- Haskell-Cafe - Haskell.org

haddock forgets parens in type sigs?
by Johannes Waldmann 12 Feb '10

12 Feb '10

The annotated type of "update" is missing parentheses: http://hackage.haskell.org/packages/archive/haskelldb/0.12/doc/html/Databas… (compare with the signature given in the source) - Best, J.W.

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Type arithmetic with ATs
by Andrew Coppin 12 Feb '10

12 Feb '10

OK, so I sat down today and tried this, but I can't figure out how. There are various examples of type-level arithmetic around the place. For example, http://www.haskell.org/haskellwiki/Type_arithmetic (This is THE first hit on Google, by the way. Haskell is apparently THAT popular!) But this does type arithmetic using functional dependencies; what I'm trying to figure out is how to do that with associated types. Any hints? (I know for a fact that other people have done this - rule 34 requires it.)

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Haskell IDEs on Windows; gtk2hs
by Alistair Bayley 12 Feb '10

12 Feb '10

I thought I'd try some of the Haskell IDEs: eclipsefp, leksah, and yi. So far, leksah requires gtk2hs (and apparently yi can use it too?), and the latest gtk2hs installer for Windows doesn't like the latest Haskell Platform, so I'm going to try building gtk2hs from source, unless anyone tells me that it's a waste of time. Building yi fails with: Yi\Prelude.hs:182:9: Duplicate instance declarations: instance Category Accessor.T -- Defined at Yi\Prelude.hs:182:9-38 instance Category Accessor.T -- Defined in data-accessor-0.2.1.2:Data.Accessor.Private cabal: Error: some packages failed to install: yi-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1 Presumably data-accessor has been updated, but yi has not? What is the easiest fix? Downgrade to an earlier version of data-accessor? So that just leaves eclipsefp. Still trying to figure out how to install eclipse plugins... Alistair

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Call for Copy: Monad.Reader Issue 16
by Brent Yorgey 12 Feb '10

12 Feb '10

Call for Copy: The Monad.Reader - Issue 16 ------------------------------------------ Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 16 will be: **Friday, April 16, 2010** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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Re: Haskell-Cafe Digest, Vol 78, Issue 14
by John Lato 12 Feb '10

12 Feb '10

Edward's reply was quite good. I'll just try to fill in a few items he didn't address. > From: Maciej Piechotka <uzytkownik2(a)gmail.com> > > I read a lot about iteratee IO and it seemed very interesting > (Unfortunately it lacks tutorial). Especially features like 'no input > yet' in network programming (with lazy IO + parsec I run into problems > as it tried to evaluate the first response character before sending > output). > > I decided first write a simple program and then attempt write a Stream > implementation for parsec. > >> {-# LANGUAGE FlexibleInstances #-} >> {-# LANGUAGE MultiParamTypeClasses #-} >> import Data.Iteratee >> import Text.Parse >> >> data Buffer a = Buffer >> instance Monad m => Stream (Buffer a) (IterateeG [] a m) a where >> uncons Buffer = IterateeG loop >> where loop (Chunk []) = >> return $! Cont (IterateeG loop) Nothing >> loop (Chunk (x:xs)) = >> return $! Done (Just (x, Buffer)) >> (Chunk xs) >> loop (EOF Nothing) = >> return $! Done Nothing (EOF Nothing) >> loop (EOF (Just e)) = >> return $! throwErr e > > > 2. Is there any package which contains this stream definition? I believe this Stream instance is incorrect. According to the parsec-3.0.1 docs, "A Stream instance is responsible for maintaining the 'position within the stream' in the stream state s. This is trivial unless you are using the monad in a non-trivial way." This is necessary for referential integrity. That is, "uncons s" needs to always evaluate to the same result for the same 's'. Your Stream instance doesn't preserve this. As an example, > testIter = let b = Buffer in uncons b >> uncons b should be an iteratee that returns a "Maybe (t, buffer)" where t is the first element in the enumeration, but with your instance it will return the second. See http://inmachina.net/~jwlato/haskell/ParsecIteratee.hs for a valid Stream instance using iteratee. Also Gregory Collins recently posted an iteratee wrapper for Attoparsec to haskell-cafe. To my knowledge these are not yet in any packages, but hackage is vast. > > 3. Why Seek FileOffset is error message? Version 3 of iteratee is somewhat experimental, one of the ideas on trial is that of resumable exceptions. This framework is perfectly suited to handle control messages as well, which is why Seek is included as an error message. I don't want to make a major release just to fix this, but both error handling and control messages will undergo a substantial cleanup in the next major version. Included in this will be a proper separation between control messages and true exceptions, most likely based upon the extensible-exceptions framework. Cheers, John

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MissingH dropped QuickCheck dep
by John Goerzen 12 Feb '10

12 Feb '10

On Fri, Feb 12, 2010 at 08:45:09AM -0800, John MacFarlane wrote: > +++ thomas hartman [Feb 11 10 21:07 ]: > > gitit on hackage is still blocked because of dependency on missingh, > > which depends on qc1. Not an easy fix -- I couldn't figure out how to > > migrate testpack to qc2. > > > > However, missingh dependency was removed from gitit head > > > > http://github.com/jgm/gitit > > > > so that's good. > > No, gitit head still depends on MissingH, via ConfigFile. > > I imagine John will update MissingH to use QuickCheck2 soon... Hey guys, I took a look at MissingH and there was no need for the main library to depend on QuickCheck in the first place. It was only needed by the tests. So I've uploaded a new MissingH 1.1.0.2 to Hackage that drops that dep. That ought to solve it for you. -- John > > John > > >

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Infix and postfix operators in Parsec
by Xinyu Jiang 12 Feb '10

12 Feb '10

I'm writing a parser for a Haskell-style language, and when I need to use the same symbol for infix, prefix and postfix operators, the combinator "buildExpressionParser" seems not to work as intended. For example, in: (1)x + y (2)x + (3)+ x If I set the priority of the postfix version of "+" to be higher than the infix version, the parser cannot recognize (1), for it stops at the end of "x +". And if the priority of postfix "+" is lower, Parsec complains about (2) by returning an error message which expects something after "+". Is there some way to get over this problem, and let me be able to still benifit from the expression mechanism in Parsec. Or should I write these stuff from scratch? Thanks.

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Haskell RPC / Cluster
by Rick R 12 Feb '10

12 Feb '10

I am preparing to embark on some serious cluster oriented coding (high availability, monitoring, failover, etc). My primary concern is conforming to standards. I would also like to aid any existing project that fall under this scope. HackPar seems currently targeted towards HPC style clustering, but the page seems to hint at future work in the cloud/high-availability area. I was looking around for RPC libs for Haskell and stumbled across this http://github.com/mariusaeriksen/bert It implements BERT, which is based on Erlang's binary serialization protocol. It seems to have quite a bit of support. Does anyone know of any other RPC modules for Haskell? In addition, can anyone recommend other cluster oriented modules for monitoring, process management, etc? If those don't exist, can anyone recommend some standards off of which to base these? SNMP seems obvious (and daunting), any others? Thanks, Rick

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Re: [Haskell-cafe] HDBC convert [SqlValue] without muchos boilerplate
by Iain Barnett 12 Feb '10

12 Feb '10

On 11 Feb 2010, at 10:05, Vasyl Pasternak wrote: > > But fromSql function could convert everything to String, so you never > need to use `show`, just simply write > > convrow2 :: [SqlValue] -> String > convrow2 (x:xs) = foldl (\i j -> i ++ " | " ++ (fromSql j)) (fromSql x) xs > But, IMO, this is more readable version of your function: > > convrow2' :: [SqlValue] -> String > convrow2' = unwords . intersperse "|" . map fromSql > On 11 Feb 2010, at 10:06, Miguel Mitrofanov wrote: > > What if you just omit the "show" function? fromSql seems to be able to convert almost anything to String. Ok, thanks, that's a big help. I'm really glad to get rid off all that extra cruft I had there. I'd forgotten about intersperse, too. The new code works fine. Regards, Iain

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parallel and distributed haskell?
by Scott A. Waterman 12 Feb '10

12 Feb '10

It looks like there was a recent hackathon focusing on implementing distributed haskell. http://hackage.haskell.org/trac/ghc/wiki/HackPar I feel there is quite a bit of latent interest in the subject here, but relatively little active development (compared to erlang, clojure, etc.) Can anyone involved give a quick overview (or pointers to one)? It would be good to hear what directions people are taking, and why, and where it's going. Personally, I'd love to know the current thinking on a variety of subjects: * erlang-like port/actor implementations v. more implicit constructs * serialization * node failure/error handling * "grid" compatibility v. various ad-hoc cluster approaches. Anyone actively looking at this kind of question who would want to talk about it? Thanks, --ts

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