ModuleMonadRandom-0.6.2Haskell2010

Control.Monad.Trans.Random.Lazy

Lazy random monads, passing a random number generator through a computation. See below for examples.

For a strict version with the same interface, see Control.Monad.Trans.Random.Strict.

3 types
21 values

PackageMonadRandom-0.6.2
Exports24
LanguageHaskell2010
LicenceBSD-3-Clause
SourceLazy.hs

The Rand monad transformer

8 declarations

typetype Rand g = RandT g Identity

A random monad parameterized by the type g of the generator to carry.

The return function leaves the generator unchanged, while >>= uses the final generator of the first computation as the initial generator of the second.

valueliftRand

:: (g -> (a, g))
pure random transformer
-> Rand g a
equivalent generator-passing computation

Construct a random monad computation from a function. (The inverse of runRand.)

valuerunRand

:: Rand g a
generator-passing computation to execute
-> g
initial generator
-> (a, g)
return value and final generator

Unwrap a random monad computation as a function. (The inverse of liftRand.)

valueevalRand

:: Rand g a
generator-passing computation to execute
-> g
initial generator
-> a
return value of the random computation

Evaluate a random computation with the given initial generator and return the final value, discarding the final generator.

```
evalRand m s = fst (runRand m s)
```

valueexecRand

:: Rand g a
generator-passing computation to execute
-> g
initial generator
-> g
final generator

Evaluate a random computation with the given initial generator and return the final generator, discarding the final value.

```
execRand m s = snd (runRand m s)
```

valuemapRand :: ((a, g) -> (b, g)) -> Rand g a -> Rand g b

Map both the return value and final generator of a computation using the given function.

```
runRand (mapRand f m) = f . runRand m
```

valuewithRand :: (g -> g) -> Rand g a -> Rand g a

withRand f m executes action m on a generator modified by applying f.

```
withRand f m = modify f >> m
```

valueevalRandIO :: 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.

The RandT monad transformer

7 declarations

newtypenewtype RandT g (m :: Type -> Type) a

A random transformer monad parameterized by:

g - The generator.
m - The inner monad.

The return function leaves the generator unchanged, while >>= uses the final generator of the first computation as the initial generator of the second.

Instances22MonadRWS, MonadError, MonadReader, MonadState, MonadWriter, MonadSplit, …

(MonadReader r m, MonadWriter w m, MonadState s m) => MonadRWS r w s (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadError e m => MonadError e (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadReader r m => MonadReader r (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadState s m => MonadState s (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadWriter w m => MonadWriter w (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
(RandomGen g, Monad m) => MonadSplit g (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadTrans (RandT g)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
(Monad m, RandomGen g) => RandomGenM (RandGen g) g (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
(Monad m, RandomGen g) => StatefulGen (RandGen g) (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
Monad m => Monad (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
Functor m => Functor (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadFix m => MonadFix (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadFail m => MonadFail (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
Monad m => Applicative (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadPlus m => Alternative (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadPlus m => MonadPlus (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadIO m => MonadIO (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
MonadCont m => MonadCont (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
PrimMonad m => PrimMonad (RandT s m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
(Monad m, RandomGen g) => MonadInterleave (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
(RandomGen g, Monad m) => MonadRandom (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
type PrimState (RandT s m) = PrimState mDefined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy

valueliftRandT

:: (g -> m (a, g))
impure random transformer
-> RandT g m a
equivalent generator-passing computation

Construct a random monad computation from an impure function. (The inverse of runRandT.)

valuerunRandT

:: RandT g m a
generator-passing computation to execute
-> g
initial generator
-> m (a, g)
return value and final generator

Unwrap a random monad computation as an impure function. (The inverse of liftRandT.)

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

Evaluate a random computation with the given initial generator and return the final value, discarding the final generator.

evalRandT m g = liftM fst (runRandT m g)

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

Evaluate a random computation with the given initial generator and return the final generator, discarding the final value.

execRandT m g = liftM snd (runRandT m g)

valuemapRandT :: (m (a, g) -> n (b, g)) -> RandT g m a -> RandT g n b

Map both the return value and final generator of a computation using the given function.

runRandT (mapRandT f m) = f . runRandT m

valuewithRandT :: (g -> g) -> RandT g m a -> RandT g m a

withRandT f m executes action m on a generator modified by applying f.

```
withRandT f m = modify f >> m
```

Lifting other operations

6 declarations

valueliftCallCC :: CallCC m (a, g) (b, g) -> CallCC (RandT g m) a b

Uniform lifting of a callCC operation to the new monad. This version rolls back to the original state on entering the continuation.

valueliftCallCC' :: CallCC m (a, g) (b, g) -> CallCC (RandT g m) a b

In-situ lifting of a callCC operation to the new monad. This version uses the current state on entering the continuation. It does not satisfy the uniformity property (see Control.Monad.Signatures).

valueliftCatch :: Catch e m (a, g) -> Catch e (RandT g m) a

Lift a catchE operation to the new monad.

valueliftListen :: Monad m => Listen w m (a, g) -> Listen w (RandT g m) a

Lift a listen operation to the new monad.

valueliftPass :: Monad m => Pass w m (a, g) -> Pass w (RandT g m) a

Lift a pass operation to the new monad.

valueevalRandTIO :: MonadIO m => RandT StdGen m a -> m a

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

StatefulGen interface

3 declarations

datadata RandGen g

A proxy that carries information about the type of generator to use with RandT monad and its StatefulGen instance.

RandGen

Instances4RandomGenM, StatefulGen

(Monad m, RandomGen g) => RandomGenM (RandGen g) g (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
(Monad m, RandomGen g) => RandomGenM (RandGen g) g (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Strict
(Monad m, RandomGen g) => StatefulGen (RandGen g) (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Lazy
(Monad m, RandomGen g) => StatefulGen (RandGen g) (RandT g m)Defined in MonadRandom-0.6.2 · Control.Monad.Trans.Random.Strict

valuewithRandGen

:: g
initial generator
-> (RandGen g -> RandT g m a)
-> m (a, g)
return value and final generator

A RandT runner that allows using it with StatefulGen restricted actions. Returns the outcome of random computation and the new pseudo-random-number generator

Example1 expression

>>> withRandGen (mkStdGen 2021) uniformM :: IO (Int, StdGen)(6070831465987696718,StdGen {unStdGen = SMGen 4687568268719557181 4805600293067301895})

valuewithRandGen_

:: Monad m
=> g
initial generator
-> (RandGen g -> RandT g m a)
-> m a
return value and final generator

Same as withRandGen, but discards the resulting generator.

Example1 expression

>>> withRandGen_ (mkStdGen 2021) uniformM :: IO Int6070831465987696718

Examples

0 declarations

Random monads

The die function simulates the roll of a die, picking a number between 1 and 6, inclusive, and returning it in the Rand monad transformer. Notice that this code will work with any random number generator 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 transformer, we can use evalRandIO.

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