2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcPat: Typechecking patterns
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module TcPat ( tcLetPat, tcLamPat, tcLamPats, tcProcPat, tcOverloadedLit,
17 addDataConStupidTheta, badFieldCon, polyPatSig ) where
19 #include "HsVersions.h"
21 import {-# SOURCE #-} TcExpr( tcSyntaxOp, tcInferRho)
46 import BasicTypes hiding (SuccessFlag(..))
57 %************************************************************************
61 %************************************************************************
64 tcLetPat :: (Name -> Maybe TcRhoType)
65 -> LPat Name -> BoxySigmaType
68 tcLetPat sig_fn pat pat_ty thing_inside
69 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
71 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state
74 -- Don't know how to deal with pattern-bound existentials yet
75 ; checkTc (null ex_tvs) (existentialExplode pat)
77 ; return (pat', res) }
80 tcLamPats :: [LPat Name] -- Patterns,
81 -> [BoxySigmaType] -- and their types
82 -> BoxyRhoType -- Result type,
83 -> (BoxyRhoType -> TcM a) -- and the checker for the body
84 -> TcM ([LPat TcId], a)
86 -- This is the externally-callable wrapper function
87 -- Typecheck the patterns, extend the environment to bind the variables,
88 -- do the thing inside, use any existentially-bound dictionaries to
89 -- discharge parts of the returning LIE, and deal with pattern type
92 -- 1. Initialise the PatState
93 -- 2. Check the patterns
95 -- 4. Check that no existentials escape
97 tcLamPats pats tys res_ty thing_inside
98 = tc_lam_pats LamPat (zipEqual "tcLamPats" pats tys)
101 tcLamPat :: LPat Name -> BoxySigmaType
102 -> BoxyRhoType -- Result type
103 -> (BoxyRhoType -> TcM a) -- Checker for body, given its result type
104 -> TcM (LPat TcId, a)
106 tcProcPat = tc_lam_pat ProcPat
107 tcLamPat = tc_lam_pat LamPat
109 tc_lam_pat ctxt pat pat_ty res_ty thing_inside
110 = do { ([pat'],thing) <- tc_lam_pats ctxt [(pat, pat_ty)] res_ty thing_inside
111 ; return (pat', thing) }
114 tc_lam_pats :: PatCtxt
115 -> [(LPat Name,BoxySigmaType)]
116 -> BoxyRhoType -- Result type
117 -> (BoxyRhoType -> TcM a) -- Checker for body, given its result type
118 -> TcM ([LPat TcId], a)
119 tc_lam_pats ctxt pat_ty_prs res_ty thing_inside
120 = do { let init_state = PS { pat_ctxt = ctxt, pat_eqs = False }
122 ; (pats', ex_tvs, res) <- do { traceTc (text "tc_lam_pats" <+> (ppr pat_ty_prs $$ ppr res_ty))
123 ; tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
124 if (pat_eqs pstate' && (not $ isRigidTy res_ty))
125 then nonRigidResult res_ty
126 else thing_inside res_ty }
128 ; let tys = map snd pat_ty_prs
129 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
131 ; return (pats', res) }
135 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
136 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
137 -> [BoxySigmaType] -- Types of the patterns
138 -> BoxyRhoType -- Type of the body of the match
139 -- Tyvars in either of these must not escape
141 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
142 -- For example, we must reject this program:
143 -- data C = forall a. C (a -> Int)
145 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
147 tcCheckExistentialPat pats [] pat_tys body_ty
148 = return () -- Short cut for case when there are no existentials
150 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
151 = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
152 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
156 pat_eqs :: Bool -- <=> there are any equational constraints
157 -- Used at the end to say whether the result
158 -- type must be rigid
163 | ProcPat -- The pattern in (proc pat -> ...)
164 -- see Note [Arrows and patterns]
165 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
167 patSigCtxt :: PatState -> UserTypeCtxt
168 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
169 patSigCtxt other = LamPatSigCtxt
174 %************************************************************************
178 %************************************************************************
181 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
182 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
183 | Just mono_ty <- lookup_sig bndr_name
184 = do { mono_name <- newLocalName bndr_name
185 ; boxyUnify mono_ty pat_ty
186 ; return (Id.mkLocalId mono_name mono_ty) }
189 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
190 ; mono_name <- newLocalName bndr_name
191 ; return (Id.mkLocalId mono_name pat_ty') }
193 tcPatBndr (PS { pat_ctxt = _lam_or_proc }) bndr_name pat_ty
194 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
195 -- We have an undecorated binder, so we do rule ABS1,
196 -- by unboxing the boxy type, forcing any un-filled-in
197 -- boxes to become monotypes
198 -- NB that pat_ty' can still be a polytype:
199 -- data T = MkT (forall a. a->a)
200 -- f t = case t of { MkT g -> ... }
201 -- Here, the 'g' must get type (forall a. a->a) from the
203 ; return (Id.mkLocalId bndr_name pat_ty') }
207 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
208 bindInstsOfPatId id thing_inside
209 | not (isOverloadedTy (idType id))
210 = do { res <- thing_inside; return (res, emptyLHsBinds) }
212 = do { (res, lie) <- getLIE thing_inside
213 ; binds <- bindInstsOfLocalFuns lie [id]
214 ; return (res, binds) }
217 unBoxPatBndrType ty name = unBoxArgType ty (ptext SLIT("The variable") <+> quotes (ppr name))
218 unBoxWildCardType ty = unBoxArgType ty (ptext SLIT("A wild-card pattern"))
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) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
426 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
427 tc_lpat pat inner_ty pstate thing_inside
428 ; return (SigPatOut pat' inner_ty, tvs, res) }
430 tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
431 = failWithTc (badTypePat pat)
433 ------------------------
434 -- Lists, tuples, arrays
435 tc_pat pstate (ListPat pats _) pat_ty thing_inside
436 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
437 ; let scoi = mkSymCoI coi
438 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
439 pats pstate thing_inside
440 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
443 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
444 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
445 ; let scoi = mkSymCoI coi
446 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
447 pats pstate thing_inside
448 ; when (null pats) (zapToMonotype pat_ty >> return ()) -- c.f. ExplicitPArr in TcExpr
449 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
452 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
453 = do { let tc = tupleTyCon boxity (length pats)
454 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
455 ; let scoi = mkSymCoI coi
456 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
459 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
460 -- so that we can experiment with lazy tuple-matching.
461 -- This is a pretty odd place to make the switch, but
462 -- it was easy to do.
463 ; let pat_ty' = mkTyConApp tc arg_tys
464 -- pat_ty /= pat_ty iff coi /= IdCo
465 unmangled_result = TuplePat pats' boxity pat_ty'
466 possibly_mangled_result
467 | opt_IrrefutableTuples &&
468 isBoxed boxity = LazyPat (noLoc unmangled_result)
469 | otherwise = unmangled_result
471 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
472 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
475 ------------------------
477 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
478 = do { data_con <- tcLookupDataCon con_name
479 ; let tycon = dataConTyCon data_con
480 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
482 ------------------------
484 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
485 = do { let lit_ty = hsLitType simple_lit
486 ; coi <- boxyUnify lit_ty pat_ty
487 -- coi is of kind: lit_ty ~ pat_ty
488 ; res <- thing_inside pstate
489 ; span <- getSrcSpanM
490 -- pattern coercions have to
491 -- be of kind: pat_ty ~ lit_ty
493 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
496 ------------------------
497 -- Overloaded patterns: n, and n+k
498 tc_pat pstate pat@(NPat over_lit mb_neg eq) pat_ty thing_inside
499 = do { let orig = LiteralOrigin over_lit
500 ; lit' <- tcOverloadedLit orig over_lit pat_ty
501 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
502 ; mb_neg' <- case mb_neg of
503 Nothing -> return Nothing -- Positive literal
504 Just neg -> -- Negative literal
505 -- The 'negate' is re-mappable syntax
506 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
507 ; return (Just neg') }
508 ; res <- thing_inside pstate
509 ; return (NPat lit' mb_neg' eq', [], res) }
511 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
512 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
513 ; let pat_ty' = idType bndr_id
514 orig = LiteralOrigin lit
515 ; lit' <- tcOverloadedLit orig lit pat_ty'
517 -- The '>=' and '-' parts are re-mappable syntax
518 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
519 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
521 -- The Report says that n+k patterns must be in Integral
522 -- We may not want this when using re-mappable syntax, though (ToDo?)
523 ; icls <- tcLookupClass integralClassName
524 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
526 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
527 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
529 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
533 %************************************************************************
535 Most of the work for constructors is here
536 (the rest is in the ConPatIn case of tc_pat)
538 %************************************************************************
540 [Pattern matching indexed data types]
541 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
542 Consider the following declarations:
544 data family Map k :: * -> *
545 data instance Map (a, b) v = MapPair (Map a (Pair b v))
547 and a case expression
549 case x :: Map (Int, c) w of MapPair m -> ...
551 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
552 worker/wrapper types for MapPair are
554 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
555 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
557 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
558 :R123Map, which means the straight use of boxySplitTyConApp would give a type
559 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
560 boxySplitTyConApp with the family tycon Map instead, which gives us the family
561 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
562 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
563 (provided by tyConFamInst_maybe together with the family tycon). This
564 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
565 the split arguments for the representation tycon :R123Map as {Int, c, w}
567 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
569 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
571 moving between representation and family type into account. To produce type
572 correct Core, this coercion needs to be used to case the type of the scrutinee
573 from the family to the representation type. This is achieved by
574 unwrapFamInstScrutinee using a CoPat around the result pattern.
576 Now it might appear seem as if we could have used the previous GADT type
577 refinement infrastructure of refineAlt and friends instead of the explicit
578 unification and CoPat generation. However, that would be wrong. Why? The
579 whole point of GADT refinement is that the refinement is local to the case
580 alternative. In contrast, the substitution generated by the unification of
581 the family type list and instance types needs to be propagated to the outside.
582 Imagine that in the above example, the type of the scrutinee would have been
583 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
584 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
585 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
586 alternatives of the case expression, whereas in the GADT case it might vary
587 between alternatives.
589 RIP GADT refinement: refinements have been replaced by the use of explicit
590 equality constraints that are used in conjunction with implication constraints
591 to express the local scope of GADT refinements.
595 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
596 -- with scrutinee of type (T ty)
598 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
599 -> BoxySigmaType -- Type of the pattern
600 -> HsConPatDetails Name -> (PatState -> TcM a)
601 -> TcM (Pat TcId, [TcTyVar], a)
602 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
603 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
604 = dataConFullSig data_con
605 skol_info = PatSkol data_con
606 origin = SigOrigin skol_info
607 full_theta = eq_theta ++ dict_theta
609 -- Instantiate the constructor type variables [a->ty]
610 -- This may involve doing a family-instance coercion, and building a
612 ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
613 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
614 pat_ty' = mkTyConApp tycon ctxt_res_tys
615 -- pat_ty' /= pat_ty iff coi /= IdCo
617 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
619 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys res_pat
621 ; traceTc $ case sym_coi of
622 IdCo -> text "sym_coi:IdCo"
623 ACo co -> text "sym_coi: ACoI" <+> ppr co
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; other -> 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 explicit or implicit (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 (checkTc (isRigidTy pat_ty)
670 (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
692 Nothing -> boxySplitTyConApp tycon pat_ty
693 Just (fam_tycon, instTys) ->
694 do { (scrutinee_arg_tys, coi) <- boxySplitTyConApp fam_tycon pat_ty
695 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
696 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
697 ; return (freshTvs, coi)
700 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
701 ppr tycon <+> ppr pat_ty
702 , text " family instance:" <+>
703 ppr (tyConFamInst_maybe tycon)
706 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
707 -- pattern if the tycon is an instance of a family.
709 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
710 unwrapFamInstScrutinee tycon args pat
711 | Just co_con <- tyConFamilyCoercion_maybe tycon
712 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
714 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
715 -- coercion is not the identity; mkCoPat is inconvenient as it
716 -- wants a located pattern.
717 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
718 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
719 pat_ty -- family inst type
724 tcConArgs :: DataCon -> [TcSigmaType]
725 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
727 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
728 = do { checkTc (con_arity == no_of_args) -- Check correct arity
729 (arityErr "Constructor" data_con con_arity no_of_args)
730 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
731 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
733 ; return (PrefixCon arg_pats', tvs, res) }
735 con_arity = dataConSourceArity data_con
736 no_of_args = length arg_pats
738 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
739 = do { checkTc (con_arity == 2) -- Check correct arity
740 (arityErr "Constructor" data_con con_arity 2)
741 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
742 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
744 ; return (InfixCon p1' p2', tvs, res) }
746 con_arity = dataConSourceArity data_con
748 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
749 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
751 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
752 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
753 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
755 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
756 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
757 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
758 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
759 ; return (HsRecField sel_id pat' pun, tvs, res) }
761 find_field_ty :: FieldLabel -> TcM (Id, TcType)
762 find_field_ty field_lbl
763 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
765 -- No matching field; chances are this field label comes from some
766 -- other record type (or maybe none). As well as reporting an
767 -- error we still want to typecheck the pattern, principally to
768 -- make sure that all the variables it binds are put into the
769 -- environment, else the type checker crashes later:
770 -- f (R { foo = (a,b) }) = a+b
771 -- If foo isn't one of R's fields, we don't want to crash when
772 -- typechecking the "a+b".
773 [] -> do { addErrTc (badFieldCon data_con field_lbl)
774 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
775 ; return (error "Bogus selector Id", bogus_ty) }
777 -- The normal case, when the field comes from the right constructor
779 ASSERT( null extras )
780 do { sel_id <- tcLookupField field_lbl
781 ; return (sel_id, pat_ty) }
783 field_tys :: [(FieldLabel, TcType)]
784 field_tys = zip (dataConFieldLabels data_con) arg_tys
785 -- Don't use zipEqual! If the constructor isn't really a record, then
786 -- dataConFieldLabels will be empty (and each field in the pattern
787 -- will generate an error below).
789 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
790 tcConArg (arg_pat, arg_ty) pstate thing_inside
791 = tc_lpat arg_pat arg_ty pstate thing_inside
795 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
796 -- Instantiate the "stupid theta" of the data con, and throw
797 -- the constraints into the constraint set
798 addDataConStupidTheta data_con inst_tys
799 | null stupid_theta = return ()
800 | otherwise = instStupidTheta origin inst_theta
802 origin = OccurrenceOf (dataConName data_con)
803 -- The origin should always report "occurrence of C"
804 -- even when C occurs in a pattern
805 stupid_theta = dataConStupidTheta data_con
806 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
807 inst_theta = substTheta tenv stupid_theta
810 Note [Arrows and patterns]
811 ~~~~~~~~~~~~~~~~~~~~~~~~~~
812 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
814 expr :: Arrow a => a () Int
815 expr = proc (y,z) -> do
819 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
820 arrow body (e.g 'term'). As things stand (bogusly) all the
821 constraints from the proc body are gathered together, so constraints
822 from 'term' will be seen by the tcPat for (y,z). But we must *not*
823 bind constraints from 'term' here, becuase the desugarer will not make
824 these bindings scope over 'term'.
826 The Right Thing is not to confuse these constraints together. But for
827 now the Easy Thing is to ensure that we do not have existential or
828 GADT constraints in a 'proc', and to short-cut the constraint
829 simplification for such vanilla patterns so that it binds no
830 constraints. Hence the 'fast path' in tcConPat; but it's also a good
831 plan for ordinary vanilla patterns to bypass the constraint
835 %************************************************************************
839 %************************************************************************
841 In tcOverloadedLit we convert directly to an Int or Integer if we
842 know that's what we want. This may save some time, by not
843 temporarily generating overloaded literals, but it won't catch all
844 cases (the rest are caught in lookupInst).
847 tcOverloadedLit :: InstOrigin
850 -> TcM (HsOverLit TcId)
851 tcOverloadedLit orig lit@(HsIntegral i fi _) res_ty
852 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
853 -- Reason: If we do, tcSimplify will call lookupInst, which
854 -- will call tcSyntaxName, which does unification,
855 -- which tcSimplify doesn't like
856 -- ToDo: noLoc sadness
857 = do { integer_ty <- tcMetaTy integerTyConName
858 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
859 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty))) res_ty) }
861 | Just expr <- shortCutIntLit i res_ty
862 = return (HsIntegral i expr res_ty)
865 = do { expr <- newLitInst orig lit res_ty
866 ; return (HsIntegral i expr res_ty) }
868 tcOverloadedLit orig lit@(HsFractional r fr _) res_ty
869 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
870 = do { rat_ty <- tcMetaTy rationalTyConName
871 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
872 -- Overloaded literals must have liftedTypeKind, because
873 -- we're instantiating an overloaded function here,
874 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
875 -- However this'll be picked up by tcSyntaxOp if necessary
876 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty))) res_ty) }
878 | Just expr <- shortCutFracLit r res_ty
879 = return (HsFractional r expr res_ty)
882 = do { expr <- newLitInst orig lit res_ty
883 ; return (HsFractional r expr res_ty) }
885 tcOverloadedLit orig lit@(HsIsString s fr _) res_ty
886 | not (fr `isHsVar` fromStringName) -- c.f. HsIntegral case
887 = do { str_ty <- tcMetaTy stringTyConName
888 ; fr' <- tcSyntaxOp orig fr (mkFunTy str_ty res_ty)
889 ; return (HsIsString s (HsApp (noLoc fr') (nlHsLit (HsString s))) res_ty) }
891 | Just expr <- shortCutStringLit s res_ty
892 = return (HsIsString s expr res_ty)
895 = do { expr <- newLitInst orig lit res_ty
896 ; return (HsIsString s expr res_ty) }
898 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
899 newLitInst orig lit res_ty -- Make a LitInst
900 = do { loc <- getInstLoc orig
901 ; res_tau <- zapToMonotype res_ty
902 ; new_uniq <- newUnique
903 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
904 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
905 tci_ty = res_tau, tci_loc = loc}
907 ; return (HsVar (instToId lit_inst)) }
911 %************************************************************************
913 Note [Pattern coercions]
915 %************************************************************************
917 In principle, these program would be reasonable:
919 f :: (forall a. a->a) -> Int
920 f (x :: Int->Int) = x 3
922 g :: (forall a. [a]) -> Bool
925 In both cases, the function type signature restricts what arguments can be passed
926 in a call (to polymorphic ones). The pattern type signature then instantiates this
927 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
928 generate the translated term
929 f = \x' :: (forall a. a->a). let x = x' Int in x 3
931 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
932 And it requires a significant amount of code to implement, becuase we need to decorate
933 the translated pattern with coercion functions (generated from the subsumption check
936 So for now I'm just insisting on type *equality* in patterns. No subsumption.
938 Old notes about desugaring, at a time when pattern coercions were handled:
940 A SigPat is a type coercion and must be handled one at at time. We can't
941 combine them unless the type of the pattern inside is identical, and we don't
942 bother to check for that. For example:
944 data T = T1 Int | T2 Bool
945 f :: (forall a. a -> a) -> T -> t
946 f (g::Int->Int) (T1 i) = T1 (g i)
947 f (g::Bool->Bool) (T2 b) = T2 (g b)
949 We desugar this as follows:
951 f = \ g::(forall a. a->a) t::T ->
953 in case t of { T1 i -> T1 (gi i)
956 in case t of { T2 b -> T2 (gb b)
959 Note that we do not treat the first column of patterns as a
960 column of variables, because the coerced variables (gi, gb)
961 would be of different types. So we get rather grotty code.
962 But I don't think this is a common case, and if it was we could
963 doubtless improve it.
965 Meanwhile, the strategy is:
966 * treat each SigPat coercion (always non-identity coercions)
968 * deal with the stuff inside, and then wrap a binding round
969 the result to bind the new variable (gi, gb, etc)
972 %************************************************************************
974 \subsection{Errors and contexts}
976 %************************************************************************
979 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
980 patCtxt (VarPat _) = Nothing
981 patCtxt (ParPat _) = Nothing
982 patCtxt (AsPat _ _) = Nothing
983 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
986 -----------------------------------------------
988 existentialExplode pat
989 = hang (vcat [text "My brain just exploded.",
990 text "I can't handle pattern bindings for existentially-quantified constructors.",
991 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
992 text "In the binding group for"])
995 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
996 = do { pat_tys' <- mapM zonkTcType pat_tys
997 ; body_ty' <- zonkTcType body_ty
998 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
999 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1000 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1002 sep [ptext SLIT("When checking an existential match that binds"),
1003 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1004 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1005 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
1008 bound_ids = collectPatsBinders pats
1009 show_ids = filter is_interesting bound_ids
1010 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1012 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1013 -- Don't zonk the types so we get the separate, un-unified versions
1015 badFieldCon :: DataCon -> Name -> SDoc
1016 badFieldCon con field
1017 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
1018 ptext SLIT("does not have field"), quotes (ppr field)]
1020 polyPatSig :: TcType -> SDoc
1022 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
1025 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
1027 existentialProcPat :: DataCon -> SDoc
1028 existentialProcPat con
1029 = hang (ptext SLIT("Illegal constructor") <+> quotes (ppr con) <+> ptext SLIT("in a 'proc' pattern"))
1030 2 (ptext SLIT("Proc patterns cannot use existentials or GADTs"))
1034 hang (ptext SLIT("A lazy (~) pattern cannot bind existential type variables"))
1035 2 (vcat (map pprSkolTvBinding tvs))
1038 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1039 2 (ptext SLIT("Solution: add a type signature"))
1041 nonRigidResult res_ty
1042 = do { env0 <- tcInitTidyEnv
1043 ; let (env1, res_ty') = tidyOpenType env0 res_ty
1044 msg = hang (ptext SLIT("GADT pattern match with non-rigid result type")
1045 <+> quotes (ppr res_ty'))
1046 2 (ptext SLIT("Solution: add a type signature"))
1047 ; failWithTcM (env1, msg) }
1050 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg