class Driver
{
public:
  virtual void connect() = 0;
};

class MysqlDriver : public Driver {}; //implements connect() using some mysql API calls.
class OracleDriver : public Driver {}; //implements connect() using some oracle API calls.

И, наконец, 
class DriverFactory
{
public:
  static Driver * createDriver( string name ) //creates necessary driver
  {
    Driver * driver = 0;
    if("mysql" == name ) driver = new MysqlDriver();
    else if( "oracle" == name ) driver = OracleDriver();
    else throw 1;
    return driver;
  }
};

let get_connect name =
  if name = "mysql" then mysql_connect else oracle_connect

let main arg =
  let connect = get_connect arg in
  bla-bla-bla

Q>let get_connect name =
Q>  if name = "mysql" then mysql_connect else oracle_connect

Q>let main arg =
Q>  let connect = get_connect arg in
Q>  bla-bla-bla
Q>

let get_close name =
if name = "mysql" then mysql_close else oracle_close

let get_commit name =
if name = "mysql" then mysql_commit else oracle_commit

Q>>let get_connect name =
Q>>  if name = "mysql" then mysql_connect else oracle_connect

Q>>let main arg =
Q>>  let connect = get_connect arg in
Q>>  bla-bla-bla
Q>>

module DB where

-- ATDs:
data DBDriver;
data DBConnection;
data DBQueryHandler;
data DBResult;
data DBValue;


-- Type synonyms:
type DSN = String -- Data source
type DBQuery = String

-- Functions
connect  :: DBDriver -> DSN -> DBConnection
prepare  :: DBConnection -> DBQuery -> DBQueryHandler
execute  :: DBQueryHandler -> [DBValue] -> DBResult
fetchAll :: DBResult -> [[DBValue]]

-- code

module Main where
import DB;
import MySql;
-- or
-- import Oracle;


getDriver :: String -> DBDriver
getDriver "mysql" = get_mysql_driver
getDriver "oracle" = get_ora_driver
getDriver x = error $ "unknown driver: " ++ x

main = let driver = getDriver "mysql"
           conn = connect driver "path:to:database"
           sth = prepare conn "SELECT * FROM test"
           result = execute [] sth
           resultSet = fetch result
           resultValue = resultSet !! 0 !! 0 -- first row / first col
       in print resultValue

А>class Driver
А>{
А>public:
А>  virtual void connect() = 0;
А>};

void main(char * arcs[])
{
    Driver * driver = DriverFactory.createDriver("mysql");
    driver->connect();
    driver->executeQuery("...");
    driver->disconnect();
}

main() ->
    Connection = driver:connect(mysql),
    Data = driver:executeQuery(Connection, "..."),
    % ... что-нибудь делаем с Data
    driver:disconnect(Connection).

class(Object) -> 
    element(0, Object).

some_generic_function(Object) ->
    Class = class(Object),
    Class:method1(Object).

А>class Driver
А>{
А>public:
А>  virtual void connect() = 0;
А>};

А>class MysqlDriver : public Driver {}; //implements connect() using some mysql API calls.
А>class OracleDriver : public Driver {}; //implements connect() using some oracle API calls.

А>И, наконец, 
А>class DriverFactory
А>{
А>public:
А>  static Driver * createDriver( string name ) //creates necessary driver
А>  {
А>    Driver * driver = 0;
А>    if("mysql" == name ) driver = new MysqlDriver();
А>    else if( "oracle" == name ) driver = OracleDriver();
А>    else throw 1;
А>    return driver;
А>  }
А>};
А>

create_driver( "mysql" ) -> create_driver( mysql_driver );
create_driver( "oracle" ) -> create_driver( oracle_driver );
create_driver( X ) -> { X, X:new() }.

map_name( "mysql" ) -> mysql_driver;
map_name( "oracle" ) -> oracle_driver;

create_driver( Name ) ->
   Module = map_name( Name ),
   { Module, Module:new() }.

%% фабрика
db_provider(mysql)  -> { mysql_connect,  mysql_query,  mysql_disconnect  };
db_provider(oracle) -> { oracle_connect, oracle_query, oracle_disconnect }.

run_db(Type,Job) ->
    {connect,queryfunc,disconnect} = db_provider(Type),
    Handle = connect("my_base"),
    Executor = function(x) -> queryfunc(Handle,x) end,
    Job(Executor),
    disconnect(Handle).

some_job(Executor) ->
    selection1 = Executor("select * from my_table"),
    selection2 = Executor("select * from something_else"),
    .....
    .

run() -> run_db(mysql, some_job).

start(Driver) ->
    start(Driver, []).
        
start(mysql, Options) ->
    [Hostname, Port, Username, Password, Database, OtherOptions] =
    lists:foldl(
      fun(Key, Acc) ->
          [proplists:lookup(Key, Options) | Acc]
      end, [],
      lists:reverse([hostname, port, username, password, database,
             options])),
    erlydb_mysql:connect(Hostname, Port, Username, Password, Database,
             OtherOptions);
    
start(_Driver, _Options) ->
    {error, driver_not_supported}.

erlydb:start(mysql).

% или там
erlydb:start(mnesia). % хотя поддержка mnesia еще не добавлена
                      % но планируется

start(Driver) ->
    case Driver of
        mysql ->
            % совершаем коннект
            {};
        mnesia ->
            % совершаем коннект
            {};
        odbc ->
            % совершаем коннект
            {}
    end.

start(Driver) when Driver == mysql ->
    % совершаем коннект
    {};
start(Driver) when Driver == mnesia ->
    % совершаем коннект
    {};
start(Driver) when Driver == odbc ->
    % совершаем коннект
    {}.

start(mysql) ->
    % совершаем коннект
    {};
start(mnesia) ->
    % совершаем коннект
    {};
start(odbc) ->
    % совершаем коннект
    {};

start(Driver) ->
    start(Driver, []).
        
start(mysql, Options) ->
    % совершаем коннект
    {};
    
start(_, _) ->
    {error, driver_not_supported}.