bug#27776: 26.0.50; `print-gensym' doesn't work as advertised

bug#27776: 26.0.50; `print-gensym' doesn't work as advertised

[Philipp]

The docstring of `print-gensym' says:

   When the uninterned symbol appears within a recursive data structure,
   and the symbol appears more than once, in addition use the #N# and #N=
   constructs as needed, so that multiple references to the same symbol are
   shared once again when the text is read back.

However, that doesn't seem to be the case, e.g.:

   emacs -Q -batch -eval '(let ((print-gensym t)) (print (quote (let ((#1=#:a 1)) #1#))))'

prints

   (let ((#:a 1)) #:a)


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bug#27776: 26.0.50; `print-gensym' doesn't work as advertised

[Philipp Stephani]

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Philipp <p.stephani2@gmail.com> schrieb am Do., 20. Juli 2017 um 23:32 Uhr:

>
> The docstring of `print-gensym' says:
>
>    When the uninterned symbol appears within a recursive data structure,
>    and the symbol appears more than once, in addition use the #N# and #N=
>    constructs as needed, so that multiple references to the same symbol are
>    shared once again when the text is read back.
>

OK, so you could read this as "the numbering is only used for recursive
data structures". But I don't understand why; it seems equally appropriate
for non-recursive data structures.

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bug#27776: 26.0.50; `print-gensym' doesn't work as advertised

[Michael Heerdegen]

Philipp Stephani <p.stephani2@gmail.com> writes:

>  The docstring of `print-gensym' says:
>
>     When the uninterned symbol appears within a recursive data structure,
>     and the symbol appears more than once, in addition use the #N# and #N=
>     constructs as needed, so that multiple references to the same symbol are
>     shared once again when the text is read back.
>
> OK, so you could read this as "the numbering is only used for
> recursive data structures". But I don't understand why; it seems
> equally appropriate for non-recursive data structures.

AFAICT...I think the docstring of `print-gensym' just fails to say that
you get that behavior only with print-circle bound non-nil.

The second problem is that docs speak about recursive data structures
when they mean data structures with shared parts.  `print-circle' is not
(only) about recursive (cyclic) structures.

In your example, it doesn't make a difference whether the list is
actually really recursive or not, as long as the same uninterned symbol
appears more than once in it.


Michael.

bug#27776: 26.0.50; `print-gensym' doesn't work as advertised

[npostavs]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> Philipp Stephani <p.stephani2@gmail.com> writes:
>
>>  The docstring of `print-gensym' says:
>>
>>     When the uninterned symbol appears within a recursive data structure,
>>     and the symbol appears more than once, in addition use the #N# and #N=
>>     constructs as needed, so that multiple references to the same symbol are
>>     shared once again when the text is read back.
>>
>> OK, so you could read this as "the numbering is only used for
>> recursive data structures". But I don't understand why; it seems
>> equally appropriate for non-recursive data structures.
>
> AFAICT...I think the docstring of `print-gensym' just fails to say that
> you get that behavior only with print-circle bound non-nil.

I agree.  Compare this note in cltl[1]

     Implementation note: Because the #: syntax does not intern the
     following symbol, it is necessary to use circular-list syntax if
     *print-circle* is not nil and the same uninterned symbol appears
     several times in an expression to be printed. For example, the
     result of

    (let ((x (make-symbol "FOO"))) (list x x))

    would be printed as

    (#:foo #:foo)

    if *print-circle* were nil, but as

    (#1=#:foo #1#)

    if *print-circle* were not nil.

[1]: https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node193.html

bug#27776: 26.0.50; `print-gensym' doesn't work as advertised

[Noam Postavsky]

tags 27776 fixed
close 27776 26.1
quit

npostavs@users.sourceforge.net writes:

> Michael Heerdegen <michael_heerdegen@web.de> writes:
>> AFAICT...I think the docstring of `print-gensym' just fails to say that
>> you get that behavior only with print-circle bound non-nil.
>
> I agree.

Fixed in emacs-26.

[1: 16022626ba]: 2018-01-30 22:14:02 -0500
  Clarify effect of print-gensym (Bug#27776)
  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=16022626ba86eb96e12b7d919edfb50912cb26bd