; dollar format
(define (take n xs)
(let loop ((n n) (xs xs) (ys '()))
(if (or (zero? n) (null? xs))
(reverse ys)
(loop (- n 1) (cdr xs)
(cons (car xs) ys)))))
(define (drop n xs)
(let loop ((n n) (xs xs))
(if (or (zero? n) (null? xs)) xs
(loop (- n 1) (cdr xs)))))
(define (digits n . args)
(let ((b (if (null? args) 10 (car args))))
(let loop ((n n) (d '()))
(if (zero? n) d
(loop (quotient n b)
(cons (modulo n b) d))))))
(define (dollar x)
(define (commas n)
(let ((xs (reverse (map number->string (digits n)))))
(let loop ((xs (drop 3 xs)) (zs (reverse (take 3 xs))))
(if (null? xs) (apply string-append zs)
(loop (drop 3 xs) (append (reverse (take 3 xs)) (list ",") zs))))))
(define (zero n)
(if (zero? n) "00"
(if (< n 10) (string-append "0" (number->string n))
(number->string n))))
(let* ((dollars (inexact->exact (floor x)))
(cents (inexact->exact (round (* 100 (- x dollars))))))
(string-append "$" (commas dollars) "." (zero cents))))
(display (dollar 1234567.8912)) (newline)