Custom error keys returned from validate function

README.md

@@ -284,16 +284,20 @@ end
 
 While storing files on S3 (rather than your harddrive) eliminates some malicious attack vectors, it is strongly encouraged to validate the extensions of uploaded files as well.
 
-Arc delegates validation to a `validate/1` function with a tuple of the file and scope.  As an example, to validate that an uploaded file conforms to popular image formats, you may use:
+Arc delegates validation to a `validate/1` function with a tuple of the file and scope. The function needs to return a tuple of two atoms `{:ok, :valid}` or `{:error, :reason}`. As an example, to validate that an uploaded file conforms to popular image formats, you may use:
 
 ```elixir
 defmodule Avatar do
   use Arc.Definition
   @extension_whitelist ~w(.jpg .jpeg .gif .png)
 
-  def validate({file, _}) do   
+  def validate({file, _}) do
     file_extension = file.file_name |> Path.extname |> String.downcase
-    Enum.member?(@extension_whitelist, file_extension)
+    if Enum.member?(@extension_whitelist, file_extension) do
+      {:ok, :valid_extension}
+    else
+      {:error, :invalid_extension}
+    end
   end
 end
 ```
@@ -431,7 +435,7 @@ defmodule Avatar do
 
   def acl(:thumb, _), do: :public_read
 
-  def validate({file, _}) do   
+  def validate({file, _}) do
     file_extension = file.file_name |> Path.extname |> String.downcase
     Enum.member?(@extension_whitelist, file_extension)
   end

lib/arc/actions/store.ex

@@ -23,8 +23,9 @@ defmodule Arc.Actions.Store do
 
   defp put(definition, {%Arc.File{}=file, scope}) do
     case definition.validate({file, scope}) do
-      true -> put_versions(definition, {file, scope})
-      _    -> {:error, :invalid_file}
+      {:ok, _} -> put_versions(definition, {file, scope})
+      {_, error}    -> {:error, error}
+      _ -> {:error, :invalid_file}
     end
   end
 

lib/arc/definition/storage.ex

@@ -5,7 +5,7 @@ defmodule Arc.Definition.Storage do
 
       def filename(_, {file, _}), do: Path.basename(file.file_name, Path.extname(file.file_name))
       def storage_dir(_, _), do: "uploads"
-      def validate(_), do: true
+      def validate(_), do: {:ok, :valid}
       def default_url(version, _), do: default_url(version)
       def default_url(_), do: nil
       def __storage, do: Arc.Storage.S3

test/actions/store_test.exs

@@ -7,7 +7,14 @@ defmodule ArcTest.Actions.Store do
     use Arc.Actions.Store
     use Arc.Definition.Storage
 
-    def validate({file, _}), do: String.ends_with?(file.file_name, ".png")
+    def validate({file, _}) do
+      if String.ends_with?(file.file_name, ".png") do
+        {:ok, :ok}
+      else
+        {:error, :invalid_file}
+      end
+    end
+
     def transform(_, _), do: :noaction
     def __versions, do: [:original, :thumb]
   end