A closure is a function that retains references to bindings from its defining lexical environment. In Clojure, fn* exposes a specialized way to create closure objects for computations known to run only once, allowing captured references to be cleared after the result returns. section 2.1.3
The once-only form differs from an ordinary fn in several practical ways:
- fn* underlies the fn macro but supports fewer features, including no pre- and post-conditions or destructuring. section 2.1.3
- A normal lambda may remain reachable for later executions, while fn* can clear references held by a once-only function after it returns. section 2.1.3
- The ^:once metadata form gives the compiler a special compile-time meaning that ordinary fn does not support. section 2.1.3
Delegating a macro to a once-only closure
A common pattern wraps an unevaluated macro body in an anonymous once-only function: (defmacro future [& body] `(future-call (^{:once true} fn* [] ~@body))). This lets the macro delegate execution to a function while avoiding unnecessary retention of captured data. section 2.1.3
