data Expr where
    EInt : Int -> Expr
    EBool : Bool -> Expr
    EAdd : Expr -> Expr -> Expr
    EAnd : Expr -> Expr -> Expr

data Val where
    VInt : Int -> Val
    VBool : Bool -> Val

data Eval where
    Just : Val -> Eval
    Nothing : Eval

interp : Expr -> Eval
interp e = case e of
    EInt i => Just (VInt i)
    EBool b => Just (VBool b)
    EAdd e1 e2 => case interp e1 of
        Just x => case x of
            VInt i => case interp e2 of
                Nothing => Nothing
                Just y => case y of
                    VInt j => Just (VInt (i + j))
                    _ => Nothing
            _ => Nothing
        Nothing => Nothing
    EAnd e1 e2 => case interp e1 of
        Just x => case x of
            VBool i => case interp e2 of
                Just y => case y of
                    VBool j => case i of 
                        True => case j of
                            True => Just (VBool True)
                            _ => Just (VBool False)
                        _ => Just (VBool False)
                Nothing => Nothing
            _ => Nothing
        Nothing => Nothing
        

readVal : Val -> Int
readVal v = case v of
    VInt i => i
    VBool a => case a of
        True => 1
        False => 0

main = case interp (EAdd (EAdd (EInt 3) (EInt 5)) (EBool True)) of
    Nothing => (0 - 1)
    Just x => case x of