[Haskell-cafe] Either instance of Monad?

Daniel Fischer daniel.is.fischer at googlemail.com
Tue Feb 22 23:33:20 CET 2011

On Tuesday 22 February 2011 23:07:05, Yves Parès wrote:
> Hello,
> When importing Control.Monad.Error, it shows me that (Either e) is an
> instance of MonadFix, but not an instance of Monad.
> Isn't there a problem?

Prelude Control.Monad.Error> :i Either
data Either a b = Left a | Right b      -- Defined in Data.Either
instance (Eq a, Eq b) => Eq (Either a b) -- Defined in Data.Either
instance Monad (Either e) -- Defined in Control.Monad.Instances
instance Functor (Either a) -- Defined in Control.Monad.Instances
instance MonadFix (Either e) -- Defined in Control.Monad.Fix

Works right here. With mtl-1.*, there's an (Error e) restriction on the 

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