Control.Monad.Random

MonadRandom-0.4.2.2: Random-number generation monad.

Copyright | 2006-2007 Cale Gibbard, Russell O'Connor, Dan Doel, Remi Turk, Eric Kidd.
License | OtherLicense
Stability | experimental
Portability | non-portable (multi-parameter type classes, undecidable instances)
Safe Haskell | None
Language | Haskell2010

Control.Monad.Random

Contents

- Special lift functions
- Example

Description

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.

Synopsis

- module System.Random
- module Control.Monad.Random.Class
- evalRandT :: Monad m => RandT g m a -> g -> m a
- runRandT :: RandT g m a -> g -> m (a, g)
- evalRand :: Rand g a -> g -> a
- runRand :: Rand g a -> g -> (a, g)
- evalRandIO :: Rand StdGen a -> IO a
- fromList :: MonadRandom m => [(a, Rational)] -> m a
- uniform :: MonadRandom m => [a] -> m a
- type Rand g = RandT g Identity
- data RandT g m a
- liftRand :: (g -> (a, g)) -> Rand g a
- liftRandT :: (g -> m (a, g)) -> RandT g m a

# 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 :: 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) |
(Functor m, MonadPlus m) => Alternative (RandT g m) |
MonadPlus m => MonadPlus (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

:: (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)