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GHC Status Report (November 2018)

GHC development continues with the release of 8.6.2, continued improvement in testing infrastructure, and a slew of new features.

Major changes in GHC 8.8

With GHC 8.6 behind us, we have started to focus on what will be GHC 8.8, which should ship with a number of great features.

Libraries, source language, and type system

  • Syntax for visible dependent quantification (Proposal #81 (closed)), allowing users to express types with visible, dependent quantifiers more directly.

  • Top-level kind signatures, allowing users to add kind signatures alongside their type declarations.

Compiler

  • The next phase of Trees That Grow, refactoring GHC's treatment of source spans in the Haskell AST.

  • TypeInType was a major increment, and we are still refactoring type inference to account for it in a robust, modular way. GHC 8.8 has a lot of consolidation in this area, especially of kind inference for type and class declarations.

  • Continued work on compiler performance

  • Support for SIMD operations in the native code generator

  • Support for sub-word sized values in the code generator and libraries

  • Further improvements to runtime performance:

    • A late-lambda lifting pass to further improve

    • A new code layout algorithm, significantly improving the

  • Many, many bug fixes.

Runtime system

  • Significantly improved Windows support with a new I/O manager (Tamar Christina).

GHC proposals

Since the launch of the GHC proposals process (https://github.com/ghc-proposals/ghc-proposals), over 171 proposals have been opened for discussion, 49 have been submitted to the committee and 38 have been accepted. Recently

At the time of writing, 10 proposals are under active discussion by the community (https://github.com/ghc-proposals/ghc-proposals/pulls?q=is\%3Aopen+is\%3Apr+no\%3Alabel) and 9 proposals (https://github.com/ghc-proposals/ghc-proposals/pulls?q=is\%3Aopen+is\%3Apr+label\%3A\%22Pending+committee+review\%22) are under review by the committee.

Looking forward: What's hot

GHC is lucky to have a large number of volunteer contributors. Many of these features will be present in the up-coming 8.8 release.

  • Matth'ias P'all Gissurarson has been adding support for significantly improved diagnostics messages for typed holes, including making the feature easier to integrate into IDE tooling.

  • Ryan Scott has been busily triaging and fixing bugs on a daily basis, and generally helps to keep things running smoothly.

  • Michal Terepeta has been performing a variety of refactoring and optimization in the backend as well as introducing support for sub-word-sized fields.

  • Abhiroop Sarkar has been working on introducing support for x86 SIMD instructions into GHC's native code generator, making these primitives applicable over a significantly wider range of settings.

  • Andreas Klebinger has been working on improving the code layout algorithms used by GHC's backend code generator. The result of his Google Summer of Code project resulted in speed-ups of between 1% and 5% on a variety of tested real-world libraries.

  • Tamar Christina has continued his work on making GHC run great on Windows. Recently he has been working to finish up a patchset enabling dynamic linking support on Windows. Tamar is also working on a rework of GHC's Windows IO manager implementation. The new implementation will take full advantage of Windows' asynchronous I/O interfaces and should solve dozens of long-standing tickets.

  • Sebastian Graf has been working on rekindling the late lambda-lifting work started by Nicholas Frisby some time ago. This transformation optimizes runtime allocations by turning free variables into call arguments. His explorations into performance this transformation on STG has resulted in extremely impressive allocations reductions on the nofib benchmark suuite.

  • Andrey Mokhov, David Eichmann, and Alp Mestanogullari have been working on the finishing the last mile of the switch to GHC's new Shake-based build system, Hadrian, which has now been merged into the GHC tree.

  • One of the larger projects in the pipeline for 8.6 is Alan Zimmerman and Shayan Najd's refactoring of GHC to use the extensible Trees That Grow AST structure.

  • Ben Gamari has been working on improving compilation time for programs making heavy use of type families. His patch fixes a long-standing performance cliff (#8095) of GHC's compilation pipeline and should significantly improve compilation times of programs with lots of fancy types.

  • Tobias Dammers and Ömer Sinan Ağacan have been working on too many projects to name, including fixing numerous bugs and improving compiler performance.

  • Kavon Farvardin has been working on numerous projects around the LLVM code generator, including working with the LLVM developers to teach LLVM about GHC's notion of proc points.

  • Ningning Xie and Richard Eisenberg have been chipping away at realizing Dependent Haskell.

  • Vladislav Zavialov, in addition to contributing a variety patches, has been invaluable in helping with code review and advising contributors.

  • Zubin Duggal has been working on a mechanism for exporting large swaths of GHC's typechecked representation for consumption by IDE tooling.

  • Simon Jakobi introduced support in GHCi for the :doc command, allowing users to view (currently unformatted) Haddock documentation from the REPL.

  • Peter Trommler has been working on improving GHC's portability to non-x86 platforms.

  • Alec Theriault has also been doing a variety of work around Template Haskell, and the numerous projects in GHC's frontend,

As always, if you are interested in contributing to any facet of GHC, be it the runtime system, type-checker, documentation, simplifier, or anything in between, please come speak to us either on IRC (#ghc on irc.freeenode.net}) or ghc-devs@haskell.org. Happy Haskelling!

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