[Haskell] RE: [Haskell-cafe] Interest in helping w/ Haskell standard

[Philippa Cowderoy]

On Fri, 14 Oct 2005, Simon Peyton-Jones wrote:

>
> | In that context, how well-understood is the combination of
> impredicative
> | types via boxy types and a proper existential quantifier at the
> moment?
> | It's certainly something that has many uses in an industrial context.
>
> Stephanie Weirich, Dimitrios Vytiniotis, and I are currently re-writing
> our paper about type inference for impredicative polymorphism.  I don't
> think there's any difficulty with existentials, at least when they are
> wrapped in a data constructor (which is the way GHC deals with
> existentials at the moment).  We'll have a draft done in a couple of
> weeks.  (Until then the paper on my home page is still approximately
> right.)
> http://research.microsoft.com/%7Esimonpj/papers/boxy
>
> Can you elaborate on what you had in mind in your last sentence above?
>

If you manage to avoid explicit boxing as required currently, you can swap 
out an existing type for an existential type covering a typeclass with 
minimal code modifications - which is wonderful for refactoring. More 
generally, they work rather nicely with typeclasses and avoid the need to 
keep track of whether you're dealing with the boxed existential type or a 
type that's an instance of the relevant class.

-- 
flippa at flippac.org

Performance anxiety leads to premature optimisation