@hackage exceptional0.1.4.2

Essentially the Maybe type with error messages.

This is a very simple type:

data Exceptional x
  = Failure String
  | Success x

It's much like Maybe, except instead of Nothing, we have Failure String.

A comparison could also be made to Either String. I made this library because I was dissatisfied with the Monad instance for Either. In this type, fail = Failure. It's rather simple.