- Glasgow-haskell-users

Records in Haskell
by Simon Peyton-Jones 28 Mar '12

28 Mar '12

Friends Provoked the (very constructive) Yesod blog post on "Limitations of Haskell", and the follow up discussion, I've started a wiki page to collect whatever ideas we have about the name spacing issue for record fields. http://hackage.haskell.org/trac/ghc/wiki/Records As Simon M said on Reddit, this is something we'd like to fix; but we need a consensus on how to fix it. Simon

41 183

ANNOUNCE: GHC version 7.2.1
by Ian Lynagh 27 Mar '12

27 Mar '12

============================================================= The (Interactive) Glasgow Haskell Compiler -- version 7.2.1 ============================================================= The GHC Team is pleased to announce a new major release of GHC, 7.2.1. The 7.2 branch is intended to be more of a "technology preview" than normal GHC stable branches; in particular, it supports a significantly improved version of DPH, as well as new features such as compiler plugins and "safe Haskell". The design of these new features may evolve as we get more experience with them. See the release notes for more details of what's new and what's changed. We are also using this branch as an opportunity to work out the best workflows to use with git. We expect the 7.2 branch to be short-lived, with 7.4.1 coming out shortly after ICFP as normal. Full release notes are here: http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/release-7-2-1.html How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: http://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces (C, whatever). GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page http://www.haskell.org/ghc/ GHC developers' home page http://hackage.haskell.org/trac/ghc/ Haskell home page http://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: http://hackage.haskell.org/trac/ghc/wiki/Contributors Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: http://hackage.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site run by Trac: http://hackage.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at http://www.haskell.org/mailman/listinfo/glasgow-haskell-users http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see http://www.haskell.org/mailman/listinfo/ Some GHC developers hang out on #haskell on IRC, too: http://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: http://www.haskell.org/ghc/reportabug

9 16

ADPfusion: Efficient, high-level dynamic programming
by Christian Höner zu Siederdissen 25 Mar '12

25 Mar '12

ADPfusion combines stream-fusion (using the stream interface provided by the vector library) and type-level programming to provide highly efficient dynamic programming (DP) combinators. You can write DP programs in a style similar to Algebraic Dynamic Programming (ADP) (Giegerich et al.), meaning symbolically without explicit indices. The symbolic operators provide a number of advantages: - take care of extracting the correct cell information from data structures (DP matrices) - no more index problems, the grammars are completely free of index calculations by the user - complex operators can be written with "next step" behaviour dependent on other information calculated during runtime - monadic interface - works with pure/monadic, boxed/unboxed, ... data structures - speed close to optimized C (between 1.8x - 3x slower currently -- compared to well-optimized C-code) Hackage: http://hackage.haskell.org/package/ADPfusion 1st example program: http://hackage.haskell.org/package/Nussinov78 Many thanks to Roman Leshchinskiy! Viele Gruesse, Christian Hoener zu Siederdissen ADP: http://bibiserv.techfak.uni-bielefeld.de/adp/adpapp.html

1 0

Stealing ideas from the latest GCC release
by Johan Tibell 24 Mar '12

24 Mar '12

Hi all, While looking at the GCC 4.7 [1] release notes I saw something that's perhaps worth stealing. Taken from the release notes: The inter-procedural constant propagation pass has been rewritten. It now performs generic function specialization. For example when compiling the following: void foo(bool flag) { if (flag) ... do something ... else ... do something else ... } void bar (void) { foo (false); foo (true); foo (false); foo (true); foo (false); foo (true); } GCC will now produce two copies of foo. One with flag being true, while other with flag being false. This leads to performance improvements previously possibly only by inlining all calls. Cloning causes a lot less code size growth. I found myself wanting something like this just today. A common pattern I see in code is this: data Options = Options { ... } defaultOptions :: Options defaultOptions = ... foo :: ... foo = fooWith defaultOptions fooWith :: Options -> ... fooWith = ... It'd be nice if we could get foo to specialize on its input arguments, without having to mark all of fooWith as INLINE. 1. http://gcc.gnu.org/gcc-4.7/changes.html -- Johan

5 5

GHCi and line numbers (with ghc-7.4.1)
by Simon Hengel 23 Mar '12

23 Mar '12

Hi, ghc --interactive now behaves different in regards to line numbers in error messages than previous versions. They are now incremented with each evaluated expression. $ ghc --interactive -ignore-dot-ghci Prelude> foo <interactive>:2:1: Not in scope: `foo' Prelude> bar <interactive>:3:1: Not in scope: `bar' Is there a way to disable this (or alternatively reset the counter for a running session)? Cheers, Simon

3 4

7.4.2 release plans
by Ian Lynagh 23 Mar '12

23 Mar '12

Hi all, Just a quick note to let you know about our release plans: We plan to put out a GHC 7.4.2 release candidate around the end of the March. The final release will probably not happen until around the end of April. Thanks Ian, on behalf of the GHC team

2 1

update to Template Haskell for rich kinds
by Richard Eisenberg 21 Mar '12

21 Mar '12

Hi all, I'm working on bringing Template Haskell up to speed with the new extensions for polymorphic kinds and promoted data kinds. This requires a few small, non-breaking changes to TH datatypes. I've made a (short) wiki page at http://hackage.haskell.org/trac/ghc/wiki/TemplateHaskellRichKinds and the official Trac feature request is hackage.haskell.org/trac/ghc/ticket/5612 Does anyone have any suggestions to what I've written? Any other comments? Thanks! Richard

2 3

How to work around GHC bug
by Volker Wysk 18 Mar '12

18 Mar '12

Hi! The following program demonstrates a (another) GHC bug. The command line isn't properly decoded to Unicode. arg.hs------------------------------ import System main = do [a] <- getArgs putStrLn (show a) -------------------------------------- When called like this: ./arg ä The program will output this: "\195\164" I'll report this as a bug in the GHC Trac. But for now, I need to work around the problem somehow. The encoders in GHC.IO.Encoding all work on buffers. How do I recode the command line, in order to get proper Unicode strings? Bye Volker

5 7

How to declare polymorphic instances for higher-kinded types?
by Herbert Valerio Riedel 16 Mar '12

16 Mar '12

Hello *, For simple-kinded type variables, instances of the type instance NFData a => NFData [a] instance NFData a => NFData (Maybe a) instance (NFData a, NFData b) => NFData (a, b) are common and can be defined effortless; now I wanted do something similiar for a type with a phantom type parameter: {-# LANGUAGE KindSignatures, TypeSynonymInstances #-} import Control.Applicative import Control.Monad data DataBase = DataBase -- specific type not relevant here data Res data Unres -- provides operation to transform an unresolved `Foo_ Unres` to a resolved `Foo_ Res` class Resolvable (e :: * -> *) where resolve :: DataBase -> e Unres -> Either String (e Res) -- trivial /resolvable/ type data Foo_ r = Foo instance Resolvable Foo_ where resolve _ x = return Foo ...it was no problem to define the polymorphic operations outside of an instance: -- Maybe (polymorphic 0 or 1 element container) resolveMaybe :: Resolvable e => DataBase -> Maybe (e Unres) -> Either String (Maybe (e Res)) resolveMaybe db (Just x) = Just <$> resolve db x resolveMaybe db Nothing = pure Nothing -- Pairs resolvePair :: (Resolvable e0, Resolvable e1) => DataBase -> (e0 Unres, e1 Unres) -> Either String (e0 Res, e1 Res) resolvePair db (x,y) = (,) <$> resolve db x <*> resolve db y ...but when I tried to wrap those into polymorphic instances in the style of the instances at the beginning of this mail, I wasn't able to convince GHC: The following attempts wouldn't work: instance Resolvable e => Resolvable (Maybe e) where resolve = resolveMaybe -- GHC fails with: -- Expecting one more argument to `e' -- In the instance declaration for `Resolvable (Maybe e)' Fair enough, but trying to workaround this by defining a type-synonym to get an (*->*)-kinded expression didn't work either, as currying doesn't seem to be supported at the type-level (is there a language-extension for that?): type Maybe_ e r = Maybe (e r) instance Resolvable e => Resolvable (Maybe_ e) where resolve = resolveMaybe -- GHC fails with: -- Type synonym `Maybe_' should have 2 arguments, but has been given 1 -- In the instance declaration for `Resolvable (Maybe_ e)' So, am I really out of luck here, wanting to define polymorphic instances in combination with phantom-types, or is there a trick I haven't thought of yet? PS: while experimenting, I accidentally triggered the following GHC exception: *** Exception: compiler/rename/RnSource.lhs:429:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars, _, SrcLoc.L _ cls, _) ...alas I lost the Haskell-code causing this; is this a known issue? Should I try harder to reproduce it again? cheers, hvr --

3 4

profiling and backtracing blues
by Ranjit Jhala 16 Mar '12

16 Mar '12

Hi all, I'm trying to use the nifty backtracing mechanism in GHC 74. AFAICT, this requires everything be built with profiling on), but as a consequence, I hit this: "You can't call hscCompileCoreExpr in a profiled compiler" Any hints on whether there are work-arounds? Thanks! Ranjit.

3 5