Poolboy example error: (Mix) Could not start application poolboy_py_elixir

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(Mix) Could not start application poolboy_py_elixir: PoolboyPyElixir.Application.start(:normal, []) returned an error: shutdown: failed to start child: :square_worker
    ** (EXIT) an exception was raised:
        ** (MatchError) no match of right hand side value:
{:error, {:EXIT, {:undef, [{PoolboyPyElixir.SquareWorker, :start_link, [[]], []}, {:supervisor, :do_start_child_i, 3, [file: 'supervisor.erl', line: 414]}, {:supervisor, :handle_call, 3, [file: 'supervisor.erl', line: 439]}, 
{:gen_server, :try_handle_call, 4, [file: 'gen_server.erl', line: 721]}, 
{:gen_server, :handle_msg, 6, [file: 'gen_server.erl', line: 750]}, 
{:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 226]}]}}}

The sample repo’s application.ex is here.

I am trying to follow this post.

This is first time trying to understand poolboy. Any help is appreciated.

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It’s probably because the start_link in your SquareWorker module expect 0 argument while the supervisor starting the process expects 1 argument.

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that is not obvious from the error, right? :sweat_smile:

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Well, it’s not really obvious, but when you see that after the error:

You see that something is undefined, and if you know about the mfa notation (module, function, arguments), namely PoolboyPyElixir.SquareWorker is the module, :start_link is the function, and [[]] are the arguments (one arg of empty list), it starts to make sense. Especially if you keep in my that in Erlang/Elixir, the arity of a function is part of its signature.

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thanks for insight on reading the error message. Kudos for the acronym mfa.

This helps me become 1% better debugger.

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