The same way that purely functional languages do input/output. (think, how do purely functional languages even write to the console, if they are in fact pure?

The same way that purely functional languages do input/output. (think, how do purely functional languages even write to the console, if they are in fact pure?). Creating the initial random generator is an effectful operation much like writing to the console; it get's handled in the Main/root part of the program. Understanding exactly how input/output in pure languages work involves Understanding the distinction between the RTS (Run Time System - the thing that runs the effects) and your Program (the recipe for what effects to run).