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.