CODEWITHSUNDEEP
erlang
cronotk
cronotk

Reputation: 156

Can't spawn with number parameter?

I'm a beginner at Erlang and I've been working through "Learn You Some Erlang For Great Good!". I use a modified version of this example code where the critic has a parameter:

critic(Count) ->
    receive
        {From, {"Rage Against the Turing Machine", "Unit Testify"}} ->
            From ! {self(), {"They are great!", Count}};
        {From, {"System of a Downtime", "Memoize"}} ->
            From ! {self(), {"They're not Johnny Crash but they're good.", Count}};
        {From, {"Johnny Crash", "The Token Ring of Fire"}} ->
                From ! {self(), {"Simply incredible.", Count}};
        {From, {_Band, _Album}} ->
            From ! {self(), {"They are terrible!", Count}}
    end,
    critic(Count).

Which is spawned like this:

restarter() ->
    process_flag(trap_exit, true),
    Pid = spawn_link(?MODULE, critic, [my_atom]),
        register(critic, Pid),
    receive
        {'EXIT', Pid, normal} -> % not a crash
                ok;
        {'EXIT', Pid, shutdown} -> % manual termination, not a crash
            ok;
        {'EXIT', Pid, _} ->
            restarter()
    end.

The module is used like this:

1> c(linkmon).                      
{ok,linkmon}
2> Monitor = linkmon:start_critic().
<0.163.0>
3> linkmon:judge("Rage Against the Turing Machine", "Unit Testify").
{"They are great!",my_atom}

Now, when I change "my_atom" to a simple number (like 255) the monitor crashes:

1> c(linkmon).                                                      
{ok,linkmon}
2> Monitor = linkmon:start_critic().                                

=ERROR REPORT==== 14-Jul-2013::20:42:20 ===
Error in process <0.173.0> with exit value: {badarg,[{erlang,register,[critic,<0.174.0>] []},{linkmon,restarter,0,[{file,"linkmon.erl"},{line,16}]}]}

However, it does work when I send [1] (so the code is "spawn(....., [[255]]).") Why can't I pass a single number? Is just skimming over the documentation of spawn/3 doesn't really tell me anything... except maybe that I missed something and a number is not an Erlang term. But then how do I pass a number?

Upvotes: 1

Views: 139

Answers (1)

RichardC
RichardC

Reputation: 10557

The error message says that the call to register(critic, Pid) on line 16 crashes due to "badarg" even though the arguments look ok. This can happen if the process referred to by Pid is already dead (if it crashes immediately, e.g. if you pass the wrong number of args), or if you already have a process around using that name. Ensure that the length of the list in the spawn(Mod,Fun,[...]) matches the number of args to your critic() function, and call "whereis(critic)" in the shell to check if there's an old process blocking the name from being reused.

Upvotes: 4

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