Hacky workaround for optparse-applicative issue with stack exec --help (

fixes #806)

Downside: usage information now looks like:

Usage: stack exec [CMD -- ARGS (e.g. stack ghc -- X.hs -o x)]

Notice how "CMD" is inside the square brackets. Still seems like a
worthwhile tradeoff to me.

ChangeLog.md

@@ -31,6 +31,7 @@ Bug fixes:
 * Create missing directories for `stack sdist`
 * Don't ignore .cabal files with extra periods [#895](https://github.com/commercialhaskell/stack/issues/895)
 * Deprecate unused `--optimizations` flag
+* Hacky workaround for optparse-applicative issue with `stack exec --help` [#806](https://github.com/commercialhaskell/stack/issues/806)
 
 ## 0.1.3.1
 

src/Stack/Options.hs

@@ -445,18 +445,19 @@ execOptsParser :: Maybe String -- ^ command
                -> Parser ExecOpts
 execOptsParser mcmd =
     ExecOpts
-        <$> maybe eoCmdParser pure mcmd
+        <$> pure mcmd
         <*> eoArgsParser
         <*> (eoPlainParser <|>
              ExecOptsEmbellished
                 <$> eoEnvSettingsParser
                 <*> eoPackagesParser)
   where
-    eoCmdParser :: Parser String
-    eoCmdParser = strArgument (metavar "CMD")
-
     eoArgsParser :: Parser [String]
-    eoArgsParser = many (strArgument (metavar "-- ARGS (e.g. stack ghc -- X.hs -o x)"))
+    eoArgsParser = many (strArgument (metavar meta))
+      where
+        meta =
+            (maybe ("CMD ") (const "") mcmd) ++
+            "-- ARGS (e.g. stack ghc -- X.hs -o x)"
 
     eoEnvSettingsParser :: Parser EnvSettings
     eoEnvSettingsParser = EnvSettings

src/Stack/Types/Config.hs

@@ -196,7 +196,10 @@ data EnvSettings = EnvSettings
     deriving (Show, Eq, Ord)
 
 data ExecOpts = ExecOpts
-    { eoCmd :: !String
+    { eoCmd :: !(Maybe String)
+    -- ^ Usage of @Maybe@ here is nothing more than a hack, to avoid some weird
+    -- bug in optparse-applicative. See:
+    -- https://github.com/commercialhaskell/stack/issues/806
     , eoArgs :: ![String]
     , eoExtra :: !ExecOptsExtra
     }

src/main/Main.hs

@@ -761,19 +761,24 @@ sdistCmd dirs go =
 
 -- | Execute a command.
 execCmd :: ExecOpts -> GlobalOpts -> IO ()
-execCmd ExecOpts {..} go@GlobalOpts{..} =
+execCmd ExecOpts {..} go@GlobalOpts{..} = do
+    (cmd, args) <-
+        case (eoCmd, eoArgs) of
+            (Just cmd, args) -> return (cmd, args)
+            (Nothing, cmd:args) -> return (cmd, args)
+            (Nothing, []) -> error "You must provide a command to exec, e.g. 'stack exec echo Hello World'"
     case eoExtra of
         ExecOptsPlain -> do
             (manager,lc) <- liftIO $ loadConfigWithOpts go
             withUserFileLock (configStackRoot $ lcConfig lc) $ \lk ->
              runStackTGlobal manager (lcConfig lc) go $
                 Docker.execWithOptionalContainer
                     (lcProjectRoot lc)
-                    (return (eoCmd, eoArgs, [], id))
+                    (return (cmd, args, [], id))
                     -- Unlock before transferring control away, whether using docker or not:
                     (Just $ liftIO $ unlockFile lk)
                     (runStackTGlobal manager (lcConfig lc) go $ do
-                        exec plainEnvSettings eoCmd eoArgs)
+                        exec plainEnvSettings cmd args)
                     Nothing
                     Nothing -- Unlocked already above.
         ExecOptsEmbellished {..} ->
@@ -784,7 +789,7 @@ execCmd ExecOpts {..} go@GlobalOpts{..} =
                        { boptsTargets = map T.pack targets
                        }
                liftIO $ unlockFile lk -- Unlock before transferring control away.
-               exec eoEnvSettings eoCmd eoArgs
+               exec eoEnvSettings cmd args
 
 -- | Run GHCi in the context of a project.
 ghciCmd :: GhciOpts -> GlobalOpts -> IO ()