gist.erl

-module(gist).
-export([authenticate/3]).

authenticate(UserName, Pass, IPAddress) ->
    Checks = [fun user_ok/1,
        fun password_ok/1,
	      fun ipaddr_ok/1
	     ],
    {Result, _} = lists:foldl(
		    fun(_CheckFun, {{error, Reason}, _}) ->
			    {{error, Reason}, undef};
		       (CheckFun, {ok, Context}) ->
			    {CheckFun(Context), Context}
		    end,
		    {ok, {UserName, Pass, IPAddress}},
		    Checks),
    Result.

user_ok({UserName, _, _}) ->
    case ext__user_exists(UserName) of
	false ->
	    {error, user_not_found};
	true ->
	    ok
    end.

password_ok({UserName, Pass, _}) ->
    case ext__user_pass_match(UserName, Pass) of
	false ->
	    {error, password_incorrect};
	true ->
	    ok
    end.

ipaddr_ok({UserName, _, IPAddress}) ->
    case ext__user_has_ip(UserName, IPAddress) of
	false ->
	    {error, ipaddress_incorrect};
	true ->
	    ok
    end.

%%%% ext

ext__user_exists(groucho) ->
    true;
ext__user_exists(_) ->
    false.

ext__user_pass_match(groucho, julius) ->
    true;
ext__user_pass_match(_, _) ->
    false.

ext__user_has_ip(groucho, '123.45.67.89') ->
    true;
ext__user_has_ip(_, _) ->
    false.

%%%% example usage
% 
% 2> gist:authenticate(groucho, julius, '123.45.67.89').  
% ok
%
% 3> gist:authenticate(chico, julius, '123.45.67.89').  
% {error,user_not_found}
%
% 4> gist:authenticate(groucho, rabbit, '123.45.67.89').
% {error,password_incorrect}
%
% 5> gist:authenticate(groucho, julius, '123.145.67.89').
% {error,ipaddress_incorrect}

%%%% caveats
% 
% the foldl in authenticate should be abstracted as a utility function