Copyright | (c) Daniel Mendler 2016 (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 |
---|---|

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

Maintainer | mail@daniel-mendler.de |

Stability | experimental |

Portability | portable |

Safe Haskell | Safe |

Language | Haskell2010 |

The strict `WriterT`

monad transformer, which adds collection of
outputs (such as a count or string output) to a given monad.

This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.

This version builds its output strictly and uses continuation-passing-style to achieve constant space usage. This transformer can be used as a drop-in replacement for Control.Monad.Trans.Writer.Strict.

## Synopsis

- type Writer w = WriterT w Identity
- writer :: (Monoid w, Monad m) => (a, w) -> WriterT w m a
- runWriter :: Monoid w => Writer w a -> (a, w)
- execWriter :: Monoid w => Writer w a -> w
- mapWriter :: (Monoid w, Monoid w') => ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
- newtype WriterT w m a = WriterT {
- unWriterT :: w -> m (a, w)

- writerT :: (Functor m, Monoid w) => m (a, w) -> WriterT w m a
- runWriterT :: Monoid w => WriterT w m a -> m (a, w)
- execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w
- mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
- tell :: (Monoid w, Monad m) => w -> WriterT w m ()
- listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w)
- listens :: (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
- pass :: (Monoid w, Monoid w', Monad m) => WriterT w m (a, w -> w') -> WriterT w' m a
- censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a
- liftCallCC :: CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b
- liftCatch :: Catch e m (a, w) -> Catch e (WriterT w m) a

# The Writer monad

writer :: (Monoid w, Monad m) => (a, w) -> WriterT w m a Source #

Construct a writer computation from a (result, output) pair.
(The inverse of `runWriter`

.)

runWriter :: Monoid w => Writer w a -> (a, w) Source #

Unwrap a writer computation as a (result, output) pair.
(The inverse of `writer`

.)

execWriter :: Monoid w => Writer w a -> w Source #

Extract the output from a writer computation.

`execWriter`

m =`snd`

(`runWriter`

m)

# The WriterT monad transformer

newtype WriterT w m a Source #

A writer 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`

.

## Instances

MonadTrans (WriterT w) Source # | |

Defined in Control.Monad.Trans.Writer.CPS.Internal | |

Monad m => Monad (WriterT w m) Source # | |

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

MonadFix m => MonadFix (WriterT w m) Source # | |

Defined in Control.Monad.Trans.Writer.CPS.Internal | |

MonadFail m => MonadFail (WriterT w m) Source # | |

Defined in Control.Monad.Trans.Writer.CPS.Internal | |

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

Defined in Control.Monad.Trans.Writer.CPS.Internal | |

MonadIO m => MonadIO (WriterT w m) Source # | |

Defined in Control.Monad.Trans.Writer.CPS.Internal | |

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

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

writerT :: (Functor m, Monoid w) => m (a, w) -> WriterT w m a Source #

The WriterT constructor is deliberately not exported in the CPS module to avoid exposing the hidden state w. writerT provides a safe way to construct a WriterT with the same api as the original WriterT.

runWriterT :: Monoid w => WriterT w m a -> m (a, w) Source #

Unwrap a writer computation.

execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w Source #

Extract the output from a writer computation.

`execWriterT`

m =`liftM`

`snd`

(`runWriterT`

m)

mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b Source #

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

`runWriterT`

(`mapWriterT`

f m) = f (`runWriterT`

m)

# Writer operations

tell :: (Monoid w, Monad m) => w -> WriterT w m () Source #

is an action that produces the output `tell`

w`w`

.

listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w) Source #

is an action that executes the action `listen`

m`m`

and adds its
output to the value of the computation.

`runWriterT`

(`listen`

m) =`liftM`

(\ (a, w) -> ((a, w), w)) (`runWriterT`

m)

listens :: (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b) Source #

is an action that executes the action `listens`

f m`m`

and adds
the result of applying `f`

to the output to the value of the computation.

`listens`

f m =`liftM`

(id *** f) (`listen`

m)`runWriterT`

(`listens`

f m) =`liftM`

(\ (a, w) -> ((a, f w), w)) (`runWriterT`

m)

pass :: (Monoid w, Monoid w', Monad m) => WriterT w m (a, w -> w') -> WriterT w' m a Source #

is an action that executes the action `pass`

m`m`

, which returns
a value and a function, and returns the value, applying the function
to the output.

`runWriterT`

(`pass`

m) =`liftM`

(\ ((a, f), w) -> (a, f w)) (`runWriterT`

m)

censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a Source #

is an action that executes the action `censor`

f m`m`

and
applies the function `f`

to its output, leaving the return value
unchanged.

`censor`

f m =`pass`

(`liftM`

(\ x -> (x,f)) m)`runWriterT`

(`censor`

f m) =`liftM`

(\ (a, w) -> (a, f w)) (`runWriterT`

m)