Merge pull request #206 from nappex/cli-help

CLI usage message

lib/cli.ex

@@ -4,9 +4,28 @@ defmodule Onigumo.CLI do
   }
 
   def main(argv) do
-    {[], [component]} = OptionParser.parse!(argv, strict: [])
-    {:ok, module} = Map.fetch(@components, String.to_atom(component))
-    root_path = File.cwd!()
-    module.main(root_path)
+    try do
+      {[], [component]} = OptionParser.parse!(argv, strict: [])
+      component
+    rescue
+      _ in [OptionParser.ParseError, MatchError] -> usage_message()
+    else
+      component ->
+        {:ok, module} = Map.fetch(@components, String.to_atom(component))
+        root_path = File.cwd!()
+        module.main(root_path)
+    end
+  end
+
+  defp usage_message() do
+    components = Enum.join(Map.keys(@components), ", ")
+
+    IO.puts("""
+    Usage: onigumo [COMPONENT]
+
+    Simple program that retrieves HTTP web content as structured data.
+
+    COMPONENT\tOnigumo component to run, available: #{components}
+    """)
   end
 end

test/onigumo_cli_test.exs

@@ -1,5 +1,6 @@
 defmodule OnigumoCLITest do
   use ExUnit.Case
+  import ExUnit.CaptureIO
   import Mox
 
   @urls [
@@ -33,15 +34,20 @@ defmodule OnigumoCLITest do
     end
 
     test("run CLI with no arguments") do
-      assert_raise(MatchError, fn -> Onigumo.CLI.main([]) end)
+      assert usage_message_printed?(fn -> Onigumo.CLI.main([]) end)
     end
 
     test("run CLI with more than one argument") do
-      assert_raise(MatchError, fn -> Onigumo.CLI.main(["Downloader", "Parser"]) end)
+      assert usage_message_printed?(fn -> Onigumo.CLI.main(["Downloader", "Parser"]) end)
     end
 
     test("run CLI with invalid switch") do
-      assert_raise(OptionParser.ParseError, fn -> Onigumo.CLI.main(["--help"]) end)
+      assert usage_message_printed?(fn -> Onigumo.CLI.main(["--invalid"]) end)
+    end
+
+    defp usage_message_printed?(function) do
+      output = capture_io(function)
+      String.starts_with?(output, "Usage: onigumo ")
     end
   end
 end