You probably won't be able to make a single function out of every outcome of a key press which is why I wouldn't use functions to begin with because what you're describing is not repeatable bits of code. I use events based on the key pressed, not detect any key pressed then pass the key pressed through a function and determine it's outcome all inside one function, seems unnecessary.
If I press 'down' then the player falls through a platform, that's one event. In a separate event if I press down and I am in the menu then other things may happen based off menu=1, I don't need functions.
You can also make use of groups. If you are in the menu then you can disable the player controls group for example.
Maybe some ******** developer has got an idea about these 'callbacks' and will respond with their input but I wouldn't use them myself.