if statement - When to use "if" and "when" in Clojure?

Question

When is one better than the other? Is one faster than the other or does the only difference is the return of false or nil?

Answer 1

Use if when you have exactly one truthy and one falsy case and you don't need an implicit do block. In contrast, when should be used when you only have to handle the truthy case and the implicit do. There is no difference in speed, it's a matter of using the most idiomatic style.

 (if (my-predicate? my-data)
     (do-something my-data)
     (do-something-else my-data))

 (when (my-predicate? my-data)
     (do-something my-data)
     (do-something-additionally my-data))

In the if case, only do-something will be run if my-predicate? returns a truthy result, whereas in the when case, both do-something and do-something-additionally are executed.