Monads in Haskell are structures used to supplement pure computations with features like state, common environment or I/O. Even though Haskell is a purely-functional language, side-effects can be conveniently simulated using monads.
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).
Most common applications of monads include:
- Representing failure using
- Nondeterminism through backtracking using
- State using
- Read-only environment using
- I/O using
All common monads are members of Monad class defined like this:
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 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.
In order to improve the look of code that uses monads Haskell provides a special syntactic sugar called
do-notation. For example, following expression:
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
can be also written using the
do-notation as follows:
do x <- thing1 y <- func1 x thing2 z <- func2 y return z
Code written using the
do-notation is transformed by the compiler to ordinary expressions that use
Monad class functions.
When using the
do-notation and a monad like
IO programs look very much like programs written in an imperative language as each line contains a statement that can change the simulated global state of the program and optionally binds a (local) variable that can be used by the statements later in the code block.
It is possible to intermix the
do-notation with regular notation.
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:
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.
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.
A collection of research papers about monads.
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.
A list of monads for various evaluation strategies and games:
- Identity monad
- Optional results
- Random values
- Read only state
- Writable state
- Unique supply
- Undoable state
- Function application
- Atomic memory transactions
- Non-deterministic evaluation
- List monad
- Concurrent threads
- Region allocation
- 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
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.