Re: [Caml-list] Polymorphic recursion

[brogoff <brogoff@speakeasy.net>]

On Tue, 3 Apr 2007, Till Varoquaux wrote:
> Therefor, if you wish to use polymorphic recursion (yep you can) you
> might want to use something where you have to specify the type; this
> includes records, objects and recursive modules. So this
>
> type 'a seq = Unit | Seq of ('a * ('a * 'a)seq)
>
> type szRec={f:'a.'a seq -> int};;
>
> let size=
>  let rec s =
>   {f=function
>     | Unit -> 0
>     | Seq(_, b) -> 1 + 2 * s.f b}
>  in
>  s.f
>
> might be what you where yearning for.

What I'd be yearning for would be more like

let rec size : 'a . 'a seq -> int =
  fun s ->
    match s with Unit -> 0 | Seq(_,b) -> 1 + 2 * (size b)

rather than having to use recursive modules or higher rank polymorphism
of record fields/polymorphic methods.

-- Brian