Re-add FunTy (big patch)
[ghc.git] / compiler / typecheck / TcValidity.hs
1 {-
2 (c) The University of Glasgow 2006
3 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 -}
5
6 {-# LANGUAGE CPP, TupleSections, ViewPatterns #-}
7
8 module TcValidity (
9 Rank, UserTypeCtxt(..), checkValidType, checkValidMonoType,
10 ContextKind(..), expectedKindInCtxt,
11 checkValidTheta, checkValidFamPats,
12 checkValidInstance, validDerivPred,
13 checkInstTermination, checkTySynRhs,
14 ClsInstInfo, checkValidCoAxiom, checkValidCoAxBranch,
15 checkValidTyFamEqn,
16 arityErr, badATErr,
17 checkValidTelescope, checkZonkValidTelescope, checkValidInferredKinds,
18 allDistinctTyVars
19 ) where
20
21 #include "HsVersions.h"
22
23 import Maybes
24
25 -- friends:
26 import TcUnify ( tcSubType_NC )
27 import TcSimplify ( simplifyAmbiguityCheck )
28 import TyCoRep
29 import TcType hiding ( sizeType, sizeTypes )
30 import TcMType
31 import PrelNames
32 import Type
33 import Coercion
34 import Kind
35 import CoAxiom
36 import Class
37 import TyCon
38
39 -- others:
40 import HsSyn -- HsType
41 import TcRnMonad -- TcType, amongst others
42 import TcHsSyn ( checkForRepresentationPolymorphism )
43 import TcEnv ( tcGetInstEnvs )
44 import FunDeps
45 import InstEnv ( ClsInst, lookupInstEnv, isOverlappable )
46 import FamInstEnv ( isDominatedBy, injectiveBranches,
47 InjectivityCheckResult(..) )
48 import FamInst ( makeInjectivityErrors )
49 import Name
50 import VarEnv
51 import VarSet
52 import UniqFM
53 import Var ( mkTyVar )
54 import ErrUtils
55 import DynFlags
56 import Util
57 import ListSetOps
58 import SrcLoc
59 import Outputable
60 import BasicTypes
61 import Module
62 import Unique ( mkAlphaTyVarUnique )
63 import qualified GHC.LanguageExtensions as LangExt
64
65 import Control.Monad
66 import Data.List ( (\\) )
67
68 {-
69 ************************************************************************
70 * *
71 Checking for ambiguity
72 * *
73 ************************************************************************
74
75 Note [The ambiguity check for type signatures]
76 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
77 checkAmbiguity is a check on *user-supplied type signatures*. It is
78 *purely* there to report functions that cannot possibly be called. So for
79 example we want to reject:
80 f :: C a => Int
81 The idea is there can be no legal calls to 'f' because every call will
82 give rise to an ambiguous constraint. We could soundly omit the
83 ambiguity check on type signatures entirely, at the expense of
84 delaying ambiguity errors to call sites. Indeed, the flag
85 -XAllowAmbiguousTypes switches off the ambiguity check.
86
87 What about things like this:
88 class D a b | a -> b where ..
89 h :: D Int b => Int
90 The Int may well fix 'b' at the call site, so that signature should
91 not be rejected. Moreover, using *visible* fundeps is too
92 conservative. Consider
93 class X a b where ...
94 class D a b | a -> b where ...
95 instance D a b => X [a] b where...
96 h :: X a b => a -> a
97 Here h's type looks ambiguous in 'b', but here's a legal call:
98 ...(h [True])...
99 That gives rise to a (X [Bool] beta) constraint, and using the
100 instance means we need (D Bool beta) and that fixes 'beta' via D's
101 fundep!
102
103 Behind all these special cases there is a simple guiding principle.
104 Consider
105
106 f :: <type>
107 f = ...blah...
108
109 g :: <type>
110 g = f
111
112 You would think that the definition of g would surely typecheck!
113 After all f has exactly the same type, and g=f. But in fact f's type
114 is instantiated and the instantiated constraints are solved against
115 the originals, so in the case an ambiguous type it won't work.
116 Consider our earlier example f :: C a => Int. Then in g's definition,
117 we'll instantiate to (C alpha) and try to deduce (C alpha) from (C a),
118 and fail.
119
120 So in fact we use this as our *definition* of ambiguity. We use a
121 very similar test for *inferred* types, to ensure that they are
122 unambiguous. See Note [Impedence matching] in TcBinds.
123
124 This test is very conveniently implemented by calling
125 tcSubType <type> <type>
126 This neatly takes account of the functional dependecy stuff above,
127 and implicit parameter (see Note [Implicit parameters and ambiguity]).
128 And this is what checkAmbiguity does.
129
130 What about this, though?
131 g :: C [a] => Int
132 Is every call to 'g' ambiguous? After all, we might have
133 intance C [a] where ...
134 at the call site. So maybe that type is ok! Indeed even f's
135 quintessentially ambiguous type might, just possibly be callable:
136 with -XFlexibleInstances we could have
137 instance C a where ...
138 and now a call could be legal after all! Well, we'll reject this
139 unless the instance is available *here*.
140
141 Note [When to call checkAmbiguity]
142 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
143 We call checkAmbiguity
144 (a) on user-specified type signatures
145 (b) in checkValidType
146
147 Conncerning (b), you might wonder about nested foralls. What about
148 f :: forall b. (forall a. Eq a => b) -> b
149 The nested forall is ambiguous. Originally we called checkAmbiguity
150 in the forall case of check_type, but that had two bad consequences:
151 * We got two error messages about (Eq b) in a nested forall like this:
152 g :: forall a. Eq a => forall b. Eq b => a -> a
153 * If we try to check for ambiguity of an nested forall like
154 (forall a. Eq a => b), the implication constraint doesn't bind
155 all the skolems, which results in "No skolem info" in error
156 messages (see Trac #10432).
157
158 To avoid this, we call checkAmbiguity once, at the top, in checkValidType.
159 (I'm still a bit worried about unbound skolems when the type mentions
160 in-scope type variables.)
161
162 In fact, because of the co/contra-variance implemented in tcSubType,
163 this *does* catch function f above. too.
164
165 Concerning (a) the ambiguity check is only used for *user* types, not
166 for types coming from inteface files. The latter can legitimately
167 have ambiguous types. Example
168
169 class S a where s :: a -> (Int,Int)
170 instance S Char where s _ = (1,1)
171 f:: S a => [a] -> Int -> (Int,Int)
172 f (_::[a]) x = (a*x,b)
173 where (a,b) = s (undefined::a)
174
175 Here the worker for f gets the type
176 fw :: forall a. S a => Int -> (# Int, Int #)
177
178
179 Note [Implicit parameters and ambiguity]
180 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
181 Only a *class* predicate can give rise to ambiguity
182 An *implicit parameter* cannot. For example:
183 foo :: (?x :: [a]) => Int
184 foo = length ?x
185 is fine. The call site will supply a particular 'x'
186
187 Furthermore, the type variables fixed by an implicit parameter
188 propagate to the others. E.g.
189 foo :: (Show a, ?x::[a]) => Int
190 foo = show (?x++?x)
191 The type of foo looks ambiguous. But it isn't, because at a call site
192 we might have
193 let ?x = 5::Int in foo
194 and all is well. In effect, implicit parameters are, well, parameters,
195 so we can take their type variables into account as part of the
196 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
197 -}
198
199 checkAmbiguity :: UserTypeCtxt -> Type -> TcM ()
200 checkAmbiguity ctxt ty
201 | wantAmbiguityCheck ctxt
202 = do { traceTc "Ambiguity check for" (ppr ty)
203 -- Solve the constraints eagerly because an ambiguous type
204 -- can cause a cascade of further errors. Since the free
205 -- tyvars are skolemised, we can safely use tcSimplifyTop
206 ; allow_ambiguous <- xoptM LangExt.AllowAmbiguousTypes
207 ; (_wrap, wanted) <- addErrCtxt (mk_msg allow_ambiguous) $
208 captureConstraints $
209 tcSubType_NC ctxt ty (mkCheckExpType ty)
210 ; simplifyAmbiguityCheck ty wanted
211
212 ; traceTc "Done ambiguity check for" (ppr ty) }
213
214 | otherwise
215 = return ()
216 where
217 mk_msg allow_ambiguous
218 = vcat [ text "In the ambiguity check for" <+> what
219 , ppUnless allow_ambiguous ambig_msg ]
220 ambig_msg = text "To defer the ambiguity check to use sites, enable AllowAmbiguousTypes"
221 what | Just n <- isSigMaybe ctxt = quotes (ppr n)
222 | otherwise = pprUserTypeCtxt ctxt
223
224 wantAmbiguityCheck :: UserTypeCtxt -> Bool
225 wantAmbiguityCheck ctxt
226 = case ctxt of -- See Note [When we don't check for ambiguity]
227 GhciCtxt -> False
228 TySynCtxt {} -> False
229 _ -> True
230
231 checkUserTypeError :: Type -> TcM ()
232 -- Check to see if the type signature mentions "TypeError blah"
233 -- anywhere in it, and fail if so.
234 --
235 -- Very unsatisfactorily (Trac #11144) we need to tidy the type
236 -- because it may have come from an /inferred/ signature, not a
237 -- user-supplied one. This is really only a half-baked fix;
238 -- the other errors in checkValidType don't do tidying, and so
239 -- may give bad error messages when given an inferred type.
240 checkUserTypeError = check
241 where
242 check ty
243 | Just msg <- userTypeError_maybe ty = fail_with msg
244 | Just (_,ts) <- splitTyConApp_maybe ty = mapM_ check ts
245 | Just (t1,t2) <- splitAppTy_maybe ty = check t1 >> check t2
246 | Just (_,t1) <- splitForAllTy_maybe ty = check t1
247 | otherwise = return ()
248
249 fail_with msg = do { env0 <- tcInitTidyEnv
250 ; let (env1, tidy_msg) = tidyOpenType env0 msg
251 ; failWithTcM (env1, pprUserTypeErrorTy tidy_msg) }
252
253
254 {- Note [When we don't check for ambiguity]
255 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
256 In a few places we do not want to check a user-specified type for ambiguity
257
258 * GhciCtxt: Allow ambiguous types in GHCi's :kind command
259 E.g. type family T a :: * -- T :: forall k. k -> *
260 Then :k T should work in GHCi, not complain that
261 (T k) is ambiguous!
262
263 * TySynCtxt: type T a b = C a b => blah
264 It may be that when we /use/ T, we'll give an 'a' or 'b' that somehow
265 cure the ambiguity. So we defer the ambiguity check to the use site.
266
267 There is also an implementation reason (Trac #11608). In the RHS of
268 a type synonym we don't (currently) instantiate 'a' and 'b' with
269 TcTyVars before calling checkValidType, so we get asertion failures
270 from doing an ambiguity check on a type with TyVars in it. Fixing this
271 would not be hard, but let's wait till there's a reason.
272
273
274 ************************************************************************
275 * *
276 Checking validity of a user-defined type
277 * *
278 ************************************************************************
279
280 When dealing with a user-written type, we first translate it from an HsType
281 to a Type, performing kind checking, and then check various things that should
282 be true about it. We don't want to perform these checks at the same time
283 as the initial translation because (a) they are unnecessary for interface-file
284 types and (b) when checking a mutually recursive group of type and class decls,
285 we can't "look" at the tycons/classes yet. Also, the checks are rather
286 diverse, and used to really mess up the other code.
287
288 One thing we check for is 'rank'.
289
290 Rank 0: monotypes (no foralls)
291 Rank 1: foralls at the front only, Rank 0 inside
292 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
293
294 basic ::= tyvar | T basic ... basic
295
296 r2 ::= forall tvs. cxt => r2a
297 r2a ::= r1 -> r2a | basic
298 r1 ::= forall tvs. cxt => r0
299 r0 ::= r0 -> r0 | basic
300
301 Another thing is to check that type synonyms are saturated.
302 This might not necessarily show up in kind checking.
303 type A i = i
304 data T k = MkT (k Int)
305 f :: T A -- BAD!
306 -}
307
308 checkValidType :: UserTypeCtxt -> Type -> TcM ()
309 -- Checks that a user-written type is valid for the given context
310 -- Assumes arguemt is fully zonked
311 -- Not used for instance decls; checkValidInstance instead
312 checkValidType ctxt ty
313 = do { traceTc "checkValidType" (ppr ty <+> text "::" <+> ppr (typeKind ty))
314 ; rankn_flag <- xoptM LangExt.RankNTypes
315 ; impred_flag <- xoptM LangExt.ImpredicativeTypes
316 ; let gen_rank :: Rank -> Rank
317 gen_rank r | rankn_flag = ArbitraryRank
318 | otherwise = r
319
320 rank1 = gen_rank r1
321 rank0 = gen_rank r0
322
323 r0 = rankZeroMonoType
324 r1 = LimitedRank True r0
325
326 rank
327 = case ctxt of
328 DefaultDeclCtxt-> MustBeMonoType
329 ResSigCtxt -> MustBeMonoType
330 PatSigCtxt -> rank0
331 RuleSigCtxt _ -> rank1
332 TySynCtxt _ -> rank0
333
334 ExprSigCtxt -> rank1
335 TypeAppCtxt | impred_flag -> ArbitraryRank
336 | otherwise -> tyConArgMonoType
337 -- Normally, ImpredicativeTypes is handled in check_arg_type,
338 -- but visible type applications don't go through there.
339 -- So we do this check here.
340
341 FunSigCtxt {} -> rank1
342 InfSigCtxt _ -> ArbitraryRank -- Inferred type
343 ConArgCtxt _ -> rank1 -- We are given the type of the entire
344 -- constructor, hence rank 1
345
346 ForSigCtxt _ -> rank1
347 SpecInstCtxt -> rank1
348 ThBrackCtxt -> rank1
349 GhciCtxt -> ArbitraryRank
350 _ -> panic "checkValidType"
351 -- Can't happen; not used for *user* sigs
352
353 ; env <- tcInitOpenTidyEnv (tyCoVarsOfTypeList ty)
354
355 -- Check the internal validity of the type itself
356 ; check_type env ctxt rank ty
357
358 ; checkUserTypeError ty
359
360 -- Check for ambiguous types. See Note [When to call checkAmbiguity]
361 -- NB: this will happen even for monotypes, but that should be cheap;
362 -- and there may be nested foralls for the subtype test to examine
363 ; checkAmbiguity ctxt ty
364
365 ; traceTc "checkValidType done" (ppr ty <+> text "::" <+> ppr (typeKind ty)) }
366
367 checkValidMonoType :: Type -> TcM ()
368 -- Assumes arguemt is fully zonked
369 checkValidMonoType ty
370 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypeList ty)
371 ; check_type env SigmaCtxt MustBeMonoType ty }
372
373 checkTySynRhs :: UserTypeCtxt -> TcType -> TcM ()
374 checkTySynRhs ctxt ty
375 | returnsConstraintKind actual_kind
376 = do { ck <- xoptM LangExt.ConstraintKinds
377 ; if ck
378 then when (isConstraintKind actual_kind)
379 (do { dflags <- getDynFlags
380 ; check_pred_ty emptyTidyEnv dflags ctxt ty })
381 else addErrTcM (constraintSynErr emptyTidyEnv actual_kind) }
382
383 | otherwise
384 = return ()
385 where
386 actual_kind = typeKind ty
387
388 -- | The kind expected in a certain context.
389 data ContextKind = TheKind Kind -- ^ a specific kind
390 | AnythingKind -- ^ any kind will do
391 | OpenKind -- ^ something of the form @TYPE _@
392
393 -- Depending on the context, we might accept any kind (for instance, in a TH
394 -- splice), or only certain kinds (like in type signatures).
395 expectedKindInCtxt :: UserTypeCtxt -> ContextKind
396 expectedKindInCtxt (TySynCtxt _) = AnythingKind
397 expectedKindInCtxt ThBrackCtxt = AnythingKind
398 expectedKindInCtxt GhciCtxt = AnythingKind
399 -- The types in a 'default' decl can have varying kinds
400 -- See Note [Extended defaults]" in TcEnv
401 expectedKindInCtxt DefaultDeclCtxt = AnythingKind
402 expectedKindInCtxt TypeAppCtxt = AnythingKind
403 expectedKindInCtxt (ForSigCtxt _) = TheKind liftedTypeKind
404 expectedKindInCtxt InstDeclCtxt = TheKind constraintKind
405 expectedKindInCtxt SpecInstCtxt = TheKind constraintKind
406 expectedKindInCtxt _ = OpenKind
407
408 {-
409 Note [Higher rank types]
410 ~~~~~~~~~~~~~~~~~~~~~~~~
411 Technically
412 Int -> forall a. a->a
413 is still a rank-1 type, but it's not Haskell 98 (Trac #5957). So the
414 validity checker allow a forall after an arrow only if we allow it
415 before -- that is, with Rank2Types or RankNTypes
416 -}
417
418 data Rank = ArbitraryRank -- Any rank ok
419
420 | LimitedRank -- Note [Higher rank types]
421 Bool -- Forall ok at top
422 Rank -- Use for function arguments
423
424 | MonoType SDoc -- Monotype, with a suggestion of how it could be a polytype
425
426 | MustBeMonoType -- Monotype regardless of flags
427
428
429 rankZeroMonoType, tyConArgMonoType, synArgMonoType, constraintMonoType :: Rank
430 rankZeroMonoType = MonoType (text "Perhaps you intended to use RankNTypes or Rank2Types")
431 tyConArgMonoType = MonoType (text "GHC doesn't yet support impredicative polymorphism")
432 synArgMonoType = MonoType (text "Perhaps you intended to use LiberalTypeSynonyms")
433 constraintMonoType = MonoType (text "A constraint must be a monotype")
434
435 funArgResRank :: Rank -> (Rank, Rank) -- Function argument and result
436 funArgResRank (LimitedRank _ arg_rank) = (arg_rank, LimitedRank (forAllAllowed arg_rank) arg_rank)
437 funArgResRank other_rank = (other_rank, other_rank)
438
439 forAllAllowed :: Rank -> Bool
440 forAllAllowed ArbitraryRank = True
441 forAllAllowed (LimitedRank forall_ok _) = forall_ok
442 forAllAllowed _ = False
443
444 -- The zonker issues errors if it zonks a representation-polymorphic binder
445 -- But sometimes it's nice to check a little more eagerly, trying to report
446 -- errors earlier.
447 representationPolymorphismForbidden :: UserTypeCtxt -> Bool
448 representationPolymorphismForbidden = go
449 where
450 go (ConArgCtxt _) = True -- A rep-polymorphic datacon won't be useful
451 go (PatSynCtxt _) = True -- Similar to previous case
452 go _ = False -- Other cases are caught by zonker
453
454 ----------------------------------------
455 -- | Fail with error message if the type is unlifted
456 check_lifted :: Type -> TcM ()
457 check_lifted _ = return ()
458
459 {- ------ Legacy comment ---------
460 The check_unlifted function seems entirely redundant. The
461 kind system should check for uses of unlifted types. So I've
462 removed the check. See Trac #11120 comment:19.
463
464 check_lifted ty
465 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypeList ty)
466 ; checkTcM (not (isUnliftedType ty)) (unliftedArgErr env ty) }
467
468 unliftedArgErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
469 unliftedArgErr env ty = (env, sep [text "Illegal unlifted type:", ppr_tidy env ty])
470 ------ End of legacy comment --------- -}
471
472
473 check_type :: TidyEnv -> UserTypeCtxt -> Rank -> Type -> TcM ()
474 -- The args say what the *type context* requires, independent
475 -- of *flag* settings. You test the flag settings at usage sites.
476 --
477 -- Rank is allowed rank for function args
478 -- Rank 0 means no for-alls anywhere
479
480 check_type env ctxt rank ty
481 | not (null tvs && null theta)
482 = do { traceTc "check_type" (ppr ty $$ ppr (forAllAllowed rank))
483 ; checkTcM (forAllAllowed rank) (forAllTyErr env rank ty)
484 -- Reject e.g. (Maybe (?x::Int => Int)),
485 -- with a decent error message
486
487 ; check_valid_theta env' SigmaCtxt theta
488 -- Allow type T = ?x::Int => Int -> Int
489 -- but not type T = ?x::Int
490
491 ; check_type env' ctxt rank tau -- Allow foralls to right of arrow
492 ; checkTcM (not (any (`elemVarSet` tyCoVarsOfType phi_kind) tvs))
493 (forAllEscapeErr env' ty tau_kind)
494 }
495 where
496 (tvs, theta, tau) = tcSplitSigmaTy ty
497 tau_kind = typeKind tau
498 (env', _) = tidyTyCoVarBndrs env tvs
499
500 phi_kind | null theta = tau_kind
501 | otherwise = liftedTypeKind
502 -- If there are any constraints, the kind is *. (#11405)
503
504 check_type _ _ _ (TyVarTy _) = return ()
505
506 check_type env ctxt rank (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
892 Note [Instance and Given overlap]. So this warning discourages uses
893 from writing simplifiable class constraints, at least unless the
894 top-level instance is explicitly declared as OVERLAPPABLE.
895 Trac #11948 provoked me to do this.
896 -}
897
898 -------------------------
899 okIPCtxt :: UserTypeCtxt -> Bool
900 -- See Note [Implicit parameters in instance decls]
901 okIPCtxt (FunSigCtxt {}) = True
902 okIPCtxt (InfSigCtxt {}) = True
903 okIPCtxt ExprSigCtxt = True
904 okIPCtxt TypeAppCtxt = True
905 okIPCtxt PatSigCtxt = True
906 okIPCtxt ResSigCtxt = True
907 okIPCtxt GenSigCtxt = True
908 okIPCtxt (ConArgCtxt {}) = True
909 okIPCtxt (ForSigCtxt {}) = True -- ??
910 okIPCtxt ThBrackCtxt = True
911 okIPCtxt GhciCtxt = True
912 okIPCtxt SigmaCtxt = True
913 okIPCtxt (DataTyCtxt {}) = True
914 okIPCtxt (PatSynCtxt {}) = True
915 okIPCtxt (TySynCtxt {}) = True -- e.g. type Blah = ?x::Int
916 -- Trac #11466
917
918 okIPCtxt (ClassSCCtxt {}) = False
919 okIPCtxt (InstDeclCtxt {}) = False
920 okIPCtxt (SpecInstCtxt {}) = False
921 okIPCtxt (RuleSigCtxt {}) = False
922 okIPCtxt DefaultDeclCtxt = False
923
924 {-
925 Note [Kind polymorphic type classes]
926 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
927 MultiParam check:
928
929 class C f where... -- C :: forall k. k -> Constraint
930 instance C Maybe where...
931
932 The dictionary gets type [C * Maybe] even if it's not a MultiParam
933 type class.
934
935 Flexibility check:
936
937 class C f where... -- C :: forall k. k -> Constraint
938 data D a = D a
939 instance C D where
940
941 The dictionary gets type [C * (D *)]. IA0_TODO it should be
942 generalized actually.
943 -}
944
945 checkThetaCtxt :: UserTypeCtxt -> ThetaType -> TidyEnv -> TcM (TidyEnv, SDoc)
946 checkThetaCtxt ctxt theta env
947 = return ( env
948 , vcat [ text "In the context:" <+> pprTheta (tidyTypes env theta)
949 , text "While checking" <+> pprUserTypeCtxt ctxt ] )
950
951 eqPredTyErr, predTupleErr, predIrredErr, predSuperClassErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
952 eqPredTyErr env pred
953 = ( env
954 , text "Illegal equational constraint" <+> ppr_tidy env pred $$
955 parens (text "Use GADTs or TypeFamilies to permit this") )
956 predTupleErr env pred
957 = ( env
958 , hang (text "Illegal tuple constraint:" <+> ppr_tidy env pred)
959 2 (parens constraintKindsMsg) )
960 predIrredErr env pred
961 = ( env
962 , hang (text "Illegal constraint:" <+> ppr_tidy env pred)
963 2 (parens constraintKindsMsg) )
964 predSuperClassErr env pred
965 = ( env
966 , hang (text "Illegal constraint" <+> quotes (ppr_tidy env pred)
967 <+> text "in a superclass context")
968 2 (parens undecidableMsg) )
969
970 predTyVarErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
971 predTyVarErr env pred
972 = (env
973 , vcat [ hang (text "Non type-variable argument")
974 2 (text "in the constraint:" <+> ppr_tidy env pred)
975 , parens (text "Use FlexibleContexts to permit this") ])
976
977 badIPPred :: TidyEnv -> PredType -> (TidyEnv, SDoc)
978 badIPPred env pred
979 = ( env
980 , text "Illegal implicit parameter" <+> quotes (ppr_tidy env pred) )
981
982 constraintSynErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
983 constraintSynErr env kind
984 = ( env
985 , hang (text "Illegal constraint synonym of kind:" <+> quotes (ppr_tidy env kind))
986 2 (parens constraintKindsMsg) )
987
988 dupPredWarn :: TidyEnv -> [[PredType]] -> (TidyEnv, SDoc)
989 dupPredWarn env dups
990 = ( env
991 , text "Duplicate constraint" <> plural primaryDups <> text ":"
992 <+> pprWithCommas (ppr_tidy env) primaryDups )
993 where
994 primaryDups = map head dups
995
996 tyConArityErr :: TyCon -> [TcType] -> SDoc
997 -- For type-constructor arity errors, be careful to report
998 -- the number of /visible/ arguments required and supplied,
999 -- ignoring the /invisible/ arguments, which the user does not see.
1000 -- (e.g. Trac #10516)
1001 tyConArityErr tc tks
1002 = arityErr (tyConFlavour tc) (tyConName tc)
1003 tc_type_arity tc_type_args
1004 where
1005 vis_tks = filterOutInvisibleTypes tc tks
1006
1007 -- tc_type_arity = number of *type* args expected
1008 -- tc_type_args = number of *type* args encountered
1009 tc_type_arity = count isVisibleBinder $ tyConBinders tc
1010 tc_type_args = length vis_tks
1011
1012 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1013 arityErr what name n m
1014 = hsep [ text "The" <+> text what, quotes (ppr name), text "should have",
1015 n_arguments <> comma, text "but has been given",
1016 if m==0 then text "none" else int m]
1017 where
1018 n_arguments | n == 0 = text "no arguments"
1019 | n == 1 = text "1 argument"
1020 | True = hsep [int n, text "arguments"]
1021
1022 {-
1023 ************************************************************************
1024 * *
1025 \subsection{Checking for a decent instance head type}
1026 * *
1027 ************************************************************************
1028
1029 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1030 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1031
1032 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1033 flag is on, or (2)~the instance is imported (they must have been
1034 compiled elsewhere). In these cases, we let them go through anyway.
1035
1036 We can also have instances for functions: @instance Foo (a -> b) ...@.
1037 -}
1038
1039 checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
1040 checkValidInstHead ctxt clas cls_args
1041 = do { dflags <- getDynFlags
1042
1043 ; mod <- getModule
1044 ; checkTc (getUnique clas `notElem` abstractClassKeys ||
1045 nameModule (getName clas) == mod)
1046 (instTypeErr clas cls_args abstract_class_msg)
1047
1048 -- Check language restrictions;
1049 -- but not for SPECIALISE instance pragmas
1050 ; let ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
1051 ; unless spec_inst_prag $
1052 do { checkTc (xopt LangExt.TypeSynonymInstances dflags ||
1053 all tcInstHeadTyNotSynonym ty_args)
1054 (instTypeErr clas cls_args head_type_synonym_msg)
1055 ; checkTc (xopt LangExt.FlexibleInstances dflags ||
1056 all tcInstHeadTyAppAllTyVars ty_args)
1057 (instTypeErr clas cls_args head_type_args_tyvars_msg)
1058 ; checkTc (xopt LangExt.MultiParamTypeClasses dflags ||
1059 length ty_args == 1 || -- Only count type arguments
1060 (xopt LangExt.NullaryTypeClasses dflags &&
1061 null ty_args))
1062 (instTypeErr clas cls_args head_one_type_msg) }
1063
1064 ; mapM_ checkValidTypePat ty_args }
1065 where
1066 spec_inst_prag = case ctxt of { SpecInstCtxt -> True; _ -> False }
1067
1068 head_type_synonym_msg = parens (
1069 text "All instance types must be of the form (T t1 ... tn)" $$
1070 text "where T is not a synonym." $$
1071 text "Use TypeSynonymInstances if you want to disable this.")
1072
1073 head_type_args_tyvars_msg = parens (vcat [
1074 text "All instance types must be of the form (T a1 ... an)",
1075 text "where a1 ... an are *distinct type variables*,",
1076 text "and each type variable appears at most once in the instance head.",
1077 text "Use FlexibleInstances if you want to disable this."])
1078
1079 head_one_type_msg = parens (
1080 text "Only one type can be given in an instance head." $$
1081 text "Use MultiParamTypeClasses if you want to allow more, or zero.")
1082
1083 abstract_class_msg =
1084 text "Manual instances of this class are not permitted."
1085
1086 tcInstHeadTyNotSynonym :: Type -> Bool
1087 -- Used in Haskell-98 mode, for the argument types of an instance head
1088 -- These must not be type synonyms, but everywhere else type synonyms
1089 -- are transparent, so we need a special function here
1090 tcInstHeadTyNotSynonym ty
1091 = case ty of -- Do not use splitTyConApp,
1092 -- because that expands synonyms!
1093 TyConApp tc _ -> not (isTypeSynonymTyCon tc)
1094 _ -> True
1095
1096 tcInstHeadTyAppAllTyVars :: Type -> Bool
1097 -- Used in Haskell-98 mode, for the argument types of an instance head
1098 -- These must be a constructor applied to type variable arguments.
1099 -- But we allow kind instantiations.
1100 tcInstHeadTyAppAllTyVars ty
1101 | Just (tc, tys) <- tcSplitTyConApp_maybe (dropCasts ty)
1102 = ok (filterOutInvisibleTypes tc tys) -- avoid kinds
1103
1104 | otherwise
1105 = False
1106 where
1107 -- Check that all the types are type variables,
1108 -- and that each is distinct
1109 ok tys = equalLength tvs tys && hasNoDups tvs
1110 where
1111 tvs = mapMaybe tcGetTyVar_maybe tys
1112
1113 dropCasts :: Type -> Type
1114 -- See Note [Casts during validity checking]
1115 -- This function can turn a well-kinded type into an ill-kinded
1116 -- one, so I've kept it local to this module
1117 -- To consider: drop only UnivCo(HoleProv) casts
1118 dropCasts (CastTy ty _) = dropCasts ty
1119 dropCasts (AppTy t1 t2) = mkAppTy (dropCasts t1) (dropCasts t2)
1120 dropCasts (FunTy t1 t2) = mkFunTy (dropCasts t1) (dropCasts t2)
1121 dropCasts (TyConApp tc tys) = mkTyConApp tc (map dropCasts tys)
1122 dropCasts (ForAllTy b ty) = ForAllTy (dropCastsB b) (dropCasts ty)
1123 dropCasts ty = ty -- LitTy, TyVarTy, CoercionTy
1124
1125 dropCastsB :: TyVarBinder -> TyVarBinder
1126 dropCastsB b = b -- Don't bother in the kind of a forall
1127
1128 abstractClassKeys :: [Unique]
1129 abstractClassKeys = [ heqTyConKey
1130 , eqTyConKey
1131 , coercibleTyConKey
1132 ] -- See Note [Equality class instances]
1133
1134 instTypeErr :: Class -> [Type] -> SDoc -> SDoc
1135 instTypeErr cls tys msg
1136 = hang (hang (text "Illegal instance declaration for")
1137 2 (quotes (pprClassPred cls tys)))
1138 2 msg
1139
1140 {- Note [Casts during validity checking]
1141 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1142 Consider the (bogus)
1143 instance Eq Char#
1144 We elaborate to 'Eq (Char# |> UnivCo(hole))' where the hole is an
1145 insoluble equality constraint for * ~ #. We'll report the insoluble
1146 constraint separately, but we don't want to *also* complain that Eq is
1147 not applied to a type constructor. So we look gaily look through
1148 CastTys here.
1149
1150 Another example: Eq (Either a). Then we actually get a cast in
1151 the middle:
1152 Eq ((Either |> g) a)
1153
1154
1155 Note [Valid 'deriving' predicate]
1156 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1157 validDerivPred checks for OK 'deriving' context. See Note [Exotic
1158 derived instance contexts] in TcDeriv. However the predicate is
1159 here because it uses sizeTypes, fvTypes.
1160
1161 It checks for three things
1162
1163 * No repeated variables (hasNoDups fvs)
1164
1165 * No type constructors. This is done by comparing
1166 sizeTypes tys == length (fvTypes tys)
1167 sizeTypes counts variables and constructors; fvTypes returns variables.
1168 So if they are the same, there must be no constructors. But there
1169 might be applications thus (f (g x)).
1170
1171 Note that tys only includes the visible arguments of the class type
1172 constructor. Including the non-vivisble arguments can cause the following,
1173 perfectly valid instance to be rejected:
1174 class Category (cat :: k -> k -> *) where ...
1175 newtype T (c :: * -> * -> *) a b = MkT (c a b)
1176 instance Category c => Category (T c) where ...
1177 since the first argument to Category is a non-visible *, which sizeTypes
1178 would count as a constructor! See Trac #11833.
1179
1180 * Also check for a bizarre corner case, when the derived instance decl
1181 would look like
1182 instance C a b => D (T a) where ...
1183 Note that 'b' isn't a parameter of T. This gives rise to all sorts of
1184 problems; in particular, it's hard to compare solutions for equality
1185 when finding the fixpoint, and that means the inferContext loop does
1186 not converge. See Trac #5287.
1187
1188 Note [Equality class instances]
1189 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1190 We can't have users writing instances for the equality classes. But we
1191 still need to be able to write instances for them ourselves. So we allow
1192 instances only in the defining module.
1193
1194 -}
1195
1196 validDerivPred :: TyVarSet -> PredType -> Bool
1197 -- See Note [Valid 'deriving' predicate]
1198 validDerivPred tv_set pred
1199 = case classifyPredType pred of
1200 ClassPred cls tys -> cls `hasKey` typeableClassKey
1201 -- Typeable constraints are bigger than they appear due
1202 -- to kind polymorphism, but that's OK
1203 || check_tys cls tys
1204 EqPred {} -> False -- reject equality constraints
1205 _ -> True -- Non-class predicates are ok
1206 where
1207 check_tys cls tys
1208 = hasNoDups fvs
1209 -- use sizePred to ignore implicit args
1210 && sizePred pred == fromIntegral (length fvs)
1211 && all (`elemVarSet` tv_set) fvs
1212 where tys' = filterOutInvisibleTypes (classTyCon cls) tys
1213 fvs = fvTypes tys'
1214
1215 {-
1216 ************************************************************************
1217 * *
1218 \subsection{Checking instance for termination}
1219 * *
1220 ************************************************************************
1221 -}
1222
1223 checkValidInstance :: UserTypeCtxt -> LHsSigType Name -> Type
1224 -> TcM ([TyVar], ThetaType, Class, [Type])
1225 checkValidInstance ctxt hs_type ty
1226 | Just (clas,inst_tys) <- getClassPredTys_maybe tau
1227 , inst_tys `lengthIs` classArity clas
1228 = do { setSrcSpan head_loc (checkValidInstHead ctxt clas inst_tys)
1229 ; traceTc "checkValidInstance {" (ppr ty)
1230 ; checkValidTheta ctxt theta
1231
1232 -- The Termination and Coverate Conditions
1233 -- Check that instance inference will terminate (if we care)
1234 -- For Haskell 98 this will already have been done by checkValidTheta,
1235 -- but as we may be using other extensions we need to check.
1236 --
1237 -- Note that the Termination Condition is *more conservative* than
1238 -- the checkAmbiguity test we do on other type signatures
1239 -- e.g. Bar a => Bar Int is ambiguous, but it also fails
1240 -- the termination condition, because 'a' appears more often
1241 -- in the constraint than in the head
1242 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1243 ; if undecidable_ok
1244 then checkAmbiguity ctxt ty
1245 else checkInstTermination inst_tys theta
1246
1247 ; traceTc "cvi 2" (ppr ty)
1248
1249 ; case (checkInstCoverage undecidable_ok clas theta inst_tys) of
1250 IsValid -> return () -- Check succeeded
1251 NotValid msg -> addErrTc (instTypeErr clas inst_tys msg)
1252
1253 ; traceTc "End checkValidInstance }" empty
1254
1255 ; return (tvs, theta, clas, inst_tys) }
1256
1257 | otherwise
1258 = failWithTc (text "Malformed instance head:" <+> ppr tau)
1259 where
1260 (tvs, theta, tau) = tcSplitSigmaTy ty
1261
1262 -- The location of the "head" of the instance
1263 head_loc = getLoc (getLHsInstDeclHead hs_type)
1264
1265 {-
1266 Note [Paterson conditions]
1267 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1268 Termination test: the so-called "Paterson conditions" (see Section 5 of
1269 "Understanding functional dependencies via Constraint Handling Rules,
1270 JFP Jan 2007).
1271
1272 We check that each assertion in the context satisfies:
1273 (1) no variable has more occurrences in the assertion than in the head, and
1274 (2) the assertion has fewer constructors and variables (taken together
1275 and counting repetitions) than the head.
1276 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1277 (which have already been checked) guarantee termination.
1278
1279 The underlying idea is that
1280
1281 for any ground substitution, each assertion in the
1282 context has fewer type constructors than the head.
1283 -}
1284
1285 checkInstTermination :: [TcType] -> ThetaType -> TcM ()
1286 -- See Note [Paterson conditions]
1287 checkInstTermination tys theta
1288 = check_preds theta
1289 where
1290 head_fvs = fvTypes tys
1291 head_size = sizeTypes tys
1292
1293 check_preds :: [PredType] -> TcM ()
1294 check_preds preds = mapM_ check preds
1295
1296 check :: PredType -> TcM ()
1297 check pred
1298 = case classifyPredType pred of
1299 EqPred {} -> return () -- See Trac #4200.
1300 IrredPred {} -> check2 pred (sizeType pred)
1301 ClassPred cls tys
1302 | isTerminatingClass cls
1303 -> return ()
1304
1305 | isCTupleClass cls -- Look inside tuple predicates; Trac #8359
1306 -> check_preds tys
1307
1308 | otherwise
1309 -> check2 pred (sizeTypes $ filterOutInvisibleTypes (classTyCon cls) tys)
1310 -- Other ClassPreds
1311
1312 check2 pred pred_size
1313 | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what)
1314 | pred_size >= head_size = addErrTc (smallerMsg what)
1315 | otherwise = return ()
1316 where
1317 what = text "constraint" <+> quotes (ppr pred)
1318 bad_tvs = fvType pred \\ head_fvs
1319
1320 smallerMsg :: SDoc -> SDoc
1321 smallerMsg what
1322 = vcat [ hang (text "The" <+> what)
1323 2 (text "is no smaller than the instance head")
1324 , parens undecidableMsg ]
1325
1326 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc
1327 noMoreMsg tvs what
1328 = vcat [ hang (text "Variable" <> plural tvs <+> quotes (pprWithCommas ppr tvs)
1329 <+> occurs <+> text "more often")
1330 2 (sep [ text "in the" <+> what
1331 , text "than in the instance head" ])
1332 , parens undecidableMsg ]
1333 where
1334 occurs = if isSingleton tvs then text "occurs"
1335 else text "occur"
1336
1337 undecidableMsg, constraintKindsMsg :: SDoc
1338 undecidableMsg = text "Use UndecidableInstances to permit this"
1339 constraintKindsMsg = text "Use ConstraintKinds to permit this"
1340
1341 {-
1342 Note [Associated type instances]
1343 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1344 We allow this:
1345 class C a where
1346 type T x a
1347 instance C Int where
1348 type T (S y) Int = y
1349 type T Z Int = Char
1350
1351 Note that
1352 a) The variable 'x' is not bound by the class decl
1353 b) 'x' is instantiated to a non-type-variable in the instance
1354 c) There are several type instance decls for T in the instance
1355
1356 All this is fine. Of course, you can't give any *more* instances
1357 for (T ty Int) elsewhere, because it's an *associated* type.
1358
1359 Note [Checking consistent instantiation]
1360 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1361 See Trac #11450 for background discussion on this check.
1362
1363 class C a b where
1364 type T a x b
1365
1366 With this class decl, if we have an instance decl
1367 instance C ty1 ty2 where ...
1368 then the type instance must look like
1369 type T ty1 v ty2 = ...
1370 with exactly 'ty1' for 'a', 'ty2' for 'b', and a variable for 'x'.
1371 For example:
1372
1373 instance C [p] Int
1374 type T [p] y Int = (p,y,y)
1375
1376 Note that
1377
1378 * We used to allow completely different bound variables in the
1379 associated type instance; e.g.
1380 instance C [p] Int
1381 type T [q] y Int = ...
1382 But from GHC 8.2 onwards, we don't. It's much simpler this way.
1383 See Trac #11450.
1384
1385 * When the class variable isn't used on the RHS of the type instance,
1386 it's tempting to allow wildcards, thus
1387 instance C [p] Int
1388 type T [_] y Int = (y,y)
1389 But it's awkward to do the test, and it doesn't work if the
1390 variable is repeated:
1391 instance C (p,p) Int
1392 type T (_,_) y Int = (y,y)
1393 Even though 'p' is not used on the RHS, we still need to use 'p'
1394 on the LHS to establish the repeated pattern. So to keep it simple
1395 we just require equality.
1396
1397 * We also check that any non-class-tyvars are instantiated with
1398 distinct tyvars. That rules out
1399 instance C [p] Int where
1400 type T [p] Bool Int = p -- Note Bool
1401 type T [p] Char Int = p -- Note Char
1402
1403 and
1404 instance C [p] Int where
1405 type T [p] p Int = p -- Note repeated 'p' on LHS
1406 It's consistent to do this because we don't allow this kind of
1407 instantiation for the class-tyvar arguments of the family.
1408
1409 Overall, we can have exactly one type instance for each
1410 associated type. If you wantmore, use an auxiliary family.
1411
1412 Implementation
1413 * Form the mini-envt from the class type variables a,b
1414 to the instance decl types [p],Int: [a->[p], b->Int]
1415
1416 * Look at the tyvars a,x,b of the type family constructor T
1417 (it shares tyvars with the class C)
1418
1419 * Apply the mini-evnt to them, and check that the result is
1420 consistent with the instance types [p] y Int
1421
1422 We make all the instance type variables scope over the
1423 type instances, of course, which picks up non-obvious kinds. Eg
1424 class Foo (a :: k) where
1425 type F a
1426 instance Foo (b :: k -> k) where
1427 type F b = Int
1428 Here the instance is kind-indexed and really looks like
1429 type F (k->k) (b::k->k) = Int
1430 But if the 'b' didn't scope, we would make F's instance too
1431 poly-kinded.
1432 -}
1433
1434 -- | Extra information about the parent instance declaration, needed
1435 -- when type-checking associated types. The 'Class' is the enclosing
1436 -- class, the [TyVar] are the type variable of the instance decl,
1437 -- and and the @VarEnv Type@ maps class variables to their instance
1438 -- types.
1439 type ClsInstInfo = (Class, [TyVar], VarEnv Type)
1440
1441 type AssocInstArgShape = (Maybe Type, Type)
1442 -- AssocInstArgShape is used only for associated family instances
1443 -- (mb_exp, actual)
1444 -- mb_exp = Just ty => this arg corresponds to a class variable
1445 -- = Nothing => it doesn't correspond to a class variable
1446 -- e.g. class C b where
1447 -- type F a b c
1448 -- instance C [x] where
1449 -- type F p [x] q
1450 -- We get [AssocInstArgShape] = [ (Nothing, p)
1451 -- , (Just [x], [x])
1452 -- , (Nothing, q)]
1453
1454 checkConsistentFamInst
1455 :: Maybe ClsInstInfo
1456 -> TyCon -- ^ Family tycon
1457 -> [TyVar] -- ^ Type variables of the family instance
1458 -> [Type] -- ^ Type patterns from instance
1459 -> TcM ()
1460 -- See Note [Checking consistent instantiation]
1461
1462 checkConsistentFamInst Nothing _ _ _ = return ()
1463 checkConsistentFamInst (Just (clas, inst_tvs, mini_env)) fam_tc _at_tvs at_tys
1464 = do { -- Check that the associated type indeed comes from this class
1465 checkTc (Just clas == tyConAssoc_maybe fam_tc)
1466 (badATErr (className clas) (tyConName fam_tc))
1467
1468 -- Check type args first (more comprehensible)
1469 ; checkTc (all check_arg type_shapes) pp_wrong_at_arg
1470 ; checkTc (check_poly_args type_shapes) pp_wrong_at_tyvars
1471
1472 -- And now kind args
1473 ; checkTc (all check_arg kind_shapes)
1474 (pp_wrong_at_arg $$ ppSuggestExplicitKinds)
1475 ; checkTc (check_poly_args kind_shapes)
1476 (pp_wrong_at_tyvars $$ ppSuggestExplicitKinds)
1477
1478 ; traceTc "cfi" (vcat [ ppr inst_tvs
1479 , ppr arg_shapes
1480 , ppr mini_env ]) }
1481 where
1482 arg_shapes :: [AssocInstArgShape]
1483 arg_shapes = [ (lookupVarEnv mini_env fam_tc_tv, at_ty)
1484 | (fam_tc_tv, at_ty) <- tyConTyVars fam_tc `zip` at_tys ]
1485
1486 (kind_shapes, type_shapes) = partitionInvisibles fam_tc snd arg_shapes
1487
1488 check_arg :: AssocInstArgShape -> Bool
1489 check_arg (Just exp_ty, at_ty) = exp_ty `tcEqType` at_ty
1490 check_arg (Nothing, _ ) = True -- Arg position does not correspond
1491 -- to a class variable
1492 check_poly_args :: [(Maybe Type,Type)] -> Bool
1493 check_poly_args arg_shapes
1494 = allDistinctTyVars (mkVarSet inst_tvs)
1495 [ at_ty | (Nothing, at_ty) <- arg_shapes ]
1496
1497 pp_wrong_at_arg
1498 = vcat [ text "Type indexes must match class instance head"
1499 , pp_exp_act ]
1500
1501 pp_wrong_at_tyvars
1502 = vcat [ text "Polymorphic type indexes of associated type" <+> quotes (ppr fam_tc)
1503 , nest 2 $ vcat [ text "(i.e. ones independent of the class type variables)"
1504 , text "must be distinct type variables" ]
1505 , pp_exp_act ]
1506
1507 pp_exp_act
1508 = vcat [ text "Expected:" <+> ppr (mkTyConApp fam_tc expected_args)
1509 , text " Actual:" <+> ppr (mkTyConApp fam_tc at_tys)
1510 , sdocWithDynFlags $ \dflags ->
1511 ppWhen (has_poly_args dflags) $
1512 vcat [ text "where the `<tv>' arguments are type variables,"
1513 , text "distinct from each other and from the instance variables" ] ]
1514
1515 expected_args = [ exp_ty `orElse` mk_tv at_ty | (exp_ty, at_ty) <- arg_shapes ]
1516 mk_tv at_ty = mkTyVarTy (mkTyVar tv_name (typeKind at_ty))
1517 tv_name = mkInternalName (mkAlphaTyVarUnique 1) (mkTyVarOcc "<tv>") noSrcSpan
1518
1519 has_poly_args dflags = any (isNothing . fst) shapes
1520 where
1521 shapes | gopt Opt_PrintExplicitKinds dflags = arg_shapes
1522 | otherwise = type_shapes
1523
1524 badATErr :: Name -> Name -> SDoc
1525 badATErr clas op
1526 = hsep [text "Class", quotes (ppr clas),
1527 text "does not have an associated type", quotes (ppr op)]
1528
1529
1530 {-
1531 ************************************************************************
1532 * *
1533 Checking type instance well-formedness and termination
1534 * *
1535 ************************************************************************
1536 -}
1537
1538 checkValidCoAxiom :: CoAxiom Branched -> TcM ()
1539 checkValidCoAxiom ax@(CoAxiom { co_ax_tc = fam_tc, co_ax_branches = branches })
1540 = do { mapM_ (checkValidCoAxBranch Nothing fam_tc) branch_list
1541 ; foldlM_ check_branch_compat [] branch_list }
1542 where
1543 branch_list = fromBranches branches
1544 injectivity = familyTyConInjectivityInfo fam_tc
1545
1546 check_branch_compat :: [CoAxBranch] -- previous branches in reverse order
1547 -> CoAxBranch -- current branch
1548 -> TcM [CoAxBranch]-- current branch : previous branches
1549 -- Check for
1550 -- (a) this branch is dominated by previous ones
1551 -- (b) failure of injectivity
1552 check_branch_compat prev_branches cur_branch
1553 | cur_branch `isDominatedBy` prev_branches
1554 = do { addWarnAt NoReason (coAxBranchSpan cur_branch) $
1555 inaccessibleCoAxBranch ax cur_branch
1556 ; return prev_branches }
1557 | otherwise
1558 = do { check_injectivity prev_branches cur_branch
1559 ; return (cur_branch : prev_branches) }
1560
1561 -- Injectivity check: check whether a new (CoAxBranch) can extend
1562 -- already checked equations without violating injectivity
1563 -- annotation supplied by the user.
1564 -- See Note [Verifying injectivity annotation] in FamInstEnv
1565 check_injectivity prev_branches cur_branch
1566 | Injective inj <- injectivity
1567 = do { let conflicts =
1568 fst $ foldl (gather_conflicts inj prev_branches cur_branch)
1569 ([], 0) prev_branches
1570 ; mapM_ (\(err, span) -> setSrcSpan span $ addErr err)
1571 (makeInjectivityErrors ax cur_branch inj conflicts) }
1572 | otherwise
1573 = return ()
1574
1575 gather_conflicts inj prev_branches cur_branch (acc, n) branch
1576 -- n is 0-based index of branch in prev_branches
1577 = case injectiveBranches inj cur_branch branch of
1578 InjectivityUnified ax1 ax2
1579 | ax1 `isDominatedBy` (replace_br prev_branches n ax2)
1580 -> (acc, n + 1)
1581 | otherwise
1582 -> (branch : acc, n + 1)
1583 InjectivityAccepted -> (acc, n + 1)
1584
1585 -- Replace n-th element in the list. Assumes 0-based indexing.
1586 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
1587 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
1588
1589
1590 -- Check that a "type instance" is well-formed (which includes decidability
1591 -- unless -XUndecidableInstances is given).
1592 --
1593 checkValidCoAxBranch :: Maybe ClsInstInfo
1594 -> TyCon -> CoAxBranch -> TcM ()
1595 checkValidCoAxBranch mb_clsinfo fam_tc
1596 (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
1597 , cab_lhs = typats
1598 , cab_rhs = rhs, cab_loc = loc })
1599 = checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1600
1601 -- | Do validity checks on a type family equation, including consistency
1602 -- with any enclosing class instance head, termination, and lack of
1603 -- polytypes.
1604 checkValidTyFamEqn :: Maybe ClsInstInfo
1605 -> TyCon -- ^ of the type family
1606 -> [TyVar] -- ^ bound tyvars in the equation
1607 -> [CoVar] -- ^ bound covars in the equation
1608 -> [Type] -- ^ type patterns
1609 -> Type -- ^ rhs
1610 -> SrcSpan
1611 -> TcM ()
1612 checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1613 = setSrcSpan loc $
1614 do { checkValidFamPats mb_clsinfo fam_tc tvs cvs typats
1615
1616 -- The argument patterns, and RHS, are all boxed tau types
1617 -- E.g Reject type family F (a :: k1) :: k2
1618 -- type instance F (forall a. a->a) = ...
1619 -- type instance F Int# = ...
1620 -- type instance F Int = forall a. a->a
1621 -- type instance F Int = Int#
1622 -- See Trac #9357
1623 ; checkValidMonoType rhs
1624 ; check_lifted rhs
1625
1626 -- We have a decidable instance unless otherwise permitted
1627 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1628 ; unless undecidable_ok $
1629 mapM_ addErrTc (checkFamInstRhs typats (tcTyFamInsts rhs)) }
1630
1631 -- Make sure that each type family application is
1632 -- (1) strictly smaller than the lhs,
1633 -- (2) mentions no type variable more often than the lhs, and
1634 -- (3) does not contain any further type family instances.
1635 --
1636 checkFamInstRhs :: [Type] -- lhs
1637 -> [(TyCon, [Type])] -- type family instances
1638 -> [MsgDoc]
1639 checkFamInstRhs lhsTys famInsts
1640 = mapMaybe check famInsts
1641 where
1642 size = sizeTypes lhsTys
1643 fvs = fvTypes lhsTys
1644 check (tc, tys)
1645 | not (all isTyFamFree tys) = Just (nestedMsg what)
1646 | not (null bad_tvs) = Just (noMoreMsg bad_tvs what)
1647 | size <= sizeTypes tys = Just (smallerMsg what)
1648 | otherwise = Nothing
1649 where
1650 what = text "type family application" <+> quotes (pprType (TyConApp tc tys))
1651 bad_tvs = fvTypes tys \\ fvs
1652
1653 checkValidFamPats :: Maybe ClsInstInfo -> TyCon -> [TyVar] -> [CoVar] -> [Type] -> TcM ()
1654 -- Patterns in a 'type instance' or 'data instance' decl should
1655 -- a) contain no type family applications
1656 -- (vanilla synonyms are fine, though)
1657 -- b) properly bind all their free type variables
1658 -- e.g. we disallow (Trac #7536)
1659 -- type T a = Int
1660 -- type instance F (T a) = a
1661 -- c) Have the right number of patterns
1662 -- d) For associated types, are consistently instantiated
1663 checkValidFamPats mb_clsinfo fam_tc tvs cvs ty_pats
1664 = do { -- A family instance must have exactly the same number of type
1665 -- parameters as the family declaration. You can't write
1666 -- type family F a :: * -> *
1667 -- type instance F Int y = y
1668 -- because then the type (F Int) would be like (\y.y)
1669 checkTc (length ty_pats == fam_arity) $
1670 wrongNumberOfParmsErr (fam_arity - count isInvisibleBinder fam_bndrs)
1671 -- report only explicit arguments
1672
1673 ; mapM_ checkValidTypePat ty_pats
1674
1675 ; let unbound_tcvs = filterOut (`elemVarSet` exactTyCoVarsOfTypes ty_pats) (tvs ++ cvs)
1676 ; checkTc (null unbound_tcvs) (famPatErr fam_tc unbound_tcvs ty_pats)
1677
1678 -- Check that type patterns match the class instance head
1679 ; checkConsistentFamInst mb_clsinfo fam_tc tvs ty_pats }
1680 where
1681 fam_arity = tyConArity fam_tc
1682 fam_bndrs = tyConBinders fam_tc
1683
1684
1685 checkValidTypePat :: Type -> TcM ()
1686 -- Used for type patterns in class instances,
1687 -- and in type/data family instances
1688 checkValidTypePat pat_ty
1689 = do { -- Check that pat_ty is a monotype
1690 checkValidMonoType pat_ty
1691 -- One could imagine generalising to allow
1692 -- instance C (forall a. a->a)
1693 -- but we don't know what all the consequences might be
1694
1695 -- Ensure that no type family instances occur a type pattern
1696 ; checkTc (isTyFamFree pat_ty) $
1697 tyFamInstIllegalErr pat_ty
1698
1699 ; check_lifted pat_ty }
1700
1701 isTyFamFree :: Type -> Bool
1702 -- ^ Check that a type does not contain any type family applications.
1703 isTyFamFree = null . tcTyFamInsts
1704
1705 -- Error messages
1706
1707 wrongNumberOfParmsErr :: Arity -> SDoc
1708 wrongNumberOfParmsErr exp_arity
1709 = text "Number of parameters must match family declaration; expected"
1710 <+> ppr exp_arity
1711
1712 inaccessibleCoAxBranch :: CoAxiom br -> CoAxBranch -> SDoc
1713 inaccessibleCoAxBranch fi_ax cur_branch
1714 = text "Type family instance equation is overlapped:" $$
1715 nest 2 (pprCoAxBranch fi_ax cur_branch)
1716
1717 tyFamInstIllegalErr :: Type -> SDoc
1718 tyFamInstIllegalErr ty
1719 = hang (text "Illegal type synonym family application in instance" <>
1720 colon) 2 $
1721 ppr ty
1722
1723 nestedMsg :: SDoc -> SDoc
1724 nestedMsg what
1725 = sep [ text "Illegal nested" <+> what
1726 , parens undecidableMsg ]
1727
1728 famPatErr :: TyCon -> [TyVar] -> [Type] -> SDoc
1729 famPatErr fam_tc tvs pats
1730 = hang (text "Family instance purports to bind type variable" <> plural tvs
1731 <+> pprQuotedList tvs)
1732 2 (hang (text "but the real LHS (expanding synonyms) is:")
1733 2 (pprTypeApp fam_tc (map expandTypeSynonyms pats) <+>
1734 text "= ..."))
1735
1736 {-
1737 ************************************************************************
1738 * *
1739 Telescope checking
1740 * *
1741 ************************************************************************
1742
1743 Note [Bad telescopes]
1744 ~~~~~~~~~~~~~~~~~~~~~
1745 Now that we can mix type and kind variables, there are an awful lot of
1746 ways to shoot yourself in the foot. Here are some.
1747
1748 data SameKind :: k -> k -> * -- just to force unification
1749
1750 1. data T1 a k (b :: k) (x :: SameKind a b)
1751
1752 The problem here is that we discover that a and b should have the same
1753 kind. But this kind mentions k, which is bound *after* a.
1754 (Testcase: dependent/should_fail/BadTelescope)
1755
1756 2. data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)
1757
1758 Note that b is not bound. Yet its kind mentions a. Because we have
1759 a nice rule that all implicitly bound variables come before others,
1760 this is bogus. (We could probably figure out to put b between a and c.
1761 But I think this is doing users a disservice, in the long run.)
1762 (Testcase: dependent/should_fail/BadTelescope4)
1763
1764 3. t3 :: forall a. (forall k (b :: k). SameKind a b) -> ()
1765
1766 This is a straightforward skolem escape. Note that a and b need to have
1767 the same kind.
1768 (Testcase: polykinds/T11142)
1769
1770 How do we deal with all of this? For TyCons, we have checkValidTyConTyVars.
1771 That function looks to see if any of the tyConTyVars are repeated, but
1772 it's really a telescope check. It works because all tycons are kind-generalized.
1773 If there is a bad telescope, the kind-generalization will end up generalizing
1774 over a variable bound later in the telescope.
1775
1776 For non-tycons, we do scope checking when we bring tyvars into scope,
1777 in tcImplicitTKBndrs and tcExplicitTKBndrs. Note that we also have to
1778 sort implicit binders into a well-scoped order whenever we have implicit
1779 binders to worry about. This is done in quantifyTyVars and in
1780 tcImplicitTKBndrs.
1781 -}
1782
1783 -- | Check a list of binders to see if they make a valid telescope.
1784 -- The key property we're checking for is scoping. For example:
1785 -- > data SameKind :: k -> k -> *
1786 -- > data X a k (b :: k) (c :: SameKind a b)
1787 -- Kind inference says that a's kind should be k. But that's impossible,
1788 -- because k isn't in scope when a is bound. This check has to come before
1789 -- general validity checking, because once we kind-generalise, this sort
1790 -- of problem is harder to spot (as we'll generalise over the unbound
1791 -- k in a's type.) See also Note [Bad telescopes].
1792 checkValidTelescope :: SDoc -- the original user-written telescope
1793 -> [TyVar] -- explicit vars (not necessarily zonked)
1794 -> SDoc -- note to put at bottom of message
1795 -> TcM ()
1796 checkValidTelescope hs_tvs orig_tvs extra
1797 = discardResult $ checkZonkValidTelescope hs_tvs orig_tvs extra
1798
1799 -- | Like 'checkZonkValidTelescope', but returns the zonked tyvars
1800 checkZonkValidTelescope :: SDoc
1801 -> [TyVar]
1802 -> SDoc
1803 -> TcM [TyVar]
1804 checkZonkValidTelescope hs_tvs orig_tvs extra
1805 = do { orig_tvs <- mapM zonkTyCoVarKind orig_tvs
1806 ; let (_, sorted_tidied_tvs) = tidyTyCoVarBndrs emptyTidyEnv $
1807 toposortTyVars orig_tvs
1808 ; unless (go [] emptyVarSet orig_tvs) $
1809 addErr $
1810 vcat [ hang (text "These kind and type variables:" <+> hs_tvs $$
1811 text "are out of dependency order. Perhaps try this ordering:")
1812 2 (sep (map pprTvBndr sorted_tidied_tvs))
1813 , extra ]
1814 ; return orig_tvs }
1815
1816 where
1817 go :: [TyVar] -- misplaced variables
1818 -> TyVarSet -> [TyVar] -> Bool
1819 go errs in_scope [] = null (filter (`elemVarSet` in_scope) errs)
1820 -- report an error only when the variable in the kind is brought
1821 -- into scope later in the telescope. Otherwise, we'll just quantify
1822 -- over it in kindGeneralize, as we should.
1823
1824 go errs in_scope (tv:tvs)
1825 = let bad_tvs = filterOut (`elemVarSet` in_scope) $
1826 tyCoVarsOfTypeList (tyVarKind tv)
1827 in go (bad_tvs ++ errs) (in_scope `extendVarSet` tv) tvs
1828
1829 -- | After inferring kinds of type variables, check to make sure that the
1830 -- inferred kinds any of the type variables bound in a smaller scope.
1831 -- This is a skolem escape check. See also Note [Bad telescopes].
1832 checkValidInferredKinds :: [TyVar] -- ^ vars to check (zonked)
1833 -> TyVarSet -- ^ vars out of scope
1834 -> SDoc -- ^ suffix to error message
1835 -> TcM ()
1836 checkValidInferredKinds orig_kvs out_of_scope extra
1837 = do { let bad_pairs = [ (tv, kv)
1838 | kv <- orig_kvs
1839 , Just tv <- map (lookupVarSet out_of_scope)
1840 (tyCoVarsOfTypeList (tyVarKind kv)) ]
1841 report (tidyTyVarOcc env -> tv, tidyTyVarOcc env -> kv)
1842 = addErr $
1843 text "The kind of variable" <+>
1844 quotes (ppr kv) <> text ", namely" <+>
1845 quotes (ppr (tyVarKind kv)) <> comma $$
1846 text "depends on variable" <+>
1847 quotes (ppr tv) <+> text "from an inner scope" $$
1848 text "Perhaps bind" <+> quotes (ppr kv) <+>
1849 text "sometime after binding" <+>
1850 quotes (ppr tv) $$
1851 extra
1852 ; mapM_ report bad_pairs }
1853
1854 where
1855 (env1, _) = tidyTyCoVarBndrs emptyTidyEnv orig_kvs
1856 (env, _) = tidyTyCoVarBndrs env1 (nonDetEltsUFM out_of_scope)
1857 -- It's OK to use nonDetEltsUFM here because it's only used for
1858 -- generating the error message
1859
1860 {-
1861 ************************************************************************
1862 * *
1863 \subsection{Auxiliary functions}
1864 * *
1865 ************************************************************************
1866 -}
1867
1868 -- Free variables of a type, retaining repetitions, and expanding synonyms
1869 fvType :: Type -> [TyCoVar]
1870 fvType ty | Just exp_ty <- coreView ty = fvType exp_ty
1871 fvType (TyVarTy tv) = [tv]
1872 fvType (TyConApp _ tys) = fvTypes tys
1873 fvType (LitTy {}) = []
1874 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1875 fvType (FunTy arg res) = fvType arg ++ fvType res
1876 fvType (ForAllTy (TvBndr tv _) ty)
1877 = fvType (tyVarKind tv) ++
1878 filter (/= tv) (fvType ty)
1879 fvType (CastTy ty co) = fvType ty ++ fvCo co
1880 fvType (CoercionTy co) = fvCo co
1881
1882 fvTypes :: [Type] -> [TyVar]
1883 fvTypes tys = concat (map fvType tys)
1884
1885 fvCo :: Coercion -> [TyCoVar]
1886 fvCo (Refl _ ty) = fvType ty
1887 fvCo (TyConAppCo _ _ args) = concatMap fvCo args
1888 fvCo (AppCo co arg) = fvCo co ++ fvCo arg
1889 fvCo (ForAllCo tv h co) = filter (/= tv) (fvCo co) ++ fvCo h
1890 fvCo (CoVarCo v) = [v]
1891 fvCo (AxiomInstCo _ _ args) = concatMap fvCo args
1892 fvCo (UnivCo p _ t1 t2) = fvProv p ++ fvType t1 ++ fvType t2
1893 fvCo (SymCo co) = fvCo co
1894 fvCo (TransCo co1 co2) = fvCo co1 ++ fvCo co2
1895 fvCo (NthCo _ co) = fvCo co
1896 fvCo (LRCo _ co) = fvCo co
1897 fvCo (InstCo co arg) = fvCo co ++ fvCo arg
1898 fvCo (CoherenceCo co1 co2) = fvCo co1 ++ fvCo co2
1899 fvCo (KindCo co) = fvCo co
1900 fvCo (SubCo co) = fvCo co
1901 fvCo (AxiomRuleCo _ cs) = concatMap fvCo cs
1902
1903 fvProv :: UnivCoProvenance -> [TyCoVar]
1904 fvProv UnsafeCoerceProv = []
1905 fvProv (PhantomProv co) = fvCo co
1906 fvProv (ProofIrrelProv co) = fvCo co
1907 fvProv (PluginProv _) = []
1908 fvProv (HoleProv h) = pprPanic "fvProv falls into a hole" (ppr h)
1909
1910 sizeType :: Type -> Int
1911 -- Size of a type: the number of variables and constructors
1912 sizeType ty | Just exp_ty <- coreView ty = sizeType exp_ty
1913 sizeType (TyVarTy {}) = 1
1914 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1915 sizeType (LitTy {}) = 1
1916 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1917 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1918 sizeType (ForAllTy _ ty) = sizeType ty
1919 sizeType (CastTy ty _) = sizeType ty
1920 sizeType (CoercionTy _) = 1
1921
1922 sizeTypes :: [Type] -> Int
1923 sizeTypes = sum . map sizeType
1924
1925 -- Size of a predicate
1926 --
1927 -- We are considering whether class constraints terminate.
1928 -- Equality constraints and constraints for the implicit
1929 -- parameter class always termiante so it is safe to say "size 0".
1930 -- (Implicit parameter constraints always terminate because
1931 -- there are no instances for them---they are only solved by
1932 -- "local instances" in expressions).
1933 -- See Trac #4200.
1934 sizePred :: PredType -> Int
1935 sizePred ty = goClass ty
1936 where
1937 goClass p = go (classifyPredType p)
1938
1939 go (ClassPred cls tys')
1940 | isTerminatingClass cls = 0
1941 | otherwise = sizeTypes (filterOutInvisibleTypes (classTyCon cls) tys')
1942 go (EqPred {}) = 0
1943 go (IrredPred ty) = sizeType ty
1944
1945 -- | When this says "True", ignore this class constraint during
1946 -- a termination check
1947 isTerminatingClass :: Class -> Bool
1948 isTerminatingClass cls
1949 = isIPClass cls
1950 || cls `hasKey` typeableClassKey
1951 || cls `hasKey` coercibleTyConKey
1952 || cls `hasKey` eqTyConKey
1953 || cls `hasKey` heqTyConKey
1954
1955 -- | Tidy before printing a type
1956 ppr_tidy :: TidyEnv -> Type -> SDoc
1957 ppr_tidy env ty = pprType (tidyType env ty)
1958
1959 allDistinctTyVars :: TyVarSet -> [KindOrType] -> Bool
1960 -- (allDistinctTyVars tvs tys) returns True if tys are
1961 -- a) all tyvars
1962 -- b) all distinct
1963 -- c) disjoint from tvs
1964 allDistinctTyVars _ [] = True
1965 allDistinctTyVars tkvs (ty : tys)
1966 = case getTyVar_maybe ty of
1967 Nothing -> False
1968 Just tv | tv `elemVarSet` tkvs -> False
1969 | otherwise -> allDistinctTyVars (tkvs `extendVarSet` tv) tys