Make arion repl work

src/haskell/exe/Main.hs

@@ -70,9 +70,9 @@ textArgument = fmap T.pack . strArgument
 parseCommand :: Parser (CommonOptions -> IO ())
 parseCommand =
   hsubparser
-    (    command "cat" (info (pure runCat) (progDesc "TODO: cat doc" <> fullDesc))
-      <> command "repl" (info (pure runRepl) (progDesc "TODO: repl doc" <> fullDesc))
-      <> command "exec" (info (pure runExec) (progDesc "TODO: exec doc" <> fullDesc))
+    (    command "cat" (info (pure runCat) (progDesc "Spit out the docker compose file as JSON" <> fullDesc))
+      <> command "repl" (info (pure runRepl) (progDesc "Start a nix repl for the whole composition" <> fullDesc))
+      -- <> command "exec" (info (pure runExec) (progDesc "TODO: exec doc" <> fullDesc))
     )
   <|>
   hsubparser
@@ -172,8 +172,18 @@ runCat co = do
   T.hPutStrLn stdout (pretty v)
 
 runRepl :: CommonOptions -> IO ()
-runRepl opts =
-  T.putStrLn "Running repl ... TODO"
+runRepl co = do
+  putErrText
+    "Launching a repl for you. To get started:\n\
+    \\n\
+    \To see deployment-wide configuration\n\
+    \  type config. and hit TAB\n\
+    \To see the services\n\
+    \  type config.docker-compose.evaluatedServices TAB or ENTER\n\
+    \To bring the top-level Nixpkgs attributes into scope\n\
+    \  type :a (config._module.args.pkgs) // { inherit config; }\n\
+    \"
+  Arion.Nix.replForComposition =<< defaultEvaluationArgs co
 
 runExec :: CommonOptions -> IO ()
 runExec opts =

src/haskell/lib/Arion/DockerCompose.hs

@@ -40,7 +40,6 @@ run args = do
 
   withCreateProcess procSpec $ \_in _out _err procHandle -> do
 
-    -- Wait for process exit and 'err' printout
     exitCode <- waitForProcess procHandle
 
     case exitCode of

src/haskell/lib/Arion/Nix.hs

@@ -2,7 +2,9 @@
 module Arion.Nix
   ( evaluateComposition
   , withEvaluatedComposition
+  , buildComposition
   , withBuiltComposition
+  , replForComposition
   , EvaluationArgs(..)
   , EvaluationMode(..)
   ) where
@@ -137,6 +139,27 @@ withBuiltComposition ea f = do
     buildComposition path ea
     f path
 
+
+replForComposition :: EvaluationArgs -> IO ()
+replForComposition ea = do
+    evalComposition <- getEvalCompositionFile
+    let args =
+          [ "repl", evalComposition ]
+          ++ argArgs ea
+          ++ map toS (evalUserArgs ea)
+        procSpec = (proc "nix" args) { cwd = evalWorkDir ea }
+    
+    withCreateProcess procSpec $ \_in _out _err procHandle -> do
+
+      exitCode <- waitForProcess procHandle
+
+      case exitCode of
+        ExitSuccess -> pass
+        ExitFailure 1 -> exitFailure
+        e@ExitFailure {} -> do
+          throwIO $ FatalError $ "nix repl failed with " <> show exitCode
+          exitWith e
+
 argArgs :: EvaluationArgs -> [[Char]]
 argArgs ea =
       [ "--argstr"