- Glasgow-haskell-users

rebinding arrow notation
by Peter Gavin 04 Sep '08

04 Sep '08

Section 8.3.5 of the documentation says: Arrow notation (see Section 8.9, “Arrow notation ”) uses whatever arr, (>>>), first, app, (|||) and loop functions are in scope. But unlike the other constructs, the types of these functions must match the Prelude types very closely. Details are in flux; if you want to use this, ask! I want to do this. Is there anything special I should know? Pete

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Arrow without `>>>'
by Valery V. Vorotyntsev 03 Sep '08

03 Sep '08

Hi, friends, I've built GHC from darcs, and... Could anybody tell me, what's the purpose of Arrow[1] not having `>>>' method? 1. http://darcs.haskell.org/packages/base/Control/Arrow.hs $ ghci GHCi, version 6.9.20080104: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m + Control.Arrow Prelude Control.Arrow> :i Arrow class (Control.Category.Category a) => Arrow a where arr :: (b -> c) -> a b c pure :: (b -> c) -> a b c first :: a b c -> a (b, d) (c, d) second :: a b c -> a (d, b) (d, c) (***) :: a b c -> a b' c' -> a (b, b') (c, c') (&&&) :: a b c -> a b c' -> a b (c, c') -- Defined in Control.Arrow instance Arrow (->) -- Defined in Control.Arrow instance (Monad m) => Arrow (Kleisli m) -- Defined in Control.Arrow Prelude Control.Arrow> Leaving GHCi. I can't build[2] arrows-0.3 package without `>>>' in Arrow. 2. vvv@fun:~/src/arrows$ darcs w { hunk ./Control/Arrow/Operations.hs 36 +import Control.Category ((>>>)) hunk ./Control/Arrow/Transformer/CoState.hs 23 +import Control.Category ((>>>)) } vvv@fun:~/src/arrows$ runhaskell Setup build Preprocessing library arrows-0.3... Building arrows-0.3... [ 3 of 12] Compiling Control.Arrow.Transformer.CoState ( Control/Arrow/Transformer/CoState.hs, dist/build/Control/Arrow/Transformer/CoState.o ) Control/Arrow/Transformer/CoState.hs:29:7: `>>>' is not a (visible) method of class `Arrow' The question arises "should I?", but this is one of lambdabot's[3] depencies. 3. http://code.haskell.org/lambdabot/ Thanks a lot! -- vvv

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*BSD GHC hackers needed...
by Simon Marlow 02 Sep '08

02 Sep '08

While perusing the tickets in the 6.10.1 milestone I spotted 3 that look to be FreeBSD-specific: 2476 internal error: awaitEvent: descriptor out of range 2502 segfault with GHC.Handle.fdToHandle' 2511 unix package doesnt load in ghci on freebsd/amd64 It's unlikely that we'll get to these in time for 6.10.1. Any FreeBSD hackers out there want to take a look? Help/advice is available as usual. There's also one NetBSD-specific ticket: 2351 NetBSD defines ELF_ST_TYPE (that one is easy), and one that I think applies to all the *BSDs: 2063 Breackage on OpenBSD due to mmap remap (actually the latter one is a top priority, because without it GHCi can't work, so we need it fixed for 6.10.1). Cheers, Simon

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GADT pattern match in non-rigid context
by Wolfgang Jeltsch 02 Sep '08

02 Sep '08

Hello, I have some code giving me the error message: “GADT pattern match in non-rigid context for … Tell GHC HQ if you'd like this to unify the context”. I reduced my code to the following example which still gives this error message: > data T a b where > > C :: T a [b] > > f :: (forall a'. T a' b) -> T a b > f t = case t of C -> C How do I work around this error? Some former e-mail discussion gave the hint of adding a type signature but this probably doesn’t work in my case. Note also that specializing the type of the argument t to T a b inside the definition of f is not an option since in the real code I need the first argument of T to be universally quantified for calling another function which needs this quantification. I use GHC 6.8.2. Don’t know whether this problem still shows up with GHC HEAD but am interested in hearing whether this is the case. I’m thankful for any help. Best wishes, Wolfgang

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Windows snapshot binaries
by Mitchell, Neil 02 Sep '08

02 Sep '08

Hi, Looking at: http://www.haskell.org/ghc/dist/current/dist/ The latest Windows binary appears to be the 22nd of June, and thus is before things such as the Control.Exception changes, and will not work with the current version of Haddock. Is it possible to get an updated binary? From what I remember, these snapshots are also used for the Windows binary release, so will have to be fixed before 6.10 can be released. Thanks Neil ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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Where STM is unstable at the moment, and how we can fix it
by Sterling Clover 01 Sep '08

01 Sep '08

This email is inspired by the discussion here: http:// hackage.haskell.org/trac/ghc/ticket/2401 As the ticket discusses, unsafeIOToSTM is, unlike unsafePerformIO or unsafeInterleaveIO, genuinely completely unsafe in that there is no way to use it such that a segfault or deadlock is not at least somewhat encouraged. The code attached to the ticket creates a deadlock solely through using it to write to stdout. But, for the same reason that unsafeIOToSTM is unstable, unsafeInterleaveIO now is very unstable as well -- conceivably, data generated from functions with lazy IO (including those in the prelude) could cause deadlocks within STM, and even segfaults. In summary, a "validation" step is performed on all threads inside atomically blocks during garbage collection. This validation step will, on encountering invalid threads (i.e. ones which should be rolled back) immediately kill them dead and retry. This is different than the implementation described in the STM paper, where rollbacks only occur on commit. However, it does add a measure of efficiency. The problem is that the validation code disregards exception handlers, since rollback is not an exception, and so anything embedded in STM that brackets an IO action, for example, can be rolled back without the final part of the exception even being called. As Simon M. notes, the obvious solution would be to turn rollbacks into regular exceptions, but this would open a number of cans of worms. A start, though not sufficient, would be for stm validation to respect blocked status -- not to block on it, obviously, but simply to refuse to rollback a transaction within it. Validation on GC is, after all, only an efficiency trick and implementation detail, and if it lets the occasional invalid transaction stand due to its blocked status, that transaction will simply be cleaned up later anyway. A more thorough solution would be, as I suggest at the end of the ticket, to add a new primitive with similar semantics to block -- blockRollback, of type STM () -> STM (). Anything that took place within blockRollback could not be stopped by validation. Finally, we could "split the difference" between block and blockRollback, by simply setting a rollbackBlocked flag on a *top level* invocation of block within STM, and thenceforth, not unsetting it until that block is exited, regardless of calls to unblock nested inside. This would effectively, without introducing a new primitive, ensure that rollback did not disrupt things terribly, and thus would be the solution that handled the lazyIO issue the best as well. There are lots of interesting applications of STM that require the ability to extend its semantics. To do this is going to require unsafeIOToSTM, just as unsafePerformIO is used on occasion as a low level tool to create safer and better things on top of (or as unsafeCoerce is, for that matter). However, the current state of STM means that writing these extensions of STM semantics safely is 100% impossible. I'm not sure which, if any, of the solutions that I'm presenting seem the most reasonable. However, without some sort of resolution for this issue, STM is far less powerful and useful than it can and should be. --Sterl.

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Version control systems
by Simon Marlow 29 Aug '08

29 Aug '08

Following lots of useful discussion and evaluation of the available DVCSs out there, the GHC team have made a decision: we're going to switch to git. It came down to two things: the degree of support available, and flexibility of the tools (git is much happier to let you modify the history than Mercurial). Speed ruled out bzr, and Windows support is less of an issue: git appears to work reasonably well on Windows these days. So we need a plan for switching. We aim to make the switch shortly before branching the repository for 6.10, which would mean we need to make the switch early September, in around 5 weeks time. Before then, the goal is to get the infrastructure to the point where we can switch with minimum fuss. We already have an up-to-date git mirror thanks to Thomas Schilling: git clone http://darcs.haskell.org/ghc.git (notice how fast that is :-) darcs-all will be able to work with either the git repository or the darcs repository (Max Bolingbroke is working on this, I believe). We can switch the automatic builds over to git as soon as darcs-all is working, and as long as the git mirror is kept up to date (Thomas: is the mirror being automatically updated now?). I'd urge people to try out the git mirror and let us know how you get on. We'll also work on updating the build documentation on the wiki and creating a page of getting-started info on using git. Cheers, Simon

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"dataflow rewriting engine"
by Chad Scherrer 27 Aug '08

27 Aug '08

Hello GHC, This page http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/IntegratedCode… mentions a to-be-developed "dataflow rewriting engine". Can someone please send a description of what this will do? Thanks! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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GHC's build system
by Simon Peyton-Jones 26 Aug '08

26 Aug '08

Friends There's been quite a bit of discussion about GHC's build system recently, and in particular about the use of Cabal. Responding to that discussion we now have a new plan, described here: http://hackage.haskell.org/trac/ghc/wiki/Design/BuildSystem If you've taken an interest in the earlier thread, now would be a good time to look at our plan and help us improve it. In concrete terms, we propose to develop the changes (which should not be a radical upheaval) on a branch, and merge them into the head *after* the 6.10 code fork. So the new build system won't be on the 6.10 branch. It'll take a while to make the new system stabilise, and we want to get 6.10 out! Some comments and suggestions are best done by email, but please feel free to clarify or amplify wording on the Wiki itself, or add lists of questions and open issues. The two media are suitable for different things. But we do want to end up with a *comprehensible* description, with long-term value, of how the new system works, so where things are obscure please help us to clarify it. All this is somewhat separate to the question of what version control system to use for what. We're still working on that question! Simon

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Re: SHA1.hs woes, was Version control systems
by C Rodrigues 22 Aug '08

22 Aug '08

I've "fixed" this problem by increasing the number of registers used on ia64 to 34. The problem will show up again if anyone finds a way to make GCC use even more registers. -heatsink >> >> Sorry, I couldn't find the rest of the preceding message. Someone >> wrote that they had to turn down cc flags to get SHA1.hs to compile on >> IA64. > > Yep. > >> What C compiler was being used, and what were the symptoms? > > GCC. > > As I recall the symptoms were that gcc used more than 32 registers and > then the mangler balked. The reason is that a registerised ia64 build > expects to only use the first 32 registers but does not take any > precautions to make sure that this is the case. It just relies on the > fact that most code coming out of the ghc backend cannot make use of > more than a handful of registers. If gcc does actually use more then the > mangler catches this. We tried some flags to make gcc restrict itself to > a subset of the registers but could not get it to obey. > > Duncan _________________________________________________________________ Talk to your Yahoo! Friends via Windows Live Messenger. Find out how. http://www.windowslive.com/explore/messenger?ocid=TXT_TAGLM_WL_messenger_ya…

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