January 2012 - Glasgow-haskell-users

Type operators in GHC
by Simon Peyton-Jones 19 Sep '12

19 Sep '12

Dear GHC users As part of beefing up the kind system, we plan to implement the "Type operators" proposal for Haskell Prime http://hackage.haskell.org/trac/haskell-prime/wiki/InfixTypeConstructors GHC has had type operators for some kind, so you can say data a :+: b = Left a | Right b but you can only do that for operators which start with ":". As part of the above wiki page you can see the proposal to broaden this to ALL operators, allowing data a + b = Left a | Right b Although this technically inconsistent the value page (as the wiki page discussed), I think the payoff is huge. (And "A foolish consistency is the hobgoblin of little minds", Emerson) This email is (a) to highlight the plan, and (b) to ask about flags. Our preferred approach is to *change* what -XTypeOperators does, to allow type operators that do not start with :. But that will mean that *some* (strange) programs will stop working. The only example I have seen in tc192 of GHC's test suite {-# LANGUAGE TypeOperators #-} comp :: Arrow (~>) => (b~>c, c~>d)~>(b~>d) comp = arr (uncurry (>>>)) Written more conventionally, the signature would look like comp :: Arrow arr => arr (arr b c, arr c d) (arr b d) comp = arr (uncurry (>>>)) or, in infix notation {-# LANGUAGE TypeOperators #-} comp :: Arrow arr => (b `arr` c, c `arr` d) `arr` (b `arr` d) comp = arr (uncurry (>>>)) But tc192 as it stands would become ILLEGAL, because (~>) would be a type *constructor* rather than (as now) a type *variable*. Of course it's easily fixed, as above, but still a breakage is a breakage. It would be possible to have two flags, so as to get - Haskell 98 behaviour - Current TypeOperator behaviuor - New TypeOperator behaviour but it turns out to be Quite Tiresome to do so, and I would much rather not. Can you live with that? http://chrisdone.com/posts/2010-10-07-haskelldb-and-typeoperator-madness.ht…

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Occasional segfault in GHCi w/ 7.4.1-rc1 and 64bit OS X
by austin seipp 17 Jul '12

17 Jul '12

Hello GHC hackers, I have been trying the recent 7.4.1-rc1 release on my OSX Snow Leopard machine. I am using the 64bit OSX .tar.bz. I used virthualenv/hsenv to set up a virtual environment to test packages with the RC, but I discovered that GHCi seems to segfault somewhat randomly when invoked. I cannot reproduce this problem with 7.0.4 (64bit,) 7.2.2 (64bit,) or 7.5.20111223 (also 64bit.) I have invoked GHCi probably a hundred times each, and have never had it crash. I can crash the RC fairly reliably however. I looked at the shell scripts that invoke GHC and ran GDB with the proper arguments. I got a backtrace like so: $ gdb /Users/a/vh-741-rc1_2/.hsenv/ghc/lib/ghc-7.4.0.20111219/ghc GNU gdb 6.3.50-20050815 (Apple version gdb-1518) (Sat Feb 12 02:52:12 UTC 2011) Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries .... done warning: Could not find object file "/Users/ian/zz64/ghc-7.4.0.20111219/rts/dist/build/libHSrts_thr.a(ffi64.o)" - no debug information available for "../src/x86/ffi64.c". warning: Could not find object file "/Users/ian/zz64/ghc-7.4.0.20111219/rts/dist/build/libHSrts_thr.a(prep_cif.o)" - no debug information available for "../src/prep_cif.c". warning: Could not find object file "/Users/ian/zz64/ghc-7.4.0.20111219/rts/dist/build/libHSrts_thr.a(types.o)" - no debug information available for "../src/types.c". (gdb) r -B/Users/a/vh-741-rc1_2/.hsenv/ghc/lib/ghc-7.4.0.20111219 --interactive Starting program: /Users/a/vh-741-rc1_2/.hsenv/ghc/lib/ghc-7.4.0.20111219/ghc -B/Users/a/vh-741-rc1_2/.hsenv/ghc/lib/ghc-7.4.0.20111219 --interactive Reading symbols for shared libraries + done GHCi, version 7.4.0.20111219: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_INVALID_ADDRESS at address: 0x00000000ae21cf68 0x00000001ae2060da in evacuate1 () (gdb) bt #0 0x00000001ae2060da in evacuate1 () #1 0x00000001ae20b868 in scavenge_block1 () Previous frame inner to this frame (gdb could not unwind past this frame) (gdb) So it looks like a GC bug at first glance, but I haven't done rigorous investigation. Again this doesn't happen every time - probably 1/3 the time it crashes, the other 2/3 the time it seems to work fine. This naturally also affects `runghc`. Is this a known issue? Or should I file a bug and investigate further? Perhaps something forgot to get something merged into the 7.4 branch? -- Regards, Austin

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Records in Haskell
by Simon Peyton-Jones 27 Mar '12

27 Mar '12

Friends Provoked the (very constructive) Yesod blog post on "Limitations of Haskell", and the follow up discussion, I've started a wiki page to collect whatever ideas we have about the name spacing issue for record fields. http://hackage.haskell.org/trac/ghc/wiki/Records As Simon M said on Reddit, this is something we'd like to fix; but we need a consensus on how to fix it. Simon

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ANNOUNCE: GHC version 7.2.1
by Ian Lynagh 27 Mar '12

27 Mar '12

============================================================= The (Interactive) Glasgow Haskell Compiler -- version 7.2.1 ============================================================= The GHC Team is pleased to announce a new major release of GHC, 7.2.1. The 7.2 branch is intended to be more of a "technology preview" than normal GHC stable branches; in particular, it supports a significantly improved version of DPH, as well as new features such as compiler plugins and "safe Haskell". The design of these new features may evolve as we get more experience with them. See the release notes for more details of what's new and what's changed. We are also using this branch as an opportunity to work out the best workflows to use with git. We expect the 7.2 branch to be short-lived, with 7.4.1 coming out shortly after ICFP as normal. Full release notes are here: http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/release-7-2-1.html How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: http://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces (C, whatever). GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page http://www.haskell.org/ghc/ GHC developers' home page http://hackage.haskell.org/trac/ghc/ Haskell home page http://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: http://hackage.haskell.org/trac/ghc/wiki/Contributors Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: http://hackage.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site run by Trac: http://hackage.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at http://www.haskell.org/mailman/listinfo/glasgow-haskell-users http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see http://www.haskell.org/mailman/listinfo/ Some GHC developers hang out on #haskell on IRC, too: http://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: http://www.haskell.org/ghc/reportabug

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partially applied type synonyms
by Arie Peterson 05 Mar '12

05 Mar '12

Dear list, Is there a good reason that partially applied type synonyms cannot be made instances of classes? - A simple example would be making the identity type 'Id' ("type Id x = x") instance of 'Monad'. This would eliminate the need for some silly occurrences of 'runIdentity', and perhaps narrow the gap between monads and monad transformers. - The reason I complain about this now is a more involved example, where introducing a newtype to work around the restriction is not an option. Well, not as far as I can see now, anyway. Partially applied type synonyms are, in a sense, the anonymous functions at the type level, and one might argue that they deserve the same 'first-class member status' as the lambda at the value level. Kind regards, Arie Peterson

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parallelizing ghc
by Evan Laforge 27 Feb '12

27 Feb '12

I recently switched from ghc --make to a parallelized build system. I was looking forward to faster builds, and while they are much faster at figuring out what has to be rebuilt (which is most of the time for a small rebuild, since ld dominates), compilation of the whole system is either the same or slightly slower than the single threaded ghc --make version. My guess is that the overhead of starting up lots of individual ghcs, each of which has to read all the .hi files all over again, just about cancels out the parallelism gains. So one way around that would be parallelizing --make, which has been a TODO for a long time. However, I believe that's never going to be satisfactory for a project involving various different languages, because ghc itself is never going to be a general purpose build system. So ghc --make provides two things: a dependency chaser and a way to keep the compiler resident as it compiles new files. Since the dependency chaser will never be as powerful as a real build system, it occurs to me that the only reasonable way forward is to split out the second part, by adding an --interactive flag to ghc. It would then read filenames on stdin, compiling each one in turn, only exiting when it sees EOF. Then a separate program, ghc-fe, can wrap ghc and acts like a drop-in replacement for ghc. It would be nice if ghc could atomically read one line from the input, then you could just start a bunch of ghcs behind a named pipe and each would steal its own work. But I don't think that's possible with unix pipes, and of course there are still a few non-unix systems out there. And I guess ghc-fe has to wait for the compilation to finish, so I guess ghc has to print a status line when it completes (or fails) a module. But it can still be done with an external distributor program that acts like a server: starts up n ghcs, distributes src files between them, and shuts them down then given the command: data Ghc = Ghc { status :: Free|Busy, in :: Handle, out :: Handle, pid :: Int } main = do origFlags <- getArgs ghcs <- mapM (startup origFlags) [0..cpus] socket <- accept while $ read socket >>= \case of Quit -> return False Compile ghcFlags src -> forkIO $ assert $ ghcFlags == origFlags result <- bracket (findFreeAndMarkBusy ghcs) markFree $ \ghc -> do tellGhc ghc src readResult ghc write socket result return True mapM_ shutdown ghcs The ghc-fe then starts a distributor if one is not running, sends a src file and waits for the response, acting like a drop-in replacement for the ghc cmdline. Build systems just call ghc-fe and have an extra responsibility to call ghc-fe --quit when they are done. And I guess if they know how many files they want to rebuild, it won't be worth it below a certain threshold. So I'm wondering, does this seem reasonable and feasible? Is there a better way to do it? Even if it could be done, would it be worth it? If the answers are "yes", "maybe not", and "maybe yes", then how hard would this be to do and where should I start looking? I'm assuming start at GhcMake.hs and work outwards from there... I'm not entirely sure it would be worth it to me even if it did make full builds, say 1.5x faster for my dual core i5, but it's interesting to think about all the same.

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Holes in GHC
by Thijs Alkemade 19 Feb '12

19 Feb '12

Hello! As mentioned in our previous mail [1], we've been working on introducing Agda's holes into GHC [2]. Our general approach is as follows: we've added a hole to the syntax, denoted here by two underscores: "__". We've introduced a new HsExpr for this, which stores the hole's source span (as this is the only thing that a user needs to identify the hole). Then, we extended the local environment of the typechecker to include a mutable Map, mapping a SrcSpan to a Type and a WantedConstraints (we need these later to produce the correct class constraints on the type). In TcExpr.lhs, whenever the tcExpr function encounters a hole, it adds the source position with the res_ty and the current tcl_lie to the map. After type checking has finished, these types can be taken out of the map and shown to the user, however, we are not sure yet where to do this. Currently the map is read in tcRnModule and tcRnExpr, so loading modules and evaluating expressions with ":t" will show the types of the holes in that module or expression. There, we call mkPiTypes, mkForAllTys (with the quantified type variables we obtained from the WantedConstraints), we zonk and tidy them all (most of this imitates how tcRnExpr modifies a type before returning it, except the tidying, which needs to pass the updated TidyEnv to the tidying of the next hole). Examples: *Main> :t [__, ()] tcRnExpr2: [(<interactive>:1:2-3, ())] [__, ()] :: [()] *Main> :t map __ __ tcRnExpr2: [(<interactive>:1:5-6, a0 -> b), (<interactive>:1:8-9, [a0])] map __ __ :: [b] Any feedback on this design would be appreciated. We would like to know if this is something that could be considered to be added to GHC, and what your requirements for that are. Also, we have a confusing problem when type checking a module. Above, we showed the result of ":t map __ __" in ghci. However, if we put "f = map __ __" in a module, we get: tcRnModule: [(f.hs:1:9-10, GHC.Prim.Any * -> b), (f.hs:1:12-13, [GHC.Prim.Any *])] If I read GHC.Prim.Any * as forall a. a, then this is not correct: the GHC.Prim.Any * in both holes should have the same type. We assume it shows up because the type that should be there does not occur in the type signature of f (as it's just [b]), but so far we've been unable to figure out why this behavior is different in interactive mode. Does someone have an idea about what to do to avoid the introduction of these GHC.Prim.Any * types? Best regards, Thijs Alkemade [1] http://www.haskell.org/pipermail/glasgow-haskell-users/2012-January/021453.… [2] https://github.com/xnyhps/ghc/commits/master

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ANNOUNCE: GHC 7.4.1 Release Candidate 2
by Ian Lynagh 15 Feb '12

15 Feb '12

We are pleased to announce the first release candidate for GHC 7.4.1: http://www.haskell.org/ghc/dist/7.4.1-rc2/ This includes the source tarball, installers for OS X and Windows, and bindists for amd64/Linux, i386/Linux, amd64/FreeBSD and i386/FreeBSD. Please test as much as possible; bugs are much cheaper if we find them before the release! We expect this to be the last release candidate, with the final release in approximately 1 week's time. Thanks Ian, on behalf of the GHC team

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ANNOUNCE: GHC 7.2.1 Release Candidate 1
by Ian Lynagh 11 Feb '12

11 Feb '12

We are pleased to announce the first release candidate for GHC 7.2.1: http://www.haskell.org/ghc/dist/7.2.1-rc1/ This includes the source and testsuite tarballs, installers for OS X and Windows, and bindists for amd64/Linux, i386/Linux, amd64/FreeBSD and i386/FreeBSD. Please test as much as possible; bugs are much cheaper if we find them before the release! Thanks Ian, on behalf of the GHC team

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Impredicative types error
by John Meacham 01 Feb '12

01 Feb '12

Hi, I am running into an issue where some code that compiled and worked under 6.12 is failing under 7.0, the offending code is class DeNameable a where deName :: Module -> a -> a getDeName :: Tc (DeNameable n => n -> n) getDeName = do mn <- asks (tcInfoModName . tcInfo) return (\n -> deName mn n) Tc is a plain typechecking monad and this returns a generic denaming function that can be used to turn haskell names back into human readable form before printing errors in jhc. I have the ImpredicativeTypes LANGUAGE feature turned on. the error I get under 7.0 is src/FrontEnd/Tc/Monad.hs:131:29: Couldn't match expected type `n -> n' with actual type `DeNameable n' In the second argument of `deName', namely `n' In the expression: deName mn n In the first argument of `return', namely `(\ n -> deName mn n)' John

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