1 files changed, 28 insertions, 7 deletions
diff --git a/lambda-calcul/haskell/src/Minilang/REPL.hs b/lambda-calcul/haskell/src/Minilang/REPL.hs
index a8d3c77..7da2042 100644
--- a/lambda-calcul/haskell/src/Minilang/REPL.hs
+++ b/lambda-calcul/haskell/src/Minilang/REPL.hs
@@ -23,6 +23,8 @@ import Minilang.REPL.Pure
 import Minilang.REPL.Purer
 import Minilang.REPL.Run
 import Minilang.REPL.Types
+import System.Environment (lookupEnv)
+import Text.Read (readMaybe)
 import System.Console.Haskeline
   ( defaultBehavior,
     defaultSettings,
@@ -30,13 +32,32 @@ import System.Console.Haskeline
     runInputTBehavior,
     setComplete,
   )
-import System.IO (Handle)
+import System.IO (Handle, stderr)
+
+-- | Read the evaluator selection from the @MINILANG_EVAL@ environment variable.
+-- Returns 'DeBruijn' when the variable is absent or unrecognised.
+evaluatorFromEnv :: IO Evaluator
+evaluatorFromEnv =
+  lookupEnv "MINILANG_EVAL" >>= \case
+    Nothing  -> pure DeBruijn
+    Just raw -> case readMaybe raw of
+      Just ev -> pure ev
+      Nothing -> do
+        Text.hPutStrLn stderr $
+          "Warning: unknown MINILANG_EVAL value " <> Text.pack (show raw)
+          <> ", falling back to DeBruijn"
+        pure DeBruijn
+
+-- | Build the initial 'REPLEnv' using the given 'Evaluator'.
+replEnvWith :: Evaluator -> REPLEnv
+replEnvWith ev = initialREPL { evaluator = ev }
 
 -- * Haskeline REPL
 
-withTerminal :: IO ()
-withTerminal =
-  runInputTBehavior defaultBehavior settings $ evalStateT (runHaskeline runREPL) initialREPL
+withTerminal :: Evaluator -> IO ()
+withTerminal ev =
+  runInputTBehavior defaultBehavior settings $
+    evalStateT (runHaskeline runREPL) (replEnvWith ev)
   where
     settings = setComplete completion $ defaultSettings {historyFile = Just "~/.minilang.history"}
 
@@ -50,12 +71,12 @@ withHandles hin hout =
   evalStateT (runConsole runREPL) (IOEnv hin hout initialREPL)
 
 -- | Simple IO REPL, used to evaluate programs from stdin or file loading
-withIO :: Handle -> Handle -> IO ()
-withIO hin hout = do
+withIO :: Evaluator -> Handle -> Handle -> IO ()
+withIO ev hin hout = do
   programText <- parseProgram <$> Text.hGetContents hin
   case programText of
     Left err -> Text.hPutStrLn hout (Text.pack (show err))
-    Right prog -> case loadProgram Untyped newEnv newContext prog of
+    Right prog -> case loadProgram ev Untyped newEnv newContext prog of
       Right (results, _, _) -> mapM_ (Text.hPutStrLn hout . render) results
       Left err -> Text.hPutStrLn hout (Text.pack $ show err)
   where