January 2015 - Glasgow-haskell-users

"Found hole"
by Volker Wysk 21 Jan '15

21 Jan '15

Hello! What is a "hole"? This program fails to compile: main = _exit 0 I get this error message: ex.hs:1:8: Found hole ‘_exit’ with type: t Where: ‘t’ is a rigid type variable bound by the inferred type of main :: t at ex.hs:1:1 Relevant bindings include main :: t (bound at ex.hs:1:1) In the expression: _exit In an equation for ‘main’: main = _exit When I replace "_exit" with "foo", it produces a "not in scope" error, as expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries. Bye Volker

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GHC 7.10 regression when using foldr
by Björn Peemöller 21 Jan '15

21 Jan '15

I just discovered that the following program compiled fine using GHC 7.8.4 but was rejected by GHC 7.10.1-rc1: ~~~ data List a = Nil | Cons a (List a) instance Read a => Read (List a) where readsPrec d s = map convert (readsPrec d s) where convert (xs, s2) = (foldr Cons Nil xs, s2) ~~~ GHC 7.10 now complains: ~~~ Read.hs:5:23: Could not deduce (Foldable t0) arising from a use of ‘convert’ from the context (Read a) bound by the instance declaration at Read.hs:4:10-32 The type variable ‘t0’ is ambiguous Note: there are several potential instances: instance Foldable (Either a) -- Defined in ‘Data.Foldable’ instance Foldable Data.Proxy.Proxy -- Defined in ‘Data.Foldable’ instance GHC.Arr.Ix i => Foldable (GHC.Arr.Array i) -- Defined in ‘Data.Foldable’ ...plus three others In the first argument of ‘map’, namely ‘convert’ In the expression: map convert (readsPrec d s) In an equation for ‘readsPrec’: readsPrec d s = map convert (readsPrec d s) where convert (xs, s2) = (foldr Cons Nil xs, s2) Read.hs:5:32: Could not deduce (Read (t0 a)) arising from a use of ‘readsPrec’ from the context (Read a) bound by the instance declaration at Read.hs:4:10-32 The type variable ‘t0’ is ambiguous Relevant bindings include readsPrec :: Int -> ReadS (List a) (bound at Read.hs:5:3) Note: there are several potential instances: instance (Read a, Read b) => Read (Either a b) -- Defined in ‘Data.Either’ instance forall (k :: BOX) (s :: k). Read (Data.Proxy.Proxy s) -- Defined in ‘Data.Proxy’ instance (GHC.Arr.Ix a, Read a, Read b) => Read (GHC.Arr.Array a b) -- Defined in ‘GHC.Read’ ...plus 18 others In the second argument of ‘map’, namely ‘(readsPrec d s)’ In the expression: map convert (readsPrec d s) In an equation for ‘readsPrec’: readsPrec d s = map convert (readsPrec d s) where convert (xs, s2) = (foldr Cons Nil xs, s2) ~~~ The reason is the usage of foldr, which changed its type from foldr :: (a -> b -> b) -> b -> [a] -> b -- GHC 7.8.4 to foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b -- GHC 7.10.1 Thus, the use of foldr is now ambiguous. I can fix this by providing a type signature convert :: ([a], String) -> (List a, String) However, is this breaking change intended? Regards, Björn

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ghc-7.10.0 type inference regression when faking injective type families
by adam vogt 20 Jan '15

20 Jan '15

Hello List, With ghc - 7.8 and 7.6 the following program is accepted: {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} class (UnF (F a) ~ a, Show a) => C a where type F a f :: F a -> a type family UnF a g :: forall a. C a => a -> String g _ = show a where a = f (undefined :: F a) -- :: a ghc-7.10.0.20141222 does not accept the program unless I uncomment the type signature (a :: a). I believe this is the main difference that prevents HList from compiling with 7.10, but I could have made a mistake in coming up with this minimal example. Regards, Adam

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Package version question with Cabal
by Volker Wysk 20 Jan '15

20 Jan '15

Hi! I've uploaded my library to Hackage, and now I'm trying to install it via cabal: ~/src/hsshellscript $ cabal install Resolving dependencies... In order, the following will be installed: hsshellscript-3.3.3 (reinstall) Warning: Note that reinstalls are always dangerous. Continuing anyway... Configuring hsshellscript-3.3.3... Building hsshellscript-3.3.3... Preprocessing library hsshellscript-3.3.3... In-place registering hsshellscript-3.3.3... Creating package registration file: /tmp/pkgConf-hsshellscript-3.37397.3 Installing library in /home/v/.cabal/lib/x86_64-linux-ghc-7.8.3/hsshellscript-3.3.3 Registering hsshellscript-3.3.3... Installed hsshellscript-3.3.3 ~/src/hsshellscript $ cabal list hsshellscript * hsshellscript Synopsis: Haskell for Unix shell scripting tasks Default available version: 3.3.2 Installed versions: 3.3.1, 3.3.2, 3.3.3 Homepage: http://www.volker-wysk.de/hsshellscript/ License: LGPL The thing wich looks like a problem is, that the new version isn't made the "Default available version". It is 3.3.3, but "Default available version" is still 3.3.2. Is this a problem? How do I register a new default version? Bye Volker

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Re: ANNOUNCE: GHC 7.10.1 Release Candidate 1 - questions on Mac OS platform
by George Colpitts 19 Jan '15

19 Jan '15

- Has anybody successfully used llvm on the Mac with 7.10.1 RC1? My problem is described below. - Which is the recommended gcc to use when building source? - GNU gcc 4.9.2 - Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn) - When using ghci with 7.10.1 RC1 I get the following errors intermittently. Is anybody else seeing these? - Too late for parseStaticFlags: call it before runGhc or runGhcT *** Exception: ExitFailure 1 - ld: library not found for -l:ghc31505_10.dylib collect2: error: ld returned 1 exit status phase `Linker' failed (exitcode = 1) Thanks On Fri, Jan 2, 2015 at 9:12 AM, George Colpitts <george.colpitts(a)gmail.com> wrote: > Only problem remaining is compiling with -fllvm and running resulting > executable > > . > .. > > > > - llvm , compiling with llvm (3.4.2) gives the following warnings: > - $ ghc -fllvm cubeFast.hs > [1 of 1] Compiling Main ( cubeFast.hs, cubeFast.o ) > clang: warning: argument unused during compilation: > '-fno-stack-protector' > clang: warning: argument unused during compilation: '-D > TABLES_NEXT_TO_CODE' > clang: warning: argument unused during compilation: '-I .' > clang: warning: argument unused during compilation: '-fno-common' > clang: warning: argument unused during compilation: '-U __PIC__' > clang: warning: argument unused during compilation: '-D __PIC__' > Linking cubeFast ... > - running the resulting executable crashes (compiling without > -fllvm gives no warnings and executable works properly) > - cat bigCube.txt | ./cubeFast > /dev/null > Segmentation fault: 11 > - Exception Type: EXC_BAD_ACCESS (SIGSEGV) > Exception Codes: KERN_INVALID_ADDRESS at 0xfffffffd5bfd8460 > > >> - ... >> >> Configuration details: >> >> >> - Mac OS 10.10.1 (Yosemite) >> - uname -a >> Darwin iMac27-5.local 14.0.0 Darwin Kernel Version 14.0.0: Fri Sep 19 >> 00:26:44 PDT 2014; root:xnu-2782.1.97~2/RELEASE_X86_64 x86_64 >> - llvm info: >> - opt --version >> LLVM (http://llvm.org/) >> LLVM version 3.4.2 >> Optimized build with assertions. >> Built Oct 31 2014 (23:14:30). >> Default target: x86_64-apple-darwin14.0.0 >> Host CPU: corei7 >> - gcc --version >> gcc (Homebrew gcc 4.9.1) 4.9.1 >> Copyright (C) 2014 Free Software Foundation, Inc. >> This is free software; see the source for copying conditions. There >> is NO >> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR >> PURPOSE. >> - /usr/bin/ghc --info >> [("Project name","The Glorious Glasgow Haskell Compilation System") >> ,("GCC extra via C opts"," -fwrapv") >> ,("C compiler command","/usr/bin/gcc") >> ,("C compiler flags"," -m64 -fno-stack-protector") >> ,("C compiler link flags"," -m64") >> ,("Haskell CPP command","/usr/bin/gcc") >> ,("Haskell CPP flags","-E -undef -traditional -Wno-invalid-pp-token >> -Wno-unicode -Wno-trigraphs") >> ,("ld command","/usr/bin/ld") >> ,("ld flags"," -arch x86_64") >> ,("ld supports compact unwind","YES") >> ,("ld supports build-id","NO") >> ,("ld supports filelist","YES") >> ,("ld is GNU ld","NO") >> ,("ar command","/usr/bin/ar") >> ,("ar flags","clqs") >> ,("ar supports at file","NO") >> ,("touch command","touch") >> ,("dllwrap command","/bin/false") >> ,("windres command","/bin/false") >> ,("libtool command","libtool") >> ,("perl command","/usr/bin/perl") >> ,("target os","OSDarwin") >> ,("target arch","ArchX86_64") >> ,("target word size","8") >> ,("target has GNU nonexec stack","False") >> ,("target has .ident directive","True") >> ,("target has subsections via symbols","True") >> ,("Unregisterised","NO") >> ,("LLVM llc command","llc") >> ,("LLVM opt command","opt") >> ,("Project version","7.8.3") >> ,("Booter version","7.6.3") >> ,("Stage","2") >> ,("Build platform","x86_64-apple-darwin") >> ,("Host platform","x86_64-apple-darwin") >> ,("Target platform","x86_64-apple-darwin") >> ,("Have interpreter","YES") >> ,("Object splitting supported","YES") >> ,("Have native code generator","YES") >> ,("Support SMP","YES") >> ,("Tables next to code","YES") >> ,("RTS ways","l debug thr thr_debug thr_l thr_p dyn debug_dyn >> thr_dyn thr_debug_dyn l_dyn thr_l_dyn") >> ,("Support dynamic-too","YES") >> ,("Support parallel --make","YES") >> ,("Dynamic by default","NO") >> ,("GHC Dynamic","YES") >> ,("Leading underscore","YES") >> ,("Debug on","False") >> >> ,("LibDir","/Library/Frameworks/GHC.framework/Versions/7.8.3-x86_64/usr/lib/ghc-7.8.3") >> ,("Global Package >> DB","/Library/Frameworks/GHC.framework/Versions/7.8.3-x86_64/usr/lib/ghc-7.8.3/package.conf.d") >> ] >> - Not sure I found the correct instructions for building from >> source, I used the following: >> - >> >> $ autoreconf >> $ ./configure >> $ make >> $ make install >> >> >> >> >> On Tue, Dec 23, 2014 at 10:36 AM, Austin Seipp <austin(a)well-typed.com> >> wrote: >> >>> We are pleased to announce the first release candidate for GHC 7.10.1: >>> >>> https://downloads.haskell.org/~ghc/7.10.1-rc1/ >>> >>> This includes the source tarball and bindists for 64bit/32bit Linux >>> and Windows. Binary builds for other platforms will be available >>> shortly. (CentOS 6.5 binaries are not available at this time like they >>> were for 7.8.x). These binaries and tarballs have an accompanying >>> SHA256SUMS file signed by my GPG key id (0x3B58D86F). >>> >>> We plan to make the 7.10.1 release sometime in February of 2015. We >>> expect another RC to occur during January of 2015. >>> >>> Please test as much as possible; bugs are much cheaper if we find them >>> before the release! >>> >>> -- >>> Regards, >>> >>> Austin Seipp, Haskell Consultant >>> Well-Typed LLP, http://www.well-typed.com/ >>> _______________________________________________ >>> ghc-devs mailing list >>> ghc-devs(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/ghc-devs >>> >> >> >

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Avoid blank line separating code and comments in Bird style?
by R Sidhu 14 Jan '15

14 Jan '15

Hi This regarding GHC behaviour for literate Haskell programs in Bird style. GHC expects a blank line between comment[1] and code. Otherwise, during the literate pre-processor stage, the error 'unlit: Program line next to comment' is reported. While above behaviour makes sense in general, there are situations where one would like to place comments adjacent to code with no intervening blank line. For example, in documenting a declaration using Haddock (in standard, non-literate Haskell), as shown in sections 3.1 <https://www.haskell.org/haddock/doc/html/markup.html#idm140354810917952> and 3.2 <http://www.haskell.org/haddock/doc/html/ch03s02.html> of the Haddock User Guide, no blank line separates comment and code. Any way to enable GHC to accept Bird style literate programs with no blank lines separating comment and code? [1] 'Comment' (in context of Bird style) refers to text on lines not beginning with '>'. Regards Sidhu

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Future of the boxes package--call for ideas
by David Feuer 13 Jan '15

13 Jan '15

I've just taken over maintainership of the boxes package, and will be making a maintenance release shortly (as soon as I figure out how and get added to the maintainers group). The package, however, currently suffers from a paucity of bug reports (no problem) and feature requests (not so great). To keep things lively, I need a bit of help from two groups of people. If you use the package but wish it could do something more for you, I want to know about it. If you considered using the package but rejected it because it couldn't quite handle your job, I want to know about that too. I'd prefer if people would open issues and pull requests at https://github.com/treeowl/boxes/issues but I will also accept requests by email. Thanks, David

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conflicting multi-parameter family instance declarations
by Michal Konečný 13 Jan '15

13 Jan '15

Dear all, The following compiles with ghc 7.6 but fails with ghc 7.8: ----- {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} module Test where class M t s where type T t s data I t = I t instance M t t where type (T t t) = () instance M t (I t) where type (T t (I t)) = () ----- The error I get with ghc 7.8.3 and 7.8.4 is: Test.hs:12:10: Conflicting family instance declarations: T t t -- Defined at Test.hs:12:10 T t (I t) -- Defined at Test.hs:15:10 I am curious if this change is an improvement or a bug. I would be grateful for help as this issue affects a fairly large library I develop. Best regards, Michal -- |-| Dr. Michal Konecny, Computer Science, Aston University |-| Room MB212D | Tel +44 121 204 3462 | Fax +44 121 204 3681 |-| http://duck.aston.ac.uk/konecnym |-| OpenPGP key http://duck.aston.ac.uk/konecnym/ki.aston

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ANNOUNCE: GHC version 7.8.4
by Austin Seipp 12 Jan '15

12 Jan '15

============================================================== The (Interactive) Glasgow Haskell Compiler -- version 7.8.4 ============================================================== The GHC Team is pleased to announce a new patchlevel release of GHC, 7.8.4. This is an important bugfix release relative to 7.8.3 (with over 30 defects fixed), so we highly recommend upgrading from the previous 7.8 releases. The full release notes are here: https://downloads.haskell.org/~ghc/7.8.4/docs/html/users_guide/release-7-8-… How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: https://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://ghc.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://ghc.haskell.org/trac/ghc/wiki/Platforms http://ghc.haskell.org/trac/ghc/wiki/CodeOwners 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://ghc.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://ghc.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 Hashes & Signatures ~~~~~~~~~~~~~~~~~ On https://downloads.haskell.org/~ghc/7.8.4/ you will find a signed copy of the SHA256 hashes for the tarballs, using my GPG key (keyid 0x3B58D86F). -- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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Equality Constraints (a ~ b)
by Dominic Steinitz 11 Jan '15

11 Jan '15

Hi, I am trying to find more background on these. They don’t exist in the Haskell 2010 Language Report, they didn’t exist in ghc 6.8.2 but make an appearance in 7.0.1. The documentation in the manual is rather sparse and doesn’t contain a reference: https://downloads.haskell.org/~ghc/latest/docs/users_guide.pdf section 7.11. Folk on #ghc referred me to http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/. I can find papers that refer to ~ in F_C (aka FC?) but as far as I can tell not in the Haskell language itself. Many thanks Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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