simpleplus2.tel

id = \x -> x

and2 = \a b -> if a then (if b then 1 else 0) else 0

not2 = \x -> if x then 0 else 1

succ2 = \x -> (x,0)

makeRecur = \n t r b -> n (\c i -> if t i then r c i else b i) b

d2c2 = \n f b -> { id
                 , \recur i  -> f (recur (left i))
                 , \i -> b
                 } n

c2d2 = \c -> c succ2 0

assert2 = \t s -> if not t then (1, s) else 0

times2 = \m n f -> m (n f)

validDigit = \x -> and2 ($47 left x) (not2 ($57 left x))
-- validDigit = \x -> not2 ($57 left x)

validInput = \x -> assert2 (and2 (validDigit (left x))
                                 (not2 ($3 right x)))
                           "invalid input"

toI = \x -> $48 left x

doAdd = \x -> c2d2 (times2 (d2c2 (toI x)) $9)

main = \input -> let inString : validInput = left input
                     x = left inString
                     r = doAdd x
                     dr = $49 succ2 r
                 in if not input
                    then ("enter two digits separated by a space", 1)
                    else (dr, 0)