Should prism and lens be defined as

[phadej]

prism :: (b -> t) -> (s -> Either t a) -> Prism s t a b
prism setter getter pab = dimap getter (either id setter) (right pab)

Then there won't be unnecessary rmap preprocessing.

lens :: (s -> a) -> (s -> b -> t) -> Lens s t a b
lens getter setter pab = dimap (\s -> (getter s, s)) (\(b, s) -> setter s b) (first' pab)

and here we won't apply setter to s before actually needed (making of pair is unavoidable)

[paf31]

That sounds like an improvement to me.