Another month, another language: pi

[jducoeur]

The latest programming language to come to my attention is rather neat: in the pi language, the entire focus is on declaring patterns -- EBNF-style declarations of syntax, and their associated meanings. See the linked page for a little more info and some cool (if simple) examples of how easy it is to define new syntax in this language.

My initial reaction is that it's a bit scary, and almost certainly *much* too easy to completely hang yourself (I'd love to see what you could do with an obfuscated-code contest in this language), but also quite powerful for certain sorts of problems. Probably extremely well-suited to defining DSLs and the like, once it gets more mature...

Comments

[baron-steffan.livejournal.com]

Interesting that they don't handle English all that well in their description, unless this is some dialect of programmer-ese of which I'm unaware.

π technically is a fully reflective dynamically both semantically and syntactically extensible programming language....

Interesting nest of adverbs there.

In addition to that, with π we provide a clean and easy mean for the design of new (experimental) languages

A mean? I don't think they intend to say that their language is average.

[jducoeur]

Yeah, I noticed those myself. I'm assuming that this is an academic project initially, and that at least half the people involved don't speak English as a first language...

[hudebnik.livejournal.com]

Reminds me somewhat of Valentin Turchin's Refal language, which my father told me about when he was taking a course from Dr. Turchin twenty-mumble years ago. It was described at the time as "a language for writing languages".

If C is an implementation of the von Neumann architecture, and Lisp & Scheme are implementations of Alonzo Church's lambda-calculus, these languages can be thought of as implementations of Emil Post's production rules.

[metahacker.livejournal.com]

I have never heard that language mentioned by its actual name! It was always mentioned as "Re-fail", and I hadn't really encountered it before. The examples in wikipedia nail for me why the moniker: each of the examples defines something in terms of itself. Equality: defined as 'if equal, return true'; if: defined as 'if (condition) then do if, else do else'...I'm all for bootstrapping but...wha?

[jducoeur]

Actually, it makes sense when you think of it as a functional pattern-matching language. In the pattern they show, I believe "Then" and "Else" are simply syntactic sugar. That is, it's saying that, if you hit a pattern of the form "x Then (e.1) Else (e.2)", and x is T (presumably the atom for "true"), then do e.1. Essentially, it's defining the usual if-then-else syntax on top of simple pattern-matching.

Similarly, the Equality pattern is saying that, if the two parameters evaluate to the same value (note the doubled use of e.1), return T; otherwise (I assume that patterns are evaluated in order), they evaluate to different values, so return F.

It seems like pretty conventional stuff nowadays -- I suspect you could write similar examples in most of the popular modern pattern-matching languages. (I'm not an expert in this area, but it looks a lot like examples I've seen in Scala.)