On 09.02.2012 18:27, Duncan Coutts wrote:
Actually it is not true that the state has to be copied. Using the
lazy ST monad we can implement this interface and internally use
mutable ST arrays.
See for example
http://web.archive.org/web/20090108050217/http://www.augustsson.net/Darcs/MT/MersenneTwister.hs
It ends up with this function that generates the infinite lazy
sequence of words from a seed word. This can be then made to fit the
next :: g -> (Int, g) interface, with g = [W].
This is very elegant trick! Ability to snapshot and restore generator
is lost.
So yes this looses a small constant factor because of the extra lazy
list (which could be reduced somewhat), so it's not suitable for the
absolutely max performance interface, but it certainly allows the use
of traditional mutable PRNGs with the nice interface with decent
performance.
Certainly none of these PRNGs support split, but that's a separate
issue. Overall, I think the Random type class is salvageable given a
few changes. In the end, the Random module probably needs both an
ST-monadic interface and a pure one. People can use the ST one if it
happens to be more convenient or if they need to squeeze the last drop
of performance out.
There is another problem. Functions like `next' aren't meant to be used
by humans. One have to thread state manually and this is tedious
and error-prone. So PRNG should be wrapped into monad. Are `next'-like
function needed at all?
I'd expect that creation of generic and useful type class would be
very nontrivial.
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