[Haskell-beginners] Who discovered the fold operation?

[Stephen Tetley]

For foldr see Olivier Danvy and Michael Spivey's "On Barron and
Strachey’s Cartesian Product Function" crediting a discovery to David
Barron and Christopher Strachey.

http://www.brics.dk/RS/07/14/BRICS-RS-07-14.pdf