Re: [Haskell-cafe] Either instance of Monad?

The latest, I think : GHC 7.0.1, mtl-2.0.1.0, base-4.3.0.0 2011/2/23 Daniel Fischer :

On Wednesday 23 February 2011 09:58:56, Yves Parès wrote:

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I am using mtl-2. Importing Control.Monad.Error doesn't work, I have to explicitly import Control.Monad.Instances.

Weird. Control.Monad.Error imports Control.Monad.Instances (), so it ought to be visible. What exact versions of mtl, base, GHC are you using?

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2011/2/22 Daniel Fischer :

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On Tuesday 22 February 2011 23:07:05, Yves Parès wrote:

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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 <snip> instance MonadFix (Either e) -- Defined in Control.Monad.Fix <snip>

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