On Friday 28 May 2010 20:44:20, Donn Cave wrote: > Quoth Vo Minh Thu <not...@gmail.com>, > ... > > > Control.Monad.Error provides an instance for Either. > > ... in the mtl transformer library, in case anyone else besides > myself didn't know that. And I see it has to be there because > it depends on the Error typeclass.
Which is considered a wart by some. (Either left) has a perfectly valid Monad instance for any type left (the only slightly difficult thing might be 'fail'). > > (So the documentation for Control.Monad.Error, is mistaken, where > at the top it says > > Example type: > Either String a > > ... which should be > > Either Error a > > ... ? No, String is an instance of Error, so (Either String) is fine. > Though I can't really be sure what the documentation is > trying to say.) > In that case, have a look at the code, perhaps that is clearer. (And bug the maintainer(s) to improve the docs.) > > Donn Cave, d...@avvanta.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
