2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcPat: Typechecking patterns
9 module TcPat ( tcLetPat, tcLamPat, tcLamPats, tcProcPat, tcOverloadedLit,
10 addDataConStupidTheta, badFieldCon, polyPatSig ) where
12 #include "HsVersions.h"
14 import {-# SOURCE #-} TcExpr( tcSyntaxOp, tcInferRho)
39 import BasicTypes hiding (SuccessFlag(..))
50 %************************************************************************
54 %************************************************************************
57 tcLetPat :: (Name -> Maybe TcRhoType)
58 -> LPat Name -> BoxySigmaType
61 tcLetPat sig_fn pat pat_ty thing_inside
62 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
64 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state
67 -- Don't know how to deal with pattern-bound existentials yet
68 ; checkTc (null ex_tvs) (existentialExplode pat)
70 ; return (pat', res) }
73 tcLamPats :: [LPat Name] -- Patterns,
74 -> [BoxySigmaType] -- and their types
75 -> BoxyRhoType -- Result type,
76 -> (BoxyRhoType -> TcM a) -- and the checker for the body
77 -> TcM ([LPat TcId], a)
79 -- This is the externally-callable wrapper function
80 -- Typecheck the patterns, extend the environment to bind the variables,
81 -- do the thing inside, use any existentially-bound dictionaries to
82 -- discharge parts of the returning LIE, and deal with pattern type
85 -- 1. Initialise the PatState
86 -- 2. Check the patterns
88 -- 4. Check that no existentials escape
90 tcLamPats pats tys res_ty thing_inside
91 = tc_lam_pats LamPat (zipEqual "tcLamPats" pats tys)
94 tcLamPat, tcProcPat :: LPat Name -> BoxySigmaType
95 -> BoxyRhoType -- Result type
96 -> (BoxyRhoType -> TcM a) -- Checker for body, given
99 tcLamPat = tc_lam_pat LamPat
100 tcProcPat = tc_lam_pat ProcPat
102 tc_lam_pat :: PatCtxt -> LPat Name -> BoxySigmaType -> BoxyRhoType
103 -> (BoxyRhoType -> TcM a) -> TcM (LPat TcId, a)
104 tc_lam_pat ctxt pat pat_ty res_ty thing_inside
105 = do { ([pat'],thing) <- tc_lam_pats ctxt [(pat, pat_ty)] res_ty thing_inside
106 ; return (pat', thing) }
109 tc_lam_pats :: PatCtxt
110 -> [(LPat Name,BoxySigmaType)]
111 -> BoxyRhoType -- Result type
112 -> (BoxyRhoType -> TcM a) -- Checker for body, given its result type
113 -> TcM ([LPat TcId], a)
114 tc_lam_pats ctxt pat_ty_prs res_ty thing_inside
115 = do { let init_state = PS { pat_ctxt = ctxt, pat_eqs = False }
117 ; (pats', ex_tvs, res) <- do { traceTc (text "tc_lam_pats" <+> (ppr pat_ty_prs $$ ppr res_ty))
118 ; tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
119 if (pat_eqs pstate' && (not $ isRigidTy res_ty))
120 then nonRigidResult res_ty
121 else thing_inside res_ty }
123 ; let tys = map snd pat_ty_prs
124 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
126 ; return (pats', res) }
130 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
131 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
132 -> [BoxySigmaType] -- Types of the patterns
133 -> BoxyRhoType -- Type of the body of the match
134 -- Tyvars in either of these must not escape
136 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
137 -- For example, we must reject this program:
138 -- data C = forall a. C (a -> Int)
140 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
142 tcCheckExistentialPat _ [] _ _
143 = return () -- Short cut for case when there are no existentials
145 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
146 = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
147 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
151 pat_eqs :: Bool -- <=> there are any equational constraints
152 -- Used at the end to say whether the result
153 -- type must be rigid
158 | ProcPat -- The pattern in (proc pat -> ...)
159 -- see Note [Arrows and patterns]
160 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
162 patSigCtxt :: PatState -> UserTypeCtxt
163 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
164 patSigCtxt _ = LamPatSigCtxt
169 %************************************************************************
173 %************************************************************************
176 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
177 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
178 | Just mono_ty <- lookup_sig bndr_name
179 = do { mono_name <- newLocalName bndr_name
180 ; boxyUnify mono_ty pat_ty
181 ; return (Id.mkLocalId mono_name mono_ty) }
184 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
185 ; mono_name <- newLocalName bndr_name
186 ; return (Id.mkLocalId mono_name pat_ty') }
188 tcPatBndr (PS { pat_ctxt = _lam_or_proc }) bndr_name pat_ty
189 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
190 -- We have an undecorated binder, so we do rule ABS1,
191 -- by unboxing the boxy type, forcing any un-filled-in
192 -- boxes to become monotypes
193 -- NB that pat_ty' can still be a polytype:
194 -- data T = MkT (forall a. a->a)
195 -- f t = case t of { MkT g -> ... }
196 -- Here, the 'g' must get type (forall a. a->a) from the
198 ; return (Id.mkLocalId bndr_name pat_ty') }
202 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
203 bindInstsOfPatId id thing_inside
204 | not (isOverloadedTy (idType id))
205 = do { res <- thing_inside; return (res, emptyLHsBinds) }
207 = do { (res, lie) <- getLIE thing_inside
208 ; binds <- bindInstsOfLocalFuns lie [id]
209 ; return (res, binds) }
212 unBoxPatBndrType :: BoxyType -> Name -> TcM TcType
213 unBoxPatBndrType ty name = unBoxArgType ty (ptext (sLit "The variable") <+> quotes (ppr name))
215 unBoxWildCardType :: BoxyType -> TcM TcType
216 unBoxWildCardType ty = unBoxArgType ty (ptext (sLit "A wild-card pattern"))
218 unBoxViewPatType :: BoxyType -> Pat Name -> TcM TcType
219 unBoxViewPatType ty pat = unBoxArgType ty (ptext (sLit "The view pattern") <+> ppr pat)
221 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
222 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
223 -- that is, it can't be an unboxed tuple. For example,
224 -- case (f x) of r -> ...
225 -- should fail if 'f' returns an unboxed tuple.
226 unBoxArgType ty pp_this
227 = do { ty' <- unBox ty -- Returns a zonked type
229 -- Neither conditional is strictly necesssary (the unify alone will do)
230 -- but they improve error messages, and allocate fewer tyvars
231 ; if isUnboxedTupleType ty' then
233 else if isSubArgTypeKind (typeKind ty') then
235 else do -- OpenTypeKind, so constrain it
236 { ty2 <- newFlexiTyVarTy argTypeKind
240 msg = pp_this <+> ptext (sLit "cannot be bound to an unboxed tuple")
244 %************************************************************************
246 The main worker functions
248 %************************************************************************
252 tcPat takes a "thing inside" over which the pattern scopes. This is partly
253 so that tcPat can extend the environment for the thing_inside, but also
254 so that constraints arising in the thing_inside can be discharged by the
257 This does not work so well for the ErrCtxt carried by the monad: we don't
258 want the error-context for the pattern to scope over the RHS.
259 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
263 type Checker inp out = forall r.
266 -> (PatState -> TcM r)
267 -> TcM (out, [TcTyVar], r)
269 tcMultiple :: Checker inp out -> Checker [inp] [out]
270 tcMultiple tc_pat args pstate thing_inside
271 = do { err_ctxt <- getErrCtxt
273 = do { res <- thing_inside pstate
274 ; return ([], [], res) }
276 loop pstate (arg:args)
277 = do { (p', p_tvs, (ps', ps_tvs, res))
278 <- tc_pat arg pstate $ \ pstate' ->
279 setErrCtxt err_ctxt $
281 -- setErrCtxt: restore context before doing the next pattern
282 -- See note [Nesting] above
284 ; return (p':ps', p_tvs ++ ps_tvs, res) }
289 tc_lpat_pr :: (LPat Name, BoxySigmaType)
291 -> (PatState -> TcM a)
292 -> TcM (LPat TcId, [TcTyVar], a)
293 tc_lpat_pr (pat, ty) = tc_lpat pat ty
298 -> (PatState -> TcM a)
299 -> TcM (LPat TcId, [TcTyVar], a)
300 tc_lpat (L span pat) pat_ty pstate thing_inside
302 maybeAddErrCtxt (patCtxt pat) $
303 do { (pat', tvs, res) <- tc_pat pstate pat pat_ty thing_inside
304 ; return (L span pat', tvs, res) }
309 -> BoxySigmaType -- Fully refined result type
310 -> (PatState -> TcM a) -- Thing inside
311 -> TcM (Pat TcId, -- Translated pattern
312 [TcTyVar], -- Existential binders
313 a) -- Result of thing inside
315 tc_pat pstate (VarPat name) pat_ty thing_inside
316 = do { id <- tcPatBndr pstate name pat_ty
317 ; (res, binds) <- bindInstsOfPatId id $
318 tcExtendIdEnv1 name id $
319 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
320 >> thing_inside pstate)
321 ; let pat' | isEmptyLHsBinds binds = VarPat id
322 | otherwise = VarPatOut id binds
323 ; return (pat', [], res) }
325 tc_pat pstate (ParPat pat) pat_ty thing_inside
326 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
327 ; return (ParPat pat', tvs, res) }
329 tc_pat pstate (BangPat pat) pat_ty thing_inside
330 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
331 ; return (BangPat pat', tvs, res) }
333 -- There's a wrinkle with irrefutable patterns, namely that we
334 -- must not propagate type refinement from them. For example
335 -- data T a where { T1 :: Int -> T Int; ... }
336 -- f :: T a -> Int -> a
338 -- It's obviously not sound to refine a to Int in the right
339 -- hand side, because the arugment might not match T1 at all!
341 -- Nor should a lazy pattern bind any existential type variables
342 -- because they won't be in scope when we do the desugaring
344 -- Note [Hopping the LIE in lazy patterns]
345 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
346 -- In a lazy pattern, we must *not* discharge constraints from the RHS
347 -- from dictionaries bound in the pattern. E.g.
349 -- We can't discharge the Num constraint from dictionaries bound by
352 -- So we have to make the constraints from thing_inside "hop around"
353 -- the pattern. Hence the getLLE and extendLIEs later.
355 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
356 = do { (pat', pat_tvs, (res,lie))
357 <- tc_lpat pat pat_ty pstate $ \ _ ->
358 getLIE (thing_inside pstate)
359 -- Ignore refined pstate', revert to pstate
361 -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
363 -- Check no existentials
364 ; if (null pat_tvs) then return ()
365 else lazyPatErr lpat pat_tvs
367 -- Check that the pattern has a lifted type
368 ; pat_tv <- newBoxyTyVar liftedTypeKind
369 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
371 ; return (LazyPat pat', [], res) }
373 tc_pat _ p@(QuasiQuotePat _) _ _
374 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
376 tc_pat pstate (WildPat _) pat_ty thing_inside
377 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
378 ; res <- thing_inside pstate
379 ; return (WildPat pat_ty', [], res) }
381 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
382 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
383 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
384 tc_lpat pat (idType bndr_id) pstate thing_inside
385 -- NB: if we do inference on:
386 -- \ (y@(x::forall a. a->a)) = e
387 -- we'll fail. The as-pattern infers a monotype for 'y', which then
388 -- fails to unify with the polymorphic type for 'x'. This could
389 -- perhaps be fixed, but only with a bit more work.
391 -- If you fix it, don't forget the bindInstsOfPatIds!
392 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
394 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
395 = do { -- morally, expr must have type
396 -- `forall a1...aN. OPT' -> B`
397 -- where overall_pat_ty is an instance of OPT'.
398 -- Here, we infer a rho type for it,
399 -- which replaces the leading foralls and constraints
400 -- with fresh unification variables.
401 (expr',expr'_inferred) <- tcInferRho expr
402 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
403 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
404 -- tcSubExp: expected first, offered second
407 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
408 -- variable to find it). this means that in an example like
409 -- (view -> f) where view :: _ -> forall b. b
410 -- we will only be able to use view at one instantation in the
412 ; (expr_coerc, pat_ty) <- tcInfer $ \ pat_ty ->
413 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
415 -- pattern must have pat_ty
416 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
417 -- this should get zonked later on, but we unBox it here
418 -- so that we do the same checks as above
419 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
420 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
422 -- Type signatures in patterns
423 -- See Note [Pattern coercions] below
424 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
425 = do { (inner_ty, tv_binds) <- 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 _ pat@(TypePat _) _ _
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 (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 -- pattern coercions have to
490 -- be of kind: pat_ty ~ lit_ty
492 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
495 ------------------------
496 -- Overloaded patterns: n, and n+k
497 tc_pat pstate (NPat over_lit mb_neg eq) pat_ty thing_inside
498 = do { let orig = LiteralOrigin over_lit
499 ; lit' <- tcOverloadedLit orig over_lit pat_ty
500 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
501 ; mb_neg' <- case mb_neg of
502 Nothing -> return Nothing -- Positive literal
503 Just neg -> -- Negative literal
504 -- The 'negate' is re-mappable syntax
505 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
506 ; return (Just neg') }
507 ; res <- thing_inside pstate
508 ; return (NPat lit' mb_neg' eq', [], res) }
510 tc_pat pstate (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
511 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
512 ; let pat_ty' = idType bndr_id
513 orig = LiteralOrigin lit
514 ; lit' <- tcOverloadedLit orig lit pat_ty'
516 -- The '>=' and '-' parts are re-mappable syntax
517 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
518 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
520 -- The Report says that n+k patterns must be in Integral
521 -- We may not want this when using re-mappable syntax, though (ToDo?)
522 ; icls <- tcLookupClass integralClassName
523 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
525 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
526 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
528 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
532 %************************************************************************
534 Most of the work for constructors is here
535 (the rest is in the ConPatIn case of tc_pat)
537 %************************************************************************
539 [Pattern matching indexed data types]
540 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
541 Consider the following declarations:
543 data family Map k :: * -> *
544 data instance Map (a, b) v = MapPair (Map a (Pair b v))
546 and a case expression
548 case x :: Map (Int, c) w of MapPair m -> ...
550 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
551 worker/wrapper types for MapPair are
553 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
554 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
556 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
557 :R123Map, which means the straight use of boxySplitTyConApp would give a type
558 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
559 boxySplitTyConApp with the family tycon Map instead, which gives us the family
560 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
561 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
562 (provided by tyConFamInst_maybe together with the family tycon). This
563 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
564 the split arguments for the representation tycon :R123Map as {Int, c, w}
566 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
568 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
570 moving between representation and family type into account. To produce type
571 correct Core, this coercion needs to be used to case the type of the scrutinee
572 from the family to the representation type. This is achieved by
573 unwrapFamInstScrutinee using a CoPat around the result pattern.
575 Now it might appear seem as if we could have used the previous GADT type
576 refinement infrastructure of refineAlt and friends instead of the explicit
577 unification and CoPat generation. However, that would be wrong. Why? The
578 whole point of GADT refinement is that the refinement is local to the case
579 alternative. In contrast, the substitution generated by the unification of
580 the family type list and instance types needs to be propagated to the outside.
581 Imagine that in the above example, the type of the scrutinee would have been
582 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
583 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
584 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
585 alternatives of the case expression, whereas in the GADT case it might vary
586 between alternatives.
588 RIP GADT refinement: refinements have been replaced by the use of explicit
589 equality constraints that are used in conjunction with implication constraints
590 to express the local scope of GADT refinements.
594 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
595 -- with scrutinee of type (T ty)
597 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
598 -> BoxySigmaType -- Type of the pattern
599 -> HsConPatDetails Name -> (PatState -> TcM a)
600 -> TcM (Pat TcId, [TcTyVar], a)
601 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
602 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
603 = dataConFullSig data_con
604 skol_info = PatSkol data_con
605 origin = SigOrigin skol_info
606 full_theta = eq_theta ++ dict_theta
608 -- Instantiate the constructor type variables [a->ty]
609 -- This may involve doing a family-instance coercion, and building a
611 ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
612 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
613 pat_ty' = mkTyConApp tycon ctxt_res_tys
614 -- pat_ty' /= pat_ty iff coi /= IdCo
616 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
618 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys res_pat
620 ; traceTc $ case sym_coi of
621 IdCo -> text "sym_coi:IdCo"
622 ACo co -> text "sym_coi: ACoI" <+> ppr co
624 -- Add the stupid theta
625 ; addDataConStupidTheta data_con ctxt_res_tys
627 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
628 -- Get location from monad, not from ex_tvs
630 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
631 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
632 arg_tys' = substTys tenv arg_tys
634 ; if null ex_tvs && null eq_spec && null full_theta
635 then do { -- The common case; no class bindings etc
636 -- (see Note [Arrows and patterns])
637 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
638 arg_pats pstate thing_inside
639 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
640 pat_tvs = [], pat_dicts = [],
641 pat_binds = emptyLHsBinds,
642 pat_args = arg_pats',
645 ; return (wrap_res_pat res_pat, inner_tvs, res) }
647 else do -- The general case, with existential, and local equality
649 { checkTc (case pat_ctxt pstate of { ProcPat -> False; _ -> True })
650 (existentialProcPat data_con)
652 -- Need to test for rigidity if *any* constraints in theta as class
653 -- constraints may have superclass equality constraints. However,
654 -- we don't want to check for rigidity if we got here only because
655 -- ex_tvs was non-null.
656 -- ; unless (null theta') $
657 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
658 -- if we explicit or implicit (by a GADT def) have equality
660 ; let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
661 theta' = substTheta tenv (eq_preds ++ full_theta)
662 -- order is *important* as we generate the list of
663 -- dictionary binders from theta'
664 no_equalities = not (any isEqPred theta')
665 pstate' | no_equalities = pstate
666 | otherwise = pstate { pat_eqs = True }
668 ; unless no_equalities (checkTc (isRigidTy pat_ty)
669 (nonRigidMatch data_con))
671 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
672 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
674 ; loc <- getInstLoc origin
675 ; dicts <- newDictBndrs loc theta'
676 ; dict_binds <- tcSimplifyCheckPat loc ex_tvs' dicts lie_req
678 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
680 pat_dicts = map instToVar dicts,
681 pat_binds = dict_binds,
682 pat_args = arg_pats', pat_ty = pat_ty' }
683 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
686 -- Split against the family tycon if the pattern constructor
687 -- belongs to a family instance tycon.
688 boxySplitTyConAppWithFamily tycon pat_ty =
690 case tyConFamInst_maybe tycon of
691 Nothing -> boxySplitTyConApp tycon pat_ty
692 Just (fam_tycon, instTys) ->
693 do { (scrutinee_arg_tys, coi) <- boxySplitTyConApp fam_tycon pat_ty
694 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
695 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
696 ; return (freshTvs, coi)
699 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
700 ppr tycon <+> ppr pat_ty
701 , text " family instance:" <+>
702 ppr (tyConFamInst_maybe tycon)
705 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
706 -- pattern if the tycon is an instance of a family.
708 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
709 unwrapFamInstScrutinee tycon args pat
710 | Just co_con <- tyConFamilyCoercion_maybe tycon
711 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
713 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
714 -- coercion is not the identity; mkCoPat is inconvenient as it
715 -- wants a located pattern.
716 = CoPat (WpCast $ mkTyConApp co_con args) -- co fam ty to repr ty
717 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
718 pat_ty -- family inst type
723 tcConArgs :: DataCon -> [TcSigmaType]
724 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
726 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
727 = do { checkTc (con_arity == no_of_args) -- Check correct arity
728 (arityErr "Constructor" data_con con_arity no_of_args)
729 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
730 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
732 ; return (PrefixCon arg_pats', tvs, res) }
734 con_arity = dataConSourceArity data_con
735 no_of_args = length arg_pats
737 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
738 = do { checkTc (con_arity == 2) -- Check correct arity
739 (arityErr "Constructor" data_con con_arity 2)
740 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
741 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
743 ; return (InfixCon p1' p2', tvs, res) }
745 con_arity = dataConSourceArity data_con
747 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
748 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
749 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
751 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
752 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
753 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
754 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
755 ; return (HsRecField sel_id pat' pun, tvs, res) }
757 find_field_ty :: FieldLabel -> TcM (Id, TcType)
758 find_field_ty field_lbl
759 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
761 -- No matching field; chances are this field label comes from some
762 -- other record type (or maybe none). As well as reporting an
763 -- error we still want to typecheck the pattern, principally to
764 -- make sure that all the variables it binds are put into the
765 -- environment, else the type checker crashes later:
766 -- f (R { foo = (a,b) }) = a+b
767 -- If foo isn't one of R's fields, we don't want to crash when
768 -- typechecking the "a+b".
769 [] -> do { addErrTc (badFieldCon data_con field_lbl)
770 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
771 ; return (error "Bogus selector Id", bogus_ty) }
773 -- The normal case, when the field comes from the right constructor
775 ASSERT( null extras )
776 do { sel_id <- tcLookupField field_lbl
777 ; return (sel_id, pat_ty) }
779 field_tys :: [(FieldLabel, TcType)]
780 field_tys = zip (dataConFieldLabels data_con) arg_tys
781 -- Don't use zipEqual! If the constructor isn't really a record, then
782 -- dataConFieldLabels will be empty (and each field in the pattern
783 -- will generate an error below).
785 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
786 tcConArg (arg_pat, arg_ty) pstate thing_inside
787 = tc_lpat arg_pat arg_ty pstate thing_inside
791 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
792 -- Instantiate the "stupid theta" of the data con, and throw
793 -- the constraints into the constraint set
794 addDataConStupidTheta data_con inst_tys
795 | null stupid_theta = return ()
796 | otherwise = instStupidTheta origin inst_theta
798 origin = OccurrenceOf (dataConName data_con)
799 -- The origin should always report "occurrence of C"
800 -- even when C occurs in a pattern
801 stupid_theta = dataConStupidTheta data_con
802 tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
803 -- NB: inst_tys can be longer than the univ tyvars
804 -- because the constructor might have existentials
805 inst_theta = substTheta tenv stupid_theta
808 Note [Arrows and patterns]
809 ~~~~~~~~~~~~~~~~~~~~~~~~~~
810 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
812 expr :: Arrow a => a () Int
813 expr = proc (y,z) -> do
817 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
818 arrow body (e.g 'term'). As things stand (bogusly) all the
819 constraints from the proc body are gathered together, so constraints
820 from 'term' will be seen by the tcPat for (y,z). But we must *not*
821 bind constraints from 'term' here, becuase the desugarer will not make
822 these bindings scope over 'term'.
824 The Right Thing is not to confuse these constraints together. But for
825 now the Easy Thing is to ensure that we do not have existential or
826 GADT constraints in a 'proc', and to short-cut the constraint
827 simplification for such vanilla patterns so that it binds no
828 constraints. Hence the 'fast path' in tcConPat; but it's also a good
829 plan for ordinary vanilla patterns to bypass the constraint
833 %************************************************************************
837 %************************************************************************
839 In tcOverloadedLit we convert directly to an Int or Integer if we
840 know that's what we want. This may save some time, by not
841 temporarily generating overloaded literals, but it won't catch all
842 cases (the rest are caught in lookupInst).
845 tcOverloadedLit :: InstOrigin
848 -> TcM (HsOverLit TcId)
849 tcOverloadedLit orig lit@(OverLit { ol_val = val, ol_rebindable = rebindable
850 , ol_witness = meth_name }) res_ty
852 -- 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 { hs_lit <- mkOverLit val
858 ; let lit_ty = hsLitType hs_lit
859 ; fi' <- tcSyntaxOp orig meth_name (mkFunTy lit_ty res_ty)
860 -- Overloaded literals must have liftedTypeKind, because
861 -- we're instantiating an overloaded function here,
862 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
863 -- However this'll be picked up by tcSyntaxOp if necessary
864 ; let witness = HsApp (noLoc fi') (noLoc (HsLit hs_lit))
865 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
867 | Just expr <- shortCutLit val res_ty
868 = return (lit { ol_witness = expr, ol_type = res_ty })
871 = do { loc <- getInstLoc orig
872 ; res_tau <- zapToMonotype res_ty
873 ; new_uniq <- newUnique
874 ; let lit_nm = mkSystemVarName new_uniq (fsLit "lit")
875 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
876 tci_ty = res_tau, tci_loc = loc}
877 witness = HsVar (instToId lit_inst)
879 ; return (lit { ol_witness = witness, ol_type = res_ty }) }
883 %************************************************************************
885 Note [Pattern coercions]
887 %************************************************************************
889 In principle, these program would be reasonable:
891 f :: (forall a. a->a) -> Int
892 f (x :: Int->Int) = x 3
894 g :: (forall a. [a]) -> Bool
897 In both cases, the function type signature restricts what arguments can be passed
898 in a call (to polymorphic ones). The pattern type signature then instantiates this
899 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
900 generate the translated term
901 f = \x' :: (forall a. a->a). let x = x' Int in x 3
903 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
904 And it requires a significant amount of code to implement, becuase we need to decorate
905 the translated pattern with coercion functions (generated from the subsumption check
908 So for now I'm just insisting on type *equality* in patterns. No subsumption.
910 Old notes about desugaring, at a time when pattern coercions were handled:
912 A SigPat is a type coercion and must be handled one at at time. We can't
913 combine them unless the type of the pattern inside is identical, and we don't
914 bother to check for that. For example:
916 data T = T1 Int | T2 Bool
917 f :: (forall a. a -> a) -> T -> t
918 f (g::Int->Int) (T1 i) = T1 (g i)
919 f (g::Bool->Bool) (T2 b) = T2 (g b)
921 We desugar this as follows:
923 f = \ g::(forall a. a->a) t::T ->
925 in case t of { T1 i -> T1 (gi i)
928 in case t of { T2 b -> T2 (gb b)
931 Note that we do not treat the first column of patterns as a
932 column of variables, because the coerced variables (gi, gb)
933 would be of different types. So we get rather grotty code.
934 But I don't think this is a common case, and if it was we could
935 doubtless improve it.
937 Meanwhile, the strategy is:
938 * treat each SigPat coercion (always non-identity coercions)
940 * deal with the stuff inside, and then wrap a binding round
941 the result to bind the new variable (gi, gb, etc)
944 %************************************************************************
946 \subsection{Errors and contexts}
948 %************************************************************************
951 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
952 patCtxt (VarPat _) = Nothing
953 patCtxt (ParPat _) = Nothing
954 patCtxt (AsPat _ _) = Nothing
955 patCtxt pat = Just (hang (ptext (sLit "In the pattern:"))
958 -----------------------------------------------
960 existentialExplode :: LPat Name -> SDoc
961 existentialExplode pat
962 = hang (vcat [text "My brain just exploded.",
963 text "I can't handle pattern bindings for existentially-quantified constructors.",
964 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
965 text "In the binding group for"])
968 sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
969 -> TcM (TidyEnv, SDoc)
970 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
971 = do { pat_tys' <- mapM zonkTcType pat_tys
972 ; body_ty' <- zonkTcType body_ty
973 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
974 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
975 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
977 sep [ptext (sLit "When checking an existential match that binds"),
978 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
979 ptext (sLit "The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
980 ptext (sLit "The body has type:") <+> ppr tidy_body_ty
983 bound_ids = collectPatsBinders pats
984 show_ids = filter is_interesting bound_ids
985 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
987 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
988 -- Don't zonk the types so we get the separate, un-unified versions
990 badFieldCon :: DataCon -> Name -> SDoc
991 badFieldCon con field
992 = hsep [ptext (sLit "Constructor") <+> quotes (ppr con),
993 ptext (sLit "does not have field"), quotes (ppr field)]
995 polyPatSig :: TcType -> SDoc
997 = hang (ptext (sLit "Illegal polymorphic type signature in pattern:"))
1000 badTypePat :: Pat Name -> SDoc
1001 badTypePat pat = ptext (sLit "Illegal type pattern") <+> ppr pat
1003 existentialProcPat :: DataCon -> SDoc
1004 existentialProcPat con
1005 = hang (ptext (sLit "Illegal constructor") <+> quotes (ppr con) <+> ptext (sLit "in a 'proc' pattern"))
1006 2 (ptext (sLit "Proc patterns cannot use existentials or GADTs"))
1008 lazyPatErr :: Pat name -> [TcTyVar] -> TcM ()
1011 hang (ptext (sLit "A lazy (~) pattern cannot bind existential type variables"))
1012 2 (vcat (map pprSkolTvBinding tvs))
1014 nonRigidMatch :: DataCon -> SDoc
1016 = hang (ptext (sLit "GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1017 2 (ptext (sLit "Solution: add a type signature"))
1019 nonRigidResult :: Type -> TcM a
1020 nonRigidResult res_ty
1021 = do { env0 <- tcInitTidyEnv
1022 ; let (env1, res_ty') = tidyOpenType env0 res_ty
1023 msg = hang (ptext (sLit "GADT pattern match with non-rigid result type")
1024 <+> quotes (ppr res_ty'))
1025 2 (ptext (sLit "Solution: add a type signature"))
1026 ; failWithTcM (env1, msg) }