mtl-2.2.1: Monad classes, using functional dependencies

Copyright (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001 BSD-style (see the file LICENSE) libraries@haskell.org experimental non-portable (multi-param classes, functional dependencies) Safe Haskell98

Description

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

Synopsis

type RWS r w s = RWST r w s Identity

A monad containing an environment of type `r`, output of type `w` and an updatable state of type `s`.

rws :: (r -> s -> (a, s, w)) -> RWS r w s a

Construct an RWS computation from a function. (The inverse of `runRWS`.)

runRWS :: RWS r w s a -> r -> s -> (a, s, w)

Unwrap an RWS computation as a function. (The inverse of `rws`.)

evalRWS

Arguments

 :: RWS r w s a RWS computation to execute -> r initial environment -> s initial value -> (a, w) final value and output

Evaluate a computation with the given initial state and environment, returning the final value and output, discarding the final state.

execRWS

Arguments

 :: RWS r w s a RWS computation to execute -> r initial environment -> s initial value -> (s, w) final state and output

Evaluate a computation with the given initial state and environment, returning the final state and output, discarding the final value.

mapRWS :: ((a, s, w) -> (b, s, w')) -> RWS r w s a -> RWS r w' s b

Map the return value, final state and output of a computation using the given function.

• ``runRWS` (`mapRWS` f m) r s = f (`runRWS` m r s)`

withRWS :: (r' -> s -> (r, s)) -> RWS r w s a -> RWS r' w s a

`withRWS f m` executes action `m` with an initial environment and state modified by applying `f`.

• ``runRWS` (`withRWS` f m) r s = `uncurry` (`runRWS` m) (f r s)`

newtype RWST r w s m a :: * -> * -> * -> (* -> *) -> * -> *

A monad transformer adding reading an environment of type `r`, collecting an output of type `w` and updating a state of type `s` to an inner monad `m`.

Constructors

 RWST FieldsrunRWST :: r -> s -> m (a, s, w)

Instances

runRWST :: RWST r w s m a -> r -> s -> m (a, s, w)

evalRWST

Arguments

 :: forall (m :: * -> *). Monad m => RWST r w s m a computation to execute -> r initial environment -> s initial value -> m (a, w) computation yielding final value and output

Evaluate a computation with the given initial state and environment, returning the final value and output, discarding the final state.

execRWST

Arguments

 :: forall (m :: * -> *). Monad m => RWST r w s m a computation to execute -> r initial environment -> s initial value -> m (s, w) computation yielding final state and output

Evaluate a computation with the given initial state and environment, returning the final state and output, discarding the final value.

mapRWST :: (m (a, s, w) -> n (b, s, w')) -> RWST r w s m a -> RWST r w' s n b

Map the inner computation using the given function.

• ``runRWST` (`mapRWST` f m) r s = f (`runRWST` m r s)`

withRWST :: (r' -> s -> (r, s)) -> RWST r w s m a -> RWST r' w s m a

`withRWST f m` executes action `m` with an initial environment and state modified by applying `f`.

• ``runRWST` (`withRWST` f m) r s = `uncurry` (`runRWST` m) (f r s)`