Re: [Caml-list] Smells like duck-typing

["Chris King" <colanderman@gmail.com>]

On 10/17/07, Dario Teixeira <darioteixeira@yahoo.com> wrote:
> c) Actually put the "Objective" part of OCaml to use.  This is the solution
>    I am using at the moment.  This is what it looks like:
>
>    class story (id, title, intro, body) =
>    object
>         val id: int option = id
>         val title: string option = title
>         val intro: string option = intro
>         val body: string option = body
>
>         method id =
>                 match id with
>                 | Some thing -> thing
>                 | None -> failwith "oops"
>
>         method title =
>                 match title with
>                 | Some thing -> thing
>                 | None -> failwith "oops"
>
>         method intro =
>                 match intro with
>                 | Some thing -> thing
>                 | None -> failwith "oops"
>
>         method body =
>                 match body with
>                 | Some thing -> thing
>                 | None -> failwith "oops"
>    end
>
>
>    class full (id, title, intro, body) =
>    object
>         inherit story (Some id, Some title, Some intro, Some body)
>    end
>
>    class blurb (id, title, intro) =
>    object
>         inherit story (Some id, Some title, Some intro, None)
>    end
>
>    class fresh (title, intro, body) =
>    object
>         inherit story (None, Some title, Some intro, Some body)
>    end
>
>
>    let print_metadata s =
>         Printf.printf "%d: %s\n" s#id s#title

Why not have different object types for each of the story types? e.g.

type full = < id: int; title: string; intro: string; body: string >
type blurb = < id: int; title: string; intro: string >
type fresh = < title: string; intro: string; body: string >

print_metadata can remain as is in your object example.  In order to
allow functions that operate differently depending on which story type
they're given, use a polymorphic variant like in your first example.
If you wanted print_metadata to take such a type, it could be written
as:

let print_metadata s =
  let s' =
    match s with
    | `Full f -> (f :> < id: int; title: string >)
    | `Blurb f -> (f :> < id: int; title: string >)
  in
  Printf.printf "%d: %s\n" s'#id s'#title

(Note the match gook is unfortunately needed to get the typing right...)

If on the other hand you wanted to ditch objects entirely, you could
do a similar thing using modules and functors.  E.g.:

module Full = struct
  type t = { id: int; title: string; intro: string; body: string }
  let id s = s.id
  let title s = s.title
  let intro s = s.intro
  let body s = s.body
end

(* etc. *)

module Print_metadata(S: sig type t val id: t -> string val title: t
-> string) = struct
  let f s = Printf.printf "%d: %s\n" (S.id s) (S.title s)
end

Of course, since calls to print_metadata now look like
"Print_metadata(Full).f story", you're essentially forced to write out
the types of everything, which is probably what you wanted to avoid
anyway.

HTH,
Chris