The I/O monad

In a purely functional language, such as Haskell, functions cannot have any externally visible side effects. Although a function cannot directly cause a side effect, it can construct a value describing a desired side effect, that the caller should apply at a convenient time. In the Haskell notation, a value of type IO a represents an action that, when performed, produces a value of type a.

We can think of a value of type IO as a function that takes as its argument the current state of the world, and will return a new world where the state has been changed according to the function’s return value. The state created in this way can be passed to another function, thus defining a series of functions which will apply in order as steps of state changes. This process is similar to how a temporal logic represents the passage of time using only declarative propositions.

— Wikipedia on Monad (functional programming)

2012.08.22 Wednesday ACHK