I wasn't able to find a definitive account of why they're separated in Ada, but I believe the idea is this: functions are for where code behaves in a roughly functionally pure way and yields a value that should not be discarded by the caller, whereas procs are for functionality with 'deep' side-effects.

The SPARK subset of Ada comes pretty close to enforcing purity of Ada functions, although it still permits them to read globals. [0]

It's not just an oversight. The core of the Ada language was designed deliberately. [1]

[0] https://learn.adacore.com/courses/intro-to-spark/chapters/01...

[1] https://en.wikipedia.org/wiki/Ada_(programming_language)#His...