[Caml-list] Restrict type to specific chars

[Sam Kuper <sampablokuper@posteo.net>]

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

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