April 2011 - Haskell-Cafe

Pattern matching on lazy bytestrings: how does it work?
by Tom Brow 23 Apr '11

23 Apr '11

I noticed today that I can pattern match against lazy bytestrings when using the OverloadedStrings extension: import Data.ByteString.Char8 () > > import Data.ByteString.Lazy.Char8 > > >> f :: ByteString -> Bool > > f "abc" = True > > f _ = False > > >> main = do > > print $ f $ fromChunks ["abc"] > > print $ f $ fromChunks ["a","bc"] > > When I run the above, I get: True > > True > > Given that pattern matching is based on data constructors, how is it possible that (Chunk "abc Empty) and (Chunk "a" (Chunk "bc" Empty)) match the same pattern? Tom

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naming convention for maybes?
by Evan Laforge 23 Apr '11

23 Apr '11

Here's a simple issue that's been with me for a while. As do many people, I use plural variable names for lists, so if a Block as called 'block' then [Block] is 'blocks'. The other pattern that comes up a lot is 'Maybe Block'. When I have to name it, I call it 'maybe_block', e.g. maybe_block <- lookup something case maybe_block of Just block -> ... However, this maybe_ prefix is rather long and unwieldy. I have considered things like 'm' or 'mb' but they don't suggest Maybe to me. An 'm' prefix or suffix is already implying 'monad'. If '?' were allowed in identifiers I could use it as a suffix. I could just append 'q' and get used to it... lispers did it with 'p' after all. I suppose 'mby' could be ok, but for some reason it just looks ugly to me. 'opt' looks ok, I suppose, but 'optional' doesn't cover the full range of Maybe's usage (i.e. it's strange to call a failed lookup result "optional"). Does anyone else have a nice convention for this? Hopefully something short but intuitive and nicely reading like the plural convention?

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Is Hugs dead?
by Robert Clausecker 23 Apr '11

23 Apr '11

Some weeks ago, I mirrored the hugs repo to github. (https://github.com/fuzxxl/Hugs) This was, when I found out, that the last commit was about 2 years ago. Also, since some of the dependencies moved, I was unable to build hugs. Now my question is: Is Hugs dead? What's the status of development of hugs? Yours, Robert Clausecker

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Re: [Haskell-cafe] Question about the Monad instance for Iteratee (from the enumerator package)
by oleg＠okmij.org 22 Apr '11

22 Apr '11

Daniel Schuessler wrote: > The thing I don't understand yet is the last line: Why is it OK to discard the > leftover input from the (f x) Iteratee and yield just the leftover input from > the first one (m0)? First of all, the question is about an older version of Iteratee. For example, the following code http://okmij.org/ftp/Haskell/Iteratee/Iteratee.hs defines Iteratee a bit differently so the question does not apply. > data Iteratee a = IE_done a > | IE_cont (Maybe ErrMsg) (Stream -> (Iteratee a,Stream)) > > instance Monad Iteratee where > return = IE_done > IE_done a >>= f = f a > IE_cont e k >>= f = IE_cont e (docase . k) > where > docase (IE_done a, stream) = case f a of > IE_cont Nothing k -> k stream > i -> (i,stream) > docase (i, s) = (i >>= f, s) No left-over is discarded any more. Your question is about the previous design, called `The second design' described in Iteratee.hs. The corresponding comment block answers your question, please search for ``Justification for the case IE_done x s >>= f''. Please see http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs for the `production' case of Iteratee in a base monad. The file IterateeM.hs talks about one more design, and its drawbacks. It's all about finding the optimal trade-off, I guess.

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ANN: hledger 0.14
by Simon Michael 22 Apr '11

22 Apr '11

I'm pleased to announce hledger 0.14. This release includes work by Dmitry Astapov, Eric Kow, Max Bolingbroke, Omari Norman and myself. Thanks especially to Dmitry for helping me with release wrangling. binaries for linux, mac, windows: http://hledger.org/DOWNLOADS.html (64-bit linux binaries to follow) or: cabal update && cabal install hledger, cabal install hledger- web, ... home and docs: http://hledger.org irc: irc://irc.freenode.net/#ledger Release notes: * remove the specific process dependency that caused too many cabal install problems * treat arguments as possibly-encoded platform strings, do not assume UTF-8 * hledger now always reads and writes data as UTF-8, ignoring the system locale (#34) * look at the LEDGER_FILE env var for the journal path, otherwise LEDGER, like ledger * handle a blank LEDGER_FILE or LEDGER value more gracefully (use the default file path) * the default journal file path is now ~/.hledger.journal, to avoid breaking mac filevault (#41) * amounts with different prices are now aggregated, like ledger * zero amounts now have no sign or commodity, like ledger * parsing: assume current year when transaction dates have no year and there is no default year * parsing: more careful validation of eg leap years in transaction dates * parsing: better international number format support, allowing comma as decimal point and flexible digit groups (#32) * parsing: support @@ syntax specifying total price * parsing: infer the conversion price in transactions involving two unpriced commodities * parsing: support per-posting cleared status * parsing: more reporting interval syntax: biweekly, bimonthly, every N days/weeks/months/quarters/years, every Nst/nd/rd/th day of month/week * add: avoid offering account names for completion in inappropriate contexts * add: remember default account even if user submits a different amount. * convert: account-field directive specifies a field containing the base account name * convert: effective-date-field directive specifies a field containing the effective date * convert: date-format directive specifies custom date formats * convert: allow amount fields containing "AMT @@ PRICE" * histogram: honour the specified start or end dates * print: don't show a trailing space when description is blank * web: allow filter patterns with spaces if quoted, like command line * web: make edit form more cross-browser compatible, fixing it in firefox (#38) * web: move hidden add/edit/import forms below main content to help text-mode browsers a bit (#33) Release contributors: Simon Michael, Dmitry Astapov, Eric Kow, Max Bolingbroke, Omari Norman. Stats: 137 days, 113 commits, 11 end-user features and 15 end-user bugfixes since last release. 189 unit & functional tests and 59% unit test coverage (hledger, hledger-lib packages). 5540 lines of code (all packages). Cheers, -Simon

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Global memoization in Haskell
by Albert Zeyer 22 Apr '11

22 Apr '11

Hi, I thought about implementing memoization which could be applied globally to Haskell as an optimization. I also asked the question (in a bit different way) here: http://stackoverflow.com/questions/5749039/automatic-memoizing-in-functiona… When you want to do that, the most important bit about the implementation is your replacement algorithm (which throws away old data -- because you need some sort of memory limit). As it was pointed out on StackOverflow, page replacement algorithms address exactly this issue. Though, when applied to memoization (esp. in pure functional languages), the problem becomes more specific and so maybe even more clever methods could be used. Earlier, I also came up with an own very simple implementation (using LRU) which can be seen here: http://www.az2000.de/docs/memoization/ I wonder if someone already tried to implement memoization as a global optimization for Haskell. And if so, how has it performed? I would be interested in implementing such thing myself as an addon for GHC and doing some benchmarks. Could you maybe: - Give me some hints where to start in GHC? I never really have worked on GHC so far. - Suggest some good benchmark code? I am mostly interesting in how this performs in huge real-world applications, so having some toy benchmark problems would be nice but some more huge benchmark problems would be better. Best would be some pool of both small and big benchmark programs. Thanks, Albert

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Question about the Monad instance for Iteratee (from the enumerator package)
by Daniel Schüssler 22 Apr '11

22 Apr '11

Hello, for reference, said instance is: > instance Monad m => Monad (Iteratee a m) where > return x = yield x (Chunks []) > > m0 >>= f = ($ m0) $ fix $ > \bind m -> Iteratee $ runIteratee m >>= \r1 -> > case r1 of > Continue k -> return (Continue (bind . k)) > Error err -> return (Error err) > Yield x (Chunks []) -> runIteratee (f x) > Yield x extra -> runIteratee (f x) >>= \r2 -> > case r2 of > Continue k -> runIteratee (k extra) > Error err -> return (Error err) > Yield x' _ -> return (Yield x' extra) The thing I don't understand yet is the last line: Why is it OK to discard the leftover input from the (f x) Iteratee and yield just the leftover input from the first one (m0)? Cheers, Daniel

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QuickCheck, (Ord a)=> [a] -> Property problem
by larry.liuxinyu 22 Apr '11

22 Apr '11

Hi, I found there is similar question as: http://groups.google.com/group/haskell-cafe/browse_thread/thread/7439262e9a… I am still think it's very strange. For example: prop_foo :: (Ord a) => [a] -> Property prop_foo xs = not (null xs) ==> maximum xs == minimum xs This is an extreme case that the property is always wrong. However, QuickCheck produces: *Main> test prop_foo OK, passed 100 tests. Why this happen? If I use verboseCheck, I can find the sample test data are as the following: *Main>verboseCheck prop_foo ... 97: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),()] 98: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),(),(),(),(),()] 99: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),()] OK, passed 100 tests. So Arbitrary () is generated as the instance of Ord a. I have to change the prototype as prop_foo :: (Num a) => [a] -> Property This works at least, However, since 'a'<'b', they are order-able, what if I want to test prop_foo works for char? I am using Haskell Platform (version 6.10.4), with QuickCheck version 1.2.0.0 Thanks -- Larry, LIU Xinyu https://sites.google.com/site/algoxy/home

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ANN: HacPhi 2011
by wagnerdm＠seas.upenn.edu 21 Apr '11

21 Apr '11

Greetings, I am very pleased to officially announce HacPhi 2011, a Haskell hackathon/get-together to be held July 29-31 at the University of Pennsylvania in Philadelphia supported by contributions from Amgen, Jane Street, and Microsoft Research. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's HacPhi was a lot of fun, drawing an international crowd, and many people have already expressed interest in coming back this year. Come meet your Haskelly brothers-in-arms! I want to stress that everyone is welcome---you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the HacPhi wiki [2]. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! - The Hac ? team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://haskell.org/haskellwiki/Hac_?/Register [2] http://haskell.org/haskellwiki/Hac_? [3] http://haskell.org/haskellwiki/Hac_?/Talks

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impoosible dependencies
by Kevin Quick 21 Apr '11

21 Apr '11

Hmmm... $ cabal update $ cabal install hakyll Resolving dependencies... cabal: dependencies conflict: ghc-6.12.3 requires unix ==2.4.0.2 however unix-2.4.0.2 was excluded because ghc-6.12.3 requires unix ==2.4.1.0 $ Any advice (other than upgrading to 7.0.3, which is not an option at the moment)? -- -KQ

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