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Fwd: rolling span and groupBy for lists
by Harendra Kumar 05 Feb '18

05 Feb '18

Hi, I am looking for the decision making authority or a place where questions about haskell libraries like in the email below can be asked or answered. ghc-devs told me that their list is not the right place for this and redirected me to haskell-cafe or libraries(a)haskell.org. I tried sending an email to libraries(a)haskell.org but that seems to be a closed list, my email got rejected, so that also does not seem like the right place. Is haskell cafe the right place? I was under the impression that haskell-cafe is for general discussions and not an owner of any of the libraries or any other haskell code. Are there any other mailing lists that I am missing? I can raise a ticket at ghc trac but I guess that cannot be the primary way to ask simple questions. -harendra ---------- Forwarded message ---------- From: <libraries-owner(a)haskell.org> Date: 5 February 2018 at 09:34 Subject: Re: rolling span and groupBy for lists To: harendra.kumar(a)gmail.com You are not allowed to post to this mailing list, and your message has been automatically rejected. If you think that your messages are being rejected in error, contact the mailing list owner at libraries-owner(a)haskell.org. ---------- Forwarded message ---------- From: Harendra Kumar <harendra.kumar(a)gmail.com> To: David Feuer <david(a)well-typed.com>, libraries <libraries(a)haskell.org> Cc: ghc-devs(a)haskell.org Bcc: Date: Mon, 5 Feb 2018 10:07:55 +0530 Subject: Re: rolling span and groupBy for lists I was mainly asking if it makes sense to include these functions in base/Data.List. Since the base package is maintained and ships along with ghc, and the issues are also raised at ghc trac I thought this is the right list. I am copying to libraries(a)haskell.org as well. -harendra On 5 February 2018 at 09:53, David Feuer <david(a)well-typed.com> wrote: > This is the wrong list. You probably meant to email haskell-cafe or > perhaps libraries(a)haskell.org. > > > > David Feuer > Well-Typed, LLP > > -------- Original message -------- > From: Harendra Kumar <harendra.kumar(a)gmail.com> > Date: 2/4/18 10:50 PM (GMT-05:00) > To: ghc-devs(a)haskell.org > Subject: rolling span and groupBy for lists > > Hi, > > For a small problem, I was looking for a groupBy like function that groups > based on a predicate on successive elements but I could not find one. I > wrote these little functions for that purpose: > > -- | Like span, but with a predicate that compares two successive elements. > The > -- span ends when the two successive elements do not satisfy the predicate. > rollingSpan :: (a -> a -> Bool) -> [a] -> ([a], [a]) > rollingSpan _ xs@[] = (xs, xs) > rollingSpan _ xs@[_] = (xs, []) > rollingSpan p (x1:xs@(x2:_)) > | p x1 x2 = > let (ys, zs) = rollingSpan p xs > in (x1 : ys, zs) > | otherwise = ([x1], xs) > > -- | Like 'groupBy' but with a predicate that compares two successive > elements. > -- A group ends when two successive elements do not satisfy the predicate. > rollingGroupBy :: (a -> a -> Bool) -> [a] -> [[a]] > rollingGroupBy _ [] = [] > rollingGroupBy cmp xs = > let (ys, zs) = rollingSpan cmp xs > in ys : rollingGroupBy cmp zs > > Are there any existing functions that serve this purpose or is there any > simpler way to achieve such functionality? If not, where is the right place > for these, if any. Can they be included in Data.List in base? > > Thanks, > Harendra >

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Call for Participation: BOB 2018 (February 23, Berlin)
by Stefan Wehr 04 Feb '18

04 Feb '18

There are several Haskell-related talks and tutorials, see below for details! ================================================================ BOB 2018 Conference “What happens if we simply use what’s best?” February 23, 2018, Berlin http://bobkonf.de/2018/ Program: http://bobkonf.de/2018/en/program.html Registration: http://bobkonf.de/2018/en/registration.html ================================================================ BOB is the conference for developers, architects and decision-makers to explore technologies beyond the mainstream in software development, and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experiences. The program features 14 talks and 8 tutorials on current topics: http://bobkonf.de/2018/en/program.html The subject range of talks includes functional programming, verticalization, formal methods, and data analytics. There will be two talks about Haskell: * Testing monadic programs using QuickCheck and state machine based models * New Hasql - a native Haskell Postgres driver faster than C The tutorials feature introductions to Haskell, Clojure, Livecoding, terminal programming in Haskell, Liquid Haskell, functional reactive programming, and domain-driven design. Leif Andersen will give the keynote talk. Registration is open online: http://bobkonf.de/2018/en/registration.html BOB cooperates with the :clojured conference on the following day. There is a registration discount available for participants of both events. http://www.clojured.de/

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MonadBaseControl and Freer
by Lana Black 02 Feb '18

02 Feb '18

Hello, I have a problem with implementing a MonadBaseControl instance for freer monad (Eff from extensible-effects). Specifically, I don't really get what the associated type StM should be. As far as I understand, MonadBaseControl class does the following: captures the current state. Performs the action passed to lifeBaseWith or other wrapper functions. Returns the result wrapped in the captured state. Here's the instance I could come up with. instance (MonadBase m (Eff r), Typeable m, SetMember Lift (Lift m) r) => MonadBaseControl m (Eff r) where type StM (Eff r) a = Eff r a liftBaseWith f = lift (f return) restoreM = id It obviously doesn't work, but I currently have no idea how to fix it, because `Eff r a' contains the state that needs to be captured and cannot be decomposed without losing data as far as I can see. The code can be found in this branch: https://github.com/greydot/extensible-effects/tree/monadbasecontrol As a side matter, I couldn't find any tests for MonadBaseControl instances, and monad-control package itself lacks any tests whatsoever. I'm curious whether there's a way to test instance correctness without plugging it into working code, e.g. something using lifted-base, and hoping for the best.

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Spectre mitigations for Haskell
by Joey Hess 01 Feb '18

01 Feb '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 31 Jan '18

31 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 31 Jan '18

31 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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What's the story behind "applicative" in "Constant Applicative Form"?
by sonne 30 Jan '18

30 Jan '18

Hi cafe. I've been trying to make the concept of CAFs get through to me, but the very name is cryptic enough to stupefy me. What does it mean for a thing to be "applicative"? Is it related to the concept of applicative functor (likely not)? What would a constant non-applicative form look like? A non-constant applicative form? An applicative non-form, in the end? I put a question on Stack Overflow about this, only to discover no one's really sure. I would appreciate either an answer put there right away or a permission to re-post or rephrase an answer there myself, but, if you do mind, I will of course keep the answer private to this mailing list. This is the link to the question: stackoverflow.com/questions/48489778 Thank you! P.S. I guess this is my first e-mail to this list, so please kindly let me know if it's unfit or in any way out of line.

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Taking over maintainership of exact-pi
by Andrew Lelechenko 28 Jan '18

28 Jan '18

I would like to take over `exact-pi` package (https://hackage.haskell.org/package/exact-pi) Unfortunately, the package became incompatible with upcoming GHC 8.4 and needs an upgrade. The relevant pull request (https://github.com/dmcclean/exact-pi/pull/5) prepared by @konn, has been open for a month without response. I also tried to reach its maintainer Douglas McClean (cced) by email, but have not heard back. I am so interested in upgrading `exact-pi`, because it is a dependency of my package `arithmoi`. — Best regards, Andrew

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LTS-10.* missing build-depends suggestion
by Rick Owens 26 Jan '18

26 Jan '18

Dear Haskell Café, I am a long-time Haskell developer with a proliferation of private projects. I typically make very heavy use of stack and LTS. Today, I tried to bootstrap my first project using LTS-10.* (with GHC-8.2.2) and I ran into an interesting problem (for me). In the past, with lts-9.* and below, when I started cranking on some code and writing a bunch of imports: > import Control.Monad.Trans.Class (MonadTrans) > import Data.Map (Map) I would try to build first without updating the cabal file build-depends, and I would get something like the following error messages: > /path/to/Module.hs:48:1: error: > Failed to load interface for ‘Data.Map’ > It is a member of the hidden package ‘containers-0.5.7.1’. > Perhaps you need to add ‘containers’ to the build-depends in your .cabal file. > Use -v to see a list of the files searched for. > > /path/to/Module.hs:21:1: error: > Failed to load interface for ‘Control.Monad.Trans.Class’ > It is a member of the hidden package ‘transformers-0.5.2.0’. > Perhaps you need to add ‘transformers’ to the build-depends in your .cabal file. > Use -v to see a list of the files searched for. This was good because the error contained a suggested solution which contained the missing package, and I could sort of lazily add the build-depends I needed without having to memorize a (Module -> Package) mapping. I even had developer tooling to do it automatically. With lts-10.*, however, the analogous error message looks like this: > /path/to/Module.hs:9:1: error: > Could not find module ‘Data.DoubleWord’ > Use -v to see a list of the files searched for. > | > 9 | import Data.DoubleWord (Word256(Word256), Word128(Word128)) > | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ I am wondering, is there a way to bring back the solution suggestion? Not having access to it has made me realize how heavily I was using that feature. What do others think about this missing bit of supplemental error information? Thanks, -Rick Owens

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Multiple positions open
by Lee Pike 25 Jan '18

25 Jan '18

We have multiple positions open doing work at the intersection of Haskell, compilers, and machine learning, as well as Haskell-based infrastructure (CI, builds, etc.) Preferably Portland, Oregon or Palo Alto, California. Email me for details! Lee

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