Hi Sam,

You can't do exactly that, because OCaml values like chars don't exist at
the type level. So you can't say e.g.

    let a : 'a' = a

...or other similar things where values would be types.

What you would usually do is make an abstract (or private) type that allows
constructing only valid values. E.g.,

    module Ab : sig
      type t = private char
      val make : char -> t option
    end = struct
      type t = char
      let make char = match char with 'a' | 'b' -> Some char | _ -> None
    end

This allows constructing only values containing 'a' or 'b', with the
guarantee provided by the module's implementation. So if you call `Ab.make
some_char`, you'll get back an `Ab.t option`, but if it's `Some`, then you
have a guarantee that it contains 'a' or 'b'.

You can convert the `Ab.t` value to a `char` using `(value :> char)`
(basically, upcasting).

Regards,

Yawar

On Tue, Jun 30, 2020 at 12:26 PM Sam Kuper <sampablokuper@posteo.net> wrote:

> Dear list,
>
> Forgive me for asking a very basic question, but I have not so far been
> able to find an answer in any of the OCaml books to which I have access,
> nor in the OCaml documentation or mailing list archive.
>
> How does one define a type whose values are restricted to one of some
> specified chars?
>
> E.g. suppose I want to define a type `ab` whose values can only be
> either 'a' or 'b'.  I imagine that should work something like this:
>
>     # type ab = Ab of char 'a' | Ab of char 'b' ;;
>     type ab = Ab of char 'a' | Ab of char 'b'
>
> and thereby give the following functionality:
>
>     # Ab 'a';;
>     - : ab = Ab 'a'
>     # Ab 'b';;
>     - : ab = Ab 'b'
>     # Ab 'c';;
>     Error: <some error>
>
> The definition above is essentially pseudo-code to illustrate what I
> would like to achieve with real, valid OCaml code.  (If I knew how to
> write valid OCaml to achieve this, then I would not be posting this
> question on the mailing list.)
>
> Here are several of my failed attempts at writing OCaml code for what I
> want to achieve:
>
>     # type ab = 'a' | 'b';;
>     Error: Syntax error
>
>     # type ab = char 'a' | char 'b';;
>     Error: Syntax error
>
>     # type ab = Ab of char 'a' | Ab of char 'b';;
>     Error: Syntax error
>
>     # type 'a ab = 'a constraint 'a = 'a' | 'b';;
>     Error: Syntax error
>
>     # type 'a ab = 'a constraint 'a = 'a' | 'a = 'b';;
>     Error: Syntax error
>
> How can I actually achieve it?
>
> Thank you in advance,
>
> Sam
>
> --
> A: When it messes up the order in which people normally read text.
> Q: When is top-posting a bad thing?
>
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