The Erlang Shell & Your First Module

3.1 The shell

Every expression ends with a period . in the shell.

Useful shell commands:

list shell commands f(). / f(X). forget all / one variable binding compile and load mod.erl h(lists, map). documentation (OTP 23+) value of command N repeat command N load record definitions print term fully pwd()., cd(Dir)., ls(). print and clear shell mailbox

Ctrl+G opens the job control mode (h for help; i interrupt; s start new shell; c connect; q quit). Ctrl+C opens the BREAK menu (a to abort).

$ erl
Erlang/OTP 27 [erts-15.2] ...
1> 1 + 2.
3
2> X = 10.
10
3> X * 2.
20
4> q().          % quit (also halt(). or Ctrl+G then q)

3.2 Comments

% Single-line comment (the only comment syntax)
%% Convention: two % for comments inside functions/sections
%%% Three % for module-level comments

3.3 Hello World module

Or from the command line:

-module(hello).          % module name must match the file name
-export([world/0]).      % public functions: name/arity

world() ->
    io:format("Hello, World!~n").

1> c(hello).
{ok,hello}
2> hello:world().
Hello, World!
ok

erlc hello.erl
erl -noshell -s hello world -s init stop

3.4 Escript

#!/usr/bin/env escript
%%! -smp enable
main(Args) ->
    io:format("Hello, args: ~p~n", [Args]).

escript hello.escript a b c

3.5 Punctuation rules (the "Erlang grammar" for beginners)

separate expressions in a sequence separate clauses (function clauses, case branches) end a function (or a shell expression)

area({square, S}) -> S * S;               % clause, more follow
area({circle, R}) ->
    Pi = 3.14159,                          % expression, more follow
    Pi * R * R.                            % end of function