It's probably a side effect if...

Subsequent calls to the same function with the same input do not return the same value

val x, y
fn f
assert f(x) == y
refute f(x) == y //why?

A function does not return a value

fn f
f //what for?

Calling one function impacts the return value of a different function

val x, y
fn f, g
assert f(x) == y
g(x)
refute f(x) == y //why?

All of these code samples are pretty strong indicators of a side effect present in function f. They also all suffer from two pretty big problems:

• They are confusing and hard to reason about
• They depend on multiple functions, multiple values, and/or execution sequence, which makes them hard to test

I find these pretty good reasons to be mindful of introducing side effects, and do it only when really necessary.