I love this. Learning a Lisp has been on my todo list for far too long. Once I saw what this was, I filed it away under "For later," but I stuck around for a bit to see if I could do a challenge first...

Totally got sucked in, and an hour or so later I had finished. Accounting for all those parens was really frustrating at first, but with the help of a good editor I eventually feel like I got into the "Lisp headspace."

I have played around with Haskell a bit. Scheme feels more verbose, but also closer to the metal^H^H^H^H^Hlambda calculus. The feature I missed the most was currying:

    >>(define sum (foldr + 0))
    [Error: too few arguments to function]

You can get around this somewhat:

    ; manually implement currying on a per-arity basis
    (define curry2 (lambda (f)
        (lambda (arg1) (lambda (arg2)
            (f arg1 arg2)))
    ))
    (define curry3 (lambda (f)
        (lambda (arg1) (lambda (arg2) (lambda (arg3)
            (f arg1 arg2 arg3))))
    ))

    ; now define a curried foldr:
    (define cfoldr (curry3 foldr))

    ; and finally:
    (define sum ((cfoldr +) 0))

The dynamic typing was also a bit foreign for me, so it took a while for me to wrap my head around "megasum." I can vaguely imagine how to implement it in Haskell, but it wouldn't be pretty!