[Haskell-cafe] Object-oriented programming, Haskell and existentials

[Lennart Augustsson]

Yes, hbc had existential types around 1993.

I've used an encoding of existentials in O'Caml (well F#), and it
works, but I find it painful.
And when a very smart but non-CS person saw it his mind boggled,
whereas he understood the existential version just fine.

On Thu, Oct 16, 2008 at 8:26 AM,  <oleg at okmij.org> wrote:
>
> Lennart Augustsson wrote:
>> We don't need them [existentials] from a theoretical perspective,
>> but in practice I'd rather use existentials than encodinging them
>> in some tricky way.
>
> If the claim that we don't need existentials theoretically is obvious,
> I don't have the argument. Still, existentials are the recurrent topic
> on the OCaml list; quite a few people perceive them as a needed
> feature and their perceived absence as a drawback of OCaml.
>
> The principle of encoding existentials is straightforward: represent
> the existential datatype by a set of its possible
> observations. Existentials demonstrate once again what I sloppily call
> initial-final dichotomy: initial encodings are easier to think of
> upfront, yet require fancier type systems (second-order types,
> dependent types, GADTs). Final encodings are elementary, yet take
> (far) longer to imagine. That difficulty may be just the matter of
> habit.
>
> BTW, wasn't hbc the first Haskell compiler to introduce existentials?
>
>
> The performance grounds for existentials are justified, for example,
> by the following paper
>
> Yasuhiko Minamide, J. Gregory Morrisett and Robert Harper
> Typed Closure Conversion, POPL 1996, pp. 271--283.
> http://www.cs.cmu.edu/~rwh/papers/closures/popl96.ps
>
> Many object encodings may be considered instances of the type closure
> conversion. On the other hand, existential elimination may be seen as
> an inverse process.
>