Mischa's adder

add.hs

import Data.Digits


carry r i x y  | i == 0                           = 0 
               | x + y + carry r (i-1) x y <= r-1 = 0
               | True                             = 1

-- currently works iff x and y have same # of digits, due to zip3
add x y = 
    sum [ (x_i + y_i + carry r i x y)*r^i | (x_i, y_i, i) <- (zip3 xs ys [0..])]  
    where r = 10
          xs = digits r x
          ys = digits r y