Use ? in predicate function name, use ! in side-effect function name

[kawas44]

Advise on function naming

Use ? at the end of predicate function name to visually hint users.
ex: nil? and empty?

use ! at the end of side-effect function name to visually hint users
ex: pop! and assoc!

[bbatsov]

There is already advice on that subject in the Naming section.

[halgari]

And to be clear, in clojure ! means that it's not STM safe (as the guide currently states), not that it's side-effect free. Notice that send and send-off have side-effects, but are STM safe, hence the lack of !

[bbatsov]

@halgari is absolutely right.