Erlang and OTP in Action.pdf - Synrc

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That’s all. (Although the spawn function has some other variants, this is the simplest.) This
will start a separate process which will print the text “erlang!” on the console, and then quit.
In chapter 2 we will get into details about the Erlang language and its syntax, but for the
time being we hope you will simply be able to get the gist of our examples without further
explanation. One of the strengths of Erlang is that it is generally pretty easy to understand
the code even if you’ve never seen the language before. Let’s see if you agree.
1.1.5 – How processes talk
Processes need to do more than spawn and run however—they need to communicate. Erlang
makes this communication quite simple. The basic operator for sending a message is !,
pronounced “bang”, and it is used on the form “Destination ! Message”. This is message
passing at its most primitive, like mailing a postcard. OTP takes process communication to
another level, and we will be diving into all that is good with OTP and messaging later on,
but for now, let’s marvel at the simplicity of communicating between two independent and
concurrent processes illustrated in the following snippet..
run() ->
Pid = spawn(fun ping/0),
Pid ! self(),
receive
pong -> ok
end.
ping() ->
receive
From -> From ! pong
end.
Take a minute and look at the code above. You can probably understand it without any
previous knowledge of Erlang. Points worth noting are: another variant of the spawn
function, that here gets just a single reference to “the function named ping that takes zero
arguments” (fun ping/0); and also the function self() that produces the identifier of the
current process, which is then sent on to the new process so that it knows where to reply.
That’s it in a nutshell: process communication. Every call to spawn yields a fresh process
identifier that uniquely identifies the new child process. This process identifier can then be
used to send messages to the child. Each process has a “process mailbox” where incoming
messages are stored as they arrive, regardless of what the process is currently busy doing,
and are kept there until it decides to look for messages. The process may then search and
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