Very often we want to debug a program by printing variables, the following macro makes it easier and more convenient.
(define-syntax dbg
(syntax-rules ()
((_ fst ...)
(begin (let loop ((lst (list fst ...))
(symbol-names (map symbol->string (quote (fst ...)))))
(if (not (null? lst))
(begin (display (string-append " " (car symbol-names) ": "))
(display (car lst))
(display ",\t")
;; when it's the last element of the list,
;; print a newline
(if (null? (cdr lst)) (newline))
(loop (cdr lst) (cdr symbol-names)))))))))
Suppose we want to print out variables var1, we can do (dbg var1). we can also print multiple variables with (dbg var1 var2 var3). See the following example.
(define (my-sqrt x)
(define (good-enough? guess previous-guess)
(< (abs (- guess previous-guess)) 0.0001))
(define (average x y)
(/ (+ x y) 2))
(define (improve guess x)
(average guess (/ x guess)))
(define (sqrt-iter guess previous-guess x iter)
(dbg iter guess)
(if (good-enough? guess previous-guess)
guess
(sqrt-iter (improve guess x) guess x (1+ iter))))
(sqrt-iter 1.0 -1 x 1))
And (in MIT/GNU scheme) evaluate (my-sqrt 42) yields
iter: 1, guess: 1.,
iter: 2, guess: 21.5,
iter: 3, guess: 11.726744186046512,
iter: 4, guess: 7.654150476761481,
iter: 5, guess: 6.570684743283659,
iter: 6, guess: 6.481356306333419,
iter: 7, guess: 6.480740727643494,
iter: 8, guess: 6.48074069840786,
;Value: 6.48074069840786