gistfile1.txt

data Op = Plus | Minus | Concat deriving Show

f l = concatMap (\r -> map (: r) l)

g = f [Plus, Minus, Concat]

allOps = iterate g [[Plus], [Minus], [Concat]] !! 7

step :: Int -> Op -> (Int, Int) -> (Int, Int)
step prev op (carry, sum) = case op of
    Concat -> (read (show prev ++ show carry) :: Int, sum)
    Plus -> (prev, carry + sum)
    Minus -> (prev, sum - carry)

output = zipWith (,)
    (map
      (\ops -> uncurry (+) $
        foldr (uncurry step) (9, 0) (zipWith (,) [1 ..] ops))
      allOps)
    allOps

answers = filter ((== 100) . fst) output