Subject: Re: backquoting: are lispworks, sbcl, clisp conformant?
From: (Rob Warnock)
Date: Sun, 18 Jan 2009 05:56:14 -0600
Newsgroups: comp.lang.lisp
Message-ID: <>
budden  <> wrote:
| > (defun ff () `(,0))
| ff
| > (eq (ff) (ff))
| this returns:
| t (in lispworks, sbcl, clisp)
| nil (in allegro, clozure cl)
| On my opinion, returning t is non-conformant.

On the contrary, what EQ returns in this case is not specified.
T is a perfectly acceptable, and so is NIL.

| Standard (section 2.4.6,
| states:
| "
|  `(x1 x2 x3 ... xn . atom) may be interpreted to mean
|      (append [ x1] [ x2] [ x3] ... [ xn] (quote atom))
|     where the brackets are used to indicate a transformation of an xj
| as follows:
|     -- [form] is interpreted as (list `form), which contains a
| backquoted form that must then be further interpreted.
|     -- [,form] is interpreted as (list form).
|   ..."
| so, `(,0) should mean (append (list 0)), but not '(0), as it is
| interpreted in first three implementations I listed

Uh... What do you think (APPEND (LIST 0)) evaluates *to*?!?

    > (append (list 0))


And what do you think `(,0) evaluates to?

    > `(,0)


So the *only* thing you're quibbling about is whether the evaluation
of the (APPEND (LIST 0)) is done at READ time, in which case the
(EQ (FF) (FF)) will likely return T [since it's returning the identical
cons cell embedded in the function definition], or whether the evaluation
of the (APPEND (LIST 0)) is done at EVAL time, in which case the
(EQ (FF) (FF)) will likely return NIL [since freshly allocated cons
cells are always different under EQ].

What you're missing is that the CLHS specifically *doesn't* specify
whether backquote evaluation is done at READ time or at EVAL time,
only that it follow the rules. From the same CLHS 2.4.6 page that
you referenced above:

    An implementation is free to interpret a backquoted form F1 as
    any form F2 that, when evaluated, will produce a result that is
    the same under EQUAL as the result implied by the above definition,
    provided that the side-effect behavior of the substitute form F2
    is also consistent with the description given above.

Read closely: It says "the same under EQUAL", *not* "the same under EQ"!!

And, indeed:

    > (equal `(,0) (append (list 0)))


*All* implementations should return T for (EQUAL (FF) (FF)).


Rob Warnock			<>
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