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

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

On 20-05-2011, Sylvain Le Gall <sylvain@le-gall.net> wrote:
> 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"
>

Another solution, using phantom type:

cond.mli:

type ('a, 'b) conditional

val get: ('a, 'b) conditional -> 'a

val solve: ('a, 'b) conditional -> ('a, [`Solved]) conditional
val get_solved: ('a, [`Solved]) conditional -> 
  [`Always 'a | `Sometimes (expr * 'a) list]

cond.ml:

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

let get cond =
 ...

let solve cond = 
  try 
    [EBool true, get cond]
  with _ ->
    cond

let get_solved =
  function
    | [EBool true, vl] -> `Always vl
    | lst -> `Sometimes lst

Cheers,
Sylvain Le Gall
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