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 ; if (null pat_tvs) then return ()
369 else lazyPatErr lpat pat_tvs
371 -- Check that the pattern has a lifted type
372 ; pat_tv <- newBoxyTyVar liftedTypeKind
373 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
375 ; return (LazyPat pat', [], res) }
377 tc_pat _ p@(QuasiQuotePat _) _ _
378 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
380 tc_pat pstate (WildPat _) pat_ty thing_inside
381 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
382 ; res <- thing_inside pstate
383 ; return (WildPat pat_ty', [], res) }
385 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
386 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
387 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
388 tc_lpat pat (idType bndr_id) pstate thing_inside
389 -- NB: if we do inference on:
390 -- \ (y@(x::forall a. a->a)) = e
391 -- we'll fail. The as-pattern infers a monotype for 'y', which then
392 -- fails to unify with the polymorphic type for 'x'. This could
393 -- perhaps be fixed, but only with a bit more work.
395 -- If you fix it, don't forget the bindInstsOfPatIds!
396 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
398 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
399 = do { -- morally, expr must have type
400 -- `forall a1...aN. OPT' -> B`
401 -- where overall_pat_ty is an instance of OPT'.
402 -- Here, we infer a rho type for it,
403 -- which replaces the leading foralls and constraints
404 -- with fresh unification variables.
405 (expr',expr'_inferred) <- tcInferRho expr
406 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
407 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
408 -- tcSubExp: expected first, offered second
411 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
412 -- variable to find it). this means that in an example like
413 -- (view -> f) where view :: _ -> forall b. b
414 -- we will only be able to use view at one instantation in the
416 ; (expr_coerc, pat_ty) <- tcInfer $ \ pat_ty ->
417 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
419 -- pattern must have pat_ty
420 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
421 -- this should get zonked later on, but we unBox it here
422 -- so that we do the same checks as above
423 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
424 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
426 -- Type signatures in patterns
427 -- See Note [Pattern coercions] below
428 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
429 = do { (inner_ty, tv_binds, coi) <- tcPatSig (patSigCtxt pstate) sig_ty
431 ; unless (isIdentityCoI coi) $
432 failWithTc (badSigPat pat_ty)
433 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
434 tc_lpat pat inner_ty pstate thing_inside
435 ; return (SigPatOut pat' inner_ty, tvs, res) }
437 tc_pat _ pat@(TypePat _) _ _
438 = failWithTc (badTypePat pat)
440 ------------------------
441 -- Lists, tuples, arrays
442 tc_pat pstate (ListPat pats _) pat_ty thing_inside
443 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
444 ; let scoi = mkSymCoI coi
445 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
446 pats pstate thing_inside
447 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
450 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
451 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
452 ; let scoi = mkSymCoI coi
453 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
454 pats pstate thing_inside
455 ; when (null pats) (zapToMonotype pat_ty >> return ()) -- c.f. ExplicitPArr in TcExpr
456 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
459 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
460 = do { let tc = tupleTyCon boxity (length pats)
461 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
462 ; let scoi = mkSymCoI coi
463 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
466 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
467 -- so that we can experiment with lazy tuple-matching.
468 -- This is a pretty odd place to make the switch, but
469 -- it was easy to do.
470 ; let pat_ty' = mkTyConApp tc arg_tys
471 -- pat_ty /= pat_ty iff coi /= IdCo
472 unmangled_result = TuplePat pats' boxity pat_ty'
473 possibly_mangled_result
474 | opt_IrrefutableTuples &&
475 isBoxed boxity = LazyPat (noLoc unmangled_result)
476 | otherwise = unmangled_result
478 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
479 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
482 ------------------------
484 tc_pat pstate (ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
485 = do { data_con <- tcLookupDataCon con_name
486 ; let tycon = dataConTyCon data_con
487 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
489 ------------------------
491 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
492 = do { let lit_ty = hsLitType simple_lit
493 ; coi <- boxyUnify lit_ty pat_ty
494 -- coi is of kind: lit_ty ~ pat_ty
495 ; res <- thing_inside pstate
496 -- pattern coercions have to
497 -- be of kind: pat_ty ~ lit_ty
499 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
502 ------------------------
503 -- Overloaded patterns: n, and n+k
504 tc_pat pstate (NPat over_lit mb_neg eq) pat_ty thing_inside
505 = do { let orig = LiteralOrigin over_lit
506 ; lit' <- tcOverloadedLit orig over_lit pat_ty
507 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
508 ; mb_neg' <- case mb_neg of
509 Nothing -> return Nothing -- Positive literal
510 Just neg -> -- Negative literal
511 -- The 'negate' is re-mappable syntax
512 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
513 ; return (Just neg') }
514 ; res <- thing_inside pstate
515 ; return (NPat lit' mb_neg' eq', [], res) }
517 tc_pat pstate (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
518 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
519 ; let pat_ty' = idType bndr_id
520 orig = LiteralOrigin lit
521 ; lit' <- tcOverloadedLit orig lit pat_ty'
523 -- The '>=' and '-' parts are re-mappable syntax
524 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
525 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
527 -- The Report says that n+k patterns must be in Integral
528 -- We may not want this when using re-mappable syntax, though (ToDo?)
529 ; icls <- tcLookupClass integralClassName
530 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
532 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
533 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
535 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
539 %************************************************************************
541 Most of the work for constructors is here
542 (the rest is in the ConPatIn case of tc_pat)
544 %************************************************************************
546 [Pattern matching indexed data types]
547 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
548 Consider the following declarations:
550 data family Map k :: * -> *
551 data instance Map (a, b) v = MapPair (Map a (Pair b v))
553 and a case expression
555 case x :: Map (Int, c) w of MapPair m -> ...
557 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
558 worker/wrapper types for MapPair are
560 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
561 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
563 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
564 :R123Map, which means the straight use of boxySplitTyConApp would give a type
565 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
566 boxySplitTyConApp with the family tycon Map instead, which gives us the family
567 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
568 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
569 (provided by tyConFamInst_maybe together with the family tycon). This
570 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
571 the split arguments for the representation tycon :R123Map as {Int, c, w}
573 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
575 Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
577 moving between representation and family type into account. To produce type
578 correct Core, this coercion needs to be used to case the type of the scrutinee
579 from the family to the representation type. This is achieved by
580 unwrapFamInstScrutinee using a CoPat around the result pattern.
582 Now it might appear seem as if we could have used the previous GADT type
583 refinement infrastructure of refineAlt and friends instead of the explicit
584 unification and CoPat generation. However, that would be wrong. Why? The
585 whole point of GADT refinement is that the refinement is local to the case
586 alternative. In contrast, the substitution generated by the unification of
587 the family type list and instance types needs to be propagated to the outside.
588 Imagine that in the above example, the type of the scrutinee would have been
589 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
590 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
591 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
592 alternatives of the case expression, whereas in the GADT case it might vary
593 between alternatives.
595 RIP GADT refinement: refinements have been replaced by the use of explicit
596 equality constraints that are used in conjunction with implication constraints
597 to express the local scope of GADT refinements.
601 -- MkT :: forall a b c. (a~[b]) => b -> c -> T a
602 -- with scrutinee of type (T ty)
604 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
605 -> BoxySigmaType -- Type of the pattern
606 -> HsConPatDetails Name -> (PatState -> TcM a)
607 -> TcM (Pat TcId, [TcTyVar], a)
608 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
609 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
610 = dataConFullSig data_con
611 skol_info = PatSkol data_con
612 origin = SigOrigin skol_info
613 full_theta = eq_theta ++ dict_theta
615 -- Instantiate the constructor type variables [a->ty]
616 -- This may involve doing a family-instance coercion, and building a
618 ; (ctxt_res_tys, coi, unwrap_ty) <- boxySplitTyConAppWithFamily tycon
620 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
621 pat_ty' = mkTyConApp tycon ctxt_res_tys
622 -- pat_ty' /= pat_ty iff coi /= IdCo
624 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
626 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys
629 -- Add the stupid theta
630 ; addDataConStupidTheta data_con ctxt_res_tys
632 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
633 -- Get location from monad, not from ex_tvs
635 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
636 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
637 arg_tys' = substTys tenv arg_tys
639 ; if null ex_tvs && null eq_spec && null full_theta
640 then do { -- The common case; no class bindings etc
641 -- (see Note [Arrows and patterns])
642 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
643 arg_pats pstate thing_inside
644 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
645 pat_tvs = [], pat_dicts = [],
646 pat_binds = emptyLHsBinds,
647 pat_args = arg_pats',
650 ; return (wrap_res_pat res_pat, inner_tvs, res) }
652 else do -- The general case, with existential, and local equality
654 { checkTc (notProcPat (pat_ctxt pstate))
655 (existentialProcPat data_con)
656 -- See Note [Arrows and patterns]
658 -- Need to test for rigidity if *any* constraints in theta as class
659 -- constraints may have superclass equality constraints. However,
660 -- we don't want to check for rigidity if we got here only because
661 -- ex_tvs was non-null.
662 -- ; unless (null theta') $
663 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
664 -- if we explicitly or implicitly (by a GADT def) have equality
666 ; let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
667 theta' = substTheta tenv (eq_preds ++ full_theta)
668 -- order is *important* as we generate the list of
669 -- dictionary binders from theta'
670 no_equalities = not (any isEqPred theta')
671 pstate' | no_equalities = pstate
672 | otherwise = pstate { pat_eqs = True }
674 ; gadts_on <- doptM Opt_GADTs
675 ; checkTc (no_equalities || gadts_on)
676 (ptext (sLit "A pattern match on a GADT requires -XGADTs"))
677 -- Trac #2905 decided that a *pattern-match* of a GADT
678 -- should require the GADT language flag
680 ; unless no_equalities $ checkTc (isRigidTy pat_ty) $
681 nonRigidMatch (pat_ctxt pstate) data_con
683 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
684 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
686 ; loc <- getInstLoc origin
687 ; dicts <- newDictBndrs loc theta'
688 ; dict_binds <- tcSimplifyCheckPat loc ex_tvs' dicts lie_req
690 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
692 pat_dicts = map instToVar dicts,
693 pat_binds = dict_binds,
694 pat_args = arg_pats', pat_ty = pat_ty' }
695 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
698 -- Split against the family tycon if the pattern constructor
699 -- belongs to a family instance tycon.
700 boxySplitTyConAppWithFamily tycon pat_ty =
702 case tyConFamInst_maybe tycon of
704 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp tycon pat_ty
705 ; return (scrutinee_arg_tys, coi1, pat_ty)
707 Just (fam_tycon, instTys) ->
708 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp fam_tycon pat_ty
709 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
710 ; let instTys' = substTys subst instTys
711 ; cois <- boxyUnifyList instTys' scrutinee_arg_tys
712 ; let coi = if isIdentityCoI coi1
713 then -- pat_ty was splittable
714 -- => boxyUnifyList had real work to do
715 mkTyConAppCoI fam_tycon instTys' cois
716 else -- pat_ty was not splittable
717 -- => scrutinee_arg_tys are fresh tvs and
718 -- boxyUnifyList just instantiated those
720 ; return (freshTvs, coi, mkTyConApp fam_tycon instTys')
722 -- iff cois is non-trivial
725 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
726 ppr tycon <+> ppr pat_ty
727 , text " family instance:" <+>
728 ppr (tyConFamInst_maybe tycon)
731 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
732 -- pattern if the tycon is an instance of a family.
734 unwrapFamInstScrutinee :: TyCon -> [Type] -> Type -> Pat Id -> Pat Id
735 unwrapFamInstScrutinee tycon args unwrap_ty pat
736 | Just co_con <- tyConFamilyCoercion_maybe tycon
737 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
739 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
740 -- coercion is not the identity; mkCoPat is inconvenient as it
741 -- wants a located pattern.
742 = CoPat (WpCast $ mkTyConApp co_con args) -- co fam ty to repr ty
743 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
744 unwrap_ty -- family inst type
748 tcConArgs :: DataCon -> [TcSigmaType]
749 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
751 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
752 = do { checkTc (con_arity == no_of_args) -- Check correct arity
753 (arityErr "Constructor" data_con con_arity no_of_args)
754 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
755 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
757 ; return (PrefixCon arg_pats', tvs, res) }
759 con_arity = dataConSourceArity data_con
760 no_of_args = length arg_pats
762 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
763 = do { checkTc (con_arity == 2) -- Check correct arity
764 (arityErr "Constructor" data_con con_arity 2)
765 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
766 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
768 ; return (InfixCon p1' p2', tvs, res) }
770 con_arity = dataConSourceArity data_con
772 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
773 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
774 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
776 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
777 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
778 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
779 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
780 ; return (HsRecField sel_id pat' pun, tvs, res) }
782 find_field_ty :: FieldLabel -> TcM (Id, TcType)
783 find_field_ty field_lbl
784 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
786 -- No matching field; chances are this field label comes from some
787 -- other record type (or maybe none). As well as reporting an
788 -- error we still want to typecheck the pattern, principally to
789 -- make sure that all the variables it binds are put into the
790 -- environment, else the type checker crashes later:
791 -- f (R { foo = (a,b) }) = a+b
792 -- If foo isn't one of R's fields, we don't want to crash when
793 -- typechecking the "a+b".
794 [] -> do { addErrTc (badFieldCon data_con field_lbl)
795 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
796 ; return (error "Bogus selector Id", bogus_ty) }
798 -- The normal case, when the field comes from the right constructor
800 ASSERT( null extras )
801 do { sel_id <- tcLookupField field_lbl
802 ; return (sel_id, pat_ty) }
804 field_tys :: [(FieldLabel, TcType)]
805 field_tys = zip (dataConFieldLabels data_con) arg_tys
806 -- Don't use zipEqual! If the constructor isn't really a record, then
807 -- dataConFieldLabels will be empty (and each field in the pattern
808 -- will generate an error below).
810 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
811 tcConArg (arg_pat, arg_ty) pstate thing_inside
812 = tc_lpat arg_pat arg_ty pstate thing_inside
816 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
817 -- Instantiate the "stupid theta" of the data con, and throw
818 -- the constraints into the constraint set
819 addDataConStupidTheta data_con inst_tys
820 | null stupid_theta = return ()
821 | otherwise = instStupidTheta origin inst_theta
823 origin = OccurrenceOf (dataConName data_con)
824 -- The origin should always report "occurrence of C"
825 -- even when C occurs in a pattern
826 stupid_theta = dataConStupidTheta data_con
827 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
828 -- NB: inst_tys can be longer than the univ tyvars
829 -- because the constructor might have existentials
830 inst_theta = substTheta tenv stupid_theta
833 Note [Arrows and patterns]
834 ~~~~~~~~~~~~~~~~~~~~~~~~~~
835 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
837 expr :: Arrow a => a () Int
838 expr = proc (y,z) -> do
842 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
843 arrow body (e.g 'term'). As things stand (bogusly) all the
844 constraints from the proc body are gathered together, so constraints
845 from 'term' will be seen by the tcPat for (y,z). But we must *not*
846 bind constraints from 'term' here, becuase the desugarer will not make
847 these bindings scope over 'term'.
849 The Right Thing is not to confuse these constraints together. But for
850 now the Easy Thing is to ensure that we do not have existential or
851 GADT constraints in a 'proc', and to short-cut the constraint
852 simplification for such vanilla patterns so that it binds no
853 constraints. Hence the 'fast path' in tcConPat; but it's also a good
854 plan for ordinary vanilla patterns to bypass the constraint
858 %************************************************************************
862 %************************************************************************
864 In tcOverloadedLit we convert directly to an Int or Integer if we
865 know that's what we want. This may save some time, by not
866 temporarily generating overloaded literals, but it won't catch all
867 cases (the rest are caught in lookupInst).
870 tcOverloadedLit :: InstOrigin
873 -> TcM (HsOverLit TcId)
874 tcOverloadedLit orig lit@(OverLit { ol_val = val, ol_rebindable = rebindable
875 , ol_witness = meth_name }) res_ty
877 -- Do not generate a LitInst for rebindable syntax.
878 -- Reason: If we do, tcSimplify will call lookupInst, which
879 -- will call tcSyntaxName, which does unification,
880 -- which tcSimplify doesn't like
881 -- ToDo: noLoc sadness
882 = do { hs_lit <- mkOverLit val
883 ; let lit_ty = hsLitType hs_lit
884 ; fi' <- tcSyntaxOp orig meth_name (mkFunTy lit_ty res_ty)
885 -- Overloaded literals must have liftedTypeKind, because
886 -- we're instantiating an overloaded function here,
887 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
888 -- However this'll be picked up by tcSyntaxOp if necessary
889 ; let witness = HsApp (noLoc fi') (noLoc (HsLit hs_lit))
890 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
892 | Just expr <- shortCutLit val res_ty
893 = return (lit { ol_witness = expr, ol_type = res_ty })
896 = do { loc <- getInstLoc orig
897 ; res_tau <- zapToMonotype res_ty
898 ; new_uniq <- newUnique
899 ; let lit_nm = mkSystemVarName new_uniq (fsLit "lit")
900 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
901 tci_ty = res_tau, tci_loc = loc}
902 witness = HsVar (instToId lit_inst)
904 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
908 %************************************************************************
910 Note [Pattern coercions]
912 %************************************************************************
914 In principle, these program would be reasonable:
916 f :: (forall a. a->a) -> Int
917 f (x :: Int->Int) = x 3
919 g :: (forall a. [a]) -> Bool
922 In both cases, the function type signature restricts what arguments can be passed
923 in a call (to polymorphic ones). The pattern type signature then instantiates this
924 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
925 generate the translated term
926 f = \x' :: (forall a. a->a). let x = x' Int in x 3
928 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
929 And it requires a significant amount of code to implement, becuase we need to decorate
930 the translated pattern with coercion functions (generated from the subsumption check
933 So for now I'm just insisting on type *equality* in patterns. No subsumption.
935 Old notes about desugaring, at a time when pattern coercions were handled:
937 A SigPat is a type coercion and must be handled one at at time. We can't
938 combine them unless the type of the pattern inside is identical, and we don't
939 bother to check for that. For example:
941 data T = T1 Int | T2 Bool
942 f :: (forall a. a -> a) -> T -> t
943 f (g::Int->Int) (T1 i) = T1 (g i)
944 f (g::Bool->Bool) (T2 b) = T2 (g b)
946 We desugar this as follows:
948 f = \ g::(forall a. a->a) t::T ->
950 in case t of { T1 i -> T1 (gi i)
953 in case t of { T2 b -> T2 (gb b)
956 Note that we do not treat the first column of patterns as a
957 column of variables, because the coerced variables (gi, gb)
958 would be of different types. So we get rather grotty code.
959 But I don't think this is a common case, and if it was we could
960 doubtless improve it.
962 Meanwhile, the strategy is:
963 * treat each SigPat coercion (always non-identity coercions)
965 * deal with the stuff inside, and then wrap a binding round
966 the result to bind the new variable (gi, gb, etc)
969 %************************************************************************
971 \subsection{Errors and contexts}
973 %************************************************************************
976 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
977 patCtxt (VarPat _) = Nothing
978 patCtxt (ParPat _) = Nothing
979 patCtxt (AsPat _ _) = Nothing
980 patCtxt pat = Just (hang (ptext (sLit "In the pattern:"))
983 -----------------------------------------------
985 existentialExplode :: LPat Name -> SDoc
986 existentialExplode pat
987 = hang (vcat [text "My brain just exploded.",
988 text "I can't handle pattern bindings for existential or GADT data constructors.",
989 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
990 text "In the binding group for"])
993 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
994 -> TcM (TidyEnv, SDoc)
995 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
996 = do { pat_tys' <- mapM zonkTcType pat_tys
997 ; body_ty' <- zonkTcType body_ty
998 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
999 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1000 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1002 sep [ptext (sLit "When checking an existential match that binds"),
1003 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1004 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1005 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
1008 bound_ids = collectPatsBinders pats
1009 show_ids = filter is_interesting bound_ids
1010 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1012 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1013 -- Don't zonk the types so we get the separate, un-unified versions
1015 badFieldCon :: DataCon -> Name -> SDoc
1016 badFieldCon con field
1017 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
1018 ptext (sLit "does not have field"), quotes (ppr field)]
1020 polyPatSig :: TcType -> SDoc
1022 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1025 badSigPat :: TcType -> SDoc
1026 badSigPat pat_ty = ptext (sLit "Pattern signature must exactly match:") <+>
1029 badTypePat :: Pat Name -> SDoc
1030 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1032 existentialProcPat :: DataCon -> SDoc
1033 existentialProcPat con
1034 = hang (ptext (sLit "Illegal constructor") <+> quotes (ppr con) <+> ptext (sLit "in a 'proc' pattern"))
1035 2 (ptext (sLit "Proc patterns cannot use existentials or GADTs"))
1037 lazyPatErr :: Pat name -> [TcTyVar] -> TcM ()
1040 hang (ptext (sLit "A lazy (~) pattern cannot match existential or GADT data constructors"))
1041 2 (vcat (map pprSkolTvBinding tvs))
1043 nonRigidMatch :: PatCtxt -> DataCon -> SDoc
1044 nonRigidMatch ctxt con
1045 = hang (ptext (sLit "GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1046 2 (ptext (sLit "Probable solution: add a type signature for") <+> what ctxt)
1048 what (APat (FunRhs f _)) = quotes (ppr f)
1049 what (APat CaseAlt) = ptext (sLit "the scrutinee of the case expression")
1050 what (APat LambdaExpr ) = ptext (sLit "the lambda expression")
1051 what (APat (StmtCtxt _)) = ptext (sLit "the right hand side of a do/comprehension binding")
1052 what _other = ptext (sLit "something")
1054 nonRigidResult :: PatCtxt -> Type -> TcM a
1055 nonRigidResult ctxt res_ty
1056 = do { env0 <- tcInitTidyEnv
1057 ; let (env1, res_ty') = tidyOpenType env0 res_ty
1058 msg = hang (ptext (sLit "GADT pattern match with non-rigid result type")
1059 <+> quotes (ppr res_ty'))
1060 2 (ptext (sLit "Solution: add a type signature for")
1062 ; failWithTcM (env1, msg) }
1064 what (APat (FunRhs f _)) = quotes (ppr f)
1065 what (APat CaseAlt) = ptext (sLit "the entire case expression")
1066 what (APat LambdaExpr) = ptext (sLit "the lambda exression")
1067 what (APat (StmtCtxt _)) = ptext (sLit "the entire do/comprehension expression")
1068 what _other = ptext (sLit "something")