[Haskell-cafe] Existential quantification problem

[Jonathan Cast]

On Thu, 2008-07-10 at 14:53 -0300, Marco Túlio Gontijo e Silva wrote:
> Hello,
> 
> how do I unbox a existential quantificated data type?

You can't.  You have to use case analysis:

  case foo of
    L l -> <whatever you wanted to do>

where none of the information your case analysis discovers about the
actual type of l can be made available outside of the scope of the case
expression.  (It can't `escape').  This is required for decidable static
typing, IIRC.

jcc

> 
> > {-# LANGUAGE ExistentialQuantification #-}
> > data L a = forall l. L (l a)
> > unboxL (L l) = l
> 
> is giving me, in GHC:
> 
>     Inferred type is less polymorphic than expected
>       Quantified type variable `l' escapes
>     When checking an existential match that binds
>         l :: l t
>     The pattern(s) have type(s): L t
>     The body has type: l t
>     In the definition of `unboxL': unboxL (L l) = l
> 
> Thanks.
>