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 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
451 pats pstate thing_inside
452 ; return (mkCoPatCoI coi (ListPat pats' elt_ty) pat_ty, pats_tvs, res) }
454 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
455 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
456 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
457 pats pstate thing_inside
458 ; ifM (null pats) (zapToMonotype pat_ty) -- c.f. ExplicitPArr in TcExpr
459 ; return (mkCoPatCoI coi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res) }
461 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
462 = do { let tc = tupleTyCon boxity (length pats)
463 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
464 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
467 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
468 -- so that we can experiment with lazy tuple-matching.
469 -- This is a pretty odd place to make the switch, but
470 -- it was easy to do.
471 ; let pat_ty' = mkTyConApp tc arg_tys
472 -- pat_ty /= pat_ty iff coi /= IdCo
473 unmangled_result = TuplePat pats' boxity pat_ty'
474 possibly_mangled_result
475 | opt_IrrefutableTuples &&
476 isBoxed boxity = LazyPat (noLoc unmangled_result)
477 | otherwise = unmangled_result
479 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
480 return (mkCoPatCoI coi possibly_mangled_result pat_ty, pats_tvs, res)
483 ------------------------
485 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
486 = do { data_con <- tcLookupDataCon con_name
487 ; let tycon = dataConTyCon data_con
488 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
490 ------------------------
492 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
493 = do { let lit_ty = hsLitType simple_lit
494 ; coi <- boxyUnify lit_ty pat_ty
495 -- coi is of kind: lit_ty ~ pat_ty
496 ; res <- thing_inside pstate
497 ; span <- getSrcSpanM
498 -- pattern coercions have to
499 -- be of kind: pat_ty ~ lit_ty
501 ; returnM (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
504 ------------------------
505 -- Overloaded patterns: n, and n+k
506 tc_pat pstate pat@(NPat over_lit mb_neg eq) pat_ty thing_inside
507 = do { let orig = LiteralOrigin over_lit
508 ; lit' <- tcOverloadedLit orig over_lit pat_ty
509 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
510 ; mb_neg' <- case mb_neg of
511 Nothing -> return Nothing -- Positive literal
512 Just neg -> -- Negative literal
513 -- The 'negate' is re-mappable syntax
514 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
515 ; return (Just neg') }
516 ; res <- thing_inside pstate
517 ; returnM (NPat lit' mb_neg' eq', [], res) }
519 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
520 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
521 ; let pat_ty' = idType bndr_id
522 orig = LiteralOrigin lit
523 ; lit' <- tcOverloadedLit orig lit pat_ty'
525 -- The '>=' and '-' parts are re-mappable syntax
526 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
527 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
529 -- The Report says that n+k patterns must be in Integral
530 -- We may not want this when using re-mappable syntax, though (ToDo?)
531 ; icls <- tcLookupClass integralClassName
532 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
534 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
535 ; returnM (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
537 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
541 %************************************************************************
543 Most of the work for constructors is here
544 (the rest is in the ConPatIn case of tc_pat)
546 %************************************************************************
548 [Pattern matching indexed data types]
549 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
550 Consider the following declarations:
552 data family Map k :: * -> *
553 data instance Map (a, b) v = MapPair (Map a (Pair b v))
555 and a case expression
557 case x :: Map (Int, c) w of MapPair m -> ...
559 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
560 worker/wrapper types for MapPair are
562 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
563 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
565 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
566 :R123Map, which means the straight use of boxySplitTyConApp would give a type
567 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
568 boxySplitTyConApp with the family tycon Map instead, which gives us the family
569 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
570 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
571 (provided by tyConFamInst_maybe together with the family tycon). This
572 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
573 the split arguments for the representation tycon :R123Map as {Int, c, w}
575 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
577 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
579 moving between representation and family type into account. To produce type
580 correct Core, this coercion needs to be used to case the type of the scrutinee
581 from the family to the representation type. This is achieved by
582 unwrapFamInstScrutinee using a CoPat around the result pattern.
584 Now it might appear seem as if we could have used the existing GADT type
585 refinement infrastructure of refineAlt and friends instead of the explicit
586 unification and CoPat generation. However, that would be wrong. Why? The
587 whole point of GADT refinement is that the refinement is local to the case
588 alternative. In contrast, the substitution generated by the unification of
589 the family type list and instance types needs to be propagated to the outside.
590 Imagine that in the above example, the type of the scrutinee would have been
591 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
592 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
593 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
594 alternatives of the case expression, whereas in the GADT case it might vary
595 between alternatives.
597 In fact, if we have a data instance declaration defining a GADT, eq_spec will
598 be non-empty and we will get a mixture of global instantiations and local
599 refinement from a single match. This neatly reflects that, as soon as we
600 have constrained the type of the scrutinee to the required type index, all
601 further type refinement is local to the alternative.
605 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
606 -- with scrutinee of type (T ty)
608 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
609 -> BoxySigmaType -- Type of the pattern
610 -> HsConPatDetails Name -> (PatState -> TcM a)
611 -> TcM (Pat TcId, [TcTyVar], a)
612 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
613 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _) = dataConFullSig data_con
614 skol_info = PatSkol data_con
615 origin = SigOrigin skol_info
616 full_theta = eq_theta ++ dict_theta
618 -- Instantiate the constructor type variables [a->ty]
619 -- This may involve doing a family-instance coercion, and building a wrapper
620 ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
621 ; let pat_ty' = mkTyConApp tycon ctxt_res_tys
622 -- pat_ty /= pat_ty iff coi /= IdCo
624 = mkCoPatCoI coi (unwrapFamInstScrutinee tycon ctxt_res_tys res_pat) pat_ty
626 -- Add the stupid theta
627 ; addDataConStupidTheta data_con ctxt_res_tys
629 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
630 -- Get location from monad, not from ex_tvs
632 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
633 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
634 arg_tys' = substTys tenv arg_tys
636 ; if null ex_tvs && null eq_spec && null full_theta
637 then do { -- The common case; no class bindings etc
638 -- (see Note [Arrows and patterns])
639 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
640 arg_pats pstate thing_inside
641 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
642 pat_tvs = [], pat_dicts = [], pat_binds = emptyLHsBinds,
643 pat_args = arg_pats', pat_ty = pat_ty' }
645 ; return (wrap_res_pat res_pat, inner_tvs, res) }
647 else do -- The general case, with existential, and local equality
649 { let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
650 theta' = substTheta tenv (full_theta ++ eq_preds)
651 ctxt = pat_ctxt pstate
652 ; checkTc (case ctxt of { ProcPat -> False; other -> True })
653 (existentialProcPat data_con)
655 -- Need to test for rigidity if *any* constraints in theta as class
656 -- constraints may have superclass equality constraints. However,
657 -- we don't want to check for rigidity if we got here only because
658 -- ex_tvs was non-null.
659 -- ; unless (null theta') $
660 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
661 -- if we explicit or implicit (by a GADT def) have equality
663 ; unless (all (not . isEqPred) theta') $
664 checkTc (isRigidTy pat_ty) (nonRigidMatch data_con)
666 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
667 tcConArgs data_con arg_tys' arg_pats pstate thing_inside
669 ; loc <- getInstLoc origin
670 ; dicts <- newDictBndrs loc theta'
671 ; dict_binds <- tcSimplifyCheckPat loc [] emptyRefinement
672 ex_tvs' dicts lie_req
674 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
676 pat_dicts = map instToVar dicts,
677 pat_binds = dict_binds,
678 pat_args = arg_pats', pat_ty = pat_ty' }
679 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
682 -- Split against the family tycon if the pattern constructor
683 -- belongs to a family instance tycon.
684 boxySplitTyConAppWithFamily tycon pat_ty =
686 case tyConFamInst_maybe tycon of
687 Nothing -> boxySplitTyConApp tycon pat_ty
688 Just (fam_tycon, instTys) ->
689 do { (scrutinee_arg_tys, coi) <- boxySplitTyConApp fam_tycon pat_ty
690 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
691 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
692 ; return (freshTvs, coi)
695 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
696 ppr tycon <+> ppr pat_ty
697 , text " family instance:" <+>
698 ppr (tyConFamInst_maybe tycon)
701 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
702 -- pattern if the tycon is an instance of a family.
704 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
705 unwrapFamInstScrutinee tycon args pat
706 | Just co_con <- tyConFamilyCoercion_maybe tycon
707 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
709 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
710 -- coercion is not the identity; mkCoPat is inconvenient as it
711 -- wants a located pattern.
712 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
713 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
714 pat_ty -- family inst type
719 tcConArgs :: DataCon -> [TcSigmaType]
720 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
722 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
723 = do { checkTc (con_arity == no_of_args) -- Check correct arity
724 (arityErr "Constructor" data_con con_arity no_of_args)
725 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
726 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
728 ; return (PrefixCon arg_pats', tvs, res) }
730 con_arity = dataConSourceArity data_con
731 no_of_args = length arg_pats
733 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
734 = do { checkTc (con_arity == 2) -- Check correct arity
735 (arityErr "Constructor" data_con con_arity 2)
736 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
737 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
739 ; return (InfixCon p1' p2', tvs, res) }
741 con_arity = dataConSourceArity data_con
743 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
744 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
746 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
747 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
748 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
750 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
751 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
752 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
753 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
754 ; return (HsRecField sel_id pat' pun, tvs, res) }
756 find_field_ty :: FieldLabel -> TcM (Id, TcType)
757 find_field_ty field_lbl
758 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
760 -- No matching field; chances are this field label comes from some
761 -- other record type (or maybe none). As well as reporting an
762 -- error we still want to typecheck the pattern, principally to
763 -- make sure that all the variables it binds are put into the
764 -- environment, else the type checker crashes later:
765 -- f (R { foo = (a,b) }) = a+b
766 -- If foo isn't one of R's fields, we don't want to crash when
767 -- typechecking the "a+b".
768 [] -> do { addErrTc (badFieldCon data_con field_lbl)
769 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
770 ; return (error "Bogus selector Id", bogus_ty) }
772 -- The normal case, when the field comes from the right constructor
774 ASSERT( null extras )
775 do { sel_id <- tcLookupField field_lbl
776 ; return (sel_id, pat_ty) }
778 field_tys :: [(FieldLabel, TcType)]
779 field_tys = zip (dataConFieldLabels data_con) arg_tys
780 -- Don't use zipEqual! If the constructor isn't really a record, then
781 -- dataConFieldLabels will be empty (and each field in the pattern
782 -- will generate an error below).
784 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
785 tcConArg (arg_pat, arg_ty) pstate thing_inside
786 = tc_lpat arg_pat arg_ty pstate thing_inside
787 -- NB: the tc_lpat will refine pat_ty if necessary
788 -- based on the current pstate, which may include
789 -- refinements from peer argument patterns to the left
793 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
794 -- Instantiate the "stupid theta" of the data con, and throw
795 -- the constraints into the constraint set
796 addDataConStupidTheta data_con inst_tys
797 | null stupid_theta = return ()
798 | otherwise = instStupidTheta origin inst_theta
800 origin = OccurrenceOf (dataConName data_con)
801 -- The origin should always report "occurrence of C"
802 -- even when C occurs in a pattern
803 stupid_theta = dataConStupidTheta data_con
804 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
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 %************************************************************************
840 refineAlt :: DataCon -- For tracing only
842 -> [TcTyVar] -- Existentials
843 -> [CoVar] -- Equational constraints
844 -> BoxySigmaType -- Pattern type
847 refineAlt con pstate ex_tvs [] pat_ty
848 | null $ dataConEqTheta con
849 = return pstate -- Common case: no equational constraints
851 refineAlt con pstate ex_tvs co_vars pat_ty
852 = do { opt_gadt <- doptM Opt_GADTs -- No type-refinement unless GADTs are on
853 ; if (not opt_gadt) then return pstate
856 { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
857 -- We are matching against a GADT constructor with non-trivial
858 -- constraints, but pattern type is wobbly. For now we fail.
859 -- We can make sense of this, however:
860 -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
861 -- (\x -> case x of { MkT v -> v })
862 -- We can infer that x must have type T [c], for some wobbly 'c'
864 -- (\(x::T [c]) -> case x of
865 -- MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
866 -- To implement this, we'd first instantiate the equational
867 -- constraints with *wobbly* type variables for the existentials;
868 -- then unify these constraints to make pat_ty the right shape;
869 -- then proceed exactly as in the rigid case
871 -- In the rigid case, we perform type refinement
872 ; case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
873 Failed msg -> failWithTc (inaccessibleAlt msg) ;
874 Succeeded reft -> do { traceTc trace_msg
875 ; return (pstate { pat_reft = reft, pat_eqs = (pat_eqs pstate || not (null $ dataConEqTheta con)) }) }
876 -- DO NOT refine the envt right away, because we
877 -- might be inside a lazy pattern. Instead, refine pstate
880 trace_msg = text "refineAlt:match" <+>
881 vcat [ ppr con <+> ppr ex_tvs,
882 ppr [(v, tyVarKind v) | v <- co_vars],
888 %************************************************************************
892 %************************************************************************
894 In tcOverloadedLit we convert directly to an Int or Integer if we
895 know that's what we want. This may save some time, by not
896 temporarily generating overloaded literals, but it won't catch all
897 cases (the rest are caught in lookupInst).
900 tcOverloadedLit :: InstOrigin
903 -> TcM (HsOverLit TcId)
904 tcOverloadedLit orig lit@(HsIntegral i fi _) res_ty
905 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
906 -- Reason: If we do, tcSimplify will call lookupInst, which
907 -- will call tcSyntaxName, which does unification,
908 -- which tcSimplify doesn't like
909 -- ToDo: noLoc sadness
910 = do { integer_ty <- tcMetaTy integerTyConName
911 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
912 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty))) res_ty) }
914 | Just expr <- shortCutIntLit i res_ty
915 = return (HsIntegral i expr res_ty)
918 = do { expr <- newLitInst orig lit res_ty
919 ; return (HsIntegral i expr res_ty) }
921 tcOverloadedLit orig lit@(HsFractional r fr _) res_ty
922 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
923 = do { rat_ty <- tcMetaTy rationalTyConName
924 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
925 -- Overloaded literals must have liftedTypeKind, because
926 -- we're instantiating an overloaded function here,
927 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
928 -- However this'll be picked up by tcSyntaxOp if necessary
929 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty))) res_ty) }
931 | Just expr <- shortCutFracLit r res_ty
932 = return (HsFractional r expr res_ty)
935 = do { expr <- newLitInst orig lit res_ty
936 ; return (HsFractional r expr res_ty) }
938 tcOverloadedLit orig lit@(HsIsString s fr _) res_ty
939 | not (fr `isHsVar` fromStringName) -- c.f. HsIntegral case
940 = do { str_ty <- tcMetaTy stringTyConName
941 ; fr' <- tcSyntaxOp orig fr (mkFunTy str_ty res_ty)
942 ; return (HsIsString s (HsApp (noLoc fr') (nlHsLit (HsString s))) res_ty) }
944 | Just expr <- shortCutStringLit s res_ty
945 = return (HsIsString s expr res_ty)
948 = do { expr <- newLitInst orig lit res_ty
949 ; return (HsIsString s expr res_ty) }
951 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
952 newLitInst orig lit res_ty -- Make a LitInst
953 = do { loc <- getInstLoc orig
954 ; res_tau <- zapToMonotype res_ty
955 ; new_uniq <- newUnique
956 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
957 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
958 tci_ty = res_tau, tci_loc = loc}
960 ; return (HsVar (instToId lit_inst)) }
964 %************************************************************************
966 Note [Pattern coercions]
968 %************************************************************************
970 In principle, these program would be reasonable:
972 f :: (forall a. a->a) -> Int
973 f (x :: Int->Int) = x 3
975 g :: (forall a. [a]) -> Bool
978 In both cases, the function type signature restricts what arguments can be passed
979 in a call (to polymorphic ones). The pattern type signature then instantiates this
980 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
981 generate the translated term
982 f = \x' :: (forall a. a->a). let x = x' Int in x 3
984 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
985 And it requires a significant amount of code to implement, becuase we need to decorate
986 the translated pattern with coercion functions (generated from the subsumption check
989 So for now I'm just insisting on type *equality* in patterns. No subsumption.
991 Old notes about desugaring, at a time when pattern coercions were handled:
993 A SigPat is a type coercion and must be handled one at at time. We can't
994 combine them unless the type of the pattern inside is identical, and we don't
995 bother to check for that. For example:
997 data T = T1 Int | T2 Bool
998 f :: (forall a. a -> a) -> T -> t
999 f (g::Int->Int) (T1 i) = T1 (g i)
1000 f (g::Bool->Bool) (T2 b) = T2 (g b)
1002 We desugar this as follows:
1004 f = \ g::(forall a. a->a) t::T ->
1006 in case t of { T1 i -> T1 (gi i)
1009 in case t of { T2 b -> T2 (gb b)
1012 Note that we do not treat the first column of patterns as a
1013 column of variables, because the coerced variables (gi, gb)
1014 would be of different types. So we get rather grotty code.
1015 But I don't think this is a common case, and if it was we could
1016 doubtless improve it.
1018 Meanwhile, the strategy is:
1019 * treat each SigPat coercion (always non-identity coercions)
1021 * deal with the stuff inside, and then wrap a binding round
1022 the result to bind the new variable (gi, gb, etc)
1025 %************************************************************************
1027 \subsection{Errors and contexts}
1029 %************************************************************************
1032 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
1033 patCtxt (VarPat _) = Nothing
1034 patCtxt (ParPat _) = Nothing
1035 patCtxt (AsPat _ _) = Nothing
1036 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
1039 -----------------------------------------------
1041 existentialExplode pat
1042 = hang (vcat [text "My brain just exploded.",
1043 text "I can't handle pattern bindings for existentially-quantified constructors.",
1044 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
1045 text "In the binding group for"])
1048 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
1049 = do { pat_tys' <- mapM zonkTcType pat_tys
1050 ; body_ty' <- zonkTcType body_ty
1051 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1052 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1053 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1055 sep [ptext SLIT("When checking an existential match that binds"),
1056 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1057 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1058 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
1061 bound_ids = collectPatsBinders pats
1062 show_ids = filter is_interesting bound_ids
1063 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1065 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1066 -- Don't zonk the types so we get the separate, un-unified versions
1068 badFieldCon :: DataCon -> Name -> SDoc
1069 badFieldCon con field
1070 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
1071 ptext SLIT("does not have field"), quotes (ppr field)]
1073 polyPatSig :: TcType -> SDoc
1075 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
1078 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
1080 existentialProcPat :: DataCon -> SDoc
1081 existentialProcPat con
1082 = hang (ptext SLIT("Illegal constructor") <+> quotes (ppr con) <+> ptext SLIT("in a 'proc' pattern"))
1083 2 (ptext SLIT("Proc patterns cannot use existentials or GADTs"))
1087 hang (ptext SLIT("A lazy (~) pattern cannot bind existential type variables"))
1088 2 (vcat (map pprSkolTvBinding tvs))
1091 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1092 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1094 nonRigidResult res_ty
1095 = hang (ptext SLIT("GADT pattern match with non-rigid result type") <+> quotes (ppr res_ty))
1096 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1099 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg