September 2010 - Haskell-Cafe

On to applicative
by michael rice 04 Sep '10

04 Sep '10

The two myAction functions below seem to be equivalent and, for this small case, show an interesting economy of code, but being far from a Haskell expert, I have to ask, is the first function as small (code wise) as it could be? Michael import Control.Applicative data Color = Red | Blue | Green | Yellow | Orange | Brown | Black | White deriving (Show, Read, Eq, Enum, Ord, Bounded) -- myAction :: IO Color -- myAction = getLine -- >>= \str -> return (read str :: Color) myAction :: IO Color myAction = read <$> getLine

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Regular expressions on Windows 7
by John Sampson 04 Sep '10

04 Sep '10

Hello - I am following the book "Real World Haskell". I am looking at the chapter on regular expressions and trying examples on WinGHCi. However, it cannot load regex-posix-0.94.2 - apparently a file regex.h is missing. I think this has been known for some time. Are there plans to have working regexes on the Windows platform? I am not enough of a hacker to be able to understand material on the Web on this subject - the problem may have been solved somewhere. Regards _John Sampson_

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Restricted type classes
by Ivan Lazar Miljenovic 04 Sep '10

04 Sep '10

When I released the first version of container-classes (which I hacked on during AusHac), some people said I should split out the various folding, etc. into duplicates of the current Foldable class, etc. rather than having large monolithic classes. I've been working on this (see my more recent email with the subject along the lines of "fighting the type system"), and I think I've worked out how to do this: * Have one version of the class (when this makes sense) for values of kind * * Have another version that's closer to the original class for kind * -> * but allowing restrictions (e.g. allowing Set to be an instance of Functor). This is based upon Ganesh Sittampalam's rmonad package (http://hackage.haskell.org/package/rmonad) Rather than my original goal of forcing all kind * -> * values to be instances of the kind * classes, my new approach is to write instances that automatically make all instances of a * -> * class to also be an instance of the kind * class, and to use a newtype wrapper with a phantom type value to allow lifting/promotion of a kind * value to a kind * -> * value (e.g. "foo :: (Word8 -> Word8) -> ByteString -> ByteString; foo f = unpromote . fmap f . Promote" is a valid usage, rather than using the kind * function of rigidMap). My goal with this is that if I have duplicated a class Foo to allow restricted values, then it should be a drop-in replacement for the original in terms of _usage_ (i.e. the class and method/function names are the same, but the type signatures are not). However, I would appreciate the communities advice on a few matters: 1) How should I name the kind * versions? For example, the kind * version of Functor is currently called Mappable with a class method of rigidMap. What should I call the kind * version of Foldable and its corresponding methods? Is there a valid system I can use for these? 2) How far should I go? Should I restrict myself to the "data-oriented" classes such as Functor, Traversable, etc. or should I try to make restricted versions of Applicative and Monad? Assuming I should: 2a) Which namespace to use? Should Monad go in Data.Restricted.Monad to keep it in the same namespace as the other classes, or should I make it closer to base with Control.Restricted.Monad? 2b) Is it OK to promote functions that use a class to being class methods? When I was discussing this in #haskell several people mentioned that defining something like liftA2 for the Set instance of (restricted) Applicative would make more sense than the default <*> (since (a -> b) isnt' an instance of Ord). 2c) Should I keep the classes as-is, or should I explicitly put in the constraints mentioned in the Typeclassopedia (e.g. make Applicative an explicit superclass of Monad, and define return = pure for compatability reasons)? If so, should I bring over Pointed, etc. from category-extras to round out the set or just stick with classes that are already in base? 3) Am I wasting my time with this? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Haddock: Documentation of instances with un-documentable type arguments
by Alexander McPhail 04 Sep '10

04 Sep '10

Hi, Here, http://hackage.haskell.org/packages/archive/graphviz/2999.10.0.1/doc/html/D…, there is an example of the internal implementation leaking. I was taught to separate interface from implementation. For example, header versus c/cc/cpp files. The privacy of typeclasses of GeneralNewTypedDeriving'ed variety, such as MonadReader and MonadState, should be hidable. I may want the programmer to know my operations, as provided in the interface, are monadic (and thus sequencable, temporally deterministic operations) but not allow them to set/get, ask, tell, fetch, push, throw, or pass my implementational details. Perhaps Haddock could exclude class instance reporting when it cannot find a documentable link to a parameter? Vivian -- --- yolar et elver. --- DISCLAIMER This transmission contains information that may be confidential. It is intended for the named addressee only. Unless you are the named addressee you may not copy or use it or disclose it to anyone else.

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TryHaskell in the classroom!
by Fritz Ruehr 04 Sep '10

04 Sep '10

I just wanted to send out a more public "Thanks!" to Chris Done for the tryhaskell.org website and to everyone else (including Chris) who was on the #haskell channel of IRC this afternoon when I "tried haskell", along with the chat feature, during my introductory functional programming class. And I want to recommend the use of tryhaskell and its tutorial to anyone else teaching Haskell in the classroom: it makes for an an especially friendly and approachable introduction to the language for students like mine, some of whom are seeing programming for the first time. I'll be using Hugs and GHC(i) for the day-to-day work through the semester, but I imagine my students will be showing their friends tryhaskell and poking around in it at odd moments from the local cafe. I had just begun my second lecture with a little introduction to the Haskell community and how friendly it was, etc., when I realized the opportunity that the chat feature represented. Sure enough, #haskellers came through for me with some fun & friendly interaction as the class and I bounced back and forth between the tutorial and their comments. It was very gratifying to have my abstract proposition about the character of the community backed up by a concrete existential witness, live in the classroom :) . Thanks again to Chris, Gwern Branwen, _why (of TryRuby) and everyone else for their efforts and interactions. -- Fritz Ruehr

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holumbus error message -- unexpected package db stack
by Roderick Ford 04 Sep '10

04 Sep '10

Hello all, I want to try out Holumbus packages, but found that they don't build when left to cabal-install. So I went to the holumbus site and did the git of the latest source...but it still wouldn't build. Surprisingly, one of the errors was that there System.IO did not export a method utf8, but alas, utf8-prelude and utf8-string are both installed. So I went into this source code tree and attempted to make minor alterations. First, of the script is the distribution package (of the three packages), and cabal wanted less than the most recent hxt package as a maximum, so I dropped that limit. This gave me a buildable make of the code (make just calls Setup for each step) but in the "make install" I get: localhost:/home/roderick/Source/hs/src/distribution # make install Installing library in /usr/local/lib/Holumbus-Distribution-0.0.2.0/ghc-6.10.4 Installing executable(s) in /usr/local/bin Registering Holumbus-Distribution-0.0.2.0... Setup.hs: internal error: unexpected package db stack make: *** [install] Error 1 So, my question is "What is the 'unexpected package db stack' caused by?" Does this give me any hints about the actual problem? Roderick

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HDBC-postgresql and safe/unsafe FFI calls
by David Powell 03 Sep '10

03 Sep '10

Greetings, I'm having an issue with the HDBC-postgresql package that requires me to manually patch it before installation for most of my use cases. All the FFI calls in this package are marked "unsafe". Unfortunately, this means that whenever I issue a slow sql query, all other processing stops. In most places that I want to use this module, I've had to manually patch it to at least mark the PQexec and PQexecParams calls as "safe". Is there any reason these calls should not be marked as "safe"? I understand that there a little extra runtime overhead with this, but I'd have thought that negligible given all the other processing that goes on with these particular calls under the hood. Cheers, -- David Powell

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Announce: lhae
by abau 03 Sep '10

03 Sep '10

lhae is a spreadsheet program. It features a simple formula language and some basic statistical methods, like descriptive statistics and pivot tables. Other features: - Import from csv files - Export to image files - Export to gnuplot diagrams (currently bar charts and box plots) - Multilingual (english, german) Here [1] is some documentation about how to build lhae and how to use the formula language. Regards, Alex [1] http://www.imn.htwk-leipzig.de/~abau/lhae/

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Missing documentation in Haskell Platform on Windows
by Arnaud Bailly 03 Sep '10

03 Sep '10

Hello, I installed (succesfully) HAskell Platform 2010.2 on windows and have a small but annoying issue: Some links in HTML documentation lead to broken links. I did not investigate all the links, but I have seen that all doc under Control.Monad.XXX is missing. What am I doing wrong ? Arnaud

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Confused about ByteString, UTF8, Data.Text and sockets, still.
by JP Moresmau 03 Sep '10

03 Sep '10

Hello all After reading the modules docs and some other discussions, I'm still not sure what's the best choice of tools for my problem. I'm looking at the scion server code base. At the moment, it's reading and writing on sockets using Lazy ByteStrings, then converting them to Haskell Strings using utf8-string. The Haskell Strings are then parsed as JSON using the JSon package. the response is in JSON, translated back with utf8-string to ByteStrings. This is efficient for small strings, but as I'm extending the API I have calls with much more data, and performance degrades significantly. Timings seem to point to the encoding of the String to UTF8. I have replaced JSon by AttoJson (there was also JSONb, which seems quite similar), which allows me to work solely with ByteStrings, bypassing the calls to utf8-string completely. Performance has improved noticeably. I'm worried that I've lost full UTF8 compatibility, though, haven't I? No double byte characters will work in that setup? Is Data.Text an alternative? Can I use that everywhere, including for dealing with sockets (the API only mentions Handle). Should I use Data.ByteString.UTF8 everywhere, rewriting the JSON parser to deal with this instead of the Word8 ByteStrings? In short, what's the fastest way to implement receiving/sending UTF8 text across sockets? Thanks for any pointer, -- JP Moresmau http://jpmoresmau.blogspot.com/

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