2.15 Defining functions over lists in practice, https://www.futurelearn.com/courses/functional-programming-erlang/3/steps/488110

two15.erl

-module(two15).

-export([product/1, product_tr/1, maximum/1, maximum_tr/1]).

product([X]) ->
    X;
product([X|Xs]) ->
    X * product(Xs).

product_tr([X|Xs]) ->
    product_tr(Xs, X).

product_tr([], Result) ->
    Result;
product_tr([X|Xs], Result_so_far) ->
    product_tr(Xs, X * Result_so_far).

maximum([X]) ->
    X;
maximum([X, Y | Zs]) ->
    maximum([max(X, Y) | Zs]).

maximum_tr([X|Xs]) ->
    maximum_tr(Xs, X).

maximum_tr([], Result) ->
    Result;
maximum_tr([X|Xs], Result_so_far) when X > Result_so_far ->
    maximum_tr(Xs, X);
maximum_tr([_X|Xs], Result_so_far) ->
    maximum_tr(Xs, Result_so_far).