List of PID's in Erlang

Question

Long story short I am trying to replicate the Sleeping barber problem in Erlang.

In my solution I decided that for all the processes that are waiting I would put them into a list. Then, once it was that processes turn, I would take that PID off of the list.

Unfortunately when I call

length(myListOfPids).

it fails, as an example:

length([<0.46.0>]).
* 2: syntax error before: '<'

is there a way to store PID's so that I can recall them and use them normally? i.e.

PID ! message

... just in case it matters here is the actual error I recieve when running my program:

=ERROR REPORT==== 1-Jul-2010::05:50:40 ===
Error in process <0.44.0> with exit value:
{badarg,[{erlang,length,[<0.46.0>]},{barber1,waitingRoom,2}]}

barber1 is my module, waitingRoom is the function that keeps track of which processes are waiting

Answer 1

You can also construct a Pid from it's three components using pid/3.

1> length([pid(0,35,0)]).

Be aware that using any of these techniques for constructing Pid goes wrong if you construct a pid on a different node than the one it was created on.

The problem your program is having is different.

{badarg,[{erlang,length,[<0.46.0>]},{barber1,waitingRoom,2}]}

A call to erlang:length/1 created a badarg. The third element of {erlang,length,[<0.46.0>]} is the list of arguments passed to erlang:length. So that's equivalent to:

1> erlang:length(pid(0,46,0)).

where you intended:

1> erlang:length([pid(0,46,0)]).

(Annoyingly, the erlang shell now hides erlang's internal representation of errors from you. Replacing the above error with:

** exception error: bad argument in function length/1 called as length(<0.35.0>)

which is much easier to understand but less useful because it gets in the way of learning the essential skill of interpreting erlang errors yourself.)