Monads in Haskell can be thought of as composable computation descriptions. The essence of monad is thus separation of composition timeline from the composed computation's execution timeline, which lends monads to supplementing pure calculations with features like state, common environment or I/O, and also to preprocessing of computations (simplification, optimization etc.).
Even though Haskell is a purely-functional language, side effects that would be performed by a computation (when and if run) can be dealt with and combined purely at the monad's composition time. The combined computation doesn't have to be impure and can be pure itself as well. Monads thus resemble programs in a particular DSL. While programs may describe impure effects and actions outside Haskell, they can still be combined and processed ("compiled") purely, inside Haskell.
Because they are very useful in practice but rather mind-twisting for the beginners, numerous tutorials that deal exclusively with monads were created (see monad tutorials).
1 Common monads
Most common applications of monads include:
- Representing failure using monadMaybe
- Nondeterminism through backtracking using monadList
- State using monadState
- Read-only environment using monadReader
- I/O using monadIO
2 Monad classMonads can be viewed as a standard programming interface to various data or control structures, which is captured by the
class Monad m where (>>=) :: m a -> (a -> m b) -> m b (>>) :: m a -> m b -> m b return :: a -> m a fail :: String -> m a
In addition to implementing the class functions, all instances of Monad should obey the following equations:
return a >>= k = k a m >>= return = m m >>= (\x -> k x >>= h) = (m >>= k) >>= h
See this intuitive explanation of why they should obey the Monad laws.
Any Monad can be made a Functor by defining
fmap ab ma = ma >>= (return . ab)
However, the Functor class is not a superclass of the Monad class. See Functor hierarchy proposal.
3 Special notationIn order to improve the look of code that uses monads Haskell provides a special syntactic sugar called
thing1 >>= (\x -> func1 x >>= (\y -> thing2 >>= (\_ -> func2 y (\z -> return z))))
which can be written more clearly by breaking it into several lines and omitting parentheses:
thing1 >>= \x -> func1 x >>= \y -> thing2 >>= \_ -> func2 y >>= \z -> return z
do x <- thing1 y <- func1 x thing2 z <- func2 y return z
4 Commutative monads
Commutative monads are monads for which the order of actions makes no difference (they commute), that is when following code:
do a <- f x b <- g y m a b
is the same as:
do b <- g y a <- f x m a b
Examples of commutative include:
5 Monad tutorials
Monads are known for being deeply confusing to lots of people, so there are plenty of tutorials specifically related to monads. Each takes a different approach to Monads, and hopefully everyone will find something useful.
See Monad tutorials.
6 Monad reference guides
An explanation of the basic Monad functions, with examples, can be found in the reference guide A tour of the Haskell Monad functions, by Henk-Jan van Tuyl.
7 Monad research
A collection of research papers about monads.
8 Monads in other languages
Implementations of monads in other languages.
- C++, doc
- CML.event ?
- Clean State monad
- Java (tar.gz)
- LINQ, more, C#, VB
- Perl6 ?
- The Unix Shell
- More monads by Oleg
- CLL: a concurrent language based on a first-order intuitionistic linear logic where all right synchronous connectives are restricted to a monad.
And possibly there exist:
- Standard ML (via modules?)
Please add them if you know of other implementations.
9 Interesting monads
A list of monads for various evaluation strategies and games:
- Identity monad
- Optional results
- Random values
- Read only state
- Writable state
- Unique supply
- ST - memory-only effects
- Global state
- Undoable state effects
- Function application
- Functions which may error
- Atomic memory transactions
- IO - unrestricted side effects
- Non-deterministic evaluation
- List monad: computations with multiple choices
- Concurrent threads
- Backtracking computations
- Region allocation effects
- LogicT: backtracking monad transformer with fair operations and pruning
- Pi calculus as a monad
- Halfs, uses a read-only and write-only monad for filesystem work.
- House's H monad for safe hardware access
- Commutable monads for parallel programming
- The Quantum computing monad
- Simple, Fair and Terminating Backtracking Monad
- Typed exceptions with call traces as a monad
- Breadth first list monad
- Continuation-based queues as monads
- Typed network protocol monad
- Non-Determinism Monad for Level-Wise Search
- Transactional state monad
- A constraint programming monad
- A probability distribution monad
There are many more interesting instance of the monad abstraction out there. Please add them as you come across each species.
- If you are tired of monads, you can easily get rid of them.