Lambda the Ultimate

π - not to be confused with the π-calculus - is a pattern-based language being developed by the Software Technology group at Technische Universität Darmstadt. Quoting from the project website:

There is only one language construct in π: the pattern. Patterns are, simply speaking, EBNF-expressions with an associated meaning; a pattern can be easiest understood as a function with a syntactically complex (context-free) "signature". The non-terminal symbols in the signature are then the parameters of the pattern. A π-program is a sequence of instruction symbols (technically, sentences), each being a sequence of (Unicode) characters. The sentences are then evaluated (executed) in the respective order.

The basic idea here seems similar to the OMeta language, previously mentioned on LtU here, but based on EBNF instead of Parsing Expression Grammars (PEGs).

Pattern definitions in π have the form

declare_pattern name ≔ syntax ⇒ type ➞ meaning;

Here's a trivial example of defining a pattern:

declare_pattern
   integer_potentiation ≔ integer:i %W- "^" %W- integer:j ⇒ integer ➞
   {
      int result = i;
      for (int k = 1; k <= j-1; k++)
         result *= i;
      return result;
   };

The resulting pattern can then be used directly in expressions, such as print(42^6);.

More information about the language, as well as the implementation, can be found at http://www.pi-programming.org. There's an OOPSLA09 paper on π as well, but I haven't been able to find an open access version of it yet.

[Update: the π team has made their OOPSLA article available here]

LP has been mentioned a number of times on LtU but never featured as a topic of discussion in its own right. On the face of it, it seems like an eminently sensible way to program. Why hasn't it taken the whole world by storm? Knuth puts forward Jon Bentley's observation as one possible answer: "a small percentage of the world's population is good at programming, and a small percentage is good at writing; apparently [Knuth is] asking everybody to be in both subsets."

To discuss this and other theories on their merits, a quick refresher on the basics of LP is in order. As usual, the relevant Wikipedia article is informative but bland. As Knuth pointed out, original sources are often best. Here are two good ones:

1. Programming Pearls: Literate Programming by Jon Bentley and Don Knuth; CACM, Vol. 29, No. 5, May 1986. (A bootleg copy available here.)
2. Programming Pearls: a Literate Program, by Jon Bentley, Don Knuth, and Doug McIlroy; CACM, Vol. 29, No. 6, June 1986. (Bootleg copies available here and here.)

The second paper is the more interesting of the two. It contains a literate program by Knuth and a review of the same by McIlroy:

Knuth has shown us here how to program intelligibly, but not wisely. I buy the discipline. I do not buy the result. He has fashioned a sort of industrial-strength Fabergé egg -- intricate, wonderfully worked, refined beyond all ordinary desires, a museum piece from the start.

I, too, buy the discipline for programming in the small but can't really see how CWEB-like systems can be adapted to and adopted by multi-hacker teams working on very large code bases written in a mixture of different languages. Ramsey's Literate Programming on a Team Project enumerates some of the problems.

Can LP be used for anything other than small-to-medium programs written by a single person in a single language?

A short comparison (plus some links) of Erlang and Clojure solutions to the simple problem of running a counter in a separate thread.

William Cook gave an interesting speech at ECOOP 2009. When discussing his career path, he touches on PLT research,

One thing I learned is that programming languages is a very small research area. As a grad student I thought we were big, because everyone in knew did it. But I found out that we are small, smaller perhaps than database research. We are also very fragmented, into functional, OO, typed and untyped, etc. This tends to lead dilute our influence on the community, I think.

differences between academia and industry

Academia is like page-rank: your value is defined by what everybody else thinks of you. And what you think of everybody else contributes to (or subtracts from!) their net academic worth. It is a closed hermetic self-referential system. The commercial value system is obviously different: it's all about money. In a way this is similar to page-rank as well. The difference is that the money comes from outside the community, from customers.

and a few other PLT morsels that might interest LtUers.

William Cook was the prinicpal lead for creating AppleScript. His HOPL paper was discussed on LtU before but I can't currently find the reference.