The Haskell Unfolder Episode 30: runST 

In Haskell, the ST type offers a restricted subset of the IO functionality: it provides mutable variables, but nothing else. The advantage is that we can use mutable storage locally, because unlike IO, ST allows us to escape from its realm via the function runST. However, runST has a so-called rank-2 type. In this episode, we will discuss why this seemingly complicated type is necessary to preserve the safety of the operation.