There are three kinds of core equality predicates in Scheme, described
below. The same kinds of comparisons arise in other functions, like
memq and friends (see List Searching).
For all three tests, objects of different types are never equal. So
for instance a list and a vector are not
equal?, even if their
contents are the same. Exact and inexact numbers are considered
different types too, and are hence not equal even if their values are
eq? tests just for the same object (essentially a pointer
comparison). This is fast, and can be used when searching for a
particular object, or when working with symbols or keywords (which are
always unique objects).
eq? to look at the value of numbers and
characters. It can for instance be used somewhat like
(see Comparison) but without an error if one operand isn't a
equal? goes further, it looks (recursively) into the contents
of lists, vectors, etc. This is good for instance on lists that have
been read or calculated in various places and are the same, just not
made up of the same pairs. Such lists look the same (when printed),
equal? will consider them the same.
(define x (vector 1 2 3)) (define y (vector 1 2 3)) (eq? x x) ⇒ #t (eq? x y) ⇒ #f
Numbers and characters are not equal to any other object, but the problem is they're not necessarily
eq?to themselves either. This is even so when the number comes directly from a variable,(let ((n (+ 2 3))) (eq? n n)) ⇒ *unspecified*
It's worth noting that end-of-list
#f, a symbol of a given name, and a keyword of a given name, are unique objects. There's just one of each, so for instance no matter how
()arises in a program, it's the same object and can be compared with
eq?,(define x (cdr '(123))) (define y (cdr '(456))) (eq? x y) ⇒ #t (define x (string->symbol "foo")) (eq? x 'foo) ⇒ #t
1when x and y are equal in the sense of
eq?, otherwise return
==operator should not be used on
SCMis a C type which cannot necessarily be compared using
==(see The SCM Type).
On objects except characters and numbers,
eqv?is the same as
eq?above, it's true if x and y are the same object.
If x and y are numbers or characters,
eqv?compares their type and value. An exact number is not
eqv?to an inexact number (even if their value is the same).(eqv? 3 (+ 1 2)) ⇒ #t (eqv? 1 1.0) ⇒ #f
For a pair, string, vector, array or structure,
equal?compares the contents, and does so using using the same
equal?recursively, so a deep structure can be traversed.(equal? (list 1 2 3) (list 1 2 3)) ⇒ #t (equal? (list 1 2 3) (vector 1 2 3)) ⇒ #f
For other objects,
equal?compares as per
eqv?above, which means characters and numbers are compared by type and value (and like
eqv?, exact and inexact numbers are not
equal?, even if their value is the same).(equal? 3 (+ 1 2)) ⇒ #t (equal? 1 1.0) ⇒ #f
Hash tables are currently only compared as per
eq?, so two different tables are not
equal?, even if their contents are the same.
equal?does not support circular data structures, it may go into an infinite loop if asked to compare two circular lists or similar.
New application-defined object types (see Defining New Types (Smobs)) have an
equalphandler which is called by
equal?. This lets an application traverse the contents or control what is considered
equal?for two objects of such a type. If there's no such handler, the default is to just compare as per