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

Philippa Cowderoy flippa at flippac.org
Sat Oct 15 14:20:07 EDT 2005

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

More information about the Haskell mailing list