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Re: [Haskell-cafe] Free monad based EDSL for writing LLVM programs.
by Matt Giles 08 Aug '14

08 Aug '14

For transforming the code, have you checked out the uniplate package? It seems like it could fit your problem pretty well. On 8 August, 2014 at 5:00:19 AM, haskell-cafe-request(a)haskell.org (haskell-cafe-request(a)haskell.org) wrote: Send Haskell-Cafe mailing list submissions to haskell-cafe(a)haskell.org To subscribe or unsubscribe via the World Wide Web, visit http://www.haskell.org/mailman/listinfo/haskell-cafe or, via email, send a message with subject or body 'help' to haskell-cafe-request(a)haskell.org You can reach the person managing the list at haskell-cafe-owner(a)haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Haskell-Cafe digest..." Today's Topics: 1. Re: Side-by-side pretty printing (J. Waldmann) 2. Free monad based EDSL for writing LLVM programs. (arrowdodger) 3. parsec: problem combining lookAhead with many1 (bug?) (silly8888) 4. Re: parsec: problem combining lookAhead with many1 (bug?) (Andreas Reuleaux) 5. Performance of StateT and best practices for debugging (Kyle Hanson) 6. [ANN] rtorrent-state 0.1.0.0 (Mateusz Kowalczyk) 7. Re: Performance of StateT and best practices for debugging (John Lato) 8. How to improve the zipwith's performance (jun zhang) 9. Re: Performance of StateT and best practices for debugging (Bardur Arantsson) 10. Visualising Haskell function execution (Jan Paul Posma) 11. Re: Performance of StateT and best practices for debugging (John Lato) ---------------------------------------------------------------------- Message: 1 Date: Thu, 7 Aug 2014 12:33:43 +0000 (UTC) From: J. Waldmann <waldmann(a)imn.htwk-leipzig.de> To: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Side-by-side pretty printing Message-ID: <loom.20140807T142947-462(a)post.gmane.org> Content-Type: text/plain; charset=us-ascii This is what I use http://autolat.imn.htwk-leipzig.de/gitweb/?p=autolib;a=blob;f=todoc/Autolib… it's of the works-but-looks-ugly-and-is-terribly-inefficient variety but since it's applied to small Docs only (like, columns of matrices), I don't really care. - J.W. ------------------------------ Message: 2 Date: Thu, 7 Aug 2014 18:16:57 +0400 From: arrowdodger <6yearold(a)gmail.com> To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Free monad based EDSL for writing LLVM programs. Message-ID: <CALH631%3DEbZ8aTZi%3DoPdfsP97J2XrRCTf%3DDpX%2BuaZ5m9%3D0mxXhw(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" Hello. I'm new with Haskell and FP, so i wanted someone to give comments on the package i've made [1]. It's, actually, my first attempt to create something more or less real, so any feedback would be welcome. I've used Free monad to create EDSL that allows writing LLVM IR code. Afterwards it could be converted into pure AST structure provided by llvm-general-pure[2] package. Currently, it supports almost every instruction, but i haven't yet come up with sensible defaults for them. Another thing that bugs me is the ability to transform the code in syb way. I want take a user-supplied function that would pattern-match instruction and produce another code block and apply this function everywhere in the code, but still can't get my head around it. I've come up with extF function, that unlike extM, would resort to wrap instead of return, but that's all i've managed to do. Thanks in advance. [1] https://bitbucket.org/arrowdodger/llvm-general-edsl [2] http://hackage.haskell.org/package/llvm-general-pure -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140807/6d973f16…> ------------------------------ Message: 3 Date: Thu, 7 Aug 2014 15:25:23 +0100 From: silly8888 <silly8888(a)gmail.com> To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] parsec: problem combining lookAhead with many1 (bug?) Message-ID: <CAMmzbfWA9S3YGjJ1iQaa72rKZyuV4psvEP3LsQuDGC3QED-YVw(a)mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Suppose that we have the following parser: p = lookAhead (char 'a') >> char 'b' If we use it like so parse p "" "a" we get the following error: Left (line 1, column 1): unexpected "a" expecting "b" What happened is that char 'a' succeeded by consuming the 'a' from the input and then lookAhead rewinded the input stream (as it does on success). Then, char 'b' tries to parse (again) the first character of the input and fails. Everything works as expected. Now let's slightly modify our parser: p' = lookAhead (many1 $ char 'a') >> char 'b' I've only added a many1. I was expecting this parser to give the same error as the previous one: many1 $ char 'a' will succeed consuming one 'a' and then lookAhead will rewind the input (as it does on success). Thus when we call char 'b' we are going to be in the beginning of the input again. Well, that doesn't happen: Left (line 1, column 2): unexpected end of input expecting "b" As you can see, lookAhead did not rewind the input as it was supposed to. ------------------------------ Message: 4 Date: Thu, 07 Aug 2014 17:32:11 +0100 From: Andreas Reuleaux <reuleaux(a)web.de> To: silly8888 <silly8888(a)gmail.com> Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] parsec: problem combining lookAhead with many1 (bug?) Message-ID: <87y4v0z2es.fsf(a)web.de> Content-Type: text/plain While I haven't tried out your example in parsec, I can at least confirm that in trifecta it does work that way you expect it, ie. there is no difference between the error messages in both of your cases: (parsec's many1 = trifecta's some) Prelude > :m +Text.Trifecta Prelude Text.Trifecta > :m +Text.Parser.LookAhead Prelude Text.Trifecta Text.Parser.LookAhead > ... Prelude Text.Trifecta Text.Parser.LookAhead > parseTest (lookAhead (char 'a') >> char 'b') "a" ... Loading package reducers-3.10.2.1 ... linking ... done. Loading package trifecta-1.5.1 ... linking ... done. (interactive):1:1: error: expected: "b" a<EOF> ^ Prelude Text.Trifecta Text.Parser.LookAhead > parseTest (lookAhead (some $ char 'a') >> char 'b') "a" (interactive):1:1: error: expected: "b" a<EOF> ^ Prelude Text.Trifecta Text.Parser.LookAhead > Hope this helps. -Andreas silly8888 <silly8888(a)gmail.com> writes: > Suppose that we have the following parser: > > p = lookAhead (char 'a') >> char 'b' > > If we use it like so > > parse p "" "a" > > we get the following error: > > Left (line 1, column 1): > unexpected "a" > expecting "b" > > What happened is that char 'a' succeeded by consuming the 'a' from the > input and then lookAhead rewinded the input stream (as it does on > success). Then, char 'b' tries to parse (again) the first character of > the input and fails. Everything works as expected. > > Now let's slightly modify our parser: > > p' = lookAhead (many1 $ char 'a') >> char 'b' > > I've only added a many1. I was expecting this parser to give the same > error as the previous one: many1 $ char 'a' will succeed consuming one > 'a' and then lookAhead will rewind the input (as it does on success). > Thus when we call char 'b' we are going to be in the beginning of the > input again. Well, that doesn't happen: > > Left (line 1, column 2): > unexpected end of input > expecting "b" > > As you can see, lookAhead did not rewind the input as it was supposed to. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe ------------------------------ Message: 5 Date: Thu, 7 Aug 2014 10:57:47 -0700 From: Kyle Hanson <me(a)khanson.io> To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Performance of StateT and best practices for debugging Message-ID: <CAMJUouBqFbSi%2BifFsbYdXpba7pPVoP8BkyZxZaUoqLNuHc7VzQ(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" Hello, I was looking at cleaning up my refactoring a core loop of template rendering to go from a loop with many parameters loop :: RenderConfig -> BlockMap -> InputBucket m -> Builder -> [Pieces] -> ExceptT StrapError m Builder to a looped state monad transformer loop :: [Pieces] -> RenderT m Builder newtype RenderT m a = RenderT { runRenderT :: ExceptT StrapError (StateT (RenderState m) m) a } deriving ( Functor, Applicative, Monad, MonadIO ) data RenderState m = RenderState { position :: SourcePos , renderConfig :: RenderConfig , blocks :: BlockMap , bucket :: InputBucket m } however, there is a big slow down (about 6-10x) using a StateT. I think it might have something to do with laziness but I am not exactly sure of where to begin in tracking it down. Swapping out the Lazy State to a Strict State helps a little (only a 5x slow down) You can find some of the processing code here: https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… With my old loop commented out. Its messy right now since I am just trying a number of different approaches. I did some more work factoring out the lifts, trying different iterations of foldlM and stuff but that didn't have that much of an effect on performance. After profiling I see in the StateT, the report has a lot more CAFs and garbage collecting. Here is the profiling report from my original version w/o StateT http://lpaste.net/108995 Slow version with StateT http://lpaste.net/108997 Here is the "makeBucket" function that is referenced (it is the same in both state and nonstate): https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… Looking at stacked overflow and the official docs I have gotten an idea of what is going on. The heaps generated between them tells me that a lot more memory is being allocated to lists. These heaps were generated running my render function against a template with nested loops and a list of elements. http://imgur.com/a/2jOIf I am hoping that maybe someone could give me a hint at what to look at next. I've played around with Strictness and refactoring loops to no avail and now am kind of stuck. Any help would be appreciated. -- Kyle Hanson -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140807/a82b99f5…> ------------------------------ Message: 6 Date: Thu, 07 Aug 2014 21:07:02 +0200 From: Mateusz Kowalczyk <fuuzetsu(a)fuuzetsu.co.uk> To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] [ANN] rtorrent-state 0.1.0.0 Message-ID: <53E3CE56.6090500(a)fuuzetsu.co.uk> Content-Type: text/plain; charset=windows-1252 Hi, rtorrent-state is a library that allows working with rtorrent state files (SOMEHASH.torrent.rtorrent) placed in your session directory. If you're an rtorrent user and ever had to manually muck around with those files, you should be able to use this library to make your life easier. For example, you can stop all torrents in your session directory with just: ?overFilesIn "rtorrent/session/dir" stopTorrent? The way it works is by parsing the session files, modifying the resulting data type and serialising it back into the file. I did not do optimisation but I had no problem with test sample of 100,000 files. I need to add IOException handling and maybe extra utility functions but otherwise I consider the library finished. Thanks -- Mateusz K. ------------------------------ Message: 7 Date: Thu, 7 Aug 2014 15:39:53 -0700 From: John Lato <jwlato(a)gmail.com> To: Kyle Hanson <me(a)khanson.io> Cc: haskell-cafe <haskell-cafe(a)haskell.org> Subject: Re: [Haskell-cafe] Performance of StateT and best practices for debugging Message-ID: <CAJ727GjPpMn3xMDs%3D4EmJGG0FpndsBwvq5Zkc97X6yQ-H21KEA(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" I haven't looked very closely, but I'm suspicious of this code from "instance Block Piece" ListLike l -> forM l (\obj -> ...) >>= (return . mconcat) The "forM" means that "l" will be traversed once and create an output list, which will then be mconcat'd together. The list has to be created because of the monadic structure imposed by forM, but if the result of the mconcat isn't demanded right away it will be retained as a thunk that references the newly-created list. I'd suggest that you replace it with something like ListLike l -> foldM (\(!acc) obj -> ... >>= return . mappend acc) mempty l Here I've justed added a bang pattern to the accumulator. If whatever is being returned has some lazy fields, you may want to change that to use deepseq instead of a bang pattern. Also, "foo >>= return . bar" is often regarded as a bit of a code smell, it can be replaced with "bar <$> foo" or "bar `liftM` foo", or sometimes something even simpler depending on circumstances (but IMHO sometimes it's more clear to just leave it alone). The heap profile does look like a space leak. The line <StrappedTemplates-0.1.1.0:Text.Strapped.Render.sat_sc1z> is a thunk (you can tell because it's in '<>' brackets), so whatever is referencing that is not strict enough. Sometimes another heap profile report, e.g. "-hc" or maybe "-hy" will give more useful information that lets you identify what exactly "sat_sc1z" is. You could also try compiling with -ddump-stg, which will dump the intermediate STG output which usually shows those names. But then you'll probably also need to re-run the profile, since the names change between compilations. Also IIRC some of values aren't named until the cmm phase, but that's harder to map back to Haskell so if you can identify the code from stg it's simpler. If you haven't seen http://blog.ezyang.com/2011/06/pinpointing-space-leaks-in-big-programs/, I'd highly recommend it if you need to track down a space leak. John L. On Thu, Aug 7, 2014 at 10:57 AM, Kyle Hanson <me(a)khanson.io> wrote: > Hello, > > I was looking at cleaning up my refactoring a core loop of template > rendering to go from a loop with many parameters > > loop :: RenderConfig -> BlockMap -> InputBucket m -> Builder -> [Pieces] > -> ExceptT StrapError m Builder > > to a looped state monad transformer > > loop :: [Pieces] -> RenderT m Builder > > newtype RenderT m a = RenderT > { runRenderT :: ExceptT StrapError (StateT (RenderState m) m) a > } deriving ( Functor, Applicative, Monad, MonadIO ) > > data RenderState m = RenderState > { position :: SourcePos > , renderConfig :: RenderConfig > , blocks :: BlockMap > , bucket :: InputBucket m > } > > however, there is a big slow down (about 6-10x) using a StateT. I think it > might have something to do with laziness but I am not exactly sure of where > to begin in tracking it down. Swapping out the Lazy State to a Strict State > helps a little (only a 5x slow down) > > You can find some of the processing code here: > > > https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… > > With my old loop commented out. > > Its messy right now since I am just trying a number of different > approaches. I did some more work factoring out the lifts, trying different > iterations of foldlM and stuff but that didn't have that much of an effect > on performance. > > After profiling I see in the StateT, the report has a lot more CAFs and > garbage collecting. > > Here is the profiling report from my original version w/o StateT > http://lpaste.net/108995 > > Slow version with StateT > http://lpaste.net/108997 > > Here is the "makeBucket" function that is referenced (it is the same in > both state and nonstate): > > > https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… > > Looking at stacked overflow and the official docs I have gotten an idea of > what is going on. The heaps generated between them tells me that a lot more > memory is being allocated to lists. These heaps were generated running my > render function against a template with nested loops and a list of elements. > > http://imgur.com/a/2jOIf > > I am hoping that maybe someone could give me a hint at what to look at > next. I've played around with Strictness and refactoring loops to no avail > and now am kind of stuck. Any help would be appreciated. > > -- > Kyle Hanson > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140807/b4c26366…> ------------------------------ Message: 8 Date: Fri, 8 Aug 2014 11:24:29 +0800 From: jun zhang <zhangjun.julian(a)gmail.com> To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] How to improve the zipwith's performance Message-ID: <CAGjcJLcT0FAmNxFLbdaZZfMFg5aEL1rZDPEWOpRxvsKJsau3ew(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" Dear All I write a code for Clustering with Data.Clustering.Hierarchical, but it's slow. I use the profiling and change some code, but I don't know why zipwith take so many time? (even I change list to vector) My code is as blow, Any one kindly give me some advices. ====================== main = do .... let cluster = dendrogram SingleLinkage vectorList getVectorDistance .... getExp2 v1 v2 = d*d where d = v1 - v2 getExp v1 v2 | v1 == v2 = 0 | otherwise = getExp2 v1 v2 tfoldl d = DV.foldl1' (+) d changeDataType:: Int -> Double changeDataType d = fromIntegral d getVectorDistance::(a,DV.Vector Int)->(a, DV.Vector Int )->Double getVectorDistance v1 v2 = fromIntegral $ tfoldl dat where l1 = snd v1 l2 = snd v2 dat = DV.zipWith getExp l1 l2 ======================================= build with ghc -prof -fprof-auto -rtsopts -O2 log_cluster.hs run with log_cluster.exe +RTS -p profiling result is log_cluster.exe +RTS -p -RTS total time = 8.43 secs (8433 ticks @ 1000 us, 1 processor) total alloc = 1,614,252,224 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc getVectorDistance.dat Main 49.4 37.8 tfoldl Main 5.7 0.0 getExp Main 4.5 0.0 getExp2 Main 0.5 1.5 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140808/d29aa298…> ------------------------------ Message: 9 Date: Fri, 08 Aug 2014 06:31:49 +0200 From: Bardur Arantsson <spam(a)scientician.net> To: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Performance of StateT and best practices for debugging Message-ID: <ls1jrl%247gp%241(a)ger.gmane.org> Content-Type: text/plain; charset=utf-8 On 2014-08-07 19:57, Kyle Hanson wrote: > Hello, > > Here is the "makeBucket" function that is referenced (it is the same in > both state and nonstate): > > https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… > Just a shot in the dark, but I notice that you're using "modify" and not "modify'" which was added in a recent version of transformers. Strict.StateT is not always "strict enough" and you may need to use modify'. At any rate, it's worth a shot, I think. Regards, ------------------------------ Message: 10 Date: Thu, 7 Aug 2014 22:30:25 -0700 From: Jan Paul Posma <me(a)janpaulposma.nl> To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Visualising Haskell function execution Message-ID: <CAPtY9n%2B6onYBYb%3DpWP4NRmLq43zY6MGXQMuhmp2HfnUzhuOiyA(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" Hey all, Last weekend my friend Steve and I did a small project for visualising Haskell function execution in the browser. It's meant to be used in education, and uses a tiny custom parser. I figured it could be of interest for anyone here learning or teaching Haskell: https://stevekrouse.github.io/hs.js/ To see it in action, scroll a bit down to the red bordered box, and click on "map", and then keep clicking on each new line. I hope it can be useful to someone. Cheers, JP -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140807/3c4dabe9…> ------------------------------ Message: 11 Date: Thu, 7 Aug 2014 23:56:43 -0700 From: John Lato <jwlato(a)gmail.com> To: Bardur Arantsson <spam(a)scientician.net> Cc: haskell-cafe <haskell-cafe(a)haskell.org> Subject: Re: [Haskell-cafe] Performance of StateT and best practices for debugging Message-ID: <CAJ727GipLL12TXfrMBeLJBX2_GdoD0W-CKJd4U1cu48CvnkrBg(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" On Thu, Aug 7, 2014 at 9:31 PM, Bardur Arantsson <spam(a)scientician.net> wrote: > On 2014-08-07 19:57, Kyle Hanson wrote: > > Hello, > > > > Here is the "makeBucket" function that is referenced (it is the same in > > both state and nonstate): > > > > > https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… > > > > Just a shot in the dark, but I notice that you're using "modify" and not > "modify'" which was added in a recent version of transformers. > > Strict.StateT is not always "strict enough" and you may need to use > modify'. > > At any rate, it's worth a shot, I think. > Good point. I think that even modify' will not be strict enough without adding strictness to RenderState as well. John L. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140807/ce617687…> ------------------------------ Subject: Digest Footer _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ------------------------------ End of Haskell-Cafe Digest, Vol 132, Issue 11 *********************************************

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Performance of StateT and best practices for debugging
by Kyle Hanson 08 Aug '14

08 Aug '14

Hello, I was looking at cleaning up my refactoring a core loop of template rendering to go from a loop with many parameters loop :: RenderConfig -> BlockMap -> InputBucket m -> Builder -> [Pieces] -> ExceptT StrapError m Builder to a looped state monad transformer loop :: [Pieces] -> RenderT m Builder newtype RenderT m a = RenderT { runRenderT :: ExceptT StrapError (StateT (RenderState m) m) a } deriving ( Functor, Applicative, Monad, MonadIO ) data RenderState m = RenderState { position :: SourcePos , renderConfig :: RenderConfig , blocks :: BlockMap , bucket :: InputBucket m } however, there is a big slow down (about 6-10x) using a StateT. I think it might have something to do with laziness but I am not exactly sure of where to begin in tracking it down. Swapping out the Lazy State to a Strict State helps a little (only a 5x slow down) You can find some of the processing code here: https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… With my old loop commented out. Its messy right now since I am just trying a number of different approaches. I did some more work factoring out the lifts, trying different iterations of foldlM and stuff but that didn't have that much of an effect on performance. After profiling I see in the StateT, the report has a lot more CAFs and garbage collecting. Here is the profiling report from my original version w/o StateT http://lpaste.net/108995 Slow version with StateT http://lpaste.net/108997 Here is the "makeBucket" function that is referenced (it is the same in both state and nonstate): https://github.com/hansonkd/StrappedTemplates/blob/321a88168d54943fc217553c… Looking at stacked overflow and the official docs I have gotten an idea of what is going on. The heaps generated between them tells me that a lot more memory is being allocated to lists. These heaps were generated running my render function against a template with nested loops and a list of elements. http://imgur.com/a/2jOIf I am hoping that maybe someone could give me a hint at what to look at next. I've played around with Strictness and refactoring loops to no avail and now am kind of stuck. Any help would be appreciated. -- Kyle Hanson

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[ANN] rtorrent-state 0.1.0.0
by Mateusz Kowalczyk 07 Aug '14

07 Aug '14

Hi, rtorrent-state is a library that allows working with rtorrent state files (SOMEHASH.torrent.rtorrent) placed in your session directory. If you're an rtorrent user and ever had to manually muck around with those files, you should be able to use this library to make your life easier. For example, you can stop all torrents in your session directory with just: “overFilesIn "rtorrent/session/dir" stopTorrent” The way it works is by parsing the session files, modifying the resulting data type and serialising it back into the file. I did not do optimisation but I had no problem with test sample of 100,000 files. I need to add IOException handling and maybe extra utility functions but otherwise I consider the library finished. Thanks -- Mateusz K.

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parsec: problem combining lookAhead with many1 (bug?)
by silly8888 07 Aug '14

07 Aug '14

Suppose that we have the following parser: p = lookAhead (char 'a') >> char 'b' If we use it like so parse p "" "a" we get the following error: Left (line 1, column 1): unexpected "a" expecting "b" What happened is that char 'a' succeeded by consuming the 'a' from the input and then lookAhead rewinded the input stream (as it does on success). Then, char 'b' tries to parse (again) the first character of the input and fails. Everything works as expected. Now let's slightly modify our parser: p' = lookAhead (many1 $ char 'a') >> char 'b' I've only added a many1. I was expecting this parser to give the same error as the previous one: many1 $ char 'a' will succeed consuming one 'a' and then lookAhead will rewind the input (as it does on success). Thus when we call char 'b' we are going to be in the beginning of the input again. Well, that doesn't happen: Left (line 1, column 2): unexpected end of input expecting "b" As you can see, lookAhead did not rewind the input as it was supposed to.

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Side-by-side pretty printing
by Michael Lazarev 07 Aug '14

07 Aug '14

Hi all, When looking into Text.PrettyPrint, I stumbled upon <> (beside) combinator. Is it true that multi-line documents can't be printed one beside other? E.g. this let doc1 = vcat $ map text ["This", "is", "a", "document"] in doc1 <> doc1 prints this: This is a documentThis is a document As well as if I would write `doc1 <> nest 50 doc1`. What I was expecting, was This This is is a a documentdocument Do I get it right that Text.PrettyPrint was not intended for such things?

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Haskell Weekly News: Issue 301
by Daniel Santa Cruz 07 Aug '14

07 Aug '14

Welcome to issue 301 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from July 27 to August 2, 2014 Quotes of the Week * bernalex: I used to have a problem then I used lens now I have an abstraction problem? * trap_exit: haskell is like the 'and they lived happily ever after' of programming languages Top Reddit Stories * The Comonad.Reader » Letter to a Young Haskell Enthusiast Domain: comonad.com, Score: 236, Comments: 56 Original: [1] http://goo.gl/Z5usp3 On Reddit: [2] http://goo.gl/CpPKnC * seL4 is now open source: the world's first end-to-end verified OS kernel Domain: github.com, Score: 87, Comments: 20 Original: [3] http://goo.gl/Uz50bj On Reddit: [4] http://goo.gl/EQK0MQ * FP Haskell Center now free for open projects Domain: fpcomplete.com, Score: 81, Comments: 23 Original: [5] http://goo.gl/ULc9ze On Reddit: [6] http://goo.gl/ZleWCk * -XOverlappingInstances and -XIncoherentInstances to be deprecated Domain: haskell.org, Score: 76, Comments: 64 Original: [7] http://goo.gl/dokkrY On Reddit: [8] http://goo.gl/0AyqA2 * First-Class "Statements": IO as data Domain: blog.jle.im, Score: 68, Comments: 32 Original: [9] http://goo.gl/JNj2D1 On Reddit: [10] http://goo.gl/h1v7yp * Foldable and Traversable in Haskell Domain: blog.jakubarnold.cz, Score: 61, Comments: 30 Original: [11] http://goo.gl/mgI6Mg On Reddit: [12] http://goo.gl/nvwj6j * Minimalist GUI toolkit in GHCi Domain: github.com, Score: 61, Comments: 7 Original: [13] http://goo.gl/9PvsUP On Reddit: [14] http://goo.gl/jQKyG7 * early adopters rejoyce! Haskell Platform 2014.2.0.0 Release Candidate 2 Domain: haskell.org, Score: 48, Comments: 27 Original: [15] http://goo.gl/AGl5po On Reddit: [16] http://goo.gl/ThS3Wc * The Modular, Functional Client Side Domain: stackbuilders.com, Score: 46, Comments: 3 Original: [17] http://goo.gl/eydkfz On Reddit: [18] http://goo.gl/z9q8hE * Beware of bracket Domain: ro-che.info, Score: 46, Comments: 18 Original: [19] http://goo.gl/UYA2mF On Reddit: [20] http://goo.gl/ksMQVV * Real World Haskell: Shipping "Carnival", a Blog Commenting Service Domain: robots.thoughtbot.com, Score: 44, Comments: 5 Original: [21] http://goo.gl/UmPyiL On Reddit: [22] http://goo.gl/4PRdpm * Flee traveller! Flee or you will be corrupted and devoured! Domain: github.com, Score: 41, Comments: 16 Original: [23] http://goo.gl/e3sA6X On Reddit: [24] http://goo.gl/RuAZ4o * Reading files from the /proc filesystem Domain: yesodweb.com, Score: 35, Comments: 4 Original: [25] http://goo.gl/mVfVyM On Reddit: [26] http://goo.gl/Bd2HBT * clckwrks CMS source now on github. A sad farewell to darcs and an optimistic look towards greater contributions from the community. Domain: clckwrks.com, Score: 33, Comments: 31 Original: [27] http://goo.gl/ZLqVWl On Reddit: [28] http://goo.gl/q0za2l * Introduction to GHC.TypeLits with base 4.7, including runtime conversion of symbols. Domain: ponies.io, Score: 32, Comments: 13 Original: [29] http://goo.gl/6FtdyO On Reddit: [30] http://goo.gl/PpmkOc * Converting Make to Shake Domain: neilmitchell.blogspot.com, Score: 23, Comments: 14 Original: [31] http://goo.gl/dOWVhV On Reddit: [32] http://goo.gl/cHYTmS * Learn Snap: Interactive Snap + Heist Examples Domain: learnsnap.chromaticleaves.com, Score: 21, Comments: 14 Original: [33] http://goo.gl/Opwt40 On Reddit: [34] http://goo.gl/06AneL * Hackage: ghc-server: A server interface to GHC. Domain: hackage.haskell.org, Score: 18, Comments: 14 Original: [35] http://goo.gl/ZjhScl On Reddit: [36] http://goo.gl/b9wBfP * Bloggy Badger: Homemade FRP: a study in following the types Domain: gelisam.blogspot.com, Score: 17, Comments: 1 Original: [37] http://goo.gl/T7ujN3 On Reddit: [38] http://goo.gl/HAV1mR Top StackOverflow Questions * IO/Monadic assign operator causing ghci to explode for infinite list votes: 16, answers: 1 Read on SO: [39] http://goo.gl/TC0Snn * A performance conundrum votes: 14, answers: 1 Read on SO: [40] http://goo.gl/b910ez * Does an unused let binding have any effect in Haskell? votes: 12, answers: 3 Read on SO: [41] http://goo.gl/KEl13j * In Haskell, are guards or matchers preferable? votes: 12, answers: 1 Read on SO: [42] http://goo.gl/LiXDjk * If MonadPlus is the “generator” class, then what is the “consumer” class? votes: 12, answers: 2 Read on SO: [43] http://goo.gl/eqRu3a * Why would using head/tail instead of pattern matching make evaluation terminate? votes: 12, answers: 3 Read on SO: [44] http://goo.gl/LOjX0V Until next time, [45]+Daniel Santa Cruz References 1. http://comonad.com/reader/2014/letter-to-a-young-haskell-enthusiast/ 2. http://www.reddit.com/r/haskell/comments/2cbbgf/the_comonadreader_letter_to… 3. https://github.com/seL4/seL4 4. http://www.reddit.com/r/haskell/comments/2c1bnp/sel4_is_now_open_source_the… 5. https://www.fpcomplete.com/business/blog/fp-haskell-center-going-free/ 6. http://www.reddit.com/r/haskell/comments/2c9u0g/fp_haskell_center_now_free_… 7. http://www.haskell.org/pipermail/ghc-devs/2014-July/005830.html 8. http://www.reddit.com/r/haskell/comments/2c136i/xoverlappinginstances_and_x… 9. http://blog.jle.im/entry/first-class-statements 10. http://www.reddit.com/r/haskell/comments/2byuhj/firstclass_statements_io_as… 11. http://blog.jakubarnold.cz/2014/07/30/foldable-and-traversable.html 12. http://www.reddit.com/r/haskell/comments/2c8b87/foldable_and_traversable_in… 13. https://github.com/divipp/lensref/wiki 14. http://www.reddit.com/r/haskell/comments/2cge6f/minimalist_gui_toolkit_in_g… 15. http://www.haskell.org/pipermail/haskell/2014-July/024276.html 16. http://www.reddit.com/r/haskell/comments/2c0ep4/early_adopters_rejoyce_hask… 17. http://www.stackbuilders.com/news/the-modular-functional-client-side 18. http://www.reddit.com/r/haskell/comments/2c21j0/the_modular_functional_clie… 19. http://ro-che.info/articles/2014-07-30-bracket.html 20. http://www.reddit.com/r/haskell/comments/2c4li4/beware_of_bracket/ 21. http://robots.thoughtbot.com/ship-you-a-haskell 22. http://www.reddit.com/r/haskell/comments/2cfdms/real_world_haskell_shipping… 23. https://github.com/haskell/bytestring/blob/2530b1c28f15d0f320a84701bf507d56… 24. http://www.reddit.com/r/haskell/comments/2cbgpz/flee_traveller_flee_or_you_… 25. http://www.yesodweb.com/blog/2014/07/reading-files-proc-filesystem 26. http://www.reddit.com/r/haskell/comments/2buufk/reading_files_from_the_proc… 27. http://www.clckwrks.com/page/view-page-slug/12/clckwrks-now-on-github 28. http://www.reddit.com/r/haskell/comments/2cd26w/clckwrks_cms_source_now_on_… 29. http://ponies.io/posts/2014-07-30-typelits.html 30. http://www.reddit.com/r/haskell/comments/2c8bfi/introduction_to_ghctypelits… 31. http://neilmitchell.blogspot.com/2014/07/converting-make-to-shake.html 32. http://www.reddit.com/r/haskell/comments/2bukaj/converting_make_to_shake/ 33. http://learnsnap.chromaticleaves.com/ 34. http://www.reddit.com/r/haskell/comments/2cdc36/learn_snap_interactive_snap… 35. http://hackage.haskell.org/package/ghc-server-1.0 36. http://www.reddit.com/r/haskell/comments/2c17zz/hackage_ghcserver_a_server_… 37. http://gelisam.blogspot.com/2014/07/homemade-frp-study-in-following-types.h… 38. http://www.reddit.com/r/haskell/comments/2c0vg4/bloggy_badger_homemade_frp_… 39. http://stackoverflow.com/questions/24986296/io-monadic-assign-operator-caus… 40. http://stackoverflow.com/questions/25089668/a-performance-conundrum 41. http://stackoverflow.com/questions/24984661/does-an-unused-let-binding-have… 42. http://stackoverflow.com/questions/25000929/in-haskell-are-guards-or-matche… 43. http://stackoverflow.com/questions/25070740/if-monadplus-is-the-generator-c… 44. http://stackoverflow.com/questions/25078598/why-would-using-head-tail-inste… 45. https://plus.google.com/105107667630152149014/about

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Dealing with encodings
by Dmitry Bogatov 06 Aug '14

06 Aug '14

Hello! How teach hxt to handle "KOI8-R" encoding of input file? And it seems that so many great packages (like hxt, feed, curl) uses String. Is it some work in progress to port them to Text? -- Best regards, Dmitry Bogatov <KAction(a)gnu.org>, Free Software supporter, esperantisto and netiquette guardian. GPG: 54B7F00D

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Applicative & Monad as interaction between categories
by Ruben Astudillo 05 Aug '14

05 Aug '14

Hi all, I recently came across of two blog posts[1][2] by Gabriel Gonzales where he shows motivation for writing compositional functions -- and thus why the concept of category is useful -- and motivation to write functors between them. I wanted to know if something similar for has been done to `Applicative' and `Monad' as design pattern and interaction between categories. I've searching but I always hit the Monad tutorials and more info on how to use them. Thanks in advance. Excuse my English. [1]: http://www.haskellforall.com/2012/08/the-category-design-pattern.html [2]: http://www.haskellforall.com/2012/09/the-functor-design-pattern.html -- Ruben Astudillo. pgp: 0x3C332311 , usala en lo posible :-) Crear un haiku, en diecisiete silabas, es complica...

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[ANN] Hayoo Relaunch
by Sebastian Philipp 05 Aug '14

05 Aug '14

Hi Cafe, Hayoo <http://hayoo.fh-wedel.de> has been relaunched. Hayoo is a search engine for Hackage packages, which allows you to search for functions, data types and packages. It is currently work in progress. Any feedback is greatly appreciated! Hayoo uses Hunt <https://github.com/hunt-framework> for indexing and searching, which is the successor of Holumbus. Hunt is a flexible, lightweight search platform with a powerful query language and JSON API. Example search requests are: * Function names: map <http://hayoo.fh-wedel.de/?query=map> * Function signatures: (a->b)->f a->f b <http://hayoo.fh-wedel.de/?query=%28a-%3Eb%29-%3Ef+a-%3Ef+b> * Module names: Control.Loop <http://hayoo.fh-wedel.de/?query=Control.Loop> Have a look at the examples on <http://hayoo.fh-wedel.de/examples> for some advances queries. The old Hayoo and Holumbus are still online at <http://holumbus-alt.fh-wedel.de/> -- Sebastian Philipp

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Fwd: Default stdout buffering of child process of createProcess
by Chris Myzie 05 Aug '14

05 Aug '14

I'm reposting this message because I think it only went to the google group and not the official haskell-cafe list: On Friday, August 1, 2014 10:06:32 AM UTC-4, Chris Myzie wrote: > > As a workaround, I am able to trick the child haskell process into > thinking it's running in an interactive terminal by wrapping it with > /usr/bin/script: > > createProcess (proc "/usr/bin/script" ["-qc","./A","/dev/null"]) { std_out > = CreatePipe } > > I still think haskell is using screwy defaults for stdout buffering.. > > On Thursday, July 31, 2014 3:24:47 PM UTC-4, Chris Myzie wrote: >> >> Hello, >> >> I'm trying to write a wrapper process that will be able to read any child >> process' stdout. The problem I'm running into is that unless I force the >> child's stdout to LineBuffering, it defaults to BlockBuffering. Is >> BlockBuffering really the default in this case? I don't want to have to >> modify all of the child processes that I want to use with this wrapper. >> >> Below is a simple test case. A.hs is the child process, and B.hs is the >> wrapper. If I run B.hs, I will get no output unless I uncomment the line >> in A.hs. >> >> Thanks, >> Chris >> >> >> ------------------------------ A.hs --------------------------- >> import Control.Concurrent >> import System.IO >> >> main :: IO () >> main = do >> -- hSetBuffering stdout LineBuffering >> putStrLn "test" >> threadDelay 1000000 >> main >> >> >> ------------------------------ B.hs --------------------------- >> import Control.Monad >> import System.IO >> import System.Process >> >> main :: IO () >> main = do >> (_,Just h,_,_) <- createProcess (proc "./A" []) { std_out = CreatePipe } >> hSetBuffering h LineBuffering >> forever $ hGetLine h >>= putStrLn >> >

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