- Libraries - Haskell.org

RE: Time Resolution
by Simon Marlow 02 Feb '05

02 Feb '05

On 02 February 2005 07:25, Ashley Yakeley wrote: > In article <ashley-EF0986.17424301022005(a)sea.gmane.org>, > Ashley Yakeley <ashley(a)semantic.org> wrote: > >> It looks like we may need three types here: >> >> * one for TAI >> >> * one for POSIX time, which is a broken encoding of UTC >> >> * one for correct encoding of UTC > > This is the sort of thing I mean: > > newtype TAITime = TAITime Integer deriving (...) > > newtype POSIXTime = POSIXTime Integer deriving (...) > > newtype DiffTime = DiffTime Integer deriving (...) > > type JulianDay = Integer > > data UTCTime = UTCTime JulianDay DiffTime > > getCurrentTime :: IO POSIXTime > > posixToUTCTime :: POSIXTime -> UTCTime Why does the user of the library care about the difference between UTC, TAI and POSIX time? The only way I can think that this really needs to be exposed to the programmer is so that they can tell the library whether the system clock is running POSIX or TAI time. For the library API, a single notion of time is sufficient, no? Cheers, Simon

2 1

Time Resolution
by Ashley Yakeley 02 Feb '05

02 Feb '05

What underlying numerical type should be used to store TAI and UTC types, and at what resolution? newtype Clocktime = Clocktime ??? deriving (Eq,Ord, ...) Here are some suggestions. Not all of them are good ones: * Integer, where 1 = 1 microsecond, nanosecond, picosecond But which? Current ClockTime is ps. POSIX "struct timeval" is micro-s, "struct timespec" is ns. TAI64NA uses attoseconds (10^-18 s). <http://cr.yp.to/libtai/tai64.html> + fast(?) + power of ten matches common practice - whatever resolution might not be enough * Integer, where 1 = 1 Planck time (c. 5*10^-44 s) + amusing + unlikely to ever need more accuracy - number of Planck times in an SI second not precisely known - not a power of ten - about 2*10^31 in a picosecond, or about 100 bits * Fixed-size integer type, where 1 = 1 microsecond or etc. 2^64 mus = 580,000 years 2^64 ns = 580 years 2^128 ps = 10^19 years + fast(?) - probably need Int128 * Rational, where 1 = 1 second + 1 = 1 second easy to use + all the resolution you need + no error when dividing, guarantee (a / b) * b == a - might be slower? * Fixed-point type, where 1 = 1 second This could be Integer at some power of ten. + 1 = 1 second easy to use - we'd have to create it - whatever resolution might not be enough * Floating-point type, where 1 = 1 second + 1 = 1 second easy to use + fair amount of resolution + can use sqrt and trig functions easily + fancy NaN values, signed zeros and infinities - resolution variable, bad use of floating point * Type parameter, where 1 = 1 second + flexible - extra complication - can't use directly with Integer except for 1-second resolution -- Ashley Yakeley, Seattle WA

7 22

Question about packages and GHCi
by Martin Erwig 01 Feb '05

01 Feb '05

I apologize if this is the wrong mailing list to turn to, but I would be grateful for any help with two problems I have when trying to add a package to GHCi. (I wasn't able to find sufficient information in the documentation for packages http://www.haskell.org/ghc/docs/latest/html/users_guide/packages.html.) (1) When I try to add a package to ghc, I receive the error message that a GHCi lib fil cannot be found. > ghc-pkg -u -i fgl.pkg Reading package info from "fgl.pkg"done. Expanding embedded variables... done. warning: can't find GHCi lib `HSfgl.o' Saving old package config file... done. Writing new package config file... done. The missing file `HSfgl.o' is located on the current directory as well as in the directory where all other .o and .a files for GHC are kept (/usr/local/lib/ghc-6.2/). (2) Maybe related to the previous problem is the following. When I start "ghci -package fgl", I receive the following error message: Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Loading package haskell-src ... linking ... done. Loading package fgl ... GHCi runtime linker: fatal error: I found a duplicate definition for symbol __module_registered whilst processing object file /usr/local/lib/ghc-6.2/HSfgl.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry. Any help is greatly appreciated. Thanks, Martin

2 1

RE: Time Resolution
by Simon Marlow 01 Feb '05

01 Feb '05

On 01 February 2005 06:23, Ashley Yakeley wrote: > In article <41FE95DF.50000(a)cql.com>, Seth Kurtzberg <seth(a)cql.com> > wrote: > >> I'm not, I hope, being pedantic here and talking about an irrelevant >> issue. But if you look in any first year engineering textbook (for >> an engineering discipline other than CS), you will find that the >> display of a value with a precision greater than the known precision >> of the inputs is a cardinal sin. It's the wrong answer. The >> roundoff error behavior is clearly going to be different. > > Well, how do you feel about using Rationals everywhere? That way > there's never any question of some irrelevant specified unused > accuracy. > > newtype ClockTime = ClockTime Rational deriving (Eq, etc.) > > I quite like the idea of using Rational for ClockTime, but I worry > that it might be slow. But it does allow you to do things such as > dividing times into n pieces and adding them all up again without > error. Not that keen, I have to say. If we had an Integral type, you can always convert to Rational for the purposes of dividing by 3 and recombining (or whatever) and then use truncate to convert back to Integral. Cheers, Simon

2 1

Data.Graph patch
by John Meacham 31 Jan '05

31 Jan '05

Here is a patch for Data.Graph which fixes an oversight in graphFromEdges. graphFromEdges returns a function which turns vertexes into nodes, but provides no way to go the other direction and figure out which vertex goes with a node. I simply return the already created function of type key -> Maybe Vertex in the tuple. I provided it as a new function graphFromEdges' and left the old functionality under the old name. if we don't care about backwards compatability, I'd recommend just replacing the one that is already there since it is a more complete interface and an easy transition for anyone currently using the library. John -- John Meacham - ⑆repetae.net⑆john⑈

1 0

RE: Time Library Organisation
by Simon Marlow 31 Jan '05

31 Jan '05

On 28 January 2005 21:54, Ketil Malde wrote: > Ashley Yakeley <ashley(a)semantic.org> writes: > >> It seems what should have been done was to create a patch to the >> kernel that allows the hardware clock to run on TAI but have >> gettimeofday return broken time as per POSIX, and have some other >> function to return TAI time. Perhaps someone's done this. > > I tried to find out, with no success. Anybody? I'm about to > reinstall my Linux laptop after a disk crash, so I could try to do > this. It also appears that support is needed in NTP, although I'm not > sure why. I don't know about the NTP support, but at least with GNU libc you can use a system clock set to TAI: > TZ=Europe/London date Mon Jan 31 09:42:47 GMT 2005 > TZ=right/Europe/London date Mon Jan 31 09:42:26 GMT 2005 when TZ is set to "right/..." glibc assumes the system clock is set to TAI seconds since the epoch. Of course, there's still no way for a user program to tell whether the system clock is supposed to be TAI or UTC. Cheers, Simon

3 2

RE: Time
by Bayley, Alistair 30 Jan '05

30 Jan '05

> From: Simon Marlow [mailto:simonmar@microsoft.com] > > I like the idea of having a single notion of absolute time, which is > independent of TAI or UTC time. You can do arithmetic on > absolute time > (add/subtract absolute units of time, find absolute time differences), > and convert to/from TAI and UTC. This is something I don't see any motivation for, from a user's point of view. AFAICT, there _is_ no notion of absolute time, just time w.r.t. a given calendar (if you view TAI and UTC as calendars). The representation of time as "units since epoch" is "just an implementation detail" :-) Seriously, though, what use is being able to find the difference between two times like this? If I want the difference between two times, I want to specify in what units the difference should be e.g. millisecs, seconds, days, years, etc. The difference between times tA and tB might be 86400 seconds, but they might still be on the same day e.g. leap seconds, so if I ask for (diffDays tA tB) I might get zero, one, or two, depending on the date. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

7 14

RE: Time Library Organisation
by Simon Marlow 28 Jan '05

28 Jan '05

On 27 January 2005 17:06, Daan Leijen wrote: > Marcin 'Qrczak' Kowalczyk wrote: >> >> You can't have gettimeofday() returning UTC and libtai returning TAI >> at the same time, because they return the same thing. This is the >> implementation from libtai: >> >> [snip] > > >> DJB (the author of libtai) disagrees with POSIX about what >> gettimeofday should return, and assumes that it actually returns >> what he wishes it returned. > > Wow, that is terrible! Well, we can not fix libraries. If libtai is > that broken, we can just as well do it ourselves: That's pretty much the conclusion I came to when I looked at libtai for implementing my library. > if we assume that > we can convert a current UTC time to TAI, we can calculate the TAI > time at the start of the program and use time_t to keep track of the > TAI delta -- here we take advantage of the time_t bug which is not > adjusted for leap seconds! You can't assume that time_t is not being adjusted for leap seconds: the host might be running NTP, for example. The best thing to do seems to be to assume that time_t is a count of seconds since the epoch minus leap seconds, and calculate TAI from that. It might be wrong by up to a second around a leap second on a host running NTP, and slightly more wrong on a host not running NTP, but the latter probably don't care too much about second-accuracy anyway. Cheers, Simon

6 6

RE: Time Library Organisation
by Simon Marlow 28 Jan '05

28 Jan '05

On 26 January 2005 08:43, Ashley Yakeley wrote: > This is one way the time libraries might be organised. I think the > functionality splits cleanly into two separate modules. > > > "Time" module (System.Time? System.DateTime.Time?) > > * TAI/UTC arithmetic and conversions, leap-second tables > > * possibly UT1 types and conversions, Julian days and dates > > * getting the current time > > > "Date" module (System.Date? System.DateTime.Date?) importing Time > module > > * time zones > > * getting the locale time zone > > * converting times to "calendrical" format, > Gregorian and possibly also Julian > > * calendrical arithmetic > e.g. one month after March 31st > > * possibly "incomplete dates" > e.g. "May 1" for all years > > * parsing and showing dates and times Looks fine to me. Maybe System.Time.Clock and System.Time.Calendar? Cheers, Simon

5 7

RE: Text.PrettyPrint.HughesPJ inserting blank lines
by Simon Peyton-Jones 28 Jan '05

28 Jan '05

I use (text ""), but I grant it's a bit of a hack. Simon | -----Original Message----- | From: libraries-bounces(a)haskell.org [mailto:libraries-bounces@haskell.org] On Behalf Of Iavor | Diatchki | Sent: 27 January 2005 18:13 | To: Christian Maeder | Cc: libraries(a)haskell.org | Subject: Re: Text.PrettyPrint.HughesPJ inserting blank lines | | Hi, | Nothing constructive here, I just like to second the suggestion, | as I often need a vertical space document for things like: | vcat (punctuate vspace lines) | or is there already a way to do that? | -Iavor | | | On Wed, 26 Jan 2005 19:51:27 +0100, Christian Maeder <maeder(a)tzi.de> wrote: | > I've found myself using "$+$ space $+$" in order to insert a blank line | > between two docs. | > | > There are probably other ways to do it. "(test "")" (and $$) instead of | > "space" (or $+$) also work in many cases (but not "empty"). Which way | > would be the best? | > | > Maybe one could add a further function "$++$" to do this and a list | > version "vsep" of "$++$"? | > | > Another idea might be something like a vertical space "vspace" so that | > several blank lines would be created by: $$ vspace $$ vspace $$ ... | > | > Cheers Christian | > _______________________________________________ | > Libraries mailing list | > Libraries(a)haskell.org | > http://www.haskell.org/mailman/listinfo/libraries | > | _______________________________________________ | Libraries mailing list | Libraries(a)haskell.org | http://www.haskell.org/mailman/listinfo/libraries

1 0