Solve calculating exponential in functional way without using loop, built-in multiply (*) and exponential function.

calculator.erl

-module (calculator).
-export ([exponential/2, multiply/2]).

exponential(_, Exp) when(Exp < 0) ->
	throw("Bad argument");

exponential(Num, Exp) when (Exp > 0) ->
	calc_exponential(Num, Num, Exp).

calc_exponential(_, _, 0) ->
    1;

calc_exponential(Result, _, 1) ->
    Result;

calc_exponential(Result, Num, Exp) ->
	calc_exponential(multiply(Result, Num), Num, Exp - 1).

multiply(Num, Multiplier) -> 
	calc_multiply(0, Num, Multiplier).

calc_multiply(Result, _, 0) ->
	Result;

calc_multiply(Result, Num, Multiplier) ->
	calc_multiply(Result + Num, Num, Multiplier - 1).