Re: [Haskell-cafe] Expressing "self-composable" functions at the type level

Hi, On 11/10/2013 03:18 AM, Dan Doel wrote:

Doing significantly better is harder, and I don't think you'll ever get to the real type you want, which is:

twice : ((a -> b) ∩ (b -> c)) -> a -> c

I.E. look up interstection types if you're into this stuff. They tend to be less popular than quantifier-based polymorphism, though.

I think the problem is that having intersection types implies subtyping, and subtyping is obviously troublesome. Essentially the same problem [as mine] is discussed in an old thread, where a partial ("value-level") solution is proposed via typeclass hackery: http://www.haskell.org/pipermail/haskell-cafe/2006-January/013868.html -- Ignas