August 2009 - Libraries

Proposal #3339: Add (+>) as a synonym for mappend
by Bryan O'Sullivan 08 Nov '11

08 Nov '11

Ticket: http://hackage.haskell.org/trac/ghc/ticket/3339 >From the ticket: This proposal was, I think, originally suggested by Jules Bean. The idea is to add two functions to the Data.Monoid module, (+>) and (<+), corresponding to different uses of mappend. These should not be methods of the Monoidtypeclass, but top-level functions. I hope (but slightly doubt) that the visual nature of the two operators might help to counter the thought that monoids are just for gluing things together. (+>) :: (Monoid a) => a -> a -> a a +> b = a `mappend` b (<+) :: (Monoid a) => a -> a -> a a <+ b = b `mappend` a infixl 4 +> infixl 4 <+ Proposed deadline: two weeks. If this looks reasonable, I'll attach darcs patches.

30 75

There are too many error handling conventions used in library code!
by dons＠cse.unsw.edu.au 13 Aug '11

13 Aug '11

This article on the 8 different error handling strategies various common Haskell libs use: http://www.randomhacks.net/articles/2007/03/10/haskell-8-ways-to-report-err… got me thinking: we need to standardise/recommend a small set of methods for library error handling. The lack of consistent error reporting between libs results in verbose code, as we're not able to use a single error handling technique when gluing together code from different libs (i.e. we can't just use Maybe or Either/ErrorT). (It's always nice when you *can* use, say, a Maybe monad to glue code from a number of sources, but eventually you reach code with a different error system, and the Maybe monad breaks down). So, some questions I think we can tackle: * can we identify error handling strategies from the list that should not be used anymore? (throwDyn?) * can we move some of the outlying libraries to a more consistent error framework? * do we need more support for gluing together our different error systems? (it should be easier to glue code using , say, Maybe/Either). * what role does MonadError play here, as a generic error handler? * can we make precise recommendations about which error strategies to use? -- Don

11 16

Proposal #2629: Data.List: Replace nub; add nubOrd, nubInt, nubWith
by Bart Massey 24 Feb '10

24 Feb '10

http://hackage.haskell.org/trac/ghc/ticket/2629 Everyone always complains about nub, but nobody ever does anything about it except (map head . group . sort), which isn't lazy and isn't always faster. :-) I've implemented a new function nubWith that takes a "stop list" as an argument and filters its target list against the stop list. I've then re-implemented nub and implemented nubOrd and nubInt in terms of nubWith: the stop list is a typeclass, so these implementations are trivial and new implementations are easily added. nubBy is left alone, since there's nothing obvious to be done about it. All of the nubs are still fully lazy. Basic QuickCheck tests are provided, and pass. Performance benchmarking code is provided. The performance of my nub implementation is quite comparable to that of the standard one. My nubOrd and nubInt implementations are dramatically faster, since they use a Set and IntSet respectively for the stop list. In particular, they are performant on long lists with long nubs, unlike the basic nub. My implementation is available via git at git://svcs.cs.pdx.edu/git/nub.git or can be browsed at http://svcs.cs.pdx.edu/gitweb?p=nub.git;a=tree and has a maybe-outdated tarball at http://svcs.cs.pdx.edu/haskell/nub.tar.gz The Nub.hs file itself is attached to the proposal. If the proposal is accepted, I will prepare a patch against current GHC library top-of-tree, but for now it seems easier for everyone to just look at the bits in their current natural habitat.

15 31

Adding an ignore function to Control.Monad
by Gwern Branwen 12 Jan '10

12 Jan '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 So while writing my wp-archivebot, I ran into the issue that forkIO requires IO () but returns IO ThreadId, and that many useful IO functions will return IO a instead of IO (). This forces some awkward contortions. Suppose I want to ping the WebCite website at a particular address, and this request makes WebCite archive a URL embedded in that address. Presumably I could venture into the depths of Network.HTTP to figure out how to ping an URL without also pulling down the server's HTML, but why do that when I already have obviously 'openURL :: String -> IO String'? Much easier to do something like 'openURL "webcite.org" ++ foo ++ "other stuff" '. But my bot needs to handle quite a few URLs; one at a time, what with all the waits and timeouts, isn't going to hack it. So for a given link, I forkIO the openURL request. But of course, forkIO demands IO (), so I toss in a '>> return ()'. Fair enough. So I examine the performance, and it's still too slow. Recent Changes has hundreds of different pages a minute. I'd better fork each page (and then fork for each link). But wait, all those forkIOs are returning IO ThreadIds, and my top-level forkIO call demands IO ()... So another >> return (). At this point, the code is starting to look pretty silly - something like '...stuff >> return ()) >> return ())'. So I see the repeated pattern, and by the rule of 3, factor it out to: - -- | Convenience function. 'forkIO' and 'forM_' demand return types of 'IO ()', but most interesting - -- IO functions don't return void. So one adds a call to 'return ()'; this just factors it out. ignore ∷ (Monad m) ⇒ m a → m () ignore x = x >> return () Not the most complex convenience function I've ever written, but not any simpler than, say Control.Monad.forever or for that matter, most of the stuff in Control.Monad. I'd think it'd be useful for more than just me. Agda is lousy with calls to '>> return ()'; and then there's ZMachine, arrayref, whim, the barracuda packages, binary, bnfc, buddha, bytestring, c2hs, cabal, chesslibrary, comas, conjure, curl, darcs, darcs-benchmark, dbus-haskell, ddc, dephd, derive, dhs, drift, easyvision, ehc, filestore, folkung, geni, geordi, gtk2hs, gnuplot, ginsu, halfs, happstack, haskeline, hback, hbeat... You get the picture. I realize the specific name of 'ignore' can be bikeshedded to death, but it's clear, it's short, Hoogle turns up one other function with ignore in its name (Distribution.ParseUtils ignoreUnrec), and it's been independently named 'ignore' by another Haskeller (lilac). Existing uses of the string 'ignore are rare - it's in a few places as a variable, cabal and cabal-install and ehc and tar have where definitions of an ignore, a test for directory defines an ignore and imports Control.Monad unqualified, fit defines an 'ignore' but doesn't seem to use it in any module that also imports Control.Monad unqualified, halipeto defines an ignore but doesn't import Control.Monad, shim/yi has a let definition of an ignore, yhc has a where definition of an ignore. And that's about it. One of directory's tests would break, and the rest might have an additional -Wall warning. - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkov5Q0ACgkQvpDo5Pfl1oIsRQCghUqynThzcT+OYV1KaYJhGFhv 6yYAnjf7CVOm0+Fg1FBa9IpdVIrRpCZm =Cd8V -----END PGP SIGNATURE-----

18 32

export toDescList from Data.Map
by Evan Laforge 25 Nov '09

25 Nov '09

So, I mentioned this a long time ago but didn't get any responses and then I got distracted. So this time I added a ticket and patch and everything: 2580 Here's the text: It's even implemented, but not exported. Without this, there's apparently no way to iterate over a map from high to low, since foldl is also not exported. Also, I implemented a toDescList for IntMap, and could do the same for Set, and could add those patches, if people agree it's a good idea. In addition, is there any problem with exporting foldl aka descending for the three modules? I implemented a foldl for IntMap and it seems to work, but I'd want someone smarter to look for problems with it. In addition, Set and Map are inconsistent with folds: Set exports 'fold' and says "no order is guaranteed", but it's just defined to foldr, which is in ascending order. Set.foldr itself is not exported. Meanwhile, Map also exports fold, but heavily implies from the doc that it's guaranteed to be ascending. Would it restrict implementation changes too much to export a descending fold for all three? Other languages that have tree oriented general purpose maps all let you start anywhere and iterate in either direction. Anyway, is 2-3 weeks ok for discussion period? The library submission wiki doesn't say how long.

6 13

Three patches for cabal
by Niklas Broberg 09 Nov '09

09 Nov '09

(Trying again since my previous patches were too big for the list.) While doing a survey[1] of the extensions registered with Cabal, I came across two warts in the list of constructors, and one constructor that should be deprecated. First there's the constructor called TransformListComp, which should really be named GeneralizedListComp, since the constructor should describe the extension and not the implementation scheme. Second there's the constructor NoMonoPatBinds, which actually describes the default Haskell 98 behavior, even if GHC has a different default. It's GHC's behavior that is the extension, so the constructor in cabal should really be named MonoPatBinds. Also, the PatternSignatures constructor has been deprecated in GHC and superceded by ScopedTypeVariables. The attached patches (three in one file) adds the proposed new constructors, deprecates the old ones, and adds documentation. Cheers, /Niklas

8 15

Proposal: add new function "check" to Control.Monad
by Jon Fairbairn 19 Sep '09

19 Sep '09

Trac ticket #3453. Two week time frame. Add check function to Control.Monad check :: (MonadPlus m) => (a -> Bool) -> a -> m a check p a | p a = return a | otherwise = mzero Rationale: The example that suggested the function to me is this: readMaybe :: Read a => String -> Maybe a readMaybe = join . fmap no_trailing_garbage . listToMaybe . reads where no_trailing_garbage = fmap fst . check (all isSpace . snd) but check is clearly more generally useful than guard. (guard = flip (check . const) ()) Discussion: I also note in the comments to check that we can define List.filter like this filter = (concat .) . map . check Now, concat is just join specialised to lists, and map [is fmap, but...] is liftM, so we would expect a function mfilter = (join .) . liftM . check to be useful. Should that also be added? -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

8 14

Recommendation for the procedure to add platform packages
by Duncan Coutts 14 Sep '09

14 Sep '09

All, The Haskell Platform steering committee have been very busy over the past two weeks drafting, discussing and redrafting. The result is our recommendation to the libraries list for a procedure for adding new packages to the Haskell Platform. Before you all go off and read it, we'd like to say what feedback we are looking for and what the next steps will be. The document we've come up with describes the procedure but only some very basic quality requirements for packages. Our aim is to have the libraries list come to an agreement on the procedure first and then we can discuss codifying a more comprehensive set of package requirements and guidelines. The main document contains: * the procedure itself, which is relatively short * a rationale, cross-linked to the procedure * a procedure to help us make decisions http://trac.haskell.org/haskell-platform/wiki/AddingPackages There is an accompanying "how to" guide to help the people who will actually be making proposals: http://trac.haskell.org/haskell-platform/wiki/AddingPackages/HowTo There is also an example proposal: http://trac.haskell.org/haskell-platform/wiki/Proposals/example So please send in your comments and of course feel free to ask questions and seek clarifications. Our hope is that we'll be able to agree this procedure stuff relatively quickly and then move onto package quality issues and indeed to actually start proposing packages. Let me also say that I've been very pleased with how the steering committee has worked. People have put in a lot of effort. It took 2 weeks, nearly 60 emails and over 100 edits to the draft. We didn't always agree but we talked things through thoroughly and turned our early draft into something much better that we're all satisfied with. Duncan, on behalf of the rest of the steering committee: Iavor Diatchki, Isaac Dupree, Thomas Schilling, Johan Tibell and Adam Wick.

6 15

Potential Network SIG
by Thomas DuBuisson 11 Sep '09

11 Sep '09

Hello All, If you are CCed it's because you are listed as a maintainer of a network-* package that I consider related to the Haskell network library. I'm hoping to roll much of the functionality of network-{bytestring, multicast, fancy} etc into a single package that the community will agree on (namely, "network"). Johan suggested starting a SIG to hammer out a design for a new Network API seeing as the current API, a straight-forward Berkeley binding, doesn't seem to please anyone in a Haskell context. If you want to partake then this e-mail if your heads up. If there is some formal method of setting up a Haskell SIG then please let me know. My thoughts on some important parts are below - I'm sure not everyone will agree as these thoughts directly contradict some designs found in current libraries. 1) Separate low level functions / bindings from high level / productive code by placing each in different modules. The low level bindings should remain available for those cases we fail to have the needed functionality in our high level packages. That said, I'm hoping to cover more than the 80% of users with any new design. 2) Maintain type safety by using type classes for most things. Unlike Network.Fancy and Network.Socket (which have IPv4 and IPv6 as constructors of the same data type), I think we should allow for the possibility that some users of the library will be limited to just one IP version without resorting to partial functions. I suggest type classes to cover this aspect (class Address, class Port, etc). 3) Use Bytestrings (and have corrosponding .Lazy modules) for efficiency. As in network-bytestring, any new API should be performance concious enough to avoid String. 4) Support more features Features such as Multicast, Header inclusion (IP_HDRINCL), address binding, etc. IOW, most the IP_ and SO_ options of socket (7) and ip (7) man pages. It would be rather nice if we were able to expose these in a friendly way - but with our cross platform concerns that might not be a good idea (e.g. I'm not familiar with windows). 5) Separate IO-less data declarations from IO and any platform dependent code in different packages. I've already got some work in this area via network-data. Not claiming its current design will stand the test of time, its just an example of keeping the data structures separate from any IO operations that will need platform specific work. 6) Integrate with the rest of hackage. This means instance of PrettyClass, Parsec, and Binary. I noticed a number of parsing utilities for IP addresses - lots of duplicated effort here. I'm currently looking at how the network library is being used - particularly when Network.Socket is invoked. So I guess I'll go sort through some of the source code for scurry, happstack, and adhoc-network. Thomas

10 31

darcs patch: Fix unportable use of sleep(1)
by Matthias Kilian 08 Sep '09

08 Sep '09

Sat Aug 29 11:37:20 CEST 2009 Matthias Kilian <kili(a)outback.escape.de> * Fix unportable use of sleep(1) `sleep 1s' is probably a GNUism; `sleep 1' should do the same in a portable way.

2 1