Re: default stack size

[Joshua Judson Rosen]

On Sat, Sep 06, 2003 at 05:33:21AM -0500, Lynn Winebarger wrote:
> 
> 
> Viktor Pavlenko wrote:
> >>>>>>"bt" == barney toma <address@hidden> writes:
> >>>>>
> >
> >    bt> I receive a stack overflow error when creating a list of 500
> >    bt> entries.
> >
> >    bt> (define (rep n)          ;(rep n) ==> (n n-1 n-2  ... 1)
> >    bt>     (cond ((= n 0) '())
> >    bt>        (t (cons n (rep (- n 1) )))))
> >    bt> (rep 500)

Tangentially: are you sure that you meant "t" here, and not "#t"?

t -is- bound to 't, in guile, while -is- true due to Scheme's `all
things other than #f are true' rule, but the canonical booleans in
Scheme, unlike in other most lisps, are #t and #f, not t and nil; in
fact, nil (which is bound to 'nil) is also `true', in guile/scheme....

You can also use the `else' syntax inside a Scheme cond, as Viktor has
demonstrated below:

> >A tail- recursive procedure will give you a list as long as you
> >wish (but in ascending order):
> >
> >(define (rep n ls)
> >  (cond ((= n 0) ls)
> >     (else (rep (- n 1) (cons n ls)))))
> >
> >(rep 10000 '())
> >
> >I think by setting the stack limit so low (389) they encourage you to
> >write tail recursive procedures (correct me if I'm wrong).

(I recall someone (either Marius or Mikael, I -think-) saying that the
stack-limit was `to be friendly to newbie programmers', because
newbies are likely to write infinitely-recursive routines and would
like prefer to have them fail sooner rather than later)

>    Yeah, but why not get it in the right order:
> 
> (define (rep n)
>   (let loop ((n n)
>              (k (lambda (v) v)))
>     (if (= n 0)
>         (k '())
>         (loop (- n 1)
>           (lambda (v)
>             (k (cons n v))))))
>
>    Though reliance on the stack may be/is probably more efficient.  All this
> version does is allocate the stack in the heap, one disconnected frame at a 
> time.

... and you can also use an `if', as Lynn has demonstrated above.

Lynn, that's certainly an interesting point, but what's wrong with
just changing the direction in which the counter progresses? :)

Like:

      (define (rep n)
        (let loop ((m 1)
                   (ls '()))
          (if (> m n)
              ls
              (loop (+ m 1) (cons m ls)))))

-- 
"The difference between theory and real life is that in theory,
 there is no difference between theory and real life, but in real life,
 there is a difference." [who originally said this?]

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