Implement commercialhaskell#3264 adding --cwd to exec

Welcome for any suggestions as to better methods to use/docs to update.

src/Stack/Options/ExecParser.hs

@@ -43,6 +43,7 @@ execOptsExtraParser = eoPlainParser <|>
                          <$> eoEnvSettingsParser
                          <*> eoPackagesParser
                          <*> eoRtsOptionsParser
+                         <*> eoCwdParser
   where
     eoEnvSettingsParser :: Parser EnvSettings
     eoEnvSettingsParser = EnvSettings
@@ -70,3 +71,9 @@ execOptsExtraParser = eoPlainParser <|>
     eoPlainParser = flag' ExecOptsPlain
                           (long "plain" <>
                            help "Use an unmodified environment (only useful with Docker)")
+
+    eoCwdParser :: Parser (Maybe FilePath)
+    eoCwdParser = optional
+                  (strOption (long "cwd"
+                             <> help "Sets the working directory before executing")
+                  )

src/Stack/Types/Config.hs

@@ -430,6 +430,7 @@ data ExecOptsExtra
         { eoEnvSettings :: !EnvSettings
         , eoPackages :: ![String]
         , eoRtsOptions :: ![String]
+        , eoCwd :: !(Maybe FilePath)
         }
     deriving (Show)
 

src/main/Main.hs

@@ -105,7 +105,7 @@ import qualified Stack.Upload as Upload
 import qualified System.Directory as D
 import           System.Environment (getProgName, getArgs, withArgs)
 import           System.Exit
-import           System.FilePath (pathSeparator)
+import           System.FilePath (isRelative, isValid, pathSeparator)
 import           System.IO (hIsTerminalDevice, stderr, stdin, stdout, hSetBuffering, BufferMode(..), hPutStrLn, hGetEncoding, hSetEncoding)
 
 -- | Change the character encoding of the given Handle to transliterate
@@ -767,7 +767,8 @@ execCmd ExecOpts {..} go@GlobalOpts{..} =
                     (ExecRunGhc, args) ->
                         getGhcCmd "run" menv eoPackages args
                 munlockFile lk -- Unlock before transferring control away.
-                exec menv cmd args
+
+                runWithPath eoCwd $ exec menv cmd args
   where
       -- return the package-id of the first package in GHC_PACKAGE_PATH
       getPkgId menv wc name = do
@@ -788,6 +789,20 @@ execCmd ExecOpts {..} go@GlobalOpts{..} =
           pkgopts <- getPkgOpts menv wc pkgs
           return (prefix ++ compilerExeName wc, pkgopts ++ args)
 
+      runWithPath path callback = case path of
+        Nothing -> callback
+        Just p | not (isValid p) -> callback
+        Just p                   ->
+          if isRelative p
+          then parseRelDir p >>= runInDirectory
+          else parseAbsDir p >>= runInDirectory
+          where
+            runInDirectory :: (Path t Dir) -> RIO EnvConfig ()
+            runInDirectory directory =
+              withUnliftIO $ \unlift ->
+                withCurrentDir directory $ unliftIO unlift callback
+
+
 -- | Evaluate some haskell code inline.
 evalCmd :: EvalOpts -> GlobalOpts -> IO ()
 evalCmd EvalOpts {..} go@GlobalOpts {..} = execCmd execOpts go