- Haskell Wiki. Fold. https://wiki.haskell.org/Fold
- Richard Bird. Sections 4.5 and 4.6 of Introduction to Functional Programming using Haskell (1998)
- Antoni Diller. Introduction to Haskell.
- Graham Hutton. A tutorial on the universality and expressiveness of fold. http://www.cs.nott.ac.uk/~gmh/fold.pdf