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[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 ( TyVarBndr(..), 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 (FunTy 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 nonDetCmpType theta
715 -- It's OK to use nonDetCmpType because dups only appears in the
716 -- warning
717
718 -------------------------
719 {- Note [Validity checking for constraints]
720 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
721 We look through constraint synonyms so that we can see the underlying
722 constraint(s). For example
723 type Foo = ?x::Int
724 instance Foo => C T
725 We should reject the instance because it has an implicit parameter in
726 the context.
727
728 But we record, in 'under_syn', whether we have looked under a synonym
729 to avoid requiring language extensions at the use site. Main example
730 (Trac #9838):
731
732 {-# LANGUAGE ConstraintKinds #-}
733 module A where
734 type EqShow a = (Eq a, Show a)
735
736 module B where
737 import A
738 foo :: EqShow a => a -> String
739
740 We don't want to require ConstraintKinds in module B.
741 -}
742
743 check_pred_ty :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
744 -- Check the validity of a predicate in a signature
745 -- See Note [Validity checking for constraints]
746 check_pred_ty env dflags ctxt pred
747 = do { check_type env SigmaCtxt constraintMonoType pred
748 ; check_pred_help False env dflags ctxt pred }
749
750 check_pred_help :: Bool -- True <=> under a type synonym
751 -> TidyEnv
752 -> DynFlags -> UserTypeCtxt
753 -> PredType -> TcM ()
754 check_pred_help under_syn env dflags ctxt pred
755 | Just pred' <- coreView pred -- Switch on under_syn when going under a
756 -- synonym (Trac #9838, yuk)
757 = check_pred_help True env dflags ctxt pred'
758 | otherwise
759 = case splitTyConApp_maybe pred of
760 Just (tc, tys)
761 | isTupleTyCon tc
762 -> check_tuple_pred under_syn env dflags ctxt pred tys
763 -- NB: this equality check must come first, because (~) is a class,
764 -- too.
765 | tc `hasKey` heqTyConKey ||
766 tc `hasKey` eqTyConKey ||
767 tc `hasKey` eqPrimTyConKey
768 -> check_eq_pred env dflags pred tc tys
769 | Just cls <- tyConClass_maybe tc
770 -> check_class_pred env dflags ctxt pred cls tys -- Includes Coercible
771 _ -> check_irred_pred under_syn env dflags ctxt pred
772
773 check_eq_pred :: TidyEnv -> DynFlags -> PredType -> TyCon -> [TcType] -> TcM ()
774 check_eq_pred env dflags pred tc tys
775 = -- Equational constraints are valid in all contexts if type
776 -- families are permitted
777 do { checkTc (length tys == tyConArity tc) (tyConArityErr tc tys)
778 ; checkTcM (xopt LangExt.TypeFamilies dflags
779 || xopt LangExt.GADTs dflags)
780 (eqPredTyErr env pred) }
781
782 check_tuple_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> [PredType] -> TcM ()
783 check_tuple_pred under_syn env dflags ctxt pred ts
784 = do { -- See Note [ConstraintKinds in predicates]
785 checkTcM (under_syn || xopt LangExt.ConstraintKinds dflags)
786 (predTupleErr env pred)
787 ; mapM_ (check_pred_help under_syn env dflags ctxt) ts }
788 -- This case will not normally be executed because without
789 -- -XConstraintKinds tuple types are only kind-checked as *
790
791 check_irred_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
792 check_irred_pred under_syn env dflags ctxt pred
793 -- The predicate looks like (X t1 t2) or (x t1 t2) :: Constraint
794 -- where X is a type function
795 = do { -- If it looks like (x t1 t2), require ConstraintKinds
796 -- see Note [ConstraintKinds in predicates]
797 -- But (X t1 t2) is always ok because we just require ConstraintKinds
798 -- at the definition site (Trac #9838)
799 failIfTcM (not under_syn && not (xopt LangExt.ConstraintKinds dflags)
800 && hasTyVarHead pred)
801 (predIrredErr env pred)
802
803 -- Make sure it is OK to have an irred pred in this context
804 -- See Note [Irreducible predicates in superclasses]
805 ; failIfTcM (is_superclass ctxt
806 && not (xopt LangExt.UndecidableInstances dflags)
807 && has_tyfun_head pred)
808 (predSuperClassErr env pred) }
809 where
810 is_superclass ctxt = case ctxt of { ClassSCCtxt _ -> True; _ -> False }
811 has_tyfun_head ty
812 = case tcSplitTyConApp_maybe ty of
813 Just (tc, _) -> isTypeFamilyTyCon tc
814 Nothing -> False
815
816 {- Note [ConstraintKinds in predicates]
817 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
818 Don't check for -XConstraintKinds under a type synonym, because that
819 was done at the type synonym definition site; see Trac #9838
820 e.g. module A where
821 type C a = (Eq a, Ix a) -- Needs -XConstraintKinds
822 module B where
823 import A
824 f :: C a => a -> a -- Does *not* need -XConstraintKinds
825
826 Note [Irreducible predicates in superclasses]
827 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
828 Allowing type-family calls in class superclasses is somewhat dangerous
829 because we can write:
830
831 type family Fooish x :: * -> Constraint
832 type instance Fooish () = Foo
833 class Fooish () a => Foo a where
834
835 This will cause the constraint simplifier to loop because every time we canonicalise a
836 (Foo a) class constraint we add a (Fooish () a) constraint which will be immediately
837 solved to add+canonicalise another (Foo a) constraint. -}
838
839 -------------------------
840 check_class_pred :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> Class -> [TcType] -> TcM ()
841 check_class_pred env dflags ctxt pred cls tys
842 | isIPClass cls
843 = do { check_arity
844 ; checkTcM (okIPCtxt ctxt) (badIPPred env pred) }
845
846 | otherwise
847 = do { check_arity
848 ; check_simplifiable_class_constraint
849 ; checkTcM arg_tys_ok (predTyVarErr env pred) }
850 where
851 check_arity = checkTc (classArity cls == length tys)
852 (tyConArityErr (classTyCon cls) tys)
853
854 -- Check the arguments of a class constraint
855 flexible_contexts = xopt LangExt.FlexibleContexts dflags
856 undecidable_ok = xopt LangExt.UndecidableInstances dflags
857 arg_tys_ok = case ctxt of
858 SpecInstCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
859 InstDeclCtxt -> checkValidClsArgs (flexible_contexts || undecidable_ok) cls tys
860 -- Further checks on head and theta
861 -- in checkInstTermination
862 _ -> checkValidClsArgs flexible_contexts cls tys
863
864 -- See Note [Simplifiable given constraints]
865 check_simplifiable_class_constraint
866 | DataTyCtxt {} <- ctxt -- Don't do this check for the "stupid theta"
867 = return () -- of a data type declaration
868 | otherwise
869 = do { instEnvs <- tcGetInstEnvs
870 ; let (matches, _, _) = lookupInstEnv False instEnvs cls tys
871 bad_matches = [ inst | (inst,_) <- matches
872 , not (isOverlappable inst) ]
873 ; warnIf (Reason Opt_WarnSimplifiableClassConstraints)
874 (not (null bad_matches))
875 (simplifiable_constraint_warn bad_matches) }
876
877 simplifiable_constraint_warn :: [ClsInst] -> SDoc
878 simplifiable_constraint_warn (match : _)
879 = vcat [ hang (text "The constraint" <+> quotes (ppr (tidyType env pred)))
880 2 (text "matches an instance declaration")
881 , ppr match
882 , hang (text "This makes type inference very fragile;")
883 2 (text "try simplifying it using the instance") ]
884 simplifiable_constraint_warn [] = pprPanic "check_class_pred" (ppr pred)
885
886 {- Note [Simplifiable given constraints]
887 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
888 A type signature like
889 f :: Eq [(a,b)] => a -> b
890 is very fragile, for reasons described at length in TcInteract
891 Note [Instance and Given overlap]. So this warning discourages uses
892 from writing simplifiable class constraints, at least unless the
893 top-level instance is explicitly declared as OVERLAPPABLE.
894 Trac #11948 provoked me to do this.
895 -}
896
897 -------------------------
898 okIPCtxt :: UserTypeCtxt -> Bool
899 -- See Note [Implicit parameters in instance decls]
900 okIPCtxt (FunSigCtxt {}) = True
901 okIPCtxt (InfSigCtxt {}) = True
902 okIPCtxt ExprSigCtxt = True
903 okIPCtxt TypeAppCtxt = True
904 okIPCtxt PatSigCtxt = True
905 okIPCtxt ResSigCtxt = True
906 okIPCtxt GenSigCtxt = True
907 okIPCtxt (ConArgCtxt {}) = True
908 okIPCtxt (ForSigCtxt {}) = True -- ??
909 okIPCtxt ThBrackCtxt = True
910 okIPCtxt GhciCtxt = True
911 okIPCtxt SigmaCtxt = True
912 okIPCtxt (DataTyCtxt {}) = True
913 okIPCtxt (PatSynCtxt {}) = True
914 okIPCtxt (TySynCtxt {}) = True -- e.g. type Blah = ?x::Int
915 -- Trac #11466
916
917 okIPCtxt (ClassSCCtxt {}) = False
918 okIPCtxt (InstDeclCtxt {}) = False
919 okIPCtxt (SpecInstCtxt {}) = False
920 okIPCtxt (RuleSigCtxt {}) = False
921 okIPCtxt DefaultDeclCtxt = False
922
923 {-
924 Note [Kind polymorphic type classes]
925 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
926 MultiParam check:
927
928 class C f where... -- C :: forall k. k -> Constraint
929 instance C Maybe where...
930
931 The dictionary gets type [C * Maybe] even if it's not a MultiParam
932 type class.
933
934 Flexibility check:
935
936 class C f where... -- C :: forall k. k -> Constraint
937 data D a = D a
938 instance C D where
939
940 The dictionary gets type [C * (D *)]. IA0_TODO it should be
941 generalized actually.
942 -}
943
944 checkThetaCtxt :: UserTypeCtxt -> ThetaType -> TidyEnv -> TcM (TidyEnv, SDoc)
945 checkThetaCtxt ctxt theta env
946 = return ( env
947 , vcat [ text "In the context:" <+> pprTheta (tidyTypes env theta)
948 , text "While checking" <+> pprUserTypeCtxt ctxt ] )
949
950 eqPredTyErr, predTupleErr, predIrredErr, predSuperClassErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
951 eqPredTyErr env pred
952 = ( env
953 , text "Illegal equational constraint" <+> ppr_tidy env pred $$
954 parens (text "Use GADTs or TypeFamilies to permit this") )
955 predTupleErr env pred
956 = ( env
957 , hang (text "Illegal tuple constraint:" <+> ppr_tidy env pred)
958 2 (parens constraintKindsMsg) )
959 predIrredErr env pred
960 = ( env
961 , hang (text "Illegal constraint:" <+> ppr_tidy env pred)
962 2 (parens constraintKindsMsg) )
963 predSuperClassErr env pred
964 = ( env
965 , hang (text "Illegal constraint" <+> quotes (ppr_tidy env pred)
966 <+> text "in a superclass context")
967 2 (parens undecidableMsg) )
968
969 predTyVarErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
970 predTyVarErr env pred
971 = (env
972 , vcat [ hang (text "Non type-variable argument")
973 2 (text "in the constraint:" <+> ppr_tidy env pred)
974 , parens (text "Use FlexibleContexts to permit this") ])
975
976 badIPPred :: TidyEnv -> PredType -> (TidyEnv, SDoc)
977 badIPPred env pred
978 = ( env
979 , text "Illegal implicit parameter" <+> quotes (ppr_tidy env pred) )
980
981 constraintSynErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
982 constraintSynErr env kind
983 = ( env
984 , hang (text "Illegal constraint synonym of kind:" <+> quotes (ppr_tidy env kind))
985 2 (parens constraintKindsMsg) )
986
987 dupPredWarn :: TidyEnv -> [[PredType]] -> (TidyEnv, SDoc)
988 dupPredWarn env dups
989 = ( env
990 , text "Duplicate constraint" <> plural primaryDups <> text ":"
991 <+> pprWithCommas (ppr_tidy env) primaryDups )
992 where
993 primaryDups = map head dups
994
995 tyConArityErr :: TyCon -> [TcType] -> SDoc
996 -- For type-constructor arity errors, be careful to report
997 -- the number of /visible/ arguments required and supplied,
998 -- ignoring the /invisible/ arguments, which the user does not see.
999 -- (e.g. Trac #10516)
1000 tyConArityErr tc tks
1001 = arityErr (tyConFlavour tc) (tyConName tc)
1002 tc_type_arity tc_type_args
1003 where
1004 vis_tks = filterOutInvisibleTypes tc tks
1005
1006 -- tc_type_arity = number of *type* args expected
1007 -- tc_type_args = number of *type* args encountered
1008 tc_type_arity = count isVisibleTyConBinder (tyConBinders tc)
1009 tc_type_args = length vis_tks
1010
1011 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1012 arityErr what name n m
1013 = hsep [ text "The" <+> text what, quotes (ppr name), text "should have",
1014 n_arguments <> comma, text "but has been given",
1015 if m==0 then text "none" else int m]
1016 where
1017 n_arguments | n == 0 = text "no arguments"
1018 | n == 1 = text "1 argument"
1019 | True = hsep [int n, text "arguments"]
1020
1021 {-
1022 ************************************************************************
1023 * *
1024 \subsection{Checking for a decent instance head type}
1025 * *
1026 ************************************************************************
1027
1028 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1029 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1030
1031 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1032 flag is on, or (2)~the instance is imported (they must have been
1033 compiled elsewhere). In these cases, we let them go through anyway.
1034
1035 We can also have instances for functions: @instance Foo (a -> b) ...@.
1036 -}
1037
1038 checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
1039 checkValidInstHead ctxt clas cls_args
1040 = do { dflags <- getDynFlags
1041
1042 ; mod <- getModule
1043 ; checkTc (getUnique clas `notElem` abstractClassKeys ||
1044 nameModule (getName clas) == mod)
1045 (instTypeErr clas cls_args abstract_class_msg)
1046
1047 -- Check language restrictions;
1048 -- but not for SPECIALISE instance pragmas
1049 ; let ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
1050 ; unless spec_inst_prag $
1051 do { checkTc (xopt LangExt.TypeSynonymInstances dflags ||
1052 all tcInstHeadTyNotSynonym ty_args)
1053 (instTypeErr clas cls_args head_type_synonym_msg)
1054 ; checkTc (xopt LangExt.FlexibleInstances dflags ||
1055 all tcInstHeadTyAppAllTyVars ty_args)
1056 (instTypeErr clas cls_args head_type_args_tyvars_msg)
1057 ; checkTc (xopt LangExt.MultiParamTypeClasses dflags ||
1058 length ty_args == 1 || -- Only count type arguments
1059 (xopt LangExt.NullaryTypeClasses dflags &&
1060 null ty_args))
1061 (instTypeErr clas cls_args head_one_type_msg) }
1062
1063 ; mapM_ checkValidTypePat ty_args }
1064 where
1065 spec_inst_prag = case ctxt of { SpecInstCtxt -> True; _ -> False }
1066
1067 head_type_synonym_msg = parens (
1068 text "All instance types must be of the form (T t1 ... tn)" $$
1069 text "where T is not a synonym." $$
1070 text "Use TypeSynonymInstances if you want to disable this.")
1071
1072 head_type_args_tyvars_msg = parens (vcat [
1073 text "All instance types must be of the form (T a1 ... an)",
1074 text "where a1 ... an are *distinct type variables*,",
1075 text "and each type variable appears at most once in the instance head.",
1076 text "Use FlexibleInstances if you want to disable this."])
1077
1078 head_one_type_msg = parens (
1079 text "Only one type can be given in an instance head." $$
1080 text "Use MultiParamTypeClasses if you want to allow more, or zero.")
1081
1082 abstract_class_msg =
1083 text "Manual instances of this class are not permitted."
1084
1085 tcInstHeadTyNotSynonym :: Type -> Bool
1086 -- Used in Haskell-98 mode, for the argument types of an instance head
1087 -- These must not be type synonyms, but everywhere else type synonyms
1088 -- are transparent, so we need a special function here
1089 tcInstHeadTyNotSynonym ty
1090 = case ty of -- Do not use splitTyConApp,
1091 -- because that expands synonyms!
1092 TyConApp tc _ -> not (isTypeSynonymTyCon tc)
1093 _ -> True
1094
1095 tcInstHeadTyAppAllTyVars :: Type -> Bool
1096 -- Used in Haskell-98 mode, for the argument types of an instance head
1097 -- These must be a constructor applied to type variable arguments.
1098 -- But we allow kind instantiations.
1099 tcInstHeadTyAppAllTyVars ty
1100 | Just (tc, tys) <- tcSplitTyConApp_maybe (dropCasts ty)
1101 = ok (filterOutInvisibleTypes tc tys) -- avoid kinds
1102
1103 | otherwise
1104 = False
1105 where
1106 -- Check that all the types are type variables,
1107 -- and that each is distinct
1108 ok tys = equalLength tvs tys && hasNoDups tvs
1109 where
1110 tvs = mapMaybe tcGetTyVar_maybe tys
1111
1112 dropCasts :: Type -> Type
1113 -- See Note [Casts during validity checking]
1114 -- This function can turn a well-kinded type into an ill-kinded
1115 -- one, so I've kept it local to this module
1116 -- To consider: drop only UnivCo(HoleProv) casts
1117 dropCasts (CastTy ty _) = dropCasts ty
1118 dropCasts (AppTy t1 t2) = mkAppTy (dropCasts t1) (dropCasts t2)
1119 dropCasts (FunTy t1 t2) = mkFunTy (dropCasts t1) (dropCasts t2)
1120 dropCasts (TyConApp tc tys) = mkTyConApp tc (map dropCasts tys)
1121 dropCasts (ForAllTy b ty) = ForAllTy (dropCastsB b) (dropCasts ty)
1122 dropCasts ty = ty -- LitTy, TyVarTy, CoercionTy
1123
1124 dropCastsB :: TyVarBinder -> TyVarBinder
1125 dropCastsB b = b -- Don't bother in the kind of a forall
1126
1127 abstractClassKeys :: [Unique]
1128 abstractClassKeys = [ heqTyConKey
1129 , eqTyConKey
1130 , coercibleTyConKey
1131 ] -- See Note [Equality class instances]
1132
1133 instTypeErr :: Class -> [Type] -> SDoc -> SDoc
1134 instTypeErr cls tys msg
1135 = hang (hang (text "Illegal instance declaration for")
1136 2 (quotes (pprClassPred cls tys)))
1137 2 msg
1138
1139 {- Note [Casts during validity checking]
1140 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1141 Consider the (bogus)
1142 instance Eq Char#
1143 We elaborate to 'Eq (Char# |> UnivCo(hole))' where the hole is an
1144 insoluble equality constraint for * ~ #. We'll report the insoluble
1145 constraint separately, but we don't want to *also* complain that Eq is
1146 not applied to a type constructor. So we look gaily look through
1147 CastTys here.
1148
1149 Another example: Eq (Either a). Then we actually get a cast in
1150 the middle:
1151 Eq ((Either |> g) a)
1152
1153
1154 Note [Valid 'deriving' predicate]
1155 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1156 validDerivPred checks for OK 'deriving' context. See Note [Exotic
1157 derived instance contexts] in TcDeriv. However the predicate is
1158 here because it uses sizeTypes, fvTypes.
1159
1160 It checks for three things
1161
1162 * No repeated variables (hasNoDups fvs)
1163
1164 * No type constructors. This is done by comparing
1165 sizeTypes tys == length (fvTypes tys)
1166 sizeTypes counts variables and constructors; fvTypes returns variables.
1167 So if they are the same, there must be no constructors. But there
1168 might be applications thus (f (g x)).
1169
1170 Note that tys only includes the visible arguments of the class type
1171 constructor. Including the non-vivisble arguments can cause the following,
1172 perfectly valid instance to be rejected:
1173 class Category (cat :: k -> k -> *) where ...
1174 newtype T (c :: * -> * -> *) a b = MkT (c a b)
1175 instance Category c => Category (T c) where ...
1176 since the first argument to Category is a non-visible *, which sizeTypes
1177 would count as a constructor! See Trac #11833.
1178
1179 * Also check for a bizarre corner case, when the derived instance decl
1180 would look like
1181 instance C a b => D (T a) where ...
1182 Note that 'b' isn't a parameter of T. This gives rise to all sorts of
1183 problems; in particular, it's hard to compare solutions for equality
1184 when finding the fixpoint, and that means the inferContext loop does
1185 not converge. See Trac #5287.
1186
1187 Note [Equality class instances]
1188 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1189 We can't have users writing instances for the equality classes. But we
1190 still need to be able to write instances for them ourselves. So we allow
1191 instances only in the defining module.
1192
1193 -}
1194
1195 validDerivPred :: TyVarSet -> PredType -> Bool
1196 -- See Note [Valid 'deriving' predicate]
1197 validDerivPred tv_set pred
1198 = case classifyPredType pred of
1199 ClassPred cls tys -> cls `hasKey` typeableClassKey
1200 -- Typeable constraints are bigger than they appear due
1201 -- to kind polymorphism, but that's OK
1202 || check_tys cls tys
1203 EqPred {} -> False -- reject equality constraints
1204 _ -> True -- Non-class predicates are ok
1205 where
1206 check_tys cls tys
1207 = hasNoDups fvs
1208 -- use sizePred to ignore implicit args
1209 && sizePred pred == fromIntegral (length fvs)
1210 && all (`elemVarSet` tv_set) fvs
1211 where tys' = filterOutInvisibleTypes (classTyCon cls) tys
1212 fvs = fvTypes tys'
1213
1214 {-
1215 ************************************************************************
1216 * *
1217 \subsection{Checking instance for termination}
1218 * *
1219 ************************************************************************
1220 -}
1221
1222 checkValidInstance :: UserTypeCtxt -> LHsSigType Name -> Type
1223 -> TcM ([TyVar], ThetaType, Class, [Type])
1224 checkValidInstance ctxt hs_type ty
1225 | Just (clas,inst_tys) <- getClassPredTys_maybe tau
1226 , inst_tys `lengthIs` classArity clas
1227 = do { setSrcSpan head_loc (checkValidInstHead ctxt clas inst_tys)
1228 ; traceTc "checkValidInstance {" (ppr ty)
1229 ; checkValidTheta ctxt theta
1230
1231 -- The Termination and Coverate Conditions
1232 -- Check that instance inference will terminate (if we care)
1233 -- For Haskell 98 this will already have been done by checkValidTheta,
1234 -- but as we may be using other extensions we need to check.
1235 --
1236 -- Note that the Termination Condition is *more conservative* than
1237 -- the checkAmbiguity test we do on other type signatures
1238 -- e.g. Bar a => Bar Int is ambiguous, but it also fails
1239 -- the termination condition, because 'a' appears more often
1240 -- in the constraint than in the head
1241 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1242 ; if undecidable_ok
1243 then checkAmbiguity ctxt ty
1244 else checkInstTermination inst_tys theta
1245
1246 ; traceTc "cvi 2" (ppr ty)
1247
1248 ; case (checkInstCoverage undecidable_ok clas theta inst_tys) of
1249 IsValid -> return () -- Check succeeded
1250 NotValid msg -> addErrTc (instTypeErr clas inst_tys msg)
1251
1252 ; traceTc "End checkValidInstance }" empty
1253
1254 ; return (tvs, theta, clas, inst_tys) }
1255
1256 | otherwise
1257 = failWithTc (text "Malformed instance head:" <+> ppr tau)
1258 where
1259 (tvs, theta, tau) = tcSplitSigmaTy ty
1260
1261 -- The location of the "head" of the instance
1262 head_loc = getLoc (getLHsInstDeclHead hs_type)
1263
1264 {-
1265 Note [Paterson conditions]
1266 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1267 Termination test: the so-called "Paterson conditions" (see Section 5 of
1268 "Understanding functional dependencies via Constraint Handling Rules,
1269 JFP Jan 2007).
1270
1271 We check that each assertion in the context satisfies:
1272 (1) no variable has more occurrences in the assertion than in the head, and
1273 (2) the assertion has fewer constructors and variables (taken together
1274 and counting repetitions) than the head.
1275 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1276 (which have already been checked) guarantee termination.
1277
1278 The underlying idea is that
1279
1280 for any ground substitution, each assertion in the
1281 context has fewer type constructors than the head.
1282 -}
1283
1284 checkInstTermination :: [TcType] -> ThetaType -> TcM ()
1285 -- See Note [Paterson conditions]
1286 checkInstTermination tys theta
1287 = check_preds theta
1288 where
1289 head_fvs = fvTypes tys
1290 head_size = sizeTypes tys
1291
1292 check_preds :: [PredType] -> TcM ()
1293 check_preds preds = mapM_ check preds
1294
1295 check :: PredType -> TcM ()
1296 check pred
1297 = case classifyPredType pred of
1298 EqPred {} -> return () -- See Trac #4200.
1299 IrredPred {} -> check2 pred (sizeType pred)
1300 ClassPred cls tys
1301 | isTerminatingClass cls
1302 -> return ()
1303
1304 | isCTupleClass cls -- Look inside tuple predicates; Trac #8359
1305 -> check_preds tys
1306
1307 | otherwise
1308 -> check2 pred (sizeTypes $ filterOutInvisibleTypes (classTyCon cls) tys)
1309 -- Other ClassPreds
1310
1311 check2 pred pred_size
1312 | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what)
1313 | pred_size >= head_size = addErrTc (smallerMsg what)
1314 | otherwise = return ()
1315 where
1316 what = text "constraint" <+> quotes (ppr pred)
1317 bad_tvs = fvType pred \\ head_fvs
1318
1319 smallerMsg :: SDoc -> SDoc
1320 smallerMsg what
1321 = vcat [ hang (text "The" <+> what)
1322 2 (text "is no smaller than the instance head")
1323 , parens undecidableMsg ]
1324
1325 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc
1326 noMoreMsg tvs what
1327 = vcat [ hang (text "Variable" <> plural tvs <+> quotes (pprWithCommas ppr tvs)
1328 <+> occurs <+> text "more often")
1329 2 (sep [ text "in the" <+> what
1330 , text "than in the instance head" ])
1331 , parens undecidableMsg ]
1332 where
1333 occurs = if isSingleton tvs then text "occurs"
1334 else text "occur"
1335
1336 undecidableMsg, constraintKindsMsg :: SDoc
1337 undecidableMsg = text "Use UndecidableInstances to permit this"
1338 constraintKindsMsg = text "Use ConstraintKinds to permit this"
1339
1340 {-
1341 Note [Associated type instances]
1342 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1343 We allow this:
1344 class C a where
1345 type T x a
1346 instance C Int where
1347 type T (S y) Int = y
1348 type T Z Int = Char
1349
1350 Note that
1351 a) The variable 'x' is not bound by the class decl
1352 b) 'x' is instantiated to a non-type-variable in the instance
1353 c) There are several type instance decls for T in the instance
1354
1355 All this is fine. Of course, you can't give any *more* instances
1356 for (T ty Int) elsewhere, because it's an *associated* type.
1357
1358 Note [Checking consistent instantiation]
1359 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1360 See Trac #11450 for background discussion on this check.
1361
1362 class C a b where
1363 type T a x b
1364
1365 With this class decl, if we have an instance decl
1366 instance C ty1 ty2 where ...
1367 then the type instance must look like
1368 type T ty1 v ty2 = ...
1369 with exactly 'ty1' for 'a', 'ty2' for 'b', and a variable for 'x'.
1370 For example:
1371
1372 instance C [p] Int
1373 type T [p] y Int = (p,y,y)
1374
1375 Note that
1376
1377 * We used to allow completely different bound variables in the
1378 associated type instance; e.g.
1379 instance C [p] Int
1380 type T [q] y Int = ...
1381 But from GHC 8.2 onwards, we don't. It's much simpler this way.
1382 See Trac #11450.
1383
1384 * When the class variable isn't used on the RHS of the type instance,
1385 it's tempting to allow wildcards, thus
1386 instance C [p] Int
1387 type T [_] y Int = (y,y)
1388 But it's awkward to do the test, and it doesn't work if the
1389 variable is repeated:
1390 instance C (p,p) Int
1391 type T (_,_) y Int = (y,y)
1392 Even though 'p' is not used on the RHS, we still need to use 'p'
1393 on the LHS to establish the repeated pattern. So to keep it simple
1394 we just require equality.
1395
1396 * We also check that any non-class-tyvars are instantiated with
1397 distinct tyvars. That rules out
1398 instance C [p] Int where
1399 type T [p] Bool Int = p -- Note Bool
1400 type T [p] Char Int = p -- Note Char
1401
1402 and
1403 instance C [p] Int where
1404 type T [p] p Int = p -- Note repeated 'p' on LHS
1405 It's consistent to do this because we don't allow this kind of
1406 instantiation for the class-tyvar arguments of the family.
1407
1408 Overall, we can have exactly one type instance for each
1409 associated type. If you wantmore, use an auxiliary family.
1410
1411 Implementation
1412 * Form the mini-envt from the class type variables a,b
1413 to the instance decl types [p],Int: [a->[p], b->Int]
1414
1415 * Look at the tyvars a,x,b of the type family constructor T
1416 (it shares tyvars with the class C)
1417
1418 * Apply the mini-evnt to them, and check that the result is
1419 consistent with the instance types [p] y Int
1420
1421 We make all the instance type variables scope over the
1422 type instances, of course, which picks up non-obvious kinds. Eg
1423 class Foo (a :: k) where
1424 type F a
1425 instance Foo (b :: k -> k) where
1426 type F b = Int
1427 Here the instance is kind-indexed and really looks like
1428 type F (k->k) (b::k->k) = Int
1429 But if the 'b' didn't scope, we would make F's instance too
1430 poly-kinded.
1431 -}
1432
1433 -- | Extra information about the parent instance declaration, needed
1434 -- when type-checking associated types. The 'Class' is the enclosing
1435 -- class, the [TyVar] are the type variable of the instance decl,
1436 -- and and the @VarEnv Type@ maps class variables to their instance
1437 -- types.
1438 type ClsInstInfo = (Class, [TyVar], VarEnv Type)
1439
1440 type AssocInstArgShape = (Maybe Type, Type)
1441 -- AssocInstArgShape is used only for associated family instances
1442 -- (mb_exp, actual)
1443 -- mb_exp = Just ty => this arg corresponds to a class variable
1444 -- = Nothing => it doesn't correspond to a class variable
1445 -- e.g. class C b where
1446 -- type F a b c
1447 -- instance C [x] where
1448 -- type F p [x] q
1449 -- We get [AssocInstArgShape] = [ (Nothing, p)
1450 -- , (Just [x], [x])
1451 -- , (Nothing, q)]
1452
1453 checkConsistentFamInst
1454 :: Maybe ClsInstInfo
1455 -> TyCon -- ^ Family tycon
1456 -> [TyVar] -- ^ Type variables of the family instance
1457 -> [Type] -- ^ Type patterns from instance
1458 -> TcM ()
1459 -- See Note [Checking consistent instantiation]
1460
1461 checkConsistentFamInst Nothing _ _ _ = return ()
1462 checkConsistentFamInst (Just (clas, inst_tvs, mini_env)) fam_tc _at_tvs at_tys
1463 = do { -- Check that the associated type indeed comes from this class
1464 checkTc (Just clas == tyConAssoc_maybe fam_tc)
1465 (badATErr (className clas) (tyConName fam_tc))
1466
1467 -- Check type args first (more comprehensible)
1468 ; checkTc (all check_arg type_shapes) pp_wrong_at_arg
1469 ; checkTc (check_poly_args type_shapes) pp_wrong_at_tyvars
1470
1471 -- And now kind args
1472 ; checkTc (all check_arg kind_shapes)
1473 (pp_wrong_at_arg $$ ppSuggestExplicitKinds)
1474 ; checkTc (check_poly_args kind_shapes)
1475 (pp_wrong_at_tyvars $$ ppSuggestExplicitKinds)
1476
1477 ; traceTc "cfi" (vcat [ ppr inst_tvs
1478 , ppr arg_shapes
1479 , ppr mini_env ]) }
1480 where
1481 arg_shapes :: [AssocInstArgShape]
1482 arg_shapes = [ (lookupVarEnv mini_env fam_tc_tv, at_ty)
1483 | (fam_tc_tv, at_ty) <- tyConTyVars fam_tc `zip` at_tys ]
1484
1485 (kind_shapes, type_shapes) = partitionInvisibles fam_tc snd arg_shapes
1486
1487 check_arg :: AssocInstArgShape -> Bool
1488 check_arg (Just exp_ty, at_ty) = exp_ty `tcEqType` at_ty
1489 check_arg (Nothing, _ ) = True -- Arg position does not correspond
1490 -- to a class variable
1491 check_poly_args :: [(Maybe Type,Type)] -> Bool
1492 check_poly_args arg_shapes
1493 = allDistinctTyVars (mkVarSet inst_tvs)
1494 [ at_ty | (Nothing, at_ty) <- arg_shapes ]
1495
1496 pp_wrong_at_arg
1497 = vcat [ text "Type indexes must match class instance head"
1498 , pp_exp_act ]
1499
1500 pp_wrong_at_tyvars
1501 = vcat [ text "Polymorphic type indexes of associated type" <+> quotes (ppr fam_tc)
1502 , nest 2 $ vcat [ text "(i.e. ones independent of the class type variables)"
1503 , text "must be distinct type variables" ]
1504 , pp_exp_act ]
1505
1506 pp_exp_act
1507 = vcat [ text "Expected:" <+> ppr (mkTyConApp fam_tc expected_args)
1508 , text " Actual:" <+> ppr (mkTyConApp fam_tc at_tys)
1509 , sdocWithDynFlags $ \dflags ->
1510 ppWhen (has_poly_args dflags) $
1511 vcat [ text "where the `<tv>' arguments are type variables,"
1512 , text "distinct from each other and from the instance variables" ] ]
1513
1514 expected_args = [ exp_ty `orElse` mk_tv at_ty | (exp_ty, at_ty) <- arg_shapes ]
1515 mk_tv at_ty = mkTyVarTy (mkTyVar tv_name (typeKind at_ty))
1516 tv_name = mkInternalName (mkAlphaTyVarUnique 1) (mkTyVarOcc "<tv>") noSrcSpan
1517
1518 has_poly_args dflags = any (isNothing . fst) shapes
1519 where
1520 shapes | gopt Opt_PrintExplicitKinds dflags = arg_shapes
1521 | otherwise = type_shapes
1522
1523 badATErr :: Name -> Name -> SDoc
1524 badATErr clas op
1525 = hsep [text "Class", quotes (ppr clas),
1526 text "does not have an associated type", quotes (ppr op)]
1527
1528
1529 {-
1530 ************************************************************************
1531 * *
1532 Checking type instance well-formedness and termination
1533 * *
1534 ************************************************************************
1535 -}
1536
1537 checkValidCoAxiom :: CoAxiom Branched -> TcM ()
1538 checkValidCoAxiom ax@(CoAxiom { co_ax_tc = fam_tc, co_ax_branches = branches })
1539 = do { mapM_ (checkValidCoAxBranch Nothing fam_tc) branch_list
1540 ; foldlM_ check_branch_compat [] branch_list }
1541 where
1542 branch_list = fromBranches branches
1543 injectivity = familyTyConInjectivityInfo fam_tc
1544
1545 check_branch_compat :: [CoAxBranch] -- previous branches in reverse order
1546 -> CoAxBranch -- current branch
1547 -> TcM [CoAxBranch]-- current branch : previous branches
1548 -- Check for
1549 -- (a) this branch is dominated by previous ones
1550 -- (b) failure of injectivity
1551 check_branch_compat prev_branches cur_branch
1552 | cur_branch `isDominatedBy` prev_branches
1553 = do { addWarnAt NoReason (coAxBranchSpan cur_branch) $
1554 inaccessibleCoAxBranch ax cur_branch
1555 ; return prev_branches }
1556 | otherwise
1557 = do { check_injectivity prev_branches cur_branch
1558 ; return (cur_branch : prev_branches) }
1559
1560 -- Injectivity check: check whether a new (CoAxBranch) can extend
1561 -- already checked equations without violating injectivity
1562 -- annotation supplied by the user.
1563 -- See Note [Verifying injectivity annotation] in FamInstEnv
1564 check_injectivity prev_branches cur_branch
1565 | Injective inj <- injectivity
1566 = do { let conflicts =
1567 fst $ foldl (gather_conflicts inj prev_branches cur_branch)
1568 ([], 0) prev_branches
1569 ; mapM_ (\(err, span) -> setSrcSpan span $ addErr err)
1570 (makeInjectivityErrors ax cur_branch inj conflicts) }
1571 | otherwise
1572 = return ()
1573
1574 gather_conflicts inj prev_branches cur_branch (acc, n) branch
1575 -- n is 0-based index of branch in prev_branches
1576 = case injectiveBranches inj cur_branch branch of
1577 InjectivityUnified ax1 ax2
1578 | ax1 `isDominatedBy` (replace_br prev_branches n ax2)
1579 -> (acc, n + 1)
1580 | otherwise
1581 -> (branch : acc, n + 1)
1582 InjectivityAccepted -> (acc, n + 1)
1583
1584 -- Replace n-th element in the list. Assumes 0-based indexing.
1585 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
1586 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
1587
1588
1589 -- Check that a "type instance" is well-formed (which includes decidability
1590 -- unless -XUndecidableInstances is given).
1591 --
1592 checkValidCoAxBranch :: Maybe ClsInstInfo
1593 -> TyCon -> CoAxBranch -> TcM ()
1594 checkValidCoAxBranch mb_clsinfo fam_tc
1595 (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
1596 , cab_lhs = typats
1597 , cab_rhs = rhs, cab_loc = loc })
1598 = checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1599
1600 -- | Do validity checks on a type family equation, including consistency
1601 -- with any enclosing class instance head, termination, and lack of
1602 -- polytypes.
1603 checkValidTyFamEqn :: Maybe ClsInstInfo
1604 -> TyCon -- ^ of the type family
1605 -> [TyVar] -- ^ bound tyvars in the equation
1606 -> [CoVar] -- ^ bound covars in the equation
1607 -> [Type] -- ^ type patterns
1608 -> Type -- ^ rhs
1609 -> SrcSpan
1610 -> TcM ()
1611 checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1612 = setSrcSpan loc $
1613 do { checkValidFamPats mb_clsinfo fam_tc tvs cvs typats
1614
1615 -- The argument patterns, and RHS, are all boxed tau types
1616 -- E.g Reject type family F (a :: k1) :: k2
1617 -- type instance F (forall a. a->a) = ...
1618 -- type instance F Int# = ...
1619 -- type instance F Int = forall a. a->a
1620 -- type instance F Int = Int#
1621 -- See Trac #9357
1622 ; checkValidMonoType rhs
1623 ; check_lifted rhs
1624
1625 -- We have a decidable instance unless otherwise permitted
1626 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1627 ; unless undecidable_ok $
1628 mapM_ addErrTc (checkFamInstRhs typats (tcTyFamInsts rhs)) }
1629
1630 -- Make sure that each type family application is
1631 -- (1) strictly smaller than the lhs,
1632 -- (2) mentions no type variable more often than the lhs, and
1633 -- (3) does not contain any further type family instances.
1634 --
1635 checkFamInstRhs :: [Type] -- lhs
1636 -> [(TyCon, [Type])] -- type family instances
1637 -> [MsgDoc]
1638 checkFamInstRhs lhsTys famInsts
1639 = mapMaybe check famInsts
1640 where
1641 size = sizeTypes lhsTys
1642 fvs = fvTypes lhsTys
1643 check (tc, tys)
1644 | not (all isTyFamFree tys) = Just (nestedMsg what)
1645 | not (null bad_tvs) = Just (noMoreMsg bad_tvs what)
1646 | size <= sizeTypes tys = Just (smallerMsg what)
1647 | otherwise = Nothing
1648 where
1649 what = text "type family application" <+> quotes (pprType (TyConApp tc tys))
1650 bad_tvs = fvTypes tys \\ fvs
1651
1652 checkValidFamPats :: Maybe ClsInstInfo -> TyCon -> [TyVar] -> [CoVar] -> [Type] -> TcM ()
1653 -- Patterns in a 'type instance' or 'data instance' decl should
1654 -- a) contain no type family applications
1655 -- (vanilla synonyms are fine, though)
1656 -- b) properly bind all their free type variables
1657 -- e.g. we disallow (Trac #7536)
1658 -- type T a = Int
1659 -- type instance F (T a) = a
1660 -- c) Have the right number of patterns
1661 -- d) For associated types, are consistently instantiated
1662 checkValidFamPats mb_clsinfo fam_tc tvs cvs ty_pats
1663 = do { -- A family instance must have exactly the same number of type
1664 -- parameters as the family declaration. You can't write
1665 -- type family F a :: * -> *
1666 -- type instance F Int y = y
1667 -- because then the type (F Int) would be like (\y.y)
1668 checkTc (length ty_pats == fam_arity) $
1669 wrongNumberOfParmsErr (fam_arity - count isInvisibleTyConBinder fam_bndrs)
1670 -- report only explicit arguments
1671
1672 ; mapM_ checkValidTypePat ty_pats
1673
1674 ; let unbound_tcvs = filterOut (`elemVarSet` exactTyCoVarsOfTypes ty_pats) (tvs ++ cvs)
1675 ; checkTc (null unbound_tcvs) (famPatErr fam_tc unbound_tcvs ty_pats)
1676
1677 -- Check that type patterns match the class instance head
1678 ; checkConsistentFamInst mb_clsinfo fam_tc tvs ty_pats }
1679 where
1680 fam_arity = tyConArity fam_tc
1681 fam_bndrs = tyConBinders fam_tc
1682
1683
1684 checkValidTypePat :: Type -> TcM ()
1685 -- Used for type patterns in class instances,
1686 -- and in type/data family instances
1687 checkValidTypePat pat_ty
1688 = do { -- Check that pat_ty is a monotype
1689 checkValidMonoType pat_ty
1690 -- One could imagine generalising to allow
1691 -- instance C (forall a. a->a)
1692 -- but we don't know what all the consequences might be
1693
1694 -- Ensure that no type family instances occur a type pattern
1695 ; checkTc (isTyFamFree pat_ty) $
1696 tyFamInstIllegalErr pat_ty
1697
1698 ; check_lifted pat_ty }
1699
1700 isTyFamFree :: Type -> Bool
1701 -- ^ Check that a type does not contain any type family applications.
1702 isTyFamFree = null . tcTyFamInsts
1703
1704 -- Error messages
1705
1706 wrongNumberOfParmsErr :: Arity -> SDoc
1707 wrongNumberOfParmsErr exp_arity
1708 = text "Number of parameters must match family declaration; expected"
1709 <+> ppr exp_arity
1710
1711 inaccessibleCoAxBranch :: CoAxiom br -> CoAxBranch -> SDoc
1712 inaccessibleCoAxBranch fi_ax cur_branch
1713 = text "Type family instance equation is overlapped:" $$
1714 nest 2 (pprCoAxBranch fi_ax cur_branch)
1715
1716 tyFamInstIllegalErr :: Type -> SDoc
1717 tyFamInstIllegalErr ty
1718 = hang (text "Illegal type synonym family application in instance" <>
1719 colon) 2 $
1720 ppr ty
1721
1722 nestedMsg :: SDoc -> SDoc
1723 nestedMsg what
1724 = sep [ text "Illegal nested" <+> what
1725 , parens undecidableMsg ]
1726
1727 famPatErr :: TyCon -> [TyVar] -> [Type] -> SDoc
1728 famPatErr fam_tc tvs pats
1729 = hang (text "Family instance purports to bind type variable" <> plural tvs
1730 <+> pprQuotedList tvs)
1731 2 (hang (text "but the real LHS (expanding synonyms) is:")
1732 2 (pprTypeApp fam_tc (map expandTypeSynonyms pats) <+>
1733 text "= ..."))
1734
1735 {-
1736 ************************************************************************
1737 * *
1738 Telescope checking
1739 * *
1740 ************************************************************************
1741
1742 Note [Bad telescopes]
1743 ~~~~~~~~~~~~~~~~~~~~~
1744 Now that we can mix type and kind variables, there are an awful lot of
1745 ways to shoot yourself in the foot. Here are some.
1746
1747 data SameKind :: k -> k -> * -- just to force unification
1748
1749 1. data T1 a k (b :: k) (x :: SameKind a b)
1750
1751 The problem here is that we discover that a and b should have the same
1752 kind. But this kind mentions k, which is bound *after* a.
1753 (Testcase: dependent/should_fail/BadTelescope)
1754
1755 2. data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)
1756
1757 Note that b is not bound. Yet its kind mentions a. Because we have
1758 a nice rule that all implicitly bound variables come before others,
1759 this is bogus. (We could probably figure out to put b between a and c.
1760 But I think this is doing users a disservice, in the long run.)
1761 (Testcase: dependent/should_fail/BadTelescope4)
1762
1763 3. t3 :: forall a. (forall k (b :: k). SameKind a b) -> ()
1764
1765 This is a straightforward skolem escape. Note that a and b need to have
1766 the same kind.
1767 (Testcase: polykinds/T11142)
1768
1769 How do we deal with all of this? For TyCons, we have checkValidTyConTyVars.
1770 That function looks to see if any of the tyConTyVars are repeated, but
1771 it's really a telescope check. It works because all tycons are kind-generalized.
1772 If there is a bad telescope, the kind-generalization will end up generalizing
1773 over a variable bound later in the telescope.
1774
1775 For non-tycons, we do scope checking when we bring tyvars into scope,
1776 in tcImplicitTKBndrs and tcExplicitTKBndrs. Note that we also have to
1777 sort implicit binders into a well-scoped order whenever we have implicit
1778 binders to worry about. This is done in quantifyTyVars and in
1779 tcImplicitTKBndrs.
1780 -}
1781
1782 -- | Check a list of binders to see if they make a valid telescope.
1783 -- The key property we're checking for is scoping. For example:
1784 -- > data SameKind :: k -> k -> *
1785 -- > data X a k (b :: k) (c :: SameKind a b)
1786 -- Kind inference says that a's kind should be k. But that's impossible,
1787 -- because k isn't in scope when a is bound. This check has to come before
1788 -- general validity checking, because once we kind-generalise, this sort
1789 -- of problem is harder to spot (as we'll generalise over the unbound
1790 -- k in a's type.) See also Note [Bad telescopes].
1791 checkValidTelescope :: SDoc -- the original user-written telescope
1792 -> [TyVar] -- explicit vars (not necessarily zonked)
1793 -> SDoc -- note to put at bottom of message
1794 -> TcM ()
1795 checkValidTelescope hs_tvs orig_tvs extra
1796 = discardResult $ checkZonkValidTelescope hs_tvs orig_tvs extra
1797
1798 -- | Like 'checkZonkValidTelescope', but returns the zonked tyvars
1799 checkZonkValidTelescope :: SDoc
1800 -> [TyVar]
1801 -> SDoc
1802 -> TcM [TyVar]
1803 checkZonkValidTelescope hs_tvs orig_tvs extra
1804 = do { orig_tvs <- mapM zonkTyCoVarKind orig_tvs
1805 ; let (_, sorted_tidied_tvs) = tidyTyCoVarBndrs emptyTidyEnv $
1806 toposortTyVars orig_tvs
1807 ; unless (go [] emptyVarSet orig_tvs) $
1808 addErr $
1809 vcat [ hang (text "These kind and type variables:" <+> hs_tvs $$
1810 text "are out of dependency order. Perhaps try this ordering:")
1811 2 (sep (map pprTvBndr sorted_tidied_tvs))
1812 , extra ]
1813 ; return orig_tvs }
1814
1815 where
1816 go :: [TyVar] -- misplaced variables
1817 -> TyVarSet -> [TyVar] -> Bool
1818 go errs in_scope [] = null (filter (`elemVarSet` in_scope) errs)
1819 -- report an error only when the variable in the kind is brought
1820 -- into scope later in the telescope. Otherwise, we'll just quantify
1821 -- over it in kindGeneralize, as we should.
1822
1823 go errs in_scope (tv:tvs)
1824 = let bad_tvs = filterOut (`elemVarSet` in_scope) $
1825 tyCoVarsOfTypeList (tyVarKind tv)
1826 in go (bad_tvs ++ errs) (in_scope `extendVarSet` tv) tvs
1827
1828 -- | After inferring kinds of type variables, check to make sure that the
1829 -- inferred kinds any of the type variables bound in a smaller scope.
1830 -- This is a skolem escape check. See also Note [Bad telescopes].
1831 checkValidInferredKinds :: [TyVar] -- ^ vars to check (zonked)
1832 -> TyVarSet -- ^ vars out of scope
1833 -> SDoc -- ^ suffix to error message
1834 -> TcM ()
1835 checkValidInferredKinds orig_kvs out_of_scope extra
1836 = do { let bad_pairs = [ (tv, kv)
1837 | kv <- orig_kvs
1838 , Just tv <- map (lookupVarSet out_of_scope)
1839 (tyCoVarsOfTypeList (tyVarKind kv)) ]
1840 report (tidyTyVarOcc env -> tv, tidyTyVarOcc env -> kv)
1841 = addErr $
1842 text "The kind of variable" <+>
1843 quotes (ppr kv) <> text ", namely" <+>
1844 quotes (ppr (tyVarKind kv)) <> comma $$
1845 text "depends on variable" <+>
1846 quotes (ppr tv) <+> text "from an inner scope" $$
1847 text "Perhaps bind" <+> quotes (ppr kv) <+>
1848 text "sometime after binding" <+>
1849 quotes (ppr tv) $$
1850 extra
1851 ; mapM_ report bad_pairs }
1852
1853 where
1854 (env1, _) = tidyTyCoVarBndrs emptyTidyEnv orig_kvs
1855 (env, _) = tidyTyCoVarBndrs env1 (nonDetEltsUFM out_of_scope)
1856 -- It's OK to use nonDetEltsUFM here because it's only used for
1857 -- generating the error message
1858
1859 {-
1860 ************************************************************************
1861 * *
1862 \subsection{Auxiliary functions}
1863 * *
1864 ************************************************************************
1865 -}
1866
1867 -- Free variables of a type, retaining repetitions, and expanding synonyms
1868 fvType :: Type -> [TyCoVar]
1869 fvType ty | Just exp_ty <- coreView ty = fvType exp_ty
1870 fvType (TyVarTy tv) = [tv]
1871 fvType (TyConApp _ tys) = fvTypes tys
1872 fvType (LitTy {}) = []
1873 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1874 fvType (FunTy arg res) = fvType arg ++ fvType res
1875 fvType (ForAllTy (TvBndr tv _) ty)
1876 = fvType (tyVarKind tv) ++
1877 filter (/= tv) (fvType ty)
1878 fvType (CastTy ty co) = fvType ty ++ fvCo co
1879 fvType (CoercionTy co) = fvCo co
1880
1881 fvTypes :: [Type] -> [TyVar]
1882 fvTypes tys = concat (map fvType tys)
1883
1884 fvCo :: Coercion -> [TyCoVar]
1885 fvCo (Refl _ ty) = fvType ty
1886 fvCo (TyConAppCo _ _ args) = concatMap fvCo args
1887 fvCo (AppCo co arg) = fvCo co ++ fvCo arg
1888 fvCo (ForAllCo tv h co) = filter (/= tv) (fvCo co) ++ fvCo h
1889 fvCo (CoVarCo v) = [v]
1890 fvCo (AxiomInstCo _ _ args) = concatMap fvCo args
1891 fvCo (UnivCo p _ t1 t2) = fvProv p ++ fvType t1 ++ fvType t2
1892 fvCo (SymCo co) = fvCo co
1893 fvCo (TransCo co1 co2) = fvCo co1 ++ fvCo co2
1894 fvCo (NthCo _ co) = fvCo co
1895 fvCo (LRCo _ co) = fvCo co
1896 fvCo (InstCo co arg) = fvCo co ++ fvCo arg
1897 fvCo (CoherenceCo co1 co2) = fvCo co1 ++ fvCo co2
1898 fvCo (KindCo co) = fvCo co
1899 fvCo (SubCo co) = fvCo co
1900 fvCo (AxiomRuleCo _ cs) = concatMap fvCo cs
1901
1902 fvProv :: UnivCoProvenance -> [TyCoVar]
1903 fvProv UnsafeCoerceProv = []
1904 fvProv (PhantomProv co) = fvCo co
1905 fvProv (ProofIrrelProv co) = fvCo co
1906 fvProv (PluginProv _) = []
1907 fvProv (HoleProv h) = pprPanic "fvProv falls into a hole" (ppr h)
1908
1909 sizeType :: Type -> Int
1910 -- Size of a type: the number of variables and constructors
1911 sizeType ty | Just exp_ty <- coreView ty = sizeType exp_ty
1912 sizeType (TyVarTy {}) = 1
1913 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1914 sizeType (LitTy {}) = 1
1915 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1916 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1917 sizeType (ForAllTy _ ty) = 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