This article introduces actegories, focusing on their role in optics and their definition through monoidal categories. It offers a Haskell implementation of monoidal categories, highlighting the notion of tensor products and constraints in type classes.
Actegories are an important concept in programming, specifically in optics, which includes lenses, prisms, and traversals. To effectively work with actegories, one must first grasp the concept of monoidal categories.
A monoidal category incorporates a tensor product, which is a functor ensuring associativity and unitality up to isomorphism. It includes an invertible associator and unit object, alongside two unitors that are also invertible and natural.
In Haskell, monoidal categories can be parameterized by the type of the tensor product. The article presents a way to define a 'MonoidalCategory' type class that incorporates these structures, specifying how to work with types in a monoidal context.
A challenge arises when encoding units as associated types due to potential circular dependencies. This can be resolved using the 'UndecidableSuperClasses' pragma, allowing the specification of types while maintaining the required constraints.
An example presented is the 'Hask' category, using cartesian products as the tensor. This implementation serves as a baseline to further explore categories within Haskell, showing the application of theoretical concepts in practical programming.
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This article introduces actegories, focusing on their role in optics and their definition through monoidal categories. It offers a Haskell implementation of monoidal categories, highlighting the notion of tensor products and constraints in type classes.