caml-list - the Caml user's mailing list
 help / Atom feed
From: Yotam Barnoy <yotambarnoy@gmail.com>
To: Yawar Amin <yawar.amin@gmail.com>
Cc: Sam Kuper <sampablokuper@posteo.net>,  "ML caml-list (ocaml discuss)" <caml-list@inria.fr>
Subject: Re: [Caml-list] Restrict type to specific chars
Date: Tue, 30 Jun 2020 13:54:58 -0400
Message-ID: <CAN6ygOkiDmUiO5EDWktESyV+Oqrtdjq1d9nufFzT3sZi=8WxyQ@mail.gmail.com> (raw)
In-Reply-To: <CAJbYVJLNcTiTwHFcKnBXgOnNURPP6OtxOsz77-jcB_m8y+24wA@mail.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?
>>
>> ()  ASCII ribbon campaign. Please avoid HTML emails & proprietary
>> /\  file formats. (Why? See e.g. https://v.gd/jrmGbS ). Thank you.

  reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-30 16:25 Sam Kuper
2020-06-30 17:34 ` Yawar Amin
2020-06-30 17:55   ` Yotam Barnoy [this message]
2020-06-30 18:07 ` Jesse Haber-Kucharsky

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAN6ygOkiDmUiO5EDWktESyV+Oqrtdjq1d9nufFzT3sZi=8WxyQ@mail.gmail.com' \
    --to=yotambarnoy@gmail.com \
    --cc=caml-list@inria.fr \
    --cc=sampablokuper@posteo.net \
    --cc=yawar.amin@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

caml-list - the Caml user's mailing list

Archives are clonable: git clone --mirror https://inbox.ocaml.org/caml-list

AGPL code for this site: git clone https://public-inbox.org/ public-inbox