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)
39 import BasicTypes hiding (SuccessFlag(..))
40 import DynFlags ( DynFlag( Opt_GADTs ) )
51 %************************************************************************
55 %************************************************************************
58 tcLetPat :: (Name -> Maybe TcRhoType)
59 -> LPat Name -> BoxySigmaType
62 tcLetPat sig_fn pat pat_ty thing_inside
63 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
65 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state
68 -- Don't know how to deal with pattern-bound existentials yet
69 ; checkTc (null ex_tvs) (existentialExplode pat)
71 ; return (pat', res) }
74 tcPats :: HsMatchContext Name
75 -> [LPat Name] -- Patterns,
76 -> [BoxySigmaType] -- and their types
77 -> BoxyRhoType -- Result type,
78 -> (BoxyRhoType -> TcM a) -- and the checker for the body
79 -> TcM ([LPat TcId], a)
81 -- This is the externally-callable wrapper function
82 -- Typecheck the patterns, extend the environment to bind the variables,
83 -- do the thing inside, use any existentially-bound dictionaries to
84 -- discharge parts of the returning LIE, and deal with pattern type
87 -- 1. Initialise the PatState
88 -- 2. Check the patterns
90 -- 4. Check that no existentials escape
92 tcPats ctxt pats tys res_ty thing_inside
93 = tc_lam_pats (APat ctxt) (zipEqual "tcLamPats" pats tys)
96 tcPat :: HsMatchContext Name
97 -> LPat Name -> BoxySigmaType
98 -> BoxyRhoType -- Result type
99 -> (BoxyRhoType -> TcM a) -- Checker for body, given
101 -> TcM (LPat TcId, a)
102 tcPat ctxt = tc_lam_pat (APat ctxt)
104 tc_lam_pat :: PatCtxt -> LPat Name -> BoxySigmaType -> BoxyRhoType
105 -> (BoxyRhoType -> TcM a) -> TcM (LPat TcId, a)
106 tc_lam_pat ctxt pat pat_ty res_ty thing_inside
107 = do { ([pat'],thing) <- tc_lam_pats ctxt [(pat, pat_ty)] res_ty thing_inside
108 ; return (pat', thing) }
111 tc_lam_pats :: PatCtxt
112 -> [(LPat Name,BoxySigmaType)]
113 -> BoxyRhoType -- Result type
114 -> (BoxyRhoType -> TcM a) -- Checker for body, given its result type
115 -> TcM ([LPat TcId], a)
116 tc_lam_pats ctxt pat_ty_prs res_ty thing_inside
117 = do { let init_state = PS { pat_ctxt = ctxt, pat_eqs = False }
119 ; (pats', ex_tvs, res) <- do { traceTc (text "tc_lam_pats" <+> (ppr pat_ty_prs $$ ppr res_ty))
120 ; tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
121 if (pat_eqs pstate' && (not $ isRigidTy res_ty))
122 then nonRigidResult ctxt res_ty
123 else thing_inside res_ty }
125 ; let tys = map snd pat_ty_prs
126 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
128 ; return (pats', res) }
132 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
133 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
134 -> [BoxySigmaType] -- Types of the patterns
135 -> BoxyRhoType -- Type of the body of the match
136 -- Tyvars in either of these must not escape
138 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
139 -- For example, we must reject this program:
140 -- data C = forall a. C (a -> Int)
142 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
144 tcCheckExistentialPat _ [] _ _
145 = return () -- Short cut for case when there are no existentials
147 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
148 = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
149 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
153 pat_eqs :: Bool -- <=> there are any equational constraints
154 -- Used at the end to say whether the result
155 -- type must be rigid
159 = APat (HsMatchContext Name)
160 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
162 notProcPat :: PatCtxt -> Bool
163 notProcPat (APat ProcExpr) = False
166 patSigCtxt :: PatState -> UserTypeCtxt
167 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
168 patSigCtxt _ = LamPatSigCtxt
173 %************************************************************************
177 %************************************************************************
180 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
181 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
182 | Just mono_ty <- lookup_sig bndr_name
183 = do { mono_name <- newLocalName bndr_name
184 ; boxyUnify mono_ty pat_ty
185 ; return (Id.mkLocalId mono_name mono_ty) }
188 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
189 ; mono_name <- newLocalName bndr_name
190 ; return (Id.mkLocalId mono_name pat_ty') }
192 tcPatBndr (PS { pat_ctxt = _lam_or_proc }) bndr_name pat_ty
193 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
194 -- We have an undecorated binder, so we do rule ABS1,
195 -- by unboxing the boxy type, forcing any un-filled-in
196 -- boxes to become monotypes
197 -- NB that pat_ty' can still be a polytype:
198 -- data T = MkT (forall a. a->a)
199 -- f t = case t of { MkT g -> ... }
200 -- Here, the 'g' must get type (forall a. a->a) from the
202 ; return (Id.mkLocalId bndr_name pat_ty') }
206 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
207 bindInstsOfPatId id thing_inside
208 | not (isOverloadedTy (idType id))
209 = do { res <- thing_inside; return (res, emptyLHsBinds) }
211 = do { (res, lie) <- getLIE thing_inside
212 ; binds <- bindInstsOfLocalFuns lie [id]
213 ; return (res, binds) }
216 unBoxPatBndrType :: BoxyType -> Name -> TcM TcType
217 unBoxPatBndrType ty name = unBoxArgType ty (ptext (sLit "The variable") <+> quotes (ppr name))
219 unBoxWildCardType :: BoxyType -> TcM TcType
220 unBoxWildCardType ty = unBoxArgType ty (ptext (sLit "A wild-card pattern"))
222 unBoxViewPatType :: BoxyType -> Pat Name -> TcM TcType
223 unBoxViewPatType ty pat = unBoxArgType ty (ptext (sLit "The view pattern") <+> ppr pat)
225 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
226 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
227 -- that is, it can't be an unboxed tuple. For example,
228 -- case (f x) of r -> ...
229 -- should fail if 'f' returns an unboxed tuple.
230 unBoxArgType ty pp_this
231 = do { ty' <- unBox ty -- Returns a zonked type
233 -- Neither conditional is strictly necesssary (the unify alone will do)
234 -- but they improve error messages, and allocate fewer tyvars
235 ; if isUnboxedTupleType ty' then
237 else if isSubArgTypeKind (typeKind ty') then
239 else do -- OpenTypeKind, so constrain it
240 { ty2 <- newFlexiTyVarTy argTypeKind
244 msg = pp_this <+> ptext (sLit "cannot be bound to an unboxed tuple")
248 %************************************************************************
250 The main worker functions
252 %************************************************************************
256 tcPat takes a "thing inside" over which the pattern scopes. This is partly
257 so that tcPat can extend the environment for the thing_inside, but also
258 so that constraints arising in the thing_inside can be discharged by the
261 This does not work so well for the ErrCtxt carried by the monad: we don't
262 want the error-context for the pattern to scope over the RHS.
263 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
267 type Checker inp out = forall r.
270 -> (PatState -> TcM r)
271 -> TcM (out, [TcTyVar], r)
273 tcMultiple :: Checker inp out -> Checker [inp] [out]
274 tcMultiple tc_pat args pstate thing_inside
275 = do { err_ctxt <- getErrCtxt
277 = do { res <- thing_inside pstate
278 ; return ([], [], res) }
280 loop pstate (arg:args)
281 = do { (p', p_tvs, (ps', ps_tvs, res))
282 <- tc_pat arg pstate $ \ pstate' ->
283 setErrCtxt err_ctxt $
285 -- setErrCtxt: restore context before doing the next pattern
286 -- See note [Nesting] above
288 ; return (p':ps', p_tvs ++ ps_tvs, res) }
293 tc_lpat_pr :: (LPat Name, BoxySigmaType)
295 -> (PatState -> TcM a)
296 -> TcM (LPat TcId, [TcTyVar], a)
297 tc_lpat_pr (pat, ty) = tc_lpat pat ty
302 -> (PatState -> TcM a)
303 -> TcM (LPat TcId, [TcTyVar], a)
304 tc_lpat (L span pat) pat_ty pstate thing_inside
306 maybeAddErrCtxt (patCtxt pat) $
307 do { (pat', tvs, res) <- tc_pat pstate pat pat_ty thing_inside
308 ; return (L span pat', tvs, res) }
313 -> BoxySigmaType -- Fully refined result type
314 -> (PatState -> TcM a) -- Thing inside
315 -> TcM (Pat TcId, -- Translated pattern
316 [TcTyVar], -- Existential binders
317 a) -- Result of thing inside
319 tc_pat pstate (VarPat name) pat_ty thing_inside
320 = do { id <- tcPatBndr pstate name pat_ty
321 ; (res, binds) <- bindInstsOfPatId id $
322 tcExtendIdEnv1 name id $
323 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
324 >> thing_inside pstate)
325 ; let pat' | isEmptyLHsBinds binds = VarPat id
326 | otherwise = VarPatOut id binds
327 ; return (pat', [], res) }
329 tc_pat pstate (ParPat pat) pat_ty thing_inside
330 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
331 ; return (ParPat pat', tvs, res) }
333 tc_pat pstate (BangPat pat) pat_ty thing_inside
334 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
335 ; return (BangPat pat', tvs, res) }
337 -- There's a wrinkle with irrefutable patterns, namely that we
338 -- must not propagate type refinement from them. For example
339 -- data T a where { T1 :: Int -> T Int; ... }
340 -- f :: T a -> Int -> a
342 -- It's obviously not sound to refine a to Int in the right
343 -- hand side, because the arugment might not match T1 at all!
345 -- Nor should a lazy pattern bind any existential type variables
346 -- because they won't be in scope when we do the desugaring
348 -- Note [Hopping the LIE in lazy patterns]
349 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
350 -- In a lazy pattern, we must *not* discharge constraints from the RHS
351 -- from dictionaries bound in the pattern. E.g.
353 -- We can't discharge the Num constraint from dictionaries bound by
356 -- So we have to make the constraints from thing_inside "hop around"
357 -- the pattern. Hence the getLLE and extendLIEs later.
359 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
360 = do { (pat', pat_tvs, (res,lie))
361 <- tc_lpat pat pat_ty pstate $ \ _ ->
362 getLIE (thing_inside pstate)
363 -- Ignore refined pstate', revert to pstate
365 -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
367 -- Check no existentials
368 ; unless (null pat_tvs) $ lazyPatErr lpat pat_tvs
370 -- Check there are no unlifted types under the lazy pattern
371 ; when (any (isUnLiftedType . idType) $ collectPatBinders pat') $
372 lazyUnliftedPatErr lpat
374 -- Check that the pattern has a lifted type
375 ; pat_tv <- newBoxyTyVar liftedTypeKind
376 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
378 ; return (LazyPat pat', [], res) }
380 tc_pat _ p@(QuasiQuotePat _) _ _
381 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
383 tc_pat pstate (WildPat _) pat_ty thing_inside
384 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
385 ; res <- thing_inside pstate
386 ; return (WildPat pat_ty', [], res) }
388 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
389 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
390 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
391 tc_lpat pat (idType bndr_id) pstate thing_inside
392 -- NB: if we do inference on:
393 -- \ (y@(x::forall a. a->a)) = e
394 -- we'll fail. The as-pattern infers a monotype for 'y', which then
395 -- fails to unify with the polymorphic type for 'x'. This could
396 -- perhaps be fixed, but only with a bit more work.
398 -- If you fix it, don't forget the bindInstsOfPatIds!
399 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
401 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
402 = do { -- morally, expr must have type
403 -- `forall a1...aN. OPT' -> B`
404 -- where overall_pat_ty is an instance of OPT'.
405 -- Here, we infer a rho type for it,
406 -- which replaces the leading foralls and constraints
407 -- with fresh unification variables.
408 (expr',expr'_inferred) <- tcInferRho expr
409 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
410 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
411 -- tcSubExp: expected first, offered second
414 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
415 -- variable to find it). this means that in an example like
416 -- (view -> f) where view :: _ -> forall b. b
417 -- we will only be able to use view at one instantation in the
419 ; (expr_coerc, pat_ty) <- tcInfer $ \ pat_ty ->
420 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
422 -- pattern must have pat_ty
423 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
424 -- this should get zonked later on, but we unBox it here
425 -- so that we do the same checks as above
426 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
427 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
429 -- Type signatures in patterns
430 -- See Note [Pattern coercions] below
431 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
432 = do { (inner_ty, tv_binds, coi) <- tcPatSig (patSigCtxt pstate) sig_ty
434 ; unless (isIdentityCoI coi) $
435 failWithTc (badSigPat pat_ty)
436 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
437 tc_lpat pat inner_ty pstate thing_inside
438 ; return (SigPatOut pat' inner_ty, tvs, res) }
440 tc_pat _ pat@(TypePat _) _ _
441 = failWithTc (badTypePat pat)
443 ------------------------
444 -- Lists, tuples, arrays
445 tc_pat pstate (ListPat pats _) pat_ty thing_inside
446 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
447 ; let scoi = mkSymCoI coi
448 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
449 pats pstate thing_inside
450 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
453 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
454 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
455 ; let scoi = mkSymCoI coi
456 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
457 pats pstate thing_inside
458 ; when (null pats) (zapToMonotype pat_ty >> return ()) -- c.f. ExplicitPArr in TcExpr
459 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
462 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
463 = do { let tc = tupleTyCon boxity (length pats)
464 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
465 ; let scoi = mkSymCoI coi
466 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
469 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
470 -- so that we can experiment with lazy tuple-matching.
471 -- This is a pretty odd place to make the switch, but
472 -- it was easy to do.
473 ; let pat_ty' = mkTyConApp tc arg_tys
474 -- pat_ty /= pat_ty iff coi /= IdCo
475 unmangled_result = TuplePat pats' boxity pat_ty'
476 possibly_mangled_result
477 | opt_IrrefutableTuples &&
478 isBoxed boxity = LazyPat (noLoc unmangled_result)
479 | otherwise = unmangled_result
481 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
482 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
485 ------------------------
487 tc_pat pstate (ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
488 = do { data_con <- tcLookupDataCon con_name
489 ; let tycon = dataConTyCon data_con
490 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
492 ------------------------
494 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
495 = do { let lit_ty = hsLitType simple_lit
496 ; coi <- boxyUnify lit_ty pat_ty
497 -- coi is of kind: lit_ty ~ pat_ty
498 ; res <- thing_inside pstate
499 -- pattern coercions have to
500 -- be of kind: pat_ty ~ lit_ty
502 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
505 ------------------------
506 -- Overloaded patterns: n, and n+k
507 tc_pat pstate (NPat over_lit mb_neg eq) pat_ty thing_inside
508 = do { let orig = LiteralOrigin over_lit
509 ; lit' <- tcOverloadedLit orig over_lit pat_ty
510 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
511 ; mb_neg' <- case mb_neg of
512 Nothing -> return Nothing -- Positive literal
513 Just neg -> -- Negative literal
514 -- The 'negate' is re-mappable syntax
515 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
516 ; return (Just neg') }
517 ; res <- thing_inside pstate
518 ; return (NPat lit' mb_neg' eq', [], res) }
520 tc_pat pstate (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
521 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
522 ; let pat_ty' = idType bndr_id
523 orig = LiteralOrigin lit
524 ; lit' <- tcOverloadedLit orig lit pat_ty'
526 -- The '>=' and '-' parts are re-mappable syntax
527 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
528 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
530 -- The Report says that n+k patterns must be in Integral
531 -- We may not want this when using re-mappable syntax, though (ToDo?)
532 ; icls <- tcLookupClass integralClassName
533 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
535 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
536 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
538 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
542 %************************************************************************
544 Most of the work for constructors is here
545 (the rest is in the ConPatIn case of tc_pat)
547 %************************************************************************
549 [Pattern matching indexed data types]
550 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
551 Consider the following declarations:
553 data family Map k :: * -> *
554 data instance Map (a, b) v = MapPair (Map a (Pair b v))
556 and a case expression
558 case x :: Map (Int, c) w of MapPair m -> ...
560 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
561 worker/wrapper types for MapPair are
563 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
564 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
566 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
567 :R123Map, which means the straight use of boxySplitTyConApp would give a type
568 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
569 boxySplitTyConApp with the family tycon Map instead, which gives us the family
570 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
571 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
572 (provided by tyConFamInst_maybe together with the family tycon). This
573 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
574 the split arguments for the representation tycon :R123Map as {Int, c, w}
576 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
578 Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
580 moving between representation and family type into account. To produce type
581 correct Core, this coercion needs to be used to case the type of the scrutinee
582 from the family to the representation type. This is achieved by
583 unwrapFamInstScrutinee using a CoPat around the result pattern.
585 Now it might appear seem as if we could have used the previous GADT type
586 refinement infrastructure of refineAlt and friends instead of the explicit
587 unification and CoPat generation. However, that would be wrong. Why? The
588 whole point of GADT refinement is that the refinement is local to the case
589 alternative. In contrast, the substitution generated by the unification of
590 the family type list and instance types needs to be propagated to the outside.
591 Imagine that in the above example, the type of the scrutinee would have been
592 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
593 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
594 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
595 alternatives of the case expression, whereas in the GADT case it might vary
596 between alternatives.
598 RIP GADT refinement: refinements have been replaced by the use of explicit
599 equality constraints that are used in conjunction with implication constraints
600 to express the local scope of GADT refinements.
604 -- MkT :: forall a b c. (a~[b]) => b -> c -> T a
605 -- with scrutinee of type (T ty)
607 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
608 -> BoxySigmaType -- Type of the pattern
609 -> HsConPatDetails Name -> (PatState -> TcM a)
610 -> TcM (Pat TcId, [TcTyVar], a)
611 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
612 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
613 = dataConFullSig data_con
614 skol_info = PatSkol data_con
615 origin = SigOrigin skol_info
616 full_theta = eq_theta ++ dict_theta
618 -- Instantiate the constructor type variables [a->ty]
619 -- This may involve doing a family-instance coercion, and building a
621 ; (ctxt_res_tys, coi, unwrap_ty) <- boxySplitTyConAppWithFamily tycon
623 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
624 pat_ty' = mkTyConApp tycon ctxt_res_tys
625 -- pat_ty' /= pat_ty iff coi /= IdCo
627 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
629 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys
632 -- Add the stupid theta
633 ; addDataConStupidTheta data_con ctxt_res_tys
635 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
636 -- Get location from monad, not from ex_tvs
638 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
639 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
640 arg_tys' = substTys tenv arg_tys
642 ; if null ex_tvs && null eq_spec && null full_theta
643 then do { -- The common case; no class bindings etc
644 -- (see Note [Arrows and patterns])
645 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
646 arg_pats pstate thing_inside
647 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
648 pat_tvs = [], pat_dicts = [],
649 pat_binds = emptyLHsBinds,
650 pat_args = arg_pats',
653 ; return (wrap_res_pat res_pat, inner_tvs, res) }
655 else do -- The general case, with existential, and local equality
657 { checkTc (notProcPat (pat_ctxt pstate))
658 (existentialProcPat data_con)
659 -- See Note [Arrows and patterns]
661 -- Need to test for rigidity if *any* constraints in theta as class
662 -- constraints may have superclass equality constraints. However,
663 -- we don't want to check for rigidity if we got here only because
664 -- ex_tvs was non-null.
665 -- ; unless (null theta') $
666 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
667 -- if we explicitly or implicitly (by a GADT def) have equality
669 ; let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
670 theta' = substTheta tenv (eq_preds ++ full_theta)
671 -- order is *important* as we generate the list of
672 -- dictionary binders from theta'
673 no_equalities = not (any isEqPred theta')
674 pstate' | no_equalities = pstate
675 | otherwise = pstate { pat_eqs = True }
677 ; gadts_on <- doptM Opt_GADTs
678 ; checkTc (no_equalities || gadts_on)
679 (ptext (sLit "A pattern match on a GADT requires -XGADTs"))
680 -- Trac #2905 decided that a *pattern-match* of a GADT
681 -- should require the GADT language flag
683 ; unless no_equalities $ checkTc (isRigidTy pat_ty) $
684 nonRigidMatch (pat_ctxt pstate) data_con
686 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
687 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
689 ; loc <- getInstLoc origin
690 ; dicts <- newDictBndrs loc theta'
691 ; dict_binds <- tcSimplifyCheckPat loc ex_tvs' dicts lie_req
693 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
695 pat_dicts = map instToVar dicts,
696 pat_binds = dict_binds,
697 pat_args = arg_pats', pat_ty = pat_ty' }
698 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
701 -- Split against the family tycon if the pattern constructor
702 -- belongs to a family instance tycon.
703 boxySplitTyConAppWithFamily tycon pat_ty =
705 case tyConFamInst_maybe tycon of
707 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp tycon pat_ty
708 ; return (scrutinee_arg_tys, coi1, pat_ty)
710 Just (fam_tycon, instTys) ->
711 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp fam_tycon pat_ty
712 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
713 ; let instTys' = substTys subst instTys
714 ; cois <- boxyUnifyList instTys' scrutinee_arg_tys
715 ; let coi = if isIdentityCoI coi1
716 then -- pat_ty was splittable
717 -- => boxyUnifyList had real work to do
718 mkTyConAppCoI fam_tycon instTys' cois
719 else -- pat_ty was not splittable
720 -- => scrutinee_arg_tys are fresh tvs and
721 -- boxyUnifyList just instantiated those
723 ; return (freshTvs, coi, mkTyConApp fam_tycon instTys')
725 -- iff cois is non-trivial
728 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
729 ppr tycon <+> ppr pat_ty
730 , text " family instance:" <+>
731 ppr (tyConFamInst_maybe tycon)
734 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
735 -- pattern if the tycon is an instance of a family.
737 unwrapFamInstScrutinee :: TyCon -> [Type] -> Type -> Pat Id -> Pat Id
738 unwrapFamInstScrutinee tycon args unwrap_ty pat
739 | Just co_con <- tyConFamilyCoercion_maybe tycon
740 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
742 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
743 -- coercion is not the identity; mkCoPat is inconvenient as it
744 -- wants a located pattern.
745 = CoPat (WpCast $ mkTyConApp co_con args) -- co fam ty to repr ty
746 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
747 unwrap_ty -- family inst type
751 tcConArgs :: DataCon -> [TcSigmaType]
752 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
754 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
755 = do { checkTc (con_arity == no_of_args) -- Check correct arity
756 (arityErr "Constructor" data_con con_arity no_of_args)
757 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
758 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
760 ; return (PrefixCon arg_pats', tvs, res) }
762 con_arity = dataConSourceArity data_con
763 no_of_args = length arg_pats
765 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
766 = do { checkTc (con_arity == 2) -- Check correct arity
767 (arityErr "Constructor" data_con con_arity 2)
768 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
769 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
771 ; return (InfixCon p1' p2', tvs, res) }
773 con_arity = dataConSourceArity data_con
775 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
776 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
777 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
779 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
780 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
781 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
782 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
783 ; return (HsRecField sel_id pat' pun, tvs, res) }
785 find_field_ty :: FieldLabel -> TcM (Id, TcType)
786 find_field_ty field_lbl
787 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
789 -- No matching field; chances are this field label comes from some
790 -- other record type (or maybe none). As well as reporting an
791 -- error we still want to typecheck the pattern, principally to
792 -- make sure that all the variables it binds are put into the
793 -- environment, else the type checker crashes later:
794 -- f (R { foo = (a,b) }) = a+b
795 -- If foo isn't one of R's fields, we don't want to crash when
796 -- typechecking the "a+b".
797 [] -> do { addErrTc (badFieldCon data_con field_lbl)
798 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
799 ; return (error "Bogus selector Id", bogus_ty) }
801 -- The normal case, when the field comes from the right constructor
803 ASSERT( null extras )
804 do { sel_id <- tcLookupField field_lbl
805 ; return (sel_id, pat_ty) }
807 field_tys :: [(FieldLabel, TcType)]
808 field_tys = zip (dataConFieldLabels data_con) arg_tys
809 -- Don't use zipEqual! If the constructor isn't really a record, then
810 -- dataConFieldLabels will be empty (and each field in the pattern
811 -- will generate an error below).
813 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
814 tcConArg (arg_pat, arg_ty) pstate thing_inside
815 = tc_lpat arg_pat arg_ty pstate thing_inside
819 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
820 -- Instantiate the "stupid theta" of the data con, and throw
821 -- the constraints into the constraint set
822 addDataConStupidTheta data_con inst_tys
823 | null stupid_theta = return ()
824 | otherwise = instStupidTheta origin inst_theta
826 origin = OccurrenceOf (dataConName data_con)
827 -- The origin should always report "occurrence of C"
828 -- even when C occurs in a pattern
829 stupid_theta = dataConStupidTheta data_con
830 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
831 -- NB: inst_tys can be longer than the univ tyvars
832 -- because the constructor might have existentials
833 inst_theta = substTheta tenv stupid_theta
836 Note [Arrows and patterns]
837 ~~~~~~~~~~~~~~~~~~~~~~~~~~
838 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
840 expr :: Arrow a => a () Int
841 expr = proc (y,z) -> do
845 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
846 arrow body (e.g 'term'). As things stand (bogusly) all the
847 constraints from the proc body are gathered together, so constraints
848 from 'term' will be seen by the tcPat for (y,z). But we must *not*
849 bind constraints from 'term' here, becuase the desugarer will not make
850 these bindings scope over 'term'.
852 The Right Thing is not to confuse these constraints together. But for
853 now the Easy Thing is to ensure that we do not have existential or
854 GADT constraints in a 'proc', and to short-cut the constraint
855 simplification for such vanilla patterns so that it binds no
856 constraints. Hence the 'fast path' in tcConPat; but it's also a good
857 plan for ordinary vanilla patterns to bypass the constraint
861 %************************************************************************
865 %************************************************************************
867 In tcOverloadedLit we convert directly to an Int or Integer if we
868 know that's what we want. This may save some time, by not
869 temporarily generating overloaded literals, but it won't catch all
870 cases (the rest are caught in lookupInst).
873 tcOverloadedLit :: InstOrigin
876 -> TcM (HsOverLit TcId)
877 tcOverloadedLit orig lit@(OverLit { ol_val = val, ol_rebindable = rebindable
878 , ol_witness = meth_name }) res_ty
880 -- Do not generate a LitInst for rebindable syntax.
881 -- Reason: If we do, tcSimplify will call lookupInst, which
882 -- will call tcSyntaxName, which does unification,
883 -- which tcSimplify doesn't like
884 -- ToDo: noLoc sadness
885 = do { hs_lit <- mkOverLit val
886 ; let lit_ty = hsLitType hs_lit
887 ; fi' <- tcSyntaxOp orig meth_name (mkFunTy lit_ty res_ty)
888 -- Overloaded literals must have liftedTypeKind, because
889 -- we're instantiating an overloaded function here,
890 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
891 -- However this'll be picked up by tcSyntaxOp if necessary
892 ; let witness = HsApp (noLoc fi') (noLoc (HsLit hs_lit))
893 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
895 | Just expr <- shortCutLit val res_ty
896 = return (lit { ol_witness = expr, ol_type = res_ty })
899 = do { loc <- getInstLoc orig
900 ; res_tau <- zapToMonotype res_ty
901 ; new_uniq <- newUnique
902 ; let lit_nm = mkSystemVarName new_uniq (fsLit "lit")
903 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
904 tci_ty = res_tau, tci_loc = loc}
905 witness = HsVar (instToId lit_inst)
907 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
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 %************************************************************************
979 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
980 patCtxt (VarPat _) = Nothing
981 patCtxt (ParPat _) = Nothing
982 patCtxt (AsPat _ _) = Nothing
983 patCtxt pat = Just (hang (ptext (sLit "In the pattern:"))
986 -----------------------------------------------
988 existentialExplode :: LPat Name -> SDoc
989 existentialExplode pat
990 = hang (vcat [text "My brain just exploded.",
991 text "I can't handle pattern bindings for existential or GADT data constructors.",
992 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
993 text "In the binding group for"])
996 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
997 -> TcM (TidyEnv, SDoc)
998 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
999 = do { pat_tys' <- mapM zonkTcType pat_tys
1000 ; body_ty' <- zonkTcType body_ty
1001 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1002 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1003 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1005 sep [ptext (sLit "When checking an existential match that binds"),
1006 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1007 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1008 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
1011 bound_ids = collectPatsBinders pats
1012 show_ids = filter is_interesting bound_ids
1013 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1015 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1016 -- Don't zonk the types so we get the separate, un-unified versions
1018 badFieldCon :: DataCon -> Name -> SDoc
1019 badFieldCon con field
1020 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
1021 ptext (sLit "does not have field"), quotes (ppr field)]
1023 polyPatSig :: TcType -> SDoc
1025 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1028 badSigPat :: TcType -> SDoc
1029 badSigPat pat_ty = ptext (sLit "Pattern signature must exactly match:") <+>
1032 badTypePat :: Pat Name -> SDoc
1033 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1035 existentialProcPat :: DataCon -> SDoc
1036 existentialProcPat con
1037 = hang (ptext (sLit "Illegal constructor") <+> quotes (ppr con) <+> ptext (sLit "in a 'proc' pattern"))
1038 2 (ptext (sLit "Proc patterns cannot use existentials or GADTs"))
1040 lazyPatErr :: Pat name -> [TcTyVar] -> TcM ()
1043 hang (ptext (sLit "A lazy (~) pattern cannot match existential or GADT data constructors"))
1044 2 (vcat (map pprSkolTvBinding tvs))
1046 lazyUnliftedPatErr :: OutputableBndr name => Pat name -> TcM ()
1047 lazyUnliftedPatErr pat
1049 hang (ptext (sLit "A lazy (~) pattern cannot contain unlifted types"))
1052 nonRigidMatch :: PatCtxt -> DataCon -> SDoc
1053 nonRigidMatch ctxt con
1054 = hang (ptext (sLit "GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1055 2 (ptext (sLit "Probable solution: add a type signature for") <+> what ctxt)
1057 what (APat (FunRhs f _)) = quotes (ppr f)
1058 what (APat CaseAlt) = ptext (sLit "the scrutinee of the case expression")
1059 what (APat LambdaExpr ) = ptext (sLit "the lambda expression")
1060 what (APat (StmtCtxt _)) = ptext (sLit "the right hand side of a do/comprehension binding")
1061 what _other = ptext (sLit "something")
1063 nonRigidResult :: PatCtxt -> Type -> TcM a
1064 nonRigidResult ctxt res_ty
1065 = do { env0 <- tcInitTidyEnv
1066 ; let (env1, res_ty') = tidyOpenType env0 res_ty
1067 msg = hang (ptext (sLit "GADT pattern match with non-rigid result type")
1068 <+> quotes (ppr res_ty'))
1069 2 (ptext (sLit "Solution: add a type signature for")
1071 ; failWithTcM (env1, msg) }
1073 what (APat (FunRhs f _)) = quotes (ppr f)
1074 what (APat CaseAlt) = ptext (sLit "the entire case expression")
1075 what (APat LambdaExpr) = ptext (sLit "the lambda exression")
1076 what (APat (StmtCtxt _)) = ptext (sLit "the entire do/comprehension expression")
1077 what _other = ptext (sLit "something")