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 ; return (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 _ p@(QuasiQuotePat _) _ _
390 = pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
392 tc_pat pstate (WildPat _) pat_ty thing_inside
393 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
394 ; res <- thing_inside pstate
395 ; return (WildPat pat_ty', [], res) }
397 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
398 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
399 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
400 tc_lpat pat (idType bndr_id) pstate thing_inside
401 -- NB: if we do inference on:
402 -- \ (y@(x::forall a. a->a)) = e
403 -- we'll fail. The as-pattern infers a monotype for 'y', which then
404 -- fails to unify with the polymorphic type for 'x'. This could
405 -- perhaps be fixed, but only with a bit more work.
407 -- If you fix it, don't forget the bindInstsOfPatIds!
408 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
410 tc_pat pstate (orig@(ViewPat expr pat _)) overall_pat_ty thing_inside
411 = do { -- morally, expr must have type
412 -- `forall a1...aN. OPT' -> B`
413 -- where overall_pat_ty is an instance of OPT'.
414 -- Here, we infer a rho type for it,
415 -- which replaces the leading foralls and constraints
416 -- with fresh unification variables.
417 (expr',expr'_inferred) <- tcInferRho expr
418 -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
419 ; let expr'_expected = \ pat_ty -> (mkFunTy overall_pat_ty pat_ty)
420 -- tcSubExp: expected first, offered second
423 -- NOTE: this forces pat_ty to be a monotype (because we use a unification
424 -- variable to find it). this means that in an example like
425 -- (view -> f) where view :: _ -> forall b. b
426 -- we will only be able to use view at one instantation in the
428 ; (expr_coerc, pat_ty) <- tcInfer $ \ pat_ty ->
429 tcSubExp ViewPatOrigin (expr'_expected pat_ty) expr'_inferred
431 -- pattern must have pat_ty
432 ; (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
433 -- this should get zonked later on, but we unBox it here
434 -- so that we do the same checks as above
435 ; annotation_ty <- unBoxViewPatType overall_pat_ty orig
436 ; return (ViewPat (mkLHsWrap expr_coerc expr') pat' annotation_ty, tvs, res) }
438 -- Type signatures in patterns
439 -- See Note [Pattern coercions] below
440 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
441 = do { (inner_ty, tv_binds) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
442 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
443 tc_lpat pat inner_ty pstate thing_inside
444 ; return (SigPatOut pat' inner_ty, tvs, res) }
446 tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
447 = failWithTc (badTypePat pat)
449 ------------------------
450 -- Lists, tuples, arrays
451 tc_pat pstate (ListPat pats _) pat_ty thing_inside
452 = do { (elt_ty, coi) <- boxySplitListTy pat_ty
453 ; let scoi = mkSymCoI coi
454 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
455 pats pstate thing_inside
456 ; return (mkCoPatCoI scoi (ListPat pats' elt_ty) pat_ty, pats_tvs, res)
459 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
460 = do { (elt_ty, coi) <- boxySplitPArrTy pat_ty
461 ; let scoi = mkSymCoI coi
462 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
463 pats pstate thing_inside
464 ; when (null pats) (zapToMonotype pat_ty >> return ()) -- c.f. ExplicitPArr in TcExpr
465 ; return (mkCoPatCoI scoi (PArrPat pats' elt_ty) pat_ty, pats_tvs, res)
468 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
469 = do { let tc = tupleTyCon boxity (length pats)
470 ; (arg_tys, coi) <- boxySplitTyConApp tc pat_ty
471 ; let scoi = mkSymCoI coi
472 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
475 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
476 -- so that we can experiment with lazy tuple-matching.
477 -- This is a pretty odd place to make the switch, but
478 -- it was easy to do.
479 ; let pat_ty' = mkTyConApp tc arg_tys
480 -- pat_ty /= pat_ty iff coi /= IdCo
481 unmangled_result = TuplePat pats' boxity pat_ty'
482 possibly_mangled_result
483 | opt_IrrefutableTuples &&
484 isBoxed boxity = LazyPat (noLoc unmangled_result)
485 | otherwise = unmangled_result
487 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
488 return (mkCoPatCoI scoi possibly_mangled_result pat_ty, pats_tvs, res)
491 ------------------------
493 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
494 = do { data_con <- tcLookupDataCon con_name
495 ; let tycon = dataConTyCon data_con
496 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
498 ------------------------
500 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
501 = do { let lit_ty = hsLitType simple_lit
502 ; coi <- boxyUnify lit_ty pat_ty
503 -- coi is of kind: lit_ty ~ pat_ty
504 ; res <- thing_inside pstate
505 ; span <- getSrcSpanM
506 -- pattern coercions have to
507 -- be of kind: pat_ty ~ lit_ty
509 ; return (mkCoPatCoI (mkSymCoI coi) (LitPat simple_lit) pat_ty,
512 ------------------------
513 -- Overloaded patterns: n, and n+k
514 tc_pat pstate pat@(NPat over_lit mb_neg eq) pat_ty thing_inside
515 = do { let orig = LiteralOrigin over_lit
516 ; lit' <- tcOverloadedLit orig over_lit pat_ty
517 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
518 ; mb_neg' <- case mb_neg of
519 Nothing -> return Nothing -- Positive literal
520 Just neg -> -- Negative literal
521 -- The 'negate' is re-mappable syntax
522 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
523 ; return (Just neg') }
524 ; res <- thing_inside pstate
525 ; return (NPat lit' mb_neg' eq', [], res) }
527 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
528 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
529 ; let pat_ty' = idType bndr_id
530 orig = LiteralOrigin lit
531 ; lit' <- tcOverloadedLit orig lit pat_ty'
533 -- The '>=' and '-' parts are re-mappable syntax
534 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
535 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
537 -- The Report says that n+k patterns must be in Integral
538 -- We may not want this when using re-mappable syntax, though (ToDo?)
539 ; icls <- tcLookupClass integralClassName
540 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
542 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
543 ; return (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
545 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
549 %************************************************************************
551 Most of the work for constructors is here
552 (the rest is in the ConPatIn case of tc_pat)
554 %************************************************************************
556 [Pattern matching indexed data types]
557 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
558 Consider the following declarations:
560 data family Map k :: * -> *
561 data instance Map (a, b) v = MapPair (Map a (Pair b v))
563 and a case expression
565 case x :: Map (Int, c) w of MapPair m -> ...
567 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
568 worker/wrapper types for MapPair are
570 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
571 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
573 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
574 :R123Map, which means the straight use of boxySplitTyConApp would give a type
575 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
576 boxySplitTyConApp with the family tycon Map instead, which gives us the family
577 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
578 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
579 (provided by tyConFamInst_maybe together with the family tycon). This
580 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
581 the split arguments for the representation tycon :R123Map as {Int, c, w}
583 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
585 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
587 moving between representation and family type into account. To produce type
588 correct Core, this coercion needs to be used to case the type of the scrutinee
589 from the family to the representation type. This is achieved by
590 unwrapFamInstScrutinee using a CoPat around the result pattern.
592 Now it might appear seem as if we could have used the existing GADT type
593 refinement infrastructure of refineAlt and friends instead of the explicit
594 unification and CoPat generation. However, that would be wrong. Why? The
595 whole point of GADT refinement is that the refinement is local to the case
596 alternative. In contrast, the substitution generated by the unification of
597 the family type list and instance types needs to be propagated to the outside.
598 Imagine that in the above example, the type of the scrutinee would have been
599 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
600 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
601 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
602 alternatives of the case expression, whereas in the GADT case it might vary
603 between alternatives.
605 In fact, if we have a data instance declaration defining a GADT, eq_spec will
606 be non-empty and we will get a mixture of global instantiations and local
607 refinement from a single match. This neatly reflects that, as soon as we
608 have constrained the type of the scrutinee to the required type index, all
609 further type refinement is local to the alternative.
613 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
614 -- with scrutinee of type (T ty)
616 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
617 -> BoxySigmaType -- Type of the pattern
618 -> HsConPatDetails Name -> (PatState -> TcM a)
619 -> TcM (Pat TcId, [TcTyVar], a)
620 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
621 = do { let (univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _)
622 = dataConFullSig data_con
623 skol_info = PatSkol data_con
624 origin = SigOrigin skol_info
625 full_theta = eq_theta ++ dict_theta
627 -- Instantiate the constructor type variables [a->ty]
628 -- This may involve doing a family-instance coercion, and building a
630 ; (ctxt_res_tys, coi) <- boxySplitTyConAppWithFamily tycon pat_ty
631 ; let sym_coi = mkSymCoI coi -- boxy split coercion oriented wrongly
632 pat_ty' = mkTyConApp tycon ctxt_res_tys
633 -- pat_ty' /= pat_ty iff coi /= IdCo
635 wrap_res_pat res_pat = mkCoPatCoI sym_coi uwScrut pat_ty
637 uwScrut = unwrapFamInstScrutinee tycon ctxt_res_tys res_pat
639 ; traceTc $ case sym_coi of
640 IdCo -> text "sym_coi:IdCo"
641 ACo co -> text "sym_coi: ACoI" <+> ppr co
643 -- Add the stupid theta
644 ; addDataConStupidTheta data_con ctxt_res_tys
646 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
647 -- Get location from monad, not from ex_tvs
649 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
650 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
651 arg_tys' = substTys tenv arg_tys
653 ; if null ex_tvs && null eq_spec && null full_theta
654 then do { -- The common case; no class bindings etc
655 -- (see Note [Arrows and patterns])
656 (arg_pats', inner_tvs, res) <- tcConArgs data_con arg_tys'
657 arg_pats pstate thing_inside
658 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
659 pat_tvs = [], pat_dicts = [],
660 pat_binds = emptyLHsBinds,
661 pat_args = arg_pats',
664 ; return (wrap_res_pat res_pat, inner_tvs, res) }
666 else do -- The general case, with existential, and local equality
668 { let eq_preds = [mkEqPred (mkTyVarTy tv, ty) | (tv, ty) <- eq_spec]
669 theta' = substTheta tenv (eq_preds ++ full_theta)
670 -- order is *important* as we generate the list of
671 -- dictionary binders from theta'
672 ctxt = pat_ctxt pstate
673 ; checkTc (case ctxt of { ProcPat -> False; other -> True })
674 (existentialProcPat data_con)
676 -- Need to test for rigidity if *any* constraints in theta as class
677 -- constraints may have superclass equality constraints. However,
678 -- we don't want to check for rigidity if we got here only because
679 -- ex_tvs was non-null.
680 -- ; unless (null theta') $
681 -- FIXME: AT THE MOMENT WE CHEAT! We only perform the rigidity test
682 -- if we explicit or implicit (by a GADT def) have equality
684 ; unless (all (not . isEqPred) theta') $
685 checkTc (isRigidTy pat_ty) (nonRigidMatch data_con)
687 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
688 tcConArgs data_con arg_tys' arg_pats pstate thing_inside
690 ; loc <- getInstLoc origin
691 ; dicts <- newDictBndrs loc theta'
692 ; dict_binds <- tcSimplifyCheckPat loc [] emptyRefinement
693 ex_tvs' dicts lie_req
695 ; let res_pat = ConPatOut { pat_con = L con_span data_con,
697 pat_dicts = map instToVar dicts,
698 pat_binds = dict_binds,
699 pat_args = arg_pats', pat_ty = pat_ty' }
700 ; return (wrap_res_pat res_pat, ex_tvs' ++ inner_tvs, res)
703 -- Split against the family tycon if the pattern constructor
704 -- belongs to a family instance tycon.
705 boxySplitTyConAppWithFamily tycon pat_ty =
707 case tyConFamInst_maybe tycon of
708 Nothing -> boxySplitTyConApp tycon pat_ty
709 Just (fam_tycon, instTys) ->
710 do { (scrutinee_arg_tys, coi) <- boxySplitTyConApp fam_tycon pat_ty
711 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
712 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
713 ; return (freshTvs, coi)
716 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
717 ppr tycon <+> ppr pat_ty
718 , text " family instance:" <+>
719 ppr (tyConFamInst_maybe tycon)
722 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
723 -- pattern if the tycon is an instance of a family.
725 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
726 unwrapFamInstScrutinee tycon args pat
727 | Just co_con <- tyConFamilyCoercion_maybe tycon
728 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
730 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
731 -- coercion is not the identity; mkCoPat is inconvenient as it
732 -- wants a located pattern.
733 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
734 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
735 pat_ty -- family inst type
740 tcConArgs :: DataCon -> [TcSigmaType]
741 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
743 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
744 = do { checkTc (con_arity == no_of_args) -- Check correct arity
745 (arityErr "Constructor" data_con con_arity no_of_args)
746 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
747 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
749 ; return (PrefixCon arg_pats', tvs, res) }
751 con_arity = dataConSourceArity data_con
752 no_of_args = length arg_pats
754 tcConArgs data_con arg_tys (InfixCon p1 p2) pstate thing_inside
755 = do { checkTc (con_arity == 2) -- Check correct arity
756 (arityErr "Constructor" data_con con_arity 2)
757 ; let [arg_ty1,arg_ty2] = arg_tys -- This can't fail after the arity check
758 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
760 ; return (InfixCon p1' p2', tvs, res) }
762 con_arity = dataConSourceArity data_con
764 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
765 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
767 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
768 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
769 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
771 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
772 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
773 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
774 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
775 ; return (HsRecField sel_id pat' pun, tvs, res) }
777 find_field_ty :: FieldLabel -> TcM (Id, TcType)
778 find_field_ty field_lbl
779 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
781 -- No matching field; chances are this field label comes from some
782 -- other record type (or maybe none). As well as reporting an
783 -- error we still want to typecheck the pattern, principally to
784 -- make sure that all the variables it binds are put into the
785 -- environment, else the type checker crashes later:
786 -- f (R { foo = (a,b) }) = a+b
787 -- If foo isn't one of R's fields, we don't want to crash when
788 -- typechecking the "a+b".
789 [] -> do { addErrTc (badFieldCon data_con field_lbl)
790 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
791 ; return (error "Bogus selector Id", bogus_ty) }
793 -- The normal case, when the field comes from the right constructor
795 ASSERT( null extras )
796 do { sel_id <- tcLookupField field_lbl
797 ; return (sel_id, pat_ty) }
799 field_tys :: [(FieldLabel, TcType)]
800 field_tys = zip (dataConFieldLabels data_con) arg_tys
801 -- Don't use zipEqual! If the constructor isn't really a record, then
802 -- dataConFieldLabels will be empty (and each field in the pattern
803 -- will generate an error below).
805 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
806 tcConArg (arg_pat, arg_ty) pstate thing_inside
807 = tc_lpat arg_pat arg_ty pstate thing_inside
808 -- NB: the tc_lpat will refine pat_ty if necessary
809 -- based on the current pstate, which may include
810 -- refinements from peer argument patterns to the left
814 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
815 -- Instantiate the "stupid theta" of the data con, and throw
816 -- the constraints into the constraint set
817 addDataConStupidTheta data_con inst_tys
818 | null stupid_theta = return ()
819 | otherwise = instStupidTheta origin inst_theta
821 origin = OccurrenceOf (dataConName data_con)
822 -- The origin should always report "occurrence of C"
823 -- even when C occurs in a pattern
824 stupid_theta = dataConStupidTheta data_con
825 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
826 inst_theta = substTheta tenv stupid_theta
829 Note [Arrows and patterns]
830 ~~~~~~~~~~~~~~~~~~~~~~~~~~
831 (Oct 07) Arrow noation has the odd property that it involves "holes in the scope".
833 expr :: Arrow a => a () Int
834 expr = proc (y,z) -> do
838 Here the 'proc (y,z)' binding scopes over the arrow tails but not the
839 arrow body (e.g 'term'). As things stand (bogusly) all the
840 constraints from the proc body are gathered together, so constraints
841 from 'term' will be seen by the tcPat for (y,z). But we must *not*
842 bind constraints from 'term' here, becuase the desugarer will not make
843 these bindings scope over 'term'.
845 The Right Thing is not to confuse these constraints together. But for
846 now the Easy Thing is to ensure that we do not have existential or
847 GADT constraints in a 'proc', and to short-cut the constraint
848 simplification for such vanilla patterns so that it binds no
849 constraints. Hence the 'fast path' in tcConPat; but it's also a good
850 plan for ordinary vanilla patterns to bypass the constraint
854 %************************************************************************
858 %************************************************************************
861 refineAlt :: DataCon -- For tracing only
863 -> [TcTyVar] -- Existentials
864 -> [CoVar] -- Equational constraints
865 -> BoxySigmaType -- Pattern type
868 refineAlt con pstate ex_tvs [] pat_ty
869 | null $ dataConEqTheta con
870 = return pstate -- Common case: no equational constraints
872 refineAlt con pstate ex_tvs co_vars pat_ty
873 = -- See Note [Flags and equational constraints]
874 do { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
875 -- We are matching against a GADT constructor with non-trivial
876 -- constraints, but pattern type is wobbly. For now we fail.
877 -- We can make sense of this, however:
878 -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
879 -- (\x -> case x of { MkT v -> v })
880 -- We can infer that x must have type T [c], for some wobbly 'c'
882 -- (\(x::T [c]) -> case x of
883 -- MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
884 -- To implement this, we'd first instantiate the equational
885 -- constraints with *wobbly* type variables for the existentials;
886 -- then unify these constraints to make pat_ty the right shape;
887 -- then proceed exactly as in the rigid case
889 -- In the rigid case, we perform type refinement
890 ; case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
891 Failed msg -> failWithTc (inaccessibleAlt msg) ;
892 Succeeded reft -> do { traceTc trace_msg
893 ; return (pstate { pat_reft = reft, pat_eqs = (pat_eqs pstate || not (null $ dataConEqTheta con)) }) }
894 -- DO NOT refine the envt right away, because we
895 -- might be inside a lazy pattern. Instead, refine pstate
898 trace_msg = text "refineAlt:match" <+>
899 vcat [ ppr con <+> ppr ex_tvs,
900 ppr [(v, tyVarKind v) | v <- co_vars],
905 Note [Flags and equational constraints]
906 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
907 If there are equational constraints, we take account of them
908 regardless of flag settings; -XGADTs etc applies only to the
909 *definition* of a data type.
911 An alternative would be also to reject a program that *used*
912 constructors with equational constraints. But want we should avoid at
913 all costs is simply to *ignore* the constraints, since that gives
914 incomprehensible errors (Trac #2004).
917 %************************************************************************
921 %************************************************************************
923 In tcOverloadedLit we convert directly to an Int or Integer if we
924 know that's what we want. This may save some time, by not
925 temporarily generating overloaded literals, but it won't catch all
926 cases (the rest are caught in lookupInst).
929 tcOverloadedLit :: InstOrigin
932 -> TcM (HsOverLit TcId)
933 tcOverloadedLit orig lit@(HsIntegral i fi _) res_ty
934 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
935 -- Reason: If we do, tcSimplify will call lookupInst, which
936 -- will call tcSyntaxName, which does unification,
937 -- which tcSimplify doesn't like
938 -- ToDo: noLoc sadness
939 = do { integer_ty <- tcMetaTy integerTyConName
940 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
941 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty))) res_ty) }
943 | Just expr <- shortCutIntLit i res_ty
944 = return (HsIntegral i expr res_ty)
947 = do { expr <- newLitInst orig lit res_ty
948 ; return (HsIntegral i expr res_ty) }
950 tcOverloadedLit orig lit@(HsFractional r fr _) res_ty
951 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
952 = do { rat_ty <- tcMetaTy rationalTyConName
953 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
954 -- Overloaded literals must have liftedTypeKind, because
955 -- we're instantiating an overloaded function here,
956 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
957 -- However this'll be picked up by tcSyntaxOp if necessary
958 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty))) res_ty) }
960 | Just expr <- shortCutFracLit r res_ty
961 = return (HsFractional r expr res_ty)
964 = do { expr <- newLitInst orig lit res_ty
965 ; return (HsFractional r expr res_ty) }
967 tcOverloadedLit orig lit@(HsIsString s fr _) res_ty
968 | not (fr `isHsVar` fromStringName) -- c.f. HsIntegral case
969 = do { str_ty <- tcMetaTy stringTyConName
970 ; fr' <- tcSyntaxOp orig fr (mkFunTy str_ty res_ty)
971 ; return (HsIsString s (HsApp (noLoc fr') (nlHsLit (HsString s))) res_ty) }
973 | Just expr <- shortCutStringLit s res_ty
974 = return (HsIsString s expr res_ty)
977 = do { expr <- newLitInst orig lit res_ty
978 ; return (HsIsString s expr res_ty) }
980 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
981 newLitInst orig lit res_ty -- Make a LitInst
982 = do { loc <- getInstLoc orig
983 ; res_tau <- zapToMonotype res_ty
984 ; new_uniq <- newUnique
985 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
986 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
987 tci_ty = res_tau, tci_loc = loc}
989 ; return (HsVar (instToId lit_inst)) }
993 %************************************************************************
995 Note [Pattern coercions]
997 %************************************************************************
999 In principle, these program would be reasonable:
1001 f :: (forall a. a->a) -> Int
1002 f (x :: Int->Int) = x 3
1004 g :: (forall a. [a]) -> Bool
1007 In both cases, the function type signature restricts what arguments can be passed
1008 in a call (to polymorphic ones). The pattern type signature then instantiates this
1009 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
1010 generate the translated term
1011 f = \x' :: (forall a. a->a). let x = x' Int in x 3
1013 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
1014 And it requires a significant amount of code to implement, becuase we need to decorate
1015 the translated pattern with coercion functions (generated from the subsumption check
1018 So for now I'm just insisting on type *equality* in patterns. No subsumption.
1020 Old notes about desugaring, at a time when pattern coercions were handled:
1022 A SigPat is a type coercion and must be handled one at at time. We can't
1023 combine them unless the type of the pattern inside is identical, and we don't
1024 bother to check for that. For example:
1026 data T = T1 Int | T2 Bool
1027 f :: (forall a. a -> a) -> T -> t
1028 f (g::Int->Int) (T1 i) = T1 (g i)
1029 f (g::Bool->Bool) (T2 b) = T2 (g b)
1031 We desugar this as follows:
1033 f = \ g::(forall a. a->a) t::T ->
1035 in case t of { T1 i -> T1 (gi i)
1038 in case t of { T2 b -> T2 (gb b)
1041 Note that we do not treat the first column of patterns as a
1042 column of variables, because the coerced variables (gi, gb)
1043 would be of different types. So we get rather grotty code.
1044 But I don't think this is a common case, and if it was we could
1045 doubtless improve it.
1047 Meanwhile, the strategy is:
1048 * treat each SigPat coercion (always non-identity coercions)
1050 * deal with the stuff inside, and then wrap a binding round
1051 the result to bind the new variable (gi, gb, etc)
1054 %************************************************************************
1056 \subsection{Errors and contexts}
1058 %************************************************************************
1061 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
1062 patCtxt (VarPat _) = Nothing
1063 patCtxt (ParPat _) = Nothing
1064 patCtxt (AsPat _ _) = Nothing
1065 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
1068 -----------------------------------------------
1070 existentialExplode pat
1071 = hang (vcat [text "My brain just exploded.",
1072 text "I can't handle pattern bindings for existentially-quantified constructors.",
1073 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
1074 text "In the binding group for"])
1077 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
1078 = do { pat_tys' <- mapM zonkTcType pat_tys
1079 ; body_ty' <- zonkTcType body_ty
1080 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1081 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
1082 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
1084 sep [ptext SLIT("When checking an existential match that binds"),
1085 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
1086 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
1087 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
1090 bound_ids = collectPatsBinders pats
1091 show_ids = filter is_interesting bound_ids
1092 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
1094 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1095 -- Don't zonk the types so we get the separate, un-unified versions
1097 badFieldCon :: DataCon -> Name -> SDoc
1098 badFieldCon con field
1099 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
1100 ptext SLIT("does not have field"), quotes (ppr field)]
1102 polyPatSig :: TcType -> SDoc
1104 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
1107 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
1109 existentialProcPat :: DataCon -> SDoc
1110 existentialProcPat con
1111 = hang (ptext SLIT("Illegal constructor") <+> quotes (ppr con) <+> ptext SLIT("in a 'proc' pattern"))
1112 2 (ptext SLIT("Proc patterns cannot use existentials or GADTs"))
1116 hang (ptext SLIT("A lazy (~) pattern cannot bind existential type variables"))
1117 2 (vcat (map pprSkolTvBinding tvs))
1120 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
1121 2 (ptext SLIT("Solution: add a type signature"))
1123 nonRigidResult res_ty
1124 = hang (ptext SLIT("GADT pattern match with non-rigid result type") <+> quotes (ppr res_ty))
1125 2 (ptext SLIT("Solution: add a type signature"))
1128 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg