What does #-3# mean?

What does #-3# mean?

[Richard Shann]

I am displaying a structure that seems to be circular, and it shows

(x_LIST . #-3#)

x_LIST is my own symbol but what is the meaning of the other member of the pair?

And how does one track it down this notation in the guile manual? I am
sure I came across this before, but cannot locate it in the manual
(again?)

Richard Shann

Re: What does #-3# mean?

[Ludovic Courtès]

Hi,

Richard Shann <richard.shann@virgin.net> writes:

> I am displaying a structure that seems to be circular, and it shows
>
> (x_LIST . #-3#)
>
> x_LIST is my own symbol but what is the meaning of the other member of the pair?

Indeed, it means it’s a circular list:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (use-modules (srfi srfi-1))
scheme@(guile-user)> (circular-list 1 2 3)
$1 = (1 2 3 . #-2#)
scheme@(guile-user)> (circular-list 1)
$2 = (1 . #0#)
--8<---------------cut here---------------end--------------->8---

> And how does one track it down this notation in the guile manual? I am
> sure I came across this before, but cannot locate it in the manual
> (again?)

It doesn’t seem to be documented but you get the idea.  ;-)

Note that ‘read’ doesn’t understand it.

Thanks,
Ludo’.

Re: What does #-3# mean?

[dskr]

I recall that the reader can be hash-extended to read these forms in a hacky way. Extend on '-'. I don't recall exactly anymore. I long ago patched the reader to read these natively. Perhaps there is some enthusiasm for making these readable in the main line?

Cheers,
  Dan

On Aug 24, 2010, at 12:38 PM, ludo@gnu.org (Ludovic Courtès) wrote:

> Hi,
> 
> Richard Shann <richard.shann@virgin.net> writes:
> 
>> I am displaying a structure that seems to be circular, and it shows
>> 
>> (x_LIST . #-3#)
>> 
>> x_LIST is my own symbol but what is the meaning of the other member of the pair?
> 
> Indeed, it means it’s a circular list:
> 
> --8<---------------cut here---------------start------------->8---
> scheme@(guile-user)> (use-modules (srfi srfi-1))
> scheme@(guile-user)> (circular-list 1 2 3)
> $1 = (1 2 3 . #-2#)
> scheme@(guile-user)> (circular-list 1)
> $2 = (1 . #0#)
> --8<---------------cut here---------------end--------------->8---
> 
>> And how does one track it down this notation in the guile manual? I am
>> sure I came across this before, but cannot locate it in the manual
>> (again?)
> 
> It doesn’t seem to be documented but you get the idea.  ;-)
> 
> Note that ‘read’ doesn’t understand it.
> 
> Thanks,
> Ludo’.
> 
> 

Re: What does #-3# mean?

[Andreas Rottmann]

"dskr@mac.com" <dskr@mac.com> writes:

> I recall that the reader can be hash-extended to read these forms in a
> hacky way. Extend on '-'. I don't recall exactly anymore. I long ago
> patched the reader to read these natively. Perhaps there is some
> enthusiasm for making these readable in the main line?
>
I'd like to implement SRFI-38, and perhaps the best way would be to
modify the core reader/writer, instead of adding something like the
reference implementation? Would this require a new reader option, or
should it be always enabled?

Regards, Rotty
-- 
Andreas Rottmann -- <http://rotty.yi.org/>

Re: What does #-3# mean?

[Ludovic Courtès]

Hello,

Andreas Rottmann <a.rottmann@gmx.at> writes:

> "dskr@mac.com" <dskr@mac.com> writes:
>
>> I recall that the reader can be hash-extended to read these forms in a
>> hacky way. Extend on '-'. I don't recall exactly anymore. I long ago
>> patched the reader to read these natively. Perhaps there is some
>> enthusiasm for making these readable in the main line?
>>
> I'd like to implement SRFI-38, and perhaps the best way would be to
> modify the core reader/writer, instead of adding something like the
> reference implementation? Would this require a new reader option, or
> should it be always enabled?

I’d do it using either ‘read-hash-extend’, which should make it possible
to implement ‘read-with-shared-structure’ without touching ‘read’ (or
I’d take this opportunity to write a new ‘read’ in Scheme using SILex.
:-))

Thanks,
Ludo’.