let & let* in Lisp

let binds all its names at once; let* builds each one on the last.

;; let binds in parallel; let* lets each binding see the ones above it
(defun cart-total (subtotal)
  (let* ((tax-rate 0.08)
         (tax (* subtotal tax-rate))
         (shipping (if (> subtotal 50) 0 5.95)))
    (+ subtotal tax shipping)))

(defparameter *basket* 40)

;; the inner binding shadows the outer one for the length of its body
(let ((discount 0.10))
  (format t "everyone gets ~a~%" discount)
  (let ((discount 0.25))
    (format t "members get ~a~%" discount))
  (format t "and back to ~a~%" discount))

(format t "cart comes to ~$~%" (cart-total *basket*))

How it works

1. Inside cart-total, let* lets tax use the tax-rate bound just above it.
2. The shipping binding waives the 5.95 charge once subtotal clears 50.
3. A nested let rebinds discount to 0.25, and the outer 0.10 returns as soon as that body ends.

Keywords and builtins used here

• defparameter
• defun
• format
• if
• let

The run, in numbers

Lines

17

Characters to type

539

Tokens

95

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 76 seconds.

Type this snippet

Step 2 of 3 in Values & bindings, step 2 of 27 in Language basics.

← Previous Next →