[mrkeen]

There isn't really a middle-ground (with regard to side-effects) despite people arguing for or against it - much like those memes where someone declares that they're not 'for' or 'against' abortion.

Either I can rely on the fact that `foo() == foo()`, or I can't.

Either I can rely on the fact that `map f . map g == map (f . g)`, or I can't.

[mrkeen]

Care to enlighten me?

[implicit]

The properties you describe regarding pure functions are correct and useful, but those ideas miss the point:

Neither OO nor pure FP are required to build high-quality software!

OCaml developers have been living in this world for decades now and they seem to get by just fine. Further, they do this purely by choice! Their language has had OO syntax for decades and they are happy ignoring it!