Typos in comments only [ci skip]
[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 UniqFM
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 [Impedence 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 arguemt 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 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 ----------------------------------------
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 arguemt 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 -- Check language restrictions;
1021 -- but not for SPECIALISE instance pragmas
1022 ; let ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
1023 ; unless spec_inst_prag $
1024 do { checkTc (xopt LangExt.TypeSynonymInstances dflags ||
1025 all tcInstHeadTyNotSynonym ty_args)
1026 (instTypeErr clas cls_args head_type_synonym_msg)
1027 ; checkTc (xopt LangExt.FlexibleInstances dflags ||
1028 all tcInstHeadTyAppAllTyVars ty_args)
1029 (instTypeErr clas cls_args head_type_args_tyvars_msg)
1030 ; checkTc (xopt LangExt.MultiParamTypeClasses dflags ||
1031 length ty_args == 1 || -- Only count type arguments
1032 (xopt LangExt.NullaryTypeClasses dflags &&
1033 null ty_args))
1034 (instTypeErr clas cls_args head_one_type_msg) }
1035
1036 ; mapM_ checkValidTypePat ty_args }
1037 where
1038 spec_inst_prag = case ctxt of { SpecInstCtxt -> True; _ -> False }
1039
1040 head_type_synonym_msg = parens (
1041 text "All instance types must be of the form (T t1 ... tn)" $$
1042 text "where T is not a synonym." $$
1043 text "Use TypeSynonymInstances if you want to disable this.")
1044
1045 head_type_args_tyvars_msg = parens (vcat [
1046 text "All instance types must be of the form (T a1 ... an)",
1047 text "where a1 ... an are *distinct type variables*,",
1048 text "and each type variable appears at most once in the instance head.",
1049 text "Use FlexibleInstances if you want to disable this."])
1050
1051 head_one_type_msg = parens (
1052 text "Only one type can be given in an instance head." $$
1053 text "Use MultiParamTypeClasses if you want to allow more, or zero.")
1054
1055 abstract_class_msg =
1056 text "Manual instances of this class are not permitted."
1057
1058 tcInstHeadTyNotSynonym :: Type -> Bool
1059 -- Used in Haskell-98 mode, for the argument types of an instance head
1060 -- These must not be type synonyms, but everywhere else type synonyms
1061 -- are transparent, so we need a special function here
1062 tcInstHeadTyNotSynonym ty
1063 = case ty of -- Do not use splitTyConApp,
1064 -- because that expands synonyms!
1065 TyConApp tc _ -> not (isTypeSynonymTyCon tc)
1066 _ -> True
1067
1068 tcInstHeadTyAppAllTyVars :: Type -> Bool
1069 -- Used in Haskell-98 mode, for the argument types of an instance head
1070 -- These must be a constructor applied to type variable arguments.
1071 -- But we allow kind instantiations.
1072 tcInstHeadTyAppAllTyVars ty
1073 | Just (tc, tys) <- tcSplitTyConApp_maybe (dropCasts ty)
1074 = ok (filterOutInvisibleTypes tc tys) -- avoid kinds
1075
1076 | otherwise
1077 = False
1078 where
1079 -- Check that all the types are type variables,
1080 -- and that each is distinct
1081 ok tys = equalLength tvs tys && hasNoDups tvs
1082 where
1083 tvs = mapMaybe tcGetTyVar_maybe tys
1084
1085 dropCasts :: Type -> Type
1086 -- See Note [Casts during validity checking]
1087 -- This function can turn a well-kinded type into an ill-kinded
1088 -- one, so I've kept it local to this module
1089 -- To consider: drop only UnivCo(HoleProv) casts
1090 dropCasts (CastTy ty _) = dropCasts ty
1091 dropCasts (AppTy t1 t2) = mkAppTy (dropCasts t1) (dropCasts t2)
1092 dropCasts (FunTy t1 t2) = mkFunTy (dropCasts t1) (dropCasts t2)
1093 dropCasts (TyConApp tc tys) = mkTyConApp tc (map dropCasts tys)
1094 dropCasts (ForAllTy b ty) = ForAllTy (dropCastsB b) (dropCasts ty)
1095 dropCasts ty = ty -- LitTy, TyVarTy, CoercionTy
1096
1097 dropCastsB :: TyVarBinder -> TyVarBinder
1098 dropCastsB b = b -- Don't bother in the kind of a forall
1099
1100 abstractClassKeys :: [Unique]
1101 abstractClassKeys = [ heqTyConKey
1102 , eqTyConKey
1103 , coercibleTyConKey
1104 ] -- See Note [Equality class instances]
1105
1106 instTypeErr :: Class -> [Type] -> SDoc -> SDoc
1107 instTypeErr cls tys msg
1108 = hang (hang (text "Illegal instance declaration for")
1109 2 (quotes (pprClassPred cls tys)))
1110 2 msg
1111
1112 {- Note [Casts during validity checking]
1113 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1114 Consider the (bogus)
1115 instance Eq Char#
1116 We elaborate to 'Eq (Char# |> UnivCo(hole))' where the hole is an
1117 insoluble equality constraint for * ~ #. We'll report the insoluble
1118 constraint separately, but we don't want to *also* complain that Eq is
1119 not applied to a type constructor. So we look gaily look through
1120 CastTys here.
1121
1122 Another example: Eq (Either a). Then we actually get a cast in
1123 the middle:
1124 Eq ((Either |> g) a)
1125
1126
1127 Note [Valid 'deriving' predicate]
1128 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1129 validDerivPred checks for OK 'deriving' context. See Note [Exotic
1130 derived instance contexts] in TcDeriv. However the predicate is
1131 here because it uses sizeTypes, fvTypes.
1132
1133 It checks for three things
1134
1135 * No repeated variables (hasNoDups fvs)
1136
1137 * No type constructors. This is done by comparing
1138 sizeTypes tys == length (fvTypes tys)
1139 sizeTypes counts variables and constructors; fvTypes returns variables.
1140 So if they are the same, there must be no constructors. But there
1141 might be applications thus (f (g x)).
1142
1143 Note that tys only includes the visible arguments of the class type
1144 constructor. Including the non-vivisble arguments can cause the following,
1145 perfectly valid instance to be rejected:
1146 class Category (cat :: k -> k -> *) where ...
1147 newtype T (c :: * -> * -> *) a b = MkT (c a b)
1148 instance Category c => Category (T c) where ...
1149 since the first argument to Category is a non-visible *, which sizeTypes
1150 would count as a constructor! See Trac #11833.
1151
1152 * Also check for a bizarre corner case, when the derived instance decl
1153 would look like
1154 instance C a b => D (T a) where ...
1155 Note that 'b' isn't a parameter of T. This gives rise to all sorts of
1156 problems; in particular, it's hard to compare solutions for equality
1157 when finding the fixpoint, and that means the inferContext loop does
1158 not converge. See Trac #5287.
1159
1160 Note [Equality class instances]
1161 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1162 We can't have users writing instances for the equality classes. But we
1163 still need to be able to write instances for them ourselves. So we allow
1164 instances only in the defining module.
1165
1166 -}
1167
1168 validDerivPred :: TyVarSet -> PredType -> Bool
1169 -- See Note [Valid 'deriving' predicate]
1170 validDerivPred tv_set pred
1171 = case classifyPredType pred of
1172 ClassPred cls tys -> cls `hasKey` typeableClassKey
1173 -- Typeable constraints are bigger than they appear due
1174 -- to kind polymorphism, but that's OK
1175 || check_tys cls tys
1176 EqPred {} -> False -- reject equality constraints
1177 _ -> True -- Non-class predicates are ok
1178 where
1179 check_tys cls tys
1180 = hasNoDups fvs
1181 -- use sizePred to ignore implicit args
1182 && sizePred pred == fromIntegral (length fvs)
1183 && all (`elemVarSet` tv_set) fvs
1184 where tys' = filterOutInvisibleTypes (classTyCon cls) tys
1185 fvs = fvTypes tys'
1186
1187 {-
1188 ************************************************************************
1189 * *
1190 \subsection{Checking instance for termination}
1191 * *
1192 ************************************************************************
1193 -}
1194
1195 checkValidInstance :: UserTypeCtxt -> LHsSigType Name -> Type
1196 -> TcM ([TyVar], ThetaType, Class, [Type])
1197 checkValidInstance ctxt hs_type ty
1198 | Just (clas,inst_tys) <- getClassPredTys_maybe tau
1199 , inst_tys `lengthIs` classArity clas
1200 = do { setSrcSpan head_loc (checkValidInstHead ctxt clas inst_tys)
1201 ; traceTc "checkValidInstance {" (ppr ty)
1202 ; checkValidTheta ctxt theta
1203
1204 -- The Termination and Coverate Conditions
1205 -- Check that instance inference will terminate (if we care)
1206 -- For Haskell 98 this will already have been done by checkValidTheta,
1207 -- but as we may be using other extensions we need to check.
1208 --
1209 -- Note that the Termination Condition is *more conservative* than
1210 -- the checkAmbiguity test we do on other type signatures
1211 -- e.g. Bar a => Bar Int is ambiguous, but it also fails
1212 -- the termination condition, because 'a' appears more often
1213 -- in the constraint than in the head
1214 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1215 ; if undecidable_ok
1216 then checkAmbiguity ctxt ty
1217 else checkInstTermination inst_tys theta
1218
1219 ; traceTc "cvi 2" (ppr ty)
1220
1221 ; case (checkInstCoverage undecidable_ok clas theta inst_tys) of
1222 IsValid -> return () -- Check succeeded
1223 NotValid msg -> addErrTc (instTypeErr clas inst_tys msg)
1224
1225 ; traceTc "End checkValidInstance }" empty
1226
1227 ; return (tvs, theta, clas, inst_tys) }
1228
1229 | otherwise
1230 = failWithTc (text "Malformed instance head:" <+> ppr tau)
1231 where
1232 (tvs, theta, tau) = tcSplitSigmaTy ty
1233
1234 -- The location of the "head" of the instance
1235 head_loc = getLoc (getLHsInstDeclHead hs_type)
1236
1237 {-
1238 Note [Paterson conditions]
1239 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1240 Termination test: the so-called "Paterson conditions" (see Section 5 of
1241 "Understanding functional dependencies via Constraint Handling Rules,
1242 JFP Jan 2007).
1243
1244 We check that each assertion in the context satisfies:
1245 (1) no variable has more occurrences in the assertion than in the head, and
1246 (2) the assertion has fewer constructors and variables (taken together
1247 and counting repetitions) than the head.
1248 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1249 (which have already been checked) guarantee termination.
1250
1251 The underlying idea is that
1252
1253 for any ground substitution, each assertion in the
1254 context has fewer type constructors than the head.
1255 -}
1256
1257 checkInstTermination :: [TcType] -> ThetaType -> TcM ()
1258 -- See Note [Paterson conditions]
1259 checkInstTermination tys theta
1260 = check_preds theta
1261 where
1262 head_fvs = fvTypes tys
1263 head_size = sizeTypes tys
1264
1265 check_preds :: [PredType] -> TcM ()
1266 check_preds preds = mapM_ check preds
1267
1268 check :: PredType -> TcM ()
1269 check pred
1270 = case classifyPredType pred of
1271 EqPred {} -> return () -- See Trac #4200.
1272 IrredPred {} -> check2 pred (sizeType pred)
1273 ClassPred cls tys
1274 | isTerminatingClass cls
1275 -> return ()
1276
1277 | isCTupleClass cls -- Look inside tuple predicates; Trac #8359
1278 -> check_preds tys
1279
1280 | otherwise
1281 -> check2 pred (sizeTypes $ filterOutInvisibleTypes (classTyCon cls) tys)
1282 -- Other ClassPreds
1283
1284 check2 pred pred_size
1285 | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what)
1286 | pred_size >= head_size = addErrTc (smallerMsg what)
1287 | otherwise = return ()
1288 where
1289 what = text "constraint" <+> quotes (ppr pred)
1290 bad_tvs = fvType pred \\ head_fvs
1291
1292 smallerMsg :: SDoc -> SDoc
1293 smallerMsg what
1294 = vcat [ hang (text "The" <+> what)
1295 2 (text "is no smaller than the instance head")
1296 , parens undecidableMsg ]
1297
1298 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc
1299 noMoreMsg tvs what
1300 = vcat [ hang (text "Variable" <> plural tvs <+> quotes (pprWithCommas ppr tvs)
1301 <+> occurs <+> text "more often")
1302 2 (sep [ text "in the" <+> what
1303 , text "than in the instance head" ])
1304 , parens undecidableMsg ]
1305 where
1306 occurs = if isSingleton tvs then text "occurs"
1307 else text "occur"
1308
1309 undecidableMsg, constraintKindsMsg :: SDoc
1310 undecidableMsg = text "Use UndecidableInstances to permit this"
1311 constraintKindsMsg = text "Use ConstraintKinds to permit this"
1312
1313 {-
1314 Note [Associated type instances]
1315 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1316 We allow this:
1317 class C a where
1318 type T x a
1319 instance C Int where
1320 type T (S y) Int = y
1321 type T Z Int = Char
1322
1323 Note that
1324 a) The variable 'x' is not bound by the class decl
1325 b) 'x' is instantiated to a non-type-variable in the instance
1326 c) There are several type instance decls for T in the instance
1327
1328 All this is fine. Of course, you can't give any *more* instances
1329 for (T ty Int) elsewhere, because it's an *associated* type.
1330
1331 Note [Checking consistent instantiation]
1332 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1333 See Trac #11450 for background discussion on this check.
1334
1335 class C a b where
1336 type T a x b
1337
1338 With this class decl, if we have an instance decl
1339 instance C ty1 ty2 where ...
1340 then the type instance must look like
1341 type T ty1 v ty2 = ...
1342 with exactly 'ty1' for 'a', 'ty2' for 'b', and a variable for 'x'.
1343 For example:
1344
1345 instance C [p] Int
1346 type T [p] y Int = (p,y,y)
1347
1348 Note that
1349
1350 * We used to allow completely different bound variables in the
1351 associated type instance; e.g.
1352 instance C [p] Int
1353 type T [q] y Int = ...
1354 But from GHC 8.2 onwards, we don't. It's much simpler this way.
1355 See Trac #11450.
1356
1357 * When the class variable isn't used on the RHS of the type instance,
1358 it's tempting to allow wildcards, thus
1359 instance C [p] Int
1360 type T [_] y Int = (y,y)
1361 But it's awkward to do the test, and it doesn't work if the
1362 variable is repeated:
1363 instance C (p,p) Int
1364 type T (_,_) y Int = (y,y)
1365 Even though 'p' is not used on the RHS, we still need to use 'p'
1366 on the LHS to establish the repeated pattern. So to keep it simple
1367 we just require equality.
1368
1369 * We also check that any non-class-tyvars are instantiated with
1370 distinct tyvars. That rules out
1371 instance C [p] Int where
1372 type T [p] Bool Int = p -- Note Bool
1373 type T [p] Char Int = p -- Note Char
1374
1375 and
1376 instance C [p] Int where
1377 type T [p] p Int = p -- Note repeated 'p' on LHS
1378 It's consistent to do this because we don't allow this kind of
1379 instantiation for the class-tyvar arguments of the family.
1380
1381 Overall, we can have exactly one type instance for each
1382 associated type. If you wantmore, use an auxiliary family.
1383
1384 Implementation
1385 * Form the mini-envt from the class type variables a,b
1386 to the instance decl types [p],Int: [a->[p], b->Int]
1387
1388 * Look at the tyvars a,x,b of the type family constructor T
1389 (it shares tyvars with the class C)
1390
1391 * Apply the mini-evnt to them, and check that the result is
1392 consistent with the instance types [p] y Int
1393
1394 We make all the instance type variables scope over the
1395 type instances, of course, which picks up non-obvious kinds. Eg
1396 class Foo (a :: k) where
1397 type F a
1398 instance Foo (b :: k -> k) where
1399 type F b = Int
1400 Here the instance is kind-indexed and really looks like
1401 type F (k->k) (b::k->k) = Int
1402 But if the 'b' didn't scope, we would make F's instance too
1403 poly-kinded.
1404 -}
1405
1406 -- | Extra information about the parent instance declaration, needed
1407 -- when type-checking associated types. The 'Class' is the enclosing
1408 -- class, the [TyVar] are the type variable of the instance decl,
1409 -- and and the @VarEnv Type@ maps class variables to their instance
1410 -- types.
1411 type ClsInstInfo = (Class, [TyVar], VarEnv Type)
1412
1413 type AssocInstArgShape = (Maybe Type, Type)
1414 -- AssocInstArgShape is used only for associated family instances
1415 -- (mb_exp, actual)
1416 -- mb_exp = Just ty => this arg corresponds to a class variable
1417 -- = Nothing => it doesn't correspond to a class variable
1418 -- e.g. class C b where
1419 -- type F a b c
1420 -- instance C [x] where
1421 -- type F p [x] q
1422 -- We get [AssocInstArgShape] = [ (Nothing, p)
1423 -- , (Just [x], [x])
1424 -- , (Nothing, q)]
1425
1426 checkConsistentFamInst
1427 :: Maybe ClsInstInfo
1428 -> TyCon -- ^ Family tycon
1429 -> [TyVar] -- ^ Type variables of the family instance
1430 -> [Type] -- ^ Type patterns from instance
1431 -> TcM ()
1432 -- See Note [Checking consistent instantiation]
1433
1434 checkConsistentFamInst Nothing _ _ _ = return ()
1435 checkConsistentFamInst (Just (clas, inst_tvs, mini_env)) fam_tc _at_tvs at_tys
1436 = do { -- Check that the associated type indeed comes from this class
1437 checkTc (Just clas == tyConAssoc_maybe fam_tc)
1438 (badATErr (className clas) (tyConName fam_tc))
1439
1440 -- Check type args first (more comprehensible)
1441 ; checkTc (all check_arg type_shapes) pp_wrong_at_arg
1442 ; checkTc (check_poly_args type_shapes) pp_wrong_at_tyvars
1443
1444 -- And now kind args
1445 ; checkTc (all check_arg kind_shapes)
1446 (pp_wrong_at_arg $$ ppSuggestExplicitKinds)
1447 ; checkTc (check_poly_args kind_shapes)
1448 (pp_wrong_at_tyvars $$ ppSuggestExplicitKinds)
1449
1450 ; traceTc "cfi" (vcat [ ppr inst_tvs
1451 , ppr arg_shapes
1452 , ppr mini_env ]) }
1453 where
1454 arg_shapes :: [AssocInstArgShape]
1455 arg_shapes = [ (lookupVarEnv mini_env fam_tc_tv, at_ty)
1456 | (fam_tc_tv, at_ty) <- tyConTyVars fam_tc `zip` at_tys ]
1457
1458 (kind_shapes, type_shapes) = partitionInvisibles fam_tc snd arg_shapes
1459
1460 check_arg :: AssocInstArgShape -> Bool
1461 check_arg (Just exp_ty, at_ty) = exp_ty `tcEqType` at_ty
1462 check_arg (Nothing, _ ) = True -- Arg position does not correspond
1463 -- to a class variable
1464 check_poly_args :: [(Maybe Type,Type)] -> Bool
1465 check_poly_args arg_shapes
1466 = allDistinctTyVars (mkVarSet inst_tvs)
1467 [ at_ty | (Nothing, at_ty) <- arg_shapes ]
1468
1469 pp_wrong_at_arg
1470 = vcat [ text "Type indexes must match class instance head"
1471 , pp_exp_act ]
1472
1473 pp_wrong_at_tyvars
1474 = vcat [ text "Polymorphic type indexes of associated type" <+> quotes (ppr fam_tc)
1475 , nest 2 $ vcat [ text "(i.e. ones independent of the class type variables)"
1476 , text "must be distinct type variables" ]
1477 , pp_exp_act ]
1478
1479 pp_exp_act
1480 = vcat [ text "Expected:" <+> ppr (mkTyConApp fam_tc expected_args)
1481 , text " Actual:" <+> ppr (mkTyConApp fam_tc at_tys)
1482 , sdocWithDynFlags $ \dflags ->
1483 ppWhen (has_poly_args dflags) $
1484 vcat [ text "where the `<tv>' arguments are type variables,"
1485 , text "distinct from each other and from the instance variables" ] ]
1486
1487 expected_args = [ exp_ty `orElse` mk_tv at_ty | (exp_ty, at_ty) <- arg_shapes ]
1488 mk_tv at_ty = mkTyVarTy (mkTyVar tv_name (typeKind at_ty))
1489 tv_name = mkInternalName (mkAlphaTyVarUnique 1) (mkTyVarOcc "<tv>") noSrcSpan
1490
1491 has_poly_args dflags = any (isNothing . fst) shapes
1492 where
1493 shapes | gopt Opt_PrintExplicitKinds dflags = arg_shapes
1494 | otherwise = type_shapes
1495
1496 badATErr :: Name -> Name -> SDoc
1497 badATErr clas op
1498 = hsep [text "Class", quotes (ppr clas),
1499 text "does not have an associated type", quotes (ppr op)]
1500
1501
1502 {-
1503 ************************************************************************
1504 * *
1505 Checking type instance well-formedness and termination
1506 * *
1507 ************************************************************************
1508 -}
1509
1510 checkValidCoAxiom :: CoAxiom Branched -> TcM ()
1511 checkValidCoAxiom ax@(CoAxiom { co_ax_tc = fam_tc, co_ax_branches = branches })
1512 = do { mapM_ (checkValidCoAxBranch Nothing fam_tc) branch_list
1513 ; foldlM_ check_branch_compat [] branch_list }
1514 where
1515 branch_list = fromBranches branches
1516 injectivity = familyTyConInjectivityInfo fam_tc
1517
1518 check_branch_compat :: [CoAxBranch] -- previous branches in reverse order
1519 -> CoAxBranch -- current branch
1520 -> TcM [CoAxBranch]-- current branch : previous branches
1521 -- Check for
1522 -- (a) this branch is dominated by previous ones
1523 -- (b) failure of injectivity
1524 check_branch_compat prev_branches cur_branch
1525 | cur_branch `isDominatedBy` prev_branches
1526 = do { addWarnAt NoReason (coAxBranchSpan cur_branch) $
1527 inaccessibleCoAxBranch ax cur_branch
1528 ; return prev_branches }
1529 | otherwise
1530 = do { check_injectivity prev_branches cur_branch
1531 ; return (cur_branch : prev_branches) }
1532
1533 -- Injectivity check: check whether a new (CoAxBranch) can extend
1534 -- already checked equations without violating injectivity
1535 -- annotation supplied by the user.
1536 -- See Note [Verifying injectivity annotation] in FamInstEnv
1537 check_injectivity prev_branches cur_branch
1538 | Injective inj <- injectivity
1539 = do { let conflicts =
1540 fst $ foldl (gather_conflicts inj prev_branches cur_branch)
1541 ([], 0) prev_branches
1542 ; mapM_ (\(err, span) -> setSrcSpan span $ addErr err)
1543 (makeInjectivityErrors ax cur_branch inj conflicts) }
1544 | otherwise
1545 = return ()
1546
1547 gather_conflicts inj prev_branches cur_branch (acc, n) branch
1548 -- n is 0-based index of branch in prev_branches
1549 = case injectiveBranches inj cur_branch branch of
1550 InjectivityUnified ax1 ax2
1551 | ax1 `isDominatedBy` (replace_br prev_branches n ax2)
1552 -> (acc, n + 1)
1553 | otherwise
1554 -> (branch : acc, n + 1)
1555 InjectivityAccepted -> (acc, n + 1)
1556
1557 -- Replace n-th element in the list. Assumes 0-based indexing.
1558 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
1559 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
1560
1561
1562 -- Check that a "type instance" is well-formed (which includes decidability
1563 -- unless -XUndecidableInstances is given).
1564 --
1565 checkValidCoAxBranch :: Maybe ClsInstInfo
1566 -> TyCon -> CoAxBranch -> TcM ()
1567 checkValidCoAxBranch mb_clsinfo fam_tc
1568 (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
1569 , cab_lhs = typats
1570 , cab_rhs = rhs, cab_loc = loc })
1571 = checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1572
1573 -- | Do validity checks on a type family equation, including consistency
1574 -- with any enclosing class instance head, termination, and lack of
1575 -- polytypes.
1576 checkValidTyFamEqn :: Maybe ClsInstInfo
1577 -> TyCon -- ^ of the type family
1578 -> [TyVar] -- ^ bound tyvars in the equation
1579 -> [CoVar] -- ^ bound covars in the equation
1580 -> [Type] -- ^ type patterns
1581 -> Type -- ^ rhs
1582 -> SrcSpan
1583 -> TcM ()
1584 checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1585 = setSrcSpan loc $
1586 do { checkValidFamPats mb_clsinfo fam_tc tvs cvs typats
1587
1588 -- The argument patterns, and RHS, are all boxed tau types
1589 -- E.g Reject type family F (a :: k1) :: k2
1590 -- type instance F (forall a. a->a) = ...
1591 -- type instance F Int# = ...
1592 -- type instance F Int = forall a. a->a
1593 -- type instance F Int = Int#
1594 -- See Trac #9357
1595 ; checkValidMonoType rhs
1596
1597 -- We have a decidable instance unless otherwise permitted
1598 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1599 ; unless undecidable_ok $
1600 mapM_ addErrTc (checkFamInstRhs typats (tcTyFamInsts rhs)) }
1601
1602 -- Make sure that each type family application is
1603 -- (1) strictly smaller than the lhs,
1604 -- (2) mentions no type variable more often than the lhs, and
1605 -- (3) does not contain any further type family instances.
1606 --
1607 checkFamInstRhs :: [Type] -- lhs
1608 -> [(TyCon, [Type])] -- type family instances
1609 -> [MsgDoc]
1610 checkFamInstRhs lhsTys famInsts
1611 = mapMaybe check famInsts
1612 where
1613 size = sizeTypes lhsTys
1614 fvs = fvTypes lhsTys
1615 check (tc, tys)
1616 | not (all isTyFamFree tys) = Just (nestedMsg what)
1617 | not (null bad_tvs) = Just (noMoreMsg bad_tvs what)
1618 | size <= sizeTypes tys = Just (smallerMsg what)
1619 | otherwise = Nothing
1620 where
1621 what = text "type family application" <+> quotes (pprType (TyConApp tc tys))
1622 bad_tvs = fvTypes tys \\ fvs
1623
1624 checkValidFamPats :: Maybe ClsInstInfo -> TyCon -> [TyVar] -> [CoVar] -> [Type] -> TcM ()
1625 -- Patterns in a 'type instance' or 'data instance' decl should
1626 -- a) contain no type family applications
1627 -- (vanilla synonyms are fine, though)
1628 -- b) properly bind all their free type variables
1629 -- e.g. we disallow (Trac #7536)
1630 -- type T a = Int
1631 -- type instance F (T a) = a
1632 -- c) Have the right number of patterns
1633 -- d) For associated types, are consistently instantiated
1634 checkValidFamPats mb_clsinfo fam_tc tvs cvs ty_pats
1635 = do { -- A family instance must have exactly the same number of type
1636 -- parameters as the family declaration. You can't write
1637 -- type family F a :: * -> *
1638 -- type instance F Int y = y
1639 -- because then the type (F Int) would be like (\y.y)
1640 checkTc (length ty_pats == fam_arity) $
1641 wrongNumberOfParmsErr (fam_arity - count isInvisibleTyConBinder fam_bndrs)
1642 -- report only explicit arguments
1643
1644 ; mapM_ checkValidTypePat ty_pats
1645
1646 ; let unbound_tcvs = filterOut (`elemVarSet` exactTyCoVarsOfTypes ty_pats) (tvs ++ cvs)
1647 ; checkTc (null unbound_tcvs) (famPatErr fam_tc unbound_tcvs ty_pats)
1648
1649 -- Check that type patterns match the class instance head
1650 ; checkConsistentFamInst mb_clsinfo fam_tc tvs ty_pats }
1651 where
1652 fam_arity = tyConArity fam_tc
1653 fam_bndrs = tyConBinders fam_tc
1654
1655
1656 checkValidTypePat :: Type -> TcM ()
1657 -- Used for type patterns in class instances,
1658 -- and in type/data family instances
1659 checkValidTypePat pat_ty
1660 = do { -- Check that pat_ty is a monotype
1661 checkValidMonoType pat_ty
1662 -- One could imagine generalising to allow
1663 -- instance C (forall a. a->a)
1664 -- but we don't know what all the consequences might be
1665
1666 -- Ensure that no type family instances occur a type pattern
1667 ; checkTc (isTyFamFree pat_ty) $
1668 tyFamInstIllegalErr pat_ty }
1669
1670 isTyFamFree :: Type -> Bool
1671 -- ^ Check that a type does not contain any type family applications.
1672 isTyFamFree = null . tcTyFamInsts
1673
1674 -- Error messages
1675
1676 wrongNumberOfParmsErr :: Arity -> SDoc
1677 wrongNumberOfParmsErr exp_arity
1678 = text "Number of parameters must match family declaration; expected"
1679 <+> ppr exp_arity
1680
1681 inaccessibleCoAxBranch :: CoAxiom br -> CoAxBranch -> SDoc
1682 inaccessibleCoAxBranch fi_ax cur_branch
1683 = text "Type family instance equation is overlapped:" $$
1684 nest 2 (pprCoAxBranch fi_ax cur_branch)
1685
1686 tyFamInstIllegalErr :: Type -> SDoc
1687 tyFamInstIllegalErr ty
1688 = hang (text "Illegal type synonym family application in instance" <>
1689 colon) 2 $
1690 ppr ty
1691
1692 nestedMsg :: SDoc -> SDoc
1693 nestedMsg what
1694 = sep [ text "Illegal nested" <+> what
1695 , parens undecidableMsg ]
1696
1697 famPatErr :: TyCon -> [TyVar] -> [Type] -> SDoc
1698 famPatErr fam_tc tvs pats
1699 = hang (text "Family instance purports to bind type variable" <> plural tvs
1700 <+> pprQuotedList tvs)
1701 2 (hang (text "but the real LHS (expanding synonyms) is:")
1702 2 (pprTypeApp fam_tc (map expandTypeSynonyms pats) <+>
1703 text "= ..."))
1704
1705 {-
1706 ************************************************************************
1707 * *
1708 Telescope checking
1709 * *
1710 ************************************************************************
1711
1712 Note [Bad telescopes]
1713 ~~~~~~~~~~~~~~~~~~~~~
1714 Now that we can mix type and kind variables, there are an awful lot of
1715 ways to shoot yourself in the foot. Here are some.
1716
1717 data SameKind :: k -> k -> * -- just to force unification
1718
1719 1. data T1 a k (b :: k) (x :: SameKind a b)
1720
1721 The problem here is that we discover that a and b should have the same
1722 kind. But this kind mentions k, which is bound *after* a.
1723 (Testcase: dependent/should_fail/BadTelescope)
1724
1725 2. data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)
1726
1727 Note that b is not bound. Yet its kind mentions a. Because we have
1728 a nice rule that all implicitly bound variables come before others,
1729 this is bogus. (We could probably figure out to put b between a and c.
1730 But I think this is doing users a disservice, in the long run.)
1731 (Testcase: dependent/should_fail/BadTelescope4)
1732
1733 3. t3 :: forall a. (forall k (b :: k). SameKind a b) -> ()
1734
1735 This is a straightforward skolem escape. Note that a and b need to have
1736 the same kind.
1737 (Testcase: polykinds/T11142)
1738
1739 How do we deal with all of this? For TyCons, we have checkValidTyConTyVars.
1740 That function looks to see if any of the tyConTyVars are repeated, but
1741 it's really a telescope check. It works because all tycons are kind-generalized.
1742 If there is a bad telescope, the kind-generalization will end up generalizing
1743 over a variable bound later in the telescope.
1744
1745 For non-tycons, we do scope checking when we bring tyvars into scope,
1746 in tcImplicitTKBndrs and tcExplicitTKBndrs. Note that we also have to
1747 sort implicit binders into a well-scoped order whenever we have implicit
1748 binders to worry about. This is done in quantifyTyVars and in
1749 tcImplicitTKBndrs.
1750 -}
1751
1752 -- | Check a list of binders to see if they make a valid telescope.
1753 -- The key property we're checking for is scoping. For example:
1754 -- > data SameKind :: k -> k -> *
1755 -- > data X a k (b :: k) (c :: SameKind a b)
1756 -- Kind inference says that a's kind should be k. But that's impossible,
1757 -- because k isn't in scope when a is bound. This check has to come before
1758 -- general validity checking, because once we kind-generalise, this sort
1759 -- of problem is harder to spot (as we'll generalise over the unbound
1760 -- k in a's type.) See also Note [Bad telescopes].
1761 checkValidTelescope :: SDoc -- the original user-written telescope
1762 -> [TyVar] -- explicit vars (not necessarily zonked)
1763 -> SDoc -- note to put at bottom of message
1764 -> TcM ()
1765 checkValidTelescope hs_tvs orig_tvs extra
1766 = discardResult $ checkZonkValidTelescope hs_tvs orig_tvs extra
1767
1768 -- | Like 'checkZonkValidTelescope', but returns the zonked tyvars
1769 checkZonkValidTelescope :: SDoc
1770 -> [TyVar]
1771 -> SDoc
1772 -> TcM [TyVar]
1773 checkZonkValidTelescope hs_tvs orig_tvs extra
1774 = do { orig_tvs <- mapM zonkTyCoVarKind orig_tvs
1775 ; let (_, sorted_tidied_tvs) = tidyTyCoVarBndrs emptyTidyEnv $
1776 toposortTyVars orig_tvs
1777 ; unless (go [] emptyVarSet orig_tvs) $
1778 addErr $
1779 vcat [ hang (text "These kind and type variables:" <+> hs_tvs $$
1780 text "are out of dependency order. Perhaps try this ordering:")
1781 2 (sep (map pprTyVar sorted_tidied_tvs))
1782 , extra ]
1783 ; return orig_tvs }
1784
1785 where
1786 go :: [TyVar] -- misplaced variables
1787 -> TyVarSet -> [TyVar] -> Bool
1788 go errs in_scope [] = null (filter (`elemVarSet` in_scope) errs)
1789 -- report an error only when the variable in the kind is brought
1790 -- into scope later in the telescope. Otherwise, we'll just quantify
1791 -- over it in kindGeneralize, as we should.
1792
1793 go errs in_scope (tv:tvs)
1794 = let bad_tvs = filterOut (`elemVarSet` in_scope) $
1795 tyCoVarsOfTypeList (tyVarKind tv)
1796 in go (bad_tvs ++ errs) (in_scope `extendVarSet` tv) tvs
1797
1798 -- | After inferring kinds of type variables, check to make sure that the
1799 -- inferred kinds any of the type variables bound in a smaller scope.
1800 -- This is a skolem escape check. See also Note [Bad telescopes].
1801 checkValidInferredKinds :: [TyVar] -- ^ vars to check (zonked)
1802 -> TyVarSet -- ^ vars out of scope
1803 -> SDoc -- ^ suffix to error message
1804 -> TcM ()
1805 checkValidInferredKinds orig_kvs out_of_scope extra
1806 = do { let bad_pairs = [ (tv, kv)
1807 | kv <- orig_kvs
1808 , Just tv <- map (lookupVarSet out_of_scope)
1809 (tyCoVarsOfTypeList (tyVarKind kv)) ]
1810 report (tidyTyVarOcc env -> tv, tidyTyVarOcc env -> kv)
1811 = addErr $
1812 text "The kind of variable" <+>
1813 quotes (ppr kv) <> text ", namely" <+>
1814 quotes (ppr (tyVarKind kv)) <> comma $$
1815 text "depends on variable" <+>
1816 quotes (ppr tv) <+> text "from an inner scope" $$
1817 text "Perhaps bind" <+> quotes (ppr kv) <+>
1818 text "sometime after binding" <+>
1819 quotes (ppr tv) $$
1820 extra
1821 ; mapM_ report bad_pairs }
1822
1823 where
1824 (env1, _) = tidyTyCoVarBndrs emptyTidyEnv orig_kvs
1825 (env, _) = tidyTyCoVarBndrs env1 (nonDetEltsUFM out_of_scope)
1826 -- It's OK to use nonDetEltsUFM here because it's only used for
1827 -- generating the error message
1828
1829 {-
1830 ************************************************************************
1831 * *
1832 \subsection{Auxiliary functions}
1833 * *
1834 ************************************************************************
1835 -}
1836
1837 -- Free variables of a type, retaining repetitions, and expanding synonyms
1838 fvType :: Type -> [TyCoVar]
1839 fvType ty | Just exp_ty <- coreView ty = fvType exp_ty
1840 fvType (TyVarTy tv) = [tv]
1841 fvType (TyConApp _ tys) = fvTypes tys
1842 fvType (LitTy {}) = []
1843 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1844 fvType (FunTy arg res) = fvType arg ++ fvType res
1845 fvType (ForAllTy (TvBndr tv _) ty)
1846 = fvType (tyVarKind tv) ++
1847 filter (/= tv) (fvType ty)
1848 fvType (CastTy ty co) = fvType ty ++ fvCo co
1849 fvType (CoercionTy co) = fvCo co
1850
1851 fvTypes :: [Type] -> [TyVar]
1852 fvTypes tys = concat (map fvType tys)
1853
1854 fvCo :: Coercion -> [TyCoVar]
1855 fvCo (Refl _ ty) = fvType ty
1856 fvCo (TyConAppCo _ _ args) = concatMap fvCo args
1857 fvCo (AppCo co arg) = fvCo co ++ fvCo arg
1858 fvCo (ForAllCo tv h co) = filter (/= tv) (fvCo co) ++ fvCo h
1859 fvCo (CoVarCo v) = [v]
1860 fvCo (AxiomInstCo _ _ args) = concatMap fvCo args
1861 fvCo (UnivCo p _ t1 t2) = fvProv p ++ fvType t1 ++ fvType t2
1862 fvCo (SymCo co) = fvCo co
1863 fvCo (TransCo co1 co2) = fvCo co1 ++ fvCo co2
1864 fvCo (NthCo _ co) = fvCo co
1865 fvCo (LRCo _ co) = fvCo co
1866 fvCo (InstCo co arg) = fvCo co ++ fvCo arg
1867 fvCo (CoherenceCo co1 co2) = fvCo co1 ++ fvCo co2
1868 fvCo (KindCo co) = fvCo co
1869 fvCo (SubCo co) = fvCo co
1870 fvCo (AxiomRuleCo _ cs) = concatMap fvCo cs
1871
1872 fvProv :: UnivCoProvenance -> [TyCoVar]
1873 fvProv UnsafeCoerceProv = []
1874 fvProv (PhantomProv co) = fvCo co
1875 fvProv (ProofIrrelProv co) = fvCo co
1876 fvProv (PluginProv _) = []
1877 fvProv (HoleProv h) = pprPanic "fvProv falls into a hole" (ppr h)
1878
1879 sizeType :: Type -> Int
1880 -- Size of a type: the number of variables and constructors
1881 sizeType ty | Just exp_ty <- coreView ty = sizeType exp_ty
1882 sizeType (TyVarTy {}) = 1
1883 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1884 sizeType (LitTy {}) = 1
1885 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1886 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1887 sizeType (ForAllTy _ ty) = sizeType ty
1888 sizeType (CastTy ty _) = sizeType ty
1889 sizeType (CoercionTy _) = 1
1890
1891 sizeTypes :: [Type] -> Int
1892 sizeTypes = sum . map sizeType
1893
1894 -- Size of a predicate
1895 --
1896 -- We are considering whether class constraints terminate.
1897 -- Equality constraints and constraints for the implicit
1898 -- parameter class always termiante so it is safe to say "size 0".
1899 -- (Implicit parameter constraints always terminate because
1900 -- there are no instances for them---they are only solved by
1901 -- "local instances" in expressions).
1902 -- See Trac #4200.
1903 sizePred :: PredType -> Int
1904 sizePred ty = goClass ty
1905 where
1906 goClass p = go (classifyPredType p)
1907
1908 go (ClassPred cls tys')
1909 | isTerminatingClass cls = 0
1910 | otherwise = sizeTypes (filterOutInvisibleTypes (classTyCon cls) tys')
1911 go (EqPred {}) = 0
1912 go (IrredPred ty) = sizeType ty
1913
1914 -- | When this says "True", ignore this class constraint during
1915 -- a termination check
1916 isTerminatingClass :: Class -> Bool
1917 isTerminatingClass cls
1918 = isIPClass cls
1919 || cls `hasKey` typeableClassKey
1920 || cls `hasKey` coercibleTyConKey
1921 || cls `hasKey` eqTyConKey
1922 || cls `hasKey` heqTyConKey
1923
1924 -- | Tidy before printing a type
1925 ppr_tidy :: TidyEnv -> Type -> SDoc
1926 ppr_tidy env ty = pprType (tidyType env ty)
1927
1928 allDistinctTyVars :: TyVarSet -> [KindOrType] -> Bool
1929 -- (allDistinctTyVars tvs tys) returns True if tys are
1930 -- a) all tyvars
1931 -- b) all distinct
1932 -- c) disjoint from tvs
1933 allDistinctTyVars _ [] = True
1934 allDistinctTyVars tkvs (ty : tys)
1935 = case getTyVar_maybe ty of
1936 Nothing -> False
1937 Just tv | tv `elemVarSet` tkvs -> False
1938 | otherwise -> allDistinctTyVars (tkvs `extendVarSet` tv) tys