Digest cycles are computations that support reading a context value (the input), accumulating a monoidal view of the context (the view), and possibly producing a final result (the result). One application of them is to implement composable forms, where the validation logic and UI are run simultaneously.

The idea is based loosely on the Formlets introduced in (The Essence of Form Abstraction)[http://www.ezrakilty.net/pubs/formlets.pdf] by Cooper, Lindley, Wadlerm and Yallop, altough this implementation includes a mechanism for failed validation as well as the ability to run computations in an arbitrary base monad via a monad transformer, DigestT.

The name Digest is taken from the (digestive-functors)[https://hackage.haskell.org/package/digestive-functors] Haskell package, though it should be noted that the design of the types and API are radically different in this library. Notably, the DigestT monad does not impose any structure on the view type beyond requiring a Monoid instance. This makes the API more general, but less domain-specific. As such, it is an appropriate choice to build domain-specific form libraries that target particular frameworks and design systems, allowing it to be tailored to the specific use case more easily.

The library provides the DigestT and Digest types, the latter of which is DigestT applied to the Identity base monad.

A Digest action can be thought of combining the abilities of Reader, Writer, and Maybe. Input can be accessed at any point in the computation, and modified locally like Reader. The view can be appended to at any point in the computation, as well as manipulated directly like Writer. Finally, the computation can short-circuit and fail with an empty result, yeilding the view accumulated so far. This ability enables things like form validation and wizards naturally.