2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcPat: Typechecking patterns
9 module TcPat ( tcLetPat, tcLamPat, tcLamPats, tcOverloadedLit,
10 addDataConStupidTheta, badFieldCon, polyPatSig ) where
12 #include "HsVersions.h"
14 import {-# SOURCE #-} TcExpr( tcSyntaxOp )
39 import BasicTypes hiding (SuccessFlag(..))
49 %************************************************************************
53 %************************************************************************
56 tcLetPat :: (Name -> Maybe TcRhoType)
57 -> LPat Name -> BoxySigmaType
60 tcLetPat sig_fn pat pat_ty thing_inside
61 = do { let init_state = PS { pat_ctxt = LetPat sig_fn,
62 pat_reft = emptyRefinement }
63 ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state (\ _ -> thing_inside)
65 -- Don't know how to deal with pattern-bound existentials yet
66 ; checkTc (null ex_tvs) (existentialExplode pat)
68 ; return (pat', res) }
71 tcLamPats :: [LPat Name] -- Patterns,
72 -> [BoxySigmaType] -- and their types
73 -> BoxyRhoType -- Result type,
74 -> ((Refinement, BoxyRhoType) -> TcM a) -- and the checker for the body
75 -> TcM ([LPat TcId], a)
77 -- This is the externally-callable wrapper function
78 -- Typecheck the patterns, extend the environment to bind the variables,
79 -- do the thing inside, use any existentially-bound dictionaries to
80 -- discharge parts of the returning LIE, and deal with pattern type
83 -- 1. Initialise the PatState
84 -- 2. Check the patterns
85 -- 3. Apply the refinement to the environment and result type
87 -- 5. Check that no existentials escape
89 tcLamPats pats tys res_ty thing_inside
90 = tc_lam_pats (zipEqual "tcLamPats" pats tys)
91 (emptyRefinement, res_ty) thing_inside
93 tcLamPat :: LPat Name -> BoxySigmaType
94 -> (Refinement,BoxyRhoType) -- Result type
95 -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
97 tcLamPat pat pat_ty res_ty thing_inside
98 = do { ([pat'],thing) <- tc_lam_pats [(pat, pat_ty)] res_ty thing_inside
99 ; return (pat', thing) }
102 tc_lam_pats :: [(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)
106 tc_lam_pats pat_ty_prs (reft, res_ty) thing_inside
107 = do { let init_state = PS { pat_ctxt = LamPat, pat_reft = reft }
109 ; (pats', ex_tvs, res) <- tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
110 refineEnvironment (pat_reft pstate') $
111 thing_inside (pat_reft pstate', res_ty)
113 ; let tys = map snd pat_ty_prs
114 ; tcCheckExistentialPat pats' ex_tvs tys res_ty
116 ; returnM (pats', res) }
120 tcCheckExistentialPat :: [LPat TcId] -- Patterns (just for error message)
121 -> [TcTyVar] -- Existentially quantified tyvars bound by pattern
122 -> [BoxySigmaType] -- Types of the patterns
123 -> BoxyRhoType -- Type of the body of the match
124 -- Tyvars in either of these must not escape
126 -- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
127 -- For example, we must reject this program:
128 -- data C = forall a. C (a -> Int)
130 -- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).
132 tcCheckExistentialPat pats [] pat_tys body_ty
133 = return () -- Short cut for case when there are no existentials
135 tcCheckExistentialPat pats ex_tvs pat_tys body_ty
136 = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
137 checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
141 pat_reft :: Refinement -- Binds rigid TcTyVars to their refinements
146 | LetPat (Name -> Maybe TcRhoType) -- Used for let(rec) bindings
148 patSigCtxt :: PatState -> UserTypeCtxt
149 patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
150 patSigCtxt other = LamPatSigCtxt
155 %************************************************************************
159 %************************************************************************
162 tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
163 tcPatBndr (PS { pat_ctxt = LamPat }) bndr_name pat_ty
164 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
165 -- We have an undecorated binder, so we do rule ABS1,
166 -- by unboxing the boxy type, forcing any un-filled-in
167 -- boxes to become monotypes
168 -- NB that pat_ty' can still be a polytype:
169 -- data T = MkT (forall a. a->a)
170 -- f t = case t of { MkT g -> ... }
171 -- Here, the 'g' must get type (forall a. a->a) from the
173 ; return (Id.mkLocalId bndr_name pat_ty') }
175 tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
176 | Just mono_ty <- lookup_sig bndr_name
177 = do { mono_name <- newLocalName bndr_name
178 ; boxyUnify mono_ty pat_ty
179 ; return (Id.mkLocalId mono_name mono_ty) }
182 = do { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
183 ; mono_name <- newLocalName bndr_name
184 ; return (Id.mkLocalId mono_name pat_ty') }
188 bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
189 bindInstsOfPatId id thing_inside
190 | not (isOverloadedTy (idType id))
191 = do { res <- thing_inside; return (res, emptyLHsBinds) }
193 = do { (res, lie) <- getLIE thing_inside
194 ; binds <- bindInstsOfLocalFuns lie [id]
195 ; return (res, binds) }
198 unBoxPatBndrType ty name = unBoxArgType ty (ptext SLIT("The variable") <+> quotes (ppr name))
199 unBoxWildCardType ty = unBoxArgType ty (ptext SLIT("A wild-card pattern"))
201 unBoxArgType :: BoxyType -> SDoc -> TcM TcType
202 -- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind;
203 -- that is, it can't be an unboxed tuple. For example,
204 -- case (f x) of r -> ...
205 -- should fail if 'f' returns an unboxed tuple.
206 unBoxArgType ty pp_this
207 = do { ty' <- unBox ty -- Returns a zonked type
209 -- Neither conditional is strictly necesssary (the unify alone will do)
210 -- but they improve error messages, and allocate fewer tyvars
211 ; if isUnboxedTupleType ty' then
213 else if isSubArgTypeKind (typeKind ty') then
215 else do -- OpenTypeKind, so constrain it
216 { ty2 <- newFlexiTyVarTy argTypeKind
220 msg = pp_this <+> ptext SLIT("cannot be bound to an unboxed tuple")
224 %************************************************************************
226 The main worker functions
228 %************************************************************************
232 tcPat takes a "thing inside" over which the pattern scopes. This is partly
233 so that tcPat can extend the environment for the thing_inside, but also
234 so that constraints arising in the thing_inside can be discharged by the
237 This does not work so well for the ErrCtxt carried by the monad: we don't
238 want the error-context for the pattern to scope over the RHS.
239 Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.
243 type Checker inp out = forall r.
246 -> (PatState -> TcM r)
247 -> TcM (out, [TcTyVar], r)
249 tcMultiple :: Checker inp out -> Checker [inp] [out]
250 tcMultiple tc_pat args pstate thing_inside
251 = do { err_ctxt <- getErrCtxt
253 = do { res <- thing_inside pstate
254 ; return ([], [], res) }
256 loop pstate (arg:args)
257 = do { (p', p_tvs, (ps', ps_tvs, res))
258 <- tc_pat arg pstate $ \ pstate' ->
259 setErrCtxt err_ctxt $
261 -- setErrCtxt: restore context before doing the next pattern
262 -- See note [Nesting] above
264 ; return (p':ps', p_tvs ++ ps_tvs, res) }
269 tc_lpat_pr :: (LPat Name, BoxySigmaType)
271 -> (PatState -> TcM a)
272 -> TcM (LPat TcId, [TcTyVar], a)
273 tc_lpat_pr (pat, ty) = tc_lpat pat ty
278 -> (PatState -> TcM a)
279 -> TcM (LPat TcId, [TcTyVar], a)
280 tc_lpat (L span pat) pat_ty pstate thing_inside
282 maybeAddErrCtxt (patCtxt pat) $
283 do { let mb_reft = refineType (pat_reft pstate) pat_ty
284 pat_ty' = case mb_reft of { Just (_, ty') -> ty'; Nothing -> pat_ty }
286 -- Make sure the result type reflects the current refinement
287 -- We must do this here, so that it correctly ``sees'' all
288 -- the refinements to the left. Example:
289 -- Suppose C :: forall a. T a -> a -> Foo
290 -- Pattern C a p1 True
291 -- So p1 might refine 'a' to True, and the True
292 -- pattern had better see it.
294 ; (pat', tvs, res) <- tc_pat pstate pat pat_ty' thing_inside
295 ; let final_pat = case mb_reft of
297 Just (co,_) -> CoPat (WpCo co) pat' pat_ty
298 ; return (L span final_pat, tvs, res) }
302 -> Pat Name -> BoxySigmaType -- Fully refined result type
303 -> (PatState -> TcM a) -- Thing inside
304 -> TcM (Pat TcId, -- Translated pattern
305 [TcTyVar], -- Existential binders
306 a) -- Result of thing inside
308 tc_pat pstate (VarPat name) pat_ty thing_inside
309 = do { id <- tcPatBndr pstate name pat_ty
310 ; (res, binds) <- bindInstsOfPatId id $
311 tcExtendIdEnv1 name id $
312 (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
313 >> thing_inside pstate)
314 ; let pat' | isEmptyLHsBinds binds = VarPat id
315 | otherwise = VarPatOut id binds
316 ; return (pat', [], res) }
318 tc_pat pstate (ParPat pat) pat_ty thing_inside
319 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
320 ; return (ParPat pat', tvs, res) }
322 tc_pat pstate (BangPat pat) pat_ty thing_inside
323 = do { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
324 ; return (BangPat pat', tvs, res) }
326 -- There's a wrinkle with irrefutable patterns, namely that we
327 -- must not propagate type refinement from them. For example
328 -- data T a where { T1 :: Int -> T Int; ... }
329 -- f :: T a -> Int -> a
331 -- It's obviously not sound to refine a to Int in the right
332 -- hand side, because the arugment might not match T1 at all!
334 -- Nor should a lazy pattern bind any existential type variables
335 -- because they won't be in scope when we do the desugaring
337 -- Note [Hopping the LIE in lazy patterns]
338 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
339 -- In a lazy pattern, we must *not* discharge constraints from the RHS
340 -- from dictionaries bound in the pattern. E.g.
342 -- We can't discharge the Num constraint from dictionaries bound by
345 -- So we have to make the constraints from thing_inside "hop around"
346 -- the pattern. Hence the getLLE and extendLIEs later.
348 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
349 = do { (pat', pat_tvs, (res,lie))
350 <- tc_lpat pat pat_ty pstate $ \ _ ->
351 getLIE (thing_inside pstate)
352 -- Ignore refined pstate', revert to pstate
354 -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
356 -- Check no existentials
357 ; if (null pat_tvs) then return ()
358 else lazyPatErr lpat pat_tvs
360 -- Check that the pattern has a lifted type
361 ; pat_tv <- newBoxyTyVar liftedTypeKind
362 ; boxyUnify pat_ty (mkTyVarTy pat_tv)
364 ; return (LazyPat pat', [], res) }
366 tc_pat pstate (WildPat _) pat_ty thing_inside
367 = do { pat_ty' <- unBoxWildCardType pat_ty -- Make sure it's filled in with monotypes
368 ; res <- thing_inside pstate
369 ; return (WildPat pat_ty', [], res) }
371 tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
372 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
373 ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
374 tc_lpat pat (idType bndr_id) pstate thing_inside
375 -- NB: if we do inference on:
376 -- \ (y@(x::forall a. a->a)) = e
377 -- we'll fail. The as-pattern infers a monotype for 'y', which then
378 -- fails to unify with the polymorphic type for 'x'. This could
379 -- perhaps be fixed, but only with a bit more work.
381 -- If you fix it, don't forget the bindInstsOfPatIds!
382 ; return (AsPat (L nm_loc bndr_id) pat', tvs, res) }
384 -- Type signatures in patterns
385 -- See Note [Pattern coercions] below
386 tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
387 = do { (inner_ty, tv_binds) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
388 ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
389 tc_lpat pat inner_ty pstate thing_inside
390 ; return (SigPatOut pat' inner_ty, tvs, res) }
392 tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
393 = failWithTc (badTypePat pat)
395 ------------------------
396 -- Lists, tuples, arrays
397 tc_pat pstate (ListPat pats _) pat_ty thing_inside
398 = do { elt_ty <- boxySplitListTy pat_ty
399 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
400 pats pstate thing_inside
401 ; return (ListPat pats' elt_ty, pats_tvs, res) }
403 tc_pat pstate (PArrPat pats _) pat_ty thing_inside
404 = do { [elt_ty] <- boxySplitTyConApp parrTyCon pat_ty
405 ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
406 pats pstate thing_inside
407 ; ifM (null pats) (zapToMonotype pat_ty) -- c.f. ExplicitPArr in TcExpr
408 ; return (PArrPat pats' elt_ty, pats_tvs, res) }
410 tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
411 = do { arg_tys <- boxySplitTyConApp (tupleTyCon boxity (length pats)) pat_ty
412 ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
415 -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
416 -- so that we can experiment with lazy tuple-matching.
417 -- This is a pretty odd place to make the switch, but
418 -- it was easy to do.
419 ; let unmangled_result = TuplePat pats' boxity pat_ty
420 possibly_mangled_result
421 | opt_IrrefutableTuples && isBoxed boxity = LazyPat (noLoc unmangled_result)
422 | otherwise = unmangled_result
424 ; ASSERT( length arg_tys == length pats ) -- Syntactically enforced
425 return (possibly_mangled_result, pats_tvs, res) }
427 ------------------------
429 tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
430 = do { data_con <- tcLookupDataCon con_name
431 ; let tycon = dataConTyCon data_con
432 ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }
434 ------------------------
436 tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
437 = do { boxyUnify (hsLitType simple_lit) pat_ty
438 ; res <- thing_inside pstate
439 ; returnM (LitPat simple_lit, [], res) }
441 ------------------------
442 -- Overloaded patterns: n, and n+k
443 tc_pat pstate pat@(NPat over_lit mb_neg eq _) pat_ty thing_inside
444 = do { let orig = LiteralOrigin over_lit
445 ; lit' <- tcOverloadedLit orig over_lit pat_ty
446 ; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
447 ; mb_neg' <- case mb_neg of
448 Nothing -> return Nothing -- Positive literal
449 Just neg -> -- Negative literal
450 -- The 'negate' is re-mappable syntax
451 do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
452 ; return (Just neg') }
453 ; res <- thing_inside pstate
454 ; returnM (NPat lit' mb_neg' eq' pat_ty, [], res) }
456 tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
457 = do { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
458 ; let pat_ty' = idType bndr_id
459 orig = LiteralOrigin lit
460 ; lit' <- tcOverloadedLit orig lit pat_ty'
462 -- The '>=' and '-' parts are re-mappable syntax
463 ; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
464 ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
466 -- The Report says that n+k patterns must be in Integral
467 -- We may not want this when using re-mappable syntax, though (ToDo?)
468 ; icls <- tcLookupClass integralClassName
469 ; instStupidTheta orig [mkClassPred icls [pat_ty']]
471 ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
472 ; returnM (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }
474 tc_pat _ _other_pat _ _ = panic "tc_pat" -- ConPatOut, SigPatOut, VarPatOut
478 %************************************************************************
480 Most of the work for constructors is here
481 (the rest is in the ConPatIn case of tc_pat)
483 %************************************************************************
485 [Pattern matching indexed data types]
486 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
487 Consider the following declarations:
489 data family Map k :: * -> *
490 data instance Map (a, b) v = MapPair (Map a (Pair b v))
492 and a case expression
494 case x :: Map (Int, c) w of MapPair m -> ...
496 As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
497 worker/wrapper types for MapPair are
499 $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
500 $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
502 So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
503 :R123Map, which means the straight use of boxySplitTyConApp would give a type
504 error. Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
505 boxySplitTyConApp with the family tycon Map instead, which gives us the family
506 type list {(Int, c), w}. To get the correct split for :R123Map, we need to
507 unify the family type list {(Int, c), w} with the instance types {(a, b), v}
508 (provided by tyConFamInst_maybe together with the family tycon). This
509 unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
510 the split arguments for the representation tycon :R123Map as {Int, c, w}
512 In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
514 Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}
516 moving between representation and family type into account. To produce type
517 correct Core, this coercion needs to be used to case the type of the scrutinee
518 from the family to the representation type. This is achieved by
519 unwrapFamInstScrutinee using a CoPat around the result pattern.
521 Now it might appear seem as if we could have used the existing GADT type
522 refinement infrastructure of refineAlt and friends instead of the explicit
523 unification and CoPat generation. However, that would be wrong. Why? The
524 whole point of GADT refinement is that the refinement is local to the case
525 alternative. In contrast, the substitution generated by the unification of
526 the family type list and instance types needs to be propagated to the outside.
527 Imagine that in the above example, the type of the scrutinee would have been
528 (Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
529 substitution [x -> (a, b), v -> w]. In contrast to GADT matching, the
530 instantiation of x with (a, b) must be global; ie, it must be valid in *all*
531 alternatives of the case expression, whereas in the GADT case it might vary
532 between alternatives.
534 In fact, if we have a data instance declaration defining a GADT, eq_spec will
535 be non-empty and we will get a mixture of global instantiations and local
536 refinement from a single match. This neatly reflects that, as soon as we
537 have constrained the type of the scrutinee to the required type index, all
538 further type refinement is local to the alternative.
542 -- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
543 -- with scrutinee of type (T ty)
545 tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon
546 -> BoxySigmaType -- Type of the pattern
547 -> HsConPatDetails Name -> (PatState -> TcM a)
548 -> TcM (Pat TcId, [TcTyVar], a)
549 tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
550 = do { let (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _) = dataConFullSig data_con
551 skol_info = PatSkol data_con
552 origin = SigOrigin skol_info
554 -- Instantiate the constructor type variables [a->ty]
555 ; ctxt_res_tys <- boxySplitTyConAppWithFamily tycon pat_ty
556 ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs -- Get location from monad,
558 ; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
559 (ctxt_res_tys ++ mkTyVarTys ex_tvs')
560 eq_spec' = substEqSpec tenv eq_spec
561 theta' = substTheta tenv theta
562 arg_tys' = substTys tenv arg_tys
564 ; co_vars <- newCoVars eq_spec' -- Make coercion variables
565 ; pstate' <- refineAlt data_con pstate ex_tvs' co_vars pat_ty
567 ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
568 tcConArgs data_con arg_tys' arg_pats pstate' thing_inside
570 ; loc <- getInstLoc origin
571 ; dicts <- newDictBndrs loc theta'
572 ; dict_binds <- tcSimplifyCheckPat loc co_vars (pat_reft pstate')
573 ex_tvs' dicts lie_req
575 ; addDataConStupidTheta data_con ctxt_res_tys
578 (unwrapFamInstScrutinee tycon ctxt_res_tys $
579 ConPatOut { pat_con = L con_span data_con,
580 pat_tvs = ex_tvs' ++ co_vars,
581 pat_dicts = map instToId dicts,
582 pat_binds = dict_binds,
583 pat_args = arg_pats', pat_ty = pat_ty },
584 ex_tvs' ++ inner_tvs, res)
587 -- Split against the family tycon if the pattern constructor
588 -- belongs to a family instance tycon.
589 boxySplitTyConAppWithFamily tycon pat_ty =
591 case tyConFamInst_maybe tycon of
592 Nothing -> boxySplitTyConApp tycon pat_ty
593 Just (fam_tycon, instTys) ->
594 do { scrutinee_arg_tys <- boxySplitTyConApp fam_tycon pat_ty
595 ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
596 ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
600 traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
601 ppr tycon <+> ppr pat_ty
602 , text " family instance:" <+>
603 ppr (tyConFamInst_maybe tycon)
606 -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
607 -- pattern if the tycon is an instance of a family.
609 unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
610 unwrapFamInstScrutinee tycon args pat
611 | Just co_con <- tyConFamilyCoercion_maybe tycon
612 -- , not (isNewTyCon tycon) -- newtypes are explicitly unwrapped by
614 -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
615 -- coercion is not the identity; mkCoPat is inconvenient as it
616 -- wants a located pattern.
617 = CoPat (WpCo $ mkTyConApp co_con args) -- co fam ty to repr ty
618 (pat {pat_ty = mkTyConApp tycon args}) -- representation type
619 pat_ty -- family inst type
624 tcConArgs :: DataCon -> [TcSigmaType]
625 -> Checker (HsConPatDetails Name) (HsConPatDetails Id)
627 tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
628 = do { checkTc (con_arity == no_of_args) -- Check correct arity
629 (arityErr "Constructor" data_con con_arity no_of_args)
630 ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
631 ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
633 ; return (PrefixCon arg_pats', tvs, res) }
635 con_arity = dataConSourceArity data_con
636 no_of_args = length arg_pats
638 tcConArgs data_con [arg_ty1,arg_ty2] (InfixCon p1 p2) pstate thing_inside
639 = do { checkTc (con_arity == 2) -- Check correct arity
640 (arityErr "Constructor" data_con con_arity 2)
641 ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
643 ; return (InfixCon p1' p2', tvs, res) }
645 con_arity = dataConSourceArity data_con
647 tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
648 = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments
650 tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) pstate thing_inside
651 = do { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
652 ; return (RecCon (HsRecFields rpats' dd), tvs, res) }
654 tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
655 tc_field (HsRecField field_lbl pat pun) pstate thing_inside
656 = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
657 ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
658 ; return (HsRecField sel_id pat' pun, tvs, res) }
660 find_field_ty :: FieldLabel -> TcM (Id, TcType)
661 find_field_ty field_lbl
662 = case [ty | (f,ty) <- field_tys, f == field_lbl] of
664 -- No matching field; chances are this field label comes from some
665 -- other record type (or maybe none). As well as reporting an
666 -- error we still want to typecheck the pattern, principally to
667 -- make sure that all the variables it binds are put into the
668 -- environment, else the type checker crashes later:
669 -- f (R { foo = (a,b) }) = a+b
670 -- If foo isn't one of R's fields, we don't want to crash when
671 -- typechecking the "a+b".
672 [] -> do { addErrTc (badFieldCon data_con field_lbl)
673 ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
674 ; return (error "Bogus selector Id", bogus_ty) }
676 -- The normal case, when the field comes from the right constructor
678 ASSERT( null extras )
679 do { sel_id <- tcLookupField field_lbl
680 ; return (sel_id, pat_ty) }
682 field_tys :: [(FieldLabel, TcType)]
683 field_tys = zip (dataConFieldLabels data_con) arg_tys
684 -- Don't use zipEqual! If the constructor isn't really a record, then
685 -- dataConFieldLabels will be empty (and each field in the pattern
686 -- will generate an error below).
688 tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
689 tcConArg (arg_pat, arg_ty) pstate thing_inside
690 = tc_lpat arg_pat arg_ty pstate thing_inside
691 -- NB: the tc_lpat will refine pat_ty if necessary
692 -- based on the current pstate, which may include
693 -- refinements from peer argument patterns to the left
697 addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
698 -- Instantiate the "stupid theta" of the data con, and throw
699 -- the constraints into the constraint set
700 addDataConStupidTheta data_con inst_tys
701 | null stupid_theta = return ()
702 | otherwise = instStupidTheta origin inst_theta
704 origin = OccurrenceOf (dataConName data_con)
705 -- The origin should always report "occurrence of C"
706 -- even when C occurs in a pattern
707 stupid_theta = dataConStupidTheta data_con
708 tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
709 inst_theta = substTheta tenv stupid_theta
713 %************************************************************************
717 %************************************************************************
720 refineAlt :: DataCon -- For tracing only
722 -> [TcTyVar] -- Existentials
723 -> [CoVar] -- Equational constraints
724 -> BoxySigmaType -- Pattern type
727 refineAlt con pstate ex_tvs [] pat_ty
728 = return pstate -- Common case: no equational constraints
730 refineAlt con pstate ex_tvs co_vars pat_ty
731 = do { opt_gadt <- doptM Opt_GADTs -- No type-refinement unless GADTs are on
732 ; if (not opt_gadt) then return pstate
735 { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
736 -- We are matching against a GADT constructor with non-trivial
737 -- constraints, but pattern type is wobbly. For now we fail.
738 -- We can make sense of this, however:
739 -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
740 -- (\x -> case x of { MkT v -> v })
741 -- We can infer that x must have type T [c], for some wobbly 'c'
743 -- (\(x::T [c]) -> case x of
744 -- MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
745 -- To implement this, we'd first instantiate the equational
746 -- constraints with *wobbly* type variables for the existentials;
747 -- then unify these constraints to make pat_ty the right shape;
748 -- then proceed exactly as in the rigid case
750 -- In the rigid case, we perform type refinement
751 ; case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
752 Failed msg -> failWithTc (inaccessibleAlt msg) ;
753 Succeeded reft -> do { traceTc trace_msg
754 ; return (pstate { pat_reft = reft }) }
755 -- DO NOT refine the envt right away, because we
756 -- might be inside a lazy pattern. Instead, refine pstate
759 trace_msg = text "refineAlt:match" <+>
760 vcat [ ppr con <+> ppr ex_tvs,
761 ppr [(v, tyVarKind v) | v <- co_vars],
767 %************************************************************************
771 %************************************************************************
773 In tcOverloadedLit we convert directly to an Int or Integer if we
774 know that's what we want. This may save some time, by not
775 temporarily generating overloaded literals, but it won't catch all
776 cases (the rest are caught in lookupInst).
779 tcOverloadedLit :: InstOrigin
782 -> TcM (HsOverLit TcId)
783 tcOverloadedLit orig lit@(HsIntegral i fi) res_ty
784 | not (fi `isHsVar` fromIntegerName) -- Do not generate a LitInst for rebindable syntax.
785 -- Reason: If we do, tcSimplify will call lookupInst, which
786 -- will call tcSyntaxName, which does unification,
787 -- which tcSimplify doesn't like
788 -- ToDo: noLoc sadness
789 = do { integer_ty <- tcMetaTy integerTyConName
790 ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
791 ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty)))) }
793 | Just expr <- shortCutIntLit i res_ty
794 = return (HsIntegral i expr)
797 = do { expr <- newLitInst orig lit res_ty
798 ; return (HsIntegral i expr) }
800 tcOverloadedLit orig lit@(HsFractional r fr) res_ty
801 | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
802 = do { rat_ty <- tcMetaTy rationalTyConName
803 ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
804 -- Overloaded literals must have liftedTypeKind, because
805 -- we're instantiating an overloaded function here,
806 -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
807 -- However this'll be picked up by tcSyntaxOp if necessary
808 ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty)))) }
810 | Just expr <- shortCutFracLit r res_ty
811 = return (HsFractional r expr)
814 = do { expr <- newLitInst orig lit res_ty
815 ; return (HsFractional r expr) }
817 tcOverloadedLit orig lit@(HsIsString s fr) res_ty
818 | not (fr `isHsVar` fromStringName) -- c.f. HsIntegral case
819 = do { str_ty <- tcMetaTy stringTyConName
820 ; fr' <- tcSyntaxOp orig fr (mkFunTy str_ty res_ty)
821 ; return (HsIsString s (HsApp (noLoc fr') (nlHsLit (HsString s)))) }
823 | Just expr <- shortCutStringLit s res_ty
824 = return (HsIsString s expr)
827 = do { expr <- newLitInst orig lit res_ty
828 ; return (HsIsString s expr) }
830 newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
831 newLitInst orig lit res_ty -- Make a LitInst
832 = do { loc <- getInstLoc orig
833 ; res_tau <- zapToMonotype res_ty
834 ; new_uniq <- newUnique
835 ; let lit_nm = mkSystemVarName new_uniq FSLIT("lit")
836 lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit,
837 tci_ty = res_tau, tci_loc = loc}
839 ; return (HsVar (instToId lit_inst)) }
843 %************************************************************************
845 Note [Pattern coercions]
847 %************************************************************************
849 In principle, these program would be reasonable:
851 f :: (forall a. a->a) -> Int
852 f (x :: Int->Int) = x 3
854 g :: (forall a. [a]) -> Bool
857 In both cases, the function type signature restricts what arguments can be passed
858 in a call (to polymorphic ones). The pattern type signature then instantiates this
859 type. For example, in the first case, (forall a. a->a) <= Int -> Int, and we
860 generate the translated term
861 f = \x' :: (forall a. a->a). let x = x' Int in x 3
863 From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
864 And it requires a significant amount of code to implement, becuase we need to decorate
865 the translated pattern with coercion functions (generated from the subsumption check
868 So for now I'm just insisting on type *equality* in patterns. No subsumption.
870 Old notes about desugaring, at a time when pattern coercions were handled:
872 A SigPat is a type coercion and must be handled one at at time. We can't
873 combine them unless the type of the pattern inside is identical, and we don't
874 bother to check for that. For example:
876 data T = T1 Int | T2 Bool
877 f :: (forall a. a -> a) -> T -> t
878 f (g::Int->Int) (T1 i) = T1 (g i)
879 f (g::Bool->Bool) (T2 b) = T2 (g b)
881 We desugar this as follows:
883 f = \ g::(forall a. a->a) t::T ->
885 in case t of { T1 i -> T1 (gi i)
888 in case t of { T2 b -> T2 (gb b)
891 Note that we do not treat the first column of patterns as a
892 column of variables, because the coerced variables (gi, gb)
893 would be of different types. So we get rather grotty code.
894 But I don't think this is a common case, and if it was we could
895 doubtless improve it.
897 Meanwhile, the strategy is:
898 * treat each SigPat coercion (always non-identity coercions)
900 * deal with the stuff inside, and then wrap a binding round
901 the result to bind the new variable (gi, gb, etc)
904 %************************************************************************
906 \subsection{Errors and contexts}
908 %************************************************************************
911 patCtxt :: Pat Name -> Maybe Message -- Not all patterns are worth pushing a context
912 patCtxt (VarPat _) = Nothing
913 patCtxt (ParPat _) = Nothing
914 patCtxt (AsPat _ _) = Nothing
915 patCtxt pat = Just (hang (ptext SLIT("In the pattern:"))
918 -----------------------------------------------
920 existentialExplode pat
921 = hang (vcat [text "My brain just exploded.",
922 text "I can't handle pattern bindings for existentially-quantified constructors.",
923 text "Instead, use a case-expression, or do-notation, to unpack the constructor.",
924 text "In the binding group for"])
927 sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
928 = do { pat_tys' <- mapM zonkTcType pat_tys
929 ; body_ty' <- zonkTcType body_ty
930 ; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
931 (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
932 (env3, tidy_body_ty) = tidyOpenType env2 body_ty'
934 sep [ptext SLIT("When checking an existential match that binds"),
935 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
936 ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
937 ptext SLIT("The body has type:") <+> ppr tidy_body_ty
940 bound_ids = collectPatsBinders pats
941 show_ids = filter is_interesting bound_ids
942 is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
944 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
945 -- Don't zonk the types so we get the separate, un-unified versions
947 badFieldCon :: DataCon -> Name -> SDoc
948 badFieldCon con field
949 = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
950 ptext SLIT("does not have field"), quotes (ppr field)]
952 polyPatSig :: TcType -> SDoc
954 = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
957 badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat
961 hang (ptext SLIT("A lazy (~) pattern cannot bind existential type variables"))
962 2 (vcat (map pprSkolTvBinding tvs))
965 = hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
966 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
969 = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg