Re: [Haskell-cafe] mixing map and mapM ?
Pierre-Etienne Meunier wrote: This way : do times-mapM PF.getFileStatus filenames = return.(map PF.modificationTime) Or also : do times-mapM (PF.getFileStatus = (return.(PF.modificationTime))) filenames let sorted=... I do not know exactly how ghc compiles the IO monad, but it seems to me that the latter would allocate a little less. FWIW, (a = (return . f)) == (liftM f a) ~= (fmap f a) Where available, the fmap version is the most efficient. The liftM function can be less efficient since it's defined generically (namely with the bind/return definition above), whereas fmap can take advantage of knowing the specific monad it's working on. But then, not everyone defines Functor instances for their monads... -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On May 9, 2010, at 07:18 , wren ng thornton wrote: Where available, the fmap version is the most efficient. The liftM function can be less efficient since it's defined generically (namely with the bind/return definition above), whereas fmap can take advantage of knowing the specific monad it's working on. But then, not everyone defines Functor instances for their monads... Arguably that deserves a bug report, as logically a Monad is an Applicative is a Functor (or read is a as subset of for pedantry). -- 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] mixing map and mapM ?
Almost - liftM modificationTime has type Status - IO EpochTime. Like other IO functions (getLine, putStrLn), it returns an IO action but accepts a pure value (the modification time) Also, I like this style: import Control.Applicative (($)) blah = do times - mapM (PF.modificationTime $ PF.getFileStatus) filenames ... The $ operator evaluates to fmap so it's a cleaner way to apply a pure function to an IO value. On Thu, May 6, 2010 at 1:20 AM, bri...@aracnet.com wrote: On Thu, 6 May 2010 15:07:30 +1000 Ivan Miljenovic ivan.miljeno...@gmail.com wrote: On 6 May 2010 15:01, bri...@aracnet.com wrote: I was doing the following: do status - mapM PF.getFileStatus filenames let times = map PF.modificationTime status let sorted = sortBy (\(_, t1) (_,t2) - compare t1 t2) (zip filenames times) times - mapM (liftM PF.modificationTime . PF.getFileStatus) filenames However, I'd be tempted to leave it as is (and hope/assume that fusion does its magic). well now it's obvious :-) I did have liftM in there, but just couldn't quite figure out how to tie things together. to be completely clear : liftM takes modificationTime from Status - EpochTime to IO Status - IO EpochTime so now it can operate on the results of getFileStatus, which returns `IO Status`. mapM gathers the [IO EpochTime] into `IO [EpochTime]` and then - gives [EpochTime]. It's a little more clear in the verbose form, isn't it ? Thanks ! Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
Bill Atkins wrote: Almost - liftM modificationTime has type Status - IO EpochTime. Like other IO functions (getLine, putStrLn), it returns an IO action but accepts a pure value (the modification time) Also, I like this style: import Control.Applicative (($)) blah = do times - mapM (PF.modificationTime $ PF.getFileStatus) filenames ... The $ operator evaluates to fmap so it's a cleaner way to apply a pure function to an IO value. That won't type-check (unless I've missed some crafty trick with the types?!); you have two functions you want to compose, but the $ operator (i.e. fmap) applies a function on the left to a functor-value on the right. You would instead need: times - mapM ((PF.modificationTime $) . PF.getFileStatus) filenames At which point I prefer Ivan's liftM version rather than the above section (or worse: using ($) prefix). The original request is a relatively common thing to want to do, so I was slightly surprised that hoogling for: (b - c) - (a - f b) - a - f c didn't turn up any relevant results. This function is a lot like (=) but with a pure rather than side-effecting function on the left-hand side. Thanks, Neil. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On Thu, May 6, 2010 at 11:51 AM, Bill Atkins watk...@alum.rpi.edu wrote: Almost - liftM modificationTime has type Status - IO EpochTime. Like other IO functions (getLine, putStrLn), it returns an IO action but accepts a pure value (the modification time) ghci :m +Control.Monad System.Posix.Files ghci :t liftM modificationTime liftM modificationTime :: (Monad m) = m FileStatus - m System.Posix.Types.EpochTime where m = IO in this case. Also, I like this style: import Control.Applicative (($)) blah = do times - mapM (PF.modificationTime $ PF.getFileStatus) filenames ... The $ operator evaluates to fmap so it's a cleaner way to apply a pure function to an IO value. Usually I'd agree but in fact PF.getFileStatus is not an IO value, but an IO function, so you need to map over its result: mapM ((PF.modificationTime $) . PF.getFileStatus) filenames but then you lose the convenience of the $ as an infix operator, so mapM (liftM PF.modificationTime . PF.getFileStatus) filenames is probably clearer in this case. Or, if you're feeling particularly silly: mapM (fmap fmap fmap modificationTime getFileStatus) filenames ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
Yep, you and Ben are both correct. Mea culpa and sorry for the bad answer. Just curious: why does getModificationTime take an IO FileStatus rather than a FileStatus? On Thu, May 6, 2010 at 7:00 AM, Neil Brown nc...@kent.ac.uk wrote: Bill Atkins wrote: Almost - liftM modificationTime has type Status - IO EpochTime. Like other IO functions (getLine, putStrLn), it returns an IO action but accepts a pure value (the modification time) Also, I like this style: import Control.Applicative (($)) blah = do times - mapM (PF.modificationTime $ PF.getFileStatus) filenames ... The $ operator evaluates to fmap so it's a cleaner way to apply a pure function to an IO value. That won't type-check (unless I've missed some crafty trick with the types?!); you have two functions you want to compose, but the $ operator (i.e. fmap) applies a function on the left to a functor-value on the right. You would instead need: times - mapM ((PF.modificationTime $) . PF.getFileStatus) filenames At which point I prefer Ivan's liftM version rather than the above section (or worse: using ($) prefix). The original request is a relatively common thing to want to do, so I was slightly surprised that hoogling for: (b - c) - (a - f b) - a - f c didn't turn up any relevant results. This function is a lot like (=) but with a pure rather than side-effecting function on the left-hand side. Thanks, Neil. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On Thu, May 6, 2010 at 12:37 PM, Bill Atkins watk...@alum.rpi.edu wrote: Just curious: why does getModificationTime take an IO FileStatus rather than a FileStatus? It doesn't. getModificationTime is a pure function (think of it like a record accessor). liftM makes it take IO FileStatus because that is what liftM is for :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
This way : do times-mapM PF.getFileStatus filenames = return.(map PF.modificationTime) Or also : do times-mapM (PF.getFileStatus = (return.(PF.modificationTime))) filenames let sorted=... I do not know exactly how ghc compiles the IO monad, but it seems to me that the latter would allocate a little less. Cheers, PE El 06/05/2010, a las 01:01, bri...@aracnet.com bri...@aracnet.com escribió: I was doing the following: do status - mapM PF.getFileStatus filenames let times = map PF.modificationTime status let sorted = sortBy (\(_, t1) (_,t2) - compare t1 t2) (zip filenames times) and I thought, surely I can combine the status and times definitions into one line, only I can't. Hint ? Thanks, Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
Pierre-Etienne Meunier pierreetienne.meun...@gmail.com writes: This way : do times-mapM PF.getFileStatus filenames = return.(map PF.modificationTime) Or also : do times-mapM (PF.getFileStatus = (return.(PF.modificationTime))) filenames let sorted=... I do not know exactly how ghc compiles the IO monad, but it seems to me that the latter would allocate a little less. List fusion probably converts them to the same core (you can always use ghc-core to verify this if you care). -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On Thu, 06 May 2010 12:00:01 +0100 Neil Brown nc...@kent.ac.uk wrote: At which point I prefer Ivan's liftM version rather than the above section (or worse: using ($) prefix). The original request is a relatively common thing to want to do, so I was slightly surprised that hoogling for: (b - c) - (a - f b) - a - f c didn't turn up any relevant results. This function is a lot like (=) but with a pure rather than side-effecting function on the left-hand side. ha ! I had actually remembered to hoogle :-) and didn't get anything either, only I wasn't sure I put the signature in correctly. Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On 6 May 2010 15:01, bri...@aracnet.com wrote: I was doing the following: do status - mapM PF.getFileStatus filenames let times = map PF.modificationTime status let sorted = sortBy (\(_, t1) (_,t2) - compare t1 t2) (zip filenames times) times - mapM (liftM PF.modificationTime . PF.getFileStatus) filenames However, I'd be tempted to leave it as is (and hope/assume that fusion does its magic). -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On Thu, 6 May 2010 15:07:30 +1000 Ivan Miljenovic ivan.miljeno...@gmail.com wrote: On 6 May 2010 15:01, bri...@aracnet.com wrote: I was doing the following: do status - mapM PF.getFileStatus filenames let times = map PF.modificationTime status let sorted = sortBy (\(_, t1) (_,t2) - compare t1 t2) (zip filenames times) times - mapM (liftM PF.modificationTime . PF.getFileStatus) filenames However, I'd be tempted to leave it as is (and hope/assume that fusion does its magic). well now it's obvious :-) I did have liftM in there, but just couldn't quite figure out how to tie things together. to be completely clear : liftM takes modificationTime from Status - EpochTime to IO Status - IO EpochTime so now it can operate on the results of getFileStatus, which returns `IO Status`. mapM gathers the [IO EpochTime] into `IO [EpochTime]` and then - gives [EpochTime]. It's a little more clear in the verbose form, isn't it ? Thanks ! Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On 6 May 2010 15:20, bri...@aracnet.com wrote: well now it's obvious :-) I did have liftM in there, but just couldn't quite figure out how to tie things together. to be completely clear : liftM takes modificationTime from Status - EpochTime to IO Status - IO EpochTime You can see it that way, yes (of course, liftM works on all monads, not just IO). so now it can operate on the results of getFileStatus, which returns `IO Status`. Operate directly on the returned values; you could always use something like (return . modificationTime) = getFileStatus file as well. mapM gathers the [IO EpochTime] into `IO [EpochTime]` and then - gives [EpochTime]. No, it's the sequence function that does that (but mapM f = sequence . map f). -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
