December 2005 - Haskell-Cafe

RE: [Haskell-cafe] Re: :t main
by Simon Peyton-Jones 07 Dec '05

07 Dec '05

I've not been following this thread, but I wanted to check: you do know about Tarmo Uustalu's stuff about comonads, don't you? http://www.cs.helsinki.fi/u/ekarttun/comonad/ summarises (link to "The essence of dataflow programming" at the bottom) Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | David Menendez | Sent: 06 December 2005 07:13 | To: Scherrer, Chad | Cc: haskell-cafe(a)haskell.org | Subject: RE: [Haskell-cafe] Re: :t main | | Scherrer, Chad writes: | | > > From: Cale Gibbard [mailto:cgibbard@gmail.com] | > > See: | > > http://haskell.org/pipermail/haskell-cafe/2003-January/003794.html | > > | > > The OI comonad as previously envisioned breaks referential | > > transparency. I/O just doesn't seem to be something which one | > > can easily do comonadically, since once coeval/extract is | > > applied, you're back to plain values, and there's no | > > imposition of sequencing. | > > | > > - Cale | > | > Hmm, I hadn't seen that. The asymmetry is pretty frustrating in that | > case. | > | > After poking around a bit more, I (re)discovered some discussion of | > this on Hawiki: | > http://www.haskell.org/hawiki/CoMonad | > | > After reading Dave Menendez's comments, I'm wondering... | > If we consider | > IO a = Realworld -> (Realworld, a) | > then wouldn't we dually have something like | > OI a = (Realworld, Realworld -> a)? | > | > Could this be what screws things up? Right now it seems like OI is | > acting like it has the same type as IO, with a different name. | | Aside from the product comonad and the exponent (reader) monad, I don't | think we'll find much overlap between monad and comonad functionality. | It's been said that monads describe effects that propagate outwards, | whereas comonads describe effects that propagate inwards. | | My guess is that comonadic IO would look more like dataflow programming. | -- | David Menendez <zednenem(a)psualum.com> | "In this house, we obey the laws | <http://www.eyrie.org/~zednenem> | of thermodynamics!" | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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New runtime crash, not sure what to make of it
by Joel Reymont 06 Dec '05

06 Dec '05

Program compiled with -threaded on Mac OSX. Crash happens when waiting in select, I think. Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_PROTECTION_FAILURE at address: 0x00000000 [Switching to process 832 thread 0x2303] 0x0011ce20 in stg_enter_info () (gdb) where #0 0x0011ce20 in stg_enter_info () #1 0x00170e7c in StgRunIsImplementedInAssembler () #2 0x001247a4 in schedule () #3 0x00124468 in taskStart () #4 0x0017f800 in startProcWrapper () #5 0x9002b200 in _pthread_body () Haskell stack trace: (gdb) p16 $r22 0x130a87c: 0x130a898 0x130a878: 0x13162c <selw_info> 0x130a874: 0x2a 0x130a870: 0x1305000 0x130a86c: 0x1b6aec <sdG0_info> 0x130a868: 0x130a884 0x130a864: 0x20e174 <GHCziIOBase_bindIO_closure> 0x130a860: 0x20e17c <GHCziIOBase_returnIO_closure> 0x130a85c: 0x1315b0 <selK_info> 0x130a858: 0x130a878 0x130a854: 0x130a86c 0x130a850: 0x20e174 <GHCziIOBase_bindIO_closure> 0x130a84c: 0x10002 <s7uY_dflt+38> 0x130a848: 0x1174c0 <stg_PAP_info> 0x130a844: 0x170114 <stg_stop_thread_info> 0x130a840: 0x170208 <stg_noforceIO_info> 0x130a83c: 0x118bd0 <stg_ap_v_info> -- http://wagerlabs.com/

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RE: [Haskell-cafe] STM and `orElse` on a few thousand TMVars
by Simon Marlow 06 Dec '05

06 Dec '05

On 06 December 2005 10:02, Joel Reymont wrote: > Is it efficient to wait on a few thousand TMVars? That depends... while waiting your thread consumes no CPU at all (of course), and simply waiting on all those TVars only imposes a small constant overhead on anyone updating one of the TVars (a TMVar is just a TVar). However, checking all those TVars is O(n), and this happens twice each time you wake up: once to find which TVar was modified, and once again to go back to sleep on all of them. I'm not sure what you're using all those TMVars for, but it sounds like it might be better to multiplex them all into a single channel, or something. > How would I write something like that, assuming that my TMVars were > in a list. perhaps something along the lines of foldr orElse (return ()) . map takeTMVar it depends what you want to return. Cheers, Simon

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Retrieving the caught signal within a handler
by Joel Reymont 06 Dec '05

06 Dec '05

Is there a way to retrieve the signal within the signal handler? I would like to know the signal that I caught. Thanks, Joel -- http://wagerlabs.com/

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Number of outstanding messages in Chan or TChan
by Joel Reymont 06 Dec '05

06 Dec '05

Is there a way to check the number of outstanding messages in a Chan or TChan? I suspect my problem is related to not reading messages fast enough and I'd like to troubleshoot this by scanning my channels once every few seconds and dumping statistics. Thanks, Joel -- http://wagerlabs.com/

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RE: [Haskell-cafe] Re: :t main
by Scherrer, Chad 06 Dec '05

06 Dec '05

> From: Simon Peyton-Jones [mailto:simonpj@microsoft.com] > > I've not been following this thread, but I wanted to check: > you do know about Tarmo Uustalu's stuff about comonads, don't you? > > http://www.cs.helsinki.fi/u/ekarttun/comonad/ summarises > (link to "The essence of dataflow programming" at the bottom) > > Simon > Thanks, Simon. I've seen it, but I haven't fully digested it yet. Dataflow programming is new to me, and it's not immediately obvious how it ties in with IO in general. I'll take another look, though. Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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Re: [Haskell-cafe] Learning Haskell
by Donn Cave 06 Dec '05

06 Dec '05

Quoth Lemmih <lemmih(a)gmail.com>: ... | You might wanna have a look at Don's FastPackedString library: | http://www.cse.unsw.edu.au/~dons/fps.html Thanks, that's actually what got me thinking about trying this again. | > | I am also hoping to use it for web development. | > | > Wonder how `links' is coming along. (Hope they figured out a better | > name, anyway.) | | Got an url for the project? http://homepages.inf.ed.ac.uk/wadler/links/ Having looked it up, of course I looked at it. Please forget I asked. Donn Cave, donn(a)drizzle.com

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RE: TChan implementation: Why TVarList
by Simon Peyton-Jones 06 Dec '05

06 Dec '05

The mutable cell is in the tail. A [TVar a] would be quite different. You can read about a very similar impl (based on MVars) in the original "Concurrent Haskell" paper (on my papers page) S | -----Original Message----- | From: Joel Reymont [mailto:joelr1@gmail.com] | Sent: 06 December 2005 11:28 | To: Simon Peyton-Jones | Cc: Haskell Cafe | Subject: TChan implementation: Why TVarList | | Simon, | | Why did you guys implement TChan on top of your own TVarList instead | of a regular list? | | -- | 'TChan' is an abstract type representing an unbounded FIFO channel. | data TChan a = TChan (TVar (TVarList a)) (TVar (TVarList a)) | | type TVarList a = TVar (TList a) | data TList a = TNil | TCons a (TVarList a) | | Thanks, Joel | | -- | http://wagerlabs.com/ | | | |

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RE: [Haskell-cafe] What's a thread doing / Erlang-style processes /Message passing
by Simon Marlow 06 Dec '05

06 Dec '05

On 06 December 2005 10:28, Joel Reymont wrote: > I'm finding myself in dire need of monitoring the Haskell runtime. > More precisely, I would like to know what each that I launch is doing > at a given point in time. > > Is there a way to obtain this information now? I would be fine with > knowing if a thread is blocking on a foreign call, MVar or something > like that. If you compile with -debug, and run with +RTS -Ds you get a scheduler trace which might help. Cheers, Simon

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RE: [Haskell-cafe] Signal 2 and -threaded
by Simon Marlow 06 Dec '05

06 Dec '05

On 06 December 2005 10:59, Joel Reymont wrote: > I'm seeing some funky behavior in my program compiled with -threaded. > It runs for a little while and then catches signal 2 and quits. This > does not happen when -threaded is not used. > > I'm compiling my library with -threaded so that is always constant. > The above applies to my main module and another module that is part > of it. > > Any ideas? I don't know any reason your program should be getting signal 2 (SIGINT, I assume), unless the program sends itself SIGINT or someone else does. Maybe on your system a process gets SIGINT when it exceeds some resource limit? Have you tried strace or equivalent (truss?)? Cheers, Simon

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