It's worth noting that the Google "MapReduce" is not type equivalent to the map (roughly 'a list -> 'b list) and reduce ('a list -> 'b) found in Lisp and func

It's worth noting that the Google "MapReduce" is not type equivalent to the map (roughly 'a list -> 'b list) and reduce ('a list -> 'b) found in Lisp and functional languages. It is actually in a way, an opposite. That is, the map (roughly 'a -> 'b list) in mapreduce is actually more like the dual/opposite of reduce (unfold). And reduce is well, a complicated beast. I think - the ideas behind MapReduce are not difficult but nor are they necessarily inevitable. The innovative part of MapReduce is Google getting to a place such that the constraints force that approach as obvious. MapReduce is impressive.