[GHC] #16311: Suggest -XExistentialQuantification for 'forall' in data declarations

[GHC]

#16311: Suggest -XExistentialQuantification for 'forall' in data declarations
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        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:                    |
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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>
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