Re: [Caml-list] Restrict type to specific chars

[Yotam Barnoy <yotambarnoy@gmail.com>]

A simpler option is to just encode chars as variants:

type t = A | B | C

and then have conversion functions

let t_of_char = function
  | 'a' -> A
  | 'b' -> B
  | 'c' -> C
  | _ -> invalid_arg "Unsupported char"

let char_of_t = function
 | A -> 'a'
 | B -> 'b'
 | C -> 'c'

On Tue, Jun 30, 2020 at 1:34 PM Yawar Amin <yawar.amin@gmail.com> wrote:
>
> 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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