January 2009 - Haskell-Cafe

Elevator pitch for functional programming
by Jim Burton 21 Jan '09

21 Jan '09

Hi, I will be a TA on a comparative PL course and I'm looking for small examples (ammunition) which motivate the use of Haskell and functional programming generally. The course is for 1st year Software Engineers, none of whom are likely to have used a functional language. They will all have experience programming Java and a little C++, with a few of them knowing Python, Ruby, PHP etc etc too. If anyone has code snippets which are the equivalent of an elevator pitch for FP, I would be very grateful to see them. What I want are some small concrete examples of idioms which are natural and powerful in Haskell but difficult or impossible in, say, Java. So I can produce examples of some of the things that make FP powerful, elegant, expressive etc: higher order functions, polymorphism, composition (ask them to write (.) in Java :-)), partial application and so on. I will point any interested souls to Hughes' great paper [1]. But I have little time and it might be hard to put across why they would want to do these things in the first place. I was looking for something that speaks directly to the kind of problems they face in languages like Java... Types are a good example because Java programmers generally already appreciate the help they get from compiler messages etc, so you can sell a more flexible, enhanced form of this. Purity might appeal to anyone who has longed to be able to reason about nastily complex code with a lot of shared state. Laziness, streams? Hard to do in Java (I presume) but also quite hard to sell the need. The existence of an O'Reilly book will help, especially one that can be sampled online, so I'll point them at RWH for extended concrete examples. They will need to be already sold before they will bother with that though. Thanks, Jim [1] http://www.cs.chalmers.se/~rjmh/Papers/whyfp.html

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haskell-src-meta Package
by George Giorgidze 21 Jan '09

21 Jan '09

Hi, In my project I needed a function that would parse a string into a template-haskell AST representing the Haskell expression. To be more precise I needed a function with the following type signature. :: String -> Either String Language.Haskell.TH.Syntax.Exp This kind of function should exist in the implementation of template-haskell, however it is not exported for public consumption from template-haskell package. There is haskell-src-exts package that provides a Haskell expression parser however it uses its own Exp data type which differs from the template haskell representation Exp. After that, I found haskell-src-meta package that provided the function I needed: parseExp :: String -> Either String TH.Exp This function uses parser from haskell-src-exts package and after that translates returned Exp representation into the template-haskell representation of Exp. So problem has been solved. However, In the light of recent changes and enhancements in the haskell-src-exts, haskell-src-meta package on Hackage is broken. I have fixed the broken package and contacted its author and maintainer (Matt Morrow). However, he did not reply. I have a number of questions: Was the use of haskell-src-meta really neccessary, I mean, is there any other package that provides the functionality of aforementioned parseExp function? Can I upload the fixed version of haskell-src-meta package to Hackage and assume its maintenance while Matt Morrow is silent? Does anyone know how to contact Matt Morrow? His is not replying on my emails sent to the email address specified in the package description. The thing is that I do not want my package to be dependent on a package that is broken on Hackage. Cheers, George -- George Giorgidze http://www.cs.nott.ac.uk/~ggg/

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Quite confused by simple transformations on this code not working
by Andrew Wagner 21 Jan '09

21 Jan '09

Strange little bit of code: http://moonpatio.com:8080/fastcgi/hpaste.fcgi/view?id=829#a829 If I do any of the following, all of which seem natural to me, it fails to typecheck: 1. move f out of the 'where' clause (with or without a type signature) 2. put the same type signature on f as is on (/\) 3. replace f with (/\) completely What's going on here?

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Type family problem
by Sjoerd Visscher 21 Jan '09

21 Jan '09

Hi all, When I try this bit of code: > class C1 a where > type F a :: * > x :: F a > y :: F a > x = y I get this error: Couldn't match expected type `F a1' against inferred type `F a' In the expression: y In the definition of `x': x = y I can't figure out what is going on or how I should fix this. -- Sjoerd Visscher sjoerd(a)w3future.com

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Re: Re[2]: [Haskell-cafe] ANN: HTTPbis / HTTP-4000.x package available
by Duncan Coutts 21 Jan '09

21 Jan '09

On Sat, 2009-01-17 at 20:34 -1000, Tim Newsham wrote: > > Speaking of proxies, I've been looking into how to find the right proxy > > to use on Windows systems. Turns out that to do it properly you need a > > JavaScript interpreter! Yes, really. > > Uhh.. What!? That's not right. I think you mean it should not be right! :-) Of course it's possible to specify a list of proxies in Windows and that info is kept in the registry. It's also common in corporate networks to use proxy auto-configuration. The client gets told the URL of the these (.pac) files and the client downloads it to work out what proxy to use. You'll notice a space to specify an auto-configuration URL (.pac file) in the proxy setting dialog of any web browser. Proxy auto-configuration files are JavaScript. It uses more or less the full JavaScript language (ECMA these days), though with a small subset of the standard library. Web browsers are ok because they've got a JavaScript interpreter. The MS WinHTTP library uses the Windows scripting host service. The open source pacparser C library links to the Mozilla SpiderMonkey JavaScript engine. Apparently we have Netscape to thank for this mess. Duncan

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ANN: HTTP-4000.0.4 released
by Sigbjorn Finne 21 Jan '09

21 Jan '09

Hi, a new release of HTTP, version 4000.0.4, is now available http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP It is also pointed at via the updated HTTP web page -- http://projects.haskell.org/http/ The main change is the addition of registering a Browser event handler for capturing state changes to the request-response processing pipeline. An experimental change, see Network.Browser.setEventHandler (Feature suggested by Yuval Kogman in order to accommodate load testing.) enjoy --sigbjorn

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ANN: HTTPbis / HTTP-4000.x package available
by Sigbjorn Finne 21 Jan '09

21 Jan '09

Hi, 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 off&on 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. Some people have reported quietly pleasing speedups as a result of this change. (If they want to report numbers, please do..) Another change/fix in this release is the _alleged_ fix to the long-standing bug in the use of absolute URIs vs absolute paths in requests (for non-proxied and proxied use.) Give it a go.. Notice that the HTTP-4000.x version will require you to make some modifications to your existing HTTP-using code -- I've tried to keep the API backwards compatible minimal despite the change in functionality and underlying types. If you do not want to deal with this right away, please introduce a <4000 dependency on the HTTP package in your .cabal files. I've also taken on the maintainership of the package, with the highly esteemed Bjorn Bingert no longer having the usual abundance of cycles to look after it (hope I'm not misrepresenting facts here, Bjorn!) However, I've yet to 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 enjoy --sigbjorn

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some ideas for Haskell', from Python
by Manlio Perillo 20 Jan '09

20 Jan '09

Hi. There are two features found in Python language, that I would like to see in Haskell. 1) In a Python string it is available the \U{name} escape, where name is a character name in the Unicode database. As an example: foo = u"abc\N{VULGAR FRACTION ONE HALF}" 2) In Python it is possible to import modules inside a function. In Haskell something like: joinPath' root name = joinPath [root, name] importing System.FilePath (joinPath) Thanks Manlio Perillo

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Another Windows question - reading the output from a process
by Günther Schmidt 20 Jan '09

20 Jan '09

Hi all, I'm using WinXP with 6.8.3. For some reason the windows version of 6.8.3 ships with a somewhat "castrated" version of package process, one that does not contain "readProcess" among other convenient functions. Is there a way to install the fully fledged version of "process", because I tried using those "rawer" functions and just don't manage. Günther

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MonadTrans lift implementation
by Daryoush Mehrtash 20 Jan '09

20 Jan '09

Is there a reason why the lift function in ReaderT's MonadTrans instance is implemented as: instance MonadTrans (ReaderT r) where lift m = ReaderT $ \_ -> m Instead of just using the monad's return function? Could "lift m" be implemented as "return m"? instance (Monad m) => Monad (ReaderT r m) where * return a = ReaderT $ \_ -> return a* m >>= k = ReaderT $ \r -> do a <- runReaderT m r runReaderT (k a) r fail msg = ReaderT $ \_ -> fail msg Thanks, Daryoush

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