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, tcPat, tcPats, tcOverloadedLit,
10 addDataConStupidTheta, badFieldCon, polyPatSig ) where
12 #include "HsVersions.h"
14 import {-# SOURCE #-} TcExpr( tcSyntaxOp, tcInferRho)
38 import BasicTypes hiding (SuccessFlag(..))
39 import DynFlags ( DynFlag( Opt_GADTs ) )
49 %************************************************************************
53 %************************************************************************
56 tcLetPat :: (Name -> Maybe TcRhoType)
57 -> LPat Name -> BoxySigmaType
60 tcLetPat sig_fn pat pat_ty thing_inside
61 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
63 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state
66 -- Don't know how to deal with pattern-bound existentials yet
67 ; checkTc (null ex_tvs) (existentialExplode pat)
69 ; return (pat', res) }
72 tcPats :: HsMatchContext Name
73 -> [LPat Name] -- Patterns,
74 -> [BoxySigmaType] -- and their types
75 -> BoxyRhoType -- Result type,
76 -> (BoxyRhoType -> TcM a) -- and the checker for the body
77 -> TcM ([LPat TcId], a)
79 -- This is the externally-callable wrapper function
80 -- Typecheck the patterns, extend the environment to bind the variables,
81 -- do the thing inside, use any existentially-bound dictionaries to
82 -- discharge parts of the returning LIE, and deal with pattern type
85 -- 1. Initialise the PatState
86 -- 2. Check the patterns
88 -- 4. Check that no existentials escape
90 tcPats ctxt pats tys res_ty thing_inside
91 = tc_lam_pats (APat ctxt) (zipEqual "tcLamPats" pats tys)
94 tcPat :: HsMatchContext Name
95 -> LPat Name -> BoxySigmaType
96 -> BoxyRhoType -- Result type
97 -> (BoxyRhoType -> TcM a) -- Checker for body, given
100 tcPat ctxt = tc_lam_pat (APat ctxt)
102 tc_lam_pat :: PatCtxt -> LPat Name -> BoxySigmaType -> BoxyRhoType
103 -> (BoxyRhoType -> TcM a) -> TcM (LPat TcId, a)
104 tc_lam_pat ctxt pat pat_ty res_ty thing_inside
105 = do { ([pat'],thing) <- tc_lam_pats ctxt [(pat, pat_ty)] res_ty thing_inside
106 ; return (pat', thing) }
109 tc_lam_pats :: PatCtxt
110 -> [(LPat Name,BoxySigmaType)]
111 -> BoxyRhoType -- Result type
112 -> (BoxyRhoType -> TcM a) -- Checker for body, given its result type
113 -> TcM ([LPat TcId], a)
114 tc_lam_pats ctxt pat_ty_prs res_ty thing_inside
115 = do { let init_state = PS { pat_ctxt = ctxt, pat_eqs = False }
117 ; (pats', ex_tvs, res) <- do { traceTc (text "tc_lam_pats" <+> (ppr pat_ty_prs $$ ppr res_ty))
118 ; tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
119 if (pat_eqs pstate' && (not $ isRigidTy res_ty))
120 then nonRigidResult ctxt res_ty
121 else thing_inside res_ty }
123 ; let tys = map snd pat_ty_prs
124 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
126 ; return (pats', res) }
130 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
131 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
132 -> [BoxySigmaType] -- Types of the patterns
133 -> BoxyRhoType -- Type of the body of the match
134 -- Tyvars in either of these must not escape
136 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
137 -- For example, we must reject this program:
138 -- data C = forall a. C (a -> Int)
140 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
142 tcCheckExistentialPat _ [] _ _
143 = return () -- Short cut for case when there are no existentials
145 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
146 = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
147 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
151 pat_eqs :: Bool -- <=> there are any equational constraints
152 -- Used at the end to say whether the result
153 -- type must be rigid
157 = APat (HsMatchContext Name)
158 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
160 notProcPat :: PatCtxt -> Bool
161 notProcPat (APat ProcExpr) = False
164 patSigCtxt :: PatState -> UserTypeCtxt
165 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
166 patSigCtxt _ = LamPatSigCtxt
171 %************************************************************************
175 %************************************************************************
178 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
179 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
180 | Just mono_ty <- lookup_sig bndr_name
181 = do { mono_name <- newLocalName bndr_name
182 ; _ <- boxyUnify mono_ty pat_ty
183 ; return (Id.mkLocalId mono_name mono_ty) }
186 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
187 ; mono_name <- newLocalName bndr_name
188 ; return (Id.mkLocalId mono_name pat_ty') }
190 tcPatBndr (PS { pat_ctxt = _lam_or_proc }) bndr_name pat_ty
191 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
192 -- We have an undecorated binder, so we do rule ABS1,
193 -- by unboxing the boxy type, forcing any un-filled-in
194 -- boxes to become monotypes
195 -- NB that pat_ty' can still be a polytype:
196 -- data T = MkT (forall a. a->a)
197 -- f t = case t of { MkT g -> ... }
198 -- Here, the 'g' must get type (forall a. a->a) from the
200 ; return (Id.mkLocalId bndr_name pat_ty') }
204 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
205 bindInstsOfPatId id thing_inside
206 | not (isOverloadedTy (idType id))
207 = do { res <- thing_inside; return (res, emptyLHsBinds) }
209 = do { (res, lie) <- getLIE thing_inside
210 ; binds <- bindInstsOfLocalFuns lie [id]
211 ; return (res, binds) }
214 unBoxPatBndrType :: BoxyType -> Name -> TcM TcType
215 unBoxPatBndrType ty name = unBoxArgType ty (ptext (sLit "The variable") <+> quotes (ppr name))
217 unBoxWildCardType :: BoxyType -> TcM TcType
218 unBoxWildCardType ty = unBoxArgType ty (ptext (sLit "A wild-card pattern"))
220 unBoxViewPatType :: BoxyType -> Pat Name -> TcM TcType
221 unBoxViewPatType ty pat = unBoxArgType ty (ptext (sLit "The view pattern") <+> ppr pat)
223 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
224 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
225 -- that is, it can't be an unboxed tuple. For example,
226 -- case (f x) of r -> ...
227 -- should fail if 'f' returns an unboxed tuple.
228 unBoxArgType ty pp_this
229 = do { ty' <- unBox ty -- Returns a zonked type
231 -- Neither conditional is strictly necesssary (the unify alone will do)
232 -- but they improve error messages, and allocate fewer tyvars
233 ; if isUnboxedTupleType ty' then
235 else if isSubArgTypeKind (typeKind ty') then
237 else do -- OpenTypeKind, so constrain it
238 { ty2 <- newFlexiTyVarTy argTypeKind
239 ; _ <- unifyType ty' ty2
242 msg = pp_this <+> ptext (sLit "cannot be bound to an unboxed tuple")
246 %************************************************************************
248 The main worker functions
250 %************************************************************************
254 tcPat takes a "thing inside" over which the pattern scopes. This is partly
255 so that tcPat can extend the environment for the thing_inside, but also
256 so that constraints arising in the thing_inside can be discharged by the
259 This does not work so well for the ErrCtxt carried by the monad: we don't
260 want the error-context for the pattern to scope over the RHS.
261 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
265 type Checker inp out = forall r.
268 -> (PatState -> TcM r)
269 -> TcM (out, [TcTyVar], r)
271 tcMultiple :: Checker inp out -> Checker [inp] [out]
272 tcMultiple tc_pat args pstate thing_inside
273 = do { err_ctxt <- getErrCtxt
275 = do { res <- thing_inside pstate
276 ; return ([], [], res) }
278 loop pstate (arg:args)
279 = do { (p', p_tvs, (ps', ps_tvs, res))
280 <- tc_pat arg pstate $ \ pstate' ->
281 setErrCtxt err_ctxt $
283 -- setErrCtxt: restore context before doing the next pattern
284 -- See note [Nesting] above
286 ; return (p':ps', p_tvs ++ ps_tvs, res) }
291 tc_lpat_pr :: (LPat Name, BoxySigmaType)
293 -> (PatState -> TcM a)
294 -> TcM (LPat TcId, [TcTyVar], a)
295 tc_lpat_pr (pat, ty) = tc_lpat pat ty
300 -> (PatState -> TcM a)
301 -> TcM (LPat TcId, [TcTyVar], a)
302 tc_lpat (L span pat) pat_ty pstate thing_inside
304 maybeAddErrCtxt (patCtxt pat) $
305 do { (pat', tvs, res) <- tc_pat pstate pat pat_ty thing_inside
306 ; return (L span pat', tvs, res) }
311 -> BoxySigmaType -- Fully refined result type
312 -> (PatState -> TcM a) -- Thing inside
313 -> TcM (Pat TcId, -- Translated pattern
314 [TcTyVar], -- Existential binders
315 a) -- Result of thing inside
317 tc_pat pstate (VarPat name) pat_ty thing_inside
318 = do { id <- tcPatBndr pstate name pat_ty
319 ; (res, binds) <- bindInstsOfPatId id $
320 tcExtendIdEnv1 name id $
321 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
322 >> thing_inside pstate)
323 ; let pat' | isEmptyLHsBinds binds = VarPat id
324 | otherwise = VarPatOut id binds
325 ; return (pat', [], res) }
327 tc_pat pstate (ParPat pat) pat_ty thing_inside
328 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
329 ; return (ParPat pat', tvs, res) }
331 tc_pat pstate (BangPat pat) pat_ty thing_inside
332 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
333 ; return (BangPat pat', tvs, res) }
335 -- There's a wrinkle with irrefutable patterns, namely that we
336 -- must not propagate type refinement from them. For example
337 -- data T a where { T1 :: Int -> T Int; ... }
338 -- f :: T a -> Int -> a
340 -- It's obviously not sound to refine a to Int in the right
341 -- hand side, because the arugment might not match T1 at all!
343 -- Nor should a lazy pattern bind any existential type variables
344 -- because they won't be in scope when we do the desugaring
346 -- Note [Hopping the LIE in lazy patterns]
347 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
348 -- In a lazy pattern, we must *not* discharge constraints from the RHS
349 -- from dictionaries bound in the pattern. E.g.
351 -- We can't discharge the Num constraint from dictionaries bound by
354 -- So we have to make the constraints from thing_inside "hop around"
355 -- the pattern. Hence the getLLE and extendLIEs later.
357 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
358 = do { (pat', pat_tvs, (res,lie))
359 <- tc_lpat pat pat_ty pstate $ \ _ ->
360 getLIE (thing_inside pstate)
361 -- Ignore refined pstate', revert to pstate
363 -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
365 -- Check no existentials
366 ; unless (null pat_tvs) $ lazyPatErr lpat pat_tvs
368 -- Check there are no unlifted types under the lazy pattern
369 ; when (any (isUnLiftedType . idType) $ collectPatBinders pat') $
370 lazyUnliftedPatErr lpat
372 -- Check that the pattern has a lifted type
373 ; pat_tv <- newBoxyTyVar liftedTypeKind
374 ; _ <- boxyUnify pat_ty (mkTyVarTy pat_tv)
376 ; return (LazyPat pat', [], res) }
378 tc_pat _ p@(QuasiQuotePat _) _ _
379 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
381 tc_pat pstate (WildPat _) pat_ty thing_inside
382 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
383 ; res <- thing_inside pstate
384 ; return (WildPat pat_ty', [], res) }
386 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
387 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
388 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
389 tc_lpat pat (idType bndr_id) pstate thing_inside
390 -- NB: if we do inference on:
391 -- \ (y@(x::forall a. a->a)) = e
392 -- we'll fail. The as-pattern infers a monotype for 'y', which then
393 -- fails to unify with the polymorphic type for 'x'. This could
394 -- perhaps be fixed, but only with a bit more work.
396 -- If you fix it, don't forget the bindInstsOfPatIds!
397 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
399 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
400 = do { -- morally, expr must have type
401 -- `forall a1...aN. OPT' -> B`
402 -- where overall_pat_ty is an instance of OPT'.
403 -- Here, we infer a rho type for it,
404 -- which replaces the leading foralls and constraints
405 -- with fresh unification variables.
406 (expr',expr'_inferred) <- tcInferRho expr
407 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
408 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
409 -- tcSubExp: expected first, offered second
412 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
413 -- variable to find it). this means that in an example like
414 -- (view -> f) where view :: _ -> forall b. b
415 -- we will only be able to use view at one instantation in the
417 ; (expr_coerc, pat_ty) <- tcInfer $ \ pat_ty ->
418 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
420 -- pattern must have pat_ty
421 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
422 -- this should get zonked later on, but we unBox it here
423 -- so that we do the same checks as above
424 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
425 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
427 -- Type signatures in patterns
428 -- See Note [Pattern coercions] below
429 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
430 = do { (inner_ty, tv_binds, coi) <- tcPatSig (patSigCtxt pstate) sig_ty
432 ; unless (isIdentityCoI coi) $
433 failWithTc (badSigPat pat_ty)
434 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
435 tc_lpat pat inner_ty pstate thing_inside
436 ; return (SigPatOut pat' inner_ty, tvs, res) }
438 tc_pat _ pat@(TypePat _) _ _
439 = failWithTc (badTypePat pat)
441 ------------------------
442 -- Lists, tuples, arrays
443 tc_pat pstate (ListPat pats _) pat_ty thing_inside
444 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
445 ; let scoi = mkSymCoI coi
446 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
447 pats pstate thing_inside
448 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
451 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
452 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
453 ; let scoi = mkSymCoI coi
454 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
455 pats pstate thing_inside
456 ; when (null pats) (zapToMonotype pat_ty >> return ()) -- c.f. ExplicitPArr in TcExpr
457 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
460 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
461 = do { let tc = tupleTyCon boxity (length pats)
462 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
463 ; let scoi = mkSymCoI coi
464 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
467 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
468 -- so that we can experiment with lazy tuple-matching.
469 -- This is a pretty odd place to make the switch, but
470 -- it was easy to do.
471 ; let pat_ty' = mkTyConApp tc arg_tys
472 -- pat_ty /= pat_ty iff coi /= IdCo
473 unmangled_result = TuplePat pats' boxity pat_ty'
474 possibly_mangled_result
475 | opt_IrrefutableTuples &&
476 isBoxed boxity = LazyPat (noLoc unmangled_result)
477 | otherwise = unmangled_result
479 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
480 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
483 ------------------------
485 tc_pat pstate (ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
486 = do { data_con <- tcLookupDataCon con_name
487 ; let tycon = dataConTyCon data_con
488 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
490 ------------------------
492 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
493 = do { let lit_ty = hsLitType simple_lit
494 ; coi <- boxyUnify lit_ty pat_ty
495 -- coi is of kind: lit_ty ~ pat_ty
496 ; res <- thing_inside pstate
497 -- pattern coercions have to
498 -- be of kind: pat_ty ~ lit_ty
500 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
503 ------------------------
504 -- Overloaded patterns: n, and n+k
505 tc_pat pstate (NPat over_lit mb_neg eq) pat_ty thing_inside
506 = do { let orig = LiteralOrigin over_lit
507 ; lit' <- tcOverloadedLit orig over_lit pat_ty
508 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
509 ; mb_neg' <- case mb_neg of
510 Nothing -> return Nothing -- Positive literal
511 Just neg -> -- Negative literal
512 -- The 'negate' is re-mappable syntax
513 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
514 ; return (Just neg') }
515 ; res <- thing_inside pstate
516 ; return (NPat lit' mb_neg' eq', [], res) }
518 tc_pat pstate (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
519 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
520 ; let pat_ty' = idType bndr_id
521 orig = LiteralOrigin lit
522 ; lit' <- tcOverloadedLit orig lit pat_ty'
524 -- The '>=' and '-' parts are re-mappable syntax
525 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
526 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
528 -- The Report says that n+k patterns must be in Integral
529 -- We may not want this when using re-mappable syntax, though (ToDo?)
530 ; icls <- tcLookupClass integralClassName
531 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
533 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
534 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
536 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
540 %************************************************************************
542 Most of the work for constructors is here
543 (the rest is in the ConPatIn case of tc_pat)
545 %************************************************************************
547 [Pattern matching indexed data types]
548 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
549 Consider the following declarations:
551 data family Map k :: * -> *
552 data instance Map (a, b) v = MapPair (Map a (Pair b v))
554 and a case expression
556 case x :: Map (Int, c) w of MapPair m -> ...
558 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
559 worker/wrapper types for MapPair are
561 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
562 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
564 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
565 :R123Map, which means the straight use of boxySplitTyConApp would give a type
566 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
567 boxySplitTyConApp with the family tycon Map instead, which gives us the family
568 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
569 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
570 (provided by tyConFamInst_maybe together with the family tycon). This
571 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
572 the split arguments for the representation tycon :R123Map as {Int, c, w}
574 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
576 Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
578 moving between representation and family type into account. To produce type
579 correct Core, this coercion needs to be used to case the type of the scrutinee
580 from the family to the representation type. This is achieved by
581 unwrapFamInstScrutinee using a CoPat around the result pattern.
583 Now it might appear seem as if we could have used the previous GADT type
584 refinement infrastructure of refineAlt and friends instead of the explicit
585 unification and CoPat generation. However, that would be wrong. Why? The
586 whole point of GADT refinement is that the refinement is local to the case
587 alternative. In contrast, the substitution generated by the unification of
588 the family type list and instance types needs to be propagated to the outside.
589 Imagine that in the above example, the type of the scrutinee would have been
590 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
591 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
592 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
593 alternatives of the case expression, whereas in the GADT case it might vary
594 between alternatives.
596 RIP GADT refinement: refinements have been replaced by the use of explicit
597 equality constraints that are used in conjunction with implication constraints
598 to express the local scope of GADT refinements.
602 -- MkT :: forall a b c. (a~[b]) => b -> c -> T a
603 -- with scrutinee of type (T ty)
605 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
606 -> BoxySigmaType -- Type of the pattern
607 -> HsConPatDetails Name -> (PatState -> TcM a)
608 -> TcM (Pat TcId, [TcTyVar], a)
609 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
610 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
611 = dataConFullSig data_con
612 skol_info = PatSkol data_con
613 origin = SigOrigin skol_info
614 full_theta = eq_theta ++ dict_theta
616 -- Instantiate the constructor type variables [a->ty]
617 -- This may involve doing a family-instance coercion, and building a
619 ; (ctxt_res_tys, coi, unwrap_ty) <- boxySplitTyConAppWithFamily tycon
621 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
622 pat_ty' = mkTyConApp tycon ctxt_res_tys
623 -- pat_ty' /= pat_ty iff coi /= IdCo
625 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
627 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys
630 -- Add the stupid theta
631 ; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys
633 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
634 -- Get location from monad, not from ex_tvs
636 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
637 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
638 arg_tys' = substTys tenv arg_tys
640 ; if null ex_tvs && null eq_spec && null full_theta
641 then do { -- The common case; no class bindings etc
642 -- (see Note [Arrows and patterns])
643 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
644 arg_pats pstate thing_inside
645 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
646 pat_tvs = [], pat_dicts = [],
647 pat_binds = emptyLHsBinds,
648 pat_args = arg_pats',
651 ; return (wrap_res_pat res_pat, inner_tvs, res) }
653 else do -- The general case, with existential, and local equality
655 { checkTc (notProcPat (pat_ctxt pstate))
656 (existentialProcPat data_con)
657 -- See Note [Arrows and patterns]
659 -- Need to test for rigidity if *any* constraints in theta as class
660 -- constraints may have superclass equality constraints. However,
661 -- we don't want to check for rigidity if we got here only because
662 -- ex_tvs was non-null.
663 -- ; unless (null theta') $
664 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
665 -- if we explicitly or implicitly (by a GADT def) have equality
667 ; let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
668 theta' = substTheta tenv (eq_preds ++ full_theta)
669 -- order is *important* as we generate the list of
670 -- dictionary binders from theta'
671 no_equalities = not (any isEqPred theta')
672 pstate' | no_equalities = pstate
673 | otherwise = pstate { pat_eqs = True }
675 ; gadts_on <- doptM Opt_GADTs
676 ; checkTc (no_equalities || gadts_on)
677 (ptext (sLit "A pattern match on a GADT requires -XGADTs"))
678 -- Trac #2905 decided that a *pattern-match* of a GADT
679 -- should require the GADT language flag
681 ; unless no_equalities $ checkTc (isRigidTy pat_ty) $
682 nonRigidMatch (pat_ctxt pstate) data_con
684 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
685 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
687 ; loc <- getInstLoc origin
688 ; dicts <- newDictBndrs loc theta'
689 ; dict_binds <- tcSimplifyCheckPat loc ex_tvs' dicts lie_req
691 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
693 pat_dicts = map instToVar dicts,
694 pat_binds = dict_binds,
695 pat_args = arg_pats', pat_ty = pat_ty' }
696 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
699 -- Split against the family tycon if the pattern constructor
700 -- belongs to a family instance tycon.
701 boxySplitTyConAppWithFamily tycon pat_ty =
703 case tyConFamInst_maybe tycon of
705 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp tycon pat_ty
706 ; return (scrutinee_arg_tys, coi1, pat_ty)
708 Just (fam_tycon, instTys) ->
709 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp fam_tycon pat_ty
710 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
711 ; let instTys' = substTys subst instTys
712 ; cois <- boxyUnifyList instTys' scrutinee_arg_tys
713 ; let coi = if isIdentityCoI coi1
714 then -- pat_ty was splittable
715 -- => boxyUnifyList had real work to do
716 mkTyConAppCoI fam_tycon instTys' cois
717 else -- pat_ty was not splittable
718 -- => scrutinee_arg_tys are fresh tvs and
719 -- boxyUnifyList just instantiated those
721 ; return (freshTvs, coi, mkTyConApp fam_tycon instTys')
723 -- iff cois is non-trivial
726 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
727 ppr tycon <+> ppr pat_ty
728 , text " family instance:" <+>
729 ppr (tyConFamInst_maybe tycon)
732 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
733 -- pattern if the tycon is an instance of a family.
735 unwrapFamInstScrutinee :: TyCon -> [Type] -> Type -> Pat Id -> Pat Id
736 unwrapFamInstScrutinee tycon args unwrap_ty pat
737 | Just co_con <- tyConFamilyCoercion_maybe tycon
738 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
740 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
741 -- coercion is not the identity; mkCoPat is inconvenient as it
742 -- wants a located pattern.
743 = CoPat (WpCast $ mkTyConApp co_con args) -- co fam ty to repr ty
744 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
745 unwrap_ty -- family inst type
749 tcConArgs :: DataCon -> [TcSigmaType]
750 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
752 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
753 = do { checkTc (con_arity == no_of_args) -- Check correct arity
754 (arityErr "Constructor" data_con con_arity no_of_args)
755 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
756 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
758 ; return (PrefixCon arg_pats', tvs, res) }
760 con_arity = dataConSourceArity data_con
761 no_of_args = length arg_pats
763 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
764 = do { checkTc (con_arity == 2) -- Check correct arity
765 (arityErr "Constructor" data_con con_arity 2)
766 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
767 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
769 ; return (InfixCon p1' p2', tvs, res) }
771 con_arity = dataConSourceArity data_con
773 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
774 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
775 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
777 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
778 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
779 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
780 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
781 ; return (HsRecField sel_id pat' pun, tvs, res) }
783 find_field_ty :: FieldLabel -> TcM (Id, TcType)
784 find_field_ty field_lbl
785 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
787 -- No matching field; chances are this field label comes from some
788 -- other record type (or maybe none). As well as reporting an
789 -- error we still want to typecheck the pattern, principally to
790 -- make sure that all the variables it binds are put into the
791 -- environment, else the type checker crashes later:
792 -- f (R { foo = (a,b) }) = a+b
793 -- If foo isn't one of R's fields, we don't want to crash when
794 -- typechecking the "a+b".
795 [] -> do { addErrTc (badFieldCon data_con field_lbl)
796 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
797 ; return (error "Bogus selector Id", bogus_ty) }
799 -- The normal case, when the field comes from the right constructor
801 ASSERT( null extras )
802 do { sel_id <- tcLookupField field_lbl
803 ; return (sel_id, pat_ty) }
805 field_tys :: [(FieldLabel, TcType)]
806 field_tys = zip (dataConFieldLabels data_con) arg_tys
807 -- Don't use zipEqual! If the constructor isn't really a record, then
808 -- dataConFieldLabels will be empty (and each field in the pattern
809 -- will generate an error below).
811 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
812 tcConArg (arg_pat, arg_ty) pstate thing_inside
813 = tc_lpat arg_pat arg_ty pstate thing_inside
817 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
818 -- Instantiate the "stupid theta" of the data con, and throw
819 -- the constraints into the constraint set
820 addDataConStupidTheta data_con inst_tys
821 | null stupid_theta = return ()
822 | otherwise = instStupidTheta origin inst_theta
824 origin = OccurrenceOf (dataConName data_con)
825 -- The origin should always report "occurrence of C"
826 -- even when C occurs in a pattern
827 stupid_theta = dataConStupidTheta data_con
828 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
829 -- NB: inst_tys can be longer than the univ tyvars
830 -- because the constructor might have existentials
831 inst_theta = substTheta tenv stupid_theta
834 Note [Arrows and patterns]
835 ~~~~~~~~~~~~~~~~~~~~~~~~~~
836 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
838 expr :: Arrow a => a () Int
839 expr = proc (y,z) -> do
843 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
844 arrow body (e.g 'term'). As things stand (bogusly) all the
845 constraints from the proc body are gathered together, so constraints
846 from 'term' will be seen by the tcPat for (y,z). But we must *not*
847 bind constraints from 'term' here, becuase the desugarer will not make
848 these bindings scope over 'term'.
850 The Right Thing is not to confuse these constraints together. But for
851 now the Easy Thing is to ensure that we do not have existential or
852 GADT constraints in a 'proc', and to short-cut the constraint
853 simplification for such vanilla patterns so that it binds no
854 constraints. Hence the 'fast path' in tcConPat; but it's also a good
855 plan for ordinary vanilla patterns to bypass the constraint
859 %************************************************************************
863 %************************************************************************
865 In tcOverloadedLit we convert directly to an Int or Integer if we
866 know that's what we want. This may save some time, by not
867 temporarily generating overloaded literals, but it won't catch all
868 cases (the rest are caught in lookupInst).
871 tcOverloadedLit :: InstOrigin
874 -> TcM (HsOverLit TcId)
875 tcOverloadedLit orig lit@(OverLit { ol_val = val, ol_rebindable = rebindable
876 , ol_witness = meth_name }) res_ty
878 -- Do not generate a LitInst for rebindable syntax.
879 -- Reason: If we do, tcSimplify will call lookupInst, which
880 -- will call tcSyntaxName, which does unification,
881 -- which tcSimplify doesn't like
882 -- ToDo: noLoc sadness
883 = do { hs_lit <- mkOverLit val
884 ; let lit_ty = hsLitType hs_lit
885 ; fi' <- tcSyntaxOp orig meth_name (mkFunTy lit_ty res_ty)
886 -- Overloaded literals must have liftedTypeKind, because
887 -- we're instantiating an overloaded function here,
888 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
889 -- However this'll be picked up by tcSyntaxOp if necessary
890 ; let witness = HsApp (noLoc fi') (noLoc (HsLit hs_lit))
891 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
893 | Just expr <- shortCutLit val res_ty
894 = return (lit { ol_witness = expr, ol_type = res_ty })
897 = do { loc <- getInstLoc orig
898 ; res_tau <- zapToMonotype res_ty
899 ; new_uniq <- newUnique
900 ; let lit_nm = mkSystemVarName new_uniq (fsLit "lit")
901 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
902 tci_ty = res_tau, tci_loc = loc}
903 witness = HsVar (instToId lit_inst)
905 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
909 %************************************************************************
911 Note [Pattern coercions]
913 %************************************************************************
915 In principle, these program would be reasonable:
917 f :: (forall a. a->a) -> Int
918 f (x :: Int->Int) = x 3
920 g :: (forall a. [a]) -> Bool
923 In both cases, the function type signature restricts what arguments can be passed
924 in a call (to polymorphic ones). The pattern type signature then instantiates this
925 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
926 generate the translated term
927 f = \x' :: (forall a. a->a). let x = x' Int in x 3
929 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
930 And it requires a significant amount of code to implement, becuase we need to decorate
931 the translated pattern with coercion functions (generated from the subsumption check
934 So for now I'm just insisting on type *equality* in patterns. No subsumption.
936 Old notes about desugaring, at a time when pattern coercions were handled:
938 A SigPat is a type coercion and must be handled one at at time. We can't
939 combine them unless the type of the pattern inside is identical, and we don't
940 bother to check for that. For example:
942 data T = T1 Int | T2 Bool
943 f :: (forall a. a -> a) -> T -> t
944 f (g::Int->Int) (T1 i) = T1 (g i)
945 f (g::Bool->Bool) (T2 b) = T2 (g b)
947 We desugar this as follows:
949 f = \ g::(forall a. a->a) t::T ->
951 in case t of { T1 i -> T1 (gi i)
954 in case t of { T2 b -> T2 (gb b)
957 Note that we do not treat the first column of patterns as a
958 column of variables, because the coerced variables (gi, gb)
959 would be of different types. So we get rather grotty code.
960 But I don't think this is a common case, and if it was we could
961 doubtless improve it.
963 Meanwhile, the strategy is:
964 * treat each SigPat coercion (always non-identity coercions)
966 * deal with the stuff inside, and then wrap a binding round
967 the result to bind the new variable (gi, gb, etc)
970 %************************************************************************
972 \subsection{Errors and contexts}
974 %************************************************************************
977 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
978 patCtxt (VarPat _) = Nothing
979 patCtxt (ParPat _) = Nothing
980 patCtxt (AsPat _ _) = Nothing
981 patCtxt pat = Just (hang (ptext (sLit "In the pattern:"))
984 -----------------------------------------------
986 existentialExplode :: LPat Name -> SDoc
987 existentialExplode pat
988 = hang (vcat [text "My brain just exploded.",
989 text "I can't handle pattern bindings for existential or GADT data constructors.",
990 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
991 text "In the binding group for"])
994 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
995 -> TcM (TidyEnv, SDoc)
996 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
997 = do { pat_tys' <- mapM zonkTcType pat_tys
998 ; body_ty' <- zonkTcType body_ty
999 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1000 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1001 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1003 sep [ptext (sLit "When checking an existential match that binds"),
1004 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1005 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1006 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
1009 bound_ids = collectPatsBinders pats
1010 show_ids = filter is_interesting bound_ids
1011 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1013 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1014 -- Don't zonk the types so we get the separate, un-unified versions
1016 badFieldCon :: DataCon -> Name -> SDoc
1017 badFieldCon con field
1018 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
1019 ptext (sLit "does not have field"), quotes (ppr field)]
1021 polyPatSig :: TcType -> SDoc
1023 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1026 badSigPat :: TcType -> SDoc
1027 badSigPat pat_ty = ptext (sLit "Pattern signature must exactly match:") <+>
1030 badTypePat :: Pat Name -> SDoc
1031 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1033 existentialProcPat :: DataCon -> SDoc
1034 existentialProcPat con
1035 = hang (ptext (sLit "Illegal constructor") <+> quotes (ppr con) <+> ptext (sLit "in a 'proc' pattern"))
1036 2 (ptext (sLit "Proc patterns cannot use existentials or GADTs"))
1038 lazyPatErr :: Pat name -> [TcTyVar] -> TcM ()
1041 hang (ptext (sLit "A lazy (~) pattern cannot match existential or GADT data constructors"))
1042 2 (vcat (map pprSkolTvBinding tvs))
1044 lazyUnliftedPatErr :: OutputableBndr name => Pat name -> TcM ()
1045 lazyUnliftedPatErr pat
1047 hang (ptext (sLit "A lazy (~) pattern cannot contain unlifted types"))
1050 nonRigidMatch :: PatCtxt -> DataCon -> SDoc
1051 nonRigidMatch ctxt con
1052 = hang (ptext (sLit "GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1053 2 (ptext (sLit "Probable solution: add a type signature for") <+> what ctxt)
1055 what (APat (FunRhs f _)) = quotes (ppr f)
1056 what (APat CaseAlt) = ptext (sLit "the scrutinee of the case expression")
1057 what (APat LambdaExpr ) = ptext (sLit "the lambda expression")
1058 what (APat (StmtCtxt _)) = ptext (sLit "the right hand side of a do/comprehension binding")
1059 what _other = ptext (sLit "something")
1061 nonRigidResult :: PatCtxt -> Type -> TcM a
1062 nonRigidResult ctxt res_ty
1063 = do { env0 <- tcInitTidyEnv
1064 ; let (env1, res_ty') = tidyOpenType env0 res_ty
1065 msg = hang (ptext (sLit "GADT pattern match with non-rigid result type")
1066 <+> quotes (ppr res_ty'))
1067 2 (ptext (sLit "Solution: add a type signature for")
1069 ; failWithTcM (env1, msg) }
1071 what (APat (FunRhs f _)) = quotes (ppr f)
1072 what (APat CaseAlt) = ptext (sLit "the entire case expression")
1073 what (APat LambdaExpr) = ptext (sLit "the lambda exression")
1074 what (APat (StmtCtxt _)) = ptext (sLit "the entire do/comprehension expression")
1075 what _other = ptext (sLit "something")