Copyright | (c) Nickolay Kudasov 2016 |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | R.Paterson@city.ac.uk |

Stability | experimental |

Portability | portable |

Safe Haskell | Safe |

Language | Haskell98 |

The lazy `AccumT`

monad transformer, which adds accumulation
capabilities (such as declarations or document patches) to a given monad.

This monad transformer provides append-only accumulation during the computation. For more general access, use Control.Monad.Trans.State instead.

## Synopsis

- type Accum w = AccumT w Identity
- accum :: Monad m => (w -> (a, w)) -> AccumT w m a
- runAccum :: Accum w a -> w -> (a, w)
- execAccum :: Accum w a -> w -> w
- evalAccum :: Monoid w => Accum w a -> w -> a
- mapAccum :: ((a, w) -> (b, w)) -> Accum w a -> Accum w b
- newtype AccumT w m a = AccumT (w -> m (a, w))
- runAccumT :: AccumT w m a -> w -> m (a, w)
- execAccumT :: Monad m => AccumT w m a -> w -> m w
- evalAccumT :: (Monad m, Monoid w) => AccumT w m a -> w -> m a
- mapAccumT :: (m (a, w) -> n (b, w)) -> AccumT w m a -> AccumT w n b
- look :: (Monoid w, Monad m) => AccumT w m w
- looks :: (Monoid w, Monad m) => (w -> a) -> AccumT w m a
- add :: Monad m => w -> AccumT w m ()
- liftCallCC :: CallCC m (a, w) (b, w) -> CallCC (AccumT w m) a b
- liftCallCC' :: CallCC m (a, w) (b, w) -> CallCC (AccumT w m) a b
- liftCatch :: Catch e m (a, w) -> Catch e (AccumT w m) a
- liftListen :: Monad m => Listen w m (a, s) -> Listen w (AccumT s m) a
- liftPass :: Monad m => Pass w m (a, s) -> Pass w (AccumT s m) a
- readerToAccumT :: (Functor m, Monoid w) => ReaderT w m a -> AccumT w m a
- writerToAccumT :: WriterT w m a -> AccumT w m a
- accumToStateT :: (Functor m, Monoid s) => AccumT s m a -> StateT s m a

# The Accum monad

accum :: Monad m => (w -> (a, w)) -> AccumT w m a Source #

Construct an accumulation computation from a (result, output) pair.
(The inverse of `runAccum`

.)

runAccum :: Accum w a -> w -> (a, w) Source #

Unwrap an accumulation computation as a (result, output) pair.
(The inverse of `accum`

.)

# The AccumT monad transformer

An accumulation monad parameterized by:

`w`

- the output to accumulate.`m`

- The inner monad.

The `return`

function produces the output `mempty`

, while `>>=`

combines the outputs of the subcomputations using `mappend`

.

This monad transformer is similar to both state and writer monad transformers. Thus it can be seen as

- a restricted append-only version of a state monad transformer or
- a writer monad transformer with the extra ability to read all previous output.

AccumT (w -> m (a, w)) |

## Instances

Monoid w => MonadTrans (AccumT w) Source # | |

(Monoid w, Functor m, Monad m) => Monad (AccumT w m) Source # | |

Functor m => Functor (AccumT w m) Source # | |

(Monoid w, Functor m, MonadFix m) => MonadFix (AccumT w m) Source # | |

Defined in Control.Monad.Trans.Accum | |

(Monoid w, MonadFail m) => MonadFail (AccumT w m) Source # | |

Defined in Control.Monad.Trans.Accum | |

(Monoid w, Functor m, Monad m) => Applicative (AccumT w m) Source # | |

Defined in Control.Monad.Trans.Accum | |

(Monoid w, Functor m, MonadIO m) => MonadIO (AccumT w m) Source # | |

Defined in Control.Monad.Trans.Accum | |

(Monoid w, Functor m, MonadPlus m) => Alternative (AccumT w m) Source # | |

(Monoid w, Functor m, MonadPlus m) => MonadPlus (AccumT w m) Source # | |

execAccumT :: Monad m => AccumT w m a -> w -> m w Source #

Extract the output from an accumulation computation.

`execAccumT`

m w =`liftM`

`snd`

(`runAccumT`

m w)

evalAccumT :: (Monad m, Monoid w) => AccumT w m a -> w -> m a Source #

Evaluate an accumulation computation with the given initial output history and return the final value, discarding the final output.

`evalAccumT`

m w =`liftM`

`fst`

(`runAccumT`

m w)

# Accum operations

look :: (Monoid w, Monad m) => AccumT w m w Source #

is an action that fetches all the previously accumulated output.`look`

looks :: (Monoid w, Monad m) => (w -> a) -> AccumT w m a Source #

is an action that retrieves a function of the previously accumulated output.`look`

# Lifting other operations

liftCallCC :: CallCC m (a, w) (b, w) -> CallCC (AccumT w m) a b Source #

Uniform lifting of a `callCC`

operation to the new monad.
This version rolls back to the original output history on entering the
continuation.

liftCallCC' :: CallCC m (a, w) (b, w) -> CallCC (AccumT w m) a b Source #

In-situ lifting of a `callCC`

operation to the new monad.
This version uses the current output history on entering the continuation.
It does not satisfy the uniformity property (see Control.Monad.Signatures).

liftCatch :: Catch e m (a, w) -> Catch e (AccumT w m) a Source #

Lift a `catchE`

operation to the new monad.

liftListen :: Monad m => Listen w m (a, s) -> Listen w (AccumT s m) a Source #

Lift a `listen`

operation to the new monad.

liftPass :: Monad m => Pass w m (a, s) -> Pass w (AccumT s m) a Source #

Lift a `pass`

operation to the new monad.

# Monad transformations

readerToAccumT :: (Functor m, Monoid w) => ReaderT w m a -> AccumT w m a Source #

Convert a read-only computation into an accumulation computation.

writerToAccumT :: WriterT w m a -> AccumT w m a Source #

Convert a writer computation into an accumulation computation.