Behaviour :gen_statem does not implement child_spec

Use GenStateMachine?

Well yes it has a default implementation for child_spec, but its not worth it since it limits the API of gen_statem

As it states in the error that is raised, you can either define the child_spec/1 function in your module or you have to pass a child specification as a map:

** (ArgumentError) The module FooBar was given as a child to a supervisor
but it does not implement child_spec/1.

If you own the given module, please define a child_spec/1 function
that receives an argument and returns a child specification as a map.
For example:

    def child_spec(opts) do
      %{
        id: __MODULE__,
        start: {__MODULE__, :start_link, [opts]},
        type: :worker,
        restart: :permanent,
        shutdown: 500
      }
    end

Note that "use Agent", "use GenServer" and so on automatically define
this function for you.

However, if you don't own the given module and it doesn't implement
child_spec/1, instead of passing the module name directly as a supervisor
child, you will have to pass a child specification as a map:

    %{
      id: FooBar,
      start: {FooBar, :start_link, [arg1, arg2]}
    }

See the Supervisor documentation for more information

gen_statem doesn’t include child_spec/1 by default. Add it manually like this:

def child_spec(args) do
  %{
    id: __MODULE__,
    start: {__MODULE__, :start_link, [args]},
    restart: :permanent,
    shutdown: 5000,
    type: :worker
  }
end

It should fix the issue

Out of interest, what’s missing?

Yes, it’s not that hard to implement. Some people are just lazy, I guess.