Re: [Haskell-cafe] Why Either = Left | Right instead of something like Result = Success | Failure

On Saturday 29 May 2010 02:26:38, Ivan Lazar Miljenovic wrote:

Daniel Fischer writes:

...

But if you want to have

instance Monad (Either ConcreteType) where ...

, you can have

fail msg = Left someDefaultValue

(or let the value depend on the message) and you can construct Left someMeaningfulValue in concrete situations.

Which happens to make it identical to Maybe ;-)

If you use only the default value. But if you use more, making your type an instance of Error would be the sensible thing (unless you're one of the mtl-haters, but probably transformers has a similar instance).