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)
47 import BasicTypes hiding (SuccessFlag(..))
58 %************************************************************************
62 %************************************************************************
65 tcLetPat :: (Name -> Maybe TcRhoType)
66 -> LPat Name -> BoxySigmaType
69 tcLetPat sig_fn pat pat_ty thing_inside
70 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
71 pat_reft = emptyRefinement,
73 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state (\ _ -> thing_inside)
75 -- Don't know how to deal with pattern-bound existentials yet
76 ; checkTc (null ex_tvs) (existentialExplode pat)
78 ; return (pat', res) }
81 tcLamPats :: [LPat Name] -- Patterns,
82 -> [BoxySigmaType] -- and their types
83 -> BoxyRhoType -- Result type,
84 -> ((Refinement, BoxyRhoType) -> TcM a) -- and the checker for the body
85 -> TcM ([LPat TcId], a)
87 -- This is the externally-callable wrapper function
88 -- Typecheck the patterns, extend the environment to bind the variables,
89 -- do the thing inside, use any existentially-bound dictionaries to
90 -- discharge parts of the returning LIE, and deal with pattern type
93 -- 1. Initialise the PatState
94 -- 2. Check the patterns
95 -- 3. Apply the refinement to the environment and result type
97 -- 5. Check that no existentials escape
99 tcLamPats pats tys res_ty thing_inside
100 = tc_lam_pats LamPat (zipEqual "tcLamPats" pats tys)
101 (emptyRefinement, res_ty) thing_inside
103 tcLamPat :: LPat Name -> BoxySigmaType
104 -> (Refinement,BoxyRhoType) -- Result type
105 -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
106 -> TcM (LPat TcId, a)
108 tcProcPat = tc_lam_pat ProcPat
109 tcLamPat = tc_lam_pat LamPat
111 tc_lam_pat ctxt pat pat_ty res_ty thing_inside
112 = do { ([pat'],thing) <- tc_lam_pats ctxt [(pat, pat_ty)] res_ty thing_inside
113 ; return (pat', thing) }
116 tc_lam_pats :: PatCtxt
117 -> [(LPat Name,BoxySigmaType)]
118 -> (Refinement,BoxyRhoType) -- Result type
119 -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
120 -> TcM ([LPat TcId], a)
121 tc_lam_pats ctxt pat_ty_prs (reft, res_ty) thing_inside
122 = do { let init_state = PS { pat_ctxt = ctxt, pat_reft = reft, pat_eqs = False }
124 ; (pats', ex_tvs, res) <- tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
125 refineEnvironment (pat_reft pstate') (pat_eqs pstate') $
126 if (pat_eqs pstate' && (not $ isRigidTy res_ty))
127 then failWithTc (nonRigidResult res_ty)
128 else thing_inside (pat_reft pstate', res_ty)
130 ; let tys = map snd pat_ty_prs
131 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
133 ; returnM (pats', res) }
137 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
138 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
139 -> [BoxySigmaType] -- Types of the patterns
140 -> BoxyRhoType -- Type of the body of the match
141 -- Tyvars in either of these must not escape
143 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
144 -- For example, we must reject this program:
145 -- data C = forall a. C (a -> Int)
147 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
149 tcCheckExistentialPat pats [] pat_tys body_ty
150 = return () -- Short cut for case when there are no existentials
152 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
153 = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
154 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
158 pat_reft :: Refinement, -- Binds rigid TcTyVars to their refinements
159 pat_eqs :: Bool -- <=> there are GADT equational constraints
165 | ProcPat -- The pattern in (proc pat -> ...)
166 -- see Note [Arrows and patterns]
167 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
169 patSigCtxt :: PatState -> UserTypeCtxt
170 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
171 patSigCtxt other = LamPatSigCtxt
176 %************************************************************************
180 %************************************************************************
183 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
184 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
185 | Just mono_ty <- lookup_sig bndr_name
186 = do { mono_name <- newLocalName bndr_name
187 ; boxyUnify mono_ty pat_ty
188 ; return (Id.mkLocalId mono_name mono_ty) }
191 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
192 ; mono_name <- newLocalName bndr_name
193 ; return (Id.mkLocalId mono_name pat_ty') }
195 tcPatBndr (PS { pat_ctxt = _lam_or_proc }) bndr_name pat_ty
196 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
197 -- We have an undecorated binder, so we do rule ABS1,
198 -- by unboxing the boxy type, forcing any un-filled-in
199 -- boxes to become monotypes
200 -- NB that pat_ty' can still be a polytype:
201 -- data T = MkT (forall a. a->a)
202 -- f t = case t of { MkT g -> ... }
203 -- Here, the 'g' must get type (forall a. a->a) from the
205 ; return (Id.mkLocalId bndr_name pat_ty') }
209 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
210 bindInstsOfPatId id thing_inside
211 | not (isOverloadedTy (idType id))
212 = do { res <- thing_inside; return (res, emptyLHsBinds) }
214 = do { (res, lie) <- getLIE thing_inside
215 ; binds <- bindInstsOfLocalFuns lie [id]
216 ; return (res, binds) }
219 unBoxPatBndrType ty name = unBoxArgType ty (ptext SLIT("The variable") <+> quotes (ppr name))
220 unBoxWildCardType ty = unBoxArgType ty (ptext SLIT("A wild-card pattern"))
221 unBoxViewPatType ty pat = unBoxArgType ty (ptext SLIT("The view pattern") <+> ppr pat)
223 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
224 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
225 -- that is, it can't be an unboxed tuple. For example,
226 -- case (f x) of r -> ...
227 -- should fail if 'f' returns an unboxed tuple.
228 unBoxArgType ty pp_this
229 = do { ty' <- unBox ty -- Returns a zonked type
231 -- Neither conditional is strictly necesssary (the unify alone will do)
232 -- but they improve error messages, and allocate fewer tyvars
233 ; if isUnboxedTupleType ty' then
235 else if isSubArgTypeKind (typeKind ty') then
237 else do -- OpenTypeKind, so constrain it
238 { ty2 <- newFlexiTyVarTy argTypeKind
242 msg = pp_this <+> ptext SLIT("cannot be bound to an unboxed tuple")
246 %************************************************************************
248 The main worker functions
250 %************************************************************************
254 tcPat takes a "thing inside" over which the pattern scopes. This is partly
255 so that tcPat can extend the environment for the thing_inside, but also
256 so that constraints arising in the thing_inside can be discharged by the
259 This does not work so well for the ErrCtxt carried by the monad: we don't
260 want the error-context for the pattern to scope over the RHS.
261 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
265 type Checker inp out = forall r.
268 -> (PatState -> TcM r)
269 -> TcM (out, [TcTyVar], r)
271 tcMultiple :: Checker inp out -> Checker [inp] [out]
272 tcMultiple tc_pat args pstate thing_inside
273 = do { err_ctxt <- getErrCtxt
275 = do { res <- thing_inside pstate
276 ; return ([], [], res) }
278 loop pstate (arg:args)
279 = do { (p', p_tvs, (ps', ps_tvs, res))
280 <- tc_pat arg pstate $ \ pstate' ->
281 setErrCtxt err_ctxt $
283 -- setErrCtxt: restore context before doing the next pattern
284 -- See note [Nesting] above
286 ; return (p':ps', p_tvs ++ ps_tvs, res) }
291 tc_lpat_pr :: (LPat Name, BoxySigmaType)
293 -> (PatState -> TcM a)
294 -> TcM (LPat TcId, [TcTyVar], a)
295 tc_lpat_pr (pat, ty) = tc_lpat pat ty
300 -> (PatState -> TcM a)
301 -> TcM (LPat TcId, [TcTyVar], a)
302 tc_lpat (L span pat) pat_ty pstate thing_inside
304 maybeAddErrCtxt (patCtxt pat) $
305 do { let mb_reft = refineType (pat_reft pstate) pat_ty
306 pat_ty' = case mb_reft of { Just (_, ty') -> ty'; Nothing -> pat_ty }
308 -- Make sure the result type reflects the current refinement
309 -- We must do this here, so that it correctly ``sees'' all
310 -- the refinements to the left. Example:
311 -- Suppose C :: forall a. T a -> a -> Foo
312 -- Pattern C a p1 True
313 -- So p1 might refine 'a' to True, and the True
314 -- pattern had better see it.
316 ; (pat', tvs, res) <- tc_pat pstate pat pat_ty' thing_inside
317 ; let final_pat = case mb_reft of
319 Just (co,_) -> CoPat (WpCo co) pat' pat_ty
320 ; return (L span final_pat, tvs, res) }
325 -> BoxySigmaType -- Fully refined result type
326 -> (PatState -> TcM a) -- Thing inside
327 -> TcM (Pat TcId, -- Translated pattern
328 [TcTyVar], -- Existential binders
329 a) -- Result of thing inside
331 tc_pat pstate (VarPat name) pat_ty thing_inside
332 = do { id <- tcPatBndr pstate name pat_ty
333 ; (res, binds) <- bindInstsOfPatId id $
334 tcExtendIdEnv1 name id $
335 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
336 >> thing_inside pstate)
337 ; let pat' | isEmptyLHsBinds binds = VarPat id
338 | otherwise = VarPatOut id binds
339 ; return (pat', [], res) }
341 tc_pat pstate (ParPat pat) pat_ty thing_inside
342 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
343 ; return (ParPat pat', tvs, res) }
345 tc_pat pstate (BangPat pat) pat_ty thing_inside
346 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
347 ; return (BangPat pat', tvs, res) }
349 -- There's a wrinkle with irrefutable patterns, namely that we
350 -- must not propagate type refinement from them. For example
351 -- data T a where { T1 :: Int -> T Int; ... }
352 -- f :: T a -> Int -> a
354 -- It's obviously not sound to refine a to Int in the right
355 -- hand side, because the arugment might not match T1 at all!
357 -- Nor should a lazy pattern bind any existential type variables
358 -- because they won't be in scope when we do the desugaring
360 -- Note [Hopping the LIE in lazy patterns]
361 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
362 -- In a lazy pattern, we must *not* discharge constraints from the RHS
363 -- from dictionaries bound in the pattern. E.g.
365 -- We can't discharge the Num constraint from dictionaries bound by
368 -- So we have to make the constraints from thing_inside "hop around"
369 -- the pattern. Hence the getLLE and extendLIEs later.
371 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
372 = do { (pat', pat_tvs, (res,lie))
373 <- tc_lpat pat pat_ty pstate $ \ _ ->
374 getLIE (thing_inside pstate)
375 -- Ignore refined pstate', revert to pstate
377 -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
379 -- Check no existentials
380 ; if (null pat_tvs) then return ()
381 else lazyPatErr lpat pat_tvs
383 -- Check that the pattern has a lifted type
384 ; pat_tv <- newBoxyTyVar liftedTypeKind
385 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
387 ; return (LazyPat pat', [], res) }
389 tc_pat pstate (WildPat _) pat_ty thing_inside
390 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
391 ; res <- thing_inside pstate
392 ; return (WildPat pat_ty', [], res) }
394 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
395 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
396 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
397 tc_lpat pat (idType bndr_id) pstate thing_inside
398 -- NB: if we do inference on:
399 -- \ (y@(x::forall a. a->a)) = e
400 -- we'll fail. The as-pattern infers a monotype for 'y', which then
401 -- fails to unify with the polymorphic type for 'x'. This could
402 -- perhaps be fixed, but only with a bit more work.
404 -- If you fix it, don't forget the bindInstsOfPatIds!
405 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
407 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
408 = do { -- morally, expr must have type
409 -- `forall a1...aN. OPT' -> B`
410 -- where overall_pat_ty is an instance of OPT'.
411 -- Here, we infer a rho type for it,
412 -- which replaces the leading foralls and constraints
413 -- with fresh unification variables.
414 (expr',expr'_inferred) <- tcInferRho expr
415 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
416 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
417 -- tcSubExp: expected first, offered second
420 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
421 -- variable to find it). this means that in an example like
422 -- (view -> f) where view :: _ -> forall b. b
423 -- we will only be able to use view at one instantation in the
425 ; (expr_coerc, pat_ty) <- tcInfer $ \ pat_ty ->
426 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
428 -- pattern must have pat_ty
429 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
430 -- this should get zonked later on, but we unBox it here
431 -- so that we do the same checks as above
432 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
433 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
435 -- Type signatures in patterns
436 -- See Note [Pattern coercions] below
437 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
438 = do { (inner_ty, tv_binds) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
439 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
440 tc_lpat pat inner_ty pstate thing_inside
441 ; return (SigPatOut pat' inner_ty, tvs, res) }
443 tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
444 = failWithTc (badTypePat pat)
446 ------------------------
447 -- Lists, tuples, arrays
448 tc_pat pstate (ListPat pats _) pat_ty thing_inside
449 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
450 ; let scoi = mkSymCoI coi
451 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
452 pats pstate thing_inside
453 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
456 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
457 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
458 ; let scoi = mkSymCoI coi
459 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
460 pats pstate thing_inside
461 ; ifM (null pats) (zapToMonotype pat_ty) -- c.f. ExplicitPArr in TcExpr
462 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
465 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
466 = do { let tc = tupleTyCon boxity (length pats)
467 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
468 ; let scoi = mkSymCoI coi
469 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
472 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
473 -- so that we can experiment with lazy tuple-matching.
474 -- This is a pretty odd place to make the switch, but
475 -- it was easy to do.
476 ; let pat_ty' = mkTyConApp tc arg_tys
477 -- pat_ty /= pat_ty iff coi /= IdCo
478 unmangled_result = TuplePat pats' boxity pat_ty'
479 possibly_mangled_result
480 | opt_IrrefutableTuples &&
481 isBoxed boxity = LazyPat (noLoc unmangled_result)
482 | otherwise = unmangled_result
484 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
485 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
488 ------------------------
490 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
491 = do { data_con <- tcLookupDataCon con_name
492 ; let tycon = dataConTyCon data_con
493 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
495 ------------------------
497 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
498 = do { let lit_ty = hsLitType simple_lit
499 ; coi <- boxyUnify lit_ty pat_ty
500 -- coi is of kind: lit_ty ~ pat_ty
501 ; res <- thing_inside pstate
502 ; span <- getSrcSpanM
503 -- pattern coercions have to
504 -- be of kind: pat_ty ~ lit_ty
506 ; returnM (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
509 ------------------------
510 -- Overloaded patterns: n, and n+k
511 tc_pat pstate pat@(NPat over_lit mb_neg eq) pat_ty thing_inside
512 = do { let orig = LiteralOrigin over_lit
513 ; lit' <- tcOverloadedLit orig over_lit pat_ty
514 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
515 ; mb_neg' <- case mb_neg of
516 Nothing -> return Nothing -- Positive literal
517 Just neg -> -- Negative literal
518 -- The 'negate' is re-mappable syntax
519 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
520 ; return (Just neg') }
521 ; res <- thing_inside pstate
522 ; returnM (NPat lit' mb_neg' eq', [], res) }
524 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
525 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
526 ; let pat_ty' = idType bndr_id
527 orig = LiteralOrigin lit
528 ; lit' <- tcOverloadedLit orig lit pat_ty'
530 -- The '>=' and '-' parts are re-mappable syntax
531 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
532 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
534 -- The Report says that n+k patterns must be in Integral
535 -- We may not want this when using re-mappable syntax, though (ToDo?)
536 ; icls <- tcLookupClass integralClassName
537 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
539 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
540 ; returnM (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
542 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
546 %************************************************************************
548 Most of the work for constructors is here
549 (the rest is in the ConPatIn case of tc_pat)
551 %************************************************************************
553 [Pattern matching indexed data types]
554 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
555 Consider the following declarations:
557 data family Map k :: * -> *
558 data instance Map (a, b) v = MapPair (Map a (Pair b v))
560 and a case expression
562 case x :: Map (Int, c) w of MapPair m -> ...
564 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
565 worker/wrapper types for MapPair are
567 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
568 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
570 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
571 :R123Map, which means the straight use of boxySplitTyConApp would give a type
572 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
573 boxySplitTyConApp with the family tycon Map instead, which gives us the family
574 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
575 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
576 (provided by tyConFamInst_maybe together with the family tycon). This
577 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
578 the split arguments for the representation tycon :R123Map as {Int, c, w}
580 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
582 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
584 moving between representation and family type into account. To produce type
585 correct Core, this coercion needs to be used to case the type of the scrutinee
586 from the family to the representation type. This is achieved by
587 unwrapFamInstScrutinee using a CoPat around the result pattern.
589 Now it might appear seem as if we could have used the existing GADT type
590 refinement infrastructure of refineAlt and friends instead of the explicit
591 unification and CoPat generation. However, that would be wrong. Why? The
592 whole point of GADT refinement is that the refinement is local to the case
593 alternative. In contrast, the substitution generated by the unification of
594 the family type list and instance types needs to be propagated to the outside.
595 Imagine that in the above example, the type of the scrutinee would have been
596 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
597 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
598 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
599 alternatives of the case expression, whereas in the GADT case it might vary
600 between alternatives.
602 In fact, if we have a data instance declaration defining a GADT, eq_spec will
603 be non-empty and we will get a mixture of global instantiations and local
604 refinement from a single match. This neatly reflects that, as soon as we
605 have constrained the type of the scrutinee to the required type index, all
606 further type refinement is local to the alternative.
610 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
611 -- with scrutinee of type (T ty)
613 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
614 -> BoxySigmaType -- Type of the pattern
615 -> HsConPatDetails Name -> (PatState -> TcM a)
616 -> TcM (Pat TcId, [TcTyVar], a)
617 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
618 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
619 = dataConFullSig data_con
620 skol_info = PatSkol data_con
621 origin = SigOrigin skol_info
622 full_theta = eq_theta ++ dict_theta
624 -- Instantiate the constructor type variables [a->ty]
625 -- This may involve doing a family-instance coercion, and building a
627 ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
628 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
629 pat_ty' = mkTyConApp tycon ctxt_res_tys
630 -- pat_ty' /= pat_ty iff coi /= IdCo
632 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
634 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys res_pat
636 ; traceTc $ case sym_coi of
637 IdCo -> text "sym_coi:IdCo"
638 ACo co -> text "sym_coi: ACoI" <+> ppr co
640 -- Add the stupid theta
641 ; addDataConStupidTheta data_con ctxt_res_tys
643 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
644 -- Get location from monad, not from ex_tvs
646 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
647 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
648 arg_tys' = substTys tenv arg_tys
650 ; if null ex_tvs && null eq_spec && null full_theta
651 then do { -- The common case; no class bindings etc
652 -- (see Note [Arrows and patterns])
653 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
654 arg_pats pstate thing_inside
655 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
656 pat_tvs = [], pat_dicts = [],
657 pat_binds = emptyLHsBinds,
658 pat_args = arg_pats',
661 ; return (wrap_res_pat res_pat, inner_tvs, res) }
663 else do -- The general case, with existential, and local equality
665 { let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
666 theta' = substTheta tenv (eq_preds ++ full_theta)
667 -- order is *important* as we generate the list of
668 -- dictionary binders from theta'
669 ctxt = pat_ctxt pstate
670 ; checkTc (case ctxt of { ProcPat -> False; other -> True })
671 (existentialProcPat data_con)
673 -- Need to test for rigidity if *any* constraints in theta as class
674 -- constraints may have superclass equality constraints. However,
675 -- we don't want to check for rigidity if we got here only because
676 -- ex_tvs was non-null.
677 -- ; unless (null theta') $
678 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
679 -- if we explicit or implicit (by a GADT def) have equality
681 ; unless (all (not . isEqPred) theta') $
682 checkTc (isRigidTy pat_ty) (nonRigidMatch data_con)
684 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
685 tcConArgs data_con arg_tys' arg_pats pstate thing_inside
687 ; loc <- getInstLoc origin
688 ; dicts <- newDictBndrs loc theta'
689 ; dict_binds <- tcSimplifyCheckPat loc [] emptyRefinement
690 ex_tvs' dicts lie_req
692 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
694 pat_dicts = map instToVar dicts,
695 pat_binds = dict_binds,
696 pat_args = arg_pats', pat_ty = pat_ty' }
697 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
700 -- Split against the family tycon if the pattern constructor
701 -- belongs to a family instance tycon.
702 boxySplitTyConAppWithFamily tycon pat_ty =
704 case tyConFamInst_maybe tycon of
705 Nothing -> boxySplitTyConApp tycon pat_ty
706 Just (fam_tycon, instTys) ->
707 do { (scrutinee_arg_tys, coi) <- boxySplitTyConApp fam_tycon pat_ty
708 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
709 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
710 ; return (freshTvs, coi)
713 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
714 ppr tycon <+> ppr pat_ty
715 , text " family instance:" <+>
716 ppr (tyConFamInst_maybe tycon)
719 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
720 -- pattern if the tycon is an instance of a family.
722 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
723 unwrapFamInstScrutinee tycon args pat
724 | Just co_con <- tyConFamilyCoercion_maybe tycon
725 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
727 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
728 -- coercion is not the identity; mkCoPat is inconvenient as it
729 -- wants a located pattern.
730 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
731 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
732 pat_ty -- family inst type
737 tcConArgs :: DataCon -> [TcSigmaType]
738 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
740 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
741 = do { checkTc (con_arity == no_of_args) -- Check correct arity
742 (arityErr "Constructor" data_con con_arity no_of_args)
743 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
744 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
746 ; return (PrefixCon arg_pats', tvs, res) }
748 con_arity = dataConSourceArity data_con
749 no_of_args = length arg_pats
751 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
752 = do { checkTc (con_arity == 2) -- Check correct arity
753 (arityErr "Constructor" data_con con_arity 2)
754 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
755 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
757 ; return (InfixCon p1' p2', tvs, res) }
759 con_arity = dataConSourceArity data_con
761 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
762 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
764 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
765 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
766 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
768 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
769 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
770 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
771 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
772 ; return (HsRecField sel_id pat' pun, tvs, res) }
774 find_field_ty :: FieldLabel -> TcM (Id, TcType)
775 find_field_ty field_lbl
776 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
778 -- No matching field; chances are this field label comes from some
779 -- other record type (or maybe none). As well as reporting an
780 -- error we still want to typecheck the pattern, principally to
781 -- make sure that all the variables it binds are put into the
782 -- environment, else the type checker crashes later:
783 -- f (R { foo = (a,b) }) = a+b
784 -- If foo isn't one of R's fields, we don't want to crash when
785 -- typechecking the "a+b".
786 [] -> do { addErrTc (badFieldCon data_con field_lbl)
787 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
788 ; return (error "Bogus selector Id", bogus_ty) }
790 -- The normal case, when the field comes from the right constructor
792 ASSERT( null extras )
793 do { sel_id <- tcLookupField field_lbl
794 ; return (sel_id, pat_ty) }
796 field_tys :: [(FieldLabel, TcType)]
797 field_tys = zip (dataConFieldLabels data_con) arg_tys
798 -- Don't use zipEqual! If the constructor isn't really a record, then
799 -- dataConFieldLabels will be empty (and each field in the pattern
800 -- will generate an error below).
802 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
803 tcConArg (arg_pat, arg_ty) pstate thing_inside
804 = tc_lpat arg_pat arg_ty pstate thing_inside
805 -- NB: the tc_lpat will refine pat_ty if necessary
806 -- based on the current pstate, which may include
807 -- refinements from peer argument patterns to the left
811 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
812 -- Instantiate the "stupid theta" of the data con, and throw
813 -- the constraints into the constraint set
814 addDataConStupidTheta data_con inst_tys
815 | null stupid_theta = return ()
816 | otherwise = instStupidTheta origin inst_theta
818 origin = OccurrenceOf (dataConName data_con)
819 -- The origin should always report "occurrence of C"
820 -- even when C occurs in a pattern
821 stupid_theta = dataConStupidTheta data_con
822 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
823 inst_theta = substTheta tenv stupid_theta
826 Note [Arrows and patterns]
827 ~~~~~~~~~~~~~~~~~~~~~~~~~~
828 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
830 expr :: Arrow a => a () Int
831 expr = proc (y,z) -> do
835 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
836 arrow body (e.g 'term'). As things stand (bogusly) all the
837 constraints from the proc body are gathered together, so constraints
838 from 'term' will be seen by the tcPat for (y,z). But we must *not*
839 bind constraints from 'term' here, becuase the desugarer will not make
840 these bindings scope over 'term'.
842 The Right Thing is not to confuse these constraints together. But for
843 now the Easy Thing is to ensure that we do not have existential or
844 GADT constraints in a 'proc', and to short-cut the constraint
845 simplification for such vanilla patterns so that it binds no
846 constraints. Hence the 'fast path' in tcConPat; but it's also a good
847 plan for ordinary vanilla patterns to bypass the constraint
851 %************************************************************************
855 %************************************************************************
858 refineAlt :: DataCon -- For tracing only
860 -> [TcTyVar] -- Existentials
861 -> [CoVar] -- Equational constraints
862 -> BoxySigmaType -- Pattern type
865 refineAlt con pstate ex_tvs [] pat_ty
866 | null $ dataConEqTheta con
867 = return pstate -- Common case: no equational constraints
869 refineAlt con pstate ex_tvs co_vars pat_ty
870 = do { opt_gadt <- doptM Opt_GADTs -- No type-refinement unless GADTs are on
871 ; if (not opt_gadt) then return pstate
874 { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
875 -- We are matching against a GADT constructor with non-trivial
876 -- constraints, but pattern type is wobbly. For now we fail.
877 -- We can make sense of this, however:
878 -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
879 -- (\x -> case x of { MkT v -> v })
880 -- We can infer that x must have type T [c], for some wobbly 'c'
882 -- (\(x::T [c]) -> case x of
883 -- MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
884 -- To implement this, we'd first instantiate the equational
885 -- constraints with *wobbly* type variables for the existentials;
886 -- then unify these constraints to make pat_ty the right shape;
887 -- then proceed exactly as in the rigid case
889 -- In the rigid case, we perform type refinement
890 ; case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
891 Failed msg -> failWithTc (inaccessibleAlt msg) ;
892 Succeeded reft -> do { traceTc trace_msg
893 ; return (pstate { pat_reft = reft, pat_eqs = (pat_eqs pstate || not (null $ dataConEqTheta con)) }) }
894 -- DO NOT refine the envt right away, because we
895 -- might be inside a lazy pattern. Instead, refine pstate
898 trace_msg = text "refineAlt:match" <+>
899 vcat [ ppr con <+> ppr ex_tvs,
900 ppr [(v, tyVarKind v) | v <- co_vars],
906 %************************************************************************
910 %************************************************************************
912 In tcOverloadedLit we convert directly to an Int or Integer if we
913 know that's what we want. This may save some time, by not
914 temporarily generating overloaded literals, but it won't catch all
915 cases (the rest are caught in lookupInst).
918 tcOverloadedLit :: InstOrigin
921 -> TcM (HsOverLit TcId)
922 tcOverloadedLit orig lit@(HsIntegral i fi _) res_ty
923 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
924 -- Reason: If we do, tcSimplify will call lookupInst, which
925 -- will call tcSyntaxName, which does unification,
926 -- which tcSimplify doesn't like
927 -- ToDo: noLoc sadness
928 = do { integer_ty <- tcMetaTy integerTyConName
929 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
930 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty))) res_ty) }
932 | Just expr <- shortCutIntLit i res_ty
933 = return (HsIntegral i expr res_ty)
936 = do { expr <- newLitInst orig lit res_ty
937 ; return (HsIntegral i expr res_ty) }
939 tcOverloadedLit orig lit@(HsFractional r fr _) res_ty
940 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
941 = do { rat_ty <- tcMetaTy rationalTyConName
942 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
943 -- Overloaded literals must have liftedTypeKind, because
944 -- we're instantiating an overloaded function here,
945 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
946 -- However this'll be picked up by tcSyntaxOp if necessary
947 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty))) res_ty) }
949 | Just expr <- shortCutFracLit r res_ty
950 = return (HsFractional r expr res_ty)
953 = do { expr <- newLitInst orig lit res_ty
954 ; return (HsFractional r expr res_ty) }
956 tcOverloadedLit orig lit@(HsIsString s fr _) res_ty
957 | not (fr `isHsVar` fromStringName) -- c.f. HsIntegral case
958 = do { str_ty <- tcMetaTy stringTyConName
959 ; fr' <- tcSyntaxOp orig fr (mkFunTy str_ty res_ty)
960 ; return (HsIsString s (HsApp (noLoc fr') (nlHsLit (HsString s))) res_ty) }
962 | Just expr <- shortCutStringLit s res_ty
963 = return (HsIsString s expr res_ty)
966 = do { expr <- newLitInst orig lit res_ty
967 ; return (HsIsString s expr res_ty) }
969 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
970 newLitInst orig lit res_ty -- Make a LitInst
971 = do { loc <- getInstLoc orig
972 ; res_tau <- zapToMonotype res_ty
973 ; new_uniq <- newUnique
974 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
975 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
976 tci_ty = res_tau, tci_loc = loc}
978 ; return (HsVar (instToId lit_inst)) }
982 %************************************************************************
984 Note [Pattern coercions]
986 %************************************************************************
988 In principle, these program would be reasonable:
990 f :: (forall a. a->a) -> Int
991 f (x :: Int->Int) = x 3
993 g :: (forall a. [a]) -> Bool
996 In both cases, the function type signature restricts what arguments can be passed
997 in a call (to polymorphic ones). The pattern type signature then instantiates this
998 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
999 generate the translated term
1000 f = \x' :: (forall a. a->a). let x = x' Int in x 3
1002 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
1003 And it requires a significant amount of code to implement, becuase we need to decorate
1004 the translated pattern with coercion functions (generated from the subsumption check
1007 So for now I'm just insisting on type *equality* in patterns. No subsumption.
1009 Old notes about desugaring, at a time when pattern coercions were handled:
1011 A SigPat is a type coercion and must be handled one at at time. We can't
1012 combine them unless the type of the pattern inside is identical, and we don't
1013 bother to check for that. For example:
1015 data T = T1 Int | T2 Bool
1016 f :: (forall a. a -> a) -> T -> t
1017 f (g::Int->Int) (T1 i) = T1 (g i)
1018 f (g::Bool->Bool) (T2 b) = T2 (g b)
1020 We desugar this as follows:
1022 f = \ g::(forall a. a->a) t::T ->
1024 in case t of { T1 i -> T1 (gi i)
1027 in case t of { T2 b -> T2 (gb b)
1030 Note that we do not treat the first column of patterns as a
1031 column of variables, because the coerced variables (gi, gb)
1032 would be of different types. So we get rather grotty code.
1033 But I don't think this is a common case, and if it was we could
1034 doubtless improve it.
1036 Meanwhile, the strategy is:
1037 * treat each SigPat coercion (always non-identity coercions)
1039 * deal with the stuff inside, and then wrap a binding round
1040 the result to bind the new variable (gi, gb, etc)
1043 %************************************************************************
1045 \subsection{Errors and contexts}
1047 %************************************************************************
1050 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
1051 patCtxt (VarPat _) = Nothing
1052 patCtxt (ParPat _) = Nothing
1053 patCtxt (AsPat _ _) = Nothing
1054 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
1057 -----------------------------------------------
1059 existentialExplode pat
1060 = hang (vcat [text "My brain just exploded.",
1061 text "I can't handle pattern bindings for existentially-quantified constructors.",
1062 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
1063 text "In the binding group for"])
1066 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
1067 = do { pat_tys' <- mapM zonkTcType pat_tys
1068 ; body_ty' <- zonkTcType body_ty
1069 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1070 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1071 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1073 sep [ptext SLIT("When checking an existential match that binds"),
1074 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1075 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1076 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
1079 bound_ids = collectPatsBinders pats
1080 show_ids = filter is_interesting bound_ids
1081 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1083 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1084 -- Don't zonk the types so we get the separate, un-unified versions
1086 badFieldCon :: DataCon -> Name -> SDoc
1087 badFieldCon con field
1088 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
1089 ptext SLIT("does not have field"), quotes (ppr field)]
1091 polyPatSig :: TcType -> SDoc
1093 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
1096 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
1098 existentialProcPat :: DataCon -> SDoc
1099 existentialProcPat con
1100 = hang (ptext SLIT("Illegal constructor") <+> quotes (ppr con) <+> ptext SLIT("in a 'proc' pattern"))
1101 2 (ptext SLIT("Proc patterns cannot use existentials or GADTs"))
1105 hang (ptext SLIT("A lazy (~) pattern cannot bind existential type variables"))
1106 2 (vcat (map pprSkolTvBinding tvs))
1109 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1110 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1112 nonRigidResult res_ty
1113 = hang (ptext SLIT("GADT pattern match with non-rigid result type") <+> quotes (ppr res_ty))
1114 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1117 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg