Hi,
On Sun, Aug 14, 2011 at 2:43 PM, Yitzchak Gale
NO please don't do that. It would break all packages that do, or soon will, use the semigroups package.
They shouldn't unless they use unqualified implicit imports. Using imports this way is asking for trouble as any time a new identifier is added to a package you depend on, your code might break. If you do use unqualified implicit imports you need to be prepared to fix breakages by adding import lists or qualified imports as needed.
That proposal is no longer relevant. Every Monoid instance should now be made an instance of Semigroup. Anyone who wants to use <> should import it from there.
This doesn't work well unless SemiGroup is a superclass of Monoid. For example, class constraints would have to include redundant information: f :: (Monoid a, SemiGroup a) => a -> ... SemiGroup is redundant here. All monoids are semigroups. Making SemiGroup a superclass of Monoid is not feasible at the moment, as we don't have a way (e.g. class aliases) to do so without breaking lots of code. I don't know of anyone who's actually working on solving this problem at the moment. In addition, I don't want to depend on the semigroup package. It needs more work. For example why does depending on the semigroups package give me a new type for natural numbers and for non-empty lists? I don't object to have a class for semigroups in the future, but we're far from where that is possible in a nice way. Cheers, Johan