- Libraries - Haskell.org

Re: Small problem in containers-0.5.0.0 docs
by Johan Tibell 01 Aug '12

01 Aug '12

Fixed. Thanks for the bug report! In the future you can file reports at https://github.com/haskell/containers/issues to make sure I don't forget about them! :) On Wed, Aug 1, 2012 at 6:30 AM, Christoph Breitkopf <chbreitkopf(a)gmail.com> wrote: > Hello, > > the haddoc sources refer to the strict/lazy modules using single quotes, > which does not work, as is is interpreted as a type reference > (.../Data-Map.html#t:Strict). Replacing the single quotes by double quotes > for a module reference fixes this. (I just had the same problem in > IntervalMap). > > I spotted the problem in Data.Map, but it's probably also present in other > modules. > > Regards, > > Chris

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Re: safe vs. unsafe (Was: Haskell Platform proposal: Add the vector package)
by Simon Marlow 30 Jul '12

30 Jul '12

On 13/07/2012 01:31, Bardur Arantsson wrote: > On 07/12/2012 11:16 PM, Simon Marlow wrote: >> Hi Thomas, >> >> All these questions are answered by the Haskell Symposium paper, which >> we'll post very shortly. > > THANK YOU! This discussion has been ridiculuously abstract and confusing > without any real (canonical) definition of the terms that are being > bandied about. Here it is: http://community.haskell.org/~simonmar/papers/safe-haskell.pdf Cheers, Simon

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Proposal: Add the missing instances for Traversable (Either b) and Traversable ((,) b)
by Edward Kmett 29 Jul '12

29 Jul '12

The following instances are missing and have only one sensible definition. I've been bitten by their lack repeatedly and there is no place outside of base that they can live without needlessly being orphaned. I would like to propose adding the following instances to Data.Foldable and Data.Traversable. instance Foldable (Either e) where foldMap f (Right m) = f m foldMap _ (Left _) = mempty instance Traversable (Either e) where traverse _ (Left e) = pure (Left e) traverse f (Right x) = Right <$> f x instance Foldable ((,) e) where foldMap f (_,x) = f x instance Traversable ((,) e) where traverse f (e ,x) = (,) e <$> f x I had thought honestly thought we'd already added them long ago. Discussion Period: 2 Weeks -Edward Kmett On Tue, Jan 3, 2012 at 7:50 PM, Ben Millwood <haskell(a)benmachine.co.uk>wrote: > Yeah, I noticed this the other day, and a couple of other instances > which aren't defined, but could be: > > http://hpaste.org/56005 > > I won't pretend I have much use for the Const instance, but it seems > like it should be there anyway, just for completeness (I think it's > possible to define Traversable instances for compositions of functors, > possibly sums and products as well, so Const could be useful in that > context). I think the Either one makes sense, though, as a natural > analogue of the Maybe instance. > > On Tue, Jan 3, 2012 at 11:41 PM, Conal Elliott <conal(a)conal.net> wrote: > > Thanks much for the answers, Conor. They all make sense to me, > particularly > > about the typical information discarding in Foldable. > > > > Regards, - Conal > > > > > > On Tue, Jan 3, 2012 at 3:30 PM, Conor McBride < > conor(a)strictlypositive.org> > > wrote: > >> > >> > >> On 3 Jan 2012, at 23:12, Conal Elliott wrote: > >> > >>> I wanted a Traversable instance for pairing, so I defined one: > >>> > instance Traversable ((,) o) where > >>> > sequenceA (o,fa) = (o,) <$> fa > >> > >> > >> That looks right. Of course, we should really have a BiTraversable > >> class of which (,) is an instance. > >> > >> > >>> > >>> However, Foldable is a superclass of Traversable, so I get an error > >>> message: > >>> > >>> Could not deduce (Foldable ((,) o)) from the context () > >>> arising from the superclasses of an instance declaration > >>> > >>> The best I've thought of is the following: > >>> > >>> > instance Foldable ((,) o) where > >>> > fold (_,m) = m > >> > >> > >> The best (upto efficiency considerations) is always > >> > >> > >> instance Foldable ((,) o) where > >> foldMap = foldMapDefault > >> > >> which amounts to what you chose. > >> > >> SHE makes this a default superclass instance. > >> > >> > >>> However, I don't like how it discards information. > >> > >> > >> But these folds always do discard information, discarding the shape > >> information and accumulating over the contents. For ((,) o), seen as > >> a functor, the first component is shape information and the second is > >> the content. > >> > >> > >>> Some questions: > >>> > >>> * Why is Foldable a superclass of Traversable? > >> > >> > >> Because the constant-monoid Applicative makes every Traversable > >> Foldable in a uniform way. > >> > >> > >>> * Is there a good choice of a Foldable instance of ((,) o)? > >> > >> > >> Yes, the one you chose. > >> > >> > >>> * Are there any other problems with the Traversable instance above > >>> (besides foldability)? > >> > >> > >> Nope. It's the Traversable instance which picks out exactly the > >> contents that correspond to the elements abstracted by the Functor. > >> > >> All the best > >> > >> Conor > >> > > > > > > _______________________________________________ > > Libraries mailing list > > Libraries(a)haskell.org > > http://www.haskell.org/mailman/listinfo/libraries > > > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries >

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Proposal: Add hasBitSize to Data.Bits.Bits
by Edward Kmett 27 Jul '12

27 Jul '12

There is currently no way to know whether or not calling Data.Bits.bitSizewill crash your program. I propose extending the Bits class to include: hasBitSize :: Bits b => b -> Bool such that it returns False for Integer and True for the other instances since the vast majority of instances are finite, it may be reasonable to set the default definition to hasBitSize _ = False Discussion Period: 2 weeks

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Haskell platform proposal: split package
by Brent Yorgey 25 Jul '12

25 Jul '12

Hello everyone, This is a proposal for the split package [1] to be included in the next major release of the Haskell platform. Everyone is invited to review this proposal, following the standard procedure [2] for proposing and reviewing packages. Review comments should be sent to the libraries mailing list by August 20 (arbitrarily chosen; there's plenty of time before the October 1 deadline [3]). The Haskell Platform wiki will be kept up-to-date with the results of the review process: http://trac.haskell.org/haskell-platform/wiki/Proposals/split [1] http://hackage.haskell.org/package/split [2] http://trac.haskell.org/haskell-platform/wiki/AddingPackages [3] http://trac.haskell.org/haskell-platform/wiki/ReleaseTimetable Credits ======= Proposal author and package maintainer: Brent Yorgey <byorgey at cis.upenn.edu> Abstract ======== The Data.List.Split module contains a wide range of strategies for splitting lists with respect to some sort of delimiter, mostly implemented through a unified combinator interface. The goal is to be a flexible yet simple alternative to the standard 'split' function found in some other mainstream languages. Documentation and tarball from the hackage page: http://hackage.haskell.org/package/split Development repo: darcs get http://code.haskell.org/~byorgey/code/split Rationale ========= Splitting a list into chunks based on some sort of delimiter(s) is a common need, and is provided in the standard libraries of several mainstream languages (e.g. Python [4], Ruby [5], Java [6]). Haskell beginners routinely ask whether such a function exists in the standard libraries. For a long time, the answer was no. Adding such a function to Haskell's standard libraries has been proposed multiple times over the years, but consensus was never reached on the design of such a function. (See, e.g. [7, 8, 9].) [4] http://docs.python.org/py3k/library/stdtypes.html?highlight=split#str.split [5] http://www.ruby-doc.org/core-1.9.3/String.html#method-i-split [6] http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#split(java.l… [7] http://www.haskell.org/pipermail/libraries/2006-July/005504.html [8] http://www.haskell.org/pipermail/libraries/2006-October/006072.html [9] http://www.haskell.org/pipermail/libraries/2008-January/008922.html In December 2008 the split package was released, implementing not just a single split method, but a wide range of splitting strategies. Since then the split package has gained wide acceptance, with almost 95 reverse dependencies [10], putting it in the top 40 for number of reverse dependencies on Hackage. [10] http://packdeps.haskellers.com/reverse/split The package is quite stable. Since the 0.1.4 release in April 2011 only very minor updates have been made. It has a large suite of QuickCheck properties [11]; to my recollection no bugs have ever been reported. [11] http://code.haskell.org/~byorgey/code/split/Properties.hs API === For a detailed description of the package API and example usage, see the Haddock documentation: http://hackage.haskell.org/packages/archive/split/0.1.4.3/doc/html/Data-Lis… Design decisions ================ Most of the library is based around a (rather simple) combinator interface. Combinators are used to build up configuration records (recording options such as whether to keep delimiters, whether to keep blank segments, etc). A configuration record is finally handed off to a function which performs a generic maximally-information-preserving splitting algorithm and then does various postprocessing steps (based on the configuration) to selectively throw information away. It is probably not the fastest way to implement these methods, but speed is explicitly not a design goal: the aim is to provide a reasonably wide range of splitting strategies which can be used simply. Blazing speed (or more complex processing), when needed, can be obtained from a proper parsing package. Open issues =========== Use of GHC.Exts --------------- At the request of a user, the 0.1.4.3 release switched from defining its own version of the standard 'build' function, to importing it from GHC.Exts. This allows GHC to do more optimization, resulting in reported speedups to uses of splitEvery, splitPlaces, and splitPlacesBlanks. However, this makes the library GHC-specific. If any reviewers think this is an issue I would be willing to go back to defining build by hand, or use CPP macros to select between build implementations based on the compiler. Missing strategies ------------------ The specific way that the generic splitting algorithm is implemented does preclude some imaginable splitting strategies. For example, a few years ago I tried adding a strategy that used a predicate on pairs of elements, splitting down the middle of any pairs that satisfy the predicate, but gave up because it simply did not fit.

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Data.ByteString.packCStringLen for Ptr Word8
by Henning Thielemann 25 Jul '12

25 Jul '12

I want to copy a piece of memory addressed by a Ptr Word8 into a ByteString. I only found packCStringLen for that purpose but it expects a Ptr CChar as input. Since ByteString represents a sequence of Word8 I thought that the most natural input for packCStringLen should be (Ptr Word8). Of course I can use castPtr but that always looks like a hack.

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split package HP proposal, round 2
by Brent Yorgey 23 Jul '12

23 Jul '12

Hello all, Thanks for the great feedback so far on the split package. I have updated the wiki page with the results of the discussion so far: http://trac.haskell.org/haskell-platform/wiki/Proposals/split In particular, I have made the following changes to the package as a result of the feedback: * Removed the GADTs extension; the package is now fully Haskell 2010. * Marked synonyms as deprecated, and excluded them from the Haddock output. Thus new users will not be confused by the synonyms anymore, and existing users will be notified that they are now deprecated, but existing code will not break since the synonyms are still exported from the package. * Many other minor cleanups and improvements to documentation etc. More details are on the wiki page. These changes are in the darcs repository but I have not released a new version to Hackage yet. * I am also looking into the possibility of using conditionals in the .cabal file to support both GHC and other compilers, by using GHC's primitive 'build' function only when GHC is the compiler, and otherwise using a generic one. More feedback on this is appreciated. Is it worth the effort/complication? Please continue responding to this thread (or the previous one) with additional feedback, upvotes, downvotes, etc. -Brent

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Re: safe vs. unsafe (Was: Haskell Platform proposal: Add the vector package)
by Simon Marlow 20 Jul '12

20 Jul '12

On 13/07/12 21:18, Heinrich Apfelmus wrote: > Simon Marlow wrote: >> Hi Thomas, >> >> All these questions are answered by the Haskell Symposium paper, which >> we'll post very shortly. FYI, the FFI is mostly safe, as long as you >> declare foreign imports to have an IO result type (otherwise it's >> unsafePerformIO, and hence unsafe). Unsafety is not viral: as soon as >> you have a safe API, you can declare its implementation to be >> Trustworthy, and then it is usable from safe code. > > How strict are the requirements for Trustworthy code? For instance, my > reactive-banana library uses observable sharing, which is inherently > Unsafe. Of course, I think that my library is still Safe, but I have no > formal proof of this "fact". I have two options: > > 1. Mark my library as Trustworthy even though I don't have sufficient > proof. This severely weakens the guarantees of Safe Haskell. > 2. Mark my library as Unsafe. But then people can't use it to write Safe > code and will complain. > > The trouble is that I have a strong incentive to solve the problem > arising from 2 by doing 1. Oops. The idea is that you do (1). All your clients get to use Safe, and nobody is obliged to use your code in a security-critical setting unless they want to. (I could write a lot more, but I've written too much about this already today. Good night!) Cheers, Simon

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Proposal: add an order-reversing newtype to Data.Ord
by ARJANEN Loïc Jean David 18 Jul '12

18 Jul '12

Hello all, I recently noticed that despite conversations about that in this mailing-list, there isn't an order-reversing newtype in Data.Ord. That newtype would be quite useful, e.g. for reverse-sorting a list or a set, so I propose we add a newtype defined thusly: newtype Reverse a = Reverse { fromReverse :: a } deriving (Eq) instance Ord a => Ord (Reverse a) where compare (Reverse x) (Reverse y) = compare y x I took the implementation from the Down newtype in GHC.Exts, so we could perhaps just re-export it. This will cause breakage to code already defining a type named Reverse, so I don't know if the damage will be important. Of course, the name and definition are open to discussion. Deadline: 12 July 2012. P.S. : A conversation about that took place here: http://haskell.1045720.n5.nabble.com/A-Down-newtype-in-Data-Ord-td5005373.h… -- ARJANEN Loïc Jean David http://blog.luigiscorner.com --- "Computer science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about beakers and test tubes. Science is not about tools. It is about how we use them, and what we find out when we do." Michael R. Fellows and Ian Parberry

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Re: safe vs. unsafe (Was: Haskell Platform proposal: Add the vector package)
by John Lato 15 Jul '12

15 Jul '12

> On Sat, Jul 14, 2012 at 3:16 AM, Henning Thielemann < > lemming(a)henning-thielemann.de> wrote: > >> On Fri, 13 Jul 2012, Brandon Allbery wrote: >> >> And now I'm having a "so what's the point?" moment? All this effort so >>> we can just mark random stuff as >>> Trusted anyway? >>> >> >> Today we have 'unsafePerformIO'. So if we praise the merits of Haskell's >> strong type system and then mention 'unsafePerformIO' the audience will ask >> "so what's the point of type safety then?" Well, the point is that >> unsafePerformIO is strongly discouraged and every use of it should be >> considered carefully. >> > > We've just been told *not* to consider carefully for purposes of marking a > module as Trustworthy; an argument based on considering carefully is not > relevant. I think the idea here is that, as a package author, it's okay to just throw Trustworthy anywhere ghc can't infer Safe on its own. As a package user, if you care about Safe Haskell, it's your responsibility to audit/consider carefully to determine if Trustworthy code is something you're willing to include in your codebase. John L.

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