Merge pull request #5786 from mpilgrem/fix809

Fix #809 Use `showHelpOnEmpty`

Author: mpilgrem

ChangeLog.md

@@ -29,6 +29,9 @@ Other enhancements:
   comment in `Script.hs` on the command line. That may help test that scripts
   compile in CI (continuous integration). See
   [#5755](https://github.com/commercialhaskell/stack/issues/5755)
+* Fuller help is provided at the command line if a subcommand is missing (for
+  example, `stack ls` now yields the equivalent of `stack ls --help`). See
+  [#809](https://github.com/commercialhaskell/stack/issues/809)
 
 Bug fixes:
 

src/Options/Applicative/Complicated.hs

@@ -48,8 +48,8 @@ complicatedOptions
   -> IO (a,b)
 complicatedOptions numericVersion stringVersion numericHpackVersion h pd footerStr commonParser mOnFailure commandParser =
   do args <- getArgs
-     (a,(b,c)) <- case execParserPure (prefs noBacktrack) parser args of
-       Failure _ | null args -> withArgs ["--help"] (execParser parser)
+     (a,(b,c)) <- let parserPrefs = prefs $ noBacktrack <> showHelpOnEmpty
+                  in  case execParserPure parserPrefs parser args of
        -- call onFailure handler if it's present and parsing options failed
        Failure f | Just onFailure <- mOnFailure -> onFailure f args
        parseResult -> handleParseResult parseResult