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(..))
46 %************************************************************************
50 %************************************************************************
53 tcLetPat :: TcSigFun -> LetBndrSpec
54 -> LPat Name -> TcSigmaType
57 tcLetPat sig_fn no_gen pat pat_ty thing_inside
58 = tc_lpat pat pat_ty penv thing_inside
60 penv = PE { pe_lazy = True
61 , pe_ctxt = LetPat sig_fn no_gen }
64 tcPats :: HsMatchContext Name
65 -> [LPat Name] -- Patterns,
66 -> [TcSigmaType] -- and their types
67 -> TcM a -- and the checker for the body
68 -> TcM ([LPat TcId], a)
70 -- This is the externally-callable wrapper function
71 -- Typecheck the patterns, extend the environment to bind the variables,
72 -- do the thing inside, use any existentially-bound dictionaries to
73 -- discharge parts of the returning LIE, and deal with pattern type
76 -- 1. Initialise the PatState
77 -- 2. Check the patterns
79 -- 4. Check that no existentials escape
81 tcPats ctxt pats pat_tys thing_inside
82 = tc_lpats penv pats pat_tys thing_inside
84 penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }
86 tcPat :: HsMatchContext Name
87 -> LPat Name -> TcSigmaType
88 -> TcM a -- Checker for body, given
91 tcPat ctxt pat pat_ty thing_inside
92 = tc_lpat pat pat_ty penv thing_inside
94 penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }
99 = PE { pe_lazy :: Bool -- True <=> lazy context, so no existentials allowed
100 , pe_ctxt :: PatCtxt -- Context in which the whole pattern appears
104 = LamPat -- Used for lambdas, case etc
105 (HsMatchContext Name)
107 | LetPat -- Used only for let(rec) bindings
108 -- See Note [Let binders]
109 TcSigFun -- Tells type sig if any
110 LetBndrSpec -- True <=> no generalisation of this let
113 = LetLclBndr -- The binder is just a local one;
114 -- an AbsBinds will provide the global version
116 | LetGblBndr TcPragFun -- There isn't going to be an AbsBinds;
117 -- here is the inline-pragma information
119 makeLazy :: PatEnv -> PatEnv
120 makeLazy penv = penv { pe_lazy = True }
122 patSigCtxt :: PatEnv -> UserTypeCtxt
123 patSigCtxt (PE { pe_ctxt = LetPat {} }) = BindPatSigCtxt
124 patSigCtxt (PE { pe_ctxt = LamPat {} }) = LamPatSigCtxt
127 type TcPragFun = Name -> [LSig Name]
128 type TcSigFun = Name -> Maybe TcSigInfo
132 sig_id :: TcId, -- *Polymorphic* binder for this value...
134 sig_scoped :: [Name], -- Scoped type variables
135 -- 1-1 correspondence with a prefix of sig_tvs
136 -- However, may be fewer than sig_tvs;
137 -- see Note [More instantiated than scoped]
138 sig_tvs :: [TcTyVar], -- Instantiated type variables
139 -- See Note [Instantiate sig]
141 sig_theta :: TcThetaType, -- Instantiated theta
143 sig_tau :: TcSigmaType, -- Instantiated tau
144 -- See Note [sig_tau may be polymorphic]
146 sig_loc :: SrcSpan -- The location of the signature
149 instance Outputable TcSigInfo where
150 ppr (TcSigInfo { sig_id = id, sig_tvs = tyvars, sig_theta = theta, sig_tau = tau})
151 = ppr id <+> ptext (sLit "::") <+> ppr tyvars <+> pprThetaArrowTy theta <+> ppr tau
154 Note [sig_tau may be polymorphic]
155 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
156 Note that "sig_tau" might actually be a polymorphic type,
157 if the original function had a signature like
158 forall a. Eq a => forall b. Ord b => ....
159 But that's ok: tcMatchesFun (called by tcRhs) can deal with that
160 It happens, too! See Note [Polymorphic methods] in TcClassDcl.
168 ...more notes to add here..
171 Note [Existential check]
172 ~~~~~~~~~~~~~~~~~~~~~~~~
173 Lazy patterns can't bind existentials. They arise in two ways:
174 * Let bindings let { C a b = e } in b
175 * Twiddle patterns f ~(C a b) = e
176 The pe_lazy field of PatEnv says whether we are inside a lazy
177 pattern (perhaps deeply)
179 If we aren't inside a lazy pattern then we can bind existentials,
180 but we need to be careful about "extra" tyvars. Consider
181 (\C x -> d) : pat_ty -> res_ty
182 When looking for existential escape we must check that the existential
183 bound by C don't unify with the free variables of pat_ty, OR res_ty
184 (or of course the environment). Hence we need to keep track of the
188 %************************************************************************
192 %************************************************************************
195 tcPatBndr :: PatEnv -> Name -> TcSigmaType -> TcM (Coercion, TcId)
196 -- (coi, xp) = tcPatBndr penv x pat_ty
197 -- Then coi : pat_ty ~ typeof(xp)
199 tcPatBndr (PE { pe_ctxt = LetPat lookup_sig no_gen}) bndr_name pat_ty
200 | Just sig <- lookup_sig bndr_name
201 = do { bndr_id <- newSigLetBndr no_gen bndr_name sig
202 ; coi <- unifyPatType (idType bndr_id) pat_ty
203 ; return (coi, bndr_id) }
206 = do { bndr_id <- newNoSigLetBndr no_gen bndr_name pat_ty
207 ; return (mkReflCo pat_ty, bndr_id) }
209 tcPatBndr (PE { pe_ctxt = _lam_or_proc }) bndr_name pat_ty
210 = do { bndr <- mkLocalBinder bndr_name pat_ty
211 ; return (mkReflCo pat_ty, bndr) }
214 newSigLetBndr :: LetBndrSpec -> Name -> TcSigInfo -> TcM TcId
215 newSigLetBndr LetLclBndr name sig
216 = do { mono_name <- newLocalName name
217 ; mkLocalBinder mono_name (sig_tau sig) }
218 newSigLetBndr (LetGblBndr prags) name sig
219 = addInlinePrags (sig_id sig) (prags name)
222 newNoSigLetBndr :: LetBndrSpec -> Name -> TcType -> TcM TcId
223 -- In the polymorphic case (no_gen = False), generate a "monomorphic version"
224 -- of the Id; the original name will be bound to the polymorphic version
226 -- In the monomorphic case there is no AbsBinds, and we use the original
228 newNoSigLetBndr LetLclBndr name ty
229 =do { mono_name <- newLocalName name
230 ; mkLocalBinder mono_name ty }
231 newNoSigLetBndr (LetGblBndr prags) name ty
232 = do { id <- mkLocalBinder name ty
233 ; addInlinePrags id (prags name) }
236 addInlinePrags :: TcId -> [LSig Name] -> TcM TcId
237 addInlinePrags poly_id prags
240 inl_sigs = filter isInlineLSig prags
241 tc_inl [] = return poly_id
242 tc_inl (L loc (InlineSig _ prag) : other_inls)
243 = do { unless (null other_inls) (setSrcSpan loc warn_dup_inline)
244 ; return (poly_id `setInlinePragma` prag) }
245 tc_inl _ = panic "tc_inl"
247 warn_dup_inline = warnPrags poly_id inl_sigs $
248 ptext (sLit "Duplicate INLINE pragmas for")
250 warnPrags :: Id -> [LSig Name] -> SDoc -> TcM ()
251 warnPrags id bad_sigs herald
252 = addWarnTc (hang (herald <+> quotes (ppr id))
253 2 (ppr_sigs bad_sigs))
255 ppr_sigs sigs = vcat (map (ppr . getLoc) sigs)
258 mkLocalBinder :: Name -> TcType -> TcM TcId
259 mkLocalBinder name ty
260 = do { checkUnboxedTuple ty $
261 ptext (sLit "The variable") <+> quotes (ppr name)
262 ; return (Id.mkLocalId name ty) }
264 checkUnboxedTuple :: TcType -> SDoc -> TcM ()
265 -- Check for an unboxed tuple type
266 -- f = (# True, False #)
267 -- Zonk first just in case it's hidden inside a meta type variable
268 -- (This shows up as a (more obscure) kind error
269 -- in the 'otherwise' case of tcMonoBinds.)
270 checkUnboxedTuple ty what
271 = do { zonked_ty <- zonkTcTypeCarefully ty
272 ; checkTc (not (isUnboxedTupleType zonked_ty))
273 (unboxedTupleErr what zonked_ty) }
276 {- Only needed if we re-add Method constraints
277 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, TcEvBinds)
278 bindInstsOfPatId id thing_inside
279 | not (isOverloadedTy (idType id))
280 = do { res <- thing_inside; return (res, emptyTcEvBinds) }
282 = do { (res, lie) <- captureConstraints thing_inside
283 ; binds <- bindLocalMethods lie [id]
284 ; return (res, binds) }
288 Note [Polymorphism and pattern bindings]
289 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
290 When is_mono holds we are not generalising
291 But the signature can still be polymoprhic!
292 data T = MkT (forall a. a->a)
295 So the no_gen flag decides whether the pattern-bound variables should
296 have exactly the type in the type signature (when not generalising) or
297 the instantiated version (when generalising)
299 %************************************************************************
301 The main worker functions
303 %************************************************************************
307 tcPat takes a "thing inside" over which the pattern scopes. This is partly
308 so that tcPat can extend the environment for the thing_inside, but also
309 so that constraints arising in the thing_inside can be discharged by the
312 This does not work so well for the ErrCtxt carried by the monad: we don't
313 want the error-context for the pattern to scope over the RHS.
314 Hence the getErrCtxt/setErrCtxt stuff in tcMultiple
318 type Checker inp out = forall r.
324 tcMultiple :: Checker inp out -> Checker [inp] [out]
325 tcMultiple tc_pat args penv thing_inside
326 = do { err_ctxt <- getErrCtxt
328 = do { res <- thing_inside
332 = do { (p', (ps', res))
334 setErrCtxt err_ctxt $
336 -- setErrCtxt: restore context before doing the next pattern
337 -- See note [Nesting] above
339 ; return (p':ps', res) }
348 -> TcM (LPat TcId, a)
349 tc_lpat (L span pat) pat_ty penv thing_inside
351 do { (pat', res) <- maybeWrapPatCtxt pat (tc_pat penv pat pat_ty)
353 ; return (L span pat', res) }
356 -> [LPat Name] -> [TcSigmaType]
358 -> TcM ([LPat TcId], a)
359 tc_lpats penv pats tys thing_inside
360 = tcMultiple (\(p,t) -> tc_lpat p t)
361 (zipEqual "tc_lpats" pats tys)
367 -> TcSigmaType -- Fully refined result type
368 -> TcM a -- Thing inside
369 -> TcM (Pat TcId, -- Translated pattern
370 a) -- Result of thing inside
372 tc_pat penv (VarPat name) pat_ty thing_inside
373 = do { (coi, id) <- tcPatBndr penv name pat_ty
374 ; res <- tcExtendIdEnv1 name id thing_inside
375 ; return (mkHsWrapPatCo coi (VarPat id) pat_ty, res) }
377 tc_pat penv (ParPat pat) pat_ty thing_inside
378 = do { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
379 ; return (ParPat pat', res) }
381 tc_pat penv (BangPat pat) pat_ty thing_inside
382 = do { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
383 ; return (BangPat pat', res) }
385 tc_pat penv lpat@(LazyPat pat) pat_ty thing_inside
386 = do { (pat', (res, pat_ct))
387 <- tc_lpat pat pat_ty (makeLazy penv) $
388 captureConstraints thing_inside
389 -- Ignore refined penv', revert to penv
391 ; emitConstraints pat_ct
392 -- captureConstraints/extendConstraints:
393 -- see Note [Hopping the LIE in lazy patterns]
395 -- Check there are no unlifted types under the lazy pattern
396 ; when (any (isUnLiftedType . idType) $ collectPatBinders pat') $
397 lazyUnliftedPatErr lpat
399 -- Check that the expected pattern type is itself lifted
400 ; pat_ty' <- newFlexiTyVarTy liftedTypeKind
401 ; _ <- unifyType pat_ty pat_ty'
403 ; return (LazyPat pat', res) }
405 tc_pat _ p@(QuasiQuotePat _) _ _
406 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
408 tc_pat _ (WildPat _) pat_ty thing_inside
409 = do { checkUnboxedTuple pat_ty $
410 ptext (sLit "A wild-card pattern")
411 ; res <- thing_inside
412 ; return (WildPat pat_ty, res) }
414 tc_pat penv (AsPat (L nm_loc name) pat) pat_ty thing_inside
415 = do { (coi, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
416 ; (pat', res) <- tcExtendIdEnv1 name bndr_id $
417 tc_lpat pat (idType bndr_id) penv thing_inside
418 -- NB: if we do inference on:
419 -- \ (y@(x::forall a. a->a)) = e
420 -- we'll fail. The as-pattern infers a monotype for 'y', which then
421 -- fails to unify with the polymorphic type for 'x'. This could
422 -- perhaps be fixed, but only with a bit more work.
424 -- If you fix it, don't forget the bindInstsOfPatIds!
425 ; return (mkHsWrapPatCo coi (AsPat (L nm_loc bndr_id) pat') pat_ty, res) }
427 tc_pat penv vpat@(ViewPat expr pat _) overall_pat_ty thing_inside
428 = do { checkUnboxedTuple overall_pat_ty $
429 ptext (sLit "The view pattern") <+> ppr vpat
431 -- Morally, expr must have type `forall a1...aN. OPT' -> B`
432 -- where overall_pat_ty is an instance of OPT'.
433 -- Here, we infer a rho type for it,
434 -- which replaces the leading foralls and constraints
435 -- with fresh unification variables.
436 ; (expr',expr'_inferred) <- tcInferRho expr
438 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
439 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
440 -- variable to find it). this means that in an example like
441 -- (view -> f) where view :: _ -> forall b. b
442 -- we will only be able to use view at one instantation in the
444 ; (expr_coi, pat_ty) <- tcInfer $ \ pat_ty ->
445 unifyPatType expr'_inferred (mkFunTy overall_pat_ty pat_ty)
447 -- pattern must have pat_ty
448 ; (pat', res) <- tc_lpat pat pat_ty penv thing_inside
450 ; return (ViewPat (mkLHsWrapCo expr_coi expr') pat' overall_pat_ty, res) }
452 -- Type signatures in patterns
453 -- See Note [Pattern coercions] below
454 tc_pat penv (SigPatIn pat sig_ty) pat_ty thing_inside
455 = do { (inner_ty, tv_binds, wrap) <- tcPatSig (patSigCtxt penv) sig_ty pat_ty
456 ; (pat', res) <- tcExtendTyVarEnv2 tv_binds $
457 tc_lpat pat inner_ty penv thing_inside
459 ; return (mkHsWrapPat wrap (SigPatOut pat' inner_ty) pat_ty, res) }
461 tc_pat _ pat@(TypePat _) _ _
462 = failWithTc (badTypePat pat)
464 ------------------------
465 -- Lists, tuples, arrays
466 tc_pat penv (ListPat pats _) pat_ty thing_inside
467 = do { (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTy pat_ty
468 ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
469 pats penv thing_inside
470 ; return (mkHsWrapPat coi (ListPat pats' elt_ty) pat_ty, res)
473 tc_pat penv (PArrPat pats _) pat_ty thing_inside
474 = do { (coi, elt_ty) <- matchExpectedPatTy matchExpectedPArrTy pat_ty
475 ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
476 pats penv thing_inside
477 ; return (mkHsWrapPat coi (PArrPat pats' elt_ty) pat_ty, res)
480 tc_pat penv (TuplePat pats boxity _) pat_ty thing_inside
481 = do { let tc = tupleTyCon boxity (length pats)
482 ; (coi, arg_tys) <- matchExpectedPatTy (matchExpectedTyConApp tc) pat_ty
483 ; (pats', res) <- tc_lpats penv pats arg_tys thing_inside
485 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
486 -- so that we can experiment with lazy tuple-matching.
487 -- This is a pretty odd place to make the switch, but
488 -- it was easy to do.
489 ; let pat_ty' = mkTyConApp tc arg_tys
490 -- pat_ty /= pat_ty iff coi /= IdCo
491 unmangled_result = TuplePat pats' boxity pat_ty'
492 possibly_mangled_result
493 | opt_IrrefutableTuples &&
494 isBoxed boxity = LazyPat (noLoc unmangled_result)
495 | otherwise = unmangled_result
497 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
498 return (mkHsWrapPat coi possibly_mangled_result pat_ty, res)
501 ------------------------
503 tc_pat penv (ConPatIn con arg_pats) pat_ty thing_inside
504 = tcConPat penv con pat_ty arg_pats thing_inside
506 ------------------------
508 tc_pat _ (LitPat simple_lit) pat_ty thing_inside
509 = do { let lit_ty = hsLitType simple_lit
510 ; coi <- unifyPatType lit_ty pat_ty
511 -- coi is of kind: pat_ty ~ lit_ty
512 ; res <- thing_inside
513 ; return ( mkHsWrapPatCo coi (LitPat simple_lit) pat_ty
516 ------------------------
517 -- Overloaded patterns: n, and n+k
518 tc_pat _ (NPat over_lit mb_neg eq) pat_ty thing_inside
519 = do { let orig = LiteralOrigin over_lit
520 ; lit' <- newOverloadedLit orig over_lit pat_ty
521 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
522 ; mb_neg' <- case mb_neg of
523 Nothing -> return Nothing -- Positive literal
524 Just neg -> -- Negative literal
525 -- The 'negate' is re-mappable syntax
526 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
527 ; return (Just neg') }
528 ; res <- thing_inside
529 ; return (NPat lit' mb_neg' eq', res) }
531 tc_pat penv (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
532 = do { (coi, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
533 ; let pat_ty' = idType bndr_id
534 orig = LiteralOrigin lit
535 ; lit' <- newOverloadedLit orig lit pat_ty'
537 -- The '>=' and '-' parts are re-mappable syntax
538 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
539 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
540 ; let pat' = NPlusKPat (L nm_loc bndr_id) lit' ge' minus'
542 -- The Report says that n+k patterns must be in Integral
543 -- We may not want this when using re-mappable syntax, though (ToDo?)
544 ; icls <- tcLookupClass integralClassName
545 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
547 ; res <- tcExtendIdEnv1 name bndr_id thing_inside
548 ; return (mkHsWrapPatCo coi pat' pat_ty, res) }
550 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut
553 unifyPatType :: TcType -> TcType -> TcM Coercion
554 -- In patterns we want a coercion from the
555 -- context type (expected) to the actual pattern type
556 -- But we don't want to reverse the args to unifyType because
557 -- that controls the actual/expected stuff in error messages
558 unifyPatType actual_ty expected_ty
559 = do { coi <- unifyType actual_ty expected_ty
560 ; return (mkSymCo coi) }
563 Note [Hopping the LIE in lazy patterns]
564 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
565 In a lazy pattern, we must *not* discharge constraints from the RHS
566 from dictionaries bound in the pattern. E.g.
568 We can't discharge the Num constraint from dictionaries bound by
571 So we have to make the constraints from thing_inside "hop around"
572 the pattern. Hence the captureConstraints and emitConstraints.
574 The same thing ensures that equality constraints in a lazy match
575 are not made available in the RHS of the match. For example
576 data T a where { T1 :: Int -> T Int; ... }
579 It's obviously not sound to refine a to Int in the right
580 hand side, because the arugment might not match T1 at all!
582 Finally, a lazy pattern should not bind any existential type variables
583 because they won't be in scope when we do the desugaring
586 %************************************************************************
588 Most of the work for constructors is here
589 (the rest is in the ConPatIn case of tc_pat)
591 %************************************************************************
593 [Pattern matching indexed data types]
594 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
595 Consider the following declarations:
597 data family Map k :: * -> *
598 data instance Map (a, b) v = MapPair (Map a (Pair b v))
600 and a case expression
602 case x :: Map (Int, c) w of MapPair m -> ...
604 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
605 worker/wrapper types for MapPair are
607 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
608 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
610 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
611 :R123Map, which means the straight use of boxySplitTyConApp would give a type
612 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
613 boxySplitTyConApp with the family tycon Map instead, which gives us the family
614 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
615 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
616 (provided by tyConFamInst_maybe together with the family tycon). This
617 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
618 the split arguments for the representation tycon :R123Map as {Int, c, w}
620 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
622 Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
624 moving between representation and family type into account. To produce type
625 correct Core, this coercion needs to be used to case the type of the scrutinee
626 from the family to the representation type. This is achieved by
627 unwrapFamInstScrutinee using a CoPat around the result pattern.
629 Now it might appear seem as if we could have used the previous GADT type
630 refinement infrastructure of refineAlt and friends instead of the explicit
631 unification and CoPat generation. However, that would be wrong. Why? The
632 whole point of GADT refinement is that the refinement is local to the case
633 alternative. In contrast, the substitution generated by the unification of
634 the family type list and instance types needs to be propagated to the outside.
635 Imagine that in the above example, the type of the scrutinee would have been
636 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
637 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
638 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
639 alternatives of the case expression, whereas in the GADT case it might vary
640 between alternatives.
642 RIP GADT refinement: refinements have been replaced by the use of explicit
643 equality constraints that are used in conjunction with implication constraints
644 to express the local scope of GADT refinements.
648 -- MkT :: forall a b c. (a~[b]) => b -> c -> T a
649 -- with scrutinee of type (T ty)
651 tcConPat :: PatEnv -> Located Name
652 -> TcRhoType -- Type of the pattern
653 -> HsConPatDetails Name -> TcM a
655 tcConPat penv (L con_span con_name) pat_ty arg_pats thing_inside
656 = do { data_con <- tcLookupDataCon con_name
657 ; let tycon = dataConTyCon data_con
658 -- For data families this is the representation tycon
659 (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _)
660 = dataConFullSig data_con
662 -- Instantiate the constructor type variables [a->ty]
663 -- This may involve doing a family-instance coercion,
664 -- and building a wrapper
665 ; (wrap, ctxt_res_tys) <- matchExpectedPatTy (matchExpectedConTy tycon) pat_ty
667 -- Add the stupid theta
668 ; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys
670 ; checkExistentials ex_tvs penv
671 ; ex_tvs' <- tcInstSuperSkolTyVars ex_tvs
672 -- Get location from monad, not from ex_tvs
674 ; let pat_ty' = mkTyConApp tycon ctxt_res_tys
675 -- pat_ty' is type of the actual constructor application
676 -- pat_ty' /= pat_ty iff coi /= IdCo
678 tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
679 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
680 arg_tys' = substTys tenv arg_tys
682 ; if null ex_tvs && null eq_spec && null theta
683 then do { -- The common case; no class bindings etc
684 -- (see Note [Arrows and patterns])
685 (arg_pats', res) <- tcConArgs data_con arg_tys'
686 arg_pats penv thing_inside
687 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
688 pat_tvs = [], pat_dicts = [],
689 pat_binds = emptyTcEvBinds,
690 pat_args = arg_pats',
693 ; return (mkHsWrapPat wrap res_pat pat_ty, res) }
695 else do -- The general case, with existential,
696 -- and local equality constraints
697 { let theta' = substTheta tenv (eqSpecPreds eq_spec ++ theta)
698 -- order is *important* as we generate the list of
699 -- dictionary binders from theta'
700 no_equalities = not (any isEqPred theta')
701 skol_info = case pe_ctxt penv of
702 LamPat mc -> PatSkol data_con mc
703 LetPat {} -> UnkSkol -- Doesn't matter
705 ; gadts_on <- xoptM Opt_GADTs
706 ; checkTc (no_equalities || gadts_on)
707 (ptext (sLit "A pattern match on a GADT requires -XGADTs"))
708 -- Trac #2905 decided that a *pattern-match* of a GADT
709 -- should require the GADT language flag
711 ; given <- newEvVars theta'
712 ; (ev_binds, (arg_pats', res))
713 <- checkConstraints skol_info ex_tvs' given $
714 tcConArgs data_con arg_tys' arg_pats penv thing_inside
716 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
719 pat_binds = ev_binds,
720 pat_args = arg_pats',
722 ; return (mkHsWrapPat wrap res_pat pat_ty, res)
725 ----------------------------
726 matchExpectedPatTy :: (TcRhoType -> TcM (Coercion, a))
727 -> TcRhoType -> TcM (HsWrapper, a)
728 -- See Note [Matching polytyped patterns]
729 -- Returns a wrapper : pat_ty ~ inner_ty
730 matchExpectedPatTy inner_match pat_ty
731 | null tvs && null theta
732 = do { (coi, res) <- inner_match pat_ty
733 ; return (coToHsWrapper (mkSymCo coi), res) }
734 -- The Sym is because the inner_match returns a coercion
735 -- that is the other way round to matchExpectedPatTy
738 = do { (_, tys, subst) <- tcInstTyVars tvs
739 ; wrap1 <- instCall PatOrigin tys (substTheta subst theta)
740 ; (wrap2, arg_tys) <- matchExpectedPatTy inner_match (TcType.substTy subst tau)
741 ; return (wrap2 <.> wrap1 , arg_tys) }
743 (tvs, theta, tau) = tcSplitSigmaTy pat_ty
745 ----------------------------
746 matchExpectedConTy :: TyCon -- The TyCon that this data
747 -- constructor actually returns
748 -> TcRhoType -- The type of the pattern
749 -> TcM (Coercion, [TcSigmaType])
750 -- See Note [Matching constructor patterns]
751 -- Returns a coercion : T ty1 ... tyn ~ pat_ty
752 -- This is the same way round as matchExpectedListTy etc
753 -- but the other way round to matchExpectedPatTy
754 matchExpectedConTy data_tc pat_ty
755 | Just (fam_tc, fam_args, co_tc) <- tyConFamInstSig_maybe data_tc
756 -- Comments refer to Note [Matching constructor patterns]
757 -- co_tc :: forall a. T [a] ~ T7 a
758 = do { (_, tys, subst) <- tcInstTyVars (tyConTyVars data_tc)
761 ; coi1 <- unifyType (mkTyConApp fam_tc (substTys subst fam_args)) pat_ty
762 -- coi1 : T (ty1,ty2) ~ pat_ty
764 ; let coi2 = mkAxInstCo co_tc tys
765 -- coi2 : T (ty1,ty2) ~ T7 ty1 ty2
767 ; return (mkTransCo (mkSymCo coi2) coi1, tys) }
770 = matchExpectedTyConApp data_tc pat_ty
771 -- coi : T tys ~ pat_ty
774 Note [Matching constructor patterns]
775 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
776 Suppose (coi, tys) = matchExpectedConType data_tc pat_ty
778 * In the simple case, pat_ty = tc tys
780 * If pat_ty is a polytype, we want to instantiate it
781 This is like part of a subsumption check. Eg
782 f :: (forall a. [a]) -> blah
785 * In a type family case, suppose we have
787 data instance T (p,q) = A p | B q
788 Then we'll have internally generated
789 data T7 p q = A p | B q
790 axiom coT7 p q :: T (p,q) ~ T7 p q
792 So if pat_ty = T (ty1,ty2), we return (coi, [ty1,ty2]) such that
793 coi = coi2 . coi1 : T7 t ~ pat_ty
794 coi1 : T (ty1,ty2) ~ pat_ty
795 coi2 : T7 ty1 ty2 ~ T (ty1,ty2)
797 For families we do all this matching here, not in the unifier,
798 because we never want a whisper of the data_tycon to appear in
799 error messages; it's a purely internal thing
802 tcConArgs :: DataCon -> [TcSigmaType]
803 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
805 tcConArgs data_con arg_tys (PrefixCon arg_pats) penv thing_inside
806 = do { checkTc (con_arity == no_of_args) -- Check correct arity
807 (arityErr "Constructor" data_con con_arity no_of_args)
808 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
809 ; (arg_pats', res) <- tcMultiple tcConArg pats_w_tys
811 ; return (PrefixCon arg_pats', res) }
813 con_arity = dataConSourceArity data_con
814 no_of_args = length arg_pats
816 tcConArgs data_con arg_tys (InfixCon p1 p2) penv thing_inside
817 = do { checkTc (con_arity == 2) -- Check correct arity
818 (arityErr "Constructor" data_con con_arity 2)
819 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
820 ; ([p1',p2'], res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
822 ; return (InfixCon p1' p2', res) }
824 con_arity = dataConSourceArity data_con
826 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) penv thing_inside
827 = do { (rpats', res) <- tcMultiple tc_field rpats penv thing_inside
828 ; return (RecCon (HsRecFields rpats' dd), res) }
830 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
831 tc_field (HsRecField field_lbl pat pun) penv thing_inside
832 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
833 ; (pat', res) <- tcConArg (pat, pat_ty) penv thing_inside
834 ; return (HsRecField sel_id pat' pun, res) }
836 find_field_ty :: FieldLabel -> TcM (Id, TcType)
837 find_field_ty field_lbl
838 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
840 -- No matching field; chances are this field label comes from some
841 -- other record type (or maybe none). As well as reporting an
842 -- error we still want to typecheck the pattern, principally to
843 -- make sure that all the variables it binds are put into the
844 -- environment, else the type checker crashes later:
845 -- f (R { foo = (a,b) }) = a+b
846 -- If foo isn't one of R's fields, we don't want to crash when
847 -- typechecking the "a+b".
848 [] -> do { addErrTc (badFieldCon data_con field_lbl)
849 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
850 ; return (error "Bogus selector Id", bogus_ty) }
852 -- The normal case, when the field comes from the right constructor
854 ASSERT( null extras )
855 do { sel_id <- tcLookupField field_lbl
856 ; return (sel_id, pat_ty) }
858 field_tys :: [(FieldLabel, TcType)]
859 field_tys = zip (dataConFieldLabels data_con) arg_tys
860 -- Don't use zipEqual! If the constructor isn't really a record, then
861 -- dataConFieldLabels will be empty (and each field in the pattern
862 -- will generate an error below).
864 tcConArg :: Checker (LPat Name, TcSigmaType) (LPat Id)
865 tcConArg (arg_pat, arg_ty) penv thing_inside
866 = tc_lpat arg_pat arg_ty penv thing_inside
870 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
871 -- Instantiate the "stupid theta" of the data con, and throw
872 -- the constraints into the constraint set
873 addDataConStupidTheta data_con inst_tys
874 | null stupid_theta = return ()
875 | otherwise = instStupidTheta origin inst_theta
877 origin = OccurrenceOf (dataConName data_con)
878 -- The origin should always report "occurrence of C"
879 -- even when C occurs in a pattern
880 stupid_theta = dataConStupidTheta data_con
881 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
882 -- NB: inst_tys can be longer than the univ tyvars
883 -- because the constructor might have existentials
884 inst_theta = substTheta tenv stupid_theta
887 Note [Arrows and patterns]
888 ~~~~~~~~~~~~~~~~~~~~~~~~~~
889 (Oct 07) Arrow noation has the odd property that it involves
890 "holes in the scope". For example:
891 expr :: Arrow a => a () Int
892 expr = proc (y,z) -> do
896 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
897 arrow body (e.g 'term'). As things stand (bogusly) all the
898 constraints from the proc body are gathered together, so constraints
899 from 'term' will be seen by the tcPat for (y,z). But we must *not*
900 bind constraints from 'term' here, becuase the desugarer will not make
901 these bindings scope over 'term'.
903 The Right Thing is not to confuse these constraints together. But for
904 now the Easy Thing is to ensure that we do not have existential or
905 GADT constraints in a 'proc', and to short-cut the constraint
906 simplification for such vanilla patterns so that it binds no
907 constraints. Hence the 'fast path' in tcConPat; but it's also a good
908 plan for ordinary vanilla patterns to bypass the constraint
911 %************************************************************************
913 Note [Pattern coercions]
915 %************************************************************************
917 In principle, these program would be reasonable:
919 f :: (forall a. a->a) -> Int
920 f (x :: Int->Int) = x 3
922 g :: (forall a. [a]) -> Bool
925 In both cases, the function type signature restricts what arguments can be passed
926 in a call (to polymorphic ones). The pattern type signature then instantiates this
927 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
928 generate the translated term
929 f = \x' :: (forall a. a->a). let x = x' Int in x 3
931 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
932 And it requires a significant amount of code to implement, becuase we need to decorate
933 the translated pattern with coercion functions (generated from the subsumption check
936 So for now I'm just insisting on type *equality* in patterns. No subsumption.
938 Old notes about desugaring, at a time when pattern coercions were handled:
940 A SigPat is a type coercion and must be handled one at at time. We can't
941 combine them unless the type of the pattern inside is identical, and we don't
942 bother to check for that. For example:
944 data T = T1 Int | T2 Bool
945 f :: (forall a. a -> a) -> T -> t
946 f (g::Int->Int) (T1 i) = T1 (g i)
947 f (g::Bool->Bool) (T2 b) = T2 (g b)
949 We desugar this as follows:
951 f = \ g::(forall a. a->a) t::T ->
953 in case t of { T1 i -> T1 (gi i)
956 in case t of { T2 b -> T2 (gb b)
959 Note that we do not treat the first column of patterns as a
960 column of variables, because the coerced variables (gi, gb)
961 would be of different types. So we get rather grotty code.
962 But I don't think this is a common case, and if it was we could
963 doubtless improve it.
965 Meanwhile, the strategy is:
966 * treat each SigPat coercion (always non-identity coercions)
968 * deal with the stuff inside, and then wrap a binding round
969 the result to bind the new variable (gi, gb, etc)
972 %************************************************************************
974 \subsection{Errors and contexts}
976 %************************************************************************
978 {- This was used to improve the error message from
979 an existential escape. Need to think how to do this.
981 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
982 -> TcM (TidyEnv, SDoc)
983 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
984 = do { pat_tys' <- mapM zonkTcType pat_tys
985 ; body_ty' <- zonkTcType body_ty
986 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
987 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
988 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
990 sep [ptext (sLit "When checking an existential match that binds"),
991 nest 2 (vcat (zipWith ppr_id show_ids tidy_tys)),
992 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
993 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
996 bound_ids = collectPatsBinders pats
997 show_ids = filter is_interesting bound_ids
998 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1000 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1001 -- Don't zonk the types so we get the separate, un-unified versions
1005 maybeWrapPatCtxt :: Pat Name -> (TcM a -> TcM b) -> TcM a -> TcM b
1006 -- Not all patterns are worth pushing a context
1007 maybeWrapPatCtxt pat tcm thing_inside
1008 | not (worth_wrapping pat) = tcm thing_inside
1009 | otherwise = addErrCtxt msg $ tcm $ popErrCtxt thing_inside
1010 -- Remember to pop before doing thing_inside
1012 worth_wrapping (VarPat {}) = False
1013 worth_wrapping (ParPat {}) = False
1014 worth_wrapping (AsPat {}) = False
1015 worth_wrapping _ = True
1016 msg = hang (ptext (sLit "In the pattern:")) 2 (ppr pat)
1018 -----------------------------------------------
1019 checkExistentials :: [TyVar] -> PatEnv -> TcM ()
1020 -- See Note [Arrows and patterns]
1021 checkExistentials [] _ = return ()
1022 checkExistentials _ (PE { pe_ctxt = LetPat {}}) = failWithTc existentialLetPat
1023 checkExistentials _ (PE { pe_ctxt = LamPat ProcExpr }) = failWithTc existentialProcPat
1024 checkExistentials _ (PE { pe_lazy = True }) = failWithTc existentialLazyPat
1025 checkExistentials _ _ = return ()
1027 existentialLazyPat :: SDoc
1029 = hang (ptext (sLit "An existential or GADT data constructor cannot be used"))
1030 2 (ptext (sLit "inside a lazy (~) pattern"))
1032 existentialProcPat :: SDoc
1034 = ptext (sLit "Proc patterns cannot use existential or GADT data constructors")
1036 existentialLetPat :: SDoc
1038 = vcat [text "My brain just exploded",
1039 text "I can't handle pattern bindings for existential or GADT data constructors.",
1040 text "Instead, use a case-expression, or do-notation, to unpack the constructor."]
1042 badFieldCon :: DataCon -> Name -> SDoc
1043 badFieldCon con field
1044 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
1045 ptext (sLit "does not have field"), quotes (ppr field)]
1047 polyPatSig :: TcType -> SDoc
1049 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1052 badTypePat :: Pat Name -> SDoc
1053 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1055 lazyUnliftedPatErr :: OutputableBndr name => Pat name -> TcM ()
1056 lazyUnliftedPatErr pat
1058 hang (ptext (sLit "A lazy (~) pattern cannot contain unlifted types:"))
1061 unboxedTupleErr :: SDoc -> Type -> SDoc
1062 unboxedTupleErr what ty
1063 = hang (what <+> ptext (sLit "cannot have an unboxed tuple type:"))