[Haskell-cafe] better way to do this?

Michael P Mossey mpm at alumni.caltech.edu
Tue Oct 6 16:22:04 EDT 2009

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?)


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