Copyright	(c) Brent Yorgey 2016
License	BSD3 (see LICENSE)
Maintainer	byorgey@gmail.com
Stability	experimental
Portability	non-portable (multi-param classes, functional dependencies, undecidable instances)
Safe Haskell	Trustworthy
Language	Haskell2010

Control.Monad.Trans.Random.Strict

Contents

The Rand monad transformer
The RandT monad transformer
Lifting other operations
Examples
- Random monads

Description

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

In this version, sequencing of computations is strict (but computations are not strict in the state unless you force it with seq or the like). For a lazy version with the same interface, see Control.Monad.Trans.Random.Lazy.

Synopsis

The Rand monad transformer

type Rand g = RandT g Identity Source #

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.

liftRand Source #

Arguments

:: (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.)

runRand Source #

Arguments

:: 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.)

evalRand Source #

Arguments

:: 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)
```

execRand Source #

Arguments

:: 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)
```

mapRand :: ((a, g) -> (b, g)) -> Rand g a -> Rand g b Source #

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

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

withRand :: (g -> g) -> Rand g a -> Rand g a Source #

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

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

evalRandIO :: Rand StdGen a -> IO a Source #

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

data RandT g m a Source #

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.

Instances

(MonadReader r m, MonadWriter w m, MonadState s m) => MonadRWS r w s (RandT g m) Source #

MonadWriter w m => MonadWriter w (RandT g m) Source #
Methods writer :: (a, w) -> RandT g m a # tell :: w -> RandT g m () # listen :: RandT g m a -> RandT g m (a, w) # pass :: RandT g m (a, w -> w) -> RandT g m a #
MonadState s m => MonadState s (RandT g m) Source #
Methods get :: RandT g m s # put :: s -> RandT g m () # state :: (s -> (a, s)) -> RandT g m a #
MonadReader r m => MonadReader r (RandT g m) Source #
Methods ask :: RandT g m r # local :: (r -> r) -> RandT g m a -> RandT g m a # reader :: (r -> a) -> RandT g m a #
MonadError e m => MonadError e (RandT g m) Source #
Methods throwError :: e -> RandT g m a # catchError :: RandT g m a -> (e -> RandT g m a) -> RandT g m a #
(RandomGen g, Monad m) => MonadSplit g (RandT g m) Source #
Methods getSplit :: RandT g m g Source #
MonadTrans (RandT g) Source #
Methods lift :: Monad m => m a -> RandT g m a #
Monad m => Monad (RandT g m) Source #
Methods (>>=) :: RandT g m a -> (a -> RandT g m b) -> RandT g m b # (>>) :: RandT g m a -> RandT g m b -> RandT g m b # return :: a -> RandT g m a # fail :: String -> RandT g m a #
Functor m => Functor (RandT g m) Source #
Methods fmap :: (a -> b) -> RandT g m a -> RandT g m b # (<$) :: a -> RandT g m b -> RandT g m a #
MonadFix m => MonadFix (RandT g m) Source #
Methods mfix :: (a -> RandT g m a) -> RandT g m a #
MonadFail m => MonadFail (RandT g m) Source #
Methods fail :: String -> RandT g m a #
Monad m => Applicative (RandT g m) Source #
Methods pure :: a -> RandT g m a # (<>) :: RandT g m (a -> b) -> RandT g m a -> RandT g m b # liftA2 :: (a -> b -> c) -> RandT g m a -> RandT g m b -> RandT g m c # (>) :: RandT g m a -> RandT g m b -> RandT g m b # (<*) :: RandT g m a -> RandT g m b -> RandT g m a #
MonadIO m => MonadIO (RandT g m) Source #
Methods liftIO :: IO a -> RandT g m a #
MonadPlus m => Alternative (RandT g m) Source #
Methods empty :: RandT g m a # (<\|>) :: RandT g m a -> RandT g m a -> RandT g m a # some :: RandT g m a -> RandT g m [a] # many :: RandT g m a -> RandT g m [a] #
MonadPlus m => MonadPlus (RandT g m) Source #
Methods mzero :: RandT g m a # mplus :: RandT g m a -> RandT g m a -> RandT g m a #
MonadCont m => MonadCont (RandT g m) Source #
Methods callCC :: ((a -> RandT g m b) -> RandT g m a) -> RandT g m a #
PrimMonad m => PrimMonad (RandT s m) Source #
Associated Types type PrimState (RandT s m :: * -> ) :: # Methods primitive :: (State# (PrimState (RandT s m)) -> (#TupleRep [RuntimeRep], LiftedRep, State# (PrimState (RandT s m)), a#)) -> RandT s m a #
(Monad m, RandomGen g) => MonadInterleave (RandT g m) Source #
Methods interleave :: RandT g m a -> RandT g m a Source #
(RandomGen g, Monad m) => MonadRandom (RandT g m) Source #
Methods getRandomR :: Random a => (a, a) -> RandT g m a Source # getRandom :: Random a => RandT g m a Source # getRandomRs :: Random a => (a, a) -> RandT g m [a] Source # getRandoms :: Random a => RandT g m [a] Source #
type PrimState (RandT s m) Source #
type PrimState (RandT s m) = PrimState m

liftRandT Source #

Arguments

:: (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.)

runRandT Source #

Arguments

:: 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.)

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

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)

execRandT :: Monad m => RandT g m a -> g -> m g Source #

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)

mapRandT :: (m (a, g) -> n (b, g)) -> RandT g m a -> RandT g n b Source #

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

runRandT (mapRandT f m) = f . runRandT m

withRandT :: (g -> g) -> RandT g m a -> RandT g m a Source #

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

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

evalRandTIO :: MonadIO m => RandT StdGen m a -> m a Source #

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

Lifting other operations

liftCallCC :: CallCC m (a, g) (b, g) -> CallCC (RandT g m) a b Source #

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

liftCallCC' :: CallCC m (a, g) (b, g) -> CallCC (RandT g m) a b Source #

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).

liftCatch :: Catch e m (a, g) -> Catch e (RandT g m) a Source #

Lift a catchE operation to the new monad.

liftListen :: Monad m => Listen w m (a, g) -> Listen w (RandT g m) a Source #

Lift a listen operation to the new monad.

liftPass :: Monad m => Pass w m (a, g) -> Pass w (RandT g m) a Source #

Lift a pass operation to the new monad.

Examples

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)