Further library additions and REPL updates

src/REPL.hs

@@ -14,15 +14,15 @@ repl env = do
   putStr "sapling < "
   hFlush stdout
   input <- getLine
-  if input == "_:exit"
-    then putStrLn "Goodbye!"
-  else if input == ""
-    then do
+  case input of
+    ":_exit" ->
+      putStrLn "Goodbye!"
+    ""       -> do
       putStrLn ""
       repl env
-    else do
+    _ -> do
       let clearEnv = Map.delete "__result" env
-      let newEnv   = evalSingle clearEnv (parseSingle input)
+          newEnv   = evalSingle clearEnv (parseSingle input)
       case Map.lookup "__result" newEnv of
         Just r  -> do
           putStrLn $ "sapling > " ++ show r
@@ -36,4 +36,4 @@ decodeResult tc =
     Right str -> "\"" ++ str ++ "\""
     Left _ -> case ofNumber tc of
       Right num -> "# " ++ show num
-      Left _ -> "Failed to decode number from Tree"
+      Left _ -> ""