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Is the CPAN river always a DAG?
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James E Keenan
Mar 27, 2018, 9:30:02 AM3/27/18
to cpan-w...@perl.org
A few weeks ago I made a presentation to the Philadelphia Perlmongers
(http://thenceforward.net/perl/talks/phlpm20180312/index.html) on the
subject "Testing CPAN against the Perl 5 Core Distribution: Where Do We
Stand?". In that presentation, I sketched the concept of the CPAN river
and described it as a directed acyclic graph (DAG)
(http://thenceforward.net/perl/talks/phlpm20180312/slide017.html), using
one of Neil Bower's images to make that point.
In the discussion afterwards, a prominent former COBOL programmer
suggested that there was nothing to exclude the possibility of circular
dependencies among CPAN distributions. A could depend on B, which
depends on C, which depends on A. If so, we would have a cyclic graph.
Wouldn't that undermine the concept of the CPAN river, he asked.
Since all I know about DAGs I got from reading Wikipedia and the
documentation to Jarkko's Graph.pm module, I didn't have a good
response. So I promised to ask the question here.
* Can CPAN be cyclic?
* If so, then does that mean that, when we speak of CPAN as a river, we
are *imposing* DAG-ness on it by means of the algorithm(s) with which we
calculate the river (e.g., https://github.com/dagolden/zzz-index-cpan-meta)?
Thank you very much.
Jim Keenan
(http://thenceforward.net/perl/talks/phlpm20180312/index.html) on the
subject "Testing CPAN against the Perl 5 Core Distribution: Where Do We
Stand?". In that presentation, I sketched the concept of the CPAN river
and described it as a directed acyclic graph (DAG)
(http://thenceforward.net/perl/talks/phlpm20180312/slide017.html), using
one of Neil Bower's images to make that point.
In the discussion afterwards, a prominent former COBOL programmer
suggested that there was nothing to exclude the possibility of circular
dependencies among CPAN distributions. A could depend on B, which
depends on C, which depends on A. If so, we would have a cyclic graph.
Wouldn't that undermine the concept of the CPAN river, he asked.
Since all I know about DAGs I got from reading Wikipedia and the
documentation to Jarkko's Graph.pm module, I didn't have a good
response. So I promised to ask the question here.
* Can CPAN be cyclic?
* If so, then does that mean that, when we speak of CPAN as a river, we
are *imposing* DAG-ness on it by means of the algorithm(s) with which we
calculate the river (e.g., https://github.com/dagolden/zzz-index-cpan-meta)?
Thank you very much.
Jim Keenan
Alberto Simoes
Mar 27, 2018, 10:00:02 AM3/27/18
to cpan-w...@perl.org
Perl-core modules. Or it would be impossible to install (ask cpan to
install A, it will download B, then C, then try again to install A).
Nevertheless, we can have A1 that depends on B that depends on A2, where
A2 exists as a core Perl module, older version than A1. Thus, B would be
happy to build based on the previous, older, version, and then A1 use B.
I think I got some similar situation some years ago, but I can't refer
whose modules they were.
Hope this can help,
ambs
Henk P. Penning
Mar 27, 2018, 10:15:02 AM3/27/18
to James E Keenan, cpan-w...@perl.org
On Tue, 27 Mar 2018, James E Keenan wrote:
> Date: Tue, 27 Mar 2018 15:17:59 +0200
> From: James E Keenan <jke...@pobox.com>
> To: cpan-w...@perl.org
> Subject: Is the CPAN river always a DAG?
and links from point A to point B iff A uses B.
Given a set of modules, any graph is possible.
It is not hard to find a cycles, if (at least one) exists ;
but if you find a cycle, it is (a priori) not clear how to break it.
> * If so, then does that mean that, when we speak of CPAN as a river, we are
> *imposing* DAG-ness on it by means of the algorithm(s) with which we
> calculate the river (e.g., https://github.com/dagolden/zzz-index-cpan-meta)?
You can always make a DAG of CPAN, by leaving out links
(dependencies) or points (modules).
It is not so easy to find a minimal set of links to leave out.
> Jim Keenan
Regards,
Henk Penning
------------------------------------------------------------ _
Henk P. Penning, ICT-beta R Uithof MG-403 _/ \_
Faculty of Science, Utrecht University T +31 30 253 4106 / \_/ \
Leuvenlaan 4, 3584CE Utrecht, NL F +31 30 253 4553 \_/ \_/
http://www.staff.science.uu.nl/~penni101/ M pen...@uu.nl \_/
> Date: Tue, 27 Mar 2018 15:17:59 +0200
> From: James E Keenan <jke...@pobox.com>
> To: cpan-w...@perl.org
> Subject: Is the CPAN river always a DAG?
>
> A few weeks ago I made a presentation to the Philadelphia Perlmongers
> (http://thenceforward.net/perl/talks/phlpm20180312/index.html) on the subject
> "Testing CPAN against the Perl 5 Core Distribution: Where Do We Stand?". In
> that presentation, I sketched the concept of the CPAN river and described it
> as a directed acyclic graph (DAG)
> (http://thenceforward.net/perl/talks/phlpm20180312/slide017.html), using one
> of Neil Bower's images to make that point.
>
> In the discussion afterwards, a prominent former COBOL programmer suggested
> that there was nothing to exclude the possibility of circular dependencies
> among CPAN distributions. A could depend on B, which depends on C, which
> depends on A. If so, we would have a cyclic graph. Wouldn't that undermine
> the concept of the CPAN river, he asked.
>
> Since all I know about DAGs I got from reading Wikipedia and the
> documentation to Jarkko's Graph.pm module, I didn't have a good response. So
> I promised to ask the question here.
>
> * Can CPAN be cyclic?
Yes ; considered as a graph, CPAN has points (modules),
> A few weeks ago I made a presentation to the Philadelphia Perlmongers
> (http://thenceforward.net/perl/talks/phlpm20180312/index.html) on the subject
> "Testing CPAN against the Perl 5 Core Distribution: Where Do We Stand?". In
> that presentation, I sketched the concept of the CPAN river and described it
> as a directed acyclic graph (DAG)
> (http://thenceforward.net/perl/talks/phlpm20180312/slide017.html), using one
> of Neil Bower's images to make that point.
>
> In the discussion afterwards, a prominent former COBOL programmer suggested
> that there was nothing to exclude the possibility of circular dependencies
> among CPAN distributions. A could depend on B, which depends on C, which
> depends on A. If so, we would have a cyclic graph. Wouldn't that undermine
> the concept of the CPAN river, he asked.
>
> Since all I know about DAGs I got from reading Wikipedia and the
> documentation to Jarkko's Graph.pm module, I didn't have a good response. So
> I promised to ask the question here.
>
> * Can CPAN be cyclic?
and links from point A to point B iff A uses B.
Given a set of modules, any graph is possible.
It is not hard to find a cycles, if (at least one) exists ;
but if you find a cycle, it is (a priori) not clear how to break it.
> * If so, then does that mean that, when we speak of CPAN as a river, we are
> *imposing* DAG-ness on it by means of the algorithm(s) with which we
> calculate the river (e.g., https://github.com/dagolden/zzz-index-cpan-meta)?
(dependencies) or points (modules).
It is not so easy to find a minimal set of links to leave out.
> Jim Keenan
Regards,
Henk Penning
------------------------------------------------------------ _
Henk P. Penning, ICT-beta R Uithof MG-403 _/ \_
Faculty of Science, Utrecht University T +31 30 253 4106 / \_/ \
Leuvenlaan 4, 3584CE Utrecht, NL F +31 30 253 4553 \_/ \_/
http://www.staff.science.uu.nl/~penni101/ M pen...@uu.nl \_/
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