From: Yotam Barnoy <firstname.lastname@example.org> To: Yawar Amin <email@example.com> Cc: Sam Kuper <firstname.lastname@example.org>, "ML caml-list (ocaml discuss)" <email@example.com> 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 <firstname.lastname@example.org> 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 <email@example.com> 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.
next prev parent 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
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