Add list command (undocumented for now)

driver/Main.hs

@@ -1,9 +1,21 @@
+{-# LANGUAGE LambdaCase #-}
 module Main (main) where
 
 import           System.Environment
 
 import qualified Hpack
-import qualified Hpack.Config as Hpack
+import           Hpack.Config
+import           Control.Exception
 
 main :: IO ()
-main = getArgs >>= Hpack.getOptions Hpack.packageConfig >>= mapM_ (uncurry Hpack.hpack)
+main = getArgs >>= \ case
+  ["list"] -> exposedModules packageConfig >>= mapM_ (putStrLn . unModule)
+  args -> Hpack.getOptions packageConfig args >>= mapM_ (uncurry Hpack.hpack)
+
+exposedModules :: FilePath -> IO [Module]
+exposedModules file = readPackageConfig defaultDecodeOptions {decodeOptionsTarget = file} >>= \ case
+  Left err -> throwIO $ ErrorCall err
+  Right result -> return $ modules result
+  where
+    modules :: DecodeResult -> [Module]
+    modules = maybe [] (libraryExposedModules . sectionData) . packageLibrary . decodeResultPackage