[Haskell-cafe] Network.CGI -- practical web programming example.
Luke Palmer
lrpalmer at gmail.com
Sat Jun 27 20:26:47 EDT 2009
Your code examples are:
On Sat, Jun 27, 2009 at 6:07 PM, Edward Ing <edward.ing at 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
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