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Ovid (2709)

Ovid
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Stuff with the Perl Foundation. A couple of patches in the Perl core. A few CPAN modules. That about sums it up.

Journal of Ovid (2709)

Monday March 22, 2004
11:52 AM

Scheme Recursion Problem

[ #18011 ]

OK, I know this place is about Perl, but I suspect a few people here are familiar with Scheme. Can anyone tell me why I get the following problem with mit-scheme?

1 ]=> (load "test.scm")

;Loading "test.scm" -- done
;Value: sqrt

1 ]=> (sqrt 4)

;Aborting!: maximum recursion depth exceeded

And test.scm looks like this:

(define (new-if predicate then-clause else-clause)
  (cond (predicate then-clause)
        (else else-clause)))

(define (sqrt x)
  (define (sqrt-iter guess x)
    (new-if (good-enough? guess x)
      guess
      (sqrt-iter (improve guess x) x)))
  (define (improve guess x)
    (average guess (/ x guess)))
  (define (average x y)
    (/ (+ x y) 2))
  (define (good-enough? guess x)
    (< (abs (- (square guess) x)) 0.001))
  (sqrt-iter 1.0 x))

That works find if the sqrt definition has new-if changed to if. If you're curious, I got the problem from Section 1.1.7 of the Wizard book.

Update: problem solved. Corion on Perlmonks pointed out that new-if will always evaluate both arguments, thereby causing infinite recursion.

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