undocumented feature in GHC-4.00?

[Jeffrey R. Lewis]

When attempting to reconstruct the syntax for existential
quantification, I tried:

newtype Groo a = Groo (Either a b)

To my surprise, using ghc-4.00, this worked - without even using
`-fglasgow-exts'.  (it doesn't work, with or without `-fglasgow-exts'
under 3.02)

Then I read the release notes ;-)  These told me about using `forall' on
`data' declarations.  But the above works, and yields the type I was
expecting, i.e. the .hi file sez:

newtype Groo $r3r = Groo (_forall_ [$r3u] = PrelEither.Either $r3r
$r3u) ;

Is this a feature or a bug?

--Jeff

Re: undocumented feature in GHC-4.00?

[Jeffrey R. Lewis]

Simon Peyton-Jones wrote:

  When attempting to reconstruct the syntax for existential
  quantification, I tried:
 
  newtype Groo a = Groo (Either a b)
 
  To my surprise, using ghc-4.00, this worked - without even using
  `-fglasgow-exts'.  (it doesn't work, with or without `-fglasgow-exts'
  under 3.02)

 Nothing about existentials here.   GHC is universally quantifying
 over the 'b'.  It's just as if you'd written

 newtype Groo a = Groo (forall b. Either a b)


Indeed - it wasn't the type I was casting about for - I was looking for
how to express:
newtype Groo a = forall b. Groo (Either a b)

This form of quantification seems to only be supported for `data' decls -
is there a reason we can't also do it with `newtype'?


 Perhaps we shouldn't do implicit universal quantification here?

  Is this a feature or a bug?



The confusion on my part was that this form of implicit universal
quantification seems to be an undocumented feature.  In the release notes,
the only comment about implicit quantification is this:


 Notice that you don't need to use a forall if there's an
 explicit context. For example in the first argument of the
 constructor MkSwizzle, an implicit "forall a." is prefixed to
 the argument type. The implicit forall quantifies all type
 variables that are not already in scope, and are mentioned in
 the type quantified over

--Jeff