s c h e m a t i c s : c o o k b o o k

(and (<= 3 x) (< x 5))

(let ((x-val x))
  (and (<= 3 x-val) (< x-val 5)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; chained comparisons syntax
;; example: (chain x < y <= z) roughly translates to
;;          (and (< x y) (<= y z)), minus the duplicated
;;           evaluation.
(define-syntax chain
  (syntax-rules ()
    ((_ x) x)
    ((_ x op y) (op x y))
    ((_ x op y rest ...)
     (let ((y-val y))
       (and (op x y-val)
            (chain y-val rest ...))))))

> (define (between-5-6? n) (chain 5 < n < 6))
> (between-5-6? 3)
#f
> (between-5-6? 5)
#f
> (between-5-6? 5.5)
#t
> (define (id* x)
    (display x)
    (newline)
    x)
> (chain (id* 3) < (id* 4) > (id* 2))
4
3
2
#t