Improve the error message when the wrong number of arguments are prov…

…ided

Currently when a user provides the wrong number of arguments, they receive a
message saying `echo is not implemented.`.

This message is confusing because it makes the user feel like they typed in the
wrong function name, when in reality they just provided the wrong number of
arguments.

This is a first draft at a fix - feedback welcome!

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
+    Enum.any?(0..20, fn arity -> Kernel.function_exported?(module, function_name, arity) end)
+  end
+
   defmacro __using__(_opts) do
     quote do
       import Expression.Callbacks.EvalHelpers

lib/expression/v2/callbacks.ex

@@ -97,12 +97,25 @@ defmodule Expression.V2.Callbacks do
       function_exported?(Standard, vargs_function_name, 2) ->
         {:vargs, Standard, vargs_function_name}
 
+      # 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
+    Enum.any?(0..20, fn arity -> Kernel.function_exported?(module, function_name, arity) end)
+  end
+
   defmacro __using__(_opts) do
     quote do
       def callback(module \\ __MODULE__, context, function_name, args)

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",