[Haskell-beginners] Combining the Rand and State monads
byorgey at seas.upenn.edu
Thu Apr 5 19:52:47 CEST 2012
On Thu, Apr 05, 2012 at 10:42:20AM +0000, Amy de Buitléir wrote:
> I'm trying to develop a very simple simulation framework. A simulation consists
> of a list of models. Models generate output events based on an input event. Here
> is what I have currently (it works fine).
> modelT :: Model g
> modelT _ = do
> s <- get
> put $ s ++ " R"
> n <- HOW DO I GET A RANDOM NUMBER????
> return [n]
I assume you are using the MonadRandom package? Getting a random
number is as simple as doing something like "getRandomR ('A','Z')" (or
using any other method from the MonadRandom class ). There is an
instance MonadRandom m => MonadRandom (StateT s m), so calls to
getRandom, getRandomR, etc. are automatically lifted into your Model
Your problem seems to be the type you have given to modelT: it claims
that it will work for *any* type g but this is not so; g must
represent a pseudorandom number generator. This works:
modelT :: RandomGen g => Model g
modelT _ = do
s <- get
put $ s ++ " R"
n <- getRandomR ('A', 'Z')
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