February 2012 - Haskell-Cafe

Re: [Haskell-cafe] Records in Haskell
by Evan Laforge 02 Mar '12

02 Mar '12

[ ccing the list because the wiki page was flawed and I made a bunch of changes, hope you don't mind ] > Thanks Evan, but I think that wiki page isn't doing your > proposal justice. There seem to be several typos in critical > places that make it hard to follow (for me at least). Sorry about the sloppy editing. I updated it and added more detailed examples. I also realized that as stated it didn't quite work for lens updates, so I extended it a little. > I think it would really help to include a record decl. to > show where `a` comes from, especially since you say that > record syntax doesn't change. Good point, added. > Could you explain what the TH does. Perhaps give an example > of what gets generated from a record decl? Sure, I added an example of that too. > And perhaps you could explain what you mean by a "type > directed function"? Aren't all overloaded functions type > directed? Can I have both a `#f` and a `f` in scope? What's > the difference? The idea is they're not overloaded functions. #f is not a symbol that can be in scope, it's special syntax to go *find* a `f` in some module. So `x = #f` is not in conflict with `x = f` provided #f is desugared to SomeModule.f. Unless of course the argument is defined in the current module, in which case it will desugar to plain `f` and then they will be the same. Since it's syntax for resolving a name, assigning it like `#f = xyz` doesn't make sense since you can't put a qualified name on the left of an `=` sign.

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Haskell showcase in 5 minutes
by Arnaud Bailly 01 Mar '12

01 Mar '12

Hello Cafe, I will be (re)presenting Haskell in a "Batlle Language" event Wednesday evening: A fun and interactive contest where various programming language champions try to attract as much followers as possible in 5 minutes. Having successfully experimented the power of live coding in a recent Haskell introduction for the Paris Scala User Group, I would like to do the same but given the time frame I need a simpler example than the music synthesizer program. So I would like to tap in the collective wisdom looking for some concise, eye-opening, mind-shaking and if possible fun example of what one can achieve in Haskell. Things that sprung to my mind are rather dull: prime factors, fibonacci numbers. Thanks in advance, Arnaud

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question about conduit source
by Alexander V Vershilov 29 Feb '12

29 Feb '12

Hello, cafe. Is it possible to read data from different concurrent sources, i.e. read data from source as soon as it become avaliable, e.g. runResourceT $ (source1 stdin $= CL.map Left) >=> (source2 handle $= CL.map Right) $= application $$ sink where >=> - stands for concurrent combining of sources It would be good if it can be sources of different types (handle or STM channel, etc..). Currently I've found no good way to handle with this situation, except of using STM Channels for collecting data source1 ---+ | | sink | output sink +---] Channel [-------> application----->] | source source2 ---+ | From this point of view application takes concurent data, but this implementation requires additional thread per data processing. Also in many cases it will require run additional runResourceT (see later example). So if there any possible simplifications? Or ideas how to make (>=>) operator. Example: So I've got next code in my network-conduit based application: main :: IO () main = do pool <- createDBPool "..." 10 let r = ServerInit pool forkIO $ forever clientConsole --read channel list and send "Left" flip runReaderT r $ runTCPServer (ServerSettings 3500 Nothing) (protoServer) myServer src sink = do ch <- liftIO $ atomically $ newTBMChan 16 initState <- lift $ ask _ <- liftIO $ fork . (flip runReaderT initState) $ runResourceT $ src $= C.sequence decode $= CL.map Right $$ sinkTBMChan ch sourceTBMChan ch $= process $= C.sequence encode $$ sinkHandle stdout But in this situation I don't know if freeing of all resources are guaranteed, because I'm running additional resourceT in main resourceT scope. So can you advice is it possible to make concurrent sources now with currenly implemented library? If it's not possible but worth of implementing, so I can make that functions? Is it correct to runResourceT inside another resourceT? -- Best regards, Alexander V Vershilov

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Use Template Haskell recover inside the IO monad
by Federico Mastellone 29 Feb '12

29 Feb '12

I'm new to Template Haskell and just started playing with it. I have Haskell platform 2011.4.0 and I get this error when I try to run this simple code. main = do code <- runQ $ recover (return []) th putStrLn (pprint code) putStrLn (show code) th :: Q [Dec] th = [d| sum a b = a + b |] Template Haskell error: Can't do `recover' in the IO monad POC.hs: user error (Template Haskell failure) How can I get declarations with the "recover" function so I can print a custom error message using "location"? If I can't do it inside the IO monad, how can I do a program to pretty print Haskell code? Thanks!

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Re: [Haskell-cafe] GSoc-2012: Lock-free Data Structures
by Sergiu Ivanov 29 Feb '12

29 Feb '12

Hello, On Tue, Feb 21, 2012 at 8:11 AM, Ryan Newton <rrnewton(a)gmail.com> wrote: > > Oops, I've attached the PDF. It's a good overview that includes a bunch of > the key ideas that appear in recent work in the area. Very cool article, thank you! I genuinely enjoyed reading it :-) > Absolutely! Feel free to make a pull request. I'm going to play with the existing structures first. However, I can't see a list of issues anywhere. Could you suggest a place where I could look for indications of some issues which I could fix for starters? Sergiu P.S. Sorry for my sluggishness :-(

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Vim plugin for ghc-mod
by Kazu Yamamoto 29 Feb '12

29 Feb '12

Hello, eagletmt implemented a Vim plugin for ghc-mod: https://github.com/eagletmt/ghcmod-vim Happy Haskell programming on Vim! --Kazu

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Haskell showcase in 5 minutes
by Doug McIlroy 29 Feb '12

29 Feb '12

Sorry, a typo in the url for the power-series example. It should have been http://www.cs.dartmouth.edu/~doug/powser.html

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Haskell showcase in 5 minutes
by Doug McIlroy 29 Feb '12

29 Feb '12

Here's an example that fits comfortably in 5 minutes--if your audience knows elementary calculus: http://www.cs.dartmouth.edu/~doug/powswer.html It depends critically on lazy evaluation, which knocks out a lot of competing languages right from the start. The five-minute version would begin with power-series addition--trivial. Then comes multiplication--an eye-opener. No subscripts! No worry about how many terms to carry in intermediate results. That's about all you have time to really derive. Go on to mention that division is about equally easy. Then allude to substitution (also called composition) and its inverse, reversion. Lots of finicky papers have been written about reversion over more than two centuries. Throw that one-liner on the screen side-by-side with Algorithm S (which is only pseudocode!) from Knuth section 4.7. That should convince the most skeptical observer. Doug

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Re: [Haskell-cafe] Theorems for free!
by Patrick Browne 28 Feb '12

28 Feb '12

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Theorems for free!
by Patrick Browne 28 Feb '12

28 Feb '12

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