[Haskell-cafe] Re: Laws and partial values
On Sat, 24 Jan 2009, Thomas Davie wrote: On 24 Jan 2009, at 21:31, Dan Doel wrote: For integers, is _|_ equal to 0? 1? 2? ... Hypothetically (as it's already been pointed out that this is not the case in Haskell), _|_ in the integers would not be known, until it became more defined. I'm coming at this from the point of view that bottom would contain all the information we could possibly know about a value while still being the least value in the set. In such a scheme, bottom for Unit would be (), as we always know that the value in that type is (); bottom for pairs would be (_|_, _|_), as all pairs look like that (this incidentally would allow fmap and second to be equal on pairs); bottom for integers would contain no information, etc. Zero- and one-constructor data types would then significantly differ from two- and more-constructor data types, wouldn't they? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Laws and partial values
On Sat, 24 Jan 2009, Thomas Davie wrote: I'm not sure why you're saying that this semantics does not capture non-termination – the only change is that computations resulting in the unit type *can't* non terminate, because we can always optimize them down to (). Of course, if you want to be able to deal with non-termination, one could use the Maybe () type! I find the current semantics better: All types are handled the same way by default and whenever you want to optimize (undefined::()) to () you can use optimizeNull :: () - () optimizeNull _ = ()___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] mapM_ - Monoid.Monad.map
I always considered the monad functions with names ending on '_' a concession to the IO monad. Would you need them for any other monad than IO? For self-written monads you would certainly use a monoid instead of monadic action, all returning (), but IO is a monad. (You could however wrap (newtype Output = Output (IO ())) and define a Monoid instance on Output.) However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) = (a - m b) - [a] - m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ and it is also safer since it connects the monadic result types of the atomic and the sequenced actions. There was a recent discussion on the topic: http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pure Haskell implementation of Float type?
Lennart Augustsson schrieb: There's the numbers package which contains BigFloat. You can pick your own precision, but it's not IEEE. It's actually base 10 floats which makes it more fun (actually, the iEEE standard will cover base 10 floats in the future). Actually, all of the arbitrary precision real number implementations may be of interest for you: http://haskell.org/haskellwiki/Applications_and_libraries/Mathematics#Number_representations The NumericPrelude implementation by me, however, sometimes uses the Double implementation for speedup. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Elevator pitch for functional programming
Jim Burton schrieb: Well, I might but they definitely do not :-) We are talking about some maths-averse people and you would not have got to the final syllable of 'fibonacci' before all hope was lost. But I am sure there are plenty of examples that rely on laziness which will communicate. I am sure I read a blog post or something on c.l.f/c.l.h recently about lazily sorting a million numbers but can't find it. Maybe they are interested in finding all equal files in a set of files. This can be done in an elegant way by sorting the files with respect to their content. Practical enough? http://hackage.haskell.org/cgi-bin/hackage-scripts/package/equal-files ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comments from OCaml Hacker Brian Hurt
On Thu, 15 Jan 2009, John Goerzen wrote: One thing that does annoy me about Haskell- naming. Say you've noticed a common pattern, a lot of data structures are similar to the difference list I described above, in that they have an empty state and the ability to append things onto the end. Now, for various reasons, you want to give this pattern a name using on Haskell's tools for expressing common idioms as general patterns (type classes, in this case). What name do you give it? I'd be inclined to call it something like Appendable. But no, Haskell calls this pattern a Monoid. I risk to repeat someones point, since I have not read the entire thread ... What I don't like about the Monoid class is, that its members are named mempty and mappend. It may be either (also respecting qualified import) Monoid(identity, op) or Appendable(empty, append) where only the first one seems reasonable, since the Sum monoid and its friends do not append anything. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comments from OCaml Hacker Brian Hurt
John Goerzen schrieb: Though if all we're talking about is naming, I would still maintain that newbie-friendly naming is a win. We can always say HEY MATHEMETICIANS: APPENDABLE MEANS MONOID in the haddock docs ;-) We already have a problem with this: Haskell 98 uses intuitive names for the numeric type classes. It introduces new names, which do not match the names of common algebraic structures. Why is a type Num (numeric?), whenever it supports number literals, (+) and (*)? Why not just number literals? Why not also division? The numeric type hierarchy of Haskell must be learned anyway, but the user learns terms he cannot use outside the Haskell world. Ring and Field are harder to learn first, but known and precise terms. (And if you don't like to learn the names, just write functions without signatures and let GHCi find out the signatures with the appropriate class constraints.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Comments from OCaml Hacker Brian Hurt
Apfelmus, Heinrich schrieb: Obviously, those who know what a monoid is have already invested years of time practicing mathematics while those that even attack the name monoid clearly lack this practice. It's like peano virtuosoes compared to beginning keyboard pressers. Aren't all Haskellers some kind of Peano virtuosos? :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Improved documentation for Bool (Was: [Haskell-cafe] Comments from OCaml Hacker Brian Hurt)
rocon...@theorem.ca schrieb: On Sun, 18 Jan 2009, Ross Paterson wrote: Anyone can check out the darcs repos for the libraries, and post suggested improvements to the documentation to librar...@haskell.org (though you have to subscribe). It doesn't even have to be a patch. Sure, it could be smoother, but there's hardly a flood of contributions. I noticed the Bool datatype isn't well documented. Since Bool is not a common English word, I figured it could use some haddock to help clarify it for newcomers. The type should be named Truth. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: Coadjute 0.0.1, generic build tool
Matti Niemenmaa schrieb: Announcing the release of Coadjute, version 0.0.1! Web site: http://iki.fi/matti.niemenmaa/coadjute/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Coadjute Coadjute is a generic build tool, intended as an easier to use and more portable replacement for make. It’s not tailored toward any particular language, and is not meant to replace tools which target a specific environment (such as ghc --make or Cabal, taking Haskell as an example). I've been sitting on this for a couple of months now and figured I might as well push it out since it seems to be in relative working order. I've used it on my web site since July and it hasn't resulted in data loss yet. How does it compare to http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: Coadjute 0.0.1, generic build tool
Matti Niemenmaa schrieb: Anyway, hake looks interesting but it's not a replacement for Coadjute; and neither is Coadjute for hake. To be completely honest I'm not sure what use case hake is meant to solve: how does it improve over plain make? It seems to be programmable, and thus may better cope with LaTeX, where the simple 'make' philosophy fails. 'latex' must be called repeatedly, interleaved with 'bibtex' and 'mkindex'. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HTTPbis / HTTP-4000.x package available
On Thu, 15 Jan 2009, Sigbjorn Finne wrote: I guess it's time to publish more widely the availability of a modernization of the venerable and trusted HTTP package, which I've been working on offon for a while. I was always afraid that a fork may happen during I work on HTTP in order to get it more lazy. That's why I started discussion on web-devel mailing list, but got only limited response. I also notified Bjorn, that my changes need still a little time and that I'm worried about darcs conflicts. Seems that we now run into a perfect conflict. I don't like to throw away the work that I have done the last weeks. My version is at http://code.haskell.org/~thielema/http/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Open unqualified imports
On Fri, 16 Jan 2009, eyal.lo...@gmail.com wrote: I believe that the last type, open-unqualified is a really bad idea, and should be discouraged or even disallowed. It should definitely not be the default, like it is today. You are not alone: http://haskell.org/haskellwiki/Import_modules_properly ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Open unqualified imports
On Fri, 16 Jan 2009, eyal.lo...@gmail.com wrote: import qualified Control.Monad as M M.for M.map and so on. http://haskell.org/haskellwiki/Qualified_names ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HTTPbis / HTTP-4000.x package available
On Fri, 16 Jan 2009, Sigbjorn Finne wrote: gain access to www.haskell.org and update http://www.haskell.org/http, so for now you may pick up a new version the lib via http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP GIT repo for HTTP-4000 / HTTPbis is here git://code.galois.com/HTTPbis.git Can you please add a motd to the old darcs repository, which tells the downloader that this repository has moved to a git repository? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] F# active patterns versus GHC's view
On Fri, 16 Jan 2009, John Van Enk wrote: 2009/1/16 Peter Verswyvelen bugf...@gmail.com [...] After a while you decide that you need to change the Bla data type, maybe give Dog more fields, maybe completely redesign it, maybe not exposing it, but you want to keep existing code backwards compatible. With F# you can write Active Patterns for the old Dog and Cat constructors, so all existing code remains compatible. At least that is the way I understand it, but I have not actually worked yet with Active Patterns, will do so soon :) You get something similar with the record syntax (though, probably still not quite as powerful as the active patterns): ... or use data-accessor package. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HTTPbis / HTTP-4000.x package available
On Thu, 15 Jan 2009, Sigbjorn Finne wrote: I guess it's time to publish more widely the availability of a modernization of the venerable and trusted HTTP package, which I've been working on offon for a while. Bunch of changes, but the headline new feature of this new version is the parameterization of the representation of payloads in both HTTP requests and responses. Two new representations are supported, strict and lazy ByteStrings. After looking into your code it seems that an easy merge will not be possible. I'm not quite ready with Lazy I/O, but close to it. I introduced a new monad which allows to get rid of explicit usage of the IO monad. I also rewrote Network.HTTP in order to handle asynchronous exceptions in a clean way, thus making Network.HTTP really lazy. See also the thread: http://www.haskell.org/pipermail/web-devel/2008/48.html I'm really frustrated now, since I invested much time into improvement of the HTTP package. I did a lot of communication in order to prevent me from doing duplicate work, which already happened in other Haskell projects, and now an official successor appears from nowhere which can almost not be merged with my effort. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HTTPbis / HTTP-4000.x package available
Sigbjorn Finne schrieb: Hi Levi, I'm guessing that you are reading something different into that than what's intended - it's client-side in the sense that it can only issue web requests and handle their responses. i.e., it doesn't handle incoming HTTP requests and issue suitable responses. Web server implementation is an interesting problem in its own right, and many packages/frameworks do an admirable job of that already, so no plans (by me) to tackle that via the HTTP package. There's however still no framework which supports both HTTP client and server functions using the same Request and Response data type, right? I don't know whether I am the only one who needs this (e.g. for the Real Monad Transformer). E.g. a proxy would need this, too. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HTTPbis / HTTP-4000.x package available
Don Stewart schrieb: lemming: On Thu, 15 Jan 2009, Sigbjorn Finne wrote: I guess it's time to publish more widely the availability of a modernization of the venerable and trusted HTTP package, which I've been working on offon for a while. I was always afraid that a fork may happen during I work on HTTP in order to get it more lazy. That's why I started discussion on web-devel mailing list, but got only limited response. I also notified Bjorn, that my changes need still a little time and that I'm worried about darcs conflicts. Seems that we now run into a perfect conflict. I don't like to throw away the work that I have done the last weeks. My version is at http://code.haskell.org/~thielema/http/ There's absolutely no need to throw away work. Hackage is a broad church, release it as http-lazy, at the very least. Somehow yes. As I said, it still needs cleanup and I wonder whether I invest that time in my version or whether I should merge my changes immediately into HTTP-4000. As it now stands, I can provide laziness and HTTP-4000 provides ByteStrings and I'm sure people wants both of them. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Sun, 11 Jan 2009, Neil Mitchell wrote: I am pleased to announce HLint version 1.2. HLint is a lint-like tool for Haskell that detects and suggests improvements for your code. HLint is compatible with most GHC extensions, and supports a wide variety of suggestions, and can be extended with additional user suggestions. To install: cabal update cabal install hlint Fails for me, because of the base-4 dependency. - I'm still using GHC-6.8.2. Can HLint suggest view-pattern-free expressions, such that the program also runs on GHC-6.8 ? :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Thu, 15 Jan 2009, Neil Mitchell wrote: Hi Henning, To install: cabal update cabal install hlint Fails for me, because of the base-4 dependency. - I'm still using GHC-6.8.2. Can HLint suggest view-pattern-free expressions, such that the program also runs on GHC-6.8 ? :-) HLint is written using view-patterns so requires GHC 6.10 to compile it. After you have it compiled its just a standard Haskell binary, so can be run on any Haskell code and will never suggest view-patterns anyway. My question was, whether HLint can help translating HLint to code without view patterns. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Thu, 15 Jan 2009, Neil Mitchell wrote: Hi My question was, whether HLint can help translating HLint to code without view patterns. Ah, I misunderstood. Yes, it could (in theory), but it can't automatically apply the hints it generates. Upgrading to GHC 6.10 is probably easier :-) Also throwing away Hugs ... (YHC too ?) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Thu, 15 Jan 2009, Neil Mitchell wrote: I normally develop in Hugs, for a change I wanted to try GHCi. It's also a project that has loads of pattern matching at a fairly complex level, so the benefits offered by view-patterns and pattern-guards were just too hard to pass up. Are you sure that you can't come up with some nice functions like 'maybe' to replace those view patterns by function calls? Did you really try? I remember the recent discussion on pattern combinators here on Haskell Cafe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness
On Tue, 13 Jan 2009, Jan Christiansen wrote: I would be very interested in functions that can be improved with respect to non-strictness as test cases for my work. See the List functions in http://hackage.haskell.org/cgi-bin/hackage-scripts/package/utility-ht Version 0.0.1 was before applying StrictCheck, 0.0.2 after processing its suggestions. I have stopped when I saw that it repeatedly shows examples where I expect, that they are unresolvable. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] version conflict on Hackage
I repeatedly encounter the following versioning problem on Hackage: There is a package A with version 1.0. I upload a package B which imports A. Thus B is bound to A-1.0 Now a new version of A is uploaded, say 1.0.1. then I upload package C which depends both on A and B. However C is bound to the new A-1.0.1, which gives conflicts with the interface of B. All packages are sane. Both B and C work with either A-1.0 or A-1.0.1, but they must use the same version. Currently I'm forced to upload many packages with different versions, although nothing really changed. Maybe the most easiest fix to this problem is to let maintainers of a package allow to trigger re-configuration and re-compilation of a package. I would also like to flag package versions that people can safely ignore, because there was a flaw in package dependencies. On the other hand there are packages that are useful, but can no longer be compiled on Hackage, e.g. compatibility packages for older versions of GHC. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness
On Mon, 12 Jan 2009, Jan Christiansen wrote: Hi, Although it seems to be overkill for a single module - How about a cabalized version on Hackage and a darcs repository? It would simplify using and updating it. I am not sure whether this would be a good idea. The original version makes a lot of suggestions which are not satisfiable but it is not at all trivial to decide which are satisfiable and which are not. I have rewritten StrictCheck from scratch to overcome this problem. But the current implementation is not presentable right now and I have no formal prove that the criterion that I am using is correct. As I said, the current version is already very useful. I have applied it to several basic functions and found a lot to improve. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Mon, 12 Jan 2009, Duncan Coutts wrote: On Mon, 2009-01-12 at 01:02 +0100, Lennart Augustsson wrote: Does GHC specialize map? If it doesn't, then hand crafted version could be faster. No because the current definition are recursive and ghc cannot inline recursive functions. map :: (a - b) - [a] - [b] map _ [] = [] map f (x:xs) = f x : map f xs It has to be manually transformed into a version that is not recursive at the top level: map :: (a - b) - [a] - [b] map f = go where go [] = [] go (x:xs) = f x : go xs Then the map can be inlined at the call site and the 'f' inlined into the body of 'go'. Maybe HLint can make such suggestions ... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Mon, 12 Jan 2009, Neil Mitchell wrote: Hi No because the current definition are recursive and ghc cannot inline recursive functions. map :: (a - b) - [a] - [b] map f = go where go [] = [] go (x:xs) = f x : go xs Then the map can be inlined at the call site and the 'f' inlined into the body of 'go'. Maybe HLint can make such suggestions ... HLint would probably just suggest you use a supercompiler*, for which specialisation such as this is easy (http://www.cs.york.ac.uk/~ndm/supero). HLint is about making code prettier, Actually, I find the 'go' variant prettier, because it clearly shows that the 'f' is not altered in the recursion. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Mon, 12 Jan 2009, Robin Green wrote: I tend to use Control.Monad.Fix.fix (which actually has nothing to do with monads, despite the package name) That's why it is also available from Data.Function now: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Function.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] HaskellWiki Upgrade Aborted
On Sun, 11 Jan 2009, Duncan Coutts wrote: We really need to upgrade the whole thing. Not an easy job given the range of stuff being run on there by lots of different people. Btw. is there a simple way to download all the Wiki content? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unfoldr [ANN: HLint 1.2]
On Mon, 12 Jan 2009, Andrew Coppin wrote: Off the top of my head, try this: convert b 0 = [] convert b n = n `mod` b : convert b (n `div` b) (Takes a number and yields the radix-B representation of it. Backwards.) convert b = unfoldr (\n - if n 0 then Just (n `mod` b, n `div` b) else Nothing) I have the nice function 'toMaybe' which simplifies this to: unfoldr (\n - toMaybe (n0) (n `mod` b, n `div` b)) Maybe HLint can also suggest non-base functions? ;-) http://hackage.haskell.org/packages/archive/utility-ht/0.0.2/doc/html/Data-Maybe-HT.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monads aren't evil? I think they are.
Apfelmus, Heinrich schrieb: Ertugrul Soeylemez wrote: Let me tell you that usually 90% of my code is monadic and there is really nothing wrong with that. I use especially State monads and StateT transformers very often, because they are convenient and are just a clean combinator frontend to what you would do manually without them: passing state. The insistence on avoiding monads by experienced Haskellers, in particular on avoiding the IO monad, is motivated by the quest for elegance. The IO and other monads make it easy to fall back to imperative programming patterns to get the job done. But do you really need to pass state around? Or is there a more elegant solution, an abstraction that makes everything fall into place automatically? Passing state is a valid implementation tool, but it's not a design principle. I collected some hints, on how to avoid at least the IO monad: http://www.haskell.org/haskellwiki/Avoiding_IO ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Monads aren't evil? I think they are.
Ertugrul Soeylemez schrieb: Apfelmus, Heinrich apfel...@quantentunnel.de wrote: The insistence on avoiding monads by experienced Haskellers, in particular on avoiding the IO monad, is motivated by the quest for elegance. The IO and other monads make it easy to fall back to imperative programming patterns to get the job done. [...] Often, the monadic solution _is_ the elegant solution. Please don't confuse monads with impure operations. I use the monadic properties of lists, often together with monad transformers, to find elegant solutions. As long as you're not abusing monads to program imperatively, I think, they are an excellent and elegant structure. I have seen several libraries where all functions of a monad have the monadic result (), e.g. Binary.Put and other writing functions. This is a clear indicator, that the Monad instance is artificial and was only chosen because of the 'do' notation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HLint 1.2
On Sun, 11 Jan 2009, Neil Mitchell wrote: HLint will automatically detect if you should have used a map, a foldr or a foldl and suggest how to change your code. In the GHC, darcs and Hoogle code bases there are no obvious map-like functions, which is a good sign :-) I found so many 'map' re-implementations in Haskell libraries, even in those, where I thought their programmers must be more experienced than me. Hm, maybe even in libraries by Neil? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Haskell BLAS bindings version 0.7
Patrick Perry schrieb: The reason I want to expose modules but hide the documentation is because there are a lot of unsafeX functions I want to provide access to, but which 99% of users don't care about. The array library does the same thing. Alternatively, the DEPRECATED pragma may prevent programmers from using these functions too extensively. However, this may be considered abuse of this pragma. I think the Unsafe module is the cleanest solution. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Syntactic Sugar (Was: Monads aren't evil)
Peter Verswyvelen schrieb: Related to this issue, I have a question here. I might be wrong, but it seems to me that some Haskellers don't like writing monads (with do notation) or arrows (with proc sugar) because of the fact they have to abandon the typical applicative syntax, which is so close to the beautiful lambda calculus core. Or is it maybe because some people choose monads were the less linear applicative style could be used instead, so the choice of monads is not always appropriate. Haskell is full of little hardcoded syntax extensions: list notation syntactic, list comprehensions, and even operator precedence that reduces the need for parentheses, etc... http://www.haskell.org/haskellwiki/Syntactic_sugar/Cons ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Infix expressions (Was: Monads aren't evil)
Peter Verswyvelen schrieb: Now, for binary operators, Thomas Davie made a nice pair of combinators on Hackage (InfixApplicative) that would allow this to become: h3 x = f x ^(+)^ g x http://www.haskell.org/haskellwiki/Infix_expressions ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] advanced class constraints in Haskell 98?
GHC accepts a class declaration like class Monad (m Maybe) = C m where ... without having any language extension switched on. But it isn't Haskell 98, is it? Hugs 2005 also accepts this. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] advanced class constraints in Haskell 98?
On Fri, 9 Jan 2009, Miguel Mitrofanov wrote: On 8 Jan 2009, at 23:59, Henning Thielemann wrote: GHC accepts a class declaration like class Monad (m Maybe) = C m where ... without having any language extension switched on. But it isn't Haskell 98, is it? It is. From Report: A class assertion has form qtycls tyvar, and indicates the membership of the type tyvar in the class qtycls. A class identifier begins with an uppercase letter. A context consists of zero or more class assertions, and has the general form ( C1 u1, ..., Cn un ) where C1, ..., Cn are class identifiers, and each of the u1, ..., un is either a type variable, or the application of type variable to one or more types. A nice. I jumped into 4.3 and found § § R 32 © ¦ 6 © ¦ 32 ¢ R ¨ © ¥ 7 ¤ ¢¨ 7 5© ¥ ¦ class =where ¢ § § § § ¥ ¦ ¡ 2 § 6 © R© 7 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] advanced class constraints in Haskell 98?
On Thu, 8 Jan 2009, Henning Thielemann wrote: On Fri, 9 Jan 2009, Miguel Mitrofanov wrote: On 8 Jan 2009, at 23:59, Henning Thielemann wrote: GHC accepts a class declaration like class Monad (m Maybe) = C m where ... without having any language extension switched on. But it isn't Haskell 98, is it? It is. From Report: A class assertion has form qtycls tyvar, and indicates the membership of the type tyvar in the class qtycls. A class identifier begins with an uppercase letter. A context consists of zero or more class assertions, and has the general form ( C1 u1, ..., Cn un ) where C1, ..., Cn are class identifiers, and each of the u1, ..., un is either a type variable, or the application of type variable to one or more types. A nice. I jumped into 4.3 and found § § R 32 © ... copying from Haskell 98 report did not only insert rubbish, but also triggered sending the e-mail. I hope it did not more damage ... scontext - simpleclass | (simpleclass_1, ..., simpleclass_n) simpleclass - qtycls tyvar So it must be 'atype' instead of 'tyvar'? Haskell 98 is really mighty.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Hypothetical Haskell job in New York
On Thu, 8 Jan 2009, Manlio Perillo wrote: Personally, I only know http://hpaste.org/, based on Server: HAppS/0.8.4 I'm using a modified HWS for the parallel webs, e.g. the Real Monad Transformer: http://www.haskell.org.monadtransformer.parallelnetz.de/haskellwiki/Category:Monad However, recently a lot of accesses made the server quite unusable. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
On Thu, 8 Jan 2009, Bardur Arantsson wrote: Thanks. For some reason I hadn't thought to use (fromIntegral x)::PortNumber I guess I got stuck on the idea of constructing a PortNum directly and didn't think beyond that. (Maybe PortNum should really be an abstract type to force indirect construction...?) I also think that a Num instance for PortNumber is not a good idea. How shall (+) and (*) be defined? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Staged evaluation, names?
wren ng thornton schrieb:
Every now and then I find myself in the position where I'd like to
define some hairy value as a CAF instead of a literal, but I'd like for
it to be fully evaluated at compile-time rather than postponed until
runtime. It'd be possible to bludgeon the CPP into doing this, but it
seems easier to use an autocannon like Template Haskell to swat this fly.
Is it really necessary to use CPP or TemplateHaskell for this kind of
optimization? Can a pragma help? Maybe {-# INLINE #-} ?
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Re: [Haskell-cafe] advanced class constraints in Haskell 98?
On Fri, 9 Jan 2009, Miguel Mitrofanov wrote: On 9 Jan 2009, at 02:47, Ross Paterson wrote: On Thu, Jan 08, 2009 at 10:27:59PM +0100, Henning Thielemann wrote: A nice. I jumped into 4.3 and found scontext - simpleclass | (simpleclass_1, ..., simpleclass_n) simpleclass - qtycls tyvar So it must be 'atype' instead of 'tyvar'? Haskell 98 is really mighty. Oh. Don't I look silly? You were absolutely right, it's not Haskell 98. Me too. I've looked at the type declaration syntax instead of instance declaration one. Maybe the report is not complete? I mean, the current behaviour of Hugs and GHC (as I observed it) is more consistent, and maybe that's what the designers had in mind. Actually Hugs does reject it without flags. Maybe you have a -98 stored somewhere? When starting, my Hugs tells Haskell 98 mode: Restart with command line option -98 to enable extensions ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Taking Exception to Exceptions
Immanuel Litzroth schrieb: Anyway, there is one more problem I have related to exceptions that is about forcing strictness. It relates to option parsing. I have an option -t that says which track from a file of tracks to print. So I go data Flags = TrackNumber !Int deriving(Read, Show, Eq) makeTrackNumber :: String - Flags makeTrackNumber str = TrackNumber $ read str options = [GetOpt.Option ['t'] [tracknumber] (GetOpt.ReqArg makeTrackNumber tracknumber) number of track to show] Now my main goes main = do args - getArgs opts - evaluate $ GetOpt.getOpt GetOpt.RequireOrder options args print done getting the opts case opts of ... which of course first prints done getting opts and then throws an exception if I give it a flag -t abc. 'evaluate' is strange here. Is it ok to put the print after the case ? But I have probably still not understood the problem. Let me point to two other issues: An elegant way to use GetOpt is: http://www.haskell.org/haskellwiki/High-level_option_handling_with_GetOpt I find it bad style to hide the exceptions, that can be raised. The type of an action should reflect what exceptions are to be expected. You can achieve this with Control.Monad.Exception.Synchronous from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/explicit-exception or Control.Monad.Error from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/transformers For an application, using these techniques, see: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/equal-files ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] advanced class constraints in Haskell 98?
On Fri, 9 Jan 2009, Ross Paterson wrote: On Fri, Jan 09, 2009 at 01:11:12AM +0100, Henning Thielemann wrote: Maybe the report is not complete? I mean, the current behaviour of Hugs and GHC (as I observed it) is more consistent, and maybe that's what the designers had in mind. I'm puzzled by the Hugs behaviour. The current version rejects it, and I downloaded and built the Mar2005 version just to check, and it also rejected it, saying Illegal Haskell 98 class constraint in class declaration You are right. I don't know, what I made different before. Btw. 2005 was the copyright year, Hugs' version is September 2006: __ __ __ __ ___ _ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Bugs: http://hackage.haskell.org/trac/hugs || || Version: September 2006 _ Haskell 98 mode: Restart with command line option -98 to enable extensions ERROR src/Data/Spreadsheet/CharSource.hs:14 - Illegal Haskell 98 class constraint in class declaration *** Constraint : Monad (a Maybe) *** Context: (Monad (a Maybe), Monad (a Identity)) So, since GHC allows this extension without an option - how can I tell Cabal, which extension I'm using? Has it a name anyway? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness
On Wed, 7 Jan 2009, Jan Christiansen wrote: The non least strict definition of (*) is taken from the module Data.Number.Natural which was published by Lennart Augustsson in the numbers package at hackage. I mention this here because I think it clearly shows that it is even hard for experienced Haskell programmers to define least strict functions. very interesting ... It's the third time within about 2 months were I was pointed to a lack of laziness in my own Peano implementation! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness
On Wed, 7 Jan 2009, Jan Christiansen wrote: This is great. I am working on the very same topic for quite a while now. My aim is to develop a tool that tells you whether a function is least strict. My work is based on an idea by Olaf Chitil: http://www.cs.kent.ac.uk/people/staff/oc/ Although it seems to be overkill for a single module - How about a cabalized version on Hackage and a darcs repository? It would simplify using and updating it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness
On Wed, 7 Jan 2009, Henning Thielemann wrote: On Wed, 7 Jan 2009, Jan Christiansen wrote: This is great. I am working on the very same topic for quite a while now. My aim is to develop a tool that tells you whether a function is least strict. My work is based on an idea by Olaf Chitil: http://www.cs.kent.ac.uk/people/staff/oc/ Although it seems to be overkill for a single module - How about a cabalized version on Hackage and a darcs repository? It would simplify using and updating it. ... I meant StrictCheck.hs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Template Haskell question
Jeff Heard schrieb: Alright... I *think* I'm nearly there, but I can't figure out how to derive a class instance using record accessors and updaters... Has this something to do with http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-accessor-template ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Portability of MonadError
On Mon, 5 Jan 2009, Luke Palmer wrote: Oh bother! My new year's resolution: think before I speak. While I do think this is the right answer, it is not the right answer in the status quo. This is because ErrorT e m is only a monad when e is an Error, which Either (and most types) are not. It will be the right answer when fail is factored out of Monad into MonadFail (which will happen someday hopefully), because then the typechecker will verify that the above composition cannot fail. But now I can't think of a good answer. Darn. In the explicit-exception package I omit 'fail' and allow an exception type without constraints.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeInterleaveIO respecting order of actions
On Sat, 3 Jan 2009, Henning Thielemann wrote:
I think I now have general Applicative functionality ...
I hope the following is a proper Monad implementation. In contrast to
Applicative a Writer for sequencing actions does no longer work, instead I
need a State monad.
newtype LazyIO a = LazyIO {runLazyIO :: StateT RunAll IO a}
data RunAll = RunAll
deriving Show
instance Monad LazyIO where
return x = LazyIO $ return x
x = f = LazyIO $
mapStateT unsafeInterleaveIO . runLazyIO . f =
mapStateT unsafeInterleaveIO (runLazyIO x)
instance MonadIO LazyIO where
liftIO m = LazyIO $ StateT $ \RunAll - fmap (\x-(x,RunAll)) m
evalLazyIO :: LazyIO a - IO a
evalLazyIO =
flip evalStateT RunAll . runLazyIO
I'll write some tests and upload it to Hackage.
Thank you for being a patient audience. ;-)
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Re: [Haskell-cafe] unsafeInterleaveIO respecting order of actions
On Thu, 1 Jan 2009, Brandon S. Allbery KF8NH wrote: On 2009 Jan 1, at 16:44, Henning Thielemann wrote: If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? I thought unsafeInterleaveIO and users of it (readFile, hGetContents) didn't guarantee the order of actions relative to independent IO actions (that is, those performed outside the unsafeInterleaveIO) and this was why it is generally disrecommended. For example the recurring situation where people try to readFile f = writeFile . someTransform and the writeFile fails with a file locked exception. Sure, it's dangerous. But for what I want to do, this situation cannot occur. I can come up with a simple example which might be generalized. It simulates what hGetContents does. liftLazy2 :: (a - b - c) - IO a - IO b - IO c liftLazy2 f x y = fmap (\ ~(xr, ~(yr,())) - f xr yr) $ unsafeInterleaveIO $ liftM2 (,) x $ unsafeInterleaveIO $ liftM2 (,) y $ return () test0, test1 :: IO String test0 = liftLazy2 (const) getLine getLine test1 = liftLazy2 (flip const) getLine getLine test0 only requests the first line, test1 expects two lines as user input. However, with liftLazy2 we have only Applicative functionality, not Monad, and it is not composable. For example: fmap (\((x,y),z) - z) $ liftLazy2A (,) (liftLazy2A (,) getLine getLine) getLine This requests only one line, but should three ones. The reason is that the first two getLines are defered even until the last one. Thus, it is not enough that liftLazy2 returns (IO c). Instead it must return (IO (c,(a,(b,() and these pair emulated lists must somehow be combined in order to preserve the order of execution. This looks somehow like a writer monad transformer. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] databases in Haskell type-safety
Gour schrieb: Hi! I'd like to use sqlite3 as application storage in my haskell project... Browsing the available database options in Haskell it seems that: a) HSQL is dead (hackage reports build-failure with 6.8 6.10) No, it is maintained by frede...@ofb.net . I have also contributed Oracle/OCI code a half year ago. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeInterleaveIO respecting order of actions
On Sat, 3 Jan 2009, Henning Thielemann wrote: On Thu, 1 Jan 2009, Brandon S. Allbery KF8NH wrote: On 2009 Jan 1, at 16:44, Henning Thielemann wrote: If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? I thought unsafeInterleaveIO and users of it (readFile, hGetContents) didn't guarantee the order of actions relative to independent IO actions (that is, those performed outside the unsafeInterleaveIO) and this was why it is generally disrecommended. For example the recurring situation where people try to readFile f = writeFile . someTransform and the writeFile fails with a file locked exception. Sure, it's dangerous. But for what I want to do, this situation cannot occur. I can come up with a simple example which might be generalized. It simulates what hGetContents does. liftLazy2 :: (a - b - c) - IO a - IO b - IO c liftLazy2 f x y = fmap (\ ~(xr, ~(yr,())) - f xr yr) $ unsafeInterleaveIO $ liftM2 (,) x $ unsafeInterleaveIO $ liftM2 (,) y $ return () I think I now have general Applicative functionality: apply :: (a - b, ()) - (a,()) - (b,()) apply (f,fs) a = let (a0,as) = case fs of () - a in (f a0, as) lazyIO :: IO a - IO (a,()) lazyIO = unsafeInterleaveIO . fmap (\x - (x,())) liftLazy2 :: (a - b - c) - IO a - IO b - IO c liftLazy2 f x y = liftM2 (\xr yr - fst $ (f,()) `apply` xr `apply` yr) (lazyIO x) (lazyIO y) The () is used to enforce the order of evaluation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Use of abbreviations in Haskell
Miguel Mitrofanov schrieb:
module Element where
import QName
import ...
data Element = Element {name :: QName, attribs :: [Attr], content ::
[Content], line :: Maybe Line}
module Attr where
import QName
import ...
data Attr = Attr {key :: QName, val :: String}
module QName where
import ...
data QName = QName {name :: String, uri :: Maybe String, prefix :: Maybe
String}
module Main where
import qualified QName as Q
import qualified Element as E
... Q.name ... E.name ...
+1 for this style
http://www.haskell.org/haskellwiki/Qualified_names
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Re: [Haskell-cafe] How do we decide on the new logo?
Fritz Ruehr schrieb: Without starting a war on the theory of voting systems, perhaps we should use a system which allows for a certain amount of secondary (etc.) preference to be expressed? Give everyone 10 points and let every voter assign these points to his favorite logos, where it is possible to give more than one of his points to the same logo. (Or give every user one point and let him choose how to divide this into fractions which can be assigned to logos. Or give every voter any number of points he want and scale them to 1 afterwards.) Another question is how to handle logos with variations. I think all logos of one idea should be grouped and considered one object and the favorite variant can be voted on later. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How do we decide on the new logo?
Achim Schneider schrieb: Step 2: Determine the winner by polling preferences, same-level preference (ambivalence) allowed (eg. place 1 for logos C and D, place 2 for A and place 3 for B) We recently had to vote for the new design of our university's website. This was done by asking every voter for an order of preference, with no equal preferences allowed. However, when the maintainer of the voting system was asked, how these answers are processed, he didn't know an answer. I think he finally converted positions to scores and added them. However, I suspect in chosing the scores for each position, he had an essential influence of the outcome of the election. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] unsafeInterleaveIO respecting order of actions
I think I have a very similar problem to the currently discussed WriterT [w] IO is not lazy in reading [w]. I want to defer IO actions, until they are needed, but they shall be executed in order. If I call unsafeInterleaveIO, they can be executed in any order. I understand that hGetContents does not defer the hGetChar operations but instead defers the call of the (:) constructors, thus preserving the order of the hGetChar calls. In the general case this is not so simple, since the result of a monadic block might not be a list and the result of particular actions might not be needed at all, e.g. (). If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANN] Haskell web server + wiki: salvia-0.0.4 + orchid-0.0.6
Judah Jacobson schrieb: On Thu, Jan 1, 2009 at 10:39 AM, Sebastiaan Visser sfvis...@cs.uu.nl wrote: On Jan 1, 2009, at 7:15 PM, Gwern Branwen wrote: On Thu, Jan 1, 2009 at 9:04 AM, Sebastiaan Visser wrote: Happy new year, you all! I'm pleased to announce three new packages on Hackage: ... Miscellaneous comments: 1) You're right about cabal-install versus runhaskell etc. There seems to be a(nother) Control.Exception issue with cabal-install using base-4: src/Network/Orchid/Backend/DarcsBackend.hs:91:29: Couldn't match expected type `IOException' against inferred type `Exception' Expected type: IO (Either IOException String) Inferred type: IO (Either Exception String) In the second argument of `()', namely `(try (U.readFile (repo /+ file)) :: IO (Either IOException String))' In the expression: eitherToMaybe (try (U.readFile (repo /+ file)) :: IO (Either IOException String)) I tried to enable building against both the old and the new Exception libraries by using some preprocessor statements. It seems this attempt failed. You may find the extensible-exceptions package from Hackage to be useful. It provides the new Exeptions API on all versions of ghc since 6.6 (and possibly earlier), so you don't have to deal with any preprocessing or cabal-install issues. Alternatively use the explicit-exception package, which works like IO (Either ...), but is more clean. Btw. although I have not looked into Pandoc, in general I find it more attracting to re-use existing software. If you think, that your document system is better than Pandoc, it would also be nice to have it as a separate package. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANN] Haskell web server + wiki: salvia-0.0.4 + orchid-0.0.6
Sebastiaan Visser schrieb: Happy new year, you all! I'm pleased to announce three new packages on Hackage: * salvia-0.0.4: A lightweight modular web server framework. Is it based on Simon Marlow's HWS, like mohws on Hackage? Does it support HTTPS? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
On Fri, 2 Jan 2009, Achim Schneider wrote: Henning Thielemann schlepp...@henning-thielemann.de wrote: If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? The question is what right order means. Let B1..Bn be some arbitrary IO-Actions. Let A1..An be some arbitrary IO Actions passed to unsafeInterleaveIO You're guaranteed that a) Bk+1 is executed after Bk b) Ak+1 is executed after Ak , all by virtue of the IO Monad. If all Ak's are defered using individual unsafeInterleaveIO's then it is not guaranteed that A[k+1] is executed after A[k]. That's my problem. Check: Prelude fmap snd $ Monad.liftM2 (,) (unsafeInterleaveIO getLine) (unsafeInterleaveIO getLine) If unsafely interleaved actions would be executed in order, then this would first ask you for the first pair member, then for the second one, then echo the second one. Actually it asks only for the second one and prints it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Don't make 'show*' functions
wren ng thornton schrieb: Thomas DuBuisson wrote: Jeff Heard proclaimed: There are multiple distinct reasons people use Show, and this gets confusing. This is exactly what I was getting at. I see four uses being discussed: Indeed, though I think the situation is even worse. It seems to me that there are a number of cross-purposes for the Read+Show classes. The two main questions at stake are: A) Who/what is the audience? Common answers: 1) The human user on the other side of the terminal no 2) The human developer trying to debug their work yes 3) The compiler, a la cutpaste yes 4) A program on the other end of the wire/disk/flux capacitor no B) What is the resolution/detail? Common answers: 1) All the gory details maybe 2) Enough for a human to get the big picture no 3) Enough for a computer to get the right value yes For complex datastructures like Map, IntMap, and Trie there are many details stored in the structure which end users need not or should not know about; but these details are essential to the developers to ensure their code is doing what they think it should be. Hm, yes, there seems to be demand for debug-levels, even different ones for different libraries Proposal 1: Combine Read and Show into a single class, enforcing @a == (read . show) a@ with the intention of capturing A3/B1 or A3/B3. While this may be serviceable for A1 or A2 uses, the intention of the class should be made clear that it is for A3. Presumably some solution should be found for types which can be read or shown but not both. functions - If you have only one field in a record, which is of function type, you may use a dummy 'show' for it, but you cannot define a 'read'. Proposal 2: Clean up Text.PrettyPrint.HughesPJ and market it heavily for covering code-oriented aspects of A1. Other visualizations like charts, graphs, or trees should be relegated elsewhere. ... TeX, HTML, ... Proposal 3: Add a generic Lisp pretty printer function to convert from the output of Proposal 1 towards output like Proposal 2. Dealing with operators makes it trickier than Lisp, but most datastructures lack operators. I'm sure this has already been written in Haskell by Yi enthusiasts, and it should reduce the cost of getting people into using Pretty. Proposal 4: Write a generic function for taking recursive types and printing them as a tree. Users should only need to serialize the here content of each node, leaving it up to the generic function to add spines and adequate spacing between nodes. This is for targeting A2/B2. While it's a time-honored tradition to implement specialized versions of this function to introduce people to recursion, making a standard version for visualizing large datastructures would alleviate some of the burden of what Show should be doing. Proposal 5: Currently a main consumer of Show is the GHCi or Hugs REPL. However, those are used both by end users and by developers, which leads to the elision between A1 and A2. I think, that a user who uses GHCi becomes a developer. For me a user is someone who calls compiled Haskell programs. Provided the previous four proposals are taken to heart, it would be nice if GHCi and Hugs had commands to select which viewing mode (show, prettyShow, lispShow, treeShow) should be used for each type. By default, when available prettyShow should be favored over lispShow which is favored over show; but this behavior could be changed on a type-by-type basis. There are complications here regarding how to recurse for each element of a type, but they seem soluble. Yes it would be nice, if the showing in GHCi could be changed. Such that e.g. matrices are shown as grids. However, this can be currently done by passing the expression to a function which does the required formatting. I remember GSLMatrix had a function like (//) :: Matrix - Precision - IO () which let you write GSLMatrix matrix // 2 /1.00 0.00\ \0.00 1.00/ I would not be surprised, if GHCi can already be configured to attach the (//2) automatically. I always think we need a type class which allows to give type specific options. However, the option type would functionally dependency on the type of the shown value, and thus the type class must be a multiparameter typeclass with functional dependencies. This way you can specify formatting options of the matrix (kind of parentheses) and its elements (precision, exponential style etc.). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Function composition
On Mon, 29 Dec 2008, Martijn van Steenbergen wrote: Conal Elliott wrote: If you think of f (here f=not) as an semantic editor (transformer) of values, then 'result', 'first', and 'second' are semantic editor combinators, which can be composed together arbitrarily. See http://conal.net/blog/semantic-editor-combinators . I recently found out Henning and Luke's Data.Accessor[1], and your editor combinators and their accessors seem very much related. Was one inspired by the other? Martijn. Well spotted - thank you for pointing out this connection! Interestingly since Conal's approach is motivated differently, he also included functions. Those are missing in data-accessor, although Accessor.Container is close to it. The main difference is, that Conal restricts to 'modify' functions, and does not support 'get' like Accessors do. You can 'set' and 'modify' all function values (or all container elements) simultaneously, but you can only 'get' one at a time. Thus an Accessor could only access a single function value, but not all of them simultaneously. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: instance Enum [Char] where ...
Justin Goguen schrieb:
My purpose is to have operations such as [aa..bc] be possible, with its
result being [aa, ab, ac ..snip.. ba, bb, bc]
... what do you want to get, if the lengths of the start and the end
string do not match?
Maybe what you are after is the Ix class:
Prelude Data.Ix.range (('a','a'), ('b','c'))
[('a','a'),('a','b'),('a','c'),('b','a'),('b','b'),('b','c')]
Prelude map (\(x,y) - x:y:[]) $ Data.Ix.range (('a','a'), ('b','c'))
[aa,ab,ac,ba,bb,bc]
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Re: [Haskell-cafe] Gitit - Encoding
On Tue, 30 Dec 2008, Arnaud Bailly wrote: Hello, I have started using Gitit and I am very happy with it and eager to start hacking. I am running into a practical problem: characters encoding. When I edit pages using accented characters (I am french), the accents get mangled when the page come back from server. The raw files are incorrectly encoded. Where Shall I look for fixing this issue ? How about http://hackage.haskell.org/cgi-bin/hackage-scripts/package/encoding ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Function composition
Luke Palmer schrieb: 2008/12/26 Oscar Picasso oscarpica...@gmail.com mailto:oscarpica...@gmail.com Hi, I can write: *Main let yes = not . not *Main :t yes yes :: Bool - Bool But not: *Main let isNotEqual = not . (==) The definition of (.): f . g = \x - f (g x) http://www.haskell.org/haskellwiki/Composing_functions_with_multiple_values ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Maintaining laziness
In case someone cares - after some battles with functions that are less lazy than expected, I have written a tutorial on how to get functions lazy and how to test, whether they are actually lazy: http://www.haskell.org/haskellwiki/Maintaining_laziness I found it especially enlightening, that one can avoid a 'force' function in a lazy parser by making the possibility of a parser failure explicit in its type. I.e. a parser that cannot fail, need no 'force'. (I learnt that in polyparse the 'force' function is hidden in 'apply': http://hackage.haskell.org/packages/archive/polyparse/1.1/doc/html/Text-ParserCombinators-PolyLazy.html#v%3Aapply ) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness
Jake McArthur schrieb: Henning Thielemann wrote: I found it especially enlightening, that one can avoid a 'force' function in a lazy parser by making the possibility of a parser failure explicit in its type. I.e. a parser that cannot fail, need no 'force'. (I learnt that in polyparse the 'force' function is hidden in 'apply': I'm curious, what is the difference between using a force function and using lazy pattern matching? Erm, nothing? Because 'force' is implemented by lazy pattern matching. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness
Miguel Mitrofanov schrieb: Seems useful. BTW, why on earth should inits undefined be undefined instead of []:undefined? I mean, come on, we all know that inits anything starts with []! I don't know the reason, but I have added this example to that article. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Infinite grid
Martijn van Steenbergen schrieb:
Hello,
I would like to construct an infinite two-dimensional grid of nodes,
where a node looks like this:
data Node = Node
{ north :: Node
, east :: Node
, south :: Node
, west :: Node
}
in such a way that for every node n in the grid it doesn't matter how I
travel to n, I always end up in the same memory location for that node.
I suspect another way of saying that is that
let f = f . north . east . south . west in f origin
should run in constant space. I hope this makes the problem clear. :-)
A dungeon game? :-)
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Re: [Haskell-cafe] Don't make 'show*' functions
On Fri, 26 Dec 2008, Thomas DuBuisson wrote: Hello cafe, This is just a small thought, but its been bugging me. We have these things called type classes for a reason (I like to think). When making a new data type 'Data', it is not productive to avoid type classes such as 'Show' and export a 'showData' function. Examples of what I'm talking about include showHtml, showTrie, showInstalledPackageInfo... I know the default derivation (and thus generally accepted) instance of Show isn't pretty, but that just means to me that we need either more methods within the Show type class or start using the prettyclass package more. If the problem is an API issue lets fix Pretty or Show. But this show* stuff should disappear in the long run. I disagree: http://www.haskell.org/haskellwiki/Slim_instance_declaration http://www.haskell.org/pipermail/libraries/2006-September/005791.html There is not much to fix in Show (except the showList issue) since it is for showing Haskell expressions. One could however blame developers of calling pretty printing functions 'show*'. :-)___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Don't make 'show*' functions
On Fri, 26 Dec 2008, Jeff Heard wrote: I don't think that making Show a type class was a mistake. I think that we have long since overloaded the meaning of Show and made it ambiguous. There are multiple distinct reasons people use Show, and this gets confusing. It would be good if we as a community tried to nail down these different meanings that people tend to attach to Show and fork out new type classes that each encompass those meanings. +1 See also http://www.haskell.org/haskellwiki/Show_and_Read_instance ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Hackage packages for indexing by Google
When testing the ShowMeta parallel web: http://hackage.haskell.org.showmeta.parallelnetz.de/packages/archive/pkg-list.html I found, that all HackageDB pages forbid crawling and indexing. Actually, the content of this page is dynamic - but it changes only incrementally. It would be very worth when I search for markov chain haskell with Google, that I will be directed to the according package page of HackageDB. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] understanding enumerator/iteratee
Jason Dusek schrieb: I'm taking a stab at composable streams, starting with cursors. I managed to make a derived cursor today -- as I work through this stuff, I hope to understand Iteratee/Enumerator better. How about a wiki page on the roles of enumerators and iteratees, best explained using a simple example? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] understanding enumerator/iteratee
Jason Dusek schrieb: Henning Thielemann schlepp...@henning-thielemann.de wrote: Jason Dusek schrieb: I'm taking a stab at composable streams, starting with cursors. I managed to make a derived cursor today -- as I work through this stuff, I hope to understand Iteratee/Enumerator better. How about a wiki page on the roles of enumerators and iteratees, best explained using a simple example? At present, I am not totally convinced of Iteratee/Enumerator. Why aren't there any functor instances anywhere? Why do filestreams and lists present distinct interfaces? A stream computation is a stream computation; the effect of pulling an item off the stream and handling it is sequenced in these computations so it seems like a monad transformer is in order. So I am just going to keep trying until I can make that transformer. I have put essentially Oleg's explanation to the Wiki: http://haskell.org/haskellwiki/Enumerator_and_iteratee ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing on multiple criteria
David Menendez schrieb: On Sun, Dec 21, 2008 at 11:20 AM, Jan-Willem Maessen jmaes...@alum.mit.edu wrote: On Dec 21, 2008, at 8:52 AM, Martijn van Steenbergen wrote: Hello all, Data.Ord has a handy function called comparing, and its documentation shows an example of its use. But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this: compareTuple = mconcat [comparing fst, comparing snd] The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!) This is a great example of why it's a bad idea to introduce new functionality with a Monoid instance. Even if you know the instance exists, mappend is so general that it's difficult or impossible to predict what it will do at a given type. There should be an explicit function for combining Ordering values lexicographically, with a note in the documentation saying that it's the basis of the Monoid instance. +1 http://haskell.org/haskellwiki/Slim_instance_declaration ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining a containing function on polymorphic list
Andrew Wagner schrieb: The problem here is even slightly deeper than you might realize. For example, what if you have a list of functions. How do you compare two functions to each other to see if they're equal? There is no good way really to do it! So, not only is == not completely polymorphic, but it CAN'T be. There is a nice solution for this, however, and it's very simple: contain :: Eq a - [a] - Bool contain x [] = False contain x (y:ys) = if x == y then True else contain x ys Would HLint jump in here and suggest: contain x (y:ys) = x == y || contain x ys ? Or even replace 'contain' by 'elem' ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] monad constraint + record update
Brent Yorgey schrieb: On Mon, Dec 22, 2008 at 06:19:07PM +0100, Peter Padawitz wrote: I'd like to define a monad Set for types in the class Eq. But how can the arguments of Set be constrained when Set is defined as an instance of Monad? instance Eq a = Monad Set where ... obviously cannot work. Is there a standard update function for fields in data types, something that OO programmers do with assignments like obj.attr := value ? Peter Note there is already a package on Hackage to do exactly this, rmonad. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rmonad And also http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-accessor ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Fwd: [Haskell-cafe] Haskell as a religion
Alberto G. Corona schrieb: But many features need other features. For example, the option to use referential transparency will be common in future languages for multicore programming purposes. This creates the problem of separating side-effect-free code from side-effect code. In C/C++ referential transparent functions code can be declared by appending a 'const' to the prototype, right? I think that once the average programmer start to use one or two of these features, he will feel a bit frustrated if its language don´t have all the others, specially if he know haskell. Probably, he will use haskell for fun. This is the best way for the takeover of the industry, because this has been so historically. Extrapolating the habit of programmers from the past to the future, I predict that Haskell can only become a mainstream language once there is a cleaner, simpler, safer and more powerful programming language than Haskell. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Time for a new logo? - Haskell logo as a stamp!
On Mon, 15 Dec 2008, Henning Thielemann wrote: On Mon, 15 Dec 2008, Don Stewart wrote: And anyone who does a version, place put it on the wiki. It'll be lost if you only post to the list. I propose we gather submissions and vote on the best for a new logo in 2009. Whatever logo someone prefers: I have written a program using HPDF which creates stamps for the German post (see http://www.internetmarke.de/) with custom images: http://code.haskell.org/~thielema/internetmarke/ Try it out now: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/internetmarke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Time for a new logo?
On Sun, 14 Dec 2008, Don Stewart wrote: I noticed a new haskell logo idea on a tshirt today, http://image.spreadshirt.net/image-server/image/configuration/13215127/producttypecolor/2/type/png Simple, clean and *pure*. Instead of the we got lots going on of the current logo. Call me conservative, but I like the current logo more than the new suggestions. Why isn't it shown big on the welcome page of haskell.org? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Time for a new logo? - Haskell logo as a stamp!
On Mon, 15 Dec 2008, Don Stewart wrote: And anyone who does a version, place put it on the wiki. It'll be lost if you only post to the list. I propose we gather submissions and vote on the best for a new logo in 2009. Whatever logo someone prefers: I have written a program using HPDF which creates stamps for the German post (see http://www.internetmarke.de/) with custom images: http://code.haskell.org/~thielema/internetmarke/ It needs a bit generalization, though, and will then be uploaded to Hackage, of course. So a dedicated Haskell stamp is close to German Haskell users! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] efficient combination of foldl' and foldr - foldl'r
I want to do a foldl' and a foldr in parallel on a list. I assumed it
would be no good idea to run foldl' and foldr separately, because then the
input list must be stored completely between the calls of foldl' and
foldr. I wanted to be clever and implemented a routine which does foldl'
and foldr in one go. But surprisingly, at least in GHCi, my clever routine
is less efficient than the naive one.
Is foldl'rNaive better than I expect, or is foldl'r worse than I hope?
module FoldLR where
import Data.List (foldl', )
import Control.Arrow (first, second, (***), )
foldl'r, foldl'rNaive ::
(b - a - b) - b - (c - d - d) - d - [(a,c)] - (b,d)
foldl'r f b0 g d0 =
first ($b0) .
foldr (\(a,c) ~(k,d) - (\b - k $! f b a, g c d)) (id,d0)
foldl'rNaive f b g d xs =
(foldl' f b *** foldr g d) $ unzip xs
test, testNaive :: (Integer, Char)
test =
second last $ foldl'r (+) 0 (:) $ replicate 100 (1,'a')
{-
*FoldLR test
(100,'a')
(2.65 secs, 237509960 bytes)
-}
testNaive =
second last $ foldl'rNaive (+) 0 (:) $ replicate 100 (1,'a')
{-
*FoldLR testNaive
(100,'a')
(0.50 secs, 141034352 bytes)
-}
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Re: [Haskell-cafe] How to define Show [MyType] ?
Jules Bean schrieb: Dmitri O.Kondratiev wrote: I am trying to define instance Show[MyType] so show (x:xs :: MyType) would return a single string where substrings corresponding to list elements will be separated by \n. This would allow pretty printing of MyType list in several lines instead of one, as default Show does for lists. You're doing it wrong. Show is not for pretty-printing. (+1) Show is for the production of haskell syntax for debugging and copy-pasting into test cases, as well as for use with 'Read'. If you want to pretty print, use a different function name. Maybe related: http://www.haskell.org/haskellwiki/List_instance ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] efficient combination of foldl' and foldr - foldl'r
On Fri, 5 Dec 2008, Ryan Ingram wrote: You're testing the interpreted code, so it's not surprising that the naive version performs better; the interpretive overhead only applies to your bit of glue code. I wanted to avoid the optimizer to do clever things itself. Alternatively, at least compile the module with optimizations before running it in ghci: ryani$ ghc -ddump-simpl -O2 -c foldlr.hs foldlr.core (This gives you functional assembly language to look at for examining code generation) ryani$ ghci foldlr.hs [...] Prelude FoldLR :set +s Prelude FoldLR test (100,'a') (0.39 secs, 70852332 bytes) Prelude FoldLR testNaive (100,'a') (0.42 secs, 105383824 bytes) There is still no clear advantage of foldl'r compared to foldl'rNaive, is it? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal
On Sun, 30 Nov 2008, Duncan Coutts wrote: On Sun, 2008-11-30 at 21:14 +0100, Henning Thielemann wrote: On Sun, 30 Nov 2008, Don Stewart wrote: lemming: Maybe you like to add a pointer in cabal-install.cabal/Homepage field to this page. Good idea. Duncan? After I finished that article, I also found: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall I'm trying to work out what the best thing is to do with the Cabal documentation. Currently it's kind of patchy and spread over about three sites. There's the http://haskell.org/cabal website, the dev wiki and trac and the pages on the main Haskell.org wiki. ... and it's divided into cabal and cabal-install documentation. Though, this separation might also be sensible. I think ideally we'd have all the user documentation on the cabal website, including the docs for cabal-install. Unless people think that's a silly idea and we should just put everything on the haskellwiki system. For me, access to HaskellWiki is the easiest, because I have no access to haskell.org/cabal I think. :-) In general people might like to add personal comments about cabal, which is best done in the wiki. E.g. if you solve a problem you can quickly add that solution to the FAQ. I don't mind having an official static site like haskell.org/cabal which points to a dynamic haskellwiki/Cabal and vice versa, but I think another page at haskell.org/trac is redundant. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combining licences
On Mon, 1 Dec 2008, Emil Axelsson wrote: Or perhaps it's better to put the cell library in its own package? I'm a bit reluctant to do this, because it means that Wired will be essentially useless on its own. It's more the question, whether a Haskell wrapper to the cell library is useful on its own. I assume yes, and thus it sounds like a good idea to make separate package for a cell library wrapper. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal
On Sun, 30 Nov 2008, Don Stewart wrote: *if* .. *might* .. *assuming* .. *potentially* .. *maybe* .. *if*.. You could have built it by now! Source: http://hackage.haskell.org/packages/archive/cabal-install/0.6.0/cabal-install-0.6.0.tar.gz Dependencies that aren't in core: http://hackage.haskell.org/packages/archive/HTTP/3001.1.5/HTTP-3001.1.5.tar.gz http://hackage.haskell.org/packages/archive/zlib/0.5.0.0/zlib-0.5.0.0.tar.gz Note the last one assumes you have zlib the C library installed. This should be straight forward to obtain. I have extended this description and put it to http://haskell.org/haskellwiki/Cabal-Install Maybe you like to add a pointer in cabal-install.cabal/Homepage field to this page. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal
On Sun, 30 Nov 2008, Don Stewart wrote: lemming: Maybe you like to add a pointer in cabal-install.cabal/Homepage field to this page. Good idea. Duncan? After I finished that article, I also found: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Extensible Exceptions
On Sun, 23 Nov 2008, Duncan Coutts wrote: On Sun, 2008-11-23 at 01:40 +0100, Henning Thielemann wrote: On Sat, 22 Nov 2008, Thomas Schilling wrote: It's a pattern match error, implemented by throwing an asynchronous exception. The idea being, that we only have one mechanism (well, an synchronous exceptions, thrown via throwIO). Yes, I know that there's a difference between error and exception, but I would argue that which is which depends on the program. For example, for most programs a pattern match error is a fatal condition, there's no sane recovery from it. OTOH, in a program like GHCi, a pattern match error in an executed statement is an exceptional condition, which we want to catch, so it doesn't kill GHCi. It's completely ok to run something in a sandbox and try to observe errors. But that's debugging and I think there is no need to do this in many places of an application. In general handling errors automatically is not possible, because an error might also be if a program loops infinitely. Thus one should not generally handle an error like an exception. In general I agree. I would advise against explicitly catching such exceptions just in the region where one is expecting them. That seems like bad design. On the other hand top level catch-all handlers that also catch such logic errors sometimes make sense. For example in a Haskell web server where we generate a page dynamically it makes a lot of sense to catch errors in the page-generation function, including pattern match errors, and produce a 500 error code response and log the error message. That's a case, rather like ghci, where some flaw in the program can and should be compartmentalised. There's no attempt to clean up the error but it is a modular system and there is a clear boundary where failures can occur without bringing down the entire system. full acknowledge ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Go Haskell! - array libraries
On Fri, 28 Nov 2008, Simon Marlow wrote: Manuel M T Chakravarty wrote: Claus Reinke: What do those folks working on parallel Haskell arrays think about the sequential Haskell array baseline performance? You won't like the answer. We are not happy with the existing array infrastructure and hence have our own. Roman recently extracted some of it as a standalone package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vector In the longer run, we would like to factor our library into DPH-specific code and general-purpose array library that you can use independent of DPH. So we have two vector libraries, vector and uvector, which have a lot in common - they are both single-dimension array types that support unboxed instances and have list-like operations with fusion. They ought to be unified, really. It's worse: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/storablevector :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Go Haskell!
On Fri, 28 Nov 2008, Johan Tibell wrote: On Fri, Nov 28, 2008 at 10:04 AM, Simon Marlow [EMAIL PROTECTED] wrote: So we have two vector libraries, vector and uvector, which have a lot in common - they are both single-dimension array types that support unboxed instances and have list-like operations with fusion. They ought to be unified, really. Yes please! Could we please have a ByteString style interface with qualified imports instead of using ad-hoc name prefixes/suffixes as a namespacing mechanism if we're going to merge the two libraries? StorableVector is organized this way. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Extensible Exceptions
On Sat, 22 Nov 2008, Thomas Schilling wrote: Be careful, though. This only works if there's a single constructor for your exception type. If there are multiple, you should write it like this: thing_to_try `catch` \(e :: MyErrorType) - case e of MyError1 _ - ..; MyError2 _ - ... If you write `catch` (MyError1 ...) and a MyError2 is thrown, you will get a pattern match error exception. A pattern match exception or pattern match error? I mean, not handling a certain pattern is a programming error not an exceptional condition at runtime. Thus there is no need to throw a pattern match exception which is catched and handled somewhere else. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Extensible Exceptions
On Sat, 22 Nov 2008, Thomas Schilling wrote: It's a pattern match error, implemented by throwing an asynchronous exception. The idea being, that we only have one mechanism (well, an synchronous exceptions, thrown via throwIO). Yes, I know that there's a difference between error and exception, but I would argue that which is which depends on the program. For example, for most programs a pattern match error is a fatal condition, there's no sane recovery from it. OTOH, in a program like GHCi, a pattern match error in an executed statement is an exceptional condition, which we want to catch, so it doesn't kill GHCi. It's completely ok to run something in a sandbox and try to observe errors. But that's debugging and I think there is no need to do this in many places of an application. In general handling errors automatically is not possible, because an error might also be if a program loops infinitely. Thus one should not generally handle an error like an exception. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] varargs zip
On Fri, 21 Nov 2008, Jason Dusek wrote: It came up on IRC last night that there is no generic zip in Haskell. I decided to write one as an example, but it only half works. I think that the ZipList type for Applicative functors is a solution. http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Applicative.html#t%3AZipList ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pattern matching on numbers?
On Tue, 18 Nov 2008, Ryan Ingram wrote: How does this work? fac n = case n of 0 - 1 _ - n * fac (n-1) ghci :t fac fac :: (Num t) = t - t The first line of fac pattern matches on 0. So how does this work over any value of the Num typeclass? I know that the 1 on the rhs of fac are replaced with (fromInteger 1), but what about numeric literals in patterns? Does it turn into a call to (==)? As far as I know, yes. It is even possible to trap into an error on pattern matching this way if fromInteger generates an 'undefined'. Should whatever technique is used be extended to other typeclasses too? It is unusual to do pattern matching on fractions, you mostly need it for recursion on natural numbers. Thus I think the cleanest solution would be to treat natural numbers like strict Peano numbers data PeanoStrict = Zero | Succ !PeanoStrict but with an efficient implementation using GMP integers, maybe using 'views', if they were part of Haskell language. Then you can implement: fac :: Integer - Integer fac Zero = 1 fac n1@(Succ n) = n1 * fac n I would then give up pattern matching on any numeric type. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: OpenGL - OGL
On Sun, 16 Nov 2008, Neal Alexander wrote: Brent Yorgey wrote: --- ANN: OpenGL with extra type safety. Neal Alexander Hopefully the code will be uploaded to Hackage as a separate package soon. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OGL-0.0.0 http://haskell.org/haskellwiki/OGL I'd prefer a more descriptive package name like safe-opengl or so. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What *not* to use Haskell for
Dan Piponi schrieb: Real time audio applications are top of my list of crazy projects I would work on if I had a month spare. I think it might work out nicely. My approach wouldn't be to talk directly to audio hardware from Haskell but instead use a framework like Lava to generate low level code from an embedded DSL. I think that would be a really elegant way to work at a high level and yet have the result execute *faster* than traditionally written C++ code. -- Dan On Tue, Nov 11, 2008 at 7:41 PM, sam lee [EMAIL PROTECTED] wrote: I haven't found multitrack audio recording applications written in Haskell. These are usually written in C++ using Jack audio or ASIO. This probably means that it is not a good idea to write real time audio applications in Haskell at the moment. So, probably avoid writing applications that use a high frequency timer and IO that should be synchronous to the timer in Haskell. I do real time audio processing based on lazy storable vectors, however I do not plan to implement another GUI driven program but want to program audio algorithms and music in Haskell. http://www.haskell.org/haskellwiki/Synthesizer Although I can do some processing in real-time I don't approach the speed of say SuperCollider so far. I must rely on GHC's optimizer and sometimes it does unexpected things. I know that one of Paul Hudak's students is working on something similar, called HasSound. The DSL approach is already implemented for CSound in Haskore (again there is also a not yet published library which encapsulates this functionality) and you can also do real time sound processing by controlling SuperCollider: http://www.haskell.org/haskellwiki/SuperCollider See also: http://www.haskell.org/haskellwiki/Category:Music ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell] The Real Monad Transformer or is Haskell.org hijacked?
On Tue, 3 Apr 2007, Henning Thielemann wrote: It was argued that people avoid Haskell because of terms from Category theory like 'Monad'. This problem can now be solved by a wrapper which presents all the WWW without monads! Start e.g. at http://saxophone.jpberlin.de/MonadTransformer?source=http%3A%2F%2Fwww%2Ehaskell%2Eorg%2Fhaskellwiki%2FCategory%3AMonadlanguage=English Of course the tool is written in Haskell, that is, Haskell helps solving problems which only exist because of Haskell. Bug reports and feature requests can be tracked at https://sourceforge.net/projects/parallelweb The produced content has not changed very much, but the interface is much more friendly now. You can now enter the world of the Real Monad Transformer e.g. at: http://www.haskell.org.monadtransformer.parallelnetz.de/haskellwiki/Category:Monad Good luck! Henning ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
