Difference between revisions of "New monads"

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(Added MaybeT.)
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View at [[New monads/MonadRandomSplittable]]
View at [[New monads/MonadRandomSplittable]]
== MaybeT ==
The Maybe monad deserves a transformer, just like the other classic monads.
View [[New monads/MaybeT]].
== MonadSupply ==
== MonadSupply ==

Revision as of 10:44, 5 February 2007

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


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.


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.


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

View New monads/MonadRandom.


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

View at New monads/MonadRandomSplittable


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

View New monads/MaybeT.


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.


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

View New monads/MonadUndo.


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

View New monads/MonadUnique.


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.


There is a 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.


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

View New monads/MonadAdvSTM.


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/


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.

Lazy and Strict variants

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


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.