# New monads

### From HaskellWiki

(Add MonadBase with liftBase to generalize liftIO) |
(Link to wiki page on monadLib instead of old galios.com pages) |
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__TOC__ | __TOC__ | ||

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+ | Remember to add a [ [ Category:Code ] ] tag to any new sub-pages. | ||

== MonadBase == | == MonadBase == | ||

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It seems that the liftIO function from MonadIO can be generalized to access whatever the base of a transformer stack happens to be. So there is no need for a liftSTM, liftST, etc. | It seems that the liftIO function from MonadIO can be generalized to access whatever the base of a transformer stack happens to be. So there is no need for a liftSTM, liftST, etc. | ||

− | + | View [[New monads/MonadBase]]. | |

− | -- | + | |

− | + | == MonadLib == | |

+ | |||

+ | [[MonadLib]] is written by Iavor S. Diatchki. | ||

+ | |||

+ | It is a new version of the mtl package with base monads: Id, and Lift, and transformers ReaderT, WriterT, StateT, ExceptionT, ChoiceT, and ContT. | ||

+ | |||

+ | It also defines BaseM which is like MonadBase above. | ||

+ | |||

+ | == MonadRandom == | ||

+ | |||

+ | A simple monad transformer to allow computations in the transformed monad to generate random values. | ||

+ | |||

+ | View [[New monads/MonadRandom]]. | ||

+ | |||

+ | ===MonadRandomSplittable=== | ||

+ | A refinement of MonadRandom to integrate RandomGen's split function. | ||

+ | |||

+ | View at [[New monads/MonadRandomSplittable]] | ||

+ | |||

+ | == MaybeT == | ||

+ | |||

+ | The Maybe monad deserves a transformer, just like the other classic monads. | ||

+ | |||

+ | View [[New monads/MaybeT]]. | ||

+ | |||

+ | == MonadSupply == | ||

+ | |||

+ | Here is a simple monad/monad transformer for computations which consume values from a (finite or infinite) supply. Note that due to pattern matching, running out of supply in a non-MonadZero monad will cause an error. | ||

+ | |||

+ | View [[New monads/MonadSupply]]. | ||

+ | |||

+ | == MonadUndo == | ||

+ | |||

+ | Here is a modified state monad transformer for keeping track of undo/redo states automatically. | ||

+ | |||

+ | View [[New monads/MonadUndo]]. | ||

+ | |||

+ | == MonadUnique == | ||

+ | |||

+ | This is a simple (trivial) monad transformer for supplying unique integer values to an algorithm. | ||

+ | |||

+ | View [[New monads/MonadUnique]]. | ||

+ | |||

+ | == MonadSTO == | ||

+ | |||

+ | Here's an extension of the ST monad in which the references are ordered and showable (they list their creation index). | ||

+ | |||

+ | View [[New monads/MonadSTO]]. | ||

+ | |||

+ | == MonadNondet == | ||

+ | |||

+ | There is a [[Sudoku#Monadic_Non-Deterministic_Solver | MonadNondet]] that when compiled with optimizations outperforms List. | ||

+ | |||

+ | == Stateful nondeterminism == | ||

+ | |||

+ | There is a [[Stateful nondeterminism]] monad for if you want to do nondeterministic computation with local states for each of your threads and a global state shared by all your threads. | ||

+ | |||

+ | == MonadAdvSTM == | ||

+ | |||

+ | Here is an extension of STM to easy interaction with IO after committing or retrying. Inspired by Simon P-J. | ||

+ | |||

+ | View [[New monads/MonadAdvSTM]]. | ||

+ | |||

+ | == TimedStateT == | ||

+ | |||

+ | A monad transformer which combines State, Reader, and Error functionality to give the effect of a StateT monad which checks clock-time and stops the current computation if a period is exceeded. | ||

+ | |||

+ | darcs get http://www.mapcar.org/haskell/TimedStateT/ | ||

+ | |||

+ | Haddocks: http://www.mapcar.org/haskell/TimedStateT/dist/doc/html/ | ||

+ | |||

+ | == MonadExit == | ||

+ | |||

+ | The Exit monad provides [[short-circuiting]] for complex program flow logic. | ||

+ | |||

+ | If you are using CPS or MonadCont only for this purpose, the Exit monad will likely simplify your program considerably. | ||

+ | |||

+ | View [[New monads/MonadExit|MonadExit]]. | ||

+ | |||

+ | == MonadSplit == | ||

+ | |||

+ | Represents the class of monads such that | ||

+ | |||

+ | <haskell>l == (msplit l >>= \(x,xs) -> return x `mplus` xs)</haskell> | ||

+ | |||

+ | In English, msplit is a counterpart to "mplus". | ||

− | + | Using this, you can redefine many of the functions which previously depended on lists: foldM, scanM, inits, tails, and some derived functions. | |

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− | + | Note: A more general form of this monad, | |

− | + | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Foldable.html Data.Foldable], is now part of the | |

+ | [http://haskell.org/ghc/docs/latest/html/libraries/ standard libraries]. | ||

− | + | View [[New monads/MonadSplit]]. | |

− | + | ||

− | + | ||

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− | + | ||

− | + | == Lazy and Strict variants == | |

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− | + | This section contains monads that have interesting Strict or Lazy properties. | |

− | + | === LazyWriterT === | |

− | + | ||

− | + | This came up on the mailing list: Why is WriterT never lazy? The answer is it does not use lazy patterns with "~". So here is a more useful [[New monads/LazyWriterT]] that add two "~" to the definition of (>>=) and renames WriterT to LazyWriterT. | |

− | + | ||

− | + | ||

− | + | ||

− | + | === Strict RWS === | |

− | + | ||

− | + | ||

− | + | ||

− | + | ||

− | + | ||

− | + | This was contribute by John Meacham on on the haskell-cafe mailing list. [[New monads/UnboxedRWS]] is an strict variant of RWS. | |

− | + | ||

− | + | [[Category:Idioms]] | |

− | + | [[Category:Monad]] | |

− | [ | + | [[Category:Proposals]] |

− | + |

## Revision as of 23:10, 7 September 2010

## Contents |

Remember to add a [ [ Category:Code ] ] tag to any new sub-pages.

## 1 MonadBase

It seems that the liftIO function from MonadIO can be generalized to access whatever the base of a transformer stack happens to be. So there is no need for a liftSTM, liftST, etc.

View New monads/MonadBase.

## 2 MonadLib

MonadLib is written by Iavor S. Diatchki.

It is a new version of the mtl package with base monads: Id, and Lift, and transformers ReaderT, WriterT, StateT, ExceptionT, ChoiceT, and ContT.

It also defines BaseM which is like MonadBase above.

## 3 MonadRandom

A simple monad transformer to allow computations in the transformed monad to generate random values.

View New monads/MonadRandom.

### 3.1 MonadRandomSplittable

A refinement of MonadRandom to integrate RandomGen's split function.

View at New monads/MonadRandomSplittable

## 4 MaybeT

The Maybe monad deserves a transformer, just like the other classic monads.

View New monads/MaybeT.

## 5 MonadSupply

Here is a simple monad/monad transformer for computations which consume values from a (finite or infinite) supply. Note that due to pattern matching, running out of supply in a non-MonadZero monad will cause an error.

View New monads/MonadSupply.

## 6 MonadUndo

Here is a modified state monad transformer for keeping track of undo/redo states automatically.

View New monads/MonadUndo.

## 7 MonadUnique

This is a simple (trivial) monad transformer for supplying unique integer values to an algorithm.

View New monads/MonadUnique.

## 8 MonadSTO

Here's an extension of the ST monad in which the references are ordered and showable (they list their creation index).

View New monads/MonadSTO.

## 9 MonadNondet

There is a MonadNondet that when compiled with optimizations outperforms List.

## 10 Stateful nondeterminism

There is a Stateful nondeterminism monad for if you want to do nondeterministic computation with local states for each of your threads and a global state shared by all your threads.

## 11 MonadAdvSTM

Here is an extension of STM to easy interaction with IO after committing or retrying. Inspired by Simon P-J.

View New monads/MonadAdvSTM.

## 12 TimedStateT

A monad transformer which combines State, Reader, and Error functionality to give the effect of a StateT monad which checks clock-time and stops the current computation if a period is exceeded.

darcs get http://www.mapcar.org/haskell/TimedStateT/

Haddocks: http://www.mapcar.org/haskell/TimedStateT/dist/doc/html/

## 13 MonadExit

The Exit monad provides short-circuiting for complex program flow logic.

If you are using CPS or MonadCont only for this purpose, the Exit monad will likely simplify your program considerably.

View MonadExit.

## 14 MonadSplit

Represents the class of monads such that

l == (msplit l >>= \(x,xs) -> return x `mplus` xs)

In English, msplit is a counterpart to "mplus".

Using this, you can redefine many of the functions which previously depended on lists: foldM, scanM, inits, tails, and some derived functions.

Note: A more general form of this monad, Data.Foldable, is now part of the standard libraries.

View New monads/MonadSplit.

## 15 Lazy and Strict variants

This section contains monads that have interesting Strict or Lazy properties.

### 15.1 LazyWriterT

This came up on the mailing list: Why is WriterT never lazy? The answer is it does not use lazy patterns with "~". So here is a more useful New monads/LazyWriterT that add two "~" to the definition of (>>=) and renames WriterT to LazyWriterT.

### 15.2 Strict RWS

This was contribute by John Meacham on on the haskell-cafe mailing list. New monads/UnboxedRWS is an strict variant of RWS.