A small module to provide memoize functions in Erlang.

memoize.erl

%% Copyright (c) 2008 Nick Gerakines <nick@gerakines.net>
%%
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%% obtaining a copy of this software and associated documentation
%% files (the "Software"), to deal in the Software without
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%% copies of the Software, and to permit persons to whom the
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%% conditions:
%%
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%% included in all copies or substantial portions of the Software.
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%% EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
%% OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
%% NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
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%% @author Nick Gerakines <nick@gerakines.net>
%% @copyright 2008 Nick Gerakines
%% @version 0.1
%% @doc A simple memoize gen_server.
-module(memoize).
-behaviour(gen_server).
-export([memoize/2, start/0]).
-export([
    init/1, terminate/2, code_change/3,
    handle_call/3, handle_cast/2, handle_info/2
]).

%% @doc the public interface to this module.
memoize(Fun, Args) -> gen_server:call(?MODULE, {Fun, Args}, infinity).

start() -> gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).

init(_) -> {ok, gb_trees:empty()}.

handle_call({Fun, Args}, _From, State) ->
    FunSig = hash(Fun, Args),
    {Resp, NewState} = case gb_trees:lookup(FunSig, State) of
        {value, Value} -> {Value, State};
        none ->
            NewResp = apply(Fun, Args),
            SavedState = gb_trees:enter(FunSig, NewResp, State),
            {NewResp, SavedState}
    end,
    {reply, Resp, NewState};
    
handle_call(stop, _From, State) -> {stop, normalStop, State};

handle_call(_, _From, State) -> {reply, error, State}.

handle_cast(_Msg, State) -> {noreply, State}.

handle_info(_Info, State) -> {noreply, State}.

terminate(_Reason, _State) -> ok.

code_change(_OldVsn, State, _Extra) -> {ok, State}.

%% @doc Create a hash based on a mfa and it's arguments.
hash(Fun, Args) ->
    {module, Module} = erlang:fun_info(Fun, module),
    {name, Name} = erlang:fun_info(Fun, name),
    {arity, Arity} = erlang:fun_info(Fun, arity),
    erlang:md5(erlang:term_to_binary({Module, Name, Arity, Args})).