[Haskell-beginners] Derived data

[Mark Carter]

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.