How a body recursive function expands in Elixir

len.ex

# This is our function definition,
# It is body (not tail) recursive, as it must wait for
# len(tail) to return before the addition can be performed
# on each iteration
def len([]), do: 0
def len([head | tail]), do: 1 + len(tail)

# If we call len([1, 2, 3]), this is what happens
len([1, 2, 3]) == 1 + len([2, 3])
len([2, 3]) == 1 + len([3])
len([3]) == 1 + len([])
len([]) == 0

# Or put another way
len([1, 2, 3]) == 1 + len([2,3]) + len([3]) + len([])
# Expanded..
len([1, 2, 3]) == 1 + 1 + 1 + 0
# Result..
len([1, 2, 3]) == 3