[Caml-list] A use-case for first-class modules... out of curiosity is there another way?

[Anthony Tavener <anthony.tavener@gmail.com>]

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Embedding "database" features into other modules.

First-class modules are allowing me to neatly unpack table implementations
into
other modules, using local types.

This is really quite basic, but I wonder if I could have done this before
first-class modules, and without leveraging the object system? I also don't
recall seeing first-class modules used for something like this.

So, is there another way to do this, aside from the object system? It's very
much like creating a basic object.

This is an extracted and simplified example...
--------------------

module Db = struct

  (* Signature for a single 'table' in the database, elements of type t. *)
  module type S = sig
    (* Full implementation relies on a Key module for different key types
than 'int' *)
    type t
    val get : int -> t
    val set : int -> t -> unit
    val del : int -> unit
    val iter : (int -> t -> unit) -> unit
    val fold : (int -> t -> 'a -> 'a) -> 'a -> 'a
  end

  (* Instantiates storage for a table, and returns FC module to interact
with the store. *)
  let create_with_default (type s) ?(size=19) default =
    (* Full implementation is parameterized by Key and Table modules *)
    let h = Hashtbl.create size in
    let module H = struct
      type t = s
      let get (id:int) =
        try Hashtbl.find h id
        with Not_found -> default
      let set id (v:t) = Hashtbl.replace h id v
      let del id = Hashtbl.remove h id
      let iter f = Hashtbl.iter f h
      let fold f init = Hashtbl.fold f h init
    end in
    (module H : S with type t = s)

end

(* An example table... *)
module Location = struct
  let unknown = "Unknown"
  include (val (Db.create_with_default unknown) : S with type t = string)
  (* Location might have a bunch of other functionality as well... *)
end

(* and basic usage... *)
# Location.get 1;;
- : Location.t = "Unknown"
# Location.set 1 "Mars";;
- : unit = ()
# Location.get 1;;
- : Location.t = "Mars"

--------------------
Some background on what this is for: (skip unless you're interested!)

I use a "component architecture" with most games -- basically a database of
properties keyed off "game object IDs". I thought this was a very powerful
feature in old MUDs/MUSHs. It's one of the first things I tried making when
I
started in OCaml, but I had some difficulties and ended up explicity
instantiating hashtables or maps in the global context of various modules.
Sloppy, but workable. (The reason I had difficulty is because I was trying
to
create a database of tables which were created at runtime -- not statically
known.)

Recently I decided to fix this mess. I had many modules, each which tended
to
have a corresponding "table". Eg. Characteristics, Size, Wounds, Faction,
Inventory, etc. So ideally I wanted a convenient way to embed "database"
functions into such modules while declaring the implementation of the
underlying datastore (hashtable, map, whatever).

This might seem a bit ugly from a functional-programming perspective, but
I've
found components to be quite powerful, and overall helps to constrain where
and how mutation happens. "Game state" is generally in flux -- well, it is
everything variable, and can be compared closely with save-game state. Most
code
which doesn't update game state can be functional. Actually, it feels
creepy to
have a variable assignment in the code, since mutation is generally to
game-state
and that's handled through a database. So the resulting style is
functional+database.

 -Tony

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