Implementation of sum in Elixir v1.3.2, taken from https://github.com/elixir-lang/elixir/blob/v1.3.2/lib/elixir/lib/enum.ex#L2096-L2122

sum.ex

@doc """
Returns the sum of all elements.

Raises `ArithmeticError` if `enumerable` contains a non-numeric value.

## Examples

    iex> Enum.sum([1, 2, 3])
    6

"""
@spec sum(t) :: number
def sum(enumerable)

def sum(first..first),
  do: first

def sum(first..last) when last < first,
  do: sum(last..first)

def sum(first..last) when last > first do
  div((last + first) * (last - first + 1), 2)
end

def sum(enumerable) do
  reduce(enumerable, 0, &+/2)
end