Transformers.hs

module Transformers where
import Control.Monad.Identity
import Control.Monad.Error
import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.Writer
import Data.Maybe
import qualified Data.Map as Map

--
-- http://bicycle1885.hatenablog.com/entry/2012/12/08/165236
--
type Name = String -- variable names
data Exp = Lit Integer -- expressions
         | Var Name
         | Plus Exp Exp
         | Abs Name Exp
         | App Exp Exp
         deriving (Show)
data Value = IntVal Integer --values
           | FunVal Env Name Exp
           deriving (Show)
type Env = Map.Map Name Value -- mapping from names to values

eval0 :: Env -> Exp -> Value
eval0 env (Lit i) = IntVal i
eval0 env (Var n) = fromJust (Map.lookup n env)
eval0 env (Plus e1 e2) = let IntVal i1 = eval0 env e1
                             IntVal i2 = eval0 env e2
                         in IntVal (i1 + i2)
eval0 env (Abs n e) = FunVal env n e
eval0 env (App e1 e2) = let val1 = eval0 env e1
                            val2 = eval0 env e2
                        in case val1 of
                            FunVal env' n body -> eval0 (Map.insert n val2 env') body