tail recursive call in elixir and default parameters

Question

I am writing a simple example in Elixir and although it works I don't really understand how.

defmodule MyList do
  def sum([],acc \\ 0), do: acc
  def sum([head | tail], acc), do: sum(tail,acc + head)
end

When I call MyList.sum I get the expected result

sum([]) => 0
sum([1,2,3]) => 6

I cannot add a default param in the second sum because the compiler throws an error

def sum/2 has default values and multiple clauses, use a separate clause for declaring defaults

So my question is, how come sum([1,2,3]) works? It does not match any of the definitions. Is the function still tail recursive?

Answer 1

When you have a multiclause with optional arguments, you can specify defaults as a body-less clause:

defmodule MyList do
  def sum(list, acc \\ 0) # sets up default arguments

  def sum([],acc), do: acc
  def sum([head | tail], acc), do: sum(tail,acc + head)
end

Regarding your example, I'm just guessing, but I think that your code amounts to something like following:

defmodule MyList do
  # implicitly generated due to default argument
  def sum(list), do: sum(list, 0)

  def sum([],acc), do: acc
  def sum([head | tail], acc), do: sum(tail,acc + head)
end

Which is why sum([1,2,3]) works as well.

Edit: The function is definitely tail recursive. If the last thing a function does is a call of another function (or itself), then it is a tail call. So in this case, when we call sum(tail, acc + head), first the arguments are calculated, and then a tail recursive call happens.