Sunday, September 7, 2014

back from Swiss Perl Workshop 2014

The second Swiss Perl Workshop just ended, and it was really nice.

I must confess that I was quite skeptical when Roman and Matthias first came up with such an idea, thinking that Swiss french people would rather go the Journées Perl in Paris, and Swiss german people would rather go to the Deutscher Perl Workshop. But I was wrong : people came, and I was very happy to encounter new local people doing Perl. The organization was tip-top : well-chosen location in the center of Switzerland, in a cosy, ancient house, nice food eaten in the garden, with good Valpolicella ripasso wine.

For this event I expanded my YAPC::EU lightning talk on virtual tables for SQlite : the expanded slides are online at http://www.slideshare.net/ldami/sq-lite-virtualtables. The other talk on App::AutoCRUD was the same as in YAPC (slides here).

Thanks again to Roman and Matthias, and see you next year.

Wednesday, July 30, 2014

Plack::App::* namespace is not for apps - so which is the proper CPAN namespace ?

OOps ... I just realized that I had misunderstood the intent of the Plack::App namespace : the top-level Plack doc explicitly says :
DO NOT USE Plack:: namespace to build a new web application or a framework. It's like naming your application under CGI:: namespace if it's supposed to run on CGI and that is a really bad choice and would confuse people badly.
and the 2009 Plack Advent Calendar goes even further with
Think twice before using Plack::App::* namespace. Plack::App namespace is for middleware components that do not act as a wrapper but rather an endpoint. Proxy, File, Cascade and URLMap are the good examples. If you write a blog application using Plack, Never call it Plack::App::Blog, okay? Name your software by what it does, not how it's written.
OK, sorry, I got this wrong when publishing Plack::App::AutoCRUD -- but to my excuse, I'm not alone, several other CPAN authors did the same.

The app is quite young, so it is still time to repair its name (even if this operation will be quite tedious, because it involves changes in all module sources, in the CPAN distribution, in the github repository name, and in the upcoming YAPC::EU::2014 talk). But if I want to be a good citizen and engage into such an operation, what should be the proper name ? The CPAN namespace is becoming a bit crowded, as already noted 2 years ago by Joel Berger. For choosing a name, there seem to be several controversial and perhaps contradictory principles :
  • CPAN is for modules, not for apps  : this was argued in 2008 in a Perlmonk discussion on the same topic ; however, many people replied in disagreement. I disagree too : publishing a Perl app on CPAN fully makes sense because we take advantage of the CPAN infrastructure for tests, dependency management, publication, etc. Furthermore, applications can be extended or forked, just like modules, so CPAN is a perfect environment for sharing.
  • publish under the App::* namespace : this is the PAUSE recommendation. But applications in the App::* namespace are mainly  command-line utilities, which is quite different from Web applications. As a matter of fact, nobody used yet the App::Web namespace -- maybe it's time to start ?
  • use a ::Web or ::WebApp suffix at the end of the module name :  I never saw this as a recommendation, but nevertheless many distributions adopted this approach. This is certainly appropriate if the main goal is to publish a functionality Foo::Bar, and by the way, there is also a web app at Foo::Bar::WebApp. But if the purpose of the whole distribution is just a web app, this approach tends to create a new top-level namespace, which is not considered good practice. Should I choose AutoCRUD::WebApp ? I think not, because other people might want to use the AutoCRUD::* namespace.
  • avoid top-level namespaces : this used to be an important recommendation, but it doesn't seem to be well respected any more :-( -- nowadays I see more and more CPAN distributions taking up top-level names. I won't cite any particular example, not to offend anybody, but it's quite obvious if you look at the list of top-level namespaces .... and unfortunately many of those top-level names give no clue whatsoever about what kind of functionality will be found in the associated distribution.
  • hide the technology underlying your app : the Plack argument above says that the app should be named from its functionality, not from its implementation technology. Well ... I'm not so sure that this is always appropriate. Many modules sit under the Tie::Hash::* namespace, just because they used the tied hash technology, for providing various kinds of functionalities.
    Concerning  "Plack", when I see that keyword in a module name, I know that a) this is Web technology, and b) this will work on any kind of web server (as opposed to modules names containing "Apache" or "Apache2"), and I consider this to be useful information for a potential user. On the opposite, I didn't want to name my module DBIx::DataModel::AutoCRUD, even if it uses DBIx::DataModel quite heavily, because that's not hardwired into the architecture and I could easily imagine a later adaptation for supporting as well DBIx::Class.
So in the end I will probably end up with something like App::Web::AutoCRUD or WebApp::AutoCRUD ... unless somebody comes up with a better suggestion !

PS : see also Catalyst::Plugin::AutoCRUD .. which can be used either as a Catalyst plugin or as an application on its own.





Friday, July 11, 2014

Perl virtual tables for DBD::SQLite : ready to test

Followup to my previous article : a first draft of Perl virtual table support for DBD::SQLite is available at https://github.com/DBD-SQLite/DBD-SQLite/tree/vtab .

This is still alpha software, but it shows the idea; I still don't know when this work will be mature enough for a CPAN publication.

Two examples of virtual table modules are bundled with the distribution :
  • FileContent : implements a virtual column that exposes file contents. This is especially useful
    in conjunction with a SQLite FTS fulltext index; see the doc in Fulltext_search.pod
  • PerlData :  binds a virtual table to a Perl array within the Perl program. This can be used for simple import/export operations, for debugging purposes, for joining data from different
    sources, etc.
I'm currently thinking of one more example , which would be fun to play with : a virtual table that would proxy to another DBI connection. Then we could join data from various sources, using SQLite's features to do the joining work. Sounds quite exciting, but at this point this is just an idea.

Thanks to Salvador Fandiño who pointed me to https://metacpan.org/pod/SQLite::VirtualTable, which is similar in idea but more meant to embed a Perl interpreter inside a sqlite application, rather than the other way around; this code helped me build the DBD::SQLite version.

Of course any comments/suggestions are welcome.

Friday, July 4, 2014

project : Perl virtual tables for DBD::SQLite

During the year I have more and more management tasks and less time for programming. So for the holidays I wanted a change and decided to engage into a really "hardcore programming"  project, namely to add support for Perl virtual tables within the DBD::SQLite database driver. SQLite has this notion of "virtual tables" which look like regular tables but are implemented through callback routines. This project  implies some C programming, using Perl XS API, and the delicate part is to design some appropriate glue between SQLite's notion of "object-oriented", through extensible C structures and callbacks, and Perl's object-oriented features.

At the beginning I wasn't even sure if such a project would be feasible, but now it is slowly taking shape and I'm pretty confident that it will eventually reach something usable. The concept is quite similar to Perl's tied variables, where a published API is reused for accessing many different kinds of data; except that here the published API is SQL instead of hashes or array operators. As a result, we could have virtual tables bound to the filesystem, to the Win32::Registry, to some configuration data, or any other accessible resource. This will open a new field for lots of creative ideas.

My main motivation for doing this work is to be able to build a framework for collections of documents, using SQLite for the fulltext index, and using the filesystem for storing document content : this will be a much more powerful replacement for my very old File::Tabular::Web::Attachment::Indexed module. That module is still heavily used at Geneva courts of law, but now we have 10 years of data, and the old architecture is clearly showing its limits.

For the virtual tables project I need some test cases, so if anybody has ideas about Perl-accessible data to be published as an SQL table, I'm interested.




Monday, June 23, 2014

Perl smartmatch : what now ?

Sorry to wake up an old discussion, but ... does anybody have a clear idea of what is going to happen to smartmatch ?

Our team maintains dozens of internal applications and modules containing "given/when" and smartmatch statements. Most of this code was written between 2007 and 2012 -- remember, at that time smartmatch was an official feature, never mentioned as being "experimental", so we happily used it in many places. The reasons for using smartmatch were quite modest :
  • match a scalar against an array
  • match 2 scalars, without a warning when one of the scalars is undef
  • more readable switch statements, thanks to "given/when"
 When 5.18 came out, I was quite worried about the regression of smartmatch to  "experimental" status, but I was confident that things would be settled in 5.20, so I decided not to upgrade (we still use 5.14). Now 5.20 is out .. and nothing has changed about smartmatch, without even a clue about how this is going to evolve.

Our servers cannot easily upgrade to 5.20, because this would throw warnings all over the place. I tried to find a way to globally turn off these warnings (like set PERL5OPT=-M-warnings=experimental::smartmatch, or  PERL5OPT=-M=experimental::smartmatch), but this doesn't work because the "no warnings" pragma is lexically scoped, so global settings are not taken into account.

So my options are :
  1. don't change anything, don't upgrade, and wait for 5.22, hoping that some reasonable form of smartmatch will be reintroduced into the core
  2. revise all source files, adding a line "use experimental qw/smartmatch/;" at the beginning of each lexical scope ... but I have no guarantee that this will still work in future versions
  3. revise all source files, removing the given/when/smartmatch statements and replacing them with plain old Perl, or with features from some CPAN modules like match::smart or Smart::Match ... but it would be a pity to engage in such work if regular smartmatch comes back in a future version of Perl.
As you can see, none of these options is really satisfactory, so I would be interested in hearing if other companies are in the same situation and how they decided to handle it.

By the way, I love the new Perl way of introducing new features as "experimental", until they become stable and official ... but this only works well when the experimental status is declared from the start. The problem with smartmatch is that it had been official for several years, before being retrograted to experimental. Agreed, the full semantics of smartmatch as published in 10.0 had inconsistencies, but throwing away the whole thing is a bit too harsh -- I'm sure that many users like me would be happy with a reasonable subset of rules for matching common cases.
  
Thanks in advance, Laurent Dami
 

Saturday, May 25, 2013

surprising interaction betwen list context and range operator

I was preparing a quiz for Perl programmers, and wanted a question on the difference between arrays and lists. So the question was : what do you get in $x and $y after

my @tab = ( 1, 3, 6..9 );
my $x   = @tab;
my $y   = ( 1, 3, 6..9 );


where I expected $x to be 6 (the number of items) and $y to be 9 (the last element), as documented in perldata. But before distributing the quiz I thought it was worth a test ... and indeed it was! To my surprise, $y turns out to be an empty string. It took me some time to find out why : with

my $y = ( 1, 3, 6, 7, 8, 9 );

the result is 9 indeed. But with

my $y = ( 1, 3, 6..9 );


the interpreter does not build the range in  list context, and then keep the last element. What happens instead is that it throws away the beginning of the list (there is even a warning about that), and then evaluates

my $y = 6..9;


in scalar context; where 6..9 is no longer a range, but a flip-flop operator. Now since the left-hand side is true, that operator is supposed to be true, right ? Well, no, because 6 is a constant, and in that case, perlop tells us that the flip-flop is  "considered true if it is equal (==) to the current input line number (the $. variable)". So $y ends up being an empty string, while

my $six  = 6;
my $nine = 9;
my $y    = $six..$nine;


would yield 1E0!

I couldn't be that nasty to the interviewed programmers, so in the end that question will not be part of the quiz.

Sunday, February 17, 2013

Slices of method calls within an object

Several years ago I complained that object accessors prevent you from using some common Perl idioms; in particular, you couldn't take a slice of several attributes within an object.

Now I just discovered the Want module; this is great for playing with lvalue subroutines. With the help of this module, I was finally able to use slices of method calls within an object : see https://metacpan.org/module/Method::Slice . There are some caveats in comparison with a usual hash slice, but nevertheless the main point is there : you can extract some values :

  my @coordinates = mslice($point, qw/x y/);

or even assign to them:

  (mslice($point, qw/x y/)) = (11, 22);

This was written just for fun ... but in the end I might use it in real code, because in some situations, I find slices to be more compact and high-level than multiple assignment statements.