finger-tree

Yesterday's Wikibender that started with this stackoverflow qestion on Comonads ended up at MarkCC's article on Finger Trees . In the article he makes extensive use of the Reduce type class. He writes about this typeclass as if it is a very common and frequently use library, but I cannot find it on hackage, nor can I find enough documentation to really understand the code. Can someone help me understand what the Reduce typeclass is doing, how the (-<) and (>-) operators work, and what should tell me about the code in the article (copied below)? Code Listing from Finger Trees Done Right (I hope

I wanted to play around with 2-3 finger trees as described in the (Haskell) paper by Hinze (see also this blog ). type Node<'a> = | Node2 of 'a * 'a | Node3 of 'a * 'a * 'a static member OfList = function | [a; b] -> Node2(a, b) | [a; b; c] -> Node3(a, b, c) | _ -> failwith "Only lists of length 2 or 3 accepted!" member me.ToList () = match me with | Node2(a, b) -> [a; b] | Node3(a, b, c) -> [a; b; c] type Digit<'a> = | One of 'a | Two of 'a * 'a | Three of 'a * 'a * 'a | Four of 'a * 'a * 'a * 'a static member OfList = function | [a] -> One(a) | [a; b] -> Two(a, b) | [a; b; c] -> Three(a, b,