Better developer experience when missing/empty style definitions files

Raise a compile error with custom message if the JSON file is empty or
absent, explaining what to do to be able to compile again.

Author: EasterPeanut

lib/ex_css_modules/ex_css_modules.ex

@@ -37,21 +37,40 @@ defmodule ExCSSModules do
   def stylesheet(definition, build_json_task), do: read_stylesheet(definition, build_json_task)
 
   defp read_stylesheet(filename, build_json_task) do
-    cond do
-      File.exists?("#{filename}.json") && !empty_file?("#{filename}.json") ->
-        "#{filename}.json"
-        |> File.read!()
-        |> json_library().decode!()
-
-      build_json_task && File.exists?(filename) ->
-        with {:ok, json_filename} <- build_json_task.run(filename: filename) do
-          json_filename
+    if File.exists?(filename) do
+      cond do
+        File.exists?("#{filename}.json") && not empty_file?("#{filename}.json") ->
+          "#{filename}.json"
           |> File.read!()
           |> json_library().decode!()
-        end
 
-      true ->
-        %{}
+        not is_nil(build_json_task) ->
+          with {:ok, json_filename} <- build_json_task.run(filename: filename) do
+            json_filename
+            |> File.read!()
+            |> json_library().decode!()
+          end
+
+        not File.exists?("#{filename}.json") ->
+          message =
+            """
+            File does not exist.
+            Error compiling ExCSSModules: Be sure to build your style definitions JSON files before compiling your application.
+            """
+
+          raise(CompileError, description: message, file: "#{filename}.json")
+
+        empty_file?(filename) ->
+          message =
+            """
+            File is empty.
+            Error compiling ExCSSModules: Remove the file and/or (re)build your style definitions JSON files before compiling your application.
+            """
+
+          raise(CompileError, description: message, file: "#{filename}.json")
+      end
+    else
+      %{}
     end
   end
 

test/ex_css_modules/view_test.exs

@@ -43,7 +43,7 @@ defmodule ExCSSModules.ViewTest do
       stylesheet:
         __ENV__.file
         |> Path.dirname()
-        |> Path.join("../support/no_stylesheet_definition.css")
+        |> Path.join("../support/no_stylesheet_definitions.css")
   end
 
   describe "stylesheet_definition/0" do
@@ -68,12 +68,28 @@ defmodule ExCSSModules.ViewTest do
       assert ViewModule.StylesheetDefinitions.EmptyObject.Test.stylesheet() == %{}
     end
 
-    test "returns an empty map if the stylesheet definitions json file is empty" do
-      assert ViewModule.StylesheetDefinitions.EmptyFile.Test.stylesheet() == %{}
+    test "raises if the stylesheet definitions json file is empty" do
+      message =
+        """
+        test/support/empty_file.css.json: File is empty.
+        Error compiling ExCSSModules: Remove the file and/or (re)build your style definitions JSON files before compiling your application.
+        """
+
+      assert_raise(CompileError, message, fn ->
+        ViewModule.StylesheetDefinitions.EmptyFile.Test.stylesheet()
+      end)
     end
 
-    test "returns an empty map if the stylesheet definitions json file does not exist" do
-      assert ViewModule.StylesheetDefinitions.NoFile.Test.stylesheet() == %{}
+    test "raises if the stylesheet definitions json file does not exist" do
+      message =
+        """
+        test/support/no_stylesheet_definitions.css.json: File does not exist.
+        Error compiling ExCSSModules: Be sure to build your style definitions JSON files before compiling your application.
+        """
+
+      assert_raise(CompileError, message, fn ->
+        ViewModule.StylesheetDefinitions.NoFile.Test.stylesheet()
+      end)
     end
   end