#16311: Suggest -XExistentialQuantification for 'forall' in data declarations -------------------------------------+------------------------------------- Reporter: int-index | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 (Parser) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I'm especially interested in this ticket because I recently implemented a prototype of the [https://github.com/ghc-proposals/ghc-proposals/pull/193 forall-always-a-keyword-in-types] proposal, and to my surprise, this caused the stderr for `rnfail053` to change even further: {{{#!diff diff -uw "rename/should_fail/rnfail053.run/rnfail053.stderr.normalised" "rename/should_fail/rnfail053.run/rnfail053.comp.stderr.normalised" --- rename/should_fail/rnfail053.run/rnfail053.stderr.normalised 2019-02-13 06:48:37.043855139 -0500 +++ rename/should_fail/rnfail053.run/rnfail053.comp.stderr.normalised 2019-02-13 06:48:37.043855139 -0500 @@ -1,5 +1,7 @@ rnfail053.hs:5:10: - Illegal symbol ‘forall’ in type - Perhaps you intended to use RankNTypes or a similar language - extension to enable explicit-forall syntax: ‘forall <tvs>. <type>’ + Data constructor ‘MkT’ has existential type variables, a context, or a specialised result type + MkT :: forall a. a -> T + (Enable ExistentialQuantification or GADTs to allow this) + In the definition of data constructor ‘MkT’ + In the data type declaration for ‘T’ }}} It wasn't clear to me at the time whether this was an upgrade or downgrade in error message quality, so I worked around it by making this change to the parser: {{{#!diff diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y index 820144d930..e3423669bf 100644 --- a/compiler/parser/Parser.y +++ b/compiler/parser/Parser.y @@ -2275,7 +2293,11 @@ constr :: { LConDecl GhcPs } (fst $ unLoc $2) } forall :: { Located ([AddAnn], Maybe [LHsTyVarBndr GhcPs]) } - : 'forall' tv_bndrs '.' { sLL $1 $> ([mu AnnForall $1,mj AnnDot $3], Just $2) } + : 'forall' tv_bndrs '.' {% do { hintExplicitForall $1 + ; pure $ sLL $1 $> + ( [ mu AnnForall $1 + , mj AnnDot $3 ] + , Just $2) } } | {- empty -} { noLoc ([], Nothing) } }}} That reverted the error message back to what it was before (mentioning `RankNTypes`). But perhaps the error message really ought to mention `ExistentialQuantification`. Some related questions: there are currently two distinct error messages that mention not enabling `ExistentialQuantification`: * There's the one in the original description of this ticket (the "`Not a data constructor: ‘forall’`" one). * There's also the one I mentioned in this comment (the "`has existential type variables, a context, or a specialised result type`" one). Do we need both? Or is there a way we could consolidate the two somehow? Would the answer change if we made `forall` permanently a keyword in types? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16311#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler