cs307 p. 103

Predicates

(symbol? x) tests whether x is a symbol.
(pair? x) tests whether x is a pair or cons cell. Since every non-empty list is made of pairs, pair? is also used to test for non-empty lists.
(null? x) tests whether x is the empty list, ().
Equality predicates: these differ in what they consider to be ``equal'' and also in cost:
- (eq? x y) tests whether x and y are the same symbol, boolean, or empty list, or are identical pointers to the same object.
```
(eq? 'a 'a)           =  #t
(eq? '(a) '(a))       =  #f
(define lst '(a))
(eq? lst lst)         =  #t
(eq? 3.0 3.0)         =  #f
```
- (eqv? x y) works like eq? and also works for numbers and characters.
```
(eqv? 3.0 3.0)        =  #t
```
- (equal? x y) works like eqv?, and works for list structures, vectors, and strings.
```
(equal? '(a) '(a))    =  #t
(equal? "foo" "foo")  =  #t
```