CSE S. Tanimoto Lisp Lisp S-Expressions: ATOMs Every Lisp object is either an ATOM or a CONS Symbols and numbers are kinds of atoms: X, APPLE, A-SYMBOL 1, 5.7, 3/5 Many other Lisp data objects are considered to be atoms (even strings and arrays are atoms!).

CSE S. Tanimoto Lisp Lisp S-Expressions: CONSes Every Lisp object is either an ATOM or a CONS A CONS represents an association or pairing of two other Lisp objects. (A. B) (APPLE. RED) (PI ) (X. (Y. Z))

CSE S. Tanimoto Lisp Lisp S-Expressions: Lists We define lists as follows: The symbol NIL is a list; it’s the empty list. This list is written in list notation as ( ) Any cons having the following structure is a list, (S1. (S2. (... (Sn. NIL)... ) ) ) where each S1 is either an atom or a cons. This list is written in list notation as (S1 S2... Sn)

CSE S. Tanimoto Lisp Examples of Lists > ’(a b c d e) (A B C D E) > () NIL > nil NIL > ’() NIL > ’(apple. (banana. (lime. nil))) (APPLE BANANA LIME)

CSE S. Tanimoto Lisp Predicates That Identify Lists > (atom ’(a b c)) NIL > (atom ’x) T > (consp ’(a b c)) T > (consp ’x) NIL

CSE S. Tanimoto Lisp List predicates (continued) > (listp ’(a b c)) T > (listp ’x) NIL > (consp ’()) ; NIL is not a cons. NIL > (listp ’()) ; NIL is a list. T > (consp ’(a. b)) T > (listp ’(a. b)) ;note listp’s limitation. T

CSE S. Tanimoto Lisp Lisp Tries to Print Conses as Lists > ’(a. (b. c)) (A B. C) > ’(a. nil) (A) > ’((a. b). (c. d)) ((A. B)C. D) > ’((nil. nil). (nil. nil)) ((NIL)NIL)

CSE S. Tanimoto Lisp Lisp Forms A form is a list whose first element is a symbol that names an operator. If the first element names a function, then the form is a functional form. > ( ) ; a functional form 6 > (functionp #’+) T > (setq x 5) ; a special form 5 > (functionp #’setq) ; Error!

CSE S. Tanimoto Lisp Evaluation of Functional Forms A functional form is evaluted as follows: If there are any arguments in the form, then they are evaluated in left-to-right order. The number of arguments is compared with the number permitted by the function named by the first element of the form. If the number is compatible, then the function is applied to the values of the arguments.

CSE S. Tanimoto Lisp QUOTE Unlike functional forms, special forms are not required to have their arguments evaluated. QUOTE is a special form that returns its argument unevaluated. > (quote ( )) ( ) > (quote x) X > ’( ) ( ) > ’x X

CSE S. Tanimoto Lisp SETQ SETQ is a special form that evaluates its second argument and assigns that value to the symbol which must be its first argument. > (setq x ( )) 6 > (setq x (* x 3)) 18 > (setq y ’( )) ( ) > (setq y (rest y)) (1 2 3) > (setq y (first y)) 1

CSE S. Tanimoto Lisp IF IF is a special form that evaluates its first argument. If the result is NIL is skips its second argument and evaluates and returns its their element if any. If result of evaluating the first element was not NIL, it evaluates the second argument and returns that. > (setq x 10) 10 > (if (> x 2) (- x 1) (+ x 1)) 9