1. LISP: Basic Functionality
S-expressions
Conses
Lists
Predicates
Evaluation and quoting
Conditional Evaluation
CSE S. Tanimoto Lisps's Basic Functionality

2. 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 Lisps's Basic Functionality

3. 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 Lisps's Basic Functionality

4. 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 S. Tanimoto Lisps's Basic Functionality

5. CSE 341 -- S. Tanimoto Lisps's Basic Functionality
Examples of Lists
> ’(a b c d e)
(A B C D E)
> ()
NIL
> nil
> ’()
> ’(apple . (banana . (lime . nil)))
(APPLE BANANA LIME)
CSE S. Tanimoto Lisps's Basic Functionality

6. Predicates That Identify Lists
> (atom ’(a b c))
NIL
> (atom ’x) T
> (consp ’(a b c))
> (consp ’x)
CSE S. Tanimoto Lisps's Basic Functionality

7. 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 S. Tanimoto Lisps's Basic Functionality

8. 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 Lisps's Basic Functionality

9. CSE 341 -- S. Tanimoto Lisps's Basic Functionality
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 Lisps's Basic Functionality

10. 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 Lisps's Basic Functionality

11. CSE 341 -- S. Tanimoto Lisps's Basic Functionality
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 S. Tanimoto Lisps's Basic Functionality

12. CSE 341 -- S. Tanimoto Lisps's Basic Functionality
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 Lisps's Basic Functionality

13. CSE 341 -- S. Tanimoto Lisps's Basic FunctionalityIF
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 value.
> (setq x 10) 10
> (if (> x 2) (- x 1) (+ x 1)) 9
CSE S. Tanimoto Lisps's Basic Functionality