November 2011 - Haskell-Cafe

Message
by Goutam Tmv 07 Nov '11

07 Nov '11

Would you ever see yourself write a web application like Twitter or Facebook in Haskell?

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[ANNOUNCE] CoreErlang-0.0.2 + maintainer change
by Alex Kropivny 07 Nov '11

07 Nov '11

Hello, With the authors blessing, I'm taking over maintenance for http://hackage.haskell.org/package/CoreErlang, a parser/printer for an intermediate language for the Erlang VM. Changes in 0.0.2: - fixed float parse bug - added Data,Typeable instances to AST Package is rather esoteric, but it's nice for static source analysis - and when someone decides they want to compile a new language to the ERTS, the package is ready... Regards, Alex Kropivny

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Time zones and IO
by Ben Gamari 07 Nov '11

07 Nov '11

Recently I've been working on an iCalendar parser implementation with support for recurrence rules. For the recurrence logic, I've been relying on Chris Heller's excellent time-recurrence package. Unfortunately, it seems we have both encountered great difficulty in cleanly handling time zones. For example, as seen in the case of the CalendarTime LocalTime instance[1], Chris does the following, instance CalendarTimeConvertible LocalTime where toCalendarTime (LocalTime day t) = CalendarTime (fromEnum $ todSec t) (todMin t) (todHour t) d (toEnum m) y weekDay yearDay tz where (y, m, d, weekDay, yearDay) = dayInfo day tz = unsafePerformIO getCurrentTimeZone Ouch. Unfortunately, I have been entirely unable to come up with any better way to deal with timezones in a referentially transparent way. Passing the current timezone as an argument seems heavy-handed and reveals the leakiness of the CalendarTimeConvertible abstraction. Short of making toCalendarTime an action, can anyone see a suitable way of refactoring CalendarTimeConvertible to avoid unsafe IO? Perhaps a TimeZoneInfo monad is in order, exposing lookup of arbitrary (through timezone-olson) as well the current timezone? It seems like this would be inconvenient and overkill, however. Ideas? - Ben [1] https://github.com/hellertime/time-recurrence/blob/master/src/Data/Time/Cal…

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Is it possible to get the information of instances of a type?
by Magicloud Magiclouds 07 Nov '11

07 Nov '11

Hi, If this was in ruby or other languages that support reflection, it won't be a question. But in Haskell, could I write a code to list the classes that a type instanced? TemplateHaskell as well. -- 竹密岂妨流水过山高哪阻野云飞

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memorize function with number parameterized types in GHC
by Bin Jin 07 Nov '11

07 Nov '11

Hi, everyone I'm recently trying to implement the Montgomery reduction algorithm[1] in Haskell, the code can be found on my Github page[2]. After doing some benchmark testing I found that the library works rather slow. With the help of `trace` function from Debug.Trace, I found that GHC is not magical enough to memorize values with the same type(well, it's actually dynamically generated number parameterized type). I used binary representation to handle all positive numbers in type system. > data One = One > data D0 a = D0 a > data D1 a = D1 a > class PostiveN a where > p2num :: (Num b, Bits b) => a -> b > instance PostiveN One ... > instance PostiveN a => PostiveN (D0 a) ... > instance PostiveN a => PostiveN (D1 a) ... Here is a function that will be called everytime by `(*)` in `Num` typeclass > montgKeys :: (PostiveN p, Integral a, Bits a) => p -> a as you can imagine, I always pass (undefined :: p) as parameter to `montgKeys`, so if it's handled well, it should be memorized for future usage. But tracing shows that both `p2num` and `montgKeys` are evaluated every time being called. So my question is, how to force GHC memorizing this kind of functions? [1]: http://en.wikipedia.org/wiki/Montgomery_reduction [2]: https://github.com/bjin/montg-reduce Regards, Bin

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Re: [Haskell-cafe] memorize function with number parameterized types in GHC
by DavidA 06 Nov '11

06 Nov '11

Bin Jin <bjin1990 <at> gmail.com> writes: > > > In order to represent Z/nZ, regular way need to store two integers, > and a data constructer. With number parameterized types, newtype+Integer > can be used, it's much more efficient. For this project, montg reduce > require to calculate a key for each modulus, but the modulus won't be > changed within Num typeclass. It's better to fix it in type system. > On Nov 7, 2011 1:38 AM, "DavidA" <polyomino <at> f2s.com> wrote: > What's the purpose of all the type trickery? > Why not just implement the algorithm using Integer? > _______________________________________________ > Haskell-Cafe mailing listHaskell-Cafe <at> haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe <at> haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > Ok, but even if you want to represent the inputs and outputs to be in Z/nZ, that doesn't mean that you have to work in Z/nZ while performing the algorithm. I think it would probably be faster to convert from Z/nZ to Integer, do the algorithm, and then convert the result back to Z/nZ. Anyway, good luck, and if you succeed in getting it fast enough, I hope you'll make the results available somewhere.

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Cabal install problem
by Victor Miller 06 Nov '11

06 Nov '11

Lately whenever I try to install a cabal package it fails with the following error message: Could not find module `Control.Monad.State': Perhaps you haven't installed the profiling libraries for package `mtl-2.0.1.0'? Use -v to see a list of the files searched for. The strange thing is that I can load Control.Monad.State from ghci. How do I fix this -- i.e. where should I look?

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[ANNOUNCE] cereal-0.3.4.0
by Trevor Elliott 06 Nov '11

06 Nov '11

Hi Everyone, I'm happy to announce version 0.3.4.0 of the cereal serialization library. This release includes a patch from Adam Foltzer to provide support for IEEE754 encoded Float/Double values. This functionality is exposed through Serialize instances for both types, and specialized get/put functions in the new Data.Serialize.IEEE754 module. Happy hacking! --trevor

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How do you describe systems in general with Haskell?
by Grigory Sarnitskiy 06 Nov '11

06 Nov '11

If you are to describe a system, which consists of several subsystems, how do you approach the problem? What types, classes, functions whatever do you introduce? I guess it is a common problem, is there a general method? Just to describe, not to solve (though if the description implies the solution then it is wonderful). Obviously this is not just a haskell-specific problem, but I think there might be people who are aware of the best current solution (category theory?). For a dummy example, how would you describe the system from a fox, goose and bag of beans puzzle (wolf, goat and cabbage)?

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New CoreErlang maintainer + patch
by Alex Kropivny 05 Nov '11

05 Nov '11

Hello, I've got a patch [1] for the CoreErlang package. It fixes a parsing bug and adds Typeable+Data instances to the syntax tree. I was unable to contact the developers/maintainers via provided email. (No response in ~3 weeks.) The package hasn't been touched for three years, since first uploaded. I would like to take over maintenance of the package, and upload the fix ASAP. Regards, Alex Kropivny [1] https://github.com/amtal/CoreErlang/tree/0.0.2

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