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, tcOverloadedLit,
10 addDataConStupidTheta, badFieldCon, polyPatSig ) where
12 #include "HsVersions.h"
14 import {-# SOURCE #-} TcExpr( tcSyntaxOp )
38 import BasicTypes hiding (SuccessFlag(..))
48 %************************************************************************
52 %************************************************************************
55 tcLetPat :: (Name -> Maybe TcRhoType)
56 -> LPat Name -> BoxySigmaType
59 tcLetPat sig_fn pat pat_ty thing_inside
60 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
61 pat_reft = emptyRefinement }
62 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state (\ _ -> thing_inside)
64 -- Don't know how to deal with pattern-bound existentials yet
65 ; checkTc (null ex_tvs) (existentialExplode pat)
67 ; return (pat', res) }
70 tcLamPats :: [LPat Name] -- Patterns,
71 -> [BoxySigmaType] -- and their types
72 -> BoxyRhoType -- Result type,
73 -> ((Refinement, BoxyRhoType) -> TcM a) -- and the checker for the body
74 -> TcM ([LPat TcId], a)
76 -- This is the externally-callable wrapper function
77 -- Typecheck the patterns, extend the environment to bind the variables,
78 -- do the thing inside, use any existentially-bound dictionaries to
79 -- discharge parts of the returning LIE, and deal with pattern type
82 -- 1. Initialise the PatState
83 -- 2. Check the patterns
84 -- 3. Apply the refinement to the environment and result type
86 -- 5. Check that no existentials escape
88 tcLamPats pats tys res_ty thing_inside
89 = tc_lam_pats (zipEqual "tcLamPats" pats tys)
90 (emptyRefinement, res_ty) thing_inside
92 tcLamPat :: LPat Name -> BoxySigmaType
93 -> (Refinement,BoxyRhoType) -- Result type
94 -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
96 tcLamPat pat pat_ty res_ty thing_inside
97 = do { ([pat'],thing) <- tc_lam_pats [(pat, pat_ty)] res_ty thing_inside
98 ; return (pat', thing) }
101 tc_lam_pats :: [(LPat Name,BoxySigmaType)]
102 -> (Refinement,BoxyRhoType) -- Result type
103 -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
104 -> TcM ([LPat TcId], a)
105 tc_lam_pats pat_ty_prs (reft, res_ty) thing_inside
106 = do { let init_state = PS { pat_ctxt = LamPat, pat_reft = reft }
108 ; (pats', ex_tvs, res) <- tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
109 refineEnvironment (pat_reft pstate') $
110 thing_inside (pat_reft pstate', res_ty)
112 ; let tys = map snd pat_ty_prs
113 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
115 ; returnM (pats', res) }
119 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
120 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
121 -> [BoxySigmaType] -- Types of the patterns
122 -> BoxyRhoType -- Type of the body of the match
123 -- Tyvars in either of these must not escape
125 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
126 -- For example, we must reject this program:
127 -- data C = forall a. C (a -> Int)
129 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
131 tcCheckExistentialPat pats [] pat_tys body_ty
132 = return () -- Short cut for case when there are no existentials
134 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
135 = addErrCtxtM (sigPatCtxt (collectPatsBinders pats) ex_tvs pat_tys body_ty) $
136 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
140 pat_reft :: Refinement -- Binds rigid TcTyVars to their refinements
145 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
147 patSigCtxt :: PatState -> UserTypeCtxt
148 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
149 patSigCtxt other = LamPatSigCtxt
154 %************************************************************************
158 %************************************************************************
161 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
162 tcPatBndr (PS { pat_ctxt = LamPat }) bndr_name pat_ty
163 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
164 -- We have an undecorated binder, so we do rule ABS1,
165 -- by unboxing the boxy type, forcing any un-filled-in
166 -- boxes to become monotypes
167 -- NB that pat_ty' can still be a polytype:
168 -- data T = MkT (forall a. a->a)
169 -- f t = case t of { MkT g -> ... }
170 -- Here, the 'g' must get type (forall a. a->a) from the
172 ; return (Id.mkLocalId bndr_name pat_ty') }
174 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
175 | Just mono_ty <- lookup_sig bndr_name
176 = do { mono_name <- newLocalName bndr_name
177 ; boxyUnify mono_ty pat_ty
178 ; return (Id.mkLocalId mono_name mono_ty) }
181 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
182 ; mono_name <- newLocalName bndr_name
183 ; return (Id.mkLocalId mono_name pat_ty') }
187 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
188 bindInstsOfPatId id thing_inside
189 | not (isOverloadedTy (idType id))
190 = do { res <- thing_inside; return (res, emptyLHsBinds) }
192 = do { (res, lie) <- getLIE thing_inside
193 ; binds <- bindInstsOfLocalFuns lie [id]
194 ; return (res, binds) }
197 unBoxPatBndrType ty name = unBoxArgType ty (ptext SLIT("The variable") <+> quotes (ppr name))
198 unBoxWildCardType ty = unBoxArgType ty (ptext SLIT("A wild-card pattern"))
200 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
201 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
202 -- that is, it can't be an unboxed tuple. For example,
203 -- case (f x) of r -> ...
204 -- should fail if 'f' returns an unboxed tuple.
205 unBoxArgType ty pp_this
206 = do { ty' <- unBox ty -- Returns a zonked type
208 -- Neither conditional is strictly necesssary (the unify alone will do)
209 -- but they improve error messages, and allocate fewer tyvars
210 ; if isUnboxedTupleType ty' then
212 else if isSubArgTypeKind (typeKind ty') then
214 else do -- OpenTypeKind, so constrain it
215 { ty2 <- newFlexiTyVarTy argTypeKind
219 msg = pp_this <+> ptext SLIT("cannot be bound to an unboxed tuple")
223 %************************************************************************
225 The main worker functions
227 %************************************************************************
231 tcPat takes a "thing inside" over which the patter scopes. This is partly
232 so that tcPat can extend the environment for the thing_inside, but also
233 so that constraints arising in the thing_inside can be discharged by the
236 This does not work so well for the ErrCtxt carried by the monad: we don't
237 want the error-context for the pattern to scope over the RHS.
238 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
242 type Checker inp out = forall r.
245 -> (PatState -> TcM r)
246 -> TcM (out, [TcTyVar], r)
248 tcMultiple :: Checker inp out -> Checker [inp] [out]
249 tcMultiple tc_pat args pstate thing_inside
250 = do { err_ctxt <- getErrCtxt
252 = do { res <- thing_inside pstate
253 ; return ([], [], res) }
255 loop pstate (arg:args)
256 = do { (p', p_tvs, (ps', ps_tvs, res))
257 <- tc_pat arg pstate $ \ pstate' ->
258 setErrCtxt err_ctxt $
260 -- setErrCtxt: restore context before doing the next pattern
261 -- See note [Nesting] above
263 ; return (p':ps', p_tvs ++ ps_tvs, res) }
268 tc_lpat_pr :: (LPat Name, BoxySigmaType)
270 -> (PatState -> TcM a)
271 -> TcM (LPat TcId, [TcTyVar], a)
272 tc_lpat_pr (pat, ty) = tc_lpat pat ty
277 -> (PatState -> TcM a)
278 -> TcM (LPat TcId, [TcTyVar], a)
279 tc_lpat (L span pat) pat_ty pstate thing_inside
281 maybeAddErrCtxt (patCtxt pat) $
282 do { let mb_reft = refineType (pat_reft pstate) pat_ty
283 pat_ty' = case mb_reft of { Just (_, ty') -> ty'; Nothing -> pat_ty }
285 -- Make sure the result type reflects the current refinement
286 -- We must do this here, so that it correctly ``sees'' all
287 -- the refinements to the left. Example:
288 -- Suppose C :: forall a. T a -> a -> Foo
289 -- Pattern C a p1 True
290 -- So p1 might refine 'a' to True, and the True
291 -- pattern had better see it.
293 ; (pat', tvs, res) <- tc_pat pstate pat pat_ty' thing_inside
294 ; let final_pat = case mb_reft of
296 Just (co,_) -> CoPat (WpCo co) pat' pat_ty
297 ; return (L span final_pat, tvs, res) }
301 -> Pat Name -> BoxySigmaType -- Fully refined result type
302 -> (PatState -> TcM a) -- Thing inside
303 -> TcM (Pat TcId, -- Translated pattern
304 [TcTyVar], -- Existential binders
305 a) -- Result of thing inside
307 tc_pat pstate (VarPat name) pat_ty thing_inside
308 = do { id <- tcPatBndr pstate name pat_ty
309 ; (res, binds) <- bindInstsOfPatId id $
310 tcExtendIdEnv1 name id $
311 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
312 >> thing_inside pstate)
313 ; let pat' | isEmptyLHsBinds binds = VarPat id
314 | otherwise = VarPatOut id binds
315 ; return (pat', [], res) }
317 tc_pat pstate (ParPat pat) pat_ty thing_inside
318 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
319 ; return (ParPat pat', tvs, res) }
321 tc_pat pstate (BangPat pat) pat_ty thing_inside
322 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
323 ; return (BangPat pat', tvs, res) }
325 -- There's a wrinkle with irrefutable patterns, namely that we
326 -- must not propagate type refinement from them. For example
327 -- data T a where { T1 :: Int -> T Int; ... }
328 -- f :: T a -> Int -> a
330 -- It's obviously not sound to refine a to Int in the right
331 -- hand side, because the arugment might not match T1 at all!
333 -- Nor should a lazy pattern bind any existential type variables
334 -- because they won't be in scope when we do the desugaring
335 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
336 = do { (pat', pat_tvs, res) <- tc_lpat pat pat_ty pstate $ \ _ ->
338 -- Ignore refined pstate',
340 -- Check no existentials
341 ; if (null pat_tvs) then return ()
342 else lazyPatErr lpat pat_tvs
344 -- Check that the pattern has a lifted type
345 ; pat_tv <- newBoxyTyVar liftedTypeKind
346 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
348 ; return (LazyPat pat', [], res) }
350 tc_pat pstate (WildPat _) pat_ty thing_inside
351 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
352 ; res <- thing_inside pstate
353 ; return (WildPat pat_ty', [], res) }
355 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
356 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
357 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
358 tc_lpat pat (idType bndr_id) pstate thing_inside
359 -- NB: if we do inference on:
360 -- \ (y@(x::forall a. a->a)) = e
361 -- we'll fail. The as-pattern infers a monotype for 'y', which then
362 -- fails to unify with the polymorphic type for 'x'. This could
363 -- perhaps be fixed, but only with a bit more work.
365 -- If you fix it, don't forget the bindInstsOfPatIds!
366 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
368 -- Type signatures in patterns
369 -- See Note [Pattern coercions] below
370 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
371 = do { (inner_ty, tv_binds) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
372 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
373 tc_lpat pat inner_ty pstate thing_inside
374 ; return (SigPatOut pat' inner_ty, tvs, res) }
376 tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
377 = failWithTc (badTypePat pat)
379 ------------------------
380 -- Lists, tuples, arrays
381 tc_pat pstate (ListPat pats _) pat_ty thing_inside
382 = do { elt_ty <- boxySplitListTy pat_ty
383 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
384 pats pstate thing_inside
385 ; return (ListPat pats' elt_ty, pats_tvs, res) }
387 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
388 = do { [elt_ty] <- boxySplitTyConApp parrTyCon pat_ty
389 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
390 pats pstate thing_inside
391 ; ifM (null pats) (zapToMonotype pat_ty) -- c.f. ExplicitPArr in TcExpr
392 ; return (PArrPat pats' elt_ty, pats_tvs, res) }
394 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
395 = do { arg_tys <- boxySplitTyConApp (tupleTyCon boxity (length pats)) pat_ty
396 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
399 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
400 -- so that we can experiment with lazy tuple-matching.
401 -- This is a pretty odd place to make the switch, but
402 -- it was easy to do.
403 ; let unmangled_result = TuplePat pats' boxity pat_ty
404 possibly_mangled_result
405 | opt_IrrefutableTuples && isBoxed boxity = LazyPat (noLoc unmangled_result)
406 | otherwise = unmangled_result
408 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
409 return (possibly_mangled_result, pats_tvs, res) }
411 ------------------------
413 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
414 = do { data_con <- tcLookupDataCon con_name
415 ; let tycon = dataConTyCon data_con
416 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
418 ------------------------
420 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
421 = do { boxyUnify (hsLitType simple_lit) pat_ty
422 ; res <- thing_inside pstate
423 ; returnM (LitPat simple_lit, [], res) }
425 ------------------------
426 -- Overloaded patterns: n, and n+k
427 tc_pat pstate pat@(NPat over_lit mb_neg eq _) pat_ty thing_inside
428 = do { let orig = LiteralOrigin over_lit
429 ; lit' <- tcOverloadedLit orig over_lit pat_ty
430 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
431 ; mb_neg' <- case mb_neg of
432 Nothing -> return Nothing -- Positive literal
433 Just neg -> -- Negative literal
434 -- The 'negate' is re-mappable syntax
435 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
436 ; return (Just neg') }
437 ; res <- thing_inside pstate
438 ; returnM (NPat lit' mb_neg' eq' pat_ty, [], res) }
440 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
441 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
442 ; let pat_ty' = idType bndr_id
443 orig = LiteralOrigin lit
444 ; lit' <- tcOverloadedLit orig lit pat_ty'
446 -- The '>=' and '-' parts are re-mappable syntax
447 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
448 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
450 -- The Report says that n+k patterns must be in Integral
451 -- We may not want this when using re-mappable syntax, though (ToDo?)
452 ; icls <- tcLookupClass integralClassName
453 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
455 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
456 ; returnM (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
458 tc_pat _ _other_pat _ _ = panic "tc_pat" -- DictPat, ConPatOut, SigPatOut, VarPatOut
462 %************************************************************************
464 Most of the work for constructors is here
465 (the rest is in the ConPatIn case of tc_pat)
467 %************************************************************************
469 [Pattern matching indexed data types]
470 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
471 Consider the following declarations:
473 data family Map k :: * -> *
474 data instance Map (a, b) v = MapPair (Map a (Pair b v))
476 and a case expression
478 case x :: Map (Int, c) w of MapPair m -> ...
480 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
481 worker/wrapper types for MapPair are
483 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
484 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
486 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
487 :R123Map, which means the straight use of boxySplitTyConApp would give a type
488 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
489 boxySplitTyConApp with the family tycon Map instead, which gives us the family
490 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
491 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
492 (provided by tyConFamInst_maybe together with the family tycon). This
493 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
494 the split arguments for the representation tycon :R123Map as {Int, c, w}
496 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
498 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
500 moving between representation and family type into account. To produce type
501 correct Core, this coercion needs to be used to case the type of the scrutinee
502 from the family to the representation type. This is achieved by
503 unwrapFamInstScrutinee using a CoPat around the result pattern.
505 Now it might appear seem as if we could have used the existing GADT type
506 refinement infrastructure of refineAlt and friends instead of the explicit
507 unification and CoPat generation. However, that would be wrong. Why? The
508 whole point of GADT refinement is that the refinement is local to the case
509 alternative. In contrast, the substitution generated by the unification of
510 the family type list and instance types needs to be propagated to the outside.
511 Imagine that in the above example, the type of the scrutinee would have been
512 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
513 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
514 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
515 alternatives of the case expression, whereas in the GADT case it might vary
516 between alternatives.
518 In fact, if we have a data instance declaration defining a GADT, eq_spec will
519 be non-empty and we will get a mixture of global instantiations and local
520 refinement from a single match. This neatly reflects that, as soon as we
521 have constrained the type of the scrutinee to the required type index, all
522 further type refinement is local to the alternative.
526 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
527 -- with scrutinee of type (T ty)
529 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
530 -> BoxySigmaType -- Type of the pattern
531 -> HsConDetails Name (LPat Name) -> (PatState -> TcM a)
532 -> TcM (Pat TcId, [TcTyVar], a)
533 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
534 = do { span <- getSrcSpanM -- Span for the whole pattern
535 ; let (univ_tvs, ex_tvs, eq_spec, theta, arg_tys) = dataConFullSig data_con
536 skol_info = PatSkol data_con span
537 origin = SigOrigin skol_info
539 -- Instantiate the constructor type variables [a->ty]
540 ; ctxt_res_tys <- boxySplitTyConAppWithFamily tycon pat_ty
541 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
542 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
543 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
544 eq_spec' = substEqSpec tenv eq_spec
545 theta' = substTheta tenv theta
546 arg_tys' = substTys tenv arg_tys
548 ; co_vars <- newCoVars eq_spec' -- Make coercion variables
549 ; pstate' <- refineAlt data_con pstate ex_tvs' co_vars pat_ty
551 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
552 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
554 ; loc <- getInstLoc origin
555 ; dicts <- newDictBndrs loc theta'
556 ; dict_binds <- tcSimplifyCheck doc ex_tvs' dicts lie_req
558 ; addDataConStupidTheta data_con ctxt_res_tys
561 (unwrapFamInstScrutinee tycon ctxt_res_tys $
562 ConPatOut { pat_con = L con_span data_con,
563 pat_tvs = ex_tvs' ++ co_vars,
564 pat_dicts = map instToId dicts,
565 pat_binds = dict_binds,
566 pat_args = arg_pats', pat_ty = pat_ty },
567 ex_tvs' ++ inner_tvs, res)
570 doc = ptext SLIT("existential context for") <+> quotes (ppr data_con)
572 -- Split against the family tycon if the pattern constructor belongs to a
573 -- representation tycon.
575 boxySplitTyConAppWithFamily tycon pat_ty =
577 case tyConFamInst_maybe tycon of
578 Nothing -> boxySplitTyConApp tycon pat_ty
579 Just (fam_tycon, instTys) ->
580 do { scrutinee_arg_tys <- boxySplitTyConApp fam_tycon pat_ty
581 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
582 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
586 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
587 ppr tycon <+> ppr pat_ty
588 , text " family instance:" <+>
589 ppr (tyConFamInst_maybe tycon)
592 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
593 -- pattern if the tycon is an instance of a family.
595 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
596 unwrapFamInstScrutinee tycon args pat
597 | Just co_con <- tyConFamilyCoercion_maybe tycon
598 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
600 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
601 -- coercion is not the identity; mkCoPat is inconvenient as it
602 -- wants a located pattern.
603 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
604 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
605 pat_ty -- family inst type
610 tcConArgs :: DataCon -> [TcSigmaType]
611 -> Checker (HsConDetails Name (LPat Name))
612 (HsConDetails Id (LPat Id))
614 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
615 = do { checkTc (con_arity == no_of_args) -- Check correct arity
616 (arityErr "Constructor" data_con con_arity no_of_args)
617 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
618 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
620 ; return (PrefixCon arg_pats', tvs, res) }
622 con_arity = dataConSourceArity data_con
623 no_of_args = length arg_pats
625 tcConArgs data_con [arg_ty1,arg_ty2] (InfixCon p1 p2) pstate thing_inside
626 = do { checkTc (con_arity == 2) -- Check correct arity
627 (arityErr "Constructor" data_con con_arity 2)
628 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
630 ; return (InfixCon p1' p2', tvs, res) }
632 con_arity = dataConSourceArity data_con
634 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
635 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
637 tcConArgs data_con arg_tys (RecCon rpats) pstate thing_inside
638 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
639 ; return (RecCon rpats', tvs, res) }
641 -- doc comments are typechecked to Nothing here
642 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
643 tc_field (HsRecField field_lbl pat _) pstate thing_inside
644 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
645 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
646 ; return (mkRecField sel_id pat', tvs, res) }
648 find_field_ty :: FieldLabel -> TcM (Id, TcType)
649 find_field_ty field_lbl
650 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
652 -- No matching field; chances are this field label comes from some
653 -- other record type (or maybe none). As well as reporting an
654 -- error we still want to typecheck the pattern, principally to
655 -- make sure that all the variables it binds are put into the
656 -- environment, else the type checker crashes later:
657 -- f (R { foo = (a,b) }) = a+b
658 -- If foo isn't one of R's fields, we don't want to crash when
659 -- typechecking the "a+b".
660 [] -> do { addErrTc (badFieldCon data_con field_lbl)
661 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
662 ; return (error "Bogus selector Id", bogus_ty) }
664 -- The normal case, when the field comes from the right constructor
666 ASSERT( null extras )
667 do { sel_id <- tcLookupField field_lbl
668 ; return (sel_id, pat_ty) }
670 field_tys :: [(FieldLabel, TcType)]
671 field_tys = zip (dataConFieldLabels data_con) arg_tys
672 -- Don't use zipEqual! If the constructor isn't really a record, then
673 -- dataConFieldLabels will be empty (and each field in the pattern
674 -- will generate an error below).
676 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
677 tcConArg (arg_pat, arg_ty) pstate thing_inside
678 = tc_lpat arg_pat arg_ty pstate thing_inside
679 -- NB: the tc_lpat will refine pat_ty if necessary
680 -- based on the current pstate, which may include
681 -- refinements from peer argument patterns to the left
685 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
686 -- Instantiate the "stupid theta" of the data con, and throw
687 -- the constraints into the constraint set
688 addDataConStupidTheta data_con inst_tys
689 | null stupid_theta = return ()
690 | otherwise = instStupidTheta origin inst_theta
692 origin = OccurrenceOf (dataConName data_con)
693 -- The origin should always report "occurrence of C"
694 -- even when C occurs in a pattern
695 stupid_theta = dataConStupidTheta data_con
696 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
697 inst_theta = substTheta tenv stupid_theta
701 %************************************************************************
705 %************************************************************************
708 refineAlt :: DataCon -- For tracing only
710 -> [TcTyVar] -- Existentials
711 -> [CoVar] -- Equational constraints
712 -> BoxySigmaType -- Pattern type
715 refineAlt con pstate ex_tvs [] pat_ty
716 = return pstate -- Common case: no equational constraints
718 refineAlt con pstate ex_tvs co_vars pat_ty
719 | not (isRigidTy pat_ty)
720 = failWithTc (nonRigidMatch con)
721 -- We are matching against a GADT constructor with non-trivial
722 -- constraints, but pattern type is wobbly. For now we fail.
723 -- We can make sense of this, however:
724 -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
725 -- (\x -> case x of { MkT v -> v })
726 -- We can infer that x must have type T [c], for some wobbly 'c'
728 -- (\(x::T [c]) -> case x of
729 -- MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
730 -- To implement this, we'd first instantiate the equational
731 -- constraints with *wobbly* type variables for the existentials;
732 -- then unify these constraints to make pat_ty the right shape;
733 -- then proceed exactly as in the rigid case
735 | otherwise -- In the rigid case, we perform type refinement
736 = case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
737 Failed msg -> failWithTc (inaccessibleAlt msg) ;
738 Succeeded reft -> do { traceTc trace_msg
739 ; return (pstate { pat_reft = reft }) }
740 -- DO NOT refine the envt right away, because we
741 -- might be inside a lazy pattern. Instead, refine pstate
744 trace_msg = text "refineAlt:match" <+>
745 vcat [ ppr con <+> ppr ex_tvs,
746 ppr [(v, tyVarKind v) | v <- co_vars],
752 %************************************************************************
756 %************************************************************************
758 In tcOverloadedLit we convert directly to an Int or Integer if we
759 know that's what we want. This may save some time, by not
760 temporarily generating overloaded literals, but it won't catch all
761 cases (the rest are caught in lookupInst).
764 tcOverloadedLit :: InstOrigin
767 -> TcM (HsOverLit TcId)
768 tcOverloadedLit orig lit@(HsIntegral i fi) res_ty
769 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
770 -- Reason: If we do, tcSimplify will call lookupInst, which
771 -- will call tcSyntaxName, which does unification,
772 -- which tcSimplify doesn't like
773 -- ToDo: noLoc sadness
774 = do { integer_ty <- tcMetaTy integerTyConName
775 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
776 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty)))) }
778 | Just expr <- shortCutIntLit i res_ty
779 = return (HsIntegral i expr)
782 = do { expr <- newLitInst orig lit res_ty
783 ; return (HsIntegral i expr) }
785 tcOverloadedLit orig lit@(HsFractional r fr) res_ty
786 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
787 = do { rat_ty <- tcMetaTy rationalTyConName
788 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
789 -- Overloaded literals must have liftedTypeKind, because
790 -- we're instantiating an overloaded function here,
791 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
792 -- However this'll be picked up by tcSyntaxOp if necessary
793 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty)))) }
795 | Just expr <- shortCutFracLit r res_ty
796 = return (HsFractional r expr)
799 = do { expr <- newLitInst orig lit res_ty
800 ; return (HsFractional r expr) }
802 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
803 newLitInst orig lit res_ty -- Make a LitInst
804 = do { loc <- getInstLoc orig
805 ; res_tau <- zapToMonotype res_ty
806 ; new_uniq <- newUnique
807 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
808 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
809 tci_ty = res_tau, tci_loc = loc}
811 ; return (HsVar (instToId lit_inst)) }
815 %************************************************************************
817 Note [Pattern coercions]
819 %************************************************************************
821 In principle, these program would be reasonable:
823 f :: (forall a. a->a) -> Int
824 f (x :: Int->Int) = x 3
826 g :: (forall a. [a]) -> Bool
829 In both cases, the function type signature restricts what arguments can be passed
830 in a call (to polymorphic ones). The pattern type signature then instantiates this
831 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
832 generate the translated term
833 f = \x' :: (forall a. a->a). let x = x' Int in x 3
835 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
836 And it requires a significant amount of code to implement, becuase we need to decorate
837 the translated pattern with coercion functions (generated from the subsumption check
840 So for now I'm just insisting on type *equality* in patterns. No subsumption.
842 Old notes about desugaring, at a time when pattern coercions were handled:
844 A SigPat is a type coercion and must be handled one at at time. We can't
845 combine them unless the type of the pattern inside is identical, and we don't
846 bother to check for that. For example:
848 data T = T1 Int | T2 Bool
849 f :: (forall a. a -> a) -> T -> t
850 f (g::Int->Int) (T1 i) = T1 (g i)
851 f (g::Bool->Bool) (T2 b) = T2 (g b)
853 We desugar this as follows:
855 f = \ g::(forall a. a->a) t::T ->
857 in case t of { T1 i -> T1 (gi i)
860 in case t of { T2 b -> T2 (gb b)
863 Note that we do not treat the first column of patterns as a
864 column of variables, because the coerced variables (gi, gb)
865 would be of different types. So we get rather grotty code.
866 But I don't think this is a common case, and if it was we could
867 doubtless improve it.
869 Meanwhile, the strategy is:
870 * treat each SigPat coercion (always non-identity coercions)
872 * deal with the stuff inside, and then wrap a binding round
873 the result to bind the new variable (gi, gb, etc)
876 %************************************************************************
878 \subsection{Errors and contexts}
880 %************************************************************************
883 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
884 patCtxt (VarPat _) = Nothing
885 patCtxt (ParPat _) = Nothing
886 patCtxt (AsPat _ _) = Nothing
887 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
890 -----------------------------------------------
892 existentialExplode pat
893 = hang (vcat [text "My brain just exploded.",
894 text "I can't handle pattern bindings for existentially-quantified constructors.",
895 text "In the binding group for"])
898 sigPatCtxt bound_ids bound_tvs pat_tys body_ty tidy_env
899 = do { pat_tys' <- mapM zonkTcType pat_tys
900 ; body_ty' <- zonkTcType body_ty
901 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
902 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
903 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
905 sep [ptext SLIT("When checking an existential match that binds"),
906 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
907 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
908 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
911 show_ids = filter is_interesting bound_ids
912 is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
914 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
915 -- Don't zonk the types so we get the separate, un-unified versions
917 badFieldCon :: DataCon -> Name -> SDoc
918 badFieldCon con field
919 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
920 ptext SLIT("does not have field"), quotes (ppr field)]
922 polyPatSig :: TcType -> SDoc
924 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
927 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
931 hang (ptext SLIT("A lazy (~) pattern connot bind existential type variables"))
932 2 (vcat (map pprSkolTvBinding tvs))
935 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
936 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
939 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg