Haskell asymptotic difference between algorithms

Question

Here we need to get all subsequences of the given length. How to calculate the asymptotic complexity of the given function?

import Data.List

subsequencesOfSize l n = [x | x <- subsequences l, length x == n]

How many "iterations" will it be? Can we speak about "iterations". Any optimizations? GHC 7.10.2. How compiler or programmer can improve this?

Answer 1

Here is a related SO question which discusses various implementations of subsequencesOfSize:

Haskell: comparison of techniques for generating combinations

and the best performing one which uses memoization:

Haskell: comparison of techniques for generating combinations

Update

Also consider this version that @user3237465 mentions in the comments:

subsequencesOfSize :: Int -> [a] -> [[a]]
subsequencesOfSize n xs | n > length xs = []
subsequencesOfSize n xs = subsequencesBySize xs !! n where
    subsequencesBySize    []  = [[[]]]
    subsequencesBySize (x:xs) = zipWith (++) ([] : map (map (x:)) next) (next ++ [[]])
        where next = subsequencesBySize xs