# Difference between revisions of "New monads"

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+ | == MonadSplit == |
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+ | Represents the class of monads such that |
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+ | <haskell>l == (msplit l >>= \(x,xs) -> return x `mplus` xs)</haskell> |
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+ | In English, msplit is a counterpart to "mplus". |
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+ | 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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+ | View [[New monads/MonadSplit]]. |
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[[Category:Idioms]] |
[[Category:Idioms]] |

## Revision as of 00:01, 17 December 2006

## Contents

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

## 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.

## MonadLib

This is by Iavor S. Diatchki and can be found at http://www.cse.ogi.edu/~diatchki/monadLib/

It is a new version of the mtl package with transformers: ReaderT WriterT StateT ExceptT SearchT ContT

It also defines BaseM which is like MonadBase above.

## 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.

## 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

## 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.

## 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/

## 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.

View New monads/MonadSplit.