Your code examples are:

On Sat, Jun 27, 2009 at 6:07 PM, Edward Ing <edward.ing@gmail.com> wrote:

saveFile n =
do cont <- (liftM fromJust) $ getInputFPS "file"
let f = uploadDir ++ "/" ++ basename n
liftIO $ BS.writeFile f cont
return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".")

Vs.

saveFile n =
do cont <- getInputFPS "file"
let f = uploadDir ++ "/" ++ basename n
liftIO $ BS.writeFile f (fromJust cont)
return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".")

Consider the line x <- y in a do expression. If y has type M a for some monad M, then x has type a.

So, let's say you have a value f :: Maybe Int, and you want to return the Int's stringification if it exists. We can write this in these two ways:

do x <- f

return (show x)

do x <- liftM show f

return x

liftM :: (a -> b) -> (M a -> M b) for any monad M. That means if you want to apply a function to a value which is currently wrapped in a monad constructor, you need to "lift" it in. liftM takes a function on ordinary values to a function on wrapped values. But after you bind, you don't need to lift anymore.

Which of the two above styles to choose is a matter of style, and, in my code at least, varies from situation to situation.

That said, you can write both of these snippets as "fmap show f" or "show <$> f" (where (<$>) is from Control.Applicative), which is how it would be done in practice.

Does that make sense?

Luke