is it safe to use side-effecting operations in a transducer on a pipeline-blocking?

as in: the side effect will only be executed once

it is complicated, what do you mean by side-effecting? there are transducers that use mutable state internally, and those generally don't work.

something like (map (fn [x] (rest-call x))) should be fine

the transducer is a composition of map and filter's

By side effecting do you mean i/o?

As @hiredman points out, stateful transducers get a bit weird because they don't coordinate across pipeline workers.

But i/o shouldn't execute multiple times.