November 2018 - Haskell-Cafe

Feedback on plan for checking code requirements in a GHC plugin
by Chris Smith 03 Apr '19

03 Apr '19

Hey everyone, I'm converging on a design for a new feature of CodeWorld, the Haskell-based educational programming environment that I teach with. I'm wondering if anyone has done something like this before. The goal is that when I give an assignment in a class (e.g., "modify this starting code to generalize the repeated pattern using a function"), I want the user to see a checklist of assignment requirements when they run the code. A prototype implementation is here: https://code.world/haskell#PpwpTF1wv3qIvwhohCfvSrQ There are all sorts of possible requirements, such as: "No lines longer than 80 characters", or "there must be a function called foo", or "all top-level definitions must have type signatures", or "your code must define at least 10 top-level variables, and use at least 3 where clauses", or "your definition of var should be equivalent to this one" (see Joachim Breitner's inspection testing work), or even "the function f must satisfy this QuickCheck property". It's been suggested that the requirements language also include the ability to match patterns in the AST, which I think is a good idea. The current prototype uses a pre-compile step that parses the code using haskell-src-exts, and doesn't implement dynamic requirements (runtime-evaluated) at all. My ultimate plan, though, is to send these requirements to GHC via a plugin, then have it evaluate the static ones at compile time, and generate code to check the dynamic ones. Finally, the plugin would add new code to the beginning of main that will invoke a configurable function with the results of the requirement check (hard-coding the static ones, and evaluating dynamic ones on the fly). (In the CodeWorld environment, this function would display the checklist in the web UI, for example.) Has anyone done anything like this before? Any wisdom to share, or ideas to contribute? Thanks, Chris

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Let's plan BayHac 2019!
by Dan Burton 04 Feb '19

04 Feb '19

It's time to start thinking about the next BayHac! I am assembling a group of organizers for next year's event. Anyone who has been involved in organizing past BayHac events will be automatically accepted. (I have BCCed a number of past organizers; please reach out to any you know in case I missed anyone.) Anyone else who is interested in helping to plan BayHac, please contact me. Depending on how many past organizers rejoin, we will discuss the applicants and accept 1 to 3 new organizers. BayHac is a weekend of learning, sharing, and hacking on a variety of projects for the Haskell community. BayHac usually occurs in San Francisco, California, Mountain View, California, or thereabouts. I'd like to aim for April/May 2019, though that is, of course, subject to change. A rough outline, in rough chronological order, of what the BayHac organizers will be doing: 0. Assemble the organizers. 1. Select date and venue for BayHac 2019. 2. Secure corporate sponsors to cover costs (mainly, food). 3. Invite talk proposals and select speakers. 4. Open online registration and publicize the event. 5. Plan details like catering, registration desk, recording, and make it happen! If any of that sounds like fun to you, then I'd love to hear from you! -- Dan Burton

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HDBC database: error on accent table name
by Damien Mattei 16 Jan '19

16 Jan '19

Hi, i have this error: *** Exception: SqlError {seState = "", seNativeError = 1064, seErrorMsg = "You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'es where Nom = 'A 20'' at line 1"} when doing this : rows_coordonnees <- quickQuery' conn "select * from sidonie.Coordonnées where Nom = 'A 20'" [] it seems tha the tabel name: Coordonnées that contain an accent is causing serious problem to the parser at some point, if i use a table name without accent it works fine. i'm at the point to rename the table which have great impact on all the project build with many other languages (Scheme) that deal correctly the table name with accent. any idea? to make accent works with haskell. Regards, Damien -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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Timing out a pure evaluation of an expression I did not write myself
by Ryan Reich 19 Dec '18

19 Dec '18

I want to time out a pure computation. My experience, and that described in various previous questions here and elsewhere (the best of which is https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html) is that this doesn't always work: for instance, >>> timeout 1 $ evaluate $ let x = 0 : x in last x does not time out because, apparently, the fact that the expression evaluates in constant space (i.e. never allocates) means that it never yields to the timeout monitor thread that would kill it. The solution that is described in the other iterations is to embed checkpoints in the expression that do allocate, giving the RTS a chance to switch contexts. However, in my application, the expression is /arbitrary/ and I do not have the freedom to inject alterations into it. (Don't argue this point, please. The expression is arbitrary.) How can I time out a tight loop like the above? Clearly, it can be done, because I can, say, alt-tab over to another terminal and kill the process, which exploits the operating system's more aggressively pre-emptive scheduling. Is there a solution using bound threads, say 'forkOS' instead of 'forkIO' in the implementation of 'timeout'? Unix signals? Some FFI-based workaround? Etc. Keep in mind that notwithstanding that comment, I don't actually want to kill the whole process, but just the one evaluation. Thanks in advance, Ryan Reich

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Re: [Haskell-cafe] static analysis of a C-like language
by Olaf Klinke 04 Dec '18

04 Dec '18

> Hello Olaf, > > to me that sounds as if you want to do an abstract interpretation with a > forward collecting semantics that employs non-relational abstract > domains for the primitive data types and summarizes the dimensions of > arrays. ... > I would start by writing a simple interpreter for the language to be > analyzed. That way you fix messy details before they bite you, e.g. the > order in which submodules are loaded and initialized. I was hoping not having to write an interpreter (because the language author wrote a translation to C++ already), but if that is the way to go, I'll do it. As I understand it, the Haskell semantics should contain just enough substance so that the errors I am after will cause hiccups in the Haskell compiler? That is indeed a compelling approach. What this does not address is the question about error reporting: How could a translation to Haskell preserve information about scope, source position and masking? Can I leverage the ghc API for that? Regards, Olaf

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semantics of concurrent program depends on -O level, -f[no-]omit-yields
by Johannes Waldmann 30 Nov '18

30 Nov '18

Dear Cafe, I am surprised by the behaviour of the program below (the interesting property is whether it will output "foo"). Behaviours (plural) actually: it seems to depend on optimisation level, on omit-yields, and on very small changes in the source code: It does print (mostly), when compiled with -O0. It does not, when compiled with -O2. With -O2 -fno-omit-yields, it will print. With -O0 -fno-omit-yields, and when I remove the two newTVar in the beginning, it will mostly not print. How come? These differences already occur with the last two lines replaced by "forever $ return ()", so the STM stuff may be inessential here. But that's the context where I came across the problem. - J.W. import Control.Concurrent.STM import Control.Concurrent ( forkIO ) import Control.Monad ( forever ) import System.IO main = do atomically $ newTVar "bar" atomically $ newTVar False forkIO $ putStrLn "foo" x <- atomically $ newTVar False forever $ atomically $ writeTVar x True

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Haskell-related summer internship in Germany
by Sandra Dylus 30 Nov '18

30 Nov '18

Our department (PL and compiler construction) again offers a summer internship in 2019 sponsored by DAAD Rise Germany [1]. Undergraduate students enrolled at North American, British and Irish universities are eligible to make an application. We'd be very happy to host a motivated student, who is keen to improve our compiler for the functional logic programming language Curry in terms of performance. For more information you may have a look at our project poster [2]. If you have any questions, feel free to mail any of the contact persons. Please also spread the news to your students and classmates. Cheers, Finn and Sandra [1]: https://www.daad.de/rise/en/rise-germany/ <https://www.daad.de/rise/en/rise-germany/> [2]: https://www-ps.informatik.uni-kiel.de/~sad/DAADRise.pdf <https://www-ps.informatik.uni-kiel.de/~sad/DAADRise.pdf>

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Re: [Haskell-cafe] WebAssembly illustrated
by Takenobu Tani 30 Nov '18

30 Nov '18

Hi Sven, Thank you for teaching me. > Also, are you slides free to use? Could you please share the source? Of course you can use it freely :) I also commit the source. (But it is PowerPoint.) Cheers, Takenobu On Fri, Nov 30, 2018 at 9:10 PM Sven Sauleau <sven(a)sauleau.com> wrote: > Hi, > > Interesting document, thanks for sharing! > > I opened a clarification issue: [1]. > > Also, are you slides free to use? Could you please share the source? > [1]: https://github.com/takenobu-hs/WebAssembly-illustrated/issues/1 > > Thanks! > Sven > > On 30/11/2018 12:45, Takenobu Tani wrote: > > Hi cafe, > > I share this even in this place. > > WebAssembly illustrated > https://takenobu-hs.github.io/downloads/WebAssembly_illustrated.pdf > > > P.S. > Haskell has backends such as asterius [1] and dhc [2]. > > [1]: https://github.com/tweag/asterius > [2]: https://github.com/dfinity/dhc > > Cheers, > Takenobu > > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to:http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. > > >

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WebAssembly illustrated
by Takenobu Tani 30 Nov '18

30 Nov '18

Hi cafe, I share this even in this place. WebAssembly illustrated https://takenobu-hs.github.io/downloads/WebAssembly_illustrated.pdf P.S. Haskell has backends such as asterius [1] and dhc [2]. [1]: https://github.com/tweag/asterius [2]: https://github.com/dfinity/dhc Cheers, Takenobu

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GHC 8.6.2 zombie processes?
by Viktor Dukhovni 29 Nov '18

29 Nov '18

It seems that GHC 8.6.2 does not reap defunct child processes in a timely manner when compiling many modules (e.g. "base" when stack is building a new sandbox). This is especially visible when using LLVM 6.0 whose optimizer is particularly CPU-intensive (read mind-numbingly slow) on my X86_84 FreeBSD system: The process: $ ps -wwwwwww -o pid,etime,args -p 98525 PID ELAPSED COMMAND 98525 55:47 /home/viktor/.local/lib/ghc-8.6.2.20181122/bin/ghc -B/home/viktor/.local/lib/ghc-8.6.2.20181122 --make -fbuilding-cabal-package -O -static -dynamic-too -dynosuf dyn_o -dynhisuf dyn_hi -outputdir .stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build -odir .stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build -hidir .stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build -stubdir .stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build -i -i.stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build -i. -i.stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build/autogen -i.stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build/global-autogen -I.stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build/autogen -I.stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build/global-autogen -I.stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build -I/usr/local/Cellar/postgresql/10.0/include -I/usr/local/include -I/usr/local/opt/icu4c/include -optP-include -optP.stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/build/autogen/cabal_macros.h -this-unit-id Cabal-2.4.1.0-IaB5GUEm19R82R9cEdbB1D -hide-all-packages -Wmissing-home-modules -no-user-package-db -package-db /usr/home/viktor/.stack/snapshots/x86_64-freebsd/nightly-2018-11-28/8.6.2.20181122/pkgdb -package-db .stack-work/dist/x86_64-freebsd/Cabal-2.4.0.1/package.conf.inplace -package-id array-0.5.3.0 -package-id base-4.12.0.0 -package-id binary-0.8.6.0 -package-id bytestring-0.10.8.2 -package-id containers-0.6.0.1 -package-id deepseq-1.4.4.0 -package-id directory-1.3.3.0 -package-id filepath-1.4.2.1 -package-id mtl-2.2.2 -package-id parsec-3.1.13.0 -package-id pretty-1.1.3.6 -package-id process-1.6.3.0 -package-id text-1.2.3.1 -package-id time-1.8.0.2 -package-id transformers-0.5.5.0 -package-id unix-2.7.2.2 -XHaskell2010 Distribution.Backpack Distribution.Backpack.Configure Distribution.Backpack.ComponentsGraph Distribution.Backpack.ConfiguredComponent Distribution.Backpack.DescribeUnitId Distribution.Backpack.FullUnitId Distribution.Backpack.LinkedComponent Distribution.Backpack.ModSubst Distribution.Backpack.ModuleShape generated multiple hundreds of "zombies": $ ps $(pgrep -P 98525) | egrep -c defunct 460 I guess the build taking an hour is far more an issue than a few hundred zombies, but this message is about the zombies. Is this a feature or a bug? -- Viktor.

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