eb96757f219227f4e7b03cc6d1f7b144d4912543
[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,
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 occCheckExpand, metaTyVarUpdateOK,
35 occCheckForErrors, OccCheckResult(..)
36
37 ) where
38
39 #include "HsVersions.h"
40
41 import GhcPrelude
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 Bag
63 import Util
64 import Pair( pFst )
65 import qualified GHC.LanguageExtensions as LangExt
66 import Outputable
67
68 import Control.Monad
69 import Control.Arrow ( second )
70
71 {-
72 ************************************************************************
73 * *
74 matchExpected functions
75 * *
76 ************************************************************************
77
78 Note [Herald for matchExpectedFunTys]
79 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
80 The 'herald' always looks like:
81 "The equation(s) for 'f' have"
82 "The abstraction (\x.e) takes"
83 "The section (+ x) expects"
84 "The function 'f' is applied to"
85
86 This is used to construct a message of form
87
88 The abstraction `\Just 1 -> ...' takes two arguments
89 but its type `Maybe a -> a' has only one
90
91 The equation(s) for `f' have two arguments
92 but its type `Maybe a -> a' has only one
93
94 The section `(f 3)' requires 'f' to take two arguments
95 but its type `Int -> Int' has only one
96
97 The function 'f' is applied to two arguments
98 but its type `Int -> Int' has only one
99
100 When visible type applications (e.g., `f @Int 1 2`, as in #13902) enter the
101 picture, we have a choice in deciding whether to count the type applications as
102 proper arguments:
103
104 The function 'f' is applied to one visible type argument
105 and two value arguments
106 but its type `forall a. a -> a` has only one visible type argument
107 and one value argument
108
109 Or whether to include the type applications as part of the herald itself:
110
111 The expression 'f @Int' is applied to two arguments
112 but its type `Int -> Int` has only one
113
114 The latter is easier to implement and is arguably easier to understand, so we
115 choose to implement that option.
116
117 Note [matchExpectedFunTys]
118 ~~~~~~~~~~~~~~~~~~~~~~~~~~
119 matchExpectedFunTys checks that a sigma has the form
120 of an n-ary function. It passes the decomposed type to the
121 thing_inside, and returns a wrapper to coerce between the two types
122
123 It's used wherever a language construct must have a functional type,
124 namely:
125 A lambda expression
126 A function definition
127 An operator section
128
129 This function must be written CPS'd because it needs to fill in the
130 ExpTypes produced for arguments before it can fill in the ExpType
131 passed in.
132
133 -}
134
135 -- Use this one when you have an "expected" type.
136 matchExpectedFunTys :: forall a.
137 SDoc -- See Note [Herald for matchExpectedFunTys]
138 -> Arity
139 -> ExpRhoType -- deeply skolemised
140 -> ([ExpSigmaType] -> ExpRhoType -> TcM a)
141 -- must fill in these ExpTypes here
142 -> TcM (a, HsWrapper)
143 -- If matchExpectedFunTys n ty = (_, wrap)
144 -- then wrap : (t1 -> ... -> tn -> ty_r) ~> ty,
145 -- where [t1, ..., tn], ty_r are passed to the thing_inside
146 matchExpectedFunTys herald arity orig_ty thing_inside
147 = case orig_ty of
148 Check ty -> go [] arity ty
149 _ -> defer [] arity orig_ty
150 where
151 go acc_arg_tys 0 ty
152 = do { result <- thing_inside (reverse acc_arg_tys) (mkCheckExpType ty)
153 ; return (result, idHsWrapper) }
154
155 go acc_arg_tys n ty
156 | Just ty' <- tcView ty = go acc_arg_tys n ty'
157
158 go acc_arg_tys n (FunTy arg_ty res_ty)
159 = ASSERT( not (isPredTy arg_ty) )
160 do { (result, wrap_res) <- go (mkCheckExpType arg_ty : acc_arg_tys)
161 (n-1) res_ty
162 ; return ( result
163 , mkWpFun idHsWrapper wrap_res arg_ty res_ty doc ) }
164 where
165 doc = text "When inferring the argument type of a function with type" <+>
166 quotes (ppr orig_ty)
167
168 go acc_arg_tys n ty@(TyVarTy tv)
169 | isMetaTyVar tv
170 = do { cts <- readMetaTyVar tv
171 ; case cts of
172 Indirect ty' -> go acc_arg_tys n ty'
173 Flexi -> defer acc_arg_tys n (mkCheckExpType ty) }
174
175 -- In all other cases we bale out into ordinary unification
176 -- However unlike the meta-tyvar case, we are sure that the
177 -- number of arguments doesn't match arity of the original
178 -- type, so we can add a bit more context to the error message
179 -- (cf Trac #7869).
180 --
181 -- It is not always an error, because specialized type may have
182 -- different arity, for example:
183 --
184 -- > f1 = f2 'a'
185 -- > f2 :: Monad m => m Bool
186 -- > f2 = undefined
187 --
188 -- But in that case we add specialized type into error context
189 -- anyway, because it may be useful. See also Trac #9605.
190 go acc_arg_tys n ty = addErrCtxtM mk_ctxt $
191 defer acc_arg_tys n (mkCheckExpType ty)
192
193 ------------
194 defer :: [ExpSigmaType] -> Arity -> ExpRhoType -> TcM (a, HsWrapper)
195 defer acc_arg_tys n fun_ty
196 = do { more_arg_tys <- replicateM n newInferExpTypeNoInst
197 ; res_ty <- newInferExpTypeInst
198 ; result <- thing_inside (reverse acc_arg_tys ++ more_arg_tys) res_ty
199 ; more_arg_tys <- mapM readExpType more_arg_tys
200 ; res_ty <- readExpType res_ty
201 ; let unif_fun_ty = mkFunTys more_arg_tys res_ty
202 ; wrap <- tcSubTypeDS AppOrigin GenSigCtxt unif_fun_ty fun_ty
203 -- Not a good origin at all :-(
204 ; return (result, wrap) }
205
206 ------------
207 mk_ctxt :: TidyEnv -> TcM (TidyEnv, MsgDoc)
208 mk_ctxt env = do { (env', ty) <- zonkTidyTcType env orig_tc_ty
209 ; let (args, _) = tcSplitFunTys ty
210 n_actual = length args
211 (env'', orig_ty') = tidyOpenType env' orig_tc_ty
212 ; return ( env''
213 , mk_fun_tys_msg orig_ty' ty n_actual arity herald) }
214 where
215 orig_tc_ty = checkingExpType "matchExpectedFunTys" orig_ty
216 -- this is safe b/c we're called from "go"
217
218 -- Like 'matchExpectedFunTys', but used when you have an "actual" type,
219 -- for example in function application
220 matchActualFunTys :: SDoc -- See Note [Herald for matchExpectedFunTys]
221 -> CtOrigin
222 -> Maybe (HsExpr GhcRn) -- the thing with type TcSigmaType
223 -> Arity
224 -> TcSigmaType
225 -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
226 -- If matchActualFunTys n ty = (wrap, [t1,..,tn], ty_r)
227 -- then wrap : ty ~> (t1 -> ... -> tn -> ty_r)
228 matchActualFunTys herald ct_orig mb_thing arity ty
229 = matchActualFunTysPart herald ct_orig mb_thing arity ty [] arity
230
231 -- | Variant of 'matchActualFunTys' that works when supplied only part
232 -- (that is, to the right of some arrows) of the full function type
233 matchActualFunTysPart :: SDoc -- See Note [Herald for matchExpectedFunTys]
234 -> CtOrigin
235 -> Maybe (HsExpr GhcRn) -- the thing with type TcSigmaType
236 -> Arity
237 -> TcSigmaType
238 -> [TcSigmaType] -- reversed args. See (*) below.
239 -> Arity -- overall arity of the function, for errs
240 -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
241 matchActualFunTysPart herald ct_orig mb_thing arity orig_ty
242 orig_old_args full_arity
243 = go arity orig_old_args orig_ty
244 -- Does not allocate unnecessary meta variables: if the input already is
245 -- a function, we just take it apart. Not only is this efficient,
246 -- it's important for higher rank: the argument might be of form
247 -- (forall a. ty) -> other
248 -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
249 -- hide the forall inside a meta-variable
250
251 -- (*) Sometimes it's necessary to call matchActualFunTys with only part
252 -- (that is, to the right of some arrows) of the type of the function in
253 -- question. (See TcExpr.tcArgs.) This argument is the reversed list of
254 -- arguments already seen (that is, not part of the TcSigmaType passed
255 -- in elsewhere).
256
257 where
258 -- This function has a bizarre mechanic: it accumulates arguments on
259 -- the way down and also builds an argument list on the way up. Why:
260 -- 1. The returns args list and the accumulated args list might be different.
261 -- The accumulated args include all the arg types for the function,
262 -- including those from before this function was called. The returned
263 -- list should include only those arguments produced by this call of
264 -- matchActualFunTys
265 --
266 -- 2. The HsWrapper can be built only on the way up. It seems (more)
267 -- bizarre to build the HsWrapper but not the arg_tys.
268 --
269 -- Refactoring is welcome.
270 go :: Arity
271 -> [TcSigmaType] -- accumulator of arguments (reversed)
272 -> TcSigmaType -- the remainder of the type as we're processing
273 -> TcM (HsWrapper, [TcSigmaType], TcSigmaType)
274 go 0 _ ty = return (idHsWrapper, [], ty)
275
276 go n acc_args ty
277 | not (null tvs && null theta)
278 = do { (wrap1, rho) <- topInstantiate ct_orig ty
279 ; (wrap2, arg_tys, res_ty) <- go n acc_args rho
280 ; return (wrap2 <.> wrap1, arg_tys, res_ty) }
281 where
282 (tvs, theta, _) = tcSplitSigmaTy ty
283
284 go n acc_args ty
285 | Just ty' <- tcView ty = go n acc_args ty'
286
287 go n acc_args (FunTy arg_ty res_ty)
288 = ASSERT( not (isPredTy arg_ty) )
289 do { (wrap_res, tys, ty_r) <- go (n-1) (arg_ty : acc_args) res_ty
290 ; return ( mkWpFun idHsWrapper wrap_res arg_ty ty_r doc
291 , arg_ty : tys, ty_r ) }
292 where
293 doc = text "When inferring the argument type of a function with type" <+>
294 quotes (ppr orig_ty)
295
296 go n acc_args ty@(TyVarTy tv)
297 | isMetaTyVar tv
298 = do { cts <- readMetaTyVar tv
299 ; case cts of
300 Indirect ty' -> go n acc_args ty'
301 Flexi -> defer n ty }
302
303 -- In all other cases we bale out into ordinary unification
304 -- However unlike the meta-tyvar case, we are sure that the
305 -- number of arguments doesn't match arity of the original
306 -- type, so we can add a bit more context to the error message
307 -- (cf Trac #7869).
308 --
309 -- It is not always an error, because specialized type may have
310 -- different arity, for example:
311 --
312 -- > f1 = f2 'a'
313 -- > f2 :: Monad m => m Bool
314 -- > f2 = undefined
315 --
316 -- But in that case we add specialized type into error context
317 -- anyway, because it may be useful. See also Trac #9605.
318 go n acc_args ty = addErrCtxtM (mk_ctxt (reverse acc_args) ty) $
319 defer n ty
320
321 ------------
322 defer n fun_ty
323 = do { arg_tys <- replicateM n newOpenFlexiTyVarTy
324 ; res_ty <- newOpenFlexiTyVarTy
325 ; let unif_fun_ty = mkFunTys arg_tys res_ty
326 ; co <- unifyType mb_thing fun_ty unif_fun_ty
327 ; return (mkWpCastN co, arg_tys, res_ty) }
328
329 ------------
330 mk_ctxt :: [TcSigmaType] -> TcSigmaType -> TidyEnv -> TcM (TidyEnv, MsgDoc)
331 mk_ctxt arg_tys res_ty env
332 = do { let ty = mkFunTys arg_tys res_ty
333 ; (env1, zonked) <- zonkTidyTcType env ty
334 -- zonking might change # of args
335 ; let (zonked_args, _) = tcSplitFunTys zonked
336 n_actual = length zonked_args
337 (env2, unzonked) = tidyOpenType env1 ty
338 ; return ( env2
339 , mk_fun_tys_msg unzonked zonked n_actual full_arity herald) }
340
341 mk_fun_tys_msg :: TcType -- the full type passed in (unzonked)
342 -> TcType -- the full type passed in (zonked)
343 -> Arity -- the # of args found
344 -> Arity -- the # of args wanted
345 -> SDoc -- overall herald
346 -> SDoc
347 mk_fun_tys_msg full_ty ty n_args full_arity herald
348 = herald <+> speakNOf full_arity (text "argument") <> comma $$
349 if n_args == full_arity
350 then text "its type is" <+> quotes (pprType full_ty) <>
351 comma $$
352 text "it is specialized to" <+> quotes (pprType ty)
353 else sep [text "but its type" <+> quotes (pprType ty),
354 if n_args == 0 then text "has none"
355 else text "has only" <+> speakN n_args]
356
357 ----------------------
358 matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
359 -- Special case for lists
360 matchExpectedListTy exp_ty
361 = do { (co, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
362 ; return (co, elt_ty) }
363
364 ----------------------
365 matchExpectedPArrTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
366 -- Special case for parrs
367 matchExpectedPArrTy exp_ty
368 = do { (co, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
369 ; return (co, elt_ty) }
370
371 ---------------------
372 matchExpectedTyConApp :: TyCon -- T :: forall kv1 ... kvm. k1 -> ... -> kn -> *
373 -> TcRhoType -- orig_ty
374 -> TcM (TcCoercionN, -- T k1 k2 k3 a b c ~N orig_ty
375 [TcSigmaType]) -- Element types, k1 k2 k3 a b c
376
377 -- It's used for wired-in tycons, so we call checkWiredInTyCon
378 -- Precondition: never called with FunTyCon
379 -- Precondition: input type :: *
380 -- Postcondition: (T k1 k2 k3 a b c) is well-kinded
381
382 matchExpectedTyConApp tc orig_ty
383 = ASSERT(tc /= funTyCon) go orig_ty
384 where
385 go ty
386 | Just ty' <- tcView ty
387 = go ty'
388
389 go ty@(TyConApp tycon args)
390 | tc == tycon -- Common case
391 = return (mkTcNomReflCo ty, args)
392
393 go (TyVarTy tv)
394 | isMetaTyVar tv
395 = do { cts <- readMetaTyVar tv
396 ; case cts of
397 Indirect ty -> go ty
398 Flexi -> defer }
399
400 go _ = defer
401
402 -- If the common case does not occur, instantiate a template
403 -- T k1 .. kn t1 .. tm, and unify with the original type
404 -- Doing it this way ensures that the types we return are
405 -- kind-compatible with T. For example, suppose we have
406 -- matchExpectedTyConApp T (f Maybe)
407 -- where data T a = MkT a
408 -- Then we don't want to instantiate T's data constructors with
409 -- (a::*) ~ Maybe
410 -- because that'll make types that are utterly ill-kinded.
411 -- This happened in Trac #7368
412 defer
413 = do { (_, arg_tvs) <- newMetaTyVars (tyConTyVars tc)
414 ; traceTc "matchExpectedTyConApp" (ppr tc $$ ppr (tyConTyVars tc) $$ ppr arg_tvs)
415 ; let args = mkTyVarTys arg_tvs
416 tc_template = mkTyConApp tc args
417 ; co <- unifyType Nothing tc_template orig_ty
418 ; return (co, args) }
419
420 ----------------------
421 matchExpectedAppTy :: TcRhoType -- orig_ty
422 -> TcM (TcCoercion, -- m a ~N orig_ty
423 (TcSigmaType, TcSigmaType)) -- Returns m, a
424 -- If the incoming type is a mutable type variable of kind k, then
425 -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.
426
427 matchExpectedAppTy orig_ty
428 = go orig_ty
429 where
430 go ty
431 | Just ty' <- tcView ty = go ty'
432
433 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
434 = return (mkTcNomReflCo orig_ty, (fun_ty, arg_ty))
435
436 go (TyVarTy tv)
437 | isMetaTyVar tv
438 = do { cts <- readMetaTyVar tv
439 ; case cts of
440 Indirect ty -> go ty
441 Flexi -> defer }
442
443 go _ = defer
444
445 -- Defer splitting by generating an equality constraint
446 defer
447 = do { ty1 <- newFlexiTyVarTy kind1
448 ; ty2 <- newFlexiTyVarTy kind2
449 ; co <- unifyType Nothing (mkAppTy ty1 ty2) orig_ty
450 ; return (co, (ty1, ty2)) }
451
452 orig_kind = typeKind orig_ty
453 kind1 = mkFunTy liftedTypeKind orig_kind
454 kind2 = liftedTypeKind -- m :: * -> k
455 -- arg type :: *
456
457 {-
458 ************************************************************************
459 * *
460 Subsumption checking
461 * *
462 ************************************************************************
463
464 Note [Subsumption checking: tcSubType]
465 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
466 All the tcSubType calls have the form
467 tcSubType actual_ty expected_ty
468 which checks
469 actual_ty <= expected_ty
470
471 That is, that a value of type actual_ty is acceptable in
472 a place expecting a value of type expected_ty. I.e. that
473
474 actual ty is more polymorphic than expected_ty
475
476 It returns a coercion function
477 co_fn :: actual_ty ~ expected_ty
478 which takes an HsExpr of type actual_ty into one of type
479 expected_ty.
480
481 These functions do not actually check for subsumption. They check if
482 expected_ty is an appropriate annotation to use for something of type
483 actual_ty. This difference matters when thinking about visible type
484 application. For example,
485
486 forall a. a -> forall b. b -> b
487 DOES NOT SUBSUME
488 forall a b. a -> b -> b
489
490 because the type arguments appear in a different order. (Neither does
491 it work the other way around.) BUT, these types are appropriate annotations
492 for one another. Because the user directs annotations, it's OK if some
493 arguments shuffle around -- after all, it's what the user wants.
494 Bottom line: none of this changes with visible type application.
495
496 There are a number of wrinkles (below).
497
498 Notice that Wrinkle 1 and 2 both require eta-expansion, which technically
499 may increase termination. We just put up with this, in exchange for getting
500 more predictable type inference.
501
502 Wrinkle 1: Note [Deep skolemisation]
503 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
504 We want (forall a. Int -> a -> a) <= (Int -> forall a. a->a)
505 (see section 4.6 of "Practical type inference for higher rank types")
506 So we must deeply-skolemise the RHS before we instantiate the LHS.
507
508 That is why tc_sub_type starts with a call to tcSkolemise (which does the
509 deep skolemisation), and then calls the DS variant (which assumes
510 that expected_ty is deeply skolemised)
511
512 Wrinkle 2: Note [Co/contra-variance of subsumption checking]
513 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
514 Consider g :: (Int -> Int) -> Int
515 f1 :: (forall a. a -> a) -> Int
516 f1 = g
517
518 f2 :: (forall a. a -> a) -> Int
519 f2 x = g x
520 f2 will typecheck, and it would be odd/fragile if f1 did not.
521 But f1 will only typecheck if we have that
522 (Int->Int) -> Int <= (forall a. a->a) -> Int
523 And that is only true if we do the full co/contravariant thing
524 in the subsumption check. That happens in the FunTy case of
525 tcSubTypeDS_NC_O, and is the sole reason for the WpFun form of
526 HsWrapper.
527
528 Another powerful reason for doing this co/contra stuff is visible
529 in Trac #9569, involving instantiation of constraint variables,
530 and again involving eta-expansion.
531
532 Wrinkle 3: Note [Higher rank types]
533 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
534 Consider tc150:
535 f y = \ (x::forall a. a->a). blah
536 The following happens:
537 * We will infer the type of the RHS, ie with a res_ty = alpha.
538 * Then the lambda will split alpha := beta -> gamma.
539 * And then we'll check tcSubType IsSwapped beta (forall a. a->a)
540
541 So it's important that we unify beta := forall a. a->a, rather than
542 skolemising the type.
543 -}
544
545
546 -- | Call this variant when you are in a higher-rank situation and
547 -- you know the right-hand type is deeply skolemised.
548 tcSubTypeHR :: CtOrigin -- ^ of the actual type
549 -> Maybe (HsExpr GhcRn) -- ^ If present, it has type ty_actual
550 -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
551 tcSubTypeHR orig = tcSubTypeDS_NC_O orig GenSigCtxt
552
553 ------------------------
554 tcSubTypeET :: CtOrigin -> UserTypeCtxt
555 -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
556 -- If wrap = tc_sub_type_et t1 t2
557 -- => wrap :: t1 ~> t2
558 tcSubTypeET orig ctxt (Check ty_actual) ty_expected
559 = tc_sub_tc_type eq_orig orig ctxt ty_actual ty_expected
560 where
561 eq_orig = TypeEqOrigin { uo_actual = ty_expected
562 , uo_expected = ty_actual
563 , uo_thing = Nothing
564 , uo_visible = True }
565
566 tcSubTypeET _ _ (Infer inf_res) ty_expected
567 = ASSERT2( not (ir_inst inf_res), ppr inf_res $$ ppr ty_expected )
568 do { co <- fillInferResult ty_expected inf_res
569 ; return (mkWpCastN (mkTcSymCo co)) }
570
571 ------------------------
572 tcSubTypeO :: CtOrigin -- ^ of the actual type
573 -> UserTypeCtxt -- ^ of the expected type
574 -> TcSigmaType
575 -> ExpRhoType
576 -> TcM HsWrapper
577 tcSubTypeO orig ctxt ty_actual ty_expected
578 = addSubTypeCtxt ty_actual ty_expected $
579 do { traceTc "tcSubTypeDS_O" (vcat [ pprCtOrigin orig
580 , pprUserTypeCtxt ctxt
581 , ppr ty_actual
582 , ppr ty_expected ])
583 ; tcSubTypeDS_NC_O orig ctxt Nothing ty_actual ty_expected }
584
585 addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a
586 addSubTypeCtxt ty_actual ty_expected thing_inside
587 | isRhoTy ty_actual -- If there is no polymorphism involved, the
588 , isRhoExpTy ty_expected -- TypeEqOrigin stuff (added by the _NC functions)
589 = thing_inside -- gives enough context by itself
590 | otherwise
591 = addErrCtxtM mk_msg thing_inside
592 where
593 mk_msg tidy_env
594 = do { (tidy_env, ty_actual) <- zonkTidyTcType tidy_env ty_actual
595 -- might not be filled if we're debugging. ugh.
596 ; mb_ty_expected <- readExpType_maybe ty_expected
597 ; (tidy_env, ty_expected) <- case mb_ty_expected of
598 Just ty -> second mkCheckExpType <$>
599 zonkTidyTcType tidy_env ty
600 Nothing -> return (tidy_env, ty_expected)
601 ; ty_expected <- readExpType ty_expected
602 ; (tidy_env, ty_expected) <- zonkTidyTcType tidy_env ty_expected
603 ; let msg = vcat [ hang (text "When checking that:")
604 4 (ppr ty_actual)
605 , nest 2 (hang (text "is more polymorphic than:")
606 2 (ppr ty_expected)) ]
607 ; return (tidy_env, msg) }
608
609 ---------------
610 -- The "_NC" variants do not add a typechecker-error context;
611 -- the caller is assumed to do that
612
613 tcSubType_NC :: UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
614 -- Checks that actual <= expected
615 -- Returns HsWrapper :: actual ~ expected
616 tcSubType_NC ctxt ty_actual ty_expected
617 = do { traceTc "tcSubType_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected])
618 ; tc_sub_tc_type origin origin ctxt ty_actual ty_expected }
619 where
620 origin = TypeEqOrigin { uo_actual = ty_actual
621 , uo_expected = ty_expected
622 , uo_thing = Nothing
623 , uo_visible = True }
624
625 tcSubTypeDS :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
626 -- Just like tcSubType, but with the additional precondition that
627 -- ty_expected is deeply skolemised (hence "DS")
628 tcSubTypeDS orig ctxt ty_actual ty_expected
629 = addSubTypeCtxt ty_actual ty_expected $
630 do { traceTc "tcSubTypeDS_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected])
631 ; tcSubTypeDS_NC_O orig ctxt Nothing ty_actual ty_expected }
632
633 tcSubTypeDS_NC_O :: CtOrigin -- origin used for instantiation only
634 -> UserTypeCtxt
635 -> Maybe (HsExpr GhcRn)
636 -> TcSigmaType -> ExpRhoType -> TcM HsWrapper
637 -- Just like tcSubType, but with the additional precondition that
638 -- ty_expected is deeply skolemised
639 tcSubTypeDS_NC_O inst_orig ctxt m_thing ty_actual ty_expected
640 = case ty_expected of
641 Infer inf_res -> fillInferResult_Inst inst_orig ty_actual inf_res
642 Check ty -> tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty
643 where
644 eq_orig = TypeEqOrigin { uo_actual = ty_actual, uo_expected = ty
645 , uo_thing = ppr <$> m_thing
646 , uo_visible = True }
647
648 ---------------
649 tc_sub_tc_type :: CtOrigin -- used when calling uType
650 -> CtOrigin -- used when instantiating
651 -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
652 -- If wrap = tc_sub_type t1 t2
653 -- => wrap :: t1 ~> t2
654 tc_sub_tc_type eq_orig inst_orig ctxt ty_actual ty_expected
655 | definitely_poly ty_expected -- See Note [Don't skolemise unnecessarily]
656 , not (possibly_poly ty_actual)
657 = do { traceTc "tc_sub_tc_type (drop to equality)" $
658 vcat [ text "ty_actual =" <+> ppr ty_actual
659 , text "ty_expected =" <+> ppr ty_expected ]
660 ; mkWpCastN <$>
661 uType TypeLevel eq_orig ty_actual ty_expected }
662
663 | otherwise -- This is the general case
664 = do { traceTc "tc_sub_tc_type (general case)" $
665 vcat [ text "ty_actual =" <+> ppr ty_actual
666 , text "ty_expected =" <+> ppr ty_expected ]
667 ; (sk_wrap, inner_wrap) <- tcSkolemise ctxt ty_expected $
668 \ _ sk_rho ->
669 tc_sub_type_ds eq_orig inst_orig ctxt
670 ty_actual sk_rho
671 ; return (sk_wrap <.> inner_wrap) }
672 where
673 possibly_poly ty
674 | isForAllTy ty = True
675 | Just (_, res) <- splitFunTy_maybe ty = possibly_poly res
676 | otherwise = False
677 -- NB *not* tcSplitFunTy, because here we want
678 -- to decompose type-class arguments too
679
680 definitely_poly ty
681 | (tvs, theta, tau) <- tcSplitSigmaTy ty
682 , (tv:_) <- tvs
683 , null theta
684 , isInsolubleOccursCheck NomEq tv tau
685 = True
686 | otherwise
687 = False
688
689 {- Note [Don't skolemise unnecessarily]
690 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
691 Suppose we are trying to solve
692 (Char->Char) <= (forall a. a->a)
693 We could skolemise the 'forall a', and then complain
694 that (Char ~ a) is insoluble; but that's a pretty obscure
695 error. It's better to say that
696 (Char->Char) ~ (forall a. a->a)
697 fails.
698
699 So roughly:
700 * if the ty_expected has an outermost forall
701 (i.e. skolemisation is the next thing we'd do)
702 * and the ty_actual has no top-level polymorphism (but looking deeply)
703 then we can revert to simple equality. But we need to be careful.
704 These examples are all fine:
705
706 * (Char -> forall a. a->a) <= (forall a. Char -> a -> a)
707 Polymorphism is buried in ty_actual
708
709 * (Char->Char) <= (forall a. Char -> Char)
710 ty_expected isn't really polymorphic
711
712 * (Char->Char) <= (forall a. (a~Char) => a -> a)
713 ty_expected isn't really polymorphic
714
715 * (Char->Char) <= (forall a. F [a] Char -> Char)
716 where type instance F [x] t = t
717 ty_expected isn't really polymorphic
718
719 If we prematurely go to equality we'll reject a program we should
720 accept (e.g. Trac #13752). So the test (which is only to improve
721 error message) is very conservative:
722 * ty_actual is /definitely/ monomorphic
723 * ty_expected is /definitely/ polymorphic
724 -}
725
726 ---------------
727 tc_sub_type_ds :: CtOrigin -- used when calling uType
728 -> CtOrigin -- used when instantiating
729 -> UserTypeCtxt -> TcSigmaType -> TcRhoType -> TcM HsWrapper
730 -- If wrap = tc_sub_type_ds t1 t2
731 -- => wrap :: t1 ~> t2
732 -- Here is where the work actually happens!
733 -- Precondition: ty_expected is deeply skolemised
734 tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty_expected
735 = do { traceTc "tc_sub_type_ds" $
736 vcat [ text "ty_actual =" <+> ppr ty_actual
737 , text "ty_expected =" <+> ppr ty_expected ]
738 ; go ty_actual ty_expected }
739 where
740 go ty_a ty_e | Just ty_a' <- tcView ty_a = go ty_a' ty_e
741 | Just ty_e' <- tcView ty_e = go ty_a ty_e'
742
743 go (TyVarTy tv_a) ty_e
744 = do { lookup_res <- lookupTcTyVar tv_a
745 ; case lookup_res of
746 Filled ty_a' ->
747 do { traceTc "tcSubTypeDS_NC_O following filled act meta-tyvar:"
748 (ppr tv_a <+> text "-->" <+> ppr ty_a')
749 ; tc_sub_type_ds eq_orig inst_orig ctxt ty_a' ty_e }
750 Unfilled _ -> unify }
751
752 -- Historical note (Sept 16): there was a case here for
753 -- go ty_a (TyVarTy alpha)
754 -- which, in the impredicative case unified alpha := ty_a
755 -- where th_a is a polytype. Not only is this probably bogus (we
756 -- simply do not have decent story for imprdicative types), but it
757 -- caused Trac #12616 because (also bizarrely) 'deriving' code had
758 -- -XImpredicativeTypes on. I deleted the entire case.
759
760 go (FunTy act_arg act_res) (FunTy exp_arg exp_res)
761 | not (isPredTy act_arg)
762 , not (isPredTy exp_arg)
763 = -- See Note [Co/contra-variance of subsumption checking]
764 do { res_wrap <- tc_sub_type_ds eq_orig inst_orig ctxt act_res exp_res
765 ; arg_wrap <- tc_sub_tc_type eq_orig given_orig ctxt exp_arg act_arg
766 ; return (mkWpFun arg_wrap res_wrap exp_arg exp_res doc) }
767 -- arg_wrap :: exp_arg ~> act_arg
768 -- res_wrap :: act-res ~> exp_res
769 where
770 given_orig = GivenOrigin (SigSkol GenSigCtxt exp_arg [])
771 doc = text "When checking that" <+> quotes (ppr ty_actual) <+>
772 text "is more polymorphic than" <+> quotes (ppr ty_expected)
773
774 go ty_a ty_e
775 | let (tvs, theta, _) = tcSplitSigmaTy ty_a
776 , not (null tvs && null theta)
777 = do { (in_wrap, in_rho) <- topInstantiate inst_orig ty_a
778 ; body_wrap <- tc_sub_type_ds
779 (eq_orig { uo_actual = in_rho
780 , uo_expected = ty_expected })
781 inst_orig ctxt in_rho ty_e
782 ; return (body_wrap <.> in_wrap) }
783
784 | otherwise -- Revert to unification
785 = inst_and_unify
786 -- It's still possible that ty_actual has nested foralls. Instantiate
787 -- these, as there's no way unification will succeed with them in.
788 -- See typecheck/should_compile/T11305 for an example of when this
789 -- is important. The problem is that we're checking something like
790 -- a -> forall b. b -> b <= alpha beta gamma
791 -- where we end up with alpha := (->)
792
793 inst_and_unify = do { (wrap, rho_a) <- deeplyInstantiate inst_orig ty_actual
794
795 -- if we haven't recurred through an arrow, then
796 -- the eq_orig will list ty_actual. In this case,
797 -- we want to update the origin to reflect the
798 -- instantiation. If we *have* recurred through
799 -- an arrow, it's better not to update.
800 ; let eq_orig' = case eq_orig of
801 TypeEqOrigin { uo_actual = orig_ty_actual }
802 | orig_ty_actual `tcEqType` ty_actual
803 , not (isIdHsWrapper wrap)
804 -> eq_orig { uo_actual = rho_a }
805 _ -> eq_orig
806
807 ; cow <- uType TypeLevel eq_orig' rho_a ty_expected
808 ; return (mkWpCastN cow <.> wrap) }
809
810
811 -- use versions without synonyms expanded
812 unify = mkWpCastN <$> uType TypeLevel eq_orig ty_actual ty_expected
813
814 -----------------
815 -- needs both un-type-checked (for origins) and type-checked (for wrapping)
816 -- expressions
817 tcWrapResult :: HsExpr GhcRn -> HsExpr GhcTcId -> TcSigmaType -> ExpRhoType
818 -> TcM (HsExpr GhcTcId)
819 tcWrapResult rn_expr = tcWrapResultO (exprCtOrigin rn_expr) rn_expr
820
821 -- | Sometimes we don't have a @HsExpr Name@ to hand, and this is more
822 -- convenient.
823 tcWrapResultO :: CtOrigin -> HsExpr GhcRn -> HsExpr GhcTcId -> TcSigmaType -> ExpRhoType
824 -> TcM (HsExpr GhcTcId)
825 tcWrapResultO orig rn_expr expr actual_ty res_ty
826 = do { traceTc "tcWrapResult" (vcat [ text "Actual: " <+> ppr actual_ty
827 , text "Expected:" <+> ppr res_ty ])
828 ; cow <- tcSubTypeDS_NC_O orig GenSigCtxt
829 (Just rn_expr) actual_ty res_ty
830 ; return (mkHsWrap cow expr) }
831
832
833 {- **********************************************************************
834 %* *
835 ExpType functions: tcInfer, fillInferResult
836 %* *
837 %********************************************************************* -}
838
839 -- | Infer a type using a fresh ExpType
840 -- See also Note [ExpType] in TcMType
841 -- Does not attempt to instantiate the inferred type
842 tcInferNoInst :: (ExpSigmaType -> TcM a) -> TcM (a, TcSigmaType)
843 tcInferNoInst = tcInfer False
844
845 tcInferInst :: (ExpRhoType -> TcM a) -> TcM (a, TcRhoType)
846 tcInferInst = tcInfer True
847
848 tcInfer :: Bool -> (ExpSigmaType -> TcM a) -> TcM (a, TcSigmaType)
849 tcInfer instantiate tc_check
850 = do { res_ty <- newInferExpType instantiate
851 ; result <- tc_check res_ty
852 ; res_ty <- readExpType res_ty
853 ; return (result, res_ty) }
854
855 fillInferResult_Inst :: CtOrigin -> TcType -> InferResult -> TcM HsWrapper
856 -- If wrap = fillInferResult_Inst t1 t2
857 -- => wrap :: t1 ~> t2
858 -- See Note [Deep instantiation of InferResult]
859 fillInferResult_Inst orig ty inf_res@(IR { ir_inst = instantiate_me })
860 | instantiate_me
861 = do { (wrap, rho) <- deeplyInstantiate orig ty
862 ; co <- fillInferResult rho inf_res
863 ; return (mkWpCastN co <.> wrap) }
864
865 | otherwise
866 = do { co <- fillInferResult ty inf_res
867 ; return (mkWpCastN co) }
868
869 fillInferResult :: TcType -> InferResult -> TcM TcCoercionN
870 -- If wrap = fillInferResult t1 t2
871 -- => wrap :: t1 ~> t2
872 fillInferResult orig_ty (IR { ir_uniq = u, ir_lvl = res_lvl
873 , ir_ref = ref })
874 = do { (ty_co, ty_to_fill_with) <- promoteTcType res_lvl orig_ty
875
876 ; traceTc "Filling ExpType" $
877 ppr u <+> text ":=" <+> ppr ty_to_fill_with
878
879 ; when debugIsOn (check_hole ty_to_fill_with)
880
881 ; writeTcRef ref (Just ty_to_fill_with)
882
883 ; return ty_co }
884 where
885 check_hole ty -- Debug check only
886 = do { let ty_lvl = tcTypeLevel ty
887 ; MASSERT2( not (ty_lvl `strictlyDeeperThan` res_lvl),
888 ppr u $$ ppr res_lvl $$ ppr ty_lvl $$
889 ppr ty <+> ppr (typeKind ty) $$ ppr orig_ty )
890 ; cts <- readTcRef ref
891 ; case cts of
892 Just already_there -> pprPanic "writeExpType"
893 (vcat [ ppr u
894 , ppr ty
895 , ppr already_there ])
896 Nothing -> return () }
897
898 {- Note [Deep instantiation of InferResult]
899 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
900 In some cases we want to deeply instantiate before filling in
901 an InferResult, and in some cases not. That's why InferReult
902 has the ir_inst flag.
903
904 * ir_inst = True: deeply instantiate
905
906 Consider
907 f x = (*)
908 We want to instantiate the type of (*) before returning, else we
909 will infer the type
910 f :: forall {a}. a -> forall b. Num b => b -> b -> b
911 This is surely confusing for users.
912
913 And worse, the monomorphism restriction won't properly. The MR is
914 dealt with in simplifyInfer, and simplifyInfer has no way of
915 instantiating. This could perhaps be worked around, but it may be
916 hard to know even when instantiation should happen.
917
918 Another reason. Consider
919 f :: (?x :: Int) => a -> a
920 g y = let ?x = 3::Int in f
921 Here want to instantiate f's type so that the ?x::Int constraint
922 gets discharged by the enclosing implicit-parameter binding.
923
924 * ir_inst = False: do not instantiate
925
926 Consider this (which uses visible type application):
927
928 (let { f :: forall a. a -> a; f x = x } in f) @Int
929
930 We'll call TcExpr.tcInferFun to infer the type of the (let .. in f)
931 And we don't want to instantite the type of 'f' when we reach it,
932 else the outer visible type application won't work
933 -}
934
935 {- *********************************************************************
936 * *
937 Promoting types
938 * *
939 ********************************************************************* -}
940
941 promoteTcType :: TcLevel -> TcType -> TcM (TcCoercion, TcType)
942 -- See Note [Promoting a type]
943 -- promoteTcType level ty = (co, ty')
944 -- * Returns ty' whose max level is just 'level'
945 -- and whose kind is ~# to the kind of 'ty'
946 -- and whose kind has form TYPE rr
947 -- * and co :: ty ~ ty'
948 -- * and emits constraints to justify the coercion
949 promoteTcType dest_lvl ty
950 = do { cur_lvl <- getTcLevel
951 ; if (cur_lvl `sameDepthAs` dest_lvl)
952 then dont_promote_it
953 else promote_it }
954 where
955 promote_it :: TcM (TcCoercion, TcType)
956 promote_it -- Emit a constraint (alpha :: TYPE rr) ~ ty
957 -- where alpha and rr are fresh and from level dest_lvl
958 = do { rr <- newMetaTyVarTyAtLevel dest_lvl runtimeRepTy
959 ; prom_ty <- newMetaTyVarTyAtLevel dest_lvl (tYPE rr)
960 ; let eq_orig = TypeEqOrigin { uo_actual = ty
961 , uo_expected = prom_ty
962 , uo_thing = Nothing
963 , uo_visible = False }
964
965 ; co <- emitWantedEq eq_orig TypeLevel Nominal ty prom_ty
966 ; return (co, prom_ty) }
967
968 dont_promote_it :: TcM (TcCoercion, TcType)
969 dont_promote_it -- Check that ty :: TYPE rr, for some (fresh) rr
970 = do { res_kind <- newOpenTypeKind
971 ; let ty_kind = typeKind ty
972 kind_orig = TypeEqOrigin { uo_actual = ty_kind
973 , uo_expected = res_kind
974 , uo_thing = Nothing
975 , uo_visible = False }
976 ; ki_co <- uType KindLevel kind_orig (typeKind ty) res_kind
977 ; let co = mkTcNomReflCo ty `mkTcCoherenceRightCo` ki_co
978 ; return (co, ty `mkCastTy` ki_co) }
979
980 {- Note [Promoting a type]
981 ~~~~~~~~~~~~~~~~~~~~~~~~~~
982 Consider (Trac #12427)
983
984 data T where
985 MkT :: (Int -> Int) -> a -> T
986
987 h y = case y of MkT v w -> v
988
989 We'll infer the RHS type with an expected type ExpType of
990 (IR { ir_lvl = l, ir_ref = ref, ... )
991 where 'l' is the TcLevel of the RHS of 'h'. Then the MkT pattern
992 match will increase the level, so we'll end up in tcSubType, trying to
993 unify the type of v,
994 v :: Int -> Int
995 with the expected type. But this attempt takes place at level (l+1),
996 rightly so, since v's type could have mentioned existential variables,
997 (like w's does) and we want to catch that.
998
999 So we
1000 - create a new meta-var alpha[l+1]
1001 - fill in the InferRes ref cell 'ref' with alpha
1002 - emit an equality constraint, thus
1003 [W] alpha[l+1] ~ (Int -> Int)
1004
1005 That constraint will float outwards, as it should, unless v's
1006 type mentions a skolem-captured variable.
1007
1008 This approach fails if v has a higher rank type; see
1009 Note [Promotion and higher rank types]
1010
1011
1012 Note [Promotion and higher rank types]
1013 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1014 If v had a higher-rank type, say v :: (forall a. a->a) -> Int,
1015 then we'd emit an equality
1016 [W] alpha[l+1] ~ ((forall a. a->a) -> Int)
1017 which will sadly fail because we can't unify a unification variable
1018 with a polytype. But there is nothing really wrong with the program
1019 here.
1020
1021 We could just about solve this by "promote the type" of v, to expose
1022 its polymorphic "shape" while still leaving constraints that will
1023 prevent existential escape. But we must be careful! Exposing
1024 the "shape" of the type is precisely what we must NOT do under
1025 a GADT pattern match! So in this case we might promote the type
1026 to
1027 (forall a. a->a) -> alpha[l+1]
1028 and emit the constraint
1029 [W] alpha[l+1] ~ Int
1030 Now the poromoted type can fill the ref cell, while the emitted
1031 equality can float or not, according to the usual rules.
1032
1033 But that's not quite right! We are exposing the arrow! We could
1034 deal with that too:
1035 (forall a. mu[l+1] a a) -> alpha[l+1]
1036 with constraints
1037 [W] alpha[l+1] ~ Int
1038 [W] mu[l+1] ~ (->)
1039 Here we abstract over the '->' inside the forall, in case that
1040 is subject to an equality constraint from a GADT match.
1041
1042 Note that we kept the outer (->) because that's part of
1043 the polymorphic "shape". And becauuse of impredicativity,
1044 GADT matches can't give equalities that affect polymorphic
1045 shape.
1046
1047 This reasoning just seems too complicated, so I decided not
1048 to do it. These higher-rank notes are just here to record
1049 the thinking.
1050 -}
1051
1052 {- *********************************************************************
1053 * *
1054 Generalisation
1055 * *
1056 ********************************************************************* -}
1057
1058 -- | Take an "expected type" and strip off quantifiers to expose the
1059 -- type underneath, binding the new skolems for the @thing_inside@.
1060 -- The returned 'HsWrapper' has type @specific_ty -> expected_ty@.
1061 tcSkolemise :: UserTypeCtxt -> TcSigmaType
1062 -> ([TcTyVar] -> TcType -> TcM result)
1063 -- ^ These are only ever used for scoped type variables.
1064 -> TcM (HsWrapper, result)
1065 -- ^ The expression has type: spec_ty -> expected_ty
1066
1067 tcSkolemise ctxt expected_ty thing_inside
1068 -- We expect expected_ty to be a forall-type
1069 -- If not, the call is a no-op
1070 = do { traceTc "tcSkolemise" Outputable.empty
1071 ; (wrap, tv_prs, given, rho') <- deeplySkolemise expected_ty
1072
1073 ; lvl <- getTcLevel
1074 ; when debugIsOn $
1075 traceTc "tcSkolemise" $ vcat [
1076 ppr lvl,
1077 text "expected_ty" <+> ppr expected_ty,
1078 text "inst tyvars" <+> ppr tv_prs,
1079 text "given" <+> ppr given,
1080 text "inst type" <+> ppr rho' ]
1081
1082 -- Generally we must check that the "forall_tvs" havn't been constrained
1083 -- The interesting bit here is that we must include the free variables
1084 -- of the expected_ty. Here's an example:
1085 -- runST (newVar True)
1086 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
1087 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
1088 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
1089 -- So now s' isn't unconstrained because it's linked to a.
1090 --
1091 -- However [Oct 10] now that the untouchables are a range of
1092 -- TcTyVars, all this is handled automatically with no need for
1093 -- extra faffing around
1094
1095 ; let tvs' = map snd tv_prs
1096 skol_info = SigSkol ctxt expected_ty tv_prs
1097
1098 ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
1099 thing_inside tvs' rho'
1100
1101 ; return (wrap <.> mkWpLet ev_binds, result) }
1102 -- The ev_binds returned by checkConstraints is very
1103 -- often empty, in which case mkWpLet is a no-op
1104
1105 -- | Variant of 'tcSkolemise' that takes an ExpType
1106 tcSkolemiseET :: UserTypeCtxt -> ExpSigmaType
1107 -> (ExpRhoType -> TcM result)
1108 -> TcM (HsWrapper, result)
1109 tcSkolemiseET _ et@(Infer {}) thing_inside
1110 = (idHsWrapper, ) <$> thing_inside et
1111 tcSkolemiseET ctxt (Check ty) thing_inside
1112 = tcSkolemise ctxt ty $ \_ -> thing_inside . mkCheckExpType
1113
1114 checkConstraints :: SkolemInfo
1115 -> [TcTyVar] -- Skolems
1116 -> [EvVar] -- Given
1117 -> TcM result
1118 -> TcM (TcEvBinds, result)
1119
1120 checkConstraints skol_info skol_tvs given thing_inside
1121 = do { (implics, ev_binds, result)
1122 <- buildImplication skol_info skol_tvs given thing_inside
1123 ; emitImplications implics
1124 ; return (ev_binds, result) }
1125
1126 buildImplication :: SkolemInfo
1127 -> [TcTyVar] -- Skolems
1128 -> [EvVar] -- Given
1129 -> TcM result
1130 -> TcM (Bag Implication, TcEvBinds, result)
1131 buildImplication skol_info skol_tvs given thing_inside
1132 = do { implication_needed <- implicationNeeded skol_tvs given
1133
1134 ; if implication_needed
1135 then do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints thing_inside
1136 ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_tvs given wanted
1137 ; return (implics, ev_binds, result) }
1138
1139 else -- Fast path. We check every function argument with
1140 -- tcPolyExpr, which uses tcSkolemise and hence checkConstraints.
1141 -- So this fast path is well-exercised
1142 do { res <- thing_inside
1143 ; return (emptyBag, emptyTcEvBinds, res) } }
1144
1145 implicationNeeded :: [TcTyVar] -> [EvVar] -> TcM Bool
1146 -- With the solver producing unlifted equalities, we need
1147 -- to have an EvBindsVar for them when they might be deferred to
1148 -- runtime. Otherwise, they end up as top-level unlifted bindings,
1149 -- which are verboten. See also Note [Deferred errors for coercion holes]
1150 -- in TcErrors. cf Trac #14149 for an example of what goes wrong.
1151 implicationNeeded skol_tvs given
1152 | null skol_tvs
1153 , null given
1154 = -- Empty skolems and givens
1155 do { tc_lvl <- getTcLevel
1156 ; if not (isTopTcLevel tc_lvl) -- No implication needed if we are
1157 then return False -- already inside an implication
1158 else
1159 do { dflags <- getDynFlags -- If any deferral can happen,
1160 -- we must build an implication
1161 ; return (gopt Opt_DeferTypeErrors dflags ||
1162 gopt Opt_DeferTypedHoles dflags ||
1163 gopt Opt_DeferOutOfScopeVariables dflags) } }
1164
1165 | otherwise -- Non-empty skolems or givens
1166 = return True -- Definitely need an implication
1167
1168 buildImplicationFor :: TcLevel -> SkolemInfo -> [TcTyVar]
1169 -> [EvVar] -> WantedConstraints
1170 -> TcM (Bag Implication, TcEvBinds)
1171 buildImplicationFor tclvl skol_info skol_tvs given wanted
1172 | isEmptyWC wanted && null given
1173 -- Optimisation : if there are no wanteds, and no givens
1174 -- don't generate an implication at all.
1175 -- Reason for the (null given): we don't want to lose
1176 -- the "inaccessible alternative" error check
1177 = return (emptyBag, emptyTcEvBinds)
1178
1179 | otherwise
1180 = ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
1181 do { ev_binds_var <- newTcEvBinds
1182 ; env <- getLclEnv
1183 ; let implic = Implic { ic_tclvl = tclvl
1184 , ic_skols = skol_tvs
1185 , ic_no_eqs = False
1186 , ic_given = given
1187 , ic_wanted = wanted
1188 , ic_status = IC_Unsolved
1189 , ic_binds = ev_binds_var
1190 , ic_env = env
1191 , ic_needed = emptyVarSet
1192 , ic_info = skol_info }
1193
1194 ; return (unitBag implic, TcEvBinds ev_binds_var) }
1195
1196 {-
1197 ************************************************************************
1198 * *
1199 Boxy unification
1200 * *
1201 ************************************************************************
1202
1203 The exported functions are all defined as versions of some
1204 non-exported generic functions.
1205 -}
1206
1207 unifyType :: Maybe (HsExpr GhcRn) -- ^ If present, has type 'ty1'
1208 -> TcTauType -> TcTauType -> TcM TcCoercionN
1209 -- Actual and expected types
1210 -- Returns a coercion : ty1 ~ ty2
1211 unifyType thing ty1 ty2 = traceTc "utype" (ppr ty1 $$ ppr ty2 $$ ppr thing) >>
1212 uType TypeLevel origin ty1 ty2
1213 where
1214 origin = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2
1215 , uo_thing = ppr <$> thing
1216 , uo_visible = True } -- always called from a visible context
1217
1218 unifyKind :: Maybe (HsType GhcRn) -> TcKind -> TcKind -> TcM CoercionN
1219 unifyKind thing ty1 ty2 = traceTc "ukind" (ppr ty1 $$ ppr ty2 $$ ppr thing) >>
1220 uType KindLevel origin ty1 ty2
1221 where origin = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2
1222 , uo_thing = ppr <$> thing
1223 , uo_visible = True } -- also always from a visible context
1224
1225 ---------------
1226 unifyPred :: PredType -> PredType -> TcM TcCoercionN
1227 -- Actual and expected types
1228 unifyPred = unifyType Nothing
1229
1230 ---------------
1231 unifyTheta :: TcThetaType -> TcThetaType -> TcM [TcCoercionN]
1232 -- Actual and expected types
1233 unifyTheta theta1 theta2
1234 = do { checkTc (equalLength theta1 theta2)
1235 (vcat [text "Contexts differ in length",
1236 nest 2 $ parens $ text "Use RelaxedPolyRec to allow this"])
1237 ; zipWithM unifyPred theta1 theta2 }
1238
1239 {-
1240 %************************************************************************
1241 %* *
1242 uType and friends
1243 %* *
1244 %************************************************************************
1245
1246 uType is the heart of the unifier.
1247 -}
1248
1249 uType, uType_defer
1250 :: TypeOrKind
1251 -> CtOrigin
1252 -> TcType -- ty1 is the *actual* type
1253 -> TcType -- ty2 is the *expected* type
1254 -> TcM Coercion
1255
1256 --------------
1257 -- It is always safe to defer unification to the main constraint solver
1258 -- See Note [Deferred unification]
1259 uType_defer t_or_k origin ty1 ty2
1260 = do { co <- emitWantedEq origin t_or_k Nominal ty1 ty2
1261
1262 -- Error trace only
1263 -- NB. do *not* call mkErrInfo unless tracing is on,
1264 -- because it is hugely expensive (#5631)
1265 ; whenDOptM Opt_D_dump_tc_trace $ do
1266 { ctxt <- getErrCtxt
1267 ; doc <- mkErrInfo emptyTidyEnv ctxt
1268 ; traceTc "utype_defer" (vcat [ debugPprType ty1
1269 , debugPprType ty2
1270 , pprCtOrigin origin
1271 , doc])
1272 ; traceTc "utype_defer2" (ppr co)
1273 }
1274 ; return co }
1275
1276 --------------
1277 uType t_or_k origin orig_ty1 orig_ty2
1278 = do { tclvl <- getTcLevel
1279 ; traceTc "u_tys" $ vcat
1280 [ text "tclvl" <+> ppr tclvl
1281 , sep [ ppr orig_ty1, text "~", ppr orig_ty2]
1282 , pprCtOrigin origin]
1283 ; co <- go orig_ty1 orig_ty2
1284 ; if isReflCo co
1285 then traceTc "u_tys yields no coercion" Outputable.empty
1286 else traceTc "u_tys yields coercion:" (ppr co)
1287 ; return co }
1288 where
1289 go :: TcType -> TcType -> TcM Coercion
1290 -- The arguments to 'go' are always semantically identical
1291 -- to orig_ty{1,2} except for looking through type synonyms
1292
1293 -- Variables; go for uVar
1294 -- Note that we pass in *original* (before synonym expansion),
1295 -- so that type variables tend to get filled in with
1296 -- the most informative version of the type
1297 go (TyVarTy tv1) ty2
1298 = do { lookup_res <- lookupTcTyVar tv1
1299 ; case lookup_res of
1300 Filled ty1 -> do { traceTc "found filled tyvar" (ppr tv1 <+> text ":->" <+> ppr ty1)
1301 ; go ty1 ty2 }
1302 Unfilled _ -> uUnfilledVar origin t_or_k NotSwapped tv1 ty2 }
1303 go ty1 (TyVarTy tv2)
1304 = do { lookup_res <- lookupTcTyVar tv2
1305 ; case lookup_res of
1306 Filled ty2 -> do { traceTc "found filled tyvar" (ppr tv2 <+> text ":->" <+> ppr ty2)
1307 ; go ty1 ty2 }
1308 Unfilled _ -> uUnfilledVar origin t_or_k IsSwapped tv2 ty1 }
1309
1310 -- See Note [Expanding synonyms during unification]
1311 go ty1@(TyConApp tc1 []) (TyConApp tc2 [])
1312 | tc1 == tc2
1313 = return $ mkReflCo Nominal ty1
1314
1315 -- See Note [Expanding synonyms during unification]
1316 --
1317 -- Also NB that we recurse to 'go' so that we don't push a
1318 -- new item on the origin stack. As a result if we have
1319 -- type Foo = Int
1320 -- and we try to unify Foo ~ Bool
1321 -- we'll end up saying "can't match Foo with Bool"
1322 -- rather than "can't match "Int with Bool". See Trac #4535.
1323 go ty1 ty2
1324 | Just ty1' <- tcView ty1 = go ty1' ty2
1325 | Just ty2' <- tcView ty2 = go ty1 ty2'
1326
1327 go (CastTy t1 co1) t2
1328 = do { co_tys <- go t1 t2
1329 ; return (mkCoherenceLeftCo co_tys co1) }
1330
1331 go t1 (CastTy t2 co2)
1332 = do { co_tys <- go t1 t2
1333 ; return (mkCoherenceRightCo co_tys co2) }
1334
1335 -- Functions (or predicate functions) just check the two parts
1336 go (FunTy fun1 arg1) (FunTy fun2 arg2)
1337 = do { co_l <- uType t_or_k origin fun1 fun2
1338 ; co_r <- uType t_or_k origin arg1 arg2
1339 ; return $ mkFunCo Nominal co_l co_r }
1340
1341 -- Always defer if a type synonym family (type function)
1342 -- is involved. (Data families behave rigidly.)
1343 go ty1@(TyConApp tc1 _) ty2
1344 | isTypeFamilyTyCon tc1 = defer ty1 ty2
1345 go ty1 ty2@(TyConApp tc2 _)
1346 | isTypeFamilyTyCon tc2 = defer ty1 ty2
1347
1348 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
1349 -- See Note [Mismatched type lists and application decomposition]
1350 | tc1 == tc2, equalLength tys1 tys2
1351 = ASSERT2( isGenerativeTyCon tc1 Nominal, ppr tc1 )
1352 do { cos <- zipWith3M (uType t_or_k) origins' tys1 tys2
1353 ; return $ mkTyConAppCo Nominal tc1 cos }
1354 where
1355 origins' = map (\is_vis -> if is_vis then origin else toInvisibleOrigin origin)
1356 (tcTyConVisibilities tc1)
1357
1358 go (LitTy m) ty@(LitTy n)
1359 | m == n
1360 = return $ mkNomReflCo ty
1361
1362 -- See Note [Care with type applications]
1363 -- Do not decompose FunTy against App;
1364 -- it's often a type error, so leave it for the constraint solver
1365 go (AppTy s1 t1) (AppTy s2 t2)
1366 = go_app (isNextArgVisible s1) s1 t1 s2 t2
1367
1368 go (AppTy s1 t1) (TyConApp tc2 ts2)
1369 | Just (ts2', t2') <- snocView ts2
1370 = ASSERT( mightBeUnsaturatedTyCon tc2 )
1371 go_app (isNextTyConArgVisible tc2 ts2') s1 t1 (TyConApp tc2 ts2') t2'
1372
1373 go (TyConApp tc1 ts1) (AppTy s2 t2)
1374 | Just (ts1', t1') <- snocView ts1
1375 = ASSERT( mightBeUnsaturatedTyCon tc1 )
1376 go_app (isNextTyConArgVisible tc1 ts1') (TyConApp tc1 ts1') t1' s2 t2
1377
1378 go (CoercionTy co1) (CoercionTy co2)
1379 = do { let ty1 = coercionType co1
1380 ty2 = coercionType co2
1381 ; kco <- uType KindLevel
1382 (KindEqOrigin orig_ty1 (Just orig_ty2) origin
1383 (Just t_or_k))
1384 ty1 ty2
1385 ; return $ mkProofIrrelCo Nominal kco co1 co2 }
1386
1387 -- Anything else fails
1388 -- E.g. unifying for-all types, which is relative unusual
1389 go ty1 ty2 = defer ty1 ty2
1390
1391 ------------------
1392 defer ty1 ty2 -- See Note [Check for equality before deferring]
1393 | ty1 `tcEqType` ty2 = return (mkNomReflCo ty1)
1394 | otherwise = uType_defer t_or_k origin ty1 ty2
1395
1396 ------------------
1397 go_app vis s1 t1 s2 t2
1398 = do { co_s <- uType t_or_k origin s1 s2
1399 ; let arg_origin
1400 | vis = origin
1401 | otherwise = toInvisibleOrigin origin
1402 ; co_t <- uType t_or_k arg_origin t1 t2
1403 ; return $ mkAppCo co_s co_t }
1404
1405 {- Note [Check for equality before deferring]
1406 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1407 Particularly in ambiguity checks we can get equalities like (ty ~ ty).
1408 If ty involves a type function we may defer, which isn't very sensible.
1409 An egregious example of this was in test T9872a, which has a type signature
1410 Proxy :: Proxy (Solutions Cubes)
1411 Doing the ambiguity check on this signature generates the equality
1412 Solutions Cubes ~ Solutions Cubes
1413 and currently the constraint solver normalises both sides at vast cost.
1414 This little short-cut in 'defer' helps quite a bit.
1415
1416 Note [Care with type applications]
1417 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1418 Note: type applications need a bit of care!
1419 They can match FunTy and TyConApp, so use splitAppTy_maybe
1420 NB: we've already dealt with type variables and Notes,
1421 so if one type is an App the other one jolly well better be too
1422
1423 Note [Mismatched type lists and application decomposition]
1424 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1425 When we find two TyConApps, you might think that the argument lists
1426 are guaranteed equal length. But they aren't. Consider matching
1427 w (T x) ~ Foo (T x y)
1428 We do match (w ~ Foo) first, but in some circumstances we simply create
1429 a deferred constraint; and then go ahead and match (T x ~ T x y).
1430 This came up in Trac #3950.
1431
1432 So either
1433 (a) either we must check for identical argument kinds
1434 when decomposing applications,
1435
1436 (b) or we must be prepared for ill-kinded unification sub-problems
1437
1438 Currently we adopt (b) since it seems more robust -- no need to maintain
1439 a global invariant.
1440
1441 Note [Expanding synonyms during unification]
1442 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1443 We expand synonyms during unification, but:
1444 * We expand *after* the variable case so that we tend to unify
1445 variables with un-expanded type synonym. This just makes it
1446 more likely that the inferred types will mention type synonyms
1447 understandable to the user
1448
1449 * We expand *before* the TyConApp case. For example, if we have
1450 type Phantom a = Int
1451 and are unifying
1452 Phantom Int ~ Phantom Char
1453 it is *wrong* to unify Int and Char.
1454
1455 * The problem case immediately above can happen only with arguments
1456 to the tycon. So we check for nullary tycons *before* expanding.
1457 This is particularly helpful when checking (* ~ *), because * is
1458 now a type synonym.
1459
1460 Note [Deferred Unification]
1461 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1462 We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
1463 and yet its consistency is undetermined. Previously, there was no way to still
1464 make it consistent. So a mismatch error was issued.
1465
1466 Now these unifications are deferred until constraint simplification, where type
1467 family instances and given equations may (or may not) establish the consistency.
1468 Deferred unifications are of the form
1469 F ... ~ ...
1470 or x ~ ...
1471 where F is a type function and x is a type variable.
1472 E.g.
1473 id :: x ~ y => x -> y
1474 id e = e
1475
1476 involves the unification x = y. It is deferred until we bring into account the
1477 context x ~ y to establish that it holds.
1478
1479 If available, we defer original types (rather than those where closed type
1480 synonyms have already been expanded via tcCoreView). This is, as usual, to
1481 improve error messages.
1482
1483
1484 ************************************************************************
1485 * *
1486 uVar and friends
1487 * *
1488 ************************************************************************
1489
1490 @uVar@ is called when at least one of the types being unified is a
1491 variable. It does {\em not} assume that the variable is a fixed point
1492 of the substitution; rather, notice that @uVar@ (defined below) nips
1493 back into @uTys@ if it turns out that the variable is already bound.
1494 -}
1495
1496 ----------
1497 uUnfilledVar :: CtOrigin
1498 -> TypeOrKind
1499 -> SwapFlag
1500 -> TcTyVar -- Tyvar 1
1501 -> TcTauType -- Type 2
1502 -> TcM Coercion
1503 -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
1504 -- It might be a skolem, or untouchable, or meta
1505
1506 uUnfilledVar origin t_or_k swapped tv1 ty2
1507 = do { ty2 <- zonkTcType ty2
1508 -- Zonk to expose things to the
1509 -- occurs check, and so that if ty2
1510 -- looks like a type variable then it
1511 -- /is/ a type variable
1512 ; uUnfilledVar1 origin t_or_k swapped tv1 ty2 }
1513
1514 ----------
1515 uUnfilledVar1 :: CtOrigin
1516 -> TypeOrKind
1517 -> SwapFlag
1518 -> TcTyVar -- Tyvar 1
1519 -> TcTauType -- Type 2, zonked
1520 -> TcM Coercion
1521 uUnfilledVar1 origin t_or_k swapped tv1 ty2
1522 | Just tv2 <- tcGetTyVar_maybe ty2
1523 = go tv2
1524
1525 | otherwise
1526 = uUnfilledVar2 origin t_or_k swapped tv1 ty2
1527
1528 where
1529 -- 'go' handles the case where both are
1530 -- tyvars so we might want to swap
1531 go tv2 | tv1 == tv2 -- Same type variable => no-op
1532 = return (mkNomReflCo (mkTyVarTy tv1))
1533
1534 | swapOverTyVars tv1 tv2 -- Distinct type variables
1535 = uUnfilledVar2 origin t_or_k (flipSwap swapped)
1536 tv2 (mkTyVarTy tv1)
1537
1538 | otherwise
1539 = uUnfilledVar2 origin t_or_k swapped tv1 ty2
1540
1541 ----------
1542 uUnfilledVar2 :: CtOrigin
1543 -> TypeOrKind
1544 -> SwapFlag
1545 -> TcTyVar -- Tyvar 1
1546 -> TcTauType -- Type 2, zonked
1547 -> TcM Coercion
1548 uUnfilledVar2 origin t_or_k swapped tv1 ty2
1549 = do { dflags <- getDynFlags
1550 ; cur_lvl <- getTcLevel
1551 ; go dflags cur_lvl }
1552 where
1553 go dflags cur_lvl
1554 | canSolveByUnification cur_lvl tv1 ty2
1555 , Just ty2' <- metaTyVarUpdateOK dflags tv1 ty2
1556 = do { co_k <- uType KindLevel kind_origin (typeKind ty2') (tyVarKind tv1)
1557 ; if isTcReflCo co_k -- only proceed if the kinds matched.
1558
1559 then do { writeMetaTyVar tv1 ty2'
1560 ; return (mkTcNomReflCo ty2') }
1561 else defer } -- this cannot be solved now.
1562 -- See Note [Equalities with incompatible kinds]
1563 -- in TcCanonical
1564
1565 | otherwise
1566 = defer
1567 -- Occurs check or an untouchable: just defer
1568 -- NB: occurs check isn't necessarily fatal:
1569 -- eg tv1 occured in type family parameter
1570
1571 ty1 = mkTyVarTy tv1
1572 kind_origin = KindEqOrigin ty1 (Just ty2) origin (Just t_or_k)
1573
1574 defer = unSwap swapped (uType_defer t_or_k origin) ty1 ty2
1575
1576 swapOverTyVars :: TcTyVar -> TcTyVar -> Bool
1577 swapOverTyVars tv1 tv2
1578 | isFmvTyVar tv1 = False -- See Note [Fmv Orientation Invariant]
1579 | isFmvTyVar tv2 = True
1580
1581 | Just lvl1 <- metaTyVarTcLevel_maybe tv1
1582 -- If tv1 is touchable, swap only if tv2 is also
1583 -- touchable and it's strictly better to update the latter
1584 -- But see Note [Avoid unnecessary swaps]
1585 = case metaTyVarTcLevel_maybe tv2 of
1586 Nothing -> False
1587 Just lvl2 | lvl2 `strictlyDeeperThan` lvl1 -> True
1588 | lvl1 `strictlyDeeperThan` lvl2 -> False
1589 | otherwise -> nicer_to_update tv2
1590
1591 -- So tv1 is not a meta tyvar
1592 -- If only one is a meta tyvar, put it on the left
1593 -- This is not because it'll be solved; but because
1594 -- the floating step looks for meta tyvars on the left
1595 | isMetaTyVar tv2 = True
1596
1597 -- So neither is a meta tyvar (including FlatMetaTv)
1598
1599 -- If only one is a flatten skolem, put it on the left
1600 -- See Note [Eliminate flat-skols]
1601 | not (isFlattenTyVar tv1), isFlattenTyVar tv2 = True
1602
1603 | otherwise = False
1604
1605 where
1606 nicer_to_update tv2
1607 = (isSigTyVar tv1 && not (isSigTyVar tv2))
1608 || (isSystemName (Var.varName tv2) && not (isSystemName (Var.varName tv1)))
1609
1610 -- @trySpontaneousSolve wi@ solves equalities where one side is a
1611 -- touchable unification variable.
1612 -- Returns True <=> spontaneous solve happened
1613 canSolveByUnification :: TcLevel -> TcTyVar -> TcType -> Bool
1614 canSolveByUnification tclvl tv xi
1615 | isTouchableMetaTyVar tclvl tv
1616 = case metaTyVarInfo tv of
1617 SigTv -> is_tyvar xi
1618 _ -> True
1619
1620 | otherwise -- Untouchable
1621 = False
1622 where
1623 is_tyvar xi
1624 = case tcGetTyVar_maybe xi of
1625 Nothing -> False
1626 Just tv -> case tcTyVarDetails tv of
1627 MetaTv { mtv_info = info }
1628 -> case info of
1629 SigTv -> True
1630 _ -> False
1631 SkolemTv {} -> True
1632 RuntimeUnk -> True
1633
1634 {- Note [Fmv Orientation Invariant]
1635 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1636 * We always orient a constraint
1637 fmv ~ alpha
1638 with fmv on the left, even if alpha is
1639 a touchable unification variable
1640
1641 Reason: doing it the other way round would unify alpha:=fmv, but that
1642 really doesn't add any info to alpha. But a later constraint alpha ~
1643 Int might unlock everything. Comment:9 of #12526 gives a detailed
1644 example.
1645
1646 WARNING: I've gone to and fro on this one several times.
1647 I'm now pretty sure that unifying alpha:=fmv is a bad idea!
1648 So orienting with fmvs on the left is a good thing.
1649
1650 This example comes from IndTypesPerfMerge. (Others include
1651 T10226, T10009.)
1652 From the ambiguity check for
1653 f :: (F a ~ a) => a
1654 we get:
1655 [G] F a ~ a
1656 [WD] F alpha ~ alpha, alpha ~ a
1657
1658 From Givens we get
1659 [G] F a ~ fsk, fsk ~ a
1660
1661 Now if we flatten we get
1662 [WD] alpha ~ fmv, F alpha ~ fmv, alpha ~ a
1663
1664 Now, if we unified alpha := fmv, we'd get
1665 [WD] F fmv ~ fmv, [WD] fmv ~ a
1666 And now we are stuck.
1667
1668 So instead the Fmv Orientation Invariant puts te fmv on the
1669 left, giving
1670 [WD] fmv ~ alpha, [WD] F alpha ~ fmv, [WD] alpha ~ a
1671
1672 Now we get alpha:=a, and everything works out
1673
1674 Note [Prevent unification with type families]
1675 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1676 We prevent unification with type families because of an uneasy compromise.
1677 It's perfectly sound to unify with type families, and it even improves the
1678 error messages in the testsuite. It also modestly improves performance, at
1679 least in some cases. But it's disastrous for test case perf/compiler/T3064.
1680 Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a.
1681 What do we do? Do we reduce F? Or do we use the given? Hard to know what's
1682 best. GHC reduces. This is a disaster for T3064, where the type's size
1683 spirals out of control during reduction. (We're not helped by the fact that
1684 the flattener re-flattens all the arguments every time around.) If we prevent
1685 unification with type families, then the solver happens to use the equality
1686 before expanding the type family.
1687
1688 It would be lovely in the future to revisit this problem and remove this
1689 extra, unnecessary check. But we retain it for now as it seems to work
1690 better in practice.
1691
1692 Note [Refactoring hazard: checkTauTvUpdate]
1693 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1694 I (Richard E.) have a sad story about refactoring this code, retained here
1695 to prevent others (or a future me!) from falling into the same traps.
1696
1697 It all started with #11407, which was caused by the fact that the TyVarTy
1698 case of defer_me didn't look in the kind. But it seemed reasonable to
1699 simply remove the defer_me check instead.
1700
1701 It referred to two Notes (since removed) that were out of date, and the
1702 fast_check code in occurCheckExpand seemed to do just about the same thing as
1703 defer_me. The one piece that defer_me did that wasn't repeated by
1704 occurCheckExpand was the type-family check. (See Note [Prevent unification
1705 with type families].) So I checked the result of occurCheckExpand for any
1706 type family occurrences and deferred if there were any. This was done
1707 in commit e9bf7bb5cc9fb3f87dd05111aa23da76b86a8967 .
1708
1709 This approach turned out not to be performant, because the expanded
1710 type was bigger than the original type, and tyConsOfType (needed to
1711 see if there are any type family occurrences) looks through type
1712 synonyms. So it then struck me that we could dispense with the
1713 defer_me check entirely. This simplified the code nicely, and it cut
1714 the allocations in T5030 by half. But, as documented in Note [Prevent
1715 unification with type families], this destroyed performance in
1716 T3064. Regardless, I missed this regression and the change was
1717 committed as 3f5d1a13f112f34d992f6b74656d64d95a3f506d .
1718
1719 Bottom lines:
1720 * defer_me is back, but now fixed w.r.t. #11407.
1721 * Tread carefully before you start to refactor here. There can be
1722 lots of hard-to-predict consequences.
1723
1724 Note [Type synonyms and the occur check]
1725 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1726 Generally speaking we try to update a variable with type synonyms not
1727 expanded, which improves later error messages, unless looking
1728 inside a type synonym may help resolve a spurious occurs check
1729 error. Consider:
1730 type A a = ()
1731
1732 f :: (A a -> a -> ()) -> ()
1733 f = \ _ -> ()
1734
1735 x :: ()
1736 x = f (\ x p -> p x)
1737
1738 We will eventually get a constraint of the form t ~ A t. The ok function above will
1739 properly expand the type (A t) to just (), which is ok to be unified with t. If we had
1740 unified with the original type A t, we would lead the type checker into an infinite loop.
1741
1742 Hence, if the occurs check fails for a type synonym application, then (and *only* then),
1743 the ok function expands the synonym to detect opportunities for occurs check success using
1744 the underlying definition of the type synonym.
1745
1746 The same applies later on in the constraint interaction code; see TcInteract,
1747 function @occ_check_ok@.
1748
1749 Note [Non-TcTyVars in TcUnify]
1750 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1751 Because the same code is now shared between unifying types and unifying
1752 kinds, we sometimes will see proper TyVars floating around the unifier.
1753 Example (from test case polykinds/PolyKinds12):
1754
1755 type family Apply (f :: k1 -> k2) (x :: k1) :: k2
1756 type instance Apply g y = g y
1757
1758 When checking the instance declaration, we first *kind-check* the LHS
1759 and RHS, discovering that the instance really should be
1760
1761 type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y
1762
1763 During this kind-checking, all the tyvars will be TcTyVars. Then, however,
1764 as a second pass, we desugar the RHS (which is done in functions prefixed
1765 with "tc" in TcTyClsDecls"). By this time, all the kind-vars are proper
1766 TyVars, not TcTyVars, get some kind unification must happen.
1767
1768 Thus, we always check if a TyVar is a TcTyVar before asking if it's a
1769 meta-tyvar.
1770
1771 This used to not be necessary for type-checking (that is, before * :: *)
1772 because expressions get desugared via an algorithm separate from
1773 type-checking (with wrappers, etc.). Types get desugared very differently,
1774 causing this wibble in behavior seen here.
1775 -}
1776
1777 data LookupTyVarResult -- The result of a lookupTcTyVar call
1778 = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
1779 | Filled TcType
1780
1781 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
1782 lookupTcTyVar tyvar
1783 | MetaTv { mtv_ref = ref } <- details
1784 = do { meta_details <- readMutVar ref
1785 ; case meta_details of
1786 Indirect ty -> return (Filled ty)
1787 Flexi -> do { is_touchable <- isTouchableTcM tyvar
1788 -- Note [Unifying untouchables]
1789 ; if is_touchable then
1790 return (Unfilled details)
1791 else
1792 return (Unfilled vanillaSkolemTv) } }
1793 | otherwise
1794 = return (Unfilled details)
1795 where
1796 details = tcTyVarDetails tyvar
1797
1798 {-
1799 Note [Unifying untouchables]
1800 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1801 We treat an untouchable type variable as if it was a skolem. That
1802 ensures it won't unify with anything. It's a slight had, because
1803 we return a made-up TcTyVarDetails, but I think it works smoothly.
1804 -}
1805
1806 -- | Breaks apart a function kind into its pieces.
1807 matchExpectedFunKind :: Outputable fun
1808 => fun -- ^ type, only for errors
1809 -> TcKind -- ^ function kind
1810 -> TcM (Coercion, TcKind, TcKind)
1811 -- ^ co :: old_kind ~ arg -> res
1812 matchExpectedFunKind hs_ty = go
1813 where
1814 go k | Just k' <- tcView k = go k'
1815
1816 go k@(TyVarTy kvar)
1817 | isMetaTyVar kvar
1818 = do { maybe_kind <- readMetaTyVar kvar
1819 ; case maybe_kind of
1820 Indirect fun_kind -> go fun_kind
1821 Flexi -> defer k }
1822
1823 go k@(FunTy arg res) = return (mkNomReflCo k, arg, res)
1824 go other = defer other
1825
1826 defer k
1827 = do { arg_kind <- newMetaKindVar
1828 ; res_kind <- newMetaKindVar
1829 ; let new_fun = mkFunTy arg_kind res_kind
1830 origin = TypeEqOrigin { uo_actual = k
1831 , uo_expected = new_fun
1832 , uo_thing = Just (ppr hs_ty)
1833 , uo_visible = True
1834 }
1835 ; co <- uType KindLevel origin k new_fun
1836 ; return (co, arg_kind, res_kind) }
1837
1838
1839 {- *********************************************************************
1840 * *
1841 Occurrence checking
1842 * *
1843 ********************************************************************* -}
1844
1845
1846 {- Note [Occurs check expansion]
1847 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1848 (occurCheckExpand tv xi) expands synonyms in xi just enough to get rid
1849 of occurrences of tv outside type function arguments, if that is
1850 possible; otherwise, it returns Nothing.
1851
1852 For example, suppose we have
1853 type F a b = [a]
1854 Then
1855 occCheckExpand b (F Int b) = Just [Int]
1856 but
1857 occCheckExpand a (F a Int) = Nothing
1858
1859 We don't promise to do the absolute minimum amount of expanding
1860 necessary, but we try not to do expansions we don't need to. We
1861 prefer doing inner expansions first. For example,
1862 type F a b = (a, Int, a, [a])
1863 type G b = Char
1864 We have
1865 occCheckExpand b (F (G b)) = Just (F Char)
1866 even though we could also expand F to get rid of b.
1867
1868 The two variants of the function are to support TcUnify.checkTauTvUpdate,
1869 which wants to prevent unification with type families. For more on this
1870 point, see Note [Prevent unification with type families] in TcUnify.
1871
1872 Note [Occurrence checking: look inside kinds]
1873 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1874 Suppose we are considering unifying
1875 (alpha :: *) ~ Int -> (beta :: alpha -> alpha)
1876 This may be an error (what is that alpha doing inside beta's kind?),
1877 but we must not make the mistake of actuallyy unifying or we'll
1878 build an infinite data structure. So when looking for occurrences
1879 of alpha in the rhs, we must look in the kinds of type variables
1880 that occur there.
1881
1882 NB: we may be able to remove the problem via expansion; see
1883 Note [Occurs check expansion]. So we have to try that.
1884
1885 Note [Checking for foralls]
1886 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1887 Unless we have -XImpredicativeTypes (which is a totally unsupported
1888 feature), we do not want to unify
1889 alpha ~ (forall a. a->a) -> Int
1890 So we look for foralls hidden inside the type, and it's convenient
1891 to do that at the same time as the occurs check (which looks for
1892 occurrences of alpha).
1893
1894 However, it's not just a question of looking for foralls /anywhere/!
1895 Consider
1896 (alpha :: forall k. k->*) ~ (beta :: forall k. k->*)
1897 This is legal; e.g. dependent/should_compile/T11635.
1898
1899 We don't want to reject it because of the forall in beta's kind,
1900 but (see Note [Occurrence checking: look inside kinds]) we do
1901 need to look in beta's kind. So we carry a flag saying if a 'forall'
1902 is OK, and sitch the flag on when stepping inside a kind.
1903
1904 Why is it OK? Why does it not count as impredicative polymorphism?
1905 The reason foralls are bad is because we reply on "seeing" foralls
1906 when doing implicit instantiation. But the forall inside the kind is
1907 fine. We'll generate a kind equality constraint
1908 (forall k. k->*) ~ (forall k. k->*)
1909 to check that the kinds of lhs and rhs are compatible. If alpha's
1910 kind had instead been
1911 (alpha :: kappa)
1912 then this kind equality would rightly complain about unifying kappa
1913 with (forall k. k->*)
1914
1915 -}
1916
1917 data OccCheckResult a
1918 = OC_OK a
1919 | OC_Bad -- Forall or type family
1920 | OC_Occurs
1921
1922 instance Functor OccCheckResult where
1923 fmap = liftM
1924
1925 instance Applicative OccCheckResult where
1926 pure = OC_OK
1927 (<*>) = ap
1928
1929 instance Monad OccCheckResult where
1930 OC_OK x >>= k = k x
1931 OC_Bad >>= _ = OC_Bad
1932 OC_Occurs >>= _ = OC_Occurs
1933
1934 occCheckForErrors :: DynFlags -> TcTyVar -> Type -> OccCheckResult ()
1935 -- Just for error-message generation; so we return OccCheckResult
1936 -- so the caller can report the right kind of error
1937 -- Check whether
1938 -- a) the given variable occurs in the given type.
1939 -- b) there is a forall in the type (unless we have -XImpredicativeTypes)
1940 occCheckForErrors dflags tv ty
1941 = case preCheck dflags True tv ty of
1942 OC_OK _ -> OC_OK ()
1943 OC_Bad -> OC_Bad
1944 OC_Occurs -> case occCheckExpand tv ty of
1945 Nothing -> OC_Occurs
1946 Just _ -> OC_OK ()
1947
1948 ----------------
1949 metaTyVarUpdateOK :: DynFlags
1950 -> TcTyVar -- tv :: k1
1951 -> TcType -- ty :: k2
1952 -> Maybe TcType -- possibly-expanded ty
1953 -- (metaTyFVarUpdateOK tv ty)
1954 -- We are about to update the meta-tyvar tv with ty
1955 -- Check (a) that tv doesn't occur in ty (occurs check)
1956 -- (b) that ty does not have any foralls
1957 -- (in the impredicative case), or type functions
1958 --
1959 -- We have two possible outcomes:
1960 -- (1) Return the type to update the type variable with,
1961 -- [we know the update is ok]
1962 -- (2) Return Nothing,
1963 -- [the update might be dodgy]
1964 --
1965 -- Note that "Nothing" does not mean "definite error". For example
1966 -- type family F a
1967 -- type instance F Int = Int
1968 -- consider
1969 -- a ~ F a
1970 -- This is perfectly reasonable, if we later get a ~ Int. For now, though,
1971 -- we return Nothing, leaving it to the later constraint simplifier to
1972 -- sort matters out.
1973 --
1974 -- See Note [Refactoring hazard: checkTauTvUpdate]
1975
1976 metaTyVarUpdateOK dflags tv ty
1977 = case preCheck dflags False tv ty of
1978 -- False <=> type families not ok
1979 -- See Note [Prevent unification with type families]
1980 OC_OK _ -> Just ty
1981 OC_Bad -> Nothing -- forall or type function
1982 OC_Occurs -> occCheckExpand tv ty
1983
1984 preCheck :: DynFlags -> Bool -> TcTyVar -> TcType -> OccCheckResult ()
1985 -- A quick check for
1986 -- (a) a forall type (unless -XImpredivativeTypes)
1987 -- (b) a type family
1988 -- (c) an occurrence of the type variable (occurs check)
1989 --
1990 -- For (a) and (b) we check only the top level of the type, NOT
1991 -- inside the kinds of variables it mentions. But for (c) we do
1992 -- look in the kinds of course.
1993
1994 preCheck dflags ty_fam_ok tv ty
1995 = fast_check ty
1996 where
1997 details = tcTyVarDetails tv
1998 impredicative_ok = canUnifyWithPolyType dflags details
1999
2000 ok :: OccCheckResult ()
2001 ok = OC_OK ()
2002
2003 fast_check :: TcType -> OccCheckResult ()
2004 fast_check (TyVarTy tv')
2005 | tv == tv' = OC_Occurs
2006 | otherwise = fast_check_occ (tyVarKind tv')
2007 -- See Note [Occurrence checking: look inside kinds]
2008
2009 fast_check (TyConApp tc tys)
2010 | bad_tc tc = OC_Bad
2011 | otherwise = mapM fast_check tys >> ok
2012 fast_check (LitTy {}) = ok
2013 fast_check (FunTy a r) = fast_check a >> fast_check r
2014 fast_check (AppTy fun arg) = fast_check fun >> fast_check arg
2015 fast_check (CastTy ty co) = fast_check ty >> fast_check_co co
2016 fast_check (CoercionTy co) = fast_check_co co
2017 fast_check (ForAllTy (TvBndr tv' _) ty)
2018 | not impredicative_ok = OC_Bad
2019 | tv == tv' = ok
2020 | otherwise = do { fast_check_occ (tyVarKind tv')
2021 ; fast_check_occ ty }
2022 -- Under a forall we look only for occurrences of
2023 -- the type variable
2024
2025 -- For kinds, we only do an occurs check; we do not worry
2026 -- about type families or foralls
2027 -- See Note [Checking for foralls]
2028 fast_check_occ k | tv `elemVarSet` tyCoVarsOfType k = OC_Occurs
2029 | otherwise = ok
2030
2031 -- For coercions, we are only doing an occurs check here;
2032 -- no bother about impredicativity in coercions, as they're
2033 -- inferred
2034 fast_check_co co | tv `elemVarSet` tyCoVarsOfCo co = OC_Occurs
2035 | otherwise = ok
2036
2037 bad_tc :: TyCon -> Bool
2038 bad_tc tc
2039 | not (impredicative_ok || isTauTyCon tc) = True
2040 | not (ty_fam_ok || isFamFreeTyCon tc) = True
2041 | otherwise = False
2042
2043 occCheckExpand :: TcTyVar -> TcType -> Maybe TcType
2044 -- See Note [Occurs check expansion]
2045 -- We may have needed to do some type synonym unfolding in order to
2046 -- get rid of the variable (or forall), so we also return the unfolded
2047 -- version of the type, which is guaranteed to be syntactically free
2048 -- of the given type variable. If the type is already syntactically
2049 -- free of the variable, then the same type is returned.
2050 occCheckExpand tv ty
2051 = go emptyVarEnv ty
2052 where
2053 go :: VarEnv TyVar -> Type -> Maybe Type
2054 -- The VarEnv carries mappings necessary
2055 -- because of kind expansion
2056 go env (TyVarTy tv')
2057 | tv == tv' = Nothing
2058 | Just tv'' <- lookupVarEnv env tv' = return (mkTyVarTy tv'')
2059 | otherwise = do { tv'' <- go_var env tv'
2060 ; return (mkTyVarTy tv'') }
2061
2062 go _ ty@(LitTy {}) = return ty
2063 go env (AppTy ty1 ty2) = do { ty1' <- go env ty1
2064 ; ty2' <- go env ty2
2065 ; return (mkAppTy ty1' ty2') }
2066 go env (FunTy ty1 ty2) = do { ty1' <- go env ty1
2067 ; ty2' <- go env ty2
2068 ; return (mkFunTy ty1' ty2') }
2069 go env ty@(ForAllTy (TvBndr tv' vis) body_ty)
2070 | tv == tv' = return ty
2071 | otherwise = do { ki' <- go env (tyVarKind tv')
2072 ; let tv'' = setTyVarKind tv' ki'
2073 env' = extendVarEnv env tv' tv''
2074 ; body' <- go env' body_ty
2075 ; return (ForAllTy (TvBndr tv'' vis) body') }
2076
2077 -- For a type constructor application, first try expanding away the
2078 -- offending variable from the arguments. If that doesn't work, next
2079 -- see if the type constructor is a type synonym, and if so, expand
2080 -- it and try again.
2081 go env ty@(TyConApp tc tys)
2082 = case mapM (go env) tys of
2083 Just tys' -> return (mkTyConApp tc tys')
2084 Nothing | Just ty' <- tcView ty -> go env ty'
2085 | otherwise -> Nothing
2086 -- Failing that, try to expand a synonym
2087
2088 go env (CastTy ty co) = do { ty' <- go env ty
2089 ; co' <- go_co env co
2090 ; return (mkCastTy ty' co') }
2091 go env (CoercionTy co) = do { co' <- go_co env co
2092 ; return (mkCoercionTy co') }
2093
2094 ------------------
2095 go_var env v = do { k' <- go env (varType v)
2096 ; return (setVarType v k') }
2097 -- Works for TyVar and CoVar
2098 -- See Note [Occurrence checking: look inside kinds]
2099
2100 ------------------
2101 go_co env (Refl r ty) = do { ty' <- go env ty
2102 ; return (mkReflCo r ty') }
2103 -- Note: Coercions do not contain type synonyms
2104 go_co env (TyConAppCo r tc args) = do { args' <- mapM (go_co env) args
2105 ; return (mkTyConAppCo r tc args') }
2106 go_co env (AppCo co arg) = do { co' <- go_co env co
2107 ; arg' <- go_co env arg
2108 ; return (mkAppCo co' arg') }
2109 go_co env co@(ForAllCo tv' kind_co body_co)
2110 | tv == tv' = return co
2111 | otherwise = do { kind_co' <- go_co env kind_co
2112 ; let tv'' = setTyVarKind tv' $
2113 pFst (coercionKind kind_co')
2114 env' = extendVarEnv env tv' tv''
2115 ; body' <- go_co env' body_co
2116 ; return (ForAllCo tv'' kind_co' body') }
2117 go_co env (FunCo r co1 co2) = do { co1' <- go_co env co1
2118 ; co2' <- go_co env co2
2119 ; return (mkFunCo r co1' co2') }
2120 go_co env (CoVarCo c) = do { c' <- go_var env c
2121 ; return (mkCoVarCo c') }
2122 go_co env (HoleCo h) = do { c' <- go_var env (ch_co_var h)
2123 ; return (HoleCo (h { ch_co_var = c' })) }
2124 go_co env (AxiomInstCo ax ind args) = do { args' <- mapM (go_co env) args
2125 ; return (mkAxiomInstCo ax ind args') }
2126 go_co env (UnivCo p r ty1 ty2) = do { p' <- go_prov env p
2127 ; ty1' <- go env ty1
2128 ; ty2' <- go env ty2
2129 ; return (mkUnivCo p' r ty1' ty2') }
2130 go_co env (SymCo co) = do { co' <- go_co env co
2131 ; return (mkSymCo co') }
2132 go_co env (TransCo co1 co2) = do { co1' <- go_co env co1
2133 ; co2' <- go_co env co2
2134 ; return (mkTransCo co1' co2') }
2135 go_co env (NthCo n co) = do { co' <- go_co env co
2136 ; return (mkNthCo n co') }
2137 go_co env (LRCo lr co) = do { co' <- go_co env co
2138 ; return (mkLRCo lr co') }
2139 go_co env (InstCo co arg) = do { co' <- go_co env co
2140 ; arg' <- go_co env arg
2141 ; return (mkInstCo co' arg') }
2142 go_co env (CoherenceCo co1 co2) = do { co1' <- go_co env co1
2143 ; co2' <- go_co env co2
2144 ; return (mkCoherenceCo co1' co2') }
2145 go_co env (KindCo co) = do { co' <- go_co env co
2146 ; return (mkKindCo co') }
2147 go_co env (SubCo co) = do { co' <- go_co env co
2148 ; return (mkSubCo co') }
2149 go_co env (AxiomRuleCo ax cs) = do { cs' <- mapM (go_co env) cs
2150 ; return (mkAxiomRuleCo ax cs') }
2151
2152 ------------------
2153 go_prov _ UnsafeCoerceProv = return UnsafeCoerceProv
2154 go_prov env (PhantomProv co) = PhantomProv <$> go_co env co
2155 go_prov env (ProofIrrelProv co) = ProofIrrelProv <$> go_co env co
2156 go_prov _ p@(PluginProv _) = return p
2157
2158 canUnifyWithPolyType :: DynFlags -> TcTyVarDetails -> Bool
2159 canUnifyWithPolyType dflags details
2160 = case details of
2161 MetaTv { mtv_info = SigTv } -> False
2162 MetaTv { mtv_info = TauTv } -> xopt LangExt.ImpredicativeTypes dflags
2163 _other -> True
2164 -- We can have non-meta tyvars in given constraints