- Haskell-Cafe - Haskell.org

Uniplate and rewriting with different types
by Chris Mears 29 Jan '13

29 Jan '13

Hi all, I have a question about the Uniplate library, regarding rewriting with transformations that have different types. With the following type, and transformation functions: data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1,t2,t3 :: Even -> Maybe Even t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 (EvenOne (OddZero x)) = Just $ EvenZero (OddOne x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing it is easy to combine the transformations into a single transformation, because they all have the same type. The result can then be passed to the Uniplate's "rewriteBi" function: allts x = t1 x `mplus` t2 x `mplus` t3 x example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBi allts example But if one of the transformations has a different type, you can't do it this way. For instance, redefine t2 to have a different type: t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing and you are stuck because the functions of different types can't be combined into a single transformation. My question is: is there a good way to combine the transformation functions if they have different types? I have come up with a possible solution (see below), but I am not sure that it is the right approach, and it is probably inefficient. Chris Mears {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-} import Control.Monad import Control.Monad.State import Data.Generics import Data.Generics.Uniplate.Data data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing -- The transformers are wrapped in an existential type. data WrappedTransformer from = forall to. (Data to, Biplate from to) => WrappedTransformer (to -> Maybe to) -- Apply a single transformation, and return "Just x" if the -- transformation fired (where x is the result of the rewriting), and -- "Nothing" if no transformation fired. rewriteBiCheck :: Biplate from to => (to -> Maybe to) -> from -> Maybe from rewriteBiCheck f e = case runState (rewriteBiM check e) False of (e', True) -> Just e' (_, False) -> Nothing where check x = case f x of Nothing -> return Nothing Just y -> put True >> return (Just y) -- Apply a list of wrapped transformations until no more -- transformations can be applied. rewriteBiList :: forall from. [WrappedTransformer from] -> from -> from rewriteBiList transformers m = go transformers m where go :: [WrappedTransformer from] -> from -> from go [] m = m go ((WrappedTransformer t):ts) m = case rewriteBiCheck t m of Just m' -> go transformers m' Nothing -> go ts m -- Test case. example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBiList [ WrappedTransformer t1 , WrappedTransformer t2 , WrappedTransformer t3 ] example

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catamorphisms and attribute grammars
by Petr P 29 Jan '13

29 Jan '13

Dear Haskellers, I read some stuff about attribute grammars recently [1] and how UUAGC [2] can be used for code generation. I felt like this should be possible inside Haskell too so I did some experiments and I realized that indeed catamorphisms can be represented in such a way that they can be combined together and all run in a single pass over a data structure. In fact, they form an applicative functor. [1] http://www.haskell.org/haskellwiki/Attribute_grammar [2] Utrecht University Attribute Grammar Compiler To give an example, let's say we want to compute the average value of a binary tree. If we compute a sum first and then count the elements, the whole tree is retained in memory (and moreover, deforestation won't happen). So it's desirable to compute both values at once during a single pass: -- Count nodes in a tree. count' :: (Num i) => CataBase (BinTree a) i count' = ... -- Sums all nodes in a tree. sum' :: (Num n) => CataBase (BinTree n) n sum' = ... -- Computes the average value of a tree. avg' :: (Fractional b) => CataBase (BinTree b) b avg' = (/) <$> sum' <*> count' Then we can compute the average in a single pass like runHylo avg' treeAnamorphism seed My experiments together with the example are available at https://github .com/ppetr/recursion-attributes I wonder, is there an existing library that expresses this idea? Best regards, Petr Pudlak

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quotRem and divMod
by Artyom Kazak 29 Jan '13

29 Jan '13

Hi! I’ve always thought that `quotRem` is faster than `quot` + `rem`, since both `quot` and `rem` are just "wrappers" that compute both the quotient and the remainder and then just throw one out. However, today I looked into the implementation of `quotRem` for `Int32` and found out that it’s not true: quotRem x@(I32# x#) y@(I32# y#) | y == 0 = divZeroError | x == minBound && y == (-1) = overflowError | otherwise = (I32# (narrow32Int# (x# `quotInt#` y#)), I32# (narrow32Int# (x# `remInt#` y#))) Why? The `DIV` instruction computes both, doesn’t it? And yet it’s being performed twice here. Couldn’t one of the experts clarify this bit?

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Mac os x (Intel, 10.6.8) problem compiling yesod-core-1.1.7.1
by jean-christophe mincke 28 Jan '13

28 Jan '13

Hello GHC version : 7.4.2 When I do a cabal-dev instal yesod-core, I get the following error: Loading package blaze-builder-conduit-0.5.0.3 ... linking ... done. Loading package hashable-1.2.0.5 ... linking ... ghc: lookupSymbol failed in resolveImports /Users/V3/windev/Haskell/Z/cabal-dev//lib/HShashable-1.2.0.5.o: unknown symbol `_hashable_siphash24_sse2' ghc: unable to load package `hashable-1.2.0.5' Has anyone encountered the same problem? Thank you J-C

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Suggestiong for inter-thread communication
by Erik de Castro Lopo 28 Jan '13

28 Jan '13

Hi all, I am in the process of writing a Haskell program consisting of two threads, one for performing a calculation and one for an Ncurses UI (haskell-ncurses). The calculation thread needs to feed a stream of numbers back to the UI thread (about 1 value per second) and the UI needs to take input from the user and will pass parameter changes to the calculation thread using an IORef and atomicModifyIORef. However, I'm not sure how to hande the UI thread. The UI thread would normally wait for Ncurses input using getEvent, but I need advice on how to the the data from the calculation thread. Any advice or things to try? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Munich Haskell Meeting
by Heinrich Hördegen 28 Jan '13

28 Jan '13

Dear Haskellers, our monthly get-togethers in Munich this year start on Thursday, 31ts of January at 19h30. Our meeting venue will be Cafe Puck once again. If you want to join, please go to http://www.haskell-munich.de/dates and click the button. Of course, you can also invite all those interested in functional programming. Have a successful week, Heinrich

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GHC.Generics and newtypes
by Roman Cheplyaka 27 Jan '13

27 Jan '13

Hi, Is it possible to generate different instances for newtypes and datatypes using GHC.Generics? Roman

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Where did the Cabal manual go?
by Richard Cobbe 27 Jan '13

27 Jan '13

I'm not able to access the cabal manual today: links to my local copy and links to the copy at haskell.org result in a 404. I'm running the Haskell Platform, 2012.4.0.0, 64-bit, OS X 10.8.2. On my first attempt this morning, I loaded file:///Library/Haskell/doc/start.html in my browser and clicked on the "Cabal" link. The browser can't find file:///Library/Haskell/doc/ghc-doc/Cabal/index.html, and indeed the directory /Library/Haskell/doc/ghc-doc/Cabal doesn't exist. Ok, I think, so my local copy's gotten corrupted. One "sudo /Library/Haskell/bin/uninstall-hs --remove thru 7.4.2" and reinstall later, I try again; no change. I also can't find any docs by doing find /Library/Haskell -iname \*cabal\* find /Library/Frameworks/GHC.framework -iname \*cabal\* either. I also get a 404 when I click on the "Cabal" link from <http://lambda.haskell.org/platform/doc/current/start.html>. I can probably recover a copy from system backups, but what's going on? Richard

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How to store Fixed data type in the database with persistent ?
by s9gf4ult＠gmail.com 27 Jan '13

27 Jan '13

All modern databases has field type NUMERIC(x, y) with arbitrary precision. I need to store financial data with absolute accuracy, and I decided to use Fixed. How can I store Fixed data type as NUMERIC ? I decided to use Snoyman's persistent, bit persistent can not use it from the box and there is a problem with custom field declaration. Here is the instance of PersistField for Fixed I wrote instance (HasResolution a) => PersistField (Fixed a) where toPersistValue a = PersistText $ T.pack $ show a -- fromPersistValue (PersistDouble d) = Right $ fromRational $ toRational d fromPersistValue (PersistText d) = case reads dpt of [(a, "")] -> Right a _ -> Left $ T.pack $ "Could not read value " ++ dpt ++ " as fixed value" where dpt = T.unpack d fromPersistValue a = Left $ T.append "Unexpected data value can not be converted to Fixed: " $ T.pack $ show a sqlType a = SqlOther $ T.pack $ "NUMERIC(" ++ (show l) ++ "," ++ (show p) ++ ")" where p = round $ (log $ fromIntegral $ resolution a) / (log 10) l = p + 15 -- FIXME: this is maybe not very good isNullable _ = False I did not found any proper PersistValue to convert into Fixed from. As well as converting Fixed to PersistValue is just a converting to string. Anyway the saving works properly, but thre reading does not - it just reads Doubles with rounding error. If you uncomment the commented string in instance you will see, that accuracy is not absolute. Here is test project to demonstrate the problem. https://github.com/s9gf4ult/xres If you launch main you will see that precission is not very good because of converting database value to Double and then converting to Fixed. How can i solve this with persistent or what other framework works well with NUMERIC database field type ?

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Use forM_ with Maybe, it's Foldable!
by Felipe Almeida Lessa 27 Jan '13

27 Jan '13

Hey, Just wanted to share this little insight I had that I don't think is a common idiom. Often times I need to define: whenJust :: Monad m => Maybe a -> (a -> m b) -> m () whenJust (Just x) f = f x whenJust Nothing _ = return () It always bugged me that you can't find this function anywhere. Actually, you can find it on at least a couple of packages [1], but you can't find it on a standard place. Or can you? A few days ago I decided to hoogle the type of whenJust [2] and what I discovered is that import Data.Foldable (forM_) whenJust = forM_ For example, instead of: mfoo <- doSomething case mfoo of Just foo -> ... Nothing -> return () You may just write: forM_ mfoo $ \foo -> ... There you go. I hope this is useful to someone =). Cheers, [1] http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=whenJust [2] http://www.haskell.org/hoogle/?hoogle=Maybe%20a%20-%3E%20(a%20-%3E%20m%20b)… -- Felipe.

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SDL and ALUT on Windows: "stdin Bad file descriptor"
by Thiago Negri 27 Jan '13

27 Jan '13

I'm trying SDL on Windows, and things are getting really weird. I can compile the code (links on the end). When I run it, if I try using `stdin` the program crashes with this message: <stdin>: hGetLine: invalid argument (Bad file descriptor) Is it something to do with SDL itself? What am I doing wrong? :( Code: http://hpaste.org/81321 Cabal file: http://hpaste.org/81322 Thanks.

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cabal install choosing an older version
by Ozgur Akgun 26 Jan '13

26 Jan '13

Hi, I noticed a weird behaviour with cabal-install. When I run `cabal install hspec --dry-run -v` cabal-install correctly picks hspec-1.4.3 (the latest version). However, when I run `cabal install ansi-terminal hspec --dry-run -v`cabal-install tries to install hspec-0.3.0 for no apparent reason. This is with a clean user package db. Following is some info about my system. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.1 $ cabal --version cabal-install version 1.16.0.2 using version 1.16.0.3 of the Cabal library $ ghc-pkg list --user $ cabal install hspec --dry-run -v Reading available packages... Choosing modular solver. Resolving dependencies... In order, the following would be installed: HUnit-1.2.5.1 (new package) ansi-terminal-0.5.5.1 (new package) hspec-expectations-0.3.0.3 (new package) random-1.0.1.1 (new package) QuickCheck-2.5.1.1 (new package) setenv-0.1.0 (new package) silently-1.2.4.1 (new package) transformers-0.3.0.0 (new package) hspec-1.4.3 (new package) $ cabal install ansi-terminal hspec --dry-run -v Reading available packages... Choosing modular solver. Resolving dependencies... In order, the following would be installed: HUnit-1.2.5.1 (new package) ansi-terminal-0.6 (new package) extensible-exceptions-0.1.1.4 (new package) random-1.0.1.1 (new package) QuickCheck-2.5 (new package) hspec-0.3.0 (new package) -- Ozgur Akgun

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Writing config files for configurator library
by Daniel Austin 26 Jan '13

26 Jan '13

Hello Haskellers, What is the typical way to write out configuration files for use by the configurator library? From asking around, seems like the library does not (obviously) support writing out config files to disc from scratch. What I want to do is write out a default config file to disc, or write out the configuration settings that a user selects. I want to avoid having the user edit the config files by hand in a text editor, as well as avoid rolling my own function to write it out if possible. Both of those solutions leave a sour taste in my mouth. Has there been any work toward this end? Thanks much, Dan

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aeson-0.6.1.0 deriveJSON error
by jims＠stuttard.org 26 Jan '13

26 Jan '13

ghc doesn't seem to be unifying deriveJSON (String->String) parameter with id :: a -> a. examples/TemplateHaskell.hs:22:14: Couldn't match expected type `Data.Aeson.TH.Options' with actual type `a0 -> a0' In the first argument of `deriveJSON', namely `id' In the expression: deriveJSON id ''Coord ghc --make: /usr/hs/ghc/7.4.1/bin/ghc failure (return code=1) ghc-7.4.1 x86_64 Centos-6 [jim@localhost aeson]$ ghc-pkg list --package-conf=../cabal-dev/packages-7.4.1.conf ../cabal-dev/packages-7.4.1.conf HTTP-4000.2.6 aeson-0.6.1.0 attoparsec-0.10.3.0 attoparsec-conduit-0.5.0.3 base-unicode-symbols-0.2.2.4 blaze-builder-0.3.1.0 blaze-builder-conduit-0.5.0.3 case-insensitive-0.4.0.4 conduit-0.5.6 dlist-0.5 hashable-1.2.0.5 http-types-0.7.3.0.1 lifted-base-0.2.0.2 monad-control-0.3.1.4 nats-0.1 network-2.4.1.0 network-conduit-0.6.2.2 parsec-3.1.3 primitive-0.5.0.1 resourcet-0.4.4 semigroups-0.9 simple-sendfile-0.2.10 transformers-base-0.4.1 unix-compat-0.4.1.0 unordered-containers-0.2.3.0 vault-0.2.0.4 vector-0.10.0.1 void-0.5.11 wai-1.3.0.1 warp-1.3.7.1

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Re: [Haskell-cafe] ANN: monad-bool 0.1
by wren ng thornton 26 Jan '13

26 Jan '13

On 1/23/13 3:00 AM, Alexander Kjeldaas wrote: > If we step back, I think the lesson here is that Haskell libraries exist, > but the concepts are far enough from what you expect to exist given > background knowledge from another programming language. > > So what is actually needed is not monad-bool, but the equivalent > documentation that makes a programmer which is not an expert in Haskell > connect the dots like you did today. Definitely. We could even set up NotOnHackage: a "package" repository just like Hackage, except the packages are just documentation on why there is no such package. Implementation-wise it's just a wiki; but the idea is about how to organize the wiki. NotOnHackage should be organized and searchable similar to Hackage itself, so that people can look there when nothing looks promising on Hackage. -- Live well, ~wren

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How to escape from typecheck error: Duplicate instance declarations ?
by s9gf4ult＠gmail.com 26 Jan '13

26 Jan '13

Hello, haskellers. I am trying to write some generic subtyping issue. Here upcast is always safe operation because of subtype is always behaves like the parrent type. downcast is not the safe becase of not every parrent type value can be converted to children type. Rangeable here is the typeclass of values in some range, so downcasting to Rang1 or Range2 or any other type, having instance for Rangeable can be done by checking if value is in proper range. The same for MultipleTo, downcasting can be done with checking if value is multiple to some value. {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies, FlexibleContexts, UndecidableInstances, OverlappingInstances, IncoherentInstances #-} class SubtypeOf a b | a -> b where upcast :: a -> b downcastSafe :: b -> Maybe a downcast :: b -> a downcast b = case downcastSafe b of Nothing -> error $ "can not downcast the value" Just a -> a class (Ord a) => Rangable t a | t -> a where lowLim :: t -> a highLim :: t -> a class Packable t a | t -> a where pack :: a -> t unpack :: t -> a class MultipleTo t a | t -> a where multiple :: t -> a instance (Num a, Ord a, Rangable range a, Packable range a) => SubtypeOf range a where upcast = unpack downcastSafe b | b >= (lowLim $ pb) && b <= (highLim $ pb) = Just $ pb | otherwise = Nothing where pb = pack b instance (Integral a, Packable range a, MultipleTo range a) => SubtypeOf range a where upcast = unpack downcastSafe b | b `mod` (multiple pb) == 0 = Just pb | otherwise = Nothing where pb = pack b newtype Range1 a = Range1 {unRange1 :: a} deriving Show instance (Num a, Ord a) => Rangable (Range1 a) a where lowLim _ = 0 highLim _ = 10 instance (Num a, Ord a) => Packable (Range1 a) a where pack = Range1 unpack = unRange1 newtype Range2 a = Range2 {unRange2 :: a} deriving Show instance (Num a, Ord a) => Rangable (Range2 a) a where lowLim _ = -10 highLim _ = 200 instance (Num a, Ord a) => Packable (Range2 a) a where pack = Range2 unpack = unRange2 but there is compilation error: Duplicate instance declarations: instance [incoherent] (Num a, Ord a, Rangable range a, Packable range a) => SubtypeOf range a -- Defined at ...:22:10 instance [incoherent] (Integral a, Packable range a, MultipleTo range a) => SubtypeOf range a -- Defined at ...:29:10 Failed, modules loaded: none. If I remove one of instances of SubtypeOf the program is compiling. How to write this instances properly, or to write proper type casting ? Thanks PS. My english is not very good, but I hope this is understandable.

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why no replace function in our regular expression libs ?
by Simon Michael 26 Jan '13

26 Jan '13

People have put a lot of work into regular expression libraries on haskell. Yet it seems very few of them provide a replace/substitute function - just regex-compat and regepr as far as I know. Why is that ? #haskell says: <sclv> iirc its because that's a really mutatey operation in the underlying c libs <sclv> should be simple enough to write a general purpose wrapper layer that uses captures to create the effect Secondly, as of today what do y'all do when you need that functionality ? -Simon

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Data.Sequence and replicateM
by Daniel Díaz Casanueva 25 Jan '13

25 Jan '13

Hi Cafe, I was coding this morning when I suddenly found something that surprised me. It's been a short time since I am really caring about the performance of my programs. Before, I was just caring about their correctness. So I am trying different things and profiling to see differences. One difference I have found surprising is that the function f is MUCH faster and less space consuming than the function g: import Control.Monad import qualified Data.Sequence as Seq type Seq = Seq.Seq f :: Monad m => Int -> m a -> m (Seq a) f n = fmap Seq.fromList . replicateM n g :: Monad m => Int -> m a -> m (Seq a) g = Seq.replicateM Maybe is just in my test case, where the Int argument is big and the monadic action short, but it looks to me that Data.Sequence.replicateM can be faster than it is right now. Regards, Daniel Díaz. -- E-mail sent by Daniel Díaz Casanueva let f x = x in x

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ANNOUNCE: hslogstash (library for working with Logstash and related tools)
by Simon Marechal 25 Jan '13

25 Jan '13

This is a library for sysadmins and/or tool writers. It provides proper types and correct serialization for Logstash messages, along with small utilities for working with ElasticSearch or Redis (using hedis). This library focus will be on safety (no messages lost). Right now, it can be used to: * send properly formated messages to Logstash * read Logstash messages from Redis or any socket * write Logstash messages to ElasticSearch or any socket This is the work of a sysadmin, thus very likely to be horrible to read. Suggestions are very welcome.

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ANNOUNCE: creatur (framework for artificial life)
by Amy de Buitléir 25 Jan '13

25 Jan '13

I'm happy to announce a new package called Créatúr (creatur) http://hackage.haskell.org/package/creatur https://github.com/mhwombat/creatur-examples/raw/master/Tutorial.pdf (tutorial) https://github.com/mhwombat/creatur (github) ABOUT CRÉATÚR: Créatúr is a software framework for automating experiments with artificial life (ALife) or other evolutionary algorithms (EA). It provides a daemon which ensures that each agent gets its turn to use the CPU. You can use other applications on the computer at the same time without fear of interfering with experiments; they will run normally (although perhaps more slowly). Créatúr also provides a library of modules to help you implement your own ALife species. Even if you aren't using the Créatúr framework, you may find some of these modules useful. A tutorial on how to use Créatúr, including lots of examples, is available at https://github.com/mhwombat/creatur-examples/raw/master/Tutorial.pdf. Créatúr (pronounced kray-toor) is an irish word meaning animal, creature, or unfortunate person.

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ANN: gray-extended (tools for working with Gray codes)
by Amy de Buitléir 25 Jan '13

25 Jan '13

I'm happy to announce a new package called gray-extended http://hackage.haskell.org/package/gray-extended-1.2 https://github.com/mhwombat/gray-extended (github) ABOUT GRAY-EXTENDED: Gray codes satisfy the property that two successive values differ in only one digit. Usually the term \"Gray code\" refers to the Binary Reflected Gray code (BRGC), but non-binary Gray codes have also been discovered.

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Manually instantiating Typeable w/DataKinds
by Uri Braun 25 Jan '13

25 Jan '13

I've read the recent posting titled "Non-derivable Typeable" (http://www.mail-archive.com/haskell-cafe@haskell.org/msg103616.html) which explains that Typeable cannot be automatically derived for cases where the kind is constrained. I'm very impressed that a solution is imminent. In the interim, can somebody kindly suggest a workaround? I'm okay with a manual instance, but I'd appreciate some help as to how to write one. I'm looking for a Typeable instance for TaggedVar for the following example below (extracted from my code). Thank you in advance! +Uri {-# LANGUAGE DeriveDataTypeable, DataKinds, KindSignatures #-} module Example where import Data.Typeable data Tag = TagV | TagE | TagA | TagL deriving Typeable newtype TaggedVar (t :: Tag) = TaggedVar Int

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hxt pickling question
by briand＠aracnet.com 25 Jan '13

25 Jan '13

Hi, Trying to understand how to write a document using a pickler but I can't make sense of the types. From the example: runX ( xunpickleDocument xpSeason [ withValidate no , withTrace 1 , withRemoveWS yes , withPreserveComment no ] "simple2.xml" >>> processSeason >>> xpickleDocument xpSeason [ withIndent yes ] "new-simple2.xml" ) So all I want to do is pickle a value directly instead of reading the value from a document. I expected to do something like: runX (someHXTPicklingFunction myValue >>> xpickleDocument ...) but I can't seem to figure out what someHXTPicklingFunction should be, it's certainly nothing obvious like pickleDoc, because that generates the wrong value. Seems like I probably have a much more fundamental problem in that I really don't understand how the arrow part of this little example really works, but I was kind of hoping that doing something "simple" like this might shed some light on that. And then was immediately stuck in type hell. Thanks, Brian

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Video - Simon Marlow on High Performance Concurrency - Skills Matter's Haskell eXchange
by Theo 24 Jan '13

24 Jan '13

Simon Marlow took the floor at the first annual Haskell eXchange last year to talk about high performance concurrency. Simon observes that the haskell community has been spending inordinate amounts of time designing beautiful programming models and APIs, whilst spending relatively less time building applications that solve real problems. In this talk Simon wants to start from an application that they want to build -- a simple concurrent chat server---and use that to inform some library design. Along the way we’ll learn useful techniques for building concurrent network applications, and show how the power of Software Transactional Memory can be brought to bear. Yet this application demonstrates some holes in the abstractions provided by Haskell as it stands, Simon shows a simple layer called “async” that fills the gap and should be widely useful for concurrent programming. To watch the podcast in full follow this link - http://bit.ly/10Bzhr1

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Haskell Weekly News: Issue 255
by Daniel Santa Cruz 24 Jan '13

24 Jan '13

Welcome to issue 255 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the weeks of January 06 to 19, 201. Quotes of the Week * ky3: Indexed monads are just categories in the category of endofunctors, what's the problem? * tac: I guess I'm just living in a fantasy world where the docs tell you how to use such a basic API :P * shachaf: 3.0 is an edwardk phase, not just a version number. It's like Picasso's Blue Period. * cmccann: Clearly the reason why edwardk uses Haskell now is because every C++ compiler has a restraining order against him. * shachaf: I'm not sure what your question is, but maybe lens is the answer. * cmccann: * cmccann has concluded that lenses are just a category whose objects are english prepositions and whose morphisms are all unsafeCoerce * shachaf: Some people, when asking about a problem in #haskell, get a link to okmij.org. Now they have two problems. * quicksilver: It returns undefined for convenience. I'm not really sure it's that convenient, but that's why. * edwardk: cmccann: let me get this straight you are saying that someone [mm_freak] who by their very nick admits to being a freak did something freakish? Top Reddit Stories * LiquidHaskell: Refinement Types in Haskell via SMT Domain: goto.ucsd.edu, Score: 71, Comments: 22 On Reddit: [1] http://goo.gl/PE2nf Original: [2] http://goo.gl/P3Gm9 * Hakyll 4.0 released Domain: jaspervdj.be, Score: 59, Comments: 19 On Reddit: [3] http://goo.gl/IQAXq Original: [4] http://goo.gl/AbLnM * When Haskell is faster than C Domain: paulspontifications.blogspot.co.uk, Score: 52, Comments: 23 On Reddit: [5] http://goo.gl/3rj8J Original: [6] http://goo.gl/N2cbA * I'm teaching an introductory 12-week class on Haskell. Join in for a journey! Domain: reddit.com, Score: 45, Comments: 15 On Reddit: [7] http://goo.gl/TsnQ5 Original: [8] http://goo.gl/EMMVR * Subtractable values are torsors Domain: ro-che.info, Score: 42, Comments: 15 On Reddit: [9] http://goo.gl/4IcMp Original: [10] http://goo.gl/6YJrY * Chordify: extract chord sequences from MP3's using HarmTrace Domain: chordify.net, Score: 42, Comments: 8 On Reddit: [11] http://goo.gl/8J7rA Original: [12] http://goo.gl/ySfR5 * Getting Started with Haskell Domain: bob.ippoli.to, Score: 41, Comments: 8 On Reddit: [13] http://goo.gl/4Evnb Original: [14] http://goo.gl/nzg9o * Using AngularJS with Haskell Snap Domain: nurpax.github.com, Score: 40, Comments: 19 On Reddit: [15] http://goo.gl/SH8e1 Original: [16] http://goo.gl/0lSyN * Down with kinds: adding dependent heterogeneous equality to to FC Domain: cis.upenn.edu, Score: 38, Comments: 14 On Reddit: [17] http://goo.gl/iEvyI Original: [18] http://goo.gl/cH0Eo * Fixing GADTs : Generic recursion schemes for GADTs using fixed points of higher-order functors Domain: timphilipwilliams.com, Score: 38, Comments: 25 On Reddit: [19] http://goo.gl/dEMq9 Original: [20] http://goo.gl/vnjhp * What's the importance of Category Theory in Haskell? Is it worth really studying? Domain: self.haskell, Score: 37, Comments: 60 On Reddit: [21] http://goo.gl/T430r Original: [22] http://goo.gl/T430r * Parsing Markdown with Parsec: How Pandoc does it Domain: youtube.com, Score: 35, Comments: 13 On Reddit: [23] http://goo.gl/TN9TJ Original: [24] http://goo.gl/zpX7f * Clckwrks, from the creator of Happstack, is developing into a clean alternative to Wordpress Domain: clckwrks.com, Score: 34, Comments: 7 On Reddit: [25] http://goo.gl/L9WXx Original: [26] http://goo.gl/FZRpU * pipes-safe-1.0 - Resource management and exception handling for pipes Domain: haskellforall.com, Score: 32, Comments: 27 On Reddit: [27] http://goo.gl/HeLWz Original: [28] http://goo.gl/1hTFc Top StackOverflow Questions * values, types, kinds,… as an infinite sequence? votes: 36, answers: 1 Read on SO: [29] http://goo.gl/lyyYc * Why is this a useful warning in Haskell when pattern matching? “Defined but not used” votes: 17, answers: 3 Read on SO: [30] http://goo.gl/iQLFq * Why are getArgs and getProgName IO actions? votes: 16, answers: 3 Read on SO: [31] http://goo.gl/5ZPYO * Can GADTs be used to prove type inequalities in GHC? [duplicate] votes: 14, answers: 2 Read on SO: [32] http://goo.gl/fDdha * Applying Denotational Semantics to design of Programs [migrated] votes: 14, answers: 0 Read on SO: [33] http://goo.gl/KLOCX * Examples where compiler-optimized functional code performs better than imperative code votes: 14, answers: 1 Read on SO: [34] http://goo.gl/Fzqdx * Why is there only one non-strict function from Int to Int? votes: 12, answers: 2 Read on SO: [35] http://goo.gl/KXcAy * Dependency injection in Haskell: solving the task idiomatically votes: 12, answers: 3 Read on SO: [36] http://goo.gl/xTcYc * What does a Haskell comment starting with `{- |` generally mean? votes: 11, answers: 1 Read on SO: [37] http://goo.gl/zNikw * Is operational really isomorphic to a free monad? votes: 10, answers: 3 Read on SO: [38] http://goo.gl/AJlgH Until next time, [39]+Daniel Santa Cruz References 1. http://goto.ucsd.edu/~rjhala/liquid/haskell/blog/blog/2013/01/01/refinement… 2. http://www.reddit.com/r/haskell/comments/16w3hp/liquidhaskell_refinement_ty… 3. http://jaspervdj.be/posts/2013-01-16-hakyll-4.0.html 4. http://www.reddit.com/r/haskell/comments/16ojtv/hakyll_40_released/ 5. http://paulspontifications.blogspot.co.uk/2013/01/when-haskell-is-faster-th… 6. http://www.reddit.com/r/haskell/comments/16tryo/when_haskell_is_faster_than… 7. http://www.reddit.com/r/UniversityofReddit/comments/169jfe/class_introducti… 8. http://www.reddit.com/r/haskell/comments/169k67/im_teaching_an_introductory… 9. http://ro-che.info/articles/2013-01-08-torsors.html 10. http://www.reddit.com/r/haskell/comments/166iju/subtractable_values_are_tor… 11. http://chordify.net/pages/how-to-use-chordify/ 12. http://www.reddit.com/r/haskell/comments/16u44u/chordify_extract_chord_sequ… 13. http://bob.ippoli.to/archives/2013/01/11/getting-started-with-haskell/ 14. http://www.reddit.com/r/haskell/comments/16fegr/getting_started_with_haskel… 15. http://nurpax.github.com/posts/2013-01-13-angularjs-and-haskell.html 16. http://www.reddit.com/r/haskell/comments/16iaiq/using_angularjs_with_haskel… 17. http://www.cis.upenn.edu/~eir/papers/2013/jmnokinds/jmnokinds.pdf 18. http://www.reddit.com/r/haskell/comments/167c21/down_with_kinds_adding_depe… 19. http://www.timphilipwilliams.com/posts/2013-01-16-fixing-gadts.html 20. http://www.reddit.com/r/haskell/comments/16qr6q/fixing_gadts_generic_recurs… 21. http://www.reddit.com/r/haskell/comments/16i322/whats_the_importance_of_cat… 22. http://www.reddit.com/r/haskell/comments/16i322/whats_the_importance_of_cat… 23. http://www.youtube.com/watch?v=nCwG9ijQMuQ&t=01m22s 24. http://www.reddit.com/r/haskell/comments/16kf0j/parsing_markdown_with_parse… 25. http://www.clckwrks.com/ 26. http://www.reddit.com/r/haskell/comments/168j2w/clckwrks_from_the_creator_o… 27. http://www.haskellforall.com/2013/01/pipes-safe-10-resource-management-and.… 28. http://www.reddit.com/r/haskell/comments/16mban/pipessafe10_resource_manage… 29. http://stackoverflow.com/questions/14401689/values-types-kinds-as-an-infini… 30. http://stackoverflow.com/questions/14309396/why-is-this-a-useful-warning-in… 31. http://stackoverflow.com/questions/14295582/why-are-getargs-and-getprogname… 32. http://stackoverflow.com/questions/14273235/can-gadts-be-used-to-prove-type… 33. http://stackoverflow.com/questions/14349232/applying-denotational-semantics… 34. http://stackoverflow.com/questions/14357918/examples-where-compiler-optimiz… 35. http://stackoverflow.com/questions/14262014/why-is-there-only-one-non-stric… 36. http://stackoverflow.com/questions/14327327/dependency-injection-in-haskell… 37. http://stackoverflow.com/questions/14270500/what-does-a-haskell-comment-sta… 38. http://stackoverflow.com/questions/14263363/is-operational-really-isomorphi… 39. https://plus.google.com/105107667630152149014/about

1 0

ANN: monad-bool 0.1
by John Wiegley 24 Jan '13

24 Jan '13

monad-bool implements a pair of Boolean monoids and monads, to support short-circuiting, value-returning computations similar to what Python and Ruby offer with their native && and || operators. For example, in Python you might see this: x = [1,2,3,0] print x[1] || x[3] -- prints "2" With this library, you can now mirror such code in Haskell: let x = [1,2,3,0] print $ (x !! 1) ||? (x !! 3) -- prints "Success 2" "Booleanness" is based on each type having an instance of the 'Control.Conditional.ToBool' type, for which only the basic types are covered (Bool, Int, Integer, Maybe a, Either a b, [a], Attempt a). If you wish to define a truth value for your own types, simply provide an instance for ToBool: instance ToBool MyType where toBool = ... The And/Or monoids use the Attempt library so that the actual type of the successful results depends on case analysis. It could be a list, a Maybe, an Either, or an exception in the IO Monad. The monad variants, AndM, AndMT, OrM and OrMT provide short-circuiting behavior in a Monad, which returns the last value returned before truth was determined. Here are two examples: Use 'onlyIf' with AndM and AndMT to guard later statements, which are only evaluated if every preceding 'onlyIf' evaluates to True. For example: foo :: AndM Int foo = do onlyIf (True == True) return 100 onlyIf (True == True) return 150 onlyIf (True == False) return 200 When run with `evalAndM foo (-1)` (where (-1) provides a default value), 'foo' returns 150. Use 'endIf' with OrM and OrMT to chain statements, which are only executed if every preceding 'endIf' evaluated to False. For example: bar :: OrM Int bar = do endIf (True == False) return 100 endIf (True == False) return 150 endIf (True == True) return 200 When run with `evalOrM bar (-1)` (where (-1) again provides a default value), 'bar' returns 150. And please, somebody let me know if this has already been done. A search for likely candidates did not turn up anything obvious in Hoogle, but as my knowledge of the whole of Hackage is minimal, I would appreciate any wiser minds that can inform me. Thank you, -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

9 17

Space leaks in function that uses Data.Vector.Mutable
by Andrey Yankin 24 Jan '13

24 Jan '13

Hi! I have a "low-level" function for insertion element into mutable vector. It looks like this: place :: (PrimMonad m) => MV.MVector (PrimState m) (Int, t) -> (Int, t) -> Int -> m () place v max@(val1,_) i = place' i where place' i = do let j = i - 1 if j < 0 then return () else do curr@(val2, _) <- MV.unsafeRead v j -- <<<<< if val2 > val1 then do MV.unsafeWrite v j max MV.unsafeWrite v i curr place' j else return () It searches a right place for the i-th element, moving it towards beginning . Surprisingly, it works, but is slow. Time profiling says this: COST CENTRE MODULE %time %alloc ticks bytes place.place' Main 40.1 77.1 9167 12169223232 And heap profiling says that most of allocations are for Int and (,). I think, binding of unsafeRead to curr and val might allocate my Precious memory more than, maybe, it is needed. Am I right? Is it some strictness that I need? Is there anything I can do in this situation? Or should I look outside of this function? PS Emergence of this piece of code is a long story. Originally I was trying to solve Project Euler's problem 411. But now it is a different issue.

2 1

ANN: hgdbmi 0.2, GDB Machine Interface
by Alexander Bernauer 23 Jan '13

23 Jan '13

Hi I am happy to announce the second release of hgdbmi, a Haskell implementation of the GDB Machine Interface. The Machine Interface is a machine-readable variant of the stdin/stdout-based interface of the GNU debugger. With hgdbmi you can control the execution of a GDB instance to drive debugging of arbitrary targets. Version 0.2 of this library is a substantial, non-compatible extension of version 0.1. The API provides a thread-safe and synchronous command-response function and supports user-defined call-backs for asynchronous notifications. All commands, responses and notifications are supported in this version. Also, for some response and notification types, we additionally provide semantic data types and corresponding translation functions. The package: http://hackage.haskell.org/package/hgdbmi-0.2 Greetings, Alex PS: This is my first serious Haskell package. If you see something that you consider odd, I might have done this simply because I didn't know better. I would appreciate if you would let me know in such a case.

2 4

The Coverage Condition of functional dependencies
by Petr P 23 Jan '13

23 Jan '13

Hi, trying to understand UndecidableInstances (and to find and answer to < http://stackoverflow.com/q/14476230/1333025>), I was trying to find out why "mtl" needs UndecidableInstances. The reason is that instances like > instance MonadState s m => MonadState s (ContT r m) where don't satisfy the Coverage Condition: "The Coverage Condition. For each functional dependency, tvsleft -> tvsright, of the class, every type variable in S(tvsright) must appear in S(tvsleft), where S is the substitution mapping each type variable in the class declaration to the corresponding type in the instance declaration. " (See http://www.haskell.org/ghc/docs/7.0.1/html/users_guide/type-class-extension… ) In other words, "s" isn't expressed using type variables in "ContT r m". But in these cases, it's actually possible. Because of the assertion "MonadState s m" and its dependency "m -> s" we know that "s" will be always deducible from "m". I wonder, would it be possible to augment the type checker to realize this? It seems reasonable: Before comparing if S(tvsright) is a subset of S(tvsleft), we'd add every type variable to S(tvsleft) that is determined from it using functional dependencies in the assertion of the instance. Best regards, Petr

1 0

Re: [Haskell-cafe] Mobile app development?
by Andrew Pennebaker 23 Jan '13

23 Jan '13

Can we un-deprecate GHC's ability to compile to C code? C may be the best option to bridge to mobile, as Android, iOS, and Windows RT do support C/C++ apps. On Jan 22, 2013 2:14 PM, "Dan Choi" <dhchoi(a)gmail.com> wrote: > > What about the option of using Haskell's Parsec or AttoParsec to implement > a Haskell-ish language that compiles down not to machine code but to > Objective C or Android Java? Like how CoffeeScript compiles down to > JavaScript. > > > > On Saturday, January 19, 2013 1:58:03 PM UTC-5, Casey Hawthorne wrote: >> >> You would need native compilers for all the platforms and/or virtual >> machine technology. >> >> Might be easier to have the browser connect to a Haskell app. >> >> >> On Sat, Jan 19, 2013 at 10:42 AM, Andrew Pennebaker >> <andrew.p...(a)gmail.com> wrote: >> > There are currently very few options, especially free and open source >> > options, when it comes to developing cross-platform mobile >> applications. >> > It's basically web apps with JavaScript, or C++. If Haskell supported >> app >> > development on Android, iOS, and Windows RT, that alone would bring in >> more >> > developers. >> > >> > Similarly, there are very few languages for mobile development that >> take >> > advantage of multiple cores and multiple CPUs. Haskell's `parmap` is an >> > amazing selling point. Can we please prioritize mobile support? I'd >> much >> > rather write everything in ML than PhoneGap. >> > >> > -- >> > Cheers, >> > >> > Andrew Pennebaker >> > www.yellosoft.us >> > >> > ______________________________**_________________ >> > Haskell-Cafe mailing list >> > Haskel...(a)haskell.org >> > http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> >> > >> >> >> >> -- >> -- >> Regards, >> KC >> >> ______________________________**_________________ >> Haskell-Cafe mailing list >> Haskel...(a)haskell.org >> http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> >> >

3 5

Re: [Haskell-cafe] ANN: Haskell bindings for the igraph C library
by George Giorgidze 23 Jan '13

23 Jan '13

Hi Jason, > How does this compare with fgl? http://hackage.haskell.org/package/fgl A couple of months ago, I was working on a project where I had to do some graph analysis. I encountered two problems with the fgl library: (1) The fgl library did not work for large graphs, even graph construction would fail with stack overflow. (2) The fgl library did not provide the graph algorithms needed in the project: closeness centrality, Brandes betweenness centrality, eigenvector centrality, pagerank and some more. The igraph library featured all functions that I needed and also worked for large graphs without problems. I was also very pleased with its performance and started writing Haskell bindings to igraph. Cheers, George

2 1

Re: [Haskell-cafe] [Haskell] ANNOUNCE: MFlow 0.2
by Alberto G. Corona 22 Jan '13

22 Jan '13

The template look is very simple but it uses a lot of dynamic code behind I changed the template to something more light. http://haskell-web.blogspot.com.es/ 2013/1/20 <briand(a)aracnet.com> > On Thu, 17 Jan 2013 16:07:41 +0100 > "Alberto G. Corona " <agocorona(a)gmail.com> wrote: > > > > > The entry in my blog, with the announcement and the philosophy behind > > http://haskell-web.blogspot.com.es/2013/01/announce-mflow-02.html > > > > FYI > That page never comes up for me. I just see spinning gears. > iceweasel 10.0.7 > > Brian > > -- Alberto.

2 1

Re: [Haskell-cafe] Mobile app development?
by KC 22 Jan '13

22 Jan '13

Then it looks as if the easier implementation would be small Haskell VM's for the various platforms with a byte code compiler. I do not believe the JVM supports all the optimizations GHC can do. Oh wait! Can the LLVM be easily ported to do this? On Sat, Jan 19, 2013 at 11:40 AM, Andrew Pennebaker <andrew.pennebaker(a)gmail.com> wrote: > >> Might be easier to have the browser connect to a Haskell app. > > > Not all apps can be run as thin clients. 3D video games and other intensive > programs aren't easily done as thin clients. Mobile Haskell would be very > powerful, because concurrency and parallelism aren't something the C-family > languages are supporting that well. > >> >> >> >> On Sat, Jan 19, 2013 at 10:42 AM, Andrew Pennebaker >> <andrew.pennebaker(a)gmail.com> wrote: >> > There are currently very few options, especially free and open source >> > options, when it comes to developing cross-platform mobile applications. >> > It's basically web apps with JavaScript, or C++. If Haskell supported >> > app >> > development on Android, iOS, and Windows RT, that alone would bring in >> > more >> > developers. >> > >> > Similarly, there are very few languages for mobile development that take >> > advantage of multiple cores and multiple CPUs. Haskell's `parmap` is an >> > amazing selling point. Can we please prioritize mobile support? I'd much >> > rather write everything in ML than PhoneGap. >> > >> > -- >> > Cheers, >> > >> > Andrew Pennebaker >> > www.yellosoft.us >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> >> >> >> -- >> -- >> Regards, >> KC > > > > > -- > Cheers, > > Andrew Pennebaker > www.yellosoft.us -- -- Regards, KC

4 3

Non-derivable Typeable
by Roman Cheplyaka 22 Jan '13

22 Jan '13

I need to declare a Typeable instance for a type which has an argument of kind * -> *. GHC refuses to derive it. What is a recommended way to go about it? In particular, if I write the instance by hand, how important is the fingerprint, and how could I generate it? Roman

2 5

[ANN] tls-extra 0.6.1 - security update, please upgrade.
by Vincent Hanquez 22 Jan '13

22 Jan '13

Hi cafe, this is a security advisory for tls-extra < 0.6.1 which are all vulnerable to bad certificate validation. Some part of the certificate validation procedure were missing (relying on the work-in-progress x509 v3 extensions), and because of this anyone with a correct end-entity certificate can issue certificate for any arbitrary domain, i.e. acting as a CA. This problem has been fixed in tls-extra 0.6.1, and I advise everyone to upgrade as soon as possible. Despite a very serious flaw in the certificate validation, I'm happy that the code is seeing some audits, and would want to thanks Ertugrul Söylemez for the findings [1]. [1] https://github.com/vincenthz/hs-tls/issues/29 -- Vincent

3 5

regular expression sub-expression matching
by Nicolas Bock 22 Jan '13

22 Jan '13

Hello list, I was wondering whether there is a way to do what the following python code does in haskell: import re result = re.compile("M$([0-9]+),([0-9]+)$ *= *([0-9.eE-]+);").search("M(1,2) = 0.1e-3;") if result: print(result.group(1), result.group(2), result.group(3)) Basically I would like to pattern match parts of a string and return the matches. I have looked at Text.Regex.Posix but (of course I might just not have understood the functions in there properly) it seems as if it does not provide anything like python's re module. Thanks already, nick

3 3

XCode Dependency for HP on Mac
by Tom Murphy 21 Jan '13

21 Jan '13

This may sound ignorant because, well, it is ignorant: I know very little about the underlying mechanics here. Installing the Haskell Platform currently requires XCode developer tools. To get XCode on my 10.6 machine, I... [*** begin ranty details (skippable) ... was told I could get a free version by registering as an Apple Developer. So I lie on the forms (phone number and address, for example, are _required_ fields!), and lie on the _required_ 2-3 page survey. I put in a code that they sent to my email (couldn't lie on that!), and log in. The page tells me, in the exact box that told me if I registered I could get XCode for free, that I... *** end ranty details] ...have to either pay to upgrade to their newer OS (10.7: Lion), or pay $99/year for a Mac OS Developer Membership. Is there a way to install HP without XCode? Could there be in the future? I'm tired of dealing with Apple's constant upgrade requirements, registration requirements, etc., and it seems like a small function that XCode actually performs in the Haskell development "toolchain." Again, I'm ignorant of the details and I'm sorry if this is ranty, but I'd love to hear your reactions. Thanks! Tom

16 26

ANN: Hquery 0.1
by Tycho Andersen 21 Jan '13

21 Jan '13

Hi cafe, I've written a library called hquery, which can be used to manipulate xmlhtml trees (and has a similar selector syntax to jquery, hence the name). This enables easy manipulation and content generation from html templates. If you're familiar with Lift's CssSels, this is basically an implementation of those in haskell. This is my first foray into haskell, so any critiques of the code or API are welcome :-) Hquery: http://hackage.haskell.org/package/hquery-0.1.0.1 Examples: http://hackage.haskell.org/packages/archive/hquery/0.1.0.1/doc/html/Text-Hq… \t

1 0

forkProcess, forkIO, and multithreaded runtime
by Michael Snoyman 21 Jan '13

21 Jan '13

Hi all, I think I have a misunderstanding of how forkProcess should be working. Ultimately this relates to some bugs in the development version of keter, but I've found some behavior in a simple test program which I wouldn't have expected either, which may or may not be related. With the program at the end of this email, I would expect that, once per second, I would get a message printed from each forkIO'd green thread, the forked process, and the master process. And if I spawn 8 or less child threads that's precisely what happens. However, as soon as I up that number to 9, the child process is never run. The process is, however, created, as can be confirmed by looking at the process table. This only occurs when using the multithreaded runtime. In other words, compiling with "ghc --make test.hs" seems to always produce the expected output, whereas "ghc --make -threaded test.hs" causes the behavior described above. Having looked through the code for the process package a bit, my initial guess is that this is being caused by a signal being sent to the child process, but I'm not familiar enough with the inner workings to confirm or disprove this guess. If anyone has any ideas on this, I'd appreciate it. Michael import System.Posix.Process (forkProcess, getProcessID) import Control.Concurrent (forkIO, threadDelay) import System.IO (hFlush, stdout) import System.Posix.Signals (signalProcess, sigKILL) import Control.Exception (finally) main :: IO () main = do mapM_ spawnChild [1..9] child <- forkProcess $ do putStrLn "starting child" hFlush stdout loop "child" 0 print ("child pid", child) hFlush stdout -- I've commented out the "finally" so that the zombie process stays alive, -- to prove that it was actually created. loop "parent" 0 -- `finally` signalProcess sigKILL child spawnChild :: Int -> IO () spawnChild i = do _ <- forkIO $ loop ("spawnChild " ++ show i) 0 return () loop :: String -> Int -> IO () loop msg i = do pid <- getProcessID print (pid, msg, i) hFlush stdout threadDelay 1000000 loop msg (i + 1)

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Up-front Unit Testing in Haskell
by Kazu Yamamoto 21 Jan '13

21 Jan '13

#Resending due to the lack of URL. Hello cafe, Just FYI: I wrote an tutorial, "Up-front Unit Testing in Haskell". It is about how to use doctest, hspec and Cabal. https://github.com/kazu-yamamoto/unit-test-example/blob/master/markdown/en/… Enjoy! --Kazu

3 2

Up-front Unit Testing in Haskell
by Kazu Yamamoto 21 Jan '13

21 Jan '13

Hello cafe, Just FYI: I wrote an tutorial, "Up-front Unit Testing in Haskell". It is about how to use doctest, hspec and Cabal. Enjoy! --Kazu

2 2

Re: [Haskell-cafe] [Haskell] Silent error
by Henning Thielemann 21 Jan '13

21 Jan '13

On Mon, 21 Jan 2013, miphis(a)meta.ua wrote: > The program looks like this: > > --*********************** > factorial :: Int->Int > factorial n = product [1..n] > > main = do > print $ factorial 50 > --*********************** > > And that yields "0" (no errors). Is it a bug or feature? :) This question is certainly better posted to haskell-cafe. The answer is: Int has limited bitsize (32 ord 64 bit depending on your machine), thus it computes factorial modulo 2^32 or 2^64. You can get the correct result by replacing Int by Integer.

1 0

"Function decoration pattern" on wiki
by Derek Elkins 20 Jan '13

20 Jan '13

I created the following page on the wiki to capture one pattern that occurs often in Edward Kmett's lens library. http://www.haskell.org/haskellwiki/Function_decoration_pattern Feel free to rename it or add comments, examples, clarifications, additional notes or extensions to the technique.

1 0

Duplicates in arbitrary instances for collections
by Tom Murphy 20 Jan '13

20 Jan '13

Every QuickCheck tutorial makes the point that, given a property on some type, the Arbitrary instance for that type is written in such a way that nearly all of the interesting edge-cases of that type are tried. So e.g. for numeric types, positive, negative, and zero numbers are tried. For collections, I think the case of duplicate elements is insufficiently covered. Say we have a list difference function: badDiff :: Eq a => [a] -> [a] -> [a] badDiff l r = filter (\e -> e `notElem` r) l And a test for it: prop_wrong :: [Int] -> [Int] -> Bool prop_wrong a b = (a `badDiff` b) == (a \\ b) `quickCheck prop_wrong` passes about 80% of the time! Obviously it shouldn't: > badDiff [1,2,3,3,3] [2,3] [1] > (\\) [1,2,3,3,3] [2,3] [1,3,3] But even with maxSuccess at 1000, this property passes at least 2/3 of the times it's run (on my machine, where (maxBound :: Int) is large). The reason is that `arbitrary` for [a] is just `sized $ \n -> choose (0,n) >>= \k -> sequence [arbitrary | _ <- [1..k]]` It would be trivial to duplicate values in the collection a certain percentage of the time (possibly sometimes adjacent). Is there a good reason not to? It shouldn't require an Eq constraint. Tom

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Container libraries
by Alexander Solla 20 Jan '13

20 Jan '13

Hi everybody, What is the state of container libraries? I am looking for a library which provides: * unordered containers (for "operational" type safety, I don't want to impose orders on things that don't have them -- unordered containers does this) * can handled nested containers (containers does that) * can be serialized easily, or better yet, has Generic instances Does such a library exist? What are you using for Set and Map needs? I have been fond of unordered-containers for a while, but found difficulty in creating HashSets of, for example, HashMaps, since there are no Hashable instances for their polymorphic types. I even dug into the code, looking at the possibility of submitting a patch, but gave up when I ran into funky #-suffixed values, which stymied my initial attempt.

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[ANNOUNCE] JuicyPixels 3.0
by Vincent Berthoux 20 Jan '13

20 Jan '13

Hello haskellers, I'm presenting the third version of JuicyPixels. As the version number indicate, this is a major version with backward compatibility break. The major reason behind this breakage is the introduction of High Dynamic Range images. High Dynamic Range images are used in photography, and in some APIs like OpenGL and DirectX for some fancy effects. Version 3.0 changelog : - Simplification of the 'Pixel' typeclass, removed many unused part. - Removal of some Storable instances for pixel types. - Amelioration of the documentation. - Support for High Dynamic range images. - Support for the Radiance file format (or RGBE, file extension .pic and .hdr). - Dropped the cereal library in favor of the last version of Binary, present in the Haskell platform. Every dependencies are now present in the platform. As usual, this version is available in Hackage and on GitHub : - Hackage : http://hackage.haskell.org/package/JuicyPixels - GitHub : https://github.com/Twinside/Juicy.Pixels Thanks Vincent Berthoux

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Inferring rewrite rules for higher order functions
by Joachim Breitner 20 Jan '13

20 Jan '13

Dear list, with rewrite rules I can tell the compiler to replace "map id" by "id", but I still have to write that rule by hand. Does anyone of you know about research into inferring such a rewrite rule automatically? I could imagine that for a function of type "f :: some -> args -> T a -> T b" and not-too-bad type constructors T it might be possible under what conditions on "some" and "args" infer when "f arg1 arg2 = id" holds, and if the conditions are fulfilled by functions like "id", generate a rewrite rule "f id id = id" automatically. Thanks, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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Why should we write "a `par` b `pseq` (f a b)" instead of "a `par` b `par` (f a b)"?
by Petr P 20 Jan '13

20 Jan '13

Dear Haskellers, I've been playing with par and pseq, and I wonder: Is there any reason to use a `par` b `pseq` (a + b) instead of a `par` b `par` (a + b) except that the second version sparks twice instead of just once (which probably degrades performance a bit)? It seems to me that the second variant would work as well: The main thread would block on one of the sparked computations, but the other one would still be evaluated in parallel. The second variant seems to have one additional advantage: If the function that combines 'a' and 'b' isn't strict (and perhaps we don't know that in advance), the main thread won't be blocked by evaluating the computation it doesn't need. Like a `par` b `pseq` (const a b) will block until both 'a' and 'b' are evaluated, but a `par` b `par` (const a b) will finish as soon as 'a' is evaluated. I found this link on SO: Why do we need 'seq' or 'pseq' with 'par' in Haskell? < http://stackoverflow.com/q/4576734/1333025> but it doesn't really solve this objection. Thanks for help, Petr Pudlak

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Mobile app development?
by Andrew Pennebaker 19 Jan '13

19 Jan '13

There are currently very few options, especially free and open source options, when it comes to developing cross-platform mobile applications. It's basically web apps with JavaScript, or C++. If Haskell supported app development on Android, iOS, and Windows RT, that alone would bring in more developers. Similarly, there are very few languages for mobile development that take advantage of multiple cores and multiple CPUs. Haskell's `parmap` is an amazing selling point. Can we please prioritize mobile support? I'd much rather write everything in ML than PhoneGap. -- Cheers, Andrew Pennebaker www.yellosoft.us

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Stackage mailing list
by Joachim Breitner 19 Jan '13

19 Jan '13

Dear Michael, I’m wondering if I missed something, but is there a mailing list for stackage? Or has one of the standard lists (-cafe, libraries) been designated for questions about stackage? What I want to know is if you plan to provide a website for stackage soon where the list of included packages and their dependencies, including the tested and approved version numbers can be seen. I’d like to take that and integrate it on http://people.debian.org/~nomeata/platform.html and http://people.debian.org/~nomeata/hackagevsdebian.html Also, once you do that, it should be possible to add Stackage to the list of Distributions listed on hackage, so that on hackage one already sees the package’s stackage status and whether it lags behind there. The integrations is rather simple, just provide a file with that format: http://people.debian.org/~nomeata/cabalDebianMap.txt and bug the hackage maintainers to add it. Greetings, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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