Monads need not be endofunctors

We introduce a generalization of monads, called relative monads, allowing for underlying functors between different categories. Examples include finite-dimensional vector spaces, untyped and typed λ-calculus syntax and indexed containers. We show that the Kleisli and Eilenberg-Moore constructions ca...

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Bibliographic Details
Main Authors: Altenkirch, Thorsten, Chapman, James, Uustalu, Tarmo
Format: Article
Published: International federation of Computer Logic 2015
Subjects:
Online Access:https://eprints.nottingham.ac.uk/30436/
Description
Summary:We introduce a generalization of monads, called relative monads, allowing for underlying functors between different categories. Examples include finite-dimensional vector spaces, untyped and typed λ-calculus syntax and indexed containers. We show that the Kleisli and Eilenberg-Moore constructions carry over to relative monads and are related to relative adjunctions. Under reasonable assumptions, relative monads are monoids in the functor category concerned and extend to monads, giving rise to a coreflection between relative monads and monads. Arrows are also an instance of relative monads.