[kaba0]

A pure function is by definition side-effect free. What you write makes 0 sense. Maybe thinking of total functions?

But not all FP languages have to be so puritan to only allow pure functions, most have escape hatches and it is just good form to prefer purity (even in more mainstream languages!). The most common way to circumvent this problem is through a Monad, which very naively put, just a description of the order of side-effecting functions, and their inter-dependencies. This will later get executed at a specific place, e.g. the main function -- the point is, that a large part of the code will be pure, and much easier to reason about.