module Env where

-- | Variable names.
type Var = String

-- | An environment mapping variables to integers.
type Env = Var -> Maybe Int

-- | An empty environment.
empty :: Env
empty = \x -> Nothing

-- | Set a variable to a value in the environment.
set :: Var -> Int -> Env -> Env
set x i m = \y -> if x == y then Just i else m y

-- | Lookup the value for a variable in the environment.
--
--   >>> get "x" (set "x" 3 (set "y" 4 empty))
--   Just 3
--
--   >>> get "y" (set "x" 3 (set "y" 4 empty))
--   Just 4
--   
--   >>> get "z" (set "x" 3 (set "y" 4 empty))
--   Nothing
--
get :: Var -> Env -> Maybe Int
get x m = m x

-- | Remove a variable from the environment.
-- 
--   >>> get "y" (unset "y" (set "x" 3 (set "y" 4 empty)))
--   Nothing
--
--   >>> get "x" (unset "y" (set "x" 3 (set "y" 4 empty)))
--   Just 3
--
unset :: Var -> Env -> Env
unset x m = \y -> if x == y then empty y else m y