March 2011 - Haskell-Cafe

Trac login problem
by Mihai Maruseac 19 Mar '11

19 Mar '11

Hi, I have a problem while trying to login to Haskell Trac instance. Entering the username and password that I know are valid gives me a login error. Trying to recover the password with the mail address that I use gives another error saying that there is no combination between the username and the mail. And, when trying to recreate the account if correctly complains that there is a valid username already. Can someone, please, look into it? Thanks, -- Mihai

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[haskell-gsoc] Haskell.org Google Summer of Code 2011
by Edward Kmett 19 Mar '11

19 Mar '11

It is that time of year again. The Google Summer of Code is upon us! *Sign Up Information* If you are a student and want to sign up to make $5,000 for hacking on the code you love over the summer or are willing to help out as a mentor, now is the time to act. Please sign up by adding your name to the list at: http://hackage.haskell.org/trac/summer-of-code/wiki/People2011 Our main wiki entry for the Summer of Code 2010 is now online: http://hackage.haskell.org/trac/summer-of-code/wiki/Soc2011 If you are interested in applying as a student, here are some guidelines: http://hackage.haskell.org/trac/summer-of-code/wiki/StudApply2011 The official deadline for student applications with Google is April 8th, but if you are interested please add your name to the People2010 page above, as soon as possible. *Interested in helping out, but not sure how? * There are a number of ideas that we've been collecting for projects available through the trac http://hackage.haskell.org/trac/summer-of-code/report/1 and which have been collecting over time in a reddit: http://www.reddit.com/r/haskell_proposals/top/ <http://www.reddit.com/r/haskell_proposals/top/>Unfortunately, reddit doesn't let you comment on topics after they become archived, and most of the topics in the haskell_proposals reddit have been. While the reddit is a great place to discuss proposals, we had to choose one mechanism to supply Google with our ideas officially, so please submit proposals <http://hackage.haskell.org/trac/summer-of-code/newticket> to the trac once you have hashed out something you like to work on, or which would be an appropriately beneficial project for a summer worth of work. *Helping Out Darcs * Haskell.org <http://haskell.org/> is going to continue to act as an umbrella organization covering darcs <http://darcs.net/> for the Summer of Code once more. If you are interested in helping them out, they also have a page full of project ideas as well: http://wiki.darcs.net/GoogleSummerOfCode Again, please submit proposals<http://hackage.haskell.org/trac/summer-of-code/newticket> as tickets to the trac if you can think of something you'd like to work on. *More Information* Here are a bunch of relevant links that may be able to answer your questions: Google's Summer of Code Homepage <http://socghop.appspot.com/> Google's Summer of Code 2011 FAQ<http://socghop.appspot.com/document/show/gsoc_program/google/gsoc2011/faqs> Google's Summer of Code 2011 Timeline<http://socghop.appspot.com/document/show/gsoc_program/google/gsoc2011/timel…> Google's Summer of Code Wiki Knowledge Base<http://code.google.com/p/google-summer-of-code/wiki/WikiStart> Of course, you can always feel free to email me <ekmett(a)gmail.com> or inquire on irc.freenode.net #haskell-soc or with the darcs folks over on #darcs for more information. If you don't have an IRC client, you can get there from: http://webchat.freenode.net Thank you all very much and we look forward to another successful Summer of Code! -Edward Kmett

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Cabal test interface, what/where is it?
by Magnus Therning 19 Mar '11

19 Mar '11

After upgrading to ghc7 and Cabal 1.10 I get the following message when running 'Setup configure' on one of my packages: Setup.hs:3:1: Warning: In the use of `runTests' (imported from Distribution.Simple, but defined in Distribution.Simple.UserHooks): Deprecated: "Please use the new testing interface instead!" I usually turn to the Cabal docs at [1], but the new testing interface isn't mentioned. Looking through the haddock API reference doesn't offer any obvious pointers. So, where should I turn to get rid of the warning? /M [1] http://www.haskell.org/ghc/docs/latest/html/Cabal/ -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

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Missing e-mail @haskell-cafe
by Mario Blažević 18 Mar '11

18 Mar '11

I've sent an e-mail to Haskell Café this morning about my troubles with Trac confirmation e-mails. The e-mail must have reached the server, because it showed up in the mailing list archive: http://www.haskell.org/pipermail/haskell-cafe/2011-March/090311.html This other archive, however, doesn't list it: http://www.mail-archive.com/haskell-cafe@haskell.org/ I thereby infer that the message was never forwarded to the mailing list. Is that correct, and does anybody have a clue why?

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Wanted: composoable parsers from haskell-src-exts
by J. Waldmann 18 Mar '11

18 Mar '11

Hi. I want to use parsers from haskell-src-exts as sub-parsers, which does not seem to work since they insist on consuming the input completely. I would need them to parse a maximal prefix, and return the (unconsumed) rest of input as well (cf. http://hackage.haskell.org/packages/archive/parsec/3.1.1/doc/html/Text-Pars… ) I figure that happy has the %partial directive for that, but the description http://www.haskell.org/happy/doc/html/sec-directives.html#sec-partial-parse… does not really tell me how to obtain the rest of the input. Any hints (or code samples) appreciated. Thanks - J.W.

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Could not deduce ... using functional dependencies with GHC7
by JP Moresmau 18 Mar '11

18 Mar '11

Hello, when moving to GHC7 a body of code that I'm not immensely familiar with, I got an error compiling with GHC 7.0.2. Can somebody kindly explain to me what it means and how to get around it? This is in the scion code base so is using some GHC types. I have a class with a functional dependency (I think :-p): class Search id a | a -> id where search :: (SrcSpan -> Bool) -> SrcSpan -> a -> SearchResults id And some instances, notably: instance Search Id Id where search _ _ i = only (FoundId i) And the error occurs on another instance: instance (Search id id) => Search id (Sig id) where search p s (IdSig i) = search p s i ... other cases here, that work The error is: Could not deduce (id ~ Id) from the context (Search id id) bound by the instance declaration at lib\Scion\Inspect\Find.hs:477:10-45 `id' is a rigid type variable bound by the instance declaration at lib\Scion\Inspect\Find.hs:477:18 When using functional dependencies to combine Search Id Id, arising from the dependency `a -> id' in the instance declaration at lib\Scion\Inspect\Find.hs:183:10 Search id Id, arising from a use of `search' at lib\Scion\Inspect\Find.hs:479:32-37 In the expression: search p s i In an equation for `search': search p s (IdSig i) = search p s i If I replace the call to search with the actual implementation of search for Id, it compiles, so at some level the code "makes sense". I much prefer when error messages end with "Possible fix:..."! Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/

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Re: [Haskell-cafe] trac.haskell.org problem
by Mario Blažević 18 Mar '11

18 Mar '11

I've tried to send the verification e-mail from trac.haskell.org to three different e-mail accounts in the last 24 hours. None arrived. Note that I'm not trying to create a new account, I'm trying to verify my e-mail address in order to edit the Wiki pages of my projects. Can anybody else confirm this problem? > Ah, I suppose that's to do with the trac server itself, that's beyond me. > > Moving to café: Does anyone know what's up with trac.haskell.org not > sending > out verification emails? > > Cheers, > > /Niklas > > ________________________________________ > From: Jurriaan Hage [jur at cs.uu.nl] > Sent: Monday, February 28, 2011 8:41 AM > To: Niklas Broberg > Subject: Re: Again a question about haskell-src-exts. > > On Feb 28, 2011, at 8:39 AM, Niklas Broberg wrote: > > > I suppose I do - to the extend I can. What's the issue? :-) > The system does not send out verification e-mails, so I can not really > register completely and submit new issues. > I have tried it with two different e-mail accounts, and somebody else has > tried too. > They also do not show up in spam folders. > > Jur >

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Data constructor synonyms
by Grigory Sarnitskiy 18 Mar '11

18 Mar '11

Hello! Assume I have a type MyType with the constructor GeneralConstructor: data MyType = GeneralConstructor [Double] but I also want to have a separate name for special case of this constructor: SpecialConstructor Double so SpecialConstructor a = GeneralConstructor (a:[]) that is SpecialConstructor 5 was exactly the same as GeneralConstructor [5]. And for example instead of writing GeneralConstructor [0] I would like to use constructor Zero. It's all just for convenience. How can I achieve this? Well, of course I always can use sed to replace SpecialConstructor 5 with GeneralConstructor [5] in program sources, but it's not convenient.

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tplot and splot - analyst's swiss army knifes for visualizing log files
by Eugene Kirpichov 18 Mar '11

18 Mar '11

Hi cafe, I've published a large presentation about two Haskell-based tools of mine - tplot and splot. Their motto is "visualize system behavior from logs with a shell one-liner". Based on my experience, they usually seem to live up to this motto. http://www.slideshare.net/jkff/two-visualization-tools [attention attractor: the presentation has *really a lot* of pictures] hackage: http://hackage.haskell.org/package/timeplot http://hackage.haskell.org/package/splot other docs: http://www.haskell.org/haskellwiki/Timeplot http://www.haskell.org/haskellwiki/Splot -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

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order of arguments matters
by Peter Padawitz 18 Mar '11

18 Mar '11

Why does only tr2 work, although the only difference between tr1 and tr2 is the order of arguments? import Data.Tree (Tree(..)) data Type a where Tree :: Type a -> Type (Tree a) Int :: Type Int String :: Type String type Traversal1 = forall a.a -> Type a -> a type Traversal2 = forall a.Type a -> a -> a tr1 :: Traversal1 tr1 (Node _ (t:_)) (Tree Int) = Node 1 [t] tr1 n Int = n+n tr1 s String = s++s tr2 :: Traversal2 tr2 (Tree Int) (Node _ (t:_)) = Node 1 [t] tr2 Int n = n+n tr2 String s = s++s Couldn't match expected type `a' against inferred type `Tree Int' `a' is a rigid type variable bound by the type signature for `tr1' at tratest.hs:9:25 In the pattern: Node _ (t : _) In the definition of `tr1': tr1 (Node _ (t : _)) (Tree Int) = Node 1 [t]

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