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Reference.org
Monad transformer
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<p>In <a href="/facts/Functional_programming/cdxqPEIM">functional programming</a>, a monad transformer is a type constructor which takes a <a href="/facts/Monads_in_functional_programming/kYazRoF1">monad</a> as an argument and returns a monad as a result. </p><p>Monad transformers can be used to compose features encapsulated by monads – such as state, <a href="/facts/Exception_handling/VrJxgb9r">exception handling</a>, 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). </p>