TcUnify: Assert precondition of matchExpectedTyConApp
[ghc.git] / compiler / typecheck / TcUnify.hs
1 {-
2 (c) The University of Glasgow 2006
3 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4
5
6 Type subsumption and unification
7 -}
8
9 {-# LANGUAGE CPP, MultiWayIf, TupleSections, ScopedTypeVariables #-}
10
11 module TcUnify (
12 -- Full-blown subsumption
13 tcWrapResult, tcWrapResultO, tcSkolemise, tcSkolemiseET,
14 tcSubTypeHR, tcSubTypeO, tcSubType_NC, tcSubTypeDS,
15 tcSubTypeDS_NC_O, tcSubTypeET,
16 checkConstraints, buildImplicationFor,
17
18 -- Various unifications
19 unifyType, unifyTheta, unifyKind, noThing,
20 uType, promoteTcType,
21 swapOverTyVars, canSolveByUnification,
22
23 --------------------------------
24 -- Holes
25 tcInferInst, tcInferNoInst,
26 matchExpectedListTy,
27 matchExpectedPArrTy,
28 matchExpectedTyConApp,
29 matchExpectedAppTy,
30 matchExpectedFunTys,
31 matchActualFunTys, matchActualFunTysPart,
32 matchExpectedFunKind,
33
34 wrapFunResCoercion,
35
36 occCheckExpand, metaTyVarUpdateOK,
37 occCheckForErrors, OccCheckResult(..)
38
39 ) where
40
41 #include "HsVersions.h"
42
43 import HsSyn
44 import TyCoRep
45 import TcMType
46 import TcRnMonad
47 import TcType
48 import Type
49 import Coercion
50 import TcEvidence
51 import Name ( isSystemName )
52 import Inst
53 import TyCon
54 import TysWiredIn
55 import TysPrim( tYPE )
56 import Var
57 import VarSet
58 import VarEnv
59 import ErrUtils
60 import DynFlags
61 import BasicTypes
62 import Name ( Name )
63 import Bag
64 import Util
65 import Pair( pFst )
66 import qualified GHC.LanguageExtensions as LangExt
67 import Outputable
68 import FastString
69
70 import Control.Monad
71 import Control.Arrow ( second )
72
73 {-
74 ************************************************************************
75 * *
76 matchExpected functions
77 * *
78 ************************************************************************
79
80 Note [Herald for matchExpectedFunTys]
81 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
82 The 'herald' always looks like:
83 "The equation(s) for 'f' have"
84 "The abstraction (\x.e) takes"
85 "The section (+ x) expects"
86 "The function 'f' is applied to"
87
88 This is used to construct a message of form
89
90 The abstraction `\Just 1 -> ...' takes two arguments
91 but its type `Maybe a -> a' has only one
92
93 The equation(s) for `f' have two arguments
94 but its type `Maybe a -> a' has only one
95
96 The section `(f 3)' requires 'f' to take two arguments
97 but its type `Int -> Int' has only one
98
99 The function 'f' is applied to two arguments
100 but its type `Int -> Int' has only one
101
102 Note [matchExpectedFunTys]
103 ~~~~~~~~~~~~~~~~~~~~~~~~~~
104 matchExpectedFunTys checks that a sigma has the form
105 of an n-ary function. It passes the decomposed type to the
106 thing_inside, and returns a wrapper to coerce between the two types
107
108 It's used wherever a language construct must have a functional type,
109 namely:
110 A lambda expression
111 A function definition
112 An operator section
113
114 This function must be written CPS'd because it needs to fill in the
115 ExpTypes produced for arguments before it can fill in the ExpType
116 passed in.
117
118 -}
119
120 -- Use this one when you have an "expected" type.
121 matchExpectedFunTys :: forall a.
122 SDoc -- See Note [Herald for matchExpectedFunTys]
123 -> Arity
124 -> ExpRhoType -- deeply skolemised
125 -> ([ExpSigmaType] -> ExpRhoType -> TcM a)
126 -- must fill in these ExpTypes here
127 -> TcM (a, HsWrapper)
128 -- If matchExpectedFunTys n ty = (_, wrap)
129 -- then wrap : (t1 -> ... -> tn -> ty_r) ~> ty,
130 -- where [t1, ..., tn], ty_r are passed to the thing_inside
131 matchExpectedFunTys herald arity orig_ty thing_inside
132 = case orig_ty of
133 Check ty -> go [] arity ty
134 _ -> defer [] arity orig_ty
135 where
136 go acc_arg_tys 0 ty
137 = do { result <- thing_inside (reverse acc_arg_tys) (mkCheckExpType ty)
138 ; return (result, idHsWrapper) }
139
140 go acc_arg_tys n ty
141 | Just ty' <- coreView ty = go acc_arg_tys n ty'
142
143 go acc_arg_tys n (FunTy arg_ty res_ty)
144 = ASSERT( not (isPredTy arg_ty) )
145 do { (result, wrap_res) <- go (mkCheckExpType arg_ty : acc_arg_tys)
146 (n-1) res_ty
147 ; return ( result
148 , mkWpFun idHsWrapper wrap_res arg_ty res_ty doc ) }
149 where
150 doc = text "When inferring the argument type of a function with type" <+>
151 quotes (ppr orig_ty)
152
153 go acc_arg_tys n ty@(TyVarTy tv)
154 | isMetaTyVar tv
155 = do { cts <- readMetaTyVar tv
156 ; case cts of
157 Indirect ty' -> go acc_arg_tys n ty'
158 Flexi -> defer acc_arg_tys n (mkCheckExpType ty) }
159
160 -- In all other cases we bale out into ordinary unification
161 -- However unlike the meta-tyvar case, we are sure that the
162 -- number of arguments doesn't match arity of the original
163 -- type, so we can add a bit more context to the error message
164 -- (cf Trac #7869).
165 --
166 -- It is not always an error, because specialized type may have
167 -- different arity, for example:
168 --
169 -- > f1 = f2 'a'
170 -- > f2 :: Monad m => m Bool
171 -- > f2 = undefined
172 --
173 -- But in that case we add specialized type into error context
174 -- anyway, because it may be useful. See also Trac #9605.
175 go acc_arg_tys n ty = addErrCtxtM mk_ctxt $
176 defer acc_arg_tys n (mkCheckExpType ty)
177
178 ------------
179 defer :: [ExpSigmaType] -> Arity -> ExpRhoType -> TcM (a, HsWrapper)
180 defer acc_arg_tys n fun_ty
181 = do { more_arg_tys <- replicateM n newInferExpTypeNoInst
182 ; res_ty <- newInferExpTypeInst
183 ; result <- thing_inside (reverse acc_arg_tys ++ more_arg_tys) res_ty
184 ; more_arg_tys <- mapM readExpType more_arg_tys
185 ; res_ty <- readExpType res_ty
186 ; let unif_fun_ty = mkFunTys more_arg_tys res_ty
187 ; wrap <- tcSubTypeDS AppOrigin GenSigCtxt unif_fun_ty fun_ty
188 -- Not a good origin at all :-(
189 ; return (result, wrap) }
190
191 ------------
192 mk_ctxt :: TidyEnv -> TcM (TidyEnv, MsgDoc)
193 mk_ctxt env = do { (env', ty) <- zonkTidyTcType env orig_tc_ty
194 ; let (args, _) = tcSplitFunTys ty
195 n_actual = length args
196 (env'', orig_ty') = tidyOpenType env' orig_tc_ty
197 ; return ( env''
198 , mk_fun_tys_msg orig_ty' ty n_actual arity herald) }
199 where
200 orig_tc_ty = checkingExpType "matchExpectedFunTys" orig_ty
201 -- this is safe b/c we're called from "go"
202
203 -- Like 'matchExpectedFunTys', but used when you have an "actual" type,
204 -- for example in function application
205 matchActualFunTys :: Outputable a
206 => SDoc -- See Note [Herald for matchExpectedFunTys]
207 -> CtOrigin
208 -> Maybe a -- the thing with type TcSigmaType
209 -> Arity
210 -> TcSigmaType
211 -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
212 -- If matchActualFunTys n ty = (wrap, [t1,..,tn], ty_r)
213 -- then wrap : ty ~> (t1 -> ... -> tn -> ty_r)
214 matchActualFunTys herald ct_orig mb_thing arity ty
215 = matchActualFunTysPart herald ct_orig mb_thing arity ty [] arity
216
217 -- | Variant of 'matchActualFunTys' that works when supplied only part
218 -- (that is, to the right of some arrows) of the full function type
219 matchActualFunTysPart :: Outputable a
220 => SDoc -- See Note [Herald for matchExpectedFunTys]
221 -> CtOrigin
222 -> Maybe a -- the thing with type TcSigmaType
223 -> Arity
224 -> TcSigmaType
225 -> [TcSigmaType] -- reversed args. See (*) below.
226 -> Arity -- overall arity of the function, for errs
227 -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
228 matchActualFunTysPart herald ct_orig mb_thing arity orig_ty
229 orig_old_args full_arity
230 = go arity orig_old_args orig_ty
231 -- Does not allocate unnecessary meta variables: if the input already is
232 -- a function, we just take it apart. Not only is this efficient,
233 -- it's important for higher rank: the argument might be of form
234 -- (forall a. ty) -> other
235 -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
236 -- hide the forall inside a meta-variable
237
238 -- (*) Sometimes it's necessary to call matchActualFunTys with only part
239 -- (that is, to the right of some arrows) of the type of the function in
240 -- question. (See TcExpr.tcArgs.) This argument is the reversed list of
241 -- arguments already seen (that is, not part of the TcSigmaType passed
242 -- in elsewhere).
243
244 where
245 -- This function has a bizarre mechanic: it accumulates arguments on
246 -- the way down and also builds an argument list on the way up. Why:
247 -- 1. The returns args list and the accumulated args list might be different.
248 -- The accumulated args include all the arg types for the function,
249 -- including those from before this function was called. The returned
250 -- list should include only those arguments produced by this call of
251 -- matchActualFunTys
252 --
253 -- 2. The HsWrapper can be built only on the way up. It seems (more)
254 -- bizarre to build the HsWrapper but not the arg_tys.
255 --
256 -- Refactoring is welcome.
257 go :: Arity
258 -> [TcSigmaType] -- accumulator of arguments (reversed)
259 -> TcSigmaType -- the remainder of the type as we're processing
260 -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
261 go 0 _ ty = return (idHsWrapper, [], ty)
262
263 go n acc_args ty
264 | not (null tvs && null theta)
265 = do { (wrap1, rho) <- topInstantiate ct_orig ty
266 ; (wrap2, arg_tys, res_ty) <- go n acc_args rho
267 ; return (wrap2 <.> wrap1, arg_tys, res_ty) }
268 where
269 (tvs, theta, _) = tcSplitSigmaTy ty
270
271 go n acc_args ty
272 | Just ty' <- coreView ty = go n acc_args ty'
273
274 go n acc_args (FunTy arg_ty res_ty)
275 = ASSERT( not (isPredTy arg_ty) )
276 do { (wrap_res, tys, ty_r) <- go (n-1) (arg_ty : acc_args) res_ty
277 ; return ( mkWpFun idHsWrapper wrap_res arg_ty ty_r doc
278 , arg_ty : tys, ty_r ) }
279 where
280 doc = text "When inferring the argument type of a function with type" <+>
281 quotes (ppr orig_ty)
282
283 go n acc_args ty@(TyVarTy tv)
284 | isMetaTyVar tv
285 = do { cts <- readMetaTyVar tv
286 ; case cts of
287 Indirect ty' -> go n acc_args ty'
288 Flexi -> defer n ty }
289
290 -- In all other cases we bale out into ordinary unification
291 -- However unlike the meta-tyvar case, we are sure that the
292 -- number of arguments doesn't match arity of the original
293 -- type, so we can add a bit more context to the error message
294 -- (cf Trac #7869).
295 --
296 -- It is not always an error, because specialized type may have
297 -- different arity, for example:
298 --
299 -- > f1 = f2 'a'
300 -- > f2 :: Monad m => m Bool
301 -- > f2 = undefined
302 --
303 -- But in that case we add specialized type into error context
304 -- anyway, because it may be useful. See also Trac #9605.
305 go n acc_args ty = addErrCtxtM (mk_ctxt (reverse acc_args) ty) $
306 defer n ty
307
308 ------------
309 defer n fun_ty
310 = do { arg_tys <- replicateM n newOpenFlexiTyVarTy
311 ; res_ty <- newOpenFlexiTyVarTy
312 ; let unif_fun_ty = mkFunTys arg_tys res_ty
313 ; co <- unifyType mb_thing fun_ty unif_fun_ty
314 ; return (mkWpCastN co, arg_tys, res_ty) }
315
316 ------------
317 mk_ctxt :: [TcSigmaType] -> TcSigmaType -> TidyEnv -> TcM (TidyEnv, MsgDoc)
318 mk_ctxt arg_tys res_ty env
319 = do { let ty = mkFunTys arg_tys res_ty
320 ; (env1, zonked) <- zonkTidyTcType env ty
321 -- zonking might change # of args
322 ; let (zonked_args, _) = tcSplitFunTys zonked
323 n_actual = length zonked_args
324 (env2, unzonked) = tidyOpenType env1 ty
325 ; return ( env2
326 , mk_fun_tys_msg unzonked zonked n_actual full_arity herald) }
327
328 mk_fun_tys_msg :: TcType -- the full type passed in (unzonked)
329 -> TcType -- the full type passed in (zonked)
330 -> Arity -- the # of args found
331 -> Arity -- the # of args wanted
332 -> SDoc -- overall herald
333 -> SDoc
334 mk_fun_tys_msg full_ty ty n_args full_arity herald
335 = herald <+> speakNOf full_arity (text "argument") <> comma $$
336 if n_args == full_arity
337 then text "its type is" <+> quotes (pprType full_ty) <>
338 comma $$
339 text "it is specialized to" <+> quotes (pprType ty)
340 else sep [text "but its type" <+> quotes (pprType ty),
341 if n_args == 0 then text "has none"
342 else text "has only" <+> speakN n_args]
343
344 ----------------------
345 matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
346 -- Special case for lists
347 matchExpectedListTy exp_ty
348 = do { (co, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
349 ; return (co, elt_ty) }
350
351 ----------------------
352 matchExpectedPArrTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
353 -- Special case for parrs
354 matchExpectedPArrTy exp_ty
355 = do { (co, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
356 ; return (co, elt_ty) }
357
358 ---------------------
359 matchExpectedTyConApp :: TyCon -- T :: forall kv1 ... kvm. k1 -> ... -> kn -> *
360 -> TcRhoType -- orig_ty
361 -> TcM (TcCoercionN, -- T k1 k2 k3 a b c ~N orig_ty
362 [TcSigmaType]) -- Element types, k1 k2 k3 a b c
363
364 -- It's used for wired-in tycons, so we call checkWiredInTyCon
365 -- Precondition: never called with FunTyCon
366 -- Precondition: input type :: *
367 -- Postcondition: (T k1 k2 k3 a b c) is well-kinded
368
369 matchExpectedTyConApp tc orig_ty
370 = ASSERT(tc /= funTyCon) go orig_ty
371 where
372 go ty
373 | Just ty' <- coreView ty
374 = go ty'
375
376 go ty@(TyConApp tycon args)
377 | tc == tycon -- Common case
378 = return (mkTcNomReflCo ty, args)
379
380 go (TyVarTy tv)
381 | isMetaTyVar tv
382 = do { cts <- readMetaTyVar tv
383 ; case cts of
384 Indirect ty -> go ty
385 Flexi -> defer }
386
387 go _ = defer
388
389 -- If the common case does not occur, instantiate a template
390 -- T k1 .. kn t1 .. tm, and unify with the original type
391 -- Doing it this way ensures that the types we return are
392 -- kind-compatible with T. For example, suppose we have
393 -- matchExpectedTyConApp T (f Maybe)
394 -- where data T a = MkT a
395 -- Then we don't want to instantate T's data constructors with
396 -- (a::*) ~ Maybe
397 -- because that'll make types that are utterly ill-kinded.
398 -- This happened in Trac #7368
399 defer
400 = do { (_, arg_tvs) <- newMetaTyVars (tyConTyVars tc)
401 ; traceTc "matchExpectedTyConApp" (ppr tc $$ ppr (tyConTyVars tc) $$ ppr arg_tvs)
402 ; let args = mkTyVarTys arg_tvs
403 tc_template = mkTyConApp tc args
404 ; co <- unifyType noThing tc_template orig_ty
405 ; return (co, args) }
406
407 ----------------------
408 matchExpectedAppTy :: TcRhoType -- orig_ty
409 -> TcM (TcCoercion, -- m a ~N orig_ty
410 (TcSigmaType, TcSigmaType)) -- Returns m, a
411 -- If the incoming type is a mutable type variable of kind k, then
412 -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.
413
414 matchExpectedAppTy orig_ty
415 = go orig_ty
416 where
417 go ty
418 | Just ty' <- coreView ty = go ty'
419
420 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
421 = return (mkTcNomReflCo orig_ty, (fun_ty, arg_ty))
422
423 go (TyVarTy tv)
424 | isMetaTyVar tv
425 = do { cts <- readMetaTyVar tv
426 ; case cts of
427 Indirect ty -> go ty
428 Flexi -> defer }
429
430 go _ = defer
431
432 -- Defer splitting by generating an equality constraint
433 defer
434 = do { ty1 <- newFlexiTyVarTy kind1
435 ; ty2 <- newFlexiTyVarTy kind2
436 ; co <- unifyType noThing (mkAppTy ty1 ty2) orig_ty
437 ; return (co, (ty1, ty2)) }
438
439 orig_kind = typeKind orig_ty
440 kind1 = mkFunTy liftedTypeKind orig_kind
441 kind2 = liftedTypeKind -- m :: * -> k
442 -- arg type :: *
443
444 {-
445 ************************************************************************
446 * *
447 Subsumption checking
448 * *
449 ************************************************************************
450
451 Note [Subsumption checking: tcSubType]
452 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
453 All the tcSubType calls have the form
454 tcSubType actual_ty expected_ty
455 which checks
456 actual_ty <= expected_ty
457
458 That is, that a value of type actual_ty is acceptable in
459 a place expecting a value of type expected_ty. I.e. that
460
461 actual ty is more polymorphic than expected_ty
462
463 It returns a coercion function
464 co_fn :: actual_ty ~ expected_ty
465 which takes an HsExpr of type actual_ty into one of type
466 expected_ty.
467
468 These functions do not actually check for subsumption. They check if
469 expected_ty is an appropriate annotation to use for something of type
470 actual_ty. This difference matters when thinking about visible type
471 application. For example,
472
473 forall a. a -> forall b. b -> b
474 DOES NOT SUBSUME
475 forall a b. a -> b -> b
476
477 because the type arguments appear in a different order. (Neither does
478 it work the other way around.) BUT, these types are appropriate annotations
479 for one another. Because the user directs annotations, it's OK if some
480 arguments shuffle around -- after all, it's what the user wants.
481 Bottom line: none of this changes with visible type application.
482
483 There are a number of wrinkles (below).
484
485 Notice that Wrinkle 1 and 2 both require eta-expansion, which technically
486 may increase termination. We just put up with this, in exchange for getting
487 more predictable type inference.
488
489 Wrinkle 1: Note [Deep skolemisation]
490 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
491 We want (forall a. Int -> a -> a) <= (Int -> forall a. a->a)
492 (see section 4.6 of "Practical type inference for higher rank types")
493 So we must deeply-skolemise the RHS before we instantiate the LHS.
494
495 That is why tc_sub_type starts with a call to tcSkolemise (which does the
496 deep skolemisation), and then calls the DS variant (which assumes
497 that expected_ty is deeply skolemised)
498
499 Wrinkle 2: Note [Co/contra-variance of subsumption checking]
500 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
501 Consider g :: (Int -> Int) -> Int
502 f1 :: (forall a. a -> a) -> Int
503 f1 = g
504
505 f2 :: (forall a. a -> a) -> Int
506 f2 x = g x
507 f2 will typecheck, and it would be odd/fragile if f1 did not.
508 But f1 will only typecheck if we have that
509 (Int->Int) -> Int <= (forall a. a->a) -> Int
510 And that is only true if we do the full co/contravariant thing
511 in the subsumption check. That happens in the FunTy case of
512 tcSubTypeDS_NC_O, and is the sole reason for the WpFun form of
513 HsWrapper.
514
515 Another powerful reason for doing this co/contra stuff is visible
516 in Trac #9569, involving instantiation of constraint variables,
517 and again involving eta-expansion.
518
519 Wrinkle 3: Note [Higher rank types]
520 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
521 Consider tc150:
522 f y = \ (x::forall a. a->a). blah
523 The following happens:
524 * We will infer the type of the RHS, ie with a res_ty = alpha.
525 * Then the lambda will split alpha := beta -> gamma.
526 * And then we'll check tcSubType IsSwapped beta (forall a. a->a)
527
528 So it's important that we unify beta := forall a. a->a, rather than
529 skolemising the type.
530 -}
531
532
533 -- | Call this variant when you are in a higher-rank situation and
534 -- you know the right-hand type is deeply skolemised.
535 tcSubTypeHR :: Outputable a
536 => CtOrigin -- ^ of the actual type
537 -> Maybe a -- ^ If present, it has type ty_actual
538 -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
539 tcSubTypeHR orig = tcSubTypeDS_NC_O orig GenSigCtxt
540
541 ------------------------
542 tcSubTypeET :: CtOrigin -> UserTypeCtxt
543 -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
544 -- If wrap = tc_sub_type_et t1 t2
545 -- => wrap :: t1 ~> t2
546 tcSubTypeET orig ctxt (Check ty_actual) ty_expected
547 = tc_sub_tc_type eq_orig orig ctxt ty_actual ty_expected
548 where
549 eq_orig = TypeEqOrigin { uo_actual = ty_expected
550 , uo_expected = ty_actual
551 , uo_thing = Nothing }
552
553 tcSubTypeET _ _ (Infer inf_res) ty_expected
554 = ASSERT2( not (ir_inst inf_res), ppr inf_res $$ ppr ty_expected )
555 do { co <- fillInferResult ty_expected inf_res
556 ; return (mkWpCastN (mkTcSymCo co)) }
557
558 ------------------------
559 tcSubTypeO :: CtOrigin -- ^ of the actual type
560 -> UserTypeCtxt -- ^ of the expected type
561 -> TcSigmaType
562 -> ExpRhoType
563 -> TcM HsWrapper
564 tcSubTypeO orig ctxt ty_actual ty_expected
565 = addSubTypeCtxt ty_actual ty_expected $
566 do { traceTc "tcSubTypeDS_O" (vcat [ pprCtOrigin orig
567 , pprUserTypeCtxt ctxt
568 , ppr ty_actual
569 , ppr ty_expected ])
570 ; tcSubTypeDS_NC_O orig ctxt noThing ty_actual ty_expected }
571
572 addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a
573 addSubTypeCtxt ty_actual ty_expected thing_inside
574 | isRhoTy ty_actual -- If there is no polymorphism involved, the
575 , isRhoExpTy ty_expected -- TypeEqOrigin stuff (added by the _NC functions)
576 = thing_inside -- gives enough context by itself
577 | otherwise
578 = addErrCtxtM mk_msg thing_inside
579 where
580 mk_msg tidy_env
581 = do { (tidy_env, ty_actual) <- zonkTidyTcType tidy_env ty_actual
582 -- might not be filled if we're debugging. ugh.
583 ; mb_ty_expected <- readExpType_maybe ty_expected
584 ; (tidy_env, ty_expected) <- case mb_ty_expected of
585 Just ty -> second mkCheckExpType <$>
586 zonkTidyTcType tidy_env ty
587 Nothing -> return (tidy_env, ty_expected)
588 ; ty_expected <- readExpType ty_expected
589 ; (tidy_env, ty_expected) <- zonkTidyTcType tidy_env ty_expected
590 ; let msg = vcat [ hang (text "When checking that:")
591 4 (ppr ty_actual)
592 , nest 2 (hang (text "is more polymorphic than:")
593 2 (ppr ty_expected)) ]
594 ; return (tidy_env, msg) }
595
596 ---------------
597 -- The "_NC" variants do not add a typechecker-error context;
598 -- the caller is assumed to do that
599
600 tcSubType_NC :: UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
601 -- Checks that actual <= expected
602 -- Returns HsWrapper :: actual ~ expected
603 tcSubType_NC ctxt ty_actual ty_expected
604 = do { traceTc "tcSubType_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected])
605 ; tc_sub_tc_type origin origin ctxt ty_actual ty_expected }
606 where
607 origin = TypeEqOrigin { uo_actual = ty_actual
608 , uo_expected = ty_expected
609 , uo_thing = Nothing }
610
611 tcSubTypeDS :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
612 -- Just like tcSubType, but with the additional precondition that
613 -- ty_expected is deeply skolemised (hence "DS")
614 tcSubTypeDS orig ctxt ty_actual ty_expected
615 = addSubTypeCtxt ty_actual ty_expected $
616 do { traceTc "tcSubTypeDS_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected])
617 ; tcSubTypeDS_NC_O orig ctxt noThing ty_actual ty_expected }
618
619 tcSubTypeDS_NC_O :: Outputable a
620 => CtOrigin -- origin used for instantiation only
621 -> UserTypeCtxt
622 -> Maybe a
623 -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
624 -- Just like tcSubType, but with the additional precondition that
625 -- ty_expected is deeply skolemised
626 tcSubTypeDS_NC_O inst_orig ctxt m_thing ty_actual ty_expected
627 = case ty_expected of
628 Infer inf_res -> fillInferResult_Inst inst_orig ty_actual inf_res
629 Check ty -> tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty
630 where
631 eq_orig = TypeEqOrigin { uo_actual = ty_actual, uo_expected = ty
632 , uo_thing = mkErrorThing <$> m_thing }
633
634 ---------------
635 tc_sub_tc_type :: CtOrigin -- used when calling uType
636 -> CtOrigin -- used when instantiating
637 -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
638 -- If wrap = tc_sub_type t1 t2
639 -- => wrap :: t1 ~> t2
640 tc_sub_tc_type eq_orig inst_orig ctxt ty_actual ty_expected
641 | is_poly ty_expected -- See Note [Don't skolemise unnecessarily]
642 , not (is_poly ty_actual)
643 = do { traceTc "tc_sub_tc_type (drop to equality)" $
644 vcat [ text "ty_actual =" <+> ppr ty_actual
645 , text "ty_expected =" <+> ppr ty_expected ]
646 ; mkWpCastN <$>
647 uType eq_orig TypeLevel ty_actual ty_expected }
648
649 | otherwise -- This is the general case
650 = do { traceTc "tc_sub_tc_type (general case)" $
651 vcat [ text "ty_actual =" <+> ppr ty_actual
652 , text "ty_expected =" <+> ppr ty_expected ]
653 ; (sk_wrap, inner_wrap) <- tcSkolemise ctxt ty_expected $
654 \ _ sk_rho ->
655 tc_sub_type_ds eq_orig inst_orig ctxt
656 ty_actual sk_rho
657 ; return (sk_wrap <.> inner_wrap) }
658 where
659 is_poly ty
660 | isForAllTy ty = True
661 | Just (_, res) <- splitFunTy_maybe ty = is_poly res
662 | otherwise = False
663 -- NB *not* tcSplitFunTy, because here we want
664 -- to decompose type-class arguments too
665
666
667 {- Note [Don't skolemise unnecessarily]
668 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
669 Suppose we are trying to solve
670 (Char->Char) <= (forall a. a->a)
671 We could skolemise the 'forall a', and then complain
672 that (Char ~ a) is insoluble; but that's a pretty obscure
673 error. It's better to say that
674 (Char->Char) ~ (forall a. a->a)
675 fails.
676
677 In general,
678 * if the RHS type an outermost forall (i.e. skolemisation
679 is the next thing we'd do)
680 * and the LHS has no top-level polymorphism (but looking deeply)
681 then we can revert to simple equality.
682 -}
683
684 ---------------
685 tc_sub_type_ds :: CtOrigin -- used when calling uType
686 -> CtOrigin -- used when instantiating
687 -> UserTypeCtxt -> TcSigmaType -> TcRhoType -> TcM HsWrapper
688 -- If wrap = tc_sub_type_ds t1 t2
689 -- => wrap :: t1 ~> t2
690 -- Here is where the work actually happens!
691 -- Precondition: ty_expected is deeply skolemised
692 tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty_expected
693 = do { traceTc "tc_sub_type_ds" $
694 vcat [ text "ty_actual =" <+> ppr ty_actual
695 , text "ty_expected =" <+> ppr ty_expected ]
696 ; go ty_actual ty_expected }
697 where
698 go ty_a ty_e | Just ty_a' <- coreView ty_a = go ty_a' ty_e
699 | Just ty_e' <- coreView ty_e = go ty_a ty_e'
700
701 go (TyVarTy tv_a) ty_e
702 = do { lookup_res <- lookupTcTyVar tv_a
703 ; case lookup_res of
704 Filled ty_a' ->
705 do { traceTc "tcSubTypeDS_NC_O following filled act meta-tyvar:"
706 (ppr tv_a <+> text "-->" <+> ppr ty_a')
707 ; tc_sub_type_ds eq_orig inst_orig ctxt ty_a' ty_e }
708 Unfilled _ -> unify }
709
710 -- Historical note (Sept 16): there was a case here for
711 -- go ty_a (TyVarTy alpha)
712 -- which, in the impredicative case unified alpha := ty_a
713 -- where th_a is a polytype. Not only is this probably bogus (we
714 -- simply do not have decent story for imprdicative types), but it
715 -- caused Trac #12616 because (also bizarrely) 'deriving' code had
716 -- -XImpredicativeTypes on. I deleted the entire case.
717
718 go (FunTy act_arg act_res) (FunTy exp_arg exp_res)
719 | not (isPredTy act_arg)
720 , not (isPredTy exp_arg)
721 = -- See Note [Co/contra-variance of subsumption checking]
722 do { res_wrap <- tc_sub_type_ds eq_orig inst_orig ctxt act_res exp_res
723 ; arg_wrap
724 <- tc_sub_tc_type eq_orig (GivenOrigin
725 (SigSkol GenSigCtxt exp_arg))
726 ctxt exp_arg act_arg
727 ; return (mkWpFun arg_wrap res_wrap exp_arg exp_res doc) }
728 -- arg_wrap :: exp_arg ~> act_arg
729 -- res_wrap :: act-res ~> exp_res
730 where
731 doc = text "When checking that" <+> quotes (ppr ty_actual) <+>
732 text "is more polymorphic than" <+> quotes (ppr ty_expected)
733
734 go ty_a ty_e
735 | let (tvs, theta, _) = tcSplitSigmaTy ty_a
736 , not (null tvs && null theta)
737 = do { (in_wrap, in_rho) <- topInstantiate inst_orig ty_a
738 ; body_wrap <- tc_sub_type_ds
739 (eq_orig { uo_actual = in_rho
740 , uo_expected = ty_expected })
741 inst_orig ctxt in_rho ty_e
742 ; return (body_wrap <.> in_wrap) }
743
744 | otherwise -- Revert to unification
745 = inst_and_unify
746 -- It's still possible that ty_actual has nested foralls. Instantiate
747 -- these, as there's no way unification will succeed with them in.
748 -- See typecheck/should_compile/T11305 for an example of when this
749 -- is important. The problem is that we're checking something like
750 -- a -> forall b. b -> b <= alpha beta gamma
751 -- where we end up with alpha := (->)
752
753 inst_and_unify = do { (wrap, rho_a) <- deeplyInstantiate inst_orig ty_actual
754
755 -- if we haven't recurred through an arrow, then
756 -- the eq_orig will list ty_actual. In this case,
757 -- we want to update the origin to reflect the
758 -- instantiation. If we *have* recurred through
759 -- an arrow, it's better not to update.
760 ; let eq_orig' = case eq_orig of
761 TypeEqOrigin { uo_actual = orig_ty_actual }
762 | orig_ty_actual `tcEqType` ty_actual
763 , not (isIdHsWrapper wrap)
764 -> eq_orig { uo_actual = rho_a }
765 _ -> eq_orig
766
767 ; cow <- uType eq_orig' TypeLevel rho_a ty_expected
768 ; return (mkWpCastN cow <.> wrap) }
769
770
771 -- use versions without synonyms expanded
772 unify = mkWpCastN <$> uType eq_orig TypeLevel ty_actual ty_expected
773
774 -----------------
775 -- needs both un-type-checked (for origins) and type-checked (for wrapping)
776 -- expressions
777 tcWrapResult :: HsExpr Name -> HsExpr TcId -> TcSigmaType -> ExpRhoType
778 -> TcM (HsExpr TcId)
779 tcWrapResult rn_expr = tcWrapResultO (exprCtOrigin rn_expr)
780
781 -- | Sometimes we don't have a @HsExpr Name@ to hand, and this is more
782 -- convenient.
783 tcWrapResultO :: CtOrigin -> HsExpr TcId -> TcSigmaType -> ExpRhoType
784 -> TcM (HsExpr TcId)
785 tcWrapResultO orig expr actual_ty res_ty
786 = do { traceTc "tcWrapResult" (vcat [ text "Actual: " <+> ppr actual_ty
787 , text "Expected:" <+> ppr res_ty ])
788 ; cow <- tcSubTypeDS_NC_O orig GenSigCtxt
789 (Just expr) actual_ty res_ty
790 ; return (mkHsWrap cow expr) }
791
792 -----------------------------------
793 wrapFunResCoercion
794 :: [TcType] -- Type of args
795 -> HsWrapper -- HsExpr a -> HsExpr b
796 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
797 wrapFunResCoercion arg_tys co_fn_res
798 | isIdHsWrapper co_fn_res
799 = return idHsWrapper
800 | null arg_tys
801 = return co_fn_res
802 | otherwise
803 = do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
804 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }
805
806
807 {- **********************************************************************
808 %* *
809 ExpType functions: tcInfer, fillInferResult
810 %* *
811 %********************************************************************* -}
812
813 -- | Infer a type using a fresh ExpType
814 -- See also Note [ExpType] in TcMType
815 -- Does not attempt to instantiate the inferred type
816 tcInferNoInst :: (ExpSigmaType -> TcM a) -> TcM (a, TcSigmaType)
817 tcInferNoInst = tcInfer False
818
819 tcInferInst :: (ExpRhoType -> TcM a) -> TcM (a, TcRhoType)
820 tcInferInst = tcInfer True
821
822 tcInfer :: Bool -> (ExpSigmaType -> TcM a) -> TcM (a, TcSigmaType)
823 tcInfer instantiate tc_check
824 = do { res_ty <- newInferExpType instantiate
825 ; result <- tc_check res_ty
826 ; res_ty <- readExpType res_ty
827 ; return (result, res_ty) }
828
829 fillInferResult_Inst :: CtOrigin -> TcType -> InferResult -> TcM HsWrapper
830 -- If wrap = fillInferResult_Inst t1 t2
831 -- => wrap :: t1 ~> t2
832 -- See Note [Deep instantiation of InferResult]
833 fillInferResult_Inst orig ty inf_res@(IR { ir_inst = instantiate_me })
834 | instantiate_me
835 = do { (wrap, rho) <- deeplyInstantiate orig ty
836 ; co <- fillInferResult rho inf_res
837 ; return (mkWpCastN co <.> wrap) }
838
839 | otherwise
840 = do { co <- fillInferResult ty inf_res
841 ; return (mkWpCastN co) }
842
843 fillInferResult :: TcType -> InferResult -> TcM TcCoercionN
844 -- If wrap = fillInferResult t1 t2
845 -- => wrap :: t1 ~> t2
846 fillInferResult orig_ty (IR { ir_uniq = u, ir_lvl = res_lvl
847 , ir_ref = ref })
848 = do { (ty_co, ty_to_fill_with) <- promoteTcType res_lvl orig_ty
849
850 ; traceTc "Filling ExpType" $
851 ppr u <+> text ":=" <+> ppr ty_to_fill_with
852
853 ; when debugIsOn (check_hole ty_to_fill_with)
854
855 ; writeTcRef ref (Just ty_to_fill_with)
856
857 ; return ty_co }
858 where
859 check_hole ty -- Debug check only
860 = do { let ty_lvl = tcTypeLevel ty
861 ; MASSERT2( not (ty_lvl `strictlyDeeperThan` res_lvl),
862 ppr u $$ ppr res_lvl $$ ppr ty_lvl $$
863 ppr ty <+> ppr (typeKind ty) $$ ppr orig_ty )
864 ; cts <- readTcRef ref
865 ; case cts of
866 Just already_there -> pprPanic "writeExpType"
867 (vcat [ ppr u
868 , ppr ty
869 , ppr already_there ])
870 Nothing -> return () }
871
872 {- Note [Deep instantiation of InferResult]
873 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
874 In some cases we want to deeply instantiate before filling in
875 an InferResult, and in some cases not. That's why InferReult
876 has the ir_inst flag.
877
878 * ir_inst = True: deeply instantantiate
879
880 Consider
881 f x = (*)
882 We want to instantiate the type of (*) before returning, else we
883 will infer the type
884 f :: forall {a}. a -> forall b. Num b => b -> b -> b
885 This is surely confusing for users.
886
887 And worse, the the monomorphism restriction won't properly. The MR is
888 dealt with in simplifyInfer, and simplifyInfer has no way of
889 instantiating. This could perhaps be worked around, but it may be
890 hard to know even when instantiation should happen.
891
892 Another reason. Consider
893 f :: (?x :: Int) => a -> a
894 g y = let ?x = 3::Int in f
895 Here want to instantiate f's type so that the ?x::Int constraint
896 gets discharged by the enclosing implicit-parameter binding.
897
898 * ir_inst = False: do not instantantiate
899
900 Consider this (which uses visible type application):
901
902 (let { f :: forall a. a -> a; f x = x } in f) @Int
903
904 We'll call TcExpr.tcInferFun to infer the type of the (let .. in f)
905 And we don't want to instantite the type of 'f' when we reach it,
906 else the outer visible type application won't work
907 -}
908
909 {- *********************************************************************
910 * *
911 Promoting types
912 * *
913 ********************************************************************* -}
914
915 promoteTcType :: TcLevel -> TcType -> TcM (TcCoercion, TcType)
916 -- See Note [Promoting a type]
917 -- promoteTcType level ty = (co, ty')
918 -- * Returns ty' whose max level is just 'level'
919 -- and whose kind is ~# to the kind of 'ty'
920 -- and whose kind has form TYPE rr
921 -- * and co :: ty ~ ty'
922 -- * and emits constraints to justify the coercion
923 promoteTcType dest_lvl ty
924 = do { cur_lvl <- getTcLevel
925 ; if (cur_lvl `sameDepthAs` dest_lvl)
926 then dont_promote_it
927 else promote_it }
928 where
929 promote_it :: TcM (TcCoercion, TcType)
930 promote_it -- Emit a constraint (alpha :: TYPE rr) ~ ty
931 -- where alpha and rr are fresh and from level dest_lvl
932 = do { rr <- newMetaTyVarTyAtLevel dest_lvl runtimeRepTy
933 ; prom_ty <- newMetaTyVarTyAtLevel dest_lvl (tYPE rr)
934 ; let eq_orig = TypeEqOrigin { uo_actual = ty
935 , uo_expected = prom_ty
936 , uo_thing = Nothing }
937
938 ; co <- emitWantedEq eq_orig TypeLevel Nominal ty prom_ty
939 ; return (co, prom_ty) }
940
941 dont_promote_it :: TcM (TcCoercion, TcType)
942 dont_promote_it -- Check that ty :: TYPE rr, for some (fresh) rr
943 = do { res_kind <- newOpenTypeKind
944 ; let ty_kind = typeKind ty
945 kind_orig = TypeEqOrigin { uo_actual = ty_kind
946 , uo_expected = res_kind
947 , uo_thing = Nothing }
948 ; ki_co <- uType kind_orig KindLevel (typeKind ty) res_kind
949 ; let co = mkTcNomReflCo ty `mkTcCoherenceRightCo` ki_co
950 ; return (co, ty `mkCastTy` ki_co) }
951
952 {- Note [Promoting a type]
953 ~~~~~~~~~~~~~~~~~~~~~~~~~~
954 Consider (Trac #12427)
955
956 data T where
957 MkT :: (Int -> Int) -> a -> T
958
959 h y = case y of MkT v w -> v
960
961 We'll infer the RHS type with an expected type ExpType of
962 (IR { ir_lvl = l, ir_ref = ref, ... )
963 where 'l' is the TcLevel of the RHS of 'h'. Then the MkT pattern
964 match will increase the level, so we'll end up in tcSubType, trying to
965 unify the type of v,
966 v :: Int -> Int
967 with the expected type. But this attempt takes place at level (l+1),
968 rightly so, since v's type could have mentioned existential variables,
969 (like w's does) and we want to catch that.
970
971 So we
972 - create a new meta-var alpha[l+1]
973 - fill in the InferRes ref cell 'ref' with alpha
974 - emit an equality constraint, thus
975 [W] alpha[l+1] ~ (Int -> Int)
976
977 That constraint will float outwards, as it should, unless v's
978 type mentions a skolem-captured variable.
979
980 This approach fails if v has a higher rank type; see
981 Note [Promotion and higher rank types]
982
983
984 Note [Promotion and higher rank types]
985 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
986 If v had a higher-rank type, say v :: (forall a. a->a) -> Int,
987 then we'd emit an equality
988 [W] alpha[l+1] ~ ((forall a. a->a) -> Int)
989 which will sadly fail because we can't unify a unification variable
990 with a polytype. But there is nothing really wrong with the program
991 here.
992
993 We could just about solve this by "promote the type" of v, to expose
994 its polymorphic "shape" while still leaving constraints that will
995 prevent existential escape. But we must be careful! Exposing
996 the "shape" of the type is precisely what we must NOT do under
997 a GADT pattern match! So in this case we might promote the type
998 to
999 (forall a. a->a) -> alpha[l+1]
1000 and emit the constraint
1001 [W] alpha[l+1] ~ Int
1002 Now the poromoted type can fill the ref cell, while the emitted
1003 equality can float or not, according to the usual rules.
1004
1005 But that's not quite right! We are exposing the arrow! We could
1006 deal with that too:
1007 (forall a. mu[l+1] a a) -> alpha[l+1]
1008 with constraints
1009 [W] alpha[l+1] ~ Int
1010 [W] mu[l+1] ~ (->)
1011 Here we abstract over the '->' inside the forall, in case that
1012 is subject to an equality constraint from a GADT match.
1013
1014 Note that we kept the outer (->) because that's part of
1015 the polymorphic "shape". And becauuse of impredicativity,
1016 GADT matches can't give equalities that affect polymorphic
1017 shape.
1018
1019 This reasoning just seems too complicated, so I decided not
1020 to do it. These higher-rank notes are just here to record
1021 the thinking.
1022 -}
1023
1024 {- *********************************************************************
1025 * *
1026 Generalisation
1027 * *
1028 ********************************************************************* -}
1029
1030 -- | Take an "expected type" and strip off quantifiers to expose the
1031 -- type underneath, binding the new skolems for the @thing_inside@.
1032 -- The returned 'HsWrapper' has type @specific_ty -> expected_ty@.
1033 tcSkolemise :: UserTypeCtxt -> TcSigmaType
1034 -> ([TcTyVar] -> TcType -> TcM result)
1035 -- ^ These are only ever used for scoped type variables.
1036 -> TcM (HsWrapper, result)
1037 -- ^ The expression has type: spec_ty -> expected_ty
1038
1039 tcSkolemise ctxt expected_ty thing_inside
1040 -- We expect expected_ty to be a forall-type
1041 -- If not, the call is a no-op
1042 = do { traceTc "tcSkolemise" Outputable.empty
1043 ; (wrap, tvs', given, rho') <- deeplySkolemise expected_ty
1044
1045 ; lvl <- getTcLevel
1046 ; when debugIsOn $
1047 traceTc "tcSkolemise" $ vcat [
1048 ppr lvl,
1049 text "expected_ty" <+> ppr expected_ty,
1050 text "inst tyvars" <+> ppr tvs',
1051 text "given" <+> ppr given,
1052 text "inst type" <+> ppr rho' ]
1053
1054 -- Generally we must check that the "forall_tvs" havn't been constrained
1055 -- The interesting bit here is that we must include the free variables
1056 -- of the expected_ty. Here's an example:
1057 -- runST (newVar True)
1058 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
1059 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
1060 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
1061 -- So now s' isn't unconstrained because it's linked to a.
1062 --
1063 -- However [Oct 10] now that the untouchables are a range of
1064 -- TcTyVars, all this is handled automatically with no need for
1065 -- extra faffing around
1066
1067 -- Use the *instantiated* type in the SkolemInfo
1068 -- so that the names of displayed type variables line up
1069 ; let skol_info = SigSkol ctxt (mkFunTys (map varType given) rho')
1070
1071 ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
1072 thing_inside tvs' rho'
1073
1074 ; return (wrap <.> mkWpLet ev_binds, result) }
1075 -- The ev_binds returned by checkConstraints is very
1076 -- often empty, in which case mkWpLet is a no-op
1077
1078 -- | Variant of 'tcSkolemise' that takes an ExpType
1079 tcSkolemiseET :: UserTypeCtxt -> ExpSigmaType
1080 -> (ExpRhoType -> TcM result)
1081 -> TcM (HsWrapper, result)
1082 tcSkolemiseET _ et@(Infer {}) thing_inside
1083 = (idHsWrapper, ) <$> thing_inside et
1084 tcSkolemiseET ctxt (Check ty) thing_inside
1085 = tcSkolemise ctxt ty $ \_ -> thing_inside . mkCheckExpType
1086
1087 checkConstraints :: SkolemInfo
1088 -> [TcTyVar] -- Skolems
1089 -> [EvVar] -- Given
1090 -> TcM result
1091 -> TcM (TcEvBinds, result)
1092
1093 checkConstraints skol_info skol_tvs given thing_inside
1094 = do { (implics, ev_binds, result)
1095 <- buildImplication skol_info skol_tvs given thing_inside
1096 ; emitImplications implics
1097 ; return (ev_binds, result) }
1098
1099 buildImplication :: SkolemInfo
1100 -> [TcTyVar] -- Skolems
1101 -> [EvVar] -- Given
1102 -> TcM result
1103 -> TcM (Bag Implication, TcEvBinds, result)
1104 buildImplication skol_info skol_tvs given thing_inside
1105 = do { tc_lvl <- getTcLevel
1106 ; deferred_type_errors <- goptM Opt_DeferTypeErrors <||>
1107 goptM Opt_DeferTypedHoles
1108 ; if null skol_tvs && null given && (not deferred_type_errors ||
1109 not (isTopTcLevel tc_lvl))
1110 then do { res <- thing_inside
1111 ; return (emptyBag, emptyTcEvBinds, res) }
1112 -- Fast path. We check every function argument with
1113 -- tcPolyExpr, which uses tcSkolemise and hence checkConstraints.
1114 -- But with the solver producing unlifted equalities, we need
1115 -- to have an EvBindsVar for them when they might be deferred to
1116 -- runtime. Otherwise, they end up as top-level unlifted bindings,
1117 -- which are verboten. See also Note [Deferred errors for coercion holes]
1118 -- in TcErrors.
1119 else
1120 do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints thing_inside
1121 ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_tvs given wanted
1122 ; return (implics, ev_binds, result) }}
1123
1124 buildImplicationFor :: TcLevel -> SkolemInfo -> [TcTyVar]
1125 -> [EvVar] -> WantedConstraints
1126 -> TcM (Bag Implication, TcEvBinds)
1127 buildImplicationFor tclvl skol_info skol_tvs given wanted
1128 | isEmptyWC wanted && null given
1129 -- Optimisation : if there are no wanteds, and no givens
1130 -- don't generate an implication at all.
1131 -- Reason for the (null given): we don't want to lose
1132 -- the "inaccessible alternative" error check
1133 = return (emptyBag, emptyTcEvBinds)
1134
1135 | otherwise
1136 = ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
1137 do { ev_binds_var <- newTcEvBinds
1138 ; env <- getLclEnv
1139 ; let implic = Implic { ic_tclvl = tclvl
1140 , ic_skols = skol_tvs
1141 , ic_no_eqs = False
1142 , ic_given = given
1143 , ic_wanted = wanted
1144 , ic_status = IC_Unsolved
1145 , ic_binds = ev_binds_var
1146 , ic_env = env
1147 , ic_needed = emptyVarSet
1148 , ic_info = skol_info }
1149
1150 ; return (unitBag implic, TcEvBinds ev_binds_var) }
1151
1152 {-
1153 ************************************************************************
1154 * *
1155 Boxy unification
1156 * *
1157 ************************************************************************
1158
1159 The exported functions are all defined as versions of some
1160 non-exported generic functions.
1161 -}
1162
1163 unifyType :: Outputable a => Maybe a -- ^ If present, has type 'ty1'
1164 -> TcTauType -> TcTauType -> TcM TcCoercionN
1165 -- Actual and expected types
1166 -- Returns a coercion : ty1 ~ ty2
1167 unifyType thing ty1 ty2 = uType origin TypeLevel ty1 ty2
1168 where
1169 origin = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2
1170 , uo_thing = mkErrorThing <$> thing }
1171
1172 -- | Use this instead of 'Nothing' when calling 'unifyType' without
1173 -- a good "thing" (where the "thing" has the "actual" type passed in)
1174 -- This has an 'Outputable' instance, avoiding amgiguity problems.
1175 noThing :: Maybe (HsExpr Name)
1176 noThing = Nothing
1177
1178 unifyKind :: Outputable a => Maybe a -> TcKind -> TcKind -> TcM CoercionN
1179 unifyKind thing ty1 ty2 = uType origin KindLevel ty1 ty2
1180 where origin = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2
1181 , uo_thing = mkErrorThing <$> thing }
1182
1183 ---------------
1184 unifyPred :: PredType -> PredType -> TcM TcCoercionN
1185 -- Actual and expected types
1186 unifyPred = unifyType noThing
1187
1188 ---------------
1189 unifyTheta :: TcThetaType -> TcThetaType -> TcM [TcCoercionN]
1190 -- Actual and expected types
1191 unifyTheta theta1 theta2
1192 = do { checkTc (equalLength theta1 theta2)
1193 (vcat [text "Contexts differ in length",
1194 nest 2 $ parens $ text "Use RelaxedPolyRec to allow this"])
1195 ; zipWithM unifyPred theta1 theta2 }
1196
1197 {-
1198 %************************************************************************
1199 %* *
1200 uType and friends
1201 %* *
1202 %************************************************************************
1203
1204 uType is the heart of the unifier.
1205 -}
1206
1207 uType, uType_defer
1208 :: CtOrigin
1209 -> TypeOrKind
1210 -> TcType -- ty1 is the *actual* type
1211 -> TcType -- ty2 is the *expected* type
1212 -> TcM Coercion
1213
1214 --------------
1215 -- It is always safe to defer unification to the main constraint solver
1216 -- See Note [Deferred unification]
1217 uType_defer origin t_or_k ty1 ty2
1218 = do { co <- emitWantedEq origin t_or_k Nominal ty1 ty2
1219
1220 -- Error trace only
1221 -- NB. do *not* call mkErrInfo unless tracing is on,
1222 -- because it is hugely expensive (#5631)
1223 ; whenDOptM Opt_D_dump_tc_trace $ do
1224 { ctxt <- getErrCtxt
1225 ; doc <- mkErrInfo emptyTidyEnv ctxt
1226 ; traceTc "utype_defer" (vcat [ppr co, ppr ty1,
1227 ppr ty2, pprCtOrigin origin, doc])
1228 }
1229 ; return co }
1230
1231 --------------
1232 uType origin t_or_k orig_ty1 orig_ty2
1233 = do { tclvl <- getTcLevel
1234 ; traceTc "u_tys" $ vcat
1235 [ text "tclvl" <+> ppr tclvl
1236 , sep [ ppr orig_ty1, text "~", ppr orig_ty2]
1237 , pprCtOrigin origin]
1238 ; co <- go orig_ty1 orig_ty2
1239 ; if isReflCo co
1240 then traceTc "u_tys yields no coercion" Outputable.empty
1241 else traceTc "u_tys yields coercion:" (ppr co)
1242 ; return co }
1243 where
1244 go :: TcType -> TcType -> TcM Coercion
1245 -- The arguments to 'go' are always semantically identical
1246 -- to orig_ty{1,2} except for looking through type synonyms
1247
1248 -- Variables; go for uVar
1249 -- Note that we pass in *original* (before synonym expansion),
1250 -- so that type variables tend to get filled in with
1251 -- the most informative version of the type
1252 go (TyVarTy tv1) ty2
1253 = do { lookup_res <- lookupTcTyVar tv1
1254 ; case lookup_res of
1255 Filled ty1 -> do { traceTc "found filled tyvar" (ppr tv1 <+> text ":->" <+> ppr ty1)
1256 ; go ty1 ty2 }
1257 Unfilled _ -> uUnfilledVar origin t_or_k NotSwapped tv1 ty2 }
1258 go ty1 (TyVarTy tv2)
1259 = do { lookup_res <- lookupTcTyVar tv2
1260 ; case lookup_res of
1261 Filled ty2 -> do { traceTc "found filled tyvar" (ppr tv2 <+> text ":->" <+> ppr ty2)
1262 ; go ty1 ty2 }
1263 Unfilled _ -> uUnfilledVar origin t_or_k IsSwapped tv2 ty1 }
1264
1265 -- See Note [Expanding synonyms during unification]
1266 go ty1@(TyConApp tc1 []) (TyConApp tc2 [])
1267 | tc1 == tc2
1268 = return $ mkReflCo Nominal ty1
1269
1270 -- See Note [Expanding synonyms during unification]
1271 --
1272 -- Also NB that we recurse to 'go' so that we don't push a
1273 -- new item on the origin stack. As a result if we have
1274 -- type Foo = Int
1275 -- and we try to unify Foo ~ Bool
1276 -- we'll end up saying "can't match Foo with Bool"
1277 -- rather than "can't match "Int with Bool". See Trac #4535.
1278 go ty1 ty2
1279 | Just ty1' <- coreView ty1 = go ty1' ty2
1280 | Just ty2' <- coreView ty2 = go ty1 ty2'
1281
1282 go (CastTy t1 co1) t2
1283 = do { co_tys <- go t1 t2
1284 ; return (mkCoherenceLeftCo co_tys co1) }
1285
1286 go t1 (CastTy t2 co2)
1287 = do { co_tys <- go t1 t2
1288 ; return (mkCoherenceRightCo co_tys co2) }
1289
1290 -- Functions (or predicate functions) just check the two parts
1291 go (FunTy fun1 arg1) (FunTy fun2 arg2)
1292 = do { co_l <- uType origin t_or_k fun1 fun2
1293 ; co_r <- uType origin t_or_k arg1 arg2
1294 ; return $ mkFunCo Nominal co_l co_r }
1295
1296 -- Always defer if a type synonym family (type function)
1297 -- is involved. (Data families behave rigidly.)
1298 go ty1@(TyConApp tc1 _) ty2
1299 | isTypeFamilyTyCon tc1 = defer ty1 ty2
1300 go ty1 ty2@(TyConApp tc2 _)
1301 | isTypeFamilyTyCon tc2 = defer ty1 ty2
1302
1303 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
1304 -- See Note [Mismatched type lists and application decomposition]
1305 | tc1 == tc2, length tys1 == length tys2
1306 = ASSERT2( isGenerativeTyCon tc1 Nominal, ppr tc1 )
1307 do { cos <- zipWithM (uType origin t_or_k) tys1 tys2
1308 ; return $ mkTyConAppCo Nominal tc1 cos }
1309
1310 go (LitTy m) ty@(LitTy n)
1311 | m == n
1312 = return $ mkNomReflCo ty
1313
1314 -- See Note [Care with type applications]
1315 -- Do not decompose FunTy against App;
1316 -- it's often a type error, so leave it for the constraint solver
1317 go (AppTy s1 t1) (AppTy s2 t2)
1318 = go_app s1 t1 s2 t2
1319
1320 go (AppTy s1 t1) (TyConApp tc2 ts2)
1321 | Just (ts2', t2') <- snocView ts2
1322 = ASSERT( mightBeUnsaturatedTyCon tc2 )
1323 go_app s1 t1 (TyConApp tc2 ts2') t2'
1324
1325 go (TyConApp tc1 ts1) (AppTy s2 t2)
1326 | Just (ts1', t1') <- snocView ts1
1327 = ASSERT( mightBeUnsaturatedTyCon tc1 )
1328 go_app (TyConApp tc1 ts1') t1' s2 t2
1329
1330 go (CoercionTy co1) (CoercionTy co2)
1331 = do { let ty1 = coercionType co1
1332 ty2 = coercionType co2
1333 ; kco <- uType (KindEqOrigin orig_ty1 (Just orig_ty2) origin
1334 (Just t_or_k))
1335 KindLevel
1336 ty1 ty2
1337 ; return $ mkProofIrrelCo Nominal kco co1 co2 }
1338
1339 -- Anything else fails
1340 -- E.g. unifying for-all types, which is relative unusual
1341 go ty1 ty2 = defer ty1 ty2
1342
1343 ------------------
1344 defer ty1 ty2 -- See Note [Check for equality before deferring]
1345 | ty1 `tcEqType` ty2 = return (mkNomReflCo ty1)
1346 | otherwise = uType_defer origin t_or_k ty1 ty2
1347
1348 ------------------
1349 go_app s1 t1 s2 t2
1350 = do { co_s <- uType origin t_or_k s1 s2
1351 ; co_t <- uType origin t_or_k t1 t2
1352 ; return $ mkAppCo co_s co_t }
1353
1354 {- Note [Check for equality before deferring]
1355 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1356 Particularly in ambiguity checks we can get equalities like (ty ~ ty).
1357 If ty involves a type function we may defer, which isn't very sensible.
1358 An egregious example of this was in test T9872a, which has a type signature
1359 Proxy :: Proxy (Solutions Cubes)
1360 Doing the ambiguity check on this signature generates the equality
1361 Solutions Cubes ~ Solutions Cubes
1362 and currently the constraint solver normalises both sides at vast cost.
1363 This little short-cut in 'defer' helps quite a bit.
1364
1365 Note [Care with type applications]
1366 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1367 Note: type applications need a bit of care!
1368 They can match FunTy and TyConApp, so use splitAppTy_maybe
1369 NB: we've already dealt with type variables and Notes,
1370 so if one type is an App the other one jolly well better be too
1371
1372 Note [Mismatched type lists and application decomposition]
1373 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1374 When we find two TyConApps, you might think that the argument lists
1375 are guaranteed equal length. But they aren't. Consider matching
1376 w (T x) ~ Foo (T x y)
1377 We do match (w ~ Foo) first, but in some circumstances we simply create
1378 a deferred constraint; and then go ahead and match (T x ~ T x y).
1379 This came up in Trac #3950.
1380
1381 So either
1382 (a) either we must check for identical argument kinds
1383 when decomposing applications,
1384
1385 (b) or we must be prepared for ill-kinded unification sub-problems
1386
1387 Currently we adopt (b) since it seems more robust -- no need to maintain
1388 a global invariant.
1389
1390 Note [Expanding synonyms during unification]
1391 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1392 We expand synonyms during unification, but:
1393 * We expand *after* the variable case so that we tend to unify
1394 variables with un-expanded type synonym. This just makes it
1395 more likely that the inferred types will mention type synonyms
1396 understandable to the user
1397
1398 * We expand *before* the TyConApp case. For example, if we have
1399 type Phantom a = Int
1400 and are unifying
1401 Phantom Int ~ Phantom Char
1402 it is *wrong* to unify Int and Char.
1403
1404 * The problem case immediately above can happen only with arguments
1405 to the tycon. So we check for nullary tycons *before* expanding.
1406 This is particularly helpful when checking (* ~ *), because * is
1407 now a type synonym.
1408
1409 Note [Deferred Unification]
1410 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1411 We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
1412 and yet its consistency is undetermined. Previously, there was no way to still
1413 make it consistent. So a mismatch error was issued.
1414
1415 Now these unifications are deferred until constraint simplification, where type
1416 family instances and given equations may (or may not) establish the consistency.
1417 Deferred unifications are of the form
1418 F ... ~ ...
1419 or x ~ ...
1420 where F is a type function and x is a type variable.
1421 E.g.
1422 id :: x ~ y => x -> y
1423 id e = e
1424
1425 involves the unification x = y. It is deferred until we bring into account the
1426 context x ~ y to establish that it holds.
1427
1428 If available, we defer original types (rather than those where closed type
1429 synonyms have already been expanded via tcCoreView). This is, as usual, to
1430 improve error messages.
1431
1432
1433 ************************************************************************
1434 * *
1435 uVar and friends
1436 * *
1437 ************************************************************************
1438
1439 @uVar@ is called when at least one of the types being unified is a
1440 variable. It does {\em not} assume that the variable is a fixed point
1441 of the substitution; rather, notice that @uVar@ (defined below) nips
1442 back into @uTys@ if it turns out that the variable is already bound.
1443 -}
1444
1445 ----------
1446 uUnfilledVar :: CtOrigin
1447 -> TypeOrKind
1448 -> SwapFlag
1449 -> TcTyVar -- Tyvar 1
1450 -> TcTauType -- Type 2
1451 -> TcM Coercion
1452 -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
1453 -- It might be a skolem, or untouchable, or meta
1454
1455 uUnfilledVar origin t_or_k swapped tv1 ty2
1456 = do { ty2 <- zonkTcType ty2
1457 -- Zonk to expose things to the
1458 -- occurs check, and so that if ty2
1459 -- looks like a type variable then it
1460 -- /is/ a type variable
1461 ; uUnfilledVar1 origin t_or_k swapped tv1 ty2 }
1462
1463 ----------
1464 uUnfilledVar1 :: CtOrigin
1465 -> TypeOrKind
1466 -> SwapFlag
1467 -> TcTyVar -- Tyvar 1
1468 -> TcTauType -- Type 2, zonked
1469 -> TcM Coercion
1470 uUnfilledVar1 origin t_or_k swapped tv1 ty2
1471 | Just tv2 <- tcGetTyVar_maybe ty2
1472 = go tv2
1473
1474 | otherwise
1475 = uUnfilledVar2 origin t_or_k swapped tv1 ty2
1476
1477 where
1478 -- 'go' handles the case where both are
1479 -- tyvars so we might want to swap
1480 go tv2 | tv1 == tv2 -- Same type variable => no-op
1481 = return (mkNomReflCo (mkTyVarTy tv1))
1482
1483 | swapOverTyVars tv1 tv2 -- Distinct type variables
1484 = uUnfilledVar2 origin t_or_k (flipSwap swapped)
1485 tv2 (mkTyVarTy tv1)
1486
1487 | otherwise
1488 = uUnfilledVar2 origin t_or_k swapped tv1 ty2
1489
1490 ----------
1491 uUnfilledVar2 :: CtOrigin
1492 -> TypeOrKind
1493 -> SwapFlag
1494 -> TcTyVar -- Tyvar 1
1495 -> TcTauType -- Type 2, zonked
1496 -> TcM Coercion
1497 uUnfilledVar2 origin t_or_k swapped tv1 ty2
1498 = do { dflags <- getDynFlags
1499 ; cur_lvl <- getTcLevel
1500 ; go dflags cur_lvl }
1501 where
1502 go dflags cur_lvl
1503 | canSolveByUnification cur_lvl tv1 ty2
1504 , Just ty2' <- metaTyVarUpdateOK dflags tv1 ty2
1505 = do { co_k <- uType kind_origin KindLevel (typeKind ty2') (tyVarKind tv1)
1506 ; co <- updateMeta tv1 ty2' co_k
1507 ; return (maybe_sym swapped co) }
1508
1509 | otherwise
1510 = unSwap swapped (uType_defer origin t_or_k) ty1 ty2
1511 -- Occurs check or an untouchable: just defer
1512 -- NB: occurs check isn't necessarily fatal:
1513 -- eg tv1 occured in type family parameter
1514
1515 ty1 = mkTyVarTy tv1
1516 kind_origin = KindEqOrigin ty1 (Just ty2) origin (Just t_or_k)
1517
1518 -- | apply sym iff swapped
1519 maybe_sym :: SwapFlag -> Coercion -> Coercion
1520 maybe_sym IsSwapped = mkSymCo
1521 maybe_sym NotSwapped = id
1522
1523 swapOverTyVars :: TcTyVar -> TcTyVar -> Bool
1524 swapOverTyVars tv1 tv2
1525 | isFmvTyVar tv1 = False -- See Note [Fmv Orientation Invariant]
1526 | isFmvTyVar tv2 = True
1527
1528 | Just lvl1 <- metaTyVarTcLevel_maybe tv1
1529 -- If tv1 is touchable, swap only if tv2 is also
1530 -- touchable and it's strictly better to update the latter
1531 -- But see Note [Avoid unnecessary swaps]
1532 = case metaTyVarTcLevel_maybe tv2 of
1533 Nothing -> False
1534 Just lvl2 | lvl2 `strictlyDeeperThan` lvl1 -> True
1535 | lvl1 `strictlyDeeperThan` lvl2 -> False
1536 | otherwise -> nicer_to_update tv2
1537
1538 -- So tv1 is not a meta tyvar
1539 -- If only one is a meta tyvar, put it on the left
1540 -- This is not because it'll be solved; but because
1541 -- the floating step looks for meta tyvars on the left
1542 | isMetaTyVar tv2 = True
1543
1544 -- So neither is a meta tyvar (including FlatMetaTv)
1545
1546 -- If only one is a flatten skolem, put it on the left
1547 -- See Note [Eliminate flat-skols]
1548 | not (isFlattenTyVar tv1), isFlattenTyVar tv2 = True
1549
1550 | otherwise = False
1551
1552 where
1553 nicer_to_update tv2
1554 = (isSigTyVar tv1 && not (isSigTyVar tv2))
1555 || (isSystemName (Var.varName tv2) && not (isSystemName (Var.varName tv1)))
1556
1557 -- @trySpontaneousSolve wi@ solves equalities where one side is a
1558 -- touchable unification variable.
1559 -- Returns True <=> spontaneous solve happened
1560 canSolveByUnification :: TcLevel -> TcTyVar -> TcType -> Bool
1561 canSolveByUnification tclvl tv xi
1562 | isTouchableMetaTyVar tclvl tv
1563 = case metaTyVarInfo tv of
1564 SigTv -> is_tyvar xi
1565 _ -> True
1566
1567 | otherwise -- Untouchable
1568 = False
1569 where
1570 is_tyvar xi
1571 = case tcGetTyVar_maybe xi of
1572 Nothing -> False
1573 Just tv -> case tcTyVarDetails tv of
1574 MetaTv { mtv_info = info }
1575 -> case info of
1576 SigTv -> True
1577 _ -> False
1578 SkolemTv {} -> True
1579 FlatSkol {} -> False
1580 RuntimeUnk -> True
1581
1582 {- Note [Fmv Orientation Invariant]
1583 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1584 * We always orient a constraint
1585 fmv ~ alpha
1586 with fmv on the left, even if alpha is
1587 a touchable unification variable
1588
1589 Reason: doing it the other way round would unify alpha:=fmv, but that
1590 really doesn't add any info to alpha. But a later constraint alpha ~
1591 Int might unlock everything. Comment:9 of #12526 gives a detailed
1592 example.
1593
1594 WARNING: I've gone to and fro on this one several times.
1595 I'm now pretty sure that unifying alpha:=fmv is a bad idea!
1596 So orienting with fmvs on the left is a good thing.
1597
1598 This example comes from IndTypesPerfMerge. (Others include
1599 T10226, T10009.)
1600 From the ambiguity check for
1601 f :: (F a ~ a) => a
1602 we get:
1603 [G] F a ~ a
1604 [WD] F alpha ~ alpha, alpha ~ a
1605
1606 From Givens we get
1607 [G] F a ~ fsk, fsk ~ a
1608
1609 Now if we flatten we get
1610 [WD] alpha ~ fmv, F alpha ~ fmv, alpha ~ a
1611
1612 Now, if we unified alpha := fmv, we'd get
1613 [WD] F fmv ~ fmv, [WD] fmv ~ a
1614 And now we are stuck.
1615
1616 So instead the Fmv Orientation Invariant puts te fmv on the
1617 left, giving
1618 [WD] fmv ~ alpha, [WD] F alpha ~ fmv, [WD] alpha ~ a
1619
1620 Now we get alpha:=a, and everything works out
1621
1622 Note [Prevent unification with type families]
1623 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1624 We prevent unification with type families because of an uneasy compromise.
1625 It's perfectly sound to unify with type families, and it even improves the
1626 error messages in the testsuite. It also modestly improves performance, at
1627 least in some cases. But it's disastrous for test case perf/compiler/T3064.
1628 Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a.
1629 What do we do? Do we reduce F? Or do we use the given? Hard to know what's
1630 best. GHC reduces. This is a disaster for T3064, where the type's size
1631 spirals out of control during reduction. (We're not helped by the fact that
1632 the flattener re-flattens all the arguments every time around.) If we prevent
1633 unification with type families, then the solver happens to use the equality
1634 before expanding the type family.
1635
1636 It would be lovely in the future to revisit this problem and remove this
1637 extra, unnecessary check. But we retain it for now as it seems to work
1638 better in practice.
1639
1640 Note [Refactoring hazard: checkTauTvUpdate]
1641 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1642 I (Richard E.) have a sad story about refactoring this code, retained here
1643 to prevent others (or a future me!) from falling into the same traps.
1644
1645 It all started with #11407, which was caused by the fact that the TyVarTy
1646 case of defer_me didn't look in the kind. But it seemed reasonable to
1647 simply remove the defer_me check instead.
1648
1649 It referred to two Notes (since removed) that were out of date, and the
1650 fast_check code in occurCheckExpand seemed to do just about the same thing as
1651 defer_me. The one piece that defer_me did that wasn't repeated by
1652 occurCheckExpand was the type-family check. (See Note [Prevent unification
1653 with type families].) So I checked the result of occurCheckExpand for any
1654 type family occurrences and deferred if there were any. This was done
1655 in commit e9bf7bb5cc9fb3f87dd05111aa23da76b86a8967 .
1656
1657 This approach turned out not to be performant, because the expanded
1658 type was bigger than the original type, and tyConsOfType (needed to
1659 see if there are any type family occurrences) looks through type
1660 synonyms. So it then struck me that we could dispense with the
1661 defer_me check entirely. This simplified the code nicely, and it cut
1662 the allocations in T5030 by half. But, as documented in Note [Prevent
1663 unification with type families], this destroyed performance in
1664 T3064. Regardless, I missed this regression and the change was
1665 committed as 3f5d1a13f112f34d992f6b74656d64d95a3f506d .
1666
1667 Bottom lines:
1668 * defer_me is back, but now fixed w.r.t. #11407.
1669 * Tread carefully before you start to refactor here. There can be
1670 lots of hard-to-predict consequences.
1671
1672 Note [Type synonyms and the occur check]
1673 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1674 Generally speaking we try to update a variable with type synonyms not
1675 expanded, which improves later error messages, unless looking
1676 inside a type synonym may help resolve a spurious occurs check
1677 error. Consider:
1678 type A a = ()
1679
1680 f :: (A a -> a -> ()) -> ()
1681 f = \ _ -> ()
1682
1683 x :: ()
1684 x = f (\ x p -> p x)
1685
1686 We will eventually get a constraint of the form t ~ A t. The ok function above will
1687 properly expand the type (A t) to just (), which is ok to be unified with t. If we had
1688 unified with the original type A t, we would lead the type checker into an infinite loop.
1689
1690 Hence, if the occurs check fails for a type synonym application, then (and *only* then),
1691 the ok function expands the synonym to detect opportunities for occurs check success using
1692 the underlying definition of the type synonym.
1693
1694 The same applies later on in the constraint interaction code; see TcInteract,
1695 function @occ_check_ok@.
1696
1697 Note [Non-TcTyVars in TcUnify]
1698 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1699 Because the same code is now shared between unifying types and unifying
1700 kinds, we sometimes will see proper TyVars floating around the unifier.
1701 Example (from test case polykinds/PolyKinds12):
1702
1703 type family Apply (f :: k1 -> k2) (x :: k1) :: k2
1704 type instance Apply g y = g y
1705
1706 When checking the instance declaration, we first *kind-check* the LHS
1707 and RHS, discovering that the instance really should be
1708
1709 type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y
1710
1711 During this kind-checking, all the tyvars will be TcTyVars. Then, however,
1712 as a second pass, we desugar the RHS (which is done in functions prefixed
1713 with "tc" in TcTyClsDecls"). By this time, all the kind-vars are proper
1714 TyVars, not TcTyVars, get some kind unification must happen.
1715
1716 Thus, we always check if a TyVar is a TcTyVar before asking if it's a
1717 meta-tyvar.
1718
1719 This used to not be necessary for type-checking (that is, before * :: *)
1720 because expressions get desugared via an algorithm separate from
1721 type-checking (with wrappers, etc.). Types get desugared very differently,
1722 causing this wibble in behavior seen here.
1723 -}
1724
1725 data LookupTyVarResult -- The result of a lookupTcTyVar call
1726 = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
1727 | Filled TcType
1728
1729 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
1730 lookupTcTyVar tyvar
1731 | MetaTv { mtv_ref = ref } <- details
1732 = do { meta_details <- readMutVar ref
1733 ; case meta_details of
1734 Indirect ty -> return (Filled ty)
1735 Flexi -> do { is_touchable <- isTouchableTcM tyvar
1736 -- Note [Unifying untouchables]
1737 ; if is_touchable then
1738 return (Unfilled details)
1739 else
1740 return (Unfilled vanillaSkolemTv) } }
1741 | otherwise
1742 = return (Unfilled details)
1743 where
1744 details = tcTyVarDetails tyvar
1745
1746 -- | Fill in a meta-tyvar
1747 updateMeta :: TcTyVar -- ^ tv to fill in, tv :: k1
1748 -> TcType -- ^ ty2 :: k2
1749 -> Coercion -- ^ kind_co :: k2 ~N k1
1750 -> TcM Coercion -- ^ :: tv ~N ty2 (= ty2 |> kind_co ~N ty2)
1751 updateMeta tv1 ty2 kind_co
1752 = do { let ty2' = ty2 `mkCastTy` kind_co
1753 ty2_refl = mkNomReflCo ty2
1754 co = mkCoherenceLeftCo ty2_refl kind_co
1755 ; writeMetaTyVar tv1 ty2'
1756 ; return co }
1757
1758 {-
1759 Note [Unifying untouchables]
1760 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1761 We treat an untouchable type variable as if it was a skolem. That
1762 ensures it won't unify with anything. It's a slight had, because
1763 we return a made-up TcTyVarDetails, but I think it works smoothly.
1764 -}
1765
1766 -- | Breaks apart a function kind into its pieces.
1767 matchExpectedFunKind :: Arity -- ^ # of args remaining, only for errors
1768 -> TcType -- ^ type, only for errors
1769 -> TcKind -- ^ function kind
1770 -> TcM (Coercion, TcKind, TcKind)
1771 -- ^ co :: old_kind ~ arg -> res
1772 matchExpectedFunKind num_args_remaining ty = go
1773 where
1774 go k | Just k' <- coreView k = go k'
1775
1776 go k@(TyVarTy kvar)
1777 | isTcTyVar kvar, isMetaTyVar kvar
1778 = do { maybe_kind <- readMetaTyVar kvar
1779 ; case maybe_kind of
1780 Indirect fun_kind -> go fun_kind
1781 Flexi -> defer k }
1782
1783 go k@(FunTy arg res) = return (mkNomReflCo k, arg, res)
1784 go other = defer other
1785
1786 defer k
1787 = do { arg_kind <- newMetaKindVar
1788 ; res_kind <- newMetaKindVar
1789 ; let new_fun = mkFunTy arg_kind res_kind
1790 thing = mkTypeErrorThingArgs ty num_args_remaining
1791 origin = TypeEqOrigin { uo_actual = k
1792 , uo_expected = new_fun
1793 , uo_thing = Just thing
1794 }
1795 ; co <- uType origin KindLevel k new_fun
1796 ; return (co, arg_kind, res_kind) }
1797
1798
1799 {- *********************************************************************
1800 * *
1801 Occurrence checking
1802 * *
1803 ********************************************************************* -}
1804
1805
1806 {- Note [Occurs check expansion]
1807 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1808 (occurCheckExpand tv xi) expands synonyms in xi just enough to get rid
1809 of occurrences of tv outside type function arguments, if that is
1810 possible; otherwise, it returns Nothing.
1811
1812 For example, suppose we have
1813 type F a b = [a]
1814 Then
1815 occCheckExpand b (F Int b) = Just [Int]
1816 but
1817 occCheckExpand a (F a Int) = Nothing
1818
1819 We don't promise to do the absolute minimum amount of expanding
1820 necessary, but we try not to do expansions we don't need to. We
1821 prefer doing inner expansions first. For example,
1822 type F a b = (a, Int, a, [a])
1823 type G b = Char
1824 We have
1825 occCheckExpand b (F (G b)) = Just (F Char)
1826 even though we could also expand F to get rid of b.
1827
1828 The two variants of the function are to support TcUnify.checkTauTvUpdate,
1829 which wants to prevent unification with type families. For more on this
1830 point, see Note [Prevent unification with type families] in TcUnify.
1831
1832 Note [Occurrence checking: look inside kinds]
1833 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1834 Suppose we are considering unifying
1835 (alpha :: *) ~ Int -> (beta :: alpha -> alpha)
1836 This may be an error (what is that alpha doing inside beta's kind?),
1837 but we must not make the mistake of actuallyy unifying or we'll
1838 build an infinite data structure. So when looking for occurrences
1839 of alpha in the rhs, we must look in the kinds of type variables
1840 that occur there.
1841
1842 NB: we may be able to remove the problem via expansion; see
1843 Note [Occurs check expansion]. So we have to try that.
1844
1845 Note [Checking for foralls]
1846 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1847 Unless we have -XImpredicativeTypes (which is a totally unsupported
1848 feature), we do not want to unify
1849 alpha ~ (forall a. a->a) -> Int
1850 So we look for foralls hidden inside the type, and it's convenient
1851 to do that at the same time as the occurs check (which looks for
1852 occurrences of alpha).
1853
1854 However, it's not just a question of looking for foralls /anywhere/!
1855 Consider
1856 (alpha :: forall k. k->*) ~ (beta :: forall k. k->*)
1857 This is legal; e.g. dependent/should_compile/T11635.
1858
1859 We don't want to reject it because of the forall in beta's kind,
1860 but (see Note [Occurrence checking: look inside kinds]) we do
1861 need to look in beta's kind. So we carry a flag saying if a 'forall'
1862 is OK, and sitch the flag on when stepping inside a kind.
1863
1864 Why is it OK? Why does it not count as impredicative polymorphism?
1865 The reason foralls are bad is because we reply on "seeing" foralls
1866 when doing implicit instantiation. But the forall inside the kind is
1867 fine. We'll generate a kind equality constraint
1868 (forall k. k->*) ~ (forall k. k->*)
1869 to check that the kinds of lhs and rhs are compatible. If alpha's
1870 kind had instead been
1871 (alpha :: kappa)
1872 then this kind equality would rightly complain about unifying kappa
1873 with (forall k. k->*)
1874
1875 -}
1876
1877 data OccCheckResult a
1878 = OC_OK a
1879 | OC_Bad -- Forall or type family
1880 | OC_Occurs
1881
1882 instance Functor OccCheckResult where
1883 fmap = liftM
1884
1885 instance Applicative OccCheckResult where
1886 pure = OC_OK
1887 (<*>) = ap
1888
1889 instance Monad OccCheckResult where
1890 OC_OK x >>= k = k x
1891 OC_Bad >>= _ = OC_Bad
1892 OC_Occurs >>= _ = OC_Occurs
1893
1894 occCheckForErrors :: DynFlags -> TcTyVar -> Type -> OccCheckResult ()
1895 -- Just for error-message generation; so we return OccCheckResult
1896 -- so the caller can report the right kind of error
1897 -- Check whether
1898 -- a) the given variable occurs in the given type.
1899 -- b) there is a forall in the type (unless we have -XImpredicativeTypes)
1900 occCheckForErrors dflags tv ty
1901 = case preCheck dflags True tv ty of
1902 OC_OK _ -> OC_OK ()
1903 OC_Bad -> OC_Bad
1904 OC_Occurs -> case occCheckExpand tv ty of
1905 Nothing -> OC_Occurs
1906 Just _ -> OC_OK ()
1907
1908 ----------------
1909 metaTyVarUpdateOK :: DynFlags
1910 -> TcTyVar -- tv :: k1
1911 -> TcType -- ty :: k2
1912 -> Maybe TcType -- possibly-expanded ty
1913 -- (metaTyFVarUpdateOK tv ty)
1914 -- We are about to update the meta-tyvar tv with ty, in
1915 -- our on-the-flyh unifier
1916 -- Check (a) that tv doesn't occur in ty (occurs check)
1917 -- (b) that ty does not have any foralls
1918 -- (in the impredicative case), or type functions
1919 --
1920 -- We have two possible outcomes:
1921 -- (1) Return the type to update the type variable with,
1922 -- [we know the update is ok]
1923 -- (2) Return Nothing,
1924 -- [the update might be dodgy]
1925 --
1926 -- Note that "Nothing" does not mean "definite error". For example
1927 -- type family F a
1928 -- type instance F Int = Int
1929 -- consider
1930 -- a ~ F a
1931 -- This is perfectly reasonable, if we later get a ~ Int. For now, though,
1932 -- we return Nothing, leaving it to the later constraint simplifier to
1933 -- sort matters out.
1934 --
1935 -- See Note [Refactoring hazard: checkTauTvUpdate]
1936
1937 metaTyVarUpdateOK dflags tv ty
1938 = case preCheck dflags False tv ty of
1939 -- False <=> type families not ok
1940 -- See Note [Prevent unification with type families]
1941 OC_OK _ -> Just ty
1942 OC_Bad -> Nothing -- forall or type function
1943 OC_Occurs -> occCheckExpand tv ty
1944
1945 preCheck :: DynFlags -> Bool -> TcTyVar -> TcType -> OccCheckResult ()
1946 -- A quick check for
1947 -- (a) a forall type (unless -XImpredivativeTypes)
1948 -- (b) a type family
1949 -- (c) an occurrence of the type variable (occurs check)
1950 --
1951 -- For (a) and (b) we check only the top level of the type, NOT
1952 -- inside the kinds of variables it mentions. But for (c) we do
1953 -- look in the kinds of course.
1954
1955 preCheck dflags ty_fam_ok tv ty
1956 = fast_check ty
1957 where
1958 details = tcTyVarDetails tv
1959 impredicative_ok = canUnifyWithPolyType dflags details
1960
1961 ok :: OccCheckResult ()
1962 ok = OC_OK ()
1963
1964 fast_check :: TcType -> OccCheckResult ()
1965 fast_check (TyVarTy tv')
1966 | tv == tv' = OC_Occurs
1967 | otherwise = fast_check_occ (tyVarKind tv')
1968 -- See Note [Occurrence checking: look inside kinds]
1969
1970 fast_check (TyConApp tc tys)
1971 | bad_tc tc = OC_Bad
1972 | otherwise = mapM fast_check tys >> ok
1973 fast_check (LitTy {}) = ok
1974 fast_check (FunTy a r) = fast_check a >> fast_check r
1975 fast_check (AppTy fun arg) = fast_check fun >> fast_check arg
1976 fast_check (CastTy ty co) = fast_check ty >> fast_check_co co
1977 fast_check (CoercionTy co) = fast_check_co co
1978 fast_check (ForAllTy (TvBndr tv' _) ty)
1979 | not impredicative_ok = OC_Bad
1980 | tv == tv' = ok
1981 | otherwise = do { fast_check_occ (tyVarKind tv')
1982 ; fast_check_occ ty }
1983 -- Under a forall we look only for occurrences of
1984 -- the type variable
1985
1986 -- For kinds, we only do an occurs check; we do not worry
1987 -- about type families or foralls
1988 -- See Note [Checking for foralls]
1989 fast_check_occ k | tv `elemVarSet` tyCoVarsOfType k = OC_Occurs
1990 | otherwise = ok
1991
1992 -- For coercions, we are only doing an occurs check here;
1993 -- no bother about impredicativity in coercions, as they're
1994 -- inferred
1995 fast_check_co co | tv `elemVarSet` tyCoVarsOfCo co = OC_Occurs
1996 | otherwise = ok
1997
1998 bad_tc :: TyCon -> Bool
1999 bad_tc tc
2000 | not (impredicative_ok || isTauTyCon tc) = True
2001 | not (ty_fam_ok || isFamFreeTyCon tc) = True
2002 | otherwise = False
2003
2004 occCheckExpand :: TcTyVar -> TcType -> Maybe TcType
2005 -- See Note [Occurs check expansion]
2006 -- We may have needed to do some type synonym unfolding in order to
2007 -- get rid of the variable (or forall), so we also return the unfolded
2008 -- version of the type, which is guaranteed to be syntactically free
2009 -- of the given type variable. If the type is already syntactically
2010 -- free of the variable, then the same type is returned.
2011 occCheckExpand tv ty
2012 = go emptyVarEnv ty
2013 where
2014 go :: VarEnv TyVar -> Type -> Maybe Type
2015 -- The VarEnv carries mappings necessary
2016 -- because of kind expansion
2017 go env (TyVarTy tv')
2018 | tv == tv' = Nothing
2019 | Just tv'' <- lookupVarEnv env tv' = return (mkTyVarTy tv'')
2020 | otherwise = do { k' <- go env (tyVarKind tv')
2021 ; return (mkTyVarTy $
2022 setTyVarKind tv' k') }
2023 -- See Note [Occurrence checking: look inside kinds]
2024
2025 go _ ty@(LitTy {}) = return ty
2026 go env (AppTy ty1 ty2) = do { ty1' <- go env ty1
2027 ; ty2' <- go env ty2
2028 ; return (mkAppTy ty1' ty2') }
2029 go env (FunTy ty1 ty2) = do { ty1' <- go env ty1
2030 ; ty2' <- go env ty2
2031 ; return (mkFunTy ty1' ty2') }
2032 go env ty@(ForAllTy (TvBndr tv' vis) body_ty)
2033 | tv == tv' = return ty
2034 | otherwise = do { ki' <- go env (tyVarKind tv')
2035 ; let tv'' = setTyVarKind tv' ki'
2036 env' = extendVarEnv env tv' tv''
2037 ; body' <- go env' body_ty
2038 ; return (ForAllTy (TvBndr tv'' vis) body') }
2039
2040 -- For a type constructor application, first try expanding away the
2041 -- offending variable from the arguments. If that doesn't work, next
2042 -- see if the type constructor is a type synonym, and if so, expand
2043 -- it and try again.
2044 go env ty@(TyConApp tc tys)
2045 = case mapM (go env) tys of
2046 Just tys' -> return (mkTyConApp tc tys')
2047 Nothing | Just ty' <- coreView ty -> go env ty'
2048 | otherwise -> Nothing
2049 -- Failing that, try to expand a synonym
2050
2051 go env (CastTy ty co) = do { ty' <- go env ty
2052 ; co' <- go_co env co
2053 ; return (mkCastTy ty' co') }
2054 go env (CoercionTy co) = do { co' <- go_co env co
2055 ; return (mkCoercionTy co') }
2056
2057 ------------------
2058 go_co env (Refl r ty) = do { ty' <- go env ty
2059 ; return (mkReflCo r ty') }
2060 -- Note: Coercions do not contain type synonyms
2061 go_co env (TyConAppCo r tc args) = do { args' <- mapM (go_co env) args
2062 ; return (mkTyConAppCo r tc args') }
2063 go_co env (AppCo co arg) = do { co' <- go_co env co
2064 ; arg' <- go_co env arg
2065 ; return (mkAppCo co' arg') }
2066 go_co env co@(ForAllCo tv' kind_co body_co)
2067 | tv == tv' = return co
2068 | otherwise = do { kind_co' <- go_co env kind_co
2069 ; let tv'' = setTyVarKind tv' $
2070 pFst (coercionKind kind_co')
2071 env' = extendVarEnv env tv' tv''
2072 ; body' <- go_co env' body_co
2073 ; return (ForAllCo tv'' kind_co' body') }
2074 go_co env (CoVarCo c) = do { k' <- go env (varType c)
2075 ; return (mkCoVarCo (setVarType c k')) }
2076 go_co env (AxiomInstCo ax ind args) = do { args' <- mapM (go_co env) args
2077 ; return (mkAxiomInstCo ax ind args') }
2078 go_co env (UnivCo p r ty1 ty2) = do { p' <- go_prov env p
2079 ; ty1' <- go env ty1
2080 ; ty2' <- go env ty2
2081 ; return (mkUnivCo p' r ty1' ty2') }
2082 go_co env (SymCo co) = do { co' <- go_co env co
2083 ; return (mkSymCo co') }
2084 go_co env (TransCo co1 co2) = do { co1' <- go_co env co1
2085 ; co2' <- go_co env co2
2086 ; return (mkTransCo co1' co2') }
2087 go_co env (NthCo n co) = do { co' <- go_co env co
2088 ; return (mkNthCo n co') }
2089 go_co env (LRCo lr co) = do { co' <- go_co env co
2090 ; return (mkLRCo lr co') }
2091 go_co env (InstCo co arg) = do { co' <- go_co env co
2092 ; arg' <- go_co env arg
2093 ; return (mkInstCo co' arg') }
2094 go_co env (CoherenceCo co1 co2) = do { co1' <- go_co env co1
2095 ; co2' <- go_co env co2
2096 ; return (mkCoherenceCo co1' co2') }
2097 go_co env (KindCo co) = do { co' <- go_co env co
2098 ; return (mkKindCo co') }
2099 go_co env (SubCo co) = do { co' <- go_co env co
2100 ; return (mkSubCo co') }
2101 go_co env (AxiomRuleCo ax cs) = do { cs' <- mapM (go_co env) cs
2102 ; return (mkAxiomRuleCo ax cs') }
2103
2104 ------------------
2105 go_prov _ UnsafeCoerceProv = return UnsafeCoerceProv
2106 go_prov env (PhantomProv co) = PhantomProv <$> go_co env co
2107 go_prov env (ProofIrrelProv co) = ProofIrrelProv <$> go_co env co
2108 go_prov _ p@(PluginProv _) = return p
2109 go_prov _ p@(HoleProv _) = return p
2110
2111 canUnifyWithPolyType :: DynFlags -> TcTyVarDetails -> Bool
2112 canUnifyWithPolyType dflags details
2113 = case details of
2114 MetaTv { mtv_info = SigTv } -> False
2115 MetaTv { mtv_info = TauTv } -> xopt LangExt.ImpredicativeTypes dflags
2116 _other -> True
2117 -- We can have non-meta tyvars in given constraints