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(..))
57 %************************************************************************
61 %************************************************************************
64 tcLetPat :: (Name -> Maybe TcRhoType)
65 -> LPat Name -> BoxySigmaType
68 tcLetPat sig_fn pat pat_ty thing_inside
69 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
70 pat_reft = emptyRefinement,
72 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state (\ _ -> thing_inside)
74 -- Don't know how to deal with pattern-bound existentials yet
75 ; checkTc (null ex_tvs) (existentialExplode pat)
77 ; return (pat', res) }
80 tcLamPats :: [LPat Name] -- Patterns,
81 -> [BoxySigmaType] -- and their types
82 -> BoxyRhoType -- Result type,
83 -> ((Refinement, BoxyRhoType) -> TcM a) -- and the checker for the body
84 -> TcM ([LPat TcId], a)
86 -- This is the externally-callable wrapper function
87 -- Typecheck the patterns, extend the environment to bind the variables,
88 -- do the thing inside, use any existentially-bound dictionaries to
89 -- discharge parts of the returning LIE, and deal with pattern type
92 -- 1. Initialise the PatState
93 -- 2. Check the patterns
94 -- 3. Apply the refinement to the environment and result type
96 -- 5. Check that no existentials escape
98 tcLamPats pats tys res_ty thing_inside
99 = tc_lam_pats LamPat (zipEqual "tcLamPats" pats tys)
100 (emptyRefinement, res_ty) thing_inside
102 tcLamPat :: LPat Name -> BoxySigmaType
103 -> (Refinement,BoxyRhoType) -- Result type
104 -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
105 -> TcM (LPat TcId, a)
107 tcProcPat = tc_lam_pat ProcPat
108 tcLamPat = tc_lam_pat LamPat
110 tc_lam_pat ctxt pat pat_ty res_ty thing_inside
111 = do { ([pat'],thing) <- tc_lam_pats ctxt [(pat, pat_ty)] res_ty thing_inside
112 ; return (pat', thing) }
115 tc_lam_pats :: PatCtxt
116 -> [(LPat Name,BoxySigmaType)]
117 -> (Refinement,BoxyRhoType) -- Result type
118 -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
119 -> TcM ([LPat TcId], a)
120 tc_lam_pats ctxt pat_ty_prs (reft, res_ty) thing_inside
121 = do { let init_state = PS { pat_ctxt = ctxt, pat_reft = reft, pat_eqs = False }
123 ; (pats', ex_tvs, res) <- tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
124 refineEnvironment (pat_reft pstate') (pat_eqs pstate') $
125 if (pat_eqs pstate' && (not $ isRigidTy res_ty))
126 then failWithTc (nonRigidResult res_ty)
127 else thing_inside (pat_reft pstate', res_ty)
129 ; let tys = map snd pat_ty_prs
130 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
132 ; returnM (pats', res) }
136 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
137 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
138 -> [BoxySigmaType] -- Types of the patterns
139 -> BoxyRhoType -- Type of the body of the match
140 -- Tyvars in either of these must not escape
142 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
143 -- For example, we must reject this program:
144 -- data C = forall a. C (a -> Int)
146 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
148 tcCheckExistentialPat pats [] pat_tys body_ty
149 = return () -- Short cut for case when there are no existentials
151 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
152 = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
153 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
157 pat_reft :: Refinement, -- Binds rigid TcTyVars to their refinements
158 pat_eqs :: Bool -- <=> there are GADT equational constraints
164 | ProcPat -- The pattern in (proc pat -> ...)
165 -- see Note [Arrows and patterns]
166 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
168 patSigCtxt :: PatState -> UserTypeCtxt
169 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
170 patSigCtxt other = LamPatSigCtxt
175 %************************************************************************
179 %************************************************************************
182 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
183 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
184 | Just mono_ty <- lookup_sig bndr_name
185 = do { mono_name <- newLocalName bndr_name
186 ; boxyUnify mono_ty pat_ty
187 ; return (Id.mkLocalId mono_name mono_ty) }
190 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
191 ; mono_name <- newLocalName bndr_name
192 ; return (Id.mkLocalId mono_name pat_ty') }
194 tcPatBndr (PS { pat_ctxt = _lam_or_proc }) bndr_name pat_ty
195 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
196 -- We have an undecorated binder, so we do rule ABS1,
197 -- by unboxing the boxy type, forcing any un-filled-in
198 -- boxes to become monotypes
199 -- NB that pat_ty' can still be a polytype:
200 -- data T = MkT (forall a. a->a)
201 -- f t = case t of { MkT g -> ... }
202 -- Here, the 'g' must get type (forall a. a->a) from the
204 ; return (Id.mkLocalId bndr_name pat_ty') }
208 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
209 bindInstsOfPatId id thing_inside
210 | not (isOverloadedTy (idType id))
211 = do { res <- thing_inside; return (res, emptyLHsBinds) }
213 = do { (res, lie) <- getLIE thing_inside
214 ; binds <- bindInstsOfLocalFuns lie [id]
215 ; return (res, binds) }
218 unBoxPatBndrType ty name = unBoxArgType ty (ptext SLIT("The variable") <+> quotes (ppr name))
219 unBoxWildCardType ty = unBoxArgType ty (ptext SLIT("A wild-card pattern"))
220 unBoxViewPatType ty pat = unBoxArgType ty (ptext SLIT("The view pattern") <+> ppr pat)
222 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
223 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
224 -- that is, it can't be an unboxed tuple. For example,
225 -- case (f x) of r -> ...
226 -- should fail if 'f' returns an unboxed tuple.
227 unBoxArgType ty pp_this
228 = do { ty' <- unBox ty -- Returns a zonked type
230 -- Neither conditional is strictly necesssary (the unify alone will do)
231 -- but they improve error messages, and allocate fewer tyvars
232 ; if isUnboxedTupleType ty' then
234 else if isSubArgTypeKind (typeKind ty') then
236 else do -- OpenTypeKind, so constrain it
237 { ty2 <- newFlexiTyVarTy argTypeKind
241 msg = pp_this <+> ptext SLIT("cannot be bound to an unboxed tuple")
245 %************************************************************************
247 The main worker functions
249 %************************************************************************
253 tcPat takes a "thing inside" over which the pattern scopes. This is partly
254 so that tcPat can extend the environment for the thing_inside, but also
255 so that constraints arising in the thing_inside can be discharged by the
258 This does not work so well for the ErrCtxt carried by the monad: we don't
259 want the error-context for the pattern to scope over the RHS.
260 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
264 type Checker inp out = forall r.
267 -> (PatState -> TcM r)
268 -> TcM (out, [TcTyVar], r)
270 tcMultiple :: Checker inp out -> Checker [inp] [out]
271 tcMultiple tc_pat args pstate thing_inside
272 = do { err_ctxt <- getErrCtxt
274 = do { res <- thing_inside pstate
275 ; return ([], [], res) }
277 loop pstate (arg:args)
278 = do { (p', p_tvs, (ps', ps_tvs, res))
279 <- tc_pat arg pstate $ \ pstate' ->
280 setErrCtxt err_ctxt $
282 -- setErrCtxt: restore context before doing the next pattern
283 -- See note [Nesting] above
285 ; return (p':ps', p_tvs ++ ps_tvs, res) }
290 tc_lpat_pr :: (LPat Name, BoxySigmaType)
292 -> (PatState -> TcM a)
293 -> TcM (LPat TcId, [TcTyVar], a)
294 tc_lpat_pr (pat, ty) = tc_lpat pat ty
299 -> (PatState -> TcM a)
300 -> TcM (LPat TcId, [TcTyVar], a)
301 tc_lpat (L span pat) pat_ty pstate thing_inside
303 maybeAddErrCtxt (patCtxt pat) $
304 do { let mb_reft = refineType (pat_reft pstate) pat_ty
305 pat_ty' = case mb_reft of { Just (_, ty') -> ty'; Nothing -> pat_ty }
307 -- Make sure the result type reflects the current refinement
308 -- We must do this here, so that it correctly ``sees'' all
309 -- the refinements to the left. Example:
310 -- Suppose C :: forall a. T a -> a -> Foo
311 -- Pattern C a p1 True
312 -- So p1 might refine 'a' to True, and the True
313 -- pattern had better see it.
315 ; (pat', tvs, res) <- tc_pat pstate pat pat_ty' thing_inside
316 ; let final_pat = case mb_reft of
318 Just (co,_) -> CoPat (WpCo co) pat' pat_ty
319 ; return (L span final_pat, tvs, res) }
324 -> BoxySigmaType -- Fully refined result type
325 -> (PatState -> TcM a) -- Thing inside
326 -> TcM (Pat TcId, -- Translated pattern
327 [TcTyVar], -- Existential binders
328 a) -- Result of thing inside
330 tc_pat pstate (VarPat name) pat_ty thing_inside
331 = do { id <- tcPatBndr pstate name pat_ty
332 ; (res, binds) <- bindInstsOfPatId id $
333 tcExtendIdEnv1 name id $
334 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
335 >> thing_inside pstate)
336 ; let pat' | isEmptyLHsBinds binds = VarPat id
337 | otherwise = VarPatOut id binds
338 ; return (pat', [], res) }
340 tc_pat pstate (ParPat pat) pat_ty thing_inside
341 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
342 ; return (ParPat pat', tvs, res) }
344 tc_pat pstate (BangPat pat) pat_ty thing_inside
345 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
346 ; return (BangPat pat', tvs, res) }
348 -- There's a wrinkle with irrefutable patterns, namely that we
349 -- must not propagate type refinement from them. For example
350 -- data T a where { T1 :: Int -> T Int; ... }
351 -- f :: T a -> Int -> a
353 -- It's obviously not sound to refine a to Int in the right
354 -- hand side, because the arugment might not match T1 at all!
356 -- Nor should a lazy pattern bind any existential type variables
357 -- because they won't be in scope when we do the desugaring
359 -- Note [Hopping the LIE in lazy patterns]
360 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
361 -- In a lazy pattern, we must *not* discharge constraints from the RHS
362 -- from dictionaries bound in the pattern. E.g.
364 -- We can't discharge the Num constraint from dictionaries bound by
367 -- So we have to make the constraints from thing_inside "hop around"
368 -- the pattern. Hence the getLLE and extendLIEs later.
370 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
371 = do { (pat', pat_tvs, (res,lie))
372 <- tc_lpat pat pat_ty pstate $ \ _ ->
373 getLIE (thing_inside pstate)
374 -- Ignore refined pstate', revert to pstate
376 -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
378 -- Check no existentials
379 ; if (null pat_tvs) then return ()
380 else lazyPatErr lpat pat_tvs
382 -- Check that the pattern has a lifted type
383 ; pat_tv <- newBoxyTyVar liftedTypeKind
384 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
386 ; return (LazyPat pat', [], res) }
388 tc_pat pstate (WildPat _) pat_ty thing_inside
389 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
390 ; res <- thing_inside pstate
391 ; return (WildPat pat_ty', [], res) }
393 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
394 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
395 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
396 tc_lpat pat (idType bndr_id) pstate thing_inside
397 -- NB: if we do inference on:
398 -- \ (y@(x::forall a. a->a)) = e
399 -- we'll fail. The as-pattern infers a monotype for 'y', which then
400 -- fails to unify with the polymorphic type for 'x'. This could
401 -- perhaps be fixed, but only with a bit more work.
403 -- If you fix it, don't forget the bindInstsOfPatIds!
404 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
406 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
407 = do { -- morally, expr must have type
408 -- `forall a1...aN. OPT' -> B`
409 -- where overall_pat_ty is an instance of OPT'.
410 -- Here, we infer a rho type for it,
411 -- which replaces the leading foralls and constraints
412 -- with fresh unification variables.
413 (expr',expr'_inferred) <- tcInferRho expr
414 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
415 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
416 -- tcSubExp: expected first, offered second
419 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
420 -- variable to find it). this means that in an example like
421 -- (view -> f) where view :: _ -> forall b. b
422 -- we will only be able to use view at one instantation in the
424 ; (expr_coerc, pat_ty) <- tcInfer (\ pat_ty -> tcSubExp (expr'_expected pat_ty) expr'_inferred)
425 -- pattern must have pat_ty
426 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
427 -- this should get zonked later on, but we unBox it here
428 -- so that we do the same checks as above
429 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
430 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
432 -- Type signatures in patterns
433 -- See Note [Pattern coercions] below
434 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
435 = do { (inner_ty, tv_binds) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
436 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
437 tc_lpat pat inner_ty pstate thing_inside
438 ; return (SigPatOut pat' inner_ty, tvs, res) }
440 tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
441 = failWithTc (badTypePat pat)
443 ------------------------
444 -- Lists, tuples, arrays
445 tc_pat pstate (ListPat pats _) pat_ty thing_inside
446 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
447 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
448 pats pstate thing_inside
449 ; return (mkCoPatCoI coi (ListPat pats' elt_ty) pat_ty, pats_tvs, res) }
451 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
452 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
453 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
454 pats pstate thing_inside
455 ; ifM (null pats) (zapToMonotype pat_ty) -- c.f. ExplicitPArr in TcExpr
456 ; return (mkCoPatCoI coi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res) }
458 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
459 = do { let tc = tupleTyCon boxity (length pats)
460 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
461 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
464 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
465 -- so that we can experiment with lazy tuple-matching.
466 -- This is a pretty odd place to make the switch, but
467 -- it was easy to do.
468 ; let pat_ty' = mkTyConApp tc arg_tys
469 -- pat_ty /= pat_ty iff coi /= IdCo
470 unmangled_result = TuplePat pats' boxity pat_ty'
471 possibly_mangled_result
472 | opt_IrrefutableTuples &&
473 isBoxed boxity = LazyPat (noLoc unmangled_result)
474 | otherwise = unmangled_result
476 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
477 return (mkCoPatCoI coi possibly_mangled_result pat_ty, pats_tvs, res)
480 ------------------------
482 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
483 = do { data_con <- tcLookupDataCon con_name
484 ; let tycon = dataConTyCon data_con
485 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
487 ------------------------
489 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
490 = do { let lit_ty = hsLitType simple_lit
491 ; coi <- boxyUnify lit_ty pat_ty
492 -- coi is of kind: lit_ty ~ pat_ty
493 ; res <- thing_inside pstate
494 ; span <- getSrcSpanM
495 -- pattern coercions have to
496 -- be of kind: pat_ty ~ lit_ty
498 ; returnM (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
501 ------------------------
502 -- Overloaded patterns: n, and n+k
503 tc_pat pstate pat@(NPat over_lit mb_neg eq) pat_ty thing_inside
504 = do { let orig = LiteralOrigin over_lit
505 ; lit' <- tcOverloadedLit orig over_lit pat_ty
506 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
507 ; mb_neg' <- case mb_neg of
508 Nothing -> return Nothing -- Positive literal
509 Just neg -> -- Negative literal
510 -- The 'negate' is re-mappable syntax
511 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
512 ; return (Just neg') }
513 ; res <- thing_inside pstate
514 ; returnM (NPat lit' mb_neg' eq', [], res) }
516 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
517 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
518 ; let pat_ty' = idType bndr_id
519 orig = LiteralOrigin lit
520 ; lit' <- tcOverloadedLit orig lit pat_ty'
522 -- The '>=' and '-' parts are re-mappable syntax
523 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
524 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
526 -- The Report says that n+k patterns must be in Integral
527 -- We may not want this when using re-mappable syntax, though (ToDo?)
528 ; icls <- tcLookupClass integralClassName
529 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
531 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
532 ; returnM (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
534 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
538 %************************************************************************
540 Most of the work for constructors is here
541 (the rest is in the ConPatIn case of tc_pat)
543 %************************************************************************
545 [Pattern matching indexed data types]
546 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
547 Consider the following declarations:
549 data family Map k :: * -> *
550 data instance Map (a, b) v = MapPair (Map a (Pair b v))
552 and a case expression
554 case x :: Map (Int, c) w of MapPair m -> ...
556 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
557 worker/wrapper types for MapPair are
559 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
560 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
562 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
563 :R123Map, which means the straight use of boxySplitTyConApp would give a type
564 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
565 boxySplitTyConApp with the family tycon Map instead, which gives us the family
566 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
567 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
568 (provided by tyConFamInst_maybe together with the family tycon). This
569 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
570 the split arguments for the representation tycon :R123Map as {Int, c, w}
572 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
574 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
576 moving between representation and family type into account. To produce type
577 correct Core, this coercion needs to be used to case the type of the scrutinee
578 from the family to the representation type. This is achieved by
579 unwrapFamInstScrutinee using a CoPat around the result pattern.
581 Now it might appear seem as if we could have used the existing GADT type
582 refinement infrastructure of refineAlt and friends instead of the explicit
583 unification and CoPat generation. However, that would be wrong. Why? The
584 whole point of GADT refinement is that the refinement is local to the case
585 alternative. In contrast, the substitution generated by the unification of
586 the family type list and instance types needs to be propagated to the outside.
587 Imagine that in the above example, the type of the scrutinee would have been
588 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
589 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
590 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
591 alternatives of the case expression, whereas in the GADT case it might vary
592 between alternatives.
594 In fact, if we have a data instance declaration defining a GADT, eq_spec will
595 be non-empty and we will get a mixture of global instantiations and local
596 refinement from a single match. This neatly reflects that, as soon as we
597 have constrained the type of the scrutinee to the required type index, all
598 further type refinement is local to the alternative.
602 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
603 -- with scrutinee of type (T ty)
605 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
606 -> BoxySigmaType -- Type of the pattern
607 -> HsConPatDetails Name -> (PatState -> TcM a)
608 -> TcM (Pat TcId, [TcTyVar], a)
609 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
610 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _) = dataConFullSig data_con
611 skol_info = PatSkol data_con
612 origin = SigOrigin skol_info
613 full_theta = eq_theta ++ dict_theta
615 -- Instantiate the constructor type variables [a->ty]
616 -- This may involve doing a family-instance coercion, and building a wrapper
617 ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
618 ; let pat_ty' = mkTyConApp tycon ctxt_res_tys
619 -- pat_ty /= pat_ty iff coi /= IdCo
621 = mkCoPatCoI coi (unwrapFamInstScrutinee tycon ctxt_res_tys res_pat) pat_ty
623 -- Add the stupid theta
624 ; addDataConStupidTheta data_con ctxt_res_tys
626 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs -- Get location from monad,
628 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
629 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
630 arg_tys' = substTys tenv arg_tys
632 ; if null ex_tvs && null eq_spec && null full_theta
633 then do { -- The common case; no class bindings etc (see Note [Arrows and patterns])
634 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
635 arg_pats pstate thing_inside
636 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
637 pat_tvs = [], pat_dicts = [], pat_binds = emptyLHsBinds,
638 pat_args = arg_pats', pat_ty = pat_ty' }
640 ; return (wrap_res_pat res_pat, inner_tvs, res) }
642 else do -- The general case, with existential, and local equality constraints
643 { let eq_spec' = substEqSpec tenv eq_spec
644 theta' = substTheta tenv full_theta
645 ctxt = pat_ctxt pstate
646 ; checkTc (case ctxt of { ProcPat -> False; other -> True })
647 (existentialProcPat data_con)
648 ; co_vars <- newCoVars eq_spec' -- Make coercion variables
649 ; traceTc (text "tcConPat: refineAlt")
650 ; pstate' <- refineAlt data_con pstate ex_tvs' co_vars pat_ty
651 ; traceTc (text "tcConPat: refineAlt done!")
653 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
654 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
656 ; loc <- getInstLoc origin
657 ; dicts <- newDictBndrs loc theta'
658 ; dict_binds <- tcSimplifyCheckPat loc co_vars (pat_reft pstate')
659 ex_tvs' dicts lie_req
661 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
662 pat_tvs = ex_tvs' ++ co_vars,
663 pat_dicts = map instToVar dicts,
664 pat_binds = dict_binds,
665 pat_args = arg_pats', pat_ty = pat_ty' }
666 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
669 -- Split against the family tycon if the pattern constructor
670 -- belongs to a family instance tycon.
671 boxySplitTyConAppWithFamily tycon pat_ty =
673 case tyConFamInst_maybe tycon of
674 Nothing -> boxySplitTyConApp tycon pat_ty
675 Just (fam_tycon, instTys) ->
676 do { (scrutinee_arg_tys, coi) <- boxySplitTyConApp fam_tycon pat_ty
677 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
678 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
679 ; return (freshTvs, coi)
682 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
683 ppr tycon <+> ppr pat_ty
684 , text " family instance:" <+>
685 ppr (tyConFamInst_maybe tycon)
688 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
689 -- pattern if the tycon is an instance of a family.
691 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
692 unwrapFamInstScrutinee tycon args pat
693 | Just co_con <- tyConFamilyCoercion_maybe tycon
694 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
696 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
697 -- coercion is not the identity; mkCoPat is inconvenient as it
698 -- wants a located pattern.
699 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
700 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
701 pat_ty -- family inst type
706 tcConArgs :: DataCon -> [TcSigmaType]
707 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
709 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
710 = do { checkTc (con_arity == no_of_args) -- Check correct arity
711 (arityErr "Constructor" data_con con_arity no_of_args)
712 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
713 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
715 ; return (PrefixCon arg_pats', tvs, res) }
717 con_arity = dataConSourceArity data_con
718 no_of_args = length arg_pats
720 tcConArgs data_con [arg_ty1,arg_ty2] (InfixCon p1 p2) pstate thing_inside
721 = do { checkTc (con_arity == 2) -- Check correct arity
722 (arityErr "Constructor" data_con con_arity 2)
723 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
725 ; return (InfixCon p1' p2', tvs, res) }
727 con_arity = dataConSourceArity data_con
729 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
730 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
732 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
733 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
734 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
736 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
737 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
738 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
739 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
740 ; return (HsRecField sel_id pat' pun, tvs, res) }
742 find_field_ty :: FieldLabel -> TcM (Id, TcType)
743 find_field_ty field_lbl
744 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
746 -- No matching field; chances are this field label comes from some
747 -- other record type (or maybe none). As well as reporting an
748 -- error we still want to typecheck the pattern, principally to
749 -- make sure that all the variables it binds are put into the
750 -- environment, else the type checker crashes later:
751 -- f (R { foo = (a,b) }) = a+b
752 -- If foo isn't one of R's fields, we don't want to crash when
753 -- typechecking the "a+b".
754 [] -> do { addErrTc (badFieldCon data_con field_lbl)
755 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
756 ; return (error "Bogus selector Id", bogus_ty) }
758 -- The normal case, when the field comes from the right constructor
760 ASSERT( null extras )
761 do { sel_id <- tcLookupField field_lbl
762 ; return (sel_id, pat_ty) }
764 field_tys :: [(FieldLabel, TcType)]
765 field_tys = zip (dataConFieldLabels data_con) arg_tys
766 -- Don't use zipEqual! If the constructor isn't really a record, then
767 -- dataConFieldLabels will be empty (and each field in the pattern
768 -- will generate an error below).
770 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
771 tcConArg (arg_pat, arg_ty) pstate thing_inside
772 = tc_lpat arg_pat arg_ty pstate thing_inside
773 -- NB: the tc_lpat will refine pat_ty if necessary
774 -- based on the current pstate, which may include
775 -- refinements from peer argument patterns to the left
779 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
780 -- Instantiate the "stupid theta" of the data con, and throw
781 -- the constraints into the constraint set
782 addDataConStupidTheta data_con inst_tys
783 | null stupid_theta = return ()
784 | otherwise = instStupidTheta origin inst_theta
786 origin = OccurrenceOf (dataConName data_con)
787 -- The origin should always report "occurrence of C"
788 -- even when C occurs in a pattern
789 stupid_theta = dataConStupidTheta data_con
790 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
791 inst_theta = substTheta tenv stupid_theta
794 Note [Arrows and patterns]
795 ~~~~~~~~~~~~~~~~~~~~~~~~~~
796 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
798 expr :: Arrow a => a () Int
799 expr = proc (y,z) -> do
803 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
804 arrow body (e.g 'term'). As things stand (bogusly) all the
805 constraints from the proc body are gathered together, so constraints
806 from 'term' will be seen by the tcPat for (y,z). But we must *not*
807 bind constraints from 'term' here, becuase the desugarer will not make
808 these bindings scope over 'term'.
810 The Right Thing is not to confuse these constraints together. But for
811 now the Easy Thing is to ensure that we do not have existential or
812 GADT constraints in a 'proc', and to short-cut the constraint
813 simplification for such vanilla patterns so that it binds no
814 constraints. Hence the 'fast path' in tcConPat; but it's also a good
815 plan for ordinary vanilla patterns to bypass the constraint
819 %************************************************************************
823 %************************************************************************
826 refineAlt :: DataCon -- For tracing only
828 -> [TcTyVar] -- Existentials
829 -> [CoVar] -- Equational constraints
830 -> BoxySigmaType -- Pattern type
833 refineAlt con pstate ex_tvs [] pat_ty
834 | null $ dataConEqTheta con
835 = return pstate -- Common case: no equational constraints
837 refineAlt con pstate ex_tvs co_vars pat_ty
838 = do { opt_gadt <- doptM Opt_GADTs -- No type-refinement unless GADTs are on
839 ; if (not opt_gadt) then return pstate
842 { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
843 -- We are matching against a GADT constructor with non-trivial
844 -- constraints, but pattern type is wobbly. For now we fail.
845 -- We can make sense of this, however:
846 -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
847 -- (\x -> case x of { MkT v -> v })
848 -- We can infer that x must have type T [c], for some wobbly 'c'
850 -- (\(x::T [c]) -> case x of
851 -- MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
852 -- To implement this, we'd first instantiate the equational
853 -- constraints with *wobbly* type variables for the existentials;
854 -- then unify these constraints to make pat_ty the right shape;
855 -- then proceed exactly as in the rigid case
857 -- In the rigid case, we perform type refinement
858 ; case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
859 Failed msg -> failWithTc (inaccessibleAlt msg) ;
860 Succeeded reft -> do { traceTc trace_msg
861 ; return (pstate { pat_reft = reft, pat_eqs = (pat_eqs pstate || not (null $ dataConEqTheta con)) }) }
862 -- DO NOT refine the envt right away, because we
863 -- might be inside a lazy pattern. Instead, refine pstate
866 trace_msg = text "refineAlt:match" <+>
867 vcat [ ppr con <+> ppr ex_tvs,
868 ppr [(v, tyVarKind v) | v <- co_vars],
874 %************************************************************************
878 %************************************************************************
880 In tcOverloadedLit we convert directly to an Int or Integer if we
881 know that's what we want. This may save some time, by not
882 temporarily generating overloaded literals, but it won't catch all
883 cases (the rest are caught in lookupInst).
886 tcOverloadedLit :: InstOrigin
889 -> TcM (HsOverLit TcId)
890 tcOverloadedLit orig lit@(HsIntegral i fi _) res_ty
891 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
892 -- Reason: If we do, tcSimplify will call lookupInst, which
893 -- will call tcSyntaxName, which does unification,
894 -- which tcSimplify doesn't like
895 -- ToDo: noLoc sadness
896 = do { integer_ty <- tcMetaTy integerTyConName
897 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
898 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty))) res_ty) }
900 | Just expr <- shortCutIntLit i res_ty
901 = return (HsIntegral i expr res_ty)
904 = do { expr <- newLitInst orig lit res_ty
905 ; return (HsIntegral i expr res_ty) }
907 tcOverloadedLit orig lit@(HsFractional r fr _) res_ty
908 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
909 = do { rat_ty <- tcMetaTy rationalTyConName
910 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
911 -- Overloaded literals must have liftedTypeKind, because
912 -- we're instantiating an overloaded function here,
913 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
914 -- However this'll be picked up by tcSyntaxOp if necessary
915 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty))) res_ty) }
917 | Just expr <- shortCutFracLit r res_ty
918 = return (HsFractional r expr res_ty)
921 = do { expr <- newLitInst orig lit res_ty
922 ; return (HsFractional r expr res_ty) }
924 tcOverloadedLit orig lit@(HsIsString s fr _) res_ty
925 | not (fr `isHsVar` fromStringName) -- c.f. HsIntegral case
926 = do { str_ty <- tcMetaTy stringTyConName
927 ; fr' <- tcSyntaxOp orig fr (mkFunTy str_ty res_ty)
928 ; return (HsIsString s (HsApp (noLoc fr') (nlHsLit (HsString s))) res_ty) }
930 | Just expr <- shortCutStringLit s res_ty
931 = return (HsIsString s expr res_ty)
934 = do { expr <- newLitInst orig lit res_ty
935 ; return (HsIsString s expr res_ty) }
937 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
938 newLitInst orig lit res_ty -- Make a LitInst
939 = do { loc <- getInstLoc orig
940 ; res_tau <- zapToMonotype res_ty
941 ; new_uniq <- newUnique
942 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
943 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
944 tci_ty = res_tau, tci_loc = loc}
946 ; return (HsVar (instToId lit_inst)) }
950 %************************************************************************
952 Note [Pattern coercions]
954 %************************************************************************
956 In principle, these program would be reasonable:
958 f :: (forall a. a->a) -> Int
959 f (x :: Int->Int) = x 3
961 g :: (forall a. [a]) -> Bool
964 In both cases, the function type signature restricts what arguments can be passed
965 in a call (to polymorphic ones). The pattern type signature then instantiates this
966 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
967 generate the translated term
968 f = \x' :: (forall a. a->a). let x = x' Int in x 3
970 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
971 And it requires a significant amount of code to implement, becuase we need to decorate
972 the translated pattern with coercion functions (generated from the subsumption check
975 So for now I'm just insisting on type *equality* in patterns. No subsumption.
977 Old notes about desugaring, at a time when pattern coercions were handled:
979 A SigPat is a type coercion and must be handled one at at time. We can't
980 combine them unless the type of the pattern inside is identical, and we don't
981 bother to check for that. For example:
983 data T = T1 Int | T2 Bool
984 f :: (forall a. a -> a) -> T -> t
985 f (g::Int->Int) (T1 i) = T1 (g i)
986 f (g::Bool->Bool) (T2 b) = T2 (g b)
988 We desugar this as follows:
990 f = \ g::(forall a. a->a) t::T ->
992 in case t of { T1 i -> T1 (gi i)
995 in case t of { T2 b -> T2 (gb b)
998 Note that we do not treat the first column of patterns as a
999 column of variables, because the coerced variables (gi, gb)
1000 would be of different types. So we get rather grotty code.
1001 But I don't think this is a common case, and if it was we could
1002 doubtless improve it.
1004 Meanwhile, the strategy is:
1005 * treat each SigPat coercion (always non-identity coercions)
1007 * deal with the stuff inside, and then wrap a binding round
1008 the result to bind the new variable (gi, gb, etc)
1011 %************************************************************************
1013 \subsection{Errors and contexts}
1015 %************************************************************************
1018 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
1019 patCtxt (VarPat _) = Nothing
1020 patCtxt (ParPat _) = Nothing
1021 patCtxt (AsPat _ _) = Nothing
1022 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
1025 -----------------------------------------------
1027 existentialExplode pat
1028 = hang (vcat [text "My brain just exploded.",
1029 text "I can't handle pattern bindings for existentially-quantified constructors.",
1030 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
1031 text "In the binding group for"])
1034 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
1035 = do { pat_tys' <- mapM zonkTcType pat_tys
1036 ; body_ty' <- zonkTcType body_ty
1037 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1038 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1039 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1041 sep [ptext SLIT("When checking an existential match that binds"),
1042 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1043 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1044 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
1047 bound_ids = collectPatsBinders pats
1048 show_ids = filter is_interesting bound_ids
1049 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1051 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1052 -- Don't zonk the types so we get the separate, un-unified versions
1054 badFieldCon :: DataCon -> Name -> SDoc
1055 badFieldCon con field
1056 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
1057 ptext SLIT("does not have field"), quotes (ppr field)]
1059 polyPatSig :: TcType -> SDoc
1061 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
1064 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
1066 existentialProcPat :: DataCon -> SDoc
1067 existentialProcPat con
1068 = hang (ptext SLIT("Illegal constructor") <+> quotes (ppr con) <+> ptext SLIT("in a 'proc' pattern"))
1069 2 (ptext SLIT("Proc patterns cannot use existentials or GADTs"))
1073 hang (ptext SLIT("A lazy (~) pattern cannot bind existential type variables"))
1074 2 (vcat (map pprSkolTvBinding tvs))
1077 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1078 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1080 nonRigidResult res_ty
1081 = hang (ptext SLIT("GADT pattern match with non-rigid result type") <+> quotes (ppr res_ty))
1082 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1085 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg