[Haskell-cafe] Efficient functional idiom for histogram

[Sterling Clover]

On Aug 1, 2009, at 2:06 PM, Paul Moore wrote:

> Is the issue with random numbers just the implementation, or is it
> something inherent in the non-pure nature of a random number generator
> that makes it hard for Haskell to implement efficiently? If the
> latter, that probably makes Haskell a somewhat poor choice for
> simulation-type programs.
>

Well, I'm not sure of the details, but in your original  
implementation, you're performing IO to pull the seed out of a ref at  
every iteration. Pekka Karjalainen's doesn't do that, which probably  
helps with the speedup. Along with that, Haskell has a fairly slow  
random implementation. As I recall however, this is partially because  
it hasn't received a great deal of optimization, but mainly because  
the algorithm itself fulfills some rather strong properties -- in  
particular it must be fairly statistically robust, and it must  
provide a "split" function which produces generators that are  
independently robust [1]. This limits algorithm choice quite a bit.

For other random numbers, with different properties (faster, but with  
tradeoffs in robustness, or ability to split, or both), you can check  
hackage for at least mersenne-random and gsl-random. There may be  
others that I don't recall.

Cheers,
Sterl.

[1] http://www.haskell.org/onlinereport/random.html