Compile-Time Resolver Validations (#16)

* Ignore tool-versions

* Add unit test for validations on validating resolver function definitions

* Validate resolver functions after compiling workflow graph

* Organize validate_resolvers/1 implementation into a pipeline

.gitignore

@@ -24,3 +24,5 @@ pacer-*.tar
 
 # Temporary files, for example, from tests.
 /tmp/
+
+/.tool-versions

lib/workflow.ex

@@ -905,10 +905,31 @@ defmodule Pacer.Workflow do
   @spec __after_compile__(Macro.Env.t(), binary()) :: :ok | no_return()
   def __after_compile__(%{module: module} = _env, _) do
     _ = validate_dependencies(module)
+    _ = validate_resolvers(module)
 
     :ok
   end
 
+  defp validate_resolvers(module) do
+    module
+    |> Module.get_attribute(:pacer_resolvers)
+    |> Enum.map(fn {field, resolver_fun} ->
+      resolver_fun
+      |> Function.info()
+      |> Map.new()
+      |> then(fn info ->
+        unless function_exported?(info.module, info.name, info.arity) do
+          raise Error, """
+          Resolver for field `#{inspect(field)}` is undefined. Ensure that the resolver you intend to use
+          has been defined and you have no mispellings.
+
+          Resolver Function: #{inspect(resolver_fun)}
+          """
+        end
+      end)
+    end)
+  end
+
   @spec ensure_no_duplicate_fields(module(), atom()) :: :ok | no_return()
   defp ensure_no_duplicate_fields(module, field_name) do
     if Enum.member?(Module.get_attribute(module, :pacer_fields), field_name) do

test/workflow_test.exs

@@ -595,6 +595,30 @@ defmodule Pacer.WorkflowTest do
         Code.eval_string(module)
       end
     end
+
+    test "alerts users when a resolver function does not exist" do
+      module = """
+      defmodule GraphWithInvalidResolver do
+        use Pacer.Workflow
+
+        graph do
+          field(:a, resolver: &NonExistentModule.mispelled_function/1, dependencies: [:b])
+          field(:b, default: "foo")
+        end
+      end
+      """
+
+      expected_error_message = """
+      Resolver for field `:a` is undefined. Ensure that the resolver you intend to use
+      has been defined and you have no mispellings.
+
+      Resolver Function: &NonExistentModule.mispelled_function/1
+      """
+
+      assert_raise Error, expected_error_message, fn ->
+        Code.eval_string(module)
+      end
+    end
   end
 
   describe "batch validations" do