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

2010/5/28 Donn Cave :

Quoth Ivan Lazar Miljenovic ,

...

  Whilst the Either type isn't officially used for errors, that is how it is usually treated in Haskell with the consensus that Left = failure and Right = success (note that due to how its defined it also has to be this way for Either's Monad instance to work).

I'm glad you mentioned that, I was going to mention how natural it would seem for Either to be an instance of Monad, given that it is used as you say by consensus for errors ... but something seems to be wrong with my libraries:

   No instance for (Monad (Either [Char]))      arising from a use of `return' at except.hs:25:24-29    Possible fix:      add an instance declaration for (Monad (Either [Char]))

So, I understand how to make a Monad instance, and I guess your point stands (as demonstrated by the expected type of (Either String)), but it's funny that Either is understood to have a Monad instance even though that's only implied, and not supplied.

[snip]

Hi, Control.Monad.Error provides an instance for Either. Cheers, Thu