Control.Monad.Random

A random number generation monad. See http://www.haskell.org/haskellwiki/NewMonads/MonadRandom for the original version of this code.

The actual interface is defined by MonadRandom.

Computation type:

Computations which consume random values.

Binding strategy:

The computation proceeds in the same fashion as the identity monad, but it carries a random number generator that may be queried to generate random values.

Useful for:

Monte Carlo algorithms and simulating random processes.

Documentation

module System.Random

module Control.Monad.Random.Class

evalRandT :: Monad m => RandT g m a -> g -> m a

Evaluate a RandT computation using the generator g. Note that the generator g is not returned, so there's no way to recover the updated version of g.

runRandT :: Monad m => RandT g m a -> g -> m (a, g)

Run a RandT computation using the generator g, returning the result and the updated generator.

evalRand :: Rand g a -> g -> a

Evaluate a random computation using the generator g. Note that the generator g is not returned, so there's no way to recover the updated version of g.

runRand :: Rand g a -> g -> (a, g)

Run a random computation using the generator g, returning the result and the updated generator.

evalRandIO :: Rand StdGen a -> IO a

Evaluate a random computation in the IO monad, splitting the global standard generator to get a new one for the computation.

fromList :: MonadRandom m => [(a, Rational)] -> m a

Sample a random value from a weighted list. The total weight of all elements must not be 0.

uniform :: MonadRandom m => [a] -> m a

Sample a value from a uniform distribution of a list of elements.

type Rand g = RandT g Identity

A basic random monad.

data RandT g m a

A monad transformer which adds a random number generator to an existing monad.

Instances

MonadReader r m => MonadReader r (RandT g m)

MonadState s m => MonadState s (RandT g m)

MonadWriter w m => MonadWriter w (RandT g m)

(Monad m, RandomGen g) => MonadSplit g (RandT g m)

MonadTrans (RandT g)

Monad m => Monad (RandT g m)

Functor m => Functor (RandT g m)

MonadFix m => MonadFix (RandT g m)

(Functor m, Monad m) => Applicative (RandT g m)

MonadIO m => MonadIO (RandT g m)

(Monad m, RandomGen g) => MonadRandom (RandT g m)

Special lift functions

liftRand

Arguments

:: (g -> (a, g))

action returning value and new generator state

-> Rand g a

Lift arbitrary action to Rand

liftRandT

Arguments

:: Monad m

=> (g -> m (a, g))

action returning value and new generator state

-> RandT g m a

Lift arbitrary action to RandT

Example

The die function simulates the roll of a die, picking a number between 1 and 6, inclusive, and returning it in the Rand monad. Notice that this code will work with any source of random numbers g.

die :: (RandomGen g) => Rand g Int
die = getRandomR (1,6)

The dice function uses replicate and sequence to simulate the roll of n dice.

dice :: (RandomGen g) => Int -> Rand g [Int]
dice n = sequence (replicate n die)

To extract a value from the Rand monad, we can can use evalRandIO.

main = do
  values <- evalRandIO (dice 2)
  putStrLn (show values)