Illustrating folds - from "Haskell Programming" (http://haskellbook.com/), who in turn borrowed the idea from from Cale Gibbard from the haskell Freenode IRC channel and on the Haskell.org wiki https://wiki.haskell.org/Fold#Examples

folds.hs

Prelude> let xs = map show [1..5]
Prelude> foldr (\x y -> concat ["(",x,"+",y,")"]) "0" xs

"(1+(2+(3+(4+(5+0)))))"

Prelude> let f = (\x y -> concat ["(",x,"+",y,")"])
Prelude> foldl f "0" (map show [1..5])

"(((((0+1)+2)+3)+4)+5)"