binding

(binding [var init ...] body...)

Rebinds each var around the body with dynamic extent: code the body calls sees the binding, and the root returns after. Only ^:dynamic vars and the clojure.core specials (*out*, *in*, *err*, *agent* and the flags such as *print-length* and *assert* -- bare or clojure.core/-qualified, since a syntax-quote qualifies them) may be bound; anything else is refused, like the oracle's non-dynamic error. A ^:dynamic defn holds its function in the var, so it rebinds too (any other defn stays refused). Inits run sequentially, like let.

*in* is *standard-input* (never nil), so rebinding it feeds readers like (.readLine *in*) -- which reads through read-line on a stream, answering nil past the end: