twitter/scalding — pull request #216 by asiegel34 (2012-10-31 23:29:54+00:00)
Title: mark Operations type parameters covariant

This is a simple change to mark the Self type parameter covariant on the following traits: FoldOperations, ReduceOperations, Sortable, StreamOperations. This makes them friendlier to type hierarchies extending GroupBuilder.

It might be valuable to parameterize GroupBuilder also, with signature

[+Self <: GroupBuilder[Self]]

so that the full spectrum of operations work as expected with GroupBuilder extensions.