Immutability of targets and pure functions

[krlmlr]

concept from FP.

[krlmlr]

@wlandau: Have you thought about the many concepts from functional programming that are picked up by drake and other workflow managers?

[wlandau-lilly]

Regarding immutability specifically, it first crossed my mind here. In a way, immutability reinforces reproducibility. And the way that drake expects commands that depend on other functions touches on FP a bit. Other parallels have not occurred to me.

[krlmlr]

I did mention purity in the workshop: Ideally, all commands should be pure or at least idempotent (repeated execution doesn't change the results). That's difficult/impossible to check for drake, it's something that the user needs to keep in mind. But we can help by capturing console output.

[wlandau]

Drake does try to counter side effects by wrapping commands in calls to local(). But I agree, other requirements of purity are difficult to check. We could add output to the metadata (alongside warnings and messages) roughly here.