- Haskell-Cafe - Haskell.org

Conduit: Where to run monad stacks?
by Niklas Hambüchen 28 Aug '12

28 Aug '12

Today I was surprised that transPipe is called for every chunk of data going through my pipe, rendering the StateT I put in useless, because it was always restarted with the initial value. It would be nice to have some explanation about this, as it makes it easy to write compiling code that has completely unexpected behaviour. I wrote this function (also on http://hpaste.org/73538) conduitWithState :: (MonadIO m) => Conduit Int (StateT Int m) String conduitWithState = do liftIO $ putStrLn $ "Counting Int->String converter ready!" awaitForever $ \x -> do i <- lift get lift $ modify (+1) liftIO $ putStrLn $ "Converting " ++ show x ++ " to a string! " ++ "Processed so far: " ++ show i yield (show x) and ran it like this: countingConverterConduit :: (MonadIO m) => Conduit Int m String countingConverterConduit = transPipe (\stateTint -> evalStateT stateTint 1) conduitWithState main :: IO () main = do stringList <- CL.sourceList [4,1,9,7,3] $= countingConverterConduit $$ CL.consume print stringList However, the output is not what I expected, but only: Processed so far:1 Processed so far:1 ... Dan Burton proposed a fix, making the whole sink-conduit-source construction run on the StateT: main = do stringList <- flip evalStateT 1 $ ... So the question is: What is the rationale for this? I was expecting that if I have an IO pipe in my main conduit, I could easily run stuff on top of that in parts of the pipe. Thanks Niklas

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OS-independent auto-monitoring of a program to do things depending on resource usage at runtime
by Alberto G. Corona 28 Aug '12

28 Aug '12

For a caching library, I need to know the runtime usage of memory of the program and the total amount of memory, the total memory used by all the programs etc. I need not do profiling or monitoring but to do different things inside my program depending on memory usage. The search is difficult because all searches go to profiling utilities which I don´t need. Are there some portable way to to this? . The various monitoring libraries indicates that there are ways to do it, but they seem not to allow runtime "internal automonitoring"

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When is a composition of catamorphisms a catamorphism?
by Sebastien Zany 28 Aug '12

28 Aug '12

>From page 3 of http://research.microsoft.com/en-us/um/people/emeijer/Papers/meijer94more.p… : it is not true in general that catamorphisms are closed under composition When is this true?

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Simple shell scripts
by Eric Tanter 27 Aug '12

27 Aug '12

Hi, Here is a simple shell script (upper.hs): import Data.Char main = interact $ map toUpper which composes fine with other scripts: bash-3.2$ yes | head -n 3 | runghc upper.hs Y Y Y but not always: bash-3.2$ yes | runghc upper.hs | head -n 3 Y Y Y <stdout>: hFlush: resource vanished (Broken pipe) Any idea why this error occurs/how to avoid it? (running just: yes | runghc upper.hs gives the expected infinite stream of Ys) Thanks! -- Éric

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map over Bijections
by Sergey Mironov 27 Aug '12

27 Aug '12

Hi. I need map equivalent for Bijection type which is defined in fclabels: data Bijection (~>) a b = Bij { fw :: a ~> b, bw :: b ~> a } instance Category (~>) => Category (Bijection (~>)) where ... I can define this function as follows: mapBij :: Bijection (->) a c -> Bijection (->) [a] [b] -> Bijection (->) [a] [c] mapBij b1 b = (map (fw b1)) `Bij` (map (bw b1)) but do I really need to do it explicitly? Can I obtain same result using some Category combinators or other common stuff? Sergey

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Compiling Haskell targetting different OS/arch
by Thiago Negri 27 Aug '12

27 Aug '12

Is it possible to compile Haskell code targetting a OS/arch that differs from the one where the compiler (GHC) is running? Thanks.

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Cabal Test-suites + custom preprocessors
by Iain Nicol 26 Aug '12

26 Aug '12

Hi café, I am successfully using a custom preprocessor to build my main executable. In particular, I am using UUAGC to preprocess .ag files into .hs files. My Setup.hs file uses 'uuagcLibUserHook' from the package uuagc-cabal to do this, which in particular overrides Cabal's 'buildHook' for me. Now I am trying to add a test-suite (of type exitcode-stdio-1.0). I want the test suite to be able to import the modules which implement the main program. Unfortunately, building the test suite fails, because the .ag files don't seem to be built into .hs files. Does anybody know if Cabal actually supports using custom preprocessors for building a test-suite? If so, is 'buildHook' still the right hook to override? Thanks, -- Iain

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Munich Haskell Meeting
by Heinrich Hördegen 26 Aug '12

26 Aug '12

Dear all, once again, it's time for our monthly get-together. Functional programmers from Munich and other cities will meet on Wed, 29th of August, at 19h30 at Max-Emanuel-Brauerei in Munich. If you plan to join, please go to http://www.haskell-munich.de/dates and click the button! With hope to meet many of you there, Heinrich

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ANN: Monad.Reader Issue 20
by Edward Z. Yang 26 Aug '12

26 Aug '12

*It’s not dead, it’s resting!* I am pleased to announce that Issue 20 of the Monad Reader is now available. http://themonadreader.files.wordpress.com/2012/08/issue20.pdf Issue 20 consists of the following three articles: - "Enumeration of Tuples with Hyperplanes" by Tillmann Vogt - "Understanding Basic Haskell Error Messages" by Jan Stolarek - "The MapReduce type of a Monad" by Julian Porter Feel free to browse the source files. You can check out the entire repository using Git: git clone https://github.com/ezyang/tmr-issue20.git If you’d like to write something for Issue 21, please get in touch! Cheers, Edward

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formal semantics
by Ramana Kumar 25 Aug '12

25 Aug '12

Dear Haskell Cafe I'm looking for information on past and current attempts to write semantics for Haskell. Features I'm particularly interested in are: - formal - mechanised - maintainable - up to date Of course, if nothing like that exists then partial attempts towards it could still be useful. My ultimate aims include: 1. Make it viable to define Haskell formally (i.e. so mechanised semantics can take over the normative role of the Haskell reports). 2. Write a verified (or verify an existing) Haskell compiler (where verified means semantics preserving). Cheers, Ramana

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