From 74ae0abd4c58835da2e664f21b7fb29313233b90 Mon Sep 17 00:00:00 2001
From: Patrick Sinclair <patrick.sinclair@remote.com>
Date: Tue, 23 Jul 2024 11:06:40 +0100
Subject: [PATCH] Improve the error message when the wrong number of arguments
 are provided

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               | 13 +++++++++++++
 lib/expression/v2/callbacks.ex            | 13 +++++++++++++
 test/expression_custom_callbacks_test.exs |  2 +-
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/lib/expression/callbacks.ex b/lib/expression/callbacks.ex
index 9d0bb1e..ca4d57d 100644
--- a/lib/expression/callbacks.ex
+++ b/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
diff --git a/lib/expression/v2/callbacks.ex b/lib/expression/v2/callbacks.ex
index d826231..1ab3b5d 100644
--- a/lib/expression/v2/callbacks.ex
+++ b/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)
diff --git a/test/expression_custom_callbacks_test.exs b/test/expression_custom_callbacks_test.exs
index f29ee2f..b3fd640 100644
--- a/test/expression_custom_callbacks_test.exs
+++ b/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",