[Haskell-cafe] Either instance of Monad?
Yves Parès
limestrael at gmail.com
Wed Feb 23 14:14:46 CET 2011
The latest, I think :
GHC 7.0.1,
mtl-2.0.1.0,
base-4.3.0.0
2011/2/23 Daniel Fischer <daniel.is.fischer at googlemail.com>:
> On Wednesday 23 February 2011 09:58:56, Yves Parès wrote:
>> 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?
>
>>
>> 2011/2/22 Daniel Fischer <daniel.is.fischer at googlemail.com>:
>> > 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
>> > <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.
>
>
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