Separate and contain effects of IO monad.
The capabilities library is an effort to make effects in Haskell more explicit by breaking the monolithic IO monad into smaller composable parts called capabilities: a use case might be an action that needs logging with current time but which should not be allowed any other IO. This exists as a pleasant middle ground between pure functions, the ST monad and the kitchen-sink IO providing a more fine-grained approach to effectful computations. Another benefit to this approach is security where a computation should only have access to resources requires to complete its job (principle of least privilege).
The implementation of the idea is based on Wouter Swierstra's Functional Pearl Data types a la carte (Journal of Functional Programming, 18(4):423-436, 2008, http://dx.doi.org/10.1017/S0956796808006758) and uses the compdata package for compositional data types.