tc_tv, at_ty) : rest)
1375 | Just inst_ty <- lookupVarEnv mini_env fam_tc_tv
1376 = case tcMatchTyX subst at_ty inst_ty of
1377 Just subst -> check_args subst rest
1378 Nothing -> False
1379
1380 | otherwise
1381 = check_args subst rest
1382
1383 check_args subst []
1384 = check_tvs subst [] at_tvs
1385
1386 check_tvs :: TCvSubst -> [TyVar] -> [TyVar] -> Bool
1387 check_tvs _ _ [] = True -- OK!!
1388 check_tvs subst acc (tv:tvs)
1389 | Just ty <- lookupTyVar subst tv
1390 = case tcGetTyVar_maybe ty of
1391 Nothing -> False
1392 Just tv' | tv' `elem` acc -> False
1393 | otherwise -> check_tvs subst (tv' : acc) tvs
1394 | otherwise
1395 = check_tvs subst acc tvs
1396
1397 {-
1398 check_arg :: (TyVar, Type) -> TCvSubst -> TcM TCvSubst
1399 check_arg (fam_tc_tv, at_ty) subst
1400 | Just inst_ty <- lookupVarEnv mini_env fam_tc_tv
1401 = case tcMatchTyX subst at_ty inst_ty of
1402 Just subst -> return subst
1403 Nothing -> failWithTc $ wrongATArgErr at_ty inst_ty
1404 -- No need to instantiate here, because the axiom
1405 -- uses the same type variables as the assocated class
1406 | otherwise
1407 = return subst -- Allow non-type-variable instantiation
1408 -- See Note [Associated type instances]
1409
1410 check_distinct :: TCvSubst -> TcM ()
1411 -- True if all the variables mapped the substitution
1412 -- map to *distinct* type *variables*
1413 check_distinct subst = go [] at_tvs
1414 where
1415 go _ [] = return ()
1416 go acc (tv:tvs) = case lookupTyVar subst tv of
1417 Nothing -> go acc tvs
1418 Just ty | Just tv' <- tcGetTyVar_maybe ty
1419 , tv' `notElem` acc
1420 -> go (tv' : acc) tvs
1421 _other -> addErrTc (dupTyVar tv)
1422 -}
1423
1424 badATErr :: Name -> Name -> SDoc
1425 badATErr clas op
1426 = hsep [text "Class", quotes (ppr clas),
1427 text "does not have an associated type", quotes (ppr op)]
1428
1429 wrongATArgErr :: TyCon -> [Type] -> [Type] -> SDoc
1430 wrongATArgErr fam_tc expected_args actual_args
1431 = vcat [ text "Type indexes must match class instance head"
1432 , text "Expected:" <+> ppr (mkTyConApp fam_tc expected_args)
1433 , text "Actual: " <+> ppr (mkTyConApp fam_tc actual_args) ]
1434
1435 {-
1436 ************************************************************************
1437 * *
1438 Checking type instance well-formedness and termination
1439 * *
1440 ************************************************************************
1441 -}
1442
1443 checkValidCoAxiom :: CoAxiom Branched -> TcM ()
1444 checkValidCoAxiom ax@(CoAxiom { co_ax_tc = fam_tc, co_ax_branches = branches })
1445 = do { mapM_ (checkValidCoAxBranch Nothing fam_tc) branch_list
1446 ; foldlM_ check_branch_compat [] branch_list }
1447 where
1448 branch_list = fromBranches branches
1449 injectivity = familyTyConInjectivityInfo fam_tc
1450
1451 check_branch_compat :: [CoAxBranch] -- previous branches in reverse order
1452 -> CoAxBranch -- current branch
1453 -> TcM [CoAxBranch]-- current branch : previous branches
1454 -- Check for
1455 -- (a) this branch is dominated by previous ones
1456 -- (b) failure of injectivity
1457 check_branch_compat prev_branches cur_branch
1458 | cur_branch `isDominatedBy` prev_branches
1459 = do { addWarnAt NoReason (coAxBranchSpan cur_branch) $
1460 inaccessibleCoAxBranch ax cur_branch
1461 ; return prev_branches }
1462 | otherwise
1463 = do { check_injectivity prev_branches cur_branch
1464 ; return (cur_branch : prev_branches) }
1465
1466 -- Injectivity check: check whether a new (CoAxBranch) can extend
1467 -- already checked equations without violating injectivity
1468 -- annotation supplied by the user.
1469 -- See Note [Verifying injectivity annotation] in FamInstEnv
1470 check_injectivity prev_branches cur_branch
1471 | Injective inj <- injectivity
1472 = do { let conflicts =
1473 fst $ foldl (gather_conflicts inj prev_branches cur_branch)
1474 ([], 0) prev_branches
1475 ; mapM_ (\(err, span) -> setSrcSpan span $ addErr err)
1476 (makeInjectivityErrors ax cur_branch inj conflicts) }
1477 | otherwise
1478 = return ()
1479
1480 gather_conflicts inj prev_branches cur_branch (acc, n) branch
1481 -- n is 0-based index of branch in prev_branches
1482 = case injectiveBranches inj cur_branch branch of
1483 InjectivityUnified ax1 ax2
1484 | ax1 `isDominatedBy` (replace_br prev_branches n ax2)
1485 -> (acc, n + 1)
1486 | otherwise
1487 -> (branch : acc, n + 1)
1488 InjectivityAccepted -> (acc, n + 1)
1489
1490 -- Replace n-th element in the list. Assumes 0-based indexing.
1491 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
1492 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
1493
1494
1495 -- Check that a "type instance" is well-formed (which includes decidability
1496 -- unless -XUndecidableInstances is given).
1497 --
1498 checkValidCoAxBranch :: Maybe ClsInfo
1499 -> TyCon -> CoAxBranch -> TcM ()
1500 checkValidCoAxBranch mb_clsinfo fam_tc
1501 (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
1502 , cab_lhs = typats
1503 , cab_rhs = rhs, cab_loc = loc })
1504 = checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1505
1506 -- | Do validity checks on a type family equation, including consistency
1507 -- with any enclosing class instance head, termination, and lack of
1508 -- polytypes.
1509 checkValidTyFamEqn :: Maybe ClsInfo
1510 -> TyCon -- ^ of the type family
1511 -> [TyVar] -- ^ bound tyvars in the equation
1512 -> [CoVar] -- ^ bound covars in the equation
1513 -> [Type] -- ^ type patterns
1514 -> Type -- ^ rhs
1515 -> SrcSpan
1516 -> TcM ()
1517 checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1518 = setSrcSpan loc $
1519 do { checkValidFamPats mb_clsinfo fam_tc tvs cvs typats
1520
1521 -- The argument patterns, and RHS, are all boxed tau types
1522 -- E.g Reject type family F (a :: k1) :: k2
1523 -- type instance F (forall a. a->a) = ...
1524 -- type instance F Int# = ...
1525 -- type instance F Int = forall a. a->a
1526 -- type instance F Int = Int#
1527 -- See Trac #9357
1528 ; checkValidMonoType rhs
1529 ; check_lifted rhs
1530
1531 -- We have a decidable instance unless otherwise permitted
1532 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1533 ; unless undecidable_ok $
1534 mapM_ addErrTc (checkFamInstRhs typats (tcTyFamInsts rhs)) }
1535
1536 -- Make sure that each type family application is
1537 -- (1) strictly smaller than the lhs,
1538 -- (2) mentions no type variable more often than the lhs, and
1539 -- (3) does not contain any further type family instances.
1540 --
1541 checkFamInstRhs :: [Type] -- lhs
1542 -> [(TyCon, [Type])] -- type family instances
1543 -> [MsgDoc]
1544 checkFamInstRhs lhsTys famInsts
1545 = mapMaybe check famInsts
1546 where
1547 size = sizeTypes lhsTys
1548 fvs = fvTypes lhsTys
1549 check (tc, tys)
1550 | not (all isTyFamFree tys) = Just (nestedMsg what)
1551 | not (null bad_tvs) = Just (noMoreMsg bad_tvs what)
1552 | size <= sizeTypes tys = Just (smallerMsg what)
1553 | otherwise = Nothing
1554 where
1555 what = text "type family application" <+> quotes (pprType (TyConApp tc tys))
1556 bad_tvs = fvTypes tys \\ fvs
1557
1558 checkValidFamPats :: Maybe ClsInfo -> TyCon -> [TyVar] -> [CoVar] -> [Type] -> TcM ()
1559 -- Patterns in a 'type instance' or 'data instance' decl should
1560 -- a) contain no type family applications
1561 -- (vanilla synonyms are fine, though)
1562 -- b) properly bind all their free type variables
1563 -- e.g. we disallow (Trac #7536)
1564 -- type T a = Int
1565 -- type instance F (T a) = a
1566 -- c) Have the right number of patterns
1567 -- d) For associated types, are consistently instantiated
1568 checkValidFamPats mb_clsinfo fam_tc tvs cvs ty_pats
1569 = do { -- A family instance must have exactly the same number of type
1570 -- parameters as the family declaration. You can't write
1571 -- type family F a :: * -> *
1572 -- type instance F Int y = y
1573 -- because then the type (F Int) would be like (\y.y)
1574 checkTc (length ty_pats == fam_arity) $
1575 wrongNumberOfParmsErr (fam_arity - count isInvisibleBinder fam_bndrs)
1576 -- report only explicit arguments
1577
1578 ; mapM_ checkValidTypePat ty_pats
1579
1580 ; let unbound_tcvs = filterOut (`elemVarSet` exactTyCoVarsOfTypes ty_pats) (tvs ++ cvs)
1581 ; checkTc (null unbound_tcvs) (famPatErr fam_tc unbound_tcvs ty_pats)
1582
1583 -- Check that type patterns match the class instance head
1584 ; checkConsistentFamInst mb_clsinfo fam_tc tvs ty_pats }
1585 where
1586 fam_arity = tyConArity fam_tc
1587 fam_bndrs = tyConBinders fam_tc
1588
1589
1590 checkValidTypePat :: Type -> TcM ()
1591 -- Used for type patterns in class instances,
1592 -- and in type/data family instances
1593 checkValidTypePat pat_ty
1594 = do { -- Check that pat_ty is a monotype
1595 checkValidMonoType pat_ty
1596 -- One could imagine generalising to allow
1597 -- instance C (forall a. a->a)
1598 -- but we don't know what all the consequences might be
1599
1600 -- Ensure that no type family instances occur a type pattern
1601 ; checkTc (isTyFamFree pat_ty) $
1602 tyFamInstIllegalErr pat_ty
1603
1604 ; check_lifted pat_ty }
1605
1606 isTyFamFree :: Type -> Bool
1607 -- ^ Check that a type does not contain any type family applications.
1608 isTyFamFree = null . tcTyFamInsts
1609
1610 -- Error messages
1611
1612 wrongNumberOfParmsErr :: Arity -> SDoc
1613 wrongNumberOfParmsErr exp_arity
1614 = text "Number of parameters must match family declaration; expected"
1615 <+> ppr exp_arity
1616
1617 inaccessibleCoAxBranch :: CoAxiom br -> CoAxBranch -> SDoc
1618 inaccessibleCoAxBranch fi_ax cur_branch
1619 = text "Type family instance equation is overlapped:" $$
1620 nest 2 (pprCoAxBranch fi_ax cur_branch)
1621
1622 tyFamInstIllegalErr :: Type -> SDoc
1623 tyFamInstIllegalErr ty
1624 = hang (text "Illegal type synonym family application in instance" <>
1625 colon) 2 $
1626 ppr ty
1627
1628 nestedMsg :: SDoc -> SDoc
1629 nestedMsg what
1630 = sep [ text "Illegal nested" <+> what
1631 , parens undecidableMsg ]
1632
1633 famPatErr :: TyCon -> [TyVar] -> [Type] -> SDoc
1634 famPatErr fam_tc tvs pats
1635 = hang (text "Family instance purports to bind type variable" <> plural tvs
1636 <+> pprQuotedList tvs)
1637 2 (hang (text "but the real LHS (expanding synonyms) is:")
1638 2 (pprTypeApp fam_tc (map expandTypeSynonyms pats) <+>
1639 text "= ..."))
1640
1641 {-
1642 ************************************************************************
1643 * *
1644 Telescope checking
1645 * *
1646 ************************************************************************
1647
1648 Note [Bad telescopes]
1649 ~~~~~~~~~~~~~~~~~~~~~
1650 Now that we can mix type and kind variables, there are an awful lot of
1651 ways to shoot yourself in the foot. Here are some.
1652
1653 data SameKind :: k -> k -> * -- just to force unification
1654
1655 1. data T1 a k (b :: k) (x :: SameKind a b)
1656
1657 The problem here is that we discover that a and b should have the same
1658 kind. But this kind mentions k, which is bound *after* a.
1659 (Testcase: dependent/should_fail/BadTelescope)
1660
1661 2. data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)
1662
1663 Note that b is not bound. Yet its kind mentions a. Because we have
1664 a nice rule that all implicitly bound variables come before others,
1665 this is bogus. (We could probably figure out to put b between a and c.
1666 But I think this is doing users a disservice, in the long run.)
1667 (Testcase: dependent/should_fail/BadTelescope4)
1668
1669 3. t3 :: forall a. (forall k (b :: k). SameKind a b) -> ()
1670
1671 This is a straightforward skolem escape. Note that a and b need to have
1672 the same kind.
1673 (Testcase: polykinds/T11142)
1674
1675 How do we deal with all of this? For TyCons, we have checkValidTyConTyVars.
1676 That function looks to see if any of the tyConTyVars are repeated, but
1677 it's really a telescope check. It works because all tycons are kind-generalized.
1678 If there is a bad telescope, the kind-generalization will end up generalizing
1679 over a variable bound later in the telescope.
1680
1681 For non-tycons, we do scope checking when we bring tyvars into scope,
1682 in tcImplicitTKBndrs and tcExplicitTKBndrs. Note that we also have to
1683 sort implicit binders into a well-scoped order whenever we have implicit
1684 binders to worry about. This is done in quantifyTyVars and in
1685 tcImplicitTKBndrs.
1686 -}
1687
1688 -- | Check a list of binders to see if they make a valid telescope.
1689 -- The key property we're checking for is scoping. For example:
1690 -- > data SameKind :: k -> k -> *
1691 -- > data X a k (b :: k) (c :: SameKind a b)
1692 -- Kind inference says that a's kind should be k. But that's impossible,
1693 -- because k isn't in scope when a is bound. This check has to come before
1694 -- general validity checking, because once we kind-generalise, this sort
1695 -- of problem is harder to spot (as we'll generalise over the unbound
1696 -- k in a's type.) See also Note [Bad telescopes].
1697 checkValidTelescope :: SDoc -- the original user-written telescope
1698 -> [TyVar] -- explicit vars (not necessarily zonked)
1699 -> SDoc -- note to put at bottom of message
1700 -> TcM () -- returns zonked tyvars
1701 checkValidTelescope hs_tvs orig_tvs extra
1702 = discardResult $ checkZonkValidTelescope hs_tvs orig_tvs extra
1703
1704 -- | Like 'checkZonkValidTelescope', but returns the zonked tyvars
1705 checkZonkValidTelescope :: SDoc
1706 -> [TyVar]
1707 -> SDoc
1708 -> TcM [TyVar]
1709 checkZonkValidTelescope hs_tvs orig_tvs extra
1710 = do { orig_tvs <- mapM zonkTyCoVarKind orig_tvs
1711 ; let (_, sorted_tidied_tvs) = tidyTyCoVarBndrs emptyTidyEnv $
1712 toposortTyVars orig_tvs
1713 ; unless (go [] emptyVarSet orig_tvs) $
1714 addErr $
1715 vcat [ hang (text "These kind and type variables:" <+> hs_tvs $$
1716 text "are out of dependency order. Perhaps try this ordering:")
1717 2 (sep (map pprTvBndr sorted_tidied_tvs))
1718 , extra ]
1719 ; return orig_tvs }
1720
1721 where
1722 go :: [TyVar] -- misplaced variables
1723 -> TyVarSet -> [TyVar] -> Bool
1724 go errs in_scope [] = null (filter (`elemVarSet` in_scope) errs)
1725 -- report an error only when the variable in the kind is brought
1726 -- into scope later in the telescope. Otherwise, we'll just quantify
1727 -- over it in kindGeneralize, as we should.
1728
1729 go errs in_scope (tv:tvs)
1730 = let bad_tvs = tyCoVarsOfType (tyVarKind tv) `minusVarSet` in_scope in
1731 go (varSetElems bad_tvs ++ errs) (in_scope `extendVarSet` tv) tvs
1732
1733 -- | After inferring kinds of type variables, check to make sure that the
1734 -- inferred kinds any of the type variables bound in a smaller scope.
1735 -- This is a skolem escape check. See also Note [Bad telescopes].
1736 checkValidInferredKinds :: [TyVar] -- ^ vars to check (zonked)
1737 -> TyVarSet -- ^ vars out of scope
1738 -> SDoc -- ^ suffix to error message
1739 -> TcM ()
1740 checkValidInferredKinds orig_kvs out_of_scope extra
1741 = do { let bad_pairs = [ (tv, kv)
1742 | kv <- orig_kvs
1743 , Just tv <- map (lookupVarSet out_of_scope)
1744 (tyCoVarsOfTypeList (tyVarKind kv)) ]
1745 report (tidyTyVarOcc env -> tv, tidyTyVarOcc env -> kv)
1746 = addErr $
1747 text "The kind of variable" <+>
1748 quotes (ppr kv) <> text ", namely" <+>
1749 quotes (ppr (tyVarKind kv)) <> comma $$
1750 text "depends on variable" <+>
1751 quotes (ppr tv) <+> text "from an inner scope" $$
1752 text "Perhaps bind" <+> quotes (ppr kv) <+>
1753 text "sometime after binding" <+>
1754 quotes (ppr tv) $$
1755 extra
1756 ; mapM_ report bad_pairs }
1757
1758 where
1759 (env1, _) = tidyTyCoVarBndrs emptyTidyEnv orig_kvs
1760 (env, _) = tidyTyCoVarBndrs env1 (varSetElems out_of_scope)
1761
1762 {-
1763 ************************************************************************
1764 * *
1765 \subsection{Auxiliary functions}
1766 * *
1767 ************************************************************************
1768 -}
1769
1770 -- Free variables of a type, retaining repetitions, and expanding synonyms
1771 fvType :: Type -> [TyCoVar]
1772 fvType ty | Just exp_ty <- coreView ty = fvType exp_ty
1773 fvType (TyVarTy tv) = [tv]
1774 fvType (TyConApp _ tys) = fvTypes tys
1775 fvType (LitTy {}) = []
1776 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1777 fvType (ForAllTy bndr ty)
1778 = fvType (binderType bndr) ++
1779 caseBinder bndr (\tv -> filter (/= tv)) (const id) (fvType ty)
1780 fvType (CastTy ty co) = fvType ty ++ fvCo co
1781 fvType (CoercionTy co) = fvCo co
1782
1783 fvTypes :: [Type] -> [TyVar]
1784 fvTypes tys = concat (map fvType tys)
1785
1786 fvCo :: Coercion -> [TyCoVar]
1787 fvCo (Refl _ ty) = fvType ty
1788 fvCo (TyConAppCo _ _ args) = concatMap fvCo args
1789 fvCo (AppCo co arg) = fvCo co ++ fvCo arg
1790 fvCo (ForAllCo tv h co) = filter (/= tv) (fvCo co) ++ fvCo h
1791 fvCo (CoVarCo v) = [v]
1792 fvCo (AxiomInstCo _ _ args) = concatMap fvCo args
1793 fvCo (UnivCo p _ t1 t2) = fvProv p ++ fvType t1 ++ fvType t2
1794 fvCo (SymCo co) = fvCo co
1795 fvCo (TransCo co1 co2) = fvCo co1 ++ fvCo co2
1796 fvCo (NthCo _ co) = fvCo co
1797 fvCo (LRCo _ co) = fvCo co
1798 fvCo (InstCo co arg) = fvCo co ++ fvCo arg
1799 fvCo (CoherenceCo co1 co2) = fvCo co1 ++ fvCo co2
1800 fvCo (KindCo co) = fvCo co
1801 fvCo (SubCo co) = fvCo co
1802 fvCo (AxiomRuleCo _ cs) = concatMap fvCo cs
1803
1804 fvProv :: UnivCoProvenance -> [TyCoVar]
1805 fvProv UnsafeCoerceProv = []
1806 fvProv (PhantomProv co) = fvCo co
1807 fvProv (ProofIrrelProv co) = fvCo co
1808 fvProv (PluginProv _) = []
1809 fvProv (HoleProv h) = pprPanic "fvProv falls into a hole" (ppr h)
1810
1811 sizeType :: Type -> Int
1812 -- Size of a type: the number of variables and constructors
1813 sizeType ty | Just exp_ty <- coreView ty = sizeType exp_ty
1814 sizeType (TyVarTy {}) = 1
1815 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1816 sizeType (LitTy {}) = 1
1817 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1818 sizeType (ForAllTy (Anon arg) res)
1819 = sizeType arg + sizeType res + 1
1820 sizeType (ForAllTy (Named {}) ty)
1821 = sizeType ty
1822 sizeType (CastTy ty _) = sizeType ty
1823 sizeType (CoercionTy _) = 1
1824
1825 sizeTypes :: [Type] -> Int
1826 sizeTypes = sum . map sizeType
1827
1828 -- Size of a predicate
1829 --
1830 -- We are considering whether class constraints terminate.
1831 -- Equality constraints and constraints for the implicit
1832 -- parameter class always termiante so it is safe to say "size 0".
1833 -- (Implicit parameter constraints always terminate because
1834 -- there are no instances for them---they are only solved by
1835 -- "local instances" in expressions).
1836 -- See Trac #4200.
1837 sizePred :: PredType -> Int
1838 sizePred ty = goClass ty
1839 where
1840 goClass p = go (classifyPredType p)
1841
1842 go (ClassPred cls tys')
1843 | isTerminatingClass cls = 0
1844 | otherwise = sizeTypes tys'
1845 go (EqPred {}) = 0
1846 go (IrredPred ty) = sizeType ty
1847
1848 -- | When this says "True", ignore this class constraint during
1849 -- a termination check
1850 isTerminatingClass :: Class -> Bool
1851 isTerminatingClass cls
1852 = isIPClass cls
1853 || cls `hasKey` typeableClassKey
1854 || cls `hasKey` coercibleTyConKey
1855 || cls `hasKey` eqTyConKey
1856 || cls `hasKey` heqTyConKey
1857
1858 -- | Tidy before printing a type
1859 ppr_tidy :: TidyEnv -> Type -> SDoc
1860 ppr_tidy env ty = pprType (tidyType env ty)