November 2012 - Haskell-Cafe

[ANNOUNCE] hashable-generics
by Clark Gaebel 07 Nov '12

07 Nov '12

Hi everybody! I have just released a handy package on Hackage that will interest you if you've ever used unordered-containers with a custom type. In order to do such a thing, you'd need to define an instance of Hashable. This process could easily be automated. And so I did. {-# LANGUAGE DeriveGeneric #-} module ThisIsPrettyNeat where import Data.Hashable.Generic import GHC.Generics data MyCoolType a = MCT0 | MCT1 (Either Int a) | MCT2 (MyCoolType a) (MyCoolType a) deriving Generic instance Hashable a => Hashable MyCoolType where hashWithSalt s x = gHashWithSalt s x {-# INLINEABLE hashWithSalt #-} and voila. You have a very performant instance of Hashable, with minimal boilerplate, no template haskell, and no subtle bugs. If you want to play with it, here it is: http://hackage.haskell.org/package/hashable-generics-1.1.6 Have fun! - Clark Oh yeah, and if anyone wants to help me figure out why it's 1.3x slower than my hand-rolled instances, that would be really helpful.

4 16

ANNOUNCE: rdf4h
by Rob Stewart 07 Nov '12

07 Nov '12

Hi, I am pleased to announce the recent release of rdf4h 1.1.0 . The Resource Description Framework (RDF) is a language for representing information about resources in the World Wide Web, and is a family of W3C specifications. RDF is a standardized machine readable data model that provides a common framework for expressing information to be exchanged between applications without loss of meaning. It is becoming a popular data-modelling language, thanks to the growing adoption of semantic web principles for publishing linked open data. This `rdf4h' is a library for working with RDF in Haskell. It includes parsers and serializers for the N-Triples and Turtle formats. It provides abilities such as RDF graph comparison, querying for triples containing a particular subject, predicate, or object, or selecting triples that satisfy an arbitrary predicate function. The library is on hackage: http://hackage.haskell.org/package/rdf4h Examples are on GitHub: https://github.com/robstewart57/rdf4h Contributions very welcome! -- Rob Stewart

1 0

Serializing with alignment
by Ben Franksen 06 Nov '12

06 Nov '12

Hi Everyone I want to implement a binary protocol that, unfortunately, has some alignment restrictions. In order to fulfill these, I need access to the "current offset" in the bytestring. The binary package does provide a function bytesRead :: Get Int64 but only for the Get monad; there is no equivalent for the Put monad. So my first question: is there a serialization library that offers something like bytesWritten :: PutM Int64 Failing that, would you think adding it to binary is a reasonable feature request? I have taken a cursory look at the implementation and it looks like this is not a matter of simply adding a missing function, but would probably need an addition to internal data structures. I could also try and wrap the PutM from binary with a StateT transformer and count the bytes myself. I will probably have to use at least a ReaderT wrapper anyway, since I have to pass the byte order as a parameter (byte order gets negotiated between client and server, it is not fixed). I was really hoping that there is some library that has built-in support for stateful serialization (alignment, byte-order, etc). Any pointers, hints, etc are much appreciated. Cheers -- Ben Franksen () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

2 2

Looking for a Full-time Haskell Programmer
by Doug Beardsley 06 Nov '12

06 Nov '12

I've posted this before but did not get a whole lot of responses, so here it is again: Karamaan Group, a principal investment firm based in Manhattan is looking for an outstanding software developer to develop tools for financial analysis and knowledge management. Candidates should have at least a bachelor's degree in computer science, math, or a related field and a good sense of coding style. This is a high-impact, high-visibility job opportunity where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. Karamaan Group is a small hedge fund, so the next hire will have a very direct influence on our software development and the firm as a whole. We are a growth oriented firm that values people who take a craftsman's pride in their work. We like to take a contrarian view of things, value experimenting, and use often overlooked technologies. We are looking for a candidate who shows strong analytical, organizational, and communication skills. We have a legacy Java code base that may need occasional maintenance, but for the past few years, most of our new development has been done with Haskell (with bits and pieces of a few other languages here and there). The ideal candidate would have at least some experience with Java and the related technologies used in our existing code, but since Haskell is our language of choice, we're looking for strong Haskell skills. We have a knowledge management system built on Snap, so experience with Snap or Haskell web development in general would help. Unlike most finance companies, we don't require previous experience in finance or business, but knowledge in those areas is definitely a plus, although candidates should already be able to work legally in the U.S. and be willing to move to New York City. Please send your CV and cover letter to recruitment at karamaan.com.

1 0

Problems updating cabal-install on Mac OS
by Harald Bögeholz 06 Nov '12

06 Nov '12

Hi Cafe, I just installed the brand new Haskell Platform 2012.4.0.0 for Mac OS X, 32 bit, after removing everything left over from the old one (I hope). Next I started cabal which said to me: > Downloading the latest package list from hackage.haskell.org > Note: there is a new version of cabal-install available. > To upgrade, run: cabal install cabal-install So next I did cabal install cabal-install which started working > Resolving dependencies... > Downloading Cabal-1.16.0.2... and finished with > Installing executable(s) in > /Users/bo/Library/Haskell/ghc-7.4.2/lib/cabal-install-1.16.0.1/bin > Updating documentation index /Users/bo/Library/Haskell/doc/index.html When I then tried cabal update it still said > Downloading the latest package list from hackage.haskell.org > Note: there is a new version of cabal-install available. > To upgrade, run: cabal install cabal-install I thought I just had done that. What could have gone wrong? > Haralds-iMac:~ bo$ which cabal > /Users/bo/Library/Haskell/bin/cabal > Haralds-iMac:~ bo$ ls -l `which cabal` > lrwxr-xr-x 1 bo staff 49 6 Nov 18:15 /Users/bo/Library/Haskell/bin/cabal -> ../ghc-7.4.2/lib/cabal-install-1.16.0.1/bin/cabal Now that I am writing this up I am noticing that the last thing that seems to be installed has version number 1.16.0.1, while it started out with 1.16.0.2. So on a second thought I am attaching the full log of the install. Am I doing anything wrong? Harald -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

2 3

Defining a Strict language pragma
by Johan Tibell 06 Nov '12

06 Nov '12

Hi all, I would like to experiment with writing some modules (e.g. low-level modules that do a lot of bit twiddling) in a strict subset of Haskell. The idea is to remove boilerplate bangs (!) and instead declare the whole module strict. I believe this would both make code that needs to be strict more declarative (you say what you want once, instead of putting bangs everywhere), less noisy, and more predictable (no need to reason about laziness in places where you know you don't want it). The idea is to introduce a new language pragma {-# LANGUAGE Strict #-} that has the above described effect. The tricky part is to define the semantics of this pragma in terms of Haskell, instead of in terms of Core. While we also need the latter, we cannot describe the feature to users in terms of Core. The hard part is to precisely define the semantics, especially in the presence of separate compilation (i.e. we might import lazy functions). I'd like to get the Haskell communities input on this. Here's a strawman: * Every function application f _|_ = _|_, if f is defined in this module [1]. This also applies to data type constructors (i.e. the code acts if all fields are preceded by a bang). * lets and where clauses act like (strict) case statements. * It's still possible to define strict arguments, using "~". In essence the Haskell lazy-by-default with opt-out via "!" is replaced with strict-by-default with opt-out via "~". Thoughts? Cheers, Johan 1. I could see the very opposite choice be made: function applications to functions defined in other modules are also strict. We should try to think about the implications of either policy.

5 5

Announce: Haskell Platform 2012.4.0.0
by Mark Lentczner 06 Nov '12

06 Nov '12

I'm pleased to announce that Haskell Platform 2012.4.0.0<http://www.haskell.org/platform/index.html#2012.4.0.0>is now available. This release contains three new packages to the platform: - async-2.0.1.3<http://lambda.haskell.org/platform/doc/current/packages/async-2.0.1.3/doc/h…> - split-0.2.1.1<http://lambda.haskell.org/platform/doc/current/packages/split-0.2.1.1/doc/h…> - vector-0.10.0.1<http://vector-0.10.0.1http://lambda.haskell.org/platform/doc/current/packag…> The remainder of the changes are small updates to the versions of GHC (now 7.4.2) and several other packages. Installers for Windows <http://www.haskell.org/platform/windows.html>, Mac OS X <http://www.haskell.org/platform/mac.html>, and the source tar ball<http://www.haskell.org/platform/linux.html>are all now available from the site. Linux distribution packages should be available shortly. On behalf of the Haskell Platform team<http://trac.haskell.org/haskell-platform/wiki/Members> , - Mark "mzero" Lentczner, Release Manager

4 4

Call for discussion: OverloadedLists extension
by Michael Snoyman 06 Nov '12

06 Nov '12

(Prettier formatting available at: https://gist.github.com/3761252) Many of us use the OverloadedStrings language extension on a regular basis. It provides the ability to keep the ease-of-use of string literal syntax, while getting the performance and correctness advantages of specialized datatypes like ByteString and Text. I think we can get the same kind of benefit by allowing another literal syntax to be overloaded, namely lists. ## Overly simple approach The simplest example I can think of is allowing easier usage of Vector: [1, 2, 3] :: Vector Int In order to allow this, we could use a typeclass approach similar to how OverloadedStrings works: class IsList a where fromList :: [b] -> a b instance IsList Vector where fromList = V.fromList foo :: Vector Int foo = fromList [1, 2, 3] ## Flaws However, such a proposal does not allow for constraints, e.g.: instance IsList Set where fromList = Set.fromList No instance for (Ord b) arising from a use of `Set.fromList' In the expression: Set.fromList In an equation for `fromList': fromList = Set.fromList In the instance declaration for `IsList Set' Additionally, it provides for no means of creating instances for datatypes like Map, where the contained value is not identical to the value contained in the original list. In other words, what I'd like to see is: [("foo", 1), ("bar", 2)] :: Map Text Int ## A little better: MPTC A simplistic approach to solve this would be to just use MultiParamTypeClasses: class IsList input output where fromList :: [input] -> output instance IsList a (Vector a) where fromList = V.fromList foo :: Vector Int foo = fromList [1, 2, 3] Unfortunately, this will fail due to too much polymorphism: No instance for (IsList input0 (Vector Int)) arising from a use of `fromList' Possible fix: add an instance declaration for (IsList input0 (Vector Int)) In the expression: fromList [1, 2, 3] In an equation for `foo': foo = fromList [1, 2, 3] This can be worked around by giving an explicit type signature on the numbers in the list, but that's not a robust solution. In order to solve this properly, I think we need either functional dependencies or type families: ## Functional dependencies class IsList input output | output -> input where fromList :: [input] -> output instance IsList a (Vector a) where fromList = V.fromList instance Ord a => IsList a (Set a) where fromList = Set.fromList instance Ord k => IsList (k, v) (Map k v) where fromList = Map.fromList foo :: Vector Int foo = fromList [1, 2, 3] bar :: Set Int bar = fromList [1, 2, 3] baz :: Map String Int baz = fromList [("foo", 1), ("bar", 2)] ## Type families class IsList a where type IsListInput a fromList :: [IsListInput a] -> a instance IsList (Vector a) where type IsListInput (Vector a) = a fromList = V.fromList instance Ord a => IsList (Set a) where type IsListInput (Set a) = a fromList = Set.fromList instance Ord k => IsList (Map k v) where type IsListInput (Map k v) = (k, v) fromList = Map.fromList foo :: Vector Int foo = fromList [1, 2, 3] bar :: Set Int bar = fromList [1, 2, 3] baz :: Map String Int baz = fromList [("foo", 1), ("bar", 2)] ## Conclusion Consider most of this proposal to be a strawman: names and techniques are completely up to debate. I'm fairly certain that our only two choices to implement this extension is a useful way is fundeps and type families, but perhaps there's another approach I'm missing. I don't have any particular recommendation here, except to say that fundeps is likely more well supported by other compilers.

10 19

Is XMonad still developed or with a current maintainer?
by Alfredo Di Napoli 06 Nov '12

06 Nov '12

Hi guys, looking at the Darcs repo it seems that something is happening, but XMonad wasn't updated in a year on Hackage and everything seems to be still. Is XMonad still actively developed? If yes, who is the current maintainer? It would be good to have him listed in the Hackage package description, in order to contact him. Atm there is one generic email that seems not to be read very often :) Cheers, A.

4 10

ANNOUNCE: vim-hs-module-name
by Roman Cheplyaka 06 Nov '12

06 Nov '12

vim-hs-module-name is a Vim plugin that infers and inserts the Haskell module name. Please see the github page for details: https://github.com/feuerbach/vim-hs-module-name Roman

1 0