(lispref.info)Equality Predicates


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Equality Predicates
===================

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

 - Function: eq OBJECT1 OBJECT2
     This function returns `t' if OBJECT1 and OBJECT2 are the same
     object, `nil' otherwise.  The "same object" means that a change in
     one will be reflected by the same change in the other.

     `eq' returns `t' if OBJECT1 and OBJECT2 are integers with the same
     value.  Also, since symbol names are normally unique, if the
     arguments are symbols with the same name, they are `eq'.  For
     other 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.

     (The `make-symbol' function returns an uninterned symbol that is
     not interned in the standard `obarray'.  When uninterned symbols
     are in use, symbol names are no longer unique.  Distinct symbols
     with the same name are not `eq'.  Note: Creating Symbols.)

          (eq 'foo 'foo)
               => t
          
          (eq 456 456)
               => t
          
          (eq "asdf" "asdf")
               => nil
          
          (eq '(1 (2 (3))) '(1 (2 (3))))
               => nil
          
          (eq [(1 2) 3] [(1 2) 3])
               => nil
          
          (eq (point-marker) (point-marker))
               => nil


 - Function: equal OBJECT1 OBJECT2
     This function returns `t' if OBJECT1 and OBJECT2 have equal
     components, `nil' otherwise.  Whereas `eq' tests if its arguments
     are the same object, `equal' looks inside nonidentical arguments
     to see if their elements 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.

          (equal "asdf" "ASDF")
               => nil

   The test for equality is implemented recursively, and circular lists
may therefore cause infinite recursion (leading to an error).


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