[karmakaze]

Pure functions are useless without inputs and outputs. The missing piece of the puzzle here is that we only want transitions between valid states. One way to define valid states is by enforcing invariants which must always be true. Armed with these, all the stuff that happens in-between can be guarded against, much like constraints in a database schema.

Contrast this with non-pure functions which incrementally mutate from a valid state, to a sequence of invalid states, and end up at a valid state again. If something goes wrong along the way, we end up with an invalid state. Think about program state like it's a journaling file system.