- Haskell-Cafe - Haskell.org

Cobbled together a version of Haskell Platform 2010.2 that works for GHC 7.1
by Ryan Newton 15 Oct '10

15 Oct '10

Well, it's not 2010.2 anymore because I had to upgrade a few packages ;-), but if you want to try out some of the new GHC 7 features this can get you a working cabal-install. Download here: http://parfunk.blogspot.com/2010/10/hacking-together-working-version-of.html Apologies if this is already available somewhere, but I didn't see it. Cheers, -Ryan

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ANNOUNCE: Facts
by Alexander Solla 15 Oct '10

15 Oct '10

The Facts hierarchy is meant to contain commonly used, relatively static facts about the "real world". The facts are meant to be encoded using relatively simple Haskell constructs. However, we do make some promises: every data type our modules export will have instances of Data, Eq, Ord, Show, andTypeable. We will use explicit module export lists to control access to internal data structures. As much of the data we are encoding is tabular, we use simple structures like lists and maps to encode the relations. This has two practical ramifications: the textual representation of the data can be very wide, but are also very easy to edit, with "block editing" tools like Vi's visual block mode. The other consequence is that the naive approach to writing queries can be tedious, and the resulting naive queries are slower than they could be. Template Haskell can eliminate much of this drudgery. Felipe Lessa has graciously donated some Template Haskell code which we have adapted. The Facts.* hierarchy currently contains modules with geographical information, such as a data type of countries, cross references to various ISO-3166-1 names for each, a list of states in the United States, and the United States address format. Please see the module hierarchy for more specifics. Patches are welcomed, though prospective contributors are encouraged to encode data structures using lists of pairs to encode bijections, all exposed data types are instances of Data, Eq, Ord, Show, and Typeable, and use explicit exports to only export queries and their atoms. For now, we will add facts to the hierarchy lazily, as our projects need them.

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ANNOUNCE: polyToMonoid
by Kevin Jardine 15 Oct '10

15 Oct '10

Polyvariadic functions that accept an indefinite number of possibly multiple typed arguments are often useful but a bit difficult to declare in Haskell. With some crucial assistance from Oleg Kiselyov, I have created a library that supplies two very general polyvariadic functions that can map their arguments into any monoid you specify. All you need to do to use these is to specify the monoid and some instances of a toMonoid function to map all the types you want to pass as arguments into the monoid. The functions can act as a sort of generalised list, but of course there are also many interesting monoids which are not lists. The library is now up on Hackage here: http://hackage.haskell.org/package/polyToMonoid-0.1 along with detailed documentation and a few examples here: http://hackage.haskell.org/packages/archive/polyToMonoid/0.1/doc/html/Data-… As you can see, the code is short and depends only upon the GHC base library. The original thread discussing the polyToMonoid idea is here: http://groups.google.com/group/haskell-cafe/browse_thread/thread/737c6cf921… Comments and suggestions for improvement are welcome! Kevin

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ANNOUNCE: type-level-natural-number and friends!
by Gregory Crosswhite 14 Oct '10

14 Oct '10

Hey everyone, I am pleased to announce the release of a family of packages for type-level natural numbers. The emphasis on these packages is minimality in order to provide simple core functionality that requires as few extensions as possible beyond Haskell-2010. The (probably foolish) hope is that this will encourage people to use these packages for their simple type-level natural number needs so that the different packages can have interoperable type-level natural numbers. === type-level-natural-numbers v1.1 === This is an update to type-level-natural-numbers, a Haskell-2010 compatible package which provides simple type-level natural numbers. (The only non-Haskell-98 extension that it requires is EmptyDataDecls.) This release adds word aliases for N1..N15 (i.e., One, Two, Three...) as well as a Typeable instance for type-level natural numbers. === type-level-natural-number-operations v1.0 === This package contains the type functions Plus and Minus for performing addition and subtraction on natural numbers; by keeping the functionality minimal I avoided the need to use UndecideableInstances, so the only extension that is required is TypeFamilies. === type-level-natural-number-induction v1.0 === This is the most interesting of the three packages. It provides high-level combinators for expressing inductive operations on data structures tagged with a natural number, which at the moment I call "inductive structures". The idea is that you express your operation in terms of a base case and an inductive case, and then call one of the combinators to do the rest for you. There are combinators for building up an inductive structure from a seed value, folding over/tearing down an inductive structure to get a result, and transforming one inductive structure into another. These combinators are supplied in both monadic and non-monadic versions. The only extension that is required to use this package is Rank2Types. === natural-number v1.0 === This package provides *value*-level natural numbers that are tagged with a type-level natural number corresponding to their value by using GADTs, as well as some simple operations on them. I also provide an instance for the EqT class in type-equality, which means that you can compare two natural numbers with possibly different tags and obtain a proof of equality if and only if they are the same type. The extensions that are required to use this package are Rank2Types and GADTs. (The package itself only uses GADTs, but it pulls in type-level-natural-number-induction which uses Rank2Types.) ======================== Feedback is always welcome! Cheers, Greg

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Installing ghc on NFS ... stripping and upx'ing executables, but what about .a/.so libs
by Ryan Newton 14 Oct '10

14 Oct '10

This gets the ghc executable itself from 42mb down to 5.6mb, which helps a lot on cruddy NFS installations. For example the libHSghc-*, which are pretty huge (49-137mb). Are these libraries that are used only by GHC for compilation? (Actually, I seem to be able to compile simple files without them... so what are they used for?) Any disadvantage to stripping them to save a factor of two? Thanks, -Ryan

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ANNOUNCE: tagged-list v1.0
by Gregory Crosswhite 14 Oct '10

14 Oct '10

Hey everyone, I am pleased to announce the release of tagged-list version 1.0, a package which provides fixed-length lists that are tagged with a phantom type-level natural number corresponding to the length. The advantage of such lists is that you can make static guarantees about them, so that for example the function addLists :: TaggedList n Int -> TaggedList n Int -> TaggedList n Int ... which adds two lists is guaranteed to get two lists of the same length and to produce a list with the same length as the inputs. Some basic operations on these lists have been provided, including instances for Applicable, Traversable, etc. One of the more interesting of these operations is the "join" function", join :: TaggedList m α → TaggedList n α → (TaggedList (Plus m n) α,TaggedList (Plus m n) β → (TaggedList m β,TaggedList n β)) This function takes two lists and appends them, and returns not only the appended list but also a function that takes a list of the same length and splits it into two lists with the same lengths as the inputs; one way of interpreting the second parameter is that it provides a "proof" that a TaggedList of type TaggedList (Plus m n) β can be broken into lists of type TaggedList m β and TaggedList n β. This function is provided because I couldn't figure out how to make a general "split" function pass the type-checker, and fortunately I found that in practice I didn't need a general such function because I was usually taking a list that was the result of some operation applied to two lists I had earlier appended and breaking the result list into sublists the lengths of the original two lists. I also provided functions to convert tagged lists to and from tuples, which can be useful at the times when the type-checker gets confused by your "a :. b :. ... :. E" expression and gives you obscure error messages. Anyway, I hope that the community finds this package useful, and welcome any feedback that you all have to offer. Cheers, Greg

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Books for review in the journal of functional programming
by Simon Thompson 14 Oct '10

14 Oct '10

Just updated: list of books for review in the journal of functional programming, available here: http://www.cs.kent.ac.uk/people/staff/sjt/JFP/available.html Regards Simon T.

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Re: [Haskell-cafe] Client-extensible heterogeneous types (Duck-typed variadic functions?)
by Brandon Moore 14 Oct '10

14 Oct '10

On Oct 13, 2010, at 7:44 PM, Jacek Generowicz <jacek.generowicz(a)cern.ch> wrote: On 2010 Oct 14, at 01:32, Evan Laforge wrote: I think I'm starting too see what my problem is. I think it boils down to hankering for Duck Typing and variadic functions. I fully appreciate that passing functions is a wonderful and powerful technique for catering for variation, but Haskell's type system cramps my style by insisting that I can't put a (Banana -> Cake) in the same container as an (Octopus -> Truffles -> DogsBreakfast). But the thing is, I don't use things like this, even in python. Shame. They're damn useful :-) How are you expecting to call the functions in that container? "for f in c: try: return f(*misc_args) except: pass"? to_do = [(call, (AuntMabel,)), (buy, ([(12*kg, sugar), (6*bushel, wheat)])), (introduce, (Romeo, Juliet))] for do,it in to_do: do(*it) What is the point of doing that? If it's just to defer execution until that loop, you should just rely on lazy evaluation, or [IO ()]. If that's not the only thing you do, then the question is still how you know enough about the structure of values In the list to do anything useful with them. If you just want to be able to inspect them interactively, you should ask about support for inspecting thunks in the ghci debugger. I suppose you haven't heard of parametricity theorems. In Haskell, values have no hair. If you don't know anything about the type of a values you can't inspect it. It's one of the major tools that helps type signatures contribute to the correctness of implementations. In Python, Java, and other similar languages there are lots of things you can do with unknown values - get a string representation, test for equality with another value, get the class it belongs to, etc. So, we won't understand the point of your example without a little more information on what you do to those heterogeneous values, and how the program can tell which things to do wi which item, In Haskell it may be fun to turn on -XGADTs and write data DelayedApp result where Base :: a -> DelayedApp a App :: DelayedApp (a -> b) -> a -> DelayedApp b but it turns out to be isomorphic to data DelayedResult r = DR a Nat - at least until you add some more data to the constructors.

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Re: [Haskell-cafe] Haskellers.com recent changes (and I need some volunteers)
by Michael Snoyman 14 Oct '10

14 Oct '10

Sorry to everyone for not getting back so quickly, I kept getting errors from postfix when I tried sending mail to the cafe. Hopefully this one will go through. As I see it, two open issues are "flagging" and "real Haskellers". Flagging: this was simply a mistake in terminology on my part. I've replaced it with "Report this user." It should be used for either inappropriate content, spam, or someone who's just clearly not part of the community (eg, "Lol, I don't know Haskell, I had Cocoa Puffs for breakfast"). I'm purposely being vague about this; if a user *thinks* there's a problem, it shouldn't take an admin more than a few seconds to investigate it. Now the more important question about real Haskellers: I think I mentioned before implementing the feature that I was a little bit nervous about doing so. The main reason I went ahead and did it anyway was we were getting some... strange gravatars showing up on the homepage. This problem was solved automatically when I added sorting by years of experience (no one can object to Simon PJ and Lennart being on the homepage of course), but the problem with that system is *anyone* can just set their start year to 1990 and get homepage status until an admin blocks him/her. So for the moment, real Haskeller is a minimal whitelisting system, simply intended to prevent people from gaming the system. I've probably chosen bad terminology, and by not explaining this upset a lot of people, my apologies. The point here is not to make the real Haskeller status exclusive, but just to give an extra level of protection. If people really think this is a bad idea, we can take it out. However, keep in mind that the community already seems to favor whitelists (the wiki requires admin intervention for an account, same on HackageDB). This also explains why Lennart and Simon PJ are not at spots 1 and 2: I haven't granted them real Haskeller status. Obviously no offense is intended, I just haven't have a chance yet. As is, I've accepted (virtually) every request for real Haskeller status: the only exception so far has been people who haven't set their full name properly (which once again is my fault for creating a slightly confusing UI). Then the question remains how to we sort people. One idea is randomly; the downside is that it will be difficult to just start browsing the way you can today. We could show newest Haskellers first, which I think definitely has a benefit. I would still recommend we use the real Haskeller status for this, and count people as "new" based on when they receive that status. This will give everyone a chance to make the rounds. And of course, the main feature that's lacking right now is a search/filter for users. I want to wait a few more days to see if we add any more features that should be accounted for in the search. This default sorting issue seems contentious enough that it might be worth doing a poll on it. If you have concrete ideas of how the algorithm should work, please send them over. Make sure to specify whether you think the real Haskeller thing should be kept, and a better name for it if you can think of one. Michael On Mon, Oct 11, 2010 at 3:53 AM, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: > The list is clearly not sorted by years of experience. I and Simon PJ > are on the second page, and I doubt many people wrote Haskell programs > before us. > > -- Lennart (iPhone) > > On Oct 10, 2010, at 13:06, Michael Snoyman <michael(a)snoyman.com> wrote: > >> Hey all, >> >> Haskellers became popular a lot faster than I'd anticipated. This has >> prompted me to need to make some changes that I was only planning on >> implementing later on. As usual, all points are up for discussion >> (this is intended to be a community-run site after all). >> >> * Pagination on homepage. We now see only 10 users at a time. >> * The sort order is different. Now, "real haskellers" (we'll discuss >> that in a second) come first, sorted by years of experience with >> Haskell and then alphabetically by full name. >> * We now have site admins. Currently, I'm the only one, but I would >> like to add some more (thus the need for volunteers). Admins currently >> have four rights/duties: >> * Grant/revoke "real haskeller" status. This status currently only >> affects sort order on home page, but might be used for more in the >> future. >> * Grant/revoke admin rights. >> * Block/unblock a user. A blocked user will not show up on the >> homepage, not be counted for public/private/unverified accounts, and >> will have a big nasty "blocked" warning on their user page. >> * Read/close admin messages. These can be created by the following features: >> * Users can request "real haskeller" status in their Edit Profile > Extras tab. >> * Users can request their account to be unblocked. >> * Anyone (logged in or not) can flag a user on their user page. >> >> You'll notice that admins are not able to make any changes to user >> data. Anyone who is interested in taking on this responsibility should >> email me about it. I think five people should easily be able to handle >> this, I'm not anticipating a high load. Then again, I didn't >> anticipate over 200 accounts created in a week either. >> >> Oh, and *please* don't email me personally to request a "real >> Haskeller" status. If you use the button on the site, it makes it much >> easier to add the status. Also, don't take it personally if an admin >> denies your "real haskeller" request; it's currently a very poorly >> defined notion, and I don't even know who the admins are going to be. >> >> Michael >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: [Haskell-cafe] Client-extensible heterogeneous types (Duck-typed variadic functions?)
by Brandon Moore 14 Oct '10

14 Oct '10

If you just want instances of questions you can keep it simple. How about something isomorphic to data Instance = Instance { question : String, answer : String, check : String -> Bool } You could make helper functions similar to your old code, like addition : (Int , Int) -> Instance You might handle problem families by taking a random number generator, or maybe using QuickCheck's Gen monad. easyMultiplication : Gen Instance

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Fuzzy time deltas
by Michael Snoyman 14 Oct '10

14 Oct '10

Hey all, Is there a library that supports fuzzy time deltas? For example, given two UTCTimes (or something like that) it could produce: 43 seconds 13 minutes 17 hours 4 days 8 months I want to use it for the news feature on Haskellers. It's not that hard to write, just wondering if it's already been done. Michael

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Make your Darcs repositories hashed?
by Christopher Done 14 Oct '10

14 Oct '10

Every Darcs repository I've pulled this year has always showed me this message: ******************************************************************************* Fetching a hashed repository would be faster. Perhaps you could persuade the maintainer to run darcs optimize --upgrade with darcs 2.4.0 or higher? ******************************************************************************* Maintainers! Please run darcs optimize --upgrade

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Re: [Haskell-cafe] Client-extensible heterogeneous types
by John Lato 14 Oct '10

14 Oct '10

> > From: Jacek Generowicz <jacek.generowicz(a)cern.ch> > > So my question is, what would be the Haskell approach to combining > heterogeneity (as provided by variant (algebraic) types) with > type-extensibility (as provided by type classes), without losing the > benefits of Haskell's type system? > > I haven't looked into Template Haskell, but i guess that it is likely > to provide a solution. > But is there a solution in plain Haskell? > It's not plain Haskell, but I'm surprised nobody mentioned the ExistentialQuantification extension, which unless I'm missing something provides exactly what you want. John

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Finite but not fixed length...
by Jason Dusek 14 Oct '10

14 Oct '10

Is there a way to write a Haskell data structure that is necessarily only one or two or seventeen items long; but that is nonetheless statically guaranteed to be of finite length? -- Jason Dusek Linux User #510144 | http://counter.li.org/

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a couple of cabal related questions
by Dmitry V'yal 14 Oct '10

14 Oct '10

Hello, recently I've cabalized my program and it brought some problems. First of all, I made use of 'import Paths_<package name>' functionality to get the path of installed data files. It works great, but now I can no longer run my program in ghci because this module is auto-generated by cabal. I tried to supply my own. This way ghci works, but my stub module gets compiled into the binary in place of auto-generated. And things break again. What's the best way to overcome it? And another question. I'd like to make my program output it's version string. Of course, I can hardcode it into sources, but I've already specified one in the cabal file. Is there any way to get it back in the run time? Thanks, Dmitry

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Haskell Weekly News: Issue 154 - October 13, 2010
by Daniel Santa Cruz 14 Oct '10

14 Oct '10

Welcome to issue 154 of the HWN, a newsletter covering developments in the Haskell community in the week of October 03 - 09. This community does not stand still! The last section of the newsletter contains a list of new or updated packages this passed week. All 87 of them! I had my doubts about listing them all, but there was encouragement from IRC folk to showcase them all, at least this time. If you are having a hard time filling your day with Haskell stories, make sure to jump into Reddit and help everyone by posting comments and voting on stories that are meaninful. If you manage to have time left after that, the volume of postings on the mailing lists is more than one person can keep up with. Send threads that you think the rest of the community would benefit from. For those of you who manage to have extra-extra time, you can use [1]twitterfall to try to keep up with the many tweets about Haskell! So, what was hot this week? Announcements Want to know what events are comming up in the community? Come and see the [2]Haskell Community Calendar. Send new events to dstcruz(a)gmail.com Michael Snoyman [3]announced and asked for advice on [4]Haskellers.com. If you are into Haskell, by all means, please stop by and make a profile on the site! A new version of the venerable Haskell XML Toolbox (version 9.0.0) [5]was announced by Uwe Schmidt. A package for tls, native TLS/SSL protocol implementation [6]was announced by Vincent Hanquez. Interesting Threads on Haskell-Cafe Petr Pudlak Asked if there are any suggested/recommended Haskell coding conventions? Perhaps something like Java has? [7]see thread caseyh commented of difficulties with GUIs working under Windows. The thread included talk about the state of Haskell GUI programming in general. [8]see thread Don Steward and Duncan Coutts presented a status report on the Haskell distribution infrastructure: Hackage, Haskell Platform and Cabal, during the Haskell Implementors Workshop held in Baltimore. You can find the [9]slides and the [10]video of the presentation. Christopher Done [11]annouced that he has been doing some refactoring on Haskell web development entries on the Wiki. Max Bolingbroke [12]told us about a proposed haskell feature: lambda-case/lambda-if. There seems to be interest from multiple people to see this feature. Some modifications to the original syntax were also proposed in the thread. Top Reddit Stories * Learn You a Haskell for Great Good! The Book! Domain: amazon.com Score: 72, Comments: 28 On Reddit: http://bit.ly/dp07AZ Original: http://amzn.to/b1JYWM * LYAH: For A Few Monads More Domain: learnyouahaskell.com Score: 64, Comments: 25 On Reddit: http://bit.ly/bLQMsD Original: http://bit.ly/dpkD0R * Real World Experiences with Long Running Haskell Servers? Domain: self.haskell Score: 34, Comments: 25 On Reddit: http://bit.ly/9NTcLo * Want professional work in Haskell? Don't forget to register your skills Domain: haskellers.com Score: 28, Comments: 21 On Reddit: http://bit.ly/9bqlhG Original: http://www.haskellers.com/ * Enumerators Tutorial Part 3: Enumeratee Domain: docs.yesodweb.com Score: 25, Comments: 1 On Reddit: http://bit.ly/d3RPP0 Original: http://bit.ly/dwEOWO * Scheduling Lazy Evaluation on Multicore : Simon Marlow : Video Domain: vimeo.com Score: 24, Comments: 5 On Reddit: http://bit.ly/cGXeFJ Original: http://vimeo.com/15573590 * Scrap Your Zippers : Michael Adams : Video Domain: vimeo.com Score: 23, Comments: On Reddit: http://bit.ly/aco7b6 Original: http://www.vimeo.com/15481513 * [The Haskell Platform] libraries process is a trackless mire Domain: haskell.org Score: 21, Comments: 7 On Reddit: http://bit.ly/cby0zw Original: http://bit.ly/a62xg0 * Angel: a daemon that runs and monitors other processes, developed by Bump Domain: github.com Score: 19, Comments: On Reddit: http://bit.ly/9P8Ccm Original: http://github.com/jamwt/Angel#readme * The HTML purification manifesto (Inside 245s) Domain: blog.ezyang.com Score: 15, Comments: On Reddit: http://bit.ly/a6Bu4N Original: http://bit.ly/duCoSz Top StackOverflow Questions * haskell regex substitution (votes: 8, answers: 1) http://bit.ly/b34nPB * What is the Haskell response to Node.js? (votes: 6, answers: 2) http://bit.ly/9YKRPj * Conventions for Stability field of Cabal packages (votes: 5, answers: 2) http://bit.ly/cxDksd * is there a lazy way to write the minus function (remove items from a list)? (votes: 4, answers: 3) http://bit.ly/c85gi9 * "maybe"-like function for Bool and List? (votes: 3, answers: 3) http://bit.ly/biWCUl Quotes of the Week * Cale: makes a fruit salad with some Abelian grapes and some Banananach spaces. * Cale: If you don't feel at sea most of the time that you're studying something, you're probably not learning fast enough. * jamy: windows Keyboard doesn't found. To rescan - press any key * kmc: we put the fun in -fundecidable * sipa: asking for experts before asking your questions prevents non-experts who know the answer as well from answering * monochrom: but yes there is hope that haskell 2039 becomes haskell 1971, all is not lost. Now or Updates Hackage Packages * anansi 0.2.1 - Simple literate programming preprocessor (JohnMillikin) * archlinux 0.3.3 - Support for working with Arch Linux packages (DonaldStewart) * aterm 0.1.0.0 - serialisation for Haskell values with sharing support (ChristianMaeder) * authenticate 0.6.6.1 - Authentication methods for Haskell web applications. (MichaelSnoyman) * bff 0.3.1.1 - Bidirectionalization for Free! (POPL'09) (DanielSeidel) * bidispec 0.1.3 - Specification of generators and parsers (StefanWehr) * bidispec-extras 0.1 - Extra helper functions for bidirectional specifications (StefanWehr) * bindings-apr 0.1 - Low level bindings to Apache Portable Runtime (APR) (MasatakeDaimon) * bindings-apr-util 0.1 - Low level bindings to Apache Portable Runtime Utility (APR Utility) (MasatakeDaimon) * bindings-libzip 0.1 - Low level bindings to libzip. (SergeyAstanin) * cabal2arch 0.7.4 - Create Arch Linux packages from Cabal packages (DonaldStewart) * certificate 0.3 - Certificates and Key Reader/Writer (VincentHanquez) * collada-output 0.2 - Generate animated 3d objects in COLLADA (TillmannVogt) * Crypto 4.2.2 - Collects together existing Haskell cryptographic functions into a package (CreightonHogg) * cuda 0.2.2 - FFI binding to the CUDA interface for programming NVIDIA GPUs (TrevorMcDonell) * dataenc 0.13.0.4 - Data encoding library (MagnusTherning) * deepseq 1.1.0.1 - Fully evaluate data structures (SimonMarlow) * ecu 0.0.4 - Tools for automotive ECU development. (TomHawkins) * enumerator 0.4.1 - Implementation of Oleg Kiselyov's left-fold enumerators (JohnMillikin) * equivalence 0.2.1 - Maintaining an equivalence relation implemented as union-find using STT. (PatrickBahr) * exception-monads-fd 0.2 - Exception monad transformer instances for monads-fd classes. (GeoffreyMainland) * exception-monads-tf 0.2 - Exception monad transformer instances for monads-tf classes. (GeoffreyMainland) * exception-mtl 0.2 - Type classes and monads for unchecked extensible exceptions. (GeoffreyMainland) * exception-transformers 0.2 - Type classes and monads for unchecked extensible exceptions. (GeoffreyMainland) * explicit-iomodes-text 0.1.0.2 - Extends explicit-iomodes with Text operations (BasVanDijk) * extemp 0.0.1 - automated printing for extemp speakers (SamAnklesaria) * fibon 0.1.0 - A reworking of the classic nofib benchmark suite (DavidPeixotto) * free-theorems-counterexamples 0.3.0.1 - Automatically Generating Counterexamples to Naive Free Theorems (DanielSeidel) * Fungi 1.0.4 - An interpreter for Funge-98 programming languages, including Befunge. (ThomasEding) * hakismet 0.1 - Akismet spam protection library (NilsSchweinsberg) * haskelldb 2.0 - A library of combinators for generating and executing SQL statements. (JustinBailey) * haskelldb-hdbc 2.0 - HaskellDB support for HDBC. (JustinBailey) * haskelldb-hdbc-odbc 2.0 - HaskellDB support for the HDBC ODBC driver. (JustinBailey) * haskelldb-hdbc-postgresql 2.0 - HaskellDB support for the HDBC PostgreSQL driver. (JustinBailey) * haskelldb-hdbc-sqlite3 2.0 - HaskellDB support for the HDBC SQLite driver. (JustinBailey) * haskelldb-th 2.0 - Template Haskell utilities for HaskellDB. (JustinBailey) * haskell-qrencode 1.0.2 - Haskell bindings for libqrencode. (JamesSanders) * haskell-src-exts 1.9.4 - Manipulating Haskell source: abstract syntax, lexer, parser, and pretty-printer (NiklasBroberg) * haskell-src-meta 0.2 - Parse source to template-haskell abstract syntax. (BenMillwood) * haskoon 0.3.1.1 - Web Application Abstraction (StefanWehr) * haskoon-httpspec 0.5.0.1 - Integrating HttpSpec with Haskoon (StefanWehr) * haskoon-salvia 0.4.0.2 - Integrating HttpSpec with Haskoon (StefanWehr) * hothasktags 0.1.1 - Generates ctags for Haskell, incorporating import lists and qualified imports (LukePalmer) * hsc3-process 0.3.1 - Create and control scsynth processes (StefanKersten) * HTF 0.5.0.0 - The Haskell Test Framework (StefanWehr) * httpspec 0.3.0.1 - Specification of HTTP request/response generators and parsers (StefanWehr) * hums 0.3.1 - Haskell UPnP Media Server (BardurArantsson) * hws 1.1.0.1 - Simple Haskell Web Server (SimonMarlow) * hxt-extras 0.4 - Extra functions for HXT (TonyMorris) * hxthelper 0.2.2 - Helper functions for HXT (StefanWehr) * language-c-comments 0.1 - Extracting comments from C code (MatthewSottile) * libpq 0.4.0 - libpq binding for Haskell (GrantMonroe) * LibZip 0.2.0 - Bindings to libzip, a library for manipulating zip archives. (SergeyAstanin) * loli 2010.10.9 - A minimum web dev DSL in Haskell (JinjingWang) * mainland-pretty 0.1.0.1 - Pretty printing designed for printing source code. (GeoffreyMainland) * memscript 0.0.1.0 - Command line utility for memorizing scriptures or any other text (KiYungAhn) * multiset 0.2.1 - The Data.MultiSet container type (TwanVanLaarhoven) * network-bytestring 0.1.3.1 - Fast, memory-efficient, low-level networking (JohanTibell) * parallel 3.1.0.1 - Parallel programming library (SimonMarlow) * persistent-sqlite 0.2.2.1 - Backend for the persistent library using sqlite3. (MichaelSnoyman) * plot 0.1.2 - A plotting library, exportable as eps/pdf/svg/png or renderable with gtk (VivianMcPhail) * polyseq 0.1.2.1 - Taming Selective Strictness (DanielSeidel) * proc 0.0.8 - Parse process information for Linux (AndyStewart) * pureMD5 2.1.0.1 - A Haskell-only implementation of the MD5 digest (hash) algorithm. (ThomasDuBuisson) * random-extras 0.18 - Additional functions for random values. (AristidBreitkreuz) * safer-file-handles-text 0.1.0.3 - Extends safer-file-handles with Text operations (BasVanDijk) * shaker 0.4.3 - simple and interactive command-line build tool (AnthoninBonnefoy) * smartGroup 0.2.1 - group strings by words in common (SamAnklesaria) * spata 2010.10.10 - brainless form validation (JinjingWang) * stream-monad 0.3 - Simple, Fair and Terminating Backtracking Monad (SebastianFischer) * tls 0.2 - TLS/SSL protocol native implementation (Server and Client) (VincentHanquez) * tree-monad 0.3 - Non-Determinism Monad for Tree Search (SebastianFischer) * uni-events 2.2.0.0 - Event handling for the uniform workbench (ChristianMaeder) * uni-graphs 2.2.0.0 - Graphs (ChristianMaeder) * uni-htk 2.2.0.0 - Graphical User Interface for Haskell Programs (ChristianMaeder) * uni-posixutil 2.2.0.0 - Posix utilities for the uniform workbench (ChristianMaeder) * uni-reactor 2.2.0.0 - Reactors for the uniform workbench (ChristianMaeder) * uni-uDrawGraph 2.2.0.0 - Graphs binding (ChristianMaeder) * uni-util 2.2.0.0 - Utilities for the uniform workbench (ChristianMaeder) * usb 0.6.0.2 - Communicate with USB devices (BasVanDijk) * wumpus-basic 0.10.0 - Common drawing utilities built on wumpus-core. (StephenTetley) * wumpus-core 0.35.0 - Pure Haskell PostScript and SVG generation. (StephenTetley) * wumpus-microprint 0.10.0 - Microprints - "greek-text" pictures. (StephenTetley) * wumpus-tree 0.9.0 - Drawing trees (StephenTetley) * xilinx-lava 5.0.1.4 - The Lava system for Xilinx FPGA design with layout combinators. (SatnamSingh) * xsd 0.3.3 - XML Schema data structures (TonyMorris) * yesod-auth 0.1.1 - Authentication for Yesod. (MichaelSnoyman) About the Haskell Weekly News New editions are posted to [13]the Haskell mailing list as well as to [14]the Haskell Sequence and [15]Planet Haskell. [16]RSS is also available, and headlines appear on [17]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz * at * gmail * dot * com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://twitterfall.com/ 2. https://sites.google.com/site/haskellcommunitycalendar/home 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81530 4. http://haskellers.com/ 5. http://osdir.com/ml/haskell-cafe@haskell.org/2010-10/msg00286.html 6. http://osdir.com/ml/haskell-cafe@haskell.org/2010-10/msg00252.html 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81083 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81225 9. http://donsbot.wordpress.com/2010/10/01/hackage-cabal-and-the-haskell-platf… 10. http://www.vimeo.com/15462768 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81345 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81366 13. http://www.haskell.org/mailman/listinfo/haskell 14. http://sequence.complete.org/ 15. http://planet.haskell.org/ 16. http://sequence.complete.org/node/feed 17. http://haskell.org/

1 0

How to fix "undefined reference" error with getDataDir?
by Andy Stewart 13 Oct '10

13 Oct '10

Hi all, I have two package A and B, and B depend A. I use below code snippets in package A: ------------------------------> code start <------------------------------ ... import Paths_manatee_ircclient import System.FilePath ... dir <- getDataDir let imagePath imageName = dir </> "icons" </> (imageName ++ ".png") ... ------------------------------> code end <------------------------------ Anyway, package A compile fine, but when i compile package B, i got below error: ------------------------------> error start <------------------------------ Linking dist/build/manatee/manatee ... /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `s6sP_info': (.text+0x3bea): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir3_closure' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `s6sP_info': (.text+0x3bf0): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir2_closure' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `s6Uf_info': (.text+0x560f): undefined reference to `__stginit_manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `r5Mp_closure': (.data+0x9d8): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir3_closure' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `r5Mp_closure': (.data+0x9e0): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir2_closure' collect2: ld returned 1 exit status cabal: Error: some packages failed to install: manatee-0.0.1 failed during the building phase. The exception was: ExitFailure 1 ------------------------------> error end <------------------------------ How to fix above error? Thanks! -- Andy Below is .cabal file for package A: ------------------------------> A.cabal start <------------------------------ name: manatee-ircclient version: 0.0.1 Cabal-Version: >= 1.6 license: GPL-3 license-file: LICENSE copyright: (c) 2009 ~ 2010 Andy Stewart synopsis: IRC client extension for Manatee. description: manatee-ircclient is IRC client extension for Manatee (Haskell/Gtk+ Integrated Live Environment) author: Andy Stewart maintainer: Andy Stewart <lazycat.manatee(a)gmail.com> stability: provisional category: Development, Other tested-with: GHC==6.12.3 build-type: Simple extra-source-files: Setup.lhs data-dir: icons data-files: angry.png confused.png crying.png embarrassed.png inlove.png kiss.png sleepy.png sad.png laugh.png smile.png surprised.png tired.png tongue.png whistling.png wink.png Library build-depends: base >= 4 && < 5, manatee-core >= 0.0.1, dbus-client >= 0.3 && < 0.4, stm >= 2.1.2.0, containers >= 0.3.0.0, gtk-serialized-event >= 0.11.0, gtk >= 0.11.0, text >= 0.7.1.0, bytestring >= 0.9.1.5, dbus-core, template-haskell, gtksourceview2 >= 0.11.0, unix >= 2.4.0.0, network, groom, fastirc >= 0.2.0, split >= 0.1.2, nano-md5 >= 0.1.2, filepath, regex-posix >= 0.94.1, array >= 0.3.0.0, GoogleTranslate >= 0.0.3, utf8-string, mtl, Cabal exposed-modules: Manatee.Extension.IrcClient Manatee.Extension.IrcClient.Types Manatee.Extension.IrcClient.DBus Manatee.Extension.IrcClient.Daemon Manatee.Extension.IrcClient.HighlightNick Manatee.Extension.IrcClient.IrcBuffer Manatee.Extension.IrcClient.IrcView Manatee.Extension.IrcClient.Smile other-modules: extensions: ghc-options: -O -fwarn-unused-matches -fwarn-unused-binds -fwarn-unused-imports -fwarn-overlapping-patterns -fwarn-duplicate-exports -threaded -fwarn-unrecognised-pragmas -fwarn-hi-shadowing Executable manatee-irc-daemon main-is: Main.hs ghc-options: -threaded ------------------------------> A.cabal end <------------------------------

2 2

Re: Re-order type
by André Batista Martins 13 Oct '10

13 Oct '10

Might have not been clear, but i will try illustrate . f:: a-> b -> c -> (b,(c,a)) f1 :: c -> a -> d input type: A B C ---------- | f | | _____ | output (B,(C,A)) C A ---------- | f1 | | _____ | output D If i want compose f and f1, i need to do a correct input to f1 from the output of f. So i want one function to convert the output of "f" to input off "f!". In this case, we do f1 fst (snd (t,(t1,t2))) snd (snd (t,(t1,t2))) But i want do this automaticaly, for type of any two function. I search for the "glue". I don't have any concern about what the function does, i only have interess on input and output types. Cheers, André No dia 9 de Outubro de 2010 22:38, André Batista Martins < andre_bm(a)netcabo.pt> escreveu: > Hello, > exists any algorithm to determine how terms can be changed to safisty the > type of one function? > > > example: > > f:: a-> b -> c -> (b,c,a) > > f1 :: c -> a -> d > > In my first function "f" i want assign the output "c" and "a" for to input > of function "f1". > I searched for any solution, but i didn't find any anything. > > One clue i have found is "minimal edit distance algorithm" for 2 strings. > Perhaps if i convert de output type of "f" to one string, and de input of > "f1" to another string and then use this algorithm , i will get one "dirty" > solution... > > I'm open to any sugestion. >

13 18

Things I would like to see in protocol-buffers
by Mads Lindstrøm 13 Oct '10

13 Oct '10

Hi I have been trying to use protocol-buffers[2], and I have some ideas which would have improved the protocol-buffers package for my usage (and maybe for others): * hprotoc should have an option to generate messages with [Char], Int and [] in stead of Utf8, Int32 and Seq. While some people may need the speed of bytestrings, for others they are just cumbersome. And if you need to convert bytestring to [Char] anyway (maybe some other library requires [Char]) you really do not gain any performance anyway. * Integration with System.IO.Handle, so that I would have a simpler interface like: readDelimitedMessage :: System.IO.Handle -> IO MessageType writeDelimitedMessage :: System.IO.Handle -> MessageType -> IO () * Union types. A common protocol buffers idiom for union types is[3]: message Foo { enum FooType { SOME_FOO_MESSAGE = 0; ANOTHER_FOO_MESSAGE = 1; } message SomeFoo { ... } message AnotherFoo { ... } required FooType fooType = 1; optional SomeFoo someFoo = 2; optional AnotherFoo anotherFoo = 3; } While the message can (statically) contain both someFoo and anotherFoo, it would be a runtime error. In reality it is a union type, that must either contain someFoo or antoherFoo. It would be nice if one could mark in the .proto file that hprotec should see Foo as a union type. Hprotoc should then generate the Foo type as: data Foo = SomeFoo ... | AnotherFoo ... Maybe the custom option mechanism[1] can be used in the .proto file, to implement this feature. What do people think of these ideas? Regards, Mads Lindstrøm [1] http://code.google.com/apis/protocolbuffers/docs/proto.html#options [2] http://hackage.haskell.org/package/protocol-buffers [3] http://code.google.com/apis/protocolbuffers/docs/techniques.html#union

1 0

Re: [Haskell-cafe] Finite but not fixed length...
by Permjacov Evgeniy 13 Oct '10

13 Oct '10

infinite value is value, that have no upper bound (see infinity definition). So, you have to provide upper bound at compile time. Tree example provides such bound. On 10/13/2010 03:27 PM, Eugene Kirpichov wrote: > Again, the question is not how to arrange that all non-bottom values > are finite: this can easily be done using strictness, as in your List > example. > > The trick is to reject infinite values in compile time. How can *this* be done? > > 13 октября 2010 г. 15:26 пользователь Permjacov Evgeniy > <permeakra(a)gmail.com> написал: >> On 10/13/2010 03:09 PM, Eugene Kirpichov wrote: >> >> 1-st scenario: If you have, for example, 2-3 tree, it definitly has a >> root. If you construct tree from list and then match over root, the >> entire tree (and entire source list) will be forced. And on every >> update, 2-3 tree's root is reconstructed in functional setting. So, if >> you'll try to build 2-3 tree from infinite list, it will fail in process >> due insuffisient memory. >> Of course, you can make the same with >> data List a = Cons a (!List a) | Nil >> >> second scenario >> data Node a = Nil | One a | Two a a >> and so Node (Node (Node (Node a))) has at most 2^4 = 16 elements. with >> some triks you'll be able to set upper bound in runtime. >> >>> I don't see how. Could you elaborate? >>> >>> 13 октября 2010 г. 14:46 пользователь Permjacov Evgeniy >>> <permeakra(a)gmail.com> написал: >>>> On 10/13/2010 12:33 PM, Eugene Kirpichov wrote: >>>> I think, tree-like structure, used as sequence (like fingertrees), will >>>> do the work. >>>>> but it's not so easy to make infinite lists fail to typecheck... >>>>> That's what I'm wondering about. >>>>> >>>>> 2010/10/13 Miguel Mitrofanov <miguelimo38(a)yandex.ru>: >>>>>> hdList :: List a n -> Maybe a >>>>>> hdList Nil = Nothing >>>>>> hdList (Cons a _) = Just a >>>>>> >>>>>> hd :: FiniteList a -> Maybe a >>>>>> hd (FL as) = hdList as >>>>>> >>>>>> *Finite> hd ones >>>>>> >>>>>> this hangs, so, my guess is that ones = _|_ >>>>>> >>>>>> >>>>>> 13.10.2010 12:13, Eugene Kirpichov пишет: >>>>>>> {-# LANGUAGE ExistentialQuantification, GADTs, EmptyDataDecls #-} >>>>>>> module Finite where >>>>>>> >>>>>>> data Zero >>>>>>> data Succ a >>>>>>> >>>>>>> class Finite a where >>>>>>> >>>>>>> instance Finite Zero >>>>>>> instance (Finite a) => Finite (Succ a) >>>>>>> >>>>>>> data List a n where >>>>>>> Nil :: List a Zero >>>>>>> Cons :: (Finite n) => a -> List a n -> List a (Succ n) >>>>>>> >>>>>>> data FiniteList a where >>>>>>> FL :: (Finite n) => List a n -> FiniteList a >>>>>>> >>>>>>> nil :: FiniteList a >>>>>>> nil = FL Nil >>>>>>> >>>>>>> cons :: a -> FiniteList a -> FiniteList a >>>>>>> cons a (FL x) = FL (Cons a x) >>>>>>> >>>>>>> list123 = cons 1 (cons 2 (cons 3 nil)) >>>>>>> >>>>>>> ones = cons 1 ones -- typechecks ok >>>>>> _______________________________________________ >>>>>> Haskell-Cafe mailing list >>>>>> Haskell-Cafe(a)haskell.org >>>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>>>> >>> >> > >

1 0

Desired behaviour of rounding etc.
by Daniel Fischer 13 Oct '10

13 Oct '10

The methods of the RealFrac class produce garbage when the value lies outside the range of the target type, e.g. Prelude GHC.Float> truncate 1.234e11 :: Int -- 32-bits -1154051584 and, in the case of truncate, different garbage when the rewrite rule fires: Prelude GHC.Float> double2Int 1.234e11 -2147483648 I'm currently working on faster implementations of properFraction, truncate, round, ceiling and floor for Float and Double, so I'd like to know - does it matter at all what garbage is returned in the above case? - if it does, what is the desired behaviour (at least for Int, I can't cater for all possibilities)? On a related note, in my benchmarks, truncFloatGen :: Integral a => Float -> a truncFloatGen = fromInteger . truncFloatInteger truncFloatInteger :: Float -> Integer truncFloatInteger x = case decodeFloat x of (m,e) | e == 0 -> m | e < 0 -> let s = -e in if m < 0 then - ((-m) `shiftR` s) else m `shiftR` s | otherwise -> m `shiftL` e is more than twice as fast as GHC.Float.float2Int, the corresponding for Double almost twice as fast as double2Int. Can anybody confirm that the above is faster than float2Int on other machines/architectures? Cheers, Daniel

4 7

Re: Polyvariadic functions operating with a monoid
by oleg＠okmij.org 13 Oct '10

13 Oct '10

Sorry, I'm still catching up. I'm replying to first few messages. > instance Show a => Monoidable a [String] where > toMonoid a = [show a] > > main = putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int)) > fails to compile. The error message points to the first problem: > No instances for (Monoidable Bool [a], > Monoidable () [a], > ... The presence of the type variable 'a' means that the type checker doesn't know list of what elements you want (in other words, the context is not specific enough to instantiate the type variable a). Thus, we need to explicitly tell that we wish a list of strings: > test3 = putStrLn $ unwrap $polyToMonoid ([]::[String]) True () (Just (5::Int)) Now we get a different error, which points to the real problem this time: the expression `unwrap ....' appears as an argument to putStrLn. That means that we are required to produce a String as a monoid. Yet we specified ([]::[String]) as mempty, which is unsuitable as mempty for the String monoid. If we desire the [String] monoid as the result, we need to change the context. For example, > test3 = mapM_ putStrLn $ unwrap $ > polyToMonoid ([]::[String]) True () (Just (5::Int)) > Another example that also fails to compile (but I cannot see why): > main = putStrLn $ show $ unwrap $ polyToMonoid (0::Int) (1::Int) > (2::Int) (3::Int) > No instance for (PolyVariadic Int (WMonoid m)) > arising from a use of `polyToMonoid' The error message is informative, mentioning the type variable, m. Whenever that happens, we know that we put a bounded polymorphic expression in the context that is not specific enough. We need some type annotations. In our case, the function 'show' can show values of many types. The type checker does not know that we wish an Int monoid specifically. So, we have to specialize the show function: > test4 = putStrLn $ (show :: Int -> String) $ > unwrap $ polyToMonoid (0::Int) (1::Int) (2::Int) (3::Int) At this point one may wonder if this is all worth it. There are too many annotations. Fortunately, if you are not afraid of one more extension, the annotations can be avoided. Your example would be accepted as it was written, see test3 and test4 below. > {-# LANGUAGE TypeSynonymInstances #-} > {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, TypeFamilies #-} > > module M where > > import Data.Monoid > > newtype WMonoid m = WMonoid{unwrap :: m} > > class Monoid m => Monoidable a m where > toMonoid :: a -> m > > class Monoid m => PolyVariadic m p where > polyToMonoid :: m -> p > > instance (Monoid m', m' ~ m) => PolyVariadic m (WMonoid m') where > polyToMonoid acc = WMonoid acc > > instance (Monoidable a m, PolyVariadic m r) => PolyVariadic m (a->r) where > polyToMonoid acc = \a -> polyToMonoid (acc `mappend` toMonoid a) > > instance Show a => Monoidable a String where > toMonoid = show > > instance Show a => Monoidable a [String] where > toMonoid a = [show a] > > test2 = putStrLn $ unwrap $ polyToMonoid "" True () (Just (5::Int)) > > test3 = mapM_ putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int)) > > instance Monoid Int where > mappend = (+) > mempty = 0 > > instance Monoidable Int Int where > toMonoid = id > > test4 = putStrLn $ show $ > unwrap $ polyToMonoid (0::Int) (1::Int) (2::Int) (3::Int) P.S. Indeed, "polyToMonoid' = unwrap . polyToMonoid" does not do what one wishes to. One should regard `unwrap' as a sort of terminator of the argument list.

2 3

Windows binary for latest darcs head
by Aaron Gray 13 Oct '10

13 Oct '10

Does anyone have a Windows binary for the latest GHC darcs head or at least October 8th they could send me. Many thanks in advance, Aaron

2 1

Protocol buffer network examples
by Mads Lindstrøm 13 Oct '10

13 Oct '10

Hi I am trying to read and write delimited messages[3] using protocol-buffers [1,2] over a socket. I have managed to write delimited messages and I was looking for examples of reading delimited messages. Maybe somebody has an example of reading delimited messages from a socket? The Haskell protocol-buffer[1] package mostly uses bytestrings, but I need to read from a socket. Of cause, I can use the bytestring package to read X number of bytes from a socket-handle. But it is impossible to know what X is, before you start parsing the message, as even integers are variable length in protocol buffers. Hope somebody can help, Mads Lindstrøm [1] http://hackage.haskell.org/package/protocol-buffers [2] http://code.google.com/p/protobuf/ [3] Delimited messages is a protocol buffers technique, where one writes the size of the message before the actual message: http://code.google.com/apis/protocolbuffers/docs/techniques.html#streaming

1 0

Re: RealFrac methods for Double and Float
by Antoine Latter 12 Oct '10

12 Oct '10

On Tue, Oct 12, 2010 at 10:02 AM, Tyson Whitehead <twhitehead(a)gmail.com> wrote: > On October 11, 2010 15:12:12 Daniel Fischer wrote: >> GHC's new code generator does some incredible stuff with the code for >> Float's RealFrac instance, properFraction has become about _seven times_ >> faster, floor and ceiling about _thirteen times_ and round about _10.5 >> times_ (values for my box, the exact numbers will differ, but the tendency >> will be the same). > > Out of curiosity, by new code generator, are you meaning just the stock GHC 7 > code path or are you meaning the new now available but disabled by default > unless you use some flag (which I forget right now) code generation path. > > Cheers! -Tyson > When I did the GHC 7 timings I simply ran the shell script provided in the linked tarball - no funny business. Antoine

2 1

Data.Judy
by Tom Doris 12 Oct '10

12 Oct '10

Hi, Are there any plans to extent the current Data.Judy package to include bindings to JudySL and JudyHS? There's a standalone binding to JudySL by Andrew Choi that is usable but it would of course be better to have the functionality in the Data.Judy package proper. Thanks Tom

2 1

Haskellers.com survey
by Michael Snoyman 12 Oct '10

12 Oct '10

Hey all, Guessing popular consensus based on what people write on the mailing list is not exactly statistically sound. So instead, I've put up some of our open questions- along with some other questions I've wanted to ask- in a survey. If you are interested in the future of Haskellers, I encourage you to fill it out. https://spreadsheets.google.com/viewform?formkey=dG8tcVhBdzRSS1lkdVV6U281em… Michael

1 0

Re: [Haskell-cafe] Haskellers.com recent changes (and I need some volunteers)
by John Lato 12 Oct '10

12 Oct '10

> > From: Michael Snoyman <michael(a)snoyman.com> > > Also, now 10 random profiles will be displayed on the homepage. Only > "verified users" will be displayed here. I'm also considering adding a > new status as well: real picture, so that only people with real images > (not cartoons, not identicons) can show up on the homepage. I think > this might give a more professional feel. Thoughts? I agree that it would be nice to use only real pictures, however I wouldn't want to leave out those who choose not to use any image at all. What about a site policy that user images must be real pictures (if they exist), and violations can be flagged/blocked? Although, I don't know how this would interact with using OpenID, which appears to be most of the users currently displaying an identicon. John

4 4

Re: [Haskell-cafe] pointers for EDSL design
by Dominique Devriese 12 Oct '10

12 Oct '10

2010/10/12 <oleg(a)okmij.org>: >> An alternative approach to model sharing at the object level is the >> technique I use for modelling context-free grammars in my PADL 2011 >> paper "Explicitly Recursive Grammar Combinators"... Using ideas from >> the Multirec generic programming library and some recent Haskell type >> system extensions (most importantly GADTs and type families), you can >> do this in a well-typed way for sets of mutually recursive >> object-level expressions. > > I guess you are using what I call `the initial embedding' of an object > language. Somehow I prefer the final embedding. No. In the library, I use both embedding styles for different purposes, but what I was referring to here (the construction of production rules) is actually implemented using what you call a typed tagless-final embedding. I see the technique as an encoding of *recursion* in a typed tagless final object language in such a way that the recursion is observable in the host language. Suppose you have the following (logically inconsistent ;)) code (in Haskell notation): term1 :: Int term1 = if term2 then 1 else 2 term2 :: Bool term2 = term1 == 2 and you want to model it in the typed tagless final encoding of simply typed lambda calculus from the examples in your online typed tagless final lecture notes [1] extended with implicit arbitrary recursion. Then you could do data Term1 data Term2 data TermDomain ix where Term1 :: TermDomain Term1 Term2 :: TermDomain Term2 data family TermVal ix newtype instance TermVal Term1 = TV1 {unTV1 :: Int} newtype instance TermVal Term2 = TV2 {unTV2 :: Bool} class ExtraSymantics repr where if_ :: repr h Bool -> repr h a -> repr h a -> repr h a eq_int :: repr h Int -> repr h Int -> repr h Bool class RecSymantics repr phi | repr -> phi where ref :: phi ix -> repr h (TermVal ix) terms :: (Functor (repr h), Symantics repr, ExtraSymantics repr, RecSymantics repr TermDomain) => TermDomain ix -> repr h (TermVal ix) terms Term1 = fmap TV1 $ if_ (fmap unTV2 (ref Term2)) (int 1) (int 2) terms Term2 = fmap TV2 $ eq_int (fmap unTV1 (ref Term1)) (int 2) In this way, the embedding models the object language recursion in such a way that the recursion remains observable in the host language because you can implement it the way you want in your RecSymantics instance. Possible needs for this observable recursion could be that you want to do some form of recursion depth-bounded evaluation or some form of static analysis or whatever... Such modifications are fundamentally impossible if you model object language recursion naively using direct host language recursion. For my parsing library, I need these techniques to get a good view on the recursion in the grammar. This allows me perform grammar transformations and analysis. Dominique Footnotes: [1] http://okmij.org/ftp/tagless-final/course/#infin1

1 0

Re: [Haskell-cafe] ANNOUNCE: tls, native TLS/SSL protocolimplementation
by Brandon Moore 12 Oct '10

12 Oct '10

While I can see your point about potentially introducing new security holes, and producing much less trusted code, I feel having tidy, pure libraries that we can all integrate into our Haskell is a benefit that far outweighs this. Especially when we have nice things like the type system, which can be used to alleviate many of the security worries. I agree in general, for code like servers and file formats, but I worry in particular about cryptographic primitives. Some side channel attacks seem to call for a very low-level language, to make it easier to verify that e.g. execution time and the memory access pattern does not depend on the key.

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[Haskell-cafe] ANNOUNCE: tls, native TLS/SSL protocol implementation
by Vincent Hanquez 12 Oct '10

12 Oct '10

Hi haskellers, I'ld like to announce the tls package [1][2], which is a native implementation of the TLS protocol, client and server. It's currently mostly supporting SSL3, TLS1.0 and TLS1.1. It's got *lots* of rough edges, and a bunch of unsupported features, but it's humming along, and at each iteration it's becoming more tighly secure and featureful. I would recommend against using this implementation in a production system just yet, or in an aggressive environment either (specially for the server side); I don't think it should necessary fail, but it's still an early implementation with probable API changes on the way. [1] http://github.com/vincenthz/hs-tls [2] http://hackage.haskell.org/package/tls -- Vincent Hanquez

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Eta-expansion destroys memoization?
by Brent Yorgey 12 Oct '10

12 Oct '10

Hi all, See below for this message from one of my students which has me stumped. Just when you think you understand Haskell... ;) I've cc'ed him on this message; please include him on any replies as I don't think he is subscribed to -cafe. -Brent ----- Forwarded message from Yue Wang <yulewang(a)gmail.com> ----- From: Yue Wang <yulewang(a)gmail.com> Date: Tue, 5 Oct 2010 21:23:57 -0400 To: Brent Yorgey <byorgey(a)seas.upenn.edu> Subject: store evaluated values Hi, Anthony (or Brent): Last time (in Anthony's TA office hour) we talked about how to store evaluated values in the program for later used. I googled for a while and wrote some code. But I still encountered two problems. Can you take a look? Thanks. First, let's write the naive fib function: naive_fib 0 = 0 naive_fib 1 = 1 naive_fib n = trace(show(n)) naive_fib (n - 1) + naive_fib (n - 2) this works good except it tries to calculate the same expression many times. So when we evaluate it ghci will show the following: *Main> naive_fib 5 5 4 3 2 2 3 2 5 Then there is a clever way to do that on haskell wiki: fib = ((map fib' [0 ..]) !!) where fib' 0 = 0 fib' 1 = 1 fib' n =trace(show(n)) fib (n - 1) + fib (n - 2) When we evaluate the same expression we can see it does not evaluate the redundant expression over and over: *Main> fib 5 5 4 3 2 5 The source code seems to be easy to read, but I don't think I understand that. For me I think if I change the first line from fib = ((map fib' [0 ..]) !!) to fib x = ((map fib' [0 ..]) !!) x It should do the same thing since I think the previous version is just an abbreviation for the second one. But After I change that, *Main> fib 5 5 4 3 2 2 3 2 5 So why does the x here matters?

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RealFrac methods for Double and Float
by Daniel Fischer 11 Oct '10

11 Oct '10

Greetings, I have put together a package to test possible implementations of the RealFrac methods for Double and Float (base-2 IEEE754) and uploaded a .tar.gz bundle to http://hackage.haskell.org/trac/ghc/ticket/2271 . On the one hand, pure Haskell implementations, on the other hand implementations calling out to rint[f], trunc[f], floor[f] and ceil[f] from math.h. Both ways go via Integer by default, with a specialised faster implementation for Int (and narrower types, but those RULES haven't yet been written) enabled by a rewrite rule. Overall, the pure Haskell implementations don't fare badly on my computer. All give a speedup compared to the current implementation, for most conversions, pure Haskell is on par with or faster than the C-call (although that would probably change if the C functions were made primops). The FFI calls are significantly faster for properFraction :: Double -> (Integer, Double) and for round (except round :: Integral a => Float -> a when compiled via C, then native and FFI are on par). Sample results for the speedups against the current implementation (note: for truncate :: x -> Int, the Prelude value is fst . properFraction, not the rewritten float2Int or double2Int) are included in the tarball. I would appreciate feedback from your tests/benchmarks on other platforms, especially 64-bit platforms (mine is x86 linux, 32 bit). To run the QuickCheck tests, you need QuickCheck-2.*, to run the benchmarks, criterion. More instructions in the README. Thanks, Daniel

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Static computation/inlining
by Alexander Solla 11 Oct '10

11 Oct '10

Hi everybody, I'm working on a module that encodes "static" facts about "the real world". For now, I'm working on an ISO 3166 compliant list of countries, country names, and country codes. I've run into a bit of an optimization issue. There is a static bijective correspondence between countries and their codes. In order to keep one just one "large" data structure representation as Haskell code, I encoded this bijection using a list. I'm looking to write queries against this list, but it is rather tedious. I figured I could make some Data.Maps to handle it for me. -- Country and ISOCountryCodes derive (Data, Eq, Ord, Show, Typeable) countries_and_iso_country_codes :: [ (Country, ISOCountryCode) ] countries_and_iso_country_codes = [ ( Afghanistan , ISOCountryCode AF AFG (isoNumericCode 004) ) , ( AlandIslands , ISOCountryCode AX ALA (isoNumericCode 248) ) , ( Albania , ISOCountryCode AL ALB (isoNumericCode 008) ) ... , ( Zimbabwe , ISOCountryCode ZW ZWE (isoNumericCode 716) ) ] map_country_to_country_code :: Map Country ISOCountryCode map_country_to_country_code = fromList countries_and_iso_country_codes map_country_code_to_country :: Map ISOCountryCode Country map_country_code_to_country = fromList . fmap (\(a,b) -> (b, a)) $ countries_and_iso_country_codes Is there anyway to instruct GHC (and maybe other compilers) to compute these maps statically? Are GHC and the other compilers smart enough to do it automatically? Although the list isn't huge, I would still rather get rid of the O(2*n) operation of turning it into maps at run- time. (Especially since some later list encodings like these might be enormous) What should I be looking into? Thanks

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pointers for EDSL design
by oleg＠okmij.org 11 Oct '10

11 Oct '10

John Lato wrote: > So here's a very simple expression: > > t1 = let v = sigGen (cnst 1) in outs v v > > which is what led to my question. I'm binding the sigGen to 'v' to > introduce sharing at the meta-level. Would it be better to introduce > support for this in the dsl? Often this is not a question of preference but that of necessity. Sharing at the meta-level may help the generator, but it does _not_ translate into the sharing at the object level. In the generated code, the code for 'sigGen (cnst 1)' shall be duplicated. It could be that two csound blocks must share the same signal source, to receive samples in parallel. Meta-level sharing (Haskell's let) would not do. We need a construct for an object-level let, for example t1 = let_ (SigGen (cnst 1)) (\v -> outs v v) (Template Haskell can improve the syntax.) The term `off-shoring' was coined by Walid Taha and collaborators, and first appeared in the paper Implicitly Heterogeneous Multi-Stage Programming. GPCE'05 (whose first draft was written in Feb 2004).

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Re: Round benchmarks
by Daniel Fischer 11 Oct '10

11 Oct '10

On Monday 11 October 2010 07:05:20 you wrote: > Hi, > > These results seem different from the samples. Thanks. The numbers are quite impressive. Although the quotients (hence speedups) are generally much larger, the ranking doesn't differ much. For round :: x -> Int, via C is a big win. For round :: x -> Integer and properFraction :: Double -> (Integer, Double), going via C doesn't seem to make much difference on 64 bits, but they do on 32. That's probably because a Double mantissa is a smallInteger on 64 bits. Overall, your timings support going via C for round and properFraction :: Double -> (Integer, Double). In two cases, it makes not much difference on 64 bits but is a big gain on 32, for round :: x -> Int, it's a big win on both. > > I was surprised that the "via Integer" results appear faster than the > Int results. No, the numbers are (Prelude time / new time), so the larger the number, the better. Sorry for not making that clear in the README. > > Vivian Cheers, Daniel

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Re: [Haskell-cafe] Haskellers.com recent changes (and I need some volunteers)
by Mark Lentczner 11 Oct '10

11 Oct '10

On Oct 10, 2010, at 10:56 AM, Michael Snoyman wrote: > I'm worried about spam accounts being featured on > the homepage. Real Haskeller is not meant to be exclusive, it's a > minimal level of oversight by the admins. A more common approach to the problem of spam accounts -- which is very real -- is to not allow accounts to appear before they have been vetted by an admin. Allowing accounts to appear on the site as they are created will, sooner or later, result in the site being over run with spam faster than you can keep up. For the community site I help run (http://www.contextfreeart.org/) had this problem a while back and we switched to all accounts being vetted. We use http://www.stopforumspam.com/ as source of information to easily help us make that determination. - Mark

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Haskellers.com recent changes (and I need some volunteers)
by Michael Snoyman 11 Oct '10

11 Oct '10

Hey all, Haskellers became popular a lot faster than I'd anticipated. This has prompted me to need to make some changes that I was only planning on implementing later on. As usual, all points are up for discussion (this is intended to be a community-run site after all). * Pagination on homepage. We now see only 10 users at a time. * The sort order is different. Now, "real haskellers" (we'll discuss that in a second) come first, sorted by years of experience with Haskell and then alphabetically by full name. * We now have site admins. Currently, I'm the only one, but I would like to add some more (thus the need for volunteers). Admins currently have four rights/duties: * Grant/revoke "real haskeller" status. This status currently only affects sort order on home page, but might be used for more in the future. * Grant/revoke admin rights. * Block/unblock a user. A blocked user will not show up on the homepage, not be counted for public/private/unverified accounts, and will have a big nasty "blocked" warning on their user page. * Read/close admin messages. These can be created by the following features: * Users can request "real haskeller" status in their Edit Profile > Extras tab. * Users can request their account to be unblocked. * Anyone (logged in or not) can flag a user on their user page. You'll notice that admins are not able to make any changes to user data. Anyone who is interested in taking on this responsibility should email me about it. I think five people should easily be able to handle this, I'm not anticipating a high load. Then again, I didn't anticipate over 200 accounts created in a week either. Oh, and *please* don't email me personally to request a "real Haskeller" status. If you use the button on the site, it makes it much easier to add the status. Also, don't take it personally if an admin denies your "real haskeller" request; it's currently a very poorly defined notion, and I don't even know who the admins are going to be. Michael

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My knight's tour does not seem to end
by C K Kashyap 11 Oct '10

11 Oct '10

Hi, I tried an implementation of knight's tour and it does not seem to terminate. I am looking for some feedback on algorithm and Haskellness of my implementation (knight.hs attached). -- Regards, Kashyap

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Polyvariadic functions operating with a monoid
by oleg＠okmij.org 11 Oct '10

11 Oct '10

Kevin Jardine wrote: > instead of passing around lists of values with these related types, I > created a polyvariadic function polyToString... > I finally figured out how to do this, but it was a bit harder to > figure this out than I expected, and I was wondering if it might be > possible to create a small utility library to help other developers do > this. > It seems to me that in the general case, we would be dealing with a > Monoid rather than a list of strings. We could have a toMonoid > function and then return > > polyToMonoid value1 value2 ... valueN = > > (toMonoid value1) `mappend` (toMonoid value2) 'mappend' ... (toMonoid > valueN) > So I tried writing the following code but GHC said it had undecidable > instances. Generally speaking, we should not be afraid of undecidable instances: it is a sufficient criterion for terminating type checking but it is not a necessary one. A longer argument can be found at http://okmij.org/ftp/Haskell/types.html#undecidable-inst-defense However, the posted code has deeper problems, I'm afraid. First, let us look at the case of Strings: > class PolyVariadic p where > polyToMonoid' :: String -> p > > instance PolyVariadic String where > polyToMonoid' acc = acc > > instance (Show a, PolyVariadic r) => PolyVariadic (a->r) where > polyToMonoid' acc = \a -> polyToMonoid' (acc ++ show a) > > polyToMonoid :: PolyVariadic p => p > polyToMonoid = polyToMonoid' mempty > > test1 = putStrLn $ polyToMonoid True () (Just (5::Int)) *M> test1 True()Just 5 Modulo the TypeSynonymInstances extension, it is Haskell98. If we now generalize it to arbitrary monoids rather than a mere String, we face several problems. First of all, if we re-write the first instance as > instance Monoid r => PolyVariadic r where > polyToMonoid' acc = acc we make it overlap with the second instance: the type variable 'r' may be instantiated to the arrow type a->r'. Now we need a more problematic overlapping instances extension. The problem is deeper however: an arrow type could possibly be an instance of Monoid (for example, functions of the type Int->Int form a monoid with mempty=id, mappend=(.)). If polyToMonoid appears in the context requiring a function type, how could type checker choose the instance of Polyvariadic? The second problem with the posted code > class Monoidable a where > toMonoid :: Monoid r => a -> r is that toMonoid has too `strong' a signature. Suppose we have an instance > instance Monoidable String where > toMonoid = \str -> ??? It means that no matter which monoid the programmer may give to us, we promise to inject a string into it. We have no idea about the details of the monoid. It means that the only thing we could do (short of divergence) is to return mempty. That is not too useful. We have little choice but to parametrise Monoidable as well as Polyvariadic with the type of the monoid. To avoid overlapping and disambiguate the contexts, we use the newtype trick. Here is the complete code. It turns out, no undecidable instances are needed. > {-# LANGUAGE TypeSynonymInstances #-} > {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} > > module M where > > import Data.Monoid > > newtype WMonoid m = WMonoid{unwrap :: m} > > class Monoid m => Monoidable a m where > toMonoid :: a -> m > > class Monoid m => PolyVariadic m p where > polyToMonoid :: m -> p > > instance Monoid m => PolyVariadic m (WMonoid m) where > polyToMonoid acc = WMonoid acc > > instance (Monoidable a m, PolyVariadic m r) => PolyVariadic m (a->r) where > polyToMonoid acc = \a -> polyToMonoid (acc `mappend` toMonoid a) > > instance Show a => Monoidable a String where > toMonoid = show > > test2 = putStrLn $ unwrap $ polyToMonoid "" True () (Just (5::Int)) The remaining problem is how to tell polyToMonoid which monoid we want. It seems simpler just to pass the appropriately specialized mempty method as the first argument, as shown in test2. Granted, a more elegant solution would be a parametrized module (functor) like those in Agda or ML: module type PolyM = functor(M:: sig type m val mempty :: m val mappend :: m -> m -> m end) = struct class Monoidable a where toMonoid :: a -> m class PolyVariadic p where polyToMonoid :: m -> p .etc end The shown solution is essentially the encoding of the above functor.

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Re-order type
by André Batista Martins 10 Oct '10

10 Oct '10

Hello, exists any algorithm to determine how terms can be changed to safisty the type of one function? example: f:: a-> b -> c -> (b,c,a) f1 :: c -> a -> d In my first function "f" i want assign the output "c" and "a" for to input of function "f1". I searched for any solution, but i didn't find any anything. One clue i have found is "minimal edit distance algorithm" for 2 strings. Perhaps if i convert de output type of "f" to one string, and de input of "f1" to another string and then use this algorithm , i will get one "dirty" solution... I'm open to any sugestion.

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Web development work
by Michael Snoyman 10 Oct '10

10 Oct '10

Hi all, Often times when trying to pitch Haskell to potential clients the concern is the lack of qualified developers willing to take on projects. As I'm sure many of you are familiar with, clients prefer not to be locked in to a single programmer: an errant bus can significantly reduce the value of their investment. So I'd like to get an idea of how many people out there would be interested in Haskell web development work. For the moment I'm just interested in doing this informally via email, though perhaps in the future it would be beneficial to the community to have this information centralized on a website. I think it would be useful to have some basic skills and experience information, including system administration abilities. I might also have one or two Yesod projects that I'll need to pass off in the future, though it's still unclear if this will happen. If you're interested in that, please let me know, I can give some more details privately. Michael

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Layered maps
by Alex Rozenshteyn 10 Oct '10

10 Oct '10

Does there exist a library which allows me to have maps whose elements are maps whose elements ... with a convenient syntax. Alternatively, does there exist a library like Data.Tree where forests are sets rather than lists? -- Alex R

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[Off-topic]Functional parsing theory
by Maurício CA 10 Oct '10

10 Oct '10

Hi, all, I've been working in a tool that reads a grammar with associated actions and act on input based on that grammar. I would like to rewrite it in a functional style, but I've not been able to find a theory that would handle any possible grammar with cyclicity and empty productions, and flexibility is more important for this tool than performance. Do you have a suggestion on that? What I'm using now is this (non-functional) article on Earley method: http://www.springerlink.com/content/602270808666074p Thanks, Maurício

5 4

Ordering vs. Order
by Christian Sternagel 09 Oct '10

09 Oct '10

Hi all, I'm not a native English speaker and recently I was wondering about the two words "order" and "ordering" (the main reason why I write this to the Haskell mailing list, is that the type class "Ordering" does exist). My dictionaries tell me that "order" (besides other meanings) denotes an ordered structure on elements and "ordering" (as only meaning) denotes some request that I made at some entity. So, to me it seems that calling the type class "Ordering" is wrong ;) However, I do know that there are many publications about "ordered structures" which use the word "ordering" (most of which I'm aware of, not by native speakers). What do native speakers have to say about that? best regards chris

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Re: [Haskell-cafe] Re: Bulletproof resource management
by Brandon Moore 09 Oct '10

09 Oct '10

On Oct 8, 2010, at 2:18 PM, Florian Weimer <fw(a)deneb.enyo.de> wrote: * Ben Franksen: You might be interested in Lightweight Monadic Regions http://okmij.org/ftp/Haskell/regions.html#light-weight which solve the problem (IMHO) in a much cleaner way, i.e. w/o explicit closing and also w/o using finalizers. Is this approach composeable in the sense that you can combine code written in this code with code from another library? I don't think so. The other library might provide something like IORef, and then it's impossible to uphold static guarantees. IMHO, this rules out such an approach (until it's part of the standard library and forced upon everyone, which seems unlikely). If you are worried about a handle being saved in the IORef to keep it outside the region, that's precisely what the library prevents. The typed handle itself can be carried outside the region, but the types make it impossible to perform any operations on it, so the resources can be released on exit from the region anyway. It's quite clever.

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Haskell Helper
by Eduardo Ribeiro 09 Oct '10

09 Oct '10

Hello, I 'm developing a new language in haskell and I need someone to help me. Anyone would like to help?

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Associated data types and contexts
by Eric Walkingshaw 08 Oct '10

08 Oct '10

The following code compiles happily in GHC: > {-# LANGUAGE TypeFamilies #-} > > class C a where > data D a > m :: D a -> Bool > > test :: C a => D a -> Bool > test = m My question is why do I need the context in the function "test"? It seems like since "D" is associated with class "C", the compiler can safely assume that any time I have a "D a", "a" must be an instance of C. But GHC complains if the context is removed. At first I thought that maybe the associated type was just syntactic sugar for a non-associated data type family. But this doesn't seem to be the case since I cannot instantiate D outside of an instance of C. Google and the type family documentation provided no other leads. Am I missing something here? Is the context assumption invalid? Or is it just an assumption that GHC doesn't make (yet)? Thanks in advance for any replies. -Eric

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Reminder: BelHac: A Hackathon in Belgium, 5-7 November
by Jasper Van der Jeugt 08 Oct '10

08 Oct '10

Hello all, We would like to remind you of BelHac, an international Hackathon taking place in Ghent, Belgium next month. All details are available on this wiki page [1]. You can register here [2]. WHEN Friday November 5: 2pm - 7pm Saturday November 6: 10am - 6pm Sunday November 7: 10am - 6pm WHERE In “The Therminal”, in Ghent, Belgium. Please see the wiki [1] for more details! SPONSORS Well-Typed: http://well-typed.com/ O’Reilly: http://oreilly.com/ Incubaid: http://incubaid.com/ We hope to see you in Belgium! [1]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHac [2]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHa… On behalf of the GhentFPG organizing committee, Kind regards, Jasper Van der Jeugt

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Re: [Haskell-cafe] Re: Re: Lambda-case / lambda-if
by Jonathan Geddes 08 Oct '10

08 Oct '10

I can honestly say that I haven't felt much pain from the status quo regarding this. Most of the time my code is structured so that case statements don't appear in do blocks. When they do, I don't see it as a big issue. The special case for operator - is a bigger wart on haskell syntax than this, imo. I would vote in favor of keeping the language simple. I do like the idea of generalizing lambda functions to include multiple cases, however. On the other hand, I almost never use lambdas now since named functions yield better "self-documenting" code. --jonathan On Oct 8, 2010 8:09 AM, "Peter Wortmann" <scpmw(a)leeds.ac.uk> wrote: On Fri, 2010-10-08 at 01:13 +0300, Lauri Alanko wrote: > Your "general" rule doesn't subsume your ... Yes, that's what I meant. Thanks for describing it properly. On Fri, 2010-10-08 at 05:41 -0700, Nicolas Pouillard wrote: > Imagine find this code: > > do s1 > ... This is roughly what I meant with "abused": Where "C" is very complex, it might become non-obvious where exactly the monad actions are supposed to happen. Hence such traps when refactoring. Also of note: Just moving sub-expressions around isn't something that is guaranteed to be save. Introducing new names and using them in "s2" would be problematic, for example: do map (\x -> (<- putStrLn x)) ["a", "b"] Obviously can't be made to work. You might have to check for this - or maybe even disallow the shorthand inside lamdbas and lets. Might be less satisfying to have such special cases, but it is still a good bit more general than what is available right now. Greetings, Peter Wortmann _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org...

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