Re: [Caml-list] use identity for default function

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2013/02/08, at 4:22, Ashish Agarwal <agarwal1975@gmail.com> wrote:

> I'd like to implement
> 
> val foo : ?f:('a -> 'b) -> 'a -> 'b
> 
> where the default for f is identity. Is there now a way to do this with all the new features of OCaml?

This is an old request (about as old as optional arguments).
The problem is clear enough: you want a different type when no argument is passed.
Interestingly, it would be possible to do that using GADTs:

type (_,_) optional =
  | Override : 'a -> ('a,'b) optional
  | Default : ('a,'a) optional

let foo : type a b. (a -> b, a -> a) optional -> a -> b = function
  | Override f -> f
  | Default -> fun x -> x

Unfortunately, GADTs were not there when optional arguments were introduced.
Also, there is the problem that the type gets more complicated.
(But there could be some sugar to print identical types only once)

Jacques Garrigue