On Tue, Jun 30, 2020 at 12:26 PM Sam Kuper 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:
>
> 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?
>
> () ASCII ribbon campaign. Please avoid HTML emails & proprietary
> /\ file formats. (Why? See e.g. https://v.gd/jrmGbS ). Thank you.
>
Hi Sam.
As a small variation of Yawar's excellent reply, consider the following
(the file is named `sam.ml`):
module AB : sig
type t = private char
val a : t
val b : t
val char : t -> char
end = struct
type t = char
let a = 'a'
let b = 'b'
let char t = t
end
let () =
let x = AB.a in
let y = AB.b in
assert (AB.char x = 'a') ;
assert (AB.char y = 'b')
I like this approach because it statically guarantees that a value of
`Ab.t` is always either 'a' or 'b', and attempting to do otherwise results
in a compilation error.
Best,
--
Jesse