Activity for Erlang course https://www.futurelearn.com/courses/functional-programming-erlang/1/steps/155134

l.erl

%%%-------------------------------------------------------------------
%%% @author Josh Rehman
%%% @copyright (C) 2017, JavaJosh Enterprises
%%% @doc
%%%
%%% @end
%%% Created : 27. Feb 2017 5:28 PM
%%%-------------------------------------------------------------------
-module(l).
-author("Josh Rehman").

%% API
-export([head/1,tail/1,second/1,sum/1, summ/1, prod/1,prodd/1, maxL/1, maxLL/1]).

% Trying out some examples from the video.
head([X|_Xs]) -> X.
tail([_X|Xs]) -> Xs.

%second(Xs) -> head(tail(Xs)).

% So the "cons" bar means "rest".
% The underscore suppresses unused parameter warning.
second([_X,Y|_Z]) -> Y.

% Calculate the sum of a list of numbers. Direct recursive.
sum([]) -> 0;
sum([X|Xs]) -> X + sum(Xs).

% Calculate the sum of a list of numbers. Indirect recursive.
summ(Xs) -> summ(Xs,0).
summ([],S) -> S;
summ([X|Xs], S) -> summ(Xs, X+S).

% Calculate the product of a list of numbers. Direct recursive.
prod([]) -> 1;
prod([X|Xs]) -> X * prod(Xs).

% Calculate the product of a list of numbers. Indirect recursive.
prodd(X) -> prodd(X, 1).
prodd([], Acc) -> Acc;
prodd([X|Xs], Acc) -> prodd(Xs, Acc * X).


% Note: the function name 'max' gave a scary warning.
%l.erl:43: Warning: ambiguous call of overridden auto-imported BIF max/2 use erlang:max/2 or "-compile({no_auto_import,[max/2]})." to resolve name clash
max2(A,B) when A>B -> A;
max2(_A,B) -> B.

% Calculate the max of a list. Direct recursive.
maxL([A]) -> A;
maxL([X|Xs]) -> max2(X, maxL(Xs)).

% Calculate the max of a list. Indirect recursive.
maxLL([X|Xs]) -> maxLL(Xs,X).

maxLL([A],Max) -> max2(A,Max);
maxLL([X|Xs], Max) when X > Max ->
  maxLL(Xs, X);
maxLL([_X|Xs], Max) ->
  maxLL(Xs, Max).