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ANSI Common Lisp  5 Data and Control Flow 5.3 Dictionary of Data and Control Flow
- Syntax:
-
equal
x y
 generalized-boolean
- Arguments and Values:
-
x - an object.
y - an object.
generalized-boolean - a generalized boolean.
- Description:
-
Returns true if x and y are structurally similar
(isomorphic) objects. Objects are treated as follows by
equal.
- Symbols, Numbers, and Characters
equal is true of two objects
if they are symbols that are eq,
if they are numbers that are eql, or
if they are characters that are eql.
- Conses
For conses, equal is defined recursively as
the two cars being equal
and the two cdrs being equal.
- Arrays
Two arrays are equal only if they are eq,
with one exception:
strings and bit vectors are compared element-by-element (using eql).
If either x or y has a fill pointer, the
fill pointer limits
the number of elements examined by equal.
Uppercase and lowercase letters in strings are considered by
equal to be different.
- Pathnames
Two pathnames are equal if and only if
all the corresponding components
(host, device, and so on) are
equivalent. Whether or not
uppercase and lowercase letters are considered equivalent
in strings appearing in components is implementation-dependent.
pathnames
that are equal should be functionally equivalent.
- Other (Structures, hash-tables, instances, ... )
Two other objects are equal only if they are eq.
equal does not descend any objects other than the
ones explicitly specified above.
The next figure summarizes the information given in the previous list.
In addition, the figure specifies the priority of the behavior of equal,
with upper
entries taking priority over lower ones.
Any two objects that are eql are also equal.
equal may fail to terminate if x or y is circular.
- Examples:
-
(equal 'a 'b) false
(equal 'a 'a) true
(equal 3 3) true
(equal 3 3.0) false
(equal 3.0 3.0) true
(equal #c(3 -4) #c(3 -4)) true
(equal #c(3 -4.0) #c(3 -4)) false
(equal (cons 'a 'b) (cons 'a 'c)) false
(equal (cons 'a 'b) (cons 'a 'b)) true
(equal #\A #\A) true
(equal #\A #\a) false
(equal "Foo" "Foo") true
(equal "Foo" (copy-seq "Foo")) true
(equal "FOO" "foo") false
(equal "This-string" "This-string") true
(equal "This-string" "this-string") false
- See Also:
-
eq, eql, equalp, =,
string=, string-equal, char=,
char-equal, tree-equal
- Notes:
-
Object equality is not a concept for which there is a uniquely
determined correct algorithm. The appropriateness of an equality
predicate can be judged only in the context of the needs of some
particular program. Although these functions take any type of
argument and their names sound very generic,
equal and equalp are
not appropriate for every application.
A rough rule of thumb is that two objects are equal
if and only if their printed representations are the same.
- Allegro CL Implementation Details:
-
None.
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