January 2005 - Haskell-Cafe

RE: [Haskell-cafe] Can't do basic time operations with System.Time
by Simon Marlow 21 Jan '05

21 Jan '05

On 21 January 2005 14:31, John Goerzen wrote: > I checked the following code into MissingH. I would be pleased if you > could take it for fptools. I should note that I am using an Integer > rather than an Int to represent seconds, since I think that is proper > given the size of values we might be encountering. If you do take it > for fptools, you could probably rewrite normalizeTimeDiff to use the > very similar code in timeDiffToSecs (shameless almost-stealing here > <g>) Have you seen this, BTW: http://www.haskell.org/~simonmar/time/NewTime.html and a prototype implemention here: http://www.haskell.org/~simonmar/time/NewTime.hsc Everything to do with TimeDiff in the current library is generally acknowledged to be broken, so I don't think we'll be doing anything with that, short of completely replacing it. See previous discussions (if you have a lot of free time...). > timelocal :: CalendarTime -> IO Integer > timelocal ct = > do guessct <- toCalendarTime guesscl > let newct = ct {ctTZ = ctTZ guessct} > return $ timegm newct > where guesscl = toClockTime ct This looks useful, given that it was what you wanted and it isn't already provided. I'd prefer to drop it into the existing library, and have it return a ClockTime, though. Perhaps: -- | converts the given CalendarTime into a ClockTime, -- ignoring the ctTZ field of CalendarTime and instead -- using the prevailing local timezone (including daylight -- savings adjustments) at the given time. localTimeToClockTime :: CalendarTime -> IO ClockTime that's more or less a direct interface to mktime(). Cheers, Simon

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Math libraries for Haskell
by Dmitri Pissarenko 21 Jan '05

21 Jan '05

Hello! Is there a math library for Haskell, using which one can calculate eigenvalues of matrices? Thanks in advance Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Reading images
by Dmitri Pissarenko 21 Jan '05

21 Jan '05

Hello! I need to write a function in Haskell, which 1) reads a greyscale image (for instance, in JPEG, PNG or the like) and 2) transforms it into a n X m matrix A, where Aij contains an integer value (in the range 0 to 255). That integer value is zero for a completely black pixel, 255 for a completely white pixel and lies between zero and 255 for a grey pixel. In my program, I need ONLY to read an image and to transform it into such matrix. Which graphics library can you recommend, if no other image manipulation operations are required? TIA Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Re: [Haskell] Implicit parallel functional programming
by Ben Lippmeier 20 Jan '05

20 Jan '05

Satnam Singh wrote: > Ample looks interesting. What license does it use? I had a quick look > over the source and can't find anything. Is there a port to Windows or > does it not do any OS specific UI/graphics? By default, we're releasing it under GPL. I guess I should place a copy of the license somewhere in the tarball... For the graphs it produces, it just emits a gnuplot source file (textfile, easy) and then automagically calls gnuplot to display that file. It would probably work straight out of the box under cygwin (or similar) with an appropriate build of gnuplot in your current path. Failing that, you (or I) could easilly comment out the "call gnuplot" stage and end up with a vanilla console-IO application. If it gives you any trouble then send me a mail and i'll fix it. ... There's also a paper that goes with it, which used to be on Clem's page but I see that the link is broken now, i'll get fixed. ... > > a (very slow) simulator you might want to poke around with. I wrote it > based around Clem Baker-Finch's "Abstract machine for parallel lazy > evaluation", which supports fully speculative implicit parallelism. > > There's a link to it on my homepage at > http://cs.anu.edu.au/people/Ben.Lippmeier >

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Can't do basic time operations with System.Time
by John Goerzen 20 Jan '05

20 Jan '05

Hi, I have a simple desire. I have a string that I need to parse as a date/time string in the local timezone, then convert it to standard seconds-since-epoch format. This is trivial in C, Perl, Python, etc. but seems impossible to do reliably in Haskell. I'm hoping someone can tell me where I've gone wrong. My initial idea was to parse the string, generate a CalendarTime object, use toClockTime to convert it to seconds-since-epoch, and then grab the value out of that. There were some problems, though: what to put in ctTZ and ctIsDST? I initially loaded them with 0 and False, though that turned out to be clearly wrong. My next idea was to take the ctTZ result from toCalendarTime, then use that for all the CalendarTime objects I'd generate. That was wrong, too, during daylight savings time. I don't think I should have to re-implement the algorithms for determining whether a given date is under DST worldwide. What I really need is just a wrapper around mktime() that just calls mktime and doesn't do anything else. All the post-processing that System.Time is doing is what's causing me trouble. I'd just like it to ignore ctTZ, ctTZName, and ctIsDST. That would yield behavior identical to the C library. -- John

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lex
by Daniel Fischer 20 Jan '05

20 Jan '05

Hi, does anybody know, why 'lex' isn't faithful to the Haskell-syntax? Since it isn't difficult to make it ignore comments, handle qualified names and recognize octal/hexadecimal literals (the system, be it hugs or ghc, does it, and I filled in the required code this evening, so for a good programmer it would be a matter of half an hour at most), I suppose there must be a reason, only I can't think of one. If you have any good ideas or even know the reason, please tell me. Thanks in advance, Daniel

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RE: [Haskell-cafe]Re: Hugsvs GHC (again)was: Re: Somerandomnewbiequestions
by Simon Marlow 20 Jan '05

20 Jan '05

On 20 January 2005 11:30, Keean Schupke wrote: > Simon Marlow wrote: > >> Yes, except that you forgot that not all foreign calls can run >> >> concurrently with Haskell code. Only the "safe" ones can. >> >> > Okay, now I understand what is going on. Why is there extra overhead > for a 'safe' call? A safe call must - save its state on the Haskell stack - save its thread state object on a queue, so GC can find it - tidy up the stack so GC can take place - release the RTS lock - possibly start a new OS thread if the pool is empty of these, releasing the RTS lock is probably the most expensive. Also, if another OS thread gets scheduled before the call returns, we have an expensive context switch on return. In contrast, an unsafe call is just an inline C call. Cheers, Simon

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Problems with compiling wxFruit
by Dmitri Pissarenko 20 Jan '05

20 Jan '05

Hello! I'm trying to compile wxFruit sample program paddleball (http://zoo.cs.yale.edu/classes/cs490/03-04b/bartholomew.robinson/) When I try to compile file paddle.hs (see below where it is availeble) using the call <call> ghc -fglasgow-exts -farrows -package wx -iC:\dapWork\haskell-learning\afrp-0.4 \src:C:\dapWork\haskell-learning paddle.hs </call> I'm getting the error message <error-message> paddle.hs:10: Failed to load interface for `WXFruit': Could not find interface file for `WXFruit' (use -v to see a list of the files searched for) paddle.hs:11: Failed to load interface for `AFRP': Could not find interface file for `AFRP' (use -v to see a list of the files searched for) paddle.hs:12: Failed to load interface for `AFRPUtilities': Could not find interface file for `AFRPUtilities' (use -v to see a list of the files searched for) </error-message> The file WXFruit.hs is located in the same directory as paddle.hs. AFRP.hs and AFRPUtilities.hs are located in the directory C:\dapWork\haskell-learning\afrp- 0.4\src. Apparently, those packages are not installed properly. What should I do in order to compile paddle.hs? Thanks in advance PS: All files related to this example (paddle.hs and wxFruit.hs) are available at http://dapissarenko.com/resources/2005_01_20_haskell_learning/wxFruitSample… in a 7 MB zip archive. -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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RE: [Haskell-cafe] Re: Hugs vs GHC (again)was: Re: Somerandomnewbiequestions
by Simon Marlow 20 Jan '05

20 Jan '05

On 11 January 2005 14:15, Gracjan Polak wrote: > Simon Marlow wrote: > > There's a big lock on File. If you want to do truly concurrent > reading, > you can make multiple FileInputStreams, each of which has > its own file > descriptor (the Unix implementation uses dup(2)). > > > > Original and descriptor returned by dup or dup2 share file pointer. *blink* You're right! Serves me right for assuming that POSIX would have sensible semantics. Perhaps this API isn't implementable, in its current state. Cheers, Simon

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RE: [Haskell-cafe]Re: Hugsvs GHC (again)was: Re: Somerandomnewbiequestions
by Simon Marlow 20 Jan '05

20 Jan '05

On 20 January 2005 10:01, Keean Schupke wrote: > Simon Marlow wrote: > >> We're getting a bit confused here. Keean: the original question was >> about whether a disk read will stop all other *Haskell* threads. >> Not OS threads. The two are quite different beasts in GHC. >> >> Cheers, >> Simon >> >> > But if GHC is running with the -threaded flag, then other > Haskell-threads can keep running using the second OS-thread, even > though one Haskell-thread (and its associated OS thread) is blocking > - right? If the call to read() was "safe" (i.e. the FFI "safe" attribute), then yes. But GHC currently does and "unsafe" call to read(), even with -threaded, which doesn't give other threads a chance to run. Perhaps it should be a safe call. That's going to hurt performance unless you really need to parallelise disk I/O, though. Maybe it wouldn't hurt too much, someone should measure it. > In other words even with disk IO (as I said the kernel would not > busy-wait - so there are > only two options, this OS-thread is put to sleep, or the data is > already in a buffer and is > returned immediately), all the other Haskell-threads should not block > (provided it is > running with -threaded) > > Have I got that right? Yes, except that you forgot that not all foreign calls can run concurrently with Haskell code. Only the "safe" ones can. Cheers, Simon

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