On 2008 Jul 13, at 19:50, Jonathan Cast wrote:
On Sun, 2008-07-13 at 18:28 -0500, Derek Elkins wrote:
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 existential types.
I know that. I didn't know implementing existential types was an end in itself, though.
No? Consider ST. Anytime you need to completely restrict access to the innards of a datum, existential types are your friend. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH