I found it reasonably readable. The only criticism I had, offhand, was that the predicates like 'is-piece-knight?' were a bit verbose: the query mark at the end is understood to label a predicate, so the 'is-piece-' was redundant. I'd have written 'knight?' instead. That's pretty nit-picky though, and I don't think the current convention really detracts from its readability.
[1] If Lisp is So Great: http://paulgraham.com/iflisp.html
On the plus side: It's minimal and doesn't spend much time building abstractions on top of the basic lists/etc. that scheme provides. However, it certainly isn't pretty.
For instance: Things like the king & knight checks are really pretty ugly brute force solutions.
Similarly, the variable & function naming really is rather unclear and very poorly commented--some of that may be use of things idiomatic to scheme that I'm unfamiliar with though.
2. Always a possibility, I certainly don't know you. ;)
3. Some people certainly have said that before and probably will again. This is really just a personal preference thing, I think.
You're fooled on this because so many languages are so similar -- if you know C++, you can trivially read Java and C# and Python, and even languages that are a bit further afield like Javascript and Perl still are heavily influenced by C, so don't seem too odd.
But Scheme is a whole new family of languages. It's like being an English reader and looking at Greek text as opposed to Spanish. It's not that Greek is harder than Spanish, it's that Spanish is closer to what you already know.
Scheme has mutable state. Scheme has unmarked side-effects. Scheme has variables, as opposed to tags applied to constant values.
Scheme was an entirely new kind of language back when lambda expressions were Heavy Deep Magic and dynamic typing was something you did when your Flexowriter was well-oiled (hyperbole intentional). These days, it's no weirder than Javascript or the more modern dialects of Java or Visual C#.Net. Hell, even C++ is getting lambdas and closures, and that's a Serious Business COBOL-Replacement Language.
Modern Algol dialects are Scheme in everything except syntax, and now that metaprogramming (Ruby, to begin with) is beginning to catch on, I fully expect language designers to re-invent some shambolic version of s-expressions when they realize that metaprogramming in Algol syntax is hard.
Scheme won. It's just that its victory was suppressed for political reasons.
Try it with an interpreter running.
1. That is badly written code. 2. I'm a bad programmer. 3. Scheme's syntax is ugly.
What do you think?