January 2018 - Haskell-Cafe

Fwd: Safe TCP accept loop
by Sumit Raja 30 May '18

30 May '18

Hi, I sent this to beginners but got no responses so I'm hoping someone here can help. I’ve looked at https://wiki.haskell.org/Concurrency_demos/Graceful_exit and used(2) Using throwTo without the use of block and unblock. It runs on 8.2.1 with my limited testing. I can’t find out much about what the > 7.x GHC replacement for block/unblock is other than mask. What does the unblock do in acceptConnections'? Does this method of handling an accept loop still need masking of async exceptions ? If so where does this need to be done? I've got a version with async running for my specific application (https://bitbucket.org/sumitraja/hvrr/src/6927216597a35a9a0d7f2e55cca83fa5b7…) but I don't know if I need to unmask at any point so was hoping for some guidance. Thanks Sumit

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Archival of community.haskell.org
by Mario Rogic 19 May '18

19 May '18

Hi all As announced back in 2015 <http://blog.haskell.org/post/7/the_future_of_community.haskell.org/> we have started archiving community.haskell.org services. We've created https://archives.haskell.org/, and the old projects mailing lists have been the first to be archived. Next on the list is the remainder of projects.haskell.org, and the code.haskell.org site. As always, you can email admin(a)h.o or hop on the #haskell-infrastructure freenode channel to provide feedback / concerns. Thanks - Mario

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Join a transformer
by Michael Snoyman 11 Apr '18

11 Apr '18

I'm wondering if this pattern exists and has a name. We have the concept of joining a Monad: join :: Monad m => m (m a) -> ma How about joining a monad transformer? joinT :: (Monad m, MonadTrans t) => t (t m) a -> t m a I believe implementing this in terms of MonadTransControl[1] might be possible, but I was wondering if there's an already existing idiom for this. Michael [1] http://haddocks.fpcomplete.com/fp/7.4.2/20130301-40/monad-control/Control-M…

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PatternSignatures (rant)
by Johannes Waldmann 06 Apr '18

06 Apr '18

Dear all, (this is a rant - please ignore) why has "PatternSignatures" been renamed to "ScopedTypeVariables" (in ghc-6.10)? show me the alleged "type variable" in this code: do x :: Int <- [ 1 .. 10 ] ; return $ x^2 and why on earth do I need to use a language extension at all to get to the most basic thing of declarative programming: to declare the type of an identifier? This seriously hurts (me, at least) when teaching Haskell. Thanks - J.W.

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Haskell to Ethereum VM ?
by Takenobu Tani 30 Mar '18

30 Mar '18

Hi cafe, Does anyone know about the code generator from Haskell's syntax to Ethereum VM language (bytecode)? That is, what corresponds to Solidity in Haskell. Although Solidity is interesting, it's difficult for me to achieve quality and safety. Does such a project already exist? Regards, Takenobu

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[ANN] brittany-0.9.0.0 (on both hackage&stackage)
by lennart spitzner 25 Mar '18

25 Mar '18

- Implement `IndentPolicyLeft`: A layouting setting that disables "hanging indentation". This indentation mode may become default in the future. Only configurable via config-file for the time being. - (!) commandline interface is reworked slightly to support multiple inputs/outputs (`brittany --write-mode=inplace *.hs`). This removes the --input/--output flags. - Ensure compatibility with stackage - Change default global config path (old configs are still respected) - Improve support for per-project config - Fix several bugs and Improve language extension support full changelog at https://hackage.haskell.org/package/brittany-0.9.0.0/changelog brittany was recently added to stackage, and should hopefully be included in the next LTS release. -- lennart

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A small milestone
by Simon Peyton Jones 11 Mar '18

11 Mar '18

Cherished friends Today is my sixtieth birthday. It is just over forty years since Phil and I called in at Yale on my way to FPCA, and floated the idea of Haskell with Paul Hudak<https://www.microsoft.com/en-us/research/publication/a-history-of-haskell-b…>. (It wasn't called Haskell then, of course.) Rather a lot of water has flowed under the bridge since then. GHC's bug tracker is up to 14,683 tickets; I have read every one of them. But the best thing is Haskell's rich community of smart, motivated, passionate, and friendly colleagues. There was a time when I knew every Haskell programmer on the planet, but we are far, far beyond that point. Now it's beyond me even to keep up with the huge wave of elegant and creative ideas, tools, libraries, and blog posts that you generate. (Kudos to Taylor - and doubtless other colleagues -- for the Haskell Weekly News<https://haskellweekly.news/>, which I love.) But despite its size, it's a community that is still characterised by a love of elegance, and a desire to distil the essence of an idea and encapsulate it in an abstraction, all tempered with respect and tolerance. We don't always live up to these ideals, but by and large we do. Thank you all. Onward and upward! Simon PS: as birthday recreation I'm working on https://ghc.haskell.org/trac/ghc/wiki/QuantifiedContexts

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Spectre mitigations for Haskell
by Joey Hess 31 Jan '18

31 Jan '18

Has anyone thought about the impact of the Spectre attack (https://spectreattack.com/spectre.pdf) on Haskell programs, and mitigations? Here's my initial thoughts, bearing in mind that low level stuff is not my forte.. There are likely to be many gadgets in the ghc RTS that could be used by a Spectre variant 2 attack, using indirect branch poisoning. Since the RTS is mostly C code, C compiler mitigations such as retpoline can avoid the indirect branch prediction problems. There's also assembly in the RTS. For example, StgCRun.c contains a "jmp eax" indirect branch which might be vulnerable. FFI code can also contain vulnerable indirect branches both C and assembly. It would be good if at least securemem wasn't vulnerable to Spectre variant 2, so that Haskell programs operating on very private data could use it to avoid the attack. But, even if securemem itself doesn't contain any vulnerable gadgets (its constant time Eq shouldn't branch, which helps..), other vulnerable gadgets in the RTS and elsewhere can still be pointed at the securemem buffer. I do have Haskell programs that I would like to harden against Spectre variant 2, since they're sometimes used in a multi-user environment and operate on private data. As for Spectre variant 1, it would need something like a haskell implementation of a javascript interpreter running untrusted code, or something like lambdabot running safe haskell code. That would then be vulnerable to conditional branch misprediction to expose whatever private data the process has access to. Perhaps a sufficiently smart compiler could do something about Spectre variant 1, but there are probably not too many vulnetable programs and it can be prevented in the program by eg avoiding exposure of high-resolution timers, the same way web browsers have been dealing with this attack. -- see shy jo

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addFinalizer in GHC 7.10
by Kazu Yamamoto 30 Jan '18

30 Jan '18

Hello cafe, We, the network library maintainers, have questions about System.Mem.Weak.addFinalizer in GHC 7.10. We are trying to make the library extensible especially for the socket type and the socket address type. For this purpose, a proposed definition for Socket in version 3.0.0.0 is a just CInt: newtype Socket = Socket CInt deriving (Eq, Show) One problem is that unreachable Socket cannot be GCed if CInt (a file descriptor) is not closed. To let GC work, we are trying to add a finalizer via addFinalizer: socket family stype protocol = do fd <- c_socket ... ... let s = Socket fd addFinalizer s $ close s ruturn s This works well for many cases. Unfortunately, we *sometime* got the following error with GHC 7.10: tests/SimpleSpec.hs:56: 1) Simple.sendMany works well uncaught exception: IOException of type InvalidArgument (threadWait: invalid argument (Bad file descriptor)) It seems to us that Socket is GCed too early in GHC 7.10. We don't see this behavior in other versions of GHC. So, here are our questions: Q1) Do we use addFinalizer correctly? Q2) Is this a bug of GHC 7.10? Q3) If this is a bug of GHC 7.10, are there any workarounds? --Kazu

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BayHac 2018 announcement
by Dan Burton 30 Jan '18

30 Jan '18

I am pleased to announce that BayHac 2018 will be held at the Takt office in San Francisco on the weekend of Friday, April 27 thru Sunday, April 29. BayHac is a Haskell hackathon weekend event for the San Francisco Bay Area and Silicon Valley. It will be a weekend of learning, hacking, and connecting with fellow Haskell enthusiasts. See the page for BayHac 2018 on the haskell.org wiki for details: https://wiki.haskell.org/BayHac2018 -- Dan Burton

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