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blame: mtl MonadReader instance for ContT
by Nicolas Frisby 01 Jan '09

01 Jan '09

This mtl instance is in conflict with a usage of shift/reset that I'm porting from ML (Sheard's type-directed partial evaluator). > instance (MonadReader r' m) => MonadReader r' (ContT r m) where > local f m = ContT $ \c -> do > r <- ask > local f (runContT m (local (const r) . c)) I need to remove the "local (const r)" bit. I think this is just a different point in the diverse design space of dynamic binding/delimited control. Since Haskell lacks visibility management for importing instances, I'm forced to duplicate the rest of the ContT code in order to replace this instance with my own. 1) I am hoping this list can recall the reason for putting this design decision into the mtl. Perhaps there's a reason to prefer this particular side-effect interaction. 2) I am suggesting that the mtl be restructured such that this instance be delegated to a separate module so that i) I can avoid importing it along with the other useful instances ii) and the mtl could provide alternative instances. An alternative to breaking out the modules would be to not provide the instance. Another, more radical alternative could involve phantom types. I don't know of any in the mtl, but this same consideration might be applicable to other libraries with "arbitrary" instances.

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Different QSem(N) (was Re: IORef vs TVar performance: 6 seconds versus 4 minutes)
by ChrisK 29 Dec '08

29 Dec '08

I think I can improve on your code. Bertram Felgenhauer wrote: > But why does it manually manage the waiters at all? MVars are fair, in > ghc at least. So this should work: > > data Sem = Sem (MVar Int) (MVar Int) I changed the above to be a data > > newSem :: Int -> IO Sem > newSem initial = liftM2 Sem (newMVar initial) newEmptyMVar > > -- | Wait for a unit to become available > waitSem :: Sem -> IO () > waitSem (Sem sem wakeup) = do > avail' <- modifyMVar sem (\avail -> return (avail-1, avail-1)) Threads can get out of order at this point. This "order bug" may be undesirable. Also, killing the thread while it waits for "wakeup" below would be bad. You need an exception handler and some kind of cleanup. > when (avail' < 0) $ takeMVar wakeup >>= putMVar sem > -- | Signal that a unit of the 'Sem' is available > signalSem :: Sem -> IO () > signalSem (Sem sem wakeup) = do > avail <- takeMVar sem > if avail < 0 then putMVar wakeup (avail+1) > else putMVar sem (avail+1) You should change this from "= do" to "= block $ do". > > (I should turn this into a library proposal.) > > Bertram If you do not need to take N at a time then the untested code below has no "order bug" and is fair. > module Sem where > > import Control.Concurrent.MVar > import Control.Monad(when,liftM2) > > data Sem = Sem { avail :: MVar Int -- ^ provides fast path and fair queue > , lock :: MVar () } -- ^ Held while signalling the queue > > -- It makes no sense here to initialize with a negative number, so > -- this is treated the same as initializing with 0. > newSem :: Int -> IO Sem > newSem init | init < 1 = liftM2 Sem newEmptyMVar (newMVar ()) > | otherwise = liftM2 Sem (newMVar init) (newMVar ()) > > waitSem :: Sem -> IO () > waitSem (Sem sem _) = block $ do > avail <- takeMVar sem > when (avail > 1) (signalSemN (pred avail)) > > signalSem :: Sem -> IO () > signalSem = signalSemN 1 > > signalSemN :: Int -> Sem -> IO () > signalSemN i (Sem sem lock) | i <= 1 = return () > | otherwise = > withMVar lock $ \ _ -> block $ do > old <- tryTakeMVar sem > case old of > Nothing -> putMVar sem i > Just v -> putMVar sem $! succ i All waitSem block in arrival order with the takeMVar in waitSem. The signalSemN avoid conflicting by serializing on the "MVar ()" lock. The above is quite fast so long as the semaphore holds no more than the value 1. Once it hold more than 1 the waiter must take time to add back the remaining value. Note that once threads are woken up in order, they may still go out of order blocking for the () lock when adding back the remaining value (in the presence of other signalers). The above is also exception safe. The only place it can die is during the takeMVar and this merely remove a blocked waiter. I see no way to add a fair waitSemN without changing Sem. But if I change Sem then I can make a fair waitSemN. The untested code is below: > module Sem where > > import Control.Concurrent.MVar > import Control.Monad(when,liftM3) > import Control.Exception.Base > > data Sem = Sem { semWait :: MVar () -- for serializing waiting threads > , semAvail :: MVar Int -- positive quantity available > , semSignal :: MVar () -- for serializing signaling threads > } > > > newSem i | i<=0 = liftM3 Sem (newMVar ()) newEmptyMVar (newMVar ()) > | otherwise = liftM3 Sem (newMVar ()) (newMVar i) (newMVar ()) > > waitSem :: Sem -> IO () > waitSem = waitSemN 1 > > waitSemN :: Int -> Sem -> IO () > waitSemN i sem@(Sem w a s) | i<=0 = return () > | otherwise = withMVar w $ \ _ -> block $ do > let go n = do > avail <- onException (takeMVar a) (signalSemN (i-n) sem) > case compare avail n of > LT -> go $! n-avail > EQ -> return () > GT -> signalSemN (avail-n) sem > go i > > signalSem :: Sem -> IO () > signalSem = signalSemN 1 > > signalSemN :: Int -> Sem -> IO () > signalSemN i (Sem _ a s) | i<=0 = return () > | otherwise = withMVar s $ \ _ -> block $ do > ma <- tryTakeMVar a > case ma of Nothing -> putMVar a i > Just v -> putMVar a $! v+i Trying for exception safety makes the above slightly tricky. It works by allowing only a single thread to get the semWait lock. This keeps all the arriving threads in the fair blocking queue for the semWait lock. The holder of the semWait lock then nibbles at semAvail's positive value until it is satisfied. Excess value is added back safely with signalSemN. Cheers, Chris Kuklewicz

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Cabal Bug / Feature and Improvement
by Dominic Steinitz 29 Dec '08

29 Dec '08

Currently if I do not want to use the default locations for cabal then the following: >dist\build\cabal\cabal.exe --config-file=urghh --remote-repo-cache=foo --verbose=3 update will create urghh but will still (silently) put the repository cache at repoLocalDir = "C:\\Documents and Settings\\steinitd\\Application Data\\cabal\\packages\\hackage.haskell.org" I've made a few changes which will put the cache at "foo". I don't have access to darcs at the moment so I've attached the changes. Let me know if a darcs patch is worth doing. Dominic. In Config.hs: loadConfig :: Verbosity -> GlobalFlags {-Flag FilePath-} -> Flag Bool -> IO SavedConfig loadConfig verbosity globalFlags {-configFileFlag-} userInstallFlag = addBaseConf $ do configFile <- maybe defaultConfigFile return (flagToMaybe (globalConfigFile globalFlags){-configFileFlag-}) minp <- readConfigFile mempty configFile case minp of Nothing -> do notice verbosity $ "Config file " ++ configFile ++ " not found." notice verbosity $ "Writing default configuration to " ++ configFile commentConf <- commentSavedConfig initialConf <- initialSavedConfig writeConfigFile configFile commentConf (initialConf `mappend` (mempty {savedGlobalFlags = globalFlags})) return initialConf The other cases of loadConfig are unchanged. In Main.hs global replace: loadConfig verbosity (globalConfigFile globalFlags) by loadConfig verbosity globalFlags

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Re: [Haskell-cafe] Cabal Install & Links to Source from Haddock Docs
by R Hayes 23 Dec '08

23 Dec '08

Thank you. As it turns out, I was aware of that recipe. What I wanted was to be able to use cabal install's nice dependency following features and still get source links in my documentation. Personally, I feel that inclusion of source and docs should be the DEFAULT for cabal, as well as for binary distributions of ghc. -rhayes On Dec 21, 2008, at 1:11 AM, Thomas Hartman wrote: > the answer: not cabal install, just cabal. > > thartman@thartman-laptop:~/haskellInstalls/smallInstalls/ > pureMD5-0.2.3> > thartman@thartman-laptop:~/haskellInstalls/smallInstalls/ > pureMD5-0.2.3>cabal > --help | grep -i doc > haddock Generate Haddock HTML documentation. > thartman@thartman-laptop:~/haskellInstalls/smallInstalls/ > pureMD5-0.2.3>cabal > haddock --help | grep -i link > --hyperlink-source Hyperlink the documentation to the source > code > thartman@thartman-laptop:~/haskellInstalls/smallInstalls/ > pureMD5-0.2.3>cabal > haddock --hyperlink-source > > 2008/12/21 R Hayes <rfhayes(a)reillyhayes.com>: >> >> Is there a way I can get Haddock Docs WITH links to source (local) >> from >> modules installed with "cabal install xxx"? >> >> Getting the docs themselves is pretty easy by changing either >> ~/.cabal/config or using --enable-documentation. >> >> Automatically generating the source (colourised or not) and >> integrated links >> eludes me. >> >> -r >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >>

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cabal-install sends invalid proxy password
by Valery V. Vorotyntsev 18 Dec '08

18 Dec '08

Dear lazy programmers, I was surprised to discover that `cabal-install' -- a popular utility for installing Hackage packages -- cannot work with HTTP proxies. Despite all the necessary code linked in. `cabal update' command returns HTTP 407 (Proxy Authentication Required) error. The problem is explained below and patches follow. * * * Our office LAN has an HTTP proxy server with authorization. And my shell environment has a variable like http_proxy='http://user:password@proxyhost:proxyport/' tcpdump-ing shows that `cabal update' sends invalid credentials in HTTP "CONNECT" request. The proper string would be "user:password" (base64-encoded) while `cabal' sends "user:password@". There's no surprise proxy server denies connection! I've traced the problem down to `userinfo' function, defined in network-2.2.0.1/Network/URI.hs:451: > -- RFC3986, section 3.2.1 > > userinfo :: URIParser String > userinfo = > do { uu <- many (uchar ";:&=+$,") > ; char '@' > ; return (concat uu ++"@") > } Let us see section 3.2 of RFC3986: > authority = [ userinfo "@" ] host [ ":" port ] > [... > ...] The user information, if present, is followed by a > commercial at-sign ("@") that delimits it from the host. There is no reason for "@" character to be a part of `userinfo' expression. * * * So, the Right Way(TM) to make cabal-install go through proxies is fixing `userinfo' function of `network' package (see the `network-2.2.0.1.patch'). Until then, we should have a workaround in `cabal-install' (cabal-install-0.6.0.patch). I do. Happy hacking! -- vvv

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Re: [Haskell] possible bug in pretty-1.0.1.0
by Duncan Coutts 18 Dec '08

18 Dec '08

On Mon, 2008-12-15 at 08:17 -0800, John MacFarlane wrote: > I noticed a difference in how "hang" works between pretty-1.0.0.0 and > pretty-1.0.1.0. I think it's a bug. If this isn't the right place to > report it, please let me know where I should. (Maintainer is listed > as libraries(a)haskell.org, but that is a closed mailing list. Perhaps > Cabal should include a report-bugs-at field?) I'd just like to advertise the fact that as of Cabal-1.6 you can put a bug-reports field in your .cabal file and it will be displayed by hackage. We would like to encourage all package authors to use this. It can be a mailto: url to a maintainer or mailing list or it can be a http: url to a bug tracker website. (Cabal-1.6 also supports specifying darcs/git/whatever repos) Duncan

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Re: possible bug in pretty-1.0.1.0
by Benedikt Huber 17 Dec '08

17 Dec '08

John MacFarlane schrieb: > I noticed a difference in how "hang" works between pretty-1.0.0.0 and > pretty-1.0.1.0. I think it's a bug. If this isn't the right place to > report it, please let me know where I should. (Maintainer is listed > as libraries(a)haskell.org, but that is a closed mailing list. Perhaps > Cabal should include a report-bugs-at field?) > > John > > GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help > ... > Prelude Text.PrettyPrint> putStrLn $ render $ hang (char '*') 4 (text "hi" $$ text "there") > Loading package pretty-1.0.0.0 ... linking ... done. > * hi > there > > GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help > ... > Prelude Text.PrettyPrint> putStrLn $ render $ hang (char '*') 4 (text "hi" $$ text "there") > Loading package pretty-1.0.1.0 ... linking ... done. > * > hi > there Hi John, I think you're right, from the perspective of 'hang' the last set of patches (http://hackage.haskell.org/trac/ghc/ticket/2393) indeed introduced a bug. Unfortunately a good description of what hang is _intended_ to mean is still missing in the documentation, but I'll assume that > hang (text "*") 4 (sep [ text "hi", text "there" ]) should either layout as > * hi there or > * hi > there If only the second layout is of interest (like in your example), then > hang' d1 n d2 = d1 $$ (nest n d2) should do the job. Related issue, we changed vcat to mean (foldr $+$ empty) instead of (foldr $$ empty) - either revert this change or update the documentation ? [1] Unfortunately, the situations is a bit trickier for hang (i.e. for sep[A, nest n B]); allowing overlap violates Invariant 7: The first line of the first layout has to be longer than the first line of the second layout because > sep [A, nest n B] ~ > oneLiner (A <+> B) `union` A $$ (nest n B) > with e.g. [A="a",B="b",n="4"] > "a b" `union` "a b" which causes the pretty printer to select an even longer! line if (A <+> B) doesn't fit on one line. I'm not sure how to resolve this - should sep [A, B] really overlap A and B if they do not fit on a line ??? Still, if it seems to be the right thing to do, it is easy to allow overlap in sep/cat. Otherwise hang might need to be changed. best regards, benedikt [1] Either hunk ./Text/PrettyPrint/HughesPJ.hs 114 - vcat is a list version of $$ + vcat is a list version of $+$ hunk ./Text/PrettyPrint/HughesPJ.hs 316 -vcat :: [Doc] -> Doc; -- ^List version of '$$'. +vcat :: [Doc] -> Doc; -- ^List version of '$+$'. or hunk ./Text/PrettyPrint/HughesPJ.hs 497 -vcat = reduceAB . foldr (above_' True) empty +vcat = reduceAB . foldr (above_' False) empty because of 505: above_' g p q = Above p g q 536: | Above Doc Bool Doc -- True <=> never overlap

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old-time missing configure script?
by Brent Yorgey 16 Dec '08

16 Dec '08

Hi all, Does anyone know what is going on with the 'old-time' package? When I try installing it from Hackage via cabal-install, I get the error Configuring old-time-1.0.0.0... Warning: The 'build-type' is 'Configure' but there is no 'configure' script. cabal: configure script not found. cabal: Error: some packages failed to install: old-time-1.0.0.0 failed during the configure step. The exception was: exit: ExitFailure 1 And indeed, the build-type is specified as Configure but there is no configure script included. -Brent

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Generics
by Simon Peyton-Jones 11 Dec '08

11 Dec '08

Jose As part of your work on the SYB library, could you spare the time to look into these two library bug reports? http://hackage.haskell.org/trac/ghc/ticket/2759 http://hackage.haskell.org/trac/ghc/ticket/2760 Many thanks Simon

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Re: [Haskell-cafe] ANNOUNCE: haskell-src-exts 0.4.4
by Niklas Broberg 10 Dec '08

10 Dec '08

(CC libraries) > So when will we simply declare that haskell-src-exts is the new > haskell-src? :-) I talked about this with Ross Paterson at one point, and his main objection (which I very much agree with) was that haskell-src-exts doesn't let you *optionally* handle extensions, so in some cases valid H98 programs will be discarded due to syntax stealing (e.g. $x from TH). I've had ideas for how to fix this shortcoming for a long while, but never really had the time to actually implement it. It is still my hope and goal that haskell-src-exts will replace the haskell-src package, when it is ready to do so. What do others think about this? Cheers, /Niklas

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