Hey,
In SICP 1.2.1 there is a function that makes a rational number, as follow:
(define (make-rat n d)
(let ((g (gcd n d)))
(cons (/ n g) (/ d g))))
I'm just curious how you can implement the same thing using lambda instead of let, without calling GCD twice. I couldn't figure it out myself.
These two things are same:
((lambda (p1 p2...) body) v1 v2...)
and
(let ((p1 v1) (p2 v2)...) body)
Looking at SICP section 1.3.2,
(let ((<var1> <exp1>)
(<var2> <exp2>)
...
(<varn> <expn>))
<body>)
is equivalent to
((lambda (<var1> ...<varn>)
<body>)
<exp1>
...
<expn>)
So your procedure,
(define (make-rat n d)
(let ((g (gcd n d)))
(cons (/ n g) (/ d g))))
should be equivalent to
(define (make-rat n d)
((lambda (g)
(cons (/ n g) (/ d g)))
(gcd n d)))
(Sorry, I don't have a Scheme interpreter in front of me to test this with.)