I guess a short explanation is in order. foldr/foldl are folding or reduction operations. Heard of MapReduce? Well, fold is the reduction part (foldl specifical

I guess a short explanation is in order. foldr/foldl are folding or reduction operations. Heard of MapReduce? Well, fold is the reduction part (foldl specifically). It takes a list, function, and an accumulator and returns a list and the final accumulator value as a result. Pretty much every functional language has one and foldr/foldl names come from S/ML and later from Haskell. You can see an example here: http://www.cse.unsw.edu.au/~en1000/haskell/hof.html http://www.cse.unsw.edu.au/~en1000/haskell/hof.html The difference between foldr and foldl is how the list is processed and in the final list (left to right or right to left) so depending on what your function is, order does matter. Also, foldr is not tail recursive so it can't be optimized nicely. These sorts of constructs are coming back to forefront of computing since they offer us a very efficient way to do distributed computing. I use MR all the time these days to deal with large data sets.