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 -- Use this when we add pattern coercions back in
439 -- return (mkCoPatCoI (mkSymCoI coi) (SigPatOut pat' inner_ty) pat_ty
442 tc_pat _ pat@(TypePat _) _ _
443 = failWithTc (badTypePat pat)
445 ------------------------
446 -- Lists, tuples, arrays
447 tc_pat pstate (ListPat pats _) pat_ty thing_inside
448 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
449 ; let scoi = mkSymCoI coi
450 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
451 pats pstate thing_inside
452 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
455 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
456 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
457 ; let scoi = mkSymCoI coi
458 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
459 pats pstate thing_inside
460 ; when (null pats) (zapToMonotype pat_ty >> return ()) -- c.f. ExplicitPArr in TcExpr
461 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
464 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
465 = do { let tc = tupleTyCon boxity (length pats)
466 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
467 ; let scoi = mkSymCoI coi
468 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
471 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
472 -- so that we can experiment with lazy tuple-matching.
473 -- This is a pretty odd place to make the switch, but
474 -- it was easy to do.
475 ; let pat_ty' = mkTyConApp tc arg_tys
476 -- pat_ty /= pat_ty iff coi /= IdCo
477 unmangled_result = TuplePat pats' boxity pat_ty'
478 possibly_mangled_result
479 | opt_IrrefutableTuples &&
480 isBoxed boxity = LazyPat (noLoc unmangled_result)
481 | otherwise = unmangled_result
483 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
484 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
487 ------------------------
489 tc_pat pstate (ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
490 = do { data_con <- tcLookupDataCon con_name
491 ; let tycon = dataConTyCon data_con
492 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
494 ------------------------
496 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
497 = do { let lit_ty = hsLitType simple_lit
498 ; coi <- boxyUnify lit_ty pat_ty
499 -- coi is of kind: lit_ty ~ pat_ty
500 ; res <- thing_inside pstate
501 -- pattern coercions have to
502 -- be of kind: pat_ty ~ lit_ty
504 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
507 ------------------------
508 -- Overloaded patterns: n, and n+k
509 tc_pat pstate (NPat over_lit mb_neg eq) pat_ty thing_inside
510 = do { let orig = LiteralOrigin over_lit
511 ; lit' <- tcOverloadedLit orig over_lit pat_ty
512 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
513 ; mb_neg' <- case mb_neg of
514 Nothing -> return Nothing -- Positive literal
515 Just neg -> -- Negative literal
516 -- The 'negate' is re-mappable syntax
517 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
518 ; return (Just neg') }
519 ; res <- thing_inside pstate
520 ; return (NPat lit' mb_neg' eq', [], res) }
522 tc_pat pstate (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
523 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
524 ; let pat_ty' = idType bndr_id
525 orig = LiteralOrigin lit
526 ; lit' <- tcOverloadedLit orig lit pat_ty'
528 -- The '>=' and '-' parts are re-mappable syntax
529 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
530 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
532 -- The Report says that n+k patterns must be in Integral
533 -- We may not want this when using re-mappable syntax, though (ToDo?)
534 ; icls <- tcLookupClass integralClassName
535 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
537 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
538 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
540 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
544 %************************************************************************
546 Most of the work for constructors is here
547 (the rest is in the ConPatIn case of tc_pat)
549 %************************************************************************
551 [Pattern matching indexed data types]
552 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
553 Consider the following declarations:
555 data family Map k :: * -> *
556 data instance Map (a, b) v = MapPair (Map a (Pair b v))
558 and a case expression
560 case x :: Map (Int, c) w of MapPair m -> ...
562 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
563 worker/wrapper types for MapPair are
565 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
566 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
568 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
569 :R123Map, which means the straight use of boxySplitTyConApp would give a type
570 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
571 boxySplitTyConApp with the family tycon Map instead, which gives us the family
572 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
573 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
574 (provided by tyConFamInst_maybe together with the family tycon). This
575 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
576 the split arguments for the representation tycon :R123Map as {Int, c, w}
578 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
580 Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
582 moving between representation and family type into account. To produce type
583 correct Core, this coercion needs to be used to case the type of the scrutinee
584 from the family to the representation type. This is achieved by
585 unwrapFamInstScrutinee using a CoPat around the result pattern.
587 Now it might appear seem as if we could have used the previous GADT type
588 refinement infrastructure of refineAlt and friends instead of the explicit
589 unification and CoPat generation. However, that would be wrong. Why? The
590 whole point of GADT refinement is that the refinement is local to the case
591 alternative. In contrast, the substitution generated by the unification of
592 the family type list and instance types needs to be propagated to the outside.
593 Imagine that in the above example, the type of the scrutinee would have been
594 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
595 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
596 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
597 alternatives of the case expression, whereas in the GADT case it might vary
598 between alternatives.
600 RIP GADT refinement: refinements have been replaced by the use of explicit
601 equality constraints that are used in conjunction with implication constraints
602 to express the local scope of GADT refinements.
606 -- MkT :: forall a b c. (a~[b]) => b -> c -> T a
607 -- with scrutinee of type (T ty)
609 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
610 -> BoxySigmaType -- Type of the pattern
611 -> HsConPatDetails Name -> (PatState -> TcM a)
612 -> TcM (Pat TcId, [TcTyVar], a)
613 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
614 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
615 = dataConFullSig data_con
616 skol_info = PatSkol data_con
617 origin = SigOrigin skol_info
618 full_theta = eq_theta ++ dict_theta
620 -- Instantiate the constructor type variables [a->ty]
621 -- This may involve doing a family-instance coercion, and building a
623 ; (ctxt_res_tys, coi, unwrap_ty) <- boxySplitTyConAppWithFamily tycon
625 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
626 pat_ty' = mkTyConApp tycon ctxt_res_tys
627 -- pat_ty' /= pat_ty iff coi /= IdCo
629 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
631 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys
634 -- Add the stupid theta
635 ; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys
637 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
638 -- Get location from monad, not from ex_tvs
640 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
641 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
642 arg_tys' = substTys tenv arg_tys
644 ; if null ex_tvs && null eq_spec && null full_theta
645 then do { -- The common case; no class bindings etc
646 -- (see Note [Arrows and patterns])
647 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
648 arg_pats pstate thing_inside
649 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
650 pat_tvs = [], pat_dicts = [],
651 pat_binds = emptyLHsBinds,
652 pat_args = arg_pats',
655 ; return (wrap_res_pat res_pat, inner_tvs, res) }
657 else do -- The general case, with existential, and local equality
659 { checkTc (notProcPat (pat_ctxt pstate))
660 (existentialProcPat data_con)
661 -- See Note [Arrows and patterns]
663 -- Need to test for rigidity if *any* constraints in theta as class
664 -- constraints may have superclass equality constraints. However,
665 -- we don't want to check for rigidity if we got here only because
666 -- ex_tvs was non-null.
667 -- ; unless (null theta') $
668 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
669 -- if we explicitly or implicitly (by a GADT def) have equality
671 ; let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
672 theta' = substTheta tenv (eq_preds ++ full_theta)
673 -- order is *important* as we generate the list of
674 -- dictionary binders from theta'
675 no_equalities = not (any isEqPred theta')
676 pstate' | no_equalities = pstate
677 | otherwise = pstate { pat_eqs = True }
679 ; gadts_on <- doptM Opt_GADTs
680 ; checkTc (no_equalities || gadts_on)
681 (ptext (sLit "A pattern match on a GADT requires -XGADTs"))
682 -- Trac #2905 decided that a *pattern-match* of a GADT
683 -- should require the GADT language flag
685 ; unless no_equalities $ checkTc (isRigidTy pat_ty) $
686 nonRigidMatch (pat_ctxt pstate) data_con
688 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
689 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
691 ; loc <- getInstLoc origin
692 ; dicts <- newDictBndrs loc theta'
693 ; dict_binds <- tcSimplifyCheckPat loc ex_tvs' dicts lie_req
695 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
697 pat_dicts = map instToVar dicts,
698 pat_binds = dict_binds,
699 pat_args = arg_pats', pat_ty = pat_ty' }
700 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
703 -- Split against the family tycon if the pattern constructor
704 -- belongs to a family instance tycon.
705 boxySplitTyConAppWithFamily tycon pat_ty =
707 case tyConFamInst_maybe tycon of
709 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp tycon pat_ty
710 ; return (scrutinee_arg_tys, coi1, pat_ty)
712 Just (fam_tycon, instTys) ->
713 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp fam_tycon pat_ty
714 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
715 ; let instTys' = substTys subst instTys
716 ; cois <- boxyUnifyList instTys' scrutinee_arg_tys
717 ; let coi = if isIdentityCoI coi1
718 then -- pat_ty was splittable
719 -- => boxyUnifyList had real work to do
720 mkTyConAppCoI fam_tycon instTys' cois
721 else -- pat_ty was not splittable
722 -- => scrutinee_arg_tys are fresh tvs and
723 -- boxyUnifyList just instantiated those
725 ; return (freshTvs, coi, mkTyConApp fam_tycon instTys')
727 -- iff cois is non-trivial
730 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
731 ppr tycon <+> ppr pat_ty
732 , text " family instance:" <+>
733 ppr (tyConFamInst_maybe tycon)
736 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
737 -- pattern if the tycon is an instance of a family.
739 unwrapFamInstScrutinee :: TyCon -> [Type] -> Type -> Pat Id -> Pat Id
740 unwrapFamInstScrutinee tycon args unwrap_ty pat
741 | Just co_con <- tyConFamilyCoercion_maybe tycon
742 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
744 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
745 -- coercion is not the identity; mkCoPat is inconvenient as it
746 -- wants a located pattern.
747 = CoPat (WpCast $ mkTyConApp co_con args) -- co fam ty to repr ty
748 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
749 unwrap_ty -- family inst type
753 tcConArgs :: DataCon -> [TcSigmaType]
754 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
756 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
757 = do { checkTc (con_arity == no_of_args) -- Check correct arity
758 (arityErr "Constructor" data_con con_arity no_of_args)
759 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
760 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
762 ; return (PrefixCon arg_pats', tvs, res) }
764 con_arity = dataConSourceArity data_con
765 no_of_args = length arg_pats
767 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
768 = do { checkTc (con_arity == 2) -- Check correct arity
769 (arityErr "Constructor" data_con con_arity 2)
770 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
771 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
773 ; return (InfixCon p1' p2', tvs, res) }
775 con_arity = dataConSourceArity data_con
777 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
778 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
779 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
781 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
782 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
783 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
784 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
785 ; return (HsRecField sel_id pat' pun, tvs, res) }
787 find_field_ty :: FieldLabel -> TcM (Id, TcType)
788 find_field_ty field_lbl
789 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
791 -- No matching field; chances are this field label comes from some
792 -- other record type (or maybe none). As well as reporting an
793 -- error we still want to typecheck the pattern, principally to
794 -- make sure that all the variables it binds are put into the
795 -- environment, else the type checker crashes later:
796 -- f (R { foo = (a,b) }) = a+b
797 -- If foo isn't one of R's fields, we don't want to crash when
798 -- typechecking the "a+b".
799 [] -> do { addErrTc (badFieldCon data_con field_lbl)
800 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
801 ; return (error "Bogus selector Id", bogus_ty) }
803 -- The normal case, when the field comes from the right constructor
805 ASSERT( null extras )
806 do { sel_id <- tcLookupField field_lbl
807 ; return (sel_id, pat_ty) }
809 field_tys :: [(FieldLabel, TcType)]
810 field_tys = zip (dataConFieldLabels data_con) arg_tys
811 -- Don't use zipEqual! If the constructor isn't really a record, then
812 -- dataConFieldLabels will be empty (and each field in the pattern
813 -- will generate an error below).
815 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
816 tcConArg (arg_pat, arg_ty) pstate thing_inside
817 = tc_lpat arg_pat arg_ty pstate thing_inside
821 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
822 -- Instantiate the "stupid theta" of the data con, and throw
823 -- the constraints into the constraint set
824 addDataConStupidTheta data_con inst_tys
825 | null stupid_theta = return ()
826 | otherwise = instStupidTheta origin inst_theta
828 origin = OccurrenceOf (dataConName data_con)
829 -- The origin should always report "occurrence of C"
830 -- even when C occurs in a pattern
831 stupid_theta = dataConStupidTheta data_con
832 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
833 -- NB: inst_tys can be longer than the univ tyvars
834 -- because the constructor might have existentials
835 inst_theta = substTheta tenv stupid_theta
838 Note [Arrows and patterns]
839 ~~~~~~~~~~~~~~~~~~~~~~~~~~
840 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
842 expr :: Arrow a => a () Int
843 expr = proc (y,z) -> do
847 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
848 arrow body (e.g 'term'). As things stand (bogusly) all the
849 constraints from the proc body are gathered together, so constraints
850 from 'term' will be seen by the tcPat for (y,z). But we must *not*
851 bind constraints from 'term' here, becuase the desugarer will not make
852 these bindings scope over 'term'.
854 The Right Thing is not to confuse these constraints together. But for
855 now the Easy Thing is to ensure that we do not have existential or
856 GADT constraints in a 'proc', and to short-cut the constraint
857 simplification for such vanilla patterns so that it binds no
858 constraints. Hence the 'fast path' in tcConPat; but it's also a good
859 plan for ordinary vanilla patterns to bypass the constraint
863 %************************************************************************
867 %************************************************************************
869 In tcOverloadedLit we convert directly to an Int or Integer if we
870 know that's what we want. This may save some time, by not
871 temporarily generating overloaded literals, but it won't catch all
872 cases (the rest are caught in lookupInst).
875 tcOverloadedLit :: InstOrigin
878 -> TcM (HsOverLit TcId)
879 tcOverloadedLit orig lit@(OverLit { ol_val = val, ol_rebindable = rebindable
880 , ol_witness = meth_name }) res_ty
882 -- Do not generate a LitInst for rebindable syntax.
883 -- Reason: If we do, tcSimplify will call lookupInst, which
884 -- will call tcSyntaxName, which does unification,
885 -- which tcSimplify doesn't like
886 -- ToDo: noLoc sadness
887 = do { hs_lit <- mkOverLit val
888 ; let lit_ty = hsLitType hs_lit
889 ; fi' <- tcSyntaxOp orig meth_name (mkFunTy lit_ty res_ty)
890 -- Overloaded literals must have liftedTypeKind, because
891 -- we're instantiating an overloaded function here,
892 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
893 -- However this'll be picked up by tcSyntaxOp if necessary
894 ; let witness = HsApp (noLoc fi') (noLoc (HsLit hs_lit))
895 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
897 | Just expr <- shortCutLit val res_ty
898 = return (lit { ol_witness = expr, ol_type = res_ty })
901 = do { loc <- getInstLoc orig
902 ; res_tau <- zapToMonotype res_ty
903 ; new_uniq <- newUnique
904 ; let lit_nm = mkSystemVarName new_uniq (fsLit "lit")
905 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
906 tci_ty = res_tau, tci_loc = loc}
907 witness = HsVar (instToId lit_inst)
909 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
913 %************************************************************************
915 Note [Pattern coercions]
917 %************************************************************************
919 In principle, these program would be reasonable:
921 f :: (forall a. a->a) -> Int
922 f (x :: Int->Int) = x 3
924 g :: (forall a. [a]) -> Bool
927 In both cases, the function type signature restricts what arguments can be passed
928 in a call (to polymorphic ones). The pattern type signature then instantiates this
929 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
930 generate the translated term
931 f = \x' :: (forall a. a->a). let x = x' Int in x 3
933 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
934 And it requires a significant amount of code to implement, becuase we need to decorate
935 the translated pattern with coercion functions (generated from the subsumption check
938 So for now I'm just insisting on type *equality* in patterns. No subsumption.
940 Old notes about desugaring, at a time when pattern coercions were handled:
942 A SigPat is a type coercion and must be handled one at at time. We can't
943 combine them unless the type of the pattern inside is identical, and we don't
944 bother to check for that. For example:
946 data T = T1 Int | T2 Bool
947 f :: (forall a. a -> a) -> T -> t
948 f (g::Int->Int) (T1 i) = T1 (g i)
949 f (g::Bool->Bool) (T2 b) = T2 (g b)
951 We desugar this as follows:
953 f = \ g::(forall a. a->a) t::T ->
955 in case t of { T1 i -> T1 (gi i)
958 in case t of { T2 b -> T2 (gb b)
961 Note that we do not treat the first column of patterns as a
962 column of variables, because the coerced variables (gi, gb)
963 would be of different types. So we get rather grotty code.
964 But I don't think this is a common case, and if it was we could
965 doubtless improve it.
967 Meanwhile, the strategy is:
968 * treat each SigPat coercion (always non-identity coercions)
970 * deal with the stuff inside, and then wrap a binding round
971 the result to bind the new variable (gi, gb, etc)
974 %************************************************************************
976 \subsection{Errors and contexts}
978 %************************************************************************
981 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
982 patCtxt (VarPat _) = Nothing
983 patCtxt (ParPat _) = Nothing
984 patCtxt (AsPat _ _) = Nothing
985 patCtxt pat = Just (hang (ptext (sLit "In the pattern:"))
988 -----------------------------------------------
990 existentialExplode :: LPat Name -> SDoc
991 existentialExplode pat
992 = hang (vcat [text "My brain just exploded.",
993 text "I can't handle pattern bindings for existential or GADT data constructors.",
994 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
995 text "In the binding group for"])
998 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
999 -> TcM (TidyEnv, SDoc)
1000 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
1001 = do { pat_tys' <- mapM zonkTcType pat_tys
1002 ; body_ty' <- zonkTcType body_ty
1003 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1004 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1005 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1007 sep [ptext (sLit "When checking an existential match that binds"),
1008 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1009 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1010 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
1013 bound_ids = collectPatsBinders pats
1014 show_ids = filter is_interesting bound_ids
1015 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1017 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1018 -- Don't zonk the types so we get the separate, un-unified versions
1020 badFieldCon :: DataCon -> Name -> SDoc
1021 badFieldCon con field
1022 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
1023 ptext (sLit "does not have field"), quotes (ppr field)]
1025 polyPatSig :: TcType -> SDoc
1027 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1030 badSigPat :: TcType -> SDoc
1031 badSigPat pat_ty = ptext (sLit "Pattern signature must exactly match:") <+>
1034 badTypePat :: Pat Name -> SDoc
1035 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1037 existentialProcPat :: DataCon -> SDoc
1038 existentialProcPat con
1039 = hang (ptext (sLit "Illegal constructor") <+> quotes (ppr con) <+> ptext (sLit "in a 'proc' pattern"))
1040 2 (ptext (sLit "Proc patterns cannot use existentials or GADTs"))
1042 lazyPatErr :: Pat name -> [TcTyVar] -> TcM ()
1045 hang (ptext (sLit "A lazy (~) pattern cannot match existential or GADT data constructors"))
1046 2 (vcat (map pprSkolTvBinding tvs))
1048 lazyUnliftedPatErr :: OutputableBndr name => Pat name -> TcM ()
1049 lazyUnliftedPatErr pat
1051 hang (ptext (sLit "A lazy (~) pattern cannot contain unlifted types"))
1054 nonRigidMatch :: PatCtxt -> DataCon -> SDoc
1055 nonRigidMatch ctxt con
1056 = hang (ptext (sLit "GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1057 2 (ptext (sLit "Probable solution: add a type signature for") <+> what ctxt)
1059 what (APat (FunRhs f _)) = quotes (ppr f)
1060 what (APat CaseAlt) = ptext (sLit "the scrutinee of the case expression")
1061 what (APat LambdaExpr ) = ptext (sLit "the lambda expression")
1062 what (APat (StmtCtxt _)) = ptext (sLit "the right hand side of a do/comprehension binding")
1063 what _other = ptext (sLit "something")
1065 nonRigidResult :: PatCtxt -> Type -> TcM a
1066 nonRigidResult ctxt res_ty
1067 = do { env0 <- tcInitTidyEnv
1068 ; let (env1, res_ty') = tidyOpenType env0 res_ty
1069 msg = hang (ptext (sLit "GADT pattern match with non-rigid result type")
1070 <+> quotes (ppr res_ty'))
1071 2 (ptext (sLit "Solution: add a type signature for")
1073 ; failWithTcM (env1, msg) }
1075 what (APat (FunRhs f _)) = quotes (ppr f)
1076 what (APat CaseAlt) = ptext (sLit "the entire case expression")
1077 what (APat LambdaExpr) = ptext (sLit "the lambda exression")
1078 what (APat (StmtCtxt _)) = ptext (sLit "the entire do/comprehension expression")
1079 what _other = ptext (sLit "something")