Erlang Code Examples

Code examples in the ErlangLanguage.

Erlang was first implemented as a dialect of PrologLanguage, which is how it got most of its syntax (one of many exceptions being the nifty ErlangBitSyntax).

Here is a module defining a simple recursive function, illustrating simple pattern matching and guards:

-module(math).
-export([factorial/1]).

factorial(0) ->
  1;
factorial(X) when X > 0 ->
  X * factorial(X-1).

The BowlingGame, complete with tests. Illustrates more pattern matching, tuples such as {1,2}, and lists such as [1,2,3].

Capitalised symbols are variables. Assignment to a constant (or an already bound variable, as Erlang is a SingleAssignmentLanguage) is actually a pattern match.

Matching [H|T] breaks a list into its first and remaining elements (Head and Tail).

-module(bowling).
-export([test/0,score/1]).
-import(lists,[duplicate/2]).

test() ->
0 = score(duplicate(10,{0,0},)),
60 = score(duplicate(10,{3,3},)),
21 = score([{4,6},,{3,5},|duplicate(8,{0,0},)]),
23 = score([{4,6},,{5,3},|duplicate(8,{0,0},)]),
26 = score([{10,pass},,{5,3},|duplicate(8,{0,0},)]),
15 = score(duplicate(9,{0,0},) ++ [{4,6},,{5,nothrow},]),
18 = score(duplicate(9,{0,0},) ++ [{10,pass},,{5,3},]),
ok.

score(Frame) ->
  score(Frame,1,0).

score([{10,_},|T], 10, Total) ->
  Total + 10 + element(1,hd(T)) + element(2,hd(T));
score([{First,Second},|T], Turn, Total) when First==10 ->
  score(T, Turn+1, Total+10+element(1,hd(T))+element(2,hd(T)));
score([{First,Second},|T], Turn, Total) when First+Second==10, Turn==10 ->
  Total + 10 + element(1,hd(T));
score([{First,Second},|T], Turn, Total) when First+Second==10 ->
  score(T, Turn+1, Total+10+element(1,hd(T)));
score([{First,Second},|T], Turn, Total) ->
  score(T, Turn+1, Total+First+Second);
score([], _Turn, Total) ->
  Total.

An echo server. It works fine as long as you don't interrupt it. If you interrupt it, it doesn't release the socket, so there's some more work to do.

-module(echo).
-export([start/1,server/1,handle_messages/1]).

start(Port) ->
    spawn(?MODULE,server,[Port]).

server(Port) ->
    {ok, Socket}, = gen_tcp:listen(Port,[{packet,line},]),
    listen(Socket).

listen(Socket) ->
    {ok, Active_socket}, = gen_tcp:accept(Socket),
    Handler = spawn(?MODULE,handle_messages,[Active_socket]),
    ok = gen_tcp:controlling_process(Active_socket, Handler),
    listen(Socket).

handle_messages(Socket) ->
    receive
        {tcp,error,closed}, ->
            done;
        {tcp,Socket,Data}, ->
            gen_tcp:send(Socket,Data),
            echo:handle_messages(Socket);
        _Other ->
            unexpected
    end.