April 2008 - Haskell-Cafe

ANN (2 Libs) -- hvac 0.1b, a lightweight web framework and HStringTemplate 0.3
by Sterling Clover 03 Jun '08

03 Jun '08

1) hvac 0.1b: transactional, declarative framework for lightweight web applications. 2) HStringTemplate 0.3 1) hvac 0.1b hvac (short for http view and controller) has been my project for the last little while, and is finally in a fairly usable state, so I'm opening up the repo (darcs get http://community.haskell.org/~sclv/ hvac/) for folks to play with and to get some feedback. While not quite yet ready for hackage, the package as provided should be fully cabal installable. Documentation is available at http:// community.haskell.org/~sclv/hvac/html_docs/hvac/ The aim of hvac is to provide an environment that makes the creation of lightweight fastcgi based web applications as simple as possible, with an emphasis on concise, declarative style code, correct concurrent transactional logic, and transparency in adding caching combinators. There are two included example programs, naturally neither of which is feature complete. They share a common login module of about 50 lines of code, excluding imports, templates, and database schema. The first program is a classic, greenspun-style message board with basic login functionality. It totals roughly 40 lines and tends to use just under 4mb of resident memory on my system. The second is a wiki based on Pandoc and the PandocWiki code. The code totals roughly 30 lines (rendering borrowed from PandocWiki aside) and uses about 5mb of memory. hvac processes all requests in the STM monad, with some bells attached to properly interleave STM with session, database and filesystem operations such that they all conceptually occur together in a single transaction per request. Currently it is only fully tested with sqlite, but it should operate, modulo a few tweaks, with any database accessible via HDBC. hvac is particularly designed to use the HStringTemplate library as an output layer, in a simple declarative fashion. As the StringTemplate grammar is explicitly sub-turing, this ensures a clean separation of program logic from presentation, while providing a nonetheless fairly powerful language to express typical display tasks. The included cache combinators, still experimental, should allow a simple and fine-grained control over the level of caching of various disk-bound operations. Phantom types are used to ensure that no functions that modify state may be cached. To give a flavor of hvac code, the following is the complete (twenty lines!) source of the wiki controller (due to sql statements, some lines are rather long): wikiController tmpl = h |/ "login" *> login_plug tmpl <|> (h |/ "wiki" |\\ \pageName -> h |// "POST" *> withValidation [ ("contents", return) ] (\ [contents] -> do pageId <- selectVal "id from pages where name=?" [toSql pageName] maybe (addErrors [("Login","must be logged in.")] >> continue) (\user -> case fromSql pageId of Just (_::Int) -> execStat "insert into page_hist (pageId,contents,author) values(?,?,?)" [pageId, toSql contents, toSql . userName $ user] Nothing -> do execStat "insert into pages (name,locked) values(?,?)" [toSql pageName, toSql (0::Int)] pid <- selectVal "max(id) from pages" [] execStat "insert into page_hist (pageId,contents,author) values(?,?,?)" [pid, toSql contents, toSql . userName $ user]) =<< getSes continue) <|> do pageId <- selectVal "id from pages where name=?" [toSql pageName] (join $ renderf (tmpl "showPage") ("pageName", pageName) <$> "pageContents" |= selectRow "* from page_hist where pageId=? order by time desc limit 1" [pageId] )) <|> (redirect . ( ++ "/wiki/Index") =<< scriptName) Future directions for work on hvac include: Stress testing for correctness of transactional logic and benchmarks. Exploration of various efficiency tweaks. Unit tests. Further development of the cache combinator API. Improvement of the example apps and addition of a few others (a blog maybe). Expansion of the library of validator functions. Exploration of transferring hvac to the standard FastCGI bindings (currently it uses a custom modified version to work properly with STM). Improvement of the database layer, particularly with regards to common paging functions. Creation of a set of simple combinators for generating CRUD (create read update delete) pages. Creation of a minimal set of standard templates (maybe). 2) HStringTemplate 0.3.1 This release of HStringTemplate (up now at Hackage) fixes a number of bugs pointed out to me by its small but growing user base (thanks, cinema, elliottt!) ranging from the minor (a particular two-level iteration pattern wasn't working properly) to the truly irritating (poor handling of file groups). It's still unfortunately skimpy on the docs, outside of the haddocks and the main StringTemplate grammar documentation at http://www.stringtemplate.org (although the examples from hvac should also prove helpful). However, it does have a set of very nice and handy new features for development. * renderf, a function similar in spirit to printf, that takes an arbitrary number of heterogeneous (String, value) tuples as arguments. This should cut down considerably on long setAttribute chains. Additionally, with custom instances (not, I'll grant, trivial to write) it can be used to declaratively chain together strings of attribute retrieval functions in arbitrary monads, as in the above code example from hvac. * dumpAttribs, a function/template that prints out the tree of the entire attribute environment a template is operating in -- extremely handy for development. * nullGroup, also for use in development, a simple way to display more information about templates that can't be found. Error messages in usafeVolatileDirectoryGroup have also been significantly improved. * getStringTemplate', a version of getStringTemplate guaranteed not to be inlined. While the optimizer will still sometimes rearrange code such that a volatile group is not updated properly, this at least helps remedy the situation (I think). * Some minor changes: For grammar reasons, dots have been removed from template names -- however, underscores and slashes are now available. Additionally, there's a much improved logic for which aspects of a local environment are overridden and preserved when a template is called from another. For both of these libraries, patches, comments, bug reports, requests, and of course contributions more than welcome! Regards, Sterl.

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HaXml and the XHTML 1.0 Strict DTD
by Peter Gammie 26 May '08

26 May '08

Is anyone using HaXml to validate XHTML Strict? The old 1.13.2 version has some bugs in how it handles attributes that stop me from using it. It handled the DTD parsing fine. The most-recent darcs version relies on a newer ByteString than I have, so it is not easy for me to test it. A recent (this year) darcs version failed to parse the DTD, yielding this error: validate: In a sequence: in content spec of ELEMENT decl: head When looking for a non-empty sequence with separators: In a sequence: Expected % but found | in file xhtml1 at line 252 col 50 when looking for a content particle when looking for a content particle This is the context:  <!ENTITY % head.misc "(script|style|meta|link|object)*">  <!ELEMENT head (%head.misc;, ((title, %head.misc;, (base, %head.misc;)?) | (base, %head.misc;, (title, %head.misc;))))> I appreciate any advice on this. cheers peter

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Starting Haskell with a web application
by Jonathan Gardner 18 May '08

18 May '08

I am sure I am missing something obvious, but since the cafe is the place to ask stupid questions... I am getting more and more excited about Haskell as I learn more and more about it. Professionally, and as a hobby, I love working on web applications. Where do I get started in writing a web app with Haskell? I am looking more for a framework like Pylons and less like Apache, if that helps. I am sure someone has already done the homework on this, so a link will be fine. -- Jonathan Gardner jgardner(a)jonathangardner.net

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Couple of formal questions
by Creighton Hogg 17 May '08

17 May '08

Hello Haskell, So there's two questions that have been bothering me lately & while they are, as usual, a little off topic I figured this might be a good forum: Where could I find a good treatment on data vs. codata & the difference between well-founded recursion & well-founded(?) corecursion? Where could I find a proof that the initial algebras & final coalgebras of CPO coincide? I saw this referenced in the "Bananas.." paper as a fact, but am not sure where this comes from. Thanks, Creighton Hogg

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Cryptographic hash uniquness (was [Haskell-cafe] Simple network client)
by Peter Verswyvelen 11 May '08

11 May '08

>winds up having a write cache, which is mutable in practice. The >interesting thing is that the block's location is the cryptographic >hash of its contents, which leads to all sorts of neat properties (as >well as requiring immutability). That's interesting. When I developed a version control system for a customer, I also used a cryptographic hash as the database key of file+content in question, but I was afraid I might have clashes (two files with different content generating the same hash)... My intuition told me that the odds of two cryptographic hashes (on meaningful content) colliding was much less than the earth being destroyed by an asteroid... But this is just intuition... What does computer science tell us about this? Thank you, Peter

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Old editions of The Monad.Reader lost
by Henk-Jan van Tuyl 10 May '08

10 May '08

L.S., Now that all hawiki pages have been removed, we have lost some valuable information. For example The Monad.Reader; on http://www.haskell.org/haskellwiki/The_Monad.Reader it says: Older editions can be found on the old Haskell wiki – they haven't been included here for licensing reasons. The page it links to, a hawiki page, has dissappeared. I propose to bring at least the The Monad.Reader pages back. Backups of these pages can be found at the Wayback Machine, but there is no guarantee that will always be there. -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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Cabalizing darcs
by Gwern Branwen 05 May '08

05 May '08

So recently I spent a bit of time working on a cabalization of Darcs. It works well for me, and is reasonably easy to apply (attached are three files; do a 'darcs get --lazy http://darcs.net' with Darcs-2 to get the latest, and copy the files into it, the usual autoconf, and it should then work as normal; if this doesn't work for you, I'd appreciate knowing). My question for y'all is: would you find an unofficial version of Darcs - one with the cabalization added and uploaded to Hackage - useful? Or would it just be frustrating and redundant and Not Good? (If yes, any suggestions for a package name? 'darcs' is out, but 'darcs-cb' and 'darcs-cabalized' strike me as horribly clunky or obscure.) -- gwern Threat WANK football Talent anarchy RIT interception Hutsul forschung ISS

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Ann: Win32-notify 0.1
by Niklas Broberg 05 May '08

05 May '08

Hi all, I wanted to use hinotify, which gives notifications on file system changes, but found (quite naturally) that it wouldn't run on my windows machine (since it's a binding to the linux kernel). So I started writing a library that would give similar functionality on Windows. This is the first release of that, more a proof-of-concept than anything useful at this stage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Win32-notify-0.1 Still, feel free to test it, and I would love feedback on everything from desired API to suggestions on how to implement the low-level bindings more efficiently. Cheers, /Niklas

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Haskell Web server
by Cetin Sert 01 May '08

01 May '08

Hi, What is the fastest way to set up a web server that can redirect requests to a haskell application and serve results returned by it? I need to demonstrate a simple visualization tool I have written for analytic tableaux on Monday and need something easy and simple. Best Regards, Cetin Sert

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approximating pi
by ry dahl 30 Apr '08

30 Apr '08

By picking points randomly from a square one can calculate pi. Keep track of how many points from the square you pick lay in the inscribed circle. Supposing the square's edges are length 2, then the inscribed circle has radius 1. Thus the area of the circle is pi*r^2 = pi. The area of the square is 4. The ratio of points picked from the circle to the total number of picked points will converge to pi / 4. What is the best way to express this algorithm in Haskell? ry

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