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

On Sun, Sep 15, 2013 at 09:31:34PM -0700, Martin DeMello wrote:
> 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

I recommend using a private type (in the module type) over an abstract
one:
    type t = private char

That way a type t instance can only be constructed in the module,
where you check the upper case, but can be used in place of a char
otherwise. No need to manually project it everywhere.

MfG
	Goswin