2 files changed, 20 insertions, 3 deletions
diff --git a/lib/elixir_math_parser.ex b/lib/elixir_math_parser.ex
index f7f916f..863ecd8 100644
--- a/lib/elixir_math_parser.ex
+++ b/lib/elixir_math_parser.ex
@@ -10,9 +10,9 @@ defmodule ElixirMathParser do
     {:ok, value <~> 1}
   end
 
-  defp reduce_to_value({:var, _line, var}, state) do
+  defp reduce_to_value({:var, line, var}, state) do
     if !Map.has_key?(state, var) do
-      {:error, "value not found for " <> to_string(var)}
+      {:error, line, "value not found for " <> to_string(var)}
     else
       {:ok, state[var]}
     end
@@ -46,9 +46,25 @@ defmodule ElixirMathParser do
     end
   end
 
-  defp evaluate_tree([{:assign, {:var, _line, lhs}, rhs} | tail], state) do
+  defp reduce_to_value({:factor_op, lhs}, state) do
+    with {:ok, op} <- reduce_to_value(lhs, state),
+         true <- Ratio.denominator(op) == 1,
+         true <- Ratio.numerator(op) >= 0 do
+      {:ok, factor(Ratio.numerator(op), 1)}
+    else
+      {:error, line, reason} -> {:error, line, reason}
+      false -> {:error, "must have a positive integer for the factorial"}
+    end
+  end
+
+  defp factor(n, acc) when n > 0, do: factor(n - 1, acc * n)
+  defp factor(0, acc), do: acc
+
+  defp evaluate_tree([{:assign, {:var, line, lhs}, rhs} | tail], state) do
     with {:ok, val} <- reduce_to_value(rhs, state) do
       evaluate_tree(tail, Map.merge(state, %{lhs => val}))
+    else
+      {:error, reason} -> {:error, line, reason}
     end
   end
 
diff --git a/lib/main.ex b/lib/main.ex
index 99c25ca..5e7cf55 100644
--- a/lib/main.ex
+++ b/lib/main.ex
@@ -11,6 +11,7 @@ defmodule ElixirMathParser.Main do
 
     case ElixirMathParser.process_tree(tree) do
       {:ok, _} -> :ok
+      {:error, line, reason} -> reason <> " (line " <> to_string(line) <> ")"
       {:error, reason} -> reason
     end
   end