Lisp: Representation of Data Atoms: Symbols, numbers, functions, strings, arrays, structures. Conses: Lists, dotted pairs. Forms: functional forms, special forms, macro forms. Evaluation of functional forms. Special forms: QUOTE, SETQ, IF CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

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 415 -- (c) S. Tanimoto, 2004 Lisp representation

Lisp S-Expressions: CONSes (Again, 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 . 3.14159) (X . (Y . Z)) CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

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 415 -- (c) S. Tanimoto, 2004 Lisp representation

CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation Examples of Lists > ’(a b c d e) (A B C D E) > () NIL > nil > ’() > ’(apple . (banana . (lime . nil))) (APPLE BANANA LIME) CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

Predicates That Identify Lists > (atom ’(a b c)) NIL > (atom ’x) T > (consp ’(a b c)) > (consp ’x) CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

List predicates (continued) > (listp ’(a b c)) T > (listp ’x) NIL > (consp ’()) ; NIL is not a cons. > (listp ’()) ; NIL is a list. > (consp ’(a . b)) > (listp ’(a . b)) ;note listp’s limitation. CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

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 415 -- (c) S. Tanimoto, 2004 Lisp representation

CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation 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. > (+ 1 2 3) ; a functional form 6 > (functionp #’+) T > (setq x 5) ; a special form 5 > (functionp #’setq) ; Error! CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

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 415 -- (c) S. Tanimoto, 2004 Lisp representation

CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation 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 (+ 1 2 3)) (+ 1 2 3) > (quote x) X > ’(+ 1 2 3) > ’x CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation 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 (+ 1 2 3)) 6 > (setq x (* x 3)) 18 > (setq y ’(+ 1 2 3)) (+ 1 2 3) > (setq y (rest y)) (1 2 3) > (setq y (first y)) 1 CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation

CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation 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 CSE 415 -- (c) S. Tanimoto, 2004 Lisp representation