August 2017 - Glasgow-haskell-users

[ANNOUNCE] GHC 8.2.1 release candidate 1
by Ben Gamari 03 Oct '17

03 Oct '17

Hello everyone, The GHC team is very pleased to announce the first candidate of the 8.2.1 release of the Glasgow Haskell Compiler. Source and binary distributions are available at https://downloads.haskell.org/~ghc/8.2.1-rc1/ This is the first of a few release candidates leading up the final 8.2.1 release. This release will feature, * A new type-indexed Typeable implementation * The long awaited Backpack * Deriving strategies for disambiguating DeriveAnyClass, GeneralizedNewtypeDeriving, and stock mechanisms * Overloaded record fields * Improved compiler performance * Better code generation through more robust tracking of join points * Compact regions for more efficient garbage collection and serialization * Better support for machines with non-uniform memory architectures * More robust support for levity (e.g. RuntimeRep) polymorphism * A simple interface for streaming eventlog data from live processes * Further refinement of DWARF support Unfortunately there are still a few known issues in this release, including a few compiler panics (#13233, #13509) and a memory leak in the simplifier (#13426) which may adversely affect heap sizes and compilation time for some modules. This memory leak unfortunately made it impossible to provide a 32-bit Windows distribution for this candidate; this will be resolved in -rc2. As always, please let us know if you have difficulty. Thanks to everyone who has contributed to this release! Happy testing, - Ben

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How to get a heap visualization
by Yitzchak Gale 20 Sep '17

20 Sep '17

I need a simple heap visualization for debugging purposes. I'm using GHC 8.0.2 to compile a large and complex yesod-based web app. What's the quickest and easiest way? Vacuum looks simple and nice. But it has some long-outstanding PRs against it to support GHC 7.10 and GHC 8.0 that were never applied. https://github.com/thoughtpolice/vacuum/issues/9 Getting ghc-vis to compile looks hopeless, for a number of reasons. The dependencies on gtk and cairo are huge. It hasn't been updated on Hackage for a year and a half. It requires base < 4.9. I need to run the visualizer either on a headless Ubuntu 16.04 server, or locally on Windows. And anyway, the fancy GUI in ghc-vis is way overkill for me. The heap scraper backend for ghc-vis, ghc-heap-view, looks usable, and better supported than vacuum. But is there a quick and simple visualizer for its output, without ghc-vis? Is there anything else? Is the best option to fork vacuum and and try to apply the PRs?

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TH Show instance not working.
by David Banas 20 Aug '17

20 Aug '17

Hi all, Does anyone know why this code: module Language.P4.UtilTest where import Language.P4.Util (mkShow) data Dummy = Bogus Char | Nonsense Int $(mkShow ''Dummy) is producing this error: Davids-Air-2:P4 dbanas$ stack ghc -- UtilTest.hs -ddump-splices [1 of 1] Compiling Language.P4.UtilTest ( UtilTest.hs, UtilTest.o ) UtilTest.hs:24:3-16: Splicing declarations mkShow ''Dummy ======> instance Show Dummy where show (Bogus x) = show x show (Nonsense x) = show x UtilTest.hs:24:3: error: Conflicting definitions for ‘show’ Bound at: UtilTest.hs:24:3-16 UtilTest.hs:24:3-16 | 24 | $(mkShow ''Dummy) | ^^^^^^^^^^^^^^ ? The TH splice expansion looks correct to me. If I comment out the second constructor (Nonsense Int), the code compiles without error. Thanks, -db

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8.2.1 changes to iface/FlagChecker.hs
by Evan Laforge 03 Aug '17

03 Aug '17

My program uses the GHC API to provide a REPL, and upgrading to 8.2.1 broke that... well, just made it really slow. Where it used to quickly load the compiled files, it now wants to recompile everything as bytecode which makes 1s startup time go to 1m. The reason was it now thinks flags have changed. I tracked it down to changes in FlagChecker.hs, documented with https://ghc.haskell.org/trac/ghc/ticket/11798 and https://ghc.haskell.org/trac/ghc/ticket/10923, which add -fhpc and -O to the tracked flags. -fhpc means loading tests in ghci can never work, because ghci rejects the -fhpc flag, and likewise -O means ghci can never load optimized files for the same reason. It seems like the easiest solution might be to just have ghci not reject -fhpc and -O. Of course we could also revert those changes, but they're justified in the trac tickets. Of course this means I'm loading optimized code together with bytecode compiled code and maybe that's a bad thing (or for tests, -fhpc with non-hpc). But I've been doing this for something like 7 years and it's been fine. Unrelatedly, I was hoping compiles would get faster, but they seem a bit slower, from around 3m20s to around 4m :( I can track down per-file timing changes if it would be interesting to someone. Also thanks to whoever put trac tickets in the comments, it makes it really easy to find out why a change was made.

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GHC release timing and future build infrastructure
by Ben Gamari 01 Aug '17

01 Aug '17

Hello everyone, I just posted a pair of posts on the GHC blog [1,2] laying out some thoughts on the GHC release cycle timing [1] and how this relates to the in-progress Jenkins build infrastructure [2]. When you have a some time feel free to give them a read and comment (either here or on the Reddit thread [3]). Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/blog/2017-release-schedule [2] https://ghc.haskell.org/trac/ghc/blog/jenkins-ci [3] https://www.reddit.com/r/haskell/comments/6qt0iv/ghc_blog_reflections_on_gh…

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