July 2013 - Haskell-Cafe

ANN: darcsden 1.1 released, darcs hub news 2013/07
by Simon Michael 20 Jul '13

20 Jul '13

Hi all, here is a combined darcsden and darcs hub update. darcsden 1.1 released ===================== darcsden 1.1 is now available on hackage! This is the updated version of darcsden which runs hub.darcs.net, so these changes are also relevant to that site's users. (More darcs hub news below.) darcsden is a web application for browsing and managing darcs repositories, issues, and users, plus a basic SSH server which lets users push changes without a system login. It is released under the BSD license. You can use it: - to browse and manage your local darcs repos with a more comfortable UI - to make your repos browsable online, optionally with issue tracking - to run a multi-user darcs hosting site, like hub.darcs.net http://hackage.haskell.org/package/darcsden - cabal package \ http://hub.darcs.net/simon/darcsden - source \ http://hub.darcs.net/simon/darcsden/issues - bug tracker Release notes for 1.1 --------------------- Fixed: * 16: Layout of links and navigation places them offscreen * 21: anchors on line numbers exist but line numbers not clickable * 28: forking then deleting a private repo makes repos unviewable * 29: darcs get to an invalid ssh repo url hangs * 46: if user kills a push, the lock file is not removed, preventing subsequent pushes New: * the signup page security question is case-insensitive ("darcs") * login redirects to the "my repos" page * a more responsive layout, with content first, buttons at top/right * many other UI updates; font, headings, borders, whitespace, robustness * more context sensitivity in buttons & links * better next/previous page controls * better support for microsoft windows, runs as a service * builds with GHC 7.6 and latest libraries * easier developer builds Brand new, from the Enhancing Darcsden GSOC (some WIP): * you can sign up, log in, and link existing accounts with your Google or Github id * you can reset your password * you can edit files through the web * you can "pack" your repositories, allowing faster darcs get Detailed change log: http://hub.darcs.net/simon/darcsden/CHANGES.md How to help ----------- darcsden is a small, clean codebase that is fun to hack on. Discussion takes place on the #darcs IRC channel, and useful changes will quickly be deployed at hub.darcs.net, providing a tight dogfooding/feedback loop. Here's how to contribute a patch there: 1. register at hub.darcs.net 2. add your ssh key in settings so you can push 3. fork your own branch: http://hub.darcs.net/simon/darcsden , fork 4. copy to your machine: darcs get http://hub.darcs.net/yourname/darcsden 5. make changes, darcs record 6. push to hub: darcs push yourname@hub.darcs.net:darcsden --set-default 7. your change will appear at http://hub.darcs.net/simon/darcsden/patches 8. discuss on #darcs, or ping me (sm, simon(a)joyful.com) to merge it Credits ------- Alex Suraci created darcsden. Simon Michael led this release, which includes contributions from Alp Mestanogullari, Jeffrey Chu, Ganesh Sittampalam, and BSRK Aditya (sponsored by Google's Summer of Code). And last time I forgot to mention two 1.0 contributors: Bertram Felgenhauer and Alex Suraci. darcsden depends on Darcs, Snap, GHC, and other fine projects from the Haskell ecosystem, as well as Twitter Bootstrap, JQuery, and many more. darcs hub news 2013/07 ====================== http://hub.darcs.net , aka darcs hub, is the darcs repository hosting site I operate. It's like a mini github, but using darcs. You can: - browse users, repos, files and changes - publish darcs repos publicly or privately - get, push and pull repos over ssh - grant push access to other members - fork repos, then view and merge upstream and downstream changes - track issues The site was announced on 2012/9/15 (http://thread.gmane.org/gmane.comp.version-control.darcs.user/26556) Since then: - The site has been deploying new darcsden work promptly; it includes all the 1.1 release improvements described above. - The server's ram has doubled from 1G to 2G (thanks Linode). This means app restarts due to excessive memory use are less frequent. - The front page's user list had become slow and has been optimised, halving the page load time. - BSRK Aditya is doing his Google Summer of Code project on enhancing darcsden and darcs hub (mentored by darcs developer Ganesh Sittampalam). Find out more at http://darcs.net/GSoC/2013-Darcsden . - The site is being used, with many small projects and a few well-known larger ones. Quick stats as of 2013/07/19: user accounts 317 repos 579 disk usage 2.5G uptime last 30 days 99.48% average response time last 30 days 1.6s - The site remains free to use, including private repos. Eventually, some kind of funding will be needed to keep it self-sustaining, and could also enable faster development. Donate button ? Gittip ? Charge for private repos ? Let's discuss. Please try it out, report problems, and contribute patches to make it better. Best, -Simon

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HaskellWiki images disappeared
by Henk-Jan van Tuyl 20 Jul '13

20 Jul '13

L.S., It looks like the HaskellWiki images have disappeared; can anybody repair this? (See for example http://www.haskell.org/haskellwiki/Special:MostLinkedFiles ) Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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List Monads and non-determinism
by Matt Ford 20 Jul '13

20 Jul '13

Hi All, I thought I'd have a go at destructing [1,2] >>= \n -> [3,4] >>= \m -> return (n,m) which results in [(1,3)(1,4),(2,3),(2,4)] I started by putting brackets in ([1,2] >>= \n -> [3,4]) >>= \m -> return (n,m) This immediately fails when evaluated: I expect it's something to do with the n value now not being seen by the final return. It seems to me that the return function is doing something more than it's definition (return x = [x]). If ignore the error introduced by the brackets I have and continue to simplify I get. [3,4,3,4] >>= \m -> return (n,m) Now this obviously won't work as there is no 'n' value. So what's happening here? Return seems to be doing way more work than lifting the result to a list, how does Haskell know to do this? Why's it not in the function definition? Are lists somehow a special case? Any pointers appreciated. Cheers, -- Matt

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Default installation path of haskell platform in windows
by Ivan Perez 20 Jul '13

20 Jul '13

Hi café, I just spent the whole day fighting ghc and cabal to get my program compiled in windows (and I haven't succeeded yet :) ) realgcc is not very happy about paths with spaces in them (I think there's an open bug about the way ghc calls realgcc, which manifests if you use extra-lib-dirs in the cabal file). I'd like to ask you: Might it be wise to change the setup file of the Haskell Platform to default to an installation path with no spaces in it, and to tell users why this is a good idea (possibly during the installation process)? Other projects are doing the same, including msys. Please be aware that I know that this "solution" is far from ideal. Ideally, it should just work (with spaces and without them). This is purely a practical decision, a workaround, aiming at making haskell more accessible to windows users, and making it easier for us to develop software that works in all platforms. Regards Ivan

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Re: [Haskell-cafe] Generating Haskell Code out of Haskell AST (GHC API)
by John Blackbox 19 Jul '13

19 Jul '13

I accidentally didn't send that email to haskell-cafe, so I'm pasting it here also: Alan - I do NOT want to generate Haskell code. I want only to generate AST and compile it. The question about generating the code was only to have a "debugging tool" - to see if the generated AST is good - I wanted to generate the Haskell code only to check if its correct, but normally I would not do it, because it makes no sense to generate AST -> code -> AST (by GHC) again etc :) Additional - I want to connect to GHC's type-checking also and translate the errors to be appropriate to my language syntax - so maybe the pure GHC API is the best way to go? 2013/7/19 John Blackbox <blackbox.dev.ml(a)gmail.com> > Additional - I want to connect to GHC's type-checking also and translate > the errors to be appropriate to my language syntax. > > > 2013/7/19 John Blackbox <blackbox.dev.ml(a)gmail.com> > >> Alan - I do NOT want to generate Haskell code. I want only to generate >> AST and compile it. >> The question about generating the code was only to have a "debugging >> tool" - to see if the generated AST is good - I wanted to generate the >> Haskell code only to check if its correct, but normally I would not do it, >> because it makes no sense to generate AST -> code -> AST (by GHC) again etc >> :) >> >> >> 2013/7/19 AlanKim Zimmerman <alan.zimm(a)gmail.com> >> >>> I have not used haskell-src-exts so I may be talking out of turn, but it >>> seems that if you want to generate an AST which you then turn into source >>> code and compile it makes more sense than using than GHC AST, which has a >>> number of wrinkles, including fields that are only valid at certain phases >>> of the compilation process. >>> >>> For my purposes, in the Haskell Refactorer, I need access to the >>> renaming and type-checking, which to my knowledge is not currently >>> available in haskell-src-exts, although there is work happening to bring it >>> in, e.g. https://github.com/haskell-suite/haskell-names. >>> >>> >>> On Fri, Jul 19, 2013 at 10:09 AM, John Blackbox < >>> blackbox.dev.ml(a)gmail.com> wrote: >>> >>>> Thank you! >>>> So, if I'm writing a compiler of custom language, which I want to >>>> generate Haskell AST and further compile it with GHC, you prefer >>>> something like haskell-src-exts over pure GHC API? >>>> >>>> >>>> 2013/7/19 Antoine Latter <aslatter(a)gmail.com> >>>> >>>>> The package haskell-src-exts is a lot less intimidating if all you are >>>>> trying to do is programmatically generate Haskell source: >>>>> >>>>> http://hackage.haskell.org/package/haskell-src-exts >>>>> >>>>> The base types are here: >>>>> >>>>> http://hackage.haskell.org/packages/archive/haskell-src-exts/1.13.5/doc/htm… >>>>> >>>>> This module has some helper function for generating parts of the AST: >>>>> >>>>> http://hackage.haskell.org/packages/archive/haskell-src-exts/1.13.5/doc/htm… >>>>> >>>>> >>>>> On Thu, Jul 18, 2013 at 1:11 PM, John Blackbox >>>>> <blackbox.dev.ml(a)gmail.com> wrote: >>>>> > Hi! >>>>> > I dont know GHC API very well, but I want to generate AST of a >>>>> program using >>>>> > GHC API. >>>>> > Is there any standard method to generate Haskell code out of it? >>>>> (something >>>>> > like "print_this_for_me_please" function? :D >>>>> > >>>>> > _______________________________________________ >>>>> > Haskell-Cafe mailing list >>>>> > Haskell-Cafe(a)haskell.org >>>>> > http://www.haskell.org/mailman/listinfo/haskell-cafe >>>>> > >>>>> >>>> >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>>> >>> >> >

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Generating Haskell Code out of Haskell AST (GHC API)
by John Blackbox 19 Jul '13

19 Jul '13

Hi! I dont know GHC API very well, but I want to generate AST of a program using GHC API. Is there any standard method to generate Haskell code out of it? (something like "print_this_for_me_please" function? :D

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Monad Transformer Space Leak
by Clark Gaebel 19 Jul '13

19 Jul '13

Hi everyone! For some reason, this leaks thunks: module Main where import Control.Monad import Control.Monad.MC -- from monte-carlo import Control.Monad.ST.Strict go :: Int -> MCT (ST s) () go k = replicateM_ k (return ()) main = print $ runST $ evalMCT (go 100000000) rng where rng = mt19937 0 while this does not: module Main where import Control.Monad import Control.Monad.MC go :: Int -> MC () go k = replicateM_ k (return ()) main = print $ evalMC (go 100000000) rng where rng = mt19937 0 Can anyone help me figure out what's going on here? Thanks, - Clark

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ghc and jump table generation
by Erik Rantapaa 18 Jul '13

18 Jul '13

I've been looking at the assembly code that ghc generates for simple pattern matching for situations like: foo :: Int -> Int foo 1 = 3 foo 2 = 10 foo 3 = 2 foo 4 = 42 -- ... other random assignments and I noticed that it doesn't seem to generate a jump table (or even a table lookup) like, for instance, gcc would for this switch statement: int foo(int x) { switch (c) { case 1: return 3; case 2: return 10; case 3: return 2; case 4: return 42; // ... etc. } } Under -O3 ghc seems to produce a binary-search of the cases. Questions: Would generating a jump/lookup table for pattern matches like this (i.e. with a suitable density of cases) be beneficial? And, if so, would it be difficult to add to ghc? And would it be a good first project for someone who wants to get into the ghc code base (perhaps with some mentoring from the community?)

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Automatic error traces
by Alberto G. Corona 18 Jul '13

18 Jul '13

It is important to have execution traces in case of error. specially in server applications that run 24/7 such are web applications Thanks to the wonderful package monadloc by Pepe Iborra, now MFlow can generate a complete execution trace in case of error. The control-monad-exception uses monadLoc to generate stack traces, but MFlow makes use of his backtracking mechanism to generate a complete execution trace. Here I explain what and how: http://haskell-web.blogspot.com.es/2013/07/automatic-error-trace-generation… The MFlow version that implements this is in gitHub. Not in hackage yet. https://github.com/agocorona/MFlow I´m quite proud of it since it is one of the things closest to magic that I have done. Feedback? I do not want to keep MFlow as a single person development. I think that it has many unique and nice features not available in other languages and frameworks, and it can be raised to a serious exploitation level by the Haskell community. This will attract people working in Industrial Web development to Haskell thanks to the edge in expressiveness and safety necessary for creating industrial solutions that Haskell has over other languages. It uses most other Haskell web developments blaze, wai, hamlet etc and there are many other things to come. monadLoc : http://hackage.haskell.org/package/monadloc -- Alberto.

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Not working examples in GHC API documentation
by John Blackbox 18 Jul '13

18 Jul '13

Hi! Please take a look here: http://www.haskell.org/haskellwiki/GHC/As_a_library The examples are not working. Even the simpelst one: import GHC import GHC.Paths ( libdir ) import DynFlags ( defaultLogAction ) main = defaultErrorHandler defaultLogAction $ do runGhc (Just libdir) $ do dflags <- getSessionDynFlags setSessionDynFlags dflags target <- guessTarget "test_main.hs" Nothing setTargets [target] load LoadAllTargets throws: $> ghc -package ghc Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Main.hs:6:25: Couldn't match type `DynFlags' with `[Char]' Expected type: DynFlags.FatalMessager Actual type: DynFlags.LogAction In the first argument of `defaultErrorHandler', namely `defaultLogAction' In the expression: defaultErrorHandler defaultLogAction In the expression: defaultErrorHandler defaultLogAction $ do { runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; .... } } Main.hs:7:7: Couldn't match expected type `DynFlags.FlushOut' with actual type `IO SuccessFlag' In a stmt of a 'do' block: runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; target <- guessTarget "test_main.hs" Nothing; setTargets [target]; .... } In the second argument of `($)', namely `do { runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; .... } }' In the expression: defaultErrorHandler defaultLogAction $ do { runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; .... } }

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