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6.25.2.1 Nil

nil in ELisp is an amalgam of Scheme’s #f and '(). It is false, and it is the end-of-list; thus it is a boolean, and a list as well.

Guile has chosen to support nil as a separate value, distinct from #f and '(). This allows existing Scheme and Elisp code to maintain their current semantics. nil, which in Elisp would just be written and read as nil, in Scheme has the external representation #nil.

This decision to have nil as a low-level distinct value facilitates interoperability between the two languages. Guile has chosen to have Scheme deal with nil as follows:

(boolean? #nil) ⇒ #t
(not #nil) ⇒ #t
(null? #nil) ⇒ #t

And in C, one has:

scm_is_bool (SCM_ELISP_NIL) ⇒ 1
scm_is_false (SCM_ELISP_NIL) ⇒ 1
scm_is_null (SCM_ELISP_NIL) ⇒ 1

In this way, a version of fold written in Scheme can correctly fold a function written in Elisp (or in fact any other language) over a nil-terminated list, as Elisp makes. The converse holds as well; a version of fold written in Elisp can fold over a '()-terminated list, as made by Scheme.

On a low level, the bit representations for #f, #t, nil, and '() are made in such a way that they differ by only one bit, and so a test for, for example, #f-or-nil may be made very efficiently. See libguile/boolean.h, for more information.

6.25.2.2 Equality

Since Scheme’s equal? must be transitive, and '() is not equal? to #f, to Scheme nil is not equal? to #f or '().

(eq? #f '()) ⇒ #f
(eq? #nil '()) ⇒ #f
(eq? #nil #f) ⇒ #f
(eqv? #f '()) ⇒ #f
(eqv? #nil '()) ⇒ #f
(eqv? #nil #f) ⇒ #f
(equal? #f '()) ⇒ #f
(equal? #nil '()) ⇒ #f
(equal? #nil #f) ⇒ #f

However, in Elisp, '(), #f, and nil are all equal (though not eq).

(defvar f (make-scheme-false))
(defvar eol (make-scheme-null))
(eq f eol) ⇒ nil
(eq nil eol) ⇒ nil
(eq nil f) ⇒ nil
(equal f eol) ⇒ t
(equal nil eol) ⇒ t
(equal nil f) ⇒ t

These choices facilitate interoperability between Elisp and Scheme code, but they are not perfect. Some code that is correct standard Scheme is not correct in the presence of a second false and null value. For example:

(define (truthiness x)
  (if (eq? x #f)
      #f
      #t))

This code seems to be meant to test a value for truth, but now that there are two false values, #f and nil, it is no longer correct.

Similarly, there is the loop:

(define (my-length l)
  (let lp ((l l) (len 0))
    (if (eq? l '())
        len
        (lp (cdr l) (1+ len)))))

Here, my-length will raise an error if l is a nil-terminated list.

Both of these examples are correct standard Scheme, but, depending on what they really want to do, they are not correct Guile Scheme. Correctly written, they would test the properties of falsehood or nullity, not the individual members of that set. That is to say, they should use not or null? to test for falsehood or nullity, not eq? or memv or the like.

Fortunately, using not and null? is in good style, so all well-written standard Scheme programs are correct, in Guile Scheme.

Here are correct versions of the above examples:

(define (truthiness* x)
  (if (not x)
      #f
      #t))
;; or: (define (t* x) (not (not x)))
;; or: (define (t** x) x)

(define (my-length* l)
  (let lp ((l l) (len 0))
    (if (null? l)
        len
        (lp (cdr l) (1+ len)))))

This problem has a mirror-image case in Elisp:

(deffn my-falsep (x)
  (if (eq x nil)
      t
      nil))

Guile can warn when compiling code that has equality comparisons with #f, '(), or nil. See Compilation, for details.


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