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Re: Language is not markup and markup is not language.
Date: 5/10/1999 10:01:00 AM
Guy_Murphy@xxxxxxxxxx wrote: > > Half of it went straight over my head, but I have a hunch that your post > here is pithy, precise, and hits the nail on the head. Just to be sure, > have you a reference to "Turing-complete[ness]" I can have a gander at <g>? > My background is informal, so I'm unfamiliar with the reference. http://plato.stanford.edu/entries/turing-machine/ http://www.geometry.net/turing.html A Turing machine is an extremely simple computational device. You can think of it as a physical machine or as a programming language. It is so simple that you could probably implement a Turing machine emulator in a few hundred lines of Python code (I've never tried it so I'm just guessing here). The interesting thing about Turing machines is that they can compute anything any other programming language can do. In other words, this simple "programming language" is as powerful as C++ or Prolog. Put yet another way: all programming languages have the same computational power -- they only differ in their I/O capabilities and their user interfaces. We say that they are all "Turing complete." You can emulate a Turing machine in them and you can emulate any of them in a Turing machine. The problem is that there are many interesting analyses of computer programs that are probably not possible. If you make a language that is Turing complete then there are certain optimizations and checks that you know that you will NEVER be able to do. For instance imagine you are serving documents through a script to publish them to the Web. Wouldn't it be nice to be able to check if the documents and the script are "compatible" before you put them on your website? A "compatible" document would be one that wouldn't cause the script to go into an infinite loop or crash. Well with a scripting language the only way to check is to run the script. If it seems to go into an infinite loop then you could wait for half an hour and say: "well, I guess it is in an infinite loop"-- but unless you know the details of the code you are just guessing. You don't know for sure. Turing proved that it is impossible to make a program that checks compatibility of other programs and documents. The best you can do is "try it and see." According to my interpretation, that's why software development is so hard and software is often so buggy -- we can't check the code with code. Even when we have AI, they will only be able to use heuristics as we do. "Declarative formats, such as context-free grammars, are formally tractable, allowing reliable document interchange and maintenance." - http://www7.scu.edu.au/programme/fullpapers/1920/com1920.htm Turing completeness is like winning the lottery: you get awesome freedom but you lose some freedom also. This is why I fought against hacks like xsl:script. On the other hand, there are many other computational categories leading up to Turing completeness and each of them has a similar cost/benefit proposition. The more powerful the language, the more constrained the optimization and static analysis techniques that implementors have available. I don't think anybody really knows what we have lost as XSL has become more and more advanced between the first draft and the most recent. Maybe there was a point at which it was feasible to prove that schemas and documents were compatible and maybe we crossed it somewhere so that we are left to use heuristics even though XSL is not fully Turing complete. It isn't as big a deal as if XML itself were Turing complete. If anyone ever tries to sell you on a Turing complete language for storing your primary data in, run far away! I will refer you again to this URL for information on why that is the wrong way to go: http://www7.scu.edu.au/programme/fullpapers/1920/com1920.htm -- Paul Prescod - ISOGEN Consulting Engineer speaking for only himself http://itrc.uwaterloo.ca/~papresco And so, in one of history's little ironies, the global triumph of bad software in the age of the PC was reversed by a surprising combination of forces: the social transformation initiated by the network, a long-discarded European theory of political economy, and a small band of programmers throughout the world mobilized by a single simple idea. - http://old.law.columbia.edu/my_pubs/anarchism.html XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list