proposal #4095: add Applicative instance for Either

Henning Thielemann lemming at
Tue May 25 13:59:37 EDT 2010

On Tue, 25 May 2010, Bas van Dijk wrote:

> On Tue, May 25, 2010 at 7:21 PM, Ross Paterson <ross at> wrote:
>> Fine by me.  Making it a non-orphan would break Haskell 98 compatibility,
>> though.
> Making it non-orphan means moving the Monad Either instance to either
> GHC.Base (which defines the Monad class) or to Data.Either (which
> defines the Either type). How exactly does this break Haskell 98
> compatibility?

Since there is no Monad Either instance in Haskell 98, Haskell 98 programs 
can safely define their own one. If we would add such an instance to the 
base library this would break those programs.

More information about the Libraries mailing list