; kind_signatures <- xoptM Opt_KindSignatures
; existential_ok <- xoptM Opt_ExistentialQuantification
; gadt_ok <- xoptM Opt_GADTs
+ ; gadtSyntax_ok <- xoptM Opt_GADTSyntax
; is_boot <- tcIsHsBoot -- Are we compiling an hs-boot file?
; let ex_ok = existential_ok || gadt_ok -- Data cons can have existential context
-- Check that we don't use GADT syntax in H98 world
- ; checkTc (gadt_ok || h98_syntax) (badGadtDecl tc_name)
+ ; checkTc (gadtSyntax_ok || h98_syntax) (badGadtDecl tc_name)
-- Check that we don't use kind signatures without Glasgow extensions
; checkTc (kind_signatures || isNothing mb_ksig) (badSigTyDecl tc_name)
-> TcM DataCon
tcConDecl unbox_strict existential_ok rep_tycon res_tmpl -- Data types
- (ConDecl {con_name =name, con_qvars = tvs, con_cxt = ctxt
+ con@(ConDecl {con_name = name, con_qvars = tvs, con_cxt = ctxt
, con_details = details, con_res = res_ty })
= addErrCtxt (dataConCtxt name) $
tcTyVarBndrs tvs $ \ tvs' -> do
{ ctxt' <- tcHsKindedContext ctxt
- ; checkTc (existential_ok || (null tvs && null (unLoc ctxt)))
+ ; checkTc (existential_ok || conRepresentibleWithH98Syntax con)
(badExistential name)
; (univ_tvs, ex_tvs, eq_preds, res_ty') <- tcResultType res_tmpl tvs' res_ty
; let
consUseH98Syntax _ = True
-- All constructors have same shape
+conRepresentibleWithH98Syntax :: ConDecl Name -> Bool
+conRepresentibleWithH98Syntax
+ (ConDecl {con_qvars = tvs, con_cxt = ctxt, con_res = ResTyH98 })
+ = null tvs && null (unLoc ctxt)
+conRepresentibleWithH98Syntax
+ (ConDecl {con_qvars = tvs, con_cxt = ctxt, con_res = ResTyGADT (L _ t) })
+ = null (unLoc ctxt) && f t (map (hsTyVarName . unLoc) tvs)
+ where -- Each type variable should be used exactly once in the
+ -- result type, and the result type must just be the type
+ -- constructor applied to type variables
+ f (HsAppTy (L _ t1) (L _ (HsTyVar v2))) vs
+ = (v2 `elem` vs) && f t1 (delete v2 vs)
+ f (HsTyVar _) [] = True
+ f _ _ = False
+
-------------------
tcConArg :: Bool -- True <=> -funbox-strict_fields
-> LHsType Name
badExistential :: Located Name -> SDoc
badExistential con_name
= hang (ptext (sLit "Data constructor") <+> quotes (ppr con_name) <+>
- ptext (sLit "has existential type variables, or a context"))
+ ptext (sLit "has existential type variables, a context, or a specialised result type"))
2 (parens $ ptext (sLit "Use -XExistentialQuantification or -XGADTs to allow this"))
badStupidTheta :: Name -> SDoc