First commit

chapter3/sum_list_tc.ex

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+defmodule ListHelper do
+  def sum(list) do
+    do_sum(0, list)
+  end
+
+  defp do_sum(current_sum, []) do
+    current_sum
+  end
+
+  defp do_sum(current_sum, [head | tail]) do
+    # More concise implementation
+    # do_sum(current_sum + head, tail)
+    new_sum = head + current_sum
+    do_sum(new_sum, tail)
+  end
+end