Impure from the start

Pure Datalog is the safe core, but most of our programs already used features beyond it:

Arithmetic did the counting: even/odd stepped with N - 1, and the parser advanced with F + 1.
Aggregates folded whole groups: the Titanic count, avg, min and max.
The JSON value type, coming up next, is one more step out.

This is what lets Datalog compute, not merely query. The catch is on the next slide.

"Impure" is not a warning; it describes most real programs. The point is knowing which guarantee you trade away, and when.