[Haskell-cafe] Existential quantification problem
derek.a.elkins at gmail.com
Sun Jul 13 19:28:10 EDT 2008
On Thu, 2008-07-10 at 10:59 -0700, Jonathan Cast wrote:
> 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.
It's not an extraneous requirement; it is part of the definition of
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