- Haskell-Cafe - Haskell.org

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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