April 2012 - Haskell-Cafe

GSOC Proposal 2012 : HDBC
by Greg Weber 04 Apr '12

04 Apr '12

Hi Pranjal, We are glad you are interested in the GSoC. Please take a look at persistent: http://www.yesodweb.com/book/persistent It performs queries and serialization based on Haskell records. It also uses native drivers rather than HDBC. I created a proposal outline [1] and there is a student submission for it [2] [1] http://hackage.haskell.org/trac/summer-of-code/ticket/1605 [2] http://hackage.haskell.org/trac/summer-of-code/ticket/1605 Let me know if you are interested in that or need help coming up with a proposal Thanks, Greg Weber

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mueval leaving behind tmp files
by Johannes Waldmann 04 Apr '12

04 Apr '12

The following program prints Right ("test","Bool","True") as it should, but it leaves behind in /tmp two files (name is a long string of digits) and an empty directory (name is ghcNNNNN_N). ... and it deletes the input file (/tmp/Main.hs). That's not nice. Ideally, I would want to read input from a String (instead of the file), and not write to disk at all. But cleaning up properly would be OK as a work-around. import Language.Haskell.Interpreter import Mueval.Interpreter import Mueval.ArgsParse main = do writeFile "/tmp/Main.hs" "test = True" result <- runInterpreter $ interpreter $ Options { timeLimit =1, modules =Just ["Prelude"], expression ="test" , loadFile ="/tmp/Main.hs", user="what", printType =True , extensions =False,namedExtensions = [] , noImports =False, rLimits =False, help=True } print result ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 ghc-pkg list| egrep 'mueval|hint' hint-0.3.3.4 mueval-0.8.1.1 uname -a Linux octopus 3.0.0-16-generic #29-Ubuntu SMP Tue Feb 14 12:48:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

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GSoC proposal: Units for GHC
by Nils Schweinsberg 04 Apr '12

04 Apr '12

Hi Haskell-Cafe & GHC-users! I'm looking to apply for the GSoC and since I've worked on GHC before I'd like to continue to do so. My proposal would be something that tempted me (as a physics student) for a while: Units for Haskell/GHC. This project has been suggested for a long time on the GHC wiki, and there is already a lot of work done for other languages like ML, F# etc[1]. I have tried to implement e.g. the unification algorithm from the "Types for Units-of-Measure in F#" talk[2] for an abstract syntax tree[3] and it was pretty much straight forward. As I see it, the project would consist of: 1.) Find appropriate rules/algorithms for unit analysis. Most (if not all?) of this should be covered in those papers/talks on [1]. 2.) Applying the rules to the Haskell syntax tree used in GHC. I have approximately 3 years of experience with Haskell, I worked for the database research group[4] at the University of Tübingen (Germany) on the Database Supported Haskell[5] library. I've done most of the coding for the monad comprehension[6] extension, which has been added to the latest GHC release version. I'm already quite familiar with the GHC internals of the compiler/typechecker, and even though I'd have to look up how exactly type interference etc. works in GHC (as I've only *used* it, but never tried to understand/modify it) I'm confident that the work on GHC itself should be doable in the given timeframe. So my questions would be: Do you think this is a appropriate GSoC project? What should I include in the application/project proposal? Anything else? Opinions, suggestions? I realize that I'm kind of late and probably should have written this email a long time ago. But there are still 2 days left for the student application and hopefully I'll get some good feedback by then. - Nils [1]: http://research.microsoft.com/en-us/um/people/akenn/units/index.html [2]: http://research.microsoft.com/en-us/um/people/akenn/units/MLWorkshop2008.pdf [3]: https://github.com/mcmaniac/units/blob/master/src/Unification.hs [4]: http://db.inf.uni-tuebingen.de/team [5]: http://hackage.haskell.org/package/DSH [6]: http://db.inf.uni-tuebingen.de/files/giorgidze/haskell2011.pdf

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Re: [Haskell-cafe] GSoC proposal: Units for GHC
by George Giorgidze 04 Apr '12

04 Apr '12

Implementing this proposal in GHC, as opposed to the library-only approach, would allow for generation of much more useful and user friendly error messages. I believe this aspect is important when it comes to type systems supporting physical units. Cheers, George > This sounds pretty cool and useful. How much of this can be implemented in a library and how much of this would need to be supported on a compiler level? Ideally, most of this would be solved on the library level.

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ANNOUNCE: fast-tags-0.0.1
by Evan Laforge 04 Apr '12

04 Apr '12

A while back I was complaining about the profusion of poorly documented tags generators. Well, there is still a profusion of poorly documented tags generators... I was able to find 5 of them. So, that said, here's my contribution to the problem: fast-tags, haskell tag generator #6. Why not use one of the other 5? Two of them use haskell-src which means they can't parse my code. Two more use haskell-src-exts, which is slow and fragile, breaks on partially edited source, and doesn't understand hsc. Then there's the venerable hasktags, but it's buggy and the source is a mess. I fixed a bug where it doesn't actually strip comments so it makes tags to things inside comments, but then decided it would be easier to just write my own. fast-tags is fast because it has a parser that's just smart enough to pick out the tags. It can tagify my entire 300 module program in about a second. But it's also incremental, so it only needs to do that the first time. I have vim's BufWrite autocommand bound to updating the tags every time a file is written, and it's fast enough that I've never noticed the delay. It understands hsc directly (that's trivial, just ignore the # lines) so there's no need to run hsc2hs before tagifying. The result is tags which are automatically up to date all the time, which is nice. If people care about lhs and emacs tags then it wouldn't be hard to support those too, and at that point I could replace hasktags and we'd be back down to 5 again. But I'm not even sure anyone uses hasktags, since surely someone would have noticed that comment bug. Anyway, it's been working great for me for a couple weeks so I uploaded it to hackage: http://hackage.haskell.org/package/fast-tags

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A Modest Records Proposal
by Gershom B 02 Apr '12

02 Apr '12

The records discussion has been really complicated and confusing. But I have a suggestion that should provide a great deal of power to records, while being mostly[1] backwards-compatible with Haskell 2010. Consider this example: data A a = A{a:a, aa::a, aaa :: a -> A (a -> a)} data B a = B{aaa :: a -> A (a -> a), a :: A} Now what is the type of this? aaaa aaaaa a aa = aaaaa{a = a, aaa = aa} Using standard Haskell typeclasses this is a difficult question to answer. The types of aaaa for A and B do not unify in an obvious way. However, while they are intensionally quite distinct, they unify trivially extensionally. The obvious thing to do is then to extend the type system with extensional equality on record functions. Back when Haskell was invented, extensional equality was thought to be hard. But purity was thought to be hard too, and so were Monads. Now, we know that function existentionality is easy. In fact, if we add the Univalence Axiom to GHC[2], then this is enough to get function existensionality. This is a well-known result of Homotopy Type Theory[3], which is a well-explored approach that has existed for at least a few years and produced more than one paper[4]. Homotopy Type Theory is so sound and well understood that it has even been formalized in Coq. Once we extend GHC with homotopies, it turns out that records reduce to mere syntactic sugar, and there is a natural proof of their soundness (Appendix A). Furthermore, there is a canonical projection for any group of fields (Appendix B). Even better, we can make "." into the identity path operator, unifying its uses in composition and projection. In fact, with extended (parenthesis-free) section rules, "." can also be used to terminate expressions, making Haskell friendly not only to programmers coming from Java, but also to those coming from Prolog! After some initial feedback, I'm going to create a page for the Homotopy Extensional Records Proposal (HERP) on trac. There are really only a few remaining questions. 1) Having introduced homotopies, why not go all the way and introduce dependent records? In fact, are HERP and Dependent Extensional Records Proposal (DERP) already isomorphic? My suspicion is that HERP is isomorphic, but DERP is not. However, I can only get away with my proof using Scott-free semantics. 2) Which trac should I post this too? Given how well understood homotopy type theory is, I'm tempted to bypass GHC entirely and propose this for haskell-prime. 3) What syntax should we use to represent homotopies? See extend discussion in Appendix C. HTH HAND, Gershom [1] To be precise, 100% of Haskell 2010 programs should, usually, be able to be rewritten to work with this proposal with a minimal set of changes[1a]. [1a] A minimal set of changes is defined as the smallest set of changes necessary to make to a Haskell 2010 program such that it works with this proposal. We can arrive at these changes by the following procedure: 1) Pick a change[1b]. 2) Is it minimal? If so keep it. 3) are we done? If not, make another change. [1b] To do this constructively, we need an order. I suggest the lo mein, since noodles give rise to a free soda. [2] I haven't looked at the source, but I would suggest putting it in the file Axioms.hs. [3] http://homotopytypetheory.org/ [4] http://arxiv.org/ *Appendix A: A Natural Proof of the Soundness of HERP Take the category of all types in HERP, with functions as morphisms. Call it C. Take the category of all sound expressions in HERP, with functions as morphisms. Call it D. Define a full functor from C to D. Call it F. Define a faithful functor on C and D. Call it G. Draw the following diagram. F(X)----F(Y) | | | | | | G(X)----G(Y) Define the arrows such that everything commutes. *Appendix B: Construction of a Canonical Projection for Any Group of Fields. 1) Take the fields along the homotopy to an n-ball. 2) Pack them loosely with newspaper and gunpowder. 3) Project them from a cannon. In an intuitionistic logic, the following simplification is possible: 1) Use your intuition. *Appendix C: Homotopy Syntax Given that we already are using the full unicode set, what syntax should we use to distinguish paths and homotopies? At first, I thought we could avoid providing any syntax for homotopies at all. Haskell is a language with type inference, so we should just be able to infer paths and homotopies behind the scenes by adding homotopies to the type system. That's a very nice answer in theory. But in the real world, when we're writing code that solves actual problems, theoretical niceties break down. What if a user wants to use a nonstandard homotopy? Why should we stop them just because we're too lazy to come up with a good syntax? I then realized that we keep running out of syntax because we've limited ourselves to unicode. Instead, I propose we add a potentially infinite universe of identifiers: youtube videos. For example, the higher inductive type for a circle can be written as: homotopyType http://www.youtube.com/watch?v=dQw4w9WgXcQ where Base ::: http://www.youtube.com/watch?v=dQw4w9WgXcQ Loop ::: http://www.youtube.com/watch?v=J---aiyznGQ Base Base Note that the urls do not use SSL. For portability reasons, I propose that SSL only be enabled as an optional extension.

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(no subject)
by Mark Snyder 02 Apr '12

02 Apr '12

<a href="http://dreadscottart.com/mynewwebsite/wp-content/plugins/extended-comment-o…"> http://dreadscottart.com/mynewwebsite/wp-content/plugins/extended-comment-o…</a>

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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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