February 2012 - Glasgow-haskell-users

Oddity with 'cabal install' in GHC 7.4.1
by Conal Elliott 05 Feb '12

05 Feb '12

Since upgrading to 7.4.1, if I 'cabal install' successfully and then 'cabal install' a second time without first doing a 'ghc-pkg unregister <package-name>', I get the following complaint: cabal: The install plan contains reinstalls which can break your GHC > installation. You can try --solver=modular for the new modular solver that > chooses such reinstalls less often and also offers the --avoid-reinstalls > option. You can also ghc-pkg unregister the affected packages and run > ghc-pkg > check to see the effect on reverse dependencies. If you know what you are > doing you can use the --force-reinstalls option to override this reinstall > check. > The only path I've found so far that's willing to rebuild or even say nothing needs rebuilding (when nothing does) is to 'ghc-pkg unregister' and then 'cabal install' again. I'm getting the same behavior on Red Hat 5 and Mac OS 10.6.8, both compiled from sources. Does anyone know what's going on here? -- Conal

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ANNOUNCE: GHC version 7.4.1
by Ian Lynagh 03 Feb '12

03 Feb '12

============================================================= The (Interactive) Glasgow Haskell Compiler -- version 7.4.1 ============================================================= The GHC Team is pleased to announce a new major release of GHC, 7.4.1. Here are some of the highlights of the 7.4 branch since 7.2 and 7.0: * The Num class no longer has Eq or Show superclasses. * There is a new feature Safe Haskell (-XSafe, -XTrustworthy, -XUnsafe). The design has changed since 7.2. * There is a new feature kind polymorphism (-XPolyKinds). A side-effect of this is that, when the extension is not enabled, in certain circumstances kinds are now defaulted to * rather than being inferred. * There is a new feature constraint kinds (-XConstraintKinds). * It is now possible to give any sort of declaration at the ghci prompt. For example, you can now declare datatypes within ghci. * The profiling and hpc implementations have been merged and overhauled. Visible changes include renaming of profiling flags, and a new semantics for the cost-centre stacks (which should in most cases result in more useful and intuitive profiles). The +RTS -xc flag now also gives a stack trace. * It is now possible to write compiler plugins. * DPH support has been significantly improved. * There is now preliminary support for registerised compilation using LLVM on the ARM platform. Full release notes are here: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/release-7-4-1.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

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Impredicative types error
by John Meacham 01 Feb '12

01 Feb '12

Hi, I am running into an issue where some code that compiled and worked under 6.12 is failing under 7.0, the offending code is class DeNameable a where deName :: Module -> a -> a getDeName :: Tc (DeNameable n => n -> n) getDeName = do mn <- asks (tcInfoModName . tcInfo) return (\n -> deName mn n) Tc is a plain typechecking monad and this returns a generic denaming function that can be used to turn haskell names back into human readable form before printing errors in jhc. I have the ImpredicativeTypes LANGUAGE feature turned on. the error I get under 7.0 is src/FrontEnd/Tc/Monad.hs:131:29: Couldn't match expected type `n -> n' with actual type `DeNameable n' In the second argument of `deName', namely `n' In the expression: deName mn n In the first argument of `return', namely `(\ n -> deName mn n)' John

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