Library function to compose a function with itself n times

Question

Is there a library function available in Haskell to compose a function with itself n times?

For example I have this function:

func :: a ->         


        

Answer 1

I'm a beginner in Haskell, currently on chapter 5 ("Higher Order Functions") of Learn You a Haskell For Great Good! so I'm not yet familiar with the functions shown in the previous replies. Given what I understand so far, I'd do it like this:

applyNTimes :: Int -> (a -> a) -> a -> a
applyNTimes n f x 
    | n == 0        = x
    | otherwise     = f (applyNTimes (n-1) f x)

Answer 2

Another solution using foldr:

\n -> flip (foldr ($)) . replicate n

Answer 3

\xs n -> iterate func xs !! n

(xs is the initial value, n is the number of times to apply func)

I don't know why, but I feel like iterate is something people aren't consistently exposed to when learning Haskell.

If you don't like !! then you could use zip and lookup as an alternative. (some people/groups/tools don't like functions that call "error" in certain cases, I'm not claiming lookup is any better in these cases)

lookup n . zip [0..] . iterate func

EDIT: Ok, so I deleted then undeleted because I agree with the other answerer - you shouldn't discount use of iterate just because it gives you more than you need.

Answer 4

A variation on trinithis' answer using the newtype package, just for fun:

(\n f -> under Endo (mconcat . replicate n) f)

Or point-free:

under Endo . (mconcat .) . replicate