September 2013 - Haskell-Cafe

Reminder to file Haddock bugs on Trac.
by Mateusz Kowalczyk 05 Sep '13

05 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 04 Sep '13

04 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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