Define a function foldr (fold right) with three parameters, op, zero_element, and lst. The parameter op itself is a two-argument function, zero_element is the zero element of the operator function (e.g., 0 for ‘+’ operator or 1 for the multiply operator), and lst is a list of elements. (The plus function takes two parameters and adds them, and the multiply function takes two parameters and multiplies them.)
The function successively applies the op function to each element of the list and the result of the previous op function (where no such results exist, the zero element is used). The following interaction log illustrates the foldr function: