March 2018 - Haskell-Cafe

missing optimization for (++)
by Ben Franksen 05 Mar '18

05 Mar '18

I found that in base [1] we have (++) [] ys = ys (++) (x:xs) ys = x : xs ++ ys I had expected there to be a clause (++) xs [] = xs at the top, which would avoid the list traversal in this common case. Is this somehow magically taken care of by the {-# RULES "++" [~1] forall xs ys. xs ++ ys = augment (\c n -> foldr c n xs) ys #-} ? No, inlining @augment g xs = g (:) xs@ gives me (\c n -> foldr c n xs) (:) ys = foldr (:) ys xs which still traverses xs even if ys=[]. Any thoughts? [1] http://hackage.haskell.org/package/base-4.10.1.0/docs/src/GHC.Base.html#%2B… Cheers Ben

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Haskell memory model
by Li-yao Xia 05 Mar '18

05 Mar '18

Hello café, What guarantees are there about shared memory concurrency in Haskell? In particular, I'm wondering how to correctly and efficiently synchronize mutable operations from the vector package, using stm or atomic-primops. - Are reads/writes on vectors sequentially consistent? As a concrete example, can the following program (a common test for relaxed memory models) print anything? import qualified Data.Vector.Mutable as MV import Control.Concurrent main :: IO () main = do v <- MV.replicate 2 0 t <- newEmptyMVar forkIO $ do MV.write v 0 1 x <- MV.read v 1 putMVar t x forkIO $ do MV.write v 1 1 x <- MV.read v 0 putMVar t x a <- takeMVar t b <- takeMVar t if a == 0 && b == 0 then putStrLn "Relaxed!" else return () - Do atomic operations (via stm or atomic-primops) imply some constraints between vector operations? As another concrete example, can the snippet linked below ever throw an exception? One thread writes to a vector, and another reads from it, and they communicate via stm to guess whether the read value is safe to evaluate (in one test case the first thread overwrites a defined value with undefined, and in the other case it overwrites undefined with a defined value). http://lpaste.net/362596 Regards, Li-yao

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Re: [Haskell-cafe] missing optimization for (++)
by Markus Läll 04 Mar '18

04 Mar '18

Hi Viktor What I think the OP is asking for is a case where the original list would be returned immediately if the second is empty -- keeping the original spine. Since that case is missing then the list is pattern-matched and then reconstructed. Whether this actually happens after compilation is the real question. On Mar 4, 2018 21:24, "Viktor Dukhovni" <ietf-dane(a)dukhovni.org> wrote: > On Mar 4, 2018, at 4:32 AM, Ben Franksen <ben.franksen(a)online.de> wrote: > > I found that in base [1] we have > > (++) [] ys = ys > (++) (x:xs) ys = x : xs ++ ys > > I had expected there to be a clause > > (++) xs [] = xs > > at the top, which would avoid the list traversal in this common case. > > [...] > > which still traverses xs even if ys=[]. > > Any thoughts? Haskell is lazy, the traversal of x happens only when the combined list is actually traversed, and only for as many elements as requested, so the construction of the concatenated list carries little additional cost. And as pointed out, needlessly forcing the first element of y can have unintended consequences. For example: GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Prelude> let x = [1..] Prelude> let y = [1..] Prelude> let z = x ++ y Prelude> take 20 z [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] -- Viktor. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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cabal: Couldn't parse the output of 'setup register --gen-pkg-config':NoParse
by Henk-Jan van Tuyl 03 Mar '18

03 Mar '18

L.S., I am trying to get wxHaskell compiled with GHC 8.4.0.20180224 and cabal-install 2.1.0.0; compilation fails with the message: wxcore-0.93.0.0-1L9rVktyK3zIYrrLxBiCr9 failed during the final install step. The exception was: dieVerbatim: user error (cabal: Couldn't parse the output of 'setup register --gen-pkg-config':NoParse "license" 5 ) I am using Windows 8.1. Can anybody tell me what to do about this? Regards, Henk-Jan van Tuyl -- Message from Stanford University: Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://foldingathome.stanford.edu/ -- http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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ANN: network-2.6.3.4
by evan＠evan-borden.com 01 Mar '18

01 Mar '18

Announcing network-2.6.3.4 * Don't touch IPv6Only when running on OpenBSD [#227](https://github.com/haskell/network/pull/227) * Do not closeFd within sendFd [#271](https://github.com/haskell/network/pull/271) * Updating examples and docs. A big thanks to the network contributors. http://hackage.haskell.org/package/network-2.6.3.4

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