December 2009 - Haskell-Cafe

Re: Pattern matching, and bugs
by John Lato 18 Dec '09

18 Dec '09

Hello, > Date: Fri, 18 Dec 2009 13:04:47 +0100 > From: Andr?s Mocs?ry <amocsy(a)gmail.com> > > > switch 1 = "Unchecked" > > switch 2 = "Checked" > > switch 3 = "Unknown" > > switch x = "Nothing" > > These general ways really avoid this particular crash, but does something > real bad to the code in my opinion. ...snip... > Which leads to the very reason I wrote to you: > > I want to propose this for Haskell prime: > > I would like to have a way for Haskell, not to crash, when my coders write > pattern matching without the above mentioned general case. I will first make a recommendation, then some comments on your proposal. My recommendation is this: if you know that a function should only be called with certain values, you should create a data type that has *only* those values, like: data ForSwitch = One | Two | Three then your switch function is switch :: ForSwitch -> String and now it can be guaranteed *at compile-time* that switch will never be called with an incorrect argument, because only arguments are available in the ForSwitch type and they're all valid. It's now impossible for another part of the program to generate invalid data. It either generates a ForSwitch, which is guaranteed to be valid, or it doesn't, in which case the code won't typecheck. Note that if you really just want to wrap Int's, you can use a newtype and a smart constructor, which if written correctly gives you the same guarantees but is in my mind less convenient. Now, as for a general response to your proposal, I'm not sure that what you're requesting is possible, and I'm quite certain it would be more trouble than it's worth. > Like having the compiler auto-include those general cases for us, > but when those cases got hit, then* instead of crashing*, it *should **report > some error* on *stdout *or *stderr*. > (It would be even nicer if it cold have been traced.) What should the program do after reporting an error? Because of Haskell's non-strict evaluation, a function is only evaluated when your program *needs the result*. The compiler can't infer a meaningful value for an arbitrary type, and the only value that can be in every type is undefined. So now the program needs a result of some type (String in your example) and gets an undefined, and then immediately crashes with an "Exception - undefined" error. As for network-corrupted data, the solution is the same. Make a type that reflects *only* the values you want. Then it's the responsibility of the serializer/deserializer to ensure that you have a correct value, using explicit error handling (Maybe or Either come to mind). Then you know that all data in your system is valid, and you only need to worry about invalid data at borders, where the problem is much easier to address. John

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lhs2tex, Haskell Platform and cygwin
by Jacques Carette 18 Dec '09

18 Dec '09

I just downloaded a fresh copy of Haskell Platform, and successfully got cabal to update itself. I installed smallcheck-0.4 successfully too. But then I tried to install lhs2tex, which 'worked' until the point of saying: Linking dist\build\lhs2TeX\lhs2TeX.exe ... Installing executable(s) in C:\Program Files (x86)\Haskell\bin setup.exe: The program mktexlsr is required but it could not be found And I do have 'mktexlsr' in my cygwin PATH (where I was running 'cabal install lhs2tex'). It's true that mktexlsr isn't in my Windows PATH. I would have thought someone else would have encountered this already, but my google searchers found other (long fixed) problems with lhs2tex under cygwin, but not this one. Jacques

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[Fwd: Termination Competition 2009 live on the web!]
by Janis Voigtländer 18 Dec '09

18 Dec '09

Hi all, This contest should be of interest to Haskellers. There is an "FP Category" in which termination of Haskell programs is automatically checked. The current state can be observed by following the corresponding "View Results" link from the page http://termcomp.uibk.ac.at/termcomp/competition/categoryList.seam?competiti… Ciao, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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openFd under -threaded gets interrupted
by Mitar 18 Dec '09

18 Dec '09

Hi! I am using such code: fd <- openFd device ReadWrite Nothing OpenFileFlags { append = False, noctty = True, exclusive = False, nonBlock = True, trunc = False } And if I compile my program with -threaded option I always get such error: interrupted (Interrupted system call) But without -threaded it works flawlessly. I am using Linux 2.6.30 amd64, GHC 6.10.4. It was the same with 6.8. Why is this? Is there any workaround? Does it work in 6.12? Mitar

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Restrictions on associated types for classes
by Stephen Lavelle 17 Dec '09

17 Dec '09

Given class MyClass k where type AssociatedType k :: * Is there a way of requiring AssociatedType be of class Eq, say?

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Haskell Platform mailing list?
by Gregory Collins 17 Dec '09

17 Dec '09

Hi all, I'm getting the following error trying to post to the haskell-platform list: haskell-platform(a)projects.haskell.org Delay reason: SMTP error from remote mail server after end of data: host community.haskell.org [72.249.126.23]: 451 Temporary local problem - please try later G -- Gregory Collins <greg(a)gregorycollins.net>

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REMINDER: Next BostonHaskell meeting TONIGHT at MIT
by Ravi Nanavati 17 Dec '09

17 Dec '09

The full details of the event are here: http://groups.google.com/group/bostonhaskell/t/792df9787155b5cc I'm just sending out this reminder to make sure more people hear about the event. If you're planning to come please respond to our attendance poll at: http://doodle.com/p2zcnca6k39iptb4 This will help us get a better count of attendees for refreshments (since my wife has volunteered to make more goodies). Please also respond to the poll if you're interested in BostonHaskell, but couldn't come this month because of scheduling or location constraints. That will help us improve planning for future meetings. I look forward to seeing many Boston-area Haskellers tonight! Thanks, - Ravi Nanavati

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Re: [Haskell-cafe] ANN: Hemkay, the 100% Haskell MOD player
by Patai Gergely 17 Dec '09

17 Dec '09

> Patai, I asked a similar question about a week ago, inquiring about > efficient buffers for audio. I got a good response: > > http://thread.gmane.org/gmane.comp.lang.haskell.cafe/67258/focus=67293 Well, the general lesson seems to be that using stream fusion or something analogous is the way to go. In this particular case I don't need any fancy processing, and I have no feedback loops. All I do in my mixing routine is to calculate a weighted (volume+panning) sum of samples and stretch them in time as dictated by their frequency. The song is flattened into a series of play states first, so all the information required for mixing is readily available. I could try rewriting the mixer to return an iterative generator function instead, which could be passed to an appropriate sound interface. The latter would have to be written once and for all. Gergely -- http://www.fastmail.fm - Does exactly what it says on the tin

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Haskell and "memoization"
by michael rice 17 Dec '09

17 Dec '09

I'm looking here at the Fibonacci stuff: http://www.haskell.org/haskellwiki/Memoization Since (I've read) Haskell never computes the value of a function more than once, I don't understand the need for memoization. Enlighten me. Michael

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Haskell arrays
by michael rice 17 Dec '09

17 Dec '09

Based upon docs I've looked at, Haskell seems to store both an array element value AND its index/indices, whereas most languages just store the value and find its location in memory through mapping calculations. Is it true? Michael

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