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haskellmonadsfunctor

Is there a better way to write this haskell function?

I just started learning Haskell again (i've already tried a few times) and i wrote a little function, that is supposed to download a file to a given filepath. If the URL is Some it should download the file and if it's Nothing it should just do nothing.

downloadFile :: URL -> FilePath -> IO ()
downloadFile url fp = ...        

maybeDownloadFile :: Maybe URL -> FilePath -> IO ()
maybeDownloadFile ( Just url ) fp = downloadFile url fp
maybeDownloadFile Nothing fp = return ()

So it essentially just wraps the downloadFile function, but with a Maybe URL instead of a URL. The thing is, I feel like this can be achieved more elegantly with a monadic operator (or maybe with functors) and without the match expression. Is there some operator like that?

Solution

  • Taking some action on Just, and doing nothing on Nothing, is exactly what traverse_ (or mapM_) does. Flip your arguments to make it even prettier.

    downloadFile :: FilePath -> URL -> IO ()
downloadFile fp url = ...

maybeDownloadFile :: FilePath -> Maybe URL -> IO ()
maybeDownloadFile = traverse_ . downloadFile