  Delta lenses are a kind of morphism between categories which are used to
model bidirectional transformations between systems. Classical state-based
lenses, also known as very well-behaved lenses, are both algebras for a monad
and coalgebras for a comonad. Delta lenses generalise state-based lenses, and
while delta lenses have been characterised as certain algebras for a
semi-monad, it is natural to ask if they also arise as coalgebras.
  This short paper establishes that delta lenses are coalgebras for a comonad,
through showing that the forgetful functor from the category of delta lenses
over a base, to the category of cofunctors over a base, is comonadic. The proof
utilises a diagrammatic approach to delta lenses, and clarifies several results
in the literature concerning the relationship between delta lenses and
cofunctors. Interestingly, while this work does not generalise the
corresponding result for state-based lenses, it does provide new avenues for
exploring lenses as coalgebras.
