Erlang Aide-mémoire

% hello.erl
-module(hello).
-export([greet/1, greet/0]).

greet() ->
    greet("World").

greet(Name) ->
    io:format("Hello, ~s!~n", [Name]).

% Compile & run in the shell:
%   c(hello).
%   hello:greet("Alice").   %=> Hello, Alice!

Name = "Alice".        % variables start with uppercase
Age = 30.               % bound once — single assignment!
% Age = 31.             % ** exception error: no match of RHS

% Atoms start with lowercase (or are quoted)
ok.                    % the ok atom (common return value)
error.                 % another atom
'This is an atom'.     % quoted atom with spaces

true, false, undefined % built-in atoms

% Pattern matching binds and asserts
{ok, Result} = {ok, 42}.   % Result = 42
{error, _} = {error, badarg}.  % _ matches and discards

% Tuples (fixed-size, positional)
Point = {point, 3, 4}.
{point, X, Y} = Point.      % X=3, Y=4
element(2, Point).          %=> point

% Records (syntactic sugar over tuples)
-record(person, {name, age, city = "Unknown"}).

P = #person{name = "Alice", age = 30}.
P#person.name.              %=> "Alice"
P2 = P#person{age = 31}.    % update field
{person, Name, Age, _} = P. % underlying tuple

[1,2,3] ++ [4,5].          %=> [1,2,3,4,5]  (append)
[1,2,3] -- [2].            %=> [1,3]        (subtract)
hd([1,2,3]).               %=> 1            (head)
tl([1,2,3]).               %=> [2,3]        (tail)

% Pattern matching on lists
[Head | Tail] = [1,2,3].   % Head=1, Tail=[2,3]
[A,B|Rest] = [1,2,3,4].    % A=1, B=2, Rest=[3,4]

% List comprehensions
[ X*2 || X <- [1,2,3,4] ].          %=> [2,4,6,8]
[ X || X <- [1..10], X rem 2 =:= 0 ]. % evens: [2,4,6,8,10]
[ {A,B} || A <- [1,2], B <- [a,b] ]. % cartesian product

-module(math_tools).
-export([factorial/1, fib/1, classify/1]).

% Multiple clauses selected by pattern matching
factorial(0) -> 1;
factorial(N) when N > 0 -> N * factorial(N - 1).

% Fibonacci with guards
fib(0) -> 0;
fib(1) -> 1;
fib(N) when N > 1 -> fib(N-1) + fib(N-2).

% Tagged returns (common Erlang idiom)
classify(X) when X < 0  -> {negative, X};
classify(0)             -> zero;
classify(X) when X > 0  -> {positive, X}.

% Create
M = #{ name => "Alice", age => 30, role => admin }.

% Lookup
maps:get(name, M).              %=> "Alice"
maps:get(city, M, "Unknown").   % default if missing
{ok, V} = maps:find(age, M).    %=> {ok, 30}

% Update / add (immutable — returns new map)
M2 = M#{ age => 31, city => "Paris" }.

% Pattern match on maps
#{ name := N } = M.            % N = "Alice"  (:= for matching, => for setting)
#{ age := A, role := R } = M.

% Select clause by structure of argument
handle({ping, From}) -> From ! pong;
handle({echo, Msg})  -> Msg;
handle({add, A, B})  -> A + B;
handle(stop)         -> bye.

% Tuple destructuring
-record(rect, {w, h}).
area(#rect{w=W, h=H}) -> W * H.

% List patterns with literal values
greet_first([H|_]) -> io:format("Hi ~p~n", [H]);
greet_first([])    -> ok.

classify_file(Size) ->
    case Size of
        0              -> empty;
        N when N < 1024 -> small;
        N when N < 1048576 -> medium;
        _              -> large
    end.

% Matching on tagged tuples
Result = case file:read_file("a.txt") of
    {ok, Bin} -> {ok, binary_to_list(Bin)};
    {error, enoent} -> {error, missing};
    {error, Reason} -> {error, Reason}
end.

% Guards go after 'when' and may use built-in guard tests (BIFs)
max(A, B) when A >= B -> A;
max(_, B)             -> B.

% Multiple guard tests: ',' = AND, ';' = OR
classify(N) when N > 0, N < 100 -> in_range;
classify(N) when N =< 0; N >= 100 -> out_of_range.

% Allowed guard BIFs include comparison, arithmetic, type tests:
% is_integer/1, is_atom/1, is_list/1, is_map/1,
% length/1, map_size/1, abs/1, element/2, hd/1, tl/1
% (No user functions — guards must be side-effect free and terminate!)

% Pack a 16-bit length + 8-bit type + payload
Bin = << Length:16, Type:8, Payload/binary >>.

% Unpack with the same pattern
<< L:16, T:8, Rest/binary >> = Bin.

% Bit fields & endianness
<< N:32/big-unsigned-integer >> = <<0,0,1,1>>.   % N = 261
<< N:32/little-signed-integer >>  = <<1,0,0,0>>. % N = 1

% IPv4 header parsing example
<< Version:4, IHL:4, _TOS:8, TotalLen:16,
   _/binary >> = Packet.

% spawn/3 starts a new lightweight process
Pid = spawn(Module, Function, ArgsList).
Pid = spawn(fun() -> timer:sleep(1000), io:format("done~n") end).

% Every process has a PID; processes share NO memory
% (BEAM processes are NOT OS threads — millions are possible)

% self() returns the current process's PID
io:format("I am ~p~n", [self()]).

% is_process_alive/1 checks liveness
case is_process_alive(Pid) of
    true  -> ok;
    false -> {error, dead}
end.

% Send a message: Pid ! Message
Pid ! {hello, self()}.
Pid ! ping.
Pid ! {compute, 1, 2, 3}.

% ! always returns the message itself, so you can broadcast:
[ P ! broadcast || P <- AllPids ].

% Send to a registered process (by atom name)
registered_name ! {request, Ref, self()}.

% Register / unregister a name for a PID
register(server, Pid).
unregister(server).
whereis(server).   %=> Pid | undefined

% receive blocks until a matching message arrives
loop() ->
    receive
        {ping, From} ->
            From ! pong,
            loop();
        {echo, Msg} ->
            io:format("echo: ~p~n", [Msg]),
            loop();
        stop ->
            ok
    end.

% receive with a timeout
receive
    {data, X} -> process(X)
after 5000 ->
    {error, timeout}
end.

% Flush mailbox (drain all messages)
flush() ->
    receive _ -> flush() after 0 -> ok end.

% Selective receive: skips non-matching messages
get_reply(Ref) ->
    receive
        {Ref, Reply} -> Reply       % only matches the reply with our Ref
    after 5000 ->
        {error, timeout}
    end.

% Problem: other messages stay in the mailbox, slowing future receives.
% For high-throughput servers use a selective receive library or
% gen_server (which keeps its mailbox clean).

% Non-selective (drain in order):
drain_all() ->
    receive
        Msg -> [Msg | drain_all()]
    after 0 ->
        []
    end.

% Client: send a tagged request, await the matching reply
call(Server, Request) ->
    Ref = make_ref(),
    Server ! {call, Ref, self(), Request},
    receive
        {Ref, Reply} -> Reply
    after 5000 ->
        exit(timeout)
    end.

% Server side:
handle({call, Ref, From, Request}, State) ->
    Reply = do_work(Request),
    From ! {Ref, Reply},
    State.

% A counter server — each message mutates state via recursion
start_counter(Init) -> spawn(fun() -> counter_loop(Init) end).

counter_loop(N) ->
    receive
        inc      -> counter_loop(N + 1);
        dec      -> counter_loop(N - 1);
        {get, From} -> From ! {value, N}, counter_loop(N);
        reset    -> counter_loop(0)
    end.

% Caller:
Pid = start_counter(0).
Pid ! inc.
Pid ! {get, self()}.
receive {value, V} -> io:format("count = ~p~n", [V]) end.

% link/1 links current process to another
Pid = spawn(fun() -> ... end),
link(Pid).

% If either dies, the other gets an exit signal.
% Default behavior: the linked partner also exits.

% spawn_link does both atomically:
Pid = spawn_link(fun() -> ... end).

% Trap exits to convert death signals into messages:
process_flag(trap_exit, true).
receive
    {'EXIT', From, Reason} -> io:format("~p died: ~p~n", [From, Reason])
end.

unlink(Pid).   % remove the link (best-effort)

% monitor/1: one-way — we observe Pid, but it doesn't observe us
Ref = monitor(process, Pid).
% If Pid dies, we receive:
%   {'DOWN', Ref, process, Pid, Reason}

receive
    {'DOWN', Ref, process, Pid, normal} -> ok;
    {'DOWN', Ref, process, Pid, Reason} -> {error, Reason}
end.

% demonitor/1 removes the monitor
demonitor(Ref).
% demonitor/2 with [flush] also drains any pending 'DOWN' message
demonitor(Ref, [flush]).

% Send an exit signal explicitly:
exit(Pid, Reason).       % tell Pid to exit with Reason

% Terminate the current process:
exit(Reason).            % Reason != normal kills linked partners

% exit/2 with the atom 'kill' is untrappable:
exit(Pid, kill).         % Pid dies even if it traps exits.
% (Only the kernel-supervised 'kill' reason cannot be trapped.)

% Reasons:
%   normal    -> linked partners do NOT die
%   shutdown  -> linked partners do NOT die (used in graceful stop)
%   {shutdown, Term} -> graceful with reason, also non-propagating
%   anything else -> linked partners also die (unless trapping)

try
    {ok, Bin} = file:read_file(Path),
    binary_to_list(Bin)
catch
    throw:Term   -> {thrown, Term};
    error:Reason -> {error, Reason, erlang:get_stacktrace()};
    exit:Reason  -> {exit, Reason}
after
    % always runs (cleanup), regardless of success/failure
    file:close(Fd)
end.

% Raise exceptions:
throw({bad, Value}).      % catchable as throw
erlang:error(badarg).     % catchable as error
exit(timeout).            % catchable as exit

% A minimal supervisor: start a child, restart on death
start_sup(Mod) ->
    process_flag(trap_exit, true),
    Pid = spawn_link(fun() -> Mod:run() end),
    sup_loop(Mod, Pid).

sup_loop(Mod, Pid) ->
    receive
        {'EXIT', Pid, Reason} ->
            io:format("child died ~p, restarting~n", [Reason]),
            NewPid = spawn_link(fun() -> Mod:run() end),
            sup_loop(Mod, NewPid);
        {stop, From} ->
            exit(Pid, shutdown),
            From ! ok
    end.

-module(counter).
-behaviour(gen_server).
-export([start_link/0, inc/0, get/0]).
-export([init/1, handle_call/3, handle_cast/2,
         handle_info/2, terminate/2, code_change/3]).

start_link() -> gen_server:start_link({local, ?MODULE}, ?MODULE, 0, []).
inc()        -> gen_server:cast(?MODULE, inc).
get()        -> gen_server:call(?MODULE, get).

init(N)              -> {ok, N}.
handle_call(get, _F, N)  -> {reply, N, N}.
handle_cast(inc, N)      -> {noreply, N + 1}.
handle_info(_, N)        -> {noreply, N}.
terminate(_, _)          -> ok.
code_change(_, N, _)     -> {ok, N}.

% Synchronous call — waits for reply, crashes on server death
{ok, Value} = gen_server:call(Server, {get, Key}).

% Asynchronous cast — fire and forget
ok = gen_server:cast(Server, {put, Key, Value}).

% Call supports timeout and monitor under the hood:
gen_server:call(Server, Req, 5000).

% Server replies in handle_call:
handle_call({get, Key}, From, State) ->
    {reply, maps:get(Key, State), State}.

% handle_cast never replies:
handle_cast({put, K, V}, State) ->
    {noreply, maps:put(K, V, State)}.

% handle_info/2 handles messages NOT from call/cast:
%   - plain messages from other processes (Pid ! Msg)
%   - 'DOWN' messages from monitors
%   - timeouts sent by gen_server:start(..., Timeout)
handle_info({'DOWN', Ref, process, Pid, Reason}, State) ->
    io:format("monitored ~p died: ~p~n", [Pid, Reason]),
    {noreply, State};
handle_info({tcp, Socket, Data}, State) ->
    %% handle incoming socket data
    {noreply, State};
handle_info(Info, State) ->
    {noreply, State}.

% terminate/2 runs on shutdown — close fds, flush, etc.
terminate(Reason, State) ->
    io:format("shutting down: ~p~n", [Reason]),
    ok.

% code_change/3 enables hot code upgrade.
code_change(_OldVsn, State, _Extra) -> {ok, State}.

% myapp.app — application resource file
{application, myapp,
 [{description, "Demo app"},
  {vsn, "1.0.0"},
  {modules, [myapp_app, myapp_sup, counter]},
  {registered, [myapp_sup]},
  {applications, [kernel, stdlib]},
  {mod, {myapp_app, []}},
  {env, [{port, 4000}]}
 ]}.

% myapp_app.erl — application behaviour
-module(myapp_app).
-behaviour(application).
-export([start/2, stop/1]).
start(_StartType, _StartArgs) -> myapp_sup:start_link().
stop(_State) -> ok.

% Start everything:
%   application:start(myapp).
% Or with release:  erl -eval 'application:ensure_all_started(myapp)'