# New monads

### From HaskellWiki

## Contents |

## 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 NewMonads/MonadBase.

## 2 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.

## 3 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 NewMonads/LazyWriterT that add two "~" to the definition of (>>=) and renames WriterT to LazyWriterT.

## 4 MonadRandom

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

View NewMonads/MonadRandom.

## 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 NewMonads/MonadSupply.

## 6 MonadUndo

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

View NewMonads/MonadUndo.

## 7 MonadUnique

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

View NewMonads/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 NewMonads/MonadSTO.