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2.8 Equality Predicates

Here we describe functions that test for equality between two objects. Other functions test equality of contents between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type.

Function: eq object1 object2

This function returns t if object1 and object2 are the same object, and nil otherwise.

If object1 and object2 are symbols with the same name, they are normally the same object—but see Creating Symbols for exceptions. For other non-numeric types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarily eq to each other: they are eq only if they are the same object, meaning that a change in the contents of one will be reflected by the same change in the contents of the other.

If object1 and object2 are numbers with differing types or values, then they cannot be the same object and eq returns nil. If they are fixnums with the same value, then they are the same object and eq returns t. If they were computed separately but happen to have the same value and the same non-fixnum numeric type, then they might or might not be the same object, and eq returns t or nil depending on whether the Lisp interpreter created one object or two.

(eq 'foo 'foo)
     ⇒ t

(eq ?A ?A)
     ⇒ t

(eq 3.0 3.0)
     ⇒ t or nil
;; Equal floats may or may not be the same object.

(eq (make-string 3 ?A) (make-string 3 ?A))
     ⇒ nil

(eq 'asdf' 'asdf')
     ⇒ t or nil
;; Equal string constants or may not be the same object.

(eq '(1 (2 (3))) '(1 (2 (3))))
     ⇒ nil

(setq foo '(1 (2 (3))))
     ⇒ (1 (2 (3)))
(eq foo foo)
     ⇒ t
(eq foo '(1 (2 (3))))
     ⇒ nil

(eq [(1 2) 3] [(1 2) 3])
     ⇒ nil

(eq (point-marker) (point-marker))
     ⇒ nil

The make-symbol function returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are not eq. See Creating Symbols.

(eq (make-symbol 'foo') 'foo)
     ⇒ nil

The Emacs Lisp byte compiler may collapse identical literal objects, such as literal strings, into references to the same object, with the effect that the byte-compiled code will compare such objects as eq, while the interpreted version of the same code will not. Therefore, your code should never rely on objects with the same literal contents being either eq or not eq, it should instead use functions that compare object contents such as equal, described below. Similarly, your code should not modify literal objects (e.g., put text properties on literal strings), since doing that might affect other literal objects of the same contents, if the byte compiler collapses them.

Function: equal object1 object2

This function returns t if object1 and object2 have equal components, and nil otherwise. Whereas eq tests if its arguments are the same object, equal looks inside nonidentical arguments to see if their elements or contents are the same. So, if two objects are eq, they are equal, but the converse is not always true.

(equal 'foo 'foo)
     ⇒ t

(equal 456 456)
     ⇒ t

(equal 'asdf' 'asdf')
     ⇒ t
(eq 'asdf' 'asdf')
     ⇒ nil

(equal '(1 (2 (3))) '(1 (2 (3))))
     ⇒ t
(eq '(1 (2 (3))) '(1 (2 (3))))
     ⇒ nil

(equal [(1 2) 3] [(1 2) 3])
     ⇒ t
(eq [(1 2) 3] [(1 2) 3])
     ⇒ nil

(equal (point-marker) (point-marker))
     ⇒ t

(eq (point-marker) (point-marker))
     ⇒ nil

Comparison of strings is case-sensitive, but does not take account of text properties—it compares only the characters in the strings. See Text Properties. Use equal-including-properties to also compare text properties. For technical reasons, a unibyte string and a multibyte string are equal if and only if they contain the same sequence of character codes and all these codes are in the range 0 through 127 (ASCII).

(equal 'asdf' 'ASDF')
     ⇒ nil

However, two distinct buffers are never considered equal, even if their textual contents are the same.

For equal, equality is defined recursively; for example, given two cons cells x and y, (equal x y) returns t if and only if both the expressions below return t:

(equal (car x) (car y))
(equal (cdr x) (cdr y))

Comparing circular lists may therefore cause deep recursion that leads to an error, and this may result in counterintuitive behavior such as (equal a b) returning t whereas (equal b a) signals an error.

Function: equal-including-properties object1 object2

This function behaves like equal in all cases but also requires that for two strings to be equal, they have the same text properties.

(equal 'asdf' (propertize 'asdf' 'asdf t))
     ⇒ t
(equal-including-properties 'asdf'
                            (propertize 'asdf' 'asdf t))
     ⇒ nil

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