On Mar 23, 2009, at 10:36 PM, Paul Drummond wrote:
>
> 2009/3/24 Joshua Fox <joshuat...@gmail.com>:
>>> Why "presumably with side effects?"
>> Otherwise you would use repeat. A pure function returns the same
>> value
>> every time, so there is no reason to call it repeatedly.
>
> Yup, that makes sense.
>
> Random numbers are side-effecting (is that the right term?) and I was
> trying to think about the (simple) problem in a pure functional way
> which was getting my simple brain all confused!
>
It's not obvious what side effect is occurring by calling rand-int,
but in fact the state of the pseudorandom generator is affected by
each call. This is from java.util.Random. Everybody calls next()
sooner or later, and it calls the compareAndSet() method:
* This is a linear congruential pseudorandom number generator, as
* defined by D. H. Lehmer and described by Donald E. Knuth in
<i>The
* Art of Computer Programming,</i> Volume 2: <i>Seminumerical
* Algorithms</i>, section 3.2.1.
protected int next(int bits) {
long oldseed, nextseed;
AtomicLong seed = this.seed;
do {
oldseed = seed.get();
nextseed = (oldseed * multiplier + addend) & mask;
} while (!seed.compareAndSet(oldseed, nextseed));
return (int)(nextseed >>> (48 - bits));
}
As Joshua pointed out, whenever a function returns different values
given the same args that's a good sign that a side effect has
occurred--some state has obviously changed.
Aloha,
David Sletten
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