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3 Equivalence Predicates

A predicate is a procedure that always returns a boolean value (#t or #f). An equivalence predicate is the computational analogue of a mathematical equivalence relation (it is symmetric, reflexive, and transitive). Of the equivalence predicates described in this section, eq? is the finest or most discriminating, and equal? is the coarsest. eqv? is slightly less discriminating than eq?.

— procedure: eqv? obj1 obj2

The eqv? procedure defines a useful equivalence relation on objects. Briefly, it returns #t if obj1 and obj2 should normally be regarded as the same object.

The eqv? procedure returns #t if:

The eqv? procedure returns #f if:

Some examples:

          (eqv? 'a 'a)                    ⇒  #t
          (eqv? 'a 'b)                    ⇒  #f
          (eqv? 2 2)                      ⇒  #t
          (eqv? '() '())                  ⇒  #t
          (eqv? 100000000 100000000)      ⇒  #t
          (eqv? (cons 1 2) (cons 1 2))    ⇒  #f
          (eqv? (lambda () 1)
                (lambda () 2))            ⇒  #f
          (eqv? #f 'nil)                  ⇒  #f
          (let ((p (lambda (x) x)))
            (eqv? p p))                   ⇒  #t

The following examples illustrate cases in which the above rules do not fully specify the behavior of eqv?. All that can be said about such cases is that the value returned by eqv? must be a boolean.

          (eqv? "" "")                    ⇒  unspecified
          (eqv? '#() '#())                ⇒  unspecified
          (eqv? (lambda (x) x)
                (lambda (x) x))           ⇒  unspecified
          (eqv? (lambda (x) x)
                (lambda (y) y))           ⇒  unspecified

The next set of examples shows the use of eqv? with procedures that have local state. gen-counter must return a distinct procedure every time, since each procedure has its own internal counter. gen-loser, however, returns equivalent procedures each time, since the local state does not affect the value or side effects of the procedures.

          (define gen-counter
            (lambda ()
              (let ((n 0))
                (lambda () (set! n (+ n 1)) n))))
          (let ((g (gen-counter)))
            (eqv? g g))                   ⇒  #t
          (eqv? (gen-counter) (gen-counter))
                                          ⇒  #f
          
          (define gen-loser
            (lambda ()
              (let ((n 0))
                (lambda () (set! n (+ n 1)) 27))))
          (let ((g (gen-loser)))
            (eqv? g g))                   ⇒  #t
          (eqv? (gen-loser) (gen-loser))
                                          ⇒  unspecified
          
          (letrec ((f (lambda () (if (eqv? f g) 'both 'f)))
                   (g (lambda () (if (eqv? f g) 'both 'g)))
            (eqv? f g))
                                          ⇒  unspecified
          
          (letrec ((f (lambda () (if (eqv? f g) 'f 'both)))
                   (g (lambda () (if (eqv? f g) 'g 'both)))
            (eqv? f g))
                                          ⇒  #f

Objects of distinct types must never be regarded as the same object.

Since it is an error to modify constant objects (those returned by literal expressions), the implementation may share structure between constants where appropriate. Thus the value of eqv? on constants is sometimes unspecified.

          (let ((x '(a)))
            (eqv? x x))                    ⇒  #t
          (eqv? '(a) '(a))                 ⇒  unspecified
          (eqv? "a" "a")                   ⇒  unspecified
          (eqv? '(b) (cdr '(a b)))         ⇒  unspecified

Rationale: The above definition of eqv? allows implementations latitude in their treatment of procedures and literals: implementations are free either to detect or to fail to detect that two procedures or two literals are equivalent to each other, and can decide whether or not to merge representations of equivalent objects by using the same pointer or bit pattern to represent both.

— procedure: eq? obj1 obj2

eq? is similar to eqv? except that in some cases it is capable of discerning distinctions finer than those detectable by eqv?.

eq? and eqv? are guaranteed to have the same behavior on symbols, booleans, the empty list, pairs, records, and non-empty strings and vectors. eq?'s behavior on numbers and characters is implementation-dependent, but it will always return either true or false, and will return true only when eqv? would also return true. eq? may also behave differently from eqv? on empty vectors and empty strings.

          (eq? 'a 'a)                     ⇒  #t
          (eq? '(a) '(a))                 ⇒  unspecified
          (eq? (list 'a) (list 'a))       ⇒  #f
          (eq? "a" "a")                   ⇒  unspecified
          (eq? "" "")                     ⇒  unspecified
          (eq? '() '())                   ⇒  #t
          (eq? 2 2)                       ⇒  unspecified
          (eq? #\A #\A)                   ⇒  unspecified
          (eq? car car)                   ⇒  #t
          (let ((n (+ 2 3)))
            (eq? n n))                    ⇒  unspecified
          (let ((x '(a)))
            (eq? x x))                    ⇒  #t
          (let ((x '#()))
            (eq? x x))                    ⇒  #t
          (let ((p (lambda (x) x)))
            (eq? p p))                    ⇒  #t

Rationale: It will usually be possible to implement eq? much more efficiently than eqv?, for example, as a simple pointer comparison instead of as some more complicated operation. One reason is that it may not be possible to compute eqv? of two numbers in constant time, whereas eq? implemented as pointer comparison will always finish in constant time. eq? may be used like eqv? in applications using procedures to implement objects with state since it obeys the same constraints as eqv?.

— procedure: equal? obj1 obj2

equal? recursively compares the contents of pairs, vectors, and strings, applying eqv? on other objects such as numbers, symbols, and records. A rule of thumb is that objects are generally equal? if they print the same. equal? may fail to terminate if its arguments are circular data structures.

          (equal? 'a 'a)                  ⇒  #t
          (equal? '(a) '(a))              ⇒  #t
          (equal? '(a (b) c)
                  '(a (b) c))             ⇒  #t
          (equal? "abc" "abc")            ⇒  #t
          (equal? 2 2)                    ⇒  #t
          (equal? (make-vector 5 'a)
                  (make-vector 5 'a))     ⇒  #t
          (equal? (lambda (x) x)
                  (lambda (y) y))         ⇒  unspecified