- Haskell-Cafe - Haskell.org

Problem with Gtk2hs
by Andrew Coppin 15 Dec '07

15 Dec '07

I just spent the evening writing a library that's a thin layer over Gtk2hs. It took an age to get it to compile, but eventually it worked. Yay! When I ran it, I got this: Test2: gtk/Graphics/UI/Gtk/Gdk/PixbufData.hs.pp:58:0: No instance nor default method for class operation Data.Array.Base.getNumElements Er... wow. OK, at this point, I am completely stumped. Any hints? (Also, the documentation for Graphics.UI.Gtk.Misc.DrawingArea suggests that you may want to draw a Pixbuf using the pixbufRenderToDrawable function - but GHC complains that this function doesn't exist. I had to use drawPixbuf instead. Wow that has a lot of parameters... In particular, the final two don't seem to be documented. Interesting. They're both 0 in the "fastdraw" demo.)

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Strict String IO now packaged
by Don Stewart 15 Dec '07

15 Dec '07

Here at Haskell.org we know you have a choice in programming languages, and a choice in evaluation strategies. To help make your decision easier, I'm pleased to announce an update to the 'strict' package providing strict file IO for Strings. You can now choose when and where to evaluate your Strings, meaning more predictable resource usage for readFile-like Strict IO. And of course you can still use lazy IO when appropriate. Find the strict IO package in System.IO.Strict, on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict Cheers, Don

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GUI
by Miguel Mitrofanov 15 Dec '07

15 Dec '07

Is there any really cross-platform GUI library for Haskell? Gtk2Hs is good (I suppose), but it requires X. OK, I have X, but it's not "native" on my Mac; some Mac users don't install it and almost all Mac users don't always run it. I was able to install wxHaskell (after some hacking - this was really painful); and Blobs editor compiled successfully, but then resisted to run. Tk-based libraries seem to be good, and Tk can be run "natively" on Mac (i.e., without X), but none of them seem to compile. Sorry if this message seems like I'm angry; I am, but that's only for a moment.

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Data.Generics: how do I get the type name of a value?
by Alistair Bayley 14 Dec '07

14 Dec '07

I've been learning/playing with Data.Generics a bit, and have a how-to question... If I say > dataTypeOf "" then I get > DataType {tycon = "Prelude.[]", datarep = AlgRep [[],(:)]} No surprises there. But I'd really like to know that I have a String, or [Char]. How do I get the name of the concrete type that the list contains? Is that a reasonable thing to ask for? I can say: > gmapQ dataTypeOf "a" to get: > [ DataType {tycon = "Prelude.Char", datarep = StringRep} > , DataType {tycon = "Prelude.[]", datarep = AlgRep [[],(:)]} > ] But if I say: > gmapQ dataTypeOf "" I get: > [] which makes sense when you consider the stucture of the List ADT, but doesn't help me determine the type of the value. Alistair

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type classes
by Peter Padawitz 14 Dec '07

14 Dec '07

I'd like to define several instances of the same type class with the same type variable instance. Only method instances differ. How can I do this without writing copies of the type class?

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-threaded
by Maurício 14 Dec '07

14 Dec '07

Hi, I see in the documentation and in many messages in this list that, if you want multithreading in your program, you need to use -threaded in ghc. Why isn't that option default? Does it imply some kind of overhead? Thanks, Maurício

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Problems with split-objs
by Magnus Therning 14 Dec '07

14 Dec '07

I've followed the instructions at [1] to create a .deb of vty[2]. It seems the helper scripts for Debian passes `--enable-split-obj' when running `./Setup.lhs configure'. This results in numerous multiple definitions of stuff. I have a few questions regarding this... I think I understand what --split-objs does, but why doesn't it work for vty? I've debianised other haskell libs using the same method without problems. The only difference I can see is that vty uses FFI. Is that what causes the problems? How do I fix this? I'd rather not go in and manually change any automatically generated files, and the file that passes `--enable-split-objs' is automatically generated :( /M [1]: http://haskell.org/haskellwiki/Creating_Debian_packages_from_Cabal_package [2]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vty-3.0.0 -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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Examples of useful functions of order >= 3?
by Ben Lippmeier 14 Dec '07

14 Dec '07

Hi all, I'm working on a type-based side effect analysis for a Haskell-like language, and need to test it against some more higher order functions. The situation is a bit icky because it uses bounded quantification and various related issues (ie covariance vs contravariance) only come into play when dealing with functions of order >= 3. Trouble is, the vast majority of useful higher order functions that I can think of are of order 2. Oh, I can certainly invent toys like: order3 f = f succ order4 b f = if b then f else order3 .. and so on, but I'm left feeling unsatisfied. I vaguely remember a paper called something like "Is there any use for a seventh order function?", but I forget why they were used, and I can't find it on Google, or ACM or any of the other likely places. Does anyone have any examples of code (in whatever language) which usefully uses functions of order >= 3?? Preferably up to 5? Thanks, Ben.

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Hoogle error
by jim burton 14 Dec '07

14 Dec '07

Hoogling (->) (>>=) gives Error, your search was invalid: Parse Error: Unexpected character '>>=)' Is there a way to escape the input so it would work? (I wasn't really expecting the right results BTW as I think hoogle searches type signatures not patterns in definitions, right?) Thanks, Jim

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Execution of external command
by Bulat Ziganshin 14 Dec '07

14 Dec '07

Hello haskell-cafe, please help me with selection of proper function to use i need to run external command with parameter and get its stdout, smth like this: output <- system "cmd param" the code should be compatible with unix and windows, and it should be possible to execute scripts (so afaiu it should execute command via cmd/sh). i use ghc 6.6.1 and it will be great if this function will not require any non-bundled libs and be compatible with later ghc versions -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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re: generics and grammars
by Greg Meredith 14 Dec '07

14 Dec '07

Ken, Thanks for the references! Have two-level types been applied to parser generation? Best wishes, --greg Greg Meredith <lgreg.meredith(a)biosimilarity.com> wrote in article <5de3f5ca0712111100l79ec280fl1d828b12125abbdc(a)mail.gmail.com > > in gmane.comp.lang.haskell.cafe: > > Here is an idea so obvious that someone else must have already thought > of it > > and worked it all out. Consider the following grammar. > > Hello! > > If I understand your basic idea correctly, it is to split a recursive > data type into two parts, a non-recursive type constructor and a > knot-tying recursive type. This idea has been christened "two-level > types" by > > Tim Sheard and Emir Pasalic. 2004. Two-level types and > parameterized modules. Journal of Functional Programming > 14(5):547-587. > > The idea dates earlier, to initial-algebra semantics and "functional > programming with bananas and lenses": > > Mark P. Jones. 1995. Functional programming with overloading and > higher-order polymorphism. In Advanced functional programming: > 1st international spring school on advanced functional programming > techniques, ed. Johan Jeuring and Erik Meijer, 97-136. Lecture > Notes in Computer Science 925. > http://web.cecs.pdx.edu/~mpj/pubs/springschool.html<http://web.cecs.pdx.edu/%7Empj/pubs/springschool.html> > > Erik Meijer, Maarten Fokkinga, and Ross Paterson. 1991. Functional > programming with bananas, lenses, envelopes and barbed wire. In > Functional programming languages and computer architecture: 5th > conference, ed. John Hughes, 124-144. Lecture Notes in Computer > Science 523. > http://research.microsoft.com/~emeijer/Papers/fpca91.pdf<http://research.microsoft.com/%7Eemeijer/Papers/fpca91.pdf> > > Cheers, > Ken > Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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parsebinaryoperations
by Ryan Bloor 14 Dec '07

14 Dec '07

hi I have a function parseInt... which needs an error guard for when the input is not an Int. parseInt :: ParserparseInt [] = []parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) in [(EInt (read digits), removeSpace rest)] Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" gives...[(Int 5, Int 2, "if")] so, op is '+' or "&&". I am unsure of how to begin... parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str Thankyou Ryan _________________________________________________________________ Celeb spotting – Play CelebMashup and win cool prizes https://www.celebmashup.com

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Not sure where to send this: delivery failure to community.haskell.org
by Magnus Therning 14 Dec '07

14 Dec '07

I received the following response when sending an email to admin(a)community.haskell.org This is an automatically generated Delivery Status Notification THIS IS A WARNING MESSAGE ONLY. YOU DO NOT NEED TO RESEND YOUR MESSAGE. Delivery to the following recipient has been delayed: support(a)community.haskell.org Message will be retried for 2 more day(s) Technical details of temporary failure: TEMP_FAILURE: Could not initiate SMTP conversation with any hosts: [community.haskell.org (1): Connection refused] The email was sent from my gmail account (magnus.therning(a)gmail.com) but with from: set to magnus(a)therning.org. I wonder if that could be the cause of the problem (I've noticed I'm unable to post to any of the haskell mailing lists that way). /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus In order to understand recursion you must first understand recursion.

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ANNOUNCE: dataenc-0.10.1
by Magnus Therning 13 Dec '07

13 Dec '07

Yesterday I uploaded a small update to the dataenc library: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc-0.10.1 Dependencies base License LGPL Copyright Magnus Therning, 2007 Author Magnus Therning Maintainer magnus(a)therning.org Exposed modules Codec.Binary.DataEncoding, Codec.Binary.Base16, Codec.Binary.Base32Hex, Codec.Binary.Base32, Codec.Binary.Uu, Codec.Binary.Base64Url, Codec.Binary.Base64 The visible change is the addition of a function, decode', that allows lazier decoding by shifting some responisility to the user. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus Time is a great teacher, but unfortunately it kills all its pupils. -- Hector Louis Berlioz

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Folding Integrals
by Mattias Bengtsson 13 Dec '07

13 Dec '07

I found myself writing this for an Euler-problem: > digits :: Int -> [Int] > digits i | i < 10 = [i] > | otherwise = i `mod` 10 : digits ( i `div` 10 ) And i realised it was quite some time ago (before this function) i had actually written any explicitly recursive function. I managed to finish the Euler problem however and i was happy about that. However it frustrated me that i couldn't find a nice way to abstract away that explicit recursion but today i managed to! :) My first thought was that the solution probably was using some function like scanr, mapAccum or unfoldr to do it (especially the name of unfoldr made me think that it would be the solution). After abstracting my digits function i realised that it wasn't anything more than a fold over the Int type (treating the Int as a sequence of digits). "i `mod` 10" and "i `div` 10" would be nothing more than the head and tail functions (that corresponds to the (:) pattern matching). This is what i came up with finally: (I'm not 100% sure on the foldr- and foldl names though. Not sure if the semantics are correct, perhaps the function names should be switched?) > module FoldIntegral (foldr, foldl) where > import Prelude hiding (foldr,foldl,head,tail) > > head, tail :: Integral a => a -> a > head i = i `mod` 10 > tail i = i `div` 10 > > foldr :: Integral a => (a -> b -> b) -> b -> a -> b > foldr f z i > | i == 0 = z > | otherwise = foldr f (h `f` z) t > where h = head i > t = tail i > > foldl :: Integral b => (a -> b -> a) -> a -> b -> a > foldl f z i > | i == 0 = z > | otherwise = (foldl f z t) `f` h > where h = head i > t = tail i Which would make the digits function a one-liner: > digits = foldr (:) [] I hope someone enjoys this. Mattias

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ANNOUNCE: A ReadP style parser for ByteStrings
by Gracjan Polak 13 Dec '07

13 Dec '07

I'm happy to announce a ReadP style parser for ByteStrings, Text.ParserCombinators.ReadP.ByteString. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestringreadp Text.ParserCombinators.ReadP.ByteString is an adaptation of Text.ParserCombinators.ReadP to work over Data.ByteString as input stream representation. This gives enormous improvements in terms of parsing speed but most significantly in memory usage. Features: * ReadP style parser over ByteString input * Drop-in replacement for Text.ParserCombinators.ReadP * Fast * Good memory usage The algorithm is slightly modified to exploit ByteString as random access data input structure. Unlike original ReadP, that stressed garbage collection very much by creating a lot of conses (:), this parser has very good memory allocation behaviour. Package works out of the box with GHC 6.8.1, with slight (cabal) modifications also with GHC 6.6.1. Thanks to everyone for their support! Happy hacking! -- Gracjan

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announcing darcs 2.0.0pre1, the first prerelease for darcs 2
by David Roundy 13 Dec '07

13 Dec '07

We are happy to announce the first prerelease version of darcs 2! Darcs 2 will feature numerous improvements, and this prerelease will also feature a few regressions, so we're looking for help, from both Haskell developers and users willing to try this release out. Read below, to see how you can benefit from this new release, and how you can help us to make the final darcs 2 release the best ever! (for the latter, see http://wiki.darcs.net/index.html/DarcsTwo/HowToHelp) (for an expanded version of this announcement, see http://wiki.darcs.net/index.html/DarcsTwo) Darcs 2 features user-visible changes two broad categories, and several under-the-hood improvements designed to improve code stability and safety. The user-visible changes are a new "hashed" repository format, and the new darcs-2 conflict handling. The new "hashed" repository format can be used in a manner that is interchangeable with older darcs--although older versions of darcs cannot read the hashed format, darcs 2 can allows you to exchange patches between repositories in new and old formats. The new conflict handling benefits from the new hashed format, but also requires a repository conversion that is not backwards-compatible, so projects switching to darcs-2 format will have require that all their users upgrade to darcs 2. === Getting darcs 2 === You can get a prerelease version of darcs 2 either by getting the latest unstable darcs darcs get http://darcs.net/repos/unstable or by downloading the prerelease tarball from http://darcs.net/darcs-2.0.0pre1.tar.gz. Once you've compiled your new darcs, you could take it for a test drive by getting a fresh copy of darcs with the hashed repository format: darcs get http://darcs.net/repos/unstabled-hashed = Hashed repository format = We expect that most testers of darcs 2 will only try the hashed repository format. While we'd prefer to also have many users testing out actual darcs-2 format repositories, the two codebases have much in common, so tests of the hashed format will greatly help us in improving darcs 2 as a whole. The hashed repository format has a number of changes that are visible to users. 1. The hashed format allows for greater atomicity of operations. This makes for greater safety and simultaneously greater efficiency. These benefits, however, have not been fully realized in this release. For instance, with a hashed repository, there is no need for darcs push to require a repository lock, so you could record patches while waiting for a push to finish (for instance, if it's waiting on the test suite). 2. The _darcs/pristine directory no longer holds the pristine cache. This disallows certain hackish short-cuts, but also dramatically reduces the danger of third-party programs (e.g. DreamWeaver) recursing into the pristine cache and corrupting darcs repositories. 3. Darcs get is now much faster, and always operates in a "lazy" fashion, meaning that patches are downloaded only when they are needed. This gives us much of the benefits of --partial repositories, without most of their disadvantages. This approach, however, does have several new dangers. First, some operations may unexpectedly require the download of large numbers of patches, which could be slow (but you could always interrupt with ^C). Secondly, if the source repository disappears, or you lose network connectivity, some operations may fail. I do not believe these dangers will prove particularly problematic, but we may need to fine-tune the user interface to make it more clear what is going on. 4. Darcs now supports caching of patches and file contents to reduce bandwidth and save disk space. See below for how to enable this. In my opinion, this is actually the most exciting new feature, as it greatly speeds up a number of operations, and is essentially transparent. The only reason we don't enable it by default is because I'm uncomfortable creating a large directory in ~/.darcs/cache without the user's explicit consent. === Creating a repository in the hashed format === Creating a hashed repository is as easy as darcs get --hashed oldrepository newrepository or alternatively you could create a fresh repository with darcs initialize --hashed You can push, pull and send patches at will between hashed and old-fashioned repositories, so you should be able to experiment with this format even on projects that you do not control. === Enabling a global cache === It is very simple to enable a global cache. Simply execute $ mkdir -p $HOME/.darcs/cache $ echo cache:$HOME/.darcs/cache > $HOME/.darcs/sources This will cause darcs to store hard links in ~/.darcs/cache. It is always safe to delete this directory. = Darcs-2 merging = The future of darcs is in the darcs-2 repository format, which features a new merge algorithm that introduces two major user-visible changes 1. It should no longer be possible to confuse darcs or freeze it indefinitely by merging conflicting changes. However, this feature '''needs to be tested''', so please, do your worst, and let us know how darcs can handle it! 2. Identical primitive changes no longer conflict. This is a long-requested feature, and has far-reaching implications. See below (the section on "new semantics") for a discussion of these implications. === Creating a repository in the darcs-2 format === Converting an existing repository to the darcs-2 format is as easy as darcs convert oldrepository newrepository However, the convert command does run rather slowly. Moreover, you should ideally only perform this command once per project, as the conversion is not reversible, and its result is dependent on the order of patches in your repository. Of course, you can experiment all you like, but projects should switch to darcs-2 format in unison, and only after the final release of darcs 2. You can also create a fresh repository with darcs initialize --darcs-2 == Changes in semantics == When using the darcs-2 format, darcs treats identical primitive patches as the''same''patch. This has dramatic implications in how darcs-2 will define dependencies. In particular, dependencies (except those explicitly created by the use with --add-deps) are always dependencies on a given''primitive''patch, not on a given named patch. This means that the change named "foo" may in effect depend on''either the change named "bar" or the change named "baz"''. This prerelease of darcs 2 has not been fully converted to always take advantage of these new semantics--it will not cause corruption, but under unusual circumstances, could exit with an error. We need to decide how to handle these semantics in the user interface. === A simple example === Let me illustrate what could happen with a story. Steve creates changes "A" and "B": steve$ echo A > foo steve$ darcs add foo steve$ darcs record -m A steve$ echo B > foo steve$ darcs record -m B Meanwhile, Monica also decides she'd like a file named foo, and she also wants it to contain A, but she also wants to make some other changes: monica$ echo A > foo monica$ darcs add foo monica$ echo Z > bar monica$ darcs add bar monica$ darcs record -m AZ At this point, Monica pulls from Steve: monica$ darcs pull ../steve but she decides she prefers her AZ change, to Steve's A change, and being a harsh person, she decides to obliterate his change: monica$ darcs obliterate --match 'exact A' --all At this point, darcs 1 would complain, pointing out that patch B depends on patch A. However, darcs 2 will happily obliterate patch A, because patch AZ provides the primitive patches that B depends upon. At this point, however, we run into the limitations of this prerelease version of darcs 2: If Steve pulls from Monica, his darcs will fail, because the common set of patches (which is only B) cannot exist without either A or AZ. I plan to fix this behavior, but the internal API for doing so is not at all clear to me, which is why I'm looking for input from others. But note that this situation can only occur if users take advantage of the new semantics, which I suspect will be relatively seldom, until we give them tools to more easily do so (see below). === A few implications === At first this may look like a regression. Certainly, it took me a long conversation (with Steve, immortalized in the above example--but in truth, Monica would never be so unkind as to obliterate Steve's change) to determine that this behavior is actually a Good Thing, and that the potential confusion among users is a relatively small danger. The main lesson regarding these new semantics is that''patches depends on primitive patches, not on named patches''. A named patch is really just a set of primitive patches. Once we train darcs to take advantage of this feature, several tantalizing possibilities open up: 1. As the above example illustrates, in certain circumstances we can obliterate patches that are depended upon by other patches. We could automate this, by enabling obliterate (perhaps given a flag?) to break apart the patch it's trying to obliterate, and leave behind only those primitive patches which are depended upon by other patches. Perhaps we could call this new feature "atomization". Of course, this applies equally well to unrecord. 2. Recognizing that amend-record is equivalent to unrecord followed by record in a clean repository, it becomes clear that with atomization we could amend even patches that later patches depend upon. I guess these two (three?) examples are all that come to mind, but they're big examples, features that have been requested many times over the years. Of course, there would be some debris left over, a portion of the patch that got atomized, but this debris would be minimal, and seems to me to be necessary. ======================================== The version of this announcement on the wiki has more discussion of implementation plans and how you can contribute. I urge you to join in the fun of testing and optimizing this new version of darcs! David

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Re: [Haskell-cafe] IO is a bad example for Monads
by Hans van Thiel 12 Dec '07

12 Dec '07

On Wed, 2007-12-12 at 16:27 +0100, Hans van Thiel wrote: > [snip] > > > > I fear those people can do vast amounts of damage. :( > > > > When inept programming yields the wrong result, it is clear (even to the > > inept) that the program is bad. > > > > When the result is correct but there are egregious time or space leaks, > > it is "clear" to everyone but the Haskell guru that it "must" be the > > programming language that is deficient, and will be duly flamed far and > > wide. This perception will be impossible to reverse when it gains > > traction (and nothing ever goes away on the Internet). > > > > Seeming "deus ex machina" code changes (perhaps helpfully offered on > > haskell-cafe) to minimize or correct the undesirable runtime behavior > > appear even to many Haskellites to be black magic, accompanied by the > > runes of profile dumps (like knowing what generation 0 and generation 1 > > garbage collection is). > I see your point, but maybe there should be better analyzing tools then, > as well as tutorials which explain that problem. > > > > >> Haskell is not a quick-and-dirty language but quite the opposite. Haskell’s > > >> unique selling propositions are features like type classes, higher order > > >> functions and lazy evaluation which make life easier in the long term. The > > >> downside of these features is that they might make life harder in the short > > >> term. > > > I don't know. In a sense Haskell is easier than, for example, C, because > > > the concept of a function definition is more natural that that of > > > assignments and loops. The idea that x = 5; x = x + 7 makes sense > > > requires a complete new way of thinking. OK, once you've been doing it > > > for a few years switching back to x = 5 + 7 is hard. > > > > I would limit that to say that *denotational* semantic intuition is easy > > to wield in Haskell. Operational semantic intuition is Haskell is very > > non-obvious to the imperative (and many functional) programmers. > > > > Making matters worse, the first is an advantage well-hyped by > > functionistas, the second hurdle is rarely admitted to. > I admit I don't understand this. > > > > >> That said, I definitely think that we should make learning the language as > > >> easy as possible. But our ultimate goal should be to primarily show > > >> newcomers the Haskell way of problem solving, not how to emulate Python or > > >> Java programming in Haskell. > > > Again, is there a danger of that happening? > > > > Yes. Those absent the necessary humility to approach haskell-cafe with > > open mind and flame-retardant dialog will fall back on what they know: > > transliterated Java/Python with a morass of do blocks and IO monads, > > then (rightly) bash how "ugly" Haskell syntax is when used in this way. > > > > This type of programmer looking to use Haskell casually should sign a > > "benefit of the doubt" contract whereby they assume that any runtime > > suboptimalities derive from their own coding and not from Haskell's > > defects. This is the innate assumption of the curious, the > > self-motivated, the clever. This is not typically the starting > > assumption of the "I'm an expert at Joe-imperative language" hacker who > > took 10 years to perfect his Java skills and expects thereby to jump to > > at least year 5 of Haskell without effort. > But that person will be used to all the help he's gotten from the Java > and/or Eclipse, with tutorials and reference implementations. Now he has > to depend on dissertations and JFP articles for anything that's less > than 10 years old, and a few helpful experts (much appreciated, I want > to add) who are willing to spend the time to answer questions. > > > > I do strongly believe in stimulating the curiosity of all comers, just > > not in giving the false impression that a quick read-through of a few > > tutorials will let you write lightning-fast code, or know when to > > abandon [Char] for something more clever, or where to insert those bangs > > and fold left instead of right, and how ad hoc and parametric > > polymorphism differ, and what Rank-n and existential means (and why you > > can just pickle any object in Python but need to know a half dozen > > abstract things including who Peano was to do the same in Haskell), and > > what the heck an infinite type is, and on and on. > It's possible, IMO, that Haskell requires a higher skill level in > information science that the imperative languages. Many working > programmers come from different backgrounds and are not experts in > computer science. But, like a skyscraper is not built just by the > architects, maybe those 'lower' skills have their place too. Maybe not, > or not in Haskell. Could be, though I don't think so, myself. > > Haskell has definitely been teaching me some serious humility! Possibly > > it is best that those not ready for that lesson might better stick with > > Python. > If they read this, I'm sure they will. > > Best Regards, > > Hans van Thiel > >

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ANNOUNCE: dataenc-0.10.1
by Magnus Therning 12 Dec '07

12 Dec '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Yesterday I uploaded a small update to the dataenc library: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc-0.10.1 Dependencies base License LGPL Copyright Magnus Therning, 2007 Author Magnus Therning Maintainer magnus(a)therning.org Exposed modules Codec.Binary.DataEncoding, Codec.Binary.Base16, Codec.Binary.Base32Hex, Codec.Binary.Base32, Codec.Binary.Uu, Codec.Binary.Base64Url, Codec.Binary.Base64 The visible change is the addition of a function, decode', that allows lazier decoding by shifting some responisility to the user. /M -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFHX9G+iMWTaatN+6QRAqclAKC6/BM7/e9jRAjVCMYYAZd7D/fAPACfe0sd EfPoHw4HW7snU3RilejfIIY= =XPUb -----END PGP SIGNATURE-----

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do
by PR Stanley 12 Dec '07

12 Dec '07

Hi I've probably asked about the do construct, if that's the right label. Unfortunately I'm still not quite sure of its role and more specifically its syntax. Something to do with generators perhaps? A description plus some examples would be most gratefully received. Thanks, Paul

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Re: [Haskell-cafe] class default method proposal
by David Menendez 12 Dec '07

12 Dec '07

On Dec 11, 2007 1:29 PM, apfelmus <apfelmus(a)quantentunnel.de> wrote: > Without the automatic search, this is already possible > > class Functor f where > fmap :: (a -> b) -> f a -> f b > > class Functor m => Monad m where > return :: a -> m a > (>>=) :: m a -> (a -> m b) -> m b > > -- aka liftM > fmapDefault :: Monad m => (a -> b) -> m a -> m b > fmapDefault f m = m >>= (return . f) > > instance Monad [] where > return x = [x] > (>>=) = flip concatMap > > instance Functor [] where > fmap = fmapDefault > > fmap is already written for you, the instance declaration is only > boilerplate. I first saw this in Data.Traversable . > This is pretty much how I define Functor and Applicative instances for my monads. It is admittedly irritating to have to write out the boilerplate, but it doesn't seem irritating enough to require a language extension to eliminate. -- Dave Menendez <dave(a)zednenem.com> <http://www.eyrie.org/~zednenem/>

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unsubscribe
by Tim- tigre11 12 Dec '07

12 Dec '07

Very suspicious extension of attachment Sender: "Gwern Branwen" <gwern0(a)gmail.com> Recipient: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Software Tools in Haskell --- avast! Antivirus: Outbound message clean. Virus Database (VPS): 071210-0, 10/12/2007 Tested on: 12/12/2007 9:33:52 AM avast! - copyright (c) 1988-2007 ALWIL Software. http://www.avast.com

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grammars and generics
by Greg Meredith 12 Dec '07

12 Dec '07

Haskellians, Here is an idea so obvious that someone else must have already thought of it and worked it all out. Consider the following grammar. N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 N1 ::= T3 N0 where Ti (0 <= i < 4) are understood to be terminals. Using generics we can translate each production independently of the others. Like so: [| N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 |] = data N0 n1 = T0 n1 | T1 n1 (N0 n1) | T2 (N0 n1) (N0 n1) deriving (Eq, Show) [| N1 ::= T3 N0 |] = data N1 n0 = T3 n0 deriving (Eq, Show) Then, we can compose the types to get the recursive grammar. data G = N0 (N1 G) deriving (Eq, Show) This approach has the apparent advantage of treating each production independently and yet being compositional. Now, let me de-obfuscate the grammar above. The first production should be very familiar. Term ::= Var Name | Abstraction Name Term | Application Term Term The generics-based translation of this grammar yields something we already know: we can use lots of different types to work as identifiers. This is something that the nominal of Gabbay, Pitts, et al, have factored out nicely. The second production can be treated independently, but composes well with the first. Name ::= Quote Term This illustrates that a particularly interesting class of names is one that requires we look no further than our original (parametric) data type. So, my question is this. Does anyone have a reference for this approach to translation of grammars? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: [Haskell-cafe] grammars and generics
by Greg Meredith 12 Dec '07

12 Dec '07

Dusan, Excellent point. To close it off, you need to add an "empty" alternative. Thus, the corrected form would be N0 ::= TEmpty | T0 N1 | T1 N1 N0 | T2 N0 N0 N1 ::= T3 N0 In the lambda calculus, this would show up as a constant term, say 0, that would have to be treated in the operational semantics. See my ltu posting<http://lambda-the-ultimate.org/node/1930>of a year ago. Best wishes, --greg On Dec 11, 2007 11:33 AM, Dušan Kolář <kolar(a)fit.vutbr.cz> wrote: > Hello, > > I can't help you with the Haskell question as I'm not really in that > much. Nevertheless, your grammar is non-generating one - that means, it > cannot generate any sentence. If the starting non-terminal is N0 then > there is no production generating a string of terminals, thus, both > non-terminals (even if reachable from the staring one) are so called > non-generating ones (they can be freely removed from the grammar and all > involved productions too). > Thus, you get empty grammar. Isn't that the problem? Shouldn't the > grammar be like: > > N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 > N1 ::= T3 T4 > > ? > > Best regards > > Dusan > > > Greg Meredith wrote: > > Haskellians, > > > > Here is an idea so obvious that someone else must have already thought > > of it and worked it all out. Consider the following grammar. > > > > N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 > > N1 ::= T3 N0 > > > > where Ti (0 <= i < 4) are understood to be terminals. > > > > Using generics we can translate each production independently of the > > others. Like so: > > > > [| N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 |] > > = > > data N0 n1 = T0 n1 | T1 n1 (N0 n1) | T2 (N0 n1) (N0 n1) deriving (Eq, > > Show) > > > > [| N1 ::= T3 N0 |] > > = > > data N1 n0 = T3 n0 deriving (Eq, Show) > > > > Then, we can compose the types to get the recursive grammar. > > > > data G = N0 (N1 G) deriving (Eq, Show) > > > > This approach has the apparent advantage of treating each production > > independently and yet being compositional. > > > > Now, let me de-obfuscate the grammar above. The first production > > should be very familiar. > > > > Term ::= Var Name | Abstraction Name Term | Application Term Term > > > > The generics-based translation of this grammar yields something we > > already know: we can use lots of different types to work as > > identifiers. This is something that the nominal of Gabbay, Pitts, et > > al, have factored out nicely. > > > > The second production can be treated independently, but composes well > > with the first. > > > > Name ::= Quote Term > > > > This illustrates that a particularly interesting class of names is one > > that requires we look no further than our original (parametric) data > > type. > > > > So, my question is this. Does anyone have a reference for this > > approach to translation of grammars? > > > > Best wishes, > > > > --greg > > > > > > -- > > L.G. Meredith > > Managing Partner > > Biosimilarity LLC > > 505 N 72nd St > > Seattle, WA 98103 > > > > +1 206.650.3740 > > > > http://biosimilarity.blogspot.com > > ------------------------------------------------------------------------ > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > -- > > Dusan Kolar tel: +420 54 114 1238 > UIFS FIT VUT Brno fax: +420 54 114 1270 > Bozetechova 2 e-mail: kolar(a)fit.vutbr.cz > Brno 612 66 > Czech Republic > > -- > > -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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c2hs and cabal
by Stefan Kersten 12 Dec '07

12 Dec '07

hi, i'm looking for a way to integrate c2hs (single .chs file) in a cabal build setup; can anybody give me some hints? thanks, <sk>

3 3

FFI: "foreign label" declaration
by Ricardo Herrmann 12 Dec '07

12 Dec '07

Hi everyone, I'm trying to get HBDD compiled under GHC 6.8.1 and now I'm stuck with a .chs file which uses a "foreign label" syntax, which seems deprecated, since the parser can't even recognize it: foreign label "bdd_reorder_stable_window3" bdd_reorder_stable_window3 :: FunPtr (BDDManager -> IO ()) Is there new syntax for this ? Or is it a deprecated functionality ? Thanks in advance, -- Ricardo Guimarães Herrmann "In a concurrent world, imperative is the wrong default!" -- Tim Sweeney (Epic Games) -- View this message in context: http://www.nabble.com/FFI%3A-%22foreign-label%22-declaration-tp14279625p142… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: distinguish functions from non-functions in a class/instances
by oleg＠pobox.com 12 Dec '07

12 Dec '07

Philipp N. wrote: > i'm trying to wrap functions (a -> b -> ... -> z) of any arity to functions > of type ([String] -> y), where list of strings replaces the typed arguments. > the problem is, that you cannot distinguish type (x->y) from z, so these > instances are overlapping. to which apfelmus replied > Exactly. What you want to do is most likely impossible, In fact, that distinction is possible. The following article How to write an instance for not-a-function http://okmij.org/ftp/Haskell/typecast.html#is-function-type specifically describes a method of writing an instance which is selected only when the type in question is NOT a function. The method is quite general and has been extensively used (for example, to implement deep monadic join).

8 18

Waiting for thread to finish
by Maurício 12 Dec '07

12 Dec '07

Hi, After I have spawned a thread with 'forkIO', how can I check if that thread work has finished already? Or wait for it? Thanks, Maurício

21 39

Need help please.
by Ryan Bloor 11 Dec '07

11 Dec '07

hi I am writing a basic Parser from scratch. So far I have functions;# removeSpaces# match - which checks if a string is a substring of another# orParser which combines two parser's abilities# Basic pasrers like... parseInt, parseTrue, parseFalse, parseBoolusing the orParser on True and False.What I want to do now is have a parseBinaryOp that recognises:parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")]So I think that I have to split the initial string into four parts."+" becomes op'(' becomes tokenF')' becomes tokenB"5" becomes e1"2" becomes e2parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) =I am not sure how to go about separating the string for how I need itusing my other functiuons. Ryan _________________________________________________________________ Celeb spotting – Play CelebMashup and win cool prizes https://www.celebmashup.com

3 2

Quickcheck generator for arrays of integers
by Paulo J. Matos 11 Dec '07

11 Dec '07

Hello all, I using an IArray to represent a matrix. I'm trying to write some properties checks with Quickcheck. Quickcheck lacks instance generators for arrays of ints for example, is there anything I can use out there or should I define it myself? (I'm asking because it seems to be something that other people already did) Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

1 0

Literate HTML
by Neil Mitchell 11 Dec '07

11 Dec '07

Hi I want literate Haskell, but where the literate bit forming a document is actually HTML, not latex. Does anyone have any idea how to go about this? For a start, how do I persuade GHC to run the file: C:\Documents\Uni\tagsoup>runhaskell index.html Warning: ignoring unrecognised input `index.html' <interactive>:1:108: attempting to use module `Main' (Main.hs) which is not loaded And leading on to that, how Cabal might be used to compile the file. Secondly, what tricks can people think of for placing the code inside - I have come up with: <code> > foo = 1 </code> And: <code>  foo = 1 \end{code} </code> I can then hide the trailing \end{code} with a margin-bottm: -2em. Does anyone have any other ideas? Thanks Neil

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[OT] A nice organized collection of threads in Haskell-Cafe
by Vimal 11 Dec '07

11 Dec '07

Hi, I am working on a product to analyze posts made in Forums, Usenet and discussion mailing lists like Haskell-Cafe. For this, I require the messages to be accessible in this format: <forum> (* example: Haskell-cafe *) [ list of - <thread> [ list of - <post> </post> ] </thread> ] </forum> as XML. However, I find that that the messages (in haskell-cafe/usenet) themselves aren't organized in this fashion. I would like to know if there is any way in which I can get the archives in this fashion. Thanks, -- ~Vimal RLE :) encode = map (length &&& head) . group decode = concatMap (uncurry replicate)

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RE: [Haskell] Nested guards?
by Simon Peyton-Jones 11 Dec '07

11 Dec '07

[redirecting to Haskell Cafe] | > It is clear that this situation must not stay this way. Bit by bit, | > disciples of Perl and Python discover Haskell and demand that Haskell will | > be plastered with syntactic sugar until the simplicity of the functional | > approach isn’t visible anymore. Sadly, they seem to be successful with this | > as syntax extensions like parallel list comprehensions show. I think it is helpful to distinguish "superficial" complexity from "deep" complexity. All of Haskell's syntactic sugar is just that: it can be translated into a small purely-functional language using straightforward translation schemes. Even something relatively complicated like the "order by/group by" extension that Phil and I proposed at this year's Haskell workshop, has an easy translation that takes a dozen or two lines to give in full detail. In contrast, something like higher order functions, or mutable state, is deep complexity. Both have a pervasive effect on the language semantics and on its implementation. (The effect of mutable state is much, much worse, but they are both deep.) Concerning Haskell, I'm quite relaxed about superficial complexity, as you'll have seen from what happens in GHC. Section 3.6 of the History of Haskell paper addresses this point specifically [1]. You may disagree with the choice made by the Haskell committee at the time, and with subsequent developments, but it was quite a conscious choice, and one not without its advantages. Simon [1] http://research.microsoft.com/%7Esimonpj/papers/history-of-haskell/history.…

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What is the role of $!?
by PR Stanley 11 Dec '07

11 Dec '07

Hi What is the role of $! ? As far as I can gather it's something to do with strict application. Could someone explain what it is meant by the term strict application please? Thanks, Paul

20 26

Software Tools in Haskell
by Tommy McGuire 11 Dec '07

11 Dec '07

In the "if anyone is interested,..." department.... For reasons that remain unclear, early this fall I started translating Brian W. Kernighan and P.J. Plaugher's classic _Software Tools in Pascal_ into Haskell. I have completed most of it, up to the second part of chapter 8 which presents a proto-m4 preprocessor. I have the code online including notes, comments, descriptions, and a few alternate approaches. Attractions include: * A fair gamut of the usual Unix suspects: proto-cat, proto-wc, proto-tr, proto-compress, proto-ar, proto-grep, etc. * A usable editor, if you consider a de-featured ed-alike to be usable. * A simple monadic regular expression engine. * Zippers, Parsec, the State monad, the StateT monad transformer, and other attempts to sully Computing Science's brightest jewels. * Lots and lots of really bad Haskell, including a fair bit that is a direct translation of 30-year old Pascal (see xindex in translit, Ch. 2, if you need to skip lunch). Programming really has advanced, you know. Anyway, the URL is: http://www.crsr.net/Programming_Languages/SoftwareTools Questions and comments would be appreciated, especially suggestions for how to make the code cleaner and more understandable. Flames and mockery are welcome, too, but only if they're funny---remember, I've been staring at Haskell, Pascal (plus my job-related Perl, CORBA, and C++) for a while; there's no telling what my mental state is like. [I had intended to wait until I had the whole thing done to make this announcement, but I recently moved and have not made much forward progress since, other than putting what I had done online.] -- Tommy M. McGuire mcguire(a)crsr.net

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Re: type class question
by Peter Padawitz 10 Dec '07

10 Dec '07

Jules Bean wrote: > Peter Padawitz wrote: > >>>> So the fundep would solve the problem. >>> >> But, actually, it doesn't :-( > > But actually, it does! Indeed... Sorry, I think I left intE out of the cycle. This might be the reason why it did not work before. > Ben Franksen's answer from yesterday compiles fine for me if I add the > missing fundep, block -> command. > > Your original code compiles without error, given the fundep. Exact > code I compiled attached at the bottom of this document. You may have > to repair long lines! > > Incidentally, I question why the "compFoo" are methods. Why not just > make them polymorphic functions? They don't look like you expect > instances to change them. The code continues to compile if I make them > functions and amend their signatures as required. I put compFoo into the class for the same reason why /= is part of the class Eq: both functions are unique as soon as the others have been instantiated. > > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} > > type Block = [Command] > data Command = Skip | Assign String IntE | Cond BoolE Block Block | > Loop BoolE Block > data IntE = IntE Int | Var String | Sub IntE IntE | Sum [IntE] | > Prod [IntE] > data BoolE = BoolE Bool | Greater IntE IntE | Not BoolE > > class Java block command intE boolE | block -> command, command -> > intE, intE -> boolE, boolE -> block > where block_ :: [command] -> block > skip :: command > assign :: String -> intE -> command > cond :: boolE -> block -> block -> command > loop :: boolE -> block -> command > intE_ :: Int -> intE > var :: String -> intE > sub :: intE -> intE -> intE > sum_ :: [intE] -> intE > prod :: [intE] -> intE > boolE_ :: Bool -> boolE > greater :: intE -> intE -> boolE > not_ :: boolE -> boolE > compBlock :: Block -> block > compBlock = block_ . map compCommand > compCommand :: Command -> command > compCommand Skip = skip > compCommand (Assign x e) = assign x (compIntE e) > compCommand (Cond be cs cs') = cond (compBoolE be) (compBlock > cs) (compBlock cs') > compCommand (Loop be cs) = loop (compBoolE be) (compBlock cs) > compIntE :: IntE -> intE > compIntE (IntE i) = intE_ i > compIntE (Var x) = var x > compIntE (Sub e e') = sub (compIntE e) (compIntE e') > compIntE (Sum es) = sum_ (map compIntE es) > compIntE (Prod es) = prod (map compIntE es) > compBoolE :: BoolE -> boolE > compBoolE (BoolE b) = boolE_ b > compBoolE (Greater e e') = greater (compIntE e) (compIntE e') > compBoolE (Not be) = not_ (compBoolE be)

3 7

help
by Ryan Bloor 10 Dec '07

10 Dec '07

hi I am writing a basic Parser from scratch. So far I have functions;# removeSpaces# match - which checks if a string is a substring of another# orParser which combines two parser's abilities# Basic pasrers like... parseInt, parseTrue, parseFalse, parseBoolusing the orParser on True and False.What I want to do now is have a parseBinaryOp that recognises:parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")]So I think that I have to split the initial string into four parts."+" becomes op'(' becomes tokenF')' becomes tokenB"5" becomes e1"2" becomes e2parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) =I am not sure how to go about separating the string for how I need itusing my other functiuons. Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

2 1

Do real programs need IO? (was IO is a bad example for Monads)
by Conal Elliott 10 Dec '07

10 Dec '07

On Dec 9, 2007 10:07 AM, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > Interactive programmes without using IO? Cool :) And how! > I think you misunderstood Lennart. Thanks for checking. In this case, I think I understood Lennart fine and that he was saying what you're saying. > Would you deny that any useful programme has to do at least some of the following: > -accept programme arguments at invocation > -get input, be it from a keyboard, mouse, reading files, pipes... > -output a result or state info, to the monitor, a file, a pipe... === If by "programme", you mean the code I write, then I'm happy to deny that my programme has to do these things. Examples below. If you include a stateful RTS, then no I don't deny it. > I think Lennart was referring to that, you HAVE to know a little IO to write > programmes, at least getArgs, getLine, putStr(Ln), readFile, writeFile, > appendFile. And therefore some use of the IO monad has to be taught > relatively early. Explicit imperative programming is just one way to deal with input & output, not the only way. As proof, see FRP, Pan, or TV programs, which contain uses of none of these functions. (Nor could they, as these libraries are functional, having IO-free types and semantics.) Moreover, use of imperative programming sacrifices some of the semantic simplicity & composability that makes FP so appealing. That's why I'd like to see this belief in its necessity dispelled. That said, I don't think the existing functional (non-IO) approaches to interaction are quite there yet with the flexibility of imperative programming. It will take more work to get them there, and that work is mostly likely to be pursued by people who doubt the necessity of IO for writing "real programs". In that sense, Lennart's and your statements are self-fulfilling prophechies, as are mine. BTW, if you haven't seen it already, please check out http://haskell.org/haskellwiki/TV . The TV (tangible values) approach includes a simple algebra of interfaces (input/output) and keeps separable from the core computation. The separability allows the interface parts to be composed in parallel with the core part. For instance, when two function-valued TVs are composed, the interfaces are peeled off, so that the core functions can be composed directly. The output half of one interface and the matching input half of the other are discarded. The remaining input and output halves are recombined into a new interface, which is used as the interface of the composed TV. The core interface algebra can be used for text stream i/o, GUIs, and many other possible styles of information passing. I mention TV, because it's an example of combining the purity & composability I love about FP with the usability a "real" app. For more about this combination, please see my Google tech talk "Tangible Functional Programming: a modern marriage of usability and composability" ( http://conal-elliott.blogspot.com/2007/11/tangible-functional-programming-m…) That talk focus on end-user composability, but the essential points apply as well to explicit programming. As I mentioned before, TV (a) is currently less flexible than imperative/IO programming, and (b) has the composability, guaranteed safety, and amenability to reasoning of pure functional programming. Cheers, - Conal Am Sonntag, 9. Dezember 2007 18:31 schrieb Conal Elliott: > > > IO is important because you can't write any real program without using > > > it. > > > > Ouch! I get awfully discouraged when I read statements like this one. > The > > more people who believe it, the more true it becomes. If you want to do > > functional programming, instead of imperative programming in a > functional > > language, you can. For instance, write real, interactive programs in > FRP, > > phooey, or TV. And if you do, you'll get semantic simplicity, powerful > & > > simpler reasoning, safety and composability. > > > > - Conal > > > On Dec 8, 2007 1:26 AM, Lennart Augustsson <lennart(a)augustsson.net> wrote: > > [...] > > IO is important because you can't write any real program without using it. > > So why not teach enough of it to get people off the ground straight away? > > People who hang around long enough to do some more Haskell programming > > will run into the other monads sooner or later. But IO is an unavoidable step to > > writing Haskell programs.

4 8

I'm translating <All about Monads> to Chinese
by Albert Lee 10 Dec '07

10 Dec '07

I have been confussed by monad for a long time. and I can't stand for it any more. so I start to translate the tutorial <All About Monads> to my mother language Chinese. My English is not good enough, so this work is only for my own study~ I know there are some other Chinese fellow in this list, wish it would be helpful. I will work for one chapter everyday. the address is: http://www.kamang.net

3 2

Re: [Haskell] IVars
by Conal Elliott 10 Dec '07

10 Dec '07

(moving to haskell-cafe) > readIVar' :: IVar a -> a > readIVar' = unsafePerformIO . readIVar > so, we do not need readIVar'. it could be a nice addition to the libraries, maybe as "unsafeReadIVar" or "unsafeReadMVar". The same argument applies any to pure function, doesn't it? For instance, a non-IO version of succ is unnecessary. My question is why make readIVar a blocking IO action rather than a blocking pure value, considering that it always returns the same value? - Conal On Dec 8, 2007 11:12 AM, Marc A. Ziegert <coeus(a)gmx.de> wrote: > many many answers, many guesses... > let's compare these semantics: > > readIVar :: IVar a -> IO a > readIVar' :: IVar a -> a > readIVar' = unsafePerformIO . readIVar > > so, we do not need readIVar'. it could be a nice addition to the > libraries, maybe as "unsafeReadIVar" or "unsafeReadMVar". > but the other way: > > readIVar v = return $ readIVar' v > > does not work. with this definition, readIVar itself does not block > anymore. it's like hGetContents. > and... > > readIVar v = return $! readIVar' v > > evaluates too much: > it wont work if the stored value evaluates to 1) undefined or 2) _|_. > it may even cause a 3) deadlock: > > do > writeIVar v (readIVar' w) > x<-readIVar v > writeIVar w "cat" > return x :: IO String > > readIVar should only return the 'reference'(internal pointer) to the read > object without evaluating it. in other words: > readIVar should wait to receive but not look into the received "box"; it > may contain a nasty undead werecat of some type. (Schrödinger's Law.) > > - marc > > > > > > Am Freitag, 7. Dezember 2007 schrieb Paul Johnson: > > Conal Elliott wrote: > > > Oh. Simple enough. Thanks. > > > > > > Another question: why the IO in readIVar :: IVar a -> IO a, instead > > > of just readIVar :: IVar a -> a? After all, won't readIVar iv yield > > > the same result (eventually) every time it's called? > > Because it won't necessarily yield the same result the next time you run > > it. This is the same reason the stuff in System.Environment returns > > values in IO. > > > > Paul. > > _______________________________________________ > > Haskell mailing list > > Haskell(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell > > > > >

4 6

ANNOUNCE: xmonad 0.5: a tiling window manager
by Don Stewart 10 Dec '07

10 Dec '07

The xmonad dev team is pleased to announce the 0.5 release of xmonad! http://xmonad.org xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard: a mouse is optional. xmonad is extensible in Haskell, allowing for powerful customisation. Custom layout algorithms, key bindings and other extensions may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several physical screens. Features: * Very stable, fast, small and simple. * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating windows * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screens custom status bars * Easy, powerful customisation and reconfiguration in Haskell * Large extension library * Extensive documentation and support for hacking Get it! More information, screenshots, documentation, tutorials and community resources are available from the xmonad home page: http://xmonad.org The 0.5 release, and its dependencies, are available from hackage.haskell.org, here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad **The headlines** xmonad 0.5 is a major new release of xmonad, allowing, for the first time, dynamic extension in Haskell without requiring recompilation. Extension and configuration of xmonad is now faster, simpler, and more flexible. All configuration is done via the ~/.xmonad/xmonad.hs file. Example config files and screenshots are available on the wiki: http://haskell.org/haskellwiki/Xmonad/Config_archive This marks the final break from its dwm origins. As a result, xmonad is now also much easier to package, distribute and extend, as recompilation of xmonad is not required to extend it. New features: * xmonad is now a library, as is the collection of xmonad extensions * xmonad is configured by editing ~/.xmonad/xmonad.hs * simpler layout hooks type and interface * small EDSL for writing window management rules * support for ghc 6.8.x * xmonad now requires Cabal 1.2 or newer * small bug fixes Extensions: xmonad comes with a huge library of extensions (now more than 5 times the size of xmonad), contributed by viewers like you. Extensions enable pretty much arbitrary window manager behaviour to be implemented by users, in Haskell, in the config files. Examples include: * ion-like window tabs * status bar support * X property and hints support * altnerative layout algorithms For more information on using and writing extensions see the webpage. The library of extensions is available from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-contrib Brought to you by the xmonad team: Spencer Janssen Don Stewart Jason Creighton Andrea Rossato David Roundy Brent Yorgey with code contributions from: Brandon Allbery Chris Mears Shachaf Ben-Kiki Eric Mertens Alec Berryman Neil Mitchell Gwern Branwen Devin Mullins Joachim Breitner Daniel Neri Alexandre Buisse Stefan O'Rear Nick Burlett Simon Peyton Jones Peter De Wachter Hans Philipp Annen Aaron Denney Karsten Schoelzel Nelson Elhage Michael Sloan Shae Erisson Ivan Tarasov Joachim Fasting Alex Tarkovsky Michael Fellinger Christian Thiemann David Glasser Joe Thornber Kai Grossjohann Matsuyama Tomohiro Dave Harrison Daniel Wagner Juraj Hercek Ferenc Wagner Sam Hughes Jamie Webb Miikka Koskinen Klaus Weidner David Lazar nornagon Lucas Mai timthelion Robert Marlow Valery V. Vorotyntsev Dougal Stanton Joel Suovaniemi Mats Jansborg Dmitry Kurochkin Clemens Fruhwirth As well as the support of many others on the #xmonad and #haskell IRC channels, and the wider Haskell and window manager communities. Thanks to everyone for their support!

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Parsing integers [was: Re: [Haskell-cafe] help]
by Tim Chevalier 10 Dec '07

10 Dec '07

On 12/9/07, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > > hi > > I have a function parseInt... which needs an error guard for when the input > is not an Int. > > parseInt :: Parser > parseInt [] = [] > parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) > in [(EInt (read digits), removeSpace rest)] > > Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" > gives...[(Int 5, Int 2, "if")] > so, op is '+' or "&&". I am unsure of how to begin... > > parseBinaryOp :: String -> String -> [(Expr, Expr, String)] > parseBinaryOp op str > Check out: http://www.haskell.org/haskellwiki/Homework_help If this isn't homework, try posting again and phrasing your question in the form of a question; saying "I am unsure of how to begin" doesn't help us help you think about the problem. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "[Teaching children to read] will make America what we want it to be, a literate country and a hopefuller country." -- George W. Bush

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Point and link
by Andrew Coppin 10 Dec '07

10 Dec '07

Hi guys. Here's a fairly basic question. I have several ideas for programs that I'd like to write. They all involve placing "units" of some kind, and then drawing "connections" between those units. How feasible is it to program such a thing in Haskell? Where would you start? (Gtk2hs is the only graphics API I'm familiar with using at present.)

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Fwd: Parsing integers [was: Re: [Haskell-cafe] help]
by Tim Chevalier 10 Dec '07

10 Dec '07

[reply forwarded to list] ---------- Forwarded message ---------- From: Ryan Bloor <ryanbloor(a)hotmail.com> hi sorry for vagueness. I am writing a basic Parser from scratch. So far I have functions; # removeSpaces # match - which checks if a string is a substring of another # orParser which combines two parser's abilities # Basic pasrers like... parseInt, parseTrue, parseFalse, parseBool using the orParser on True and False. What I want to do now is have a parseBinaryOp that recognises: parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")] So I think that I have to split the initial string into four parts. "+" becomes op '(' becomes tokenF ')' becomes tokenB "5" becomes e1 "2" becomes e2 parseBinaryOp :: String -> String -> [(Expr, Expr, String)] parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) = I am not sure how to go about separating the string for how I need it using my other functiuons. Ryan ________________________________ > Date: Sun, 9 Dec 2007 13:07:23 -0800 > From: catamorphism(a)gmail.com > To: ryanbloor(a)hotmail.com > Subject: Parsing integers [was: Re: [Haskell-cafe] help] > CC: haskell-cafe(a)haskell.org > > On 12/9/07, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > > > > hi > > > > I have a function parseInt... which needs an error guard for when the input > > is not an Int. > > > > parseInt :: Parser > > parseInt [] = [] > > parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) > > in [(EInt (read digits), removeSpace rest)] > > > > Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" > > gives...[(Int 5, Int 2, "if")] > > so, op is '+' or "&&". I am unsure of how to begin... > > > > parseBinaryOp :: String -> String -> [(Expr, Expr, String)] > > parseBinaryOp op str > > > > Check out: > http://www.haskell.org/haskellwiki/Homework_help > > If this isn't homework, try posting again and phrasing your question > in the form of a question; saying "I am unsure of how to begin" > doesn't help us help you think about the problem. > > Cheers, > Tim > > -- > Tim Chevalier * catamorphism.org * Often in error, never in doubt > "[Teaching children to read] will make America what we want it to be, > a literate country and a hopefuller country." -- George W. Bush ________________________________ Are you the Quizmaster? Play BrainBattle with a friend now!

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help
by Ryan Bloor 10 Dec '07

10 Dec '07

hi I have a function parseInt... which needs an error guard for when the input is not an Int. parseInt :: ParserparseInt [] = []parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) in [(EInt (read digits), removeSpace rest)] Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" gives...[(Int 5, Int 2, "if")]so, op is '+' or "&&". I am unsure of how to begin... parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str Thankyou Ryan _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

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ARM back end?
by Greg Fitzgerald 10 Dec '07

10 Dec '07

Anybody know of an ARM back end for any of the Haskell compilers? Thanks, Greg

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ST Monad - what's wrong?
by Nicu Ionita 10 Dec '07

10 Dec '07

Hi, I'm trying to use the ST monad in order to turn an almost pure function into a pure one: reading a precalculated list of primes into a prime set. But the following code brings an error: > primes :: Set Integer > primes = runST $ getPrimes "primes10h7.txt" > getPrimes :: String -> (forall s. ST s (Set Integer)) > getPrimes file = > do cont <- unsafeIOToST (readFile file) > let set = fromList $ map read $ lines cont > return set And here is the error: Couldn't match expected type `forall s. ST s a' against inferred type `ST s (Set Integer)' In the second argument of `($)', namely `getPrimes "primes10h7.txt"' In the expression: runST $ (getPrimes "primes10h7.txt") In the definition of `primes': primes = runST $ (getPrimes "primes10h7.txt") Compiled with GHC 6.6.1 with extensions activated. Nicu

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AW: ST Monad - what's wrong?
by Nicu Ionita 09 Dec '07

09 Dec '07

Ok, solved, the use of ($) was the problem. I changed it > primes :: Set Integer > primes = runST (getPrimes "primes10h7.txt") and it compiles (and works too). Nicu > -----Ursprüngliche Nachricht----- > Von: Nicu Ionita [mailto:nionita@lycos.de] > Gesendet: Sonntag, 9. Dezember 2007 12:12 > An: 'haskell-cafe(a)haskell.org' > Betreff: ST Monad - what's wrong? > > > Hi, > > I'm trying to use the ST monad in order to turn an almost > pure function into a pure one: reading a precalculated list > of primes into a prime set. But the following code brings an error: > > > primes :: Set Integer > > primes = runST $ getPrimes "primes10h7.txt" > > > getPrimes :: String -> (forall s. ST s (Set Integer)) > getPrimes file = > > do cont <- unsafeIOToST (readFile file) > > let set = fromList $ map read $ lines cont > > return set > > And here is the error: > > Couldn't match expected type `forall s. ST s a' > against inferred type `ST s (Set Integer)' > In the second argument of `($)', namely > `getPrimes "primes10h7.txt"' > In the expression: runST $ (getPrimes "primes10h7.txt") > In the definition of `primes': > primes = runST $ (getPrimes "primes10h7.txt") > > Compiled with GHC 6.6.1 with extensions activated. > > Nicu >

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Re: [Haskell-cafe] do... error
by Felipe Lessa 09 Dec '07

09 Dec '07

On Dec 9, 2007 1:01 AM, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > But what is the right way to indent...? It is so annoying, why does it > matter so much! :( You may read http://en.wikibooks.org/wiki/Haskell/Indentation which tries to explain in a very simple language. -- Felipe.

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do... error
by Ryan Bloor 09 Dec '07

09 Dec '07

hi test :: Parser (Char,Char) test = do x <- item item y <- item return (x,y) How come this brings an error saying that after do {....} it must end with an expression. Ryan _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

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