README

Either monads support either a L (left) or a R (right) value. L is traditionally used as an error carrier, and any Either monad carrying an L will short-cut any bind operations and return the L (kinda like an Exception). However, they're not only for that, and can be used to carry an alternative value which could be mapped using BiMap, or MapLeft.

You can construct an Either using the constructor functions in the Prelude:

Either<string, int> ma = Left<string, int>("this is an error");
Either<string, int> mb = Right<string, int>(123);

There are also convenience types called Left and Right, that don't need both generics providing. They can often make it a little easier to work with Either:

Either<string, int> ma = Left("this is an error");
Either<string, int> mb = Right(123);

It uses implicit casts to work out what the type should be. Note, if you're having trouble getting the types resolved, just specify the type parameters.