Re: [Haskell-cafe] pure Haskell database
On Thu, Sep 25, 2008 at 11:09 AM, Manlio Perillo [EMAIL PROTECTED]wrote: Rich Neswold ha scritto: On Wed, Sep 24, 2008 at 4:17 PM, Manlio Perillo [EMAIL PROTECTED]mailto: [EMAIL PROTECTED] wrote: I need a simple, concurrent safe, database, written in Haskell. A database with the interface of Data.Map would be great, since what I need to to is atomically increment some integer values, and I would like to avoid to use SQLite. How about MVar (Map k Int)? or even Map k (MVar Int)? Yes, it is a solution; and I can run a thread that every N seconds writes the database to a file. But this works only if the database is used by only one process. Ah. When you said concurrent safe, I thought you meant within the application. You're looking for something like thishttp://hackage.haskell.org/cgi-bin/hackage-scripts/package/anydbm . -- Rich LOI: https://www.google.com/reader/shared/00900594587109808626 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] pure Haskell database
On Wed, Sep 24, 2008 at 4:17 PM, Manlio Perillo [EMAIL PROTECTED]wrote: I need a simple, concurrent safe, database, written in Haskell. A database with the interface of Data.Map would be great, since what I need to to is atomically increment some integer values, and I would like to avoid to use SQLite. How about MVar (Map k Int)? or even Map k (MVar Int)? -- Rich LOI: https://www.google.com/reader/shared/00900594587109808626 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MonadPlus
On Fri, May 9, 2008 at 2:39 PM, Andrew Coppin [EMAIL PROTECTED] wrote: [In a somewhat unrelated question... I saw some code the other day that used Either as if it were a monad. And yet, I don't see an instance given in the standard libraries - even though there should be one. I can see Functor (Either a), but not Monad (Either a) or even Monad (Either String)...] It's used in the Error Monad. -- Rich JID: [EMAIL PROTECTED] LOI: https://www.google.com/reader/shared/00900594587109808626 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help using CGIT
On 8/24/07, Bjorn Bringert [EMAIL PROTECTED] wrote: On Aug 23, 2007, at 3:34 , Rich Neswold wrote: Bingo! Method #3 works beautifully! I missed the using-lift-with- the-constructor permutation. Thanks for your help! I started writing a tutorial for Haskell web programming with the cgi package a while back, but haven't worked on it for a while, see http://www.haskell.org/haskellwiki/Practical_web_programming_in_Haskell I haven't added it to the list of tutorials yet, since it's still rather incomplete. The section on using CGIT is just a stub, perhaps you would like to contribute to it? See http://www.haskell.org/haskellwiki/ Practical_web_programming_in_Haskell#Extending_the_CGI_monad_with_monad_ transformers Done. As I learn more about using CGIT, I'll try to remember to add more content. Any suggestions to improve it are welcome! -- Rich JID: [EMAIL PROTECTED] AIM: rnezzy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Help using CGIT
Hello!
I've been having a tough time trying to use the CGI monad transformer
(CGIT). Hopefully someone can show me my misstep(s).
I have a CGI script that's evolving. Early on, it needed a single
database access. Now it's doing two accesses (and it looks like I'll
be adding more.) Rather than making a connection for each access, the
script needs to connect once. To do this, I want to combine the CGI
monad with the Reader monad.
This version compiles cleanly:
module AppMonad (App (..), runApp)
where
import Control.Exception (bracket)
import Control.Monad.Reader
import Network.CGI.Monad
import Network.CGI.Protocol
import System.IO (stdin, stdout)
import Database.HSQL.PostgreSQL
newtype App a = App (ReaderT Connection (CGIT IO) a)
deriving (Monad, MonadIO, MonadReader Connection)
runApp :: App CGIResult - IO ()
runApp (App a) =
bracket (connect host dbname user password)
disconnect
(\c - do { env - getCGIVars
; hRunCGI env stdin stdout (runCGIT (runReaderT a c))
; return () } )
Unfortunately, when another module tries to actually use the monad, I
get warnings about No instance for (MonadCGI App). I tried making an
instance:
instance MonadCGI App where
cgiAddHeader = ?
cgiGet = ?
But I don't know how to define these functions. I tried various
'lift'ing combinations, but couldn't come up with a solution that
would compile. I'm also disappointed that I had to break apart
'runCGI' (by cut-and-pasting its source) because I couldn't make it
believe my monad looked enough like MonadCGI.
My previous experiment with monad transformers was successful. It
didn't use CGIT, however, so the 'run*' functions were simpler.
Does anyone have an example of using CGIT (I didn't find any from
Google)? Shouldn't I be able to use 'runCGI' with my monad? CGIT users
shouldn't be required to re-implement 'runCGI, right?
Any help or ideas is appreciated!
--
Rich
JID: [EMAIL PROTECTED]
AIM: rnezzy
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help using CGIT
On 8/22/07, Ian Lynagh [EMAIL PROTECTED] wrote: On Wed, Aug 22, 2007 at 01:27:00PM -0500, Rich Neswold wrote: newtype App a = App (ReaderT Connection (CGIT IO) a) deriving (Monad, MonadIO, MonadReader Connection) Unfortunately, when another module tries to actually use the monad, I get warnings about No instance for (MonadCGI App). I tried making an instance: instance MonadCGI App where cgiAddHeader = ? cgiGet = ? You have three choices: 1: 2: 3: Provide a single instance for App that does the whole thing: instance MonadCGI App where cgiAddHeader n v = App $ lift $ cgiAddHeader n v cgiGet x = App $ lift $ cgiGet x This one you would obviously have to change if you added a StateT. Bingo! Method #3 works beautifully! I missed the using-lift-with-the-constructor permutation. Thanks for your help! -- Rich JID: [EMAIL PROTECTED] AIM: rnezzy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Bi-directional Maps
On 8/20/07, apfelmus [EMAIL PROTECTED] wrote:
Andrew Wagner wrote:
It occurred to me that it would be useful to explicitly
have a Bi-directional Map, which does the maintenance of keeping the
Maps synchronized behind the scenes. Thus, Bimap was born!
... most of the map functions (including update above) probably won't
work anyway, what should
left_insertWith (\new old - new) 'a' 1 (fromList [('a',2),('b',1)])
do? I can't yield
fromList [('a',1),('b',1)]
since 1 has two keys now.
Exactly. For this to work there needs to be the constraint that there's a
one-to-one mapping in each direction. The Bimap should have the uniqueness
promise that Set (k, v) gives. Yet you should be able to search on either
tuple value.
--
Rich
JID: [EMAIL PROTECTED]
AIM: rnezzy
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] interrupting an accept()ing thread
On 7/7/07, Lukas Mai [EMAIL PROTECTED] wrote: If I understand this correctly, spin should be written as: spin = do block $ do (t, _) - accept s unblock (forkIO $ doStuff t) `finally` sClose t spin I think the `finally` portion should be done in the forked process context. Otherwise once the process is forked, the socket gets closed by the parent process. Something more along the lines of: unblock (forkIO $ doStuff t `finally` sClose t) -- Rich JID: [EMAIL PROTECTED] AIM: rnezzy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] interrupting an accept()ing thread
On 7/5/07, Lukas Mai [EMAIL PROTECTED] wrote: Hello, cafe! I have the following code (paraphrased): ... forkIO spin ... spin = do (t, _) - accept s -- (*) forkIO $ dealWith t -- (**) spin My problem is that I want to stop spin from another thread. The obvious solution would be to throw it an exception. However, that leaks a socket (t) if the exception arrives between (*) and (**). I could wrap the whole thing in block, but from looking at the source of Network.Socket it seems that accept itself is not exception safe; so no matter what I do, I can't use asynchronous exceptions to make spin exit. What about using bracketOnError? nextClient s = bracketOnError (fst . accept s) sClose spin = do nextClient s (\s' - forkIO $ dealWith s') spin If bracketOnError leaks the resource in the event of an exception, then it needs to be fixed. -- Rich JID: [EMAIL PROTECTED] AIM: rnezzy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Telling the time
On 6/21/07, Andrew Coppin [EMAIL PROTECTED] wrote: Is there a standard library function anywhere which will parse a string into some kind of date/time representation? In Data.Time.Format, there's parseTime. parseTime takes a format string that describes the layout. Since you have varying layouts in your files (hopefully consistent in the same file!), you simply change the format string for each file. -- Rich JID: [EMAIL PROTECTED] AIM: rnezzy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [IO Int] - IO [Int]
On 5/4/07, Phlex [EMAIL PROTECTED] wrote: Hello all, I'm trying to learn haskell, so here's is my first newbie question. I hope this list is appropriate for such help requests. I'm trying to write a function with the signature [IO Int] - IO [Int] Control.Monad has a function (called sequence) that does this for you. In fact, sequence is a more generic solution since its signature is (Monadhttp://haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html#t%3AMonadm = [m a] - m [a]). As a newbie, I found it educational to peruse the various modules in http://haskell.org/ghc/docs/latest/html/libraries/;. Just start looking at modules that sound interesting and see what has already been defined. Some modules at first may be too advanced, but if you go back to them in a few days (weeks?), they'll start making more sense, too. -- Rich AIM : rnezzy ICQ : 174908475 Jabber: [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Tracking characters and a timestamp ?
On 4/5/07, Bulat Ziganshin [EMAIL PROTECTED] wrote: you definitely should read http://haskell.org/haskellwiki/IO_inside Thanks for mentioning this link -- I wasn't aware of it. I wish it existed when I first started learning Haskell... -- Rich AIM : rnezzy ICQ : 174908475 Jabber: [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] split string into n parts
On 10/23/06, jim burton [EMAIL PROTECTED] wrote: I want to split a string into 5 parts of equal length, with the last fifth padded if necessary, but can't get it right I got this:fifths :: String - Stringfifths xs = let len = (length xs + 4) `div` 5 padded = take (len * 5) (xs ++ ) in unwords $ nth len padded where nth _ [] = [] nth n xs = (take n xs) : (nth n $ drop n xs) *Main fifths IDOLIKETOBEBESIDETHESEASIDE IDOLI KETOBE BESIDE THESEA SIDEXX *Main fifths 12345 1 23 45This gives the following results:IDOLIK ETOBEB ESIDET HESEAS IDE and1 2 3 4 5But it also gives this result, which may or may not be correct for your problem: *Main fifths 12345612 34 56 -- RichAIM : rnezzyICQ : 174908475 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Receiving multicasts
Hello, Has anyone figured out a way to receive multicasts in a Haskell program? It doesn't appear that Network.Socket.setSocketOption provides enough information to join a multicast address. Any information would be appreciated. -- Rich AIM : rnezzy ICQ : 174908475 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Constructor constraints
Hello,I've looked through the two tutorials and the Report, but couldn't find help on this topic. My question is whether you can place constraints on new data types. For instance, I want to make a new type that is a 4 element tuple where each element is greater than or equal to the previous entry. Is this possible? i.e.data Category = Membership a a a aI'd like to be able to prevent invalid Category data from being created. Any information would be appreciated. -- RichAIM : rnezzyICQ : 174908475 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constructor constraints
On 9/20/05, Malcolm Wallace [EMAIL PROTECTED] wrote: You can make a 'smart' constructor function, and hide the real dataconstructor so that it cannot be used:Thanks! I'll give your solution a try.-- RichAIM : rnezzy ICQ : 174908475 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
