I think I've answered part of my question. There is a function that lives in the
IO monad, newStdGen. This gives you a new generator with random initial state.
That's the first part of the story. Then, I'm not sure, but perhaps you can call
newStdGen many times during a program and get good pseudorandom behavior. For
example:
shuffle :: RandomGen g, Random a => g -> [a] -> [a]
shuffle = ...
main = do
g1 <- newStdGen
print (shuffle g1 [1,2,3,4,5] :: [Int])
g2 <- newStdGen
print (shuffle g2 [6,7,8,9,0] :: [Int])
Does calling newStdGen twice in a row give good behavior? Is it a bad idea? Is
it system-dependent?
Also note that I added the Random class constraint to 'shuffle'. I haven't
tested this but it might be necessary. Or not?
Thanks,
Mike
Michael P Mossey wrote:
Duncan Coutts wrote:
So you end up with pure functions like:
shuffle :: RandomGen g => g -> [x] -> [x]
Thanks for the help, Duncan. I'm confused on one point. Don't you always
need the new state of the generator back? So wouldn't this need to be:
shuffle :: RandomGen g => g -> [x] -> (g,[x])
Another approach is to hide the 'g' inside a monad. That's what
MonadRandom is all about. eg:
shuffle :: [x] -> Rand [x]
One tutorial mentions the class Gen in the Test.QuickCheck module. Is
"Rand" a different class defined somewhere else?
The tutorials above explain about the other random functions, for
getting values of different types (not just Int) and restricted ranges
of number etc.
Of course at some point you want to seed the random number generator
with some initial genuinely random value (not like the 12345 we used
above). That is the only place in your random-handling code that needs
to do IO. All the rest of it can be pure.
What function gives you that initial random seed? (Grabs it from the
system clock?)
Thanks,
Mike
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
