bind.hs

-----------------------------                                             -- 001
-- Simple Functional Language                                             -- 002
-----------------------------                                             -- 003
   
-- variables are just names                                               -- 006
type Var = String                                                         -- 007
                                                                          -- 008
-- values are integers and functions                                      -- 009
data Value = ValInt Integer                                               -- 010
           | ValFunc (Value -> Value)                                     -- 011
           | ValError String                                              -- 012
                                                                          -- 013
                                                                          -- 014
-- an Environment maps variables to Values                                -- 015
type Env = Var -> Value      

-- An empty Environment                                                 
emptyEnv :: Env                                                           
emptyEnv v = ValError ("undefined variable " ++ v)

-- bind a new value in an environment                                     -- 029
bind :: Var -> Value -> Env -> Env                                        -- 030
bind var val env = \v -> if var == v then val else env v