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Editorial Team
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Asked: 2026-05-14T23:38:33+00:00

In SICP 1.2.1 there is a function that makes a rational number, as follow:

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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.

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1 Answer

  1. Editorial Team

    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)))
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