News aggregator

Error when trying to use Hashable

haskell-cafe - Mon, 05/12/2014 - 1:58am
Hi Haskell Cafe, I have some code that compiles when running on GHC on OS X, but not on Ubuntu: No instance for (hashable- (GHC.Generics.Rep Point)) arising from a use of `hashable-$gdmhashWithSalt' Possible fix: add an instance declaration for (hashable- (GHC.Generics.Rep Point)) In the expression: (hashable-$gdmhashWithSalt) In an equation for `hashWithSalt': hashWithSalt = (hashable-$gdmhashWithSalt) In the instance declaration for `Hashable Point' Anyone know what's happening here? Cheers, -John
Categories: Offsite Discussion

Installation of package text failing installation onghc 7.6.3

haskell-cafe - Sun, 05/11/2014 - 9:46pm
I just downloaded the latest Haskell platform (I realize that ghc might not be up-to-date in this) for my macbook. I installed it, which appeared to go without problems. However, when I tried to update the package text, I get the errors below. I suspect that the first error (not recognizing ' in a comment) cascades through the rest. How do you suggest fixing this (other than getting Bryan do drop the apostrophe :-))? Victor Configuring text- Building text- Preprocessing library text- Data/Text.hs:9:52: warning: missing terminating ' character [-Winvalid-pp-token]
Categories: Offsite Discussion

ANNOUNCE: jhc-0.8.1

General haskell list - Sun, 05/11/2014 - 9:20pm
After a hiatus, jhc 0.8.1 is released. - New license, jhc is now released under a permissive BSD style licence rather than the GPL. The license is compatible with that of ghc allowing code mixing between them. - New library layout based around the standards, there are now haskell98 and haskell2010 packages that are guarenteed to be future proof strictly compatible with the respective standards. A package haskell-extras contains the additonal libraries from ghc's base. - Native support for complex and vector SIMD primitives, exposed via type functions. for instance 'foo :: Complex_ Float32_' for hardware accelerated complex 32 bit floats for instance. These are unboxed only for now, full library Num support in the works. - support for android as a target, you must install the android NDK to use this. - Support for embedded ARM architectures imported from Kiwamu Okabe's branch allowing targeting bare hardware with no OS. - user defined kinds, introduced
Categories: Incoming News

Why doesn't cabal compile once and manage binary packages?

Haskell on Reddit - Sun, 05/11/2014 - 5:19pm

Someone was asking about cabal on #haskell just earlier tonight, specifically what it does do and what it does not do, and a discussion ensued. I commented that once I understood why sandboxes were needed at all, I had somewhat of an epiphany in the sense that now I cannot begin to fathom why cabal was implemented the way it was to begin with.

That is to say, as cabal is a tool that is (with caveats) used for downloading libraries to use in development of software, it makes practically 0 sense for it to install packages to a global repository, when the very nature of software development means that you might need to be working on different programs that require different versions of the same package.

One guy in IRC(timthelion) pointed out that there had been a project called hellno( that manages cabal packages by only building them once and then copying binary packages of the correct versions around as needed.

So, I was wondering: why doesn't cabal do this? Is there an effort to do something like this, etc? I ask here because it wouldn't surprise me there are some parts of the picture I might be missing completely, as I don't really know much about the intricacies of package management. I know that with sandboxes, we basically get the same thing, but we have to build the packages from scratch. But IMO, that is more than a minor inconvenience, considering the dependency graphs of some packages.

submitted by IceDane
[link] [22 comments]
Categories: Incoming News

cabal install lens fails

haskell-cafe - Sun, 05/11/2014 - 3:29pm
Hi Haskell Cafe, Anyone know about this error? Installing library in /Users/johnky/Library/Haskell/ghc-7.6.3/lib/reflection-1.4/lib Registering reflection-1.4... Installed reflection-1.4 Configuring mtl- Building mtl- Preprocessing library mtl- [ 1 of 21] Compiling Control.Monad.Writer.Class ( Control/Monad/Writer/Class.hs, dist/build/Control/Monad/Writer/Class.o ) [ 2 of 21] Compiling Control.Monad.State.Class ( Control/Monad/State/Class.hs, dist/build/Control/Monad/State/Class.o ) [ 3 of 21] Compiling Control.Monad.Reader.Class ( Control/Monad/Reader/Class.hs, dist/build/Control/Monad/Reader/Class.o ) [ 4 of 21] Compiling Control.Monad.RWS.Class ( Control/Monad/RWS/Class.hs, dist/build/Control/Monad/RWS/Class.o ) [ 5 of 21] Compiling Control.Monad.Identity ( Control/Monad/Identity.hs, dist/build/Control/Monad/Identity.o ) [ 6 of 21] Compiling Control.Monad.Error.Class ( Control/Monad/Error/Class.hs, dist/build/Control/Monad/Error/Class.o ) Control/Monad/Error/Class.hs:93:18
Categories: Offsite Discussion

Dominic Steinitz: Hölder’s and Minkowski’s Inequalities

Planet Haskell - Sun, 05/11/2014 - 2:45pm

I have seen Hölder’s inequality and Minkowski’s inequality proved in several ways but this seems the most perspicuous (to me at any rate).

Young’s Inequality

If and such that


A and satisfying the premise are known as conjugate indices.


Since is convex we have

Substituting in appropriate values gives


Now take exponents.

Hölders’s Inequality

Let and be conjugate indices with and let and then and


By Young’s inequality

By applying a counting measure to we also obtain

Minkowski’s Inequality


By Hölder’s inequality


and is finite since is a vector space.

Categories: Offsite Blogs

Solutions with respect to conditional compilation, CPP, K&R, ANSI, and clang?

Haskell on Reddit - Sun, 05/11/2014 - 2:02pm

Goal: Maintainable cross-platform code that is compatible with multiple versions of its dependencies.

Conventional Haskell solution: Use CPP in traditional (aka K&R style) mode to preprocess Haskell files to adapt to platform, environment, and dependency specifics. Traditional CPP is used so that CPP doesn't make too many assumptions about C syntax. The cabal user guide gives CPP examples further giving the impression that this is the preferred and standard approach.

Problem: clang authors prefer ANSI CPP and, reluctantly, only support some bits of traditional CPP. This is creating problems on OSX Mavericks where clang is the default and clang's CPP rejects some Haskell sources that previously worked with gcc's CPP. As several people have pointed out to me, it is a hack to use CPP for Haskell in the first place. Our hack is bad and we should feel bad :)

Question: What is a solid and practical alternative to CPP? I want a solution that is solid and known good. I'm optimistically looking for suggestions that have worked over several ghc releases and on OSX, Windows, and Linux.

Below is a list of alternatives that I'm vaguely aware of. I haven't actually tried them yet and so I may have some of the details wrong. Suggestions, corrections, experience reports, and pros&cons lists are all greatly appreciated:

  • Supply a custom preprocessor with each package that needs CPP support and feed that to ghc. This could work as long as the preprocessor is fairly general. cpphs been around for a while, but when lens tried to use it they hit some rough edges.
  • Put cpp-options: -traditional in the cabal file: lens-4.1.1 and newer uses this, but it may not do anything? clang's cpp --help lists -traditional-cpp but not -traditional. Does it accept both for compatibility with gcc? Also, I checked ghc-7.6.1 and ghc-7.6.3 (linux and OSX Mavericks, respectively) and both are passing -traditional to CPP already.
  • Continue (ab)using CPP and:
    • test each release with clang's CPP. This could get messy as it would likely require isolating CPP bits into standalone modules with very controlled used of syntax to reduce the risk of CPP hitting a syntax error.
    • require gcc's CPP. I believe this is the current direction that ghc is moving in. It requires that OSX installs of ghc are a bit more involved. It also has a tendency to generate more bug reports as anyone who uses the wrong CPP runs the risk of thinking a particular package is broken.

Thank you for your time!


I understand my immediate CPP issue a bit better now. Traditional CPP doesn't understand # and ## (the stringification and token concatenation operations defined in ANSI CPP). Furthermore, gcc's CPP strips out comments and spaces as it goes. So you can get token concatenation this way:

#define C(a,b) a/**/b

C(Foo,Bar) would become the token FooBar. clang on the other hand, tokenizes a and b while treating the comment as a token separator. So it generates Foo Bar.

I rewrote the macro to work with ANSI CPP, but now I can't get ghc to invoke CPP without -traditional. I tried adding -optP-ansi but it gets ignored because -traditional is also on the command line. When I use -pgmP cpp, it fails because clang's CPP doesn't use the same command line options as gcc's CPP.

I can get rid of -traditional by using -pgmP gcc -optP-E -optP-ansi, but that is wrong because it may not be the same gcc that ghc is using.

Update 2:

I rewrote the macros to work with both traditional and ansi CPP. Not my favorite solution. It means more boilerplate.

submitted by dagit
[link] [15 comments]
Categories: Incoming News

Code Review please? Things aren't working as I expect.

Haskell on Reddit - Sun, 05/11/2014 - 2:00pm

I wrote a simple server that I could connect to over telnet to run some basic commands, but things are breaking and throwing exceptions and it's not telling me why.

submitted by all_you_need_to_know
[link] [3 comments]
Categories: Incoming News

Quickcheck2 vs Smallcheck vs Smartcheck, etc

Haskell on Reddit - Sun, 05/11/2014 - 9:51am

I am learning how to do testing in Haskell. What quickcheck/smallcheck style framework do you guys recommend I use? I'm having trouble understanding the differences. Many of the pages I've seen that make comparisons talk about Quickcheck, but they dont establish whether they are referring to Quickcheck1 or Quickcheck2.

submitted by cessationoftime
[link] [9 comments]
Categories: Incoming News

ANN: th-alpha

haskell-cafe - Sun, 05/11/2014 - 12:55am
Hello Cafe, I just uploaded th-alpha to hackage [1]. This package primarily provides one function, 'areExpAEq', which compares expressions for alpha equivalence (equivalence modulo renaming of bound variables): True Pretty unambitious, but I found myself needing this when testing another library that uses Template Haskell, and it might be useful to others in that same situation. Cheers, Julian [1] _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

Deadline extended: Workshop on Generic Programming (WGP)2014

General haskell list - Sat, 05/10/2014 - 7:03pm
Hello all, Apologies for multiple postings. We've extended the submission deadline for this year's Workshop on Generic Programming until coming Thursday (the 15th of May). This year we also welcome short papers with case studies, tool demos, and generic pearls up to six pages; it's not too late to consider submitting! The call for papers is attached; you'll find more information on the workshop's website: All the best, Pedro ====================================================================== CALL FOR PAPERS WGP 2014 10th ACM SIGPLAN Workshop on Generic Programming Gothenburg, Sweden Sunday, August 31, 2014 Co-located with the International Conference on Functional Programming (ICFP 2014) ====================================================================
Categories: Incoming News

Using mutable array after an unsafeFreezeArray, and GC details

glasgow-user - Fri, 05/09/2014 - 7:21pm
A couple of updates: Edward Yang responded here, confirming the sort of track I was thinking on: And I can report that: 1) cloning a frozen array doesn't provide the benefits of creating a new array and freezing 2) and anyway, I'm seeing some segfaults when cloning, freezing, reading then writing in my library I'd love to learn if there are any other approaches I might take, e.g. maybe with my own CMM primop variants? Thanks, Brandon
Categories: Offsite Discussion

Using mutable array after an unsafeFreezeArray, and GC details

glasgow-user - Fri, 05/09/2014 - 12:18am
I have an unusual application with some unusual performance problems and I'm trying to understand how I might use unsafeFreezeArray to help me, as well as understand in detail what's going on with boxed mutable arrays and GC. I'm using the interface from 'primitive' below. First some basic questions, then a bit more background 1) What happens when I do `newArray s x >>= \a-> unsafeFreezeArray a 2) And what if a do a `cloneMutableArray` on `a` and likewise use the resulting array? Background: I've been looking into an issue [1] in a library in which as more mutable arrays are allocated, GC dominates (I think I verified this?) and all code gets slower in proportion to the number of mutable arrays that are hanging around. I've been trying to understand how this is working internally. I don't quite understand how the "remembered set" works with respect to MutableArray. As best I understand: the remembered set in generation G points to certain objects in older generations, which objects hold references to ob
Categories: Offsite Discussion

New gtk2hs 0.12.4 release

gtk2hs - Wed, 11/21/2012 - 12:56pm

Thanks to John Lato and Duncan Coutts for the latest bugfix release! The latest packages should be buildable on GHC 7.6, and the cairo package should behave a bit nicer in ghci on Windows. Thanks to all!


Categories: Incoming News