wiki:Status/Oct10

Version 3 (modified by simonpj, 3 years ago) (diff)

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GHC Status October 2010

GHC is humming along. We are currently deep into the release cycle for GHC 7.0. We have finally bumped the major version number, becuase GHC 7.0 has quite a bit of new stuff

  • As long promised, Simon PJ and Dimitrios have spent a good chunk of the summer doing a complete rewrite of the constraint solver in the type inference engine. Because of GHC's myriad type-system extensions, especially GADTs and type famlies, the old engine had begun to resemble the final stages of a game of Jenga. It was a delicately-balanced pile of blocks that lived in constant danger of complete collapse, and had become extremely different to modify (or even to understand). The new inference engine is much more modular and robust; it is described in detail in our paper http://haskell.org/haskellwiki/Simonpj/Talk:OutsideIn OutsideIn.

As a result we have closed dozens of open type inference bugs, especially related to GADTs and type families.

  • There is a new, robust implementation of INLINE pragmas, that behaves much more intuitively. GHC now captures the original RHS of an INLINE function, and keeps it more-or-less pristine, ready to inline at call sites. Separately, the original RHS is optimised in the usual way. Suppose you say
    {-# INLINE f #-}
    f x = ...blah...
    
    g1 y = f y + 1
    g2 ys = map f ys
    
    Here, f will be inlined into g1 as you'd expect, but obviously not into g2 (since it's not applied to anything). However f's right hand side will be optimised (sepraately from the copy retained for inlining) so that the call from g2 runs optimised code.
  • David Terei implemented a new back end for GHC using LLVM. David: a few sentences more; include a citation.
  • Bryan O’Sullivan and Johan Tibell and implemented a new, highly-concurrent IO library. This allows GHC to support thousands of I/O connections, using epoll. Johan: a few sentences more?.
  • In joint work with Phil Trinder and his colleagues at Herriot Watt, Simon M designed implemented a new parallel strategies library, described in their 2010 Haskell Symposium paper [Seq].
  • Simon M did a lot of work on the runtime system. In particular he substantially improved the way that thunks are handled in a concurrent programs. Simon: a little more info?
  • Simon M designed and implmented a new API for asynchronous exceptions. Simon: describe

We are fortunate to have a growing team of people willing to roll up their sleeves and help us with GHC. Particular thanks to:

  • Daniel Fischer, who worked on improving the performance of the numeric libraries
  • Milan Straka, for great work improving the performance of the widely-used containers package [Containers]
  • Greg Wright is leading a strike team to make GHC work better on Macs.
  • Sam Anklesaria implemented rebindable syntax for conditionals
  • ..who else..?

At GHC HQ we are having way too much fun; if you wait for us to do something you have to wait a long time. So don't wait; join in!

Language developments, especially types

GHC continues to act as an incubator for interesting new language developments. Here's a selection that we know about.

  • Pedro Magalhaes is implementing the "derivable type classes" mechanism described in his 2010 Hsakell Symposium paper [Derivable]. I plan for this to replace GHC's current derivable-type-class mechanism, which has a poor power-to-weight ratio and is little used.
  • Stephanie Weirich and Steve Zdancewic had a great sabbatical year at Cambridge. One of the things we worked on, with Brent Yorgey who came as an intern, was to close the embarrassing hole in the type system concerning "newtype deriving" (see Trac bug #1496). I have delayed fixing until I could figure out a Decent Solution, but now we know; see our 2011 POPL paper [Newtype]. Brent is working on some infrastructal changes to GHC's Core language, and then we'll be ready to tackle the main issue.
  • Next after that is a mechanism for promotimg types to become kinds, and data constructors to become types, so that you can do typed functional programming at the type level. Conor McBride's SHE prototype is the inspiration here [SHE]. Currently it is, embarrassingly, essentially untyped.
  • Iavor Diatchki plans to add numeric types, so that you can have a type like Bus 8, and do simple arithmetic at the type level. You can encode this stuff, but it's easier to use and more powerful to do it directly.
  • David Mazieres at Stanford wants to implement "Safe Haskell", a flag for GHC that will guarantee that your program does not use unsafePerformIO, foreign calls, RULES, and other stuff stuff. This is part of his projet to ... David pls fill in.

Packges and the runtime system

Simon M

  • Independent parallel garbage collection [Simon M]
  • Better package management (esp wrt profiling)
  • Glorious new back end -fuse-new-codegen

The Parallel Haskell Project

Duncan to write

Data Parallel Haskell

Manuel to fill in. Mention upcoming release.

Infrastructure and the build system

Ian: anything to say?

Bibliography

  • [Derivable] "A generic deriving mechanism for Haskell", Magalhães, Dijkstra, Jeuring and Löh, Haskell Symposium 2010, www.dreixel.net/research/pdf/gdmh_nocolor.pdf.