December 2006 - Haskell-Cafe

Well-typed functions with nonexisting signatures [Was: type variable question]
by oleg＠pobox.com 16 Dec '06

16 Dec '06

Nicolas Frisby wrote > The commented out signature is the signature that GHC infers for the > function. When I uncomment that signature, it will no longer type > check. Why does this happen? Even with the forall and the explicit > signatures in the where clause, it chokes. Alas, this is the property of Haskell, even of Haskell98. Half a year ago I even wanted to write a message on the Haskell' list, pointing out the embarrassment. One one hand, writing signatures is highly recommended. On the other hand, there exist, even in Haskell98, well-typed functions for which signatures simply do not exist. The number of such exceptions multiply when we move to higher-rank types (where such anomalies are to be expected). The reason for the anomalies is the difference between the 'internal' language that expresses inferred types and the external type language. > Is there a canonical example example that exhibits this behavior? Yes, it was discussed back in 2003. Here's the canonical form: g::(Show a,Show b) => a->b g = undefined --h :: Show a => b -> a h x = g(h x) Both Hugs and GHC (in the pure Haskell98 mode!) are happy with h. Both compilers infer the type for h, shown in the comment. If you give h the type signature -- the one that is inferred by the compilers themselves -- both compilers complain. This example has been distilled from practical problems (the earliest one was by Levent Erkok reported back on 2000) http://www.mail-archive.com/haskell@haskell.org/msg06754.html The following thread gives more detail and discussion: http://www.haskell.org/pipermail/haskell/2003-May/011730.html The final solution to the problem is in http://www.haskell.org/pipermail/haskell/2003-May/011762.html I have actually managed to give h the type signature. The following message indicates that the problem is actually quite common: http://www.haskell.org/pipermail/haskell-cafe/2003-May/004302.html Here's an example of an anomaly with higher-rank types: http://www.haskell.org/pipermail/haskell-cafe/2004-October/007207.html http://www.haskell.org/pipermail/haskell-cafe/2004-October/007212.html we can define functions Foo and Bar (data constructors are functions) such that we can write > myFoo :: Int -> Foo > myFoo i = Foo (Bar run op) where > run :: Monad m => StateT Int m a -> m a > run prog = do (a, s) <- runStateT prog i > return a > op :: Monad m => StateT Int m Int > op = get but we cannot write the composition of Foo and Bar more directly: > foo run op = Foo (Bar run op) The latter is not accepted: it must have a signature but no signature can ever be given to this function.

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Why is this array use wrong?
by Maurício 16 Dec '06

16 Dec '06

Hi, I'm trying this in ghci: let b = array (1,33) [(i,False) | i <- [1..33]] after :m Data.Array.IArray. It gives me that error message: <interactive>:1:8: No instance for (IArray a Bool) arising from use of `array' at <interactive>:1:8-12 Probable fix: add an instance declaration for (IArray a Bool) In the definition of `b': b = array (1, 33) ([(i, False) | i <- [1 .. 33]]) Why? Thanks, Maurício

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GHC package lang not found
by Aditya Siram 16 Dec '06

16 Dec '06

Hi all, I am trying to install Hare but I cannot build it. GHC complains that package lang is not found. Here is the output: .... editors/GenEditorInterfaces pfeRefactoringCmds > refactorer/PfeRefactoringCmds.hs cd refactorer; rm -f hidir/`uname`/Main.hi; ./myghc--make -o pfe pfe.hs 2>&1 | tee log.txt ghc-6.6: unknown package: lang cd refactorer; rm -f hidir/`uname`/Main.hi; ./myghc--make -o pfe_client pfe_client.hs 2>&1 | tee -a log.txt ghc-6.6: unknown package: lang I've searched the net for lang but I cannot find it. Here is my ghc-pkg list: /usr/local/lib/ghc-6.6/package.conf: Cabal-1.1.6, HUnit-1.1, QuickCheck-1.0, base-2.0, cgi-2006.9.6, fgl-5.2, (ghc-6.6), haskell-src-1.0, haskell98-1.0, html-1.0, mtl-1.0, network-2.0, parsec-2.0, readline-1.0, regex-base-0.71, regex-compat-0.71, regex-posix-0.71, rts-1.0, stm-2.0, template-haskell-2.0, time-1.0, unix-1.0, xhtml-2006.9.13 /home/deech/.ghc/i386-linux-6.6/package.conf: (hat-2.4), (hat-2.5) What am I doing wrong? Thanks... Deech _________________________________________________________________ WIN up to $10,000 in cash or prizes enter the Microsoft Office Live Sweepstakes http://clk.atdmt.com/MRT/go/aub0050001581mrt/direct/01/

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[ANN] JoinCabal makes its brave entry into the world
by Dougal Stanton 15 Dec '06

15 Dec '06

Okay, after a bit of a wait (I believe it was several weeks, in fact) there's an alternative to Don Stewart's mkcabal. JoinCabal is available from here: <http://brokenhut.no-ip.org/~dougal/darcs/joincabal/> or <http://brokenhut.no-ip.org/~dougal/cgi-bin/darcsweb.cgi?r=JoinCabal;a=summa…> It follows on from my post yesterday about the default licences in cabal, in that it'll create a stub source file with a licence header and description relevant to the licence you chose. (Apart from the BSD licences, which are not well documented. I'll get round to it.) If you use the [L]GPL a lot then this'll save you a wee bit of time. In future I also hope to download licences where I can, straight into the LICENSE file. Full usage can be found in the README. I hope someone finds it useful. Cheers, D. -- Dougal Stanton

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Re[2]: [Haskell] Haskell alternatives: functional programming in rich IDEs
by Bulat Ziganshin 15 Dec '06

15 Dec '06

Hello Martin, Friday, December 15, 2006, 3:51:24 PM, you wrote: >>1) F#, is Ocaml dialect integrated in .NET environment >>2) Clean, very Haskell-like language with a commercial IDE, GUI libs and so on >>3) Business Objects, integration of Haskell-like language into Java > 4) Scala, which has a nice eclipse plugin, and I believe there's a > plugin for netbeans as well. is Scala FP language or imperative/OOP language with FP extensions like C#, for example? in other words, how it looks compared to F#, at least? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Building the community
by dons＠cse.unsw.edu.au 15 Dec '06

15 Dec '06

One of the great aspects of programming Haskell is the community. We should aim to have the strongest, friendliest, most productive community we can. We're doing a good job so far, can we do even better? Reading this: "How to Build a User Community" http://headrush.typepad.com/creating_passionate_users/2006/12/how_to_build_… has inspired me to make some suggestions of what *you* can do to help make Haskell the language and community people want to spend time on. * Always encourage newbies to ask questions + I rate us at 8/10 * Encourage newer users--especially those who've been active askers--to start trying to answer questions + we can do more here. If you know the solutoin to a problem, dive in and help out. Canonical, type correct answers preferred! + if you're at the 'intermediate' stage, you can help *now* by contributing to the wiki, solving new user problems, blogging your ideas and code and so on. * Give tips on how to answer questions + Ok. we can put up an article here. Some suggestions: - No questions are bad questions - Code should come with examples of how to run it - Solutions with unsafePerformIO should be discouraged (moreso ;) - Be polite! (we're good at this) * Adopt a near-zero-tolerance "Be Nice" policy when people answer questions + 8/10, we can do even better I think * Teach and encourage the more advanced users (including moderators) how to correct a wrong answer while maintaining the original answerer's dignity. + Pretty good. I encourage people to point out (privately) and encourage grumpy people to be nicer So dive in! Help out new users, collaborate, contribute to the wiki, release your code, and help make Haskell the language people want to use. -- Don

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Aim Of Haskell
by Kaveh Shahbazian 15 Dec '06

15 Dec '06

What is the aim of Haskell? This question may seems too stupid to answer. But I think it needs a re-clarifying attempt. Why? i.e recently there was a project not that critical, but an internal tool for a group. There were three candidates : Haskell, Clean and Erlang. In this company all projects are in C# and this was the first one for using a new approach in development. After all Erlang was chosen, in second place Clean and in 3rd Haskell. Because those two ones has some standards, and Erlang licensing was the chosen one. But Haskell seems to be buzz-full research platform. Now again to the top : what is the aim of Haskell project? If it is going to be used in real world applications it needs more attention to real world application developers and their needs. And if not; and it is just for research, then there could be a global advise : do not use this for doing your tasks, because there is strange variations in syntax, and strange behaviors in components. (Even clean has a simple GUI. Is it that hard to provide a simple GUI like that to be installed by default?) And real applications are not those that really worked in research! Real applications are those that works in real world (That mean-level day to day developing life!). So please do not talk about Lispiry things - that are totally ananymous and virtual. And still there is ambition in licensing GHC and licensing of programs that are developed in that environment. Thanks to all.

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Re: [Haskell] Haskell alternatives: functional programming in rich IDEs
by Jason Dagit 15 Dec '06

15 Dec '06

Moving this to Haskell-Cafe which seems more appropriate given my question. On 12/15/06, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > Hello haskell, > > it was several times mentioned that Haskell doesn't provide rich > environments for productive work, including lack of IDEs, bundled > "real-world" libs and so on. i want to mention alternative environments > that is better in these areas > > 1) F#, is Ocaml dialect integrated in .NET environment > 2) Clean, very Haskell-like language with a commercial IDE, GUI libs and so on > 3) Business Objects, integration of Haskell-like language into Java > environment, http://labs.businessobjects.com/cal Can anyone comment about their experiences with these alternatives? Jason

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Book "Haskell in real world"
by Bulat Ziganshin 15 Dec '06

15 Dec '06

Hello haskell-cafe, first, i should say that i'm real-world programmer with 15 years experience. and my experience taught me that programming language in first place should allow to express complex algorithms easily, factor out any code patterns, and facilitate development of error-proof programs. from several tens of languages that i know, Haskell is best in these areas and therefore i think that it is the best language for professional program development i've written several medium-sized apps and libs in Haskell so i somewhat know how to write real-world programs here, and have written a few pages that may help other haskellers to grok tricky parts of Haskell the note on lambdadot mentioned here don't contain anything new for us - the problems are the same as for any other not-so-popular language. there is even Wadler's paper "Why Haskell will NOT become popular" which said exactly the same: while language itself is great, we lack in areas of libraries, tools/IDEs, and information support (teaching, books, training...); i call all these things "infrastructure". Moreover, for any other language not in "top 5" list, you will found just the same problems Is this means that developers of C# and Java are only smart guys and all other are suckers? No, it's just chicken-and-egg problem - development of rich infrastructure needs large number of developers and gathering large number of interesting developers needs rich infrastructure. the only thing we cam do now is to find "critical points", where small improvements may give significant overall changes and address these problems one of such points may be the book for commercial programmers that will show them how their real programs can be written Haskell and convince them that Haskell really cuts off development cycle and increase software reliability a few pages that i already wrote may be considered as precursors of such book and i will be glad to take part in this work but only if this book will be online-available. not everyone prefers (or even can) order the paper book. it should be allowed to translate book to other languages too next question - how this book should be organized. i think that it should contain two parts: first, we should look which concepts are already described in YAHT and other introductory books, and cover all remaining concepts that will be required for professional programming. second part will describe process of development real program(s) which use all above-mentioned concepts. we can even start with second part just to find which concepts will be required in first part. of course, first part should also contain small examples for each described detail, because it's better one time to look rather than 100 times to hear my a few pages: http://haskell.org/haskellwiki/Modern_array_libraries http://haskell.org/haskellwiki/OOP_vs_type_classes http://haskell.org/haskellwiki/IO_inside http://www.haskell.org/bz/th3.htm -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Draft MissingH Reorg Plan
by John Goerzen 15 Dec '06

15 Dec '06

Please tell me if I should just go away or go to another list here. Thanks again for all the feedback you've sent. I've got the new MissingH website getting started, and I've posted there the draft reorganization, module rename, and package split plan here: http://software.complete.org/missingh/wiki/TransitionPlanning Your comments (and edits! -- must register/login first) are welcome. I have not yet audited the plan for dependency sanity. This could complicate things a few places, but hopefully not too many. -- John

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