Improve warnings for use of "forall" without -XExistentialQuantification

[dmhouse]

Here's a GHCi session:

Prelude> let foo :: forall a. a; foo = undefined

<interactive>:1:19:
    Illegal operator `.' in type `forall a . a'
      (Use -XTypeOperators to allow operators in types)
Prelude> let foo :: forall a. Show a => a; foo = undefined
<interactive>:1:28: parse error on input `=>'

Both of these errors could be improved, I think, by detecting the use of 'forall' as a type variable name, and suggesting -XExistentialQuantification if it's being used.

[simonpj]

Good idea.

Prelude> let foo :: forall a. a; foo = undefined

<interactive>:1:19:
    Illegal operator `.' in type `forall a . a'
      Perhaps you intended to use -XRankNTypes or similar flag
      to enable explicit-forall syntax: forall <tvs>. <type>
Prelude> 

Patch is this

Thu Aug 21 13:35:02 GMT Daylight Time 2008  simonpj@microsoft.com
  * Improve error message when 'forall' is not a keyword

Simon