module test

import StdEnv

class arith v
where
	lit :: a -> v a | toString a
	plus :: (v a) (v a) -> v a | + a

class rtrn v :: (v a) -> v (TaskValue a)

:: Print a = P String
unPrint :: (Print a) -> String
unPrint (P a) = a

:: Eval a = E a
unEval :: (Eval a) -> a
unEval (E a) = a

:: TaskValue a = NoValue | Value a Bool

instance arith Print
where
	lit a = P (toString a)
	plus (P a) (P b) = P (a +++ "+" +++ b)

instance arith Eval
where
	lit a = E a
	plus (E a) (E b) = E (a + b)

instance rtrn Print where rtrn (P a) = P ("Task (" +++ a +++ ")")
instance rtrn Eval where rtrn (E a) = E (Value a True)

class default a :: a
instance default Int where default = 42
instance default (TaskValue a) where default = NoValue

listItem ::
	(A.v: a -> v (TaskValue b) | rtrn, arith v)
	-> (String, b)
	| +, toString, default b & default a
listItem mtask =
	( unPrint (mtask default)
	, default //unEval (mtask default)
	)

Start :: (String, Int)
Start = listItem \i->rtrn (lit i)