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

[Martin DeMello <martindemello@gmail.com>]

Ah, thanks, that will do very nicely. I just wanted a type of char map
I couldn't insert anything else other than A-Z into, that the
typechecker would prevent me from mixing with the regular char map.

martin

On Sun, Sep 15, 2013 at 9:20 PM, Ivan Gotovchits <ivg@ieee.org> wrote:
> 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
>
> --
>          (__)
>          (oo)
>    /------\/
>   / |    ||
>  *  /\---/\
>     ~~   ~~
> ...."Have you mooed today?"...