[Haskell-beginners] Derived data

I read inputs from files into various data types. Not all of the information is known at input time, so some of the data for a type needs to be derived. The question is: what is the best way of "guarding" between derived and underived data? By that I mean, there is a function which will derive missing data, and other functions can only work on derived data. Working on underived data would be a mistake.