Which internal DSL are there in Perl? (Domain Specific Languages

Taking some definitions from Wikipedia and Martin Fowlers Book: (emphasizes added)

DSLs are small languages, focused on a particular aspect of a software system. You can't build a whole program with a DSL, but you often use multiple DSLs in a system mainly written in a general purpose language.
DSLs come in two main forms: external and internal
An external DSL is a language that's parsed independently of the host general purpose language: good examples include regular expressions and CSS
Internal DSLs are a particular form of API in a host general purpose language, often referred to as a fluent interface.
They are represented within the syntax of a general-purpose language. It’s a stylized use of that language for a domain-specific purpose.
People find DSLs valuable because a well designed DSL can be much easier to program with than a traditional library.
This improves programmer productivity , which is always valuable. In particular it may also improve communication with domain experts , which is an important tool for tackling one of the hardest problems in software development.

Now there are some more frequently used examples in Perl which come to mind like SQL and Template languages like in Template::Toolkit2

[% FOREACH file=files %]
    [% IF ( matches = file.date.match('dd/mm/(\d+)') ) %]
        <a href='#'>[% matches.0 %]</a>
    [% END %]
[% END %]
[download]

Both qualify as are for sure as DSLs.

They are sub-languages which can be used by "domain experts" without any knowledge of Perl.

And they are external, because neither SQL nor TT commands are implemented as Perl functions or follow Perl syntax.

SQL appears as embedded string inside Perl code is only parsed after leaving Perl thru DBI
And TT may be parsed with Perl but is not parsed as Perl.

So far for external DSLs , but what are good examples for internal DSLs in Perl?

Well designed traditional modules and classes are sometimes close to represent a sub-language but they still come with a lot of boilerplate.

For instance an expert in the domain of TCL/TK may (probably?) not care much if the "host" language is Perl or Python or Ruby as long as he can call the commonly known methods. (see http://www.tkdocs.com/ for comparison of snippets in various languages).

But the necessity to constantly repeat an $obj-> or PACK::AGE:: on the left hand side of a command doesn't make it a completely "fluent" approach.

(On a side note: avoiding to repeat this LHS for method calls seems to be the principal mechanism in Ruby for transforming a simple class to what Rubyists like to call a "DSL", a bunch of function calls inside a block are somehow interpreted as method calls on the same object, hence very similar to the with(obj) {} mechanism in JavaScript - please correct me if not)

Now, what are examples for internal DSLs in Perl?

The only widely known one which comes to my mind is CGI::HTML::Functions which used to be part of CGI

For example, here's one way to make an ordered list:

   print ul(
             li({-type=>'disc'},['Sneezy','Doc','Sleepy','Happy'])
           );
[download]

This example will result in HTML output that looks like this:

   <ul>
     <li type="disc">Sneezy</li>
     <li type="disc">Doc</li>
     <li type="disc">Sleepy</li>
     <li type="disc">Happy</li>
   </ul>
[download]

Nice but IMHO it could be even more fluent.

But are there others?

There is for instance Rex which I suppose was inspired by Ruby's Puppet

use Rex -feature => ['1.0'];
desc "Get Uptime";
task "uptime", sub {
  say run "uptime";
}
[download]

And I think I could theoretically implement most Assembler languages as Perl code just with some minor changes. I just need to implement ASM-commands as uppercase subs, quote literals and adjust the comment symbol.

         CPX '3'          ;# Year starts in March to bypass
         BCS 'MARCH'      ;# leap year problem
         DEY              ;# If Jan or Feb, decrement year
MARCH:   EOR '$7F'        ;# Invert A so carry works right
...
[download]

(original example)

But there must be more examples of internal DSLs in Perl ...

Could you please help me to find and classify them?

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)

Je suis Charlie!}

Comment on Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) Select or Download Code

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Re: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by shmem (Chancellor) on Aug 03, 2017 at 07:17 UTC
The meditation Embedding a mini-language for XML construction into Perl by tmoertel immediately comes to mind (XML domain), and a derived HTML::Writer module. Do these qualify as internal DSL, since code written in them is executed as perl? Update: tmoertel gave a talk on the subject for the Pittsburgh Perl Mongers. The slides are still available via the Wayback Machine. perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply]
Re^2: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by LanX (Saint) on Aug 03, 2017 at 11:43 UTC
Absolutely, but why didn't you publish? FWIW I worked on a similar project, but took some other design decisions. (Attributes are not proper functions) Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply]
Re^3: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by shmem (Chancellor) on Aug 03, 2017 at 12:29 UTC
Absolutely, but why didn't you publish? Because, well... tests, pod, you name it... then, mainly because HTML::Writer is more a proof of concept than of productive use: because it is quite slow and because of inherent namespace pollution - all those functions derived from a DTD, yecch! security... I foresaw a lot of issues with that, and since chromatic pointed me to Technical Debt, I didn't dare to. Had somebody expressed interest to work that out with me into a module proper, with defined use cases and what not, making it more generic and so on... but no. At that time I had to move on, and other things to do. It rests in the vault of unfinished projects. <update> If you (as in "anybody") want to grab that code and make it into a CPAN module, feel free to do so. But please include references to tmoertel and me. And I would appreciate if you contact me (via PM here or my CPAN mail handle shmem-at-cpan.org) to discuss things. Thanks. </update> <update2> The module is now on github and will eventually make its way to CPAN. </update2> FWIW I worked on a similar project, but took some other design decisions. (Attributes are not proper functions) Publish or show, or it didn't happen ;-) perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply]
Re^4: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by LanX (Saint) on Aug 03, 2017 at 18:51 UTC
Re^5: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by shmem (Chancellor) on Aug 04, 2017 at 10:28 UTC
Some notes below your chosen depth have not been shown here
Re^5: Which internal DSL are there in Perl? (HTML::Writer as pragma) by shmem (Chancellor) on Aug 07, 2017 at 09:00 UTC
Re: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by hippo (Bishop) on Aug 03, 2017 at 08:03 UTC
Surely the plethora of OO helpers (Moose, Mouse, Moo, Mo, etc.) qualify? I'm not entirely clear whether PDL would count as internal or external.	[reply]
Re^2: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by LanX (Saint) on Aug 03, 2017 at 11:41 UTC
> Surely the plethora of OO helpers I already thought about them, but I'd rather try to classify them as syntax extensions or syntactic sugar, because the domain is part of the host language. But the distinction is fuzzy since techniques and requirements are similar. For comparison LINQ is often called a DSL, while it's integral part of C#. (Language Integrated Query), at the same time there are ports for other host languages as external libraries, which are not fully equivalent, but surely a DSL. (Basically an expert in the LINQ domain could help using them, without knowing the host language in depth) I'm not aware about a port of any M* lib to another language. ... > I'm not entirely clear whether PDL would count as internal or external. I had a quick look into the docs and it's really too wast to tell. I can see operator overloading (internal) and a dependency to Filter::Simple (external) And despite the L in the name, it doesn't look independent enough to be a DSL. maybe I'm missing a good tutorial, it certainly has elements of all in its large code base. You see im struggling to find objective definitions, maybe it's better to concentrate on the applied techniques... :) Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply]
Re^3: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by ikegami (Patriarch) on Aug 04, 2017 at 21:18 UTC
I already thought about them, I'd rather try to classify them as syntax extensions or syntactic sugar I would include them. They form a language to describe classes and attributes, and their relations.	[reply]
Re^3: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by etj (Deacon) on May 09, 2022 at 13:40 UTC
I'd say that PDL::NiceSlice (the Filter::Simple-using module) which creates the "new" syntax `$pdl1(0:4,2;-)` is definitely a DSL, particularly if LINQ counts.	[reply] [d/l]
Re: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by Anonymous Monk on Aug 03, 2017 at 05:34 UTC
perlre (obvious), perlform, sprintf, pack, perlpod all have their own special little syntax	[reply]
Re: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by ikegami (Patriarch) on Aug 04, 2017 at 20:54 UTC
Quoting: `q`: Escaping rules `qq` (incl `qx`, `qr`, `m`, `s` (search part)): Escaping rules and interpolation Formatting: `sprintf` `strftime` forms Perl6::Form Template languages (e.g. Template-Toolkit) `pack` Query languages: SQL XPath CGI::HTML::Functions Find::File::Rule Regular expressions Regular expressions sub-languages: Character classes Extended bracketed character class Properties etc Parsing: Various grammar languages (e.g. Parse::RecDescent) `strptime` `unpack` Other: `tr///` POD Another category is parts of the language that don't match the rest, such as the flags for `m` and other operators. There are also small "languages" too small to mention, such as `use overload`.	[reply] [d/l] [select]
Re^2: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by LanX (Saint) on Aug 04, 2017 at 21:05 UTC
Which of them are internal DSLs? I have the impression most except CGI::HTML::Functions (which I already mentioned) Find::File::Rule seems to be a nice example for an OO interface, which comes close thru cascading. Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply]
Re^3: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by ikegami (Patriarch) on Aug 04, 2017 at 21:09 UTC
ah, you just want internal DSLs. I just added File::Find::Rule (a query language parsed as Perl), then.	[reply]
Re^4: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by ikegami (Patriarch) on Aug 04, 2017 at 21:22 UTC
Re^5: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by LanX (Saint) on Aug 04, 2017 at 22:20 UTC
Some notes below your chosen depth have not been shown here
Re: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by Anonymous Monk on Aug 03, 2017 at 05:36 UTC
Could you please help me to find and classify them? Are we writing your talk next week for you? http://act.perlconference.org/tpc-2017-amsterdam/talk/7079 :-P	[reply]
Re^2: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by LanX (Saint) on Aug 03, 2017 at 10:01 UTC
Absolutely! :)	[reply]
Re^3: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by coetry (Novice) on Aug 03, 2017 at 17:54 UTC
Hahah that's so neat! If I see a youtube video of your talk, I'll know where some of the content came from!	[reply]
Re^4: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by LanX (Saint) on Aug 03, 2017 at 18:07 UTC
Re: Which internal DSL are there in Perl? (Domain Specific Languages - Part 1) by melezhik (Monk) on Aug 03, 2017 at 18:31 UTC
Hi! Here is the DSL I use in Outthentic framework - https://metacpan.org/pod/Outthentic::DSL - language to verify (un)structured text.	[reply]
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Which internal DSL are there in Perl? (Domain Specific Languages - Part 1)