Errata for hyperspec?

[Steve Haflich]

On Thu, Dec 14, 2017 at 5:49 PM, Pierpaolo Bernardi <olopierpa at gmail.com>
wrote:

>
> Spec is not explicit, and currently different implementations do
> different things.
>
>  (let ((*print-circle* t)
>         (name "q"))
>     (format nil "~A ~A" name name))      ==>  ?
>

It may be true that implementations do different things, but IMO the
standard permits only one result.  #n= notion aught not be produced.  See
the first paragraph of the dictionary page for *print-circle*:

    Controls the attempt to detect circularity and sharing in an *object*
<http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_o.htm#object>
being
printed.

FORMAT is not a function that prints an *object*.  Instead, it interprets a
format control.  A call to FORMAT may print zero objects or any number of
objects.

The functions that are specified to print an object are WRITE, PRINT,
PRINC, PRIN1, and PPRINT (which might all be conceptually defined to make
the obvious call to WRITE on that object, although this is not required),
their mumble-TO-STRING versions, and PPRINT-LINEAR, PPRINT-FILL, and
PPRINT-TABULAR.  The printer is recursive, but is the outermost entry to
one of these functions that defines the boundary for circularity
detection.  (PRINT-OBJECT might be included in this set, but it is not
necessary since it ought only be called dynamically inside one of the
others.)

~A, ~S, ~W and various others (e.g. when a numeric format control is given
a non-number) act as if they make the obvious call to WRITE.  I maintain
that the ANS is firm that the following results (here in sbcl) are correct
and required.  *PRINT-CIRCLE* is true:

* (let ((g "a")) (format nil "~a ~a" g g))
"a a"
* (let ((g "a")) (format nil "~a" (list g g)))
"(#1=a #1#)"
* (let ((g "a")) (format nil "~{~a ~}" (list g g)))
"a a "

The first two examples are clear from the above argument.  The third
follows from the description of ~{, which specifies that it behaves like a
recursive format control, and therefore also does not itself "print an
object".

So while the ANS is not ambiguous here, this issue can be hard to
understand and a clarification could indeed be useful.  However, it can
only be a clarification, neither a change nor resolution of an ambiguity.
It bears repetition that the X3J13 goal for the ANS was a *specification*.
Ease of understanding was of course desirable, but a very secondary
consideration.  There are a great many tangled or distant interrelated
passages; one needs be intimately familiar with the entire document even to
rememberwhere to look for the other end of a tangle.  Also, it is
inconceivable to me that someone not already familiar with some Lisp
dialect could learn CL from the ANS.
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