How can I resist this request to talk about ancient code? Even if it seems off-topic?
My own long-lived examples are a type-theoretical program verifier, and an Algol 68 compiler. In 1972 I started an Algol 68 conpiler, worked on it for a few years at the University of Alberta, decided to abandon the project when it went over two and a half years (I had planned ono two) and my PDF expired, went on to work at the Mathematical Centre in Amsterdam , where against my better judgement I was persuaded to resue work on it. The facilities there were much worse than the ones I had had in Alberta. When that time came to a close I had a compier that handles about half of a very demanding test suite. But that meant it was probably about 90% complete, but you really needed closer to 99% to make the thing useful. I backed it up on magnetic tape, left one copy in a friend's sock drawer and took teh other with me. Over the years the one I had got copied a few times, because reel-to-reel 9-track tape drives were getting scarce. Unfortunately, one time that copy was not done correctly, although it appeared to have been done correctly. Then a few years ago I found that someone on the other side of the world had written an Algol W compiler (the language in which I had written my compiler), and I tried to use it to resurrect my Algol 68 compiler. When I discovered the bit rot I felt sick. But my friend in Amsterdm had contacts within IBM, and someone there took it on as a personal project. It turned out he was working on long-term data archiving, and he thought it was just *great* that he gos a 30-year-old magnetic tape to try to recover data from. For all I know that tape might have been mentioned in some archiving conference proceedings. Anyway, I got the enntire compiler and a bunch of other data sent to me in 8-bit EBCDIC code and proceeded to decode it. I ended up with the source code to the compiler. It was really fun to be looking through it again. Most of hte deficiencies I find looking at it are caused by (1) It wasn't finished (2) the limitations of Algol W. Algol W was a pretty good tool for the time, with garbage collection and data structures, but its limitations were severe. No decent modern modules and separate compilation mechanisms (although procedures could be separately compiled if they didnt need any global variables) and limita on the size of procedures, on the number of blocks in a compilation, and on the number of different record types that could be used in a complete program (16 of them). There limitations rather warped the program from its original design. I'd love to do the whole thing over, except that it might take a few more years, and I don't think anyone would be interested in the resultt. Nevertheless, I go back to work on it every now and then in one-or-two-month spurts of energy. Doingg this is kind of like playing a video game of extreme complexity. Each new test case passed is line beating a boss. A lot of code in the code generator got tossed or commented out, because that class of machine just isn't around any more. But I do keep the old commented-out code around until I'm ready to ocmmit to the new. If anyone is interested in looking at the code, it's available from the monotone repository at http://mtn-host.prjek.net/. The algol W compiler in that repository has bugs; it's *not* the version I'm using, and I'm currently wondering how it's related to the one I *am* using. Every now and then I wake up in the middle of the night having figured out once again how I should have approached the whole project. They vary a lot in detail, but the common threads are: (1) I should have started with a simplified, significantly unoptimizing code generator, then wrote test cases for it. (2) I should have also started implementing a sublanguage -- a sublanguage that contained only the features I really needed for writing the compiler. Actually, it wouldn't really have been a sublanguage -- it would have had a subset of the semantics, but drastically simplified syntax, and almost no implicit type conversions. Kind of a compromise betwen a programming language and a test generator for the code generator. (3) write the real compiler in the sublanguage, extending the sublanguage whenever implementing a feature was faster than coding around its absence. The history of the verifier is a tale for another day. -- hendrik ____________________ Racket Users list: http://lists.racket-lang.org/users
