working on interpreter

src/Interpreter.hs

@@ -0,0 +1,67 @@
+{-# LANGUAGE FlexibleContexts #-}
+module Interpreter where
+import           Control.Monad.Reader
+import           Control.Monad.State
+
+data Exp a =
+      Var String
+    | Op (Operator a) (Exp a) (Exp a)
+    | Let String (Exp a) (Exp a)
+    | Val a
+
+type Operator a =  a -> a -> a
+
+type Env a = [(String, a)]
+
+-- using a Reader Monad to thread the environment. The Environment can be accessed by ask.
+--eval :: Real a => Exp a -> Env a -> a
+eval (Var x)       = asks (fetch x)
+eval (Val i)       = return i
+eval (Op op e1 e2) = liftM2 op (eval e1) (eval e2)
+eval (Let x e1 e2) = asks (eval e1) >>= \v -> local ((x,v):) (eval e2)
+
+    
+{--
+    env <- ask
+    v   <- eval e1
+    local ((x,v):) (eval e2)
+--}
+
+{--
+--eval5 :: Num a => Exp a -> Env a -> a
+eval5 :: (MonadState (Env a) m, Num a) => Exp a -> m a
+eval5 (Var x)   = 
+    gets (fetch x)
+eval5 (Def k v) = do
+    env <- get
+    put ((k,v):env)
+    return v
+eval5 (Val i)   = return i
+eval5 (Add p q) = liftM2 (+) (eval5 p) (eval5 q)
+eval5 (Mul p q) = liftM2 (*) (eval5 p) (eval5 q)
+--}
+
+-- simple environment lookup
+fetch :: String -> Env a -> a
+fetch x []        = error $ "variable " ++ x ++ " is not defined"
+fetch x ((y,v):ys)
+    | x == y    = v
+    | otherwise = fetch x ys
+
+interpreterDemo :: IO ()
+interpreterDemo = do
+    putStrLn "Interpreter -> Reader Monad + ADTs + pattern matching"
+    let exp = Let "x" 
+                (Let "y" 
+                    (Op (+) (Val 5) (Val 6))
+                    (Op (/) (Var "y") (Val 5)))
+                (Op (*) (Val 3) (Var "x"))
+        env = [("pi", pi)]
+    print $ eval exp env
+    --print $ runReader (eval exp) env
+    --print $ eval5 exp env
+
+
+    --print $ eval5 exp (put env :: State (Env Double) ())
+
+    putStrLn ""

src/Singleton.hs

@@ -1,12 +1,9 @@
 {-# LANGUAGE FlexibleContexts #-}
 module Singleton where
 import           IdiomBrackets
-import           Control.Monad.Reader
-import           Control.Monad.State
 
 data Exp a =
       Var String
-    | Def String a
     | Val a
     | Add (Exp a) (Exp a)
     | Mul (Exp a) (Exp a) deriving (Show)
@@ -58,26 +55,6 @@ eval3 (Val i)   = iI i Ii
 eval3 (Add p q) = iI (+) (eval3 p) (eval3 q) Ii
 eval3 (Mul p q) = iI (*) (eval3 p) (eval3 q) Ii
 
--- using a Reader Monad to thread the environment. The Environment can be accessed by ask.
-eval4 :: Num a => Exp a -> Env a -> a
-eval4 (Var x)   = ask >>= return $ fetch x
-eval4 (Val i)   = return i
-eval4 (Add p q) = liftM2 (+) (eval4 p) (eval4 q)
-eval4 (Mul p q) = liftM2 (*) (eval4 p) (eval4 q)
-
---eval5 :: Num a => Exp a -> Env a -> a
-eval5 :: (MonadState (Env a) m, Num a) => Exp a -> m a
-eval5 (Var x)   = do 
-    env <- get 
-    return $ fetch x env
-eval5 (Def k v) = do
-    env <- get
-    put ((k,v):env)
-    return v
-eval5 (Val i)   = return i
-eval5 (Add p q) = liftM2 (+) (eval5 p) (eval5 q)
-eval5 (Mul p q) = liftM2 (*) (eval5 p) (eval5 q)
-
 -- simple environment lookup
 fetch :: String -> Env a -> a
 fetch x []        = error $ "variable " ++ x ++ " is not defined"
@@ -85,9 +62,6 @@ fetch x ((y,v):ys)
     | x == y    = v
     | otherwise = fetch x ys
 
-exp1 = Mul (Add (Def "pi" pi) (Val 1))
-    (Mul (Val 2) (Var "pi"))
-
 singletonDemo :: IO ()
 singletonDemo = do
     putStrLn "Singleton -> Applicative Functor (and let in general)"
@@ -99,10 +73,4 @@ singletonDemo = do
     print $ eval2 exp env
     print $ eval3 exp env
 
-    print $ eval4 exp env
-
-    let exp1 = Mul (Add (Def "pi" 4) (Val 1))
-                  (Mul (Val 2) (Var "pi"))
-   -- print $ eval5 exp (state env)
-
     putStrLn ""