e885e9873d6f1283f8a03bf50c3714016ceadd91
[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,
14 ClsInfo, checkValidCoAxiom, checkValidCoAxBranch,
15 checkValidTyFamEqn,
16 checkConsistentFamInst,
17 arityErr, badATErr,
18 checkValidTelescope, checkZonkValidTelescope, checkValidInferredKinds
19 ) where
20
21 #include "HsVersions.h"
22
23 -- friends:
24 import TcUnify ( tcSubType_NC )
25 import TcSimplify ( simplifyAmbiguityCheck )
26 import TyCoRep
27 import TcType hiding ( sizeType, sizeTypes )
28 import TcMType
29 import PrelNames
30 import Type
31 import Coercion
32 import Unify( tcMatchTyX )
33 import Kind
34 import CoAxiom
35 import Class
36 import TyCon
37
38 -- others:
39 import HsSyn -- HsType
40 import TcRnMonad -- TcType, amongst others
41 import FunDeps
42 import FamInstEnv ( isDominatedBy, injectiveBranches,
43 InjectivityCheckResult(..) )
44 import FamInst ( makeInjectivityErrors )
45 import Name
46 import VarEnv
47 import VarSet
48 import ErrUtils
49 import DynFlags
50 import Util
51 import ListSetOps
52 import SrcLoc
53 import Outputable
54 import BasicTypes
55 import Module
56 import qualified GHC.LanguageExtensions as LangExt
57
58 import Control.Monad
59 import Data.Maybe
60 import Data.List ( (\\) )
61
62 {-
63 ************************************************************************
64 * *
65 Checking for ambiguity
66 * *
67 ************************************************************************
68
69 Note [The ambiguity check for type signatures]
70 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
71 checkAmbiguity is a check on *user-supplied type signatures*. It is
72 *purely* there to report functions that cannot possibly be called. So for
73 example we want to reject:
74 f :: C a => Int
75 The idea is there can be no legal calls to 'f' because every call will
76 give rise to an ambiguous constraint. We could soundly omit the
77 ambiguity check on type signatures entirely, at the expense of
78 delaying ambiguity errors to call sites. Indeed, the flag
79 -XAllowAmbiguousTypes switches off the ambiguity check.
80
81 What about things like this:
82 class D a b | a -> b where ..
83 h :: D Int b => Int
84 The Int may well fix 'b' at the call site, so that signature should
85 not be rejected. Moreover, using *visible* fundeps is too
86 conservative. Consider
87 class X a b where ...
88 class D a b | a -> b where ...
89 instance D a b => X [a] b where...
90 h :: X a b => a -> a
91 Here h's type looks ambiguous in 'b', but here's a legal call:
92 ...(h [True])...
93 That gives rise to a (X [Bool] beta) constraint, and using the
94 instance means we need (D Bool beta) and that fixes 'beta' via D's
95 fundep!
96
97 Behind all these special cases there is a simple guiding principle.
98 Consider
99
100 f :: <type>
101 f = ...blah...
102
103 g :: <type>
104 g = f
105
106 You would think that the definition of g would surely typecheck!
107 After all f has exactly the same type, and g=f. But in fact f's type
108 is instantiated and the instantiated constraints are solved against
109 the originals, so in the case an ambiguous type it won't work.
110 Consider our earlier example f :: C a => Int. Then in g's definition,
111 we'll instantiate to (C alpha) and try to deduce (C alpha) from (C a),
112 and fail.
113
114 So in fact we use this as our *definition* of ambiguity. We use a
115 very similar test for *inferred* types, to ensure that they are
116 unambiguous. See Note [Impedence matching] in TcBinds.
117
118 This test is very conveniently implemented by calling
119 tcSubType <type> <type>
120 This neatly takes account of the functional dependecy stuff above,
121 and implicit parameter (see Note [Implicit parameters and ambiguity]).
122 And this is what checkAmbiguity does.
123
124 What about this, though?
125 g :: C [a] => Int
126 Is every call to 'g' ambiguous? After all, we might have
127 intance C [a] where ...
128 at the call site. So maybe that type is ok! Indeed even f's
129 quintessentially ambiguous type might, just possibly be callable:
130 with -XFlexibleInstances we could have
131 instance C a where ...
132 and now a call could be legal after all! Well, we'll reject this
133 unless the instance is available *here*.
134
135 Note [When to call checkAmbiguity]
136 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
137 We call checkAmbiguity
138 (a) on user-specified type signatures
139 (b) in checkValidType
140
141 Conncerning (b), you might wonder about nested foralls. What about
142 f :: forall b. (forall a. Eq a => b) -> b
143 The nested forall is ambiguous. Originally we called checkAmbiguity
144 in the forall case of check_type, but that had two bad consequences:
145 * We got two error messages about (Eq b) in a nested forall like this:
146 g :: forall a. Eq a => forall b. Eq b => a -> a
147 * If we try to check for ambiguity of an nested forall like
148 (forall a. Eq a => b), the implication constraint doesn't bind
149 all the skolems, which results in "No skolem info" in error
150 messages (see Trac #10432).
151
152 To avoid this, we call checkAmbiguity once, at the top, in checkValidType.
153 (I'm still a bit worried about unbound skolems when the type mentions
154 in-scope type variables.)
155
156 In fact, because of the co/contra-variance implemented in tcSubType,
157 this *does* catch function f above. too.
158
159 Concerning (a) the ambiguity check is only used for *user* types, not
160 for types coming from inteface files. The latter can legitimately
161 have ambiguous types. Example
162
163 class S a where s :: a -> (Int,Int)
164 instance S Char where s _ = (1,1)
165 f:: S a => [a] -> Int -> (Int,Int)
166 f (_::[a]) x = (a*x,b)
167 where (a,b) = s (undefined::a)
168
169 Here the worker for f gets the type
170 fw :: forall a. S a => Int -> (# Int, Int #)
171
172
173 Note [Implicit parameters and ambiguity]
174 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
175 Only a *class* predicate can give rise to ambiguity
176 An *implicit parameter* cannot. For example:
177 foo :: (?x :: [a]) => Int
178 foo = length ?x
179 is fine. The call site will supply a particular 'x'
180
181 Furthermore, the type variables fixed by an implicit parameter
182 propagate to the others. E.g.
183 foo :: (Show a, ?x::[a]) => Int
184 foo = show (?x++?x)
185 The type of foo looks ambiguous. But it isn't, because at a call site
186 we might have
187 let ?x = 5::Int in foo
188 and all is well. In effect, implicit parameters are, well, parameters,
189 so we can take their type variables into account as part of the
190 "tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
191 -}
192
193 checkAmbiguity :: UserTypeCtxt -> Type -> TcM ()
194 checkAmbiguity ctxt ty
195 | wantAmbiguityCheck ctxt
196 = do { traceTc "Ambiguity check for" (ppr ty)
197 -- Solve the constraints eagerly because an ambiguous type
198 -- can cause a cascade of further errors. Since the free
199 -- tyvars are skolemised, we can safely use tcSimplifyTop
200 ; allow_ambiguous <- xoptM LangExt.AllowAmbiguousTypes
201 ; (_wrap, wanted) <- addErrCtxt (mk_msg allow_ambiguous) $
202 captureConstraints $
203 tcSubType_NC ctxt ty ty
204 ; simplifyAmbiguityCheck ty wanted
205
206 ; traceTc "Done ambiguity check for" (ppr ty) }
207
208 | otherwise
209 = return ()
210 where
211 mk_msg allow_ambiguous
212 = vcat [ text "In the ambiguity check for" <+> what
213 , ppUnless allow_ambiguous ambig_msg ]
214 ambig_msg = text "To defer the ambiguity check to use sites, enable AllowAmbiguousTypes"
215 what | Just n <- isSigMaybe ctxt = quotes (ppr n)
216 | otherwise = pprUserTypeCtxt ctxt
217
218 wantAmbiguityCheck :: UserTypeCtxt -> Bool
219 wantAmbiguityCheck ctxt
220 = case ctxt of
221 GhciCtxt -> False -- Allow ambiguous types in GHCi's :kind command
222 -- E.g. type family T a :: * -- T :: forall k. k -> *
223 -- Then :k T should work in GHCi, not complain that
224 -- (T k) is ambiguous!
225 _ -> True
226
227
228 checkUserTypeError :: Type -> TcM ()
229 -- Check to see if the type signature mentions "TypeError blah"
230 -- anywhere in it, and fail if so.
231 --
232 -- Very unsatisfactorily (Trac #11144) we need to tidy the type
233 -- because it may have come from an /inferred/ signature, not a
234 -- user-supplied one. This is really only a half-baked fix;
235 -- the other errors in checkValidType don't do tidying, and so
236 -- may give bad error messages when given an inferred type.
237 checkUserTypeError = check
238 where
239 check ty
240 | Just msg <- userTypeError_maybe ty = fail_with msg
241 | Just (_,ts) <- splitTyConApp_maybe ty = mapM_ check ts
242 | Just (t1,t2) <- splitAppTy_maybe ty = check t1 >> check t2
243 | Just (_,t1) <- splitForAllTy_maybe ty = check t1
244 | otherwise = return ()
245
246 fail_with msg = do { env0 <- tcInitTidyEnv
247 ; let (env1, tidy_msg) = tidyOpenType env0 msg
248 ; failWithTcM (env1, pprUserTypeErrorTy tidy_msg) }
249
250
251 {-
252 ************************************************************************
253 * *
254 Checking validity of a user-defined type
255 * *
256 ************************************************************************
257
258 When dealing with a user-written type, we first translate it from an HsType
259 to a Type, performing kind checking, and then check various things that should
260 be true about it. We don't want to perform these checks at the same time
261 as the initial translation because (a) they are unnecessary for interface-file
262 types and (b) when checking a mutually recursive group of type and class decls,
263 we can't "look" at the tycons/classes yet. Also, the checks are rather
264 diverse, and used to really mess up the other code.
265
266 One thing we check for is 'rank'.
267
268 Rank 0: monotypes (no foralls)
269 Rank 1: foralls at the front only, Rank 0 inside
270 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
271
272 basic ::= tyvar | T basic ... basic
273
274 r2 ::= forall tvs. cxt => r2a
275 r2a ::= r1 -> r2a | basic
276 r1 ::= forall tvs. cxt => r0
277 r0 ::= r0 -> r0 | basic
278
279 Another thing is to check that type synonyms are saturated.
280 This might not necessarily show up in kind checking.
281 type A i = i
282 data T k = MkT (k Int)
283 f :: T A -- BAD!
284 -}
285
286 checkValidType :: UserTypeCtxt -> Type -> TcM ()
287 -- Checks that a user-written type is valid for the given context
288 -- Assumes arguemt is fully zonked
289 -- Not used for instance decls; checkValidInstance instead
290 checkValidType ctxt ty
291 = do { traceTc "checkValidType" (ppr ty <+> text "::" <+> ppr (typeKind ty))
292 ; rankn_flag <- xoptM LangExt.RankNTypes
293 ; impred_flag <- xoptM LangExt.ImpredicativeTypes
294 ; let gen_rank :: Rank -> Rank
295 gen_rank r | rankn_flag = ArbitraryRank
296 | otherwise = r
297
298 rank1 = gen_rank r1
299 rank0 = gen_rank r0
300
301 r0 = rankZeroMonoType
302 r1 = LimitedRank True r0
303
304 rank
305 = case ctxt of
306 DefaultDeclCtxt-> MustBeMonoType
307 ResSigCtxt -> MustBeMonoType
308 PatSigCtxt -> rank0
309 RuleSigCtxt _ -> rank1
310 TySynCtxt _ -> rank0
311
312 ExprSigCtxt -> rank1
313 TypeAppCtxt | impred_flag -> ArbitraryRank
314 | otherwise -> tyConArgMonoType
315 -- Normally, ImpredicativeTypes is handled in check_arg_type,
316 -- but visible type applications don't go through there.
317 -- So we do this check here.
318
319 FunSigCtxt {} -> rank1
320 InfSigCtxt _ -> ArbitraryRank -- Inferred type
321 ConArgCtxt _ -> rank1 -- We are given the type of the entire
322 -- constructor, hence rank 1
323
324 ForSigCtxt _ -> rank1
325 SpecInstCtxt -> rank1
326 ThBrackCtxt -> rank1
327 GhciCtxt -> ArbitraryRank
328 _ -> panic "checkValidType"
329 -- Can't happen; not used for *user* sigs
330
331 ; env <- tcInitOpenTidyEnv (tyCoVarsOfType ty)
332
333 -- Check the internal validity of the type itself
334 ; check_type env ctxt rank ty
335
336 -- Check that the thing has kind Type, and is lifted if necessary.
337 -- Do this *after* check_type, because we can't usefully take
338 -- the kind of an ill-formed type such as (a~Int)
339 ; check_kind env ctxt ty
340
341 ; checkUserTypeError ty
342
343 -- Check for ambiguous types. See Note [When to call checkAmbiguity]
344 -- NB: this will happen even for monotypes, but that should be cheap;
345 -- and there may be nested foralls for the subtype test to examine
346 ; checkAmbiguity ctxt ty
347
348 ; traceTc "checkValidType done" (ppr ty <+> text "::" <+> ppr (typeKind ty)) }
349
350 checkValidMonoType :: Type -> TcM ()
351 -- Assumes arguemt is fully zonked
352 checkValidMonoType ty
353 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfType ty)
354 ; check_type env SigmaCtxt MustBeMonoType ty }
355
356 check_kind :: TidyEnv -> UserTypeCtxt -> TcType -> TcM ()
357 -- Check that the type's kind is acceptable for the context
358 check_kind env ctxt ty
359 | TySynCtxt {} <- ctxt
360 , returnsConstraintKind actual_kind
361 = do { ck <- xoptM LangExt.ConstraintKinds
362 ; if ck
363 then when (isConstraintKind actual_kind)
364 (do { dflags <- getDynFlags
365 ; check_pred_ty env dflags ctxt ty })
366 else addErrTcM (constraintSynErr env actual_kind) }
367
368 | otherwise
369 = case expectedKindInCtxt ctxt of
370 TheKind k -> checkTcM (tcEqType actual_kind k) (kindErr env actual_kind)
371 OpenKind -> checkTcM (classifiesTypeWithValues actual_kind) (kindErr env actual_kind)
372 AnythingKind -> return ()
373 where
374 actual_kind = typeKind ty
375
376 -- | The kind expected in a certain context.
377 data ContextKind = TheKind Kind -- ^ a specific kind
378 | AnythingKind -- ^ any kind will do
379 | OpenKind -- ^ something of the form @TYPE _@
380
381 -- Depending on the context, we might accept any kind (for instance, in a TH
382 -- splice), or only certain kinds (like in type signatures).
383 expectedKindInCtxt :: UserTypeCtxt -> ContextKind
384 expectedKindInCtxt (TySynCtxt _) = AnythingKind
385 expectedKindInCtxt ThBrackCtxt = AnythingKind
386 expectedKindInCtxt GhciCtxt = AnythingKind
387 -- The types in a 'default' decl can have varying kinds
388 -- See Note [Extended defaults]" in TcEnv
389 expectedKindInCtxt DefaultDeclCtxt = AnythingKind
390 expectedKindInCtxt TypeAppCtxt = AnythingKind
391 expectedKindInCtxt (ForSigCtxt _) = TheKind liftedTypeKind
392 expectedKindInCtxt InstDeclCtxt = TheKind constraintKind
393 expectedKindInCtxt SpecInstCtxt = TheKind constraintKind
394 expectedKindInCtxt _ = OpenKind
395
396 {-
397 Note [Higher rank types]
398 ~~~~~~~~~~~~~~~~~~~~~~~~
399 Technically
400 Int -> forall a. a->a
401 is still a rank-1 type, but it's not Haskell 98 (Trac #5957). So the
402 validity checker allow a forall after an arrow only if we allow it
403 before -- that is, with Rank2Types or RankNTypes
404 -}
405
406 data Rank = ArbitraryRank -- Any rank ok
407
408 | LimitedRank -- Note [Higher rank types]
409 Bool -- Forall ok at top
410 Rank -- Use for function arguments
411
412 | MonoType SDoc -- Monotype, with a suggestion of how it could be a polytype
413
414 | MustBeMonoType -- Monotype regardless of flags
415
416 rankZeroMonoType, tyConArgMonoType, synArgMonoType :: Rank
417 rankZeroMonoType = MonoType (text "Perhaps you intended to use RankNTypes or Rank2Types")
418 tyConArgMonoType = MonoType (text "GHC doesn't yet support impredicative polymorphism")
419 synArgMonoType = MonoType (text "Perhaps you intended to use LiberalTypeSynonyms")
420
421 funArgResRank :: Rank -> (Rank, Rank) -- Function argument and result
422 funArgResRank (LimitedRank _ arg_rank) = (arg_rank, LimitedRank (forAllAllowed arg_rank) arg_rank)
423 funArgResRank other_rank = (other_rank, other_rank)
424
425 forAllAllowed :: Rank -> Bool
426 forAllAllowed ArbitraryRank = True
427 forAllAllowed (LimitedRank forall_ok _) = forall_ok
428 forAllAllowed _ = False
429
430 ----------------------------------------
431 -- | Fail with error message if the type is unlifted
432 check_lifted :: TidyEnv -> Type -> TcM ()
433 check_lifted _ _ = return ()
434
435 {- ------ Legacy comment ---------
436 The check_unlifted function seems entirely redundant. The
437 kind system should check for uses of unlifted types. So I've
438 removed the check. See Trac #11120 comment:19.
439
440 check_lifted env ty
441 = checkTcM (not (isUnLiftedType ty)) (unliftedArgErr env ty)
442
443 unliftedArgErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
444 unliftedArgErr env ty = (env, sep [text "Illegal unlifted type:", ppr_tidy env ty])
445 ------ End of legacy comment --------- -}
446
447
448 check_type :: TidyEnv -> UserTypeCtxt -> Rank -> Type -> TcM ()
449 -- The args say what the *type context* requires, independent
450 -- of *flag* settings. You test the flag settings at usage sites.
451 --
452 -- Rank is allowed rank for function args
453 -- Rank 0 means no for-alls anywhere
454
455 check_type env ctxt rank ty
456 | not (null tvs && null theta)
457 = do { checkTcM (forAllAllowed rank) (forAllTyErr env' rank ty)
458 -- Reject e.g. (Maybe (?x::Int => Int)),
459 -- with a decent error message
460
461 ; check_valid_theta env' SigmaCtxt theta
462 -- Allow type T = ?x::Int => Int -> Int
463 -- but not type T = ?x::Int
464
465 ; check_type env' ctxt rank tau -- Allow foralls to right of arrow
466 ; checkTcM (not (any (`elemVarSet` tyCoVarsOfType phi_kind) tvs))
467 (forAllEscapeErr env' ty tau_kind)
468 }
469 where
470 (tvs, theta, tau) = tcSplitSigmaTy ty
471 tau_kind = typeKind tau
472
473 phi_kind | null theta = tau_kind
474 | otherwise = liftedTypeKind
475 -- If there are any constraints, the kind is *. (#11405)
476
477 (env', _) = tidyTyCoVarBndrs env tvs
478
479 check_type _ _ _ (TyVarTy _) = return ()
480
481 check_type env ctxt rank (ForAllTy (Anon arg_ty) res_ty)
482 = do { check_type env ctxt arg_rank arg_ty
483 ; check_type env ctxt res_rank res_ty }
484 where
485 (arg_rank, res_rank) = funArgResRank rank
486
487 check_type env ctxt rank (AppTy ty1 ty2)
488 = do { check_arg_type env ctxt rank ty1
489 ; check_arg_type env ctxt rank ty2 }
490
491 check_type env ctxt rank ty@(TyConApp tc tys)
492 | isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
493 = check_syn_tc_app env ctxt rank ty tc tys
494 | isUnboxedTupleTyCon tc = check_ubx_tuple env ctxt ty tys
495 | otherwise = mapM_ (check_arg_type env ctxt rank) tys
496
497 check_type _ _ _ (LitTy {}) = return ()
498
499 check_type env ctxt rank (CastTy ty _) = check_type env ctxt rank ty
500
501 check_type _ _ _ ty = pprPanic "check_type" (ppr ty)
502
503 ----------------------------------------
504 check_syn_tc_app :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType
505 -> TyCon -> [KindOrType] -> TcM ()
506 -- Used for type synonyms and type synonym families,
507 -- which must be saturated,
508 -- but not data families, which need not be saturated
509 check_syn_tc_app env ctxt rank ty tc tys
510 | tc_arity <= length tys -- Saturated
511 -- Check that the synonym has enough args
512 -- This applies equally to open and closed synonyms
513 -- It's OK to have an *over-applied* type synonym
514 -- data Tree a b = ...
515 -- type Foo a = Tree [a]
516 -- f :: Foo a b -> ...
517 = do { -- See Note [Liberal type synonyms]
518 ; liberal <- xoptM LangExt.LiberalTypeSynonyms
519 ; if not liberal || isTypeFamilyTyCon tc then
520 -- For H98 and synonym families, do check the type args
521 mapM_ check_arg tys
522
523 else -- In the liberal case (only for closed syns), expand then check
524 case coreView ty of
525 Just ty' -> check_type env ctxt rank ty'
526 Nothing -> pprPanic "check_tau_type" (ppr ty) }
527
528 | GhciCtxt <- ctxt -- Accept under-saturated type synonyms in
529 -- GHCi :kind commands; see Trac #7586
530 = mapM_ check_arg tys
531
532 | otherwise
533 = failWithTc (tyConArityErr tc tys)
534 where
535 tc_arity = tyConArity tc
536 check_arg | isTypeFamilyTyCon tc = check_arg_type env ctxt rank
537 | otherwise = check_type env ctxt synArgMonoType
538
539 ----------------------------------------
540 check_ubx_tuple :: TidyEnv -> UserTypeCtxt -> KindOrType
541 -> [KindOrType] -> TcM ()
542 check_ubx_tuple env ctxt ty tys
543 = do { ub_tuples_allowed <- xoptM LangExt.UnboxedTuples
544 ; checkTcM ub_tuples_allowed (ubxArgTyErr env ty)
545
546 ; impred <- xoptM LangExt.ImpredicativeTypes
547 ; let rank' = if impred then ArbitraryRank else tyConArgMonoType
548 -- c.f. check_arg_type
549 -- However, args are allowed to be unlifted, or
550 -- more unboxed tuples, so can't use check_arg_ty
551 ; mapM_ (check_type env ctxt rank') tys }
552
553 ----------------------------------------
554 check_arg_type :: TidyEnv -> UserTypeCtxt -> Rank -> KindOrType -> TcM ()
555 -- The sort of type that can instantiate a type variable,
556 -- or be the argument of a type constructor.
557 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
558 -- Other unboxed types are very occasionally allowed as type
559 -- arguments depending on the kind of the type constructor
560 --
561 -- For example, we want to reject things like:
562 --
563 -- instance Ord a => Ord (forall s. T s a)
564 -- and
565 -- g :: T s (forall b.b)
566 --
567 -- NB: unboxed tuples can have polymorphic or unboxed args.
568 -- This happens in the workers for functions returning
569 -- product types with polymorphic components.
570 -- But not in user code.
571 -- Anyway, they are dealt with by a special case in check_tau_type
572
573 check_arg_type _ _ _ (CoercionTy {}) = return ()
574
575 check_arg_type env ctxt rank ty
576 = do { impred <- xoptM LangExt.ImpredicativeTypes
577 ; let rank' = case rank of -- Predictive => must be monotype
578 MustBeMonoType -> MustBeMonoType -- Monotype, regardless
579 _other | impred -> ArbitraryRank
580 | otherwise -> tyConArgMonoType
581 -- Make sure that MustBeMonoType is propagated,
582 -- so that we don't suggest -XImpredicativeTypes in
583 -- (Ord (forall a.a)) => a -> a
584 -- and so that if it Must be a monotype, we check that it is!
585
586 ; check_type env ctxt rank' ty
587 ; check_lifted env ty }
588 -- NB the isUnLiftedType test also checks for
589 -- T State#
590 -- where there is an illegal partial application of State# (which has
591 -- kind * -> #); see Note [The kind invariant] in TyCoRep
592
593 ----------------------------------------
594 forAllTyErr :: TidyEnv -> Rank -> Type -> (TidyEnv, SDoc)
595 forAllTyErr env rank ty
596 = ( env
597 , vcat [ hang (text "Illegal polymorphic or qualified type:") 2 (ppr_tidy env ty)
598 , suggestion ] )
599 where
600 suggestion = case rank of
601 LimitedRank {} -> text "Perhaps you intended to use RankNTypes or Rank2Types"
602 MonoType d -> d
603 _ -> Outputable.empty -- Polytype is always illegal
604
605 forAllEscapeErr :: TidyEnv -> Type -> Kind -> (TidyEnv, SDoc)
606 forAllEscapeErr env ty tau_kind
607 = ( env
608 , hang (vcat [ text "Quantified type's kind mentions quantified type variable"
609 , text "(skolem escape)" ])
610 2 (vcat [ text " type:" <+> ppr_tidy env ty
611 , text "of kind:" <+> ppr_tidy env tau_kind ]) )
612
613 ubxArgTyErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
614 ubxArgTyErr env ty = (env, sep [text "Illegal unboxed tuple type as function argument:", ppr_tidy env ty])
615
616 kindErr :: TidyEnv -> Kind -> (TidyEnv, SDoc)
617 kindErr env kind = (env, sep [text "Expecting an ordinary type, but found a type of kind", ppr_tidy env kind])
618
619 {-
620 Note [Liberal type synonyms]
621 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
622 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
623 doing validity checking. This allows us to instantiate a synonym defn
624 with a for-all type, or with a partially-applied type synonym.
625 e.g. type T a b = a
626 type S m = m ()
627 f :: S (T Int)
628 Here, T is partially applied, so it's illegal in H98. But if you
629 expand S first, then T we get just
630 f :: Int
631 which is fine.
632
633 IMPORTANT: suppose T is a type synonym. Then we must do validity
634 checking on an appliation (T ty1 ty2)
635
636 *either* before expansion (i.e. check ty1, ty2)
637 *or* after expansion (i.e. expand T ty1 ty2, and then check)
638 BUT NOT BOTH
639
640 If we do both, we get exponential behaviour!!
641
642 data TIACons1 i r c = c i ::: r c
643 type TIACons2 t x = TIACons1 t (TIACons1 t x)
644 type TIACons3 t x = TIACons2 t (TIACons1 t x)
645 type TIACons4 t x = TIACons2 t (TIACons2 t x)
646 type TIACons7 t x = TIACons4 t (TIACons3 t x)
647
648
649 ************************************************************************
650 * *
651 \subsection{Checking a theta or source type}
652 * *
653 ************************************************************************
654
655 Note [Implicit parameters in instance decls]
656 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
657 Implicit parameters _only_ allowed in type signatures; not in instance
658 decls, superclasses etc. The reason for not allowing implicit params in
659 instances is a bit subtle. If we allowed
660 instance (?x::Int, Eq a) => Foo [a] where ...
661 then when we saw
662 (e :: (?x::Int) => t)
663 it would be unclear how to discharge all the potential uses of the ?x
664 in e. For example, a constraint Foo [Int] might come out of e, and
665 applying the instance decl would show up two uses of ?x. Trac #8912.
666 -}
667
668 checkValidTheta :: UserTypeCtxt -> ThetaType -> TcM ()
669 -- Assumes arguemt is fully zonked
670 checkValidTheta ctxt theta
671 = do { env <- tcInitOpenTidyEnv (tyCoVarsOfTypes theta)
672 ; addErrCtxtM (checkThetaCtxt ctxt theta) $
673 check_valid_theta env ctxt theta }
674
675 -------------------------
676 check_valid_theta :: TidyEnv -> UserTypeCtxt -> [PredType] -> TcM ()
677 check_valid_theta _ _ []
678 = return ()
679 check_valid_theta env ctxt theta
680 = do { dflags <- getDynFlags
681 ; warnTcM (wopt Opt_WarnDuplicateConstraints dflags &&
682 notNull dups) (dupPredWarn env dups)
683 ; traceTc "check_valid_theta" (ppr theta)
684 ; mapM_ (check_pred_ty env dflags ctxt) theta }
685 where
686 (_,dups) = removeDups cmpType theta
687
688 -------------------------
689 {- Note [Validity checking for constraints]
690 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
691 We look through constraint synonyms so that we can see the underlying
692 constraint(s). For example
693 type Foo = ?x::Int
694 instance Foo => C T
695 We should reject the instance because it has an implicit parameter in
696 the context.
697
698 But we record, in 'under_syn', whether we have looked under a synonym
699 to avoid requiring language extensions at the use site. Main example
700 (Trac #9838):
701
702 {-# LANGUAGE ConstraintKinds #-}
703 module A where
704 type EqShow a = (Eq a, Show a)
705
706 module B where
707 import A
708 foo :: EqShow a => a -> String
709
710 We don't want to require ConstraintKinds in module B.
711 -}
712
713 check_pred_ty :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
714 -- Check the validity of a predicate in a signature
715 -- See Note [Validity checking for constraints]
716 check_pred_ty env dflags ctxt pred
717 = do { check_type env SigmaCtxt MustBeMonoType pred
718 ; check_pred_help False env dflags ctxt pred }
719
720 check_pred_help :: Bool -- True <=> under a type synonym
721 -> TidyEnv
722 -> DynFlags -> UserTypeCtxt
723 -> PredType -> TcM ()
724 check_pred_help under_syn env dflags ctxt pred
725 | Just pred' <- coreView pred -- Switch on under_syn when going under a
726 -- synonym (Trac #9838, yuk)
727 = check_pred_help True env dflags ctxt pred'
728 | otherwise
729 = case splitTyConApp_maybe pred of
730 Just (tc, tys)
731 | isTupleTyCon tc
732 -> check_tuple_pred under_syn env dflags ctxt pred tys
733 -- NB: this equality check must come first, because (~) is a class,
734 -- too.
735 | tc `hasKey` heqTyConKey ||
736 tc `hasKey` eqTyConKey ||
737 tc `hasKey` eqPrimTyConKey
738 -> check_eq_pred env dflags pred tc tys
739 | Just cls <- tyConClass_maybe tc
740 -> check_class_pred env dflags ctxt pred cls tys -- Includes Coercible
741 _ -> check_irred_pred under_syn env dflags ctxt pred
742
743 check_eq_pred :: TidyEnv -> DynFlags -> PredType -> TyCon -> [TcType] -> TcM ()
744 check_eq_pred env dflags pred tc tys
745 = -- Equational constraints are valid in all contexts if type
746 -- families are permitted
747 do { checkTc (length tys == tyConArity tc) (tyConArityErr tc tys)
748 ; checkTcM (xopt LangExt.TypeFamilies dflags
749 || xopt LangExt.GADTs dflags)
750 (eqPredTyErr env pred) }
751
752 check_tuple_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> [PredType] -> TcM ()
753 check_tuple_pred under_syn env dflags ctxt pred ts
754 = do { -- See Note [ConstraintKinds in predicates]
755 checkTcM (under_syn || xopt LangExt.ConstraintKinds dflags)
756 (predTupleErr env pred)
757 ; mapM_ (check_pred_help under_syn env dflags ctxt) ts }
758 -- This case will not normally be executed because without
759 -- -XConstraintKinds tuple types are only kind-checked as *
760
761 check_irred_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> TcM ()
762 check_irred_pred under_syn env dflags ctxt pred
763 -- The predicate looks like (X t1 t2) or (x t1 t2) :: Constraint
764 -- where X is a type function
765 = do { -- If it looks like (x t1 t2), require ConstraintKinds
766 -- see Note [ConstraintKinds in predicates]
767 -- But (X t1 t2) is always ok because we just require ConstraintKinds
768 -- at the definition site (Trac #9838)
769 failIfTcM (not under_syn && not (xopt LangExt.ConstraintKinds dflags)
770 && hasTyVarHead pred)
771 (predIrredErr env pred)
772
773 -- Make sure it is OK to have an irred pred in this context
774 -- See Note [Irreducible predicates in superclasses]
775 ; failIfTcM (is_superclass ctxt
776 && not (xopt LangExt.UndecidableInstances dflags)
777 && has_tyfun_head pred)
778 (predSuperClassErr env pred) }
779 where
780 is_superclass ctxt = case ctxt of { ClassSCCtxt _ -> True; _ -> False }
781 has_tyfun_head ty
782 = case tcSplitTyConApp_maybe ty of
783 Just (tc, _) -> isTypeFamilyTyCon tc
784 Nothing -> False
785
786 {- Note [ConstraintKinds in predicates]
787 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
788 Don't check for -XConstraintKinds under a type synonym, because that
789 was done at the type synonym definition site; see Trac #9838
790 e.g. module A where
791 type C a = (Eq a, Ix a) -- Needs -XConstraintKinds
792 module B where
793 import A
794 f :: C a => a -> a -- Does *not* need -XConstraintKinds
795
796 Note [Irreducible predicates in superclasses]
797 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
798 Allowing type-family calls in class superclasses is somewhat dangerous
799 because we can write:
800
801 type family Fooish x :: * -> Constraint
802 type instance Fooish () = Foo
803 class Fooish () a => Foo a where
804
805 This will cause the constraint simplifier to loop because every time we canonicalise a
806 (Foo a) class constraint we add a (Fooish () a) constraint which will be immediately
807 solved to add+canonicalise another (Foo a) constraint. -}
808
809 -------------------------
810 check_class_pred :: TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> Class -> [TcType] -> TcM ()
811 check_class_pred env dflags ctxt pred cls tys
812 | isIPClass cls
813 = do { check_arity
814 ; checkTcM (okIPCtxt ctxt) (badIPPred env pred) }
815
816 | otherwise
817 = do { check_arity
818 ; checkTcM arg_tys_ok (env, predTyVarErr (tidyType env pred)) }
819 where
820 check_arity = checkTc (classArity cls == length tys)
821 (tyConArityErr (classTyCon cls) tys)
822 flexible_contexts = xopt LangExt.FlexibleContexts dflags
823 undecidable_ok = xopt LangExt.UndecidableInstances dflags
824
825 arg_tys_ok = case ctxt of
826 SpecInstCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
827 InstDeclCtxt -> checkValidClsArgs (flexible_contexts || undecidable_ok) cls tys
828 -- Further checks on head and theta
829 -- in checkInstTermination
830 _ -> checkValidClsArgs flexible_contexts cls tys
831
832 -------------------------
833 okIPCtxt :: UserTypeCtxt -> Bool
834 -- See Note [Implicit parameters in instance decls]
835 okIPCtxt (FunSigCtxt {}) = True
836 okIPCtxt (InfSigCtxt {}) = True
837 okIPCtxt ExprSigCtxt = True
838 okIPCtxt TypeAppCtxt = True
839 okIPCtxt PatSigCtxt = True
840 okIPCtxt ResSigCtxt = True
841 okIPCtxt GenSigCtxt = True
842 okIPCtxt (ConArgCtxt {}) = True
843 okIPCtxt (ForSigCtxt {}) = True -- ??
844 okIPCtxt ThBrackCtxt = True
845 okIPCtxt GhciCtxt = True
846 okIPCtxt SigmaCtxt = True
847 okIPCtxt (DataTyCtxt {}) = True
848 okIPCtxt (PatSynCtxt {}) = True
849 okIPCtxt (TySynCtxt {}) = True -- e.g. type Blah = ?x::Int
850 -- Trac #11466
851
852 okIPCtxt (ClassSCCtxt {}) = False
853 okIPCtxt (InstDeclCtxt {}) = False
854 okIPCtxt (SpecInstCtxt {}) = False
855 okIPCtxt (RuleSigCtxt {}) = False
856 okIPCtxt DefaultDeclCtxt = False
857
858 badIPPred :: TidyEnv -> PredType -> (TidyEnv, SDoc)
859 badIPPred env pred
860 = ( env
861 , text "Illegal implicit parameter" <+> quotes (ppr_tidy env pred) )
862
863 {-
864 Note [Kind polymorphic type classes]
865 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
866 MultiParam check:
867
868 class C f where... -- C :: forall k. k -> Constraint
869 instance C Maybe where...
870
871 The dictionary gets type [C * Maybe] even if it's not a MultiParam
872 type class.
873
874 Flexibility check:
875
876 class C f where... -- C :: forall k. k -> Constraint
877 data D a = D a
878 instance C D where
879
880 The dictionary gets type [C * (D *)]. IA0_TODO it should be
881 generalized actually.
882 -}
883
884 checkThetaCtxt :: UserTypeCtxt -> ThetaType -> TidyEnv -> TcM (TidyEnv, SDoc)
885 checkThetaCtxt ctxt theta env
886 = return ( env
887 , vcat [ text "In the context:" <+> pprTheta (tidyTypes env theta)
888 , text "While checking" <+> pprUserTypeCtxt ctxt ] )
889
890 eqPredTyErr, predTupleErr, predIrredErr, predSuperClassErr :: TidyEnv -> PredType -> (TidyEnv, SDoc)
891 eqPredTyErr env pred
892 = ( env
893 , text "Illegal equational constraint" <+> ppr_tidy env pred $$
894 parens (text "Use GADTs or TypeFamilies to permit this") )
895 predTupleErr env pred
896 = ( env
897 , hang (text "Illegal tuple constraint:" <+> ppr_tidy env pred)
898 2 (parens constraintKindsMsg) )
899 predIrredErr env pred
900 = ( env
901 , hang (text "Illegal constraint:" <+> ppr_tidy env pred)
902 2 (parens constraintKindsMsg) )
903 predSuperClassErr env pred
904 = ( env
905 , hang (text "Illegal constraint" <+> quotes (ppr_tidy env pred)
906 <+> text "in a superclass context")
907 2 (parens undecidableMsg) )
908
909 predTyVarErr :: PredType -> SDoc -- type is already tidied!
910 predTyVarErr pred
911 = vcat [ hang (text "Non type-variable argument")
912 2 (text "in the constraint:" <+> ppr pred)
913 , parens (text "Use FlexibleContexts to permit this") ]
914
915 constraintSynErr :: TidyEnv -> Type -> (TidyEnv, SDoc)
916 constraintSynErr env kind
917 = ( env
918 , hang (text "Illegal constraint synonym of kind:" <+> quotes (ppr_tidy env kind))
919 2 (parens constraintKindsMsg) )
920
921 dupPredWarn :: TidyEnv -> [[PredType]] -> (TidyEnv, SDoc)
922 dupPredWarn env dups
923 = ( env
924 , text "Duplicate constraint" <> plural primaryDups <> text ":"
925 <+> pprWithCommas (ppr_tidy env) primaryDups )
926 where
927 primaryDups = map head dups
928
929 tyConArityErr :: TyCon -> [TcType] -> SDoc
930 -- For type-constructor arity errors, be careful to report
931 -- the number of /visible/ arguments required and supplied,
932 -- ignoring the /invisible/ arguments, which the user does not see.
933 -- (e.g. Trac #10516)
934 tyConArityErr tc tks
935 = arityErr (tyConFlavour tc) (tyConName tc)
936 tc_type_arity tc_type_args
937 where
938 vis_tks = filterOutInvisibleTypes tc tks
939
940 -- tc_type_arity = number of *type* args expected
941 -- tc_type_args = number of *type* args encountered
942 tc_type_arity = count isVisibleBinder $ fst $ splitPiTys (tyConKind tc)
943 tc_type_args = length vis_tks
944
945 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
946 arityErr what name n m
947 = hsep [ text "The" <+> text what, quotes (ppr name), text "should have",
948 n_arguments <> comma, text "but has been given",
949 if m==0 then text "none" else int m]
950 where
951 n_arguments | n == 0 = text "no arguments"
952 | n == 1 = text "1 argument"
953 | True = hsep [int n, text "arguments"]
954
955 {-
956 ************************************************************************
957 * *
958 \subsection{Checking for a decent instance head type}
959 * *
960 ************************************************************************
961
962 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
963 it must normally look like: @instance Foo (Tycon a b c ...) ...@
964
965 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
966 flag is on, or (2)~the instance is imported (they must have been
967 compiled elsewhere). In these cases, we let them go through anyway.
968
969 We can also have instances for functions: @instance Foo (a -> b) ...@.
970 -}
971
972 checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
973 checkValidInstHead ctxt clas cls_args
974 = do { dflags <- getDynFlags
975
976 ; mod <- getModule
977 ; checkTc (getUnique clas `notElem` abstractClassKeys ||
978 nameModule (getName clas) == mod)
979 (instTypeErr clas cls_args abstract_class_msg)
980
981 -- Check language restrictions;
982 -- but not for SPECIALISE instance pragmas
983 ; let ty_args = filterOutInvisibleTypes (classTyCon clas) cls_args
984 ; unless spec_inst_prag $
985 do { checkTc (xopt LangExt.TypeSynonymInstances dflags ||
986 all tcInstHeadTyNotSynonym ty_args)
987 (instTypeErr clas cls_args head_type_synonym_msg)
988 ; checkTc (xopt LangExt.FlexibleInstances dflags ||
989 all tcInstHeadTyAppAllTyVars ty_args)
990 (instTypeErr clas cls_args head_type_args_tyvars_msg)
991 ; checkTc (xopt LangExt.MultiParamTypeClasses dflags ||
992 length ty_args == 1 || -- Only count type arguments
993 (xopt LangExt.NullaryTypeClasses dflags &&
994 null ty_args))
995 (instTypeErr clas cls_args head_one_type_msg) }
996
997 -- May not contain type family applications
998 ; mapM_ checkTyFamFreeness ty_args
999
1000 ; mapM_ checkValidMonoType ty_args
1001 -- For now, I only allow tau-types (not polytypes) in
1002 -- the head of an instance decl.
1003 -- E.g. instance C (forall a. a->a) is rejected
1004 -- One could imagine generalising that, but I'm not sure
1005 -- what all the consequences might be
1006
1007 -- We can't have unlifted type arguments.
1008 -- check_arg_type is redundant with checkValidMonoType
1009 ; env <- tcInitOpenTidyEnv (tyCoVarsOfTypes ty_args)
1010 ; mapM_ (check_lifted env) ty_args
1011 }
1012
1013 where
1014 spec_inst_prag = case ctxt of { SpecInstCtxt -> True; _ -> False }
1015
1016 head_type_synonym_msg = parens (
1017 text "All instance types must be of the form (T t1 ... tn)" $$
1018 text "where T is not a synonym." $$
1019 text "Use TypeSynonymInstances if you want to disable this.")
1020
1021 head_type_args_tyvars_msg = parens (vcat [
1022 text "All instance types must be of the form (T a1 ... an)",
1023 text "where a1 ... an are *distinct type variables*,",
1024 text "and each type variable appears at most once in the instance head.",
1025 text "Use FlexibleInstances if you want to disable this."])
1026
1027 head_one_type_msg = parens (
1028 text "Only one type can be given in an instance head." $$
1029 text "Use MultiParamTypeClasses if you want to allow more, or zero.")
1030
1031 abstract_class_msg =
1032 text "Manual instances of this class are not permitted."
1033
1034 tcInstHeadTyNotSynonym :: Type -> Bool
1035 -- Used in Haskell-98 mode, for the argument types of an instance head
1036 -- These must not be type synonyms, but everywhere else type synonyms
1037 -- are transparent, so we need a special function here
1038 tcInstHeadTyNotSynonym ty
1039 = case ty of -- Do not use splitTyConApp,
1040 -- because that expands synonyms!
1041 TyConApp tc _ -> not (isTypeSynonymTyCon tc)
1042 _ -> True
1043
1044 tcInstHeadTyAppAllTyVars :: Type -> Bool
1045 -- Used in Haskell-98 mode, for the argument types of an instance head
1046 -- These must be a constructor applied to type variable arguments.
1047 -- But we allow kind instantiations.
1048 tcInstHeadTyAppAllTyVars ty
1049 | Just (tc, tys) <- tcSplitTyConApp_maybe (dropCasts ty)
1050 = ok (filterOutInvisibleTypes tc tys) -- avoid kinds
1051
1052 | otherwise
1053 = False
1054 where
1055 -- Check that all the types are type variables,
1056 -- and that each is distinct
1057 ok tys = equalLength tvs tys && hasNoDups tvs
1058 where
1059 tvs = mapMaybe tcGetTyVar_maybe tys
1060
1061 dropCasts :: Type -> Type
1062 -- See Note [Casts during validity checking]
1063 -- This function can turn a well-kinded type into an ill-kinded
1064 -- one, so I've kept it local to this module
1065 -- To consider: drop only UnivCo(HoleProv) casts
1066 dropCasts (CastTy ty _) = dropCasts ty
1067 dropCasts (AppTy t1 t2) = mkAppTy (dropCasts t1) (dropCasts t2)
1068 dropCasts (TyConApp tc tys) = mkTyConApp tc (map dropCasts tys)
1069 dropCasts (ForAllTy b ty) = ForAllTy (dropCastsB b) (dropCasts ty)
1070 dropCasts ty = ty -- LitTy, TyVarTy, CoercionTy
1071
1072 dropCastsB :: TyBinder -> TyBinder
1073 dropCastsB (Anon ty) = Anon (dropCasts ty)
1074 dropCastsB b = b -- Don't bother in the kind of a forall
1075
1076 abstractClassKeys :: [Unique]
1077 abstractClassKeys = [ heqTyConKey
1078 , eqTyConKey
1079 , coercibleTyConKey
1080 ] -- See Note [Equality class instances]
1081
1082 instTypeErr :: Class -> [Type] -> SDoc -> SDoc
1083 instTypeErr cls tys msg
1084 = hang (hang (text "Illegal instance declaration for")
1085 2 (quotes (pprClassPred cls tys)))
1086 2 msg
1087
1088 {- Note [Casts during validity checking]
1089 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1090 Consider the (bogus)
1091 instance Eq Char#
1092 We elaborate to 'Eq (Char# |> UnivCo(hole))' where the hole is an
1093 insoluble equality constraint for * ~ #. We'll report the insoluble
1094 constraint separately, but we don't want to *also* complain that Eq is
1095 not applied to a type constructor. So we look gaily look through
1096 CastTys here.
1097
1098 Another example: Eq (Either a). Then we actually get a cast in
1099 the middle:
1100 Eq ((Either |> g) a)
1101
1102
1103 Note [Valid 'deriving' predicate]
1104 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1105 validDerivPred checks for OK 'deriving' context. See Note [Exotic
1106 derived instance contexts] in TcDeriv. However the predicate is
1107 here because it uses sizeTypes, fvTypes.
1108
1109 It checks for three things
1110
1111 * No repeated variables (hasNoDups fvs)
1112
1113 * No type constructors. This is done by comparing
1114 sizeTypes tys == length (fvTypes tys)
1115 sizeTypes counts variables and constructors; fvTypes returns variables.
1116 So if they are the same, there must be no constructors. But there
1117 might be applications thus (f (g x)).
1118
1119 * Also check for a bizarre corner case, when the derived instance decl
1120 would look like
1121 instance C a b => D (T a) where ...
1122 Note that 'b' isn't a parameter of T. This gives rise to all sorts of
1123 problems; in particular, it's hard to compare solutions for equality
1124 when finding the fixpoint, and that means the inferContext loop does
1125 not converge. See Trac #5287.
1126
1127 Note [Equality class instances]
1128 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1129 We can't have users writing instances for the equality classes. But we
1130 still need to be able to write instances for them ourselves. So we allow
1131 instances only in the defining module.
1132
1133 -}
1134
1135 validDerivPred :: TyVarSet -> PredType -> Bool
1136 -- See Note [Valid 'deriving' predicate]
1137 validDerivPred tv_set pred
1138 = case classifyPredType pred of
1139 ClassPred cls _ -> cls `hasKey` typeableClassKey
1140 -- Typeable constraints are bigger than they appear due
1141 -- to kind polymorphism, but that's OK
1142 || check_tys
1143 EqPred {} -> False -- reject equality constraints
1144 _ -> True -- Non-class predicates are ok
1145 where
1146 check_tys = hasNoDups fvs
1147 -- use sizePred to ignore implicit args
1148 && sizePred pred == fromIntegral (length fvs)
1149 && all (`elemVarSet` tv_set) fvs
1150
1151 fvs = fvType pred
1152
1153 {-
1154 ************************************************************************
1155 * *
1156 \subsection{Checking instance for termination}
1157 * *
1158 ************************************************************************
1159 -}
1160
1161 checkValidInstance :: UserTypeCtxt -> LHsSigType Name -> Type
1162 -> TcM ([TyVar], ThetaType, Class, [Type])
1163 checkValidInstance ctxt hs_type ty
1164 | Just (clas,inst_tys) <- getClassPredTys_maybe tau
1165 , inst_tys `lengthIs` classArity clas
1166 = do { setSrcSpan head_loc (checkValidInstHead ctxt clas inst_tys)
1167 ; checkValidTheta ctxt theta
1168
1169 -- The Termination and Coverate Conditions
1170 -- Check that instance inference will terminate (if we care)
1171 -- For Haskell 98 this will already have been done by checkValidTheta,
1172 -- but as we may be using other extensions we need to check.
1173 --
1174 -- Note that the Termination Condition is *more conservative* than
1175 -- the checkAmbiguity test we do on other type signatures
1176 -- e.g. Bar a => Bar Int is ambiguous, but it also fails
1177 -- the termination condition, because 'a' appears more often
1178 -- in the constraint than in the head
1179 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1180 ; traceTc "cvi" (ppr undecidable_ok $$ ppr ty)
1181 ; if undecidable_ok
1182 then checkAmbiguity ctxt ty
1183 else checkInstTermination inst_tys theta
1184
1185 ; case (checkInstCoverage undecidable_ok clas theta inst_tys) of
1186 IsValid -> return () -- Check succeeded
1187 NotValid msg -> addErrTc (instTypeErr clas inst_tys msg)
1188
1189 ; return (tvs, theta, clas, inst_tys) }
1190
1191 | otherwise
1192 = failWithTc (text "Malformed instance head:" <+> ppr tau)
1193 where
1194 (tvs, theta, tau) = tcSplitSigmaTy ty
1195
1196 -- The location of the "head" of the instance
1197 head_loc = case splitLHsInstDeclTy hs_type of
1198 (_, _, L loc _) -> loc
1199
1200 {-
1201 Note [Paterson conditions]
1202 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1203 Termination test: the so-called "Paterson conditions" (see Section 5 of
1204 "Understanding functional dependencies via Constraint Handling Rules,
1205 JFP Jan 2007).
1206
1207 We check that each assertion in the context satisfies:
1208 (1) no variable has more occurrences in the assertion than in the head, and
1209 (2) the assertion has fewer constructors and variables (taken together
1210 and counting repetitions) than the head.
1211 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1212 (which have already been checked) guarantee termination.
1213
1214 The underlying idea is that
1215
1216 for any ground substitution, each assertion in the
1217 context has fewer type constructors than the head.
1218 -}
1219
1220 checkInstTermination :: [TcType] -> ThetaType -> TcM ()
1221 -- See Note [Paterson conditions]
1222 checkInstTermination tys theta
1223 = check_preds theta
1224 where
1225 head_fvs = fvTypes tys
1226 head_size = sizeTypes tys
1227
1228 check_preds :: [PredType] -> TcM ()
1229 check_preds preds = mapM_ check preds
1230
1231 check :: PredType -> TcM ()
1232 check pred
1233 = case classifyPredType pred of
1234 EqPred {} -> return () -- See Trac #4200.
1235 IrredPred {} -> check2 pred (sizeType pred)
1236 ClassPred cls tys
1237 | isTerminatingClass cls
1238 -> return ()
1239
1240 | isCTupleClass cls -- Look inside tuple predicates; Trac #8359
1241 -> check_preds tys
1242
1243 | otherwise
1244 -> check2 pred (sizeTypes $ filterOutInvisibleTypes (classTyCon cls) tys)
1245 -- Other ClassPreds
1246
1247 check2 pred pred_size
1248 | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what)
1249 | pred_size >= head_size = addErrTc (smallerMsg what)
1250 | otherwise = return ()
1251 where
1252 what = text "constraint" <+> quotes (ppr pred)
1253 bad_tvs = fvType pred \\ head_fvs
1254
1255 smallerMsg :: SDoc -> SDoc
1256 smallerMsg what
1257 = vcat [ hang (text "The" <+> what)
1258 2 (text "is no smaller than the instance head")
1259 , parens undecidableMsg ]
1260
1261 noMoreMsg :: [TcTyVar] -> SDoc -> SDoc
1262 noMoreMsg tvs what
1263 = vcat [ hang (text "Variable" <> plural tvs <+> quotes (pprWithCommas ppr tvs)
1264 <+> occurs <+> text "more often")
1265 2 (sep [ text "in the" <+> what
1266 , text "than in the instance head" ])
1267 , parens undecidableMsg ]
1268 where
1269 occurs = if isSingleton tvs then text "occurs"
1270 else text "occur"
1271
1272 undecidableMsg, constraintKindsMsg :: SDoc
1273 undecidableMsg = text "Use UndecidableInstances to permit this"
1274 constraintKindsMsg = text "Use ConstraintKinds to permit this"
1275
1276 {-
1277 Note [Associated type instances]
1278 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1279 We allow this:
1280 class C a where
1281 type T x a
1282 instance C Int where
1283 type T (S y) Int = y
1284 type T Z Int = Char
1285
1286 Note that
1287 a) The variable 'x' is not bound by the class decl
1288 b) 'x' is instantiated to a non-type-variable in the instance
1289 c) There are several type instance decls for T in the instance
1290
1291 All this is fine. Of course, you can't give any *more* instances
1292 for (T ty Int) elsewhere, because it's an *associated* type.
1293
1294 Note [Checking consistent instantiation]
1295 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1296 class C a b where
1297 type T a x b
1298
1299 instance C [p] Int
1300 type T [p] y Int = (p,y,y) -- Induces the family instance TyCon
1301 -- type TR p y = (p,y,y)
1302
1303 So we
1304 * Form the mini-envt from the class type variables a,b
1305 to the instance decl types [p],Int: [a->[p], b->Int]
1306
1307 * Look at the tyvars a,x,b of the type family constructor T
1308 (it shares tyvars with the class C)
1309
1310 * Apply the mini-evnt to them, and check that the result is
1311 consistent with the instance types [p] y Int
1312
1313 We do *not* assume (at this point) the the bound variables of
1314 the associated type instance decl are the same as for the parent
1315 instance decl. So, for example,
1316
1317 instance C [p] Int
1318 type T [q] y Int = ...
1319
1320 would work equally well. Reason: making the *kind* variables line
1321 up is much harder. Example (Trac #7282):
1322 class Foo (xs :: [k]) where
1323 type Bar xs :: *
1324
1325 instance Foo '[] where
1326 type Bar '[] = Int
1327 Here the instance decl really looks like
1328 instance Foo k ('[] k) where
1329 type Bar k ('[] k) = Int
1330 but the k's are not scoped, and hence won't match Uniques.
1331
1332 So instead we just match structure, with tcMatchTyX, and check
1333 that distinct type variables match 1-1 with distinct type variables.
1334
1335 HOWEVER, we *still* make the instance type variables scope over the
1336 type instances, to pick up non-obvious kinds. Eg
1337 class Foo (a :: k) where
1338 type F a
1339 instance Foo (b :: k -> k) where
1340 type F b = Int
1341 Here the instance is kind-indexed and really looks like
1342 type F (k->k) (b::k->k) = Int
1343 But if the 'b' didn't scope, we would make F's instance too
1344 poly-kinded.
1345 -}
1346
1347 -- | Extra information needed when type-checking associated types. The 'Class' is
1348 -- the enclosing class, and the @VarEnv Type@ maps class variables to their
1349 -- instance types.
1350 type ClsInfo = (Class, VarEnv Type)
1351
1352 checkConsistentFamInst
1353 :: Maybe ClsInfo
1354 -> TyCon -- ^ Family tycon
1355 -> [TyVar] -- ^ Type variables of the family instance
1356 -> [Type] -- ^ Type patterns from instance
1357 -> TcM ()
1358 -- See Note [Checking consistent instantiation]
1359
1360 checkConsistentFamInst Nothing _ _ _ = return ()
1361 checkConsistentFamInst (Just (clas, mini_env)) fam_tc at_tvs at_tys
1362 = do { -- Check that the associated type indeed comes from this class
1363 checkTc (Just clas == tyConAssoc_maybe fam_tc)
1364 (badATErr (className clas) (tyConName fam_tc))
1365
1366 -- See Note [Checking consistent instantiation] in TcTyClsDecls
1367 -- Check right to left, so that we spot type variable
1368 -- inconsistencies before (more confusing) kind variables
1369 ; discardResult $ foldrM check_arg emptyTCvSubst $
1370 tyConTyVars fam_tc `zip` at_tys }
1371 where
1372 check_arg :: (TyVar, Type) -> TCvSubst -> TcM TCvSubst
1373 check_arg (fam_tc_tv, at_ty) subst
1374 | Just inst_ty <- lookupVarEnv mini_env fam_tc_tv
1375 = case tcMatchTyX subst at_ty inst_ty of
1376 Just subst | all_distinct subst -> return subst
1377 _ -> failWithTc $ wrongATArgErr at_ty inst_ty
1378 -- No need to instantiate here, because the axiom
1379 -- uses the same type variables as the assocated class
1380 | otherwise
1381 = return subst -- Allow non-type-variable instantiation
1382 -- See Note [Associated type instances]
1383
1384 all_distinct :: TCvSubst -> Bool
1385 -- True if all the variables mapped the substitution
1386 -- map to *distinct* type *variables*
1387 all_distinct subst = go [] at_tvs
1388 where
1389 go _ [] = True
1390 go acc (tv:tvs) = case lookupTyVar subst tv of
1391 Nothing -> go acc tvs
1392 Just ty | Just tv' <- tcGetTyVar_maybe ty
1393 , tv' `notElem` acc
1394 -> go (tv' : acc) tvs
1395 _other -> False
1396
1397 badATErr :: Name -> Name -> SDoc
1398 badATErr clas op
1399 = hsep [text "Class", quotes (ppr clas),
1400 text "does not have an associated type", quotes (ppr op)]
1401
1402 wrongATArgErr :: Type -> Type -> SDoc
1403 wrongATArgErr ty instTy =
1404 sep [ text "Type indexes must match class instance head"
1405 , text "Found" <+> quotes (ppr ty)
1406 <+> text "but expected" <+> quotes (ppr instTy)
1407 ]
1408
1409 {-
1410 ************************************************************************
1411 * *
1412 Checking type instance well-formedness and termination
1413 * *
1414 ************************************************************************
1415 -}
1416
1417 checkValidCoAxiom :: CoAxiom Branched -> TcM ()
1418 checkValidCoAxiom ax@(CoAxiom { co_ax_tc = fam_tc, co_ax_branches = branches })
1419 = do { mapM_ (checkValidCoAxBranch Nothing fam_tc) branch_list
1420 ; foldlM_ check_branch_compat [] branch_list }
1421 where
1422 branch_list = fromBranches branches
1423 injectivity = familyTyConInjectivityInfo fam_tc
1424
1425 check_branch_compat :: [CoAxBranch] -- previous branches in reverse order
1426 -> CoAxBranch -- current branch
1427 -> TcM [CoAxBranch]-- current branch : previous branches
1428 -- Check for
1429 -- (a) this branch is dominated by previous ones
1430 -- (b) failure of injectivity
1431 check_branch_compat prev_branches cur_branch
1432 | cur_branch `isDominatedBy` prev_branches
1433 = do { addWarnAt (coAxBranchSpan cur_branch) $
1434 inaccessibleCoAxBranch ax cur_branch
1435 ; return prev_branches }
1436 | otherwise
1437 = do { check_injectivity prev_branches cur_branch
1438 ; return (cur_branch : prev_branches) }
1439
1440 -- Injectivity check: check whether a new (CoAxBranch) can extend
1441 -- already checked equations without violating injectivity
1442 -- annotation supplied by the user.
1443 -- See Note [Verifying injectivity annotation] in FamInstEnv
1444 check_injectivity prev_branches cur_branch
1445 | Injective inj <- injectivity
1446 = do { let conflicts =
1447 fst $ foldl (gather_conflicts inj prev_branches cur_branch)
1448 ([], 0) prev_branches
1449 ; mapM_ (\(err, span) -> setSrcSpan span $ addErr err)
1450 (makeInjectivityErrors ax cur_branch inj conflicts) }
1451 | otherwise
1452 = return ()
1453
1454 gather_conflicts inj prev_branches cur_branch (acc, n) branch
1455 -- n is 0-based index of branch in prev_branches
1456 = case injectiveBranches inj cur_branch branch of
1457 InjectivityUnified ax1 ax2
1458 | ax1 `isDominatedBy` (replace_br prev_branches n ax2)
1459 -> (acc, n + 1)
1460 | otherwise
1461 -> (branch : acc, n + 1)
1462 InjectivityAccepted -> (acc, n + 1)
1463
1464 -- Replace n-th element in the list. Assumes 0-based indexing.
1465 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
1466 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
1467
1468
1469 -- Check that a "type instance" is well-formed (which includes decidability
1470 -- unless -XUndecidableInstances is given).
1471 --
1472 checkValidCoAxBranch :: Maybe ClsInfo
1473 -> TyCon -> CoAxBranch -> TcM ()
1474 checkValidCoAxBranch mb_clsinfo fam_tc
1475 (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
1476 , cab_lhs = typats
1477 , cab_rhs = rhs, cab_loc = loc })
1478 = checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1479
1480 -- | Do validity checks on a type family equation, including consistency
1481 -- with any enclosing class instance head, termination, and lack of
1482 -- polytypes.
1483 checkValidTyFamEqn :: Maybe ClsInfo
1484 -> TyCon -- ^ of the type family
1485 -> [TyVar] -- ^ bound tyvars in the equation
1486 -> [CoVar] -- ^ bound covars in the equation
1487 -> [Type] -- ^ type patterns
1488 -> Type -- ^ rhs
1489 -> SrcSpan
1490 -> TcM ()
1491 checkValidTyFamEqn mb_clsinfo fam_tc tvs cvs typats rhs loc
1492 = setSrcSpan loc $
1493 do { checkValidFamPats fam_tc tvs cvs typats
1494
1495 -- The argument patterns, and RHS, are all boxed tau types
1496 -- E.g Reject type family F (a :: k1) :: k2
1497 -- type instance F (forall a. a->a) = ...
1498 -- type instance F Int# = ...
1499 -- type instance F Int = forall a. a->a
1500 -- type instance F Int = Int#
1501 -- See Trac #9357
1502 ; env <- tcInitOpenTidyEnv (tyCoVarsOfTypes (rhs : typats))
1503 ; mapM_ checkValidMonoType typats
1504 ; mapM_ (check_lifted env) typats
1505 ; checkValidMonoType rhs
1506 ; check_lifted env rhs
1507
1508 -- We have a decidable instance unless otherwise permitted
1509 ; undecidable_ok <- xoptM LangExt.UndecidableInstances
1510 ; unless undecidable_ok $
1511 mapM_ addErrTc (checkFamInstRhs typats (tcTyFamInsts rhs))
1512
1513 -- Check that type patterns match the class instance head
1514 ; checkConsistentFamInst mb_clsinfo fam_tc tvs typats }
1515
1516 -- Make sure that each type family application is
1517 -- (1) strictly smaller than the lhs,
1518 -- (2) mentions no type variable more often than the lhs, and
1519 -- (3) does not contain any further type family instances.
1520 --
1521 checkFamInstRhs :: [Type] -- lhs
1522 -> [(TyCon, [Type])] -- type family instances
1523 -> [MsgDoc]
1524 checkFamInstRhs lhsTys famInsts
1525 = mapMaybe check famInsts
1526 where
1527 size = sizeTypes lhsTys
1528 fvs = fvTypes lhsTys
1529 check (tc, tys)
1530 | not (all isTyFamFree tys) = Just (nestedMsg what)
1531 | not (null bad_tvs) = Just (noMoreMsg bad_tvs what)
1532 | size <= sizeTypes tys = Just (smallerMsg what)
1533 | otherwise = Nothing
1534 where
1535 what = text "type family application" <+> quotes (pprType (TyConApp tc tys))
1536 bad_tvs = fvTypes tys \\ fvs
1537
1538 checkValidFamPats :: TyCon -> [TyVar] -> [CoVar] -> [Type] -> TcM ()
1539 -- Patterns in a 'type instance' or 'data instance' decl should
1540 -- a) contain no type family applications
1541 -- (vanilla synonyms are fine, though)
1542 -- b) properly bind all their free type variables
1543 -- e.g. we disallow (Trac #7536)
1544 -- type T a = Int
1545 -- type instance F (T a) = a
1546 -- c) Have the right number of patterns
1547 checkValidFamPats fam_tc tvs cvs ty_pats
1548 = do { -- A family instance must have exactly the same number of type
1549 -- parameters as the family declaration. You can't write
1550 -- type family F a :: * -> *
1551 -- type instance F Int y = y
1552 -- because then the type (F Int) would be like (\y.y)
1553 checkTc (length ty_pats == fam_arity) $
1554 wrongNumberOfParmsErr (fam_arity - count isInvisibleBinder fam_bndrs)
1555 -- report only explicit arguments
1556
1557 ; mapM_ checkTyFamFreeness ty_pats
1558 ; let unbound_tcvs = filterOut (`elemVarSet` exactTyCoVarsOfTypes ty_pats) (tvs ++ cvs)
1559 ; checkTc (null unbound_tcvs) (famPatErr fam_tc unbound_tcvs ty_pats) }
1560 where fam_arity = tyConArity fam_tc
1561 fam_bndrs = take fam_arity $ fst $ splitPiTys (tyConKind fam_tc)
1562
1563 wrongNumberOfParmsErr :: Arity -> SDoc
1564 wrongNumberOfParmsErr exp_arity
1565 = text "Number of parameters must match family declaration; expected"
1566 <+> ppr exp_arity
1567
1568 -- Ensure that no type family instances occur in a type.
1569 checkTyFamFreeness :: Type -> TcM ()
1570 checkTyFamFreeness ty
1571 = checkTc (isTyFamFree ty) $
1572 tyFamInstIllegalErr ty
1573
1574 -- Check that a type does not contain any type family applications.
1575 --
1576 isTyFamFree :: Type -> Bool
1577 isTyFamFree = null . tcTyFamInsts
1578
1579 -- Error messages
1580
1581 inaccessibleCoAxBranch :: CoAxiom br -> CoAxBranch -> SDoc
1582 inaccessibleCoAxBranch fi_ax cur_branch
1583 = text "Type family instance equation is overlapped:" $$
1584 nest 2 (pprCoAxBranch fi_ax cur_branch)
1585
1586 tyFamInstIllegalErr :: Type -> SDoc
1587 tyFamInstIllegalErr ty
1588 = hang (text "Illegal type synonym family application in instance" <>
1589 colon) 2 $
1590 ppr ty
1591
1592 nestedMsg :: SDoc -> SDoc
1593 nestedMsg what
1594 = sep [ text "Illegal nested" <+> what
1595 , parens undecidableMsg ]
1596
1597 famPatErr :: TyCon -> [TyVar] -> [Type] -> SDoc
1598 famPatErr fam_tc tvs pats
1599 = hang (text "Family instance purports to bind type variable" <> plural tvs
1600 <+> pprQuotedList tvs)
1601 2 (hang (text "but the real LHS (expanding synonyms) is:")
1602 2 (pprTypeApp fam_tc (map expandTypeSynonyms pats) <+>
1603 text "= ..."))
1604
1605 {-
1606 ************************************************************************
1607 * *
1608 Telescope checking
1609 * *
1610 ************************************************************************
1611
1612 Note [Bad telescopes]
1613 ~~~~~~~~~~~~~~~~~~~~~
1614 Now that we can mix type and kind variables, there are an awful lot of
1615 ways to shoot yourself in the foot. Here are some.
1616
1617 data SameKind :: k -> k -> * -- just to force unification
1618
1619 1. data T1 a k (b :: k) (x :: SameKind a b)
1620
1621 The problem here is that we discover that a and b should have the same
1622 kind. But this kind mentions k, which is bound *after* a.
1623 (Testcase: dependent/should_fail/BadTelescope)
1624
1625 2. data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)
1626
1627 Note that b is not bound. Yet its kind mentions a. Because we have
1628 a nice rule that all implicitly bound variables come before others,
1629 this is bogus. (We could probably figure out to put b between a and c.
1630 But I think this is doing users a disservice, in the long run.)
1631 (Testcase: dependent/should_fail/BadTelescope4)
1632
1633 3. t3 :: forall a. (forall k (b :: k). SameKind a b) -> ()
1634
1635 This is a straightforward skolem escape. Note that a and b need to have
1636 the same kind.
1637 (Testcase: polykinds/T11142)
1638
1639 How do we deal with all of this? For TyCons, we have checkValidTyConTyVars.
1640 That function looks to see if any of the tyConTyVars are repeated, but
1641 it's really a telescope check. It works because all tycons are kind-generalized.
1642 If there is a bad telescope, the kind-generalization will end up generalizing
1643 over a variable bound later in the telescope.
1644
1645 For non-tycons, we do scope checking when we bring tyvars into scope,
1646 in tcImplicitTKBndrs and tcExplicitTKBndrs. Note that we also have to
1647 sort implicit binders into a well-scoped order whenever we have implicit
1648 binders to worry about. This is done in quantifyTyVars and in
1649 tcImplicitTKBndrs.
1650 -}
1651
1652 -- | Check a list of binders to see if they make a valid telescope.
1653 -- The key property we're checking for is scoping. For example:
1654 -- > data SameKind :: k -> k -> *
1655 -- > data X a k (b :: k) (c :: SameKind a b)
1656 -- Kind inference says that a's kind should be k. But that's impossible,
1657 -- because k isn't in scope when a is bound. This check has to come before
1658 -- general validity checking, because once we kind-generalise, this sort
1659 -- of problem is harder to spot (as we'll generalise over the unbound
1660 -- k in a's type.) See also Note [Bad telescopes].
1661 checkValidTelescope :: SDoc -- the original user-written telescope
1662 -> [TyVar] -- explicit vars (not necessarily zonked)
1663 -> SDoc -- note to put at bottom of message
1664 -> TcM () -- returns zonked tyvars
1665 checkValidTelescope hs_tvs orig_tvs extra
1666 = discardResult $ checkZonkValidTelescope hs_tvs orig_tvs extra
1667
1668 -- | Like 'checkZonkValidTelescope', but returns the zonked tyvars
1669 checkZonkValidTelescope :: SDoc
1670 -> [TyVar]
1671 -> SDoc
1672 -> TcM [TyVar]
1673 checkZonkValidTelescope hs_tvs orig_tvs extra
1674 = do { orig_tvs <- mapM zonkTyCoVarKind orig_tvs
1675 ; let (_, sorted_tidied_tvs) = tidyTyCoVarBndrs emptyTidyEnv $
1676 toposortTyVars orig_tvs
1677 ; unless (go [] emptyVarSet orig_tvs) $
1678 addErr $
1679 vcat [ hang (text "These kind and type variables:" <+> hs_tvs $$
1680 text "are out of dependency order. Perhaps try this ordering:")
1681 2 (sep (map pprTvBndr sorted_tidied_tvs))
1682 , extra ]
1683 ; return orig_tvs }
1684
1685 where
1686 go :: [TyVar] -- misplaced variables
1687 -> TyVarSet -> [TyVar] -> Bool
1688 go errs in_scope [] = null (filter (`elemVarSet` in_scope) errs)
1689 -- report an error only when the variable in the kind is brought
1690 -- into scope later in the telescope. Otherwise, we'll just quantify
1691 -- over it in kindGeneralize, as we should.
1692
1693 go errs in_scope (tv:tvs)
1694 = let bad_tvs = tyCoVarsOfType (tyVarKind tv) `minusVarSet` in_scope in
1695 go (varSetElems bad_tvs ++ errs) (in_scope `extendVarSet` tv) tvs
1696
1697 -- | After inferring kinds of type variables, check to make sure that the
1698 -- inferred kinds any of the type variables bound in a smaller scope.
1699 -- This is a skolem escape check. See also Note [Bad telescopes].
1700 checkValidInferredKinds :: [TyVar] -- ^ vars to check (zonked)
1701 -> TyVarSet -- ^ vars out of scope
1702 -> SDoc -- ^ suffix to error message
1703 -> TcM ()
1704 checkValidInferredKinds orig_kvs out_of_scope extra
1705 = do { let bad_pairs = [ (tv, kv)
1706 | kv <- orig_kvs
1707 , Just tv <- map (lookupVarSet out_of_scope)
1708 (tyCoVarsOfTypeList (tyVarKind kv)) ]
1709 report (tidyTyVarOcc env -> tv, tidyTyVarOcc env -> kv)
1710 = addErr $
1711 text "The kind of variable" <+>
1712 quotes (ppr kv) <> text ", namely" <+>
1713 quotes (ppr (tyVarKind kv)) <> comma $$
1714 text "depends on variable" <+>
1715 quotes (ppr tv) <+> text "from an inner scope" $$
1716 text "Perhaps bind" <+> quotes (ppr kv) <+>
1717 text "sometime after binding" <+>
1718 quotes (ppr tv) $$
1719 extra
1720 ; mapM_ report bad_pairs }
1721
1722 where
1723 (env1, _) = tidyTyCoVarBndrs emptyTidyEnv orig_kvs
1724 (env, _) = tidyTyCoVarBndrs env1 (varSetElems out_of_scope)
1725
1726 {-
1727 ************************************************************************
1728 * *
1729 \subsection{Auxiliary functions}
1730 * *
1731 ************************************************************************
1732 -}
1733
1734 -- Free variables of a type, retaining repetitions, and expanding synonyms
1735 fvType :: Type -> [TyCoVar]
1736 fvType ty | Just exp_ty <- coreView ty = fvType exp_ty
1737 fvType (TyVarTy tv) = [tv]
1738 fvType (TyConApp _ tys) = fvTypes tys
1739 fvType (LitTy {}) = []
1740 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1741 fvType (ForAllTy bndr ty)
1742 = fvType (binderType bndr) ++
1743 caseBinder bndr (\tv -> filter (/= tv)) (const id) (fvType ty)
1744 fvType (CastTy ty co) = fvType ty ++ fvCo co
1745 fvType (CoercionTy co) = fvCo co
1746
1747 fvTypes :: [Type] -> [TyVar]
1748 fvTypes tys = concat (map fvType tys)
1749
1750 fvCo :: Coercion -> [TyCoVar]
1751 fvCo (Refl _ ty) = fvType ty
1752 fvCo (TyConAppCo _ _ args) = concatMap fvCo args
1753 fvCo (AppCo co arg) = fvCo co ++ fvCo arg
1754 fvCo (ForAllCo tv h co) = filter (/= tv) (fvCo co) ++ fvCo h
1755 fvCo (CoVarCo v) = [v]
1756 fvCo (AxiomInstCo _ _ args) = concatMap fvCo args
1757 fvCo (UnivCo p _ t1 t2) = fvProv p ++ fvType t1 ++ fvType t2
1758 fvCo (SymCo co) = fvCo co
1759 fvCo (TransCo co1 co2) = fvCo co1 ++ fvCo co2
1760 fvCo (NthCo _ co) = fvCo co
1761 fvCo (LRCo _ co) = fvCo co
1762 fvCo (InstCo co arg) = fvCo co ++ fvCo arg
1763 fvCo (CoherenceCo co1 co2) = fvCo co1 ++ fvCo co2
1764 fvCo (KindCo co) = fvCo co
1765 fvCo (SubCo co) = fvCo co
1766 fvCo (AxiomRuleCo _ cs) = concatMap fvCo cs
1767
1768 fvProv :: UnivCoProvenance -> [TyCoVar]
1769 fvProv UnsafeCoerceProv = []
1770 fvProv (PhantomProv co) = fvCo co
1771 fvProv (ProofIrrelProv co) = fvCo co
1772 fvProv (PluginProv _) = []
1773 fvProv (HoleProv h) = pprPanic "fvProv falls into a hole" (ppr h)
1774
1775 sizeType :: Type -> Int
1776 -- Size of a type: the number of variables and constructors
1777 sizeType ty | Just exp_ty <- coreView ty = sizeType exp_ty
1778 sizeType (TyVarTy {}) = 1
1779 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1780 sizeType (LitTy {}) = 1
1781 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1782 sizeType (ForAllTy (Anon arg) res)
1783 = sizeType arg + sizeType res + 1
1784 sizeType (ForAllTy (Named {}) ty)
1785 = sizeType ty
1786 sizeType (CastTy ty _) = sizeType ty
1787 sizeType (CoercionTy _) = 1
1788
1789 sizeTypes :: [Type] -> Int
1790 sizeTypes = sum . map sizeType
1791
1792 -- Size of a predicate
1793 --
1794 -- We are considering whether class constraints terminate.
1795 -- Equality constraints and constraints for the implicit
1796 -- parameter class always termiante so it is safe to say "size 0".
1797 -- (Implicit parameter constraints always terminate because
1798 -- there are no instances for them---they are only solved by
1799 -- "local instances" in expressions).
1800 -- See Trac #4200.
1801 sizePred :: PredType -> Int
1802 sizePred ty = goClass ty
1803 where
1804 goClass p = go (classifyPredType p)
1805
1806 go (ClassPred cls tys')
1807 | isTerminatingClass cls = 0
1808 | otherwise = sizeTypes tys'
1809 go (EqPred {}) = 0
1810 go (IrredPred ty) = sizeType ty
1811
1812 -- | When this says "True", ignore this class constraint during
1813 -- a termination check
1814 isTerminatingClass :: Class -> Bool
1815 isTerminatingClass cls
1816 = isIPClass cls
1817 || cls `hasKey` typeableClassKey
1818 || cls `hasKey` coercibleTyConKey
1819 || cls `hasKey` eqTyConKey
1820 || cls `hasKey` heqTyConKey
1821
1822 -- | Tidy before printing a type
1823 ppr_tidy :: TidyEnv -> Type -> SDoc
1824 ppr_tidy env ty = pprType (tidyType env ty)