Friday, September 12, 2014

Elixir spawn/1 and spawn/3

Since I spent an afternoon banging my head against this, I thought I'd blog it.

If you have elixir code that dies with this message

17:39:25.661 [error] Error in process <0.60.0> with exit value: {undef,[{'Elixir.MyFunModule',fun_stuff,[],[]}]}

when you do this

  pid = spawn( MyFunModule, :fun_stuff, [] )

But runs just fine when you do this

 pid = spawn( fn -> MyFunModule.funstuff([]) end )

The problem is the arity of the function you are using in the spawn/3 version.
Elixir uses the count of the elements in the third argument to spawn/3 to
find the function to spawn in the subprocess. In the first version it is looking
for a MyFunModule.fun_stuff/0 which does not exist. If you change

  pid = spawn( MyFunModule, :fun_stuff, [] )

  pid = spawn( MyFunModule, :fun_stuff, [[]] )
it will then run without the process error above.
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