[Haskell-cafe] Manual type-checking in graphs: Avoidable?

[Jeffrey Brown]

Yes, that is my point. Existentials cannot be unwrapped.

On Sat, Feb 20, 2016 at 4:18 AM, Kosyrev Serge <_deepfire at feelingofgreen.ru>
wrote:

> Jeffrey Brown <jeffbrown.the at gmail.com> writes:
> > After further study I believe existentials are not (at least alone)
> > enough to solve the problem.
> ..
> > getInt :: ShowBox -> Int
> > getInt (SB i) = i
> >
> > will not compile, because it cannot infer that i is an Int:
>
> You take a value of an existentially quantified type (which means it
> can be anything at all, absent some extra context) and *proclaim* it
> is an integer.
>
> On what grounds should the compiler accept your optimistic restriction?
>
> --
> с уважениeм / respectfully,
> Косырев Сергей
>



-- 
Jeffrey Benjamin Brown
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