# Valid XML PIs for ERB **URL:** \ **Category:** Ruby\ **Created:** [January 26, 2006, 12:16pm UTC](https://www.ruby-forum.com/t/valid-xml-pis-for-erb/53139 "2006-01-26T12:16:00Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [January 26, 2006, 12:16pm UTC](https://www.ruby-forum.com/t/valid-xml-pis-for-erb/53139/1 "2006-01-26T12:16:00Z")
Hi. Why doesn’t ERB work with valid XML processing instructions, as suggested by lots of people[1]? Is there any good reason we’re stuck with invalid, nonstandard, Microsoft-inspired “\<%…%\>” tags? It shouldn’t be difficult to allow both syntaxes (someone even made a patch[2]), and .rhtml files could finally be valid XHTML. References: [1] [http://groups.google.com/group/comp.lang.ruby/browse\_thread/thread/936cf51a2b8b01c0/aed5dbbdac99cdfb?q=erb+xml&rnum=9#aed5dbbdac99cdfb](http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/936cf51a2b8b01c0/aed5dbbdac99cdfb?q=erb+xml&rnum=9#aed5dbbdac99cdfb) [2] [http://groups.google.com/group/comp.lang.ruby/browse\_thread/thread/602c5f90e980dffb/9f9c6229a3313e73?q=erb+xml&rnum=2](http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/602c5f90e980dffb/9f9c6229a3313e73?q=erb+xml&rnum=2) ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [January 27, 2006, 9:57am UTC](https://www.ruby-forum.com/t/valid-xml-pis-for-erb/53139/2 "2006-01-27T09:57:25Z")
On Thu, 2006-01-26 at 12:13, removed\_email\_address@domain.invalid wrote: > Hi. Why doesn’t ERB work with valid XML processing instructions, as > suggested by lots of people[1]? Is there any good reason we’re stuck > with invalid, nonstandard, Microsoft-inspired “\<%…%\>” tags? It > shouldn’t be difficult to allow both syntaxes (someone even made a > patch[2]), and .rhtml files could finally be valid XHTML. Historical reasons, mostly. HTML started out as inspired by SGML, rather than compliant with SGML. The people who built the first web servers didn’t know much about SGML, and so they reinvented processing instructions in an annoyingly incompatible manner. Ever since, web frameworks have been built on a solid foundation of don’t-bother-me-with-the-basics-of-SGML/XML-processing. Quite successfully too, which really grates my cheese. 🙂 In addition to the non-compliant syntax there are problems with how the \<%…%\> tags are used. It is very common for them to contain either procedural instructions, such as in PHP and Rails, or to be tightly coupled to procedures, such as [ASP.NET](http://ASP.NET) and Java tag libraries. None of this is good from an SGML/XML point of view. The SGML/XML paradigm says “separate information from processing so that the same information can be processed by many different applications”. This is quite different from the OO idea of “bunch data and operations together in an object”. (Apologies for the vast oversimplification.) A processing instruction, when used as originally intended, indicates that something should be done, and what that something is. It should not indicate which piece of software that will perform the action. A well written processing system (again, from an SGML/XML point of view) uses templates that are independent of the templating system, so that they can be processed by other software if need be, just as you suggest. I am no Rails expert, but as I understand it, Rails is one of the few, MVC based web application framework that can easily be taught to understand processing instructions. It is possible to add new templating systems fairly easily. If someone added an XML compliant templating system, Rails would have a strong competitive advantage with SGML/XML aware organizations, for example in the automotive, telecommunications, and electronics industries. An alternative is to skip the MVC framework bit altogether, and build Transformation View based applications instead. Not much framework support that I know of, but a paradigm that fits the problem better than MVC in many cases. With a Transformation View based application, you would most likely deal with XML parsers and an XML transformation system directly, and the problem on non-compliant tagging never arises. /Henrik – [http://kallokain.blogspot.com/](http://kallokain.blogspot.com/) - Blogging from the trenches of software development [http://www.henrikmartensson.org/](http://www.henrikmartensson.org/) - Reflections on software development [http://testunitxml.rubyforge.org/](http://testunitxml.rubyforge.org/) - The Test::Unit::XML Home Page [http://declan.rubyforge.org/](http://declan.rubyforge.org/) - The Declan Home Page ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [January 27, 2006, 4:17pm UTC](https://www.ruby-forum.com/t/valid-xml-pis-for-erb/53139/3 "2006-01-27T16:17:39Z")
Henrik M. wrote: > Historical reasons, mostly. HTML started out as inspired by SGML, rather > than compliant with SGML. The people who built the first web servers > didn’t know much about SGML, and so they reinvented processing > instructions in an annoyingly incompatible manner. Ever since, web > frameworks have been built on a solid foundation of > don’t-bother-me-with-the-basics-of-SGML/XML-processing. Quite > successfully too, which really grates my cheese. 🙂 Do you have any references for this? I’m pretty sure Tim Berners-Lee, Marc Andreessen, etc. knew about SGML, and I do not believe that HTML ever had PIs. Also, the xml-dev list is a good place to read varying, but informed, opinions on the use of PIs. For example: [http://lists.xml.org/archives/xml-dev/200505/msg00159.html](http://lists.xml.org/archives/xml-dev/200505/msg00159.html) Thanks, ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [January 27, 2006, 10:23pm UTC](https://www.ruby-forum.com/t/valid-xml-pis-for-erb/53139/4 "2006-01-27T22:23:43Z")
On Fri, 2006-01-27 at 16:15, James B. wrote: > > Historical reasons, mostly. HTML started out as inspired by SGML, rather > > than compliant with SGML. The people who built the first web servers > > didn’t know much about SGML, and so they reinvented processing > > instructions in an annoyingly incompatible manner. Ever since, web > > frameworks have been built on a solid foundation of > > don’t-bother-me-with-the-basics-of-SGML/XML-processing. Quite > > successfully too, which really grates my cheese. 🙂 > > Do you have any references for this? Tim Berners-Lee certainly knew about SGML. The original specification explicitly mentions it. See [Tags used in HTML](http://www.w3.org/History/19921103-hypertext/hypertext/WWW/MarkUp/Tags.html). However, HTML was not fully SGML compliant. For example, there is a sentence in the original spec that says: “Currently HTML documents are transmitted without the normal SGML framing tags, but if these are included parsers will ignore them.” There was also an original test dataset, including this file: [Hypertext HTML formatting example](http://www.w3.org/History/19921103-hypertext/hypertext/WWW/Test/test.html). If you look at the source, you can see that it is not fully SGML compliant. For starters, there is no Doctype. Also, there are tags that contain formatted text that is not wrapped in a CDATA section. Neither is allowed in SGML. An interesting thing to note is that the tag was used to indicate the end of a paragraph in the test document, though the original spec said was a paragraph start tag. I remember that all my early HTML books said was an end tag. Unfortunately, I threw those books away years ago. ☹ Also, I believe the first HTML DTD was for version 2.0, written in 1995. Here is the link: [http://www.w3.org/MarkUp/html-spec/html.dtd](http://www.w3.org/MarkUp/html-spec/html.dtd). Since there was no DTD for version 1.0, it could not have been SGML compliant. In all fairness, I could be wrong about there being no HTML 1.0 DTD. There are notes from 1992 that talk about the future of HTML and “a new DTD”, which indicates the existence of an old one. It’s just that I haven’t found it. Even so, the lack of a requirement for a Doctype would be enough to render HTML non-compliant. More importantly, it would not be parseable by SGML parsers. At the time, loosing the Doctype and CDATA sections, and not supporting hierarchical chapter and section structures, was probably the right decision. HTML had to be very simple, or people would not have used it. If the design had been “better”, we might not have had a web today. > I’m pretty sure Tim Berners-Lee, > Marc Andreessen, etc. knew about SGML, and I do not believe that HTML > ever had PIs. I have never seen a HTML spec that mentions processing instructions. Nor is there any need to. Processing instructions can be defined by anyone who designs a processing application, they are not tied to a specific DTD or SGML application. (Well, except that some specifications explicitly defines some PIs, but there is nothing that prevents users of the DTD to specify more of them.) I can’t prove that the people who wrote the first web servers did not know about PIs, but I think it is likely. If they had known, what possible reason could they have had for deliberately doing something that was not SGML compliant? (Browser wars and vendor lock in didn’t become major issues until later.) > Also, the xml-dev list is a good place to read varying, but informed, > opinions on the use of PIs. > > For example: > > [xml-dev - Well-established uses of processing instructions?](http://lists.xml.org/archives/xml-dev/200505/msg00159.html) I follow the list, though not as carefully now as I did a couple of years ago. In addition to the applications mentioned in the thread you refer to, XML editors, like XMetaL and Arbortext Editor make use of processing instructions. So does many proprietary SGML/XML processing systems. /Henrik – [http://kallokain.blogspot.com/](http://kallokain.blogspot.com/) - Blogging from the trenches of software development [http://www.henrikmartensson.org/](http://www.henrikmartensson.org/) - Reflections on software development [http://testunitxml.rubyforge.org/](http://testunitxml.rubyforge.org/) - The Test::Unit::XML Home Page [http://declan.rubyforge.org/](http://declan.rubyforge.org/) - The Declan Home Page ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [January 27, 2006, 11:36pm UTC](https://www.ruby-forum.com/t/valid-xml-pis-for-erb/53139/5 "2006-01-27T23:36:26Z")
Henrik M. wrote: … > > I’m pretty sure Tim Berners-Lee, > > Marc Andreessen, etc. knew about SGML, and I do not believe that HTML > > ever had PIs. … > I can’t prove that the people who wrote the first web servers did not > know about PIs, but I think it is likely. If they had known, what > possible reason could they have had for deliberately doing something > that was not SGML compliant? (Browser wars and vendor lock in didn’t > become major issues until later.) Tim Berners-Lee arguably wrote the first Web server (if Wikipedia is to be believed). I’m doubtful that it had anything to do with processing instructions; Web servers generally don’t worry if the pages they serve up are SGML compliant or otherwise; that tends to be left to the client. The issues with funky page-generation syntax, at least for the Ruby Web tools I’ve used, arise prior to interaction with any Web server. There seem to be (again, generally speaking) two camps: Those who want to treat the page templating or page generation source as a consistent set of a single markup language (e.g. SGML or XML), and those who want a special syntax such that an editor or other tool might readily distinguish between document markup and programming language markup. I’m guessing the latter describes the PHP/ASP/Erb path. Meanwhile, Nitro, Amrita, Sean Russell’s xml-tmpl, some others I can’t recall, allow for the use of either XML elements or processing instructions or both. I tend to prefer the PI-only approach, as it is (for me) less intrusive for various XML tools such as tidy, and seems to make it easier to check for certain page validation errors prior to full rendering (though at some point that needs to be checked as well). Luckily, Erb lends itself to some basic hacking to allow the use of PI syntax. But I can see why people might want a syntax that was orthogonal to any particular output format; Erb need not only be used to create Web pages. It can generate Ruby or postscript or whatever, and a PI syntax would mean little then. – James B. “Blanket statements are over-rated”