Unused variable warnings when variables created in map pattern match

[carusso]

I have some function headers like so:

defcall set_open_device(%{"path" => path, "baud" => speed}), state: state do

"path" and "speed" are used in the function, but I see the following compiler warnings:

warning: variable "path" is unused
  lib/rni/dispatch/fmt.ex:53

warning: variable "speed" is unused
  lib/rni/dispatch/fmt.ex:53

[sasa1977]

Hi,

Unfortunately, this is the consequence of implicit generation of two functions. Thedefcall expression in your example will lead to the following functions being generated:

def set_open_device(server, %{"path" => path, "baud" => speed} = payload) do 
  GenServer.call(server, {:set_open_device, payload})
end

def handle_call({set_open_device, %{"path" => path, "baud" => speed}}, _from, state) do
  # ...
end

So the warning happens because path and speed are not used in the interface function.

A simple fix could be to pattern match in the body:

defcall set_open_device(payload), state: state do
  %{"path" => path, "baud" => speed} = payload
  # ...
end

This is one of the reasons why I personally don't use ExActor anymore, nor recommend using it. My advice is to stick to plain GenServer. It might mean a bit more typing, but at least it's explicit, with no magic in between, and you're in complete control of the code.