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

Einar Karttunen ekarttun at cs.helsinki.fi
Wed May 10 09:13:20 EDT 2006

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

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