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, tcLamPat, tcLamPats, tcProcPat, tcOverloadedLit,
10 addDataConStupidTheta, badFieldCon, polyPatSig ) where
12 #include "HsVersions.h"
14 import {-# SOURCE #-} TcExpr( tcSyntaxOp, tcInferRho)
39 import BasicTypes hiding (SuccessFlag(..))
50 %************************************************************************
54 %************************************************************************
57 tcLetPat :: (Name -> Maybe TcRhoType)
58 -> LPat Name -> BoxySigmaType
61 tcLetPat sig_fn pat pat_ty thing_inside
62 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
64 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state
67 -- Don't know how to deal with pattern-bound existentials yet
68 ; checkTc (null ex_tvs) (existentialExplode pat)
70 ; return (pat', res) }
73 tcLamPats :: [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 tcLamPats pats tys res_ty thing_inside
91 = tc_lam_pats LamPat (zipEqual "tcLamPats" pats tys)
94 tcLamPat, tcProcPat :: LPat Name -> BoxySigmaType
95 -> BoxyRhoType -- Result type
96 -> (BoxyRhoType -> TcM a) -- Checker for body, given
99 tcLamPat = tc_lam_pat LamPat
100 tcProcPat = tc_lam_pat ProcPat
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 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
158 | ProcPat -- The pattern in (proc pat -> ...)
159 -- see Note [Arrows and patterns]
160 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
162 patSigCtxt :: PatState -> UserTypeCtxt
163 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
164 patSigCtxt _ = LamPatSigCtxt
169 %************************************************************************
173 %************************************************************************
176 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
177 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
178 | Just mono_ty <- lookup_sig bndr_name
179 = do { mono_name <- newLocalName bndr_name
180 ; boxyUnify mono_ty pat_ty
181 ; return (Id.mkLocalId mono_name mono_ty) }
184 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
185 ; mono_name <- newLocalName bndr_name
186 ; return (Id.mkLocalId mono_name pat_ty') }
188 tcPatBndr (PS { pat_ctxt = _lam_or_proc }) bndr_name pat_ty
189 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
190 -- We have an undecorated binder, so we do rule ABS1,
191 -- by unboxing the boxy type, forcing any un-filled-in
192 -- boxes to become monotypes
193 -- NB that pat_ty' can still be a polytype:
194 -- data T = MkT (forall a. a->a)
195 -- f t = case t of { MkT g -> ... }
196 -- Here, the 'g' must get type (forall a. a->a) from the
198 ; return (Id.mkLocalId bndr_name pat_ty') }
202 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
203 bindInstsOfPatId id thing_inside
204 | not (isOverloadedTy (idType id))
205 = do { res <- thing_inside; return (res, emptyLHsBinds) }
207 = do { (res, lie) <- getLIE thing_inside
208 ; binds <- bindInstsOfLocalFuns lie [id]
209 ; return (res, binds) }
212 unBoxPatBndrType :: BoxyType -> Name -> TcM TcType
213 unBoxPatBndrType ty name = unBoxArgType ty (ptext (sLit "The variable") <+> quotes (ppr name))
215 unBoxWildCardType :: BoxyType -> TcM TcType
216 unBoxWildCardType ty = unBoxArgType ty (ptext (sLit "A wild-card pattern"))
218 unBoxViewPatType :: BoxyType -> Pat Name -> TcM TcType
219 unBoxViewPatType ty pat = unBoxArgType ty (ptext (sLit "The view pattern") <+> ppr pat)
221 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
222 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
223 -- that is, it can't be an unboxed tuple. For example,
224 -- case (f x) of r -> ...
225 -- should fail if 'f' returns an unboxed tuple.
226 unBoxArgType ty pp_this
227 = do { ty' <- unBox ty -- Returns a zonked type
229 -- Neither conditional is strictly necesssary (the unify alone will do)
230 -- but they improve error messages, and allocate fewer tyvars
231 ; if isUnboxedTupleType ty' then
233 else if isSubArgTypeKind (typeKind ty') then
235 else do -- OpenTypeKind, so constrain it
236 { ty2 <- newFlexiTyVarTy argTypeKind
240 msg = pp_this <+> ptext (sLit "cannot be bound to an unboxed tuple")
244 %************************************************************************
246 The main worker functions
248 %************************************************************************
252 tcPat takes a "thing inside" over which the pattern scopes. This is partly
253 so that tcPat can extend the environment for the thing_inside, but also
254 so that constraints arising in the thing_inside can be discharged by the
257 This does not work so well for the ErrCtxt carried by the monad: we don't
258 want the error-context for the pattern to scope over the RHS.
259 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
263 type Checker inp out = forall r.
266 -> (PatState -> TcM r)
267 -> TcM (out, [TcTyVar], r)
269 tcMultiple :: Checker inp out -> Checker [inp] [out]
270 tcMultiple tc_pat args pstate thing_inside
271 = do { err_ctxt <- getErrCtxt
273 = do { res <- thing_inside pstate
274 ; return ([], [], res) }
276 loop pstate (arg:args)
277 = do { (p', p_tvs, (ps', ps_tvs, res))
278 <- tc_pat arg pstate $ \ pstate' ->
279 setErrCtxt err_ctxt $
281 -- setErrCtxt: restore context before doing the next pattern
282 -- See note [Nesting] above
284 ; return (p':ps', p_tvs ++ ps_tvs, res) }
289 tc_lpat_pr :: (LPat Name, BoxySigmaType)
291 -> (PatState -> TcM a)
292 -> TcM (LPat TcId, [TcTyVar], a)
293 tc_lpat_pr (pat, ty) = tc_lpat pat ty
298 -> (PatState -> TcM a)
299 -> TcM (LPat TcId, [TcTyVar], a)
300 tc_lpat (L span pat) pat_ty pstate thing_inside
302 maybeAddErrCtxt (patCtxt pat) $
303 do { (pat', tvs, res) <- tc_pat pstate pat pat_ty thing_inside
304 ; return (L span pat', tvs, res) }
309 -> BoxySigmaType -- Fully refined result type
310 -> (PatState -> TcM a) -- Thing inside
311 -> TcM (Pat TcId, -- Translated pattern
312 [TcTyVar], -- Existential binders
313 a) -- Result of thing inside
315 tc_pat pstate (VarPat name) pat_ty thing_inside
316 = do { id <- tcPatBndr pstate name pat_ty
317 ; (res, binds) <- bindInstsOfPatId id $
318 tcExtendIdEnv1 name id $
319 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
320 >> thing_inside pstate)
321 ; let pat' | isEmptyLHsBinds binds = VarPat id
322 | otherwise = VarPatOut id binds
323 ; return (pat', [], res) }
325 tc_pat pstate (ParPat pat) pat_ty thing_inside
326 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
327 ; return (ParPat pat', tvs, res) }
329 tc_pat pstate (BangPat pat) pat_ty thing_inside
330 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
331 ; return (BangPat pat', tvs, res) }
333 -- There's a wrinkle with irrefutable patterns, namely that we
334 -- must not propagate type refinement from them. For example
335 -- data T a where { T1 :: Int -> T Int; ... }
336 -- f :: T a -> Int -> a
338 -- It's obviously not sound to refine a to Int in the right
339 -- hand side, because the arugment might not match T1 at all!
341 -- Nor should a lazy pattern bind any existential type variables
342 -- because they won't be in scope when we do the desugaring
344 -- Note [Hopping the LIE in lazy patterns]
345 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
346 -- In a lazy pattern, we must *not* discharge constraints from the RHS
347 -- from dictionaries bound in the pattern. E.g.
349 -- We can't discharge the Num constraint from dictionaries bound by
352 -- So we have to make the constraints from thing_inside "hop around"
353 -- the pattern. Hence the getLLE and extendLIEs later.
355 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
356 = do { (pat', pat_tvs, (res,lie))
357 <- tc_lpat pat pat_ty pstate $ \ _ ->
358 getLIE (thing_inside pstate)
359 -- Ignore refined pstate', revert to pstate
361 -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
363 -- Check no existentials
364 ; if (null pat_tvs) then return ()
365 else lazyPatErr lpat pat_tvs
367 -- Check that the pattern has a lifted type
368 ; pat_tv <- newBoxyTyVar liftedTypeKind
369 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
371 ; return (LazyPat pat', [], res) }
373 tc_pat _ p@(QuasiQuotePat _) _ _
374 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
376 tc_pat pstate (WildPat _) pat_ty thing_inside
377 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
378 ; res <- thing_inside pstate
379 ; return (WildPat pat_ty', [], res) }
381 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
382 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
383 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
384 tc_lpat pat (idType bndr_id) pstate thing_inside
385 -- NB: if we do inference on:
386 -- \ (y@(x::forall a. a->a)) = e
387 -- we'll fail. The as-pattern infers a monotype for 'y', which then
388 -- fails to unify with the polymorphic type for 'x'. This could
389 -- perhaps be fixed, but only with a bit more work.
391 -- If you fix it, don't forget the bindInstsOfPatIds!
392 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
394 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
395 = do { -- morally, expr must have type
396 -- `forall a1...aN. OPT' -> B`
397 -- where overall_pat_ty is an instance of OPT'.
398 -- Here, we infer a rho type for it,
399 -- which replaces the leading foralls and constraints
400 -- with fresh unification variables.
401 (expr',expr'_inferred) <- tcInferRho expr
402 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
403 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
404 -- tcSubExp: expected first, offered second
407 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
408 -- variable to find it). this means that in an example like
409 -- (view -> f) where view :: _ -> forall b. b
410 -- we will only be able to use view at one instantation in the
412 ; (expr_coerc, pat_ty) <- tcInfer $ \ pat_ty ->
413 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
415 -- pattern must have pat_ty
416 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
417 -- this should get zonked later on, but we unBox it here
418 -- so that we do the same checks as above
419 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
420 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
422 -- Type signatures in patterns
423 -- See Note [Pattern coercions] below
424 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
425 = do { (inner_ty, tv_binds, coi) <- tcPatSig (patSigCtxt pstate) sig_ty
427 ; unless (isIdentityCoercion coi) $
428 failWithTc (badSigPat pat_ty)
429 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
430 tc_lpat pat inner_ty pstate thing_inside
431 ; return (SigPatOut pat' inner_ty, tvs, res) }
433 tc_pat _ pat@(TypePat _) _ _
434 = failWithTc (badTypePat pat)
436 ------------------------
437 -- Lists, tuples, arrays
438 tc_pat pstate (ListPat pats _) pat_ty thing_inside
439 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
440 ; let scoi = mkSymCoI coi
441 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
442 pats pstate thing_inside
443 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
446 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
447 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
448 ; let scoi = mkSymCoI coi
449 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
450 pats pstate thing_inside
451 ; when (null pats) (zapToMonotype pat_ty >> return ()) -- c.f. ExplicitPArr in TcExpr
452 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
455 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
456 = do { let tc = tupleTyCon boxity (length pats)
457 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
458 ; let scoi = mkSymCoI coi
459 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
462 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
463 -- so that we can experiment with lazy tuple-matching.
464 -- This is a pretty odd place to make the switch, but
465 -- it was easy to do.
466 ; let pat_ty' = mkTyConApp tc arg_tys
467 -- pat_ty /= pat_ty iff coi /= IdCo
468 unmangled_result = TuplePat pats' boxity pat_ty'
469 possibly_mangled_result
470 | opt_IrrefutableTuples &&
471 isBoxed boxity = LazyPat (noLoc unmangled_result)
472 | otherwise = unmangled_result
474 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
475 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
478 ------------------------
480 tc_pat pstate (ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
481 = do { data_con <- tcLookupDataCon con_name
482 ; let tycon = dataConTyCon data_con
483 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
485 ------------------------
487 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
488 = do { let lit_ty = hsLitType simple_lit
489 ; coi <- boxyUnify lit_ty pat_ty
490 -- coi is of kind: lit_ty ~ pat_ty
491 ; res <- thing_inside pstate
492 -- pattern coercions have to
493 -- be of kind: pat_ty ~ lit_ty
495 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
498 ------------------------
499 -- Overloaded patterns: n, and n+k
500 tc_pat pstate (NPat over_lit mb_neg eq) pat_ty thing_inside
501 = do { let orig = LiteralOrigin over_lit
502 ; lit' <- tcOverloadedLit orig over_lit pat_ty
503 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
504 ; mb_neg' <- case mb_neg of
505 Nothing -> return Nothing -- Positive literal
506 Just neg -> -- Negative literal
507 -- The 'negate' is re-mappable syntax
508 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
509 ; return (Just neg') }
510 ; res <- thing_inside pstate
511 ; return (NPat lit' mb_neg' eq', [], res) }
513 tc_pat pstate (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
514 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
515 ; let pat_ty' = idType bndr_id
516 orig = LiteralOrigin lit
517 ; lit' <- tcOverloadedLit orig lit pat_ty'
519 -- The '>=' and '-' parts are re-mappable syntax
520 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
521 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
523 -- The Report says that n+k patterns must be in Integral
524 -- We may not want this when using re-mappable syntax, though (ToDo?)
525 ; icls <- tcLookupClass integralClassName
526 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
528 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
529 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
531 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
535 %************************************************************************
537 Most of the work for constructors is here
538 (the rest is in the ConPatIn case of tc_pat)
540 %************************************************************************
542 [Pattern matching indexed data types]
543 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
544 Consider the following declarations:
546 data family Map k :: * -> *
547 data instance Map (a, b) v = MapPair (Map a (Pair b v))
549 and a case expression
551 case x :: Map (Int, c) w of MapPair m -> ...
553 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
554 worker/wrapper types for MapPair are
556 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
557 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
559 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
560 :R123Map, which means the straight use of boxySplitTyConApp would give a type
561 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
562 boxySplitTyConApp with the family tycon Map instead, which gives us the family
563 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
564 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
565 (provided by tyConFamInst_maybe together with the family tycon). This
566 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
567 the split arguments for the representation tycon :R123Map as {Int, c, w}
569 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
571 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
573 moving between representation and family type into account. To produce type
574 correct Core, this coercion needs to be used to case the type of the scrutinee
575 from the family to the representation type. This is achieved by
576 unwrapFamInstScrutinee using a CoPat around the result pattern.
578 Now it might appear seem as if we could have used the previous GADT type
579 refinement infrastructure of refineAlt and friends instead of the explicit
580 unification and CoPat generation. However, that would be wrong. Why? The
581 whole point of GADT refinement is that the refinement is local to the case
582 alternative. In contrast, the substitution generated by the unification of
583 the family type list and instance types needs to be propagated to the outside.
584 Imagine that in the above example, the type of the scrutinee would have been
585 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
586 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
587 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
588 alternatives of the case expression, whereas in the GADT case it might vary
589 between alternatives.
591 RIP GADT refinement: refinements have been replaced by the use of explicit
592 equality constraints that are used in conjunction with implication constraints
593 to express the local scope of GADT refinements.
597 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
598 -- with scrutinee of type (T ty)
600 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
601 -> BoxySigmaType -- Type of the pattern
602 -> HsConPatDetails Name -> (PatState -> TcM a)
603 -> TcM (Pat TcId, [TcTyVar], a)
604 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
605 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
606 = dataConFullSig data_con
607 skol_info = PatSkol data_con
608 origin = SigOrigin skol_info
609 full_theta = eq_theta ++ dict_theta
611 -- Instantiate the constructor type variables [a->ty]
612 -- This may involve doing a family-instance coercion, and building a
614 ; (ctxt_res_tys, coi, unwrap_ty) <- boxySplitTyConAppWithFamily tycon
616 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
617 pat_ty' = mkTyConApp tycon ctxt_res_tys
618 -- pat_ty' /= pat_ty iff coi /= IdCo
620 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
622 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys
625 -- Add the stupid theta
626 ; addDataConStupidTheta data_con ctxt_res_tys
628 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
629 -- Get location from monad, not from ex_tvs
631 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
632 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
633 arg_tys' = substTys tenv arg_tys
635 ; if null ex_tvs && null eq_spec && null full_theta
636 then do { -- The common case; no class bindings etc
637 -- (see Note [Arrows and patterns])
638 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
639 arg_pats pstate thing_inside
640 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
641 pat_tvs = [], pat_dicts = [],
642 pat_binds = emptyLHsBinds,
643 pat_args = arg_pats',
646 ; return (wrap_res_pat res_pat, inner_tvs, res) }
648 else do -- The general case, with existential, and local equality
650 { checkTc (case pat_ctxt pstate of { ProcPat -> False; _ -> True })
651 (existentialProcPat data_con)
653 -- Need to test for rigidity if *any* constraints in theta as class
654 -- constraints may have superclass equality constraints. However,
655 -- we don't want to check for rigidity if we got here only because
656 -- ex_tvs was non-null.
657 -- ; unless (null theta') $
658 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
659 -- if we explicitly or implicitly (by a GADT def) have equality
661 ; let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
662 theta' = substTheta tenv (eq_preds ++ full_theta)
663 -- order is *important* as we generate the list of
664 -- dictionary binders from theta'
665 no_equalities = not (any isEqPred theta')
666 pstate' | no_equalities = pstate
667 | otherwise = pstate { pat_eqs = True }
669 ; unless no_equalities $
670 checkTc (isRigidTy pat_ty) (nonRigidMatch data_con)
672 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
673 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
675 ; loc <- getInstLoc origin
676 ; dicts <- newDictBndrs loc theta'
677 ; dict_binds <- tcSimplifyCheckPat loc ex_tvs' dicts lie_req
679 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
681 pat_dicts = map instToVar dicts,
682 pat_binds = dict_binds,
683 pat_args = arg_pats', pat_ty = pat_ty' }
684 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
687 -- Split against the family tycon if the pattern constructor
688 -- belongs to a family instance tycon.
689 boxySplitTyConAppWithFamily tycon pat_ty =
691 case tyConFamInst_maybe tycon of
693 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp tycon pat_ty
694 ; return (scrutinee_arg_tys, coi1, pat_ty)
696 Just (fam_tycon, instTys) ->
697 do { (scrutinee_arg_tys, coi1) <- boxySplitTyConApp fam_tycon pat_ty
698 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
699 ; let instTys' = substTys subst instTys
700 ; cois <- boxyUnifyList instTys' scrutinee_arg_tys
701 ; let coi = if isIdentityCoercion coi1
702 then -- pat_ty was splittable
703 -- => boxyUnifyList had real work to do
704 mkTyConAppCoI fam_tycon instTys' cois
705 else -- pat_ty was not splittable
706 -- => scrutinee_arg_tys are fresh tvs and
707 -- boxyUnifyList just instantiated those
709 ; return (freshTvs, coi, mkTyConApp fam_tycon instTys')
711 -- iff cois is non-trivial
714 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
715 ppr tycon <+> ppr pat_ty
716 , text " family instance:" <+>
717 ppr (tyConFamInst_maybe tycon)
720 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
721 -- pattern if the tycon is an instance of a family.
723 unwrapFamInstScrutinee :: TyCon -> [Type] -> Type -> Pat Id -> Pat Id
724 unwrapFamInstScrutinee tycon args unwrap_ty pat
725 | Just co_con <- tyConFamilyCoercion_maybe tycon
726 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
728 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
729 -- coercion is not the identity; mkCoPat is inconvenient as it
730 -- wants a located pattern.
731 = CoPat (WpCast $ mkTyConApp co_con args) -- co fam ty to repr ty
732 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
733 unwrap_ty -- family inst type
737 tcConArgs :: DataCon -> [TcSigmaType]
738 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
740 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
741 = do { checkTc (con_arity == no_of_args) -- Check correct arity
742 (arityErr "Constructor" data_con con_arity no_of_args)
743 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
744 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
746 ; return (PrefixCon arg_pats', tvs, res) }
748 con_arity = dataConSourceArity data_con
749 no_of_args = length arg_pats
751 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
752 = do { checkTc (con_arity == 2) -- Check correct arity
753 (arityErr "Constructor" data_con con_arity 2)
754 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
755 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
757 ; return (InfixCon p1' p2', tvs, res) }
759 con_arity = dataConSourceArity data_con
761 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
762 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
763 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
765 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
766 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
767 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
768 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
769 ; return (HsRecField sel_id pat' pun, tvs, res) }
771 find_field_ty :: FieldLabel -> TcM (Id, TcType)
772 find_field_ty field_lbl
773 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
775 -- No matching field; chances are this field label comes from some
776 -- other record type (or maybe none). As well as reporting an
777 -- error we still want to typecheck the pattern, principally to
778 -- make sure that all the variables it binds are put into the
779 -- environment, else the type checker crashes later:
780 -- f (R { foo = (a,b) }) = a+b
781 -- If foo isn't one of R's fields, we don't want to crash when
782 -- typechecking the "a+b".
783 [] -> do { addErrTc (badFieldCon data_con field_lbl)
784 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
785 ; return (error "Bogus selector Id", bogus_ty) }
787 -- The normal case, when the field comes from the right constructor
789 ASSERT( null extras )
790 do { sel_id <- tcLookupField field_lbl
791 ; return (sel_id, pat_ty) }
793 field_tys :: [(FieldLabel, TcType)]
794 field_tys = zip (dataConFieldLabels data_con) arg_tys
795 -- Don't use zipEqual! If the constructor isn't really a record, then
796 -- dataConFieldLabels will be empty (and each field in the pattern
797 -- will generate an error below).
799 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
800 tcConArg (arg_pat, arg_ty) pstate thing_inside
801 = tc_lpat arg_pat arg_ty pstate thing_inside
805 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
806 -- Instantiate the "stupid theta" of the data con, and throw
807 -- the constraints into the constraint set
808 addDataConStupidTheta data_con inst_tys
809 | null stupid_theta = return ()
810 | otherwise = instStupidTheta origin inst_theta
812 origin = OccurrenceOf (dataConName data_con)
813 -- The origin should always report "occurrence of C"
814 -- even when C occurs in a pattern
815 stupid_theta = dataConStupidTheta data_con
816 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
817 -- NB: inst_tys can be longer than the univ tyvars
818 -- because the constructor might have existentials
819 inst_theta = substTheta tenv stupid_theta
822 Note [Arrows and patterns]
823 ~~~~~~~~~~~~~~~~~~~~~~~~~~
824 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
826 expr :: Arrow a => a () Int
827 expr = proc (y,z) -> do
831 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
832 arrow body (e.g 'term'). As things stand (bogusly) all the
833 constraints from the proc body are gathered together, so constraints
834 from 'term' will be seen by the tcPat for (y,z). But we must *not*
835 bind constraints from 'term' here, becuase the desugarer will not make
836 these bindings scope over 'term'.
838 The Right Thing is not to confuse these constraints together. But for
839 now the Easy Thing is to ensure that we do not have existential or
840 GADT constraints in a 'proc', and to short-cut the constraint
841 simplification for such vanilla patterns so that it binds no
842 constraints. Hence the 'fast path' in tcConPat; but it's also a good
843 plan for ordinary vanilla patterns to bypass the constraint
847 %************************************************************************
851 %************************************************************************
853 In tcOverloadedLit we convert directly to an Int or Integer if we
854 know that's what we want. This may save some time, by not
855 temporarily generating overloaded literals, but it won't catch all
856 cases (the rest are caught in lookupInst).
859 tcOverloadedLit :: InstOrigin
862 -> TcM (HsOverLit TcId)
863 tcOverloadedLit orig lit@(OverLit { ol_val = val, ol_rebindable = rebindable
864 , ol_witness = meth_name }) res_ty
866 -- Do not generate a LitInst for rebindable syntax.
867 -- Reason: If we do, tcSimplify will call lookupInst, which
868 -- will call tcSyntaxName, which does unification,
869 -- which tcSimplify doesn't like
870 -- ToDo: noLoc sadness
871 = do { hs_lit <- mkOverLit val
872 ; let lit_ty = hsLitType hs_lit
873 ; fi' <- tcSyntaxOp orig meth_name (mkFunTy lit_ty res_ty)
874 -- Overloaded literals must have liftedTypeKind, because
875 -- we're instantiating an overloaded function here,
876 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
877 -- However this'll be picked up by tcSyntaxOp if necessary
878 ; let witness = HsApp (noLoc fi') (noLoc (HsLit hs_lit))
879 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
881 | Just expr <- shortCutLit val res_ty
882 = return (lit { ol_witness = expr, ol_type = res_ty })
885 = do { loc <- getInstLoc orig
886 ; res_tau <- zapToMonotype res_ty
887 ; new_uniq <- newUnique
888 ; let lit_nm = mkSystemVarName new_uniq (fsLit "lit")
889 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
890 tci_ty = res_tau, tci_loc = loc}
891 witness = HsVar (instToId lit_inst)
893 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
897 %************************************************************************
899 Note [Pattern coercions]
901 %************************************************************************
903 In principle, these program would be reasonable:
905 f :: (forall a. a->a) -> Int
906 f (x :: Int->Int) = x 3
908 g :: (forall a. [a]) -> Bool
911 In both cases, the function type signature restricts what arguments can be passed
912 in a call (to polymorphic ones). The pattern type signature then instantiates this
913 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
914 generate the translated term
915 f = \x' :: (forall a. a->a). let x = x' Int in x 3
917 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
918 And it requires a significant amount of code to implement, becuase we need to decorate
919 the translated pattern with coercion functions (generated from the subsumption check
922 So for now I'm just insisting on type *equality* in patterns. No subsumption.
924 Old notes about desugaring, at a time when pattern coercions were handled:
926 A SigPat is a type coercion and must be handled one at at time. We can't
927 combine them unless the type of the pattern inside is identical, and we don't
928 bother to check for that. For example:
930 data T = T1 Int | T2 Bool
931 f :: (forall a. a -> a) -> T -> t
932 f (g::Int->Int) (T1 i) = T1 (g i)
933 f (g::Bool->Bool) (T2 b) = T2 (g b)
935 We desugar this as follows:
937 f = \ g::(forall a. a->a) t::T ->
939 in case t of { T1 i -> T1 (gi i)
942 in case t of { T2 b -> T2 (gb b)
945 Note that we do not treat the first column of patterns as a
946 column of variables, because the coerced variables (gi, gb)
947 would be of different types. So we get rather grotty code.
948 But I don't think this is a common case, and if it was we could
949 doubtless improve it.
951 Meanwhile, the strategy is:
952 * treat each SigPat coercion (always non-identity coercions)
954 * deal with the stuff inside, and then wrap a binding round
955 the result to bind the new variable (gi, gb, etc)
958 %************************************************************************
960 \subsection{Errors and contexts}
962 %************************************************************************
965 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
966 patCtxt (VarPat _) = Nothing
967 patCtxt (ParPat _) = Nothing
968 patCtxt (AsPat _ _) = Nothing
969 patCtxt pat = Just (hang (ptext (sLit "In the pattern:"))
972 -----------------------------------------------
974 existentialExplode :: LPat Name -> SDoc
975 existentialExplode pat
976 = hang (vcat [text "My brain just exploded.",
977 text "I can't handle pattern bindings for existentially-quantified constructors.",
978 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
979 text "In the binding group for"])
982 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
983 -> TcM (TidyEnv, SDoc)
984 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
985 = do { pat_tys' <- mapM zonkTcType pat_tys
986 ; body_ty' <- zonkTcType body_ty
987 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
988 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
989 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
991 sep [ptext (sLit "When checking an existential match that binds"),
992 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
993 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
994 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
997 bound_ids = collectPatsBinders pats
998 show_ids = filter is_interesting bound_ids
999 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1001 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1002 -- Don't zonk the types so we get the separate, un-unified versions
1004 badFieldCon :: DataCon -> Name -> SDoc
1005 badFieldCon con field
1006 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
1007 ptext (sLit "does not have field"), quotes (ppr field)]
1009 polyPatSig :: TcType -> SDoc
1011 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1014 badSigPat :: TcType -> SDoc
1015 badSigPat pat_ty = ptext (sLit "Pattern signature must exactly match:") <+>
1018 badTypePat :: Pat Name -> SDoc
1019 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1021 existentialProcPat :: DataCon -> SDoc
1022 existentialProcPat con
1023 = hang (ptext (sLit "Illegal constructor") <+> quotes (ppr con) <+> ptext (sLit "in a 'proc' pattern"))
1024 2 (ptext (sLit "Proc patterns cannot use existentials or GADTs"))
1026 lazyPatErr :: Pat name -> [TcTyVar] -> TcM ()
1029 hang (ptext (sLit "A lazy (~) pattern cannot bind existential type variables"))
1030 2 (vcat (map pprSkolTvBinding tvs))
1032 nonRigidMatch :: DataCon -> SDoc
1034 = hang (ptext (sLit "GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1035 2 (ptext (sLit "Solution: add a type signature"))
1037 nonRigidResult :: Type -> TcM a
1038 nonRigidResult res_ty
1039 = do { env0 <- tcInitTidyEnv
1040 ; let (env1, res_ty') = tidyOpenType env0 res_ty
1041 msg = hang (ptext (sLit "GADT pattern match with non-rigid result type")
1042 <+> quotes (ppr res_ty'))
1043 2 (ptext (sLit "Solution: add a type signature"))
1044 ; failWithTcM (env1, msg) }