A side effect is an observable action or state change that occurs while a computation runs, apart from the value it returns. Printing, advancing a stream’s read position, and updating a transient collection are examples. Clojure favors value-producing transformations, but provides iteration forms for work whose purpose is an effect. section 3.4.4 section 3.4.5 section 20.5
Choose an iteration form according to whether you need results. `dotimes` executes a body a fixed number of times and always returns `nil`; a count of zero or less skips the body. `repeatedly` instead builds a lazy sequence from calls to a no-argument function. Its calls can produce different values or launch work, but occur as the sequence is consumed. section 3.4.5 section 9.2.1
Effects versus collected values
`(dotimes [i 3] (println i))` prints `0`, `1`, and `2`, then returns `nil`. By contrast, `(take 3 (repeatedly rand))` yields three random values. Use `dotimes` when there are no results to collect; use `repeatedly` when the generated values matter. section 3.4.5 section 9.2.1
`while` is useful when each test must observe changing conditions. Reading from a Java stream, for example, advances its position; successive tests can therefore detect the end of the stream. For ordinary iteration, prefer approaches that do not require effects to make progress, such as sequence transformations. section 3.4.4
