Re: [Caml-list] Type Constraints and .mli

[Nick Lucaroni <nicholas.r.lucaroni@gmail.com>]

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I had come to the same conclusion.
On Aug 6, 2014 8:14 AM, "Trevor Smith" <trevorsummerssmith@gmail.com> wrote:

> Hello,
>
> I have a question about using .mli files for increased readability. I
> think my question boils down to: "Can one tersely add type constraints to a
> signature defined in a .mli in that same .mli file?"
>
> Detailed problem: You want to have a read interface and a write interface
> for the same implementation.
>
> We'll use a trivial example with a character and a name.
>
> module type CharacterSig = sig
>   val t
>   val create : string -> t
>   val name : t -> string
> end
>
> module type MutableCharacterSig = sig
>   val t
>   val create : string -> t
>   val name : t -> string
>   val set_name : t -> string -> unit
> end
>
> module CharacterImpl = struct
>   type t = {name : string ref}
>   let create name  =
>     {name = ref name }
>   let name c = !(c.name)
>   let set_name c name =
>     c.name := name
> end
>
> module Character = (CharacterImpl : CharacterSig with type t =
> CharacterImpl.t)
> module MutableCharacter = (CharacterImpl : MutableCharacterSig with type t
> = CharacterImpl.t)
>
> But what I would like is to specify the read and write signatures in .mli
> files for a more readable codebase.
>
> So:
>
> character.mli:
>   val t
>   val create : string -> t
>   val name : t -> string
>
> mCharacter.mli:
>   val t
>   val create : string -> t
>   val name : t -> string
>   val set_name : t -> string -> unit
>
> characterImpl.ml (* ... implementation as above ... *)
>
> However, it is not clear to me that there is a way to attach the type
> constraint to character.mli and mCharacter.mli, while keeping the terse
> readability of the .mli file. One idea for a solution, would be to
> reference a "this" so that the interface could show that it was being
> implemented by CharacterImpl, and include the type constraint.
>
> The solution I've come to use, that is still pretty readable, is to define
> the signature in the .ml file (so no .mli file) and then defining an
> internal module which I include (so that I can still reference file name as
> the module). So:
>
> character.ml
>
> module type CharacterSig = sig
>     type t
>     val create : string -> t
>     val name : t -> string
> end
>
> module T = (CharacterImpl : CharacterSig with type t = CharacterImpl.t)
> include T
>
> However, it seems like there could be a slightly more readable way of
> doing this.
>
> Thoughts? Thank you.
>
> Trevor
>

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