July 2020 - Haskell-Cafe

Test on identity?
by Dušan Kolář 08 Jul '20

08 Jul '20

Dear Café, I'm trying to build a DAG from a binary tree. I don't think there's a big trouble. Nevertheless, I do even some transformations. Thus, I would like to know it is still a DAG, not adding, accidentally, a node. Is there any way, if I have data like data Ex = Val Int | Add Ex Ex so that I can test that some value Val i === Val i ? I mean, the pointers go to the same data box? I could do that via some IORefs, AFAIK, but I don't think it is feasible. Maybe to tune the algorithm... Best regards, Dusan

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FMBC 2020 - Call for Participation
by Bruno Bernardo 07 Jul '20

07 Jul '20

[ Please distribute, apologies for multiple postings. ] ======================================================================== 2nd Workshop on Formal Methods for Blockchains (FMBC) 2020 - Call for Participation https://fmbc.gitlab.io/2020 July 20 and 21, 2020, Online, 6AM-8AM PDT Co-located with the 32nd International Conference on Computer-Aided Verification (CAV 2020) http://i-cav.org/2020/ --------------------------------------------------------- The FMBC workshop is a forum to identify theoretical and practical approaches of formal methods for Blockchain technology. Topics include, but are not limited to: * Formal models of Blockchain applications or concepts * Formal methods for consensus protocols * Formal methods for Blockchain-specific cryptographic primitives or protocols * Design and implementation of Smart Contract languages * Verification of Smart Contracts The list of lightning talks and conditionally accecpted papers is available on the FMBC 2020 website: https://fmbc.gitlab.io/2020/program.html There will be one keynote by Grigore Rosu, Professor at University of Illinois at Urbana-Champaign, USA and Founder of Runtime Verification. Registration Registration to FMBC 2020 is free but required. It is done through the CAV 2020 registration form: http://i-cav.org/2020/attending/ Please register before *July 10, 2020*.

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ekg-core/RTS : bogus GC stats when running on Google Cloud Run
by Marco Zocca 04 Jul '20

04 Jul '20

Hi all, I have two services deployed on Google cloud infrastructure; Service 1 runs on Compute Engine and Service 2 on Cloud Run and I'd like to log their memory usage via the `ekg-core` library (https://hackage.haskell.org/package/ekg-core-0.1.1.7/docs/System-Metrics.ht…) (which is just a thin wrapper around GHC.Stats ). The logging bracket is basically this : mems <- newStore registerGcMetrics mems void $ concurrently io (loop mems) where loop ms = do m <- sampleAll ms ... (lookup the gauges from m and log their values) threadDelay dt loop ms I'm very puzzled by this: both rts.gc.current_bytes_used and rts.gc.max_bytes_used gauges return constant 0 in the case of Service 2 (the Cloud Run one), even though I'm using the same sampling/logging functionality and build options for both services. This is about where my knowledge ends; could this behaviour be due to the Google Cloud Run hypervisor ("gRun") implementing certain syscalls in a non-standard way (gRun syscall guide : https://gvisor.dev/docs/user_guide/compatibility/linux/amd64/) ? Thank you for any pointers to guides/manuals/computer wisdom. Details : Both are built with these options : -optl-pthread -optc-Os -threaded -rtsopts -with-rtsopts=-N -with-rtsopts=-T the only difference is that Service 2 has an additional flag -with-rtsopts=-M2G since Cloud Run services must work with 2 GB of memory at most. The container OS in both cases is Debian 10.4 ("Buster").

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Is there a handy refl or refl generator for converting GADT types?
by chris done 02 Jul '20

02 Jul '20

Hi all, I have stages in my compiler that convert e.g. Global Renamed -> Global Generated, etc. where certain constructors are available in certain stages. data GlobalRef s where FromIntegerGlobal :: GlobalRef s FromDecimalGlobal :: GlobalRef s InstanceGlobal :: !InstanceName -> GlobalRef Resolved E.g. after type class resolution is done, the InstanceGlobal constructor is available. But instances can't appear in the AST at other stages. In three stages, I've had to copy/paste this code: + refl = + case name of + FromIntegerGlobal -> FromIntegerGlobal + FromDecimalGlobal -> FromDecimalGlobal Because if I just put `name` then GHC will complain that the types are different, which is correct. But a straight-forward pattern match gives GHC the proof that they can be converted. Is there a handy way already implemented that could derive either this code or this proof for me? Cheers, Chris

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