- Haskell-Cafe - Haskell.org

generics and sql
by nadine.and.henry＠pobox.com 24 Apr '11

24 Apr '11

Dear Group, Greetings. I have a feeling that what I am trying to do is easy, I just don't know how to say it in Haskell. Let's start with: > {-# LANGUAGE > DeriveDataTypeable, GeneralizedNewtypeDeriving #-} > > import Database.HDBC > import Data.Typeable (Typeable) > import Data.Data > data C = C { str :: String, dbl:: Double } > deriving (Eq, Ord, Typeable, Data) > > a :: C > a = C "twelve" 12.0 > Now I load this up in ghci and I can do the following: toSql . str $ a -- result: SqlString "twelve" toSql . dbl $ a -- result: SqlDouble 12.0 but what I would really like to do is something like: gmapQ toSql $ a which results in: <interactive>:1:7: Could not deduce (Convertible d SqlValue) arising from a use of `toSql' from the context (Data d) bound by a type expected by the context: Data d => d -> SqlValue at <interactive>:1:1-11 Possible fix: add (Convertible d SqlValue) to the context of a type expected by the context: Data d => d -> SqlValue or add an instance declaration for (Convertible d SqlValue) In the first argument of `gmapQ', namely `toSql' In the expression: gmapQ toSql In other words, I'm looking for a function with a signature: (Whatever Instances I neeed here) => a -> [SqlValue] I have tried various incantations of type signatures, but thus far I can't get it right. Can someone point me in the right direction? Thanks. Henry Laxen -- Nadine & Henry Laxen Belle, Venus, Aphrodite 10580 N. McCarran Blvd. Adonis, Miss Parker & Jarod Suite 115-396 Via Alta # 6 Reno, Nevada Chapala, Jalisco, Mexico 89503-1896 CP 45900 The rest is silence. (Hamlet)

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How to use cabal's data-files feature and run in-place?
by Richard Cobbe 24 Apr '11

24 Apr '11

I'm running Haskell Platform 2011.2.0.1 on a MacOS 10.6.7 machine (the machine is 64-bit, but I'm running the 32-bit platform). I'm writing an application for personal use, and I'd like to use Cabal to package it up and handle installation. This way, when I'm working on the program, I won't break the reasonably stable installed version and can continue to use it. (I'm not committed to cabal by any means; suggestions for other ways to accomplish the same thing are more than welcome!) I've got some data files I'd like to package with the application, and Cabal's data-files field is the obvious way to do that. The immediate problem, then, is how to access the files -- I can use the generated Paths_pkg module for the "installed" copy of the app, but then I can't run the thing in-place in ghci in the development directory. I did some googling and came across a blog post (http://neilmitchell.blogspot.com/2008/02/adding-data-files-using-cabal.html) which suggested that I provide my own Paths_pkg.hs file that points to the files' location in the development copy. When I build with Cabal, the generated form of this module would override my hand-written one, and it should therefore work in the installed case as well. Unfortunately, that's not happening. Cabal is clearly generating the module; I can see it in dist/build/autogen. But my copy is overriding the autogenerated one, even for cabal builds -- at least, that's what I'm seeing when I run the binary out of dist/build/<package>/<executable>. Is there a way to be able to use data files in both contexts? FWIW, I'm running cabal as runhaskell Setup.hs configure --user runhaskell Setup.hs build Thanks much, Richard

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http://trac.haskell.org/ not redering correctly
by Erik de Castro Lopo 24 Apr '11

24 Apr '11

Hi all, Trac pages aren't rendering correctly. It seems the HTTP server can't find the CSS files. See below. Cheers, Erik erikd > wget --proxy=off http://trac.haskell.org/trac/chrome/common/css/ticket.css -O /dev/null --2011-04-17 20:11:53-- http://trac.haskell.org/trac/chrome/common/css/ticket.css Resolving trac.haskell.org... 178.63.91.44, 2a01:4f8:121:6::1:10 Connecting to trac.haskell.org|178.63.91.44|:80... connected. HTTP request sent, awaiting response... 404 Not Found 2011-04-17 20:11:56 ERROR 404: Not Found. erikd > wget --proxy=off http://trac.haskell.org/trac/chrome/common/css/trac.css -O /dev/null --2011-04-17 20:28:36-- http://trac.haskell.org/trac/chrome/common/css/trac.css Resolving trac.haskell.org... 178.63.91.44, 2a01:4f8:121:6::1:10 Connecting to trac.haskell.org|178.63.91.44|:80... connected. HTTP request sent, awaiting response... 404 Not Found 2011-04-17 20:28:40 ERROR 404: Not Found. -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Berlin Haskell Meeting
by Sönke Hahn 23 Apr '11

23 Apr '11

Hi all! The next Berlin Haskell Meeting will be next wednesday: Date: Wednesday, April 27th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin I hope, this is not on too short notice. Cheers, Sönke

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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 23 Apr '11

23 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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