- Haskell-Cafe - Haskell.org

Polyvariadic composition
by Artyom Kazak 31 Jul '12

31 Jul '12

Hello, I have accidentally written my version of polyvariadic composition combinator, `mcomp`. It differs from Oleg’s version ( http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp ) in three aspects: a) it is simpler, b) it works without enumerating basic cases (all existing types, in other words), and c) it needs more type extensions. > {-# LANGUAGE > MultiParamTypeClasses > , FunctionalDependencies > , FlexibleInstances > , UndecidableInstances > , TypeFamilies , OverlappingInstances > #-} > > class Mcomp a ar b br | a br -> b where > mcomp :: a -> (ar -> br) -> b > > instance (a ~ ar, b ~ br) => Mcomp a ar b br where > mcomp a f = f a > > instance (Mcomp a ar b br) => Mcomp (x -> a) ar (x -> b) br where > mcomp a f = \x -> mcomp (a x) f My question is: why doesn’t it work when I replace instance (a ~ ar, b ~ br) => Mcomp a ar b br with instance Mcomp a a b b ? I thought that equal letters mean equal types…

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why does a foldr not have a space leak effect?
by Qi Qi 30 Jul '12

30 Jul '12

Hi, I wonder why a foldr does not have a space leak effect? Any hints? Thanks. Qi

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AI - machine learning
by miro 30 Jul '12

30 Jul '12

Hi All, recently I started to take a look at haskell, especially at AI. I can see some email addresses of interested people there but not so much of other activity behind. Does it exist some mailing group especially for AI? Basically I'm interested in trying some machine learning algorithms. Start with reinforcement learning and value-based), and go towards AGI (Artificial General Intelligence). Does anybody know about some already existing haskell approaches, or is there anybody working on this? Cheers, m.

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Logging pure code
by Marco Túlio Gontijo e Silva 30 Jul '12

30 Jul '12

Hi. I'm using the hslogger library for logging pure code, using the following function: uLog :: Priority -> String -> a -> a uLog pri str x = unsafePerformIO $ logM rootLoggerName pri str >> return x Everything was working fine, till I used uLog on a very accessed function. Then the program started to fail with: thread blocked indefinitely in an MVar operation I've changed uLog to: uLog :: Priority -> String -> a -> a uLog pri str x = unsafePerformIO $ hPutStrLn stderr (show str) >> return x And I noticed on the log that two messages were sent "at the same time" on the point where the version with hslogger fails: "canonicate 4 "canonicate 1 \r\n\t\t\t\t\t\t\t\tRio Branco = rio branco" "canonicate 2 AC\r\n\t\t\t\t\t\t\t\t = ac" ac = ac" They were sent as (in any order): "canonicate 4 ac = ac" "canonicate 1 \r\n\t\t\t\t\t\t\t\tRio Branco = rio branco" "canonicate 2 AC\r\n\t\t\t\t\t\t\t\t = ac" I tried changing hslogger to avoid calling modifyMVar when getting logger for rootLoggerName, and using withMVar instead, but the problem persisted. I tried to create a simple test case to reproduce the problem, but I couldn't, all test cases I created worked as expected. As I was with no other ideas, I tried some things that make little sense. This happens with optimization or without it. Following http://hackage.haskell.org/trac/ghc/ticket/5859 I tried using -fno-state-hack, but the problem persisted. I tried adding NOINLINE to uLog, without success. I also tried changing unsafePerformIO to unsafeDupablePerformIO. I thought that the only thing I needed to take care while using unsafePerformIO was knowing that the time of execution is undetermined and that it could even run more than once. This is not a problem for my logging. Is there something else I should be aware while using unsafePerformIO? Greetings. -- marcot http://marcot.eti.br/

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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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