fn

(fn name? [params...] body...) (fn name? ([params...] body...)+

Builds a lambda. The optional name binds itself for self-calls (a labels self-binding, so the recursive call is direct). Several arities are one lambda dispatching on the argument count, the same way a multi-arity defn dispatches; clauses close over the outer scope (there are no local functions). Parameters destructure, vector and map patterns alike. A recur in the body jumps back to the enclosing clause with new argument values.

The reader's #(...) form reads as the oracle's reader reads it, (fn* [p1__N# ...] (body...)): % and %1 are the first parameter, %N the Nth, %& the rest, and the parameter vector runs up to the highest %N used, so a call with any other count signals. The body forms wrap as ONE call -- multi-form bodies need an explicit do. A #(...) inside another is refused. Quoted, read with read-string or passed to a macro, it is that fn* form.