2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcPat]{Typechecking patterns}
7 module TcPat ( tcPat, tcPats, PatCtxt(..), badFieldCon, polyPatSig, refineTyVars ) where
9 #include "HsVersions.h"
11 import HsSyn ( Pat(..), LPat, HsConDetails(..), HsLit(..), HsOverLit(..),
12 HsExpr(..), LHsBinds, emptyLHsBinds, isEmptyLHsBinds )
14 import TcHsSyn ( TcId, hsLitType )
16 import Inst ( InstOrigin(..),
17 newMethodFromName, newOverloadedLit, newDicts,
18 instToId, tcInstStupidTheta, tcSyntaxName
20 import Id ( Id, idType, mkLocalId )
22 import TcSimplify ( tcSimplifyCheck, bindInstsOfLocalFuns )
23 import TcEnv ( newLocalName, tcExtendIdEnv1, tcExtendTyVarEnv3,
24 tcLookupClass, tcLookupDataCon, tcLookupId )
25 import TcMType ( newTyFlexiVarTy, arityErr, tcSkolTyVars, readMetaTyVar )
26 import TcType ( TcType, TcTyVar, TcSigmaType, TcTauType, zipTopTvSubst,
27 SkolemInfo(PatSkol), isSkolemTyVar, pprSkolemTyVar,
28 TvSubst, mkTvSubst, substTyVar, substTy, MetaDetails(..),
29 mkTyVarTys, mkClassPred, mkTyConApp, isOverloadedTy )
30 import VarEnv ( mkVarEnv ) -- ugly
31 import Kind ( argTypeKind, liftedTypeKind )
32 import TcUnify ( tcSubPat, Expected(..), zapExpectedType,
33 zapExpectedTo, zapToListTy, zapToTyConApp )
34 import TcHsType ( UserTypeCtxt(..), TcSigInfo( sig_tau ), TcSigFun, tcHsPatSigType )
35 import TysWiredIn ( stringTy, parrTyCon, tupleTyCon )
36 import Unify ( MaybeErr(..), gadtRefineTys, gadtMatchTys )
37 import Type ( substTys, substTheta )
38 import CmdLineOpts ( opt_IrrefutableTuples )
39 import TyCon ( TyCon )
40 import DataCon ( DataCon, dataConTyCon, isVanillaDataCon, dataConInstOrigArgTys,
41 dataConFieldLabels, dataConSourceArity, dataConSig )
42 import PrelNames ( eqStringName, eqName, geName, negateName, minusName,
44 import BasicTypes ( isBoxed )
45 import SrcLoc ( Located(..), SrcSpan, noLoc, unLoc )
46 import Maybes ( catMaybes )
47 import ErrUtils ( Message )
53 %************************************************************************
57 %************************************************************************
61 tcPat takes a "thing inside" over which the patter scopes. This is partly
62 so that tcPat can extend the environment for the thing_inside, but also
63 so that constraints arising in the thing_inside can be discharged by the
66 This does not work so well for the ErrCtxt carried by the monad: we don't
67 want the error-context for the pattern to scope over the RHS.
68 Hence the getErrCtxt/setErrCtxt stuff in tcPat.
72 -> LPat Name -> Expected TcSigmaType
73 -> TcM a -- Thing inside
74 -> TcM (LPat TcId, -- Translated pattern
75 [TcTyVar], -- Existential binders
76 a) -- Result of thing inside
78 tcPat ctxt pat exp_ty thing_inside
79 = do { err_ctxt <- getErrCtxt
80 ; maybeAddErrCtxt (patCtxt (unLoc pat)) $
81 tc_lpat ctxt pat exp_ty $
82 setErrCtxt err_ctxt thing_inside }
83 -- Restore error context before doing thing_inside
84 -- See note [Nesting] above
89 -> [Expected TcSigmaType] -- Excess types discarded
91 -> TcM ([LPat TcId], [TcTyVar], a)
93 tcPats ctxt [] _ thing_inside
94 = do { res <- thing_inside
95 ; return ([], [], res) }
97 tcPats ctxt (p:ps) (ty:tys) thing_inside
98 = do { (p', p_tvs, (ps', ps_tvs, res))
100 tcPats ctxt ps tys thing_inside
101 ; return (p':ps', p_tvs ++ ps_tvs, res) }
104 tcCheckPats :: PatCtxt
105 -> [LPat Name] -> [TcSigmaType]
107 -> TcM ([LPat TcId], [TcTyVar], a)
108 tcCheckPats ctxt pats tys thing_inside -- A trivial wrapper
109 = tcPats ctxt pats (map Check tys) thing_inside
113 %************************************************************************
117 %************************************************************************
120 data PatCtxt = LamPat Bool -- Used for lambda, case, do-notation etc
121 | LetPat TcSigFun -- Used for let(rec) bindings
122 -- True <=> we are checking the case expression,
123 -- so can do full-blown refinement
124 -- False <=> inferring, do no refinement
127 tcPatBndr :: PatCtxt -> Name -> Expected TcSigmaType -> TcM TcId
128 tcPatBndr (LamPat _) bndr_name pat_ty
129 = do { pat_ty' <- zapExpectedType pat_ty argTypeKind
130 -- If pat_ty is Expected, this returns the appropriate
131 -- SigmaType. In Infer mode, we create a fresh type variable.
132 -- Note the SigmaType: we can get
133 -- data T = MkT (forall a. a->a)
134 -- f t = case t of { MkT g -> ... }
135 -- Here, the 'g' must get type (forall a. a->a) from the
137 ; return (mkLocalId bndr_name pat_ty') }
139 tcPatBndr (LetPat lookup_sig) bndr_name pat_ty
140 | Just sig <- lookup_sig bndr_name
141 = do { let mono_ty = sig_tau sig
142 ; mono_name <- newLocalName bndr_name
143 ; tcSubPat mono_ty pat_ty
144 ; return (mkLocalId mono_name mono_ty) }
147 = do { mono_name <- newLocalName bndr_name
148 ; pat_ty' <- zapExpectedType pat_ty argTypeKind
149 ; return (mkLocalId mono_name pat_ty') }
153 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
154 bindInstsOfPatId id thing_inside
155 | not (isOverloadedTy (idType id))
156 = do { res <- thing_inside; return (res, emptyLHsBinds) }
158 = do { (res, lie) <- getLIE thing_inside
159 ; binds <- bindInstsOfLocalFuns lie [id]
160 ; return (res, binds) }
164 %************************************************************************
166 tc_pat: the main worker function
168 %************************************************************************
172 -> LPat Name -> Expected TcSigmaType
173 -> TcM a -- Thing inside
174 -> TcM (LPat TcId, -- Translated pattern
175 [TcTyVar], -- Existential binders
176 a) -- Result of thing inside
178 tc_lpat ctxt (L span pat) pat_ty thing_inside
180 -- It's OK to keep setting the SrcSpan;
181 -- it just overwrites the previous value
182 do { (pat', tvs, res) <- tc_pat ctxt pat pat_ty thing_inside
183 ; return (L span pat', tvs, res) }
185 ---------------------
186 tc_pat ctxt (VarPat name) pat_ty thing_inside
187 = do { id <- tcPatBndr ctxt name pat_ty
188 ; (res, binds) <- bindInstsOfPatId id $
189 tcExtendIdEnv1 name id $
190 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
192 ; let pat' | isEmptyLHsBinds binds = VarPat id
193 | otherwise = VarPatOut id binds
194 ; return (pat', [], res) }
196 tc_pat ctxt (ParPat pat) pat_ty thing_inside
197 = do { (pat', tvs, res) <- tc_lpat ctxt pat pat_ty thing_inside
198 ; return (ParPat pat', tvs, res) }
200 -- There's a wrinkle with irrefuatable patterns, namely that we
201 -- must not propagate type refinement from them. For example
202 -- data T a where { T1 :: Int -> T Int; ... }
203 -- f :: T a -> Int -> a
205 -- It's obviously not sound to refine a to Int in the right
206 -- hand side, because the arugment might not match T1 at all!
208 -- Nor should a lazy pattern bind any existential type variables
209 -- because they won't be in scope when we do the desugaring
210 tc_pat ctxt lpat@(LazyPat pat) pat_ty thing_inside
211 = do { reft <- getTypeRefinement
212 ; (pat', pat_tvs, res) <- tc_lpat ctxt pat pat_ty $
213 setTypeRefinement reft thing_inside
214 ; if (null pat_tvs) then return ()
215 else lazyPatErr lpat pat_tvs
216 ; return (LazyPat pat', [], res) }
218 tc_pat ctxt (WildPat _) pat_ty thing_inside
219 = do { pat_ty' <- zapExpectedType pat_ty argTypeKind
220 -- Note argTypeKind, so that
222 -- is rejected when f applied to an unboxed tuple
223 -- However, this means that
224 -- (case g x of _ -> ...)
225 -- is rejected g returns an unboxed tuple, which is perhpas
226 -- annoying. I suppose we could pass the context into tc_pat...
227 ; res <- thing_inside
228 ; return (WildPat pat_ty', [], res) }
230 tc_pat ctxt (AsPat (L nm_loc name) pat) pat_ty thing_inside
231 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr ctxt name pat_ty)
232 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
233 tc_lpat ctxt pat (Check (idType bndr_id)) thing_inside
234 -- NB: if we do inference on:
235 -- \ (y@(x::forall a. a->a)) = e
236 -- we'll fail. The as-pattern infers a monotype for 'y', which then
237 -- fails to unify with the polymorphic type for 'x'. This could
238 -- perhaps be fixed, but only with a bit more work.
240 -- If you fix it, don't forget the bindInstsOfPatIds!
241 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
243 tc_pat ctxt (SigPatIn pat sig) pat_ty thing_inside
244 = do { -- See Note [Pattern coercions] below
245 (sig_tvs, sig_ty) <- tcHsPatSigType PatSigCtxt sig
246 ; tcSubPat sig_ty pat_ty
247 ; subst <- refineTyVars sig_tvs -- See note [Type matching]
248 ; let tv_binds = [(tv, substTyVar subst tv) | tv <- sig_tvs]
249 sig_ty' = substTy subst sig_ty
251 <- tcExtendTyVarEnv3 tv_binds $
252 tc_lpat ctxt pat (Check sig_ty') thing_inside
254 ; return (SigPatOut pat' sig_ty, tvs, res) }
256 tc_pat ctxt pat@(TypePat ty) pat_ty thing_inside
257 = failWithTc (badTypePat pat)
259 ------------------------
260 -- Lists, tuples, arrays
261 tc_pat ctxt (ListPat pats _) pat_ty thing_inside
262 = do { elem_ty <- zapToListTy pat_ty
263 ; (pats', pats_tvs, res) <- tcCheckPats ctxt pats (repeat elem_ty) thing_inside
264 ; return (ListPat pats' elem_ty, pats_tvs, res) }
266 tc_pat ctxt (PArrPat pats _) pat_ty thing_inside
267 = do { [elem_ty] <- zapToTyConApp parrTyCon pat_ty
268 ; (pats', pats_tvs, res) <- tcCheckPats ctxt pats (repeat elem_ty) thing_inside
269 ; return (PArrPat pats' elem_ty, pats_tvs, res) }
271 tc_pat ctxt (TuplePat pats boxity) pat_ty thing_inside
272 = do { let arity = length pats
273 tycon = tupleTyCon boxity arity
274 ; arg_tys <- zapToTyConApp tycon pat_ty
275 ; (pats', pats_tvs, res) <- tcCheckPats ctxt pats arg_tys thing_inside
277 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
278 -- so that we can experiment with lazy tuple-matching.
279 -- This is a pretty odd place to make the switch, but
280 -- it was easy to do.
281 ; let unmangled_result = TuplePat pats' boxity
282 possibly_mangled_result
283 | opt_IrrefutableTuples && isBoxed boxity = LazyPat (noLoc unmangled_result)
284 | otherwise = unmangled_result
286 ; ASSERT( length arg_tys == arity ) -- Syntactically enforced
287 return (possibly_mangled_result, pats_tvs, res) }
289 ------------------------
291 tc_pat ctxt pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
292 = do { data_con <- tcLookupDataCon con_name
293 ; let tycon = dataConTyCon data_con
294 ; ty_args <- zapToTyConApp tycon pat_ty
295 ; (pat', tvs, res) <- tcConPat ctxt con_span data_con tycon ty_args arg_pats thing_inside
296 ; return (pat', tvs, res) }
299 ------------------------
301 tc_pat ctxt pat@(LitPat lit@(HsString _)) pat_ty thing_inside
302 = do { -- Strings are mapped to NPatOuts, which have a guard expression
303 zapExpectedTo pat_ty stringTy
304 ; eq_id <- tcLookupId eqStringName
305 ; res <- thing_inside
306 ; returnM (NPatOut lit stringTy (nlHsVar eq_id `HsApp` nlHsLit lit), [], res) }
308 tc_pat ctxt (LitPat simple_lit) pat_ty thing_inside
309 = do { -- All other simple lits
310 zapExpectedTo pat_ty (hsLitType simple_lit)
311 ; res <- thing_inside
312 ; returnM (LitPat simple_lit, [], res) }
314 ------------------------
315 -- Overloaded patterns: n, and n+k
316 tc_pat ctxt pat@(NPatIn over_lit mb_neg) pat_ty thing_inside
317 = do { pat_ty' <- zapExpectedType pat_ty liftedTypeKind
318 ; let origin = LiteralOrigin over_lit
319 ; pos_lit_expr <- newOverloadedLit origin over_lit pat_ty'
320 ; eq <- newMethodFromName origin pat_ty' eqName
321 ; lit_expr <- case mb_neg of
322 Nothing -> returnM pos_lit_expr -- Positive literal
323 Just neg -> -- Negative literal
324 -- The 'negate' is re-mappable syntax
325 do { (_, neg_expr) <- tcSyntaxName origin pat_ty'
326 (negateName, HsVar neg)
327 ; returnM (mkHsApp (noLoc neg_expr) pos_lit_expr) }
329 ; let -- The literal in an NPatIn is always positive...
330 -- But in NPatOut, the literal is used to find identical patterns
331 -- so we must negate the literal when necessary!
332 lit' = case (over_lit, mb_neg) of
333 (HsIntegral i _, Nothing) -> HsInteger i pat_ty'
334 (HsIntegral i _, Just _) -> HsInteger (-i) pat_ty'
335 (HsFractional f _, Nothing) -> HsRat f pat_ty'
336 (HsFractional f _, Just _) -> HsRat (-f) pat_ty'
338 ; res <- thing_inside
339 ; returnM (NPatOut lit' pat_ty' (HsApp (nlHsVar eq) lit_expr), [], res) }
341 tc_pat ctxt pat@(NPlusKPatIn (L nm_loc name) lit@(HsIntegral i _) minus_name) pat_ty thing_inside
342 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr ctxt name pat_ty)
343 ; let pat_ty' = idType bndr_id
344 origin = LiteralOrigin lit
345 ; over_lit_expr <- newOverloadedLit origin lit pat_ty'
346 ; ge <- newMethodFromName origin pat_ty' geName
348 -- The '-' part is re-mappable syntax
349 ; (_, minus_expr) <- tcSyntaxName origin pat_ty' (minusName, HsVar minus_name)
351 -- The Report says that n+k patterns must be in Integral
352 -- We may not want this when using re-mappable syntax, though (ToDo?)
353 ; icls <- tcLookupClass integralClassName
354 ; dicts <- newDicts origin [mkClassPred icls [pat_ty']]
357 ; res <- tcExtendIdEnv1 name bndr_id thing_inside
358 ; returnM (NPlusKPatOut (L nm_loc bndr_id) i
359 (SectionR (nlHsVar ge) over_lit_expr)
360 (SectionR (noLoc minus_expr) over_lit_expr),
365 %************************************************************************
367 Most of the work for constructors is here
368 (the rest is in the ConPatIn case of tc_pat)
370 %************************************************************************
373 tcConPat :: PatCtxt -> SrcSpan -> DataCon -> TyCon -> [TcTauType]
374 -> HsConDetails Name (LPat Name) -> TcM a
375 -> TcM (Pat TcId, [TcTyVar], a)
376 tcConPat ctxt span data_con tycon ty_args arg_pats thing_inside
377 | isVanillaDataCon data_con
378 = do { let arg_tys = dataConInstOrigArgTys data_con ty_args
379 ; tcInstStupidTheta data_con ty_args
380 ; traceTc (text "tcConPat" <+> vcat [ppr data_con, ppr ty_args, ppr arg_tys])
381 ; (arg_pats', tvs, res) <- tcConArgs ctxt data_con arg_pats arg_tys thing_inside
382 ; return (ConPatOut (L span data_con) [] [] emptyLHsBinds
383 arg_pats' (mkTyConApp tycon ty_args),
386 | otherwise -- GADT case
387 = do { let (tvs, theta, arg_tys, _, res_tys) = dataConSig data_con
388 ; span <- getSrcSpanM
389 ; let rigid_info = PatSkol data_con span
390 ; tvs' <- tcSkolTyVars rigid_info tvs
391 ; let tv_tys' = mkTyVarTys tvs'
392 tenv = zipTopTvSubst tvs tv_tys'
393 theta' = substTheta tenv theta
394 arg_tys' = substTys tenv arg_tys
395 res_tys' = substTys tenv res_tys
396 ; dicts <- newDicts (SigOrigin rigid_info) theta'
398 -- Do type refinement!
399 ; traceTc (text "tcGadtPat" <+> vcat [ppr data_con, ppr tvs', ppr arg_tys', ppr res_tys',
400 text "ty-args:" <+> ppr ty_args ])
401 ; refineAlt ctxt data_con tvs' ty_args res_tys' $ do
403 { ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
404 do { tcInstStupidTheta data_con tv_tys'
405 -- The stupid-theta mentions the newly-bound tyvars, so
406 -- it must live inside the getLIE, so that the
407 -- tcSimplifyCheck will apply the type refinement to it
408 ; tcConArgs ctxt data_con arg_pats arg_tys' thing_inside }
410 ; dict_binds <- tcSimplifyCheck doc tvs' dicts lie_req
412 ; return (ConPatOut (L span data_con)
413 tvs' (map instToId dicts) dict_binds
414 arg_pats' (mkTyConApp tycon ty_args),
415 tvs' ++ inner_tvs, res) } }
417 doc = ptext SLIT("existential context for") <+> quotes (ppr data_con)
419 tcConArgs :: PatCtxt -> DataCon
420 -> HsConDetails Name (LPat Name) -> [TcSigmaType]
422 -> TcM (HsConDetails TcId (LPat Id), [TcTyVar], a)
424 tcConArgs ctxt data_con (PrefixCon arg_pats) arg_tys thing_inside
425 = do { checkTc (con_arity == no_of_args) -- Check correct arity
426 (arityErr "Constructor" data_con con_arity no_of_args)
427 ; (arg_pats', tvs, res) <- tcCheckPats ctxt arg_pats arg_tys thing_inside
428 ; return (PrefixCon arg_pats', tvs, res) }
430 con_arity = dataConSourceArity data_con
431 no_of_args = length arg_pats
433 tcConArgs ctxt data_con (InfixCon p1 p2) arg_tys thing_inside
434 = do { checkTc (con_arity == 2) -- Check correct arity
435 (arityErr "Constructor" data_con con_arity 2)
436 ; ([p1',p2'], tvs, res) <- tcCheckPats ctxt [p1,p2] arg_tys thing_inside
437 ; return (InfixCon p1' p2', tvs, res) }
439 con_arity = dataConSourceArity data_con
441 tcConArgs ctxt data_con (RecCon rpats) arg_tys thing_inside
442 = do { (rpats', tvs, res) <- tc_fields rpats thing_inside
443 ; return (RecCon rpats', tvs, res) }
445 tc_fields :: [(Located Name, LPat Name)] -> TcM a
446 -> TcM ([(Located TcId, LPat TcId)], [TcTyVar], a)
447 tc_fields [] thing_inside
448 = do { res <- thing_inside
449 ; return ([], [], res) }
451 tc_fields (rpat : rpats) thing_inside
452 = do { (rpat', tvs1, (rpats', tvs2, res))
453 <- tc_field rpat (tc_fields rpats thing_inside)
454 ; return (rpat':rpats', tvs1 ++ tvs2, res) }
456 tc_field (field_lbl, pat) thing_inside
457 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
458 ; (pat', tvs, res) <- tcPat ctxt pat (Check pat_ty) thing_inside
459 ; return ((sel_id, pat'), tvs, res) }
461 find_field_ty field_lbl
462 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
464 -- No matching field; chances are this field label comes from some
465 -- other record type (or maybe none). As well as reporting an
466 -- error we still want to typecheck the pattern, principally to
467 -- make sure that all the variables it binds are put into the
468 -- environment, else the type checker crashes later:
469 -- f (R { foo = (a,b) }) = a+b
470 -- If foo isn't one of R's fields, we don't want to crash when
471 -- typechecking the "a+b".
472 [] -> do { addErrTc (badFieldCon data_con field_lbl)
473 ; bogus_ty <- newTyFlexiVarTy liftedTypeKind
474 ; return (error "Bogus selector Id", bogus_ty) }
476 -- The normal case, when the field comes from the right constructor
478 ASSERT( null extras )
479 do { sel_id <- tcLookupId field_lbl
480 ; return (sel_id, pat_ty) }
482 field_tys = zip (dataConFieldLabels data_con) arg_tys
483 -- Don't use zipEqual! If the constructor isn't really a record, then
484 -- dataConFieldLabels will be empty (and each field in the pattern
485 -- will generate an error below).
489 %************************************************************************
493 %************************************************************************
496 refineAlt :: PatCtxt -> DataCon
497 -> [TcTyVar] -- Freshly bound type variables
498 -> [TcType] -- Types from the scrutinee (context)
499 -> [TcType] -- Types from the pattern
501 refineAlt ctxt con ex_tvs ctxt_tys pat_tys thing_inside
502 = do { old_subst <- getTypeRefinement
503 ; let refiner | can_i_refine ctxt = gadtRefineTys
504 | otherwise = gadtMatchTys
505 ; case refiner ex_tvs old_subst pat_tys ctxt_tys of
506 Failed msg -> failWithTc (inaccessibleAlt msg)
507 Succeeded new_subst -> do {
508 traceTc (text "refineTypes:match" <+> ppr con <+> ppr new_subst)
509 ; setTypeRefinement new_subst thing_inside } }
512 can_i_refine (LamPat can_refine) = can_refine
513 can_i_refine other_ctxt = False
518 This little function @refineTyVars@ is a little tricky. Suppose we have a pattern type
520 f = \(x :: Term a) -> body
521 Then 'a' should be bound to a wobbly type. But if we have
522 f :: Term b -> some-type
523 f = \(x :: Term a) -> body
524 then 'a' should be bound to the rigid type 'b'. So we want to
525 * instantiate the type sig with fresh meta tyvars (e.g. \alpha)
526 * unify with the type coming from the context
527 * read out whatever information has been gleaned
528 from that unificaiton (e.g. unifying \alpha with 'b')
529 * and replace \alpha by 'b' in the type, when typechecking the
530 pattern inside the type sig (x in this case)
531 It amounts to combining rigid info from the context and from the sig.
533 Exactly the same thing happens for 'smart function application'.
536 refineTyVars :: [TcTyVar] -- Newly instantiated meta-tyvars of the function
537 -> TcM TvSubst -- Substitution mapping any of the meta-tyvars that
538 -- has been unifies to what it was instantiated to
539 -- Just one level of de-wobblification though. What a hack!
541 = do { mb_prs <- mapM mk_pr tvs
542 ; return (mkTvSubst (mkVarEnv (catMaybes mb_prs))) }
544 mk_pr tv = do { details <- readMetaTyVar tv
546 Indirect ty -> return (Just (tv,ty))
547 other -> return Nothing
551 %************************************************************************
553 Note [Pattern coercions]
555 %************************************************************************
557 In principle, these program would be reasonable:
559 f :: (forall a. a->a) -> Int
560 f (x :: Int->Int) = x 3
562 g :: (forall a. [a]) -> Bool
565 In both cases, the function type signature restricts what arguments can be passed
566 in a call (to polymorphic ones). The pattern type signature then instantiates this
567 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
568 generate the translated term
569 f = \x' :: (forall a. a->a). let x = x' Int in x 3
571 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
572 And it requires a significant amount of code to implement, becuase we need to decorate
573 the translated pattern with coercion functions (generated from the subsumption check
576 So for now I'm just insisting on type *equality* in patterns. No subsumption.
578 Old notes about desugaring, at a time when pattern coercions were handled:
580 A SigPat is a type coercion and must be handled one at at time. We can't
581 combine them unless the type of the pattern inside is identical, and we don't
582 bother to check for that. For example:
584 data T = T1 Int | T2 Bool
585 f :: (forall a. a -> a) -> T -> t
586 f (g::Int->Int) (T1 i) = T1 (g i)
587 f (g::Bool->Bool) (T2 b) = T2 (g b)
589 We desugar this as follows:
591 f = \ g::(forall a. a->a) t::T ->
593 in case t of { T1 i -> T1 (gi i)
596 in case t of { T2 b -> T2 (gb b)
599 Note that we do not treat the first column of patterns as a
600 column of variables, because the coerced variables (gi, gb)
601 would be of different types. So we get rather grotty code.
602 But I don't think this is a common case, and if it was we could
603 doubtless improve it.
605 Meanwhile, the strategy is:
606 * treat each SigPat coercion (always non-identity coercions)
608 * deal with the stuff inside, and then wrap a binding round
609 the result to bind the new variable (gi, gb, etc)
612 %************************************************************************
614 \subsection{Errors and contexts}
616 %************************************************************************
619 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
620 patCtxt (VarPat _) = Nothing
621 patCtxt (ParPat _) = Nothing
622 patCtxt (AsPat _ _) = Nothing
623 patCtxt pat = Just (hang (ptext SLIT("When checking the pattern:"))
626 badFieldCon :: DataCon -> Name -> SDoc
627 badFieldCon con field
628 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
629 ptext SLIT("does not have field"), quotes (ppr field)]
631 polyPatSig :: TcType -> SDoc
633 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
636 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
640 hang (ptext SLIT("A lazy (~) pattern connot bind existential type variables"))
641 2 (vcat (map get tvs))
643 get tv = ASSERT( isSkolemTyVar tv ) pprSkolemTyVar tv
646 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg