I want to replace lines with the content of a file if the line is a filename

> {-# LANGUAGE LambdaCase #-}

> test = do

>   putStrLn $ concatMap (\l -> l ++ "\n")

>            $ map (\case l

>                           | (isPrefixOf "./" l) -> readFile l

>                           | otherwise           -> l)

But since l is a String and readFile gives an IO String this doesn't work.

How should I get around this?

Kind regards,

Jona