A bit more tcTrace
[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' <- tcView 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' <- tcView 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' <- tcView 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' <- tcView 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' <- tcView ty_a = go ty_a' ty_e
699 | Just ty_e' <- tcView 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 <- tc_sub_tc_type eq_orig given_orig ctxt exp_arg act_arg
724 ; return (mkWpFun arg_wrap res_wrap exp_arg exp_res doc) }
725 -- arg_wrap :: exp_arg ~> act_arg
726 -- res_wrap :: act-res ~> exp_res
727 where
728 given_orig = GivenOrigin (SigSkol GenSigCtxt exp_arg [])
729 doc = text "When checking that" <+> quotes (ppr ty_actual) <+>
730 text "is more polymorphic than" <+> quotes (ppr ty_expected)
731
732 go ty_a ty_e
733 | let (tvs, theta, _) = tcSplitSigmaTy ty_a
734 , not (null tvs && null theta)
735 = do { (in_wrap, in_rho) <- topInstantiate inst_orig ty_a
736 ; body_wrap <- tc_sub_type_ds
737 (eq_orig { uo_actual = in_rho
738 , uo_expected = ty_expected })
739 inst_orig ctxt in_rho ty_e
740 ; return (body_wrap <.> in_wrap) }
741
742 | otherwise -- Revert to unification
743 = inst_and_unify
744 -- It's still possible that ty_actual has nested foralls. Instantiate
745 -- these, as there's no way unification will succeed with them in.
746 -- See typecheck/should_compile/T11305 for an example of when this
747 -- is important. The problem is that we're checking something like
748 -- a -> forall b. b -> b <= alpha beta gamma
749 -- where we end up with alpha := (->)
750
751 inst_and_unify = do { (wrap, rho_a) <- deeplyInstantiate inst_orig ty_actual
752
753 -- if we haven't recurred through an arrow, then
754 -- the eq_orig will list ty_actual. In this case,
755 -- we want to update the origin to reflect the
756 -- instantiation. If we *have* recurred through
757 -- an arrow, it's better not to update.
758 ; let eq_orig' = case eq_orig of
759 TypeEqOrigin { uo_actual = orig_ty_actual }
760 | orig_ty_actual `tcEqType` ty_actual
761 , not (isIdHsWrapper wrap)
762 -> eq_orig { uo_actual = rho_a }
763 _ -> eq_orig
764
765 ; cow <- uType eq_orig' TypeLevel rho_a ty_expected
766 ; return (mkWpCastN cow <.> wrap) }
767
768
769 -- use versions without synonyms expanded
770 unify = mkWpCastN <$> uType eq_orig TypeLevel ty_actual ty_expected
771
772 -----------------
773 -- needs both un-type-checked (for origins) and type-checked (for wrapping)
774 -- expressions
775 tcWrapResult :: HsExpr Name -> HsExpr TcId -> TcSigmaType -> ExpRhoType
776 -> TcM (HsExpr TcId)
777 tcWrapResult rn_expr = tcWrapResultO (exprCtOrigin rn_expr)
778
779 -- | Sometimes we don't have a @HsExpr Name@ to hand, and this is more
780 -- convenient.
781 tcWrapResultO :: CtOrigin -> HsExpr TcId -> TcSigmaType -> ExpRhoType
782 -> TcM (HsExpr TcId)
783 tcWrapResultO orig expr actual_ty res_ty
784 = do { traceTc "tcWrapResult" (vcat [ text "Actual: " <+> ppr actual_ty
785 , text "Expected:" <+> ppr res_ty ])
786 ; cow <- tcSubTypeDS_NC_O orig GenSigCtxt
787 (Just expr) actual_ty res_ty
788 ; return (mkHsWrap cow expr) }
789
790 -----------------------------------
791 wrapFunResCoercion
792 :: [TcType] -- Type of args
793 -> HsWrapper -- HsExpr a -> HsExpr b
794 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
795 wrapFunResCoercion arg_tys co_fn_res
796 | isIdHsWrapper co_fn_res
797 = return idHsWrapper
798 | null arg_tys
799 = return co_fn_res
800 | otherwise
801 = do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
802 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }
803
804
805 {- **********************************************************************
806 %* *
807 ExpType functions: tcInfer, fillInferResult
808 %* *
809 %********************************************************************* -}
810
811 -- | Infer a type using a fresh ExpType
812 -- See also Note [ExpType] in TcMType
813 -- Does not attempt to instantiate the inferred type
814 tcInferNoInst :: (ExpSigmaType -> TcM a) -> TcM (a, TcSigmaType)
815 tcInferNoInst = tcInfer False
816
817 tcInferInst :: (ExpRhoType -> TcM a) -> TcM (a, TcRhoType)
818 tcInferInst = tcInfer True
819
820 tcInfer :: Bool -> (ExpSigmaType -> TcM a) -> TcM (a, TcSigmaType)
821 tcInfer instantiate tc_check
822 = do { res_ty <- newInferExpType instantiate
823 ; result <- tc_check res_ty
824 ; res_ty <- readExpType res_ty
825 ; return (result, res_ty) }
826
827 fillInferResult_Inst :: CtOrigin -> TcType -> InferResult -> TcM HsWrapper
828 -- If wrap = fillInferResult_Inst t1 t2
829 -- => wrap :: t1 ~> t2
830 -- See Note [Deep instantiation of InferResult]
831 fillInferResult_Inst orig ty inf_res@(IR { ir_inst = instantiate_me })
832 | instantiate_me
833 = do { (wrap, rho) <- deeplyInstantiate orig ty
834 ; co <- fillInferResult rho inf_res
835 ; return (mkWpCastN co <.> wrap) }
836
837 | otherwise
838 = do { co <- fillInferResult ty inf_res
839 ; return (mkWpCastN co) }
840
841 fillInferResult :: TcType -> InferResult -> TcM TcCoercionN
842 -- If wrap = fillInferResult t1 t2
843 -- => wrap :: t1 ~> t2
844 fillInferResult orig_ty (IR { ir_uniq = u, ir_lvl = res_lvl
845 , ir_ref = ref })
846 = do { (ty_co, ty_to_fill_with) <- promoteTcType res_lvl orig_ty
847
848 ; traceTc "Filling ExpType" $
849 ppr u <+> text ":=" <+> ppr ty_to_fill_with
850
851 ; when debugIsOn (check_hole ty_to_fill_with)
852
853 ; writeTcRef ref (Just ty_to_fill_with)
854
855 ; return ty_co }
856 where
857 check_hole ty -- Debug check only
858 = do { let ty_lvl = tcTypeLevel ty
859 ; MASSERT2( not (ty_lvl `strictlyDeeperThan` res_lvl),
860 ppr u $$ ppr res_lvl $$ ppr ty_lvl $$
861 ppr ty <+> ppr (typeKind ty) $$ ppr orig_ty )
862 ; cts <- readTcRef ref
863 ; case cts of
864 Just already_there -> pprPanic "writeExpType"
865 (vcat [ ppr u
866 , ppr ty
867 , ppr already_there ])
868 Nothing -> return () }
869
870 {- Note [Deep instantiation of InferResult]
871 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
872 In some cases we want to deeply instantiate before filling in
873 an InferResult, and in some cases not. That's why InferReult
874 has the ir_inst flag.
875
876 * ir_inst = True: deeply instantantiate
877
878 Consider
879 f x = (*)
880 We want to instantiate the type of (*) before returning, else we
881 will infer the type
882 f :: forall {a}. a -> forall b. Num b => b -> b -> b
883 This is surely confusing for users.
884
885 And worse, the the monomorphism restriction won't properly. The MR is
886 dealt with in simplifyInfer, and simplifyInfer has no way of
887 instantiating. This could perhaps be worked around, but it may be
888 hard to know even when instantiation should happen.
889
890 Another reason. Consider
891 f :: (?x :: Int) => a -> a
892 g y = let ?x = 3::Int in f
893 Here want to instantiate f's type so that the ?x::Int constraint
894 gets discharged by the enclosing implicit-parameter binding.
895
896 * ir_inst = False: do not instantantiate
897
898 Consider this (which uses visible type application):
899
900 (let { f :: forall a. a -> a; f x = x } in f) @Int
901
902 We'll call TcExpr.tcInferFun to infer the type of the (let .. in f)
903 And we don't want to instantite the type of 'f' when we reach it,
904 else the outer visible type application won't work
905 -}
906
907 {- *********************************************************************
908 * *
909 Promoting types
910 * *
911 ********************************************************************* -}
912
913 promoteTcType :: TcLevel -> TcType -> TcM (TcCoercion, TcType)
914 -- See Note [Promoting a type]
915 -- promoteTcType level ty = (co, ty')
916 -- * Returns ty' whose max level is just 'level'
917 -- and whose kind is ~# to the kind of 'ty'
918 -- and whose kind has form TYPE rr
919 -- * and co :: ty ~ ty'
920 -- * and emits constraints to justify the coercion
921 promoteTcType dest_lvl ty
922 = do { cur_lvl <- getTcLevel
923 ; if (cur_lvl `sameDepthAs` dest_lvl)
924 then dont_promote_it
925 else promote_it }
926 where
927 promote_it :: TcM (TcCoercion, TcType)
928 promote_it -- Emit a constraint (alpha :: TYPE rr) ~ ty
929 -- where alpha and rr are fresh and from level dest_lvl
930 = do { rr <- newMetaTyVarTyAtLevel dest_lvl runtimeRepTy
931 ; prom_ty <- newMetaTyVarTyAtLevel dest_lvl (tYPE rr)
932 ; let eq_orig = TypeEqOrigin { uo_actual = ty
933 , uo_expected = prom_ty
934 , uo_thing = Nothing }
935
936 ; co <- emitWantedEq eq_orig TypeLevel Nominal ty prom_ty
937 ; return (co, prom_ty) }
938
939 dont_promote_it :: TcM (TcCoercion, TcType)
940 dont_promote_it -- Check that ty :: TYPE rr, for some (fresh) rr
941 = do { res_kind <- newOpenTypeKind
942 ; let ty_kind = typeKind ty
943 kind_orig = TypeEqOrigin { uo_actual = ty_kind
944 , uo_expected = res_kind
945 , uo_thing = Nothing }
946 ; ki_co <- uType kind_orig KindLevel (typeKind ty) res_kind
947 ; let co = mkTcNomReflCo ty `mkTcCoherenceRightCo` ki_co
948 ; return (co, ty `mkCastTy` ki_co) }
949
950 {- Note [Promoting a type]
951 ~~~~~~~~~~~~~~~~~~~~~~~~~~
952 Consider (Trac #12427)
953
954 data T where
955 MkT :: (Int -> Int) -> a -> T
956
957 h y = case y of MkT v w -> v
958
959 We'll infer the RHS type with an expected type ExpType of
960 (IR { ir_lvl = l, ir_ref = ref, ... )
961 where 'l' is the TcLevel of the RHS of 'h'. Then the MkT pattern
962 match will increase the level, so we'll end up in tcSubType, trying to
963 unify the type of v,
964 v :: Int -> Int
965 with the expected type. But this attempt takes place at level (l+1),
966 rightly so, since v's type could have mentioned existential variables,
967 (like w's does) and we want to catch that.
968
969 So we
970 - create a new meta-var alpha[l+1]
971 - fill in the InferRes ref cell 'ref' with alpha
972 - emit an equality constraint, thus
973 [W] alpha[l+1] ~ (Int -> Int)
974
975 That constraint will float outwards, as it should, unless v's
976 type mentions a skolem-captured variable.
977
978 This approach fails if v has a higher rank type; see
979 Note [Promotion and higher rank types]
980
981
982 Note [Promotion and higher rank types]
983 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
984 If v had a higher-rank type, say v :: (forall a. a->a) -> Int,
985 then we'd emit an equality
986 [W] alpha[l+1] ~ ((forall a. a->a) -> Int)
987 which will sadly fail because we can't unify a unification variable
988 with a polytype. But there is nothing really wrong with the program
989 here.
990
991 We could just about solve this by "promote the type" of v, to expose
992 its polymorphic "shape" while still leaving constraints that will
993 prevent existential escape. But we must be careful! Exposing
994 the "shape" of the type is precisely what we must NOT do under
995 a GADT pattern match! So in this case we might promote the type
996 to
997 (forall a. a->a) -> alpha[l+1]
998 and emit the constraint
999 [W] alpha[l+1] ~ Int
1000 Now the poromoted type can fill the ref cell, while the emitted
1001 equality can float or not, according to the usual rules.
1002
1003 But that's not quite right! We are exposing the arrow! We could
1004 deal with that too:
1005 (forall a. mu[l+1] a a) -> alpha[l+1]
1006 with constraints
1007 [W] alpha[l+1] ~ Int
1008 [W] mu[l+1] ~ (->)
1009 Here we abstract over the '->' inside the forall, in case that
1010 is subject to an equality constraint from a GADT match.
1011
1012 Note that we kept the outer (->) because that's part of
1013 the polymorphic "shape". And becauuse of impredicativity,
1014 GADT matches can't give equalities that affect polymorphic
1015 shape.
1016
1017 This reasoning just seems too complicated, so I decided not
1018 to do it. These higher-rank notes are just here to record
1019 the thinking.
1020 -}
1021
1022 {- *********************************************************************
1023 * *
1024 Generalisation
1025 * *
1026 ********************************************************************* -}
1027
1028 -- | Take an "expected type" and strip off quantifiers to expose the
1029 -- type underneath, binding the new skolems for the @thing_inside@.
1030 -- The returned 'HsWrapper' has type @specific_ty -> expected_ty@.
1031 tcSkolemise :: UserTypeCtxt -> TcSigmaType
1032 -> ([TcTyVar] -> TcType -> TcM result)
1033 -- ^ These are only ever used for scoped type variables.
1034 -> TcM (HsWrapper, result)
1035 -- ^ The expression has type: spec_ty -> expected_ty
1036
1037 tcSkolemise ctxt expected_ty thing_inside
1038 -- We expect expected_ty to be a forall-type
1039 -- If not, the call is a no-op
1040 = do { traceTc "tcSkolemise" Outputable.empty
1041 ; (wrap, tv_prs, given, rho') <- deeplySkolemise expected_ty
1042
1043 ; lvl <- getTcLevel
1044 ; when debugIsOn $
1045 traceTc "tcSkolemise" $ vcat [
1046 ppr lvl,
1047 text "expected_ty" <+> ppr expected_ty,
1048 text "inst tyvars" <+> ppr tv_prs,
1049 text "given" <+> ppr given,
1050 text "inst type" <+> ppr rho' ]
1051
1052 -- Generally we must check that the "forall_tvs" havn't been constrained
1053 -- The interesting bit here is that we must include the free variables
1054 -- of the expected_ty. Here's an example:
1055 -- runST (newVar True)
1056 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
1057 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
1058 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
1059 -- So now s' isn't unconstrained because it's linked to a.
1060 --
1061 -- However [Oct 10] now that the untouchables are a range of
1062 -- TcTyVars, all this is handled automatically with no need for
1063 -- extra faffing around
1064
1065 ; let tvs' = map snd tv_prs
1066 skol_info = SigSkol ctxt expected_ty tv_prs
1067
1068 ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
1069 thing_inside tvs' rho'
1070
1071 ; return (wrap <.> mkWpLet ev_binds, result) }
1072 -- The ev_binds returned by checkConstraints is very
1073 -- often empty, in which case mkWpLet is a no-op
1074
1075 -- | Variant of 'tcSkolemise' that takes an ExpType
1076 tcSkolemiseET :: UserTypeCtxt -> ExpSigmaType
1077 -> (ExpRhoType -> TcM result)
1078 -> TcM (HsWrapper, result)
1079 tcSkolemiseET _ et@(Infer {}) thing_inside
1080 = (idHsWrapper, ) <$> thing_inside et
1081 tcSkolemiseET ctxt (Check ty) thing_inside
1082 = tcSkolemise ctxt ty $ \_ -> thing_inside . mkCheckExpType
1083
1084 checkConstraints :: SkolemInfo
1085 -> [TcTyVar] -- Skolems
1086 -> [EvVar] -- Given
1087 -> TcM result
1088 -> TcM (TcEvBinds, result)
1089
1090 checkConstraints skol_info skol_tvs given thing_inside
1091 = do { (implics, ev_binds, result)
1092 <- buildImplication skol_info skol_tvs given thing_inside
1093 ; emitImplications implics
1094 ; return (ev_binds, result) }
1095
1096 buildImplication :: SkolemInfo
1097 -> [TcTyVar] -- Skolems
1098 -> [EvVar] -- Given
1099 -> TcM result
1100 -> TcM (Bag Implication, TcEvBinds, result)
1101 buildImplication skol_info skol_tvs given thing_inside
1102 = do { tc_lvl <- getTcLevel
1103 ; deferred_type_errors <- goptM Opt_DeferTypeErrors <||>
1104 goptM Opt_DeferTypedHoles
1105 ; if null skol_tvs && null given && (not deferred_type_errors ||
1106 not (isTopTcLevel tc_lvl))
1107 then do { res <- thing_inside
1108 ; return (emptyBag, emptyTcEvBinds, res) }
1109 -- Fast path. We check every function argument with
1110 -- tcPolyExpr, which uses tcSkolemise and hence checkConstraints.
1111 -- But with the solver producing unlifted equalities, we need
1112 -- to have an EvBindsVar for them when they might be deferred to
1113 -- runtime. Otherwise, they end up as top-level unlifted bindings,
1114 -- which are verboten. See also Note [Deferred errors for coercion holes]
1115 -- in TcErrors.
1116 else
1117 do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints thing_inside
1118 ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_tvs given wanted
1119 ; return (implics, ev_binds, result) }}
1120
1121 buildImplicationFor :: TcLevel -> SkolemInfo -> [TcTyVar]
1122 -> [EvVar] -> WantedConstraints
1123 -> TcM (Bag Implication, TcEvBinds)
1124 buildImplicationFor tclvl skol_info skol_tvs given wanted
1125 | isEmptyWC wanted && null given
1126 -- Optimisation : if there are no wanteds, and no givens
1127 -- don't generate an implication at all.
1128 -- Reason for the (null given): we don't want to lose
1129 -- the "inaccessible alternative" error check
1130 = return (emptyBag, emptyTcEvBinds)
1131
1132 | otherwise
1133 = ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
1134 do { ev_binds_var <- newTcEvBinds
1135 ; env <- getLclEnv
1136 ; let implic = Implic { ic_tclvl = tclvl
1137 , ic_skols = skol_tvs
1138 , ic_no_eqs = False
1139 , ic_given = given
1140 , ic_wanted = wanted
1141 , ic_status = IC_Unsolved
1142 , ic_binds = ev_binds_var
1143 , ic_env = env
1144 , ic_needed = emptyVarSet
1145 , ic_info = skol_info }
1146
1147 ; return (unitBag implic, TcEvBinds ev_binds_var) }
1148
1149 {-
1150 ************************************************************************
1151 * *
1152 Boxy unification
1153 * *
1154 ************************************************************************
1155
1156 The exported functions are all defined as versions of some
1157 non-exported generic functions.
1158 -}
1159
1160 unifyType :: Outputable a => Maybe a -- ^ If present, has type 'ty1'
1161 -> TcTauType -> TcTauType -> TcM TcCoercionN
1162 -- Actual and expected types
1163 -- Returns a coercion : ty1 ~ ty2
1164 unifyType thing ty1 ty2 = traceTc "utype" (ppr ty1 $$ ppr ty2 $$ ppr thing) >>
1165 uType origin TypeLevel ty1 ty2
1166 where
1167 origin = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2
1168 , uo_thing = mkErrorThing <$> thing }
1169
1170 -- | Use this instead of 'Nothing' when calling 'unifyType' without
1171 -- a good "thing" (where the "thing" has the "actual" type passed in)
1172 -- This has an 'Outputable' instance, avoiding amgiguity problems.
1173 noThing :: Maybe (HsExpr Name)
1174 noThing = Nothing
1175
1176 unifyKind :: Outputable a => Maybe a -> TcKind -> TcKind -> TcM CoercionN
1177 unifyKind thing ty1 ty2 = traceTc "ukind" (ppr ty1 $$ ppr ty2 $$ ppr thing) >>
1178 uType origin KindLevel ty1 ty2
1179 where origin = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2
1180 , uo_thing = mkErrorThing <$> thing }
1181
1182 ---------------
1183 unifyPred :: PredType -> PredType -> TcM TcCoercionN
1184 -- Actual and expected types
1185 unifyPred = unifyType noThing
1186
1187 ---------------
1188 unifyTheta :: TcThetaType -> TcThetaType -> TcM [TcCoercionN]
1189 -- Actual and expected types
1190 unifyTheta theta1 theta2
1191 = do { checkTc (equalLength theta1 theta2)
1192 (vcat [text "Contexts differ in length",
1193 nest 2 $ parens $ text "Use RelaxedPolyRec to allow this"])
1194 ; zipWithM unifyPred theta1 theta2 }
1195
1196 {-
1197 %************************************************************************
1198 %* *
1199 uType and friends
1200 %* *
1201 %************************************************************************
1202
1203 uType is the heart of the unifier.
1204 -}
1205
1206 uType, uType_defer
1207 :: CtOrigin
1208 -> TypeOrKind
1209 -> TcType -- ty1 is the *actual* type
1210 -> TcType -- ty2 is the *expected* type
1211 -> TcM Coercion
1212
1213 --------------
1214 -- It is always safe to defer unification to the main constraint solver
1215 -- See Note [Deferred unification]
1216 uType_defer origin t_or_k ty1 ty2
1217 = do { co <- emitWantedEq origin t_or_k Nominal ty1 ty2
1218
1219 -- Error trace only
1220 -- NB. do *not* call mkErrInfo unless tracing is on,
1221 -- because it is hugely expensive (#5631)
1222 ; whenDOptM Opt_D_dump_tc_trace $ do
1223 { ctxt <- getErrCtxt
1224 ; doc <- mkErrInfo emptyTidyEnv ctxt
1225 ; traceTc "utype_defer" (vcat [ppr co, ppr ty1,
1226 ppr ty2, pprCtOrigin origin, doc])
1227 }
1228 ; return co }
1229
1230 --------------
1231 uType origin t_or_k orig_ty1 orig_ty2
1232 = do { tclvl <- getTcLevel
1233 ; traceTc "u_tys" $ vcat
1234 [ text "tclvl" <+> ppr tclvl
1235 , sep [ ppr orig_ty1, text "~", ppr orig_ty2]
1236 , pprCtOrigin origin]
1237 ; co <- go orig_ty1 orig_ty2
1238 ; if isReflCo co
1239 then traceTc "u_tys yields no coercion" Outputable.empty
1240 else traceTc "u_tys yields coercion:" (ppr co)
1241 ; return co }
1242 where
1243 go :: TcType -> TcType -> TcM Coercion
1244 -- The arguments to 'go' are always semantically identical
1245 -- to orig_ty{1,2} except for looking through type synonyms
1246
1247 -- Variables; go for uVar
1248 -- Note that we pass in *original* (before synonym expansion),
1249 -- so that type variables tend to get filled in with
1250 -- the most informative version of the type
1251 go (TyVarTy tv1) ty2
1252 = do { lookup_res <- lookupTcTyVar tv1
1253 ; case lookup_res of
1254 Filled ty1 -> do { traceTc "found filled tyvar" (ppr tv1 <+> text ":->" <+> ppr ty1)
1255 ; go ty1 ty2 }
1256 Unfilled _ -> uUnfilledVar origin t_or_k NotSwapped tv1 ty2 }
1257 go ty1 (TyVarTy tv2)
1258 = do { lookup_res <- lookupTcTyVar tv2
1259 ; case lookup_res of
1260 Filled ty2 -> do { traceTc "found filled tyvar" (ppr tv2 <+> text ":->" <+> ppr ty2)
1261 ; go ty1 ty2 }
1262 Unfilled _ -> uUnfilledVar origin t_or_k IsSwapped tv2 ty1 }
1263
1264 -- See Note [Expanding synonyms during unification]
1265 go ty1@(TyConApp tc1 []) (TyConApp tc2 [])
1266 | tc1 == tc2
1267 = return $ mkReflCo Nominal ty1
1268
1269 -- See Note [Expanding synonyms during unification]
1270 --
1271 -- Also NB that we recurse to 'go' so that we don't push a
1272 -- new item on the origin stack. As a result if we have
1273 -- type Foo = Int
1274 -- and we try to unify Foo ~ Bool
1275 -- we'll end up saying "can't match Foo with Bool"
1276 -- rather than "can't match "Int with Bool". See Trac #4535.
1277 go ty1 ty2
1278 | Just ty1' <- tcView ty1 = go ty1' ty2
1279 | Just ty2' <- tcView ty2 = go ty1 ty2'
1280
1281 go (CastTy t1 co1) t2
1282 = do { co_tys <- go t1 t2
1283 ; return (mkCoherenceLeftCo co_tys co1) }
1284
1285 go t1 (CastTy t2 co2)
1286 = do { co_tys <- go t1 t2
1287 ; return (mkCoherenceRightCo co_tys co2) }
1288
1289 -- Functions (or predicate functions) just check the two parts
1290 go (FunTy fun1 arg1) (FunTy fun2 arg2)
1291 = do { co_l <- uType origin t_or_k fun1 fun2
1292 ; co_r <- uType origin t_or_k arg1 arg2
1293 ; return $ mkFunCo Nominal co_l co_r }
1294
1295 -- Always defer if a type synonym family (type function)
1296 -- is involved. (Data families behave rigidly.)
1297 go ty1@(TyConApp tc1 _) ty2
1298 | isTypeFamilyTyCon tc1 = defer ty1 ty2
1299 go ty1 ty2@(TyConApp tc2 _)
1300 | isTypeFamilyTyCon tc2 = defer ty1 ty2
1301
1302 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
1303 -- See Note [Mismatched type lists and application decomposition]
1304 | tc1 == tc2, length tys1 == length tys2
1305 = ASSERT2( isGenerativeTyCon tc1 Nominal, ppr tc1 )
1306 do { cos <- zipWithM (uType origin t_or_k) tys1 tys2
1307 ; return $ mkTyConAppCo Nominal tc1 cos }
1308
1309 go (LitTy m) ty@(LitTy n)
1310 | m == n
1311 = return $ mkNomReflCo ty
1312
1313 -- See Note [Care with type applications]
1314 -- Do not decompose FunTy against App;
1315 -- it's often a type error, so leave it for the constraint solver
1316 go (AppTy s1 t1) (AppTy s2 t2)
1317 = go_app s1 t1 s2 t2
1318
1319 go (AppTy s1 t1) (TyConApp tc2 ts2)
1320 | Just (ts2', t2') <- snocView ts2
1321 = ASSERT( mightBeUnsaturatedTyCon tc2 )
1322 go_app s1 t1 (TyConApp tc2 ts2') t2'
1323
1324 go (TyConApp tc1 ts1) (AppTy s2 t2)
1325 | Just (ts1', t1') <- snocView ts1
1326 = ASSERT( mightBeUnsaturatedTyCon tc1 )
1327 go_app (TyConApp tc1 ts1') t1' s2 t2
1328
1329 go (CoercionTy co1) (CoercionTy co2)
1330 = do { let ty1 = coercionType co1
1331 ty2 = coercionType co2
1332 ; kco <- uType (KindEqOrigin orig_ty1 (Just orig_ty2) origin
1333 (Just t_or_k))
1334 KindLevel
1335 ty1 ty2
1336 ; return $ mkProofIrrelCo Nominal kco co1 co2 }
1337
1338 -- Anything else fails
1339 -- E.g. unifying for-all types, which is relative unusual
1340 go ty1 ty2 = defer ty1 ty2
1341
1342 ------------------
1343 defer ty1 ty2 -- See Note [Check for equality before deferring]
1344 | ty1 `tcEqType` ty2 = return (mkNomReflCo ty1)
1345 | otherwise = uType_defer origin t_or_k ty1 ty2
1346
1347 ------------------
1348 go_app s1 t1 s2 t2
1349 = do { co_s <- uType origin t_or_k s1 s2
1350 ; co_t <- uType origin t_or_k t1 t2
1351 ; return $ mkAppCo co_s co_t }
1352
1353 {- Note [Check for equality before deferring]
1354 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1355 Particularly in ambiguity checks we can get equalities like (ty ~ ty).
1356 If ty involves a type function we may defer, which isn't very sensible.
1357 An egregious example of this was in test T9872a, which has a type signature
1358 Proxy :: Proxy (Solutions Cubes)
1359 Doing the ambiguity check on this signature generates the equality
1360 Solutions Cubes ~ Solutions Cubes
1361 and currently the constraint solver normalises both sides at vast cost.
1362 This little short-cut in 'defer' helps quite a bit.
1363
1364 Note [Care with type applications]
1365 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1366 Note: type applications need a bit of care!
1367 They can match FunTy and TyConApp, so use splitAppTy_maybe
1368 NB: we've already dealt with type variables and Notes,
1369 so if one type is an App the other one jolly well better be too
1370
1371 Note [Mismatched type lists and application decomposition]
1372 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1373 When we find two TyConApps, you might think that the argument lists
1374 are guaranteed equal length. But they aren't. Consider matching
1375 w (T x) ~ Foo (T x y)
1376 We do match (w ~ Foo) first, but in some circumstances we simply create
1377 a deferred constraint; and then go ahead and match (T x ~ T x y).
1378 This came up in Trac #3950.
1379
1380 So either
1381 (a) either we must check for identical argument kinds
1382 when decomposing applications,
1383
1384 (b) or we must be prepared for ill-kinded unification sub-problems
1385
1386 Currently we adopt (b) since it seems more robust -- no need to maintain
1387 a global invariant.
1388
1389 Note [Expanding synonyms during unification]
1390 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1391 We expand synonyms during unification, but:
1392 * We expand *after* the variable case so that we tend to unify
1393 variables with un-expanded type synonym. This just makes it
1394 more likely that the inferred types will mention type synonyms
1395 understandable to the user
1396
1397 * We expand *before* the TyConApp case. For example, if we have
1398 type Phantom a = Int
1399 and are unifying
1400 Phantom Int ~ Phantom Char
1401 it is *wrong* to unify Int and Char.
1402
1403 * The problem case immediately above can happen only with arguments
1404 to the tycon. So we check for nullary tycons *before* expanding.
1405 This is particularly helpful when checking (* ~ *), because * is
1406 now a type synonym.
1407
1408 Note [Deferred Unification]
1409 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1410 We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
1411 and yet its consistency is undetermined. Previously, there was no way to still
1412 make it consistent. So a mismatch error was issued.
1413
1414 Now these unifications are deferred until constraint simplification, where type
1415 family instances and given equations may (or may not) establish the consistency.
1416 Deferred unifications are of the form
1417 F ... ~ ...
1418 or x ~ ...
1419 where F is a type function and x is a type variable.
1420 E.g.
1421 id :: x ~ y => x -> y
1422 id e = e
1423
1424 involves the unification x = y. It is deferred until we bring into account the
1425 context x ~ y to establish that it holds.
1426
1427 If available, we defer original types (rather than those where closed type
1428 synonyms have already been expanded via tcCoreView). This is, as usual, to
1429 improve error messages.
1430
1431
1432 ************************************************************************
1433 * *
1434 uVar and friends
1435 * *
1436 ************************************************************************
1437
1438 @uVar@ is called when at least one of the types being unified is a
1439 variable. It does {\em not} assume that the variable is a fixed point
1440 of the substitution; rather, notice that @uVar@ (defined below) nips
1441 back into @uTys@ if it turns out that the variable is already bound.
1442 -}
1443
1444 ----------
1445 uUnfilledVar :: CtOrigin
1446 -> TypeOrKind
1447 -> SwapFlag
1448 -> TcTyVar -- Tyvar 1
1449 -> TcTauType -- Type 2
1450 -> TcM Coercion
1451 -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
1452 -- It might be a skolem, or untouchable, or meta
1453
1454 uUnfilledVar origin t_or_k swapped tv1 ty2
1455 = do { ty2 <- zonkTcType ty2
1456 -- Zonk to expose things to the
1457 -- occurs check, and so that if ty2
1458 -- looks like a type variable then it
1459 -- /is/ a type variable
1460 ; uUnfilledVar1 origin t_or_k swapped tv1 ty2 }
1461
1462 ----------
1463 uUnfilledVar1 :: CtOrigin
1464 -> TypeOrKind
1465 -> SwapFlag
1466 -> TcTyVar -- Tyvar 1
1467 -> TcTauType -- Type 2, zonked
1468 -> TcM Coercion
1469 uUnfilledVar1 origin t_or_k swapped tv1 ty2
1470 | Just tv2 <- tcGetTyVar_maybe ty2
1471 = go tv2
1472
1473 | otherwise
1474 = uUnfilledVar2 origin t_or_k swapped tv1 ty2
1475
1476 where
1477 -- 'go' handles the case where both are
1478 -- tyvars so we might want to swap
1479 go tv2 | tv1 == tv2 -- Same type variable => no-op
1480 = return (mkNomReflCo (mkTyVarTy tv1))
1481
1482 | swapOverTyVars tv1 tv2 -- Distinct type variables
1483 = uUnfilledVar2 origin t_or_k (flipSwap swapped)
1484 tv2 (mkTyVarTy tv1)
1485
1486 | otherwise
1487 = uUnfilledVar2 origin t_or_k swapped tv1 ty2
1488
1489 ----------
1490 uUnfilledVar2 :: CtOrigin
1491 -> TypeOrKind
1492 -> SwapFlag
1493 -> TcTyVar -- Tyvar 1
1494 -> TcTauType -- Type 2, zonked
1495 -> TcM Coercion
1496 uUnfilledVar2 origin t_or_k swapped tv1 ty2
1497 = do { dflags <- getDynFlags
1498 ; cur_lvl <- getTcLevel
1499 ; go dflags cur_lvl }
1500 where
1501 go dflags cur_lvl
1502 | canSolveByUnification cur_lvl tv1 ty2
1503 , Just ty2' <- metaTyVarUpdateOK dflags tv1 ty2
1504 = do { co_k <- uType kind_origin KindLevel (typeKind ty2') (tyVarKind tv1)
1505 ; co <- updateMeta tv1 ty2' co_k
1506 ; return (maybe_sym swapped co) }
1507
1508 | otherwise
1509 = unSwap swapped (uType_defer origin t_or_k) ty1 ty2
1510 -- Occurs check or an untouchable: just defer
1511 -- NB: occurs check isn't necessarily fatal:
1512 -- eg tv1 occured in type family parameter
1513
1514 ty1 = mkTyVarTy tv1
1515 kind_origin = KindEqOrigin ty1 (Just ty2) origin (Just t_or_k)
1516
1517 -- | apply sym iff swapped
1518 maybe_sym :: SwapFlag -> Coercion -> Coercion
1519 maybe_sym IsSwapped = mkSymCo
1520 maybe_sym NotSwapped = id
1521
1522 swapOverTyVars :: TcTyVar -> TcTyVar -> Bool
1523 swapOverTyVars tv1 tv2
1524 | isFmvTyVar tv1 = False -- See Note [Fmv Orientation Invariant]
1525 | isFmvTyVar tv2 = True
1526
1527 | Just lvl1 <- metaTyVarTcLevel_maybe tv1
1528 -- If tv1 is touchable, swap only if tv2 is also
1529 -- touchable and it's strictly better to update the latter
1530 -- But see Note [Avoid unnecessary swaps]
1531 = case metaTyVarTcLevel_maybe tv2 of
1532 Nothing -> False
1533 Just lvl2 | lvl2 `strictlyDeeperThan` lvl1 -> True
1534 | lvl1 `strictlyDeeperThan` lvl2 -> False
1535 | otherwise -> nicer_to_update tv2
1536
1537 -- So tv1 is not a meta tyvar
1538 -- If only one is a meta tyvar, put it on the left
1539 -- This is not because it'll be solved; but because
1540 -- the floating step looks for meta tyvars on the left
1541 | isMetaTyVar tv2 = True
1542
1543 -- So neither is a meta tyvar (including FlatMetaTv)
1544
1545 -- If only one is a flatten skolem, put it on the left
1546 -- See Note [Eliminate flat-skols]
1547 | not (isFlattenTyVar tv1), isFlattenTyVar tv2 = True
1548
1549 | otherwise = False
1550
1551 where
1552 nicer_to_update tv2
1553 = (isSigTyVar tv1 && not (isSigTyVar tv2))
1554 || (isSystemName (Var.varName tv2) && not (isSystemName (Var.varName tv1)))
1555
1556 -- @trySpontaneousSolve wi@ solves equalities where one side is a
1557 -- touchable unification variable.
1558 -- Returns True <=> spontaneous solve happened
1559 canSolveByUnification :: TcLevel -> TcTyVar -> TcType -> Bool
1560 canSolveByUnification tclvl tv xi
1561 | isTouchableMetaTyVar tclvl tv
1562 = case metaTyVarInfo tv of
1563 SigTv -> is_tyvar xi
1564 _ -> True
1565
1566 | otherwise -- Untouchable
1567 = False
1568 where
1569 is_tyvar xi
1570 = case tcGetTyVar_maybe xi of
1571 Nothing -> False
1572 Just tv -> case tcTyVarDetails tv of
1573 MetaTv { mtv_info = info }
1574 -> case info of
1575 SigTv -> True
1576 _ -> False
1577 SkolemTv {} -> True
1578 FlatSkol {} -> False
1579 RuntimeUnk -> True
1580
1581 {- Note [Fmv Orientation Invariant]
1582 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1583 * We always orient a constraint
1584 fmv ~ alpha
1585 with fmv on the left, even if alpha is
1586 a touchable unification variable
1587
1588 Reason: doing it the other way round would unify alpha:=fmv, but that
1589 really doesn't add any info to alpha. But a later constraint alpha ~
1590 Int might unlock everything. Comment:9 of #12526 gives a detailed
1591 example.
1592
1593 WARNING: I've gone to and fro on this one several times.
1594 I'm now pretty sure that unifying alpha:=fmv is a bad idea!
1595 So orienting with fmvs on the left is a good thing.
1596
1597 This example comes from IndTypesPerfMerge. (Others include
1598 T10226, T10009.)
1599 From the ambiguity check for
1600 f :: (F a ~ a) => a
1601 we get:
1602 [G] F a ~ a
1603 [WD] F alpha ~ alpha, alpha ~ a
1604
1605 From Givens we get
1606 [G] F a ~ fsk, fsk ~ a
1607
1608 Now if we flatten we get
1609 [WD] alpha ~ fmv, F alpha ~ fmv, alpha ~ a
1610
1611 Now, if we unified alpha := fmv, we'd get
1612 [WD] F fmv ~ fmv, [WD] fmv ~ a
1613 And now we are stuck.
1614
1615 So instead the Fmv Orientation Invariant puts te fmv on the
1616 left, giving
1617 [WD] fmv ~ alpha, [WD] F alpha ~ fmv, [WD] alpha ~ a
1618
1619 Now we get alpha:=a, and everything works out
1620
1621 Note [Prevent unification with type families]
1622 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1623 We prevent unification with type families because of an uneasy compromise.
1624 It's perfectly sound to unify with type families, and it even improves the
1625 error messages in the testsuite. It also modestly improves performance, at
1626 least in some cases. But it's disastrous for test case perf/compiler/T3064.
1627 Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a.
1628 What do we do? Do we reduce F? Or do we use the given? Hard to know what's
1629 best. GHC reduces. This is a disaster for T3064, where the type's size
1630 spirals out of control during reduction. (We're not helped by the fact that
1631 the flattener re-flattens all the arguments every time around.) If we prevent
1632 unification with type families, then the solver happens to use the equality
1633 before expanding the type family.
1634
1635 It would be lovely in the future to revisit this problem and remove this
1636 extra, unnecessary check. But we retain it for now as it seems to work
1637 better in practice.
1638
1639 Note [Refactoring hazard: checkTauTvUpdate]
1640 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1641 I (Richard E.) have a sad story about refactoring this code, retained here
1642 to prevent others (or a future me!) from falling into the same traps.
1643
1644 It all started with #11407, which was caused by the fact that the TyVarTy
1645 case of defer_me didn't look in the kind. But it seemed reasonable to
1646 simply remove the defer_me check instead.
1647
1648 It referred to two Notes (since removed) that were out of date, and the
1649 fast_check code in occurCheckExpand seemed to do just about the same thing as
1650 defer_me. The one piece that defer_me did that wasn't repeated by
1651 occurCheckExpand was the type-family check. (See Note [Prevent unification
1652 with type families].) So I checked the result of occurCheckExpand for any
1653 type family occurrences and deferred if there were any. This was done
1654 in commit e9bf7bb5cc9fb3f87dd05111aa23da76b86a8967 .
1655
1656 This approach turned out not to be performant, because the expanded
1657 type was bigger than the original type, and tyConsOfType (needed to
1658 see if there are any type family occurrences) looks through type
1659 synonyms. So it then struck me that we could dispense with the
1660 defer_me check entirely. This simplified the code nicely, and it cut
1661 the allocations in T5030 by half. But, as documented in Note [Prevent
1662 unification with type families], this destroyed performance in
1663 T3064. Regardless, I missed this regression and the change was
1664 committed as 3f5d1a13f112f34d992f6b74656d64d95a3f506d .
1665
1666 Bottom lines:
1667 * defer_me is back, but now fixed w.r.t. #11407.
1668 * Tread carefully before you start to refactor here. There can be
1669 lots of hard-to-predict consequences.
1670
1671 Note [Type synonyms and the occur check]
1672 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1673 Generally speaking we try to update a variable with type synonyms not
1674 expanded, which improves later error messages, unless looking
1675 inside a type synonym may help resolve a spurious occurs check
1676 error. Consider:
1677 type A a = ()
1678
1679 f :: (A a -> a -> ()) -> ()
1680 f = \ _ -> ()
1681
1682 x :: ()
1683 x = f (\ x p -> p x)
1684
1685 We will eventually get a constraint of the form t ~ A t. The ok function above will
1686 properly expand the type (A t) to just (), which is ok to be unified with t. If we had
1687 unified with the original type A t, we would lead the type checker into an infinite loop.
1688
1689 Hence, if the occurs check fails for a type synonym application, then (and *only* then),
1690 the ok function expands the synonym to detect opportunities for occurs check success using
1691 the underlying definition of the type synonym.
1692
1693 The same applies later on in the constraint interaction code; see TcInteract,
1694 function @occ_check_ok@.
1695
1696 Note [Non-TcTyVars in TcUnify]
1697 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1698 Because the same code is now shared between unifying types and unifying
1699 kinds, we sometimes will see proper TyVars floating around the unifier.
1700 Example (from test case polykinds/PolyKinds12):
1701
1702 type family Apply (f :: k1 -> k2) (x :: k1) :: k2
1703 type instance Apply g y = g y
1704
1705 When checking the instance declaration, we first *kind-check* the LHS
1706 and RHS, discovering that the instance really should be
1707
1708 type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y
1709
1710 During this kind-checking, all the tyvars will be TcTyVars. Then, however,
1711 as a second pass, we desugar the RHS (which is done in functions prefixed
1712 with "tc" in TcTyClsDecls"). By this time, all the kind-vars are proper
1713 TyVars, not TcTyVars, get some kind unification must happen.
1714
1715 Thus, we always check if a TyVar is a TcTyVar before asking if it's a
1716 meta-tyvar.
1717
1718 This used to not be necessary for type-checking (that is, before * :: *)
1719 because expressions get desugared via an algorithm separate from
1720 type-checking (with wrappers, etc.). Types get desugared very differently,
1721 causing this wibble in behavior seen here.
1722 -}
1723
1724 data LookupTyVarResult -- The result of a lookupTcTyVar call
1725 = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
1726 | Filled TcType
1727
1728 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
1729 lookupTcTyVar tyvar
1730 | MetaTv { mtv_ref = ref } <- details
1731 = do { meta_details <- readMutVar ref
1732 ; case meta_details of
1733 Indirect ty -> return (Filled ty)
1734 Flexi -> do { is_touchable <- isTouchableTcM tyvar
1735 -- Note [Unifying untouchables]
1736 ; if is_touchable then
1737 return (Unfilled details)
1738 else
1739 return (Unfilled vanillaSkolemTv) } }
1740 | otherwise
1741 = return (Unfilled details)
1742 where
1743 details = tcTyVarDetails tyvar
1744
1745 -- | Fill in a meta-tyvar
1746 updateMeta :: TcTyVar -- ^ tv to fill in, tv :: k1
1747 -> TcType -- ^ ty2 :: k2
1748 -> Coercion -- ^ kind_co :: k2 ~N k1
1749 -> TcM Coercion -- ^ :: tv ~N ty2 (= ty2 |> kind_co ~N ty2)
1750 updateMeta tv1 ty2 kind_co
1751 = do { let ty2' = ty2 `mkCastTy` kind_co
1752 ty2_refl = mkNomReflCo ty2
1753 co = mkCoherenceLeftCo ty2_refl kind_co
1754 ; writeMetaTyVar tv1 ty2'
1755 ; return co }
1756
1757 {-
1758 Note [Unifying untouchables]
1759 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1760 We treat an untouchable type variable as if it was a skolem. That
1761 ensures it won't unify with anything. It's a slight had, because
1762 we return a made-up TcTyVarDetails, but I think it works smoothly.
1763 -}
1764
1765 -- | Breaks apart a function kind into its pieces.
1766 matchExpectedFunKind :: Arity -- ^ # of args remaining, only for errors
1767 -> TcType -- ^ type, only for errors
1768 -> TcKind -- ^ function kind
1769 -> TcM (Coercion, TcKind, TcKind)
1770 -- ^ co :: old_kind ~ arg -> res
1771 matchExpectedFunKind num_args_remaining ty = go
1772 where
1773 go k | Just k' <- tcView k = go k'
1774
1775 go k@(TyVarTy kvar)
1776 | isTcTyVar kvar, isMetaTyVar kvar
1777 = do { maybe_kind <- readMetaTyVar kvar
1778 ; case maybe_kind of
1779 Indirect fun_kind -> go fun_kind
1780 Flexi -> defer k }
1781
1782 go k@(FunTy arg res) = return (mkNomReflCo k, arg, res)
1783 go other = defer other
1784
1785 defer k
1786 = do { arg_kind <- newMetaKindVar
1787 ; res_kind <- newMetaKindVar
1788 ; let new_fun = mkFunTy arg_kind res_kind
1789 thing = mkTypeErrorThingArgs ty num_args_remaining
1790 origin = TypeEqOrigin { uo_actual = k
1791 , uo_expected = new_fun
1792 , uo_thing = Just thing
1793 }
1794 ; co <- uType origin KindLevel k new_fun
1795 ; return (co, arg_kind, res_kind) }
1796
1797
1798 {- *********************************************************************
1799 * *
1800 Occurrence checking
1801 * *
1802 ********************************************************************* -}
1803
1804
1805 {- Note [Occurs check expansion]
1806 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1807 (occurCheckExpand tv xi) expands synonyms in xi just enough to get rid
1808 of occurrences of tv outside type function arguments, if that is
1809 possible; otherwise, it returns Nothing.
1810
1811 For example, suppose we have
1812 type F a b = [a]
1813 Then
1814 occCheckExpand b (F Int b) = Just [Int]
1815 but
1816 occCheckExpand a (F a Int) = Nothing
1817
1818 We don't promise to do the absolute minimum amount of expanding
1819 necessary, but we try not to do expansions we don't need to. We
1820 prefer doing inner expansions first. For example,
1821 type F a b = (a, Int, a, [a])
1822 type G b = Char
1823 We have
1824 occCheckExpand b (F (G b)) = Just (F Char)
1825 even though we could also expand F to get rid of b.
1826
1827 The two variants of the function are to support TcUnify.checkTauTvUpdate,
1828 which wants to prevent unification with type families. For more on this
1829 point, see Note [Prevent unification with type families] in TcUnify.
1830
1831 Note [Occurrence checking: look inside kinds]
1832 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1833 Suppose we are considering unifying
1834 (alpha :: *) ~ Int -> (beta :: alpha -> alpha)
1835 This may be an error (what is that alpha doing inside beta's kind?),
1836 but we must not make the mistake of actuallyy unifying or we'll
1837 build an infinite data structure. So when looking for occurrences
1838 of alpha in the rhs, we must look in the kinds of type variables
1839 that occur there.
1840
1841 NB: we may be able to remove the problem via expansion; see
1842 Note [Occurs check expansion]. So we have to try that.
1843
1844 Note [Checking for foralls]
1845 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1846 Unless we have -XImpredicativeTypes (which is a totally unsupported
1847 feature), we do not want to unify
1848 alpha ~ (forall a. a->a) -> Int
1849 So we look for foralls hidden inside the type, and it's convenient
1850 to do that at the same time as the occurs check (which looks for
1851 occurrences of alpha).
1852
1853 However, it's not just a question of looking for foralls /anywhere/!
1854 Consider
1855 (alpha :: forall k. k->*) ~ (beta :: forall k. k->*)
1856 This is legal; e.g. dependent/should_compile/T11635.
1857
1858 We don't want to reject it because of the forall in beta's kind,
1859 but (see Note [Occurrence checking: look inside kinds]) we do
1860 need to look in beta's kind. So we carry a flag saying if a 'forall'
1861 is OK, and sitch the flag on when stepping inside a kind.
1862
1863 Why is it OK? Why does it not count as impredicative polymorphism?
1864 The reason foralls are bad is because we reply on "seeing" foralls
1865 when doing implicit instantiation. But the forall inside the kind is
1866 fine. We'll generate a kind equality constraint
1867 (forall k. k->*) ~ (forall k. k->*)
1868 to check that the kinds of lhs and rhs are compatible. If alpha's
1869 kind had instead been
1870 (alpha :: kappa)
1871 then this kind equality would rightly complain about unifying kappa
1872 with (forall k. k->*)
1873
1874 -}
1875
1876 data OccCheckResult a
1877 = OC_OK a
1878 | OC_Bad -- Forall or type family
1879 | OC_Occurs
1880
1881 instance Functor OccCheckResult where
1882 fmap = liftM
1883
1884 instance Applicative OccCheckResult where
1885 pure = OC_OK
1886 (<*>) = ap
1887
1888 instance Monad OccCheckResult where
1889 OC_OK x >>= k = k x
1890 OC_Bad >>= _ = OC_Bad
1891 OC_Occurs >>= _ = OC_Occurs
1892
1893 occCheckForErrors :: DynFlags -> TcTyVar -> Type -> OccCheckResult ()
1894 -- Just for error-message generation; so we return OccCheckResult
1895 -- so the caller can report the right kind of error
1896 -- Check whether
1897 -- a) the given variable occurs in the given type.
1898 -- b) there is a forall in the type (unless we have -XImpredicativeTypes)
1899 occCheckForErrors dflags tv ty
1900 = case preCheck dflags True tv ty of
1901 OC_OK _ -> OC_OK ()
1902 OC_Bad -> OC_Bad
1903 OC_Occurs -> case occCheckExpand tv ty of
1904 Nothing -> OC_Occurs
1905 Just _ -> OC_OK ()
1906
1907 ----------------
1908 metaTyVarUpdateOK :: DynFlags
1909 -> TcTyVar -- tv :: k1
1910 -> TcType -- ty :: k2
1911 -> Maybe TcType -- possibly-expanded ty
1912 -- (metaTyFVarUpdateOK tv ty)
1913 -- We are about to update the meta-tyvar tv with ty
1914 -- Check (a) that tv doesn't occur in ty (occurs check)
1915 -- (b) that ty does not have any foralls
1916 -- (in the impredicative case), or type functions
1917 --
1918 -- We have two possible outcomes:
1919 -- (1) Return the type to update the type variable with,
1920 -- [we know the update is ok]
1921 -- (2) Return Nothing,
1922 -- [the update might be dodgy]
1923 --
1924 -- Note that "Nothing" does not mean "definite error". For example
1925 -- type family F a
1926 -- type instance F Int = Int
1927 -- consider
1928 -- a ~ F a
1929 -- This is perfectly reasonable, if we later get a ~ Int. For now, though,
1930 -- we return Nothing, leaving it to the later constraint simplifier to
1931 -- sort matters out.
1932 --
1933 -- See Note [Refactoring hazard: checkTauTvUpdate]
1934
1935 metaTyVarUpdateOK dflags tv ty
1936 = case preCheck dflags False tv ty of
1937 -- False <=> type families not ok
1938 -- See Note [Prevent unification with type families]
1939 OC_OK _ -> Just ty
1940 OC_Bad -> Nothing -- forall or type function
1941 OC_Occurs -> occCheckExpand tv ty
1942
1943 preCheck :: DynFlags -> Bool -> TcTyVar -> TcType -> OccCheckResult ()
1944 -- A quick check for
1945 -- (a) a forall type (unless -XImpredivativeTypes)
1946 -- (b) a type family
1947 -- (c) an occurrence of the type variable (occurs check)
1948 --
1949 -- For (a) and (b) we check only the top level of the type, NOT
1950 -- inside the kinds of variables it mentions. But for (c) we do
1951 -- look in the kinds of course.
1952
1953 preCheck dflags ty_fam_ok tv ty
1954 = fast_check ty
1955 where
1956 details = tcTyVarDetails tv
1957 impredicative_ok = canUnifyWithPolyType dflags details
1958
1959 ok :: OccCheckResult ()
1960 ok = OC_OK ()
1961
1962 fast_check :: TcType -> OccCheckResult ()
1963 fast_check (TyVarTy tv')
1964 | tv == tv' = OC_Occurs
1965 | otherwise = fast_check_occ (tyVarKind tv')
1966 -- See Note [Occurrence checking: look inside kinds]
1967
1968 fast_check (TyConApp tc tys)
1969 | bad_tc tc = OC_Bad
1970 | otherwise = mapM fast_check tys >> ok
1971 fast_check (LitTy {}) = ok
1972 fast_check (FunTy a r) = fast_check a >> fast_check r
1973 fast_check (AppTy fun arg) = fast_check fun >> fast_check arg
1974 fast_check (CastTy ty co) = fast_check ty >> fast_check_co co
1975 fast_check (CoercionTy co) = fast_check_co co
1976 fast_check (ForAllTy (TvBndr tv' _) ty)
1977 | not impredicative_ok = OC_Bad
1978 | tv == tv' = ok
1979 | otherwise = do { fast_check_occ (tyVarKind tv')
1980 ; fast_check_occ ty }
1981 -- Under a forall we look only for occurrences of
1982 -- the type variable
1983
1984 -- For kinds, we only do an occurs check; we do not worry
1985 -- about type families or foralls
1986 -- See Note [Checking for foralls]
1987 fast_check_occ k | tv `elemVarSet` tyCoVarsOfType k = OC_Occurs
1988 | otherwise = ok
1989
1990 -- For coercions, we are only doing an occurs check here;
1991 -- no bother about impredicativity in coercions, as they're
1992 -- inferred
1993 fast_check_co co | tv `elemVarSet` tyCoVarsOfCo co = OC_Occurs
1994 | otherwise = ok
1995
1996 bad_tc :: TyCon -> Bool
1997 bad_tc tc
1998 | not (impredicative_ok || isTauTyCon tc) = True
1999 | not (ty_fam_ok || isFamFreeTyCon tc) = True
2000 | otherwise = False
2001
2002 occCheckExpand :: TcTyVar -> TcType -> Maybe TcType
2003 -- See Note [Occurs check expansion]
2004 -- We may have needed to do some type synonym unfolding in order to
2005 -- get rid of the variable (or forall), so we also return the unfolded
2006 -- version of the type, which is guaranteed to be syntactically free
2007 -- of the given type variable. If the type is already syntactically
2008 -- free of the variable, then the same type is returned.
2009 occCheckExpand tv ty
2010 = go emptyVarEnv ty
2011 where
2012 go :: VarEnv TyVar -> Type -> Maybe Type
2013 -- The VarEnv carries mappings necessary
2014 -- because of kind expansion
2015 go env (TyVarTy tv')
2016 | tv == tv' = Nothing
2017 | Just tv'' <- lookupVarEnv env tv' = return (mkTyVarTy tv'')
2018 | otherwise = do { k' <- go env (tyVarKind tv')
2019 ; return (mkTyVarTy $
2020 setTyVarKind tv' k') }
2021 -- See Note [Occurrence checking: look inside kinds]
2022
2023 go _ ty@(LitTy {}) = return ty
2024 go env (AppTy ty1 ty2) = do { ty1' <- go env ty1
2025 ; ty2' <- go env ty2
2026 ; return (mkAppTy ty1' ty2') }
2027 go env (FunTy ty1 ty2) = do { ty1' <- go env ty1
2028 ; ty2' <- go env ty2
2029 ; return (mkFunTy ty1' ty2') }
2030 go env ty@(ForAllTy (TvBndr tv' vis) body_ty)
2031 | tv == tv' = return ty
2032 | otherwise = do { ki' <- go env (tyVarKind tv')
2033 ; let tv'' = setTyVarKind tv' ki'
2034 env' = extendVarEnv env tv' tv''
2035 ; body' <- go env' body_ty
2036 ; return (ForAllTy (TvBndr tv'' vis) body') }
2037
2038 -- For a type constructor application, first try expanding away the
2039 -- offending variable from the arguments. If that doesn't work, next
2040 -- see if the type constructor is a type synonym, and if so, expand
2041 -- it and try again.
2042 go env ty@(TyConApp tc tys)
2043 = case mapM (go env) tys of
2044 Just tys' -> return (mkTyConApp tc tys')
2045 Nothing | Just ty' <- tcView ty -> go env ty'
2046 | otherwise -> Nothing
2047 -- Failing that, try to expand a synonym
2048
2049 go env (CastTy ty co) = do { ty' <- go env ty
2050 ; co' <- go_co env co
2051 ; return (mkCastTy ty' co') }
2052 go env (CoercionTy co) = do { co' <- go_co env co
2053 ; return (mkCoercionTy co') }
2054
2055 ------------------
2056 go_co env (Refl r ty) = do { ty' <- go env ty
2057 ; return (mkReflCo r ty') }
2058 -- Note: Coercions do not contain type synonyms
2059 go_co env (TyConAppCo r tc args) = do { args' <- mapM (go_co env) args
2060 ; return (mkTyConAppCo r tc args') }
2061 go_co env (AppCo co arg) = do { co' <- go_co env co
2062 ; arg' <- go_co env arg
2063 ; return (mkAppCo co' arg') }
2064 go_co env co@(ForAllCo tv' kind_co body_co)
2065 | tv == tv' = return co
2066 | otherwise = do { kind_co' <- go_co env kind_co
2067 ; let tv'' = setTyVarKind tv' $
2068 pFst (coercionKind kind_co')
2069 env' = extendVarEnv env tv' tv''
2070 ; body' <- go_co env' body_co
2071 ; return (ForAllCo tv'' kind_co' body') }
2072 go_co env (FunCo r co1 co2) = do { co1' <- go_co env co1
2073 ; co2' <- go_co env co2
2074 ; return (mkFunCo r co1' co2') }
2075 go_co env (CoVarCo c) = do { k' <- go env (varType c)
2076 ; return (mkCoVarCo (setVarType c k')) }
2077 go_co env (AxiomInstCo ax ind args) = do { args' <- mapM (go_co env) args
2078 ; return (mkAxiomInstCo ax ind args') }
2079 go_co env (UnivCo p r ty1 ty2) = do { p' <- go_prov env p
2080 ; ty1' <- go env ty1
2081 ; ty2' <- go env ty2
2082 ; return (mkUnivCo p' r ty1' ty2') }
2083 go_co env (SymCo co) = do { co' <- go_co env co
2084 ; return (mkSymCo co') }
2085 go_co env (TransCo co1 co2) = do { co1' <- go_co env co1
2086 ; co2' <- go_co env co2
2087 ; return (mkTransCo co1' co2') }
2088 go_co env (NthCo n co) = do { co' <- go_co env co
2089 ; return (mkNthCo n co') }
2090 go_co env (LRCo lr co) = do { co' <- go_co env co
2091 ; return (mkLRCo lr co') }
2092 go_co env (InstCo co arg) = do { co' <- go_co env co
2093 ; arg' <- go_co env arg
2094 ; return (mkInstCo co' arg') }
2095 go_co env (CoherenceCo co1 co2) = do { co1' <- go_co env co1
2096 ; co2' <- go_co env co2
2097 ; return (mkCoherenceCo co1' co2') }
2098 go_co env (KindCo co) = do { co' <- go_co env co
2099 ; return (mkKindCo co') }
2100 go_co env (SubCo co) = do { co' <- go_co env co
2101 ; return (mkSubCo co') }
2102 go_co env (AxiomRuleCo ax cs) = do { cs' <- mapM (go_co env) cs
2103 ; return (mkAxiomRuleCo ax cs') }
2104
2105 ------------------
2106 go_prov _ UnsafeCoerceProv = return UnsafeCoerceProv
2107 go_prov env (PhantomProv co) = PhantomProv <$> go_co env co
2108 go_prov env (ProofIrrelProv co) = ProofIrrelProv <$> go_co env co
2109 go_prov _ p@(PluginProv _) = return p
2110 go_prov _ p@(HoleProv _) = return p
2111
2112 canUnifyWithPolyType :: DynFlags -> TcTyVarDetails -> Bool
2113 canUnifyWithPolyType dflags details
2114 = case details of
2115 MetaTv { mtv_info = SigTv } -> False
2116 MetaTv { mtv_info = TauTv } -> xopt LangExt.ImpredicativeTypes dflags
2117 _other -> True
2118 -- We can have non-meta tyvars in given constraints