Coding up the monad laws from a category-theoretic perspective
https://medium.com/@brettwines/implementing-a-category-theoretic-hask-monad-in-haskell-7bf662f2e98b
MonadMath.hs exposes the three components of a monad: the functor F and the natural transformations µ and η.
test-monadmath.hs contains tests of naturality (µ y . F h = F h . µ x and η y . I h = F h . η x) and monad law tests (µµf = µfµ and µfη = I_F = µηf).
(Thanks to Stefan Klinger's excellent monad guide)