let

let

This opens a new scope with new variables.
(let (arg1 val1 ...) body ...) is syntactic sugar for ((λ (arg1 arg2 ...) body ...) val1 val2 ...).

(let (x 1 y 2)
    ...)

-> ((λ (x y) ...) 1 2)

A let operator can have a label, allowing for recursion without adding a name to the enclosing environment.

(def things '(1 2 3))

(let loop (x things.head xs things.tail acc ())
    (if xs
        (loop xs.head xs.tail (acc.cons x))
        (acc.cons x)))

-> (3 2 1)