GHC Status Report, October 2013

A big event occurred earlier this year in July: the news that Ian Lynagh would be leaving Well-Typed - and consequently, GHC HQ - to move onto new challenges. Ian worked on GHC for 7 years full-time, helped write our new build system, redesigned large parts of the compiler, and fixed innumerable other issues over the course of his work with us. According to some statistics for the past 7 years or so, Ian quite clearly is one of the biggest contributors we've ever had. And we should take the time to say what is rightfully deserved: thank you for everything you've done for us, Ian!

While Ian has not completely disappeared, GHC and Haskell are not his day-job anymore. As a result, this means everyone - including you, our friend and reader - have a huge opportunity to help continue making GHC even better. Luckily, the past few months of activity points towards a tremendous surge in community involvement. GHC is a community project, and the community is what keeps us afloat. A great thanks goes to all those who have helped us come so far!

In light of this, Well-Typed has added two people to GHC HQ to keep things moving, and help the development process going forward: Edsko de Vries and Austin Seipp.

The GHC 7.8 release is in its final stages, and will be released in late November according to our plans. There's a tremendous amount of exciting changes coming very soon, described below.

Source language and Type System

Type natural solver - Iavor Diatchki implemented a basic constraint solver for the type naturals extension, meaning that GHC can now infer and understand basic identities such as (x + 2) ~ 5 , which implies x = 3 . The constraint solver does only basic things for now; there is more to come.

Closed type families - Richard Eisenberg & co. have implemented support for closed type families in GHC, allowing you to write a type family where no instances can be made beyond the ones in the definition (wiki page [CTF-wiki] and paper [CTF-paper]). This allows a host of new programs to be expressed, as we now know certain invariants can hold. For example, we may now write: type family Flip p :: * type family Flip a where Flip Even = Odd Flip Odd = Even to express that Flip may have no more instances written, meaning nefarious users can no longer write silly instances like Flip Bool = Int .

Role support - Richard Eisenberg implemented support for role checking in GHC, fixing a long standing bug #1496 (closed) where GeneralizedNewtypeDeriving could be used to derive unsafe instances for a newtype (wiki [Roles-wiki] and paper [Roles-paper]).

New and improved I/O manager - Earlier this year, Andreas Voellmy and Kazu Yamamoto worked on a host of improvements to our I/O manager, making it scale significantly better on multicore machines. Since then, it's seen some other performance tweaks, and many bugfixes. As a result, the new I/O manager should scale linearly up to about 40 cores. Andreas reports their McNettle Software-defined-network (SDN) implementation can now achieve over twenty million connections per second, making it the fastest SDN implementation around - an incredible feat!

Type Holes - Thijs Alkemade and Simon PJ got an implementation of TypeHoles in GHC, meaning it's possible to tell GHC there is a 'hole' in a program, and have the compiler spit out an error stating what types are in scope. As a trivial example Prelude> :set -XTypeHoles Prelude> let f :: a -> a; f x = _ <interactive>:6:24: Found hole ‛_’ with type: a Where: ‛a’ is a rigid type variable bound by the type signature for f :: a -> a at <interactive>:6:10 Relevant bindings include x :: a (bound at <interactive>:6:20) f :: a -> a (bound at <interactive>:6:18) In the expression: _ In an equation for ‛f’: f x = _ GHC now tells us that the term f has a hole of type a , and there is a term x :: a in scope. So the definition is clear: f x = x . Holes are originally a concept borrowed from Agda, and we hope they will be useful to Haskell programmers too!

New Template Haskell - Geoffrey Mainland implemented support for New Template Haskell, fixing a lot of long-standing bugs with the TH implementation, while making it significantly more expressive, including support for typed quotes, pattern splices and more. This allows us to write for example a typed, staged power function. For details and examples, you can see Geoff's blog [NewTH1] and the GHC wiki pages describing the design [NewTH2].

Newtype coercions . Joachim Breitner spent time earlier this year implementing a new, fancy Coercible design in GHC, which helps eliminate the cases in which newtypes are 'not free'. This occurs for example, when we say something like newtype Age = MkAge Int and then map MkAge [0..10] - while newtype is supposed to be free, this particular example has a runtime cost! While Coercible is still in flux [NTC], we hope it will be useful as a 'preview technology' in the 7.8 release.

A small menagerie of various other language improvements and extensions, including: -XOverloadedLists which allows overloading list literals (Achim Krause, George Giorgidze, Weijers Jeroen) -XNumDecimals allowing a compact floating-point syntax for integrals, e.g. 1.2e6 :: Integer (Shachaf Ben-Kiki) Support for empty case statements (via -XEmptyCase , by Richard Eisenberg) -XIncoherentInstances became even more liberal (Joachim Breitner) A new pragma for specifying the minimal complete definition of a typeclass, via {-# MINIMAL #-} (Twan van Laarhoven)



Back-end and runtime system

New code generator - As previously reported, the New Code Generator is live and switched on by default. There have been a host of bugfixes and stability improvements, meaning it should be solid for the 7.8 release.

SSE/AVX support - Geoffrey Mainland implemented support for SSE/AVX intrinsics in the compiler backend, making it possible to exploit hardware-accelerated SIMD operations in your code on Intel/AMD machines. It's currently only enabled for the LLVM backend, however.

LLVM back end . Peter Wortmann spent time earlier this year doing a significant refactoring of the LLVM backend, which means it should be easier in the future to extend the backend and perform long-term maintenance.

Code generation . Jan Stolarek had an internship at Microsoft Research during the summer, and as part of this he implemented an array of improvements to the code generator and backend, including a new loopification pass to turn tail-recursive calls into loops, and a refactoring of our Bool based primops to return unboxed Int# values (making them much faster, with a sizeable performance improvement in some cases [PrimBool].)

Unloading object code . Simon Marlow implemented support for unloading object code at runtime in the GHC linker. Previously, while GHC's linker could load object code dynamically, there was no facility to unload it - meaning long running applications would continuously suffer a memory leak as they reloaded more code.

Library constructors . Edward Yang implemented support for running library constructors in GHCi, making it possible to use foreign libraries which depend on constructors being run at load time.

Atomic and prefetch primops. There are new primops for all backends, including new atomic memory operations (by Ryan Newton) and support for low-level prefetch instructions in the processor, allowing you to guide cache decisions (by Carter Schonwald.)

Frontend, build-system, and miscellaneous changes

Dynamic-by-default - In 7.8, we're hoping to make GHCi use the system linker by default on supported platforms, eliminating a host of bugs in the current home-grown linker. Eventually we hope to remove the old linker completely. Until then, GHC now supports compiling files statically and dynamically at the same time (with the -dynamic-too flag,) meaning you can switch between static/dynamic builds much more easily.

Compiler hooks - Luite Stegeman and Edsko de Vries did a significant amount of work to improve hooking support in the GHC API. This new API makes it possible for users to plug in their own pipeline machinery to the compiler, suitable for implementing new frontend features (like QuasiQuoting hooks) or new backends (like a JavaScript target, as part of the new GHCJS.)

Parallel --make - as part of the haskell.org 2013 GSoC, Patrick Palka implemented a new parallel compilation driver, a long-requested feature. This allows GHC to build multiple modules in parallel when using --make by adding a -j flag, while having almost no overhead in the single-threaded case.

Clang support - Austin Seipp added support for GHC to use Clang as the C compiler, instead of GCC. While the work is still rough, it should be a boon to modern OS X users, who no longer have GCC available by default.

iOS support & cross compilation - After many years of work by Ian, Stephen Blackheath, Gabor Greif and friends Luke Iannini and Maxwell Swadling, GHC now has full support for iOS cross-compilation. As of GHC 7.8, you'll really be able to write iOS apps in your favorite programming language!

Better ARM support - Ben Gamari and Austin Seipp have some final changes to make for the ARM story, which should make it significantly more stable and usable, including GHCi support. We hope to have ARMv7 binary releases for GHC 7.8 available for download.

Future plans

After the 7.8 release, there are some improvements scheduled we plan on integrating:

Applicative as a superclass of Monad - A long-standing proposal, GHC 7.10 will finally make Applicative a superclass of Monad . GHC 7.8 features warnings to ensure users know where their code will break as a result of this API change.

Kinds without data - Trevor Elliott, Eric Mertens, and Iavor Diatchki have began implementing support for "data kind" declarations, described in more detail on the GHC wiki [KD]. The idea is to allow a new form of declaration that introduces a new kind, whose members are described by the (type) constructors in the declaration. This is similar to promoting data declarations, except that no new value-level-constructors are declared, and it also allows the constructors to mention other kinds that do not have corresponding type-level representation (e.g., *).

**Overloaded record fields ** - Adam Gundry implemented the overloaded records field proposal as part of the Haskell.org 2013 GSoC. This work will make it possible to not only have overloaded record field names, but also enable a wealth of other nice features, like polymorphic update/lookup, and automatically turning record fields into lens. More detail can be found on the GHC wiki [ORF].

Pattern synonyms - Gergö Érdi worked on an implementation of pattern synonyms for GHC, which will finally give us the power to abstract over patterns and give them names. While the design is not final (see the wiki for details[PS]), the results look promising, and will hopefully fix a long-standing 'abstraction hole' in the term language for Haskell.

Explicit Type Application - Stephanie Weirich, Richard Eisenberg and Hamidhasan Ahmed have been working on adding explicit type applications to GHC. This allows the programmer to specify the types that should be instantiated for arguments to a function application, where normally they would be inferred. While this capability already exists in GHC's internal language, System FC -- indeed, every FC-pro program has function application with explicitly applied types -- it has not been available in Haskell itself. While a lot of the syntax and design is not quite final, there are some details about the design available on the wiki [TA].

Git management changes - For a long time, GHC has used a loosely coupled set of repositories during development. However, as we've added more contributors, this practice has become increasingly problematic, preventing us from using useful tools like git bisect to track down bugs. Our plans after the 7.8 release are to sort this out, and hopefully have stable, reproducible GHC builds for all.

Continuous Integration overhaul - GHC has historically been very spotty about continuous integration. Part of the problem is our git management, mentioned above. In the 7.10 timeframe, we hope to spend a significant amount of time overhauling our buildbots and machines. To this end, Rackspace has graciously donated services as part of its Open Source support initiatives.

But we're not sure what else might happen. It's a great time to step up to the plate and do something fun!

Development updates, joining in and a big Thank You!

In the past several months, GHC has seen a surge of community involvement, and a great deal of new contributors. We now have 37 committers, with 14 added in 2013 alone - it's an exciting time to help out!

Amongst those who have rolled up their sleeves and helped us recently:

Herbert Valerio Riedel has stepped up to the plate to help maintain http://ghc.haskell.org, all of our git repositories, and has become increasingly involved in library maintenance and development.

has stepped up to the plate to help maintain http://ghc.haskell.org, all of our git repositories, and has become increasingly involved in library maintenance and development. Takano Akio has submitted many great bug reports and patches recently, and has a keen eye for reproducing challenging or tricky bugs.

has submitted many great bug reports and patches recently, and has a keen eye for reproducing challenging or tricky bugs. Reid Barton has submitted some excellent patches to fix some awkward bugs, and help us as we move towards Android cross compiler support.

has submitted some excellent patches to fix some awkward bugs, and help us as we move towards Android cross compiler support. Kazu Yamamoto helped Andreas Voellmy in the improvements to the I/O manager, and has been extremely helpful in pointing out and reproducing bugs, and helping us find solutions.

helped Andreas Voellmy in the improvements to the I/O manager, and has been extremely helpful in pointing out and reproducing bugs, and helping us find solutions. Joachim Breitner implemented Coercible, but has also stepped up to merge patches, fix more bugs, and help set up some Continuous-Integration for GHC.

implemented Coercible, but has also stepped up to merge patches, fix more bugs, and help set up some Continuous-Integration for GHC. Carter Schonwald has been involved in talking with many who wish to get involved and inspiring them to get their hands dirty!

has been involved in talking with many who wish to get involved and inspiring them to get their hands dirty! Krzysztof Gogolewski has fixed many bugs, and like Takano, has helped reproduce and track down issues with fantastic test cases.

has fixed many bugs, and like Takano, has helped reproduce and track down issues with fantastic test cases. Jan Stolarek has helped improve the documentation of the low-level parts of the compiler, as well as implementing optimizations for us too.

has helped improve the documentation of the low-level parts of the compiler, as well as implementing optimizations for us too. Muhaimin Ahsan has been helping with infrastructure, writing patches, and generally offering help to us who are already busy.

As ever, there is a ton of stuff in the future for us to do. If you want something done - don't wait, it might take a while. You should join us instead!

References

[CTF-wiki] Closed type families with overlapping equations - http://ghc.haskell.org/trac/ghc/wiki/NewAxioms

[CTF-paper] Closed type families with overlapping equations, Richard Eisenberg, Dimitrios Vytiniotis, Simon Peyton Jones, Stephanie Weirich, POPL 2014 - http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/

[KD] Kinds without Data - http://ghc.haskell.org/trac/ghc/wiki/GhcKinds/KindsWithoutData

[NewTH1] Runtime codegen with typed Template Haskell - http://gmainland.blogspot.com/2013/05/type-safe-runtime-code-generation-with.html

[NewTH2] Major proposed Template Haskell revision - http://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/BlogPostChanges

[NTC] Newtype wrappers - http://ghc.haskell.org/trac/ghc/wiki/NewtypeWrappers

[PrimBool] New comparison primitives - http://ghc.haskell.org/trac/ghc/wiki/PrimBool

[ORF] Overloaded record fields - http://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Plan

[PS] Pattern synonyms - http://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms

[Roles-wiki] Roles - http://ghc.haskell.org/trac/ghc/wiki/Roles

[Roles-paper] Generative Type Abstraction and Type-level Computation, Stephanie Wirich, Dimitrios Vytiniotis, Simon Peyton Jones, and Steve Zdancewic, POPL 2011 - http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/

[TA] Explicit type application - http://ghc.haskell.org/trac/ghc/wiki/ExplicitTypeApplication