improvement: add warnings to DSL transformer returns

improvement: warn on empty registries

closes #359

lib/ash/dsl/dsl.ex

@@ -169,7 +169,8 @@ defmodule Ash.Dsl do
                 |> Ash.Dsl.Extension.run_transformers(
                   transformers_to_run,
                   Module.get_attribute(__MODULE__, :ash_dsl_config),
-                  false
+                  false,
+                  __ENV__
                 )
               end
             end

lib/ash/dsl/extension.ex

@@ -479,12 +479,13 @@ defmodule Ash.Dsl.Extension do
       |> Ash.Dsl.Extension.run_transformers(
         transformers_to_run,
         ash_dsl_config,
-        true
+        true,
+        __ENV__
       )
     end
   end
 
-  def run_transformers(mod, transformers, ash_dsl_config, store?) do
+  def run_transformers(mod, transformers, ash_dsl_config, store?, env) do
     Enum.reduce_while(transformers, ash_dsl_config, fn transformer, dsl ->
       result =
         try do
@@ -506,6 +507,17 @@ defmodule Ash.Dsl.Extension do
         :halt ->
           {:halt, dsl}
 
+        {:warn, new_dsl, warnings} ->
+          warnings
+          |> List.wrap()
+          |> Enum.each(&IO.warn(&1, Macro.Env.stacktrace(env)))
+
+          if store? do
+            Module.put_attribute(mod, :ash_dsl_config, new_dsl)
+          end
+
+          {:cont, new_dsl}
+
         {:ok, new_dsl} ->
           if store? do
             Module.put_attribute(mod, :ash_dsl_config, new_dsl)

lib/ash/dsl/transformer.ex

@@ -14,7 +14,12 @@ defmodule Ash.Dsl.Transformer do
   point in returning a new dsl structure from `transform/2` if `after_compile/0` is defined. Instead,
   simply return `:ok` or `{:error, error}`
   """
-  @callback transform(module, map) :: :ok | {:ok, map} | {:error, term} | :halt
+  @callback transform(module, map) ::
+              :ok
+              | {:ok, map}
+              | {:error, term}
+              | {:warn, map, String.t() | list(String.t())}
+              | :halt
   @callback before?(module) :: boolean
   @callback after?(module) :: boolean
   @callback after_compile?() :: boolean

lib/ash/registry/dsl.ex

@@ -30,14 +30,27 @@ defmodule Ash.Registry.Dsl do
     ],
     entities: [
       @entry
+    ],
+    schema: [
+      warn_on_empty?: [
+        type: :boolean,
+        doc: "Set to `false` to ignore warnings about an empty registry",
+        default: true
+      ]
     ]
   }
 
   @sections [@entries]
 
+  @transformers [Ash.Registry.Transformers.WarnOnEmpty]
+
   @moduledoc """
   A small DSL for declaring an `Ash.Registry`.
   """
 
-  use Ash.Dsl.Extension, sections: @sections
+  use Ash.Dsl.Extension, sections: @sections, transformers: @transformers
+
+  def warn_on_empty?(registry) do
+    Extension.get_opt(registry, [:entries], :warn_on_empty?, false, true)
+  end
 end

lib/ash/registry/transformers/warn_on_empty.ex

@@ -0,0 +1,13 @@
+defmodule Ash.Registry.Transformers.WarnOnEmpty do
+  use Ash.Dsl.Transformer
+
+  def transform(registry, dsl) do
+    case Ash.Registry.entries(registry) do
+      [] ->
+        {:warn, dsl, "#{inspect(registry)} has no entries."}
+
+      _ ->
+        {:ok, dsl}
+    end
+  end
+end