- Haskell-Cafe - Haskell.org

GHC 7.0.1 developer challenges
by John D. Ramsdell 24 Dec '10

24 Dec '10

A quick review of GHC 7.0.1 revealed two challenges for developers. I downloaded the GHC 7.0.1 sources, configured for a home directory install, and built and installed the compiler. Very close to the end, my machine froze, perhaps due to memory exhaustion. In any event, a reboot allowed me to complete the installation. I tested the new compiler on an application I distribute, CPSA. The algorithm implemented by the program is not guaranteed to terminate, and it's hard to predict when non-termination is inevitable. Sometimes the way it terminates is to allocate all the memory in swap space and freeze the machine. It's an ugly crash. So in my documentation, I recommend people supply a runtime flag limiting memory usage. Due to a security concern, GHC 7.0.1 disables all runtime flags unless a new flag is provided during linking. Since limiting memory usage is so important, many developers will modify their cabal files to add the linker flag or prepare for complaints from users that the developer's program caused their machine to freeze and lose their work. The irony of this situation is deep. CPSA is a program that analyzes cryptographic protocols in an effort to expose security flaws. To ensure that the program does not crash a user's machine, I have to use a linker option that may expose the user to some security problems. There is one more disappointment in GHC 7.0.1 for developers. Cabal sdist is still hosed. Since ten months ago, cabal sdist fails to preserve the file mode bits of the source files put into the tarball being generated (Ticket #627 reported by draconx). An executable shell script and a source file world readable both have a mode of 600 in the tarball! So developers, continue to keep GHC 6.10.4 around so you can create source distributions. That's what I do. John

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ANN: xmobar 0.12 released
by Jose A. Ortega Ruiz 24 Dec '10

24 Dec '10

I'm happy to announce a new relase of xmobar, the text-based monitor bar. You can read everything about xmobar and this release at our new web address (http://projects.haskell.org/xmobar/) or see below for a summary of what's new (cf. http://projects.haskell.org/xmobar/releases.html) ## Version 0.12 xmobar has a new [maintainer], a new [website], a new [mailing list] and uses a new [source code repository]. Many thanks to Andrea Rossato, xombar's author and maintainer so far, for creating xmobar in the first place, and for giving me the chance to become its maintainer. And a big thanks to Ben Boeckel, Petr Rockai and Norbert Zeh for their patches. [website]: http://projects.haskell.org/xmobar/ [mailing list]: http://projects.haskell.org/cgi-bin/mailman/listinfo/xmobar [source code repository]: https://github.com/jaor/xmobar [maintainer]: http://hacks-galore.org/jao/ _New features_ - Window borders: configuration options `border` and `borderColor` allow drawing borders around xmobar's window. - New monitor, `Uptime`, showing the system uptime. - New monitor argument (`-S`) to enable displaying the `%` symbol in percentages or other suffixes (e.g., units in Uptime and Network); the symbol is now never included by default. - New 'run once' commands, by specifying a 0 refresh rate in `Run Com` ([issue 26]). - MPD monitor: updated to libmpd 1.5. New fields `ppos` (playlist position) and `remaining` (remaining time). New configuration options to specify MPD's host, user name and password. - Battery monitor: new `watts` and `timeleft` fields (Petr Rockai), and specific arguments to control coloring and thresholds of the former. - MultiCPU monitor: new `auto*` fields that automatically detect all present CPUs (Ben Boeckel). - CpuFreq monitor uses just one decimal digit for GHz values (Petr Rockai). - Mail plugin expands paths starting with "~/" (Ben Boeckel). Ditto MBox. - Weather monitor now skips not retrieved fields, instead of displaying a long error message. - New compilation flag, `all_extensions`. - Documentation and website updates. _Bug fixes_ - [issue 23]: Wireless monitor is now compatible with iwlib 29. - [issue 24]: Swap monitor's used ratio display fixed. - [issue 25]: Percentages only include `%` if requested using `-P`. - [issue 31]: MPD monitor now respects `-W` argument. - Fixes in CPU frequency formatting, string alignment and colour boxes in monitors (Norbert Zeh). - TopMem and TopProc now use the `-L` and `-H` options correctly for memory template fields. - MBox skips non-existent mbox paths instead of hanging.

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List of numbers to list of ranges
by C K Kashyap 24 Dec '10

24 Dec '10

Hi all, Here's my attempt to convert a list of integers to a list of range tuples - Given [1,2,3,6,8,9,10], I need [(1,3),(6,6),8,10)] My attempt using foldl yields me the output in reverse. I can ofcourse reverse the result, but what would be a better way? f xs = foldl ff [] xs where [] `ff` i = [(i,i)] ((s,e):ns) `ff` i = if i == e+1 then (s,i):ns else (i,i):(s,e):ns Regards, Kashyap

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Do we need Monad fail (or MonadFail)?
by John Smith 23 Dec '10

23 Dec '10

Monads seem to use fail in one of three ways: -Regular monads leave it at the default definition of error -MonadPlus sometimes redefines it to mzero -IO redefines it to failIO Are there any other definitions of fail? If not, does the special case of IO really need a class-level definition, or could there be another way of dealing with failed pattern matches?

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Why is Haskell flagging this?
by michael rice 23 Dec '10

23 Dec '10

I don't understand this error message. Haskell appears not to understand that 1 is a Num. Prelude> :t 1 1 :: (Num t) => t Prelude> :t [1,2,3,4,5] [1,2,3,4,5] :: (Num t) => [t] Prelude> Michael =================== f :: [Int] -> IO [Int] f lst = do return lst main = do let lst = f [1,2,3,4,5] fmap (+1) lst =============================== Prelude> :l test [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:5:17: No instance for (Num [Int]) arising from the literal `1' at test.hs:5:17 Possible fix: add an instance declaration for (Num [Int]) In the second argument of `(+)', namely `1' In the first argument of `fmap', namely `(+ 1)' In the expression: fmap (+ 1) lst Failed, modules loaded: none. Prelude>

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Help!! Cannot get RandT and liftIO to work together...
by JP Moresmau 23 Dec '10

23 Dec '10

Hello all, sorry I must have taken my stupid pills this morning, I cannot get the following code to compile, what am I missing? data MyState=MyState Integer newState:: (RandomGen g) => RandT g IO MyState newState = do time<-liftIO getCPUTime rand<-getRandomR (1,6) return $ MyState (time+rand) newStateIO :: IO MyState newStateIO = do r<-getStdGen evalRandT newState r -> Could not deduce (MonadIO (RandT g IO)) from the context (RandomGen g) arising from a use of `liftIO' at ...:4:15-31 Possible fix: add (MonadIO (RandT g IO)) to the context of the type signature for `newState' or add an instance declaration for (MonadIO (RandT g IO)) Pirates.hs /Pirates/src line 167 Problem But the MonadRandom docs say: Instances: MonadIO<http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/C…> m => MonadIO<http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/C…> (RandT<http://hackage.haskell.org/packages/archive/MonadRandom/0.1.6/doc/html/Cont…> g m) And the MonadIO docs say: Instances: MonadIO<http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/C…> IO<http://hackage.haskell.org/packages/archive/base/4.3.1.0/doc/html/System-IO…> So it looks to me it should work! I've also tried to replace IO by MonadIO m=> in the signature, or remove all signatures. No joy. Thanks for any help. -- JP Moresmau http://jpmoresmau.blogspot.com/

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Re: [Haskell-cafe] Functor => Applicative => Monad
by Simon Marlow 23 Dec '10

23 Dec '10

On 22/12/10 19:17, John Smith wrote: > On 22/12/2010 19:03, Simon Marlow wrote: >> On 14/12/2010 08:35, Isaac Dupree wrote: >>> On 12/14/10 03:13, John Smith wrote: >>>> I would like to formally propose that Monad become a subclass of >>>> Applicative, with a call for consensus by 1 February. The change is >>>> described on the wiki at >>>> http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal, >>> >>> That page isn't written as a proposal yet, it's written as a bunch of >>> ideas. I would be happy to see something along the lines of Bas van >>> Dijk's work >>> http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/14740 . >> >> This is a proposal with far-reaching consequences, and with several >> alternative designs. I'm not sure I understand all >> the tradeoffs. Some parts of the proposal are orthogonal to the rest >> (e.g. changing fmap to map), and should probably be >> considered separately. >> >> Could someone please write a detailed proposal, enumerating all the >> pros and cons, and the rationale for this design >> compared to other designs? >> >> Cheers, >> Simon > > The wiki page was admittedly too broad of scope when I first wrote it, > and it's getting broader with time. > > The ticket at http://hackage.haskell.org/trac/ghc/ticket/4834 has > patches for only one change, making Monad a subclass of Applicative. (I > would update the description to make this clear, but I can't edit the > ticket description, so this has been relegated to a comment.) Dealing with one issue at a time is great. But even with this single change, we need to weight up the advantages and disadvantages - it's difficult to make an assessment without trawling through long email threads, and yet I don't want to let the proposal pass without comment. So it would help a lot if someone could take the time to explain the design space and the tradeoffs. I know for example I have lots of code that defines a Monad instance and doesn't need an Applicative instance, so this change will force me to jump through hoops that I don't need to. Cheers, Simon

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non_lazy_ptr ld error?
by Evan Laforge 23 Dec '10

23 Dec '10

Lately I've been seeing this error occasionally when recompiling: Undefined symbols: "_UiziTrackC_d26DB", referenced from: _UiziTrackC_d26DB$non_lazy_ptr in TrackC.o ld: symbol(s) not found If I delete the .o file for the mentioned module and recompile, it links fine. This makes me think something in the dependency analysis is mixed up and not recompiling something that should be recompiled. But I'm using --make and letting ghc figure out the deps. Does this error look familiar to anyone? This is ghc 6.12.3 on OS X 10.5.8. On a probably unrelated note, a few times lately I've had a startup segfault on a program which was previously fine. The stack trace on the crashing thread is: Thread 2 Crashed: 0 libSystem.B.dylib 0x954f9fa0 strcmp + 80 1 elaforge.seq.seq 0x0143f7dd lookupHashTable + 131 2 elaforge.seq.seq 0x01444243 ghciInsertStrHashTable + 24 3 elaforge.seq.seq 0x01445a87 ocGetNames_MachO + 1803 4 elaforge.seq.seq 0x014447bf loadObj + 500 5 elaforge.seq.seq 0x007064a4 sYy_info + 12 This is a little worrying because I've never seen this before and I don't know why it's started. It seems to happen consistently until I make clean and recompile. My app uses hint and I suspect this is part of the hint initialization and has to do with inconsistent .o files or something... anyone seen this either?

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Yet Another Haskell Tutorial missing (maybe lost on server move?)
by Henning Thielemann 23 Dec '10

23 Dec '10

The page http://www.haskell.org/haskellwiki/Yet_Another_Haskell_Tutorial says that there is a darcs repository at http://darcs.haskell.org/yaht/ but it is not. Also in the canonical replacement location http://code.haskell.org/yaht/ I cannot find it. The Wikiboook https://secure.wikimedia.org/wikibooks/en/wiki/Haskell/YAHT mentions the same URL and additionally a PDF at http://darcs.haskell.org/yaht/yaht.pdf which is missing, too. Can someone please tell me, where it is, update HaskellWiki and Wikibook and add a forward from darcs.haskell.org to the new location?

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New simplified primes; no VIP necessary.
by Will Ness 23 Dec '10

23 Dec '10

Hi, For those who remember the discussion about this about a year ago, it turns out there was a simpler version after all lurking somewhere in there (or is it _out_?). I've just posted it to the haskellwiki's Prime Numbers page: primes = 2: primes' where primes' = 3: 5: [7,9..] `minus` tfold [ [p*p,p*p+2*p..] | p <- primes' ] tfold ((x:xs):t) = x : xs `union` tfold (pairs t) pairs ((x:xs):ys:t) = (x: union xs ys) : pairs t The full code with double-feed, wheel and better folding structure is also there. Speed and complexity weren't changed by this. But it is rather _short_. :) Cheers,

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