September 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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Multiple C threads -> one unthreaded GHC RTS
by Albert Y. C. Lai 28 Sep '17

28 Sep '17

Suppose I have a C program with multiple C threads calling Haskell functions, maybe the same Haskell function too, and the GHC RTS used is the unthreaded one. What will go wrong? I ask because Cabal/cabal-install has recently added building of export-to-C shared libraries, i.e., the "foreign-library" section. It sees to all the technical details, very nice, I'm very happy about this addition, but it chooses the unthreaded GHC RTS. Thanks.

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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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Spurious recompilations when using a compiler plugin
by Conal Elliott 19 Sep '17

19 Sep '17

It appears that use of GHC plugins causes client code to get needlessly recompiled. (See Trac issues 12567 <https://ghc.haskell.org/trac/ghc/ticket/12567> and 7414 <https://ghc.haskell.org/trac/ghc/ticket/7414>.) It’s becoming more of a problem for usability of the plugin <http://conal.net/papers/compiling-to-categories> I’ve been developing, and I’m wondering what can be done. Some questions: - Is there any work in progress on fixing this situation? - Are there serious obstacles to fixing it? - Do plugin writers or users have any workarounds? Other insights appreciated. – Conal

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Inhibiting the specialiser?
by Conal Elliott 15 Sep '17

15 Sep '17

Is there a GHC flag for inhibiting the specializer (but not all optimizations)? I'm seeing huge output from the Specialise phase killed at 4GB and growing. The output starts as follows: Result size of Specialise = {terms: 29,639, types: 10,921,552, coercions: 4,425,185} Sounds like a lot to me. Is it?. I get this behavior with -O2 and with -O. -- Conal

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