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] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon university KF8NH
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