From 1cc5b0b03a408cd197974c19d407d4fc28dece74 Mon Sep 17 00:00:00 2001
From: darkf <lw9k123@gmail.com>
Date: Thu, 17 Oct 2013 18:58:41 -0700
Subject: [PATCH] initial commit

---
 interp.hs | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)
 create mode 100644 interp.hs

diff --git a/interp.hs b/interp.hs
new file mode 100644
index 0000000..71c0c27
--- /dev/null
+++ b/interp.hs
@@ -0,0 +1,53 @@
+import Control.Monad.State (State, runState, evalState, get, put)
+import qualified Data.Map as M
+
+data AST = Add AST AST
+		 | Def String AST
+		 | Var String
+		 | StrConst String
+		 | IntConst Int
+		 deriving (Show, Eq)
+
+data Value = IntV Int
+		   | StrV String
+		   deriving (Show, Eq)
+
+type Env = M.Map String Value -- an environment
+type InterpState = State Env -- interpreter state (pass along the global environment)
+
+(IntV l) +$ (IntV r) = IntV (l + r)
+(StrV l) +$ (StrV r) = StrV (l ++ r)
+l +$ r = error $ "cannot + " ++ show l ++ " and " ++ show r
+
+eval :: AST -> InterpState Value
+
+eval (IntConst i) = return $ IntV i
+eval (StrConst s) = return $ StrV s
+
+eval (Var var) = get >>= \m ->
+  case M.lookup var m of
+    Just v -> return v
+    Nothing -> error $ "unbound variable " ++ var
+
+eval (Def name v') = do
+	v <- eval v'
+	m <- get
+	put $ M.insert name v m
+	return v
+
+eval (Add l r) = do
+	l <- eval l
+	r <- eval r
+	return $ l +$ r
+
+evalProgram :: [AST] -> Value -- fold the state from each node and return the result
+evalProgram nodes = evalState (foldr1 (>>) $ map eval nodes) M.empty
+
+main = do
+	print $ evalProgram prg
+	print $ evalProgram prg2
+	where
+		prg = [Def "x" (IntConst 5),
+			   Def "y" (IntConst 3),
+			   Add (Var "x") (Var "y")]
+		prg2 = [Add (StrConst "hi ") (StrConst "there")]
\ No newline at end of file