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.
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() ASCII ribbon campaign. Please avoid HTML emails & proprietary
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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