Monad

Haskell as a pure functional language has only pure functions. A key feature of these pure functions is that they always return the same result when given the same arguments. Thanks to this property called referential transparency a Haskell function cannot have side effects; therefore, Haskell has a conceptional issue. The world is full of calculations that have side effects. These are calculations that can fail, that can return an unknown number of results, or that are dependent on the environment. To solve this conceptional issue, Haskell uses monads and embeds them in the pure functional language.

The classical monads encapsulate one side effect:

• I/O monad: Calculations that deal with input and output.
• Maybe monad: Calculations that maybe return a result.
• Error monad: Calculations that can fail.
• List monad: Calculations that can have an arbitrary number of results.
• State monad: Calculations that build a state.
• Reader monad: Calculations that read from the...