急増加関数もどき

pfgh.hs

type I = Integer

pfgh :: [I] -> I -> I
pfgh list x | null l2    = x+1
            | null l1    = fpow (pfgh (head list -1 : tail l2)) x x
            | otherwise  = pfgh (init l1 ++ [x] ++ (head l2 -1) : tail l2) x
  where
    (l1,l2) = span (<=0) list

fpow :: (a -> a) -> I -> (a -> a)
fpow _ 0 = id
fpow f x = f. fpow f (x-1)

toOI :: I -> I
toOI x = pfgh (replicate (fromIntegral x) x) x

main :: IO()
main = print $ fpow toOI 64 3