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

[Trevor Smith <trevorsummerssmith@gmail.com>]

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Edwin,

Thank you for your response.

You are correct that the "val t" in my example should read "type t".

My main goal is one of readability: I want to have large swathes of the
codebase only use the immutable interface. A few, but very few, parts of
the codebase will use the mutable interface. Ideally, I want the developer
reading the .mli files to quickly and easily understand what interfaces do
what. The standard way of adding the type constraints add a lot of syntax
and make it (slightly) harder to read. My entire question is sort of a
nitpick.

I am interested in your use of polymorphic variants to deal with this
issue: that would actually give better compiler errors than what I am
currently implemented. Thank you for the suggestion. I will mull that and
see if I think of any downsides to that approach.

Trevor


On Wed, Aug 6, 2014 at 11:36 AM, Török Edwin <edwin+ml-ocaml@etorok.net>
wrote:

> On 08/06/2014 03:14 PM, Trevor Smith 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 <http://c.name/>)
> >   let set_name c name =
> >     c.name <http://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
>
> shouldn't this be a type?
>
> >   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.
>
> Not sure I understood exactly what you want to do, but using 'include
> module type of' and 'type t = CharacterImpl.t' should work:
>
> character.mli
> type t = CharacterImpl.t
> val create : string -> t
> val name : t -> string
>
> character.ml:
> include CharacterImpl
>
> mCharacter.mli:
> include module type of Character
> val set_name : t -> string -> unit
>
> mCharacter.ml:
> include CharacterImpl
>
> characterImpl.ml:
> type t = {name : string ref}
> let create name  =
>   {name = ref name }
> let name c = !(c.name)
> let set_name c name =
>   c.name := name
>
> However in this case Character.t = MCharacter.t = CharacterImpl.t, so you
> won't get the type safety you want
> (A Character.t can still be modified by MCharacter.set_name).
>
> Perhaps it'd be better to use different types, though to_character is not
> the identity function:
>
> character.mli:
> type t
> val create : string -> t
> val name : t -> string
>
> mCharacter.mli:
> include module type of Character
> val set_name : t -> string -> unit
> val to_character : t -> Character.t
>
> character.ml:
> include MCharacter
>
> mCharacter.ml:
> type t = {name : string ref}
> let create name  =
>   {name = ref name }
> let name c = !(c.name)
> let set_name c name =
>   c.name := name
>
> let to_character x = Character.create (name x)
>
> In fact you should probably take a look at String and Bytes type in OCaml
> 4.02 (and the ocaml-bytes compatibility lib for <4.0.2).
>
> I'd prefer something simpler though:
>
> character.ml:
> type u = {name : string ref}
> type 'a t = u
>
> let create name  =
>   {name = ref name }
> let create_ro = create
> let create_rw = create
> let name c = !(c.name)
> let set_name c name =
>   c.name := name
> let readonly x = x
>
> character.mli:
> type 'a t constraint 'a = [< `W | `R]
> val create_ro : string -> [`R] t
> val create_rw : string -> [`R | `W] t
> val name : 'a t -> string
> val set_name : [> `W] t -> string -> unit
> val readonly : [> `R] t -> [`R] t
>
> Best regards,
> --Edwin
>
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