Adjust old type checker to new syntax, and refactor lambda lifter to use typed AST

src/Auxiliary.hs

@@ -1,4 +1,4 @@
-
+{-# LANGUAGE LambdaCase #-}
 module Auxiliary (module Auxiliary) where
 import           Control.Monad.Error.Class (liftEither)
 import           Control.Monad.Except      (MonadError)
@@ -9,3 +9,13 @@ snoc x xs = xs ++ [x]
 
 maybeToRightM :: MonadError l m => l -> Maybe r -> m r
 maybeToRightM err = liftEither . maybeToRight err
+
+mapAccumM :: Monad m => (s -> a -> m (s, b)) -> s -> [a] -> m (s, [b])
+mapAccumM f = go
+  where
+    go acc = \case
+      [] -> pure (acc, [])
+      x:xs -> do
+        (acc', x') <- f acc x
+        (acc'', xs') <- go acc' xs
+        pure (acc'', x':xs')