not evaluating in hnix executable, only parsing

hnix.cabal

@@ -59,7 +59,7 @@ Library
 
 Executable hnix
   Default-language: Haskell2010
-  Main-is: Nix.hs
+  Main-is: Main.hs
   Hs-source-dirs: main
   Default-extensions:
     DataKinds

main/Main.hs

@@ -0,0 +1,31 @@
+{-# LANGUAGE LambdaCase #-}
+module Main where
+
+import Nix.Parser
+import Nix.Pretty
+
+import System.Environment
+import System.IO
+import Text.PrettyPrint.ANSI.Leijen
+
+nix :: FilePath -> IO ()
+nix path = do
+  res <- parseNixFile path
+  case res of
+    Failure e -> hPutStrLn stderr $ "Parse failed: " ++ show e
+    Success n -> do
+      displayIO stdout $ renderPretty 0.4 80 (prettyNix n)
+
+main :: IO ()
+main = do
+  let usageStr = "Parses a nix file and prints to stdout.\n\
+                 \\n\
+                 \Usage:\n\
+                 \  hnix --help\n\
+                 \  hnix <path>\n"
+  let argErr msg = error $ "Invalid arguments: " ++ msg ++ "\n" ++ usageStr
+  getArgs >>= \case
+    "--help":_ -> putStrLn usageStr
+    ('-':_):_ -> argErr "Provide a path to a nix file."
+    path:_ -> nix path
+    _ -> argErr "Provide a path to a nix file."

main/Nix.hs

@@ -1,28 +0,0 @@
-module Main where
-
-import Nix.Parser
-import Nix.Pretty
-import Nix.Eval
-import Nix.Types
-
-import Data.Fix
-import System.Environment
-import System.IO
-import Text.PrettyPrint.ANSI.Leijen
-
-import qualified Data.Map as Map
-
-nix :: FilePath -> IO ()
-nix path = do
-  res <- parseNixFile path
-  case res of
-    Failure e -> hPutStrLn stderr $ "Parse failed: " ++ show e
-    Success n -> do
-      displayIO stdout $ renderPretty 0.4 80 (prettyNix n)
-      putStrLn ""
-      evalExpr n (Fix $ NVSet Map.empty) >>= print
-
-main :: IO ()
-main = do
-    [path] <- getArgs
-    nix path