diff --git a/ast.hs b/ast.hs
new file mode 100644
index 0000000..cf216ea
--- /dev/null
+++ b/ast.hs
@@ -0,0 +1,22 @@
+module AST where
+
+data AST = Add AST AST
+		 | Mul AST AST
+		 | Block [AST]
+		 | FunDef String (Pattern, AST)
+		 | Def String AST
+		 | Var String
+		 | Lambda [(Pattern, [AST])]
+		 | Call String [AST]
+		 | UnitConst
+		 | ListConst [AST]
+		 | StrConst String
+		 | IntConst Integer
+		 deriving (Show, Eq)
+
+data Pattern = VarP String
+			 | IntP Integer
+			 | UnitP
+			 | ConsP Pattern Pattern
+			 | ListP [Pattern]
+	 deriving (Show, Eq)
\ No newline at end of file
diff --git a/interp.hs b/interp.hs
index 26fef7e..620df2a 100644
--- a/interp.hs
+++ b/interp.hs
@@ -1,24 +1,11 @@
 import Control.Monad.State (State, runState, evalState, get, put)
 import qualified Data.Map as M
+import AST
+import Parser (parseProgram)
 
-data AST = Add AST AST
-		 | Def String AST
-		 | Var String
-		 | Lambda [(Pattern, [AST])]
-		 | Call String [AST]
-		 | ListConst [AST]
-		 | StrConst String
-		 | IntConst Int
-		 deriving (Show, Eq)
-
-data Pattern = VarP String
-			 | IntP Int
-			 | ConsP Pattern Pattern
-			 | ListP [Pattern]
-	 deriving (Show, Eq)
-
-data Value = IntV Int
+data Value = IntV Integer
 		   | StrV String
+		   | UnitV
 		   | ListV [Value]
 		   | FnV [(Pattern, [AST])] -- pattern->body bindings
 		   deriving (Show, Eq)
@@ -37,6 +24,8 @@ eval :: AST -> InterpState Value
 eval (IntConst i) = return $ IntV i
 eval (StrConst s) = return $ StrV s
 
+eval UnitConst = return UnitV
+
 eval (ListConst v) =
 	mapM eval v >>= \xs ->
 		return $ ListV xs
@@ -76,6 +65,9 @@ patternBindings (IntP n) (IntV v)
 	| otherwise = Nothing
 patternBindings (IntP n) _ = Nothing
 
+patternBindings UnitP UnitV = Just M.empty
+patternBindings UnitP _ = Nothing
+
 patternBindings (ConsP x (ListP [])) (ListV (y:[])) = patternBindings x y
 patternBindings (ConsP _ _) (ListV (_:[])) = Nothing
 patternBindings (ConsP xp xsp) (ListV (x:xs)) =