- Haskell-Cafe - Haskell.org

Db and STRef
by Brandon Moore 16 Jan '06

16 Jan '06

John Goerzen wrote: ... > I didn't honestly follow the STRef discussion, or how something so > I/O-based could work there. > > -- John The only message I've found was Oleg mentioning the typing that keeps an STRef limited to the scope of the runST it came from. I don't think database access could live in ST either. I think the proposal was to build something like Oleg's Monadic Regions http://okmij.org/ftp/Haskell/types.html#monadic-regions on top of IO I'm assuming you understand how the type on runST and the STRef operations ensure that, even though you *can* smuggle out an STRef in the result from runST, you will never be able to use it again. The idea was to do the equivalent thing with databases: use fancy types to ensure that handle can only be used inside to origination withDB or withCursor or whatever, and the bracketing function can release the resource on the way out, without worrying about it being used again. Brandon > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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Shootout rankings
by dons＠cse.unsw.edu.au 16 Jan '06

16 Jan '06

:D Haskell now ranked 2nd overall, only a point or so behind C: http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=all And still a bit more we can squeeze out... -- Don

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Re: What's the current bug report procedure?
by Simon Marlow 16 Jan '06

16 Jan '06

Seth Kurtzberg wrote: > All, > > I have a (minor) but to report in the networking (Network and Network.Socket) libraries. If the /etc/protocols file is missing, an error message is printed saying that a service could not be found. This occurs even when no service is involved (the port number is specified explicitly using the constructor PortNumber). > > I'm aware that the procedure changed; can you point me at the right place to report this and/or the right people/bug tracker/whatever? > > Thanks, > Seth Kurtzberg I believe that's been fixed, but the place to report bugs in the network library is currently the GHC bug tracker, at http://www.haskell.org/ghc/reportabug. Cheers, Simon

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please send your cabal bug reports, wishlist, and patches!
by Isaac Jones 16 Jan '06

16 Jan '06

Greetings, I'm trying hard to get a better hold on the Cabal[1] project, and a more clear idea of all the outstanding work that needs to be done. I've gone through my mailbox to dig up stuff like this, but no doubt some has slipped between the cracks. I started a bug tracker / wiki a few weeks ago, and would like your help to make sure that it reflects _all_ of the work that needs to be done in Cabal. If you've sent me a feature request or bug report that hasn't been added to Cabal, can you please check if it is on the wiki, and if not, create a "ticket" (bug report) for it: Cabal Wiki & Bug Tracker: http://hackage.haskell.org/trac/hackage/ If you're not sure, or don't want to use the bug tracker, just email me. This bug tracker is a great place to make more complex feature requests, as it has full wiki markup. See this ticket for a nice example: http://hackage.haskell.org/trac/hackage/ticket/27 Also, if you've sent me any patches that are not yet applied, please re-send them to me. peace, isaac [1] http://www.haskell.org/cabal

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formal methods & functional programming
by Abigail 16 Jan '06

16 Jan '06

Hi, I have been searching papers about tha raltionship between formal methods in software engineering and functinal programmming, but i haven't found enough information. can u hel me?. Thanks Abigail. __________________________________________________ Correo Yahoo! Espacio para todos tus mensajes, antivirus y antispam ¡gratis! Regístrate ya - http://correo.espanol.yahoo.com/

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HDBC Transactions with Sqlite3
by Tom Hawkins 15 Jan '06

15 Jan '06

I have HDBC running with Sqlite3, but I'm getting a SqlError due to a locked table. Please excuse my SQL ignorance, but what may be causing the problem? In SQL, are we not allowed to select, update, and delete from a table within a single transaction? If not, what are the rules for transactions? Thanks for any help! -Tom SELECT * FROM freeKey; SELECT * FROM nextKey; INSERT INTO nextKey VALUES (1); SELECT * FROM freeKey; SELECT * FROM nextKey; UPDATE nextKey SET key = 2 WHERE key = 1; SELECT * FROM freeKey; SELECT * FROM nextKey; UPDATE nextKey SET key = 3 WHERE key = 2; INSERT INTO freeKey VALUES (0); INSERT INTO freeKey VALUES (1); SELECT * FROM freeKey; DELETE FROM freeKey WHERE key = 0; SqlError {seState = "", seNativeError = 1, seErrorMsg = "step: database table is locked"}

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using cabal? add your package to the list!
by Isaac Jones 15 Jan '06

15 Jan '06

I started a list of packages using cabal (including those uplaoded to Hackage). Please either upload your packages to hackage, or add it here. I want this list so that I can try to figure out things like how much a given modification to Cabal will break stuff: http://hackage.haskell.org/trac/hackage/wiki/CabalPackages To add your package to the list, hit "Edit Page" at the bottom of the page. Feel free to make the page into a table, add links for the packages already there, reorganize, etc. peace, isaac

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Inheritance without OOHaskell
by John Goerzen 14 Jan '06

14 Jan '06

Hello, I'd like to put forth two situations in which I miss the ability to use inheritance in Haskell, and then see if maybe somebody has some insight that I'm missing out on. Situation #1: In HDBC, there is a Connection type that is more or less equivolent to a class on a OOP language. In Haskell, we use a record with named fields that represent functions. Closures are used to permit those functions to access internal state. That works well enough. But there is no way at all to extend this. Say a database such as PostgreSQL has some extra features -- it would be nice for the PostgreSQL objects to support additional functions, whereas Connection objects from other databases might not support those functions. Any Haskell function could expect a Connection object (in which case it could access only the standard functions) or a PostgreSQL object (in which case it could access the standard plus the enhanced functions). The internal state of a Connection object is DB-specific, so there can be no general function to expose it. Situation #2: In Python, every exception is an object, and every object can be extended. Therefore, I could write an exception handler for, say, an IO error, and have it work for anything that's a subclass of the generic IO error -- even if these subclasses weren't known at the time the program was written. Other handlers could be as specific or as general as desired. In Haskell, it seems that all of this has to be anticipated in advance; it's not very easy to extend things. So my questions are: 1. I have sometimes used typeclasses instead of data records. This provides some of what I'm searching for, but has the unfortunate side-effect that one can't very easily build a list of objects that may not come from the same place but are nonetheless part of the class. For instance, had I used typeclasses for HDBC, I couldn't have a list of Connection objects where some are from MySQL, some from PostgreSQL, etc. 2. As a library designer, what is the most friendly way around these problems that adheres to the principle of least surprise for Haskell programmers? 3. How does one choose between a type class and a data record of functions when both would meet the general needs? 4. Is there a way to solve these inheritance problems without resorting to a library such as OOHaskell, which many Haskell programmers are not familiar with? Thanks, -- John -- John Goerzen <jgoerzen(a)complete.org> GPG: 0x8A1D9A1F www.complete.org "Value your freedom, or you will lose it, teaches history. `Don't bother us with politics,' respond those who don't want to learn."

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powerful type checking & false expectations that a program is correct
by Isaac Gouy 14 Jan '06

14 Jan '06

Programmers who use languages without static type checking sometimes claim that static type checking gives folk the false impression that once the program passes type checking the program is correct. That always seemed silly to me, but I'm starting to wonder ;-) Of course, the shootout programs are a silly entertainment; but after taking the time to optimise and write the programs, I'm a little surprised that there have been Haskell contributions which just don't give 'the correct answer' in a way that visual inspection with the expected answer would detect. For example ackermann Haskell GHC program regex-dna (new) Haskell GHC #2 program Oh well, they are just easy to fix formating bugs :-) __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Formated output with ghci
by Frank 14 Jan '06

14 Jan '06

A 'stupid' question (sort of an embarassing one): I want to do formated output by converting data types to strings and the bring them in a formated way together. I use ghci 6.4 and when I want to see this text then it is shown as a string (i.e. with "... " added around) and the formating characters in the text (e.g. \n) are quoted and not executed. (if I apply show to the text string, then the above is done twice!) This does occur if I use a defined function (something like 'toString' or 'text') and does not occur if I use show (but I have other reasons not to use show). Is there a cure to have output formated with ghci without using show to individually convert data types into strings? Here an example output: text s2 "last id used #2\n\n(#2,EntType {unEntType = \"Point\"})\n(#1,EntType {unEntType = \"Point \"})\n\n\n deleted\nRelX empty\n\nHCons (Region2start \"start of region\",RelX empty\n) ( HCons (Region2end \"end of region\",RelX empty\n) (HCons (Point2coord \"Coordinates of Poi nt\",\n(#2,\n[NV 0.0 100.0])\n(#1,\n[NV 0.0 0.0])\n\n) HNil))" Wrapping the text string into a data type and then apply show to this data type gives also the desired result. Is there an easier way? Help is highly appreciated module OutputTest where data X = X Int Int Float deriving (Show) x1 = X 3 4 4.5 text :: X -> String text (X a b c) = show a ++ show b ++ " \n" ++ show c -- example output -- >text x1 --"34 \n4.5" data T a = T a instance Show a => Show (T a) where show (T a) = "Tx " ++ show a tx1 = T x1 instance Show (T X) where show (T a) = text a -------- works: -- >tx1 --34 --4.5

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