Monad transformer: Difference between revisions

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{{Wikibooks|Haskell|Monad transformers}}

In [[functional programming]], a '''monad transformer''' is essentially a type constructor which takes a [[monads in functional programming|monad]] as an argument, and returns a monad as a result.

Monad transformers can be used to compose features encapsulated by monads - such as state, exception handling, and I/O - in a modular way. Typically, a monad transformer is created by generalising an existing monad; applying the resulting monad transformer to the identity monad yields a monad which is equivalent to the original monad (ignoring any necessary boxing and unboxing).