Spoilers for Advent of Code 2019 follow.
Fuel required to launch a given module is based on its mass. Specifically, to find the fuel required for a module, take its mass, divide by three, round down, and subtract 2.
This describes a simple function. There seems to be an oversight in the problem statement that modules with very low mass have a negative fuel requirement. I'm going to assume that's not right, and that instead of integer subtraction, we want natural number subtraction (sometimes called "monus"). In Unison, we can use the Nat type instead of integers, so we don't have to consider negatives. The subtraction operation is called drop: