a56e7a24187f4266491abbf7dcb9fbbc02af3ba6
[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 TcEnv ( tcGetInstEnvs )
43 import FunDeps
44 import InstEnv ( ClsInst, lookupInstEnv, isOverlappable )
45 import FamInstEnv ( isDominatedBy, injectiveBranches,
46 InjectivityCheckResult(..) )
47 import FamInst ( makeInjectivityErrors )
48 import Name
49 import VarEnv
50 import VarSet
51 import UniqSet
52 import Var ( TyVarBndr(..), mkTyVar )
53 import ErrUtils
54 import DynFlags
55 import Util
56 import ListSetOps
57 import SrcLoc
58 import Outputable
59 import BasicTypes
60 import Module
61 import Unique ( mkAlphaTyVarUnique )
62 import qualified GHC.LanguageExtensions as LangExt
63
64 import Control.Monad
65 import Data.List ( (\\) )
66
67 {-
68 ************************************************************************
69 * *
70 Checking for ambiguity
71 * *
72 ************************************************************************
73
74 Note [The ambiguity check for type signatures]
75 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
76 checkAmbiguity is a check on *user-supplied type signatures*. It is
77 *purely* there to report functions that cannot possibly be called. So for
78 example we want to reject:
79 f :: C a => Int
80 The idea is there can be no legal calls to 'f' because every call will
81 give rise to an ambiguous constraint. We could soundly omit the
82 ambiguity check on type signatures entirely, at the expense of
83 delaying ambiguity errors to call sites. Indeed, the flag
84 -XAllowAmbiguousTypes switches off the ambiguity check.
85
86 What about things like this:
87 class D a b | a -> b where ..
88 h :: D Int b => Int
89 The Int may well fix 'b' at the call site, so that signature should
90 not be rejected. Moreover, using *visible* fundeps is too
91 conservative. Consider
92 class X a b where ...
93 class D a b | a -> b where ...
94 instance D a b => X [a] b where...
95 h :: X a b => a -> a
96 Here h's type looks ambiguous in 'b', but here's a legal call:
97 ...(h [True])...
98 That gives rise to a (X [Bool] beta) constraint, and using the
99 instance means we need (D Bool beta) and that fixes 'beta' via D's
100 fundep!
101
102 Behind all these special cases there is a simple guiding principle.
103 Consider
104
105 f :: <type>
106 f = ...blah...
107
108 g :: <type>
109 g = f
110
111 You would think that the definition of g would surely typecheck!
112 After all f has exactly the same type, and g=f. But in fact f's type
113 is instantiated and the instantiated constraints are solved against
114 the originals, so in the case an ambiguous type it won't work.
115 Consider our earlier example f :: C a => Int. Then in g's definition,
116 we'll instantiate to (C alpha) and try to deduce (C alpha) from (C a),
117 and fail.
118
119 So in fact we use this as our *definition* of ambiguity. We use a
120 very similar test for *inferred* types, to ensure that they are
121 unambiguous. See Note [Impedance matching] in TcBinds.
122
123 This test is very conveniently implemented by calling
124 tcSubType <type> <type>
125 This neatly takes account of the functional dependecy stuff above,
126 and implicit parameter (see Note [Implicit parameters and ambiguity]).
127 And this is what checkAmbiguity does.
128
129 What about this, though?
130 g :: C [a] => Int
131 Is every call to 'g' ambiguous? After all, we might have
132 instance C [a] where ...
133 at the call site. So maybe that type is ok! Indeed even f's
134 quintessentially ambiguous type might, just possibly be callable:
135 with -XFlexibleInstances we could have
136 instance C a where ...
137 and now a call could be legal after all! Well, we'll reject this
138 unless the instance is available *here*.
139
140 Note [When to call checkAmbiguity]
141 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
142 We call checkAmbiguity
143 (a) on user-specified type signatures
144 (b) in checkValidType
145
146 Conncerning (b), you might wonder about nested foralls. What about
147 f :: forall b. (forall a. Eq a => b) -> b
148 The nested forall is ambiguous. Originally we called checkAmbiguity
149 in the forall case of check_type, but that had two bad consequences:
150 * We got two error messages about (Eq b) in a nested forall like this:
151 g :: forall a. Eq a => forall b. Eq b => a -> a
152 * If we try to check for ambiguity of an nested forall like
153 (forall a. Eq a => b), the implication constraint doesn't bind
154 all the skolems, which results in "No skolem info" in error
155 messages (see Trac #10432).
156
157 To avoid this, we call checkAmbiguity once, at the top, in checkValidType.
158 (I'm still a bit worried about unbound skolems when the type mentions
159 in-scope type variables.)
160
161 In fact, because of the co/contra-variance implemented in tcSubType,
162 this *does* catch function f above. too.
163
164 Concerning (a) the ambiguity check is only used for *user* types, not
165 for types coming from inteface files. The latter can legitimately
166 have ambiguous types. Example
167
168 class S a where s :: a -> (Int,Int)
169 instance S Char where s _ = (1,1)
170 f:: S a => [a] -> Int -> (Int,Int)
171 f (_::[a]) x = (a*x,b)
172 where (a,b) = s (undefined::a)
173
174 Here the worker for f gets the type
175 fw :: forall a. S a => Int -> (# Int, Int #)
176
177
178 Note [Implicit parameters and ambiguity]
179 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
180 Only a *class* predicate can give rise to ambiguity
181 An *implicit parameter* cannot. For example:
182 foo :: (?x :: [a]) => Int
183 foo = length ?x
184 is fine. The call site will supply a particular 'x'
185
186 Furthermore, the type variables fixed by an implicit parameter
187 propagate to the others. E.g.
188 foo :: (Show a, ?x::[a]) => Int
189 foo = show (?x++?x)
190 The type of foo looks ambiguous. But it isn't, because at a call site
191 we might have
192 let ?x = 5::Int in foo
193 and all is well. In effect, implicit parameters are, well, parameters,
194 so we can take their type variables into account as part of the
195 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
196 -}
197
198 checkAmbiguity :: UserTypeCtxt -> Type -> TcM ()
199 checkAmbiguity ctxt ty
200 | wantAmbiguityCheck ctxt
201 = do { traceTc "Ambiguity check for" (ppr ty)
202 -- Solve the constraints eagerly because an ambiguous type
203 -- can cause a cascade of further errors. Since the free
204 -- tyvars are skolemised, we can safely use tcSimplifyTop
205 ; allow_ambiguous <- xoptM LangExt.AllowAmbiguousTypes
206 ; (_wrap, wanted) <- addErrCtxt (mk_msg allow_ambiguous) $
207 captureConstraints $
208 tcSubType_NC ctxt ty ty
209 ; simplifyAmbiguityCheck ty wanted
210
211 ; traceTc "Done ambiguity check for" (ppr ty) }
212
213 | otherwise
214 = return ()
215 where
216 mk_msg allow_ambiguous
217 = vcat [ text "In the ambiguity check for" <+> what
218 , ppUnless allow_ambiguous ambig_msg ]
219 ambig_msg = text "To defer the ambiguity check to use sites, enable AllowAmbiguousTypes"
220 what | Just n <- isSigMaybe ctxt = quotes (ppr n)
221 | otherwise = pprUserTypeCtxt ctxt
222
223 wantAmbiguityCheck :: UserTypeCtxt -> Bool
224 wantAmbiguityCheck ctxt
225 = case ctxt of -- See Note [When we don't check for ambiguity]
226 GhciCtxt -> False
227 TySynCtxt {} -> False
228 _ -> True
229
230 checkUserTypeError :: Type -> TcM ()
231 -- Check to see if the type signature mentions "TypeError blah"
232 -- anywhere in it, and fail if so.
233 --
234 -- Very unsatisfactorily (Trac #11144) we need to tidy the type
235 -- because it may have come from an /inferred/ signature, not a
236 -- user-supplied one. This is really only a half-baked fix;
237 -- the other errors in checkValidType don't do tidying, and so
238 -- may give bad error messages when given an inferred type.
239 checkUserTypeError = check
240 where
241 check ty
242 | Just msg <- userTypeError_maybe ty = fail_with msg
243 | Just (_,ts) <- splitTyConApp_maybe ty = mapM_ check ts
244 | Just (t1,t2) <- splitAppTy_maybe ty = check t1 >> check t2
245 | Just (_,t1) <- splitForAllTy_maybe ty = check t1
246 | otherwise = return ()
247
248 fail_with msg = do { env0 <- tcInitTidyEnv
249 ; let (env1, tidy_msg) = tidyOpenType env0 msg
250 ; failWithTcM (env1, pprUserTypeErrorTy tidy_msg) }
251
252
253 {- Note [When we don't check for ambiguity]
254 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
255 In a few places we do not want to check a user-specified type for ambiguity
256
257 * GhciCtxt: Allow ambiguous types in GHCi's :kind command
258 E.g. type family T a :: * -- T :: forall k. k -> *
259 Then :k T should work in GHCi, not complain that
260 (T k) is ambiguous!
261
262 * TySynCtxt: type T a b = C a b => blah
263 It may be that when we /use/ T, we'll give an 'a' or 'b' that somehow
264 cure the ambiguity. So we defer the ambiguity check to the use site.
265
266 There is also an implementation reason (Trac #11608). In the RHS of
267 a type synonym we don't (currently) instantiate 'a' and 'b' with
268 TcTyVars before calling checkValidType, so we get asertion failures
269 from doing an ambiguity check on a type with TyVars in it. Fixing this
270 would not be hard, but let's wait till there's a reason.
271
272
273 ************************************************************************
274 * *
275 Checking validity of a user-defined type
276 * *
277 ************************************************************************
278
279 When dealing with a user-written type, we first translate it from an HsType
280 to a Type, performing kind checking, and then check various things that should
281 be true about it. We don't want to perform these checks at the same time
282 as the initial translation because (a) they are unnecessary for interface-file
283 types and (b) when checking a mutually recursive group of type and class decls,
284 we can't "look" at the tycons/classes yet. Also, the checks are rather
285 diverse, and used to really mess up the other code.
286
287 One thing we check for is 'rank'.
288
289 Rank 0: monotypes (no foralls)
290 Rank 1: foralls at the front only, Rank 0 inside
291 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
292
293 basic ::= tyvar | T basic ... basic
294
295 r2 ::= forall tvs. cxt => r2a
296 r2a ::= r1 -> r2a | basic
297 r1 ::= forall tvs. cxt => r0
298 r0 ::= r0 -> r0 | basic
299
300 Another thing is to check that type synonyms are saturated.
301 This might not necessarily show up in kind checking.
302 type A i = i
303 data T k = MkT (k Int)
304 f :: T A -- BAD!
305 -}
306
307 checkValidType :: UserTypeCtxt -> Type -> TcM ()
308 -- Checks that a user-written type is valid for the given context
309 -- Assumes argument is fully zonked
310 -- Not used for instance decls; checkValidInstance instead
311 checkValidType ctxt ty
312 = do { traceTc "checkValidType" (ppr ty <+> text "::" <+> ppr (typeKind ty))
313 ; rankn_flag <- xoptM LangExt.RankNTypes
314 ; impred_flag <- xoptM LangExt.ImpredicativeTypes
315 ; let gen_rank :: Rank -> Rank
316 gen_rank r | rankn_flag = ArbitraryRank
317 | otherwise = r
318
319 rank1 = gen_rank r1
320 rank0 = gen_rank r0
321
322 r0 = rankZeroMonoType
323 r1 = LimitedRank True r0
324
325 rank
326 = case ctxt of
327 DefaultDeclCtxt-> MustBeMonoType
328 ResSigCtxt -> MustBeMonoType
329 PatSigCtxt -> rank0
330 RuleSigCtxt _ -> rank1
331 TySynCtxt _ -> rank0
332
333 ExprSigCtxt -> rank1
334 TypeAppCtxt | impred_flag -> ArbitraryRank
335 | otherwise -> tyConArgMonoType
336 -- Normally, ImpredicativeTypes is handled in check_arg_type,
337 -- but visible type applications don't go through there.
338 -- So we do this check here.
339
340 FunSigCtxt {} -> rank1
341 InfSigCtxt _ -> ArbitraryRank -- Inferred type
342 ConArgCtxt _ -> rank1 -- We are given the type of the entire
343 -- constructor, hence rank 1
344 PatSynCtxt _ -> rank1
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 argument 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 ----------------------------------------
445 check_type :: TidyEnv -> UserTypeCtxt -> Rank -> Type -> TcM ()
446 -- The args say what the *type context* requires, independent
447 -- of *flag* settings. You test the flag settings at usage sites.
448 --
449 -- Rank is allowed rank for function args
450 -- Rank 0 means no for-alls anywhere
451
452 check_type env ctxt rank ty
453 | not (null tvs && null theta)
454 = do { traceTc "check_type" (ppr ty $$ ppr (forAllAllowed rank))
455 ; checkTcM (forAllAllowed rank) (forAllTyErr env rank ty)
456 -- Reject e.g. (Maybe (?x::Int => Int)),
457 -- with a decent error message
458
459 ; check_valid_theta env' SigmaCtxt theta
460 -- Allow type T = ?x::Int => Int -> Int
461 -- but not type T = ?x::Int
462
463 ; check_type env' ctxt rank tau -- Allow foralls to right of arrow
464 ; checkTcM (not (any (`elemVarSet` tyCoVarsOfType phi_kind) tvs))
465 (forAllEscapeErr env' ty tau_kind)
466 }
467 where
468 (tvs, theta, tau) = tcSplitSigmaTy ty
469 tau_kind = typeKind tau
470 (env', _) = tidyTyCoVarBndrs env tvs
471
472 phi_kind | null theta = tau_kind
473 | otherwise = liftedTypeKind
474 -- If there are any constraints, the kind is *. (#11405)
475
476 check_type _ _ _ (TyVarTy _) = return ()
477
478 check_type env ctxt rank (FunTy arg_ty res_ty)
479 = do { check_type env ctxt arg_rank arg_ty
480 ; check_type env ctxt res_rank res_ty }
481 where
482 (arg_rank, res_rank) = funArgResRank rank
483
484 check_type env ctxt rank (AppTy ty1 ty2)
485 = do { check_arg_type env ctxt rank ty1
486 ; check_arg_type env ctxt rank ty2 }
487
488 check_type env ctxt rank ty@(TyConApp tc tys)
489 | isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
490 = check_syn_tc_app env ctxt rank ty tc tys
491 | isUnboxedTupleTyCon tc = check_ubx_tuple env ctxt ty tys
492 | otherwise = mapM_ (check_arg_type env ctxt rank) tys
493
494 check_type _ _ _ (LitTy {}) = return ()
495
496 check_type env ctxt rank (CastTy ty _) = check_type env ctxt rank ty
497
498 check_type _ _ _ ty = pprPanic "check_type" (ppr ty)
499
500 ----------------------------------------
501 check_syn_tc_app :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType
502 -> TyCon -> [KindOrType] -> TcM ()
503 -- Used for type synonyms and type synonym families,
504 -- which must be saturated,
505 -- but not data families, which need not be saturated
506 check_syn_tc_app env ctxt rank ty tc tys
507 | tc_arity <= length tys -- Saturated
508 -- Check that the synonym has enough args
509 -- This applies equally to open and closed synonyms
510 -- It's OK to have an *over-applied* type synonym
511 -- data Tree a b = ...
512 -- type Foo a = Tree [a]
513 -- f :: Foo a b -> ...
514 = do { -- See Note [Liberal type synonyms]
515 ; liberal <- xoptM LangExt.LiberalTypeSynonyms
516 ; if not liberal || isTypeFamilyTyCon tc then
517 -- For H98 and synonym families, do check the type args
518 mapM_ check_arg tys
519
520 else -- In the liberal case (only for closed syns), expand then check
521 case coreView ty of
522 Just ty' -> check_type env ctxt rank ty'
523 Nothing -> pprPanic "check_tau_type" (ppr ty) }
524
525 | GhciCtxt <- ctxt -- Accept under-saturated type synonyms in
526 -- GHCi :kind commands; see Trac #7586
527 = mapM_ check_arg tys
528
529 | otherwise
530 = failWithTc (tyConArityErr tc tys)
531 where
532 tc_arity = tyConArity tc
533 check_arg | isTypeFamilyTyCon tc = check_arg_type env ctxt rank
534 | otherwise = check_type env ctxt synArgMonoType
535
536 ----------------------------------------
537 check_ubx_tuple :: TidyEnv -> UserTypeCtxt -> KindOrType
538 -> [KindOrType] -> TcM ()
539 check_ubx_tuple env ctxt ty tys
540 = do { ub_tuples_allowed <- xoptM LangExt.UnboxedTuples
541 ; checkTcM ub_tuples_allowed (ubxArgTyErr env ty)
542
543 ; impred <- xoptM LangExt.ImpredicativeTypes
544 ; let rank' = if impred then ArbitraryRank else tyConArgMonoType
545 -- c.f. check_arg_type
546 -- However, args are allowed to be unlifted, or
547 -- more unboxed tuples, so can't use check_arg_ty
548 ; mapM_ (check_type env ctxt rank') tys }
549
550 ----------------------------------------
551 check_arg_type :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType -> TcM ()
552 -- The sort of type that can instantiate a type variable,
553 -- or be the argument of a type constructor.
554 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
555 -- Other unboxed types are very occasionally allowed as type
556 -- arguments depending on the kind of the type constructor
557 --
558 -- For example, we want to reject things like:
559 --
560 -- instance Ord a => Ord (forall s. T s a)
561 -- and
562 -- g :: T s (forall b.b)
563 --
564 -- NB: unboxed tuples can have polymorphic or unboxed args.
565 -- This happens in the workers for functions returning
566 -- product types with polymorphic components.
567 -- But not in user code.
568 -- Anyway, they are dealt with by a special case in check_tau_type
569
570 check_arg_type _ _ _ (CoercionTy {}) = return ()
571
572 check_arg_type env ctxt rank ty
573 = do { impred <- xoptM LangExt.ImpredicativeTypes
574 ; let rank' = case rank of -- Predictive => must be monotype
575 MustBeMonoType -> MustBeMonoType -- Monotype, regardless
576 _other | impred -> ArbitraryRank
577 | otherwise -> tyConArgMonoType
578 -- Make sure that MustBeMonoType is propagated,
579 -- so that we don't suggest -XImpredicativeTypes in
580 -- (Ord (forall a.a)) => a -> a
581 -- and so that if it Must be a monotype, we check that it is!
582
583 ; check_type env ctxt rank' ty }
584
585 ----------------------------------------
586 forAllTyErr :: TidyEnv -> Rank -> Type -> (TidyEnv, SDoc)
587 forAllTyErr env rank ty
588 = ( env
589 , vcat [ hang herald 2 (ppr_tidy env ty)
590 , suggestion ] )
591 where
592 (tvs, _theta, _tau) = tcSplitSigmaTy ty
593 herald | null tvs = text "Illegal qualified type:"
594 | otherwise = text "Illegal polymorphic type:"
595 suggestion = case rank of
596 LimitedRank {} -> text "Perhaps you intended to use RankNTypes or Rank2Types"
597 MonoType d -> d
598 _ -> Outputable.empty -- Polytype is always illegal
599
600 forAllEscapeErr :: TidyEnv -> Type -> Kind -> (TidyEnv, SDoc)
601 forAllEscapeErr env ty tau_kind
602 = ( env
603 , hang (vcat [ text "Quantified type's kind mentions quantified type variable"
604 , text "(skolem escape)" ])
605 2 (vcat [ text " type:" <+> ppr_tidy env ty
606 , text "of kind:" <+> ppr_tidy env tau_kind ]) )
607
608 ubxArgTyErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
609 ubxArgTyErr env ty = (env, sep [text "Illegal unboxed tuple type as function argument:", ppr_tidy env ty])
610
611 {-
612 Note [Liberal type synonyms]
613 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
614 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
615 doing validity checking. This allows us to instantiate a synonym defn
616 with a for-all type, or with a partially-applied type synonym.
617 e.g. type T a b = a
618 type S m = m ()
619 f :: S (T Int)
620 Here, T is partially applied, so it's illegal in H98. But if you
621 expand S first, then T we get just
622 f :: Int
623 which is fine.
624
625 IMPORTANT: suppose T is a type synonym. Then we must do validity
626 checking on an appliation (T ty1 ty2)
627
628 *either* before expansion (i.e. check ty1, ty2)
629 *or* after expansion (i.e. expand T ty1 ty2, and then check)
630 BUT NOT BOTH
631
632 If we do both, we get exponential behaviour!!
633
634 data TIACons1 i r c = c i ::: r c
635 type TIACons2 t x = TIACons1 t (TIACons1 t x)
636 type TIACons3 t x = TIACons2 t (TIACons1 t x)
637 type TIACons4 t x = TIACons2 t (TIACons2 t x)
638 type TIACons7 t x = TIACons4 t (TIACons3 t x)
639
640
641 ************************************************************************
642 * *
643 \subsection{Checking a theta or source type}
644 * *
645 ************************************************************************
646
647 Note [Implicit parameters in instance decls]
648 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
649 Implicit parameters _only_ allowed in type signatures; not in instance
650 decls, superclasses etc. The reason for not allowing implicit params in
651 instances is a bit subtle. If we allowed
652 instance (?x::Int, Eq a) => Foo [a] where ...
653 then when we saw
654 (e :: (?x::Int) => t)
655 it would be unclear how to discharge all the potential uses of the ?x
656 in e. For example, a constraint Foo [Int] might come out of e, and
657 applying the instance decl would show up two uses of ?x. Trac #8912.
658 -}
659
660 checkValidTheta :: UserTypeCtxt -> ThetaType -> TcM ()
661 -- Assumes argument is fully zonked
662 checkValidTheta ctxt theta
663 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypesList theta)
664 ; addErrCtxtM (checkThetaCtxt ctxt theta) $
665 check_valid_theta env ctxt theta }
666
667 -------------------------
668 check_valid_theta :: TidyEnv -> UserTypeCtxt -> [PredType] -> TcM ()
669 check_valid_theta _ _ []
670 = return ()
671 check_valid_theta env ctxt theta
672 = do { dflags <- getDynFlags
673 ; warnTcM (Reason Opt_WarnDuplicateConstraints)
674 (wopt Opt_WarnDuplicateConstraints dflags && notNull dups)
675 (dupPredWarn env dups)
676 ; traceTc "check_valid_theta" (ppr theta)
677 ; mapM_ (check_pred_ty env dflags ctxt) theta }
678 where
679 (_,dups) = removeDups nonDetCmpType theta
680 -- It's OK to use nonDetCmpType because dups only appears in the
681 -- warning
682
683 -------------------------
684 {- Note [Validity checking for constraints]
685 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
686 We look through constraint synonyms so that we can see the underlying
687 constraint(s). For example
688 type Foo = ?x::Int
689 instance Foo => C T
690 We should reject the instance because it has an implicit parameter in
691 the context.
692
693 But we record, in 'under_syn', whether we have looked under a synonym
694 to avoid requiring language extensions at the use site. Main example
695 (Trac #9838):
696
697 {-# LANGUAGE ConstraintKinds #-}
698 module A where
699 type EqShow a = (Eq a, Show a)
700
701 module B where
702 import A
703 foo :: EqShow a => a -> String
704
705 We don't want to require ConstraintKinds in module B.
706 -}
707
708 check_pred_ty :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
709 -- Check the validity of a predicate in a signature
710 -- See Note [Validity checking for constraints]
711 check_pred_ty env dflags ctxt pred
712 = do { check_type env SigmaCtxt constraintMonoType pred
713 ; check_pred_help False env dflags ctxt pred }
714
715 check_pred_help :: Bool -- True <=> under a type synonym
716 -> TidyEnv
717 -> DynFlags -> UserTypeCtxt
718 -> PredType -> TcM ()
719 check_pred_help under_syn env dflags ctxt pred
720 | Just pred' <- coreView pred -- Switch on under_syn when going under a
721 -- synonym (Trac #9838, yuk)
722 = check_pred_help True env dflags ctxt pred'
723 | otherwise
724 = case splitTyConApp_maybe pred of
725 Just (tc, tys)
726 | isTupleTyCon tc
727 -> check_tuple_pred under_syn env dflags ctxt pred tys
728 -- NB: this equality check must come first, because (~) is a class,
729 -- too.
730 | tc `hasKey` heqTyConKey ||
731 tc `hasKey` eqTyConKey ||
732 tc `hasKey` eqPrimTyConKey
733 -> check_eq_pred env dflags pred tc tys
734 | Just cls <- tyConClass_maybe tc
735 -> check_class_pred env dflags ctxt pred cls tys -- Includes Coercible
736 _ -> check_irred_pred under_syn env dflags ctxt pred
737
738 check_eq_pred :: TidyEnv -> DynFlags -> PredType -> TyCon -> [TcType] -> TcM ()
739 check_eq_pred env dflags pred tc tys
740 = -- Equational constraints are valid in all contexts if type
741 -- families are permitted
742 do { checkTc (length tys == tyConArity tc) (tyConArityErr tc tys)
743 ; checkTcM (xopt LangExt.TypeFamilies dflags
744 || xopt LangExt.GADTs dflags)
745 (eqPredTyErr env pred) }
746
747 check_tuple_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> [PredType] -> TcM ()
748 check_tuple_pred under_syn env dflags ctxt pred ts
749 = do { -- See Note [ConstraintKinds in predicates]
750 checkTcM (under_syn || xopt LangExt.ConstraintKinds dflags)
751 (predTupleErr env pred)
752 ; mapM_ (check_pred_help under_syn env dflags ctxt) ts }
753 -- This case will not normally be executed because without
754 -- -XConstraintKinds tuple types are only kind-checked as *
755
756 check_irred_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
757 check_irred_pred under_syn env dflags ctxt pred
758 -- The predicate looks like (X t1 t2) or (x t1 t2) :: Constraint
759 -- where X is a type function
760 = do { -- If it looks like (x t1 t2), require ConstraintKinds
761 -- see Note [ConstraintKinds in predicates]
762 -- But (X t1 t2) is always ok because we just require ConstraintKinds
763 -- at the definition site (Trac #9838)
764 failIfTcM (not under_syn && not (xopt LangExt.ConstraintKinds dflags)
765 && hasTyVarHead pred)
766 (predIrredErr env pred)
767
768 -- Make sure it is OK to have an irred pred in this context
769 -- See Note [Irreducible predicates in superclasses]
770 ; failIfTcM (is_superclass ctxt
771 && not (xopt LangExt.UndecidableInstances dflags)
772 && has_tyfun_head pred)
773 (predSuperClassErr env pred) }
774 where
775 is_superclass ctxt = case ctxt of { ClassSCCtxt _ -> True; _ -> False }
776 has_tyfun_head ty
777 = case tcSplitTyConApp_maybe ty of
778 Just (tc, _) -> isTypeFamilyTyCon tc
779 Nothing -> False
780
781 {- Note [ConstraintKinds in predicates]
782 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
783 Don't check for -XConstraintKinds under a type synonym, because that
784 was done at the type synonym definition site; see Trac #9838
785 e.g. module A where
786 type C a = (Eq a, Ix a) -- Needs -XConstraintKinds
787 module B where
788 import A
789 f :: C a => a -> a -- Does *not* need -XConstraintKinds
790
791 Note [Irreducible predicates in superclasses]
792 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
793 Allowing type-family calls in class superclasses is somewhat dangerous
794 because we can write:
795
796 type family Fooish x :: * -> Constraint
797 type instance Fooish () = Foo
798 class Fooish () a => Foo a where
799
800 This will cause the constraint simplifier to loop because every time we canonicalise a
801 (Foo a) class constraint we add a (Fooish () a) constraint which will be immediately
802 solved to add+canonicalise another (Foo a) constraint. -}
803
804 -------------------------
805 check_class_pred :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> Class -> [TcType] -> TcM ()
806 check_class_pred env dflags ctxt pred cls tys
807 | isIPClass cls
808 = do { check_arity
809 ; checkTcM (okIPCtxt ctxt) (badIPPred env pred) }
810
811 | otherwise
812 = do { check_arity
813 ; check_simplifiable_class_constraint
814 ; checkTcM arg_tys_ok (predTyVarErr env pred) }
815 where
816 check_arity = checkTc (classArity cls == length tys)
817 (tyConArityErr (classTyCon cls) tys)
818
819 -- Check the arguments of a class constraint
820 flexible_contexts = xopt LangExt.FlexibleContexts dflags
821 undecidable_ok = xopt LangExt.UndecidableInstances dflags
822 arg_tys_ok = case ctxt of
823 SpecInstCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
824 InstDeclCtxt -> checkValidClsArgs (flexible_contexts || undecidable_ok) cls tys
825 -- Further checks on head and theta
826 -- in checkInstTermination
827 _ -> checkValidClsArgs flexible_contexts cls tys
828
829 -- See Note [Simplifiable given constraints]
830 check_simplifiable_class_constraint
831 | xopt LangExt.MonoLocalBinds dflags
832 = return ()
833 | DataTyCtxt {} <- ctxt -- Don't do this check for the "stupid theta"
834 = return () -- of a data type declaration
835 | otherwise
836 = do { instEnvs <- tcGetInstEnvs
837 ; let (matches, _, _) = lookupInstEnv False instEnvs cls tys
838 bad_matches = [ inst | (inst,_) <- matches
839 , not (isOverlappable inst) ]
840 ; warnIf (Reason Opt_WarnSimplifiableClassConstraints)
841 (not (null bad_matches))
842 (simplifiable_constraint_warn bad_matches) }
843
844 simplifiable_constraint_warn :: [ClsInst] -> SDoc
845 simplifiable_constraint_warn (match : _)
846 = vcat [ hang (text "The constraint" <+> quotes (ppr (tidyType env pred)))
847 2 (text "matches an instance declaration")
848 , ppr match
849 , hang (text "This makes type inference for inner bindings fragile;")
850 2 (text "either use MonoLocalBinds, or simplify it using the instance") ]
851 simplifiable_constraint_warn [] = pprPanic "check_class_pred" (ppr pred)
852
853 {- Note [Simplifiable given constraints]
854 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
855 A type signature like
856 f :: Eq [(a,b)] => a -> b
857 is very fragile, for reasons described at length in TcInteract
858 Note [Instance and Given overlap]. As that Note discusses, for the
859 most part the clever stuff in TcInteract means that we don't use a
860 top-level instance if a local Given might fire, so there is no
861 fragility. But if we /infer/ the type of a local let-binding, things
862 can go wrong (Trac #11948 is an example, discussed in the Note).
863
864 So this warning is switched on only if we have NoMonoLocalBinds; in
865 that case the warning discourages uses from writing simplifiable class
866 constraints, at least unless the top-level instance is explicitly
867 declared as OVERLAPPABLE.
868 -}
869
870 -------------------------
871 okIPCtxt :: UserTypeCtxt -> Bool
872 -- See Note [Implicit parameters in instance decls]
873 okIPCtxt (FunSigCtxt {}) = True
874 okIPCtxt (InfSigCtxt {}) = True
875 okIPCtxt ExprSigCtxt = True
876 okIPCtxt TypeAppCtxt = True
877 okIPCtxt PatSigCtxt = True
878 okIPCtxt ResSigCtxt = True
879 okIPCtxt GenSigCtxt = True
880 okIPCtxt (ConArgCtxt {}) = True
881 okIPCtxt (ForSigCtxt {}) = True -- ??
882 okIPCtxt ThBrackCtxt = True
883 okIPCtxt GhciCtxt = True
884 okIPCtxt SigmaCtxt = True
885 okIPCtxt (DataTyCtxt {}) = True
886 okIPCtxt (PatSynCtxt {}) = True
887 okIPCtxt (TySynCtxt {}) = True -- e.g. type Blah = ?x::Int
888 -- Trac #11466
889
890 okIPCtxt (ClassSCCtxt {}) = False
891 okIPCtxt (InstDeclCtxt {}) = False
892 okIPCtxt (SpecInstCtxt {}) = False
893 okIPCtxt (RuleSigCtxt {}) = False
894 okIPCtxt DefaultDeclCtxt = False
895
896 {-
897 Note [Kind polymorphic type classes]
898 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
899 MultiParam check:
900
901 class C f where... -- C :: forall k. k -> Constraint
902 instance C Maybe where...
903
904 The dictionary gets type [C * Maybe] even if it's not a MultiParam
905 type class.
906
907 Flexibility check:
908
909 class C f where... -- C :: forall k. k -> Constraint
910 data D a = D a
911 instance C D where
912
913 The dictionary gets type [C * (D *)]. IA0_TODO it should be
914 generalized actually.
915 -}
916
917 checkThetaCtxt :: UserTypeCtxt -> ThetaType -> TidyEnv -> TcM (TidyEnv, SDoc)
918 checkThetaCtxt ctxt theta env
919 = return ( env
920 , vcat [ text "In the context:" <+> pprTheta (tidyTypes env theta)
921 , text "While checking" <+> pprUserTypeCtxt ctxt ] )
922
923 eqPredTyErr, predTupleErr, predIrredErr, predSuperClassErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
924 eqPredTyErr env pred
925 = ( env
926 , text "Illegal equational constraint" <+> ppr_tidy env pred $$
927 parens (text "Use GADTs or TypeFamilies to permit this") )
928 predTupleErr env pred
929 = ( env
930 , hang (text "Illegal tuple constraint:" <+> ppr_tidy env pred)
931 2 (parens constraintKindsMsg) )
932 predIrredErr env pred
933 = ( env
934 , hang (text "Illegal constraint:" <+> ppr_tidy env pred)
935 2 (parens constraintKindsMsg) )
936 predSuperClassErr env pred
937 = ( env
938 , hang (text "Illegal constraint" <+> quotes (ppr_tidy env pred)
939 <+> text "in a superclass context")
940 2 (parens undecidableMsg) )
941
942 predTyVarErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
943 predTyVarErr env pred
944 = (env
945 , vcat [ hang (text "Non type-variable argument")
946 2 (text "in the constraint:" <+> ppr_tidy env pred)
947 , parens (text "Use FlexibleContexts to permit this") ])
948
949 badIPPred :: TidyEnv -> PredType -> (TidyEnv, SDoc)
950 badIPPred env pred
951 = ( env
952 , text "Illegal implicit parameter" <+> quotes (ppr_tidy env pred) )
953
954 constraintSynErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
955 constraintSynErr env kind
956 = ( env
957 , hang (text "Illegal constraint synonym of kind:" <+> quotes (ppr_tidy env kind))
958 2 (parens constraintKindsMsg) )
959
960 dupPredWarn :: TidyEnv -> [[PredType]] -> (TidyEnv, SDoc)
961 dupPredWarn env dups
962 = ( env
963 , text "Duplicate constraint" <> plural primaryDups <> text ":"
964 <+> pprWithCommas (ppr_tidy env) primaryDups )
965 where
966 primaryDups = map head dups
967
968 tyConArityErr :: TyCon -> [TcType] -> SDoc
969 -- For type-constructor arity errors, be careful to report
970 -- the number of /visible/ arguments required and supplied,
971 -- ignoring the /invisible/ arguments, which the user does not see.
972 -- (e.g. Trac #10516)
973 tyConArityErr tc tks
974 = arityErr (tyConFlavour tc) (tyConName tc)
975 tc_type_arity tc_type_args
976 where
977 vis_tks = filterOutInvisibleTypes tc tks
978
979 -- tc_type_arity = number of *type* args expected
980 -- tc_type_args = number of *type* args encountered
981 tc_type_arity = count isVisibleTyConBinder (tyConBinders tc)
982 tc_type_args = length vis_tks
983
984 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
985 arityErr what name n m
986 = hsep [ text "The" <+> text what, quotes (ppr name), text "should have",
987 n_arguments <> comma, text "but has been given",
988 if m==0 then text "none" else int m]
989 where
990 n_arguments | n == 0 = text "no arguments"
991 | n == 1 = text "1 argument"
992 | True = hsep [int n, text "arguments"]
993
994 {-
995 ************************************************************************
996 * *
997 \subsection{Checking for a decent instance head type}
998 * *
999 ************************************************************************
1000
1001 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1002 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1003
1004 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1005 flag is on, or (2)~the instance is imported (they must have been
1006 compiled elsewhere). In these cases, we let them go through anyway.
1007
1008 We can also have instances for functions: @instance Foo (a -> b) ...@.
1009 -}
1010
1011 checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
1012 checkValidInstHead ctxt clas cls_args
1013 = do { dflags <- getDynFlags
1014
1015 ; mod <- getModule
1016 ; checkTc (getUnique clas `notElem` abstractClassKeys ||
1017 nameModule (getName clas) == mod)
1018 (instTypeErr clas cls_args abstract_class_msg)
1019
1020 ; when (clas `hasKey` hasFieldClassNameKey) $
1021 checkHasFieldInst clas cls_args
1022
1023 -- Check language restrictions;
1024 -- but not for SPECIALISE instance pragmas
1025 ; let ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
1026 ; unless spec_inst_prag $
1027 do { checkTc (xopt LangExt.TypeSynonymInstances dflags ||
1028 all tcInstHeadTyNotSynonym ty_args)
1029 (instTypeErr clas cls_args head_type_synonym_msg)
1030 ; checkTc (xopt LangExt.FlexibleInstances dflags ||
1031 all tcInstHeadTyAppAllTyVars ty_args)
1032 (instTypeErr clas cls_args head_type_args_tyvars_msg)
1033 ; checkTc (xopt LangExt.MultiParamTypeClasses dflags ||
1034 length ty_args == 1 || -- Only count type arguments
1035 (xopt LangExt.NullaryTypeClasses dflags &&
1036 null ty_args))
1037 (instTypeErr clas cls_args head_one_type_msg) }
1038
1039 ; mapM_ checkValidTypePat ty_args }
1040 where
1041 spec_inst_prag = case ctxt of { SpecInstCtxt -> True; _ -> False }
1042
1043 head_type_synonym_msg = parens (
1044 text "All instance types must be of the form (T t1 ... tn)" $$
1045 text "where T is not a synonym." $$
1046 text "Use TypeSynonymInstances if you want to disable this.")
1047
1048 head_type_args_tyvars_msg = parens (vcat [
1049 text "All instance types must be of the form (T a1 ... an)",
1050 text "where a1 ... an are *distinct type variables*,",
1051 text "and each type variable appears at most once in the instance head.",
1052 text "Use FlexibleInstances if you want to disable this."])
1053
1054 head_one_type_msg = parens (
1055 text "Only one type can be given in an instance head." $$
1056 text "Use MultiParamTypeClasses if you want to allow more, or zero.")
1057
1058 abstract_class_msg =
1059 text "Manual instances of this class are not permitted."
1060
1061 tcInstHeadTyNotSynonym :: Type -> Bool
1062 -- Used in Haskell-98 mode, for the argument types of an instance head
1063 -- These must not be type synonyms, but everywhere else type synonyms
1064 -- are transparent, so we need a special function here
1065 tcInstHeadTyNotSynonym ty
1066 = case ty of -- Do not use splitTyConApp,
1067 -- because that expands synonyms!
1068 TyConApp tc _ -> not (isTypeSynonymTyCon tc)
1069 _ -> True
1070
1071 tcInstHeadTyAppAllTyVars :: Type -> Bool
1072 -- Used in Haskell-98 mode, for the argument types of an instance head
1073 -- These must be a constructor applied to type variable arguments.
1074 -- But we allow kind instantiations.
1075 tcInstHeadTyAppAllTyVars ty
1076 | Just (tc, tys) <- tcSplitTyConApp_maybe (dropCasts ty)
1077 = ok (filterOutInvisibleTypes tc tys) -- avoid kinds
1078
1079 | otherwise
1080 = False
1081 where
1082 -- Check that all the types are type variables,
1083 -- and that each is distinct
1084 ok tys = equalLength tvs tys && hasNoDups tvs
1085 where
1086 tvs = mapMaybe tcGetTyVar_maybe tys
1087
1088 dropCasts :: Type -> Type
1089 -- See Note [Casts during validity checking]
1090 -- This function can turn a well-kinded type into an ill-kinded
1091 -- one, so I've kept it local to this module
1092 -- To consider: drop only UnivCo(HoleProv) casts
1093 dropCasts (CastTy ty _) = dropCasts ty
1094 dropCasts (AppTy t1 t2) = mkAppTy (dropCasts t1) (dropCasts t2)
1095 dropCasts (FunTy t1 t2) = mkFunTy (dropCasts t1) (dropCasts t2)
1096 dropCasts (TyConApp tc tys) = mkTyConApp tc (map dropCasts tys)
1097 dropCasts (ForAllTy b ty) = ForAllTy (dropCastsB b) (dropCasts ty)
1098 dropCasts ty = ty -- LitTy, TyVarTy, CoercionTy
1099
1100 dropCastsB :: TyVarBinder -> TyVarBinder
1101 dropCastsB b = b -- Don't bother in the kind of a forall
1102
1103 abstractClassKeys :: [Unique]
1104 abstractClassKeys = [ heqTyConKey
1105 , eqTyConKey
1106 , coercibleTyConKey
1107 ] -- See Note [Equality class instances]
1108
1109 instTypeErr :: Class -> [Type] -> SDoc -> SDoc
1110 instTypeErr cls tys msg
1111 = hang (hang (text "Illegal instance declaration for")
1112 2 (quotes (pprClassPred cls tys)))
1113 2 msg
1114
1115 -- | See Note [Validity checking of HasField instances]
1116 checkHasFieldInst :: Class -> [Type] -> TcM ()
1117 checkHasFieldInst cls tys@[_k_ty, x_ty, r_ty, _a_ty] =
1118 case splitTyConApp_maybe r_ty of
1119 Nothing -> whoops (text "Record data type must be specified")
1120 Just (tc, _)
1121 | isFamilyTyCon tc
1122 -> whoops (text "Record data type may not be a data family")
1123 | otherwise -> case isStrLitTy x_ty of
1124 Just lbl
1125 | isJust (lookupTyConFieldLabel lbl tc)
1126 -> whoops (ppr tc <+> text "already has a field"
1127 <+> quotes (ppr lbl))
1128 | otherwise -> return ()
1129 Nothing
1130 | null (tyConFieldLabels tc) -> return ()
1131 | otherwise -> whoops (ppr tc <+> text "has fields")
1132 where
1133 whoops = addErrTc . instTypeErr cls tys
1134 checkHasFieldInst _ tys = pprPanic "checkHasFieldInst" (ppr tys)
1135
1136 {- Note [Casts during validity checking]
1137 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1138 Consider the (bogus)
1139 instance Eq Char#
1140 We elaborate to 'Eq (Char# |> UnivCo(hole))' where the hole is an
1141 insoluble equality constraint for * ~ #. We'll report the insoluble
1142 constraint separately, but we don't want to *also* complain that Eq is
1143 not applied to a type constructor. So we look gaily look through
1144 CastTys here.
1145
1146 Another example: Eq (Either a). Then we actually get a cast in
1147 the middle:
1148 Eq ((Either |> g) a)
1149
1150
1151 Note [Validity checking of HasField instances]
1152 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1153 The HasField class has magic constraint solving behaviour (see Note
1154 [HasField instances] in TcInteract). However, we permit users to
1155 declare their own instances, provided they do not clash with the
1156 built-in behaviour. In particular, we forbid:
1157
1158 1. `HasField _ r _` where r is a variable
1159
1160 2. `HasField _ (T ...) _` if T is a data family
1161 (because it might have fields introduced later)
1162
1163 3. `HasField x (T ...) _` where x is a variable,
1164 if T has any fields at all
1165
1166 4. `HasField "foo" (T ...) _` if T has a "foo" field
1167
1168 The usual functional dependency checks also apply.
1169
1170
1171 Note [Valid 'deriving' predicate]
1172 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1173 validDerivPred checks for OK 'deriving' context. See Note [Exotic
1174 derived instance contexts] in TcDeriv. However the predicate is
1175 here because it uses sizeTypes, fvTypes.
1176
1177 It checks for three things
1178
1179 * No repeated variables (hasNoDups fvs)
1180
1181 * No type constructors. This is done by comparing
1182 sizeTypes tys == length (fvTypes tys)
1183 sizeTypes counts variables and constructors; fvTypes returns variables.
1184 So if they are the same, there must be no constructors. But there
1185 might be applications thus (f (g x)).
1186
1187 Note that tys only includes the visible arguments of the class type
1188 constructor. Including the non-vivisble arguments can cause the following,
1189 perfectly valid instance to be rejected:
1190 class Category (cat :: k -> k -> *) where ...
1191 newtype T (c :: * -> * -> *) a b = MkT (c a b)
1192 instance Category c => Category (T c) where ...
1193 since the first argument to Category is a non-visible *, which sizeTypes
1194 would count as a constructor! See Trac #11833.
1195
1196 * Also check for a bizarre corner case, when the derived instance decl
1197 would look like
1198 instance C a b => D (T a) where ...
1199 Note that 'b' isn't a parameter of T. This gives rise to all sorts of
1200 problems; in particular, it's hard to compare solutions for equality
1201 when finding the fixpoint, and that means the inferContext loop does
1202 not converge. See Trac #5287.
1203
1204 Note [Equality class instances]
1205 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1206 We can't have users writing instances for the equality classes. But we
1207 still need to be able to write instances for them ourselves. So we allow
1208 instances only in the defining module.
1209
1210 -}
1211
1212 validDerivPred :: TyVarSet -> PredType -> Bool
1213 -- See Note [Valid 'deriving' predicate]
1214 validDerivPred tv_set pred
1215 = case classifyPredType pred of
1216 ClassPred cls tys -> cls `hasKey` typeableClassKey
1217 -- Typeable constraints are bigger than they appear due
1218 -- to kind polymorphism, but that's OK
1219 || check_tys cls tys
1220 EqPred {} -> False -- reject equality constraints
1221 _ -> True -- Non-class predicates are ok
1222 where
1223 check_tys cls tys
1224 = hasNoDups fvs
1225 -- use sizePred to ignore implicit args
1226 && sizePred pred == fromIntegral (length fvs)
1227 && all (`elemVarSet` tv_set) fvs
1228 where tys' = filterOutInvisibleTypes (classTyCon cls) tys
1229 fvs = fvTypes tys'
1230
1231 {-
1232 ************************************************************************
1233 * *
1234 \subsection{Checking instance for termination}
1235 * *
1236 ************************************************************************
1237 -}
1238
1239 {- Note [Instances and constraint synonyms]
1240 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1241 Currently, we don't allow instances for constraint synonyms at all.
1242 Consider these (Trac #13267):
1243 type C1 a = Show (a -> Bool)
1244 instance C1 Int where -- I1
1245 show _ = "ur"
1246
1247 This elicits "show is not a (visible) method of class C1", which isn't
1248 a great message. But it comes from the renamer, so it's hard to improve.
1249
1250 This needs a bit more care:
1251 type C2 a = (Show a, Show Int)
1252 instance C2 Int -- I2
1253
1254 If we use (splitTyConApp_maybe tau) in checkValidInstance to decompose
1255 the instance head, we'll expand the synonym on fly, and it'll look like
1256 instance (%,%) (Show Int, Show Int)
1257 and we /really/ don't want that. So we carefully do /not/ expand
1258 synonyms, by matching on TyConApp directly.
1259 -}
1260
1261 checkValidInstance :: UserTypeCtxt -> LHsSigType Name -> Type
1262 -> TcM ([TyVar], ThetaType, Class, [Type])
1263 checkValidInstance ctxt hs_type ty
1264 | not is_tc_app
1265 = failWithTc (text "Instance head is not headed by a class")
1266
1267 | isNothing mb_cls
1268 = failWithTc (vcat [ text "Illegal instance for a" <+> text (tyConFlavour tc)
1269 , text "A class instance must be for a class" ])
1270
1271 | not arity_ok
1272 = failWithTc (text "Arity mis-match in instance head")
1273
1274 | otherwise
1275 = do { setSrcSpan head_loc (checkValidInstHead ctxt clas inst_tys)
1276 ; traceTc "checkValidInstance {" (ppr ty)
1277 ; checkValidTheta ctxt theta
1278
1279 -- The Termination and Coverate Conditions
1280 -- Check that instance inference will terminate (if we care)
1281 -- For Haskell 98 this will already have been done by checkValidTheta,
1282 -- but as we may be using other extensions we need to check.
1283 --
1284 -- Note that the Termination Condition is *more conservative* than
1285 -- the checkAmbiguity test we do on other type signatures
1286 -- e.g. Bar a => Bar Int is ambiguous, but it also fails
1287 -- the termination condition, because 'a' appears more often
1288 -- in the constraint than in the head
1289 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1290 ; if undecidable_ok
1291 then checkAmbiguity ctxt ty
1292 else checkInstTermination inst_tys theta
1293
1294 ; traceTc "cvi 2" (ppr ty)
1295
1296 ; case (checkInstCoverage undecidable_ok clas theta inst_tys) of
1297 IsValid -> return () -- Check succeeded
1298 NotValid msg -> addErrTc (instTypeErr clas inst_tys msg)
1299
1300 ; traceTc "End checkValidInstance }" empty
1301
1302 ; return (tvs, theta, clas, inst_tys) }
1303 where
1304 (tvs, theta, tau) = tcSplitSigmaTy ty
1305 is_tc_app = case tau of { TyConApp {} -> True; _ -> False }
1306 TyConApp tc inst_tys = tau -- See Note [Instances and constraint synonyms]
1307 mb_cls = tyConClass_maybe tc
1308 Just clas = mb_cls
1309 arity_ok = inst_tys `lengthIs` classArity clas
1310
1311 -- The location of the "head" of the instance
1312 head_loc = getLoc (getLHsInstDeclHead hs_type)
1313
1314 {-
1315 Note [Paterson conditions]
1316 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1317 Termination test: the so-called "Paterson conditions" (see Section 5 of
1318 "Understanding functional dependencies via Constraint Handling Rules,
1319 JFP Jan 2007).
1320
1321 We check that each assertion in the context satisfies:
1322 (1) no variable has more occurrences in the assertion than in the head, and
1323 (2) the assertion has fewer constructors and variables (taken together
1324 and counting repetitions) than the head.
1325 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1326 (which have already been checked) guarantee termination.
1327
1328 The underlying idea is that
1329
1330 for any ground substitution, each assertion in the
1331 context has fewer type constructors than the head.
1332 -}
1333
1334 checkInstTermination :: [TcType] -> ThetaType -> TcM ()
1335 -- See Note [Paterson conditions]
1336 checkInstTermination tys theta
1337 = check_preds theta
1338 where
1339 head_fvs = fvTypes tys
1340 head_size = sizeTypes tys
1341
1342 check_preds :: [PredType] -> TcM ()
1343 check_preds preds = mapM_ check preds
1344
1345 check :: PredType -> TcM ()
1346 check pred
1347 = case classifyPredType pred of
1348 EqPred {} -> return () -- See Trac #4200.
1349 IrredPred {} -> check2 pred (sizeType pred)
1350 ClassPred cls tys
1351 | isTerminatingClass cls
1352 -> return ()
1353
1354 | isCTupleClass cls -- Look inside tuple predicates; Trac #8359
1355 -> check_preds tys
1356
1357 | otherwise
1358 -> check2 pred (sizeTypes $ filterOutInvisibleTypes (classTyCon cls) tys)
1359 -- Other ClassPreds
1360
1361 check2 pred pred_size
1362 | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what)
1363 | pred_size >= head_size = addErrTc (smallerMsg what)
1364 | otherwise = return ()
1365 where
1366 what = text "constraint" <+> quotes (ppr pred)
1367 bad_tvs = fvType pred \\ head_fvs
1368
1369 smallerMsg :: SDoc -> SDoc
1370 smallerMsg what
1371 = vcat [ hang (text "The" <+> what)
1372 2 (text "is no smaller than the instance head")
1373 , parens undecidableMsg ]
1374
1375 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc
1376 noMoreMsg tvs what
1377 = vcat [ hang (text "Variable" <> plural tvs <+> quotes (pprWithCommas ppr tvs)
1378 <+> occurs <+> text "more often")
1379 2 (sep [ text "in the" <+> what
1380 , text "than in the instance head" ])
1381 , parens undecidableMsg ]
1382 where
1383 occurs = if isSingleton tvs then text "occurs"
1384 else text "occur"
1385
1386 undecidableMsg, constraintKindsMsg :: SDoc
1387 undecidableMsg = text "Use UndecidableInstances to permit this"
1388 constraintKindsMsg = text "Use ConstraintKinds to permit this"
1389
1390 {-
1391 Note [Associated type instances]
1392 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1393 We allow this:
1394 class C a where
1395 type T x a
1396 instance C Int where
1397 type T (S y) Int = y
1398 type T Z Int = Char
1399
1400 Note that
1401 a) The variable 'x' is not bound by the class decl
1402 b) 'x' is instantiated to a non-type-variable in the instance
1403 c) There are several type instance decls for T in the instance
1404
1405 All this is fine. Of course, you can't give any *more* instances
1406 for (T ty Int) elsewhere, because it's an *associated* type.
1407
1408 Note [Checking consistent instantiation]
1409 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1410 See Trac #11450 for background discussion on this check.
1411
1412 class C a b where
1413 type T a x b
1414
1415 With this class decl, if we have an instance decl
1416 instance C ty1 ty2 where ...
1417 then the type instance must look like
1418 type T ty1 v ty2 = ...
1419 with exactly 'ty1' for 'a', 'ty2' for 'b', and some type 'v' for 'x'.
1420 For example:
1421
1422 instance C [p] Int
1423 type T [p] y Int = (p,y,y)
1424
1425 Note that
1426
1427 * We used to allow completely different bound variables in the
1428 associated type instance; e.g.
1429 instance C [p] Int
1430 type T [q] y Int = ...
1431 But from GHC 8.2 onwards, we don't. It's much simpler this way.
1432 See Trac #11450.
1433
1434 * When the class variable isn't used on the RHS of the type instance,
1435 it's tempting to allow wildcards, thus
1436 instance C [p] Int
1437 type T [_] y Int = (y,y)
1438 But it's awkward to do the test, and it doesn't work if the
1439 variable is repeated:
1440 instance C (p,p) Int
1441 type T (_,_) y Int = (y,y)
1442 Even though 'p' is not used on the RHS, we still need to use 'p'
1443 on the LHS to establish the repeated pattern. So to keep it simple
1444 we just require equality.
1445
1446 * For variables in associated type families that are not bound by the class
1447 itself, we do _not_ check if they are over-specific. In other words,
1448 it's perfectly acceptable to have an instance like this:
1449
1450 instance C [p] Int where
1451 type T [p] (Maybe x) Int = x
1452
1453 While the first and third arguments to T are required to be exactly [p] and
1454 Int, respectively, since they are bound by C, the second argument is allowed
1455 to be more specific than just a type variable. Furthermore, it is permissible
1456 to define multiple equations for T that differ only in the non-class-bound
1457 argument:
1458
1459 instance C [p] Int where
1460 type T [p] (Maybe x) Int = x
1461 type T [p] (Either x y) Int = x -> y
1462
1463 We once considered requiring that non-class-bound variables in associated
1464 type family instances be instantiated with distinct type variables. However,
1465 that requirement proved too restrictive in practice, as there were examples
1466 of extremely simple associated type family instances that this check would
1467 reject, and fixing them required tiresome boilerplate in the form of
1468 auxiliary type families. For instance, you would have to define the above
1469 example as:
1470
1471 instance C [p] Int where
1472 type T [p] x Int = CAux x
1473
1474 type family CAux x where
1475 CAux (Maybe x) = x
1476 CAux (Either x y) = x -> y
1477
1478 We decided that this restriction wasn't buying us much, so we opted not
1479 to pursue that design (see also GHC Trac #13398).
1480
1481 Implementation
1482 * Form the mini-envt from the class type variables a,b
1483 to the instance decl types [p],Int: [a->[p], b->Int]
1484
1485 * Look at the tyvars a,x,b of the type family constructor T
1486 (it shares tyvars with the class C)
1487
1488 * Apply the mini-evnt to them, and check that the result is
1489 consistent with the instance types [p] y Int. (where y can be any type, as
1490 it is not scoped over the class type variables.
1491
1492 We make all the instance type variables scope over the
1493 type instances, of course, which picks up non-obvious kinds. Eg
1494 class Foo (a :: k) where
1495 type F a
1496 instance Foo (b :: k -> k) where
1497 type F b = Int
1498 Here the instance is kind-indexed and really looks like
1499 type F (k->k) (b::k->k) = Int
1500 But if the 'b' didn't scope, we would make F's instance too
1501 poly-kinded.
1502 -}
1503
1504 -- | Extra information about the parent instance declaration, needed
1505 -- when type-checking associated types. The 'Class' is the enclosing
1506 -- class, the [TyVar] are the type variable of the instance decl,
1507 -- and and the @VarEnv Type@ maps class variables to their instance
1508 -- types.
1509 type ClsInstInfo = (Class, [TyVar], VarEnv Type)
1510
1511 type AssocInstArgShape = (Maybe Type, Type)
1512 -- AssocInstArgShape is used only for associated family instances
1513 -- (mb_exp, actual)
1514 -- mb_exp = Just ty => this arg corresponds to a class variable
1515 -- = Nothing => it doesn't correspond to a class variable
1516 -- e.g. class C b where
1517 -- type F a b c
1518 -- instance C [x] where
1519 -- type F p [x] q
1520 -- We get [AssocInstArgShape] = [ (Nothing, p)
1521 -- , (Just [x], [x])
1522 -- , (Nothing, q)]
1523
1524 checkConsistentFamInst
1525 :: Maybe ClsInstInfo
1526 -> TyCon -- ^ Family tycon
1527 -> [TyVar] -- ^ Type variables of the family instance
1528 -> [Type] -- ^ Type patterns from instance
1529 -> TcM ()
1530 -- See Note [Checking consistent instantiation]
1531
1532 checkConsistentFamInst Nothing _ _ _ = return ()
1533 checkConsistentFamInst (Just (clas, inst_tvs, mini_env)) fam_tc _at_tvs at_tys
1534 = do { -- Check that the associated type indeed comes from this class
1535 checkTc (Just clas == tyConAssoc_maybe fam_tc)
1536 (badATErr (className clas) (tyConName fam_tc))
1537
1538 -- Check type args first (more comprehensible)
1539 ; checkTc (all check_arg type_shapes) pp_wrong_at_arg
1540
1541 -- And now kind args
1542 ; checkTc (all check_arg kind_shapes)
1543 (pp_wrong_at_arg $$ ppSuggestExplicitKinds)
1544
1545 ; traceTc "cfi" (vcat [ ppr inst_tvs
1546 , ppr arg_shapes
1547 , ppr mini_env ]) }
1548 where
1549 arg_shapes :: [AssocInstArgShape]
1550 arg_shapes = [ (lookupVarEnv mini_env fam_tc_tv, at_ty)
1551 | (fam_tc_tv, at_ty) <- tyConTyVars fam_tc `zip` at_tys ]
1552
1553 (kind_shapes, type_shapes) = partitionInvisibles fam_tc snd arg_shapes
1554
1555 check_arg :: AssocInstArgShape -> Bool
1556 check_arg (Just exp_ty, at_ty) = exp_ty `tcEqType` at_ty
1557 check_arg (Nothing, _ ) = True -- Arg position does not correspond
1558 -- to a class variable
1559
1560 pp_wrong_at_arg
1561 = vcat [ text "Type indexes must match class instance head"
1562 , pp_exp_act ]
1563
1564 pp_exp_act
1565 = vcat [ text "Expected:" <+> ppr (mkTyConApp fam_tc expected_args)
1566 , text " Actual:" <+> ppr (mkTyConApp fam_tc at_tys)
1567 , sdocWithDynFlags $ \dflags ->
1568 ppWhen (has_poly_args dflags) $
1569 vcat [ text "where the `<tv>' arguments are type variables,"
1570 , text "distinct from each other and from the instance variables" ] ]
1571
1572 expected_args = [ exp_ty `orElse` mk_tv at_ty | (exp_ty, at_ty) <- arg_shapes ]
1573 mk_tv at_ty = mkTyVarTy (mkTyVar tv_name (typeKind at_ty))
1574 tv_name = mkInternalName (mkAlphaTyVarUnique 1) (mkTyVarOcc "<tv>") noSrcSpan
1575
1576 has_poly_args dflags = any (isNothing . fst) shapes
1577 where
1578 shapes | gopt Opt_PrintExplicitKinds dflags = arg_shapes
1579 | otherwise = type_shapes
1580
1581 badATErr :: Name -> Name -> SDoc
1582 badATErr clas op
1583 = hsep [text "Class", quotes (ppr clas),
1584 text "does not have an associated type", quotes (ppr op)]
1585
1586
1587 {-
1588 ************************************************************************
1589 * *
1590 Checking type instance well-formedness and termination
1591 * *
1592 ************************************************************************
1593 -}
1594
1595 checkValidCoAxiom :: CoAxiom Branched -> TcM ()
1596 checkValidCoAxiom ax@(CoAxiom { co_ax_tc = fam_tc, co_ax_branches = branches })
1597 = do { mapM_ (checkValidCoAxBranch Nothing fam_tc) branch_list
1598 ; foldlM_ check_branch_compat [] branch_list }
1599 where
1600 branch_list = fromBranches branches
1601 injectivity = familyTyConInjectivityInfo fam_tc
1602
1603 check_branch_compat :: [CoAxBranch] -- previous branches in reverse order
1604 -> CoAxBranch -- current branch
1605 -> TcM [CoAxBranch]-- current branch : previous branches
1606 -- Check for
1607 -- (a) this branch is dominated by previous ones
1608 -- (b) failure of injectivity
1609 check_branch_compat prev_branches cur_branch
1610 | cur_branch `isDominatedBy` prev_branches
1611 = do { addWarnAt NoReason (coAxBranchSpan cur_branch) $
1612 inaccessibleCoAxBranch ax cur_branch
1613 ; return prev_branches }
1614 | otherwise
1615 = do { check_injectivity prev_branches cur_branch
1616 ; return (cur_branch : prev_branches) }
1617
1618 -- Injectivity check: check whether a new (CoAxBranch) can extend
1619 -- already checked equations without violating injectivity
1620 -- annotation supplied by the user.
1621 -- See Note [Verifying injectivity annotation] in FamInstEnv
1622 check_injectivity prev_branches cur_branch
1623 | Injective inj <- injectivity
1624 = do { let conflicts =
1625 fst $ foldl (gather_conflicts inj prev_branches cur_branch)
1626 ([], 0) prev_branches
1627 ; mapM_ (\(err, span) -> setSrcSpan span $ addErr err)
1628 (makeInjectivityErrors ax cur_branch inj conflicts) }
1629 | otherwise
1630 = return ()
1631
1632 gather_conflicts inj prev_branches cur_branch (acc, n) branch
1633 -- n is 0-based index of branch in prev_branches
1634 = case injectiveBranches inj cur_branch branch of
1635 InjectivityUnified ax1 ax2
1636 | ax1 `isDominatedBy` (replace_br prev_branches n ax2)
1637 -> (acc, n + 1)
1638 | otherwise
1639 -> (branch : acc, n + 1)
1640 InjectivityAccepted -> (acc, n + 1)
1641
1642 -- Replace n-th element in the list. Assumes 0-based indexing.
1643 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
1644 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
1645
1646
1647 -- Check that a "type instance" is well-formed (which includes decidability
1648 -- unless -XUndecidableInstances is given).
1649 --
1650 checkValidCoAxBranch :: Maybe ClsInstInfo
1651 -> TyCon -> CoAxBranch -> TcM ()
1652 checkValidCoAxBranch mb_clsinfo fam_tc
1653 (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
1654 , cab_lhs = typats
1655 , cab_rhs = rhs, cab_loc = loc })
1656 = checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1657
1658 -- | Do validity checks on a type family equation, including consistency
1659 -- with any enclosing class instance head, termination, and lack of
1660 -- polytypes.
1661 checkValidTyFamEqn :: Maybe ClsInstInfo
1662 -> TyCon -- ^ of the type family
1663 -> [TyVar] -- ^ bound tyvars in the equation
1664 -> [CoVar] -- ^ bound covars in the equation
1665 -> [Type] -- ^ type patterns
1666 -> Type -- ^ rhs
1667 -> SrcSpan
1668 -> TcM ()
1669 checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1670 = setSrcSpan loc $
1671 do { checkValidFamPats mb_clsinfo fam_tc tvs cvs typats
1672
1673 -- The argument patterns, and RHS, are all boxed tau types
1674 -- E.g Reject type family F (a :: k1) :: k2
1675 -- type instance F (forall a. a->a) = ...
1676 -- type instance F Int# = ...
1677 -- type instance F Int = forall a. a->a
1678 -- type instance F Int = Int#
1679 -- See Trac #9357
1680 ; checkValidMonoType rhs
1681
1682 -- We have a decidable instance unless otherwise permitted
1683 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1684 ; unless undecidable_ok $
1685 mapM_ addErrTc (checkFamInstRhs typats (tcTyFamInsts rhs)) }
1686
1687 -- Make sure that each type family application is
1688 -- (1) strictly smaller than the lhs,
1689 -- (2) mentions no type variable more often than the lhs, and
1690 -- (3) does not contain any further type family instances.
1691 --
1692 checkFamInstRhs :: [Type] -- lhs
1693 -> [(TyCon, [Type])] -- type family instances
1694 -> [MsgDoc]
1695 checkFamInstRhs lhsTys famInsts
1696 = mapMaybe check famInsts
1697 where
1698 size = sizeTypes lhsTys
1699 fvs = fvTypes lhsTys
1700 check (tc, tys)
1701 | not (all isTyFamFree tys) = Just (nestedMsg what)
1702 | not (null bad_tvs) = Just (noMoreMsg bad_tvs what)
1703 | size <= sizeTypes tys = Just (smallerMsg what)
1704 | otherwise = Nothing
1705 where
1706 what = text "type family application" <+> quotes (pprType (TyConApp tc tys))
1707 bad_tvs = fvTypes tys \\ fvs
1708
1709 checkValidFamPats :: Maybe ClsInstInfo -> TyCon -> [TyVar] -> [CoVar] -> [Type] -> TcM ()
1710 -- Patterns in a 'type instance' or 'data instance' decl should
1711 -- a) contain no type family applications
1712 -- (vanilla synonyms are fine, though)
1713 -- b) properly bind all their free type variables
1714 -- e.g. we disallow (Trac #7536)
1715 -- type T a = Int
1716 -- type instance F (T a) = a
1717 -- c) Have the right number of patterns
1718 -- d) For associated types, are consistently instantiated
1719 checkValidFamPats mb_clsinfo fam_tc tvs cvs ty_pats
1720 = do { -- A family instance must have exactly the same number of type
1721 -- parameters as the family declaration. You can't write
1722 -- type family F a :: * -> *
1723 -- type instance F Int y = y
1724 -- because then the type (F Int) would be like (\y.y)
1725 checkTc (length ty_pats == fam_arity) $
1726 wrongNumberOfParmsErr (fam_arity - count isInvisibleTyConBinder fam_bndrs)
1727 -- report only explicit arguments
1728
1729 ; mapM_ checkValidTypePat ty_pats
1730
1731 ; let unbound_tcvs = filterOut (`elemVarSet` exactTyCoVarsOfTypes ty_pats) (tvs ++ cvs)
1732 ; checkTc (null unbound_tcvs) (famPatErr fam_tc unbound_tcvs ty_pats)
1733
1734 -- Check that type patterns match the class instance head
1735 ; checkConsistentFamInst mb_clsinfo fam_tc tvs ty_pats }
1736 where
1737 fam_arity = tyConArity fam_tc
1738 fam_bndrs = tyConBinders fam_tc
1739
1740
1741 checkValidTypePat :: Type -> TcM ()
1742 -- Used for type patterns in class instances,
1743 -- and in type/data family instances
1744 checkValidTypePat pat_ty
1745 = do { -- Check that pat_ty is a monotype
1746 checkValidMonoType pat_ty
1747 -- One could imagine generalising to allow
1748 -- instance C (forall a. a->a)
1749 -- but we don't know what all the consequences might be
1750
1751 -- Ensure that no type family instances occur a type pattern
1752 ; checkTc (isTyFamFree pat_ty) $
1753 tyFamInstIllegalErr pat_ty }
1754
1755 isTyFamFree :: Type -> Bool
1756 -- ^ Check that a type does not contain any type family applications.
1757 isTyFamFree = null . tcTyFamInsts
1758
1759 -- Error messages
1760
1761 wrongNumberOfParmsErr :: Arity -> SDoc
1762 wrongNumberOfParmsErr exp_arity
1763 = text "Number of parameters must match family declaration; expected"
1764 <+> ppr exp_arity
1765
1766 inaccessibleCoAxBranch :: CoAxiom br -> CoAxBranch -> SDoc
1767 inaccessibleCoAxBranch fi_ax cur_branch
1768 = text "Type family instance equation is overlapped:" $$
1769 nest 2 (pprCoAxBranch fi_ax cur_branch)
1770
1771 tyFamInstIllegalErr :: Type -> SDoc
1772 tyFamInstIllegalErr ty
1773 = hang (text "Illegal type synonym family application in instance" <>
1774 colon) 2 $
1775 ppr ty
1776
1777 nestedMsg :: SDoc -> SDoc
1778 nestedMsg what
1779 = sep [ text "Illegal nested" <+> what
1780 , parens undecidableMsg ]
1781
1782 famPatErr :: TyCon -> [TyVar] -> [Type] -> SDoc
1783 famPatErr fam_tc tvs pats
1784 = hang (text "Family instance purports to bind type variable" <> plural tvs
1785 <+> pprQuotedList tvs)
1786 2 (hang (text "but the real LHS (expanding synonyms) is:")
1787 2 (pprTypeApp fam_tc (map expandTypeSynonyms pats) <+>
1788 text "= ..."))
1789
1790 {-
1791 ************************************************************************
1792 * *
1793 Telescope checking
1794 * *
1795 ************************************************************************
1796
1797 Note [Bad telescopes]
1798 ~~~~~~~~~~~~~~~~~~~~~
1799 Now that we can mix type and kind variables, there are an awful lot of
1800 ways to shoot yourself in the foot. Here are some.
1801
1802 data SameKind :: k -> k -> * -- just to force unification
1803
1804 1. data T1 a k (b :: k) (x :: SameKind a b)
1805
1806 The problem here is that we discover that a and b should have the same
1807 kind. But this kind mentions k, which is bound *after* a.
1808 (Testcase: dependent/should_fail/BadTelescope)
1809
1810 2. data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)
1811
1812 Note that b is not bound. Yet its kind mentions a. Because we have
1813 a nice rule that all implicitly bound variables come before others,
1814 this is bogus. (We could probably figure out to put b between a and c.
1815 But I think this is doing users a disservice, in the long run.)
1816 (Testcase: dependent/should_fail/BadTelescope4)
1817
1818 3. t3 :: forall a. (forall k (b :: k). SameKind a b) -> ()
1819
1820 This is a straightforward skolem escape. Note that a and b need to have
1821 the same kind.
1822 (Testcase: polykinds/T11142)
1823
1824 How do we deal with all of this? For TyCons, we have checkValidTyConTyVars.
1825 That function looks to see if any of the tyConTyVars are repeated, but
1826 it's really a telescope check. It works because all tycons are kind-generalized.
1827 If there is a bad telescope, the kind-generalization will end up generalizing
1828 over a variable bound later in the telescope.
1829
1830 For non-tycons, we do scope checking when we bring tyvars into scope,
1831 in tcImplicitTKBndrs and tcExplicitTKBndrs. Note that we also have to
1832 sort implicit binders into a well-scoped order whenever we have implicit
1833 binders to worry about. This is done in quantifyTyVars and in
1834 tcImplicitTKBndrs.
1835 -}
1836
1837 -- | Check a list of binders to see if they make a valid telescope.
1838 -- The key property we're checking for is scoping. For example:
1839 -- > data SameKind :: k -> k -> *
1840 -- > data X a k (b :: k) (c :: SameKind a b)
1841 -- Kind inference says that a's kind should be k. But that's impossible,
1842 -- because k isn't in scope when a is bound. This check has to come before
1843 -- general validity checking, because once we kind-generalise, this sort
1844 -- of problem is harder to spot (as we'll generalise over the unbound
1845 -- k in a's type.) See also Note [Bad telescopes].
1846 checkValidTelescope :: SDoc -- the original user-written telescope
1847 -> [TyVar] -- explicit vars (not necessarily zonked)
1848 -> SDoc -- note to put at bottom of message
1849 -> TcM ()
1850 checkValidTelescope hs_tvs orig_tvs extra
1851 = discardResult $ checkZonkValidTelescope hs_tvs orig_tvs extra
1852
1853 -- | Like 'checkZonkValidTelescope', but returns the zonked tyvars
1854 checkZonkValidTelescope :: SDoc
1855 -> [TyVar]
1856 -> SDoc
1857 -> TcM [TyVar]
1858 checkZonkValidTelescope hs_tvs orig_tvs extra
1859 = do { orig_tvs <- mapM zonkTyCoVarKind orig_tvs
1860 ; let (_, sorted_tidied_tvs) = tidyTyCoVarBndrs emptyTidyEnv $
1861 toposortTyVars orig_tvs
1862 ; unless (go [] emptyVarSet orig_tvs) $
1863 addErr $
1864 vcat [ hang (text "These kind and type variables:" <+> hs_tvs $$
1865 text "are out of dependency order. Perhaps try this ordering:")
1866 2 (sep (map pprTyVar sorted_tidied_tvs))
1867 , extra ]
1868 ; return orig_tvs }
1869
1870 where
1871 go :: [TyVar] -- misplaced variables
1872 -> TyVarSet -> [TyVar] -> Bool
1873 go errs in_scope [] = null (filter (`elemVarSet` in_scope) errs)
1874 -- report an error only when the variable in the kind is brought
1875 -- into scope later in the telescope. Otherwise, we'll just quantify
1876 -- over it in kindGeneralize, as we should.
1877
1878 go errs in_scope (tv:tvs)
1879 = let bad_tvs = filterOut (`elemVarSet` in_scope) $
1880 tyCoVarsOfTypeList (tyVarKind tv)
1881 in go (bad_tvs ++ errs) (in_scope `extendVarSet` tv) tvs
1882
1883 -- | After inferring kinds of type variables, check to make sure that the
1884 -- inferred kinds any of the type variables bound in a smaller scope.
1885 -- This is a skolem escape check. See also Note [Bad telescopes].
1886 checkValidInferredKinds :: [TyVar] -- ^ vars to check (zonked)
1887 -> TyVarSet -- ^ vars out of scope
1888 -> SDoc -- ^ suffix to error message
1889 -> TcM ()
1890 checkValidInferredKinds orig_kvs out_of_scope extra
1891 = do { let bad_pairs = [ (tv, kv)
1892 | kv <- orig_kvs
1893 , Just tv <- map (lookupVarSet out_of_scope)
1894 (tyCoVarsOfTypeList (tyVarKind kv)) ]
1895 report (tidyTyVarOcc env -> tv, tidyTyVarOcc env -> kv)
1896 = addErr $
1897 text "The kind of variable" <+>
1898 quotes (ppr kv) <> text ", namely" <+>
1899 quotes (ppr (tyVarKind kv)) <> comma $$
1900 text "depends on variable" <+>
1901 quotes (ppr tv) <+> text "from an inner scope" $$
1902 text "Perhaps bind" <+> quotes (ppr kv) <+>
1903 text "sometime after binding" <+>
1904 quotes (ppr tv) $$
1905 extra
1906 ; mapM_ report bad_pairs }
1907
1908 where
1909 (env1, _) = tidyTyCoVarBndrs emptyTidyEnv orig_kvs
1910 (env, _) = tidyTyCoVarBndrs env1 (nonDetEltsUniqSet out_of_scope)
1911 -- It's OK to use nonDetEltsUniqSet here because it's only used for
1912 -- generating the error message
1913
1914 {-
1915 ************************************************************************
1916 * *
1917 \subsection{Auxiliary functions}
1918 * *
1919 ************************************************************************
1920 -}
1921
1922 -- Free variables of a type, retaining repetitions, and expanding synonyms
1923 fvType :: Type -> [TyCoVar]
1924 fvType ty | Just exp_ty <- coreView ty = fvType exp_ty
1925 fvType (TyVarTy tv) = [tv]
1926 fvType (TyConApp _ tys) = fvTypes tys
1927 fvType (LitTy {}) = []
1928 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1929 fvType (FunTy arg res) = fvType arg ++ fvType res
1930 fvType (ForAllTy (TvBndr tv _) ty)
1931 = fvType (tyVarKind tv) ++
1932 filter (/= tv) (fvType ty)
1933 fvType (CastTy ty co) = fvType ty ++ fvCo co
1934 fvType (CoercionTy co) = fvCo co
1935
1936 fvTypes :: [Type] -> [TyVar]
1937 fvTypes tys = concat (map fvType tys)
1938
1939 fvCo :: Coercion -> [TyCoVar]
1940 fvCo (Refl _ ty) = fvType ty
1941 fvCo (TyConAppCo _ _ args) = concatMap fvCo args
1942 fvCo (AppCo co arg) = fvCo co ++ fvCo arg
1943 fvCo (ForAllCo tv h co) = filter (/= tv) (fvCo co) ++ fvCo h
1944 fvCo (FunCo _ co1 co2) = fvCo co1 ++ fvCo co2
1945 fvCo (CoVarCo v) = [v]
1946 fvCo (AxiomInstCo _ _ args) = concatMap fvCo args
1947 fvCo (UnivCo p _ t1 t2) = fvProv p ++ fvType t1 ++ fvType t2
1948 fvCo (SymCo co) = fvCo co
1949 fvCo (TransCo co1 co2) = fvCo co1 ++ fvCo co2
1950 fvCo (NthCo _ co) = fvCo co
1951 fvCo (LRCo _ co) = fvCo co
1952 fvCo (InstCo co arg) = fvCo co ++ fvCo arg
1953 fvCo (CoherenceCo co1 co2) = fvCo co1 ++ fvCo co2
1954 fvCo (KindCo co) = fvCo co
1955 fvCo (SubCo co) = fvCo co
1956 fvCo (AxiomRuleCo _ cs) = concatMap fvCo cs
1957
1958 fvProv :: UnivCoProvenance -> [TyCoVar]
1959 fvProv UnsafeCoerceProv = []
1960 fvProv (PhantomProv co) = fvCo co
1961 fvProv (ProofIrrelProv co) = fvCo co
1962 fvProv (PluginProv _) = []
1963 fvProv (HoleProv h) = pprPanic "fvProv falls into a hole" (ppr h)
1964
1965 sizeType :: Type -> Int
1966 -- Size of a type: the number of variables and constructors
1967 sizeType ty | Just exp_ty <- coreView ty = sizeType exp_ty
1968 sizeType (TyVarTy {}) = 1
1969 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1970 sizeType (LitTy {}) = 1
1971 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1972 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1973 sizeType (ForAllTy _ ty) = sizeType ty
1974 sizeType (CastTy ty _) = sizeType ty
1975 sizeType (CoercionTy _) = 1
1976
1977 sizeTypes :: [Type] -> Int
1978 sizeTypes = sum . map sizeType
1979
1980 -- Size of a predicate
1981 --
1982 -- We are considering whether class constraints terminate.
1983 -- Equality constraints and constraints for the implicit
1984 -- parameter class always termiante so it is safe to say "size 0".
1985 -- (Implicit parameter constraints always terminate because
1986 -- there are no instances for them---they are only solved by
1987 -- "local instances" in expressions).
1988 -- See Trac #4200.
1989 sizePred :: PredType -> Int
1990 sizePred ty = goClass ty
1991 where
1992 goClass p = go (classifyPredType p)
1993
1994 go (ClassPred cls tys')
1995 | isTerminatingClass cls = 0
1996 | otherwise = sizeTypes (filterOutInvisibleTypes (classTyCon cls) tys')
1997 go (EqPred {}) = 0
1998 go (IrredPred ty) = sizeType ty
1999
2000 -- | When this says "True", ignore this class constraint during
2001 -- a termination check
2002 isTerminatingClass :: Class -> Bool
2003 isTerminatingClass cls
2004 = isIPClass cls
2005 || cls `hasKey` typeableClassKey
2006 || cls `hasKey` coercibleTyConKey
2007 || cls `hasKey` eqTyConKey
2008 || cls `hasKey` heqTyConKey
2009
2010 -- | Tidy before printing a type
2011 ppr_tidy :: TidyEnv -> Type -> SDoc
2012 ppr_tidy env ty = pprType (tidyType env ty)
2013
2014 allDistinctTyVars :: TyVarSet -> [KindOrType] -> Bool
2015 -- (allDistinctTyVars tvs tys) returns True if tys are
2016 -- a) all tyvars
2017 -- b) all distinct
2018 -- c) disjoint from tvs
2019 allDistinctTyVars _ [] = True
2020 allDistinctTyVars tkvs (ty : tys)
2021 = case getTyVar_maybe ty of
2022 Nothing -> False
2023 Just tv | tv `elemVarSet` tkvs -> False
2024 | otherwise -> allDistinctTyVars (tkvs `extendVarSet` tv) tys