- Libraries - Haskell.org

bug in printf, and patch
by Evan Laforge 28 Nov '12

28 Nov '12

Text.Printf.printf has a bug where it doesn't like negative numbers for * modifiers. I opened a ticket along with a patch at http://hackage.haskell.org/trac/ghc/ticket/7457 I feel like it shouldn't need a discussion since it's just a bug fix, but if someone wants to speak up go ahead. It will change behaviour for people using negative numbers, but since the previous behaviour was "doesn't work" I hope they won't mind :)

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Proposal: Add Eq instance for Control.Exception.ErrorCall
by Simon Hengel 28 Nov '12

28 Nov '12

Hi, I propose to add an Eq instance for ErrorCall. The main motivation is to make it more convenient to construct predicates that select specific exceptions. My current use case is testing for expected exceptions. In Hspec[1] we use predicates for that, e.g.: evaluate (head []) `shouldThrow` (== ErrorCall "Prelude.head: empty list") But I think it can be useful in other situations and it is consistent with the fact that other common exception types have an Eq instance (e.g. ArithException, IOException, ExitCode). Discussion period: 3 Weeks Cheers, Simon [1] http://hspec.github.com/

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Cabal and GHC
by Simon Peyton-Jones 28 Nov '12

28 Nov '12

Michael Snoyman wrote a blog post about Solving Cabal Hall, which came across via the peerless Haskell Weekly News. http://www.yesodweb.com/blog/2012/11/solving-cabal-hell But I'd be unlikely to return there so I'm posting this to the libraries and cabal-devel lists. I'm not deep in Cabal lore, but it seems to me that there is a fairly easy way to do a Lot Better than we are now. Let's go back to "Identifying the Problem". Michael doesn't say, but I'm guessing that Fay depends on yesod-platform. Very well, so Cabal sees I'm installing Fay, which depends on yesod-platform, which dpends on data-default and depends directly on data-default If the package database was empty, Cabal would try to figure out a version of data-default that is acceptable to both yesod-platform and to Fay. And that's what we want! Suppose that Cabal figures out that yesod-platform-2.7 and data-default-0,4 would work. Then, in an empty package database, it could just go ahead and install those. But in your example, the database isn't empty; we have already isntalled yesod-platform-2.7, dependin gon data-default-0.5. And (here's the rub) you can only have yesod-platform-2.7 installed once. One solution might be 1. *un* install yesod-platform-2.7 (depending on data-default-0.5) and 2. *re* install it (depending on data-default-0.4). But that would break any programs that use both yesod-platform-2.7 and data-default-0.5. The solution is obvious: we should make it possible to instally yesod-platform-2.7 twice, once version depending on data-default-0.4 once version depending on data-default-0.5. Now, if Cabal can figure out a plan based on an empty database, it can deliver on that plan even in a non-empty database, without messing up any existing installations. Of course, if Fay depends exclusively on data-default-0.4, and yesod-platform depends exclusively on data-default-0.5, then they genuinely are incompatible, and Cabal can and should say so. That's not Cabal Hell. That's just saying that the package authors have *specified* that they are incompatible. Summary. I may be way off beam here, but I think we can easily make things way better than they are. By "easily" I mean with just design and implementation work. That's not free, and no one has any time.... but it's well within reach. I'd even been wondering about trying to crowd-fund this development so that Well Typed can do it. But let's first see if it'd solve the problem. There must be a wiki page somewhere that articulates the challenge, points to blog posts about it, and discusses solutions. Simon

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portable encoding/decoding without going via a handle
by Ganesh Sittampalam 28 Nov '12

28 Nov '12

Hi, I need to convert directly between different string encodings, rather than just using a particular encoding when reading from/writing to a Handle. I'm aware of the following options, but they have a few problems: - text-icu: not easily usable on Windows as it requires libicu - text: just handles utf8/16/32 - iconv: POSIX only It seems like GHC's TextEncoding has the necessary low-level functionality (http://hackage.haskell.org/packages/archive/base/latest/doc/html/GHC-IO-Enc…) but I can't find any high-level interface for directly transcoding between String/Bytestring/Text. Am I missing something, or would this be a useful addition as a separate library? Cheers, Ganesh

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Non-monotonic releases and package clobbering
by Edward Z. Yang 27 Nov '12

27 Nov '12

Hello all, I recently noticed that there is a subset of packages on Hackage which have had a non-monotonic release in their history. Non-monotonic releases occur for various reasons; for example, http://hackage.haskell.org/package/QuickCheck-1.2.0.1 was made in 2010 (2.1 was released in 2008), ostensibly as a minor bugfix release. However, I found at least one case where a non-monotonic release was due to package clobbering: http://hackage.haskell.org/package/Adaptive-0.1 http://hackage.haskell.org/package/Adaptive-0.22 It's easy to see that these are two different packages. 0.1 is the "later" release, but it is completely shadowed by the old 0.22 release. I'm not sure if I have any concrete recommendations (since this problem is mostly subsumed by the "people shouldn't be able to upload arbitrary stuff to Hackage problem); but I thought this might be interesting information to someone. Edward

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Changes to Typeable
by Simon Peyton-Jones 26 Nov '12

26 Nov '12

Friends (Ian pointed out that I should have sent this to the libraries list): The page describes an improved implementation of the Typeable class, making use of polymorphic kinds. Technically it is straightforward, but it represents a non-backward-compatible change to a widely used library, so we need to make a plan for the transition. http://hackage.haskell.org/trac/ghc/wiki/GhcKinds/PolyTypeable Comments? You can fix typos or add issues directly in the wiki page, or discuss by email Simon

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Re: [Haskell] ANN: time-1.4.0.2
by José Pedro Magalhães 26 Nov '12

26 Nov '12

Hi Ashley, Sorry to repeat this question, but is there any particular reason why you give handwritten instances of Typeable, instead of deriving them? Thanks, Pedro On Sun, Nov 25, 2012 at 12:14 PM, Ashley Yakeley <ashley(a)semantic.org>wrote: > time-1.4.0.2 is now on Hackage. > > 1.4.0.2 fixes bugs and infelicities in the formatting and parsing in > Data.Time.Format > > For formatting: > > * %0Y, %_Y, %0G and %_G now pad to four characters. > > * Likewise %0C, %_C, %0f and %_f pad to two characters. > > For parsing: > > * Runs of n whitespace characters now match n or more whitespace > characters. Formerly it matched zero or more whitespace characters. > > * %Y, %G, %C and %f match one or more digits > > * %0Y and %0G match exactly four digits. %0C and %0f match exactly two > digits > > General: > > * Now uses mkTyCon3 instead of deprecated mkTyCon (contributed by Chris > Moline) > > Testing: > > * Various infrastructure improvements (Chris Moline and me) > > * QuickCheck properties now tested > > I tried using Cabal's "detailed" test-suite, but it doesn't play well with > QuickCheck and is very slow with large numbers of small test cases. I'm now > making use of the test-framework package, which does play well with > QuickCheck. > > -- Ashley Yakeley > > > ______________________________**_________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/**mailman/listinfo/haskell<http://www.haskell.org/mailman/listinfo/haskell> >

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can't understand the Cabal manual. flag, manual
by Roman Beslik 24 Nov '12

24 Nov '12

Hello. http://www.haskell.org/cabal/users-guide/developing-packages.html#package-d… {{{ manual: boolean (default: False) By default, Cabal will first try to satisfy dependencies with the default flag value and then, if that is not possible, with the negated value. However, if the flag is manual, then the default value (which can be overridden by commandline flags) will be used. }}} Can I interpret this as {{{ First, Cabal tries to satisfy dependencies with the default flag value. Second, if the flag is manual, Cabal tries to satisfy dependencies with the negated flag value. }}} ? {{{ This file should contain of a number global property descriptions and several sections. }}} Maybe it's my English, but I can only think of "descriptions of global properties of a number" which does not make sense.

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Adding swap/compose functions to Data.Tuple and Data.Function
by John Wiegley 23 Nov '12

23 Nov '12

Moving discussion from http://hackage.haskell.org/trac/ghc/ticket/7435: I propose adding the following three functions to Data.Tuple and Data.Function, respectively: swap :: (a,b) -> (b,a) swap = snd &&& fst Fairly obvious, but strangely missing. compose :: [(a -> a)] -> a -> a compose = foldr (.) id composeM :: [(a -> m a)] -> a -> m a composeM = foldr (<=<) return This can be done with Endo, of course, but the result is awkward. The idiom of composing a list of endomorphisms occurs often enough that you can Google for it. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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Proposal: Allow gunfold for Data.Map, Data.IntMap, etc.
by Edward Kmett 22 Nov '12

22 Nov '12

I would like to propose improving the Data instances for a number of currently completely opaque data types in the containers package, by using virtual constructors. The instance for Data.Map already uses fromList for gfoldl, it just stops there. Extending it to be able to gunfold and mention the name of that constructor would enable generic traversal libraries like uniplate, etc. to work over the contents of the Map, rather than bailing out in fear or crashing at the sight of a mkNoRepType. An example of the changes for Data.Map are highlighted below. instance (Data k, Data a, Ord k) => Data (Map k a) where gfoldl f z m = z fromList `f` toList m toConstr _ = fromListConstr gunfold k z c = case constrIndex c of 1 -> k (z fromList) _ -> error "gunfold" dataTypeOf _ = mapDataType dataCast2 f = gcast2 f fromListConstr :: Constr fromListConstr = mkConstr mapDataType "fromList" [] Prefix mapDataType :: DataType mapDataType = mkDataType "Data.Map.Map" [fromListConstr] I've used this approach for years on my own libraries to great effect. Discussion Period: 3 Weeks (I added a week for ICFP) -Edward

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