October 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.

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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

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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.

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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-----

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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.

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Proposal: System.FilePath: current directory should be ".", not ""
by Simon Marlow 15 Nov '09

15 Nov '09

See http://hackage.haskell.org/trac/ghc/ticket/2034 to which I have attached a fairly simple patch. All the tests pass. The main change is that splitFileName "foo" = (".", "foo") and takeDirectory "foo" = "." and "." </> x = x Discussion period: 2 weeks (5 October) Cheers, Simon

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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

traceM and traceShowM
by Martijn van Steenbergen 05 Nov '09

05 Nov '09

Hello, I propose the addition of the following two functions to module Debug.Trace: > traceM :: Monad m => String -> m () > traceM msg = trace msg (return ()) > > traceShowM :: (Show a, Monad m) => a -> m () > traceShowM = traceM . show These functions allow tracing in any do-block. I often define them myself; I think they are useful in general. Deadline: 23 October 2009. Martijn.

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Network library broken on Windows with GHC 6.12rc1
by ndmitchell＠gmail.com 04 Nov '09

04 Nov '09

Hi, The subject says it all really. Using: network-2.2.1.5 from Cabal Windows XP, Admin mode Cygwin 1.6 The Glorious Glasgow Haskell Compilation System, version 6.12.0.20091010 $ runhaskell Setup configure Setup.hs:3:0: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you n eed Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Warning: defaultUserHooks in Setup script is deprecated. Configuring network-2.2.1.5... checking build system type... i686-pc-cygwin checking host system type... i686-pc-cygwin checking for gcc... gcc checking for C compiler default output file name... a.exe checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... .exe checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking for an ANSI C-conforming const... yes checking how to run the C preprocessor... gcc -E checking for grep that handles long lines and -e... /usr/bin/grep checking for egrep... /usr/bin/grep -E checking for ANSI C header files... yes checking for sys/types.h... yes checking for sys/stat.h... yes checking for stdlib.h... yes checking for string.h... yes checking for memory.h... yes checking for strings.h... yes checking for inttypes.h... yes checking for stdint.h... yes checking for unistd.h... yes checking fcntl.h usability... yes checking fcntl.h presence... yes checking for fcntl.h... yes checking limits.h usability... yes checking limits.h presence... yes checking for limits.h... yes checking for stdlib.h... (cached) yes checking for sys/types.h... (cached) yes checking for unistd.h... (cached) yes checking winsock2.h usability... no checking winsock2.h presence... yes configure: WARNING: winsock2.h: present but cannot be compiled configure: WARNING: winsock2.h: check for missing prerequisite headers? configure: WARNING: winsock2.h: see the Autoconf documentation configure: WARNING: winsock2.h: section "Present But Cannot Be Compiled" configure: WARNING: winsock2.h: proceeding with the preprocessor's result configure: WARNING: winsock2.h: in the future, the compiler will take precedence configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for winsock2.h... yes checking ws2tcpip.h usability... no checking ws2tcpip.h presence... yes configure: WARNING: ws2tcpip.h: present but cannot be compiled configure: WARNING: ws2tcpip.h: check for missing prerequisite headers? configure: WARNING: ws2tcpip.h: see the Autoconf documentation configure: WARNING: ws2tcpip.h: section "Present But Cannot Be Compiled" configure: WARNING: ws2tcpip.h: proceeding with the preprocessor's result configure: WARNING: ws2tcpip.h: in the future, the compiler will take precedence configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for ws2tcpip.h... yes checking wspiapi.h usability... no checking wspiapi.h presence... no checking for wspiapi.h... no checking arpa/inet.h usability... no checking arpa/inet.h presence... yes configure: WARNING: arpa/inet.h: present but cannot be compiled configure: WARNING: arpa/inet.h: check for missing prerequisite headers? configure: WARNING: arpa/inet.h: see the Autoconf documentation configure: WARNING: arpa/inet.h: section "Present But Cannot Be Compiled" configure: WARNING: arpa/inet.h: proceeding with the preprocessor's result configure: WARNING: arpa/inet.h: in the future, the compiler will take precedenc e configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for arpa/inet.h... yes checking netdb.h usability... no checking netdb.h presence... yes configure: WARNING: netdb.h: present but cannot be compiled configure: WARNING: netdb.h: check for missing prerequisite headers? configure: WARNING: netdb.h: see the Autoconf documentation configure: WARNING: netdb.h: section "Present But Cannot Be Compiled" configure: WARNING: netdb.h: proceeding with the preprocessor's result configure: WARNING: netdb.h: in the future, the compiler will take precedence configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for netdb.h... yes checking netinet/in.h usability... no checking netinet/in.h presence... yes configure: WARNING: netinet/in.h: present but cannot be compiled configure: WARNING: netinet/in.h: check for missing prerequisite headers? configure: WARNING: netinet/in.h: see the Autoconf documentation configure: WARNING: netinet/in.h: section "Present But Cannot Be Compiled" configure: WARNING: netinet/in.h: proceeding with the preprocessor's result configure: WARNING: netinet/in.h: in the future, the compiler will take preceden ce configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for netinet/in.h... yes checking netinet/tcp.h usability... no checking netinet/tcp.h presence... yes configure: WARNING: netinet/tcp.h: present but cannot be compiled configure: WARNING: netinet/tcp.h: check for missing prerequisite headers? configure: WARNING: netinet/tcp.h: see the Autoconf documentation configure: WARNING: netinet/tcp.h: section "Present But Cannot Be Compiled" configure: WARNING: netinet/tcp.h: proceeding with the preprocessor's result configure: WARNING: netinet/tcp.h: in the future, the compiler will take precede nce configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for netinet/tcp.h... yes checking sys/socket.h usability... no checking sys/socket.h presence... yes configure: WARNING: sys/socket.h: present but cannot be compiled configure: WARNING: sys/socket.h: check for missing prerequisite headers? configure: WARNING: sys/socket.h: see the Autoconf documentation configure: WARNING: sys/socket.h: section "Present But Cannot Be Compiled" configure: WARNING: sys/socket.h: proceeding with the preprocessor's result configure: WARNING: sys/socket.h: in the future, the compiler will take preceden ce configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for sys/socket.h... yes checking sys/uio.h usability... no checking sys/uio.h presence... yes configure: WARNING: sys/uio.h: present but cannot be compiled configure: WARNING: sys/uio.h: check for missing prerequisite headers? configure: WARNING: sys/uio.h: see the Autoconf documentation configure: WARNING: sys/uio.h: section "Present But Cannot Be Compiled" configure: WARNING: sys/uio.h: proceeding with the preprocessor's result configure: WARNING: sys/uio.h: in the future, the compiler will take precedence configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for sys/uio.h... yes checking sys/un.h usability... no checking sys/un.h presence... yes configure: WARNING: sys/un.h: present but cannot be compiled configure: WARNING: sys/un.h: check for missing prerequisite headers? configure: WARNING: sys/un.h: see the Autoconf documentation configure: WARNING: sys/un.h: section "Present But Cannot Be Compiled" configure: WARNING: sys/un.h: proceeding with the preprocessor's result configure: WARNING: sys/un.h: in the future, the compiler will take precedence configure: WARNING: ## ------------------------------------ ## configure: WARNING: ## Report this to libraries(a)haskell.org ## configure: WARNING: ## ------------------------------------ ## checking for sys/un.h... yes checking for readlink... yes checking for symlink... yes checking for struct msghdr.msg_control... yes checking for struct msghdr.msg_accrights... no checking for struct sockaddr.sa_len... no checking for in_addr_t in netinet/in.h... yes checking for SO_PEERCRED and struct ucred in sys/socket.h... yes checking for _head_libws2_32_a in -lws2_32... yes checking for getaddrinfo... no checking for getaddrinfo if WINVER is 0x0501... no checking for gai_strerror... no checking whether AI_ADDRCONFIG is declared... no checking whether AI_ALL is declared... no checking whether AI_NUMERICSERV is declared... no checking whether AI_V4MAPPED is declared... no checking for sendfile in sys/sendfile.h... no checking for sendfile in sys/socket.h... no checking for gethostent... no configure: creating ./config.status config.status: creating network.buildinfo config.status: creating include/HsNetworkConfig.h $ runhaskell Setup build Setup.hs:3:0: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you n eed Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Preprocessing library network-2.2.1.5... Network\Socket.hsc: In function `main': Network\Socket.hsc:994: error: invalid application of `sizeof' to incomplete typ e `ucred' Network\Socket.hsc:994: error: invalid application of `sizeof' to incomplete typ e `ucred' Network\Socket.hsc:994: error: invalid application of `sizeof' to incomplete typ e `ucred' Network\Socket.hsc:999: error: `SO_PEERCRED' undeclared (first use in this funct ion) Network\Socket.hsc:999: error: (Each undeclared identifier is reported only once Network\Socket.hsc:999: error: for each function it appears in.) Network\Socket.hsc:1000: error: dereferencing pointer to incomplete type Network\Socket.hsc:1001: error: dereferencing pointer to incomplete type Network\Socket.hsc:1002: error: dereferencing pointer to incomplete type compiling dist\build\Network\Socket_hsc_make.c failed command was: C:\ghc\ghc-6.12.0.20091010\mingw\bin\gcc.exe -c -BC:\ghc\ghc-6.12.0 .20091010\gcc-lib -IC:\ghc\ghc-6.12.0.20091010\include\mingw -D__GLASGOW_HASKELL __=612 -Iinclude -DCALLCONV=ccall -DBASE4 -IC:\ghc\ghc-6.12.0.20091010\base-4.2. 0.0\include -IC:\ghc\ghc-6.12.0.20091010/include -IC:\ghc\ghc-6.12.0.20091010/in clude -IC:/ghc/ghc-6.12.0.20091010/lib/include/ dist\build\Network\Socket_hsc_ma ke.c -o dist\build\Network\Socket_hsc_make.o Thanks, Neil

7 9

Data.Sequence 'index' documentation may be incorrect.
by iæfai 28 Oct '09

28 Oct '09

In the documentation for Data.Sequence.index: index :: Seq a -> Int -> a O(log(min(i,n-i))). The element at the specified position, which should be a positive integer less than the size of the sequence. If the position is out of range, index fails with an error. == 'a positive integer' refers to an number in the set of {1, 2, 3, …} but through some code discovered that what it really means is a non- negative integer, {0, 1, 2, …} Although it would be more immediately known if it just said it was a 0 based index. - iæfai

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