Common first class function: fold

There are two versions of the fold operator: a right-fold and a left-fold. The fold operator takes as its arguments a so called zero-value (Z), a function (f) and a list.

f takes as its arguments two values and returns one value.

Fold-right:

fold_right(Z, f, [v1, v2, v3]) == f(v1, f(v2, f(v3, Z)))

Fold-left:

fold_left(Z, f, [v1, v2, v3]) == f(f(f(Z, v1), v2), v3)