- Haskell-Cafe - Haskell.org

Fw: [darcs-devel] "Inferred type is less polymorphic than expected" and type witnesses
by Rob Hoelz 12 Jan '09

12 Jan '09

Forwarding to Haskell Cafe per Eric's suggestion. Begin forwarded message: Date: Sun, 11 Jan 2009 23:01:31 -0600 From: Rob Hoelz <rob(a)hoelzro.net> To: darcs-devel(a)darcs.net Subject: [darcs-devel] "Inferred type is less polymorphic than expected" and type witnesses Hello again, Darcs users and developers, As I mentioned in my last e-mail, I'm working on http://bugs.darcs.net/issue291. It's actually gone pretty well, and I feel I'm just about finished (I've done all but sorting out the changes after leaving the editor), only I've encountered the compiler error you see in the subject of this message. This error only appears when compiling with witnesses. Here's the source for the function that it's complaining about: compare_changes_with_old (x :>: xs) (y :>: ys) = case compare_changes_with_old xs ys of nx :> ny -> if unsafeCompare x y then ((x :>: nx) :> ny) else (NilFL :> (y :>: ys)) compare_changes_with_old NilFL NilFL = (NilFL :> NilFL) compare_changes_with_old NilFL ys@(_ :>: _) = (NilFL :> ys) compare_changes_with_old x@(_ :>: _) NilFL = (NilFL :> NilFL) Now, I have two questions: 1) What exactly does this error mean, and how do I get around it? 2) What are witness types, and what are they used for? I will gladly accept links to fine manuals as answers to either question, but simple explanations would be nice. =D I thought I understood Haskell pretty well, but existentially qualified types have thrown me for a loop. Thanks much, Rob Hoelz _______________________________________________ darcs-devel mailing list (AUTOMATIC POSTINGS ONLY PLEASE!) darcs-devel(a)darcs.net http://lists.osuosl.org/mailman/listinfo/darcs-devel

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haskell-platform mailing list
by Duncan Coutts 12 Jan '09

12 Jan '09

Hi all, If you are interested in helping out with the haskell platform project then we invite you to subscribe to the haskell-platform mailing list. http://projects.haskell.org/cgi-bin/mailman/listinfo/haskell-platform This mailing list is for discussing practical stuff. We expect to discuss policy questions on the libraries mailing list. Issues like which packages to pick, what package QA standards to use etc would be discussed on the libraries mailing list. Those issues are for the community as a whole. Building and coordinating the actual releases will be done via the haskell-platform mailing list. Duncan

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Re: [Haskell] HaskellWiki Upgrade Aborted
by Duncan Coutts 12 Jan '09

12 Jan '09

On Sat, 2009-01-10 at 19:02 -0800, Ashley Yakeley wrote: > I have given up attempting to upgrade HaskellWiki, and resumed the > site's previous normal operation. > > I wrote: > > I will also need up upgrade both PHP and MySQL. > > It turns out it would be hard to upgrade MySQL, as various things depend > on the old version: For example the gtk2hs website is using mysql and php too. I think we'd need to upgrade that to have it work with mysql-5. Perhaps gtk2hs should move to projects.haskell.org. > $ sudo rpm -U MySQL-server-community-5.1.30-0.rhel3.i386.rpm > warning: MySQL-server-community-5.1.30-0.rhel3.i386.rpm: V3 DSA > signature: NOKEY, key ID 5072e1f5 > error: Failed dependencies: > libmysqlclient.so.10 is needed by (installed) mod_auth_mysql-20030510-2.ent > libmysqlclient.so.10 is needed by (installed) perl-DBD-MySQL-2.1021-4.EL3 > libmysqlclient.so.10 is needed by (installed) php-mysql-4.3.2-48.ent > > I don't know what mod_auth_mysql and perl-DBD-MySQL are being used for, > so I don't believe I can upgrade them safely. > > The machine is running RHEL AS 3 update 9 with Linux 2.4.21. We really need to upgrade the whole thing. Not an easy job given the range of stuff being run on there by lots of different people. Duncan

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Generalized partial application (again)
by Peter Verswyvelen 12 Jan '09

12 Jan '09

Please correct me if I'm wrong in any of the reasoning below. Haskell provides the ability the perform partial application on the rightmost arguments. I learned from some pretty smart guys that partial application cannot be emulated with lambas, because they behave differently operationally, e.g. e = trace "e" op = (+) a1 = let f = (op (e 1)) in (f 10, f 100) a2 = let f = (\x -> op (e 1) x) in (f 10, f 100) a1 and a2 are operationally not the same: a1 will evaluate (e 1) once, a2 will evaluate it twice. However, Haskell also allows to partially apply the second argument of binary functions (using sections) b1 = let f = (`op` (e 1)) in (f 10, f 100) b2 = let f = (\x -> (e 1) `op` x) in (f 10, f 100) Again, b1 evaluates (e 1) once, b2 twice. b1 is actually the same as c1 = let f = ((flip op) (e 1)) in (f 10, f 100) So for ternary functions we could write more permutations of flip: flip3_12 f a1 a2 a3 = f a2 a1 a3 flip3_13 f a1 a2 a3 = f a3 a2 a1 flip3_23 f a1 a2 a3 = f a1 a3 a2 And use these to partially apply any argument e1 x = trace "e1" x e2 x = trace "e2" x op2 :: Int -> Int -> Int -> Int op2 x y z = x+10*y+100*z z1 = let f = (flip3_12 op2) (e1 1) (e2 2) in (f 3, f 4) z2 = let f = (flip3_13 op2) (e1 1) (e2 2) in (f 3, f 4) z3 = let f = (flip3_23 op2) (e1 1) (e2 2) in (f 3, f 4) This could be extended to N-ary functions. Haskell doesn't give any special syntactic support for this, but it surely is possible. Am I missing something here? Is there any easier way to perform "generalized partial application"?

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Question about touchForeignPtr
by Patrick Perry 12 Jan '09

12 Jan '09

I have the following code: IOVector n e = IOVector !ConjEnum !Int (ForeignPtr e)! (Ptr e)! Int! newtype Vector n e = IOVector n e unsafeAtVector :: Vector n e -> Int -> e unsafeAtVector (Vector (IOVector c _ f p inc)) i = let g = if c == Conj then conjugate else id in inlinePerformIO $ do e <- peekElemOff p (i*inc) io <- touchForeignPtr f let e' = g e e' `seq` io `seq` return e' {-# INLINE unsafeAtVector #-} The Ptr, 'p' is derived from the ForeignPtr, 'f'. For some offset, 'o', it is defined as: p = unsafeForeignPtrToPtr f `advancePtr` o The "touchForeignPtr" is there to keep the garbage collector from deallocating the memory before we have a chance to read 'e'. My question is the following: Is the `seq` on `io` necessary (from a safety standpoint)? Or am I just being paranoid? Thanks in advance for any help, Patrick

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data, util, and lang packages
by rodrigo.bonifacio 12 Jan '09

12 Jan '09

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version conflict on Hackage
by Henning Thielemann 12 Jan '09

12 Jan '09

I repeatedly encounter the following versioning problem on Hackage: There is a package A with version 1.0. I upload a package B which imports A. Thus B is bound to A-1.0 Now a new version of A is uploaded, say 1.0.1. then I upload package C which depends both on A and B. However C is bound to the new A-1.0.1, which gives conflicts with the interface of B. All packages are sane. Both B and C work with either A-1.0 or A-1.0.1, but they must use the same version. Currently I'm forced to upload many packages with different versions, although nothing really changed. Maybe the most easiest fix to this problem is to let maintainers of a package allow to trigger re-configuration and re-compilation of a package. I would also like to flag package versions that people can safely ignore, because there was a flaw in package dependencies. On the other hand there are packages that are useful, but can no longer be compiled on Hackage, e.g. compatibility packages for older versions of GHC.

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Error building Sdf2Haskell
by rodrigo.bonifacio 12 Jan '09

12 Jan '09

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Computer time, independent of date
by Mauricio 12 Jan '09

12 Jan '09

Hi, I'm writing a program where I need to know elapsed times between some events, but I can't depende on Data.Time since ntpd or user can change that while my program is running. Is there an alternative? Something like how much time has passed since the program has started would be great. Thanks, Maurício

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[ANNOUNCE] First release candidate of darcs 2.2.
by Eric Kow 12 Jan '09

12 Jan '09

Dear Haskellers, I would like to forward this announcement on behalf of Petr Ročkai (the current darcs Release Manager). Darcs 2.2 is scheduled for release in 2009-01-15, only three days from now! This is the first of our biannual time-based releases. We hope you'll like it, and to help us ensure that you will like it, please help us test our the release candidate. Thanks! Eric Petr's words follow: ----------------8<------------------------------------------------------------- I am pleased to announce that darcs 2.2 is coming along nicely. I would like to ask everyone to give a ride to darcs 2.2, release candidate 1. This release again comes in two flavours: 1) The source tarball, http://repos.mornfall.net/darcs/darcs-2.2.0rc1.tar.gz, which can be built using the traditional autoconf-based buildsystem. This is the fully supported version. After downloading and unpacking, you can issue: $ ./configure $ make $ ./darcs --version or # make install More detailed instructions inside the tarball (file README). 2) Cabalised source. You can either download a tarball from http://repos.mornfall.net/darcs/darcs-2.1.99.0.tar.gz and build manually (see the build instructions in README inside the tarball), or, alternatively, you can use cabal-install to obtain a copy (the release candidate is now available on hackage): $ cabal update $ cabal install darcs This should give you a darcs binary in ~/.cabal/bin -- you should probably add that to your PATH. This is a preliminary changelog since version 2.1.2 (released last November): * In interactive record, it is now possible to get a list of currently selected hunks (command 'l'). (Christian Kellermann) * It is now possible to specify --in-reply-to when using darcs send, to generate correct references in the mail header. (Pavel Shramov) * New repositories with --no-pristine-tree can no longer be created. This only has effect on legacy darcs-1 repositories. * Improvements in Windows support. (Salvatore Insalaco) * Performance improvements in `darcs repair` and robustness improvements in `darcs check`. (Petr Ročkai) * Extensive manual and online help improvements. (Trent W. Buck) * Support for GHC 6.10. * Overhaul of the make-based build system. (Trent W. Buck) * Cabal is now supported as a build method for darcs. (Duncan Coutts, Petr Ročkai, Gwern Branwen) * First stab at libdarcs -- when building through Cabal, all of darcs implementation is now exposed in modules. No API guarantees whatsoever. (Eric Kow) * Additions to Haddock documentation of the existing darcs modules for improved development experience. * Improvements in the testing infrastructure. (Christian Kellermann, Gwern Branwen) * Low-level optimisations in filesystem code. (Ganesh Sittampalam) * Numerous major and minor bug fixes, refactorings and cleanups by David Roundy, Eric Kow, Jason Dagit, Dmitry Kurochkin, Thorkil Naur, Salvatore Insalaco, Christian Kellerman, Florent Becker, Duncan Coutts, Reinier Lamers, Ganesh Sittampalam, Petr Ročkai. Preliminary list of issues that have been fixed in darcs since version 2.1.2: 1223 sporadic init.sh test failure (2.1.1rc2+472) 525 amend-record => darcs patches show duplicate additions 1247 make TAGS is broken 1273 renameFile: does not exist (No such file or directory) 1165 get should print last gotten tag 1249 2.1.2 (+ 342 patches) local drive detection on Windows error 1238 wish: darcs help setpref should list all prefs 1199 Backup files darcs added after external merge 1043 pull => mergeAfterConflicting failed in geteff (2.0.2+) 1117 Whatsnew should warn on non-recorded files 1101 darcs send --cc recipient not included in success message Thanks to Thorkil Naur for compiling this list. I would like to thank all contributors -- developers, testers, bystanders -- for helping darcs get along further. It's been hard times recently for darcs, as many of you probably know. Nevertheless, we are regaining confidence in future darcs development. No way are we going to leave darcs fall by the road. I am sure that this one time, I speak for everyone in our developer and user community. Yours, Petr. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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