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.
#t if x and y are the same object, except
for numbers and characters. For example,
(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*
eqv? below should be used when comparing numbers or
= (see Comparison) or
(see Characters) can be used too.
It’s worth noting that end-of-list
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
(define x (cdr '(123))) (define y (cdr '(456))) (eq? x y) ⇒ #t (define x (string->symbol "foo")) (eq? x 'foo) ⇒ #t
1 when x and y are equal in the sense of
eq?, otherwise return
== operator should not be used on
SCM values, an
SCM is a C type which cannot necessarily be compared using
== (see The SCM Type).
#t if x and y are the same object, or for
characters and numbers the same value.
On objects except characters and numbers,
eqv? is the same as
eq? above, it’s true if x and y are the same
If x and y are numbers or characters,
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
#t if x and y are the same type, and their
contents or value are equal.
For a pair, string, vector, array or structure,
equal? compares the
contents, and does so using the same
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
which means characters and numbers are compared by type and value (and
eqv?, exact and inexact numbers are not
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
equal? does not support circular data structures, it may go
into an infinite loop if asked to compare two circular lists or
New application-defined object types (see Defining New Types (Smobs)) have an
equalp handler 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