[Caml-list] Types for solved and unsolved conditional values

[Sylvain Le Gall <sylvain@le-gall.net>]

Hello all,

I would like to express a conditional value that can be either solved or
unsolved and be able to quickly match the result, in the case of a
solved value. The value returned should be 'a.

Some code:

type ('a, 'b) conditional

type 'a unsolved = ('a, (expr, 'a) list) conditional
type 'a solved = ('a, [`Always 'a | `NotSolved]) conditional

(* Unsolved conditional *)
let myval = 
  [EFlag "foo", "foo"; 
   EBool true, "nofoo"]

(* Solved conditional *)
let myval' = 
  `Always "foo"

A basic solution would be to make types independent, this way:

type ('a, 'b) conditional = 'a, (expr * 'b) list

type 'a unsolved = ('a, unit) conditional 
type 'a solved = ('a, [`Always of int | `NotSolved]) conditional

let myval = 
  (),
  [EFlag "foo", "foo"; 
   EBool true, "nofoo"]

let myval' = 
  `Always 0,
  [EFlag "foo", "foo"; 
   EBool true, "nofoo"]

But I don't feel it is the right solution. Is there any way to do it
better ?

Cheers,
Sylvain Le Gall
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