The update monad is a generalized (and pretty abstract) monad which can be specialized to implement any of Reader, Writer, or State! Check out this paper for a description.
This repo implements both the update monad transformer (UpdateT) and an
experiment called FreeUpdateT which forgoes the traditional requirement of
using a monoid as the 'action' of the update monad by substituting a free
monoid (i.e. [action]). It also defers the choice of an
act :: action -> state -> state function until the last second when the monad
is run (rather than using a typeclass); this conveniently allows you to
substitute out the act function for testing or alternate behaviour without
needing to rewrite or transform the monadic values themselves. Nifty!
Yes; I know I could implement Update in terms of UpdateT Identity; but implementing
them separately is clearer for people learning how it works.