Show as "hpack-dhall", empty list of warnings for decodeDhall, NixOS#374

.

src/Distribution/Nixpkgs/Haskell/PackageSourceSpec.hs

@@ -49,7 +49,7 @@ data CabalGen
 
 instance Show CabalGen where
   show CabalGenHpack = "hpack"
-  show CabalGenDhall = "dhall"
+  show CabalGenDhall = "hpack-dhall"
 
 getPackage :: Maybe CabalGen
            -- ^ Whether and how to regenerate the cabal file.
@@ -227,7 +227,7 @@ hpackDirectory' :: CabalGen
 hpackDirectory' gen optsDecode = do
   mPackage <- liftIO $ Hpack.readPackageConfig optsDecode
   case mPackage of
-    Left err -> liftIO $ hPutStrLn stderr ("*** hpack error: " ++ show err ++ ". Exiting.") >> exitFailure
+    Left err -> liftIO $ hPutStrLn stderr ("*** " ++ show gen ++ " error: " ++ show err ++ ". Exiting.") >> exitFailure
     Right r -> do
       let hpackOutput = encodeUtf8 $ Hpack.renderPackage [] (Hpack.decodeResultPackage r)
           hash = printSHA256 $ digest (digestByName "sha256") hpackOutput

src/Hpack/Dhall.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE NoMonomorphismRestriction #-}
+{-# LANGUAGE TupleSections #-}
 module Hpack.Dhall where
 
 import Control.Monad.Trans.Except
@@ -12,11 +13,12 @@ import qualified Dhall.TypeCheck
 import qualified Dhall.JSON
 
 -- SEE: https://github.com/sol/hpack-dhall/blob/master/src/Hpack/Dhall.hs
-decodeDhall :: FilePath -> IO (Either String Value)
+decodeDhall :: FilePath -> IO (Either String ([String], Value))
 decodeDhall file = runExceptT $ do
   expr <- readInput >>= parseExpr >>= liftIO . Dhall.Import.load
   _ <- liftResult $ Dhall.TypeCheck.typeOf expr
-  liftResult $ Dhall.JSON.dhallToJSON expr
+  -- NOTE: dhallToJSON returns no warnings hence the empty list of warnings.
+  liftResult $ ([],) <$> Dhall.JSON.dhallToJSON expr
   where
     readInput = liftIO (T.readFile file)
     parseExpr = liftResult . Dhall.Parser.exprFromText file