Stephen Tetley wrote:
There is that formulation, though usually I find I need to do it with an alternative instead: altconcat alt [] = alt altconcat _ (a:as) = go a as where go acc [] = acc go acc (b:bs) = go (acc <> b) bs
But the whole reason we need this as a method is for the case that consecutive appends is inefficient.
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. Another approach would be to depend on one of the packages that provides a non-empty list type, such as the NonEmptyList package. But I don't think this simple case justifies another dependency. You can wrap Holger's function in one of those types easily enough if you need to. Thanks, Yitz