- Haskell-Cafe - Haskell.org

interactive-diagrams: call for suggestions
by Daniil Frumin 06 Sep '13

06 Sep '13

Good time of day, everyone! As some of you may know, I am developing an interactive-diagrams pastebin as part of the GSoC program [1]. The demo is live at <http://paste.hskll.org>, feel free to play around with it. This week I plan to migrate to the PostgreSQL database (right now all the information is stored using sqlite). And since I am already going to write code for migration I might as well implement some changes to the DB schema. That's why I am asking you to provide any suggestions on what (front-end) features would you like to see, that are most likely require to change the DB schema. Right now I plan on implementing changes that would allow the following: - Ability to annotate pastes. Pastes may have parent pastes and child pastes. - Logins (via GitHub), in addition to pseudonymous-pastes where user can just enter any text as his nickname. This will also allow user to make collections of his pastes. - Stars. Logged in users may "star" pastes they like. Please report your suggestions here or on GitHub [2]. Thanks! [1] https://gist.github.com/co-dan/437e9694563c0c13dd8e [2] https://github.com/co-dan/interactive-diagrams/issues/16 -- Sincerely yours, -- Daniil Frumin

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ghc-mod v3.0.0
by Kazu Yamamoto 06 Sep '13

06 Sep '13

Hi all, I have just uploaded ghc-mod v3.0.0 to Hackage. In this version, ghc-mod supports the sandbox feature of "cabal-install". Instead, it stopped supporting "cabal-dev". If you want to use ghc-mod v3.0.0, I would recommand to install cabal-install 1.18. The sandbox in your package is automatically detected by ghc-mod if exists. --Kazu

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Reminder to file Haddock bugs on Trac.
by Mateusz Kowalczyk 06 Sep '13

06 Sep '13

This is just a friendly reminder that if you have any issues with Haddock that you would like to get looked at, you should probably be making your way to the Haddock Trac[1] now. If you already have open tickets, you can probably get them looked at if you reply to this thread. Unfortunately there are a lot of old tickets so it's difficult to tell what's still desired, what's still a problem, what has been solved outside of Trac and not marked and so on. Thanks! [1]: http://trac.haskell.org/haddock/ -- Mateusz K.

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Performance of delete-and-return-last-element
by Lucas Paul 05 Sep '13

05 Sep '13

Suppose I need to get an element from a data structure, and also modify the data structure. For example, I might need to get and delete the last element of a list: darle xs = ((last xs), (rmlast xs)) where rmlast [_] = [] rmlast (y:ys) = y:(rmlast ys) There are probably other and better ways to write rmlast, but I want to focus on the fact that darle here, for lack of a better name off the top of my head, appears to traverse the list twice. Once to get the element, and once to remove it to produce a new list. This seems bad. Especially for large data structures, I don't want to be traversing twice to do what ought to be one operation. To fix it, I might be tempted to write something like: darle' [a] = (a, []) darle' (x:xs) = let (a, ys) = darle' xs in (a, (x:ys)) But this version has lost its elegance. It was also kind of harder to come up with, and for more complex data structures (like the binary search tree) the simpler expression is really desirable. Can a really smart compiler transform/optimize the first definition into something that traverses the data structure only once? Can GHC? - Lucas

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Unexpected behaviour with send and send-buffer setting
by Simon Yarde 05 Sep '13

05 Sep '13

I'm new to Haskell and have reached an impasse in understanding the behaviour of sockets. I see that under the hood Network.Socket sockets are set to non-blocking. Presumably, when a non-blocking socket's buffer is full it should immediately return 0 bytes. I've found that setting the send buffer size causes send to truncate the ByteString to the buffer size, but that successive sends continue to succeed when the buffer should be full. In the server code I set the send buffer to 1, then attempted to overflow it: handleConnection conn = do setSocketOption conn SendBuffer 1 s1 <- send conn "abc" putStrLn $ "Bytes sent: " ++ show s1 s2 <- send conn "def" putStrLn $ "Bytes sent: " ++ show s2 s3 <- send conn "ghi" putStrLn $ "Bytes sent: " ++ show s3 close conn And in the client I delay the recv by 1 second: setSocketOption sk RecvBuffer 1 threadDelay (1 * 10^6) b1 <- recv sk 1 B8.putStrLn b1 b2 <- recv sk 1 B8.putStrLn b2 b3 <- recv sk 1 B8.putStrLn b3 The server immediately outputs: Bytes sent: 1 Bytes sent: 1 Bytes sent: 1 The client waits a for second and then outputs: a d g What's going on? I expected the second and third send operation to return 0 bytes sent, because the send buffer can only hold 1 byte. The crux of my line of enquiry is this; how can my application know when to pause in generating its chunked output if send doesn't block and the current non-blocking send behaviour apparently succeeds when the send buffer should be full? More generally, isn't polling sockets using system calls to be avoided in favour of blocking and lightweight haskell threads? Hope someone can help clear up the confusion.

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 121, Issue 8
by Gregory Guthrie 05 Sep '13

05 Sep '13

> Hi all, > > On behalf of the cabal maintainers and contributors I'm proud to announce the Cabal (and > cabal-install) 1.18.0 release. To install run > > cabal update && cabal install Cabal-1.18.0 cabal-install-1.18.0 > > With 854 commits since the last release there are two many improvements and bug fixes to > list them here, but two highlights are: > > * Hermetic builds using sandboxes. This should reduce the number of "dependency hell" and > broken package DB problems. > [>>] I was quite happy to hear of the new cabal - and the chance to escape the various mazes of cabal-dependency problems. But I seem to have a bootstrapping problem, the new cabal won't install to help me get out of its messes! I tried to install what I thought were its complaints (below), and they seemed OK - any hints? The key problem seems to be: " mtl-2.1.2-5337caef659244e51e2f5fb2e944d97f is shadowed by package mtl-2.1.2-678980f6077e9f83a86186556aa2a425" Not sure how this happened, or how to remedy it. Cabal FAQ says to unregister the user database version, but that notes that it would break about 20+ other packages, but do it anyway. So yes, I did and now have a bigger mess it seems. Now mtl install gives: Building mtl-2.1.2... Preprocessing library mtl-2.1.2... Bad interface file: E:\Plang\Haskell Platform\lib\extralibs\transformers-0.3.0.0\ghc-7.4.2\Control\Monad\Trans\Error.hi mismatched interface file versions (wanted "7063", got "7042") Failed to install mtl-2.1.2 cabal: Error: some packages failed to install: mtl-2.1.2 failed during the building phase. The exception was: ExitFailure 1 I am reluctant to do a clean restart install, since it was so much trouble building some packages like wx and other graphics .dll packages. Argh... --------------------------------------------------------------------------------------------------- C:\Users\guthrie>cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install C:\Users\guthrie>cabal install cabal-install Resolving dependencies... Downloading Cabal-1.18.0... [ 1 of 70] Compiling Distribution.PackageDescription.Utils ... [70 of 70] Compiling Main ( C:\Users\guthrie\AppData\Local\Temp\Cabal-1.18.0-13108\Cabal-1.18.0\Setup.hs, C:\Users\guthrie\AppData\Local\Temp\Cabal-1.18.0-13108\Cabal-1.18.0\dist\setup\Main.o ) Linking C:\Users\guthrie\AppData\Local\Temp\Cabal-1.18.0-13108\Cabal-1.18.0\dist\setup\setup.exe ... Configuring Cabal-1.18.0... Building Cabal-1.18.0... Preprocessing library Cabal-1.18.0... [ 1 of 72] Compiling Paths_Cabal ... Distribution\Version.hs:125:1: Warning: Orphan instance: instance Data Version In-place registering Cabal-1.18.0... Installing library in C:\Users\guthrie\AppData\Roaming\cabal\i386-windows-ghc-7.6.3\Cabal-1.18.0 Registering Cabal-1.18.0... Installed Cabal-1.18.0 Downloading cabal-install-1.18.0... Configuring cabal-install-1.18.0... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. ... package mtl-2.1.2 requires transformers-0.3.0.0 package mtl-2.1.2 requires transformers-0.3.0.0 Building cabal-install-1.18.0... Preprocessing executable 'cabal' for cabal-install-1.18.0... <command line>: cannot satisfy -package-id mtl-2.1.2-5337caef659244e51e2f5fb2e944d97f: mtl-2.1.2-5337caef659244e51e2f5fb2e944d97f is shadowed by package mtl-2.1.2-678980f6077e9f83a86186556aa2a425 (use -v for more information) Failed to install cabal-install-1.18.0 cabal: Error: some packages failed to install: cabal-install-1.18.0 failed during the building phase. The exception was:ExitFailure 1 C:\Users\guthrie>cabal install mtl Resolving dependencies... All the requested packages are already installed: mtl-2.1.2 Use --reinstall if you want to reinstall anyway. C:\Users\guthrie>cabal install transformers Resolving dependencies... All the requested packages are already installed: transformers-0.3.0.0 Use --reinstall if you want to reinstall anyway.

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Tutorial on JS with Haskell: Fay or GHCJS?
by Alejandro Serrano Mena 05 Sep '13

05 Sep '13

Hi, I'm currently writing a tutorial on web applications using Haskell. I know the pros and cons of each server-side library (Yesod, Snap, Scotty, Warp, Happstack), but I'm looking for the right choice for client-side programming that converts Haskell to JavaScript. I've finally come to Fay vs. GHCJS, and would like your opinion on what's the best to tackle. My current list of pros and cons is: Fay === Pros: - Does not need GHC 7.8 - Easy FFI with JS - Has libraries for integration with Yesod and Snap Cons: - Only supports a subset of GHC (in particular, no type classes) GHCJS ====== Pros: - Supports full GHC - Easy FFI with JS - Highly opinionated point: will stay longer than Fay (but it's very important for not having a tutorial that is old in few months) Cons: - Needs GHC 7.8 (but provides a Vagrant image)

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How to read a file and return a String?
by yi lu 05 Sep '13

05 Sep '13

I want to read a text file, and store it in a *String*. But readFile will get *IO String*. I search with google and they tell me it is not necessarily to do so. Can you explain to me why is this? Furthermore, How to read a file and store it in a String? In fact, I want to read a file and split it into *[String]* with *lines*function. Here is what I have tried. link<https://github.com/eccstartup/findDict/blob/master/src/Data/Dict/Utils.hs>I think I could directly operate on *[String]*. But it is not true for me. [1] http://www.haskell.org/hoogle/?hoogle=IO+String+-%3E+String Yi Lu

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Efficient matrix multiply using accelerate
by Morten Olsen Lysgaard 05 Sep '13

05 Sep '13

I've been trying to get some speed out of the accelerate library today. What I want to implement is something as simple as a matrix multiply. I'd like it to be fast and memory efficient. Given the equation C = AB where A is nxr B is rxm C is nxm it seem reasonable to allocate three arrays on the GPU wiht n*r, r*m and n*m elements respectively. Anyone know how to achieve this with accelerate? My first thought was to use the generate function to create the new C array, but I didn't manage to wrap my head around all the fancy type features that pop up when you want to return an array C that has dimensions dependent on the dimensions of it's inputs, A and B. I've search around a bit and found this [1] example implementation but it is just as slow as a simple sequential algorithm in C. I would be very thankful for any advice for working with accelerate! Here's a snippet of what I have tried to make. There are several errors in there. Maybe I'm approaching the problem from the wrong angle. matMul' arr brr = let dotProd shp = let (Z :. rowsA :. _) = unlift (shape arr) :: (Z :. Exp Int :. Exp Int) (Z :. _ :. colsB) = unlift (shape brr) :: (Z :. Exp Int :. Exp Int) (Z :. i :. j) = unlift shp :: (Z :. Exp Int :. Exp Int) rs = lift (Z :. All :.) (unlift i) cs = (lift (Z :.) (unlift j)) (:. All) in the $ A.fold1All (+) $ A.zipWith (+) (flatten (slice arr rs)) (flatten (slice brr cs)) in A.generate (lift (Z :. rowsA :. colsB)) dotProd [1] http://www.mail-archive.com/haskell-cafe@haskell.org/msg102782.html

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type constructor section for (-> Bool), _not_ ((->) Bool)
by AntC 05 Sep '13

05 Sep '13

I'm probably being dumb, but Hoogle nor the wiki are helping me. I want an instance and type improvement constraint of the form instance (f ~ (-> Bool)) => C Foo (f b) where ... The first arg to C is driving type improvement, for example: instance (f ~ []) => C Bar (f b) where ... (The real instances are more complex, and involve overlap, of course.) I'm trying to write a section to get the improved type (b -> Bool), but (-> Bool) or ((-> Bool) b) is invalid syntax. This is valid, but wrong: ((->) Bool) b -- gives (Bool -> b). I could do: data FlipFun b -- abstract instance (f ~ FlipFun) => C Foo (f b) where ... And a type function inside the class to generate the type. But then I'd have to apply the type function for all instances, and in most places it'd be id. ??

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ANNOUNCE: Haskell Refactorer (HaRe) version 0.7.0.0
by AlanKim Zimmerman 04 Sep '13

04 Sep '13

I am please to announce an alpha release of the Haskell Refactorer making use of the GHC API. It is a work in progress, but currently supports the following refactorings iftocase Convert an if expression to a case expression dupdef Duplicate a definition liftToTopLevel Lift a declaration to the top level liftOneLevel Lift a declaration one level demote Move a declaration down one level rename Rename an identifier It offers emacs integration only at this point, based on the Wrangler elisp. The underlying environment in terms of cabal file, sandbox, etc are managed by ghc-mod via the internals exposed in version 2.1.2 There are some known issues (and bound to be plenty of unknown ones too). The most damaging of these is that layout is not adjusted after renaming an identifier if the new name is a different length. This affects layout-sensitive code. Installation instructions can be found at https://github.com/alanz/HaRe/blob/master/README.markdown Regards Alan

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Haskell / Functional Programmers Group in Madrid (Spain)?
by Alejandro Serrano Mena 04 Sep '13

04 Sep '13

Hi, Is there any Haskell or functional programmers user group in the region of Madrid? If not, I think it could be a great idea to get to know each other, and share experiences. Furthermore, we could try to organize some meetings with presentations, hackatons and so on (my personal ambition is to be able to organize another "ekmett Workshop" as the Japanese one - http://comonad.com/reader/2013/japanese-workshop-1/) I know there is some locations along Madrid dedicated to open source promotion, like http://madridonrails.com/, and which we could use for these meetings. I'm really looking forward to hearing from you! PS: Maybe a bare "Haskell" group is not big enough (at least according to Haskellers.com) but a "functional programmers" one may well be.

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Fwd: Can I use String without "" in ghci?
by Rustom Mody 04 Sep '13

04 Sep '13

On Mon, Sep 2, 2013 at 10:43 AM, Richard A. O'Keefe <ok(a)cs.otago.ac.nz>wrote: > > On 2/09/2013, at 3:55 PM, Rustom Mody wrote: > > > On Mon, Sep 2, 2013 at 5:43 AM, Richard A. O'Keefe wrote: > > > > > A slogan I have programmed by since I first met C and recognised > > how vastly superior to PL/I it was for text manipulation _because_ > > it didn't have a proper string type is "Strings are Wrong!". > > > > C rode to fame on the back of Unix. And Unix's innovation – one of many – > is that at the OS level the string type was made common fare – a universal > type. So everything from file names to file contents to IPC is a string. > > The idea of file names being strings was no innovation. > Yes, in crippled monstrosities like TOPS-10 file names were > weird records -- I can still remember too much of the details -- > and every ruddy TOPS-10 program had to do its own file name > parsing and it seemed as if they all did it differently. But > the B6700 MCP interfaces treated file names as strings before > UNIX was dreamed of. > > File contents in UNIX are *not* strings and never have been -- > NUL termination is no part of files and binary files have been > commonplace since the beginning (an a.out file is not a string!). > They are *byte arrays*. > > As for IPC, since when have System V shared memory, semaphores, > or message queues had anything to do with strings? > (Hint: the 'name' of a System V shared memory segment is a > key_t, and that's an integral type, not a string. > Hint: the 'name' of a System V semaphore is also a key_t > integer, not a string. > Hint: the 'name' of a System V message queue is also a key_t > integer, not a string. > Hint: messages sent using msgsnd are not strings, they are > byte arrays with a separate count parameter. > ) > > Whoops! my bad -- I was *thinking* 'pipes' but ended up *writing* 'IPC' :-) So let me restate more explicitly what I intended -- pipes, FIFOs, sockets, etc. IOW read/write/send/recv calls and the mathematical model represented by the (non-firstclass) pair of C data structures in those functions: <buf, len> (or count). As an aside: modern usage types the buf as void * . The version 7 unix manuals on which I grew up (and first edition of K&R), there was no void; buf would be just 'char *buf; ' > Classic UNIX uses strings for file names, and really, that's it. > (The command line argv[] is not really an exception, because it > was used for file names as well as options, and in fact mixing > the two up caused endless problems.) > Everything else in V7, S3, or SysV was identified by a *number*. > Plan 9 has exit(string) but Unix has exit(byte). > > From the perspective of someone who used UNIX v6 in 1979, > *POSIX* IPC -- with its IPC objects *might* be in the file > system but then again might *not* be so their names are > sorta-kinda-like file names but not really) -- and /proc are > recent innovations. > > The idea that 'string' was even remotely like a "universal type" > in UNIX is bizarre. > > Heck, UNIX never even used 'string' for *lines* in text files! > > > Of course when instructing a beginning programmer your basic premise > 'Strings are Wrong!' is most likely right. > > No, I'm talking about experienced programmers writing high performance > programs. > > > However if programs are seen as entities interacting with an 'external' > world, the currency at the portals is invariably string. > > - The currency at the portals is *not* invariably string. > Learn PowerShell. > - "Text" is one thing and "string" is another. This was the > B6700 lesson (well, really the B5500 lesson): for many purposes > you want a text *stream* not a text *string* at the interface. > It's also the what-Smalltalk-got-right-and-Java-got-wrong > lesson: the right way to convert objects to text is via a > *stream* interface, not a *string* interface. > I realize this is a terminology issue: My usage of terminology like string/file are evidently more aligned to http://en.wikipedia.org/wiki/Vienna_Development_Method#Collections:_Sets.2C… file(chap 4): http://red.cs.nott.ac.uk/~rxq/files/SpecificationCaseStudies.pdf Contrariwise 'file' can mean http://en.wikipedia.org/wiki/Data_set_%28IBM_mainframe%29 So coming back from terminology to principles... > And more than just noob programmers have got this wrong – think of the > precious one-byte opcodes that Intel wastes on ascii and decimal arithmetic. > > Hang on, they are there in order to *support* the "numbers are text" model. > You can't have it both ways. > So let me restate (actually I didn't state it earlier!) my point in this example: When Intel introduced these instructions in 8008 (or whatever) decades ago, it seemed like a good idea to help programmers and reduce their burden by allowing them to do some minimal arithmetic on data without burdensome conversion-to-binary functions. 4 decades on and (Intel's very own Gordon) Moore's law ensuring our machines and networks some 7 orders of magnitude larger, the cost-equations look different. printf and scanf are a basic given in any C library so optimizing them out does not optimize anything. On the other hand having instructions --that too 1-byte instructions -- that are almost never used is terribly inefficient: - the extra transistors in the millions of CPUs that are never used - the instructions that are used become fatter. Multiply by the GBs per installation multipled by millions of installations. And so Intel made (makes) the same mistake that the typical programmer-noob makes, which you capture pithily in your 'Strings are wrong!' Put slightly more verbosely: Strings (or byte-arrays if you prefer) are invariably what come into and go out of your program. Keep them that way and you reduce your work in the immediate term but increase it in the long term since it is almost always a bad fit for both problem-domain and machine-model. Building appropriate models is the central business of programmers Brings me to the OPs question: I want to know if it is possible that I use strings without "". > > If I type > *Prelude>foo bar* > which actually I mean > *Prelude>foo "bar"* > However I don't want to type ""s. > > I have noticed if *bar* is predefined or it is a number, it can be used > as arguments. But can other strings be used this way? Like in bash, we can > use *ping 127.0.0.1* where *127.0.0.1* is an argument. > > If not, can *foo* be defined as a function so that it recognize arguments > like *bar* as *"bar"*? > Its not clear what your use-case is. 1. In the simplest and most common case, a few data declarations will give you what you want. eg if in place of bar you have (a few) colors like red, green etc you can do data Color = Red | Green... and your strings like "red" "green" will disappear and become Red Green etc IOW if your "bar" and like strings is a small enumerate-able set then make a corresponding enumerated type. Thereafter haskell will do the quoting for you. For the IP example you dont even need a data; just type type IP = (Int,Int,Int,Int) or if you prefer type IP = (Word8,Word8,Word8,Word8) and then the shell 127.0.0.1 would be the haskell (127,0,0,1) Sometimes though one may prefer to be a little pedantic and write newtype IP = IP (Word8,Word8,Word8,Word8) Note the exact fit of the Word8 to the IP spec. And the corresponding non-exact fit with the string form: What happens when you see the string, "500.1000.1.2" ? You need to decide... Beyond that, as others have shown you may want to consider building your own DSL perhaps using template haskell Beyond that... are you sure shell is not what you want? ie if you need full haskell power, and typical shell quoting behavior and you have thought through a design that is sound and consistent (note Albert's refs above to $ usage in shell) then I (and I guess many others here) will want to hear of it!! However for starters you probably dont want to go beyond just defining well-fitting datas and types and many of the ".." will vanish from your code Rusi -- http://www.the-magus.in http://blog.languager.org

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Traversals of monomorphic containers (was: Re: mapM_ for bytestring)
by Nicolas Trangez 04 Sep '13

04 Sep '13

# Redirected to haskell-cafe On Sun, 2013-09-01 at 14:58 +0400, Artyom Kazak wrote: > Would this be an appropriate place to propose adding mapM_ (and then > possibly mapM) to bytestring library? > > Was it suggested before? If yes, why was it rejected? This got me wondering: there are several type-classes useful for polymorphic container types, e.g. Functor, Foldable & Traversable which all apply to some type of kind (* -> *). Are there related things for monomorphic containers, like ByteString, Text or some newtype'd Vector with fixed element type, e.g. class MFunctor f a where mfmap :: (a -> a) -> f -> f instance MFunctor ByteString Word8 where mfmap = ByteString.map or (maybe even better) class MFunctor f where type Elem mfmap :: (Elem -> Elem) -> f -> f instance MFunctor ByteString where type Elem = Word8 mfmap = ByteString.map and similar for other classes. Nicolas

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Fwd: World's First Commercial Haskell IDE and Deployment Platform, FP Haskell Center Launches Today
by Natalia Muska 04 Sep '13

04 Sep '13

FP Complete Has Officially Launched the World's First Commercial Haskell IDE! http://www.i-newswire.com/fp-complete-launches-fp-haskell/237230 We greatly appreciate the support we've received from the Haskell community. Natalia Muska Marketing Manager FP Complete, Inc. Natalia(a)fpcomplete.com 865-506-6513 skype: natalia.s.muska -- Natalia Muska Marketing Manager FP Complete, Inc. Natalia(a)fpcomplete.com 865-506-6513 skype: natalia.s.muska

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On Markdown in Haddock and why it's not going to happen
by Mateusz Kowalczyk 03 Sep '13

03 Sep '13

Greetings café, Perhaps some saddening news for Markdown fans out there. As you might remember, there was a fair amount of push for having Markdown as an alternate syntax for Haddock. Unfortunately, this is probably not going to happen for reasons listed on the post I just published at [1]. This thread is meant to be for discussion about the post as many people, myself included, felt that Markdown would be a nice thing to have. I feel like I covered the topic fairly well on the post but feel free to give suggestions or ask questions. I would also like to remind you that if there's something that you'd like to see in Haddock or something that you feel is broken, a good way express this is to make a ticket on the Haddock Trac[2]. I will close the relevant Markdown ticket on the Trac[3] in about 3 days, unless someone can come up with a reasonable solution that meets the initial intent of this part of the project: a widely known markup format that could be used as an alternate syntax for Haddock so that it's possible to write the documentation without learning the vanilla syntax itself. [1]: http://fuuzetsu.co.uk/blog/posts/2013-08-30-why-Markdown-in-Haddock-can't-happen.html [2]: http://trac.haskell.org/haddock [3]: http://trac.haskell.org/haddock/ticket/244 -- Mateusz K.

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Fwd: MVar problem in acid-state?
by Corentin Dupont 03 Sep '13

03 Sep '13

Hi the list, I have compiled my application on my PC, it works fine, but when I copy it on my server (same architecture), I get: Nomyx: thread blocked indefinitely in an MVar operation I don't use MVars in my application, is it possible that it's coming from acid-state? Thanks, Corentin

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Re: [Haskell-cafe] stream interface vs string interface: references
by oleg＠okmij.org 03 Sep '13

03 Sep '13

> For lazy I/O, using shows in Haskell is a good analogue of using > #printOn: in Smalltalk. The basic form is "include this as PART of > a stream", with "convert this to a whole string" as a derived form. > > What the equivalent of this would be for Iteratees I don't yet > understand. Why not to try simple generators first, which are simpler, truly. It seems generators reproduce the Smalltalk printing patterns pretty well -- even simpler since we don't have to specify any stream. The printing takes linear time in input size. The same `printing' generator can be used even if we don't actually want to see any output -- rather, we only want the statistics (e.g., number of characters printed, or number of lines, etc). Likewise, the same printing generator print_yield can be used if we are to encode the output somehow (e.g., compress it). The entire pipeline can run in constant space (if encoding is in constant space). Here is the code module PrintYield where -- http://okmij.org/ftp/continuations/PPYield/ import GenT import Data.Set as S import Data.Foldable import Control.Monad.State.Strict type Producer m e = GenT e m () class PrintYield a where print_yield :: Monad m => a -> Producer m String instance PrintYield Int where print_yield = yield . show instance (PrintYield a, PrintYield b) => PrintYield (Either a b) where print_yield (Left x) = yield "Left " >> print_yield x print_yield (Right x) = yield "Right " >> print_yield x instance (PrintYield a) => PrintYield (Set a) where print_yield x = do yield "{" let f True x = print_yield x >> return False f False x = yield ", " >> print_yield x >> return False foldlM f True x yield "}" instance PrintYield ISet where print_yield (ISet x) = print_yield x newtype ISet = ISet (Either Int (Set ISet)) deriving (Eq, Ord) set1 :: Set ISet set1 = Prelude.foldr (\e s -> S.fromList [ISet (Left e), ISet (Right s)]) S.empty [1..200000] -- Real printing print_set :: Set ISet -> IO () print_set s = print_yield s `runGenT` putStr t1 = print_set set1 -- Counting the number of characters -- Could use Writer but the Writer is too lazy, may leak memory count_printed :: Set ISet -> Integer count_printed s = (print_yield s `runGenT` counter) `execState` 0 where counter _ = get >>= put . succ_strict succ_strict x = x `seq` succ x -- According to GHCi statistics, counting is linear in time -- (space is harder to estimate: it is not clear what GHCi prints -- for memory statistics; we need max bytes allocated rather than -- total bytes allocated) t2 = count_printed set1 -- Doesn't do anything but ensures the set is constructed t3 :: IO () t3 = print_yield set1 `runGenT` (\x -> return ())

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stream interface vs string interface: references
by damodar kulkarni 03 Sep '13

03 Sep '13

In a reply to the question on the thread titled "Can I use String without "" in ghci?", Richard A. O'Keefe has noted that, It's also the what-Smalltalk-got-right-and-Java-got-wrong lesson: the right > way to convert objects to text is via a *stream* interface, not a *string* > interface. > I didn't want to clutter that thread so I am asking a question here. Where do I find foundational and/or other good references on the topic of "stream interface vs string interface to convert objects to text"? I tried google but failed. I am looking for pointers that explain the rational behind the above claim made by O'Keefe. and what is the take of Haskell on this topic? Thanks and regards, -Damodar Kulkarni

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Fw: Re: On Markdown in Haddock and why it's not going to happen
by John MacFarlane 03 Sep '13

03 Sep '13

+++ Mateusz Kowalczyk [Sep 02 13 21:34 ]: > On 02/09/13 19:43, John MacFarlane wrote: > > When the proposal was first being discussed, I suggested that instead of > > adding markdown support to haddock, one might enhance the existing > > haddock markup, making it more expressive, so that it could encode the same > > range of structural features as markdown. If I'm not mistaken, currently > > haddock doesn't allow list items with multiple paragraphs or other block > > elements, or nested lists, or images, or blockquotes. > Paragraph level elements are indeed not allowed in lists. The list > contents are currently (that is, on my working branch) allowed to be: > monospace (delimited by @), anchors, identifiers, module names, > pictures, URLs, bold, emphasis, HTML escape codes, regular strings. Note > that if a monospaced string is the only element of the list content, it > gets turned into a code block. > > I'm not sure what you mean by block elements. I mean things like paragraphs, blockquotes, code blocks, other lists. In markdown you can have a list item that contains these things: 1. This is my first list item. Second paragraph of first list item. Some code: foo = bar <$> baz <*> 2.0 2. This is my second list item. * Sublist item one * Sublist item two 3. This is my third list item. Someone said: > We want markdown support in haddock! In Haddock, last I looked, you couldn't do any of this... In your original message, you emphasized features of Haddock that don't exist in standard markdown and would require extensions (definition lists, automatic links to Haskell identifiers, etc.). But in many other ways, markdown is much more expressive than Haddock markup, as the example above illustrates. The changes that would be needed to Haddock to allow it to express what can be expressed in markdown are much bigger, I think, than the changes that would be needed to markdown to allow it to express what can be expressed in Haddock.

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Proposal: Generic conditions for 'if' and 'case'
by Wvv 02 Sep '13

02 Sep '13

I think it is an old idea, but nevertheless. Now we have next functions: if (a :: Bool) then x else y case b of a1 :: Bool -> x1 a2 :: Bool -> x2 ... Let we have generic conditions for 'if' and 'case': class Boolean a where toBool :: a -> Bool instance Boolean Bool where toBool = id instance Boolean [a] where toBool [] = False toBool _ = True instance Boolean (Maybe a) where toBool Nothing = False toBool _ = True instance Boolean Int where toBool 0 = False toBool _ = True if' (a :: Boolean b) then x else y case' d of a1 :: Boolean b1 -> x1 a2 :: Boolean b2 -> x2 ... It is very easy to implement to desugar: if' a then ... == if toBool ( a ) then ... -- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Generic-conditions-for-if-and… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANNOUNCE: Ajhc Haskell Compiler 0.8.0.8 Release
by Kiwamu Okabe 02 Sep '13

02 Sep '13

We are happy to announce Ajhc 0.8.0.8. This version is a point release to fix some BUGs, and support new platforms. You can get Ajhc using "cabal install ajhc" command. The usage is found at Ajhc's project web site http://ajhc.metasepi.org/. The source code at https://github.com/ajhc/ajhc/tags. Welcome sending any bugs or your ideas to https://github.com/ajhc/ajhc/issues. ## Support new platforms ### mbed http://mbed.org/ The mbed platform is a single-board microcontroller using ARM Cortex-M3 CPU. It has only 32kB RAM for application. >From this release, Ajhc support mbed and Text LCD device driver. You can find the demo source code at https://github.com/ajhc/demo-cortex-m3, and watch the demo movie at http://www.youtube.com/watch?v=fOsjEfNmTkY. ### ChibiOS/RT http://www.chibios.org/ The ChibiOS/RT is embedded real time OS. >From this release, Ajhc can compile Haskell application running on ChibiOS/RT. The application is able to use forkOS API with ChibiOS/RT's thread. You will find the demo source code at https://github.com/metasepi/chibios-arafura. ## Other changes * Use cpphs command for C preprocessor. * Don't insert (void) cast for function call. * Add --targetsini option to specify the targets.ini file. * Data.Bits support CInt. * Retry download when some error occured on getting tar ball from Hackage DB. * Fix BUGs * Don't increment megablock count, when re-use the megablock in free list. * Fix IORef compile BUG. Enjoy! :) - - - Metasepi team

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ANN: th-desugar simplifies Template Haskell processing
by Richard Eisenberg 02 Sep '13

02 Sep '13

I've just uploaded my new th-desugar package, which enables easier processing of Template Haskell source syntax by desugaring it into a much simpler core language. The meaning of the code after desugaring is identical to before desugaring, but the syntax is much simpler. To wit, th-desugar simplifies out all of the following constructs: - guarded expressions in both functions and case statements - "where" declarations - do syntax - list/monad comprehensions - record creation / updates - as patterns - non-trivial patterns in a lambda expression - lambda-case - multi-way if - several more If you are writing a library that manipulates Template Haskell syntax, you may wish to consider if th-desugar will make your job easier by forcing you to consider fewer cases. The one source Haskell construct supported by Template Haskell but not supported by th-desguar is view patterns, mostly because these interact quite non-trivially with pattern binders. It's possible this hole will be closed in a future version. Enjoy! Richard

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Can I use String without "" in ghci?
by yi lu 02 Sep '13

02 Sep '13

I want to know if it is possible that I use strings without "". If I type *Prelude>foo bar* which actually I mean *Prelude>foo "bar"* However I don't want to type ""s. I have noticed if *bar* is predefined or it is a number, it can be used as arguments. But can other strings be used this way? Like in bash, we can use *ping 127.0.0.1* where *127.0.0.1* is an argument. If not, can *foo* be defined as a function so that it recognize arguments like *bar* as *"bar"*? Thanks, Yi Lu

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Re: [Haskell-cafe] function arithmetic?
by Christopher Howard 02 Sep '13

02 Sep '13

On 08/31/2013 09:27 PM, Charlie Paul wrote: > I believe that this is what you want: > http://www.haskell.org/haskellwiki/Num_instance_for_functions > > On Sat, Aug 31, 2013 at 10:01 PM, Christopher Howard > <christopher.howard(a)frigidcode.com> wrote: The author seemed to be subtly mocking the idea. It seemed to be suggesting that a Num instance for functions would imply the need for constant number functions, which leads to difficulties. But I don't see why one would have to take it that far. In any case, I just tried the NumInstances package from Hackage and it seems to work great.

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Proposal: Polymorphic typeclass and Records
by Wvv 02 Sep '13

02 Sep '13

Let we have data in one module as this: data Person = Person { personId :: Int, name :: String } data Address a = Address { personId :: Int, address :: String , way :: a} It was discussed a lot in topics "OverloadedRecordFields" This is an alternative: Let we have polymorphic typeclass: class Record{f} a b | a -> b where f :: a -> b so, compiler could create instances for our data as: instance Record{personId} Person Int where personId (Person x _) = x instance Record{personId} (Address a) Int where personId (Address x _ _) = x instance Record{way} (Address Int) Int where way (Address _ _ x) = x and we could use this: p:: Record {personId} r Int => r -> Int p = personId What do you think about this? -- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Polymorphic-typeclass-and-Rec… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Compiler stops at SpecConstr optimization
by Daniel Díaz Casanueva 02 Sep '13

02 Sep '13

Hello. While hacking in one of my projects, one of my modules stopped to compile for apparently no reason. The compiler just freezes (like if it where in an infinite loop) while trying to compile that particular module. Since I had this problem I have been trying to reduce the problem as much as I could, and I came out with this small piece of code: > module Blah (foo) where > import Data.Vector (Vector) > import qualified Data.Vector as V > foo :: (a -> a) -> Vector a -> Vector a > foo f = V.fromList . V.foldl (\xs x -> f x : xs) [] The module I am importing comes from package vector-0.10.0.1. The compiler I am using is ghc-7.6.3. The problem arises when compiling using the following flags: ghc Blah.hs -O1 -fspec-constr If I compile it verbosely (with -v), I can see that the compiler stops in the SpecConstr optimization step. I have tried to reduce the -O1 flag to a particular set of flags, but I didn't manage yet to find the complete set of flags that -O1 translates to (optLevelFlags [1] seems to give not enough information). Since this problem can be OS-dependent, my system is Debian 7 and I didn't try yet to reproduce my problem in other systems (mainly because I don't have access to other systems at the moment). I am posting this here to, with the help of the Haskell Community, check first if this is indeed a bug and, in such case, to find out where that bug comes from (GHC, vector library, something else?). Then, if necessary, I would write a bug report where appropriate. Thank you, Daniel Díaz. -- References [1] - https://github.com/ghc/ghc/blob/ghc-7.6.3-release/compiler/main/DynFlags.hs…

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Hoogle vs Hayoo
by jabolopes＠google.com 01 Sep '13

01 Sep '13

Hi, I noticed Hayoo appears as a link in the toolbox of http://hackage.haskell.org and also that Hayoo seems to display better results than Hoogle. For example, if you search for 'PublicKey' in Hayoo, you will get several results from Hackage libraries, such as, 'crypto-pubkey' and 'crypto-api'. However, the same query in Hoogle displays no results. Is Hayoo the default Hackage search engine ? Is Hoogle deprecated ? What the status ? Thank you, Jose

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function arithmetic?
by Christopher Howard 01 Sep '13

01 Sep '13

Hi. I was just curious about something. In one of my math textbooks I see expressions like this f + g or (f + g)(a) where f and g are functions. What is meant is f(a) + g(a) Is there a way in Haskell you can make use of syntax like that (i.e., expressions like f + g and f * g to create a new function), perhaps by loading a module or something?

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Tasty not compiling
by Thiago Negri 30 Aug '13

30 Aug '13

I can't install tasty with cabal. Anyone with the same issue or a fix? $ cabal install tasty ... Test\Tasty\Core.hs:147:11: Not in scope: `witness'

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Debugging ByteString and Data.Binary.Get memory usage
by Kyle Hanson 30 Aug '13

30 Aug '13

OK I have a bunch of BSON documents that I convert to ByteStrings, put in a Map, and write to a socket based on the response. I noticed some high memory usage (in the GBs) so I decided to investigate. I simplified my problem into a small program that demonstrates clearer what is happening. I wrote two versions, one with a Lazy Map and Lazy ByteStrings and one with a Strict Map and Strict ByteStrings. Both share the same memory behavior (except the lazy BS one is faster) Here is the strict version: http://lpaste.net/92298 And here is the lazy version: http://lpaste.net/92299 I wrote this and compared the memory and speed behavior of ByteStrings generated by converting it from a BSON document and ByteStrings generated more purely. The length of the ByteString from a BSON document is 68k and the length of the "pure" BS is 70k. This is my weird memory behavior, both BSON and "pure" methods use the same amount of memory after inserting 10k of them (90mb) However when I go to lookup a value, the BSON Map explodes the memory to over 250mb. Even if I lookup just 1 value. Looking up any number of values in the "pure BS" keeps the memory usage stable (90mb). I am hoping someone can help me understand this. I have read some posts about Temporary ByteStrings causing memory issues but I don't know how to get started debugging. -- Kyle Hanson

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Announcing GHC iOS
by Luke Iannini 30 Aug '13

30 Aug '13

Hi all! (in case you don't read /r/haskell : )) Stephen Blackheath and I are extremely happy to report that as of today, GHC can natively build binaries for iOS devices and the iOS Simulator. You'll find everything you need here: http://ghc.haskell.org/trac/ghc/wiki/Building/CrossCompiling/iOS The short of it is you can use "ghc-ios myFiles.hs" to get a universal ARMv7/ARMv7s/i386 static library to drop straight into an Xcode project. Stephen's already working on some wonderful FRP-powered games, I'm working on a project called SpaceTime that I'll announce shortly, and you may now all begin creating insanely great iOS software using the finest language in the world : ). Stephen deserves the lion's share of the credit for this project — it's been over 3 years in the making! Many thanks to Maxwell Swadling for generalizing static library builds into a -staticlib flag for GHC, Carter Schonwald for compilation guidance, and the venerable Austin Seipp for reviewing and merging our patches.

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Template Haskell and Unit
by Jose A. Lopes 29 Aug '13

29 Aug '13

Hi, I am positive about the following situation, but I can't find any concrete answer on the Web. Can anyone confirm this ? In template-haskell-2.7.0, the following quote [t| () |] appears as a (ConT name), where "name" is the name for unit. However, in template-haskell-2.8.0, the same quote appears as (TupleT 0). Is this the expected behaviour ? Thanks, Jose -- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370

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Building recent Cabal/cabal-install
by Mateusz Kowalczyk 29 Aug '13

29 Aug '13

Greetings café, There are some problems in Haddock to do with Template Haskell that I believe are being caused by Cabal. These were apparently addressed in 1.18 which came out recently. ‘Great!’, I thought. My problem is that I'm unsure how to use 1.18. I'm using GHC HEAD (well, 3 days old now) which is meant to come with Cabal 1.18 (the library) and in fact it seems to ship with it. Running ‘cabal --version’ however: > cabal-install version 1.17.0 > using version 1.17.0 of the Cabal library I'm unsure how to get it to use 1.18. I tried to build cabal-install from git but that just complains about dependencies not being possible to resolve: it depends on HTTP which depends on base <4.7 but 4.7 comes with GHC HEAD! I am baffled as to how I should go about using 1.18 and would love to hear from some people who have this up and running. -- Mateusz K.

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enumerators: exception that can't be catched
by Yuras Shumovich 29 Aug '13

29 Aug '13

Hello, I'm debugging an issue in "websockets" package, https://github.com/jaspervdj/websockets/issues/42 I'm not familiar with "enumerator" package (websockets are based on it), so I'm looking for help. The exception is throws inside "enumSocket" enumerator using "throwError" ( http://hackage.haskell.org/packages/archive/network-enumerator/0.1.5/doc/ht… ), but I can't catch it with "catchError". It is propagated to "run" function: <interactive>: recv: resource vanished (Connection reset by peer) The question is: how is it possible? could it be a bug in "enumerator" package? Thanks, Yuras

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Haskell Weekly News: Issue 278
by Daniel Santa Cruz 29 Aug '13

29 Aug '13

Welcome to issue 278 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 18 to 24, 2013. Quotes of the Week * johnw: finger trees must be related to palm trees somehow * monochrom: "do, or undo. there is no redo." :) * danharaj: arrows are just strong monads in the 2-category of profunctors. what's the problem? * BMeph: If only they knew the POWER...of the darcs-IDE! * geekosaur: "tekmo and the pipes list" sounds like some kind of postmodernist band Top Reddit Stories * Building an Asteroids Clone in Haskell using Netwire Domain: ocharles.org.uk, Score: 88, Comments: 22 On Reddit: [1] http://goo.gl/HZBtcK Original: [2] http://goo.gl/Gp8TVw * What's new in Cabal 1.18 - sandboxes, REPL, cross-compilation and more! Domain: coldwa.st, Score: 75, Comments: 18 On Reddit: [3] http://goo.gl/8eSChe Original: [4] http://goo.gl/VrHkMY * An Introduction to Cabal sandboxes Domain: coldwa.st, Score: 64, Comments: 33 On Reddit: [5] http://goo.gl/oFGzfK Original: [6] http://goo.gl/ByA6Ru * Reasoning about space leaks with space invariants Domain: apfelmus.nfshost.com, Score: 59, Comments: 48 On Reddit: [7] http://goo.gl/cKlPrM Original: [8] http://goo.gl/E4kIjD * Bound: Making de Bruijn Succ Less on School of Haskell by Edward Kmett Domain: fpcomplete.com, Score: 48, Comments: 14 On Reddit: [9] http://goo.gl/v7tgWO Original: [10] http://goo.gl/glTA3N * PatternSynonyms - ghc trac Domain: ghc.haskell.org, Score: 35, Comments: 14 On Reddit: [11] http://goo.gl/8zuy1T Original: [12] http://goo.gl/5Bw6FN * Revisiting Matrix Multiplication, Part III: Customizing Vector by Edward Kmett Domain: fpcomplete.com, Score: 33, Comments: 1 On Reddit: [13] http://goo.gl/hfoJJ7 Original: [14] http://goo.gl/yLUSWP * Great new platform to increase *practical* code literacy (with Haskell support). Domain: mdswanson.com, Score: 29, Comments: 13 On Reddit: [15] http://goo.gl/sFLBQm Original: [16] http://goo.gl/TIJPTi * Bartosz' basic Haskell: Error Handling Domain: fpcomplete.com, Score: 28, Comments: 4 On Reddit: [17] http://goo.gl/1jBaWG Original: [18] http://goo.gl/zZrPxH * A Pragmatic Case for Static Typing with Brian Hurt on Vimeo (at NY-Haskell) Domain: vimeo.com, Score: 27, Comments: 11 On Reddit: [19] http://goo.gl/uECaaB Original: [20] http://goo.gl/59c8Wo * Write yourself a Brainfuck in an hour Domain: github.com, Score: 26, Comments: 13 On Reddit: [21] http://goo.gl/lAfzRf Original: [22] http://goo.gl/cbJZ7t Top StackOverflow Questions * Is it possible to lazily traverse a recursive data-structure with O(1) memory usage, tail-call optimized? votes: 10, answers: 4 Read on SO: [23] http://goo.gl/1qsBKq * pattern matching of the form: Option{..} <- votes: 10, answers: 1 Read on SO: [24] http://goo.gl/HY3FWY Until next time, [25]+Daniel Santa Cruz References 1. http://ocharles.org.uk/blog/posts/2013-08-18-asteroids-in-netwire.html 2. http://www.reddit.com/r/haskell/comments/1kmes7/building_an_asteroids_clone… 3. http://coldwa.st/e/blog/2013-08-21-Cabal-1-18.html 4. http://www.reddit.com/r/haskell/comments/1kslyi/whats_new_in_cabal_118_sand… 5. http://coldwa.st/e/blog/2013-08-20-Cabal-sandbox.html 6. http://www.reddit.com/r/haskell/comments/1kpynl/an_introduction_to_cabal_sa… 7. http://apfelmus.nfshost.com/blog/2013/08/21-space-invariants.html 8. http://www.reddit.com/r/haskell/comments/1ksu0v/reasoning_about_space_leaks… 9. https://www.fpcomplete.com/user/edwardk/bound 10. http://www.reddit.com/r/haskell/comments/1knvz2/bound_making_de_bruijn_succ… 11. http://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms 12. http://www.reddit.com/r/haskell/comments/1kmods/patternsynonyms_ghc_trac/ 13. https://www.fpcomplete.com/user/edwardk/revisiting-matrix-multiplication-pa… 14. http://www.reddit.com/r/haskell/comments/1kmvs2/revisiting_matrix_multiplic… 15. http://mdswanson.com/blog/2013/08/19/how-to-practice-code-reviews.html 16. http://www.reddit.com/r/haskell/comments/1krc6t/great_new_platform_to_incre… 17. https://www.fpcomplete.com/user/bartosz/basics-of-haskell/10_Error_Handling 18. http://www.reddit.com/r/haskell/comments/1ksjt4/bartosz_basic_haskell_error… 19. https://vimeo.com/72870631 20. http://www.reddit.com/r/haskell/comments/1kusbt/a_pragmatic_case_for_static… 21. https://github.com/quchen/articles/blob/master/write_yourself_a_brainfuck.md 22. http://www.reddit.com/r/haskell/comments/1kz39d/write_yourself_a_brainfuck_… 23. http://stackoverflow.com/questions/18345734/is-it-possible-to-lazily-traver… 24. http://stackoverflow.com/questions/18372188/pattern-matching-of-the-form-op… 25. https://plus.google.com/105107667630152149014/about

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cpphs calls error when it finds an #error declaration
by Niklas Hambüchen 29 Aug '13

29 Aug '13

Hi, after some debugging of a higher-level tool I found out that when I use cpphs as a library and the `runCpphs` function that is to produce the preprocessed output, when it comes across the #error directive it will terminate my program. This is because handling #error is implemented with Haskell's `error`. I find that slightly unfortunate since it means I cannot distinguish betwen an #error written in the input file and a programming error inside cpphs. @Malcolm, would you mind a change towards throwing an exception that is different from error so that it can be easily caught, or even better, a change from runCpphs :: ... -> IO String to runCpphs :: ... -> IO (Either String String) or similar? If an exception based interface is kept, it would be nice to add some haddock to `runCpphs`; not knowing about the existence of #error, it is easy to assume that the IO is only used for accessing the FilePath passed in.

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Re: [Haskell-cafe] sequence causing stack overflow on pretty small lists
by John Lato 28 Aug '13

28 Aug '13

IMHO it's perfectly reasonable to expect sequence/replicateM/mapM to be able to handle a list of ~1e6 elements in the Unescapable Monad (i.e. IO). All the alternate implementations in the world won't be as handy as Prelude.sequence, and no amount of documentation will prevent people from running into this headlong*. So unless there's a downside to upping the stack size limitation I'm unaware of, +1 to that suggestion from me. John [1] Most people are physically incapable of reading documents that explain why what they want to do won't work. Even if people did read the documentation, I suspect that the people most in need of the information would be the least likely to understand how it applies to their situation. On Tue, Aug 27, 2013 at 9:19 PM, John Alfred Nathanael Chee < cheecheeo(a)gmail.com> wrote: > This is somewhat related: http://ghc.haskell.org/trac/ghc/ticket/4219 > > This also solves the concrete problem you gave in your original post > (in reverse order): > > import Control.Monad > import System.Random > > sequencel :: Monad m => [m a] -> m [a] > sequencel = foldM (\tail m -> (\x -> return $ x : tail) =<< m) [] > > main :: IO () > main = print =<< sequencel (replicate 1000000 (randomIO :: IO Integer)) > > Following on Reid's point, maybe it's worth noting in the > documentation that replicateM, mapM, and sequence are not tail > recursive for Monads that define (>>=) as strict in the first > argument? > > On Tue, Aug 27, 2013 at 6:07 AM, Niklas Hambüchen <mail(a)nh2.me> wrote: > > On 27/08/13 20:37, Patrick Palka wrote: > >> You can use ContT to force the function to use heap instead of stack > >> space, e.g. runContT (replicateM 1000000 (lift randomIO)) return > > > > That is interesting, and works. > > > > Unfortunately its pure existence will not fix sequence, mapM etc. in > base. > > > > _______________________________________________ > > Libraries mailing list > > Libraries(a)haskell.org > > http://www.haskell.org/mailman/listinfo/libraries > > > > -- > Love in Jesus Christ, John Alfred Nathanael Chee > http://www.biblegateway.com/ > http://web.cecs.pdx.edu/~chee/ > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries >

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Template Haskell: let statement in a splice put in the "main = do" part of a program?
by TP 28 Aug '13

28 Aug '13

Hi, I continue to test Template Haskell, and I have some difficulties to use a splice $() in a "do" contained in the "main" part of a program. Here is an example. I want to make a splice that does `let a="a"` in my code. $ cat MakeLetStatement.hs ---------------------------- {-# LANGUAGE TemplateHaskell #-} module MakeLetStatement where import Language.Haskell.TH makeLetStatement :: String -> ExpQ makeLetStatement s = return $ DoE $ [ LetS $ [ ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) [] ]] ---------------------------- $ cat test_MakeLetStatement.hs ---------------------------- {-# LANGUAGE TemplateHaskell #-} import MakeLetStatement main = do $(makeLetStatement "a") -- print a ---------------------------- Note I have commented "print a" because otherwise I obtain "Not in scope: `a'" that shows that `a` has not been defined correctly, but does not show whether my splice has been correctly expanded (I use --dump-splices GHC option, but it seems it is not working for splices in the "main = do" part). I obtain: $ runghc -ddump-splices test_MakeLetStatement.hs test_MakeLetStatement.hs:7:3: Illegal last statement of a 'do' block: let a = "a" (It should be an expression.) When splicing a TH expression: do let a = "a" In a stmt of a 'do' block: $(makeLetStatement "a") In the expression: do { $(makeLetStatement "a") } In an equation for `main': main = do { $(makeLetStatement "a") } That shows that my splice has been correctly expanded: we have `let a = "a"`. However, what happens is the same as in the following dummy script, we have in fact defined a "do" inside the first "do" (with DoE), and so we obtain an error because the last statement in a do block should be an expression. ---------------------------- main = do do let a = "a" print a ---------------------------- So my code does not work, without surprise, but in fact my problem is to transform a LetS statement: http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell-2.8.… that has type Stmt, in an ExpQ that seems to be the only thing that we can put in a splice. I have found that it can only be done by doE (or DoE) and compE (or CompE) according to http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell-2.8.… But doE is not a solution as we have seen above, and compE is to construct list comprehensions, which is a different thing. So, is there any solution to my problem? Thanks in advance, TP

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Template Haskell
by Jose A. Lopes 27 Aug '13

27 Aug '13

Hi, Is it possible to retrieve all definitions contained in a module using Template Haskell ? Thanks, Jose -- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370

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Re: [Haskell-cafe] Extending Type Classes
by Henning Thielemann 27 Aug '13

27 Aug '13

The problem of refinement of type classes annoys me from time to time when I work on the NumericPrelude. It is an experimental type class hierarchy for mathematical types. Sometimes a new data type T shall be implemented and it turns out that you can implement only a part of all methods of a certain class. Then a natural step is to split the class into two classes A and B: 'A' contains the methods we can implement for T and 'B' contains the remaining methods and 'B' is a sub-class of 'A'. First, this means that all client code has to be rewritten. Second, code for instances becomes very lengthy, because over the time code tends to contain one instances for every method. However the many small instances actually carry information: Every instance has its specialised constraints. E.g. you would certainly try to use only Applicative constraints in an Applicative instance and not Monad constraints. However, if there is a way to define Applicative and Monad instances in one go, the Applicative instance may get Monad constraints.

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Re: [Haskell-cafe] sequence causing stack overflow on pretty small lists
by Niklas Hambüchen 27 Aug '13

27 Aug '13

Maybe an unlimited stack size should be the default? As far as I understand, the only negative effect would be that some programming mistakes would not result in a stack overflow. However, I doubt the usefulness of that: * It already depends a lot on the optimisation level * If you do the same thing in a slightly different way, and you allocate on the heap instead of on the stack you will not get it either

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definition of the term "combinator"
by damodar kulkarni 27 Aug '13

27 Aug '13

Hello, The word "combinator" is used several times in the Haskell community. e.g. parser combinator, combinator library etc. Is it exactly the same term that is used in the "combinatory logic" ? A combinator is a higher-order function that uses *only function application* and earlier defined combinators to define a result from its arguments. [1] It seems, the term combinator as in, say, "parser combinator", doesn't have much to do with the "*only function application*" requirement of the "combinatory logic", per se. If the above observation holds, is the term combinator as used in the Haskell community, properly defined? In other words: Where can I find a formal and precise definition of the term "combinator", as a term used by the Haskell community to describe "something"? Ref: http://en.wikipedia.org/wiki/Combinatory_logic Thanks and regards, -Damodar Kulkarni

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TypeLits & Typeable
by Nicolas Trangez 26 Aug '13

26 Aug '13

Hello Cafe, I was playing around with TypeLits in combination with Typeable (using GHC 7.7.7.20130812 FWIW), but was surprised to find Symbols aren't Typeable, and as such the following doesn't work. Is this intentional, or am I missing something? Thanks, Nicolas {-# LANGUAGE DataKinds, KindSignatures, DeriveFunctor, DeriveDataTypeable #-} module Main where import Data.Typeable import GHC.TypeLits data NoSymbol n a b = NoSymbol a b deriving (Typeable) data WithSymbol (n :: Symbol) a b = WithSymbol a b deriving (Typeable) data Sym deriving (Typeable) main :: IO () main = do print $ typeOf (undefined :: NoSymbol Sym Int Int) let d = undefined :: WithSymbol "sym" Int Int {- print $ typeOf d No instance for (Typeable Symbol "sym") arising from a use of ‛typeOf’ -} return ()

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Proposal: Hackage's packages should be seperated by buildable
by He-chien Tsai 26 Aug '13

26 Aug '13

I'm sick for checking whether package is obsolete or not. I think packages build failed long time ago should be collected and moved to another page until someone fix them, or hackage pages should have a filter for checking obsolete packages.

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xmonad (+ mate) evince problem?
by Johannes Waldmann 26 Aug '13

26 Aug '13

Hi. I am using xmonad + mate (on fedora 19) and evince (PDF viewer) seems unresponsive: it reacts to my (mouse) input only after switching to another screen and back. Any hints? - Thanks, J. W.

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Conduit : is it possible to write this function?
by Erik de Castro Lopo 26 Aug '13

26 Aug '13

Hi all Using the Conduit library is it possible to write the function: eitherSrc :: MonadResource m => Source m a -> Source m b -> Source m (Either a b) which combines two sources into new output source such that data being produced aysnchronously by the original two sources will be returned as either a Left or Right of tne new source? If so, how? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Renumbered mailing list posts
by Henning Thielemann 26 Aug '13

26 Aug '13

Recently I found that links from Google search results to archive Haskell-Cafe messages are invalid. The messages are still there, but got a different number. E.g. the search result says: http://www.haskell.org/pipermail/haskell-cafe/2011-February/089455.html But the message is at http://www.haskell.org/pipermail/haskell-cafe/2011-February/088146.html Also links from Haskell-Wiki articles to the Haskell-Cafe archive are invalid now. This is very very very bad, since I used tons of such URLs in Wiki articles and it is very hard to find the message I referred to, if the URL does not work anymore.

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reasons why Template Haskell does not propose something similar to Python exec() or eval()
by TP 25 Aug '13

25 Aug '13

Hi everybody, I continue to learn and test Template Haskell (one more time thanks to John Lato for his post at: http://www.mail-archive.com/haskell-cafe@haskell.org/msg106696.html that made me understand a lot of things). I have a question about the way Template Haskell is working. Why Template Haskell does not propose something similar to Python (or bash) exec() or eval(), i.e. does not offer the possibility to take a (quoted) string in input, to make abstract syntax in output (to be executed later in a splice $()). For example, in Python, to make an affectation 'a="a"' programatically, I can simply do (at runtime; even if I am here only concerned with what Template Haskell could do, i.e. at compile time): > def f(s): return '%s = \'%s\'' % (s,s) > exec(f("a")) > a 'a' With Template Haskell, I am compelled to make a function returning the abstract syntax corresponding to variable declaration: ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) (see complete example in Post Scriptum). This works fine, but it is less direct. I am sure that the Template Haskell approach has many advantages, but I am unable to list them. I think it is important to have the reasons in mind. Could you help me? Thanks in advance, TP PS: the complete Haskell example: ----------------------------------- module MakeVard where import Language.Haskell.TH makeVard :: Monad m => String -> m [Dec] -- Equivalent to "%s = \"%s\"" makeVard s = return [ ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) [] ] ----------------------------------- tested by ----------------------------------- {-# LANGUAGE TemplateHaskell #-} import MakeVard $(makeVard "a") main = do print a ----------------------------------- resulting in $ runghc -ddump-splices test.hs test_makeVar.hs:1:1: Splicing declarations makeVard "a" ======> test_makeVar.hs:4:3-14 a = "a" "a"

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GHC flags: optghc
by jabolopes＠google.com 24 Aug '13

24 Aug '13

Hi, I am using GHC version 6.12.1. What is optghc ? I can't find that information anywhere... Thanks, Jose -- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370

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