2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcPat: Typechecking patterns
9 module TcPat ( tcLetPat, TcSigFun, TcSigInfo(..), TcPragFun
10 , LetBndrSpec(..), addInlinePrags, warnPrags
11 , tcPat, tcPats, newNoSigLetBndr, newSigLetBndr
12 , addDataConStupidTheta, badFieldCon, polyPatSig ) where
14 #include "HsVersions.h"
16 import {-# SOURCE #-} TcExpr( tcSyntaxOp, tcInferRho)
36 import BasicTypes hiding (SuccessFlag(..))
47 %************************************************************************
51 %************************************************************************
54 tcLetPat :: TcSigFun -> LetBndrSpec
55 -> LPat Name -> TcSigmaType
58 tcLetPat sig_fn no_gen pat pat_ty thing_inside
59 = tc_lpat pat pat_ty penv thing_inside
61 penv = PE { pe_lazy = True
62 , pe_ctxt = LetPat sig_fn no_gen }
65 tcPats :: HsMatchContext Name
66 -> [LPat Name] -- Patterns,
67 -> [TcSigmaType] -- and their types
68 -> TcM a -- and the checker for the body
69 -> TcM ([LPat TcId], a)
71 -- This is the externally-callable wrapper function
72 -- Typecheck the patterns, extend the environment to bind the variables,
73 -- do the thing inside, use any existentially-bound dictionaries to
74 -- discharge parts of the returning LIE, and deal with pattern type
77 -- 1. Initialise the PatState
78 -- 2. Check the patterns
80 -- 4. Check that no existentials escape
82 tcPats ctxt pats pat_tys thing_inside
83 = tc_lpats penv pats pat_tys thing_inside
85 penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }
87 tcPat :: HsMatchContext Name
88 -> LPat Name -> TcSigmaType
89 -> TcM a -- Checker for body, given
92 tcPat ctxt pat pat_ty thing_inside
93 = tc_lpat pat pat_ty penv thing_inside
95 penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }
100 = PE { pe_lazy :: Bool -- True <=> lazy context, so no existentials allowed
101 , pe_ctxt :: PatCtxt -- Context in which the whole pattern appears
105 = LamPat -- Used for lambdas, case etc
106 (HsMatchContext Name)
108 | LetPat -- Used only for let(rec) bindings
109 -- See Note [Let binders]
110 TcSigFun -- Tells type sig if any
111 LetBndrSpec -- True <=> no generalisation of this let
114 = LetLclBndr -- The binder is just a local one;
115 -- an AbsBinds will provide the global version
117 | LetGblBndr TcPragFun -- There isn't going to be an AbsBinds;
118 -- here is the inline-pragma information
120 makeLazy :: PatEnv -> PatEnv
121 makeLazy penv = penv { pe_lazy = True }
123 patSigCtxt :: PatEnv -> UserTypeCtxt
124 patSigCtxt (PE { pe_ctxt = LetPat {} }) = BindPatSigCtxt
125 patSigCtxt (PE { pe_ctxt = LamPat {} }) = LamPatSigCtxt
128 type TcPragFun = Name -> [LSig Name]
129 type TcSigFun = Name -> Maybe TcSigInfo
133 sig_id :: TcId, -- *Polymorphic* binder for this value...
135 sig_scoped :: [Name], -- Scoped type variables
136 -- 1-1 correspondence with a prefix of sig_tvs
137 -- However, may be fewer than sig_tvs;
138 -- see Note [More instantiated than scoped]
139 sig_tvs :: [TcTyVar], -- Instantiated type variables
140 -- See Note [Instantiate sig]
142 sig_theta :: TcThetaType, -- Instantiated theta
144 sig_tau :: TcSigmaType, -- Instantiated tau
145 -- See Note [sig_tau may be polymorphic]
147 sig_loc :: SrcSpan -- The location of the signature
150 instance Outputable TcSigInfo where
151 ppr (TcSigInfo { sig_id = id, sig_tvs = tyvars, sig_theta = theta, sig_tau = tau})
152 = ppr id <+> ptext (sLit "::") <+> ppr tyvars <+> pprThetaArrow theta <+> ppr tau
155 Note [sig_tau may be polymorphic]
156 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
157 Note that "sig_tau" might actually be a polymorphic type,
158 if the original function had a signature like
159 forall a. Eq a => forall b. Ord b => ....
160 But that's ok: tcMatchesFun (called by tcRhs) can deal with that
161 It happens, too! See Note [Polymorphic methods] in TcClassDcl.
169 ...more notes to add here..
172 Note [Existential check]
173 ~~~~~~~~~~~~~~~~~~~~~~~~
174 Lazy patterns can't bind existentials. They arise in two ways:
175 * Let bindings let { C a b = e } in b
176 * Twiddle patterns f ~(C a b) = e
177 The pe_lazy field of PatEnv says whether we are inside a lazy
178 pattern (perhaps deeply)
180 If we aren't inside a lazy pattern then we can bind existentials,
181 but we need to be careful about "extra" tyvars. Consider
182 (\C x -> d) : pat_ty -> res_ty
183 When looking for existential escape we must check that the existential
184 bound by C don't unify with the free variables of pat_ty, OR res_ty
185 (or of course the environment). Hence we need to keep track of the
189 %************************************************************************
193 %************************************************************************
196 tcPatBndr :: PatEnv -> Name -> TcSigmaType -> TcM (CoercionI, TcId)
197 -- (coi, xp) = tcPatBndr penv x pat_ty
198 -- Then coi : pat_ty ~ typeof(xp)
200 tcPatBndr (PE { pe_ctxt = LetPat lookup_sig no_gen}) bndr_name pat_ty
201 | Just sig <- lookup_sig bndr_name
202 = do { bndr_id <- newSigLetBndr no_gen bndr_name sig
203 ; coi <- unifyPatType (idType bndr_id) pat_ty
204 ; return (coi, bndr_id) }
207 = do { bndr_id <- newNoSigLetBndr no_gen bndr_name pat_ty
208 ; return (IdCo pat_ty, bndr_id) }
210 tcPatBndr (PE { pe_ctxt = _lam_or_proc }) bndr_name pat_ty
211 = do { bndr <- mkLocalBinder bndr_name pat_ty
212 ; return (IdCo pat_ty, bndr) }
215 newSigLetBndr :: LetBndrSpec -> Name -> TcSigInfo -> TcM TcId
216 newSigLetBndr LetLclBndr name sig
217 = do { mono_name <- newLocalName name
218 ; mkLocalBinder mono_name (sig_tau sig) }
219 newSigLetBndr (LetGblBndr prags) name sig
220 = addInlinePrags (sig_id sig) (prags name)
223 newNoSigLetBndr :: LetBndrSpec -> Name -> TcType -> TcM TcId
224 -- In the polymorphic case (no_gen = False), generate a "monomorphic version"
225 -- of the Id; the original name will be bound to the polymorphic version
227 -- In the monomorphic case there is no AbsBinds, and we use the original
229 newNoSigLetBndr LetLclBndr name ty
230 =do { mono_name <- newLocalName name
231 ; mkLocalBinder mono_name ty }
232 newNoSigLetBndr (LetGblBndr prags) name ty
233 = do { id <- mkLocalBinder name ty
234 ; addInlinePrags id (prags name) }
237 addInlinePrags :: TcId -> [LSig Name] -> TcM TcId
238 addInlinePrags poly_id prags
241 inl_sigs = filter isInlineLSig prags
242 tc_inl [] = return poly_id
243 tc_inl (L loc (InlineSig _ prag) : other_inls)
244 = do { unless (null other_inls) (setSrcSpan loc warn_dup_inline)
245 ; return (poly_id `setInlinePragma` prag) }
246 tc_inl _ = panic "tc_inl"
248 warn_dup_inline = warnPrags poly_id inl_sigs $
249 ptext (sLit "Duplicate INLINE pragmas for")
251 warnPrags :: Id -> [LSig Name] -> SDoc -> TcM ()
252 warnPrags id bad_sigs herald
253 = addWarnTc (hang (herald <+> quotes (ppr id))
254 2 (ppr_sigs bad_sigs))
256 ppr_sigs sigs = vcat (map (ppr . getLoc) sigs)
259 mkLocalBinder :: Name -> TcType -> TcM TcId
260 mkLocalBinder name ty
261 = do { checkUnboxedTuple ty $
262 ptext (sLit "The variable") <+> quotes (ppr name)
263 ; return (Id.mkLocalId name ty) }
265 checkUnboxedTuple :: TcType -> SDoc -> TcM ()
266 -- Check for an unboxed tuple type
267 -- f = (# True, False #)
268 -- Zonk first just in case it's hidden inside a meta type variable
269 -- (This shows up as a (more obscure) kind error
270 -- in the 'otherwise' case of tcMonoBinds.)
271 checkUnboxedTuple ty what
272 = do { zonked_ty <- zonkTcTypeCarefully ty
273 ; checkTc (not (isUnboxedTupleType zonked_ty))
274 (unboxedTupleErr what zonked_ty) }
277 {- Only needed if we re-add Method constraints
278 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, TcEvBinds)
279 bindInstsOfPatId id thing_inside
280 | not (isOverloadedTy (idType id))
281 = do { res <- thing_inside; return (res, emptyTcEvBinds) }
283 = do { (res, lie) <- captureConstraints thing_inside
284 ; binds <- bindLocalMethods lie [id]
285 ; return (res, binds) }
289 Note [Polymorphism and pattern bindings]
290 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
291 When is_mono holds we are not generalising
292 But the signature can still be polymoprhic!
293 data T = MkT (forall a. a->a)
296 So the no_gen flag decides whether the pattern-bound variables should
297 have exactly the type in the type signature (when not generalising) or
298 the instantiated version (when generalising)
300 %************************************************************************
302 The main worker functions
304 %************************************************************************
308 tcPat takes a "thing inside" over which the pattern scopes. This is partly
309 so that tcPat can extend the environment for the thing_inside, but also
310 so that constraints arising in the thing_inside can be discharged by the
313 This does not work so well for the ErrCtxt carried by the monad: we don't
314 want the error-context for the pattern to scope over the RHS.
315 Hence the getErrCtxt/setErrCtxt stuff in tcMultiple
319 type Checker inp out = forall r.
325 tcMultiple :: Checker inp out -> Checker [inp] [out]
326 tcMultiple tc_pat args penv thing_inside
327 = do { err_ctxt <- getErrCtxt
329 = do { res <- thing_inside
333 = do { (p', (ps', res))
335 setErrCtxt err_ctxt $
337 -- setErrCtxt: restore context before doing the next pattern
338 -- See note [Nesting] above
340 ; return (p':ps', res) }
349 -> TcM (LPat TcId, a)
350 tc_lpat (L span pat) pat_ty penv thing_inside
352 maybeAddErrCtxt (patCtxt pat) $
353 do { (pat', res) <- tc_pat penv pat pat_ty thing_inside
354 ; return (L span pat', res) }
357 -> [LPat Name] -> [TcSigmaType]
359 -> TcM ([LPat TcId], a)
360 tc_lpats penv pats tys thing_inside
361 = tcMultiple (\(p,t) -> tc_lpat p t)
362 (zipEqual "tc_lpats" pats tys)
368 -> TcSigmaType -- Fully refined result type
369 -> TcM a -- Thing inside
370 -> TcM (Pat TcId, -- Translated pattern
371 a) -- Result of thing inside
373 tc_pat penv (VarPat name) pat_ty thing_inside
374 = do { (coi, id) <- tcPatBndr penv name pat_ty
375 ; res <- tcExtendIdEnv1 name id thing_inside
376 ; return (mkHsWrapPatCoI coi (VarPat id) pat_ty, res) }
378 {- Need this if we re-add Method constraints
379 ; (res, binds) <- bindInstsOfPatId id $
380 tcExtendIdEnv1 name id $
381 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
383 ; let pat' | isEmptyTcEvBinds binds = VarPat id
384 | otherwise = VarPatOut id binds
385 ; return (mkHsWrapPatCoI coi pat' pat_ty, res) }
388 tc_pat penv (ParPat pat) pat_ty thing_inside
389 = do { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
390 ; return (ParPat pat', res) }
392 tc_pat penv (BangPat pat) pat_ty thing_inside
393 = do { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
394 ; return (BangPat pat', res) }
396 tc_pat penv lpat@(LazyPat pat) pat_ty thing_inside
397 = do { (pat', (res, pat_ct))
398 <- tc_lpat pat pat_ty (makeLazy penv) $
399 captureConstraints thing_inside
400 -- Ignore refined penv', revert to penv
402 ; emitConstraints pat_ct
403 -- captureConstraints/extendConstraints:
404 -- see Note [Hopping the LIE in lazy patterns]
406 -- Check there are no unlifted types under the lazy pattern
407 ; when (any (isUnLiftedType . idType) $ collectPatBinders pat') $
408 lazyUnliftedPatErr lpat
410 -- Check that the expected pattern type is itself lifted
411 ; pat_ty' <- newFlexiTyVarTy liftedTypeKind
412 ; _ <- unifyType pat_ty pat_ty'
414 ; return (LazyPat pat', res) }
416 tc_pat _ p@(QuasiQuotePat _) _ _
417 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
419 tc_pat _ (WildPat _) pat_ty thing_inside
420 = do { checkUnboxedTuple pat_ty $
421 ptext (sLit "A wild-card pattern")
422 ; res <- thing_inside
423 ; return (WildPat pat_ty, res) }
425 tc_pat penv (AsPat (L nm_loc name) pat) pat_ty thing_inside
426 = do { (coi, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
427 ; (pat', res) <- tcExtendIdEnv1 name bndr_id $
428 tc_lpat pat (idType bndr_id) penv thing_inside
429 -- NB: if we do inference on:
430 -- \ (y@(x::forall a. a->a)) = e
431 -- we'll fail. The as-pattern infers a monotype for 'y', which then
432 -- fails to unify with the polymorphic type for 'x'. This could
433 -- perhaps be fixed, but only with a bit more work.
435 -- If you fix it, don't forget the bindInstsOfPatIds!
436 ; return (mkHsWrapPatCoI coi (AsPat (L nm_loc bndr_id) pat') pat_ty, res) }
438 tc_pat penv vpat@(ViewPat expr pat _) overall_pat_ty thing_inside
439 = do { checkUnboxedTuple overall_pat_ty $
440 ptext (sLit "The view pattern") <+> ppr vpat
442 -- Morally, expr must have type `forall a1...aN. OPT' -> B`
443 -- where overall_pat_ty is an instance of OPT'.
444 -- Here, we infer a rho type for it,
445 -- which replaces the leading foralls and constraints
446 -- with fresh unification variables.
447 ; (expr',expr'_inferred) <- tcInferRho expr
449 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
450 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
451 -- variable to find it). this means that in an example like
452 -- (view -> f) where view :: _ -> forall b. b
453 -- we will only be able to use view at one instantation in the
455 ; (expr_coi, pat_ty) <- tcInfer $ \ pat_ty ->
456 unifyPatType expr'_inferred (mkFunTy overall_pat_ty pat_ty)
458 -- pattern must have pat_ty
459 ; (pat', res) <- tc_lpat pat pat_ty penv thing_inside
461 ; return (ViewPat (mkLHsWrapCoI expr_coi expr') pat' overall_pat_ty, res) }
463 -- Type signatures in patterns
464 -- See Note [Pattern coercions] below
465 tc_pat penv (SigPatIn pat sig_ty) pat_ty thing_inside
466 = do { (inner_ty, tv_binds, wrap) <- tcPatSig (patSigCtxt penv) sig_ty pat_ty
467 ; (pat', res) <- tcExtendTyVarEnv2 tv_binds $
468 tc_lpat pat inner_ty penv thing_inside
470 ; return (mkHsWrapPat wrap (SigPatOut pat' inner_ty) pat_ty, res) }
472 tc_pat _ pat@(TypePat _) _ _
473 = failWithTc (badTypePat pat)
475 ------------------------
476 -- Lists, tuples, arrays
477 tc_pat penv (ListPat pats _) pat_ty thing_inside
478 = do { (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTy pat_ty
479 ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
480 pats penv thing_inside
481 ; return (mkHsWrapPat coi (ListPat pats' elt_ty) pat_ty, res)
484 tc_pat penv (PArrPat pats _) pat_ty thing_inside
485 = do { (coi, elt_ty) <- matchExpectedPatTy matchExpectedPArrTy pat_ty
486 ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
487 pats penv thing_inside
488 ; return (mkHsWrapPat coi (PArrPat pats' elt_ty) pat_ty, res)
491 tc_pat penv (TuplePat pats boxity _) pat_ty thing_inside
492 = do { let tc = tupleTyCon boxity (length pats)
493 ; (coi, arg_tys) <- matchExpectedPatTy (matchExpectedTyConApp tc) pat_ty
494 ; (pats', res) <- tc_lpats penv pats arg_tys thing_inside
496 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
497 -- so that we can experiment with lazy tuple-matching.
498 -- This is a pretty odd place to make the switch, but
499 -- it was easy to do.
500 ; let pat_ty' = mkTyConApp tc arg_tys
501 -- pat_ty /= pat_ty iff coi /= IdCo
502 unmangled_result = TuplePat pats' boxity pat_ty'
503 possibly_mangled_result
504 | opt_IrrefutableTuples &&
505 isBoxed boxity = LazyPat (noLoc unmangled_result)
506 | otherwise = unmangled_result
508 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
509 return (mkHsWrapPat coi possibly_mangled_result pat_ty, res)
512 ------------------------
514 tc_pat penv (ConPatIn con arg_pats) pat_ty thing_inside
515 = tcConPat penv con pat_ty arg_pats thing_inside
517 ------------------------
519 tc_pat _ (LitPat simple_lit) pat_ty thing_inside
520 = do { let lit_ty = hsLitType simple_lit
521 ; coi <- unifyPatType lit_ty pat_ty
522 -- coi is of kind: pat_ty ~ lit_ty
523 ; res <- thing_inside
524 ; return ( mkHsWrapPatCoI coi (LitPat simple_lit) pat_ty
527 ------------------------
528 -- Overloaded patterns: n, and n+k
529 tc_pat _ (NPat over_lit mb_neg eq) pat_ty thing_inside
530 = do { let orig = LiteralOrigin over_lit
531 ; lit' <- newOverloadedLit orig over_lit pat_ty
532 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
533 ; mb_neg' <- case mb_neg of
534 Nothing -> return Nothing -- Positive literal
535 Just neg -> -- Negative literal
536 -- The 'negate' is re-mappable syntax
537 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
538 ; return (Just neg') }
539 ; res <- thing_inside
540 ; return (NPat lit' mb_neg' eq', res) }
542 tc_pat penv (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
543 = do { (coi, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
544 ; let pat_ty' = idType bndr_id
545 orig = LiteralOrigin lit
546 ; lit' <- newOverloadedLit orig lit pat_ty'
548 -- The '>=' and '-' parts are re-mappable syntax
549 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
550 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
551 ; let pat' = NPlusKPat (L nm_loc bndr_id) lit' ge' minus'
553 -- The Report says that n+k patterns must be in Integral
554 -- We may not want this when using re-mappable syntax, though (ToDo?)
555 ; icls <- tcLookupClass integralClassName
556 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
558 ; res <- tcExtendIdEnv1 name bndr_id thing_inside
559 ; return (mkHsWrapPatCoI coi pat' pat_ty, res) }
561 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
564 unifyPatType :: TcType -> TcType -> TcM CoercionI
565 -- In patterns we want a coercion from the
566 -- context type (expected) to the actual pattern type
567 -- But we don't want to reverse the args to unifyType because
568 -- that controls the actual/expected stuff in error messages
569 unifyPatType actual_ty expected_ty
570 = do { coi <- unifyType actual_ty expected_ty
571 ; return (mkSymCoI coi) }
574 Note [Hopping the LIE in lazy patterns]
575 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
576 In a lazy pattern, we must *not* discharge constraints from the RHS
577 from dictionaries bound in the pattern. E.g.
579 We can't discharge the Num constraint from dictionaries bound by
582 So we have to make the constraints from thing_inside "hop around"
583 the pattern. Hence the captureConstraints and emitConstraints.
585 The same thing ensures that equality constraints in a lazy match
586 are not made available in the RHS of the match. For example
587 data T a where { T1 :: Int -> T Int; ... }
590 It's obviously not sound to refine a to Int in the right
591 hand side, because the arugment might not match T1 at all!
593 Finally, a lazy pattern should not bind any existential type variables
594 because they won't be in scope when we do the desugaring
597 %************************************************************************
599 Most of the work for constructors is here
600 (the rest is in the ConPatIn case of tc_pat)
602 %************************************************************************
604 [Pattern matching indexed data types]
605 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
606 Consider the following declarations:
608 data family Map k :: * -> *
609 data instance Map (a, b) v = MapPair (Map a (Pair b v))
611 and a case expression
613 case x :: Map (Int, c) w of MapPair m -> ...
615 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
616 worker/wrapper types for MapPair are
618 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
619 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
621 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
622 :R123Map, which means the straight use of boxySplitTyConApp would give a type
623 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
624 boxySplitTyConApp with the family tycon Map instead, which gives us the family
625 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
626 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
627 (provided by tyConFamInst_maybe together with the family tycon). This
628 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
629 the split arguments for the representation tycon :R123Map as {Int, c, w}
631 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
633 Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
635 moving between representation and family type into account. To produce type
636 correct Core, this coercion needs to be used to case the type of the scrutinee
637 from the family to the representation type. This is achieved by
638 unwrapFamInstScrutinee using a CoPat around the result pattern.
640 Now it might appear seem as if we could have used the previous GADT type
641 refinement infrastructure of refineAlt and friends instead of the explicit
642 unification and CoPat generation. However, that would be wrong. Why? The
643 whole point of GADT refinement is that the refinement is local to the case
644 alternative. In contrast, the substitution generated by the unification of
645 the family type list and instance types needs to be propagated to the outside.
646 Imagine that in the above example, the type of the scrutinee would have been
647 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
648 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
649 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
650 alternatives of the case expression, whereas in the GADT case it might vary
651 between alternatives.
653 RIP GADT refinement: refinements have been replaced by the use of explicit
654 equality constraints that are used in conjunction with implication constraints
655 to express the local scope of GADT refinements.
659 -- MkT :: forall a b c. (a~[b]) => b -> c -> T a
660 -- with scrutinee of type (T ty)
662 tcConPat :: PatEnv -> Located Name
663 -> TcRhoType -- Type of the pattern
664 -> HsConPatDetails Name -> TcM a
666 tcConPat penv (L con_span con_name) pat_ty arg_pats thing_inside
667 = do { data_con <- tcLookupDataCon con_name
668 ; let tycon = dataConTyCon data_con
669 -- For data families this is the representation tycon
670 (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
671 = dataConFullSig data_con
673 -- Instantiate the constructor type variables [a->ty]
674 -- This may involve doing a family-instance coercion,
675 -- and building a wrapper
676 ; (wrap, ctxt_res_tys) <- matchExpectedPatTy (matchExpectedConTy tycon) pat_ty
678 -- Add the stupid theta
679 ; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys
681 ; checkExistentials ex_tvs penv
682 ; let skol_info = case pe_ctxt penv of
683 LamPat mc -> PatSkol data_con mc
684 LetPat {} -> UnkSkol -- Doesn't matter
685 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
686 -- Get location from monad, not from ex_tvs
688 ; let pat_ty' = mkTyConApp tycon ctxt_res_tys
689 -- pat_ty' is type of the actual constructor application
690 -- pat_ty' /= pat_ty iff coi /= IdCo
692 tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
693 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
694 arg_tys' = substTys tenv arg_tys
695 full_theta = eq_theta ++ dict_theta
697 ; if null ex_tvs && null eq_spec && null full_theta
698 then do { -- The common case; no class bindings etc
699 -- (see Note [Arrows and patterns])
700 (arg_pats', res) <- tcConArgs data_con arg_tys'
701 arg_pats penv thing_inside
702 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
703 pat_tvs = [], pat_dicts = [],
704 pat_binds = emptyTcEvBinds,
705 pat_args = arg_pats',
708 ; return (mkHsWrapPat wrap res_pat pat_ty, res) }
710 else do -- The general case, with existential,
711 -- and local equality constraints
712 { let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
713 theta' = substTheta tenv (eq_preds ++ full_theta)
714 -- order is *important* as we generate the list of
715 -- dictionary binders from theta'
716 no_equalities = not (any isEqPred theta')
718 ; gadts_on <- xoptM Opt_GADTs
719 ; checkTc (no_equalities || gadts_on)
720 (ptext (sLit "A pattern match on a GADT requires -XGADTs"))
721 -- Trac #2905 decided that a *pattern-match* of a GADT
722 -- should require the GADT language flag
724 ; given <- newEvVars theta'
725 ; (ev_binds, (arg_pats', res))
726 <- checkConstraints skol_info ex_tvs' given $
727 tcConArgs data_con arg_tys' arg_pats penv thing_inside
729 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
732 pat_binds = ev_binds,
733 pat_args = arg_pats',
735 ; return (mkHsWrapPat wrap res_pat pat_ty, res)
738 ----------------------------
739 matchExpectedPatTy :: (TcRhoType -> TcM (CoercionI, a))
740 -> TcRhoType -> TcM (HsWrapper, a)
741 -- See Note [Matching polytyped patterns]
742 -- Returns a wrapper : pat_ty ~ inner_ty
743 matchExpectedPatTy inner_match pat_ty
744 | null tvs && null theta
745 = do { (coi, res) <- inner_match pat_ty
746 ; return (coiToHsWrapper (mkSymCoI coi), res) }
747 -- The Sym is because the inner_match returns a coercion
748 -- that is the other way round to matchExpectedPatTy
751 = do { (_, tys, subst) <- tcInstTyVars tvs
752 ; wrap1 <- instCall PatOrigin tys (substTheta subst theta)
753 ; (wrap2, arg_tys) <- matchExpectedPatTy inner_match (substTy subst tau)
754 ; return (wrap2 <.> wrap1 , arg_tys) }
756 (tvs, theta, tau) = tcSplitSigmaTy pat_ty
758 ----------------------------
759 matchExpectedConTy :: TyCon -- The TyCon that this data
760 -- constructor actually returns
761 -> TcRhoType -- The type of the pattern
762 -> TcM (CoercionI, [TcSigmaType])
763 -- See Note [Matching constructor patterns]
764 -- Returns a coercion : T ty1 ... tyn ~ pat_ty
765 -- This is the same way round as matchExpectedListTy etc
766 -- but the other way round to matchExpectedPatTy
767 matchExpectedConTy data_tc pat_ty
768 | Just (fam_tc, fam_args, co_tc) <- tyConFamInstSig_maybe data_tc
769 -- Comments refer to Note [Matching constructor patterns]
770 -- co_tc :: forall a. T [a] ~ T7 a
771 = do { (_, tys, subst) <- tcInstTyVars (tyConTyVars data_tc)
774 ; coi1 <- unifyType (mkTyConApp fam_tc (substTys subst fam_args)) pat_ty
775 -- coi1 : T (ty1,ty2) ~ pat_ty
777 ; let coi2 = ACo (mkTyConApp co_tc tys)
778 -- coi2 : T (ty1,ty2) ~ T7 ty1 ty2
780 ; return (mkTransCoI (mkSymCoI coi2) coi1, tys) }
783 = matchExpectedTyConApp data_tc pat_ty
784 -- coi : T tys ~ pat_ty
788 Note [Matching constructor patterns]
789 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
790 Suppose (coi, tys) = matchExpectedConType data_tc pat_ty
792 * In the simple case, pat_ty = tc tys
794 * If pat_ty is a polytype, we want to instantiate it
795 This is like part of a subsumption check. Eg
796 f :: (forall a. [a]) -> blah
799 * In a type family case, suppose we have
801 data instance T (p,q) = A p | B q
802 Then we'll have internally generated
803 data T7 p q = A p | B q
804 axiom coT7 p q :: T (p,q) ~ T7 p q
806 So if pat_ty = T (ty1,ty2), we return (coi, [ty1,ty2]) such that
807 coi = coi2 . coi1 : T7 t ~ pat_ty
808 coi1 : T (ty1,ty2) ~ pat_ty
809 coi2 : T7 ty1 ty2 ~ T (ty1,ty2)
811 For families we do all this matching here, not in the unifier,
812 because we never want a whisper of the data_tycon to appear in
813 error messages; it's a purely internal thing
816 tcConArgs :: DataCon -> [TcSigmaType]
817 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
819 tcConArgs data_con arg_tys (PrefixCon arg_pats) penv thing_inside
820 = do { checkTc (con_arity == no_of_args) -- Check correct arity
821 (arityErr "Constructor" data_con con_arity no_of_args)
822 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
823 ; (arg_pats', res) <- tcMultiple tcConArg pats_w_tys
825 ; return (PrefixCon arg_pats', res) }
827 con_arity = dataConSourceArity data_con
828 no_of_args = length arg_pats
830 tcConArgs data_con arg_tys (InfixCon p1 p2) penv thing_inside
831 = do { checkTc (con_arity == 2) -- Check correct arity
832 (arityErr "Constructor" data_con con_arity 2)
833 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
834 ; ([p1',p2'], res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
836 ; return (InfixCon p1' p2', res) }
838 con_arity = dataConSourceArity data_con
840 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) penv thing_inside
841 = do { (rpats', res) <- tcMultiple tc_field rpats penv thing_inside
842 ; return (RecCon (HsRecFields rpats' dd), res) }
844 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
845 tc_field (HsRecField field_lbl pat pun) penv thing_inside
846 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
847 ; (pat', res) <- tcConArg (pat, pat_ty) penv thing_inside
848 ; return (HsRecField sel_id pat' pun, res) }
850 find_field_ty :: FieldLabel -> TcM (Id, TcType)
851 find_field_ty field_lbl
852 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
854 -- No matching field; chances are this field label comes from some
855 -- other record type (or maybe none). As well as reporting an
856 -- error we still want to typecheck the pattern, principally to
857 -- make sure that all the variables it binds are put into the
858 -- environment, else the type checker crashes later:
859 -- f (R { foo = (a,b) }) = a+b
860 -- If foo isn't one of R's fields, we don't want to crash when
861 -- typechecking the "a+b".
862 [] -> do { addErrTc (badFieldCon data_con field_lbl)
863 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
864 ; return (error "Bogus selector Id", bogus_ty) }
866 -- The normal case, when the field comes from the right constructor
868 ASSERT( null extras )
869 do { sel_id <- tcLookupField field_lbl
870 ; return (sel_id, pat_ty) }
872 field_tys :: [(FieldLabel, TcType)]
873 field_tys = zip (dataConFieldLabels data_con) arg_tys
874 -- Don't use zipEqual! If the constructor isn't really a record, then
875 -- dataConFieldLabels will be empty (and each field in the pattern
876 -- will generate an error below).
878 tcConArg :: Checker (LPat Name, TcSigmaType) (LPat Id)
879 tcConArg (arg_pat, arg_ty) penv thing_inside
880 = tc_lpat arg_pat arg_ty penv thing_inside
884 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
885 -- Instantiate the "stupid theta" of the data con, and throw
886 -- the constraints into the constraint set
887 addDataConStupidTheta data_con inst_tys
888 | null stupid_theta = return ()
889 | otherwise = instStupidTheta origin inst_theta
891 origin = OccurrenceOf (dataConName data_con)
892 -- The origin should always report "occurrence of C"
893 -- even when C occurs in a pattern
894 stupid_theta = dataConStupidTheta data_con
895 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
896 -- NB: inst_tys can be longer than the univ tyvars
897 -- because the constructor might have existentials
898 inst_theta = substTheta tenv stupid_theta
901 Note [Arrows and patterns]
902 ~~~~~~~~~~~~~~~~~~~~~~~~~~
903 (Oct 07) Arrow noation has the odd property that it involves
904 "holes in the scope". For example:
905 expr :: Arrow a => a () Int
906 expr = proc (y,z) -> do
910 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
911 arrow body (e.g 'term'). As things stand (bogusly) all the
912 constraints from the proc body are gathered together, so constraints
913 from 'term' will be seen by the tcPat for (y,z). But we must *not*
914 bind constraints from 'term' here, becuase the desugarer will not make
915 these bindings scope over 'term'.
917 The Right Thing is not to confuse these constraints together. But for
918 now the Easy Thing is to ensure that we do not have existential or
919 GADT constraints in a 'proc', and to short-cut the constraint
920 simplification for such vanilla patterns so that it binds no
921 constraints. Hence the 'fast path' in tcConPat; but it's also a good
922 plan for ordinary vanilla patterns to bypass the constraint
925 %************************************************************************
927 Note [Pattern coercions]
929 %************************************************************************
931 In principle, these program would be reasonable:
933 f :: (forall a. a->a) -> Int
934 f (x :: Int->Int) = x 3
936 g :: (forall a. [a]) -> Bool
939 In both cases, the function type signature restricts what arguments can be passed
940 in a call (to polymorphic ones). The pattern type signature then instantiates this
941 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
942 generate the translated term
943 f = \x' :: (forall a. a->a). let x = x' Int in x 3
945 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
946 And it requires a significant amount of code to implement, becuase we need to decorate
947 the translated pattern with coercion functions (generated from the subsumption check
950 So for now I'm just insisting on type *equality* in patterns. No subsumption.
952 Old notes about desugaring, at a time when pattern coercions were handled:
954 A SigPat is a type coercion and must be handled one at at time. We can't
955 combine them unless the type of the pattern inside is identical, and we don't
956 bother to check for that. For example:
958 data T = T1 Int | T2 Bool
959 f :: (forall a. a -> a) -> T -> t
960 f (g::Int->Int) (T1 i) = T1 (g i)
961 f (g::Bool->Bool) (T2 b) = T2 (g b)
963 We desugar this as follows:
965 f = \ g::(forall a. a->a) t::T ->
967 in case t of { T1 i -> T1 (gi i)
970 in case t of { T2 b -> T2 (gb b)
973 Note that we do not treat the first column of patterns as a
974 column of variables, because the coerced variables (gi, gb)
975 would be of different types. So we get rather grotty code.
976 But I don't think this is a common case, and if it was we could
977 doubtless improve it.
979 Meanwhile, the strategy is:
980 * treat each SigPat coercion (always non-identity coercions)
982 * deal with the stuff inside, and then wrap a binding round
983 the result to bind the new variable (gi, gb, etc)
986 %************************************************************************
988 \subsection{Errors and contexts}
990 %************************************************************************
992 {- This was used to improve the error message from
993 an existential escape. Need to think how to do this.
995 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
996 -> TcM (TidyEnv, SDoc)
997 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
998 = do { pat_tys' <- mapM zonkTcType pat_tys
999 ; body_ty' <- zonkTcType body_ty
1000 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1001 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1002 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1004 sep [ptext (sLit "When checking an existential match that binds"),
1005 nest 2 (vcat (zipWith ppr_id show_ids tidy_tys)),
1006 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1007 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
1010 bound_ids = collectPatsBinders pats
1011 show_ids = filter is_interesting bound_ids
1012 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1014 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1015 -- Don't zonk the types so we get the separate, un-unified versions
1019 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
1020 patCtxt (VarPat _) = Nothing
1021 patCtxt (ParPat _) = Nothing
1022 patCtxt (AsPat _ _) = Nothing
1023 patCtxt pat = Just (hang (ptext (sLit "In the pattern:"))
1026 -----------------------------------------------
1027 checkExistentials :: [TyVar] -> PatEnv -> TcM ()
1028 -- See Note [Arrows and patterns]
1029 checkExistentials [] _ = return ()
1030 checkExistentials _ (PE { pe_ctxt = LetPat {}}) = failWithTc existentialLetPat
1031 checkExistentials _ (PE { pe_ctxt = LamPat ProcExpr }) = failWithTc existentialProcPat
1032 checkExistentials _ (PE { pe_lazy = True }) = failWithTc existentialLazyPat
1033 checkExistentials _ _ = return ()
1035 existentialLazyPat :: SDoc
1037 = hang (ptext (sLit "An existential or GADT data constructor cannot be used"))
1038 2 (ptext (sLit "inside a lazy (~) pattern"))
1040 existentialProcPat :: SDoc
1042 = ptext (sLit "Proc patterns cannot use existential or GADT data constructors")
1044 existentialLetPat :: SDoc
1046 = vcat [text "My brain just exploded",
1047 text "I can't handle pattern bindings for existential or GADT data constructors.",
1048 text "Instead, use a case-expression, or do-notation, to unpack the constructor."]
1050 badFieldCon :: DataCon -> Name -> SDoc
1051 badFieldCon con field
1052 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
1053 ptext (sLit "does not have field"), quotes (ppr field)]
1055 polyPatSig :: TcType -> SDoc
1057 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1060 badTypePat :: Pat Name -> SDoc
1061 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1063 lazyUnliftedPatErr :: OutputableBndr name => Pat name -> TcM ()
1064 lazyUnliftedPatErr pat
1066 hang (ptext (sLit "A lazy (~) pattern cannot contain unlifted types:"))
1069 unboxedTupleErr :: SDoc -> Type -> SDoc
1070 unboxedTupleErr what ty
1071 = hang (what <+> ptext (sLit "cannot have an unboxed tuple type:"))