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Pattern matching but no construction?
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Date: -- (:)
From: brogoff <brogoff@s...>
Subject: Re: [Caml-list] Pattern matching but no construction?
private types do exactly what you want. How lucky. See where I inserted private
below, that should do it.

On Thu, 28 Oct 2004, Harrison, John R wrote:

> Is there a way to use the OCaml module system to declare an
> abstract type with an implementation as a recursive type in
> such a way that:
>
>  * You can use the constructors to pattern-match against
>
>  * You cannot use the constructors to construct values
>
> For example, suppose I do the following:
>
>   module type Wibble =
>     sig type thing = Integer of int | Boolean of bool
                      ^
                      --- private
>         val mk_thing : int -> thing
>         val dest_thing: thing -> int
>     end;;
>
>   module Thing : Wibble = struct
>     type thing = Integer of int | Boolean of bool
>          let mk_thing i = Integer i
>          let dest_thing t = match t with
>            Integer i -> i
>          | Boolean b -> if b then 1 else 0
>     end;;
>
>   include Thing;;
>
> I can now define functions by pattern-matching, which I want:
>
>   fun (Boolean b) -> b;;
>
> but I can also use the constructors to construct, which I don't:
>
>   Integer(3);;
>
> On the other hand, if I change the signature to just
>
>   module type Wibble =
>     sig type thing
>         val mk_thing : int -> thing
>         val dest_thing: thing -> int
>     end;;
>
> then I can do neither. Is there any way to get one and not the
> other?
>
> John.
>
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