[Haskell-cafe] what is a difference between existential quantification and polymorhic field?
Hello haskell-cafe, now i'm reading Haskell' proposals and found that these two things considered as different: http://hackage.haskell.org/trac/haskell-prime/wiki/ExistentialQuantification http://hackage.haskell.org/trac/haskell-prime/wiki/PolymorphicComponents can you please explain me what is the difference between data Ex = forall a. Num a = Ex a and data Po = Po (forall a. Num a = a) ? -- Best regards, Bulat mailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web forum
As requested, I continue here the thread on the proposal for a web forum. You will soon enough find out what I think of web forums. Let me first show you what this mailing list looks like to me using software of my choice: ! [ 192: Mark Carroll] [Haskell-cafe] Card trick ! [ 37: Thomas Conway ] Re: [Haskell-cafe] How can we detect and fix E [ 19: Albert Lai ] Re: [Haskell-cafe] Either e Monad E [ 19: Deokhwan Kim] [ 62: Bas van Dijk] E [ 47: [EMAIL PROTECTED] [Haskell-cafe] Re: Optimization problem [ 51: Ross Paterson ] E [ 42: Conor McBride ] [ 56: Robert Dockins ] [ 42: Conor McBride ] [ 61: [EMAIL PROTECTED] [Haskell-cafe] Re: Optimization prob [ 14: Ross Paterson ] [ 154: Jan-Willem Maessen ] Re: [Haskell-cafe] Traversing a graph in STM [ 52: Josef Svenningsson ] [ 18: Bulat Ziganshin ] Re[2]: [Haskell-cafe] Traversing a graph [ 26: Sebastian Sylvan] [ 22: Ashley Yakeley ] [Haskell-cafe] Re: Wiki contact An elaboration of the cool features I have always enjoyed is in order: On browsing threads, both inter-thread jumps and intra-thread navigation, the screenshot speaks for itself. (If for any perverted reason I want the messages listed chronologically rather than well-organized, the software can do it too.) The messages marked with E are messages I have just read. With this mark, they will not show up next time I browse this mailing list again. Take note that I can mark and unmark any subset of messages, unlike certain people's linear-time idea that one single timestamp distinguishes already read from new across the board. (I have always heard that non-linear thinking is superior to linear thinking. I think I believe it for at least this application.) (Is this an example of what is meant when someone said that a web forum requires less technical knowledge, i.e., programmers on a web forum will not need to know about subsets?) (Speaking of which, is the thread-browsing part also an example, i.e., programmers on a web forum will not need to know about trees and forests?) Though the marked messages will not show up next time, they are not thrown away yet. They are kept on my disk for quite a while. (I get to set how many days they stay.) The software offers several ways to show them, but I think the most useful one is this. Suppose I look at apfelmus's message on Optimization problem (two are shown, I'm referring to the first one), and wonder, gee, what is it replying to? To find out, I position myself to that message, then press a button, then the software will show the desired parent message. In fact, the updated screen looks like this (I just include the thread in question): E [ 18: Ross Paterson ] Re: [Haskell-cafe] Re: Optimization problem E [ 47: [EMAIL PROTECTED] [ 51: Ross Paterson ] E [ 42: Conor McBride ] [ 56: Robert Dockins ] [ 42: Conor McBride ] [ 61: [EMAIL PROTECTED] [Haskell-cafe] Re: Optimizatio [ 14: Ross Paterson ] So even if apfelmus's message quotes nothing from Ross's, I can still find out, provided it is recently enough to be still on my disk. Take note that, on a linear-thinking web forum, there could be any number of intervening messages in the same thread between Ross's and apfelmus's, and even though it is true that Ross's is immortalized in the database of the forum, the real question is how to fish for it. (ObRant again about how web forums are friendly to programmers who have no clue about trees.) Here is a feature unrelated to threading, and in fact it is much more fundamental and pervasive (since almost all email software, not just mine, provides this), and in fact it has much to do with pervasive computing too. Here in Canada, in cafes one can get wireless Internet access, but it has to be paid by the minutes. So suppose I want to visit haskell-cafe when I visit a Starbucks cafe, and there are 30 messages I want to read, and I can read them at the rate of half a minute per message. If haskell-cafe were on a web forum, I would end up keeping my wireless connection for 15 minutes. (Could I disconnect and reconnect between messages? First of all that's really a pain. Secondly, let's say I do that, the billing is still such that I connect for 2 seconds and I'm still charged for 1 minute.) But since haskell-cafe is on a mailing list, I can just connect for a few seconds to download the messages, then disconnect and read offline; I'm charged for at most 1 minute. (Alternatively but equivalently, suppose I want to read haskell-cafe while riding subway - underground trains. Here in Toronto the subway doesn't provide wireless Internet access yet, and probably won't for another decade. If haskell-cafe
Re: [Haskell-cafe] what is a difference between existential quantification and polymorhic field?
[apologies to Bulat for the repeat posting] On Thu, Sep 21, 2006 at 12:05:23PM +0400, Bulat Ziganshin wrote: now i'm reading Haskell' proposals and found that these two things considered as different: http://hackage.haskell.org/trac/haskell-prime/wiki/ExistentialQuantification http://hackage.haskell.org/trac/haskell-prime/wiki/PolymorphicComponents can you please explain me what is the difference between data Ex = forall a. Num a = Ex a and data Po = Po (forall a. Num a = a) Consider the types of the constructors: Ex :: forall a. (Num a) = a - Ex Po :: (forall a. (Num a) = a) - Po ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie] How to test this function?
Hello Bruno, Thursday, September 21, 2006, 8:28:23 AM, you wrote: First, how do I fix the identation of the if then else? http://haskell.org/haskellwiki/IO_inside contains a lot of information about using IO monad Second, I want to test this function, without hitting the filesystem. In C++ I would use a istringstream. I couldn't find a function that returns a Handle from a String. http://haskell.org/haskellwiki/Library/Streams although you will need to slightly change your program using v* operations instead of h*, in return you will get ability to use String as Stream: h1 - openFile test ReadMode vGetChar h1 h2 - newStringReader input vGetChar h2 -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with simple embedding of shell in haskell
Hello George, Thursday, September 21, 2006, 7:44:22 AM, you wrote: sh :: String - String - IO String sh cmd = \input - do (stdin, stdout, _, pid) - runInteractiveCommand cmd forkIO $ hPutStr stdin input hClose stdin -- get exit status to prevent zombie accumulation forkIO $ waitForProcess pid return () hGetContents stdout 1. when 'main' finishes, all the threads created by forkIO, silently dies. typically you need to wait until they are finished. ghc libraries don't provide any provision for it, they are too low-level. you can consider writing your own simple wrapper using MVar: myFork action = do v - newEmptyMVar forIO $ action = putMVar v return v finishHim v = takeMVar v which also returns value returned by action. usage: subProcess - myFork $ do-something-here ... finishHim subProcess or you can use some simple libraries which use more developed solutions of this problem: http://freearc.narod.ru/Process.tar.gz (my own) http://www-i2.informatik.rwth-aachen.de/~stolz/Haskell/CA.hs 2. afaiu, you don't want to wait for process finishing before calling getContents? in this case you need to collect all 'pids' returned by myFork function in some list and finish them all before main returns. or, better, develop more complex interface that will allow to kill processes whose output is no more required, in order to help 'head'-like processes also look at http://www.haskell.org/~petersen/haskell/popenhs/popenhs-1.00.0.tar.gz http://www.volker-wysk.de/hsshellscript/dist/hsshellscript-2.5.0.tar.gz and file i attached also, look at Tackling the awkward squad: monadic input/output, concurrency, exceptions, and foreign-language calls in Haskell http://research.microsoft.com/Users/simonpj/papers/marktoberdorf/marktoberdorf.ps.gz paper, which is a definitive source of information about GHC concurrency implementation -- Best regards, Bulatmailto:[EMAIL PROTECTED]Following the release early, release often motto, I am happy to announce version 0.1.0 of HSH, the Haskell shell. You may obtain it with: darcs get --tag 0.1.0 http://darcs.complete.org/hsh Things are still very rough in many ways, but this version already lets you: * Run commands * Pipe things between commands * Pipe command input/output into and out of pure Haskell functions * Pure Haskell functions are as much a first-class citizen as is grep or cat Here is an example session: $ ghci -fglasgow-exts HSH *HSH run $ (ls, [.]) COPYINGHSHHSH.hsTODOannouncements testsrc COPYRIGHT HSH.cabal Makefile _darcs test.hs *HSH run $ (ls, [-l]) -|- (wc, [-l]) 12 *HSH :m +Text.Printf *HSH Text.Printf let countLines = (zipWith (\i line - printf %-5d %s i line) [(1::Int)..])::([String] - [String]) *HSH Text.Printf run $ (ls, [-l]) -|- countLines -|- (grep, [hs$]) 6 -rw-r--r-- 1 jgoerzen jgoerzen 1285 Jun 6 09:43 HSH.hs 11-rw-r--r-- 1 jgoerzen jgoerzen 565 Jun 6 09:43 test.hs *HSH Text.Printf :m +Data.List *HSH Text.Printf Data.List run $ (ls, [-l]) -|- countLines -|- filter (isSuffixOf hs) 6 -rw-r--r-- 1 jgoerzen jgoerzen 1285 Jun 6 09:43 HSH.hs 11-rw-r--r-- 1 jgoerzen jgoerzen 565 Jun 6 09:43 test.hs *HSH Text.Printf Data.List run $ (ls, [-l]) -|- countLines -|- filter (isSuffixOf hs) -|- (tr, [a-z, A-Z]) 6 -RW-R--R-- 1 JGOERZEN JGOERZEN 1285 JUN 6 09:43 HSH.HS 11-RW-R--R-- 1 JGOERZEN JGOERZEN 565 JUN 6 09:43 TEST.HS *HSH Text.Printf Data.List let generator = \(_::String) - unlines . map show $ [1..20] *HSH Text.Printf Data.List generator 1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n12\n13\n14\n15\n16\n17\n18\n19\n20\n *HSH Text.Printf Data.List run $ generator -|- (grep, [1]) 1 10 11 12 13 14 15 16 17 18 19 Future versions will likely simplify syntax to make it easier to write scripts and introduce a sh to hsh converter. I also plan to add pure Haskell tools for some common shell-ish things that one could do in Haskell. -- John___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] what is a difference between existential quantification and polymorhic field?
Hello Ross, Thursday, September 21, 2006, 12:55:40 PM, you wrote: data Ex = forall a. Num a = Ex a and data Po = Po (forall a. Num a = a) Consider the types of the constructors: Ex :: forall a. (Num a) = a - Ex Po :: (forall a. (Num a) = a) - Po sorry, Ross, can you give me a more detailed explanation? it seems that Po argument is existential by itself, but i don't understand that all this mean also, ghc66 adds impredicative polymorphism. how it differs from unqualified existentials? -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell web forum
On Thu, 2006-09-21 at 04:55 -0400, Albert Lai wrote: As requested, I continue here the thread on the proposal for a web forum. You will soon enough find out what I think of web forums. [snip] I don't understand one thing: it looks like web-forums should exclude mailing lists and vice versa, but Bulat just wrote: ...we need now to create web forum. ?? So, why the two cannot co-exist and let users decide which one to use? Gentoo community is nice example of it. Sincerely, Gour signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] what is a difference between existential quantification and polymorhic field?
Hello Bullat, now i'm reading Haskell' proposals and found that these two things considered as different: http://hackage.haskell.org/trac/haskell-prime/wiki/ExistentialQuantification http://hackage.haskell.org/trac/haskell-prime/wiki/PolymorphicComponents can you please explain me what is the difference between data Ex = forall a. Num a = Ex a and data Po = Po (forall a. Num a = a) With existencial types you know what what the type of the thing you are packing is: t = Ex (3 :: Int) and you forget about it once it is packed. However, with polymophic components the following is a type error t = Po ( 3 :: Int) because you are required to provide a polymorphic value (forall a . Num a = a) and you have given it a value Int. However, the following is valid: t1 = Po 3 since (3 :: forall a . Num a = a). So, perhaps an easy way to think about existencials is that they are almost like: data Ex a = Ex a except that the type "a" is lost as soon as you construct such a value. Where does this make a difference? Try the following two definitions: addPo :: Po - Po - Po addPo (Po x) (Po y) = Po (x + y) addEx :: Ex - Ex - Ex addEx (Ex x) (Ex y) = Ex (x + y) The first one works, the second one doesn't. The reason that the first works is because "x" and "y" are polymorphic and thus they can be unified. This is more/less equivallent to: addPo' :: (forall a . Num a = a) - (forall a . Num a = a) - (forall a . Num a = a) addPo' x y = x + y The second does *not* work because when you created the values for the existencials you assumed some concrete types. So, "x" could be an Integer and "y" could be a Float and therefore, you should not be allowed to perform this operation. also, ghc66 adds impredicative polymorphism. how it differs from unqualified existentials? I have not tried ghc66, but I think one of the things you should be able to do and that is perhaps helpful for understanding existencial is: myList :: [forall a . Num a = a] myList = [3 :: Int, 4 :: Float, 6 :: Integer] which in previous versions of GHC would need to be written as: myList :: [Ex] myList = [Ex (3 ::Int), Ex (4 :: Float), Ex (6 :: Integer)] Hope this helps. Cheers, Bruno Oliveira ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell web forum
On 9/21/06, Gour [EMAIL PROTECTED] wrote: So, why the two cannot co-exist and let users decide which one to use? Gentoo community is nice example of it. I was just about to ask the same thing. Clearly, some prefer email and some prefer web forums. This is undoubtably true of new Haskell users with questions, as well. Let those interested in a web forum set one up and run it. Those interested in email can ignore the web forum. The Perl community has a similar setup with the popular Perlmonks web forum, and their hundreds of mailing lists. There are many users who read and post on both, and many who do not. I believe Bulat was just trying to find out if anyone else would like to help set one up, not suggesting that it replace the mailing lists. Kurt ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell web forum
I have only recently started accessing some web fora, but I've noticed that some of those powered by phpBB are vulnerable to spamming, whereas the news groups seem to be less so. For example, the python-forum has nearly lost it's General forum to spammers. Maybe the experts know better engines, better ways to set up a forum, or better ways to administer them after they're up, but it is a concern. -- Bill Wood ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell web forum
Hallo, On 9/21/06, Bill Wood [EMAIL PROTECTED] wrote: I have only recently started accessing some web fora, but I've noticed that some of those powered by phpBB are vulnerable to spamming, whereas the news groups seem to be less so. For example, the python-forum has nearly lost it's General forum to spammers. Maybe the experts know better engines, better ways to set up a forum, or better ways to administer them after they're up, but it is a concern. I only hope this mailing list will continue. I can't stand the slowness/spams/avatar/size of web fora. -- -alex http://www.ventonegro.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell web forum
Kurt Hutchinson wrote: Let those interested in a web forum set one up and run it. Those interested in email can ignore the web forum. My concern about introducing a web forum would be that it is yet another place I have to search every time I need information (besides the haskell report, compiler docs and tracker, 2 wikis and the mailing lists :) So setting up a web forum would only be good if it can do something a mailing list cannot do. Following things were mentioned before: - ease of starting new topics. Maybe. I personally end up searching through all topics in forums anyway, because people often tend to post in a wrong topic :) Christian Neumann before mentioned that mailman might support topics as well. Also IMHO the current division of mailing lists (general, café, libraries, etc.) is exactly right for organizing and separating information. - searchability. I wouldn't agree - I can download the whole contents of a mailing list from gmane and search it in my mail client - goes way faster than in a forum. - ease of access. But preventing spam would probably require subscription just as for a mailing list. Any more advantages of a forum? Cheers, Misha ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] what is a difference between existential quantification and polymorhic field?
Hello Bullat, also, ghc66 adds impredicative polymorphism. how it differs from unqualified existentials? I have not tried ghc66, but I think one of the things you should be able to do and that is perhaps helpful for understanding existencial is: myList :: [forall a . Num a = a] myList = [3 :: Int, 4 :: Float, 6 :: Integer] which in previous versions of GHC would need to be written as: myList :: [Ex] myList = [Ex (3 ::Int), Ex (4 :: Float), Ex (6 :: Integer)] I took a look at the documentation and I think I told you the wrong thing here. I think this should be equivallent to: myList :: [Po] myList = [Po 3, Po 4, Po 6] which, in this case wouldn't be too useful (would it?). Having structures with polymorphic components is more useful when you have functions. The example they give is: f :: Maybe (forall a. [a] - [a]) - Maybe ([Int], [Char]) f (Just g) = Just (g [3], g hello) f Nothing = Nothing However, the following (which would be the right way to express my example) would be handy to have as well: myList :: [exists a . Num a = a] myList = [3 :: Int, 4 :: Float, 6 :: Integer] but I don't think this is available in GHC 6.6. Can anyone confirm this? Cheers, Bruno Oliveira ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [newbie] How to test this function?
On Thu, 21 Sep 2006 01:52:38 -0300, Donald Bruce Stewart [EMAIL PROTECTED] wrote: First, how do I fix the identation of the if then else? getList = find 5 where find 0 = return [] find n = do ch - getChar if ch `elem` ['a'..'e'] then do tl - find (n-1) return (ch : tl) else find n OK. Thanks. I didn't find that one because it's not offered as an identation option in emacs haskell mode. Second, I want to test this function, without hitting the filesystem. In C++ I would use a istringstream. I couldn't find a function that returns a Handle from a String. The closer thing that may work that I could find was making a pipe and convertind the file descriptor. Can I simplify that function to take it out of the IO monad? How? I thought about getContents, but that eats all input. Refactor! The reason your getList is hard to test, is that you're mixing side effecting monadic code with pure computations. Let's untangle that, and then test the the referentially transparent parts simply with QuickCheck. And remember that since getContents uses lazy IO, it only eats as much input as you ask it to. So let's refactor this, partitioning off the side effecting IO code: getList :: IO [Char] getList = take5 `fmap` getContents -- a thin IO skin take5 :: [Char] - [Char] take5 = take 5 . filter (`elem` ['a'..'e']) -- the actual worker The problem is that getContents 'eats' stdin, which can't be used again. I'm forced to change all my IO code to the getContents way, which is not possible. About QuickCheck: I'm going to check it out. Thanks for the pointers. Bruno ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Re: [newbie] How to test this function?
On Thu, 21 Sep 2006 11:20:33 -0300, Henning Thielemann [EMAIL PROTECTED] wrote: On Thu, 21 Sep 2006, Bruno Martínez wrote: getList :: IO [Char] getList = take5 `fmap` getContents -- a thin IO skin take5 :: [Char] - [Char] take5 = take 5 . filter (`elem` ['a'..'e']) -- the actual worker The problem is that getContents 'eats' stdin, which can't be used again. You can process the result of getContents as many times as you want. But I cannot process input again without getContents. This function's use of getContents forces me to use only getContents everywhere. For example, I can't use hWaitForInput in another IO action. I'm forced to change all my IO code to the getContents way, which is not possible. If your program contains so much IO code you should seriously consider refactoring. I try to minimize the amount of IO I do, but there's a point I don't know how to strip my functions of their remaining IOness. That's why I ask :) Bruno ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie] How to test this function?
On Thu, 21 Sep 2006 05:30:34 -0300, Bulat Ziganshin [EMAIL PROTECTED] wrote: Hello Bruno, Thursday, September 21, 2006, 8:28:23 AM, you wrote: First, how do I fix the identation of the if then else? http://haskell.org/haskellwiki/IO_inside contains a lot of information about using IO monad That page is very good. Second, I want to test this function, without hitting the filesystem. In C++ I would use a istringstream. I couldn't find a function that returns a Handle from a String. http://haskell.org/haskellwiki/Library/Streams I'll give this a try. Thanks, Bruno Conectese mas rapido y ahorre hasta un 50% Tel. 0909.2030 = $0,15 IVA incluido el minuto __ http://www.internet.com.uy - En Uruguay somos internet ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell web forum
On 21/09/06, Misha Aizatulin [EMAIL PROTECTED] wrote: My concern about introducing a web forum would be that it is yet another place I have to search every time I need information (besides the haskell report, compiler docs and tracker, 2 wikis and the mailing lists :) So let's have a single unified search box. This is what was done on wordpress.org -- a single glorified interface searched the wiki, support forums and bug tracker. Everything's currently hosted by the umbrella domain name haskell.org, so all the information's at least theoretically in the same place, so getting the various tools interoperating with one another shouldn't be impossible. +1 for the introduction of forums. -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Non-existant existential?
Consider the following: data SimpExist a = SimpExist (forall x . x - a) f :: SimpExist Bool f = SimpExist (const True) g = SimpExist id What is the type of g? In a similar example, GHC tells me it is of type SimpExist c. Yet, I can't unify it with any other SimpExist c'. It seems to me that this is something like exists x . SimpExist x, and is similar to: data ExistWrap = forall a . ExistWrap (forall x . x - a) Jim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Trying to write a TCP proxy
On 20/09/06, John Ky [EMAIL PROTECTED] wrote: So Monads don't actually eliminate laziness? Monads and laziness are completely orthagonal issues; they have nothing to do with one another. I think you're getting confused with purity. Monads can be used to encapsulate effects, to allow you to write code as if the code had side effects, while your program remains technically pure. -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell web forum
On 9/21/06, Misha Aizatulin [EMAIL PROTECTED] wrote: My concern about introducing a web forum would be that it is yet another place I have to search every time I need information (besides the haskell report, compiler docs and tracker, 2 wikis and the mailing lists :) That probably speaks to an effort for better organization of the information rather than banning a way to create more. So setting up a web forum would only be good if it can do something a mailing list cannot do. Well there's one thing that a web forum can do that a mailing list can never do, which is provide a mechanism for those who like web forums better than mailing lists. How important that is really depends on how many people would rather use a web forum than a mailing list. Many of the responses so far have decried web forums in preference to mailing lists, but this is, after all, a mailing list. - searchability. I wouldn't agree - I can download the whole contents of a mailing list from gmane and search it in my mail client - goes way faster than in a forum. A similar mechanism could be implemented for a web forum. The data is just sitting in a database, after all. An format for easy downloading and searching would probably not be difficult. I personally don't care either way. I'm happy with the mailing list. But I know of many people that do prefer web forums, and they seem to be perceived as a lower barrier to entry, even if that isn't actually true. I would probably read both if both existed. Kurt ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Non-existant existential?
Hello, Consider the following: data SimpExist a = SimpExist (forall x . x - a) f :: SimpExist Bool f = SimpExist (const True) g = SimpExist id What is the type of g? In a similar example, GHC tells me it is of type SimpExist c. Yet, I can't unify it with any other SimpExist c'. Have you tried to type check this example (the g)? It does not type check in my GHC. There are not many functions with the type forall x . x - a when a is also polymorphic --- it is the type of unsafeCoerce. It seems to me that this is something like exists x . SimpExist x, and is similar to: data ExistWrap = forall a . ExistWrap (forall x . x - a) Sure, you should read a forall on the left side of a constructor as exists. Look at section 7.4.1.4.1 of: http://www.haskell.org/ghc/docs/6.4.2/html/users_guide/type-extensions.html The design decision here was to avoid introducing a new exist construct. I believe that the justification for this option is that in Logic a forall in a contravariant position has the same effect as an existential. Cheers, Bruno ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web forum
Moved to haskell-cafe from haskell-general. On 2006-09-20, Niklas Broberg [EMAIL PROTECTED] wrote: On 9/20/06, Aaron Denney [EMAIL PROTECTED] wrote: And I disagree with you. Web forums are usenet reinvented poorly. It's impossible to keep track of what's new, threading is either poor or nonexistent. Mailing lists with searchable archives work well. gmane provides a nice usenet interface to mailing lists. I don't recognize the forums I frequent in your description at all. I have absolutely no problem keeping up with what's new. In fact I find it a lot easier since many of the topics take place in subforums that I know don't interest me, so I don't even need to go there to check. And half of the time messages are miscategorized, because conversations are miscategorized, so I do need to go check. With this mailing list, I have to manually mark as read about 2/3 of all incoming mails because they don't (from the title) interest me, and I have to read a few that don't interest me because I couldn't tell from the title. Where as an entire page of messages under a particular thread (it's not a thread, because they almost never have the messages showing the exact post they're responding to, just the general list of messages in this topic.) Then I need to go locate the tiny link that gets me to the next page. They're nearly unuseable. Threading also works as well as could be expected, better than threading in gmail for instance, so I don't see the problem here either. That has never been my experience. Threads branch annd mutate and drift. And this is a good thing. The rigid categorizations can't handle this, whereas I can easily ignore entire threads and subthreads with a client designed for it. It's all there, all the time. Yes, that's part of the problem. How is that? You mean because all the old stuff gets in the way of the new? Then you're just using a bad forum software that can't properly point out the new stuff for you. I agree that not all forums are good, but there are definitely those that are. Exactly. but the problem with forums is that everynone must use the same forum software because it's sitting on a website. With usenet or mailing lists, everyone gets their own software that provides the features they want, with the interface they want. It is also easy to create sub-groups/forums for specific projects, This is the one semi-useful thing. Of course, what you end up with is another not-so-useful forum. This is probably the one most useful thing yes. I wouldn't call it semi-useful though. Just remember all the responses that the HCAR gets each year of the form wow how many cool projects are out there that I had no clue existed. What if all (or many of) those projects were actually there, on the forums, where everyone knew where to look? Forums are another place to look, instead of on the wiki, on the mailing lists, etc. It fragments the community. And your last sentence, I just don't understand. not-so-useful just because it's a forum, or did you mean somethine else as well? Yes, because it's a forum. Almost any feature can be implemented in a web-forum. And many will, but almost none will be done well, because (a) it's hard to get something that will please everyone and it has to please everyone because everyone must use the same software (b) The experience isn't there. Mail usenet software has been worked on for over 25 years. It works. Your mail client sucks? Get a new one. There are lots, and one should surely work for you. I don't have the option of using different web-forum software when something about it annoys the hell out of me. The biggest thing I see for them is web accessibility. But we already have that. We have archives. We have gmane. Tons of other gatewaying software exist, and could be easily set up, because mailing lists are designed to be used by multiple clients. Web forums are only pull. I have to go check each individual one, on a semi-regular basis. Yes, there is RSS. It's another hacky workaround for something that mail inherently has. Mailiing lists can be either pull or push. I can dump the list mail in my inbox and get it regularly, or segregate it out to another mailbox, or several mailboxes, or whatever. -- Aaron Denney -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [newbie] How to test this function?
Bruno Martínez wrote: On Thu, 21 Sep 2006 01:52:38 -0300, Donald Bruce Stewart [EMAIL PROTECTED] wrote: First, how do I fix the identation of the if then else? getList = find 5 where find 0 = return [] find n = do ch - getChar if ch `elem` ['a'..'e'] then do tl - find (n-1) return (ch : tl) else find n OK. Thanks. I didn't find that one because it's not offered as an identation option in emacs haskell mode. Emacs is evil! It also inserts random tab characters into your code just to save a few space bytes. Tends to completely trash indentation e.g. when pasting code into mails etc. Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Re: Haskell web forum
I'll just add myself onto the list of webforum haters. I find gmane over a news reader much more comfortable that any webforums I have used. Perhaps gmane can be used over a web interface? Then those that want to use a web forum have one, and I can continue to use my news reader. When I want to read the lists when I am away from the newsreader then I use www.netvibes.com. A very nice web interface for reading in my opinion. Alas no interface for posting. Rene. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Re[2]: what is a difference between existential quantification and polymorhic field?
Bulat Ziganshin wrote: Hello Ross, Thursday, September 21, 2006, 12:55:40 PM, you wrote: data Ex = forall a. Num a = Ex a and data Po = Po (forall a. Num a = a) Consider the types of the constructors: Ex :: forall a. (Num a) = a - Ex Po :: (forall a. (Num a) = a) - Po sorry, Ross, can you give me a more detailed explanation? it seems that Po argument is existential by itself, The Po argument is a polymorphic function, not an existential. As I understand it, the constructor Ex _is_ a polymorphic function, whereas the constructor Po (only) _takes_as_argument_ a polymorphic function. In the latter case you store a polymorphic function inside your data, whereas in the former case you construct (monomorphic) data in a polymorphic way, which means that during the process you loose the concrete type of the argument. Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Re: Haskell web forum
On 9/21/06, Rene de Visser [EMAIL PROTECTED] wrote: I'll just add myself onto the list of webforum haters. I find gmane over a news reader much more comfortable that any webforums I have used. Just to be clear. Is it worth debating which is better? Some people prefer one over the other, fine. The original topic was do we need a web forum, not which do you personally prefer. I think it's quite clear that web forums are more used than mailing lists by the general public (perhaps not by academics and professionals, though), so it certainly wouldn't hurt to have one -- regardless of any personal preferences people may have. I do think we should try to do better than the usual suspects that are available though. Maybe even write something in Haskell? Are we allowed to install basically anything we want on the server, or is it restricted somehow? /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Re: Haskell web forum
On 21/09/06, Sebastian Sylvan [EMAIL PROTECTED] wrote: I do think we should try to do better than the usual suspects that are available though. Maybe even write something in Haskell? This sounds like a good test of maturity for Haskell web platforms. I'd love to be involved in a collaboration for this, should one be drummed together. I asked the #haskell IRC channel whether such software already existed: shapr dmhouse: The comment plugin for Hope would be easily hacked into forum software. shapr You'd only need to change it so that a top level message would be a comment that's not attached to anything. Although if the design requires it, it might be easier to write one from scratch. -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] [newbie] How to test this function?
Hello Bruno, Thursday, September 21, 2006, 6:16:49 PM, you wrote: Second, I want to test this function, without hitting the filesystem. In C++ I would use a istringstream. I couldn't find a function that returns a Handle from a String. http://haskell.org/haskellwiki/Library/Streams I'll give this a try. btw, although this may be considered as bug, it's possible to use Streams to combine getContents and other I/O on the same stream: h - openFD test ReadMode str - vGetContents h print (take 5 str) c - vGetChar h print с print (take 5 str) :) so it's possible to use Streams to simplify your program. of course, you should understand how it works in order to use it in safe manner and that means that you should understand how unsafeInterleaveIO works. look at my today message in haskell list and expalanation of unsafePerformIO in IO inside -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [newbie] How to test this function?
On 21/09/06, Benjamin Franksen [EMAIL PROTECTED] wrote: Emacs is evil! I'll ignore the throwaway flaimbait there ;) It also inserts random tab characters into your code just to save a few space bytes. Tends to completely trash indentation e.g. when pasting code into mails etc. Not once happened to me when writing Haskell code. It does annoy you when writing Lisp, but it takes a single line of config to turn it off. -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with simple embedding of shell in haskell
On Thu, 21 Sep 2006, George Brewster wrote: I'm just tried writing a function to allow convenient embedding of shell commands, but I'm running into behavior I don't really understand somewhere at the intersection of lazy evaluation, IO, and threading. You may also find some unwelcome surprises in the area of pipes and buffered I/O, that aren't specifically about Haskell. Are you on a UNIX platform? I rewrote your function by removing both instances of forkIO, and it worked about like I expected. (The last one encounters an error broken pipe when it tries to write there to the echo hi shell process, because that process exits instead of reading from its input.) I can't say whether you really need forkIO, or whether it's really going to do what you need - not only do I not know enough about the thread model, neither do I know what you're really trying to do. Donn Cave, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Non-existant existential?
On 9/21/06, Bruno Oliveira [EMAIL PROTECTED] wrote: Have you tried to type check this example (the g)? No. Please excuse me, as I wasn't by my GHC at the time. Let's try: data SimpExist a = Base a | SimpExist (SimpExist (forall x . x - a)) g :: SimpExist (forall a . a - a) g = Base id h = SimpExist g data WrapExist = forall a . WrapExist (SimpExist a) i = WrapExist h I'm familiar with the use for forall to mean exists, but I am baffled by h's ineffable type! Jim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Polymorphism/monomorphism (and the restriction)
Hi, I think the following might help a little in understanding the monomorphic restriction (which I don't fully understand myself). I'm a bit of a newbie so apologies in advance if I've made a mistake or if my description isn't as useful to others as it seems to me. I've been following a thread on haskell@haskell.org and I think the below might help. I used GHCi, version 6.2.2 (it fails in hugs but that seems to be because of hugs non-compliance with the standard in this case). First off, I'm guessing that I'm getting Haskell98 behaviour here and not some GHCi extension. Please tell me if this is not the case. Run the code listing at the bottom of this email and you should get the output which I've also listed below. This code experiments with Int, Float and (Num a) = a, and I tried to print x*2 and x/2 for each. (4::Int)/2 isn't allowed because / isn't defined for Ints. You can see that kN :: (Num a) = a took two different types depending on what method ( / or * ) was applied to it. kN / 2 = 2.0 kN * 2 = 8 kN/2 is a Float (it can't use Int as / isn't defined for Int, so it uses Float, for which / is defined). kN*2 is an Int. The above outputs demonstrates polymorphism, doesn't it? i.e. Not only has the compiler got a variety of types to choose from, but a variety of types can be used at runtime? The output for kI and kF is obvious. The interesting thing is that k behaves as a Float in both cases. This is monomorphism isn't it? i.e. the compiler may have a variety of types to choose from, but it picks one and sticks to it for every usage. In summary, k didn't give the same outputs as kN. And the monomorphism restriction is a rule which means that sometimes things are forced to a monomorphic type (like k as Float here) when it could have given it a polymorphic type like kN :: (Num a) = a I'm fairly new to these lists, so apologies if I'm covering old ground again. My first aim is to understand exactly what polymorphism and monomorphism is and demonstrate corresponding results, before thinking about the restriction. Thanks, Aaron -- The code kI :: Int kI = 4 kF :: Float kF = 4 kN :: (Num a) = a kN = 4 k = 4 main = do p kI * 2 $ kI * 2 p kF / 2 $ kF / 2 p kF * 2 $ kF * 2 p kN / 2 $ kN / 2 p kN * 2 $ kN * 2 p k / 2 $ k / 2 p k * 2 $ k * 2 p :: (Show a) = String - a - IO () p s = putStrLn.(s++).( = ++).show -- the output - remember kI / 2 is not possible. kI * 2 = 8 kF / 2 = 2.0 kF * 2 = 8.0 kN / 2 = 2.0 kN * 2 = 8 k / 2 = 2.0 k * 2 = 8.0 -- PS: If you delete the k / 2 line from the program, then k * 2 becomes simply 8 (not 8.0). It uses Int if possible, and Float if that's not available. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Non-existant existential?
When I look at the generated core, I see that both h and same = Base undefined have the same type: %forall a . main:Fonly.SimpExist a I'm using GHC 6.5.20060819. If this is a bug, I actually find it kind of useful, for reasons I can elaborate later. Jim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Trouble with simple embedding of shell in haskell
Donn Cave wrote: On Thu, 21 Sep 2006, George Brewster wrote: I'm just tried writing a function to allow convenient embedding of shell commands, but I'm running into behavior I don't really understand somewhere at the intersection of lazy evaluation, IO, and threading. You may also find some unwelcome surprises in the area of pipes and buffered I/O, that aren't specifically about Haskell. Are you on a UNIX platform? I rewrote your function by removing both instances of forkIO, and it worked about like I expected. (The last one encounters an error broken pipe when it tries to write there to the echo hi shell process, because that process exits instead of reading from its input.) I can't say whether you really need forkIO, or whether it's really going to do what you need - not only do I not know enough about the thread model, neither do I know what you're really trying to do. Donn Cave, [EMAIL PROTECTED] Yup, I'm on linux. My goal is to have a function which gives me a lazy string which is the output of a shell command, and takes a string as input. In particular, I'd like the shell command to behave lazily (read only as much input as is needed, and write only as much output as needed), so things like this work (should terminate and print a few lines of hi): main = sh yes hi there = sh head = putStrLn I tried your suggestion out (remove forkIO), and when I do that, this example returns but doesn't print anything. Interestingly, my original implementation usually behaves this way also, but occasionally works as I would like. I had forked a seperate thread for writing the input string to the command so that the writing wouldn't block if the process filled up its write buffer and read buffer before we were done writing the string to it -- this doesn't seem to really have worked for me though. For reference, here is the funciton in question: sh :: String - String - IO String sh cmd = \input - do (stdin, stdout, _, pid) - runInteractiveCommand cmd forkIO $ hPutStr stdin input hClose stdin forkIO $ waitForProcess pid return () hGetContents stdout -George ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [newbie] How to test this function?
On Thu, 21 Sep 2006 15:12:07 -0300, Benjamin Franksen [EMAIL PROTECTED] wrote: OK. Thanks. I didn't find that one because it's not offered as an identation option in emacs haskell mode. Emacs is evil! I'm open to alternatives. I use Windows, so went out of the way to have emacs. What do you use? I don't care much for colors, but automatic identation is very handy (when it works :D). Bruno ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Trouble with simple embedding of shell in haskell
On Thu, 21 Sep 2006, George Brewster wrote: ... Yup, I'm on linux. My goal is to have a function which gives me a lazy string which is the output of a shell command, and takes a string as input. In particular, I'd like the shell command to behave lazily (read only as much input as is needed, and write only as much output as needed), so things like this work (should terminate and print a few lines of hi): main = sh yes hi there = sh head = putStrLn Good luck! I tried your suggestion out (remove forkIO), and when I do that, this example returns but doesn't print anything. For me, it hangs - since I left the wait in. If I omit the wait, it's broken pipe, I think because we're trying to write data to head after it exits. Donn Cave, [EMAIL PROTECTED] sh :: String - String - IO String sh cmd = \input - do (stdin, stdout, _, pid) - runInteractiveCommand cmd forkIO $ hPutStr stdin input hClose stdin forkIO $ waitForProcess pid return () hGetContents stdout ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
