DB.ml (in first-class module)

module type DB = sig
  type t

  type result

  type query

  val connect : string -> t

  val create_query : string -> query

  val select : t -> query -> result option

  val insert : t -> query -> result option

  val update : t -> query -> result option

end

exception Unimplemented

module SomeSQL : DB = struct

  type t = Connection of string

  type result = bytes

  type query = string

  let connect s = Connection s

  let select t q = raise Unimplemented

  let insert t q = raise Unimplemented

  let update t q = raise Unimplemented

  let create_query s = s
end


module ExtDB(DBImpl: DB) = struct
  include DBImpl

  let upsert t qs qi qu =
    match (DBImpl.select t qs) with
    | Some x -> DBImpl.update t qu
    | None -> DBImpl.insert t qi

end

module ExtSomeSQL = ExtDB(SomeSQL)


let () =

  let conn = ExtSomeSQL.connect "conn" in
  let result = ExtSomeSQL.upsert conn (ExtSomeSQL.create_query "select")
            (ExtSomeSQL.create_query "on insert")
            (ExtSomeSQL.create_query "on update") in
    ignore (result)




let sql_impls = [
  ("some_sql", (module SomeSQL : DB))
]

let get_sql_impl label =
  let (_, impl) = List.find (fun (_label, _) -> label = _label) sql_impls
  in
  impl


let () =
  let module D = (val (get_sql_impl "some_sql"): DB) in (* module D = SomeSQL *)
  let module E = ExtDB(D) in
  let conn = E.connect "conn" in
  let result = E.upsert conn (E.create_query "select")
            (E.create_query "on insert")
            (E.create_query "on update") in
    ignore (result)