> As "observable state goes from a to b" You might also notice that this is equivalent to a function that accepts a and returns b. Which lets one use equivalen

> As "observable state goes from a to b" You might also notice that this is equivalent to a function that accepts a and returns b. Which lets one use equivalence partitioning to plan the tests: https://en.wikipedia.org/wiki/Equivalence_partitioning https://en.wikipedia.org/wiki/Equivalence_partitioning Except that with singletons, ‘context objects’ or similar quasi-global state, the effect might be non-isolated, and effects may be silently introduced that the calling code doesn't know about.