[Haskell-cafe] Efficient functional idiom for histogram

wren ng thornton wren at freegeek.org
Wed Aug 5 19:57:37 EDT 2009

Paul Moore wrote:
> 2009/8/5 Yitzchak Gale <gale at sefer.org>:
>> Or is this with an alternate RNG? Although I think even that
>> would be fair, since Python uses Mersenne.
> I got the impression Dmitry was using Haskell's standard RNG, not
> Mersenne Twister. If so, then we'd get further improvements with MT,
> but that's still a hit against Haskell, as I'd interpret it as meaning
> that Haskell supplies as default a PRNG which costs noticeable
> performance in order to provide guarantees that "ordinary" programs
> don't need.

I'm not sure how fair that is. For most "ordinary" programs I've written 
that use randomness anywhere, they have many different places that want 
randomness. Having a single global seed can suffice for this, but it 
introduces severe constraints that are seldom noticed.

Namely, these are in fact *P*RNGs. If every place needing randomness is 
given its own seed, then it becomes possible to store all those seeds, 
allowing for replay. Replay can be good for capturing the history of a 
particular run of a simulation/game ---which is difficult at best in 
non-Haskell approaches. And more critically the seeds can be stored in a 
"core dump" allowing replay as a debugging tool, allowing reproduction 
of 'random' bugs which are otherwise quite difficult to track down.

Imperative languages could use the same algorithms as Haskell, but they 
do not; which means these benefits of PRNGs are seldom even thought of. 
By separating concerns, the Haskell approach not only leads to cleaner 
code, but that separation can also be used to add new capabilities. The 
invisible manacles of imperativism should not be looked upon lightly.

Live well,

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