April 2002 - Glasgow-haskell-users

How to make Claessen's Refs Ord-able?
by Mike Gunter 07 Apr '02

07 Apr '02

I'd like to extend the Ref type for observable sharing of Koen Claessen's Ph.D. thesis: > import IOExts ( IORef, newIORef, readIORef, writeIORef, unsafePerformIO ) > > newtype Ref a = MkRef (IORef a) > > ref :: a -> Ref a > ref x = MkRef (unsafePerformIO (newIORef x)) > > deref :: Ref a -> a > deref (MkRef r) = unsafePerformIO (readIORef r) > > (<==>) :: Ref a -> Ref a -> Bool > (MkRef ref1) <==> (MkRef ref2) = ref1 == ref2 to make it possible for the Refs to be the keys for efficient finite maps. I.e. I want something with at least an efficient Ord-like compare and maybe also a hash function. The closest I've got is: > type UniqTy = Integer > newtype RefWInt a = RefWInt (UniqTy, a) deriving (Show) > > curUniqInt :: IORef UniqTy > curUniqInt = unsafePerformIO (newIORef 0) > newRef a = do > v <- readIORef curUniqInt > writeIORef curUniqInt (v + 1) > return (v, a) > > refWInt x = RefWInt (unsafePerformIO (newRef x)) which works properly for this program alone: > tri_1 = (a, b, b1) > where a = refWInt "a" > b = refWInt "b" > b1 = refWInt "b" yielding (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (2,"b")). But if I add another copy of the same definition: > tri_2 = (a, b, b1) > where a = refWInt "a" > b = refWInt "b" > b1 = refWInt "b" and use it by uncommenting the second line here: > main = print tri_1 > -- >> print tri_2 I get: (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (1,"b")) (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (1,"b")) with ghc. (I get the desired (RefWInt (0,"a"),RefWInt (1,"b"),RefWInt (2,"b")) (RefWInt (3,"a"),RefWInt (4,"b"),RefWInt (5,"b")) with ghci.) Can I write code to always get the desired behavior? How? thanks, mike

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GHC 4.08.2 binaries for OpenBSD i386 available
by dons＠cse.unsw.edu.au 05 Apr '02

05 Apr '02

Hi, I've finished a port of GHC 4.08.2 to OpenBSD i386. The binary package (12M) can be ftp'd from: ftp://ftp.cse.unsw.edu.au/pub/users/dons/ghc/ghc-4.08.2.tgz This can be installed with the command (as root): pkg_add ghc-4.08.2.tgz You can remove the package with: pkg_delete ghc-4.08.2.tgz If you want to see how the package was built, or just like building from source, an OpenBSD ports system "port" (6k) is also available: ftp://ftp.cse.unsw.edu.au/pub/users/dons/ghc/ghc-4.08.2-port.tgz Making sure you have a reasonably recent OpenBSD ports tree on the system, download the port to a convenient place (with >150M free) and: tar xvzf ghc-4.08.2-port.tgz cd ghc then, as root: make && make install This should pull the sources from the Haskell site, build the binary package and then install it. It takes around 4 hours on my P3 700 machine. You can uninstall this with pkg_delete, as above. Note that you will need both bzip2 and gmake to do the build. The port was built using the 2.9 branch of the ports tree so feedback from 3.0 or current ports tree users would be welcome. Send any bugs or suggestions to me at <dons(a)cse.unsw.edu.au>. I will work on getting 5.02.2 and 5.03 built and out soon. Thanks, Don

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Fundep/Existential Types in 5.03
by Ashley Yakeley 05 Apr '02

05 Apr '02

Consider this: module Test3 where class C a b | a -> b where m :: a -> b data D a = forall b. (C a b) => MkD a f :: (C a b) => D a -> b f (MkD a) = m a This compiles fine under GHC 5.02.2. But under 5.03, it gives an error: Model/Test3.hs:9: Inferred type is less polymorphic than expected Quantified type variable `b' escapes When checking an existential match that binds and whose type is D a -> b1 In the definition of `f': f (MkD a) = m a I consider that the 5.02.2 behaviour is preferable, and that this is a perfectly good program. 'b' does not escape because it is fundep on 'a', which is specified in the type-signature. There can be only one. What was changed in 5.03 and why? -- Ashley Yakeley, Seattle WA

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Dynamic loading with GHCi Linker
by Keean 05 Apr '02

05 Apr '02

[Firstly, appologies if this is a duplicate, my first mail did not appear on the list.] Summary so far: I want to dynamic load haskell code into a host haskell application (plug-in style) I have, as suggested, put together an interface for Linker.h, and written some little wrapper functions. I have a few questions though... (is there any documentation?) What are the integer return values from loadObj/resolveObjs/unloadObj ? What is the void* pointer returned from lookupSymbol - and how do I apply it if its a function. (my first guess was to import rts_apply) Finally, when I load the module there are unresolved symbols - will it link against the main program? Is there any way to get it to do this? (I would like to avoid dynamic loading dependancies if the code is already used by the main program) Keean

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RE: explicitly quantified classes in functions
by Simon Peyton-Jones 05 Apr '02

05 Apr '02

| Ah yes, silly me. What I had in mind, I suppose, was | something more along the lines of: | | foo :: Double -> (exists q . Foo q => q) Correct. Currently you can only express this by wrapping the existential in a constructor: data R = forall q. Foo q => MkR q foo :: Double -> R Mark Shields and I wrote a paper about doing more fully fledged existentials, but it's quite a significant chunk of work to implement. http://research.microsoft.com/~simonpj/Papers/first-class-modules Simon

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Re: explicitly quantified classes in functions
by John Hughes 04 Apr '02

04 Apr '02

Why can I not define the following (in ghc): > class Foo p where > instance Foo Double where > foo :: Double -> (forall q . Foo q => q) > foo p = p From my humble (lack of) knowledge, there seems to be nothing wrong here, but ghc (5.03) complains about unifying q with Double. Well, of course! The result has to have type (forall q . Foo q => q), but p actually has type Double. Of course GHC complains. The question is, why YOU think it is OK. And the answer is that you reason: there is only one instance of Foo, so when I say Foo q, that means q must be Double. But you're making the "closed world assumption", that you know all of the instances of Foo. The compiler doesn't, and for good reason. Suppose you were to export these definitions from the module where they appear, into another module which defines instance Foo Integer Now, of course, it should be OK to use Foo with the type Double -> Integer (since Foo Integer holds in this context). Yet clearly, the *definition* you gave cannot be used with this type. Hence the type is wrong. Of course, you could argue that if the definition of foo is *not* exported, then the compiler should use the closed world assumption and the program should be accepted. But it would be really weird if whether a definition is well-typed or not depended on whether or not it was exported... John

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explicitly quantified classes in functions
by Hal Daume III 04 Apr '02

04 Apr '02

Why can I not define the following (in ghc): > class Foo p where > instance Foo Double where > foo :: Double -> (forall q . Foo q => q) > foo p = p >From my humble (lack of) knowledge, there seems to be nothing wrong here, but ghc (5.03) complains about unifying q with Double. I *can* write: > class Foo p where > instance Foo Double where > data T = forall q . Foo q => T q > foo :: Double -> T > foo p = T p which is very similar, except that the explicit universal quantification is happening in in the datatype and not the function type. why is the former disallowed? -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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Re: Cygwin & GHC
by George Russell 04 Apr '02

04 Apr '02

Simon Peyton Jones wrote: > I am therefore deeply reluctant to provide both GHC-for-mingw32 > and GHC-for-cygwin. One build on Win32 is enough! We ended > up with a mingw32 basis because it meant we could make GHC=20 > completely self-contained -- no dependence on cygwin1.dll etc. > This was *huge* step forward: GHC installs and runs with no problem > on Windows now. I agree about it being a huge step forward. I do not think I would be in favour of undoing this and going back to having to install cygwin (in the right version) to get GHC to run. > 1. GHC does not understand cygwin paths in the file names passed > to it on the command line. > > 2. GHC on Win32 does not come with a Posix library. If we used a > Cygwin basis, Posix would be easy because cygwin does all the hard work. > > 3. I/O on Win32 is *blocking*. A blocking input operation freezes all > the > other Haskell threads. I am a little but not hugely bothered by (1) and (2), as they can all be worked around in various ways. The real problem is (3), since it means that not just IO events but almost all events from the non-Haskell world cannot be waited on without bringing the Haskell world to a halt. Is it possible to do something about (3) without switching back to Cygwin?

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RE: [nhc-bugs] Cygwin and GHC
by Simon Peyton-Jones 04 Apr '02

04 Apr '02

| Can you comment on how much longer the GHC core team will exist? Indefinitely. Meaning that (a) There is no time limit: Simon and I have permanent posts Reuben and Julian left when the grant ran out; we knew that was coming, but there is no correponding future event known. (b) But (as has been the case for the last 15 yrs) "indefinite" does not mean "forever". One day I'll get old and die. The more people that help look after GHC, the healthier it will be. It's fun, it's rewarding, and it's not as demanding as you might think. There's even a lovely commentary to explain how it works http://www.cse.unsw.edu.au/~chak/haskell/ghc/comm/ Simon

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Fw: Cygwin and GHC
by Claus Reinke 04 Apr '02

04 Apr '02

[interesting; postfix at haskell.org claims rightly that there is no ghc-users list there. so how did Simon's mail reach me in the first place? well, here we go again] ----- Original Message ----- From: "Claus Reinke" <claus.reinke(a)talk21.com> To: "Simon Peyton-Jones" <simonpj(a)microsoft.com>; <ghc-users(a)haskell.org>; <nhc-bugs(a)haskell.org> Cc: "C.Reinke" <C.Reinke(a)ukc.ac.uk> Sent: Thursday, April 04, 2002 1:45 PM Subject: Re: Cygwin and GHC > >I am therefore deeply reluctant to provide both GHC-for-mingw32 > >and GHC-for-cygwin. One build on Win32 is enough! We ended > >up with a mingw32 basis because it meant we could make GHC > >completely self-contained -- no dependence on cygwin1.dll etc. > > Some comments from a binary-only GHC user who tends to depend > a lot on cygwin while using windows: > > >This was *huge* step forward: GHC installs and runs with no problem > >on Windows now. > > This *is* a huge step forward, and together with GHCi and HOpenGL, > has even tempted me to use GHC a bit more:-) > > Having to wrestle with two different GHC installations on one platform > would seem a step backward. So, I'd not only agree with one build > only (GHC should be compilable under cygwin, though, for those who > absolutely need to go the other way, or who want to track the CVS > version), I'd like one build to work both in native and in cygwin mode. > As a Haskell user, I'm interested in: > > - a standalone GHC, producing standalone executables and dlls, with > good FFI interfaces to the non-Haskell world > - portability across platforms, with as few code changes or restrictions > as possible > > My approach to keeping windows/unix differences small is mostly based > on cygwin, so I need to be able to use GHC and it's executables under > cygwin, as I would use it under unix, in combination with other (windows/ > cygwin) software. That doesn't mean that GHC-generated executables or > libraries need to be cygwin-dependent, and cygwin is, by design, able to > use windows executables (mixing of libraries is probably another story). > > >1. GHC does not understand cygwin paths in the file names passed > >to it on the command line. > > Making GHC understand cygwin paths makes software more system > dependent, not more portable. And what about the executables produced > by GHC? Most of the cygwin path problems could, in theory, be solved > without changing GHC, but with a lot of accumulated UNIX makefiles, > that can be unpractical. > > As far as I understand, GHC can cope with both relative unix-style and > relative and absolute windows-styles paths, so the remaining problem I > tend to encounter are "absolute" unix-style paths which are really relative > to the cygwin root directory. (I also seem to recall someone mentioning > problems related to GHC passing "normalised" paths to other tools, but > if GHC uses it's own toolchain, that seems unlikely?) > > My suggestion would be a --prefix <path> option, or GHC_PATH_PREFIX > variable for GHC-produced executables (including GHC itself), telling them > that any absolute, unix-style paths are to be interpreted relative to <path> > (e.g., in cygwin makefiles, default installation, HC=ghc --prefix c:/cygwin). > That wouldn't be platform-specific and might also come in handy for other > purposes. > > >2. GHC on Win32 does not come with a Posix library. If we used a > >Cygwin basis, Posix would be easy because cygwin does all the hard work. > > That looks like a real bugger to me as it impacts on portability of Haskell > programs. Going from incomplete posix support to even less posix support > was a step backwards. > > >3. I/O on Win32 is *blocking*. A blocking input operation freezes all > >the other Haskell threads. > > No experience with that one, but in general, establishing consistent I/O > behaviour across platforms would be a very useful asset. > > >Are there any other problems? > > perhaps: > > 4. File I/O on windows differs from I/O in unix (locking of files instead > of implicit maintainance of hidden handles, I think??). cygwin tries to > smooth things over, but fails for more complex cases (open a file for > reading, remove it, open it for writing, copy from read handle to write > handle).We just traced a problem in building nhc on cygwin down to > that one.. How does mingw fare in that respect? Better? Or even worse? > > 5. I assume that GHC and it's executables interface rather well with the > windows world. What about interfacing to software ported from unix > that depends on cygwin, though? > > >1. GHC already fudges filenames to take account of the Win32/Unix > >conventions. We could add more fudges, to change /cygwin/c/foo > >to c:\foo, for example. Perhaps controlled by a -cygwin flag to tell > >GHC-for-win32 whether to use cygwin fudges or not. Heuristic, yes; > >but might solve the problem for 99% of customers. > > No heuristic, please, just some more flexibility for makefile authors > (/cygwin/c/foo tends to be /cygdrive/c/foo these days, and sometimes > is //c/foo, but c:/foo tends to work as well - do you want to track > cygwin's mount table in GHC?-). > > >2. Mingw32 provides quite a lot of Posix, so if someone was prepared > >to put in a bit of work we could get a good part of the Posix library > >available on Win32, still via mingw. Any volunteers? > > I step back to make space for the volunteers, but mingw being minimalistic, > what are the chances that it's posix support is a suitable alternative to > cygwin's, which isn't complete, either? > > >3. Non-blocking I/O is a soluble problem: Win32 provides suitable > >primitives. But they are different to the Unix primitives, so there is > >work to do in the runtime system to make it work. This is harder > >for a volunteer to do because it's in the runtime system, but not > >impossible. > > As far as I know, cygwin (the *larger* cousin of mingw) is still waiting > for someone to contribute solutions to I/O problems on top of windows.. > > Thanks for raising this issue, > Claus > > PS. > >The GHC core team is now down to Simon M and me. Sigbjorn > >heroically helps out on Win32 stuff, but it isn't his job. So we have > >strictly limited effort available. > > [I have to admit that I'm a bit concerned about the overall number of > people involved in Haskell implementations. Fortunately, the licenses > are very open, and fortunately, we do have those heroic volunteers, > but the Haskell implementation core team isn't going to be much (if > any) bigger than the GHC core team soon. Given the interests of some > (well, at least one;) companies, and the volunteers working for those, > perhaps they could acknowledge the work done by those volunteers > as being essential for their business? And given the wide-spread use > in academia as a research and teaching platform, does anyone know > of a way of securing some kind of international platform funding, to > keep the current implementations alive and to develop them further > (as an academic complement to Galois' (?) and MS' contributions > in terms of man-power)?] > >

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