2010/5/28 Donn Cave <d...@avvanta.com>: > Quoth Ivan Lazar Miljenovic <ivan.miljeno...@gmail.com>, > >> Whilst the Either type isn't officially used for >> errors, that is how it is usually treated in Haskell with the consensus >> that Left = failure and Right = success (note that due to how its >> defined it also has to be this way for Either's Monad instance to >> work). > > I'm glad you mentioned that, I was going to mention how natural it > would seem for Either to be an instance of Monad, given that it is > used as you say by consensus for errors ... but something seems to > be wrong with my libraries: > > No instance for (Monad (Either [Char])) > arising from a use of `return' at except.hs:25:24-29 > Possible fix: > add an instance declaration for (Monad (Either [Char])) > > So, I understand how to make a Monad instance, and I guess your > point stands (as demonstrated by the expected type of (Either String)), > but it's funny that Either is understood to have a Monad instance > even though that's only implied, and not supplied. > > [snip]
Hi, Control.Monad.Error provides an instance for Either. Cheers, Thu _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
