#14320: Brackets change meaning of value-constructor type -------------------------------------+------------------------------------- Reporter: nr | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Keywords: GADT, Resolution: | existential type Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Just to make sure: your file has the following GADT: {{{#!hs newtype TypedExpToo :: * -> * where TEBad :: (forall a . (Exp -> (TypedExp a))) }}} Did you mean for `TEBad`'s return type to actually be `TypedExpToo a`? (I'll assume the answer is "yes".) This is another oddity caused by the fact that we validity check GADT constructor type signatures on source syntax instead of Core. In this case, the `HsParTy` constructor is throwing a wrench into things. Supporting this use case doesn't seem like it would be particularly difficult: we'd just need to have `splitLHsForAllTy` and `splitLHsQualTy` looks through parentheses: {{{#!diff diff --git a/compiler/hsSyn/HsTypes.hs b/compiler/hsSyn/HsTypes.hs index b9cd946..1864aa2 100644 --- a/compiler/hsSyn/HsTypes.hs +++ b/compiler/hsSyn/HsTypes.hs @@ -1053,10 +1053,12 @@ splitLHsSigmaTy ty splitLHsForAllTy :: LHsType pass -> ([LHsTyVarBndr pass], LHsType pass) splitLHsForAllTy (L _ (HsForAllTy { hst_bndrs = tvs, hst_body = body })) = (tvs, body) +splitLHsForAllTy (L _ (HsParTy t)) = splitLHsForAllTy t splitLHsForAllTy body = ([], body) splitLHsQualTy :: LHsType pass -> (LHsContext pass, LHsType pass) splitLHsQualTy (L _ (HsQualTy { hst_ctxt = ctxt, hst_body = body })) = (ctxt, body) +splitLHsQualTy (L _ (HsParTy t)) = splitLHsQualTy t splitLHsQualTy body = (noLoc [], body) splitLHsInstDeclTy :: LHsSigType GhcRn }}} This makes `Badgadt.hs` typecheck (after correcting the return type of `TEBad`). Do others think this is reasonable? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14320#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler