1. 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 341, S. Tanimoto Lisp 2 -

2. 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 341, S. Tanimoto Lisp 2 -

3. 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 S Sn)
CSE 341, S. Tanimoto Lisp 2 -

4. Examples of Lists > ’(a b c d e) (A B C D E) > () NIL > nil
> ’()
> ’(apple . (banana . (lime . nil)))
(APPLE BANANA LIME)
CSE 341, S. Tanimoto Lisp 2 -

5. Predicates That Identify Lists
> (atom ’(a b c))
NIL
> (atom ’x) T
> (consp ’(a b c))
> (consp ’x)
CSE 341, S. Tanimoto Lisp 2 -

6. 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 341, S. Tanimoto Lisp 2 -

7. 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 341, S. Tanimoto Lisp 2 -

8. 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 341, S. Tanimoto Lisp 2 -

9. 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 341, S. Tanimoto Lisp 2 -

10. 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
CSE 341, S. Tanimoto Lisp 2 -

11. 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 341, S. Tanimoto Lisp 2 -

12. IF
IF is a special form that evaluates its first argument. If the result is NIL it skips its second argument and evaluates and returns its third element if any. If the 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 341, S. Tanimoto Lisp 2 -