Organize code into a Lambda module

interactive.hs

@@ -0,0 +1,26 @@
+import qualified Lambda as L
+import Data.Maybe
+
+-- Helper type for using arbitrary strings as symbols
+newtype StrSymbol = StrSymbol String
+
+instance Show StrSymbol where
+  show (StrSymbol s) = s
+
+instance Eq StrSymbol where
+  (StrSymbol a) == (StrSymbol b) = a == b
+
+linewise :: (String -> String) -> String -> String
+linewise f = unlines . map f . lines
+
+evaluate :: String -> String
+evaluate s =
+  let result = return
+             . map (L.display True)
+             . L.evaluate
+             . fmap StrSymbol
+      l = fromMaybe ["Error: Could not parse expression."]
+        $ L.parseMaybe s >>= result
+  in  unlines l
+
+main = interact $ linewise evaluate