[Haskell-cafe] Existentially-quantified constructors: Hugs is fine, GHC is not?

[Einar Karttunen]

On 10.05 13:27, Otakar Smrz wrote:
>    data ... = ... | forall b . FMap (b -> a) (Mapper s b)
> 
>    ... where FMap qf qc = stripFMap f q
> 
> the GHC compiler as well as GHCi (6.4.2 and earlier) issue an error
> 
>     My brain just exploded.
>     I can't handle pattern bindings for existentially-quantified
>     constructors.

You can rewrite the code in a way that GHC accepts it. Just
avoid pattern binding your variables. I had the same problem
in HAppS code and needed to lift some code to the top
level to solve it.

- Einar Karttunen