These notes explain the logic monad by solving solve n-queens with it: https://github.com/sshastry/queenslogic Despite its name, it's more of a search monad, it doesn't perform unification or anything like that.

Another interesting paper is "Monad transformers for backtracking search" https://arxiv.org/abs/1406.2058

The Select monad described in the later paper can be found in the "transformers" library http://hackage.haskell.org/package/transformers-0.5.4.0/docs...