Re: [Caml-list] "subclassing" a char map

[Ivan Gotovchits <ivg@ieee.org>]

Martin DeMello <martindemello@gmail.com> writes:

> I have a char multiset defined via
>
> module MultiSet = Map.Make(struct type t = char let compare = compare end)
>
> Now I would like to split off a distinct type that can only contain
> A-Z as keys. What's the best way to do this?
>
> martin

Not sure, that I correctly understood your needs... but you can
implement «A-Z keys» as an abstract type, contained in a module with the
following signature:

module type Caps =  sig
  type t
  val create: char -> t option
  val compare: t -> t -> int
  val project: t -> char
end

Next, you can instatiate a Map from abstract type Cap.t to 'a:

module CapMap = Map.Make(Caps)

where Caps is an implementation, conforming to the signature Caps. For
example like this:

module Caps : Caps = struct
    type t = char
    let create = function
      | 'A'..'Z' as ch -> Some ch
      | otherwise      -> None
    let compare = compare
    let project ch = ch
end

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