- Haskell-Cafe - Haskell.org

Current state of garbage collection in Haskell
by C K Kashyap 30 Jul '12

30 Jul '12

Hi, I was looking at a video that talks about GC pauses. That got me curious about the current state of GC in Haskell - say ghc 7.4.1. Would it suffer from lengthy pauses when we talk about memory in the range of 500M +? What would be a good way to keep abreast with the progress on haskell GC? Regards, Kashyap

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Capturing the parent element as I parse XML using parsec
by C K Kashyap 30 Jul '12

30 Jul '12

Hi, With the help of the cafe I've been able to write up the xml parser using parsec - https://github.com/ckkashyap/really-simple-xml-parser/blob/master/RSXP.hs I am struggling with an idea though - How can I capture the parent element of each element as I parse? Is it possible or would I have to do a second pass to do the fixup? Regards, Kashyap

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Munich Haskell Meeting
by Heinrich Hördegen 29 Jul '12

29 Jul '12

Dear all, next Tuesday, 31st of July, at 19h30, Munich's functional programming enthusiasts will meet at Max-Emanuell-Brauerei. If the weather is fine, we will sit outside. If you plan to join, please go to: http://www.haskell-munich.de/dates and click the button. Heinrich

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Explaining instance declarations
by Patrick Browne 29 Jul '12

29 Jul '12

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MonadPlus m => Maybe a -> m a
by Thiago Negri 28 Jul '12

28 Jul '12

I'm solving this exercise: http://www.haskell.org/haskellwiki/All_About_Monads#Exercise_4:_Using_the_M… I'm missing a function to transform a Maybe a into a MonadPlus m => m a. I did search on Hoogle with no luck. There is no standard definition for the "g" function I'm defining? My take on the exercise: data Sheep = Sheep { mother :: Maybe Sheep, father :: Maybe Sheep, name :: String } instance Show Sheep where -- for testing show = name g :: (MonadPlus m) => Maybe a -> m a g Nothing = mzero g (Just a) = return a mother' :: (MonadPlus m) => Sheep -> m Sheep mother' = g . mother father' :: (MonadPlus m) => Sheep -> m Sheep father' = g . father parent'' :: (MonadPlus m) => Sheep -> m Sheep parent'' s = mother' s `mplus` father' s grandparent'' :: (MonadPlus m) => Sheep -> m Sheep grandparent'' s = parent'' s >>= parent''

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every Friday the 13th at midnight...
by Corentin Dupont 27 Jul '12

27 Jul '12

Hello! I'm looking for a library to be able to express, store and retrieve recurring appointments, like "every Monday at midnight". I saw Data.Time.Recurrence, which has a nice language, but how to store the appointments? A command like: *now <- getCurrentTime recur daily `begin` now* produces an infinite list of UTCTime. Best, Corentin

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Haskell on Mac OS X Mountain Lion
by mrbuchmann＠googlemail.com 26 Jul '12

26 Jul '12

Hi all, does the Haskell Platform (2012.2.0.0 I suppose) work on 10.8. http://hackage.haskell.org/platform/mac.html only says something about 10.7. I don't want to break my development system due to upgrading to 10.8, so if anybody has any experience regarding this topic, please let me know. Best wishes

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FP Developers Conference
by Bartosz Milewski 26 Jul '12

26 Jul '12

Alex Miller is the organizer of a very successful Strange Loop developers conference in St Luis. He also happens to be a Closure programmer. He wants to organize a Functional Programming Developers Conference in the US (stress on _developers_). He will probably succeed, so it's very important that Haskell be well represented there. I would like to encourage everybody to take a minute and fill out a short preliminary poll at http://tech.puredanger.com/2012/07/24/functional-programming-conference/ . This is to give Alex the idea how many people are interested, what language they prefer, and what area of development they're interested in. -- [:Bartosz:]

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GHC type error pretty printing
by Christopher Done 26 Jul '12

26 Jul '12

Is it me or is this output not right? The \ syntax doesn't indicate a newline. chris@midnight:~$ ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> "abc\n123" :: () <interactive>:2:1: Couldn't match expected type `()' with actual type `[Char]' In the expression: "abc\ \123" :: () In an equation for `it': it = "abc\ \123" :: ()

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Applicative functors with branch/choice ?
by Евгений Пермяков 26 Jul '12

26 Jul '12

Let assume, that some computation takes argument and produces value Either a b. This computation may be represented in for different forms ==== computePure :: a -> Either b c computeMonad :: a -> m (Either b c) computeApplicative :: app a -> app (Either b c) computeArrow :: arr a (Either b c) ===== And now, having result, we need to execute several actions, making a choice, what actions to perform for Left and Right tags of Either. Pure function and monads are easy, as there is way to pattern-match on value and take actions depending on result. There is an extension to Arrow class that do the job -- ArrowChoice. However, I cannot find any way to make choice for Applicative. It seems that both Applicative and Alternative are not suited for it. So, it seems for me, that Applicative API should be extended with typeclass for making choice what actions to execute depending on result of some test (pattern matching). Is there any reasonable definition of such typeclass or clear explanation, why such typeclass is unneeded? The possible extension may look somehow like this: class Applicative a => Branching a where branch :: a (Either b c) -> (a b -> a d) -> (a c -> a d) -> a d

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