Recently, a fellow in category land discovered a fact that we in Haskell land have actually known for a while (in addition to things most of us probably don't). Specifically, given two categories $\mathcal{C}$ and $\mathcal{D}$, a functor $G : \mathcal{C} \rightarrow \mathcal{D}$, and provided some conditions in $\mathcal{D}$ hold, there exists a monad $T^G$, the codensity monad of $G$.

In category theory, the codensity monad is given by the rather frightening expression:

$ T^G(a) = \int_r \left[\mathcal{D}(a, Gr), Gr\right] $