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Is scanl' safe for foldr/build fusion?
by David Feuer 24 Jul '14

24 Jul '14

In a comment on trac #9345, nomeata proposed the implementation of Data.List.inits below. He indicates that the performance is very good, and wrote it in this fashion so it could fuse. My concern is that since seq is used in the argument to build, we need to make sure that fusion on the left will be safe. If I did the calculations right, this specifically requires that for all f, cons, nil, and bs, and all a≠_|_, the following "correct" expression e1 = a `cons` foldr cons nil $ foldr (\b g x -> (let b' = f x b in b' `seq` (b' : g b'))) (\b -> b `seq` []) bs a gives the same result as the fused expression e2 = a `cons` foldr (\b g x -> (let b' = f x b in b' `seq` (b' `cons` g b'))) (\b -> b `seq` nil) bs a I haven't been able to figure out how to prove this or give a counterexample, so far, but I have very little experience with such. myScanl' :: (b -> a -> b) -> b -> [a] -> [b] myScanl' f a bs = build $ \c n -> a `seq` a `c` foldr (\b g x -> (let b' = f x b in b' `seq` (b' `c` g b'))) (\b -> b `seq` n) bs a initsQ2 :: [a] -> [[a]] initsQ2 = map toListQ . myScanl' snocQ emptyQ

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Re: At long last: 2014.2.0.0 Release Candidate 1
by Brandon Allbery 24 Jul '14

24 Jul '14

On Thu, Jul 24, 2014 at 8:09 AM, George Colpitts <george.colpitts(a)gmail.com> wrote: > Installed on the Mac, looks good, not sure if following is significant: > > ghc-pkg check > Warning: library-dirs: /opt/local/lib/ doesn't exist or isn't a directory > Warning: include-dirs: /opt/local/include/ doesn't exist or isn't a > directory > Warning: haddock-interfaces: > /Users/gcolpitts/Library/Haskell/ghc-7.8.3/lib/Cabal-1.20.0.1/doc/html/Cabal.haddock > doesn't exist or isn't a file > > did cabal install -j3 of threadscope, criterion, hlint, ghc-mod, > quickcheck, smallcheck and smartcheck > > Errrr. Whoever is making the Mac bindist should take care to avoid any MacPorts (or for that matter Homebrew) components slipping in to it; this will cause problems down the road. (The same goes for users; if you use MacPorts or Homebrew, wait for the Platform to be updated there and install from there, or you will end up with iconv etc. conflicts. Possibly a warning about this should be included in the install docs.) -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: At long last: 2014.2.0.0 Release Candidate 1
by Mark Lentczner 24 Jul '14

24 Jul '14

On Thu, Jul 24, 2014 at 5:09 AM, George Colpitts <george.colpitts(a)gmail.com> wrote: > Installed on the Mac, looks good, ... > did cabal install -j3 of threadscope, criterion, hlint, ghc-mod, > quickcheck, smallcheck and smartcheck > > That's all a good sign... not sure if following is significant: > > ghc-pkg check > Warning: library-dirs: /opt/local/lib/ doesn't exist or isn't a directory > Warning: include-dirs: /opt/local/include/ doesn't exist or isn't a > directory > Warning: haddock-interfaces: > /Users/gcolpitts/Library/Haskell/ghc-7.8.3/lib/Cabal-1.20.0.1/doc/html/Cabal.haddock > doesn't exist or isn't a file > > Something is amiss in how things ended up on your machine. I've just double checked the contents of the Mac distro and /opt doesn't appear in any of the package registrations. Furthermore, unless you built Cabal-1.20.0.1, it shouldn't be in your package db. My rough guess is that you've got a 7.8.3 compatible package db left over from a prior install somewhere. Can you e-mail me directly the output of: which cabal cabal --version which ghc ghc --version which ghc-pkg ghc-pkg --version ghc-pkg list ghc-pkg dump Thanks!

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GHCi: Behave nicely on `-e`, like `ghc` and other programs
by Andrew Pennebaker 24 Jul '14

24 Jul '14

Like many programming language environments, GHC offers a handy `-e` option for evaluating an expression, then returning to the shell. $ ghc -e '2 + 2' 4 One would expect the interpreter, GHCi, to offer a similar flag, but it surprisingly rejects it. ghci -e '2 + 2' ghc: on the commandline: cannot use `--interactive' with `-e' Usage: For basic information, try the `--help' option. I think this behavior is quite unintuitive--when I pass `-e <exp>` to ghci, or pass `--interactive -e <exp>` to ghc, I expect the expression to be evaluated as the leading expression in an interactive interpreter session. Could we please tweak ghc like this to make it slightly more intuitive when these flags are used together? -- Cheers, Andrew Pennebaker www.yellosoft.us

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Looking for help writing some GHC RULES
by David Feuer 24 Jul '14

24 Jul '14

I'm looking for some help writing some translate/untranslate rules to implement fusion for a couple of basic functions (the ones I'm looking at right now are takeWhile and scanr, but there may be others). I have a translation for takeWhile that seems to be at least decent, and one for scanr that is completely untested. I've never mucked about with GHC RULES, however, so I don't even know where to begin. Thanks in advance, David Feuer

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Status of Haskell Platform 2014.2.0.0
by Mark Lentczner 22 Jul '14

22 Jul '14

The status is: *Good-to-Go!* The new-build branch of Haskell Platform is in pretty great shape: - One consistent build system using Shake - Builds source tarball - Builds linux distribution tarball - Builds Mac installer - Builds in one command line from a GHC bindist to end-user installer! - We have a running travis-ci instance <https://travis-ci.org/haskell/haskell-platform>* (currently red as we await a 7.8.3 bindist)* As soon as GHC 7.8.3 is out, I'll be running an alpha (or is this rc1?) or Haskell Platform (well, after I build the bindists for GHC for Mac... so it'll be a few hours on my puny MacBook Air...) *Shout out to: Yitzchak Gale, Bob Ipoloito, and Randy Polen for lots of help with the new build; Carter Schonwald for Mac and GHC build issues; Robert Lefkowitz for wiki and issue converstion (now on github!); Neil Mitchell for consultations on Shake.* The platform is now in a place that we'll be able to turn it much more quickly. This means we can track GHC release more closely, handle important library fixes when needed. And more importantly, spend time improving the platform itself, rather than sapping our energies build it! What you need to do now: - Check the version list in Release2014.hs <https://github.com/haskell/haskell-platform/blob/new-build/hptool/src/Relea…> - If you see a problem with the version of your library, let us know - If you are a packager for one of the linux distros - please try out the build and see how it works, and how it meshes with your packaging process for the distro. Come talk to me if you need help or brainstorming w.r.t. to the new-build. - If you have a Mac, you can try building the platform. *Build note: The repo is in sync with head of GHC 7.8 and we've been working with bindists we build ourselves off of head. If you want to try out the HP build with a 7.8.2 bindist, you'll need to change the version of base back to 4.7.0.0.* What you can do to help There are few aspects in progress, but none of these are show stoppers: - Windows installer using the new build being written by Randy Polen. (Existing windows build from the source tarball is still possible) - contact him if you'd like to help. - Platform website is being revamped by Erin Depew in concert with the general Haskell redesign by Chris Done - Platform website is template generated... but the pages haven't been templated much. if you're interested, e-mail and I can supply more details and hook up interested parties and Erin. - The travis-ci instance is enabled for Mac builds, but isn't configured to do them right. - There is a TO DO small task list in BUILD-NEW <https://github.com/haskell/haskell-platform/blob/new-build/notes/BUILD-NEW#…> in the repo - I'd like to see the As and Bs done. If you want to tackle one just do it and send me a merge request. - Many people have asked for a "server edition" of the platform w/o packages that don't make sense on a server (such as the OpenGL stuff). The new-build could easily be extended to do this now. - I'd love to see a version of Simon Hengel's Haskell Platform Versions Comparison Chart <http://sol.github.io/haskell-platform-versions-comparison-chart/> as part of the platform website. Now that it is templatized and the raw data is in Haskell, it should be possible. (I have prior year's data in the right format, just not checked in, ask me...) - It would be wonderful if we could get tests incorporated: Both running the tests that are part of the included libraries, and perhaps some "big integration tests" (is compiling Pandoc enough?). It would be great if these tests can be run "in-place" after the platform is built (for travis-ci), and if they could be run on the target machine (post-installation). — Mark "is it July already?" Lentczner

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Let's make takeWhile fuse!
by David Feuer 21 Jul '14

21 Jul '14

`takeWhile` doesn't do the stream fusion thing. This definition seems to fix that, at least to a great extent. I don't know how to write the necessary rules to make this definition be used in the right places. takeWhile :: forall a . (a -> Bool) -> [a] -> [a] takeWhile p xs = build tw' where tw' :: forall b . (a -> b -> b) -> b -> b tw' kons knil = foldr go knil xs where go x rest | p x = x `kons` rest | otherwise = knil Tests (performed on 7.8.3, but the definition hasn't changed in HEAD): In the trivial main = print $ length $ takeWhile (< 10000000) [(1::Int) .. 20000000] it fused completely, allocating virtually nothing, whereas using Prelude.takeWhile (on 7.8.3) required a whopping 1,360,049,352 In the more complex main = print $ length $ filter (\x -> x `rem` 3 == 0) $ takeWhile (< 10000000) $ filter even [(1::Int) .. 20000000] it fused partially, allocating 266,716,048 bytes. The current GHC definition allocates 746,716,000 bytes in this case (strangely, less than it used for the simple test)! David Feuer

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Proposal: Use (.) and id from Control.Category in Prelude
by Carter Charbonneau 21 Jul '14

21 Jul '14

(.) and id are generalized in the Category class in Control.Category. In the past, many libraries have instructed users to use (.) and id from Control.Category for composition of their types. Importing these types into Prelude shouldn't break existing code, and makes libraries that use Category easier to use. Discussion period: 4 weeks

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Planning a Haskell Platfrom Release Candidate this weekend
by Mark Lentczner 21 Jul '14

21 Jul '14

Here's my remaining list before putting out a realse candidate. I'm aiming for this weekend: *-- General --* [ ] haddock linkage from HP packages to GHC core packages is broken [ ] decision on QuickCheck? *-- Source Tarball --* [ ] missing platform.sh scripts [ ] missing os-extras in hptool/src *-- Mac --* [ ] start.html needs a once over updating [ ] uninstall-hs should clean out /usr/local/bin symlinks (as it does with /usr/bin) *-- Windows --* [ ] GLUT issue in ghci *-- Website --* [ ] templatize info on pages [ ] add build mode to bptool Main - needs to take file of downloads & hashes Anyone have anything else? Otherwise... we're almost good to go!!!! - Mark

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Proposal: add uncons (and unsnoc) to Data.List
by David Feuer 20 Jul '14

20 Jul '14

Alexander Berntsen indicates that he has a branch all ready to go. Henning Thienemann seems to prefer the term viewL. Alexander says that Edward Kmett says that uncons/unsnoc is the more common term. Personally, I find uncons and unsnoc to be more intuitive names. Details: uncons :: [a] -> Maybe (a, [a]) uncons [] = Nothing uncons (a:as) = Just (a,as) -- Henning's implementation of "viewR", renamed, looks like unsnoc :: [a] -> Maybe ([a], a) unsnoc = foldr (\x -> Just . forcePair . maybe ([],x) (mapFst (x:))) Nothing I wonder if it would be possible to tease that apart a bit to get the maybe out of the loop, which I think might be prettier. The really tricky part of unsnoc is making it lazy enough—messing around with it a bit suggested that it's very easy to mess up the pair lifting. The tough rule seems to be this mouthful, if I'm not mistaken: unsnoc (xs ++ (y : _|_)) = Just (xs ++ _|_, _|_) David

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