[Haskell-beginners] How do I map a String and a IO String?

[Francesco Ariis]

On Thu, Jun 29, 2017 at 02:33:22PM +0200, Jona Ekenberg wrote:
> I want to replace lines with the content of a file if the line is a filename

Hello Jonas, you want to user `return`:

    λ> :t return
    return :: Monad m => a -> m a

which lifts a simple value inside a monad (in this case, a -> IO a), like
this:

    lineOrIO :: String -> IO String
    lineOrIO cs | (isPrefixOf "./" cs) = readFile cs
                | otherwise            = return cs

If this is not a school assignment, consider replacing `isPrefixOf "./"`
with something from `System.Directory`.

Does this help?