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New Assistant Professorship in Nottingham
by Graham Hutton 23 Jun '14

23 Jun '14

Dear all, The School of Computer Science at the University of Nottingham in the UK is seeking to appoint a new Assistant Professor: http://www.nottingham.ac.uk/jobs/currentvacancies/ref/SCI181514 Applications in the area of the Functional Programming (FP) lab would be most welcome. The FP lab is keen to receive applications from candidates with an excellent publication record, experience in combining theory with practice, and the ability to secure external funding to support their research. Further information about the FP lab is available from: http://fp.cs.nott.ac.uk The deadline for applications is Friday 22nd August 2014. The advert mentions computer vision, but the position is open to applicants from any area of Computer Science. Best wishes, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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hayoo doesn't find the Kleisli arrow?
by Erik Rantapaa 23 Jun '14

23 Jun '14

I just happen to notice that this query comes back empty in Hayoo: (a -> m b) -> (b -> m c) -> (a -> m c) Hoogle is able to find (>=>), and I'm wondering if its a bug or...?

2 1

Pathological overloaded function
by Simon Yarde 23 Jun '14

23 Jun '14

Could anyone chip in with examples of a pathological overloaded functions in relation to the GHC Guide 6.2: > Overloaded functions are not your friend: > Haskell's overloading (using type classes) is elegant, neat, etc., etc., but it is death to performance if left to linger in an inner loop. How can you squash it? > > Give explicit type signatures: > Signatures are the basic trick; putting them on exported, top-level functions is good software-engineering practice, anyway. (Tip: using -fwarn-missing-signatures can help enforce good signature-practice). http://lambda.haskell.org/platform/doc/current/ghc-doc/users_guide/faster.h… Best Regards, Simon

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cabal-install failure
by Magicloud Magiclouds 23 Jun '14

23 Jun '14

Hi, This is a box that used to work well. But today, I found that I could not install anything by cabal. I removed ~/.ghc and ~/.cabal to start all userspace thing from the beginning. I used Setup.hs to install Cabal, and bootstrap.sh to install cabal-install. All succeeded. Then I tried to install data-default-class, I got following. Notice the ld fail near the end. Versions: OS: Ubuntu Precise; ghc 7.8.2; Cabal/cabal-install: latest from github. % cabal install -v3 data-default-class Searching for ghc in path. Found ghc at /usr/local/bin/ghc ("/usr/local/bin/ghc",["--numeric-version"]) /usr/local/bin/ghc is version 7.8.2 looking for tool ghc-pkg near compiler in /usr/local/bin found ghc-pkg in /usr/local/bin/ghc-pkg ("/usr/local/bin/ghc-pkg",["--version"]) /usr/local/bin/ghc-pkg is version 7.8.2 ("/usr/local/bin/ghc",["--supported-languages"]) ("/usr/local/bin/ghc",["--info"]) Reading installed packages... ("/usr/local/bin/ghc-pkg",["dump","--global","-v0"]) ("/usr/local/bin/ghc-pkg",["dump","--user","-v0"]) ("/usr/local/bin/ghc",["--print-libdir"]) Reading available packages... Warning: The package list for the local repo '/home/local/ANT/shida/.cabal/packages/cabal-src/' is missing. The repo is invalid. Resolving dependencies... targets: data-default-class constraints: base installed ghc-prim installed integer-gmp installed stanzas data-default-class preferences: strategy: PreferAllLatest [__0] trying: data-default-class-0.0.1 (user goal) [__1] trying: base-4.7.0.0/installed-018... (dependency of data-default-class-0.0.1) [__2] trying: rts-1.0/installedbuil... (dependency of base-4.7.0.0/installed-018...) [__3] trying: integer-gmp-0.5.1.0/installed-dc4... (dependency of base-4.7.0.0/installed-018...) [__4] next goal: ghc-prim (dependency of base-4.7.0.0/installed-018...) [__4] trying: ghc-prim-0.3.1.0/installed-948... [__5] done Ready to install data-default-class-0.0.1 Waiting for install task to finish... Extracting /home/local/ANT/shida/.cabal/packages/ hackage.haskell.org/data-default-class/0.0.1/data-default-class-0.0.1.tar.gz to /tmp/data-default-class-0.0.1-19534... Updating data-default-class.cabal with the latest revision from the index. Using internal setup method with build-type Simple and args: ["configure","--verbose=3","--ghc","--prefix=/home/local/ANT/shida/.cabal","--bindir=/home/local/ANT/shida/.cabal/bin","--libdir=/home/local/ANT/shida/.cabal/lib","--libsubdir=x86_64-linux-ghc-7.8.2/data-default-class-0.0.1","--libexecdir=/home/local/ANT/shida/.cabal/libexec","--datadir=/home/local/ANT/shida/.cabal/share","--datasubdir=x86_64-linux-ghc-7.8.2/data-default-class-0.0.1","--docdir=/home/local/ANT/shida/.cabal/share/doc/x86_64-linux-ghc-7.8.2/data-default-class-0.0.1","--htmldir=/home/local/ANT/shida/.cabal/share/doc/x86_64-linux-ghc-7.8.2/data-default-class-0.0.1/html","--haddockdir=/home/local/ANT/shida/.cabal/share/doc/x86_64-linux-ghc-7.8.2/data-default-class-0.0.1/html","--sysconfdir=/home/local/ANT/shida/.cabal/etc","--enable-split-objs","--user","--dependency=base=base-4.7.0.0-018311399e3b6350d5be3a16b144df9b","--disable-tests","--exact-configuration","--disable-benchmarks"] Configuring data-default-class-0.0.1... creating dist Searching for ghc in path. Found ghc at /usr/local/bin/ghc ("/usr/local/bin/ghc",["--numeric-version"]) /usr/local/bin/ghc is version 7.8.2 looking for tool ghc-pkg near compiler in /usr/local/bin found ghc-pkg in /usr/local/bin/ghc-pkg ("/usr/local/bin/ghc-pkg",["--version"]) /usr/local/bin/ghc-pkg is version 7.8.2 ("/usr/local/bin/ghc",["--supported-languages"]) ("/usr/local/bin/ghc",["--info"]) Reading installed packages... ("/usr/local/bin/ghc-pkg",["dump","--global","-v0"]) ("/usr/local/bin/ghc-pkg",["dump","--user","-v0"]) ("/usr/local/bin/ghc",["--print-libdir"]) Dependency base ==4.7.0.0: using base-4.7.0.0 Searching for alex in path. Cannot find alex on the path Searching for ar in path. Found ar at /usr/bin/ar Searching for c2hs in path. Cannot find c2hs on the path Searching for cpphs in path. Cannot find cpphs on the path Searching for ffihugs in path. Cannot find ffihugs on the path Searching for gcc in path. Found gcc at /usr/bin/gcc ("/usr/bin/gcc",["-dumpversion"]) /usr/bin/gcc is version 4.6 Searching for greencard in path. Cannot find greencard on the path Searching for haddock in path. Found haddock at /usr/local/bin/haddock ("/usr/local/bin/haddock",["--version"]) /usr/local/bin/haddock is version 2.14.2 Searching for happy in path. Cannot find happy on the path Searching for hmake in path. Cannot find hmake on the path Searching for hpc in path. Found hpc at /usr/local/bin/hpc ("/usr/local/bin/hpc",["version"]) /usr/local/bin/hpc is version 0.67 looking for tool hsc2hs near compiler in /usr/local/bin found hsc2hs in /usr/local/bin/hsc2hs ("/usr/local/bin/hsc2hs",["--version"]) /usr/local/bin/hsc2hs is version 0.67 Searching for HsColour in path. Cannot find HsColour on the path Searching for hugs in path. Cannot find hugs on the path Searching for jhc in path. Cannot find jhc on the path Searching for ld in path. Found ld at /usr/bin/ld Environment: [("COLORFGBG","12;default;8"),("COLORTERM","rxvt-xpm"),("DBUS_SESSION_BUS_ADDRESS","unix:abstract=/tmp/dbus-5El0U8sCi8,guid=09290436e309789f9814dffc00000406"),("DEFAULTS_PATH","/usr/share/gconf/xmonad.default.path"),("DESKTOP_SESSION","xmonad"),("DISPLAY"," 10.69.208.185:0"),("EDITOR","vim"),("GDMSESSION","xmonad"),("GPG_AGENT_INFO","/tmp/gpg-KKH3WT/S.gpg-agent:5467:1"),("GREP_COLOR","1;32"),("GREP_OPTIONS","--color=auto --exclude-dir=.cvs --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn"),("GTK_IM_MODULE","xim"),("HOME","/home/local/ANT/shida"),("LANG","C.UTF-8"),("LC_ALL","C.UTF-8"),("LC_CTYPE","C.UTF-8"),("LESS","-R"),("LOGNAME","shida"),("LSCOLORS","Gxfxcxdxbxegedabagacad"),("MANDATORY_PATH","/usr/share/gconf/xmonad.mandatory.path"),("NODE_PATH","/usr/lib/nodejs:/usr/lib/node_modules:/usr/share/javascript"),("OLDPWD","/home/local/ANT/shida/src/git/cabal/cabal-install"),("PAGER","less"),("PATH","/usr/local/heroku/bin:.:/home/local/ANT/shida/.cabal/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/lib/lightdm/lightdm:/usr/local/bin:/usr/bin:/bin"),("PULSE_SERVER","10.69.208.185"),("PWD","/home/local/ANT/shida"),("QT_IM_MODULE","xim"),("SHELL","/usr/bin/zsh"),("SHLVL","1"),("SSH_AGENT_PID","14748"),("SSH_AUTH_SOCK","/tmp/ssh-NeRVrYb14746/agent.14746"),("TERM","rxvt-unicode-256color"),("UBUNTU_MENUPROXY","libappmenu.so"),("USER","shida"),("VISUAL","vim"),("WINDOWID","14683293"),("XAUTHORITY","/home/local/ANT/shida/.Xauthority"),("XDG_CONFIG_DIRS","/etc/xdg/xdg-xmonad:/etc/xdg"),("XDG_DATA_DIRS","/usr/share/xmonad:/usr/local/share/:/usr/share/"),("XDG_SEAT_PATH","/org/freedesktop/DisplayManager/Seat1"),("XDG_SESSION_COOKIE","05ca21f0ee7e6a10e18260ac00001d83-1402975930.974487-843139146"),("XDG_SESSION_PATH","/org/freedesktop/DisplayManager/Session0"),("XMODIFIERS","@im=ibus"),("ZSH","/home/local/ANT/shida/.oh-my-zsh"),("_","/home/local/ANT/shida/.cabal/bin/cabal")] ("/usr/local/bin/ghc",["-c","/tmp/19534.c","-o","/tmp/19534.o"]) ("/usr/bin/ld",["-x","-r","/tmp/19534.o","-o","/tmp/19535.o"]) Searching for lhc in path. Cannot find lhc on the path Searching for lhc-pkg in path. Cannot find lhc-pkg on the path Searching for nhc98 in path. Cannot find nhc98 on the path Searching for pkg-config in path. Found pkg-config at /usr/bin/pkg-config ("/usr/bin/pkg-config",["--version"]) /usr/bin/pkg-config is version 0.26 Searching for strip in path. Found strip at /usr/bin/strip Searching for tar in path. Found tar at /bin/tar Searching for uhc in path. Cannot find uhc on the path Using Cabal-1.21.0.0 compiled by ghc-7.8 Using compiler: ghc-7.8.2 Using install prefix: /home/local/ANT/shida/.cabal Binaries installed in: /home/local/ANT/shida/.cabal/bin Libraries installed in: /home/local/ANT/shida/.cabal/lib/x86_64-linux-ghc-7.8.2/data-default-class-0.0.1 Private binaries installed in: /home/local/ANT/shida/.cabal/libexec Data files installed in: /home/local/ANT/shida/.cabal/share/x86_64-linux-ghc-7.8.2/data-default-class-0.0.1 Documentation installed in: /home/local/ANT/shida/.cabal/share/doc/x86_64-linux-ghc-7.8.2/data-default-class-0.0.1 Configuration files installed in: /home/local/ANT/shida/.cabal/etc No alex found Using ar found on system at: /usr/bin/ar No c2hs found No cpphs found No ffihugs found Using gcc version 4.6 found on system at: /usr/bin/gcc Using ghc version 7.8.2 found on system at: /usr/local/bin/ghc Using ghc-pkg version 7.8.2 found on system at: /usr/local/bin/ghc-pkg No greencard found Using haddock version 2.14.2 found on system at: /usr/local/bin/haddock No happy found Using haskell-suite found on system at: haskell-suite-dummy-location Using haskell-suite-pkg found on system at: haskell-suite-pkg-dummy-location No hmake found Using hpc version 0.67 found on system at: /usr/local/bin/hpc Using hsc2hs version 0.67 found on system at: /usr/local/bin/hsc2hs No hscolour found No hugs found No jhc found Using ld found on system at: /usr/bin/ld No lhc found No lhc-pkg found No nhc98 found Using pkg-config version 0.26 found on system at: /usr/bin/pkg-config Using strip found on system at: /usr/bin/strip Using tar found on system at: /bin/tar No uhc found ("/usr/bin/gcc",["/tmp/19534.c","-o","/tmp/19534","-D__GLASGOW_HASKELL__=708","-Dlinux_BUILD_OS=1","-Dx86_64_BUILD_ARCH=1","-Dlinux_HOST_OS=1","-Dx86_64_HOST_ARCH=1","-Idist/build/autogen","-I.","-I/usr/local/lib/ghc-7.8.2/base-4.7.0.0/include","-I/usr/local/lib/ghc-7.8.2/integer-gmp-0.5.1.0/include","-I/usr/local/lib/ghc-7.8.2/include","-L/usr/local/lib/ghc-7.8.2/base-4.7.0.0","-L/usr/local/lib/ghc-7.8.2/integer-gmp-0.5.1.0","-L/usr/local/lib/ghc-7.8.2/ghc-prim-0.3.1.0","-L/usr/local/lib/ghc-7.8.2/rts-1.0"]) Using internal setup method with build-type Simple and args: ["build","--verbose=3"] Component build order: library creating dist/build creating dist/build/autogen Building data-default-class-0.0.1... Preprocessing library data-default-class-0.0.1... Building library... creating dist/build Environment: [("COLORFGBG","12;default;8"),("COLORTERM","rxvt-xpm"),("DBUS_SESSION_BUS_ADDRESS","unix:abstract=/tmp/dbus-5El0U8sCi8,guid=09290436e309789f9814dffc00000406"),("DEFAULTS_PATH","/usr/share/gconf/xmonad.default.path"),("DESKTOP_SESSION","xmonad"),("DISPLAY"," 10.69.208.185:0"),("EDITOR","vim"),("GDMSESSION","xmonad"),("GPG_AGENT_INFO","/tmp/gpg-KKH3WT/S.gpg-agent:5467:1"),("GREP_COLOR","1;32"),("GREP_OPTIONS","--color=auto --exclude-dir=.cvs --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn"),("GTK_IM_MODULE","xim"),("HOME","/home/local/ANT/shida"),("LANG","C.UTF-8"),("LC_ALL","C.UTF-8"),("LC_CTYPE","C.UTF-8"),("LESS","-R"),("LOGNAME","shida"),("LSCOLORS","Gxfxcxdxbxegedabagacad"),("MANDATORY_PATH","/usr/share/gconf/xmonad.mandatory.path"),("NODE_PATH","/usr/lib/nodejs:/usr/lib/node_modules:/usr/share/javascript"),("OLDPWD","/home/local/ANT/shida/src/git/cabal/cabal-install"),("PAGER","less"),("PATH","/usr/local/heroku/bin:.:/home/local/ANT/shida/.cabal/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/lib/lightdm/lightdm:/usr/local/bin:/usr/bin:/bin"),("PULSE_SERVER","10.69.208.185"),("PWD","/home/local/ANT/shida"),("QT_IM_MODULE","xim"),("SHELL","/usr/bin/zsh"),("SHLVL","1"),("SSH_AGENT_PID","14748"),("SSH_AUTH_SOCK","/tmp/ssh-NeRVrYb14746/agent.14746"),("TERM","rxvt-unicode-256color"),("UBUNTU_MENUPROXY","libappmenu.so"),("USER","shida"),("VISUAL","vim"),("WINDOWID","14683293"),("XAUTHORITY","/home/local/ANT/shida/.Xauthority"),("XDG_CONFIG_DIRS","/etc/xdg/xdg-xmonad:/etc/xdg"),("XDG_DATA_DIRS","/usr/share/xmonad:/usr/local/share/:/usr/share/"),("XDG_SEAT_PATH","/org/freedesktop/DisplayManager/Seat1"),("XDG_SESSION_COOKIE","05ca21f0ee7e6a10e18260ac00001d83-1402975930.974487-843139146"),("XDG_SESSION_PATH","/org/freedesktop/DisplayManager/Session0"),("XMODIFIERS","@im=ibus"),("ZSH","/home/local/ANT/shida/.oh-my-zsh"),("_","/home/local/ANT/shida/.cabal/bin/cabal")] ("/usr/local/bin/ghc",["--make","-v","-fbuilding-cabal-package","-O","-split-objs","-static","-dynamic-too","-dynosuf","dyn_o","-dynhisuf","dyn_hi","-outputdir","dist/build","-odir","dist/build","-hidir","dist/build","-stubdir","dist/build","-i","-idist/build","-i.","-idist/build/autogen","-Idist/build/autogen","-Idist/build","-optP-include","-optPdist/build/autogen/cabal_macros.h","-package-name","data-default-class-0.0.1","-hide-all-packages","-package-db","dist/package.conf.inplace","-package-id","base-4.7.0.0-018311399e3b6350d5be3a16b144df9b","-XHaskell98","Data.Default.Class"]) Glasgow Haskell Compiler, Version 7.8.2, stage 2 booted by GHC version 7.8.1 Using binary package database: /usr/local/lib/ghc-7.8.2/package.conf.d/package.cache Using binary package database: /home/local/ANT/shida/.ghc/x86_64-linux-7.8.2/package.conf.d/package.cache Using package config file: dist/package.conf.inplace wired-in package ghc-prim mapped to ghc-prim-0.3.1.0-948744e1f99cc8bcc7c7d3ba60c7c2d8 wired-in package integer-gmp mapped to integer-gmp-0.5.1.0-dc47f6b546fc171f67a7f7d311684a99 wired-in package base mapped to base-4.7.0.0-018311399e3b6350d5be3a16b144df9b wired-in package rts mapped to builtin_rts wired-in package template-haskell mapped to template-haskell-2.9.0.0-dcc8c210fb02937e104bc1784d7b0f06 wired-in package dph-seq not found. wired-in package dph-par not found. Hsc static flags: wired-in package ghc-prim mapped to ghc-prim-0.3.1.0-948744e1f99cc8bcc7c7d3ba60c7c2d8 wired-in package integer-gmp mapped to integer-gmp-0.5.1.0-dc47f6b546fc171f67a7f7d311684a99 wired-in package base mapped to base-4.7.0.0-018311399e3b6350d5be3a16b144df9b wired-in package rts mapped to builtin_rts wired-in package template-haskell mapped to template-haskell-2.9.0.0-dcc8c210fb02937e104bc1784d7b0f06 wired-in package dph-seq not found. wired-in package dph-par not found. *** Chasing dependencies: Chasing modules from: *Data.Default.Class Stable obj: [] Stable BCO: [] Ready for upsweep [NONREC ModSummary { ms_hs_date = 2014-06-23 03:29:30.794067064 UTC ms_mod = data-default-class-0.0.1:Data.Default.Class, ms_textual_imps = [import (implicit) Prelude] ms_srcimps = [] }] *** Deleting temp files: Deleting: compile: input file ./Data/Default/Class.hs Created temporary directory: /tmp/ghc19596_0 *** Checking old interface for data-default-class-0.0.1:Data.Default.Class: [1 of 1] Compiling Data.Default.Class ( Data/Default/Class.hs, dist/build/Data/Default/Class.o ) *** Parser: *** Renamer/typechecker: *** Desugar: Result size of Desugar (after optimization) = {terms: 0, types: 0, coercions: 0} *** Simplifier: Result size of Simplifier = {terms: 0, types: 0, coercions: 0} *** Specialise: Result size of Specialise = {terms: 0, types: 0, coercions: 0} *** Float out(FOS {Lam = Just 0, Consts = True, PAPs = False}): Result size of Float out(FOS {Lam = Just 0, Consts = True, PAPs = False}) = {terms: 0, types: 0, coercions: 0} *** Float inwards: Result size of Float inwards = {terms: 0, types: 0, coercions: 0} *** Simplifier: Result size of Simplifier = {terms: 0, types: 0, coercions: 0} *** Simplifier: Result size of Simplifier = {terms: 0, types: 0, coercions: 0} *** Simplifier: Result size of Simplifier = {terms: 0, types: 0, coercions: 0} *** Demand analysis: Result size of Demand analysis = {terms: 0, types: 0, coercions: 0} *** Worker Wrapper binds: Result size of Worker Wrapper binds = {terms: 0, types: 0, coercions: 0} *** Simplifier: Result size of Simplifier = {terms: 0, types: 0, coercions: 0} *** Float out(FOS {Lam = Just 0, Consts = True, PAPs = True}): Result size of Float out(FOS {Lam = Just 0, Consts = True, PAPs = True}) = {terms: 0, types: 0, coercions: 0} *** Common sub-expression: Result size of Common sub-expression = {terms: 0, types: 0, coercions: 0} *** Float inwards: Result size of Float inwards = {terms: 0, types: 0, coercions: 0} *** Simplifier: Result size of Simplifier = {terms: 0, types: 0, coercions: 0} *** Tidy Core: Result size of Tidy Core = {terms: 4, types: 7, coercions: 2} writeBinIface: 2 Names writeBinIface: 16 dict entries writeBinIface: 2 Names writeBinIface: 16 dict entries *** CorePrep: Result size of CorePrep = {terms: 4, types: 7, coercions: 2} *** Stg2Stg: *** CodeOutput: *** New CodeGen: *** CPSZ: *** CPSZ: *** Splitter: /usr/local/lib/ghc-7.8.2/ghc-split /tmp/ghc19596_0/ghc19596_2.split_s /tmp/ghc19596_0/ghc19596_4.split /tmp/ghc19596_0/ghc19596_4.split *** Assembler: /usr/bin/gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -c -o dist/build/Data/Default/Class_o_split/Class__1.o /tmp/ghc19596_0/ghc19596_4.split__1.s *** Assembler: /usr/bin/gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -c -o dist/build/Data/Default/Class_o_split/Class__2.o /tmp/ghc19596_0/ghc19596_4.split__2.s *** Linker: /usr/bin/gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -nostdlib -Wl,-r -nodefaultlibs '-Wl,--build-id=none' -o dist/build/Data/Default/Class.o /tmp/ghc19596_0/ghc19596_5.ldscript /usr/bin/ld: error: cannot find dist/build/Data/Default/Class_o_split/Class__1.o /usr/bin/ld: error: cannot find dist/build/Data/Default/Class_o_split/.o::Class(void) collect2: ld returned 1 exit status *** Deleting temp files: Deleting: /tmp/ghc19596_0/ghc19596_5.ldscript /tmp/ghc19596_0/ghc19596_4.split__1.s /tmp/ghc19596_0/ghc19596_4.split__2.s /tmp/ghc19596_0/ghc19596_4.split /tmp/ghc19596_0/ghc19596_3.c /tmp/ghc19596_0/ghc19596_2.split_s /tmp/ghc19596_0/ghc19596_1.split_s Warning: deleting non-existent /tmp/ghc19596_0/ghc19596_3.c Warning: deleting non-existent /tmp/ghc19596_0/ghc19596_1.split_s *** Deleting temp dirs: Deleting: /tmp/ghc19596_0 /usr/local/bin/ghc returned ExitFailure 1 Failed to install data-default-class-0.0.1 Updating world file... cabal: Error: some packages failed to install: data-default-class-0.0.1 failed during the building phase. The exception was: ExitFailure 1 -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

1 1

Problems with Parallel N Queens using Control.Parallel.Strategy
by bruce li 22 Jun '14

22 Jun '14

Hello, Cafe, I'm currently reading Parallel and Concurrent Programming in Haskell. I tried to implement an n-queens problem as an exercise, but a little bit overwhelmed by the strategy mystery. The code is listed at http://pastebin.com/ySgc3VSR. I implemented the higher order skeleton of search and try to add parallelism. My original idea was spawning a few threads while the search tree was small and each thread handles the following expansion of the tree. Hence, I gave a "maxDepth" variable to restrict the parallelism. But this did not go very well if I set the maxDepth to very small value, say, 2, 3, 4 etc. The speed up was very small. Viewing in threadscope, it seems there was very small amount of parallelism. Then I tried to let it go, which means I set the tree depth to a value larger than the maximum possible depth, say 20. Then it speeds up about 1.4 times on 2-cores. Viewing in threadscope, it seems the 2 cores are fully utilized. But the report is a bit annoying: SPARKS: 4686728 (100456 converted, 3461 overflowed, 0 dud, 4540817 GC'd, 41994 fizzled) with no restrictions on the parallelism, a lot of sparks are not converted and are GC'd. I wonder if there is some way to further improve the results, reducing un-convereted sparks and achieving better speed-up ratio. Thanks.

1 0

Re: [Haskell-cafe] ANN: set-cover solves Sudoku, Soma cube, 8 Queens etc.
by Johannes Waldmann 22 Jun '14

22 Jun '14

Henning Thielemann <lemming <at> henning-thielemann.de> writes: > .. package set-cover for solving exact set cover problems. > http://hackage.haskell.org/package/set-cover It's hard to evaluate whether one could use the library because there's essentially no visible documentation. E.g., what does Math.SetCover.Exact.search do? Its type refers to "State" which seems implementation-dependent, and hides the connection to the specification of the set cover problem. Putting some text and examples into the haddocks might help. But of course putting the specification there would be even better. Also, care to explain what algorithm your solver uses, and give some performance data (e.g., N-queens for N=10,20,40,..)? - J.W.

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Re: [Haskell-cafe] diagrams
by Dominic Steinitz 22 Jun '14

22 Jun '14

> Message: 3 > Date: Fri, 20 Jun 2014 13:30:29 -0700 > From: Joachim Breitner <mail(a)joachim-breitner.de> > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] diagrams > Message-ID: <1403296229.16156.3.camel@kirk> > Content-Type: text/plain; charset="utf-8" > > Hi, > > Am Mittwoch, den 18.06.2014, 23:44 -0400 schrieb Anthony Cowley: > >>> I have no doubt that diagrams can make the diagrams themselves. What >>> I wonder about is the animation capabilities---ability to have >>> elements move or cross-fade (dissolve), in particular. > > I very recently created this animation with diagrams, which does move > and fade stuff: > http://joachim-breitner.de/publications/haskell_bytes_portland_2014-06-13.w… > > But the ride was not as smooth as it should have been. See > https://gist.github.com/nomeata/cde96a2e693a23cca8ee for the code. The > function positionSpec contains a ?script?, i.e. a sequence of animated > modifications to the image, using lenses to position the change. > > I guess with the right combinators, this can be greatly simplified, but > it is hard to come up with the right combinators. Especially as our code > is one-dimensional, and we want to compose in three dimensions (x, y and > time). It’s still a nice example though - thanks for posting it. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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diagrams
by Dennis Raddle 20 Jun '14

20 Jun '14

I'm a math tutor and I want to create videos to explain certain math concepts, and I'm wondering if the diagrams library would help me. There are two questions--first, whether it has the capability to do what I want, and second how hard it would to integrate the animations into YouTube videos with voice-over. To describe the capability, I'm looking first to typeset equations. Then, second, to represent manipulation of equations by having elements of equations move locations or cross-fade. Also "elements" could mean pictures or non-mathematical notation. I have no doubt that diagrams can make the diagrams themselves. What I wonder about is the animation capabilities---ability to have elements move or cross-fade (dissolve), in particular. As far as the ability to integrate these into YouTube videos, that is probably beyond the scope of this list, but maybe someone happens to have done this and knows the answer. Mike

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Do you recognize this type?
by Corentin Dupont 20 Jun '14

20 Jun '14

Hi guys, I am making a DSL for event composition (inspired from digestive-functor & reactive-banana) and I find myself wanting a primitive like that: -- given a list of events, evaluate the function on those that have already fired. -- returns an event yelding the result when ready. ListEvent :: [Event a] -> ([a] -> Maybe b) -> Event b Does this type tell you something? Is there a better abstraction that can encompass it? My DSL is like that: -- | Composable events data Event a where SumEvent :: Event a -> Event a -> Event a -- The first event to fire will be returned AppEvent :: Event (a -> b) -> Event a -> Event b -- Both events should fire, and then the result is returned PureEvent :: a -> Event a -- Create a fake event. The result is useable with no delay. EmptyEvent :: Event a -- An event that is never fired. BaseEvent :: (Typeable a) => Field a -> Event a -- Embed a base event It's easy to define instances for Applicative and Alternative. But then I have a use case that I cannot solve: In the case of votations (which use this DSL), sometime you can end prematuraly the voting, even if not everybody voted yet: for example if the majority is already reached. In this case no need to wait for everybody, we can end the vote. This does not seem to be possible with Applicative or Monad interfaces: things have to be sequenced in a monad, they cannot be evaluated in parralel. For example: do v1 <- getVote1 v2 <- getVote2 v3 <- getVote3 evalMajority(v1, v2, v3) In this example I have to wait for all 3 votes to be completed, but in fact only two would suffice to achieve majority. That's why I added "ListEvent" to the DSL above which allow to achieve that but I'm wondering if there is a better/bigger abstraction to put it in. Thanks!! Corentin

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How to represent a board?
by Rafael Almeida 20 Jun '14

20 Jun '14

Hello, There are many examples of games which use boards: chess, checkers, go, etc. Many people starting out at programming and/or game programming are very much tempted to code a board game. It looks like such a game would be easy to build at first sight. Even more sophisticated games sometimes use a grid system, which looks a lot like boards as well. In a grid system the movements of a given agent are bound do discrete positions. Although the user may not see the board underneath pretty graphics, it is a board game. Having in mind the sort of operations such games have to make on boards, I ask: what are the best representations for a board in Haskell? In many languages a NxN array seems like a good pick. In Haskell, most would translate that into lists of lists. I know I have. However, traversing those lists to get a position, calculate where a piece or agent could go, etc., feels awkward and unefficient. Beside the point already made, we have no type safe guarantee that our 64x64 won't become a 63x63 + 65x1 board due to some misbehaving function. It strikes me that list of lists is not a great board representation. I think STArrays are a better pick; from the perspective of translating an NxN array into Haskell, at least. However, maybe there is an even more elegant way to deal with a board in Haskell. I hope you can help me out figuring it out. []'s Rafael

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