diff --git a/pkgs/ruby/lib/kernaux/file.rb b/pkgs/ruby/lib/kernaux/file.rb
index b2c9ee8..426c6e0 100644
--- a/pkgs/ruby/lib/kernaux/file.rb
+++ b/pkgs/ruby/lib/kernaux/file.rb
@@ -2,18 +2,20 @@
 
 # rubocop:disable Lint/EmptyClass
 
-module KernAux
-  ##
-  # File simulator.
-  #
-  class File
+if defined? KernAux::File
+  module KernAux
     ##
-    # @!method initialize(out)
-    # Create a file.
+    # File simulator.
     #
-    # @param out [Proc] writing method
+    class File
+      ##
+      # @!method initialize(out)
+      # Create a file.
+      #
+      # @param out [Proc] writing method
 
-    # @!parse [ruby]
+      # @!parse [ruby]
+    end
   end
 end
 
diff --git a/pkgs/ruby/spec/lib/kernaux/file_spec.rb b/pkgs/ruby/spec/lib/kernaux/file_spec.rb
new file mode 100644
index 0000000..f21903a
--- /dev/null
+++ b/pkgs/ruby/spec/lib/kernaux/file_spec.rb
@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+defined? KernAux::File and RSpec.describe KernAux::File do
+  subject(:our_file) { described_class.new method :out }
+
+  def out(str)
+    (@buffer ||= +'') << String(str)
+  end
+
+  specify do
+    expect(our_file.instance_variable_get(:@out)).to eq method :out
+  end
+end