import qualified Data.Vector as V

import Data.List

f ∷ String → (String → Int → Char) → [Int] → String

f str g idx = map (g str) idx

h ∷ String → Int → Char

h s i = (V.fromList $ sort s) V.! i

slow ∷ String → [Int] → String

slow str = f str h 

fast ∷ String → [Int] → String

fast str = map ((V.fromList $ sort str) V.!)

main = do

    let testString = replicate 100000 'a'

        iterations = replicate 1000 100

    putStrLn $ fast testString iterations

    putStrLn $ slow testString iterations

Without inline (remove the inline pragma), "slow" would be much faster. I suspect this is because ghc can build a "persistent structure" for the partial applied function. After inline, each call of "g" will try to build a new vector. How can I tell ghc not to inline some specific functions? Or are there other ways to resolve this issue?