Haskell Language Monads


A monad is a data type of composable actions. Monad is the class of type constructors whose values represent such actions. Perhaps IO is the most recognizable one: a value of IO a is a "recipe for retrieving an a value from the real world".

We say a type constructor m (such as [] or Maybe) forms a monad if there is an instance Monad m satisfying certain laws about composition of actions. We can then reason about m a as an "action whose result has type a".