0.2.0

Includes better error handling, additional tests, parsing and lexing
fixes to match the desired behavior defined by the new tests, and a very
basic REPL implementation.

src/REPL.hs

@@ -0,0 +1,25 @@
+module REPL where
+
+import Eval
+import Lexer
+import Parser
+import Research
+
+import Control.Monad         (void)
+import qualified Data.Map as Map
+import System.IO            (hFlush, stdout)
+
+repl :: Map.Map String T -> IO ()
+repl env = do
+  putStr "sapling > "
+  hFlush stdout
+  input <- getLine
+  if input == "_:exit"
+    then putStrLn "Goodbye!"
+    else do
+      let clearEnv = Map.delete "__result" env
+      let newEnv   = evalSingle clearEnv (parseSingle input)
+      case Map.lookup "__result" newEnv of
+        Just r  -> putStrLn $ "sapling < " ++ show r
+        Nothing -> pure ()
+      repl newEnv