Needs more combinators! It's already got Y, so why not SKI (http://en.wikipedia.org/wiki/SKI_combinator_calculus) or BCKW (http://en.wikipedia.org/wiki/B,C,K,W_system)?

(I'm not sure if it's kosher to link to it—if not, then please let me know and I'll remove it—but I implemented a SKI combinator reducer in Perl regexes a while back: http://perlmonks.org/?node_id=809842 .)

Nice! I'll have to read that in full later tonight - thanks for sharing!

This was mostly just me trying to wrap my head around Church encodings and such, but I could surely do with a couple more combinators in there :).

Just a warning: at least for me, once I got started, it became addictive to try, Oulipo-like (http://en.wikipedia.org/wiki/Oulipo), to write simple programmes in these ever-more-restrictive languages. I was helped along immensely by Hindley–Seldin (http://www.cambridge.org/us/academic/subjects/computer-scien...) and, for, as one would expect, a more friendly but still thoroughly mathematical introduction, Smullyan (http://en.wikipedia.org/wiki/To_Mock_a_Mockingbird). I am not alone in my admiration for the latter; see, for example, http://www.angelfire.com/tx4/cus/combinator/birds.html.

EDIT to add: Speaking of 'Mockingbird' articles, you may be interested in Appendix A of http://dkeenan.com/Lambda/index.htm, whose author discusses another interpretation, attributed there to Barendregt, of Booleans and numerals. For someone who regards the Church encoding as 'intuitive', they were very surprising!

Esoterrorism is at the end of http://codon.com/programming-with-nothing - nice FizzBuzz version...

What is YARV? :)

Yet Another Ruby VM - the "official" Ruby interpreter, if you will (also known as Matz's Ruby, or MRI). http://en.wikipedia.org/wiki/YARV

Yeah, Tom Stuart is great - that's where I got the idea to create this.