module skeleton12

import StdEnv
import StdMaybe

:: BM a b = {t :: a -> b, f :: b -> a}                                          
bm :: BM a a
bm = {f=id, t=id}
:: Expr a
	= Lit a
	| (+.) infixl 6 (Expr a) (Expr a)
	| (*.) infixl 6 (Expr a) (Expr a)
	| Read
	| Write (Expr Int)
	| XOR (Expr Bool) (Expr Bool)
	| Not (Expr Bool)
	| E.b: Eq (Expr b) ( Expr b) & == b
	| Throw
	| Try (Expr a) (Expr a)

class show a :: a [String] -> [String]
instance show (Expr a) where
	show (Lit x) l = [toString x:l]
	show (x1 +. x2) l = show x1 ["+":show x2 l]
	show (x1 *. x2) l = show x1 ["*":show x2 l]
	show Read l = ["Read":l]
	show (Write x) l = ["Write (":show x [")":l]]
	show (XOR x1 x2) l = show x1 ["XOR":show x2 l]
	show (Not x) l = ["Not":show x l]
	show (Eq x1 x2) l = show x1 ["==":show x2 l]
	show Throw l = ["throw":l]
	show (Try x1 x2) l = ["Try":show x1 ["Except":show x2 l]]