August 2019 - Glasgow-haskell-users

[ANNOUNCE] GHC 8.8.1 is now available
by Ben Gamari 06 Sep '19

06 Sep '19

Hello everyone, The GHC team is pleased to announce the release candidate for GHC 8.8.1. The source distribution, binary distributions, and documentation are available at https://downloads.haskell.org/ghc/8.8.1 This release is the culmination of over 3000 commits by over one hundred contributors and has several new features and numerous bug fixes relative to GHC 8.6: * Visible kind applications are now supported (GHC Proposal #15) * Profiling now works correctly on 64-bit Windows (although still may be problematic on 32-bit Windows due to platform limitations; see #15934) * A new code layout algorithm for amd64's native code generator significantly improving the runtime performance of some kernels * The introduction of a late lambda-lifting pass which may reduce allocations significantly for some programs. * Further work on Trees That Grow, enabling improved code re-use of the Haskell AST in tooling * Users can write `forall` in more contexts (GHC Proposal #7) * The pattern-match checker is now more precise in the presence of strict fields with uninhabited types. * A comprehensive audit of GHC's memory ordering barriers has been performed, resulting in a number of fixes that should significantly improve the reliability of programs on architectures with weakly-ordered memory models (e.g. PowerPC, many ARM and AArch64 implementations). * A long-standing linker limitation rendering GHCi unusable with projects with cyclic symbol dependencies has been fixed (#13786) * Further work on the Hadrian build system * Countless miscellaneous bug-fixes Unfortunately, due to a build issue (#17108) found late in the release process i386 Windows builds are currently unavailable. These will be provided in the coming weeks. As always, if anything looks amiss do let us know. Happy compiling! Cheers, - Ben [1] https://downloads.haskell.org/ghc/8.8.1/docs/html/users_guide/8.8.1-notes.h…

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exhausted simplifier ticks and hs-boot files
by Ganesh Sittampalam 30 Aug '19

30 Aug '19

Hi, The code below (also attached - unzip and run go.sh) triggers the GHC panic "Simplifier ticks exhausted", and I'm unsure whether I should view it as an instance of the known infelicity in the inliner (https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#b…) My code does have a recursive datatype, but the recursion isn't contravariant, which is the case described in "Secrets of the GHC inliner" (https://www.microsoft.com/en-us/research/wp-content/uploads/2002/07/inline.…, section 4). It's cut down from some real code where I have a mutually recursive datatype that I want to define across two modules for code structuring reasons, meaning I need a .hs-boot file. I haven't been able to reproduce it without the .hs-boot file - if I put everything in one module it stops happening. I've tried with a range of GHC versions from 8.2.x to an early version of 8.8. It happens with -O1 and not -O0, but I haven't managed to find a specific optimisation that triggers it. Is this just an instance of the known problem in a different guise, or is it something different and worth a bug report? Cheers, Ganesh T2.hs-boot ----------- module T2 where data T2 mapP_T2 :: (Int -> Int) -> T2 -> T2 T1.hs ----- module T1 where import {-# SOURCE #-} T2 data T1 = T1 T2 mapP_T1 :: (Int -> Int) -> T1 -> T1 mapP_T1 _ (T1 xs) = T1 (mapP_T2 id xs) T2.hs ----- module T2 where import T1 data T2 = T2 T1 mapP_T2 :: (Int -> Int) -> T2 -> T2 mapP_T2 f (T2 t) = T2 (mapP_T1 f t) go :: T1 -> T1 go = mapP_T1 id GHC output ---------- $ ghc --make T2.hs -O1 -fsimpl-tick-factor=1000 -ddump-simpl-stats) [...] ghc.exe: panic! (the 'impossible' happened) (GHC version 8.2.2 for x86_64-unknown-mingw32): Simplifier ticks exhausted When trying UnfoldingDone mapP_T2 To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed Total ticks: 61203 24481 PreInlineUnconditionally 6121 ds_i17h 6120 f_a16p 6120 ds_d17d 6120 ds1_i17i 12241 UnfoldingDone 6121 mapP_T1 6120 mapP_T2 24481 BetaReduction 6121 ds_i17h 6120 f_a16p 6120 ds_d17d 6120 ds1_i17i

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`StablePtr` in `ST`
by Matthew Farkas-Dyck 21 Aug '19

21 Aug '19

I have been doing some work where i want `StablePtr`, but also to not be confined to `IO`. I saw the following comment in "compiler/prelude/PrimOp.hs": Question: Why @RealWorld@ - won't any instance of @_ST@ do the job? [ADR] It has been there for 20 years. What is the answer? If it is safe i'll send the patch generalizing these operations.

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