Creative uses of monads

Question

I\'m looking for creative uses of monads to learn from. I\'ve read somewhere that monads have been used for example in AI, but being a monad newbie, I fail to see how.

Answer 1

I'd like to list a couple of monads not yet mentioned in other answers.

Enumerate and weighted search monads

The Omega monad can be used to productively traverse infinite lists of results. Compare:

>>> take 10 $ liftM2 (,) [0..] [0..]
[(0,0),(0,1),(0,2),(0,3),(0,4),(0,5),(0,6),(0,7),(0,8),(0,9)]

>>> take 10 $ runOmega $ liftM2 (,) (each' [0..]) (each' [0..])
[(0,0),(0,1),(1,0),(0,2),(1,1),(2,0),(0,3),(1,2),(2,1),(3,0)]

With a bit more advanced WeightedSearch monad it is also possible to assign weights to computations so that results of computations with lower weights would appear first in the output.

Accumulating errors monad

A useful These data type forms a Monad similar to Either, but able to accumulate errors rather. The package also defines MonadChronicle class as well as ChronicleT monad transformer based on These.