Improve the error message when the wrong number of arguments are provided

lib/expression/callbacks.ex

@@ -93,12 +93,25 @@ defmodule Expression.Callbacks do
       function_exported?(Standard, vargs_function_name, 2) ->
         {:vargs, Standard, vargs_function_name, 2}
 
+      # Check if the wrong number of arguments was provided
+      wrong_arity_but_function_exists?(module, exact_function_name) ->
+        {:error, "wrong number of arguments to #{function_name}."}
+
+      # Check if the wrong number of arguments was provided
+      wrong_arity_but_function_exists?(Standard, exact_function_name) ->
+        {:error, "wrong number of arguments to #{function_name}."}
+
       # Otherwise fail
       true ->
         {:error, "#{function_name} is not implemented."}
     end
   end
 
+  defp wrong_arity_but_function_exists?(module, function_name)
+       when is_atom(module) and is_atom(function_name) do
+    module.__info__(:functions)[function_name] != nil
+  end
+
   defmacro __using__(_opts) do
     quote do
       import Expression.Callbacks.EvalHelpers

test/expression_custom_callbacks_test.exs

@@ -44,7 +44,7 @@ defmodule ExpressionCustomCallbacksTest do
     end
 
     test "wrong arity functions" do
-      assert {:error, "echo is not implemented."} ==
+      assert {:error, "wrong number of arguments to echo."} ==
                Expression.Callbacks.implements(
                  ExpressionCustomCallbacksTest.CustomCallback,
                  "echo",