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 ->
425 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
427 -- pattern must have pat_ty
428 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
429 -- this should get zonked later on, but we unBox it here
430 -- so that we do the same checks as above
431 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
432 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
434 -- Type signatures in patterns
435 -- See Note [Pattern coercions] below
436 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
437 = do { (inner_ty, tv_binds) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
438 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
439 tc_lpat pat inner_ty pstate thing_inside
440 ; return (SigPatOut pat' inner_ty, tvs, res) }
442 tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
443 = failWithTc (badTypePat pat)
445 ------------------------
446 -- Lists, tuples, arrays
447 tc_pat pstate (ListPat pats _) pat_ty thing_inside
448 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
449 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
450 pats pstate thing_inside
451 ; return (mkCoPatCoI coi (ListPat pats' elt_ty) pat_ty, pats_tvs, res) }
453 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
454 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
455 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
456 pats pstate thing_inside
457 ; ifM (null pats) (zapToMonotype pat_ty) -- c.f. ExplicitPArr in TcExpr
458 ; return (mkCoPatCoI coi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res) }
460 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
461 = do { let tc = tupleTyCon boxity (length pats)
462 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
463 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
466 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
467 -- so that we can experiment with lazy tuple-matching.
468 -- This is a pretty odd place to make the switch, but
469 -- it was easy to do.
470 ; let pat_ty' = mkTyConApp tc arg_tys
471 -- pat_ty /= pat_ty iff coi /= IdCo
472 unmangled_result = TuplePat pats' boxity pat_ty'
473 possibly_mangled_result
474 | opt_IrrefutableTuples &&
475 isBoxed boxity = LazyPat (noLoc unmangled_result)
476 | otherwise = unmangled_result
478 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
479 return (mkCoPatCoI coi possibly_mangled_result pat_ty, pats_tvs, res)
482 ------------------------
484 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
485 = do { data_con <- tcLookupDataCon con_name
486 ; let tycon = dataConTyCon data_con
487 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
489 ------------------------
491 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
492 = do { let lit_ty = hsLitType simple_lit
493 ; coi <- boxyUnify lit_ty pat_ty
494 -- coi is of kind: lit_ty ~ pat_ty
495 ; res <- thing_inside pstate
496 ; span <- getSrcSpanM
497 -- pattern coercions have to
498 -- be of kind: pat_ty ~ lit_ty
500 ; returnM (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
503 ------------------------
504 -- Overloaded patterns: n, and n+k
505 tc_pat pstate pat@(NPat over_lit mb_neg eq) pat_ty thing_inside
506 = do { let orig = LiteralOrigin over_lit
507 ; lit' <- tcOverloadedLit orig over_lit pat_ty
508 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
509 ; mb_neg' <- case mb_neg of
510 Nothing -> return Nothing -- Positive literal
511 Just neg -> -- Negative literal
512 -- The 'negate' is re-mappable syntax
513 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
514 ; return (Just neg') }
515 ; res <- thing_inside pstate
516 ; returnM (NPat lit' mb_neg' eq', [], res) }
518 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
519 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
520 ; let pat_ty' = idType bndr_id
521 orig = LiteralOrigin lit
522 ; lit' <- tcOverloadedLit orig lit pat_ty'
524 -- The '>=' and '-' parts are re-mappable syntax
525 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
526 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
528 -- The Report says that n+k patterns must be in Integral
529 -- We may not want this when using re-mappable syntax, though (ToDo?)
530 ; icls <- tcLookupClass integralClassName
531 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
533 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
534 ; returnM (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
536 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
540 %************************************************************************
542 Most of the work for constructors is here
543 (the rest is in the ConPatIn case of tc_pat)
545 %************************************************************************
547 [Pattern matching indexed data types]
548 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
549 Consider the following declarations:
551 data family Map k :: * -> *
552 data instance Map (a, b) v = MapPair (Map a (Pair b v))
554 and a case expression
556 case x :: Map (Int, c) w of MapPair m -> ...
558 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
559 worker/wrapper types for MapPair are
561 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
562 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
564 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
565 :R123Map, which means the straight use of boxySplitTyConApp would give a type
566 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
567 boxySplitTyConApp with the family tycon Map instead, which gives us the family
568 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
569 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
570 (provided by tyConFamInst_maybe together with the family tycon). This
571 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
572 the split arguments for the representation tycon :R123Map as {Int, c, w}
574 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
576 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
578 moving between representation and family type into account. To produce type
579 correct Core, this coercion needs to be used to case the type of the scrutinee
580 from the family to the representation type. This is achieved by
581 unwrapFamInstScrutinee using a CoPat around the result pattern.
583 Now it might appear seem as if we could have used the existing GADT type
584 refinement infrastructure of refineAlt and friends instead of the explicit
585 unification and CoPat generation. However, that would be wrong. Why? The
586 whole point of GADT refinement is that the refinement is local to the case
587 alternative. In contrast, the substitution generated by the unification of
588 the family type list and instance types needs to be propagated to the outside.
589 Imagine that in the above example, the type of the scrutinee would have been
590 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
591 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
592 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
593 alternatives of the case expression, whereas in the GADT case it might vary
594 between alternatives.
596 In fact, if we have a data instance declaration defining a GADT, eq_spec will
597 be non-empty and we will get a mixture of global instantiations and local
598 refinement from a single match. This neatly reflects that, as soon as we
599 have constrained the type of the scrutinee to the required type index, all
600 further type refinement is local to the alternative.
604 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
605 -- with scrutinee of type (T ty)
607 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
608 -> BoxySigmaType -- Type of the pattern
609 -> HsConPatDetails Name -> (PatState -> TcM a)
610 -> TcM (Pat TcId, [TcTyVar], a)
611 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
612 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _) = dataConFullSig data_con
613 skol_info = PatSkol data_con
614 origin = SigOrigin skol_info
615 full_theta = eq_theta ++ dict_theta
617 -- Instantiate the constructor type variables [a->ty]
618 -- This may involve doing a family-instance coercion, and building a wrapper
619 ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
620 ; let pat_ty' = mkTyConApp tycon ctxt_res_tys
621 -- pat_ty /= pat_ty iff coi /= IdCo
623 = mkCoPatCoI coi (unwrapFamInstScrutinee tycon ctxt_res_tys res_pat) pat_ty
625 -- Add the stupid theta
626 ; addDataConStupidTheta data_con ctxt_res_tys
628 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs -- Get location from monad,
630 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
631 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
632 arg_tys' = substTys tenv arg_tys
634 ; if null ex_tvs && null eq_spec && null full_theta
635 then do { -- The common case; no class bindings etc (see Note [Arrows and patterns])
636 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
637 arg_pats pstate thing_inside
638 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
639 pat_tvs = [], pat_dicts = [], pat_binds = emptyLHsBinds,
640 pat_args = arg_pats', pat_ty = pat_ty' }
642 ; return (wrap_res_pat res_pat, inner_tvs, res) }
644 else do -- The general case, with existential, and local equality constraints
645 { let eq_spec' = substEqSpec tenv eq_spec
646 theta' = substTheta tenv full_theta
647 ctxt = pat_ctxt pstate
648 ; checkTc (case ctxt of { ProcPat -> False; other -> True })
649 (existentialProcPat data_con)
650 ; co_vars <- newCoVars eq_spec' -- Make coercion variables
651 ; traceTc (text "tcConPat: refineAlt")
652 ; pstate' <- refineAlt data_con pstate ex_tvs' co_vars pat_ty
653 ; traceTc (text "tcConPat: refineAlt done!")
655 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
656 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
658 ; loc <- getInstLoc origin
659 ; dicts <- newDictBndrs loc theta'
660 ; dict_binds <- tcSimplifyCheckPat loc co_vars (pat_reft pstate')
661 ex_tvs' dicts lie_req
663 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
664 pat_tvs = ex_tvs' ++ co_vars,
665 pat_dicts = map instToVar dicts,
666 pat_binds = dict_binds,
667 pat_args = arg_pats', pat_ty = pat_ty' }
668 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
671 -- Split against the family tycon if the pattern constructor
672 -- belongs to a family instance tycon.
673 boxySplitTyConAppWithFamily tycon pat_ty =
675 case tyConFamInst_maybe tycon of
676 Nothing -> boxySplitTyConApp tycon pat_ty
677 Just (fam_tycon, instTys) ->
678 do { (scrutinee_arg_tys, coi) <- boxySplitTyConApp fam_tycon pat_ty
679 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
680 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
681 ; return (freshTvs, coi)
684 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
685 ppr tycon <+> ppr pat_ty
686 , text " family instance:" <+>
687 ppr (tyConFamInst_maybe tycon)
690 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
691 -- pattern if the tycon is an instance of a family.
693 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
694 unwrapFamInstScrutinee tycon args pat
695 | Just co_con <- tyConFamilyCoercion_maybe tycon
696 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
698 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
699 -- coercion is not the identity; mkCoPat is inconvenient as it
700 -- wants a located pattern.
701 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
702 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
703 pat_ty -- family inst type
708 tcConArgs :: DataCon -> [TcSigmaType]
709 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
711 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
712 = do { checkTc (con_arity == no_of_args) -- Check correct arity
713 (arityErr "Constructor" data_con con_arity no_of_args)
714 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
715 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
717 ; return (PrefixCon arg_pats', tvs, res) }
719 con_arity = dataConSourceArity data_con
720 no_of_args = length arg_pats
722 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
723 = do { checkTc (con_arity == 2) -- Check correct arity
724 (arityErr "Constructor" data_con con_arity 2)
725 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
726 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
728 ; return (InfixCon p1' p2', tvs, res) }
730 con_arity = dataConSourceArity data_con
732 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
733 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
735 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
736 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
737 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
739 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
740 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
741 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
742 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
743 ; return (HsRecField sel_id pat' pun, tvs, res) }
745 find_field_ty :: FieldLabel -> TcM (Id, TcType)
746 find_field_ty field_lbl
747 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
749 -- No matching field; chances are this field label comes from some
750 -- other record type (or maybe none). As well as reporting an
751 -- error we still want to typecheck the pattern, principally to
752 -- make sure that all the variables it binds are put into the
753 -- environment, else the type checker crashes later:
754 -- f (R { foo = (a,b) }) = a+b
755 -- If foo isn't one of R's fields, we don't want to crash when
756 -- typechecking the "a+b".
757 [] -> do { addErrTc (badFieldCon data_con field_lbl)
758 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
759 ; return (error "Bogus selector Id", bogus_ty) }
761 -- The normal case, when the field comes from the right constructor
763 ASSERT( null extras )
764 do { sel_id <- tcLookupField field_lbl
765 ; return (sel_id, pat_ty) }
767 field_tys :: [(FieldLabel, TcType)]
768 field_tys = zip (dataConFieldLabels data_con) arg_tys
769 -- Don't use zipEqual! If the constructor isn't really a record, then
770 -- dataConFieldLabels will be empty (and each field in the pattern
771 -- will generate an error below).
773 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
774 tcConArg (arg_pat, arg_ty) pstate thing_inside
775 = tc_lpat arg_pat arg_ty pstate thing_inside
776 -- NB: the tc_lpat will refine pat_ty if necessary
777 -- based on the current pstate, which may include
778 -- refinements from peer argument patterns to the left
782 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
783 -- Instantiate the "stupid theta" of the data con, and throw
784 -- the constraints into the constraint set
785 addDataConStupidTheta data_con inst_tys
786 | null stupid_theta = return ()
787 | otherwise = instStupidTheta origin inst_theta
789 origin = OccurrenceOf (dataConName data_con)
790 -- The origin should always report "occurrence of C"
791 -- even when C occurs in a pattern
792 stupid_theta = dataConStupidTheta data_con
793 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
794 inst_theta = substTheta tenv stupid_theta
797 Note [Arrows and patterns]
798 ~~~~~~~~~~~~~~~~~~~~~~~~~~
799 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
801 expr :: Arrow a => a () Int
802 expr = proc (y,z) -> do
806 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
807 arrow body (e.g 'term'). As things stand (bogusly) all the
808 constraints from the proc body are gathered together, so constraints
809 from 'term' will be seen by the tcPat for (y,z). But we must *not*
810 bind constraints from 'term' here, becuase the desugarer will not make
811 these bindings scope over 'term'.
813 The Right Thing is not to confuse these constraints together. But for
814 now the Easy Thing is to ensure that we do not have existential or
815 GADT constraints in a 'proc', and to short-cut the constraint
816 simplification for such vanilla patterns so that it binds no
817 constraints. Hence the 'fast path' in tcConPat; but it's also a good
818 plan for ordinary vanilla patterns to bypass the constraint
822 %************************************************************************
826 %************************************************************************
829 refineAlt :: DataCon -- For tracing only
831 -> [TcTyVar] -- Existentials
832 -> [CoVar] -- Equational constraints
833 -> BoxySigmaType -- Pattern type
836 refineAlt con pstate ex_tvs [] pat_ty
837 | null $ dataConEqTheta con
838 = return pstate -- Common case: no equational constraints
840 refineAlt con pstate ex_tvs co_vars pat_ty
841 = do { opt_gadt <- doptM Opt_GADTs -- No type-refinement unless GADTs are on
842 ; if (not opt_gadt) then return pstate
845 { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
846 -- We are matching against a GADT constructor with non-trivial
847 -- constraints, but pattern type is wobbly. For now we fail.
848 -- We can make sense of this, however:
849 -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
850 -- (\x -> case x of { MkT v -> v })
851 -- We can infer that x must have type T [c], for some wobbly 'c'
853 -- (\(x::T [c]) -> case x of
854 -- MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
855 -- To implement this, we'd first instantiate the equational
856 -- constraints with *wobbly* type variables for the existentials;
857 -- then unify these constraints to make pat_ty the right shape;
858 -- then proceed exactly as in the rigid case
860 -- In the rigid case, we perform type refinement
861 ; case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
862 Failed msg -> failWithTc (inaccessibleAlt msg) ;
863 Succeeded reft -> do { traceTc trace_msg
864 ; return (pstate { pat_reft = reft, pat_eqs = (pat_eqs pstate || not (null $ dataConEqTheta con)) }) }
865 -- DO NOT refine the envt right away, because we
866 -- might be inside a lazy pattern. Instead, refine pstate
869 trace_msg = text "refineAlt:match" <+>
870 vcat [ ppr con <+> ppr ex_tvs,
871 ppr [(v, tyVarKind v) | v <- co_vars],
877 %************************************************************************
881 %************************************************************************
883 In tcOverloadedLit we convert directly to an Int or Integer if we
884 know that's what we want. This may save some time, by not
885 temporarily generating overloaded literals, but it won't catch all
886 cases (the rest are caught in lookupInst).
889 tcOverloadedLit :: InstOrigin
892 -> TcM (HsOverLit TcId)
893 tcOverloadedLit orig lit@(HsIntegral i fi _) res_ty
894 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
895 -- Reason: If we do, tcSimplify will call lookupInst, which
896 -- will call tcSyntaxName, which does unification,
897 -- which tcSimplify doesn't like
898 -- ToDo: noLoc sadness
899 = do { integer_ty <- tcMetaTy integerTyConName
900 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
901 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty))) res_ty) }
903 | Just expr <- shortCutIntLit i res_ty
904 = return (HsIntegral i expr res_ty)
907 = do { expr <- newLitInst orig lit res_ty
908 ; return (HsIntegral i expr res_ty) }
910 tcOverloadedLit orig lit@(HsFractional r fr _) res_ty
911 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
912 = do { rat_ty <- tcMetaTy rationalTyConName
913 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
914 -- Overloaded literals must have liftedTypeKind, because
915 -- we're instantiating an overloaded function here,
916 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
917 -- However this'll be picked up by tcSyntaxOp if necessary
918 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty))) res_ty) }
920 | Just expr <- shortCutFracLit r res_ty
921 = return (HsFractional r expr res_ty)
924 = do { expr <- newLitInst orig lit res_ty
925 ; return (HsFractional r expr res_ty) }
927 tcOverloadedLit orig lit@(HsIsString s fr _) res_ty
928 | not (fr `isHsVar` fromStringName) -- c.f. HsIntegral case
929 = do { str_ty <- tcMetaTy stringTyConName
930 ; fr' <- tcSyntaxOp orig fr (mkFunTy str_ty res_ty)
931 ; return (HsIsString s (HsApp (noLoc fr') (nlHsLit (HsString s))) res_ty) }
933 | Just expr <- shortCutStringLit s res_ty
934 = return (HsIsString s expr res_ty)
937 = do { expr <- newLitInst orig lit res_ty
938 ; return (HsIsString s expr res_ty) }
940 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
941 newLitInst orig lit res_ty -- Make a LitInst
942 = do { loc <- getInstLoc orig
943 ; res_tau <- zapToMonotype res_ty
944 ; new_uniq <- newUnique
945 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
946 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
947 tci_ty = res_tau, tci_loc = loc}
949 ; return (HsVar (instToId lit_inst)) }
953 %************************************************************************
955 Note [Pattern coercions]
957 %************************************************************************
959 In principle, these program would be reasonable:
961 f :: (forall a. a->a) -> Int
962 f (x :: Int->Int) = x 3
964 g :: (forall a. [a]) -> Bool
967 In both cases, the function type signature restricts what arguments can be passed
968 in a call (to polymorphic ones). The pattern type signature then instantiates this
969 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
970 generate the translated term
971 f = \x' :: (forall a. a->a). let x = x' Int in x 3
973 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
974 And it requires a significant amount of code to implement, becuase we need to decorate
975 the translated pattern with coercion functions (generated from the subsumption check
978 So for now I'm just insisting on type *equality* in patterns. No subsumption.
980 Old notes about desugaring, at a time when pattern coercions were handled:
982 A SigPat is a type coercion and must be handled one at at time. We can't
983 combine them unless the type of the pattern inside is identical, and we don't
984 bother to check for that. For example:
986 data T = T1 Int | T2 Bool
987 f :: (forall a. a -> a) -> T -> t
988 f (g::Int->Int) (T1 i) = T1 (g i)
989 f (g::Bool->Bool) (T2 b) = T2 (g b)
991 We desugar this as follows:
993 f = \ g::(forall a. a->a) t::T ->
995 in case t of { T1 i -> T1 (gi i)
998 in case t of { T2 b -> T2 (gb b)
1001 Note that we do not treat the first column of patterns as a
1002 column of variables, because the coerced variables (gi, gb)
1003 would be of different types. So we get rather grotty code.
1004 But I don't think this is a common case, and if it was we could
1005 doubtless improve it.
1007 Meanwhile, the strategy is:
1008 * treat each SigPat coercion (always non-identity coercions)
1010 * deal with the stuff inside, and then wrap a binding round
1011 the result to bind the new variable (gi, gb, etc)
1014 %************************************************************************
1016 \subsection{Errors and contexts}
1018 %************************************************************************
1021 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
1022 patCtxt (VarPat _) = Nothing
1023 patCtxt (ParPat _) = Nothing
1024 patCtxt (AsPat _ _) = Nothing
1025 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
1028 -----------------------------------------------
1030 existentialExplode pat
1031 = hang (vcat [text "My brain just exploded.",
1032 text "I can't handle pattern bindings for existentially-quantified constructors.",
1033 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
1034 text "In the binding group for"])
1037 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
1038 = do { pat_tys' <- mapM zonkTcType pat_tys
1039 ; body_ty' <- zonkTcType body_ty
1040 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1041 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1042 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1044 sep [ptext SLIT("When checking an existential match that binds"),
1045 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1046 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1047 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
1050 bound_ids = collectPatsBinders pats
1051 show_ids = filter is_interesting bound_ids
1052 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1054 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1055 -- Don't zonk the types so we get the separate, un-unified versions
1057 badFieldCon :: DataCon -> Name -> SDoc
1058 badFieldCon con field
1059 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
1060 ptext SLIT("does not have field"), quotes (ppr field)]
1062 polyPatSig :: TcType -> SDoc
1064 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
1067 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
1069 existentialProcPat :: DataCon -> SDoc
1070 existentialProcPat con
1071 = hang (ptext SLIT("Illegal constructor") <+> quotes (ppr con) <+> ptext SLIT("in a 'proc' pattern"))
1072 2 (ptext SLIT("Proc patterns cannot use existentials or GADTs"))
1076 hang (ptext SLIT("A lazy (~) pattern cannot bind existential type variables"))
1077 2 (vcat (map pprSkolTvBinding tvs))
1080 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1081 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1083 nonRigidResult res_ty
1084 = hang (ptext SLIT("GADT pattern match with non-rigid result type") <+> quotes (ppr res_ty))
1085 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
1088 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg