- Haskell-Cafe - Haskell.org

HDBC PostgreSQL
by Patrick Brannan 10 Oct '09

10 Oct '09

Hello, I'm getting back into Haskell after an absence of a few years. I'm in the process of trying to connect to postgresql with hdbc-postgresql on a Windows XP box. Seemingly, things installed without a problem, but at the command prompt I get the following: Prelude> :module Database.HDBC Database.HDBC.PostgreSQL module `Database.HDBC.PostgreSQL' is not loaded If I run ghci this way with package options here's what I get: C:\work\haskell\HDBC-postgresql-2.1.0.0>ghci -package HDBC -package HDBC-postgresql GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package HUnit-1.2.0.3 ... linking ... done. Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package QuickCheck-1.2.0.0 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package Win32-2.2.0.0 ... linking ... done. Loading package time-1.1.4 ... linking ... done. Loading package convertible-1.0.5 ... linking ... done. Loading package filepath-1.1.0.2 ... linking ... done. Loading package directory-1.0.0.3 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package testpack-1.0.2 ... linking ... done. Loading package utf8-string-0.3.5 ... linking ... done. Loading package HDBC-2.1.1 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package HDBC-postgresql-2.1.0.0 ... linking ... done. Prelude> module HDBC HDBC.PostgreSQL Now I would think that the line "Loading package HDBC-postgresql-2.1.0.0 ... linking ... done." means that the module is installed correctly, but I still can't execute the :module statement. Does anyone have any ideas about where I should start? It's probably something stupid, but I'm a little worn out on searching. Thank you, Patrick

4 5

How to add use custom preprocessor in cabal
by Bernd Brassel 10 Oct '09

10 Oct '09

Hi folks, I am trying to integrate my own preprocessor into a cabal build process. But there are several points that I get stuck with. Could someone help me, please? A simplification of my problem: I have files "Abc.foo" and each of them should be transformed into "Abc.hs" by calling function "transform". The resulting "Abc.hs" files should then be exposed as Haskell library modules. 1. Question: How do I tell cabal that the file Abc.foo belongs to the package? After reading the User's Guide I had the impression that I should write something like Library Eposed-Modules: Abc.foo But this leads to a parse error. Adding a Data-Files: Abc.foo to the heading, on the other hand, does not seem to do anything. 2. Question: How to add the preprocessor? I have tried main = defaultMainWithHooks simpleUserHooks{hookedPreProcessors=[("foo",transformation)]} transformation :: BuildInfo -> LocalBuildInfo -> PreProcessor But under which circumstances will this function be called? Up to now I have not succeeded in making cabal call this function. Thanks for your time! Bernd

2 1

ANN: Extensions to Vec uploaded (useful for GPipe programs)
by Tobias Bexelius 10 Oct '09

10 Oct '09

Hi, I've added two extensions to the Vec package: Vec-Transform and Vec-Boolean. These two packages are more or less essential when programming GPipe programs, but can also be useful for other applications as well. * Vec-Transform provides some 4x4 transform matrices such as perspective projection and rotation. * Vec-Boolean provides Data.Boolan instances for the Vec data types. Cheers /Tobias _________________________________________________________________ Windows Live: Håll dina vänner uppdaterade om vad du gör online. http://www.microsoft.com/windows/windowslive/see-it-in-action/social-networ…

1 0

Ghci :ctags or hasktags print to standard out instead of file
by Johannes 10 Oct '09

10 Oct '09

Hi, I want to use Vim with Haskell. Therefore ctags generation is really useful. The TagList plugin for Vim reads the ctags info from the command line instead of from the file. I could not figure out how to make ghci :ctags or hasktasks to print the ctags info to the command line. Is there a way to do that? Any hints? Kind regards Johannes

2 1

What does "mkUsageInfo: internal name?" mean? (ghc 6.10.1, Template Haskell)
by Serguey Zefirov 10 Oct '09

10 Oct '09

I get a bunch of messages like "mkUsageInfo: internal name? WritebackCmd{tc a1eZY}" while transforming my program using Template Haskell. It displayed for every data type and for every constructor - I generate some instances for my data types.

1 0

Re: Applicative do?
by oleg＠okmij.org 10 Oct '09

10 Oct '09

It seems the applicative do is almost identical to the form ML and Scheme has had since the beginning. Perhaps that semantic similarity might inform the syntactic debate. > do a <- f > g > b <- h > pure $ foo a b > > into this: > > (\a b -> pure $ foo a b) <*> (f <*> g *> h) This form, although very similar in appearance to the monadic do, is very different in the scoping of bound identifiers. To me, that is quite unfortunate, greatly increasing confusion. In the monadic do, do x <- e1 y <- e2 e3 the identifier 'x' may appear in e2 and e3. To be more precise, the scope of each bound identifier extends to the rest of the do-block and can be used on the right-hand-side of other bindings in the rest of the block. In the proposed applicative do above, the identifier 'a' may not appear within g and h. Such a significant difference in scoping calls for a different name at the very least; furthermore, it calls for a compiler supports so that errors, such as using the identifier 'a' in 'h' by mistake (common during editing and refactoring) could be reported with an intelligent error message. Also, in monadic-do, it makes sense to bind the same identifier several times, as in do {x <- e1; x <- e2; e3}. It makes little sense doing that in applicative-do, and probably should be disallowed. Bob Atkey wrote: > applicatively > let x = foo > let y = bar > in <pure stuff> But OCaml already has the right syntax -- and so does Scheme. That syntax is let. In Scheme expression (let ((x e1) (y e2) (_ e11)) e3) only the expression e3 is in scope of x and y. Emphatically, e2 is not in scope in x; so the value bound to x cannot be used during the evaluation of e2. The expression e11 is evaluated only for its side-effect (underscore is just an identifier, I could've used `dummy'). Again, the expression e11 is not in scope of x and y. All identifiers bound by the let-clause must be distinct. The equivalent OCaml construction is let x = e1 and y = e2 and _ = e11 in e3 If, by mistake, we do mention x or y in e11 (and there are no global binding to x and y), OCaml will report the error about the unbound identifier. It seems the parallel let of Scheme or OCaml fall short of Applicative in one respect: the order of evaluating binding is not specified. If we agreed that the binders of a parallel let must be evaluated in order, we would get exactly the desired `applicative-do'. Perhaps, a better form in OCaml would be let seq x = foo and y = bar in e3 or let x = foo andalso y = bar in e3 Perhaps it may make sense to use the keyword `let' rather than 'do' in Haskell as well? P.S. For completeness, both Scheme and OCaml have the exact equivalent of monadic do. Haskell's do x <- e1 y <- e2 x <- e3 e4 e5 can be written in Scheme as (let* ((x e1) (y e2) (x e3) (_ e4)) e5) and in OCaml as let x = e1 in let y = e2 in let x = e3 in let () = e4 in e5 The latter pattern is very common in OCaml.

2 1

Haskell Weekly News
by Joe Fredette 09 Oct '09

09 Oct '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091003 Issue 134 - October 03, 2009 --------------------------------------------------------------------------- Welcome to issue 134 of HWN, a newsletter covering developments in the [1]Haskell community. I have a nasty sinus infection this week, so we're somewhat light on content. Lots of good discussion about DSL related stuff this week. Bryan O'Sullivan also release 'Criterion' this week, a new benchmarking library that Don Stewart described (on reddit) as 'awesome and game changing.' A new TMR editor -- someone familiar -- was announced. Also, there was some talk about homework policies on the mailinglists and in the irc channels. There is a [2]page on the Haskell wiki about this, but to sum it up in a maxim, remember, 'Help, don't do'. Until next week, the Haskell Weekly News! Announcements New TMR editor. Wouter Swierstra [3]announced that he would be stepping down from the editorship of 'The Monad Reader', with former HWN editor Brent Yorgey taking his place. Much thanks for Wouter's hard work and good luck to Brent on his new editor job! SourceGraph 0.5.{0,1,2}.0. Ivan Lazar Miljenovic [4]announced three new releases of the SourceGraph packages, this links to the latest release. json-b-0.0.4. Jason Dusek [5]announced a new version of the json-b package, which fixes defective handling of empty objects and arrays. rss2irc 0.4 released. Simon Michael [6]announced a new release of rss2irc, with many new improvements and features. vty-ui 0.1. Jonathan Daugherty [7]announced vty-ui, which is an extensible library of user interface widgets for composing and laying out Vty user interfaces. atom-0.1.1. Tom Hawkins [8]announced Atom, a Haskell DSL for designing hard real-time embedded applications. Graphalyze-0.7.0.0. Ivan Lazar Miljenovic [9]announced (in an apparent effort to take over hackage by submitting dozens of quality packages at absurdly high speed), Graphalyze, a library for using graph-theoretic techniques to analyse the relationships inherent within discrete data. Criterion. Bryan O'Sullivan [10]announced (without tacking on an 'ANN' tag, I might add, I almost missed it!) Criterion, a benchmarking library he describes [11]here. ListTree 0.1. yairchu(a)gmail.com [12]announced ListTree, a package for combinatorial search and pruning of trees. usb-0.1. Bas van Dijk [13]announced a library for interacting with usb modules from userspace. (Deadline extended to October 5th) APLAS 2009 Call for Posters. Kiminori Matsuzaki [14]announced a deadline extension to the call for posters for the APLAS conference. graphviz-2999.6.0.0. Ivan Lazar Miljenovic [15]announced a new version of the graphviz library, which features various new features and small changes. Discussion Testing polymorphic properties with QuickCheck. Jean-Philippe Bernardy [16]gave an excellent overview about how to use QuickCheck to test polymorphic properties. Designing a DSL? Gunther Schmidt [17]asked about different methods employed for designing a DSL. DSL and GUI Toolkits. Gunther Schmidt [18]also asked about different DSLs for working with GUIs error on "--++ bla bla bla". Hong Yang [19]asked about why '--++' wasn't being parsed in the way he thought it was. Haskell for Physicists. edgar [20]requested name suggestions for the talk he is giving about Physics and Haskell. Blog noise [21]Haskell news from the [22]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Sean Leather: [23]'Extensibility and type safety in formatting: the design of xformat' at the Dutch HUG. * Martijn van Steenbergen: [24]let 5 = 6. * Lee Pike: [25]Writer's unblock. * Manuel M T Chakravarty: [26]NVIDIAs next generation GPU architecture has a lot for HPC to love. * David Amos: [27]Finite geometries, part 4: Lines in PG(n,Fq). * Bryan O'Sullivan: [28]New criterion release works on Macs. * Neil Brown: [29]Poison: Concurrent Termination. * The GHC Team: [30]Heads up: what you need to know about Unicode I/O in GHC 6.12.1. * Galois, Inc: [31]Tech Talk: Roll Your Own Test Bed for Embedded Real-Time Protocols: A Haskell Experience. * Bryan O'Sullivan: [32]Criterion, a new benchmarking library for Haskell. * Tom Schrijvers: [33]Monadic Constraint Programming. * Neil Brown: [34]Growing Sort Pump. Quotes of the Week * dekudekuplex: (Unfortunately (unless intentional)) the preceding (by ksf (in the 'Quotes of the Week' section)) quote had mismatched (one too many opening) parentheses (although it was still funny (even though it could have been edited (to make the parentheses match (even though that is not an important issue)))). * pozic: I think if you want to contact dons, you have to say that you found a bug in ByteString. * Veinor: [about dibblego kicking a whole bunch of spammers] crouching dibblego, hidden op * allbery_b: [on UndecidableInstances] 'this exceeds my easy threshold, turn on wizard mode' (at which point it becomes a lot smarter but may start contemplating its navel without warning) * byorgey: a bus error? try recompiling with -fsubway, perhaps * jafet: 'Zygomorphism' sounds like a reproductive disorder About the Haskell Weekly News New editions are posted to [35]the Haskell mailing list as well as to [36]the Haskell Sequence and [37]Planet Haskell. [38]RSS is also available, and headlines appear on [39]haskell.org. To help create new editions of this newsletter, please see the information on [40]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [41]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.haskell.org/haskellwiki/Homework_help 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64213 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64259 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64254 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63838 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64070 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64035 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63989 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63985 11. http://www.serpentine.com/blog/2009/09/29/criterion-a-new-benchmarking-libr… 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63853 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64206 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17529 15. http://article.gmane.org/gmane.comp.lang.haskell.general/17520 16. http://thread.gmane.org/gmane.comp.lang.haskell.general/17535 17. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64267 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64244 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64161 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64128 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://feedproxy.google.com/~r/splonderzoek/~3/BHqNHIZFF7U/extensibility-an… 24. http://martijn.van.steenbergen.nl/journal/2009/10/02/let-5-6/ 25. http://leepike.wordpress.com/2009/09/30/writers-unblock/ 26. http://justtesting.org/post/201508784 27. http://haskellformaths.blogspot.com/2009/09/finite-geometries-part-4-lines-… 28. http://www.serpentine.com/blog/2009/09/30/new-criterion-release-works-on-ma… 29. http://chplib.wordpress.com/2009/09/30/poison-concurrent-termination/ 30. http://ghcmutterings.wordpress.com/2009/09/30/heads-up-what-you-need-to-kno… 31. http://www.galois.com/blog/2009/09/29/pike-haskell0/ 32. http://www.serpentine.com/blog/2009/09/29/criterion-a-new-benchmarking-libr… 33. http://tomschrijvers.blogspot.com/2009/01/monadic-constraint-programming.ht… 34. http://chplib.wordpress.com/2009/09/28/growing-sort-pump/ 35. http://www.haskell.org/mailman/listinfo/haskell 36. http://sequence.complete.org/ 37. http://planet.haskell.org/ 38. http://sequence.complete.org/node/feed 39. http://haskell.org/ 40. http://haskell.org/haskellwiki/HWN 41. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

3 3

Non-traversable foldables
by Martijn van Steenbergen 09 Oct '09

09 Oct '09

Hallo café, Can anyone think of datatypes that are Foldable but not Traversable? If not, what is the purpose of having a separate Foldable class? Thanks, Martijn.

4 5

Different semantics in "identical" do statement?
by staafmeister 09 Oct '09

09 Oct '09

In my program do x <- do blah <- someFunc return blah return $ Constructor x behaves differently from do blah <- someFunc return $ Constructor blah where the dots are identical. I would think that these programs should behave identically, by the monad laws. The result of my program is that the second gives correct behaviour, while the first loops forever. Greetings, Gerben -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

5 7

Applicative do?
by Philippa Cowderoy 09 Oct '09

09 Oct '09

I do a lot of work with parsers, and want to do more using Applicatives. That said, I'm finding it a little tedious being forced to use pointless style for a task that's well-suited to having a few names around. The idea of an applicative do notation's been kicked around on #haskell a few times (I can't find any trace of it on the mailing list, but I confess to not having searched too hard), so I thought I'd propose it here. The basic idea is to turn this: do a <- f g b <- h pure $ foo a b into this: (\a b -> pure $ foo a b) <*> (f <*> g *> h) Aside from changing >>= and >> into <*> and *>, the most significant difference from monadic do is that all the generated lambda abstractions go in front of the final "return" statement which is then fmapped across the rest of the code. Bindings are thus only in scope in the "return" statement. I believe sugared let statements can be handled similarly so long as they respect the binding discipline. This leads us to the bikeshed topic: what's the concrete syntax? The obvious way is to replace do with a new keyword - for example, ado for "applicative do". There's a nice alternative though: we can check whether a do statement meets the binding rules for an applicative block and treat it as one if so, or a monadic one if not. While not all Monads are Applicatives, code can readily be changed back using the WrappedMonad newtype - whereas existing code needn't turn on the appropriate extension in the first place. Thoughts, comments? -- flippa(a)flippac.org

6 8