[Haskell-cafe] Re: Existencial quantification and polymorphic datatypes (actually, components...)

[Jake McArthur]

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Gleb Alexeyev wrote:
| Mauricio wrote:
|
|> data SomeNum = SN (forall a. a)
|
| [...]
|
| you cannot do anything to the value you extract

Maybe. Say you construct (SN x). If you later extract x, you can observe
that it terminates (using seq, perhaps), assuming that it does
terminate. You still can't really do anything with the value, but you
can at least observe something about its computation. The uses of this
may be obscure (I can't think of any right now), but I wouldn't say they
necessarily don't exist.

- - Jake
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