Move the constraint-kind validity check
[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, checkTySynRhs,
14 ClsInstInfo, checkValidCoAxiom, checkValidCoAxBranch,
15 checkValidTyFamEqn,
16 arityErr, badATErr,
17 checkValidTelescope, checkZonkValidTelescope, checkValidInferredKinds,
18 allDistinctTyVars
19 ) where
20
21 #include "HsVersions.h"
22
23 import Maybes
24
25 -- friends:
26 import TcUnify ( tcSubType_NC )
27 import TcSimplify ( simplifyAmbiguityCheck )
28 import TyCoRep
29 import TcType hiding ( sizeType, sizeTypes )
30 import TcMType
31 import PrelNames
32 import Type
33 import Coercion
34 import Kind
35 import CoAxiom
36 import Class
37 import TyCon
38
39 -- others:
40 import HsSyn -- HsType
41 import TcRnMonad -- TcType, amongst others
42 import TcHsSyn ( checkForRepresentationPolymorphism )
43 import TcEnv ( tcGetInstEnvs )
44 import FunDeps
45 import InstEnv ( ClsInst, lookupInstEnv, isOverlappable )
46 import FamInstEnv ( isDominatedBy, injectiveBranches,
47 InjectivityCheckResult(..) )
48 import FamInst ( makeInjectivityErrors )
49 import Name
50 import VarEnv
51 import VarSet
52 import UniqFM
53 import Var ( mkTyVar )
54 import ErrUtils
55 import DynFlags
56 import Util
57 import ListSetOps
58 import SrcLoc
59 import Outputable
60 import BasicTypes
61 import Module
62 import Unique ( mkAlphaTyVarUnique )
63 import qualified GHC.LanguageExtensions as LangExt
64
65 import Control.Monad
66 import Data.List ( (\\) )
67
68 {-
69 ************************************************************************
70 * *
71 Checking for ambiguity
72 * *
73 ************************************************************************
74
75 Note [The ambiguity check for type signatures]
76 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
77 checkAmbiguity is a check on *user-supplied type signatures*. It is
78 *purely* there to report functions that cannot possibly be called. So for
79 example we want to reject:
80 f :: C a => Int
81 The idea is there can be no legal calls to 'f' because every call will
82 give rise to an ambiguous constraint. We could soundly omit the
83 ambiguity check on type signatures entirely, at the expense of
84 delaying ambiguity errors to call sites. Indeed, the flag
85 -XAllowAmbiguousTypes switches off the ambiguity check.
86
87 What about things like this:
88 class D a b | a -> b where ..
89 h :: D Int b => Int
90 The Int may well fix 'b' at the call site, so that signature should
91 not be rejected. Moreover, using *visible* fundeps is too
92 conservative. Consider
93 class X a b where ...
94 class D a b | a -> b where ...
95 instance D a b => X [a] b where...
96 h :: X a b => a -> a
97 Here h's type looks ambiguous in 'b', but here's a legal call:
98 ...(h [True])...
99 That gives rise to a (X [Bool] beta) constraint, and using the
100 instance means we need (D Bool beta) and that fixes 'beta' via D's
101 fundep!
102
103 Behind all these special cases there is a simple guiding principle.
104 Consider
105
106 f :: <type>
107 f = ...blah...
108
109 g :: <type>
110 g = f
111
112 You would think that the definition of g would surely typecheck!
113 After all f has exactly the same type, and g=f. But in fact f's type
114 is instantiated and the instantiated constraints are solved against
115 the originals, so in the case an ambiguous type it won't work.
116 Consider our earlier example f :: C a => Int. Then in g's definition,
117 we'll instantiate to (C alpha) and try to deduce (C alpha) from (C a),
118 and fail.
119
120 So in fact we use this as our *definition* of ambiguity. We use a
121 very similar test for *inferred* types, to ensure that they are
122 unambiguous. See Note [Impedence matching] in TcBinds.
123
124 This test is very conveniently implemented by calling
125 tcSubType <type> <type>
126 This neatly takes account of the functional dependecy stuff above,
127 and implicit parameter (see Note [Implicit parameters and ambiguity]).
128 And this is what checkAmbiguity does.
129
130 What about this, though?
131 g :: C [a] => Int
132 Is every call to 'g' ambiguous? After all, we might have
133 intance C [a] where ...
134 at the call site. So maybe that type is ok! Indeed even f's
135 quintessentially ambiguous type might, just possibly be callable:
136 with -XFlexibleInstances we could have
137 instance C a where ...
138 and now a call could be legal after all! Well, we'll reject this
139 unless the instance is available *here*.
140
141 Note [When to call checkAmbiguity]
142 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
143 We call checkAmbiguity
144 (a) on user-specified type signatures
145 (b) in checkValidType
146
147 Conncerning (b), you might wonder about nested foralls. What about
148 f :: forall b. (forall a. Eq a => b) -> b
149 The nested forall is ambiguous. Originally we called checkAmbiguity
150 in the forall case of check_type, but that had two bad consequences:
151 * We got two error messages about (Eq b) in a nested forall like this:
152 g :: forall a. Eq a => forall b. Eq b => a -> a
153 * If we try to check for ambiguity of an nested forall like
154 (forall a. Eq a => b), the implication constraint doesn't bind
155 all the skolems, which results in "No skolem info" in error
156 messages (see Trac #10432).
157
158 To avoid this, we call checkAmbiguity once, at the top, in checkValidType.
159 (I'm still a bit worried about unbound skolems when the type mentions
160 in-scope type variables.)
161
162 In fact, because of the co/contra-variance implemented in tcSubType,
163 this *does* catch function f above. too.
164
165 Concerning (a) the ambiguity check is only used for *user* types, not
166 for types coming from inteface files. The latter can legitimately
167 have ambiguous types. Example
168
169 class S a where s :: a -> (Int,Int)
170 instance S Char where s _ = (1,1)
171 f:: S a => [a] -> Int -> (Int,Int)
172 f (_::[a]) x = (a*x,b)
173 where (a,b) = s (undefined::a)
174
175 Here the worker for f gets the type
176 fw :: forall a. S a => Int -> (# Int, Int #)
177
178
179 Note [Implicit parameters and ambiguity]
180 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
181 Only a *class* predicate can give rise to ambiguity
182 An *implicit parameter* cannot. For example:
183 foo :: (?x :: [a]) => Int
184 foo = length ?x
185 is fine. The call site will supply a particular 'x'
186
187 Furthermore, the type variables fixed by an implicit parameter
188 propagate to the others. E.g.
189 foo :: (Show a, ?x::[a]) => Int
190 foo = show (?x++?x)
191 The type of foo looks ambiguous. But it isn't, because at a call site
192 we might have
193 let ?x = 5::Int in foo
194 and all is well. In effect, implicit parameters are, well, parameters,
195 so we can take their type variables into account as part of the
196 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
197 -}
198
199 checkAmbiguity :: UserTypeCtxt -> Type -> TcM ()
200 checkAmbiguity ctxt ty
201 | wantAmbiguityCheck ctxt
202 = do { traceTc "Ambiguity check for" (ppr ty)
203 -- Solve the constraints eagerly because an ambiguous type
204 -- can cause a cascade of further errors. Since the free
205 -- tyvars are skolemised, we can safely use tcSimplifyTop
206 ; allow_ambiguous <- xoptM LangExt.AllowAmbiguousTypes
207 ; (_wrap, wanted) <- addErrCtxt (mk_msg allow_ambiguous) $
208 captureConstraints $
209 tcSubType_NC ctxt ty (mkCheckExpType ty)
210 ; simplifyAmbiguityCheck ty wanted
211
212 ; traceTc "Done ambiguity check for" (ppr ty) }
213
214 | otherwise
215 = return ()
216 where
217 mk_msg allow_ambiguous
218 = vcat [ text "In the ambiguity check for" <+> what
219 , ppUnless allow_ambiguous ambig_msg ]
220 ambig_msg = text "To defer the ambiguity check to use sites, enable AllowAmbiguousTypes"
221 what | Just n <- isSigMaybe ctxt = quotes (ppr n)
222 | otherwise = pprUserTypeCtxt ctxt
223
224 wantAmbiguityCheck :: UserTypeCtxt -> Bool
225 wantAmbiguityCheck ctxt
226 = case ctxt of -- See Note [When we don't check for ambiguity]
227 GhciCtxt -> False
228 TySynCtxt {} -> False
229 _ -> True
230
231 checkUserTypeError :: Type -> TcM ()
232 -- Check to see if the type signature mentions "TypeError blah"
233 -- anywhere in it, and fail if so.
234 --
235 -- Very unsatisfactorily (Trac #11144) we need to tidy the type
236 -- because it may have come from an /inferred/ signature, not a
237 -- user-supplied one. This is really only a half-baked fix;
238 -- the other errors in checkValidType don't do tidying, and so
239 -- may give bad error messages when given an inferred type.
240 checkUserTypeError = check
241 where
242 check ty
243 | Just msg <- userTypeError_maybe ty = fail_with msg
244 | Just (_,ts) <- splitTyConApp_maybe ty = mapM_ check ts
245 | Just (t1,t2) <- splitAppTy_maybe ty = check t1 >> check t2
246 | Just (_,t1) <- splitForAllTy_maybe ty = check t1
247 | otherwise = return ()
248
249 fail_with msg = do { env0 <- tcInitTidyEnv
250 ; let (env1, tidy_msg) = tidyOpenType env0 msg
251 ; failWithTcM (env1, pprUserTypeErrorTy tidy_msg) }
252
253
254 {- Note [When we don't check for ambiguity]
255 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
256 In a few places we do not want to check a user-specified type for ambiguity
257
258 * GhciCtxt: Allow ambiguous types in GHCi's :kind command
259 E.g. type family T a :: * -- T :: forall k. k -> *
260 Then :k T should work in GHCi, not complain that
261 (T k) is ambiguous!
262
263 * TySynCtxt: type T a b = C a b => blah
264 It may be that when we /use/ T, we'll give an 'a' or 'b' that somehow
265 cure the ambiguity. So we defer the ambiguity check to the use site.
266
267 There is also an implementation reason (Trac #11608). In the RHS of
268 a type synonym we don't (currently) instantiate 'a' and 'b' with
269 TcTyVars before calling checkValidType, so we get asertion failures
270 from doing an ambiguity check on a type with TyVars in it. Fixing this
271 would not be hard, but let's wait till there's a reason.
272
273
274 ************************************************************************
275 * *
276 Checking validity of a user-defined type
277 * *
278 ************************************************************************
279
280 When dealing with a user-written type, we first translate it from an HsType
281 to a Type, performing kind checking, and then check various things that should
282 be true about it. We don't want to perform these checks at the same time
283 as the initial translation because (a) they are unnecessary for interface-file
284 types and (b) when checking a mutually recursive group of type and class decls,
285 we can't "look" at the tycons/classes yet. Also, the checks are rather
286 diverse, and used to really mess up the other code.
287
288 One thing we check for is 'rank'.
289
290 Rank 0: monotypes (no foralls)
291 Rank 1: foralls at the front only, Rank 0 inside
292 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
293
294 basic ::= tyvar | T basic ... basic
295
296 r2 ::= forall tvs. cxt => r2a
297 r2a ::= r1 -> r2a | basic
298 r1 ::= forall tvs. cxt => r0
299 r0 ::= r0 -> r0 | basic
300
301 Another thing is to check that type synonyms are saturated.
302 This might not necessarily show up in kind checking.
303 type A i = i
304 data T k = MkT (k Int)
305 f :: T A -- BAD!
306 -}
307
308 checkValidType :: UserTypeCtxt -> Type -> TcM ()
309 -- Checks that a user-written type is valid for the given context
310 -- Assumes arguemt is fully zonked
311 -- Not used for instance decls; checkValidInstance instead
312 checkValidType ctxt ty
313 = do { traceTc "checkValidType" (ppr ty <+> text "::" <+> ppr (typeKind ty))
314 ; rankn_flag <- xoptM LangExt.RankNTypes
315 ; impred_flag <- xoptM LangExt.ImpredicativeTypes
316 ; let gen_rank :: Rank -> Rank
317 gen_rank r | rankn_flag = ArbitraryRank
318 | otherwise = r
319
320 rank1 = gen_rank r1
321 rank0 = gen_rank r0
322
323 r0 = rankZeroMonoType
324 r1 = LimitedRank True r0
325
326 rank
327 = case ctxt of
328 DefaultDeclCtxt-> MustBeMonoType
329 ResSigCtxt -> MustBeMonoType
330 PatSigCtxt -> rank0
331 RuleSigCtxt _ -> rank1
332 TySynCtxt _ -> rank0
333
334 ExprSigCtxt -> rank1
335 TypeAppCtxt | impred_flag -> ArbitraryRank
336 | otherwise -> tyConArgMonoType
337 -- Normally, ImpredicativeTypes is handled in check_arg_type,
338 -- but visible type applications don't go through there.
339 -- So we do this check here.
340
341 FunSigCtxt {} -> rank1
342 InfSigCtxt _ -> ArbitraryRank -- Inferred type
343 ConArgCtxt _ -> rank1 -- We are given the type of the entire
344 -- constructor, hence rank 1
345
346 ForSigCtxt _ -> rank1
347 SpecInstCtxt -> rank1
348 ThBrackCtxt -> rank1
349 GhciCtxt -> ArbitraryRank
350 _ -> panic "checkValidType"
351 -- Can't happen; not used for *user* sigs
352
353 ; env <- tcInitOpenTidyEnv (tyCoVarsOfTypeList ty)
354
355 -- Check the internal validity of the type itself
356 ; check_type env ctxt rank ty
357
358 ; checkUserTypeError ty
359
360 -- Check for ambiguous types. See Note [When to call checkAmbiguity]
361 -- NB: this will happen even for monotypes, but that should be cheap;
362 -- and there may be nested foralls for the subtype test to examine
363 ; checkAmbiguity ctxt ty
364
365 ; traceTc "checkValidType done" (ppr ty <+> text "::" <+> ppr (typeKind ty)) }
366
367 checkValidMonoType :: Type -> TcM ()
368 -- Assumes arguemt is fully zonked
369 checkValidMonoType ty
370 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypeList ty)
371 ; check_type env SigmaCtxt MustBeMonoType ty }
372
373 checkTySynRhs :: UserTypeCtxt -> TcType -> TcM ()
374 checkTySynRhs ctxt ty
375 | returnsConstraintKind actual_kind
376 = do { ck <- xoptM LangExt.ConstraintKinds
377 ; if ck
378 then when (isConstraintKind actual_kind)
379 (do { dflags <- getDynFlags
380 ; check_pred_ty emptyTidyEnv dflags ctxt ty })
381 else addErrTcM (constraintSynErr emptyTidyEnv actual_kind) }
382
383 | otherwise
384 = return ()
385 where
386 actual_kind = typeKind ty
387
388 -- | The kind expected in a certain context.
389 data ContextKind = TheKind Kind -- ^ a specific kind
390 | AnythingKind -- ^ any kind will do
391 | OpenKind -- ^ something of the form @TYPE _@
392
393 -- Depending on the context, we might accept any kind (for instance, in a TH
394 -- splice), or only certain kinds (like in type signatures).
395 expectedKindInCtxt :: UserTypeCtxt -> ContextKind
396 expectedKindInCtxt (TySynCtxt _) = AnythingKind
397 expectedKindInCtxt ThBrackCtxt = AnythingKind
398 expectedKindInCtxt GhciCtxt = AnythingKind
399 -- The types in a 'default' decl can have varying kinds
400 -- See Note [Extended defaults]" in TcEnv
401 expectedKindInCtxt DefaultDeclCtxt = AnythingKind
402 expectedKindInCtxt TypeAppCtxt = AnythingKind
403 expectedKindInCtxt (ForSigCtxt _) = TheKind liftedTypeKind
404 expectedKindInCtxt InstDeclCtxt = TheKind constraintKind
405 expectedKindInCtxt SpecInstCtxt = TheKind constraintKind
406 expectedKindInCtxt _ = OpenKind
407
408 {-
409 Note [Higher rank types]
410 ~~~~~~~~~~~~~~~~~~~~~~~~
411 Technically
412 Int -> forall a. a->a
413 is still a rank-1 type, but it's not Haskell 98 (Trac #5957). So the
414 validity checker allow a forall after an arrow only if we allow it
415 before -- that is, with Rank2Types or RankNTypes
416 -}
417
418 data Rank = ArbitraryRank -- Any rank ok
419
420 | LimitedRank -- Note [Higher rank types]
421 Bool -- Forall ok at top
422 Rank -- Use for function arguments
423
424 | MonoType SDoc -- Monotype, with a suggestion of how it could be a polytype
425
426 | MustBeMonoType -- Monotype regardless of flags
427
428
429 rankZeroMonoType, tyConArgMonoType, synArgMonoType, constraintMonoType :: Rank
430 rankZeroMonoType = MonoType (text "Perhaps you intended to use RankNTypes or Rank2Types")
431 tyConArgMonoType = MonoType (text "GHC doesn't yet support impredicative polymorphism")
432 synArgMonoType = MonoType (text "Perhaps you intended to use LiberalTypeSynonyms")
433 constraintMonoType = MonoType (text "A constraint must be a monotype")
434
435 funArgResRank :: Rank -> (Rank, Rank) -- Function argument and result
436 funArgResRank (LimitedRank _ arg_rank) = (arg_rank, LimitedRank (forAllAllowed arg_rank) arg_rank)
437 funArgResRank other_rank = (other_rank, other_rank)
438
439 forAllAllowed :: Rank -> Bool
440 forAllAllowed ArbitraryRank = True
441 forAllAllowed (LimitedRank forall_ok _) = forall_ok
442 forAllAllowed _ = False
443
444 -- The zonker issues errors if it zonks a representation-polymorphic binder
445 -- But sometimes it's nice to check a little more eagerly, trying to report
446 -- errors earlier.
447 representationPolymorphismForbidden :: UserTypeCtxt -> Bool
448 representationPolymorphismForbidden = go
449 where
450 go (ConArgCtxt _) = True -- A rep-polymorphic datacon won't be useful
451 go (PatSynCtxt _) = True -- Similar to previous case
452 go _ = False -- Other cases are caught by zonker
453
454 ----------------------------------------
455 -- | Fail with error message if the type is unlifted
456 check_lifted :: Type -> TcM ()
457 check_lifted _ = return ()
458
459 {- ------ Legacy comment ---------
460 The check_unlifted function seems entirely redundant. The
461 kind system should check for uses of unlifted types. So I've
462 removed the check. See Trac #11120 comment:19.
463
464 check_lifted ty
465 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypeList ty)
466 ; checkTcM (not (isUnliftedType ty)) (unliftedArgErr env ty) }
467
468 unliftedArgErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
469 unliftedArgErr env ty = (env, sep [text "Illegal unlifted type:", ppr_tidy env ty])
470 ------ End of legacy comment --------- -}
471
472
473 check_type :: TidyEnv -> UserTypeCtxt -> Rank -> Type -> TcM ()
474 -- The args say what the *type context* requires, independent
475 -- of *flag* settings. You test the flag settings at usage sites.
476 --
477 -- Rank is allowed rank for function args
478 -- Rank 0 means no for-alls anywhere
479
480 check_type env ctxt rank ty
481 | not (null tvs && null theta)
482 = do { traceTc "check_type" (ppr ty $$ ppr (forAllAllowed rank))
483 ; checkTcM (forAllAllowed rank) (forAllTyErr env rank ty)
484 -- Reject e.g. (Maybe (?x::Int => Int)),
485 -- with a decent error message
486
487 ; check_valid_theta env' SigmaCtxt theta
488 -- Allow type T = ?x::Int => Int -> Int
489 -- but not type T = ?x::Int
490
491 ; check_type env' ctxt rank tau -- Allow foralls to right of arrow
492 ; checkTcM (not (any (`elemVarSet` tyCoVarsOfType phi_kind) tvs))
493 (forAllEscapeErr env' ty tau_kind)
494 }
495 where
496 (tvs, theta, tau) = tcSplitSigmaTy ty
497 tau_kind = typeKind tau
498 (env', _) = tidyTyCoVarBndrs env tvs
499
500 phi_kind | null theta = tau_kind
501 | otherwise = liftedTypeKind
502 -- If there are any constraints, the kind is *. (#11405)
503
504 check_type _ _ _ (TyVarTy _) = return ()
505
506 check_type env ctxt rank (ForAllTy (Anon arg_ty) res_ty)
507 = do { check_type env ctxt arg_rank arg_ty
508 ; when (representationPolymorphismForbidden ctxt) $
509 checkForRepresentationPolymorphism empty arg_ty
510 ; check_type env ctxt res_rank res_ty }
511 where
512 (arg_rank, res_rank) = funArgResRank rank
513
514 check_type env ctxt rank (AppTy ty1 ty2)
515 = do { check_arg_type env ctxt rank ty1
516 ; check_arg_type env ctxt rank ty2 }
517
518 check_type env ctxt rank ty@(TyConApp tc tys)
519 | isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
520 = check_syn_tc_app env ctxt rank ty tc tys
521 | isUnboxedTupleTyCon tc = check_ubx_tuple env ctxt ty tys
522 | otherwise = mapM_ (check_arg_type env ctxt rank) tys
523
524 check_type _ _ _ (LitTy {}) = return ()
525
526 check_type env ctxt rank (CastTy ty _) = check_type env ctxt rank ty
527
528 check_type _ _ _ ty = pprPanic "check_type" (ppr ty)
529
530 ----------------------------------------
531 check_syn_tc_app :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType
532 -> TyCon -> [KindOrType] -> TcM ()
533 -- Used for type synonyms and type synonym families,
534 -- which must be saturated,
535 -- but not data families, which need not be saturated
536 check_syn_tc_app env ctxt rank ty tc tys
537 | tc_arity <= length tys -- Saturated
538 -- Check that the synonym has enough args
539 -- This applies equally to open and closed synonyms
540 -- It's OK to have an *over-applied* type synonym
541 -- data Tree a b = ...
542 -- type Foo a = Tree [a]
543 -- f :: Foo a b -> ...
544 = do { -- See Note [Liberal type synonyms]
545 ; liberal <- xoptM LangExt.LiberalTypeSynonyms
546 ; if not liberal || isTypeFamilyTyCon tc then
547 -- For H98 and synonym families, do check the type args
548 mapM_ check_arg tys
549
550 else -- In the liberal case (only for closed syns), expand then check
551 case coreView ty of
552 Just ty' -> check_type env ctxt rank ty'
553 Nothing -> pprPanic "check_tau_type" (ppr ty) }
554
555 | GhciCtxt <- ctxt -- Accept under-saturated type synonyms in
556 -- GHCi :kind commands; see Trac #7586
557 = mapM_ check_arg tys
558
559 | otherwise
560 = failWithTc (tyConArityErr tc tys)
561 where
562 tc_arity = tyConArity tc
563 check_arg | isTypeFamilyTyCon tc = check_arg_type env ctxt rank
564 | otherwise = check_type env ctxt synArgMonoType
565
566 ----------------------------------------
567 check_ubx_tuple :: TidyEnv -> UserTypeCtxt -> KindOrType
568 -> [KindOrType] -> TcM ()
569 check_ubx_tuple env ctxt ty tys
570 = do { ub_tuples_allowed <- xoptM LangExt.UnboxedTuples
571 ; checkTcM ub_tuples_allowed (ubxArgTyErr env ty)
572
573 ; impred <- xoptM LangExt.ImpredicativeTypes
574 ; let rank' = if impred then ArbitraryRank else tyConArgMonoType
575 -- c.f. check_arg_type
576 -- However, args are allowed to be unlifted, or
577 -- more unboxed tuples, so can't use check_arg_ty
578 ; mapM_ (check_type env ctxt rank') tys }
579
580 ----------------------------------------
581 check_arg_type :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType -> TcM ()
582 -- The sort of type that can instantiate a type variable,
583 -- or be the argument of a type constructor.
584 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
585 -- Other unboxed types are very occasionally allowed as type
586 -- arguments depending on the kind of the type constructor
587 --
588 -- For example, we want to reject things like:
589 --
590 -- instance Ord a => Ord (forall s. T s a)
591 -- and
592 -- g :: T s (forall b.b)
593 --
594 -- NB: unboxed tuples can have polymorphic or unboxed args.
595 -- This happens in the workers for functions returning
596 -- product types with polymorphic components.
597 -- But not in user code.
598 -- Anyway, they are dealt with by a special case in check_tau_type
599
600 check_arg_type _ _ _ (CoercionTy {}) = return ()
601
602 check_arg_type env ctxt rank ty
603 = do { impred <- xoptM LangExt.ImpredicativeTypes
604 ; let rank' = case rank of -- Predictive => must be monotype
605 MustBeMonoType -> MustBeMonoType -- Monotype, regardless
606 _other | impred -> ArbitraryRank
607 | otherwise -> tyConArgMonoType
608 -- Make sure that MustBeMonoType is propagated,
609 -- so that we don't suggest -XImpredicativeTypes in
610 -- (Ord (forall a.a)) => a -> a
611 -- and so that if it Must be a monotype, we check that it is!
612
613 ; check_type env ctxt rank' ty
614 ; check_lifted ty }
615 -- NB the isUnliftedType test also checks for
616 -- T State#
617 -- where there is an illegal partial application of State# (which has
618 -- kind * -> #); see Note [The kind invariant] in TyCoRep
619
620 ----------------------------------------
621 forAllTyErr :: TidyEnv -> Rank -> Type -> (TidyEnv, SDoc)
622 forAllTyErr env rank ty
623 = ( env
624 , vcat [ hang herald 2 (ppr_tidy env ty)
625 , suggestion ] )
626 where
627 (tvs, _theta, _tau) = tcSplitSigmaTy ty
628 herald | null tvs = text "Illegal qualified type:"
629 | otherwise = text "Illegal polymorphic type:"
630 suggestion = case rank of
631 LimitedRank {} -> text "Perhaps you intended to use RankNTypes or Rank2Types"
632 MonoType d -> d
633 _ -> Outputable.empty -- Polytype is always illegal
634
635 forAllEscapeErr :: TidyEnv -> Type -> Kind -> (TidyEnv, SDoc)
636 forAllEscapeErr env ty tau_kind
637 = ( env
638 , hang (vcat [ text "Quantified type's kind mentions quantified type variable"
639 , text "(skolem escape)" ])
640 2 (vcat [ text " type:" <+> ppr_tidy env ty
641 , text "of kind:" <+> ppr_tidy env tau_kind ]) )
642
643 ubxArgTyErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
644 ubxArgTyErr env ty = (env, sep [text "Illegal unboxed tuple type as function argument:", ppr_tidy env ty])
645
646 {-
647 Note [Liberal type synonyms]
648 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
649 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
650 doing validity checking. This allows us to instantiate a synonym defn
651 with a for-all type, or with a partially-applied type synonym.
652 e.g. type T a b = a
653 type S m = m ()
654 f :: S (T Int)
655 Here, T is partially applied, so it's illegal in H98. But if you
656 expand S first, then T we get just
657 f :: Int
658 which is fine.
659
660 IMPORTANT: suppose T is a type synonym. Then we must do validity
661 checking on an appliation (T ty1 ty2)
662
663 *either* before expansion (i.e. check ty1, ty2)
664 *or* after expansion (i.e. expand T ty1 ty2, and then check)
665 BUT NOT BOTH
666
667 If we do both, we get exponential behaviour!!
668
669 data TIACons1 i r c = c i ::: r c
670 type TIACons2 t x = TIACons1 t (TIACons1 t x)
671 type TIACons3 t x = TIACons2 t (TIACons1 t x)
672 type TIACons4 t x = TIACons2 t (TIACons2 t x)
673 type TIACons7 t x = TIACons4 t (TIACons3 t x)
674
675
676 ************************************************************************
677 * *
678 \subsection{Checking a theta or source type}
679 * *
680 ************************************************************************
681
682 Note [Implicit parameters in instance decls]
683 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
684 Implicit parameters _only_ allowed in type signatures; not in instance
685 decls, superclasses etc. The reason for not allowing implicit params in
686 instances is a bit subtle. If we allowed
687 instance (?x::Int, Eq a) => Foo [a] where ...
688 then when we saw
689 (e :: (?x::Int) => t)
690 it would be unclear how to discharge all the potential uses of the ?x
691 in e. For example, a constraint Foo [Int] might come out of e, and
692 applying the instance decl would show up two uses of ?x. Trac #8912.
693 -}
694
695 checkValidTheta :: UserTypeCtxt -> ThetaType -> TcM ()
696 -- Assumes arguemt is fully zonked
697 checkValidTheta ctxt theta
698 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypesList theta)
699 ; addErrCtxtM (checkThetaCtxt ctxt theta) $
700 check_valid_theta env ctxt theta }
701
702 -------------------------
703 check_valid_theta :: TidyEnv -> UserTypeCtxt -> [PredType] -> TcM ()
704 check_valid_theta _ _ []
705 = return ()
706 check_valid_theta env ctxt theta
707 = do { dflags <- getDynFlags
708 ; warnTcM (Reason Opt_WarnDuplicateConstraints)
709 (wopt Opt_WarnDuplicateConstraints dflags && notNull dups)
710 (dupPredWarn env dups)
711 ; traceTc "check_valid_theta" (ppr theta)
712 ; mapM_ (check_pred_ty env dflags ctxt) theta }
713 where
714 (_,dups) = removeDups cmpType theta
715
716 -------------------------
717 {- Note [Validity checking for constraints]
718 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
719 We look through constraint synonyms so that we can see the underlying
720 constraint(s). For example
721 type Foo = ?x::Int
722 instance Foo => C T
723 We should reject the instance because it has an implicit parameter in
724 the context.
725
726 But we record, in 'under_syn', whether we have looked under a synonym
727 to avoid requiring language extensions at the use site. Main example
728 (Trac #9838):
729
730 {-# LANGUAGE ConstraintKinds #-}
731 module A where
732 type EqShow a = (Eq a, Show a)
733
734 module B where
735 import A
736 foo :: EqShow a => a -> String
737
738 We don't want to require ConstraintKinds in module B.
739 -}
740
741 check_pred_ty :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
742 -- Check the validity of a predicate in a signature
743 -- See Note [Validity checking for constraints]
744 check_pred_ty env dflags ctxt pred
745 = do { check_type env SigmaCtxt constraintMonoType pred
746 ; check_pred_help False env dflags ctxt pred }
747
748 check_pred_help :: Bool -- True <=> under a type synonym
749 -> TidyEnv
750 -> DynFlags -> UserTypeCtxt
751 -> PredType -> TcM ()
752 check_pred_help under_syn env dflags ctxt pred
753 | Just pred' <- coreView pred -- Switch on under_syn when going under a
754 -- synonym (Trac #9838, yuk)
755 = check_pred_help True env dflags ctxt pred'
756 | otherwise
757 = case splitTyConApp_maybe pred of
758 Just (tc, tys)
759 | isTupleTyCon tc
760 -> check_tuple_pred under_syn env dflags ctxt pred tys
761 -- NB: this equality check must come first, because (~) is a class,
762 -- too.
763 | tc `hasKey` heqTyConKey ||
764 tc `hasKey` eqTyConKey ||
765 tc `hasKey` eqPrimTyConKey
766 -> check_eq_pred env dflags pred tc tys
767 | Just cls <- tyConClass_maybe tc
768 -> check_class_pred env dflags ctxt pred cls tys -- Includes Coercible
769 _ -> check_irred_pred under_syn env dflags ctxt pred
770
771 check_eq_pred :: TidyEnv -> DynFlags -> PredType -> TyCon -> [TcType] -> TcM ()
772 check_eq_pred env dflags pred tc tys
773 = -- Equational constraints are valid in all contexts if type
774 -- families are permitted
775 do { checkTc (length tys == tyConArity tc) (tyConArityErr tc tys)
776 ; checkTcM (xopt LangExt.TypeFamilies dflags
777 || xopt LangExt.GADTs dflags)
778 (eqPredTyErr env pred) }
779
780 check_tuple_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> [PredType] -> TcM ()
781 check_tuple_pred under_syn env dflags ctxt pred ts
782 = do { -- See Note [ConstraintKinds in predicates]
783 checkTcM (under_syn || xopt LangExt.ConstraintKinds dflags)
784 (predTupleErr env pred)
785 ; mapM_ (check_pred_help under_syn env dflags ctxt) ts }
786 -- This case will not normally be executed because without
787 -- -XConstraintKinds tuple types are only kind-checked as *
788
789 check_irred_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
790 check_irred_pred under_syn env dflags ctxt pred
791 -- The predicate looks like (X t1 t2) or (x t1 t2) :: Constraint
792 -- where X is a type function
793 = do { -- If it looks like (x t1 t2), require ConstraintKinds
794 -- see Note [ConstraintKinds in predicates]
795 -- But (X t1 t2) is always ok because we just require ConstraintKinds
796 -- at the definition site (Trac #9838)
797 failIfTcM (not under_syn && not (xopt LangExt.ConstraintKinds dflags)
798 && hasTyVarHead pred)
799 (predIrredErr env pred)
800
801 -- Make sure it is OK to have an irred pred in this context
802 -- See Note [Irreducible predicates in superclasses]
803 ; failIfTcM (is_superclass ctxt
804 && not (xopt LangExt.UndecidableInstances dflags)
805 && has_tyfun_head pred)
806 (predSuperClassErr env pred) }
807 where
808 is_superclass ctxt = case ctxt of { ClassSCCtxt _ -> True; _ -> False }
809 has_tyfun_head ty
810 = case tcSplitTyConApp_maybe ty of
811 Just (tc, _) -> isTypeFamilyTyCon tc
812 Nothing -> False
813
814 {- Note [ConstraintKinds in predicates]
815 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
816 Don't check for -XConstraintKinds under a type synonym, because that
817 was done at the type synonym definition site; see Trac #9838
818 e.g. module A where
819 type C a = (Eq a, Ix a) -- Needs -XConstraintKinds
820 module B where
821 import A
822 f :: C a => a -> a -- Does *not* need -XConstraintKinds
823
824 Note [Irreducible predicates in superclasses]
825 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
826 Allowing type-family calls in class superclasses is somewhat dangerous
827 because we can write:
828
829 type family Fooish x :: * -> Constraint
830 type instance Fooish () = Foo
831 class Fooish () a => Foo a where
832
833 This will cause the constraint simplifier to loop because every time we canonicalise a
834 (Foo a) class constraint we add a (Fooish () a) constraint which will be immediately
835 solved to add+canonicalise another (Foo a) constraint. -}
836
837 -------------------------
838 check_class_pred :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> Class -> [TcType] -> TcM ()
839 check_class_pred env dflags ctxt pred cls tys
840 | isIPClass cls
841 = do { check_arity
842 ; checkTcM (okIPCtxt ctxt) (badIPPred env pred) }
843
844 | otherwise
845 = do { check_arity
846 ; check_simplifiable_class_constraint
847 ; checkTcM arg_tys_ok (predTyVarErr env pred) }
848 where
849 check_arity = checkTc (classArity cls == length tys)
850 (tyConArityErr (classTyCon cls) tys)
851
852 -- Check the arguments of a class constraint
853 flexible_contexts = xopt LangExt.FlexibleContexts dflags
854 undecidable_ok = xopt LangExt.UndecidableInstances dflags
855 arg_tys_ok = case ctxt of
856 SpecInstCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
857 InstDeclCtxt -> checkValidClsArgs (flexible_contexts || undecidable_ok) cls tys
858 -- Further checks on head and theta
859 -- in checkInstTermination
860 _ -> checkValidClsArgs flexible_contexts cls tys
861
862 -- See Note [Simplifiable given constraints]
863 check_simplifiable_class_constraint
864 | DataTyCtxt {} <- ctxt -- Don't do this check for the "stupid theta"
865 = return () -- of a data type declaration
866 | otherwise
867 = do { instEnvs <- tcGetInstEnvs
868 ; let (matches, _, _) = lookupInstEnv False instEnvs cls tys
869 bad_matches = [ inst | (inst,_) <- matches
870 , not (isOverlappable inst) ]
871 ; warnIf (Reason Opt_WarnSimplifiableClassConstraints)
872 (not (null bad_matches))
873 (simplifiable_constraint_warn bad_matches) }
874
875 simplifiable_constraint_warn :: [ClsInst] -> SDoc
876 simplifiable_constraint_warn (match : _)
877 = vcat [ hang (text "The constraint" <+> quotes (ppr (tidyType env pred)))
878 2 (text "matches an instance declaration")
879 , ppr match
880 , hang (text "This makes type inference very fragile;")
881 2 (text "try simplifying it using the instance") ]
882 simplifiable_constraint_warn [] = pprPanic "check_class_pred" (ppr pred)
883
884 {- Note [Simplifiable given constraints]
885 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
886 A type signature like
887 f :: Eq [(a,b)] => a -> b
888 is very fragile, for reasons described at length in TcInteract
889
890 Note [Instance and Given overlap]. So this warning discourages uses
891 from writing simplifiable class constraints, at least unless the
892 top-level instance is explicitly declared as OVERLAPPABLE.
893 Trac #11948 provoked me to do this.
894 -}
895
896 -------------------------
897 okIPCtxt :: UserTypeCtxt -> Bool
898 -- See Note [Implicit parameters in instance decls]
899 okIPCtxt (FunSigCtxt {}) = True
900 okIPCtxt (InfSigCtxt {}) = True
901 okIPCtxt ExprSigCtxt = True
902 okIPCtxt TypeAppCtxt = True
903 okIPCtxt PatSigCtxt = True
904 okIPCtxt ResSigCtxt = True
905 okIPCtxt GenSigCtxt = True
906 okIPCtxt (ConArgCtxt {}) = True
907 okIPCtxt (ForSigCtxt {}) = True -- ??
908 okIPCtxt ThBrackCtxt = True
909 okIPCtxt GhciCtxt = True
910 okIPCtxt SigmaCtxt = True
911 okIPCtxt (DataTyCtxt {}) = True
912 okIPCtxt (PatSynCtxt {}) = True
913 okIPCtxt (TySynCtxt {}) = True -- e.g. type Blah = ?x::Int
914 -- Trac #11466
915
916 okIPCtxt (ClassSCCtxt {}) = False
917 okIPCtxt (InstDeclCtxt {}) = False
918 okIPCtxt (SpecInstCtxt {}) = False
919 okIPCtxt (RuleSigCtxt {}) = False
920 okIPCtxt DefaultDeclCtxt = False
921
922 {-
923 Note [Kind polymorphic type classes]
924 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
925 MultiParam check:
926
927 class C f where... -- C :: forall k. k -> Constraint
928 instance C Maybe where...
929
930 The dictionary gets type [C * Maybe] even if it's not a MultiParam
931 type class.
932
933 Flexibility check:
934
935 class C f where... -- C :: forall k. k -> Constraint
936 data D a = D a
937 instance C D where
938
939 The dictionary gets type [C * (D *)]. IA0_TODO it should be
940 generalized actually.
941 -}
942
943 checkThetaCtxt :: UserTypeCtxt -> ThetaType -> TidyEnv -> TcM (TidyEnv, SDoc)
944 checkThetaCtxt ctxt theta env
945 = return ( env
946 , vcat [ text "In the context:" <+> pprTheta (tidyTypes env theta)
947 , text "While checking" <+> pprUserTypeCtxt ctxt ] )
948
949 eqPredTyErr, predTupleErr, predIrredErr, predSuperClassErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
950 eqPredTyErr env pred
951 = ( env
952 , text "Illegal equational constraint" <+> ppr_tidy env pred $$
953 parens (text "Use GADTs or TypeFamilies to permit this") )
954 predTupleErr env pred
955 = ( env
956 , hang (text "Illegal tuple constraint:" <+> ppr_tidy env pred)
957 2 (parens constraintKindsMsg) )
958 predIrredErr env pred
959 = ( env
960 , hang (text "Illegal constraint:" <+> ppr_tidy env pred)
961 2 (parens constraintKindsMsg) )
962 predSuperClassErr env pred
963 = ( env
964 , hang (text "Illegal constraint" <+> quotes (ppr_tidy env pred)
965 <+> text "in a superclass context")
966 2 (parens undecidableMsg) )
967
968 predTyVarErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
969 predTyVarErr env pred
970 = (env
971 , vcat [ hang (text "Non type-variable argument")
972 2 (text "in the constraint:" <+> ppr_tidy env pred)
973 , parens (text "Use FlexibleContexts to permit this") ])
974
975 badIPPred :: TidyEnv -> PredType -> (TidyEnv, SDoc)
976 badIPPred env pred
977 = ( env
978 , text "Illegal implicit parameter" <+> quotes (ppr_tidy env pred) )
979
980 constraintSynErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
981 constraintSynErr env kind
982 = ( env
983 , hang (text "Illegal constraint synonym of kind:" <+> quotes (ppr_tidy env kind))
984 2 (parens constraintKindsMsg) )
985
986 dupPredWarn :: TidyEnv -> [[PredType]] -> (TidyEnv, SDoc)
987 dupPredWarn env dups
988 = ( env
989 , text "Duplicate constraint" <> plural primaryDups <> text ":"
990 <+> pprWithCommas (ppr_tidy env) primaryDups )
991 where
992 primaryDups = map head dups
993
994 tyConArityErr :: TyCon -> [TcType] -> SDoc
995 -- For type-constructor arity errors, be careful to report
996 -- the number of /visible/ arguments required and supplied,
997 -- ignoring the /invisible/ arguments, which the user does not see.
998 -- (e.g. Trac #10516)
999 tyConArityErr tc tks
1000 = arityErr (tyConFlavour tc) (tyConName tc)
1001 tc_type_arity tc_type_args
1002 where
1003 vis_tks = filterOutInvisibleTypes tc tks
1004
1005 -- tc_type_arity = number of *type* args expected
1006 -- tc_type_args = number of *type* args encountered
1007 tc_type_arity = count isVisibleBinder $ tyConBinders tc
1008 tc_type_args = length vis_tks
1009
1010 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1011 arityErr what name n m
1012 = hsep [ text "The" <+> text what, quotes (ppr name), text "should have",
1013 n_arguments <> comma, text "but has been given",
1014 if m==0 then text "none" else int m]
1015 where
1016 n_arguments | n == 0 = text "no arguments"
1017 | n == 1 = text "1 argument"
1018 | True = hsep [int n, text "arguments"]
1019
1020 {-
1021 ************************************************************************
1022 * *
1023 \subsection{Checking for a decent instance head type}
1024 * *
1025 ************************************************************************
1026
1027 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1028 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1029
1030 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1031 flag is on, or (2)~the instance is imported (they must have been
1032 compiled elsewhere). In these cases, we let them go through anyway.
1033
1034 We can also have instances for functions: @instance Foo (a -> b) ...@.
1035 -}
1036
1037 checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
1038 checkValidInstHead ctxt clas cls_args
1039 = do { dflags <- getDynFlags
1040
1041 ; mod <- getModule
1042 ; checkTc (getUnique clas `notElem` abstractClassKeys ||
1043 nameModule (getName clas) == mod)
1044 (instTypeErr clas cls_args abstract_class_msg)
1045
1046 -- Check language restrictions;
1047 -- but not for SPECIALISE instance pragmas
1048 ; let ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
1049 ; unless spec_inst_prag $
1050 do { checkTc (xopt LangExt.TypeSynonymInstances dflags ||
1051 all tcInstHeadTyNotSynonym ty_args)
1052 (instTypeErr clas cls_args head_type_synonym_msg)
1053 ; checkTc (xopt LangExt.FlexibleInstances dflags ||
1054 all tcInstHeadTyAppAllTyVars ty_args)
1055 (instTypeErr clas cls_args head_type_args_tyvars_msg)
1056 ; checkTc (xopt LangExt.MultiParamTypeClasses dflags ||
1057 length ty_args == 1 || -- Only count type arguments
1058 (xopt LangExt.NullaryTypeClasses dflags &&
1059 null ty_args))
1060 (instTypeErr clas cls_args head_one_type_msg) }
1061
1062 ; mapM_ checkValidTypePat ty_args }
1063 where
1064 spec_inst_prag = case ctxt of { SpecInstCtxt -> True; _ -> False }
1065
1066 head_type_synonym_msg = parens (
1067 text "All instance types must be of the form (T t1 ... tn)" $$
1068 text "where T is not a synonym." $$
1069 text "Use TypeSynonymInstances if you want to disable this.")
1070
1071 head_type_args_tyvars_msg = parens (vcat [
1072 text "All instance types must be of the form (T a1 ... an)",
1073 text "where a1 ... an are *distinct type variables*,",
1074 text "and each type variable appears at most once in the instance head.",
1075 text "Use FlexibleInstances if you want to disable this."])
1076
1077 head_one_type_msg = parens (
1078 text "Only one type can be given in an instance head." $$
1079 text "Use MultiParamTypeClasses if you want to allow more, or zero.")
1080
1081 abstract_class_msg =
1082 text "Manual instances of this class are not permitted."
1083
1084 tcInstHeadTyNotSynonym :: Type -> Bool
1085 -- Used in Haskell-98 mode, for the argument types of an instance head
1086 -- These must not be type synonyms, but everywhere else type synonyms
1087 -- are transparent, so we need a special function here
1088 tcInstHeadTyNotSynonym ty
1089 = case ty of -- Do not use splitTyConApp,
1090 -- because that expands synonyms!
1091 TyConApp tc _ -> not (isTypeSynonymTyCon tc)
1092 _ -> True
1093
1094 tcInstHeadTyAppAllTyVars :: Type -> Bool
1095 -- Used in Haskell-98 mode, for the argument types of an instance head
1096 -- These must be a constructor applied to type variable arguments.
1097 -- But we allow kind instantiations.
1098 tcInstHeadTyAppAllTyVars ty
1099 | Just (tc, tys) <- tcSplitTyConApp_maybe (dropCasts ty)
1100 = ok (filterOutInvisibleTypes tc tys) -- avoid kinds
1101
1102 | otherwise
1103 = False
1104 where
1105 -- Check that all the types are type variables,
1106 -- and that each is distinct
1107 ok tys = equalLength tvs tys && hasNoDups tvs
1108 where
1109 tvs = mapMaybe tcGetTyVar_maybe tys
1110
1111 dropCasts :: Type -> Type
1112 -- See Note [Casts during validity checking]
1113 -- This function can turn a well-kinded type into an ill-kinded
1114 -- one, so I've kept it local to this module
1115 -- To consider: drop only UnivCo(HoleProv) casts
1116 dropCasts (CastTy ty _) = dropCasts ty
1117 dropCasts (AppTy t1 t2) = mkAppTy (dropCasts t1) (dropCasts t2)
1118 dropCasts (TyConApp tc tys) = mkTyConApp tc (map dropCasts tys)
1119 dropCasts (ForAllTy b ty) = ForAllTy (dropCastsB b) (dropCasts ty)
1120 dropCasts ty = ty -- LitTy, TyVarTy, CoercionTy
1121
1122 dropCastsB :: TyBinder -> TyBinder
1123 dropCastsB (Anon ty) = Anon (dropCasts ty)
1124 dropCastsB b = b -- Don't bother in the kind of a forall
1125
1126 abstractClassKeys :: [Unique]
1127 abstractClassKeys = [ heqTyConKey
1128 , eqTyConKey
1129 , coercibleTyConKey
1130 ] -- See Note [Equality class instances]
1131
1132 instTypeErr :: Class -> [Type] -> SDoc -> SDoc
1133 instTypeErr cls tys msg
1134 = hang (hang (text "Illegal instance declaration for")
1135 2 (quotes (pprClassPred cls tys)))
1136 2 msg
1137
1138 {- Note [Casts during validity checking]
1139 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1140 Consider the (bogus)
1141 instance Eq Char#
1142 We elaborate to 'Eq (Char# |> UnivCo(hole))' where the hole is an
1143 insoluble equality constraint for * ~ #. We'll report the insoluble
1144 constraint separately, but we don't want to *also* complain that Eq is
1145 not applied to a type constructor. So we look gaily look through
1146 CastTys here.
1147
1148 Another example: Eq (Either a). Then we actually get a cast in
1149 the middle:
1150 Eq ((Either |> g) a)
1151
1152
1153 Note [Valid 'deriving' predicate]
1154 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1155 validDerivPred checks for OK 'deriving' context. See Note [Exotic
1156 derived instance contexts] in TcDeriv. However the predicate is
1157 here because it uses sizeTypes, fvTypes.
1158
1159 It checks for three things
1160
1161 * No repeated variables (hasNoDups fvs)
1162
1163 * No type constructors. This is done by comparing
1164 sizeTypes tys == length (fvTypes tys)
1165 sizeTypes counts variables and constructors; fvTypes returns variables.
1166 So if they are the same, there must be no constructors. But there
1167 might be applications thus (f (g x)).
1168
1169 Note that tys only includes the visible arguments of the class type
1170 constructor. Including the non-vivisble arguments can cause the following,
1171 perfectly valid instance to be rejected:
1172 class Category (cat :: k -> k -> *) where ...
1173 newtype T (c :: * -> * -> *) a b = MkT (c a b)
1174 instance Category c => Category (T c) where ...
1175 since the first argument to Category is a non-visible *, which sizeTypes
1176 would count as a constructor! See Trac #11833.
1177
1178 * Also check for a bizarre corner case, when the derived instance decl
1179 would look like
1180 instance C a b => D (T a) where ...
1181 Note that 'b' isn't a parameter of T. This gives rise to all sorts of
1182 problems; in particular, it's hard to compare solutions for equality
1183 when finding the fixpoint, and that means the inferContext loop does
1184 not converge. See Trac #5287.
1185
1186 Note [Equality class instances]
1187 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1188 We can't have users writing instances for the equality classes. But we
1189 still need to be able to write instances for them ourselves. So we allow
1190 instances only in the defining module.
1191
1192 -}
1193
1194 validDerivPred :: TyVarSet -> PredType -> Bool
1195 -- See Note [Valid 'deriving' predicate]
1196 validDerivPred tv_set pred
1197 = case classifyPredType pred of
1198 ClassPred cls tys -> cls `hasKey` typeableClassKey
1199 -- Typeable constraints are bigger than they appear due
1200 -- to kind polymorphism, but that's OK
1201 || check_tys cls tys
1202 EqPred {} -> False -- reject equality constraints
1203 _ -> True -- Non-class predicates are ok
1204 where
1205 check_tys cls tys
1206 = hasNoDups fvs
1207 -- use sizePred to ignore implicit args
1208 && sizePred pred == fromIntegral (length fvs)
1209 && all (`elemVarSet` tv_set) fvs
1210 where tys' = filterOutInvisibleTypes (classTyCon cls) tys
1211 fvs = fvTypes tys'
1212
1213 {-
1214 ************************************************************************
1215 * *
1216 \subsection{Checking instance for termination}
1217 * *
1218 ************************************************************************
1219 -}
1220
1221 checkValidInstance :: UserTypeCtxt -> LHsSigType Name -> Type
1222 -> TcM ([TyVar], ThetaType, Class, [Type])
1223 checkValidInstance ctxt hs_type ty
1224 | Just (clas,inst_tys) <- getClassPredTys_maybe tau
1225 , inst_tys `lengthIs` classArity clas
1226 = do { setSrcSpan head_loc (checkValidInstHead ctxt clas inst_tys)
1227 ; traceTc "checkValidInstance {" (ppr ty)
1228 ; checkValidTheta ctxt theta
1229
1230 -- The Termination and Coverate Conditions
1231 -- Check that instance inference will terminate (if we care)
1232 -- For Haskell 98 this will already have been done by checkValidTheta,
1233 -- but as we may be using other extensions we need to check.
1234 --
1235 -- Note that the Termination Condition is *more conservative* than
1236 -- the checkAmbiguity test we do on other type signatures
1237 -- e.g. Bar a => Bar Int is ambiguous, but it also fails
1238 -- the termination condition, because 'a' appears more often
1239 -- in the constraint than in the head
1240 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1241 ; if undecidable_ok
1242 then checkAmbiguity ctxt ty
1243 else checkInstTermination inst_tys theta
1244
1245 ; traceTc "cvi 2" (ppr ty)
1246
1247 ; case (checkInstCoverage undecidable_ok clas theta inst_tys) of
1248 IsValid -> return () -- Check succeeded
1249 NotValid msg -> addErrTc (instTypeErr clas inst_tys msg)
1250
1251 ; traceTc "End checkValidInstance }" empty
1252
1253 ; return (tvs, theta, clas, inst_tys) }
1254
1255 | otherwise
1256 = failWithTc (text "Malformed instance head:" <+> ppr tau)
1257 where
1258 (tvs, theta, tau) = tcSplitSigmaTy ty
1259
1260 -- The location of the "head" of the instance
1261 head_loc = getLoc (getLHsInstDeclHead hs_type)
1262
1263 {-
1264 Note [Paterson conditions]
1265 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1266 Termination test: the so-called "Paterson conditions" (see Section 5 of
1267 "Understanding functional dependencies via Constraint Handling Rules,
1268 JFP Jan 2007).
1269
1270 We check that each assertion in the context satisfies:
1271 (1) no variable has more occurrences in the assertion than in the head, and
1272 (2) the assertion has fewer constructors and variables (taken together
1273 and counting repetitions) than the head.
1274 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1275 (which have already been checked) guarantee termination.
1276
1277 The underlying idea is that
1278
1279 for any ground substitution, each assertion in the
1280 context has fewer type constructors than the head.
1281 -}
1282
1283 checkInstTermination :: [TcType] -> ThetaType -> TcM ()
1284 -- See Note [Paterson conditions]
1285 checkInstTermination tys theta
1286 = check_preds theta
1287 where
1288 head_fvs = fvTypes tys
1289 head_size = sizeTypes tys
1290
1291 check_preds :: [PredType] -> TcM ()
1292 check_preds preds = mapM_ check preds
1293
1294 check :: PredType -> TcM ()
1295 check pred
1296 = case classifyPredType pred of
1297 EqPred {} -> return () -- See Trac #4200.
1298 IrredPred {} -> check2 pred (sizeType pred)
1299 ClassPred cls tys
1300 | isTerminatingClass cls
1301 -> return ()
1302
1303 | isCTupleClass cls -- Look inside tuple predicates; Trac #8359
1304 -> check_preds tys
1305
1306 | otherwise
1307 -> check2 pred (sizeTypes $ filterOutInvisibleTypes (classTyCon cls) tys)
1308 -- Other ClassPreds
1309
1310 check2 pred pred_size
1311 | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what)
1312 | pred_size >= head_size = addErrTc (smallerMsg what)
1313 | otherwise = return ()
1314 where
1315 what = text "constraint" <+> quotes (ppr pred)
1316 bad_tvs = fvType pred \\ head_fvs
1317
1318 smallerMsg :: SDoc -> SDoc
1319 smallerMsg what
1320 = vcat [ hang (text "The" <+> what)
1321 2 (text "is no smaller than the instance head")
1322 , parens undecidableMsg ]
1323
1324 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc
1325 noMoreMsg tvs what
1326 = vcat [ hang (text "Variable" <> plural tvs <+> quotes (pprWithCommas ppr tvs)
1327 <+> occurs <+> text "more often")
1328 2 (sep [ text "in the" <+> what
1329 , text "than in the instance head" ])
1330 , parens undecidableMsg ]
1331 where
1332 occurs = if isSingleton tvs then text "occurs"
1333 else text "occur"
1334
1335 undecidableMsg, constraintKindsMsg :: SDoc
1336 undecidableMsg = text "Use UndecidableInstances to permit this"
1337 constraintKindsMsg = text "Use ConstraintKinds to permit this"
1338
1339 {-
1340 Note [Associated type instances]
1341 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1342 We allow this:
1343 class C a where
1344 type T x a
1345 instance C Int where
1346 type T (S y) Int = y
1347 type T Z Int = Char
1348
1349 Note that
1350 a) The variable 'x' is not bound by the class decl
1351 b) 'x' is instantiated to a non-type-variable in the instance
1352 c) There are several type instance decls for T in the instance
1353
1354 All this is fine. Of course, you can't give any *more* instances
1355 for (T ty Int) elsewhere, because it's an *associated* type.
1356
1357 Note [Checking consistent instantiation]
1358 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1359 See Trac #11450 for background discussion on this check.
1360
1361 class C a b where
1362 type T a x b
1363
1364 With this class decl, if we have an instance decl
1365 instance C ty1 ty2 where ...
1366 then the type instance must look like
1367 type T ty1 v ty2 = ...
1368 with exactly 'ty1' for 'a', 'ty2' for 'b', and a variable for 'x'.
1369 For example:
1370
1371 instance C [p] Int
1372 type T [p] y Int = (p,y,y)
1373
1374 Note that
1375
1376 * We used to allow completely different bound variables in the
1377 associated type instance; e.g.
1378 instance C [p] Int
1379 type T [q] y Int = ...
1380 But from GHC 8.2 onwards, we don't. It's much simpler this way.
1381 See Trac #11450.
1382
1383 * When the class variable isn't used on the RHS of the type instance,
1384 it's tempting to allow wildcards, thus
1385 instance C [p] Int
1386 type T [_] y Int = (y,y)
1387 But it's awkward to do the test, and it doesn't work if the
1388 variable is repeated:
1389 instance C (p,p) Int
1390 type T (_,_) y Int = (y,y)
1391 Even though 'p' is not used on the RHS, we still need to use 'p'
1392 on the LHS to establish the repeated pattern. So to keep it simple
1393 we just require equality.
1394
1395 * We also check that any non-class-tyvars are instantiated with
1396 distinct tyvars. That rules out
1397 instance C [p] Int where
1398 type T [p] Bool Int = p -- Note Bool
1399 type T [p] Char Int = p -- Note Char
1400
1401 and
1402 instance C [p] Int where
1403 type T [p] p Int = p -- Note repeated 'p' on LHS
1404 It's consistent to do this because we don't allow this kind of
1405 instantiation for the class-tyvar arguments of the family.
1406
1407 Overall, we can have exactly one type instance for each
1408 associated type. If you wantmore, use an auxiliary family.
1409
1410 Implementation
1411 * Form the mini-envt from the class type variables a,b
1412 to the instance decl types [p],Int: [a->[p], b->Int]
1413
1414 * Look at the tyvars a,x,b of the type family constructor T
1415 (it shares tyvars with the class C)
1416
1417 * Apply the mini-evnt to them, and check that the result is
1418 consistent with the instance types [p] y Int
1419
1420 We make all the instance type variables scope over the
1421 type instances, of course, which picks up non-obvious kinds. Eg
1422 class Foo (a :: k) where
1423 type F a
1424 instance Foo (b :: k -> k) where
1425 type F b = Int
1426 Here the instance is kind-indexed and really looks like
1427 type F (k->k) (b::k->k) = Int
1428 But if the 'b' didn't scope, we would make F's instance too
1429 poly-kinded.
1430 -}
1431
1432 -- | Extra information about the parent instance declaration, needed
1433 -- when type-checking associated types. The 'Class' is the enclosing
1434 -- class, the [TyVar] are the type variable of the instance decl,
1435 -- and and the @VarEnv Type@ maps class variables to their instance
1436 -- types.
1437 type ClsInstInfo = (Class, [TyVar], VarEnv Type)
1438
1439 type AssocInstArgShape = (Maybe Type, Type)
1440 -- AssocInstArgShape is used only for associated family instances
1441 -- (mb_exp, actual)
1442 -- mb_exp = Just ty => this arg corresponds to a class variable
1443 -- = Nothing => it doesn't correspond to a class variable
1444 -- e.g. class C b where
1445 -- type F a b c
1446 -- instance C [x] where
1447 -- type F p [x] q
1448 -- We get [AssocInstArgShape] = [ (Nothing, p)
1449 -- , (Just [x], [x])
1450 -- , (Nothing, q)]
1451
1452 checkConsistentFamInst
1453 :: Maybe ClsInstInfo
1454 -> TyCon -- ^ Family tycon
1455 -> [TyVar] -- ^ Type variables of the family instance
1456 -> [Type] -- ^ Type patterns from instance
1457 -> TcM ()
1458 -- See Note [Checking consistent instantiation]
1459
1460 checkConsistentFamInst Nothing _ _ _ = return ()
1461 checkConsistentFamInst (Just (clas, inst_tvs, mini_env)) fam_tc _at_tvs at_tys
1462 = do { -- Check that the associated type indeed comes from this class
1463 checkTc (Just clas == tyConAssoc_maybe fam_tc)
1464 (badATErr (className clas) (tyConName fam_tc))
1465
1466 -- Check type args first (more comprehensible)
1467 ; checkTc (all check_arg type_shapes) pp_wrong_at_arg
1468 ; checkTc (check_poly_args type_shapes) pp_wrong_at_tyvars
1469
1470 -- And now kind args
1471 ; checkTc (all check_arg kind_shapes)
1472 (pp_wrong_at_arg $$ ppSuggestExplicitKinds)
1473 ; checkTc (check_poly_args kind_shapes)
1474 (pp_wrong_at_tyvars $$ ppSuggestExplicitKinds)
1475
1476 ; traceTc "cfi" (vcat [ ppr inst_tvs
1477 , ppr arg_shapes
1478 , ppr mini_env ]) }
1479 where
1480 arg_shapes :: [AssocInstArgShape]
1481 arg_shapes = [ (lookupVarEnv mini_env fam_tc_tv, at_ty)
1482 | (fam_tc_tv, at_ty) <- tyConTyVars fam_tc `zip` at_tys ]
1483
1484 (kind_shapes, type_shapes) = partitionInvisibles fam_tc snd arg_shapes
1485
1486 check_arg :: AssocInstArgShape -> Bool
1487 check_arg (Just exp_ty, at_ty) = exp_ty `tcEqType` at_ty
1488 check_arg (Nothing, _ ) = True -- Arg position does not correspond
1489 -- to a class variable
1490 check_poly_args :: [(Maybe Type,Type)] -> Bool
1491 check_poly_args arg_shapes
1492 = allDistinctTyVars (mkVarSet inst_tvs)
1493 [ at_ty | (Nothing, at_ty) <- arg_shapes ]
1494
1495 pp_wrong_at_arg
1496 = vcat [ text "Type indexes must match class instance head"
1497 , pp_exp_act ]
1498
1499 pp_wrong_at_tyvars
1500 = vcat [ text "Polymorphic type indexes of associated type" <+> quotes (ppr fam_tc)
1501 , nest 2 $ vcat [ text "(i.e. ones independent of the class type variables)"
1502 , text "must be distinct type variables" ]
1503 , pp_exp_act ]
1504
1505 pp_exp_act
1506 = vcat [ text "Expected:" <+> ppr (mkTyConApp fam_tc expected_args)
1507 , text " Actual:" <+> ppr (mkTyConApp fam_tc at_tys)
1508 , sdocWithDynFlags $ \dflags ->
1509 ppWhen (has_poly_args dflags) $
1510 vcat [ text "where the `<tv>' arguments are type variables,"
1511 , text "distinct from each other and from the instance variables" ] ]
1512
1513 expected_args = [ exp_ty `orElse` mk_tv at_ty | (exp_ty, at_ty) <- arg_shapes ]
1514 mk_tv at_ty = mkTyVarTy (mkTyVar tv_name (typeKind at_ty))
1515 tv_name = mkInternalName (mkAlphaTyVarUnique 1) (mkTyVarOcc "<tv>") noSrcSpan
1516
1517 has_poly_args dflags = any (isNothing . fst) shapes
1518 where
1519 shapes | gopt Opt_PrintExplicitKinds dflags = arg_shapes
1520 | otherwise = type_shapes
1521
1522 badATErr :: Name -> Name -> SDoc
1523 badATErr clas op
1524 = hsep [text "Class", quotes (ppr clas),
1525 text "does not have an associated type", quotes (ppr op)]
1526
1527
1528 {-
1529 ************************************************************************
1530 * *
1531 Checking type instance well-formedness and termination
1532 * *
1533 ************************************************************************
1534 -}
1535
1536 checkValidCoAxiom :: CoAxiom Branched -> TcM ()
1537 checkValidCoAxiom ax@(CoAxiom { co_ax_tc = fam_tc, co_ax_branches = branches })
1538 = do { mapM_ (checkValidCoAxBranch Nothing fam_tc) branch_list
1539 ; foldlM_ check_branch_compat [] branch_list }
1540 where
1541 branch_list = fromBranches branches
1542 injectivity = familyTyConInjectivityInfo fam_tc
1543
1544 check_branch_compat :: [CoAxBranch] -- previous branches in reverse order
1545 -> CoAxBranch -- current branch
1546 -> TcM [CoAxBranch]-- current branch : previous branches
1547 -- Check for
1548 -- (a) this branch is dominated by previous ones
1549 -- (b) failure of injectivity
1550 check_branch_compat prev_branches cur_branch
1551 | cur_branch `isDominatedBy` prev_branches
1552 = do { addWarnAt NoReason (coAxBranchSpan cur_branch) $
1553 inaccessibleCoAxBranch ax cur_branch
1554 ; return prev_branches }
1555 | otherwise
1556 = do { check_injectivity prev_branches cur_branch
1557 ; return (cur_branch : prev_branches) }
1558
1559 -- Injectivity check: check whether a new (CoAxBranch) can extend
1560 -- already checked equations without violating injectivity
1561 -- annotation supplied by the user.
1562 -- See Note [Verifying injectivity annotation] in FamInstEnv
1563 check_injectivity prev_branches cur_branch
1564 | Injective inj <- injectivity
1565 = do { let conflicts =
1566 fst $ foldl (gather_conflicts inj prev_branches cur_branch)
1567 ([], 0) prev_branches
1568 ; mapM_ (\(err, span) -> setSrcSpan span $ addErr err)
1569 (makeInjectivityErrors ax cur_branch inj conflicts) }
1570 | otherwise
1571 = return ()
1572
1573 gather_conflicts inj prev_branches cur_branch (acc, n) branch
1574 -- n is 0-based index of branch in prev_branches
1575 = case injectiveBranches inj cur_branch branch of
1576 InjectivityUnified ax1 ax2
1577 | ax1 `isDominatedBy` (replace_br prev_branches n ax2)
1578 -> (acc, n + 1)
1579 | otherwise
1580 -> (branch : acc, n + 1)
1581 InjectivityAccepted -> (acc, n + 1)
1582
1583 -- Replace n-th element in the list. Assumes 0-based indexing.
1584 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
1585 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
1586
1587
1588 -- Check that a "type instance" is well-formed (which includes decidability
1589 -- unless -XUndecidableInstances is given).
1590 --
1591 checkValidCoAxBranch :: Maybe ClsInstInfo
1592 -> TyCon -> CoAxBranch -> TcM ()
1593 checkValidCoAxBranch mb_clsinfo fam_tc
1594 (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
1595 , cab_lhs = typats
1596 , cab_rhs = rhs, cab_loc = loc })
1597 = checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1598
1599 -- | Do validity checks on a type family equation, including consistency
1600 -- with any enclosing class instance head, termination, and lack of
1601 -- polytypes.
1602 checkValidTyFamEqn :: Maybe ClsInstInfo
1603 -> TyCon -- ^ of the type family
1604 -> [TyVar] -- ^ bound tyvars in the equation
1605 -> [CoVar] -- ^ bound covars in the equation
1606 -> [Type] -- ^ type patterns
1607 -> Type -- ^ rhs
1608 -> SrcSpan
1609 -> TcM ()
1610 checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1611 = setSrcSpan loc $
1612 do { checkValidFamPats mb_clsinfo fam_tc tvs cvs typats
1613
1614 -- The argument patterns, and RHS, are all boxed tau types
1615 -- E.g Reject type family F (a :: k1) :: k2
1616 -- type instance F (forall a. a->a) = ...
1617 -- type instance F Int# = ...
1618 -- type instance F Int = forall a. a->a
1619 -- type instance F Int = Int#
1620 -- See Trac #9357
1621 ; checkValidMonoType rhs
1622 ; check_lifted rhs
1623
1624 -- We have a decidable instance unless otherwise permitted
1625 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1626 ; unless undecidable_ok $
1627 mapM_ addErrTc (checkFamInstRhs typats (tcTyFamInsts rhs)) }
1628
1629 -- Make sure that each type family application is
1630 -- (1) strictly smaller than the lhs,
1631 -- (2) mentions no type variable more often than the lhs, and
1632 -- (3) does not contain any further type family instances.
1633 --
1634 checkFamInstRhs :: [Type] -- lhs
1635 -> [(TyCon, [Type])] -- type family instances
1636 -> [MsgDoc]
1637 checkFamInstRhs lhsTys famInsts
1638 = mapMaybe check famInsts
1639 where
1640 size = sizeTypes lhsTys
1641 fvs = fvTypes lhsTys
1642 check (tc, tys)
1643 | not (all isTyFamFree tys) = Just (nestedMsg what)
1644 | not (null bad_tvs) = Just (noMoreMsg bad_tvs what)
1645 | size <= sizeTypes tys = Just (smallerMsg what)
1646 | otherwise = Nothing
1647 where
1648 what = text "type family application" <+> quotes (pprType (TyConApp tc tys))
1649 bad_tvs = fvTypes tys \\ fvs
1650
1651 checkValidFamPats :: Maybe ClsInstInfo -> TyCon -> [TyVar] -> [CoVar] -> [Type] -> TcM ()
1652 -- Patterns in a 'type instance' or 'data instance' decl should
1653 -- a) contain no type family applications
1654 -- (vanilla synonyms are fine, though)
1655 -- b) properly bind all their free type variables
1656 -- e.g. we disallow (Trac #7536)
1657 -- type T a = Int
1658 -- type instance F (T a) = a
1659 -- c) Have the right number of patterns
1660 -- d) For associated types, are consistently instantiated
1661 checkValidFamPats mb_clsinfo fam_tc tvs cvs ty_pats
1662 = do { -- A family instance must have exactly the same number of type
1663 -- parameters as the family declaration. You can't write
1664 -- type family F a :: * -> *
1665 -- type instance F Int y = y
1666 -- because then the type (F Int) would be like (\y.y)
1667 checkTc (length ty_pats == fam_arity) $
1668 wrongNumberOfParmsErr (fam_arity - count isInvisibleBinder fam_bndrs)
1669 -- report only explicit arguments
1670
1671 ; mapM_ checkValidTypePat ty_pats
1672
1673 ; let unbound_tcvs = filterOut (`elemVarSet` exactTyCoVarsOfTypes ty_pats) (tvs ++ cvs)
1674 ; checkTc (null unbound_tcvs) (famPatErr fam_tc unbound_tcvs ty_pats)
1675
1676 -- Check that type patterns match the class instance head
1677 ; checkConsistentFamInst mb_clsinfo fam_tc tvs ty_pats }
1678 where
1679 fam_arity = tyConArity fam_tc
1680 fam_bndrs = tyConBinders fam_tc
1681
1682
1683 checkValidTypePat :: Type -> TcM ()
1684 -- Used for type patterns in class instances,
1685 -- and in type/data family instances
1686 checkValidTypePat pat_ty
1687 = do { -- Check that pat_ty is a monotype
1688 checkValidMonoType pat_ty
1689 -- One could imagine generalising to allow
1690 -- instance C (forall a. a->a)
1691 -- but we don't know what all the consequences might be
1692
1693 -- Ensure that no type family instances occur a type pattern
1694 ; checkTc (isTyFamFree pat_ty) $
1695 tyFamInstIllegalErr pat_ty
1696
1697 ; check_lifted pat_ty }
1698
1699 isTyFamFree :: Type -> Bool
1700 -- ^ Check that a type does not contain any type family applications.
1701 isTyFamFree = null . tcTyFamInsts
1702
1703 -- Error messages
1704
1705 wrongNumberOfParmsErr :: Arity -> SDoc
1706 wrongNumberOfParmsErr exp_arity
1707 = text "Number of parameters must match family declaration; expected"
1708 <+> ppr exp_arity
1709
1710 inaccessibleCoAxBranch :: CoAxiom br -> CoAxBranch -> SDoc
1711 inaccessibleCoAxBranch fi_ax cur_branch
1712 = text "Type family instance equation is overlapped:" $$
1713 nest 2 (pprCoAxBranch fi_ax cur_branch)
1714
1715 tyFamInstIllegalErr :: Type -> SDoc
1716 tyFamInstIllegalErr ty
1717 = hang (text "Illegal type synonym family application in instance" <>
1718 colon) 2 $
1719 ppr ty
1720
1721 nestedMsg :: SDoc -> SDoc
1722 nestedMsg what
1723 = sep [ text "Illegal nested" <+> what
1724 , parens undecidableMsg ]
1725
1726 famPatErr :: TyCon -> [TyVar] -> [Type] -> SDoc
1727 famPatErr fam_tc tvs pats
1728 = hang (text "Family instance purports to bind type variable" <> plural tvs
1729 <+> pprQuotedList tvs)
1730 2 (hang (text "but the real LHS (expanding synonyms) is:")
1731 2 (pprTypeApp fam_tc (map expandTypeSynonyms pats) <+>
1732 text "= ..."))
1733
1734 {-
1735 ************************************************************************
1736 * *
1737 Telescope checking
1738 * *
1739 ************************************************************************
1740
1741 Note [Bad telescopes]
1742 ~~~~~~~~~~~~~~~~~~~~~
1743 Now that we can mix type and kind variables, there are an awful lot of
1744 ways to shoot yourself in the foot. Here are some.
1745
1746 data SameKind :: k -> k -> * -- just to force unification
1747
1748 1. data T1 a k (b :: k) (x :: SameKind a b)
1749
1750 The problem here is that we discover that a and b should have the same
1751 kind. But this kind mentions k, which is bound *after* a.
1752 (Testcase: dependent/should_fail/BadTelescope)
1753
1754 2. data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)
1755
1756 Note that b is not bound. Yet its kind mentions a. Because we have
1757 a nice rule that all implicitly bound variables come before others,
1758 this is bogus. (We could probably figure out to put b between a and c.
1759 But I think this is doing users a disservice, in the long run.)
1760 (Testcase: dependent/should_fail/BadTelescope4)
1761
1762 3. t3 :: forall a. (forall k (b :: k). SameKind a b) -> ()
1763
1764 This is a straightforward skolem escape. Note that a and b need to have
1765 the same kind.
1766 (Testcase: polykinds/T11142)
1767
1768 How do we deal with all of this? For TyCons, we have checkValidTyConTyVars.
1769 That function looks to see if any of the tyConTyVars are repeated, but
1770 it's really a telescope check. It works because all tycons are kind-generalized.
1771 If there is a bad telescope, the kind-generalization will end up generalizing
1772 over a variable bound later in the telescope.
1773
1774 For non-tycons, we do scope checking when we bring tyvars into scope,
1775 in tcImplicitTKBndrs and tcExplicitTKBndrs. Note that we also have to
1776 sort implicit binders into a well-scoped order whenever we have implicit
1777 binders to worry about. This is done in quantifyTyVars and in
1778 tcImplicitTKBndrs.
1779 -}
1780
1781 -- | Check a list of binders to see if they make a valid telescope.
1782 -- The key property we're checking for is scoping. For example:
1783 -- > data SameKind :: k -> k -> *
1784 -- > data X a k (b :: k) (c :: SameKind a b)
1785 -- Kind inference says that a's kind should be k. But that's impossible,
1786 -- because k isn't in scope when a is bound. This check has to come before
1787 -- general validity checking, because once we kind-generalise, this sort
1788 -- of problem is harder to spot (as we'll generalise over the unbound
1789 -- k in a's type.) See also Note [Bad telescopes].
1790 checkValidTelescope :: SDoc -- the original user-written telescope
1791 -> [TyVar] -- explicit vars (not necessarily zonked)
1792 -> SDoc -- note to put at bottom of message
1793 -> TcM ()
1794 checkValidTelescope hs_tvs orig_tvs extra
1795 = discardResult $ checkZonkValidTelescope hs_tvs orig_tvs extra
1796
1797 -- | Like 'checkZonkValidTelescope', but returns the zonked tyvars
1798 checkZonkValidTelescope :: SDoc
1799 -> [TyVar]
1800 -> SDoc
1801 -> TcM [TyVar]
1802 checkZonkValidTelescope hs_tvs orig_tvs extra
1803 = do { orig_tvs <- mapM zonkTyCoVarKind orig_tvs
1804 ; let (_, sorted_tidied_tvs) = tidyTyCoVarBndrs emptyTidyEnv $
1805 toposortTyVars orig_tvs
1806 ; unless (go [] emptyVarSet orig_tvs) $
1807 addErr $
1808 vcat [ hang (text "These kind and type variables:" <+> hs_tvs $$
1809 text "are out of dependency order. Perhaps try this ordering:")
1810 2 (sep (map pprTvBndr sorted_tidied_tvs))
1811 , extra ]
1812 ; return orig_tvs }
1813
1814 where
1815 go :: [TyVar] -- misplaced variables
1816 -> TyVarSet -> [TyVar] -> Bool
1817 go errs in_scope [] = null (filter (`elemVarSet` in_scope) errs)
1818 -- report an error only when the variable in the kind is brought
1819 -- into scope later in the telescope. Otherwise, we'll just quantify
1820 -- over it in kindGeneralize, as we should.
1821
1822 go errs in_scope (tv:tvs)
1823 = let bad_tvs = filterOut (`elemVarSet` in_scope) $
1824 tyCoVarsOfTypeList (tyVarKind tv)
1825 in go (bad_tvs ++ errs) (in_scope `extendVarSet` tv) tvs
1826
1827 -- | After inferring kinds of type variables, check to make sure that the
1828 -- inferred kinds any of the type variables bound in a smaller scope.
1829 -- This is a skolem escape check. See also Note [Bad telescopes].
1830 checkValidInferredKinds :: [TyVar] -- ^ vars to check (zonked)
1831 -> TyVarSet -- ^ vars out of scope
1832 -> SDoc -- ^ suffix to error message
1833 -> TcM ()
1834 checkValidInferredKinds orig_kvs out_of_scope extra
1835 = do { let bad_pairs = [ (tv, kv)
1836 | kv <- orig_kvs
1837 , Just tv <- map (lookupVarSet out_of_scope)
1838 (tyCoVarsOfTypeList (tyVarKind kv)) ]
1839 report (tidyTyVarOcc env -> tv, tidyTyVarOcc env -> kv)
1840 = addErr $
1841 text "The kind of variable" <+>
1842 quotes (ppr kv) <> text ", namely" <+>
1843 quotes (ppr (tyVarKind kv)) <> comma $$
1844 text "depends on variable" <+>
1845 quotes (ppr tv) <+> text "from an inner scope" $$
1846 text "Perhaps bind" <+> quotes (ppr kv) <+>
1847 text "sometime after binding" <+>
1848 quotes (ppr tv) $$
1849 extra
1850 ; mapM_ report bad_pairs }
1851
1852 where
1853 (env1, _) = tidyTyCoVarBndrs emptyTidyEnv orig_kvs
1854 (env, _) = tidyTyCoVarBndrs env1 (nonDetEltsUFM out_of_scope)
1855 -- It's OK to use nonDetEltsUFM here because it's only used for
1856 -- generating the error message
1857
1858 {-
1859 ************************************************************************
1860 * *
1861 \subsection{Auxiliary functions}
1862 * *
1863 ************************************************************************
1864 -}
1865
1866 -- Free variables of a type, retaining repetitions, and expanding synonyms
1867 fvType :: Type -> [TyCoVar]
1868 fvType ty | Just exp_ty <- coreView ty = fvType exp_ty
1869 fvType (TyVarTy tv) = [tv]
1870 fvType (TyConApp _ tys) = fvTypes tys
1871 fvType (LitTy {}) = []
1872 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1873 fvType (ForAllTy bndr ty)
1874 = fvType (binderType bndr) ++
1875 caseBinder bndr (\tv -> filter (/= tv)) (const id) (fvType ty)
1876 fvType (CastTy ty co) = fvType ty ++ fvCo co
1877 fvType (CoercionTy co) = fvCo co
1878
1879 fvTypes :: [Type] -> [TyVar]
1880 fvTypes tys = concat (map fvType tys)
1881
1882 fvCo :: Coercion -> [TyCoVar]
1883 fvCo (Refl _ ty) = fvType ty
1884 fvCo (TyConAppCo _ _ args) = concatMap fvCo args
1885 fvCo (AppCo co arg) = fvCo co ++ fvCo arg
1886 fvCo (ForAllCo tv h co) = filter (/= tv) (fvCo co) ++ fvCo h
1887 fvCo (CoVarCo v) = [v]
1888 fvCo (AxiomInstCo _ _ args) = concatMap fvCo args
1889 fvCo (UnivCo p _ t1 t2) = fvProv p ++ fvType t1 ++ fvType t2
1890 fvCo (SymCo co) = fvCo co
1891 fvCo (TransCo co1 co2) = fvCo co1 ++ fvCo co2
1892 fvCo (NthCo _ co) = fvCo co
1893 fvCo (LRCo _ co) = fvCo co
1894 fvCo (InstCo co arg) = fvCo co ++ fvCo arg
1895 fvCo (CoherenceCo co1 co2) = fvCo co1 ++ fvCo co2
1896 fvCo (KindCo co) = fvCo co
1897 fvCo (SubCo co) = fvCo co
1898 fvCo (AxiomRuleCo _ cs) = concatMap fvCo cs
1899
1900 fvProv :: UnivCoProvenance -> [TyCoVar]
1901 fvProv UnsafeCoerceProv = []
1902 fvProv (PhantomProv co) = fvCo co
1903 fvProv (ProofIrrelProv co) = fvCo co
1904 fvProv (PluginProv _) = []
1905 fvProv (HoleProv h) = pprPanic "fvProv falls into a hole" (ppr h)
1906
1907 sizeType :: Type -> Int
1908 -- Size of a type: the number of variables and constructors
1909 sizeType ty | Just exp_ty <- coreView ty = sizeType exp_ty
1910 sizeType (TyVarTy {}) = 1
1911 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1912 sizeType (LitTy {}) = 1
1913 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1914 sizeType (ForAllTy (Anon arg) res)
1915 = sizeType arg + sizeType res + 1
1916 sizeType (ForAllTy (Named {}) ty)
1917 = sizeType ty
1918 sizeType (CastTy ty _) = sizeType ty
1919 sizeType (CoercionTy _) = 1
1920
1921 sizeTypes :: [Type] -> Int
1922 sizeTypes = sum . map sizeType
1923
1924 -- Size of a predicate
1925 --
1926 -- We are considering whether class constraints terminate.
1927 -- Equality constraints and constraints for the implicit
1928 -- parameter class always termiante so it is safe to say "size 0".
1929 -- (Implicit parameter constraints always terminate because
1930 -- there are no instances for them---they are only solved by
1931 -- "local instances" in expressions).
1932 -- See Trac #4200.
1933 sizePred :: PredType -> Int
1934 sizePred ty = goClass ty
1935 where
1936 goClass p = go (classifyPredType p)
1937
1938 go (ClassPred cls tys')
1939 | isTerminatingClass cls = 0
1940 | otherwise = sizeTypes (filterOutInvisibleTypes (classTyCon cls) tys')
1941 go (EqPred {}) = 0
1942 go (IrredPred ty) = sizeType ty
1943
1944 -- | When this says "True", ignore this class constraint during
1945 -- a termination check
1946 isTerminatingClass :: Class -> Bool
1947 isTerminatingClass cls
1948 = isIPClass cls
1949 || cls `hasKey` typeableClassKey
1950 || cls `hasKey` coercibleTyConKey
1951 || cls `hasKey` eqTyConKey
1952 || cls `hasKey` heqTyConKey
1953
1954 -- | Tidy before printing a type
1955 ppr_tidy :: TidyEnv -> Type -> SDoc
1956 ppr_tidy env ty = pprType (tidyType env ty)
1957
1958 allDistinctTyVars :: TyVarSet -> [KindOrType] -> Bool
1959 -- (allDistinctTyVars tvs tys) returns True if tys are
1960 -- a) all tyvars
1961 -- b) all distinct
1962 -- c) disjoint from tvs
1963 allDistinctTyVars _ [] = True
1964 allDistinctTyVars tkvs (ty : tys)
1965 = case getTyVar_maybe ty of
1966 Nothing -> False
1967 Just tv | tv `elemVarSet` tkvs -> False
1968 | otherwise -> allDistinctTyVars (tkvs `extendVarSet` tv) tys