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by Mark Snyder 02 Apr '12

02 Apr '12

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Regarding simple xml string parser code review
by rajendra prasad 02 Apr '12

02 Apr '12

Hi, I am trying to fetch the values under tags in an xml string given the tag string and xml string. Actually, I don't want to use xml parser since I need to parse very small xml strings. Also, I dont want to use anything else other than haskell platform. I wrote some routines and want to know whether there is any better option available. Please review them and the comments will be highly appreciated. please find routines posted at http://hpaste.org/66357 Thanks in advance. Regards, Rajendra

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Towards a single, unified API for incremental data processing
by John Millikin 02 Apr '12

02 Apr '12

There are currently several APIs for processing strict monoidal values as if they were pieces of a larger, lazy value. Some of the most popular are based on Oleg's left-fold enumerators, including the "iteratee", "enumerator", "iterIO". Other choices include "comonads", "conduits", and "pipes". Despite having various internal implementations and semantics, these libraries generally export a similar-looking API. This is a terrible duplication of effort, and it causes dependant packages to be strongly tied to the underlying implementation. I propose that a new package, "tzinorot", be written to provide a single API based on Data.List. It should be pretty easy to use, requiring only a few common extensions to the type system. For example, the enumerator package's 'mapM' function could be generalized for use in tzinorot through a few simple modifications to the type signature: ---------- -- enumerator mapM mapM :: Monad m => (ao -> m ai) -> Enumeratee ao ai m b -- tzinorot mapM mapM :: (Monad m, Tzinorot t, ListLike l1 a1, ListLike l2 a2) => (l1 a1 -> m (l2 a2)) -> t Void s (TzinorotItems (l1 a1)) (TzinorotItems (l2 a2)) m r ---------- To make it easier to install and use the tzinorot package, it will depend on all of its supported implementations (iteratee, enumerator, conduits, pipes, etc), and use Michael Snoyman's "cabala" tool to manage dependency versions. See the cabala announcement for details on use: http://www.yesodweb.com/blog/2012/04/replacing-cabal

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ANN: acme-http
by Jeremy Shaw 01 Apr '12

01 Apr '12

Hello, As we all know, the true measure of performance for a web server is the classic PONG test. And, so the Happstack team is pleased to announce the release of the new acme-http server! hackage: http://hackage.haskell.org/package/acme-http source: http://patch-tag.com/r/stepcut/acme-http When testing on my laptop with +RTS -N4 using the classic PONG test: $ httperf --hog -v --server 127.0.0.1 --port 8000 --uri / --num-conns=1000 --num-calls=1000 --burst-length=20 --rate=1000 acme-http delivered 221,693.0 req/s, making it the fastest Haskell web server on the planet. By comparison, warp delivered 51,346.6 req/s on this machine. The secret to acme-http's success is that it large avoids doing anything not required to win the PONG benchmark. It does not support timeouts, it does not check quotas, it assumes the client is HTTP 1.1, it does not catch exceptions, and it responds to every single request with PONG. The goal of acme-http is two fold: 1. determine the upper-bound on Haskell web-server performance 2. push that upper bound even higher In regards to #1, we have now established the current upper limit at 221,693.0 req/s. In regards to #2, I believe acme-http will be useful as a place to investigate performance bottlenecks. It is very small, only 250 lines of code or so. And many of those lines deal with pretty-printing, and other non-performance related tasks. Additionally, it works in the plain IO monad. It does not use conduits, enumerators, pipes, or even lazy IO. As, a result, it should be very easy to understand, profile, and benchmark. In providing such a simple environment and avoiding as much extra work as possible we should be able to more easily answer questions like "Why is so much RAM required?", "What is limiting the number of connections per second", etc. As we address these issues in acme-http, we can hopefully bring solutions back to practical frameworks, or to the underlying GHC implementation itself. If performance tuning is your thing, I invite you to check out acme-http and see if you can raise the limit even higher! - jeremy

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Re: [Haskell-cafe] hint and type synonyms
by Claude Heiland-Allen 01 Apr '12

01 Apr '12

Hi Daniel, cafe, On 31/03/12 17:47, Daniel Gorín wrote: > Could you provide a short example of the code you'd like to write but gives you problems? I'm not able to infer it from your workaround alone... This problem originally came up on #haskell, where Rc43 had a problem making a library with a common module that re-exports several other modules: http://hpaste.org/66281 My personal interest is somewhat secondary, having not yet used hint in a real project, but code I would like to write at some point in the future is much like the 'failure' below, unrolled it looks like: main = (print =<<) . runInterpreter $ do setImports ["Prelude"] interpret "1/5" (as :: Rational) -- fails Having to remember that Rational is defined as type Rational = Ratio Integer and that Ratio is defined in Data.Ratio and then to add that to the setImports list is a little inconvenient: main = (print =<<) . runInterpreter $ do setImports ["Prelude", "Data.Ratio" ] interpret "1/5" (as :: Rational) -- works But for my own purposes this is probably much saner in the long run than my newtype wrapping approach below. However, this is not always possible: supposing Ratio itself was defined as a type synonym of Ratio2, and Ratio2 was not exported. Perhaps this is what Rc43 was experiencing, but I shouldn't speculate, as this is all getting a bit theoretical - I should try out hint in the real world to see if this problem makes things impractical for me - sorry for the noise! Thanks, Claude > Thanks, > Daniel > > > On Mar 31, 2012, at 6:19 PM, Claude Heiland-Allen wrote: > >> Hi all, >> >> What's the recommended way to get hint[0] to play nice with type synonyms[1]? >> >> A problem occurs with in scope type synonyms involving types not in scope. >> >> I came up with this after looking at the source[2], but it makes me feel ill: >> >> --8<-- >> -- hint and type synonyms don't play nice >> module Main where >> >> import Language.Haskell.Interpreter >> >> import Data.Typeable as T >> import Data.Typeable.Internal >> import GHC.Fingerprint.Type >> >> main = failure>> success >> >> test t = (print =<<) . runInterpreter $ do >> setImports [ "Prelude" ] >> interpret "1/5" t >> >> failure = test (as :: Rational) >> -- Left (WontCompile [GhcError {errMsg = "Not in scope: type constructor or class `Ratio'"}]) >> >> success = test (as :: Q) >> -- Right (1 % 5) >> >> newtype Q = Q Rational >> >> instance Show Q where >> show (Q a) = show a >> showsPrec n (Q a) = showsPrec n a >> >> instance Typeable Q where >> typeOf _ = TypeRep (Fingerprint 0 0) (T.mkTyCon "Rational") [] >> --8<-- >> >> Thanks, >> >> >> Claude >> >> [0] http://hackage.haskell.org/package/hint >> [1] http://www.haskell.org/onlinereport/decls.html#type-synonym-decls >> [2] http://hackage.haskell.org/packages/archive/hint/0.3.3.4/doc/html/src/Hint-… >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: [Haskell-cafe] Google Summer of Code - Lock-free data
by John Lato 01 Apr '12

01 Apr '12

Slightly related: I think it would be interesting to compare a Disruptor-based concurrency communications mechanism and compare it to e.g. TChans 1. Disruptor - http://code.google.com/p/disruptor/ > From: Ryan Newton <rrnewton(a)gmail.com> > >> I think so. Atomically reading and writing a single memory location >>> (which CAS does) is just a very simple transaction. But using a CAS >>> instruction should be more efficient, since STM has to maintain a >>> transaction log and commit transactions, which creates some overhead. >>> >> >> Ah, I see. In that case, it may be worthwhile to implement the CAS >> instruction in terms of STM as well and measure the performance difference >> this makes for the final data structure. After all, STM is a lot more >> compositional than CAS, so I'd like to know whether the loss of >> expressiveness is worth the gain in performance. > > > There's one annoying hitch with doing apples-to-apples comparisons here. > > The problem is that CAS falls short of being a hardware-accelerated version > of a simple transaction (read TVar, (==) against expected value, > conditionally update TVar), because CAS is based on pointer equality rather > than true equality. (eq? rather than equal? for any Schemers out there.) > > For this reason our "Fake" version of CAS -- for older GHCs and for > performance comparison -- has to use reallyUnsafePointerEquality#: > > > http://hackage.haskell.org/packages/archive/IORefCAS/0.2/doc/html/Data-CAS-… > > But we do provide a "CASable" type class in that package which is precisely > for comparing the performance of: > > - Hardware CAS > - Fake CAS -- atomicModifyIORef + ptrEquality > - Foreign CAS -- gcc intrinsic + function call

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get a string representation (show) of a function argument
by TP 31 Mar '12

31 Mar '12

Hi, I don't known the advanced features and extensions of GHC at all. Look at the following program: ---------------- f :: Integer -> Integer -> IO Integer f a b = do print $ "first argument=" ++ (show a) print $ "second argument=" ++ (show b) print $ a+b return (a+b) main = do k <- f 3 5 f 2 k ---------------- It yields: ---------------- "first argument=3" "second argument=5" 8 "first argument=2" "second argument=8" 10 10 ---------------- I am wondering if there is any means to get "f 3 5" instead of "8" in the output of this program. My idea would be to write some generic test function: I give it an expression (probably a function call) and the expected result (as a string), and then: * it should check that the obtained result is identical to the expected one. * it should also print the expression that yielded the first argument of the test function (i.e. the function call), under an understandable form (show). This is the feature that is missing to my mind in some test suites: for example in HUnit, it is not automatic: in the first argument of assertEqual, we may give a string corresponding to the executed command, see the documentation of Test.HUnit: test1 = TestCase (assertEqual "for (foo 3)," (1,2) (foo 3)) > runTestTT tests # Failure in: 0:test1 for (foo 3), expected: (1,2) but got: (1,3) Cases: 2 Tried: 2 Errors: 0 Failures: 1 So, we have duplication of code: "foo 3". On the other hand, I guess that the needed features to do that are not possible in Haskell (related to some kind of metaprogramming), but I prefer to ask people about that. Thanks in advance, TP

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ANNOUNCE: fix-imports 1.0.0
by Evan Laforge 31 Mar '12

31 Mar '12

I just uploaded a new version to hackage. This improves the heuristics for guessing the right module quite a bit, and adds a local config file where you can prioritize or de-prioritize packages or module prefixes. It's also a bit faster. >From the hackage description: A small standalone program to manage the import block of a haskell program. It will try to add import lines for qualified names with no corresponding import, remove unused import lines, and sort the import block according to some heuristic you can define. This only works for qualified imports! Unqualified imports are left untouched. It's most convenient if bound to an editor key. Recent major changes: version 1.0.0 Change name from FixImports to fix-imports, which is more unixy. Change ghc-pkg parsing from String to Text. It's noticeably faster. Add a more flexible system for prioritizing imports. When there are several possibilities for a module name, they are all given to a single function to decide. The config file moved from fix-imports-priority to .fix-imports and can now specify sort orders for packages and modules by prefix. Make -i includes for non-existent dirs ignored instead of causing an error.

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hint and type synonyms
by Claude Heiland-Allen 31 Mar '12

31 Mar '12

Hi all, What's the recommended way to get hint[0] to play nice with type synonyms[1]? A problem occurs with in scope type synonyms involving types not in scope. I came up with this after looking at the source[2], but it makes me feel ill: --8<-- -- hint and type synonyms don't play nice module Main where import Language.Haskell.Interpreter import Data.Typeable as T import Data.Typeable.Internal import GHC.Fingerprint.Type main = failure >> success test t = (print =<<) . runInterpreter $ do setImports [ "Prelude" ] interpret "1/5" t failure = test (as :: Rational) -- Left (WontCompile [GhcError {errMsg = "Not in scope: type constructor or class `Ratio'"}]) success = test (as :: Q) -- Right (1 % 5) newtype Q = Q Rational instance Show Q where show (Q a) = show a showsPrec n (Q a) = showsPrec n a instance Typeable Q where typeOf _ = TypeRep (Fingerprint 0 0) (T.mkTyCon "Rational") [] --8<-- Thanks, Claude [0] http://hackage.haskell.org/package/hint [1] http://www.haskell.org/onlinereport/decls.html#type-synonym-decls [2] http://hackage.haskell.org/packages/archive/hint/0.3.3.4/doc/html/src/Hint-…

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New in haskell for old-timers?
by Mats Rauhala 31 Mar '12

31 Mar '12

I ended up discussing in university with the person next to me about Haskell. He's an old-timer who's even been to conferences, but it's a long time since he's looked at Haskell. He asked me to write "what's new in Haskell", but I'm not too experienced myself. I didn't realize asking how long it's been for him, but he mentioned about new fronts in optimizing compilers and specifically mentioned a compiler that could compile to legible C. I've been following Haskell only for a couple of years, so my intuition tells me that he means `-fvia-c`, and I mentioned to him that it's about to be deprecated and replaced with the llvm back-end. I also mentioned hackage, but he hadn't heard of it. He mentioned that he'd like to use Haskell for his master's thesis, but we got a interrupted when he said what it was about. Formal something or other. I trust you fellow Haskellers have better understanding of the time frame, and can think of something interesting new features, libraries, communities etc. that's since been emerged. I ask you to give me something interesting to give him and reinstate his interest. -- Mats Rauhala MasseR

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