Higher_order_funcs.erl

%% This is the functions used to find min, max number and to calculate sum of a list without use fold.

%% find the maximum number of the list
max([H|T]) -> max2(T, H).

max2([], Max) -> Max;
max2([H|T], Max) when H > Max -> max2(T, H);
max2([_|T], Max) -> max2(T, Max).


%% find the minium number of the list
min([H|T]) -> min2(T,H).

min2([], Min) -> Min;
min2([H|T], Min) when H < Min -> min2(T,H);
min([_|T], Min) -> min2(T, Min).


%% find the sum of the list. Tail recursion.
sum(L) -> sum(L,0).
sum([], Sum) -> Sum;
sum([H|T], Sum) -> sum(T, Sum+H).


%% =================================================
%% Use fold here.
fold(_, Start, []) -> Start.
fold(F, Start, [H|T]) -> fold(F, F(H, Start), T).

%% Base fold function, we can re-write the function to find min, max number or to calculate sum of a list.
min([H|T]) -> fold(fun(A, B) when A > B -> A; (_, B) -> B end, H, T).
max([H|T]) -> fold(fun(A, B) when A < B -> A; (_, B) -> B end, H, T).
sum(L) -> fold(fun(A, B) -> A + B end, 0, L).