6.11.1 Equality

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 section 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 the same.

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).

eqv? extends eq? to look at the value of numbers and characters. It can for instance be used somewhat like = (see section Comparison Predicates) but without an error if one operand isn’t a number.

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), and equal? will consider them the same.

Scheme Procedure: eq? x y

C Function: scm_eq_p (x, y)

Return #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*

Generally eqv? below should be used when comparing numbers or characters. = (see section Comparison Predicates) or char=? (see section Characters) can be used too.

It’s worth noting that end-of-list (), #t, #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

C Function: int scm_is_eq (SCM x, SCM y)

Return 1 when x and y are equal in the sense of eq?, otherwise return 0.

The == operator should not be used on SCM values, an SCM is a C type which cannot necessarily be compared using == (see section The SCM Type).

Scheme Procedure: eqv? x y

C Function: scm_eqv_p (x, y)

Return #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 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

Scheme Procedure: equal? x y

C Function: scm_equal_p (x, y)

Return #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 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 section 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 per eq?.

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