monad-validate
A Haskell library providing the ValidateT monad transformer, designed for writing data validations that provide high-quality error reporting without much effort. ValidateT automatically exploits the data dependencies of your program—as encoded implicitly in uses of fmap, <*>, and >>=—to report as many errors as possible upon failure instead of completely aborting at the first one.
See the documentation on Hackage for more information and examples.