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Re: Network.CGI -- practical web programming example.
by Brandon S. Allbery KF8NH
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On Jun 27, 2009, at 20:07 , Edward Ing 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 +++ ".")
>
> 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 +
> ++ ".")
>
> 1) Why did the author choose to insert "liftM" in function saveFile?
It's because of where fromJust is being called. In yours, it's being
used at a place that expects a normal value, so you can just go ahead
and use it.
The original is applying the fromJust inside of a monadic computation,
as indicated by the (<-), so it needs to be lifted. Some Haskell
programmers use fmap (because most Monads are also Functors), others
use liftM. Both have the same effect: given a monadic computation "m
a", "liftM f" turns "f" into a function that operates on the enclosed
"a" instead of the entire "m a".
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@...
system administrator [openafs,heimdal,too many hats] allbery@...
electrical and computer engineering, carnegie mellon university KF8NH
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@...
http://www.haskell.org/mailman/listinfo/haskell-cafe
> 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 +++ ".")
>
> 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 +
> ++ ".")
>
> 1) Why did the author choose to insert "liftM" in function saveFile?
It's because of where fromJust is being called. In yours, it's being
used at a place that expects a normal value, so you can just go ahead
and use it.
The original is applying the fromJust inside of a monadic computation,
as indicated by the (<-), so it needs to be lifted. Some Haskell
programmers use fmap (because most Monads are also Functors), others
use liftM. Both have the same effect: given a monadic computation "m
a", "liftM f" turns "f" into a function that operates on the enclosed
"a" instead of the entire "m a".
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@...
system administrator [openafs,heimdal,too many hats] allbery@...
electrical and computer engineering, carnegie mellon university KF8NH
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@...
http://www.haskell.org/mailman/listinfo/haskell-cafe
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