Specs

map_key(key :: atom(), value :: integer(), keys :: keys(), state :: any()) ::
  {:ok, nil, state :: any()}
  | {:ok, codepoint :: String.t(), state :: any()}
  | {:error, msg :: String.t(), state :: any()}

Translate a key to a codepoint, which is really just a string.

The first time this is called, state is nil. After that you can return any state that makes sense and it will be passed back on the next call.

If the mapping is successful, i.e. the key press results in a valid character, Then this function should return { :ok, codepoint, state }. The returned codepoint will be sent on to the ViewPort as a codepoint input event.

If the key press does not map to a string (this is common), then the function should return { :ok, nil, state }. This will not result in a codepoint input being sent to the ViewPort.

If the data makes no sense at all, then you can return { :error, error_msg, state }. This will not send a codepoint input, but will log the error message, which should be a string.