Difference between revisions of "New monads"
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__TOC__ 
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−  +  Remember to add a [ [ Category:Code ] ] tag to any new subpages. 

== MonadBase == 
== MonadBase == 

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== MonadLib == 
== MonadLib == 

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

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

−  It is a new version of the mtl package with 
+  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. 
It also defines BaseM which is like MonadBase above. 

−  
⚫  
−  
⚫  
== MonadRandom == 
== MonadRandom == 

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View [[New monads/MonadRandom]]. 
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 == 
== MonadSupply == 

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

+  
+  == MonadNondet == 

+  
+  There is a [[Sudoku#Monadic_NonDeterministic_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 PJ. 

+  
+  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 clocktime 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 [[shortcircuiting]] for complex program flow logic. 

+  
+  If you are using CPS (either explicitly, or in a CPSbased monad such as MonadCont or LogicT) only for this purpose, the Exit monad will likely simplify your program considerably. 

+  
+  '''Note:''' Now that a restriction on the Left type has been removed, the standard <hask>Either</hask> type can be used for this purpose. No separate Exit monad is needed anymore. For a monad transformer, use the version of EitherT defined in the [http://hackage.haskell.org/package/either either] package. 

+  
+  View [[New monads/MonadExitMonadExit]]. 

+  
+  == 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. 

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

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

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

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

+  
+  == Lazy and Strict variants == 

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

+  
⚫  
+  
⚫  
+  
+  === Strict RWS === 

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

[[Category:Idioms]] 
[[Category:Idioms]] 

[[Category:Monad]] 
[[Category:Monad]] 

+  [[Category:Proposals]] 
Latest revision as of 09:43, 9 December 2014
Contents
Remember to add a [ [ Category:Code ] ] tag to any new subpages.
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
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 nonMonadZero 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 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 PJ.
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 clocktime 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 shortcircuiting for complex program flow logic.
If you are using CPS (either explicitly, or in a CPSbased monad such as MonadCont or LogicT) only for this purpose, the Exit monad will likely simplify your program considerably.
Note: Now that a restriction on the Left type has been removed, the standard Either
type can be used for this purpose. No separate Exit monad is needed anymore. For a monad transformer, use the version of EitherT defined in the either package.
View MonadExit.
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.
Lazy and Strict variants
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 haskellcafe mailing list. New monads/UnboxedRWS is an strict variant of RWS.