[Haskell-cafe] better way to do this?

Michael P Mossey mpm at alumni.caltech.edu
Tue Oct 6 16:52:30 EDT 2009

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 

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?


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
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