On Sat, Sep 28, 2013 at 1:09 PM, Ryan Newton <rrnewton@gmail.com> wrote:

Hi all,

We all know and love Data.Foldable and are familiar with left folds and right folds. But what you want in a parallel program is a balanced fold over a tree. Fortunately, many of our datatypes (Sets, Maps) actually ARE balanced trees. Hmm, but how do we expose that?

It seems like it would be nice to have a standard class that allows you to split a datatype into roughly even halves, until you get down to the leaves. This goes along with Guy Steele's argument that we should use "append lists" as primitive rather than "cons-lists", and it's why we added append-lists within the monad-par library.

Interestingly, in my Fortress days we looked at both using a split-like interface and at a more foldMap / reduce - like interface, and it seemed like the latter worked better – it requires a lot less boilerplate for controlling recursion, and better matches the fanout of whatever structure you're actually using underneath.

So I'd just go with a hand-written Foldable instance here.

But I'd love to hear if you've come up with an application that requires split itself, and that *isn't* zip. I recall we decided zip was better done with element-and-index iteration over one of the structures and indexing into the other since most tree structures don't actually zip properly anyway.

-Jan-Willem Maessen