Re: [Haskell-cafe] Network.CGI -- practical web programming example.
Brandon S. Allbery KF8NH wrote: 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. That is, given the theory behind it all, every monad is a functor (note the lower case); from which it follows that liftM == fmap. For historical reasons the Monad typeclass does not require a Functor instance, however, and so it's not the case that every Monad is also a Functor (note the upper case). The function liftM can be defined generically given definitions for return and (=), so some prefer to use liftM to avoid the extra Functor dependency. The function fmap can be given specialized definitions due to overloading, so others prefer to use it for efficiency reasons. The ($) function is just a symbolic name for fmap. You'll also see the Applicative typeclass for applicative functors. Applicative does require a Functor instance, which is good. (And actually, every monad is an applicative functor; though the Monad class doesn't require Applicative either.) The function liftA can be defined generically given definitions for pure and (*), and liftA == fmap as well. The only reason anyone should use liftA is for defining a Functor instance when they're too lazy to give a specialized implementation. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Network.CGI -- practical web programming example.
On Jul 2, 2009, at 17:59 , wren ng thornton wrote: Brandon S. Allbery KF8NH wrote: 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. That is, given the theory behind it all, every monad is a functor (note the lower case); from Yeh, I decided to bypass the whole all monads are functors, but for Hysterical Raisins not all Monads are Functors morass. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Network.CGI -- practical web programming example.
Your code examples are: On Sat, Jun 27, 2009 at 6:07 PM, Edward Ing edward@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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Network.CGI -- practical web programming example.
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] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
