- Haskell-Cafe - Haskell.org

Munich Haskell Meeting, 2019-09-30 @ 19:30
by Leah Neukirchen 27 Sep '19

27 Sep '19

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, September 30 at Augustiner-Gaststätte Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-sep-2019/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

1 0

Stack overflow with ReadP
by Henry Laxen 25 Sep '19

25 Sep '19

Dear Haskell gurus, There has to be something very simple that I am misunderstanding here. I have tried sticking in trace statement, using the ghci debugger, but it just makes no sense to me. Please tell me what I am missing. Best wishes, Henry Laxen -------------------------------------------------------------------------------- module S where import Text.ParserCombinators.ReadP import Data.Char data Exp = Num Int | Add Exp Exp deriving (Eq, Show) expr :: ReadP Exp expr = do e <- (parseNumber +++ parseAdd) return e parseAdd :: ReadP Exp parseAdd = do e1 <- expr _ <- char '+' e2 <- expr return (Add e1 e2) parseNumber :: ReadP Exp parseNumber = do ds <- (munch1 isDigit) return . Num . read $ ds parse s = let parses = (readP_to_S expr) s in case parses of (p : _) -> fst (last parses) _ -> error "parse error" main = do print $ parse "1" λ> main *** Exception: stack overflow λ> parse "1+2" *** Exception: stack overflow -------------------------------------------------------------------------------- -- Nadine and Henry Laxen The rest is silence Villa Alta #6 Calle Gaviota #10 Never try to teach a pig to sing Chapala It wastes your time +52 (376) 765-3181 And it annoys the pig

5 4

Foldable, Traversable and choosing an order
by Juan Casanova 25 Sep '19

25 Sep '19

Hello again, haskell-cafe. I was trying to implement the Unifiable typeclass from Control.Unification on one of my types, and did so fine, but then realized I needed a Traversable instance. So I thought about it, and concluded: Yes, my type definitely is Traversable, and I implemented Traversable. But then I realized I needed a Foldable instance for that. And then I wasn't so sure anymore. I *can* implement a Foldable instance, but it requires a choice of the order in which my structure is traversed that I don't really want to make? A.k.a: I can think of at least two different ways of implementing Foldable that would give different results if the function passed to foldr was not associative. So I looked online a bit, and it seems this is a topic people have talked about, but maybe not precisely in the context of the order. What I have gathered is that: yes, if a functor is Traversable, then there is at least one way to implement a Foldable in it. And I understand this, but then, I don't really grasp how that works in my specific case. Because my Traversable instance does not make a choice of this order (at least I think so), and I do need to make that choice to implement Foldable (at least I think so), and I don't see how the "default" implementation of foldMap in terms of traverse makes that choice. But the choice must be made somewhere if it is made in the Foldable. So what is going on? Of course, here's the code. My types: data SOTermPF fn p f = ConstF fn | Proj Int | CompF p [f] deriving Eq newtype SOTermF fn f = SOF (SOTermPF fn f f) deriving Eq My Traversable instance (with comments on intermediate types because otherwise it can get pretty obscure): instance Traversable (SOTermF fn) where traverse f (SOF (ConstF c)) = pure (SOF (ConstF c)) traverse f (SOF (Proj idx)) = pure (SOF (Proj idx)) -- f g :: f b -- map f sargs = [f b] -- CompF :: a -> ([a] -> SOTermPF fn p a) -- (\h -> \ts -> SOF (CompF h ts)) :: ([a] -> SOTermF fn a) -- fmap (\h -> \ts -> SOF (CompF h ts)) (f g) :: f ([b] -> SOTermF fn b) -- traverse :: (aa -> ff bb) -> [aa] -> ff [bb] -- traverse :: (f b -> f b) -> [f b] -> f [b] -- ((fmap (\h -> \ts -> SOF (CompF h ts)) (f g)) <*>) :: f [b] -> f (SOTermF fn b) -- traverse id (map f sargs) :: f [b] -- (fmap (\h -> \ts -> SOF (CompF h ts)) (f g)) <*> (traverse id (map f sargs)) :: f (SOTermF fn b) traverse f (SOF (CompF g sargs)) = (fmap (\h -> \ts -> SOF (CompF h ts)) (f g)) <*> (traverse id (map f sargs)) But to implement Foldable I need to choose an order because in the case where I do have elements under the functor (CompF), I have them in two shapes: The "head" and the "arguments" (the p and the [f] in CompF p [f]). So, of course, the arguments are traversed in the order of the list, but the order that I need to choose is: Do I apply the head first and the arguments later or do I apply the arguments first and the head later? But my traverse seems so natural. Am I making that choice there? Maybe if I made the fmap over the arguments (while traversing the list) instead of over the head, and then did a flipped application of <*>? Does that mean my traverse instance has implicitly assumed that the head will be done first? All of this is really out of curiosity and making sure I know what I'm doing: I will not use foldr on my structure and I'm pretty sure that the traversal that Control.Unification needs is associative. But I don't like seeing arbitrary choices appear out of nowhere in my code, I want to make sure of why I made them. :P Thanks in advance, Juan. -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

6 9

JOB | Web Architect (London, UK)
by James Tobin 25 Sep '19

25 Sep '19

Hello, I'm working with an employer that is looking to hire a permanent front end architect to join their London office. You should have experience with Javascript and frameworks such as (but not only) React. Consequently, I had hoped that some members of this mailing list may like to discuss further off-list using "JamesBTobin (at) Gmail (dot) Com". Kind regards, James

2 1

ANN: network 3.1.1.0
by Evan Borden 23 Sep '19

23 Sep '19

I'm happy to announce the release of network 3.1.1.0. http://hackage.haskell.org/package/network-3.1.1.0 Changes: - A new API: gracefulClose. #417 <https://github.com/haskell/network/pull/417> Also see Kazu's blog post on the design choices in this function https://kazu-yamamoto.hatenablog.jp/entry/2019/09/20/165939 - touchSocket, unsafeFdSocket: Allow direct access to a socket's file descriptor while providing tools to prevent it from being garbage collected. This also deprecated fdSocket in favor of unsafeFdSocket and withFdSocket. #423 <https://github.com/haskell/network/pull/423> - socketToFd: Duplicates a socket as a file descriptor and closes the source socket. #424 <https://github.com/haskell/network/pull/424> As always thank you to network's contributors.

1 0

FMBC 2019 Call for Participation - Porto (Portugal), October 11
by Bruno Bernardo 23 Sep '19

23 Sep '19

[Please accept our apologies for duplicates.] ===================================================== Call for Participation 1st Workshop on Formal Methods for Blockchains (FMBC) 2019 https://sites.google.com/view/fmbc/home Porto, Portugal, October 11 Part of the 3rd World Congress on Formal Methods http://formalmethods2019.inesctec.pt/ ===================================================== About FMBC ---------------- Blockchains are decentralized transactional ledgers that rely on cryptographic hash functions for guaranteeing the integrity of the stored data. Participants on the network reach agreement on what valid transactions are through consensus algorithms. Blockchains may also provide support for Smart Contracts. Smart Contracts are scripts of an ad-hoc programming language that are stored in the blockchain and that run on the network. They can interact with the ledger’s data and update its state. These scripts can express the logic of possibly complex contracts between users of the blockchain. Thus, Smart Contracts can facilitate the economic activity of blockchain participants. With the emergence and increasing popularity of cryptocurrencies such as Bitcoin and Ethereum, it is now of utmost importance to have strong guarantees of the behavior of blockchain so ware. These guarantees can be brought by using Formal Methods. Indeed, Blockchain software encompasses many topics of computer science where using Formal Methods techniques and tools are relevant: consensus algorithms to ensure the liveness and the security of the data on the chain, programming languages specifically designed to write smart contracts, cryptographic protocols, such as zero-knowledge proofs, used to ensure privacy, etc. This workshop is a forum to identify theoretical and practical approaches of formal methods for blockchain technology. Topics include, but are not limited to: * Design and implementation of Smar Contract languages * Formal models of blockchain applications or concepts * Formal methods for consensus protocols * Formal methods for blockchain-specific cryptographic primitives or protocols * Formal languages for Smart Contracts * Verification of Smart Contracts Invited Speaker -------------------- Ilya Sergey (Yale-NUS College / NUS School of Computing) Contributed papers -------------------------- See the workshop program at: https://sites.google.com/view/fmbc/program Registration ---------------- Registration is shared for all FM events: http://formalmethods2019.inesctec.pt/?page_id=2363 Attending ------------- See the FM webpage at: http://formalmethods2019.inesctec.pt/?page_id=140 Contact ---------- mailto:fmbc19@easychair.org

1 0

Re: [Haskell-cafe] How are we helping? Climate Strike / Earth Strike fri 20, fri 27
by Olaf Klinke 22 Sep '19

22 Sep '19

Haskeller working in the renewable energy sector here. Richard O'Keefe has good points, it is hard for the average programmer to have a real impact on climate change. Power consumption of computing equipment, though, is indeed a huge part of our electricity usage, so any progress will be very welcome in this sector. Ask your cloud computing vendor where the data center sources its power. <shameless advertising> It ought to look like this. [*] </shameless advertising> My suggestion is to help make renewables the cheapest power source in the market. It already is in some countries, and both China and India are investing massively in wind and solar power, not beause of morale, but beause it is cheap. So go ahead and develop good SCADA software, help the grid operators balance their volatile loads, use deep learning and so-called AI to make maintenance cheaper and life cycles longer. And, but this has nothing to do with programming, keep pestering your first-world governments. Even if your country's share in the global emissions is minute, showing that even industrialized countries can reduce their emissions hopefully leads to others following. But this is only going to work if saving the environment is also cheaper compared to going on as usual. Olaf [*] http://windcores.de/en/homepage/#!/who

1 0

How are we helping ? Climate Strike / Earth Strike fri 20, fri 27
by Simon Michael 22 Sep '19

22 Sep '19

Hi all, There's world-wide "strike" event, to raise awareness and stimulate climate action, happening tomorrow and also on friday 27th. Some folks will observe one, the other, both, or the whole period between. We probably don't all share the same awareness or even concern about climate change.. but this is changing rapidly. I wanted to start a discussion and figure out something we could do to support these events.. earlier. But I'm working, preparing for a trip, etc. etc. and "ran out of time". I'll send this now at least. If the site goes green or something, this is the reason. Re the sep 20 and sep 27 events: Thanks to Open Collective, for their leadership and blog post: https://blog.opencollective.com/digital-climate-strike hledger's opencollective, though we're not really using it yet, is opted in for tomorrow: https://opencollective.com/hledger Origin and recent history of this event: https://en.wikipedia.org/wiki/School_strike_for_climate#'Intergenerational'_Global_Climate_Strikes_in_September_2019 There are probably a bunch of "home" sites for this, here are two: https://www.fridaysforfuture.org https://www.earth-strike.com Going forward: I think "How are we helping?" is a standard question every individual, project, institution should be answering. For the hledger project, I've had a few ideas for a long time but haven't managed to manifest them yet. Looking into the near future, some days it feels like working on hledger is a waste of time. Probably many of you can relate. If you are working on anything in this direction I'd love to hear more. Best! -Simon PS I ran out of time even more for crafting a mail to haskell-cafe; cc'ing this anyway.

2 1

Changing a function definition causes an error at link-time
by Vanessa McHale 21 Sep '19

21 Sep '19

Hi all, I have a function ft :: CMode -> FileType ft i = toEnum (fromIntegral i) which is used to marshal a function defined with c2hs, viz. {# fun archive_entry_filetype as ^ { `ArchiveEntryPtr' } -> `FileType' ft #} This all works fine under GHC 8.8.1-GHC-8.2.2. When I try to compile the test suite with GHC 8.0.2 I get Linking /home/vanessa/programming/haskell/done/libarchive/dist-newstyle/build/x86_64-linux/ghc-8.0.2/libarchive-2.0.0.0/t/libarchive-test/build/libarchive-test/libarchive-test ... /usr/bin/ld: /home/vanessa/programming/haskell/done/libarchive/dist-newstyle/build/x86_64-linux/ghc-8.0.2/libarchive-2.0.0.0/build/libHSlibarchive-2.0.0.0-inplace.a(ArchiveEntry.o): in function `rg6i_info': (.text+0x9cd6): undefined reference to `archive_entry_acl_next_w' /usr/bin/ld: /home/vanessa/programming/haskell/done/libarchive/dist-newstyle/build/x86_64-linux/ghc-8.0.2/libarchive-2.0.0.0/build/libHSlibarchive-2.0.0.0-inplace.a(ArchiveEntry.o): in function `cjVN_info': (.text+0x128c6): undefined reference to `archive_entry_acl_next_w' collect2: error: ld returned 1 exit status `gcc' failed in phase `Linker'. (Exit code: 1) When I define the marshaling function with ft :: CMode -> FileType ft 0 = error "here" ft i = toEnum (fromIntegral i) I can no longer compile the test suite; it fails with a linker error, even with GHC 8.8.1-GHC-8.2.2. I get Linking /home/vanessa/programming/haskell/done/libarchive/dist-newstyle/build/x86_64-linux/ghc-8.8.1/libarchive-2.0.0.0/t/libarchive-test/build/libarchive-test/libarchive-test ... /home/vanessa/programming/haskell/done/libarchive/dist-newstyle/build/x86_64-linux/ghc-8.8.1/libarchive-2.0.0.0/build/libHSlibarchive-2.0.0.0-inplace.a(ArchiveEntry.o)(.text+0x9d86): error: undefined reference to 'archive_entry_acl_next_w' /home/vanessa/programming/haskell/done/libarchive/dist-newstyle/build/x86_64-linux/ghc-8.8.1/libarchive-2.0.0.0/build/libHSlibarchive-2.0.0.0-inplace.a(ArchiveEntry.o):function libarchivezm2zi0zi0zi0zminplace_CodecziArchiveziForeignziArchiveEntry_zdwarchiveEntryAclNextW_info: error: undefined reference to 'archive_entry_acl_next_w' collect2: error: ld returned 1 exit status `gcc' failed in phase `Linker'. (Exit code: 1) You can have a look at some of some of the code in this branch here: https://github.com/vmchale/libarchive/blob/ghc-potential-bug/src/Codec/Arch… and here: https://github.com/vmchale/libarchive/blob/ghc-potential-bug/src/Codec/Arch… Cheers, Vanessa McHale

1 0

Re: [Haskell-cafe] [Haskell] ANNOUNCE: mm2: The library that can be used for optimization of multiple (Ord a) => a -> b transformations
by Henning Thielemann 21 Sep '19

21 Sep '19

On Sat, 21 Sep 2019, olexandr543--- via Haskell wrote: > My library that can help to optimize using 'case ... of ...' construction if there are multiple (more than at > least 5) variants. > mm2: The library that can be used for optimization of multiple (Ord a) => a -> b transformations Isn't this a problem you would solve using Data.Map?

2 2