grid :: [(Integer, Integer)]
grid = f (0,0)
    where
      f (x,y) = (x,y) : f next
          where
            next | x == 0 && y == 0                   = right
                 | x >  0 && y <  0 && abs x <= abs y = left
                 | x >  0 && y <= 0                   = down
                 | x <  0 && y <  0 && abs x >= abs y = up
                 | x <= 0 && y <  0                   = left
                 | x <  0 && y >  0 && abs x <= abs y = right
                 | x <  0 && y >= 0                   = up
                 | x >  0 && y >  0 && abs x > abs y  = down
                 | x >= 0 && y >  0                   = right
            left  = (x-1,y)
            right = (x+1,y)
            up    = (x,y+1)
            down  = (x,y-1)

gridInCircle :: Integer -> [(Integer, Integer)]
gridInCircle r | r > 0 = [p | p@(x,y) <- takeWhile ((r,r)/=) grid, x^2+y^2<=r^2]
               | otherwise = []

main :: IO ()
main = do
    print $ take 25 grid
    print $ gridInCircle 2