Older blog entries for chalst (starting at number 59)

21 Jan 2003 (updated 21 Jan 2003 at 18:00 UTC) »

First answer, to question #3: mglazer rated all of their diary entries at 1; incidentally he also rated the diary entries of ladypine, moshez, shlomif, nixnut, and ChrisMcDonough at 10. Congratulations to bgeiger who got it. I think it is reasonable to assume that mglazer is not interested in reading any criticism of his anti-arab hate campaign.

Second answer, to question #1: Bohemian Hall and Park is the place. Thanks to wlach who gave me the lead so I could find it using google.

19 Jan 2003 (updated 21 Jan 2003 at 17:11 UTC) »

Three questions:

  1. I have heard that there is a German-style Biergarten in New York, but I was not been able to find it on my last two visits there. Would anyone know how to find the address of the place?
  2. I'm assembling a list of what I think are the seminal high-level programming languages, the languages that were based on truly radical ideas about how to make computers do what you want, rather than just adding layers upon existing languages. So far I have:
    FORTRAN, LISP, Algol 60, CPL, GPM, PROLOG, the UNIX shell, APL, Forth, Janus, Smalltalk, Scheme, ML, FP, Occam, Elephant 2k.

    Does anyone think these are not right, or have other suggestions for this list?

  3. What did auspex, zenalot, lmjohns3, tk, rasmus, whytheluckystiff, graydon, RossBurton, ciphergoth, sad, bratsche, NoWhereMan, seanc, kwoo, magsilva, zhaoway, davidw, davidu, vab, badvogato, sisob, mikl, Bram, amars, dmerrill, kilmo, Cantanker, daniels, async, Stevey, mibus, aes, chakie, ajh, logic, TheCorruptor, bjf, yeupou, Uche, chalst, thomasvs, deekayen, MichaelCrawford, bgeiger, Uraeus, purcell, nymia, kbreit, chipx86, Archit, dyork, BenFrantzDale, raph, mjcox, vorlon, kai, movement, bytesplit, crhodes, ncm, Mondragon, lyb, Jordi, mwh, and garym have in common on the 17th January? ...answer here </ol>

Postscript This diary entry:

  • has generated a lot of interest: sad, mwh, bjf, lmjohns3, kwoo, aes, dyork, and, of course bgeiger have all weighed in.
  • renders horribly on konqueror and dillo, but lynx can cope with it. Apparently the krufty parsers in many browsers can't handle anchor reference points nested within lists...
18 Jan 2003 (updated 21 Jan 2003 at 11:53 UTC) »
Dialectology
I've recently been thinking about getting languages to talk to each other, mostly in the context of the LISP family, and have been struck by how difficult it is to give clear, language independent, information in a program text about what language you are writing in, and what coding conventions you are using. I've thought of a simple device to correct this, a "dialect" directive, that could easily be integrated into any programming language. Here are a few examples to illustrate what I am talking about:
For C:
#dialect C gnu-cpp linux-style

For python:
dialect python lexical-scope indent-4-spaces

For the LISP family:
(dialect scheme
case-sensitive-ids scheme48-package multiline-comments)

For Java:
/** ...
* @dialect java sun-codeconv gcj-code
*/

I think having a reasonably coherent, cross-language, machine readable set of conventions for indicating the dialect of a source text is useful, particularly for allowing syntax-directed editors to reliably determine how to format program text, and to figure what identifiers are introduced and how they are used.

An issue is: should dialect directives extend the language? My opinion is they should, since they could be used to influence the semantics of the program, but they should be permitted to occur in comments, so ensuring backwards compatibility.

Another issue concerns UNIX #! directives; see SRFI 22: Running Scheme Scripts on Unix for some discussion of important issues here.

I'd like to write an advogato article about this, probably in a couple of weeks time, but I have to hammer down a few ideas first. In the meantime I am interested in any comments or references to similar ideas.

Postscript
Withdrew my certification of robocoder as apprentice due to his (cynical|confused) certification of mglazer as master.

solovay: Welcome to advogato!

mglazer: Hypothetical question -- Would you feel any regret for your anti-arab invective if you heard that it motivated a vicious hate crime against innocent arabs?

28 Dec 2002 (updated 29 Dec 2002 at 08:49 UTC) »
Stressful month:
The month so far from my last diary entry has been the most stressful month of a year with many stressful months: I feel relieved just to have enough space and emotional energy to be typing this diary entry. To give a taste of my month: on or around the 17th me and my wife were challenged by four thugs on the U1 Berlin underground line, one of who kicked me in the eye, sending me to hospital for two days. That was the second most stressful event of this month... I'm much better now, and am satisfied to say the police have a very good chance of catching them: there are three eye-witnesses, and the police (who have been great; the Berlin police have a not entirely undeserved bad reputation in Germany) want me to come in and look at photos.

Web based proof exchange systems:
I am still very enthusiastic about raph's work here, although I am disappointed he has used python for the implementation. Apart from that, I think raph's writings show excellent taste and insight. What I think he has got right:

  • Metamath is probably the simplest system good enough to do the job. I don't think Hilbert systems are the best possible systems from a proof theoretic point of view, but the problem with the more sophisticated frameworks is that none of them are yet ``the right thing'';
  • Definitions are the hardest thing to get right in a Hilbert system based framework, and I think here is the place where it is easiest to criticise metamath. Raph seems to have a pretty good grasp of the issues here, which are riddled with subtleties, and doesn't seem bound by doing things the way the experts have always done them, which ensures that his work will be interesting whether or not it is successful.

There have been lots of interesting comments appearing on recentlog, but I've not had a chance to write any responses. I'll try to do this in the next few weeks.

graydon:
Great post!

lkcl:
Nice series of articles on windows and its free rivals. Just a little point: just because POSIX specifies a file system that doesn't support interesting permissions models doesn't mean that linux and FreeBSD can't: both of the have VFS layers that allow them to do just this sort of thing.

Wishing all advogatans a Happy New Year!

3 Dec 2002 (updated 3 Dec 2002 at 12:25 UTC) »
dan: Alan Cox's diary, November the 27th, "strange lisp people turn up from Oxford" -- that wouldn't be you by any chance, would it?

Can you put a price on heroism?

Yes! And you can manage it sensibly or wastefully too! I've been growing irritated with the Economist over recent years (since the departure of Rupert Pennant-Rea, I think...), and I hardly buy it anymore, but the occasional article like this one keeps me checking the website...

25 Nov 2002 (updated 25 Nov 2002 at 23:13 UTC) »

There's an article by Thierry Coquand, A new paradox in type theory that gives a good explanation of Reynold's argument that polymorphism is not set-theoretic. I think this goes some way towards making important ideas about the nature of the polymorphic lambda-calculus accessible purely through online references, though a lot of important references are from the 70s and 80s and would benefit from good online summaries. I'd be grateful to hear of more references. BTW mail sent to cas@janeway.inf.tu-dresden.de will get to me, I'm quite happy that my main email address has been lost from the internet, since I don't have to spend so much time deleting spam...

jameson: Were you in Thomas Streicher's group when you were at Darmstadt? Who are you working with at Boulder? Despite my reservations, Girard's book is already something of a classic: I don't think you would have any problems convincing the librarian to try to get a copy.

Postscript: The last `Communications of the ACM' issue has as it main focus an event that I think will prove to be of great importance to computer science: The ACM has rejected licensing of software engineers essentially because code construction is not a normal engineering task. Interesting also in this light is Phil Greenspun's article reflecting on management at Ars Digita. Conclusions? I don't think software will *ever* be a `normal' engineering discipline, the way that civil engineering is today. I do think that the `Wild West' nature of the disipline will not last, however, because I think that in the next <n> decades it will become feasible to construct large systems that are proven to satisfy a formal specification.

Thanks to raph, Michael Norrish was able to get in touch with me. Here is his message (it took me rather a long time to post this, but better late than never) which clarifies the status of HOL:

Dear Charles,

I got to your Advogato diary entries through those of Raph Levien's. Just a few things about HOL:

  • It's not based on System F, but a polymorphic version of Church's simple type theory, making its type system even simpler than that of ML. This also means that it's easy to give it a set theoretic semantics.
  • HOL is a direct descendent of LCF, the project that invented ML. HOL's first developer was Mike Gordon was one of the people that worked on LCF with Robin Milner in Edinburgh and Stanford. LF is later, mainly unrelated, project.
Regards,

Michael.

raph: I wish it was normal for most university professors to apply the same intellectual energy and curiosity that you do to fields outside their main area of enquiry... To answer the questions you ask:

  • A theory of functions doesn't have a consequence relation, but Goedel showed, in his Dialectica paper, how it is possible for a calculus of functions to have the same mathematical strength as a proof system. This technique is called functional interpretation, and it is a very important technique in proof theory. Under this system, the pure predicate calculus (ie. FOL) can be captured by the simply-typed lambda calculus, Peano Arithmetic by the extension of the simply-typed lambda calculus by an iterator, and Z_2 by the polymorphic lambda calculus.

    Your question was: how is it that the polymorphic lambda calculus (ie. System F) can have the same strength as Z_2? It's hard to explain without reference to Girard's proof of strong-normalisation for system F, but the essential idea is that (i) the impredicative quantification over types possessed by system F is powerful because a lambda-term possessing this form of type abstraction is abstracted over its *own* type; (ii) this quantification doesn't introduce vicious circles because we can stratify the terms of the calculus by their types, assigning a rank to them based on how deeply their most deeply nested type quantifier occurs, and the type system forbids application of terms of given rank to terms of equal or higher rank. In this scheme applications of Pi^1_k comprehension in Z_2 correspond to terms of rank k. This is a difficult idea: if you really want to understand it you should read Girard's book.

  • In describing the above I avoided the details about classical vs. intuitionistic systems. With classical proof systems, the functional interpretation does not proceed as directly as for intuitionistic systems, and we need to do a bit of meta-mathematical massaging to get our correspondence. The intuitionistic logics, such as the calculus of constructions that lies behind Coq, that are in direct correspondence with lambda calculi of system F-like strength obviously do not have the principle of the excluded middle; rather less obviously it is not straightforward to add the principle without making the calculus inconsistent. So in fact classical vs. intuitionistic turns out to be an important distinction. Total vs. partial functions I think is not so important, since a good definitional scheme will allow you to move between the two representations.
How important is all this? It can become important if you either want to have automatic program extraction from proofs, or if you want a family of proof systems of radically different strength with facilities to move between representations. If you fix on an adequately powerful consequence relation expressed for a tractable language, then I think you can avoid the need for a complex metamathamatical infrastructure. For usual mathematical demonstrations Z_2 is more than enough, and I think Z_2 has a better language for mathematics than ZFC. The point is that with a powerful object proof theory, it becomes possible to capture sophisticated abstractions in the object language, so minimising the need for sophisticated transformations of definitions.
raph: Two points:
  1. If you don't check that the translation is good, then there is every chance that either:
    • the proofs you carry out in your rich system do not correspond naturally to proofs in Z_2;
    • your rich system does not share the same consequence relation as Z_2;
    (btw I use SOA and Z_2 interchangeably to mean `a reasonable axiomatisation of the usual SOA consequence relation').
  2. I can't say what the best books are, since I picked up most of what I know from fairly obscure sources. The text I have read that gives the best feel of what life is like in a a proof theory based on the lambda calculus is Jean-Yves Girard's `Proofs and Types' (with appendices by Paul Taylor and Yves Lafont). This text gives many of the most important results, in particular the proof of strong normalisation for System F and Martin-Loef's demonstration that this SN result is equivalent to the consistency of SOA, but be warned that this text is challenging, cavalier about definitions and has a very non-mainstream agenda. Also the book does not explain why polymorphism is not set-theoretic; for this I recommend John Reynold's orginal paper.

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