Xdeon

(* a stream is a thunk that when called produces a pair
   #1 of the pair will be of type 'a and #2 of the pair will be another stream *)
(* can only have recursive type using datatype binding *)
datatype 'a StreamPair = Stream of 'a * (unit -> 'a StreamPair)
			    
(* a ones stream *)
fun ones () = Stream (1, ones)
		   
(* a fibonacci number stream *)
fun fibs () =

Xdeon

%% Based on code from 
%%   Erlang Programming
%%   Francecso Cesarini and Simon Thompson
%%   O'Reilly, 2008
%%   http://oreilly.com/catalog/9780596518189/
%%   http://www.erlangprogramming.org/
%%   (c) Francesco Cesarini and Simon Thompson

-module(gf).
-behaviour(gen_server).

Xdeon

-module(frequency_front).
-export([start/0, allocate/0, deallocate/1, stop/0]).
-export([init/0]).

%% API
allocate() -> 
    send_receive(allocate).

deallocate(Freq) -> 
   send_receive({deallocate, Freq}).

Xdeon

1> % first compile frequency using contents of frequency2.erl
1> c(frequency).
{ok,frequency}
2> frequency:start().
true
3> frequency:allocate().
{ok,10}
4> frequency:allocate().
{ok,11}

Xdeon

-module(exceptions).
-export([eval/2, wrap/2]).

% simple model of arithmetical expressions, like
% {add, {num, 0}, {mul, {num,1}, {var,a2}}}

% environment gives value of a variable:
% [ {a,2}, {b,1} ]

eval(_Env, {num, N}) -> N;

Xdeon

-module(frequency).
-export([start/0, allocate/0, deallocate/1, stop/0, init/0]).

start() ->
    Pid = spawn(?MODULE, init, []),
    register(?MODULE, Pid),
    Pid.

allocate() ->
    send_receive(allocate).

Xdeon

-module(echo).
-export([listener/0]).

listener() ->
    receive
        {Pid, M} ->
            io:format("~w echoed.~n", [M]),
            Pid ! M,
            listener()
    end.

Xdeon

-module(frequency_hardened).
-export([start/0, allocate/0, deallocate/1, stop/0, init/0]).

start() ->
    Pid = spawn(?MODULE, init, []),
    register(?MODULE, Pid),
    Pid.

allocate() ->
    ?MODULE ! {request, self(), allocate},

Xdeon

-module(frequency).
-export([start/0, allocate/0, deallocate/1, stop/0, clear/0, clear_with_print/0, init/0]).

start() ->
    Pid = spawn(?MODULE, init, []),
    register(?MODULE, Pid),
    Pid.

%% Add clear at beginning of each API call to eliminate unexpected messages in mailbox
%% better use combination of erlang:make_ref() and "let it fail"

Xdeon

-module(frequency).
-export([start/0, init/0]).

start() ->
    Pid = spawn(?MODULE, init, []),
    register(?MODULE, Pid),
    Pid.

init() ->
    Frequencies = {get_frequencies(), []},