Quoting in Lisp

A quote is the difference between running (+ 2 3) and holding on to it.

;; quote holds a form back instead of running it
(defparameter *sum* (+ 2 3))
(defparameter *unrun* '(+ 2 3))
(format t "evaluated ~a, quoted ~a~%" *sum* *unrun*)

;; symbols are ordinary data once quoted, not just names of variables
(defparameter *weekdays* '(monday tuesday wednesday))
(format t "second day: ~a~%" (second *weekdays*))
(format t "spelled out: ~a~%" (symbol-name 'friday))

;; eq asks whether two things are the same object, equal compares shape
(format t "eq on symbols: ~a~%" (eq 'monday 'monday))
(format t "eq on fresh lists: ~a~%" (eq (list 1 2) (list 1 2)))
(format t "equal on fresh lists: ~a~%" (equal (list 1 2) (list 1 2)))

How it works

1. (+ 2 3) evaluates to 5 while '(+ 2 3) stays a three-element list.
2. '(monday tuesday wednesday) treats symbols as data, reachable with second and readable with symbol-name.
3. eq is true for two references to 'monday but false for two fresh lists that equal accepts.

Keywords and builtins used here

• defparameter
• eq
• equal
• format
• list
• second

The run, in numbers

Lines

14

Characters to type

651

Tokens

103

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 88 seconds.

Type this snippet

Step 3 of 3 in Lists & conses, step 6 of 27 in Language basics.

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