#4159: move Monad and MonadFix instances for Either from mtl to base

[Henning Thielemann]

On Tue, 29 Jun 2010, Claus Reinke wrote:

> "Ross Paterson" <ross at soi.city.ac.uk> schrieb im Newsbeitrag 
> news:20100629092649.GA3460 at soi.city.ac.uk...
>> The proposal is to move the Monad and MonadFix instances for Either
>> (currently in the mtl package) to Control.Monad.Instances and
>> Control.Monad.Fix respectively (both in the base package).  The Monad
>> instance is still an orphan, to retain Haskell 98 compatibility, but the
>> MonadFix instance is together with its class.  The Error constraint is
>> removed from both instances, and the default definition of fail is used.
>> 
>> Discussion deadline: 20th July 2010
>
> -1, because the default definition of fail is error, which would
> render the Monad instance useless (unless I'm missing something?).

You would just use Left instead of 'fail' if you want to show failure in 
an Either value.