On 3 May 2011 13:26, Yitzchak Gale
Both are "kind of, sort of" bringing you up to a Monoid though...
altconcat and sconcatMaybe are doing that, because you need to decide what to do with an empty list when you define the instance. Holger's interface is not doing that, because the type does not require you to say anything about the case of an empty list in the instance.
Holger's interface is bringing you "kind of, sort of" up to a Monoid but it allows neutral of whatever value you fancy at the time. At which point, either you're working at directly at a type - so you don't really need the "idea" of a semigroup just its pretty (<>) operator, or you do actually have a neutral and thus were working with a Monoid all along - again you just wanted the pretty (<>) operator. My real contention is that Semigroup doesn't have a proper concat operation[*], though notationally it is seductive - I do use both altconcat and Holger's version in my own code. [*] I could be persuaded otherwise, but I can't see how it would be an analogue mconcat in Monoid.