module InterpA where

-- The interpreter monad
-- (Generated by MonadLab)
import InterpMonadA

-- Language syntax
type Name = String
data Term = Var Name | Con Int | Add Term Term
          | Lam Name Term | App Term Term

mkfun :: Name -> M V -> M V
mkfun x phi =
   rdEnvM >>= \ e ->
     return (Fun $ \ arg -> inEnvM ((x,arg):e) phi)

appEnv :: Name -> M V
appEnv x = rdEnvM >>= \ e ->
           case lookup x e of
              Nothing  -> return Wrong
              (Just v) -> return v

apply :: V -> V -> M V
apply (Fun k) a = k a
apply _ _       = return Wrong

-- The interpreter
add :: V -> V -> M V
add (Num i) (Num j) = return (Num (i+j))
add _ _             = return Wrong

interp :: Term -> M V
interp (Var x)   = appEnv x
interp (Con i)   = return (Num i)
interp (Add u v) = interp u >>= \ a ->
                   interp v >>= \ b ->
                      add a b
interp (Lam x v) = mkfun x (interp v)
interp (App t u) = interp t >>= \ f -> 
                   interp u >>= \ a -> 
                      apply f a
                      
test :: Term -> V
test t = runM (interp t) initEnv
         where initEnv = []