The Essence of the Iterator pattern

The Essence of the Iterator pattern treats iterating over collections as two problems, which exhibit traversal of collections (and modifying the content) and accumulating values based on the contents. Jeremy Gibbons and Bruno C.d.S. Oliveira show how Applicative Functors and related type classes can be used in functional programming to solve these problems.

Paper: http://www.cs.ox.ac.uk/jeremy.gibbons/publications/iterator.pdf

The paper is widely quoted amongst the functional programming community and illustrates nicely how recent acadamic research (Applicative Functors, McBride, 2008) finds its way into language design and application of functional programming languages such as Scala or Haskell.