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test
by Gershom B 30 May '18

30 May '18

Testing this list, due to a report. -g

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Munich Haskell Meeting, 2018-05-28 @ 19:30
by Leah Neukirchen 26 May '18

26 May '18

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, May 28 at Hirschgarten (self-service area) at 19h30. (The weather should be fine for sitting outside, else can move inside.) 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-may-2018/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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Optimizing nearest-k code
by Richard Evans 25 May '18

25 May '18

Dear Haskell Cafe, Given a set of sets, and a particular target set, I want to find the sets that are nearest (in terms of Hamming distance) to the target set. I am using the following code: import Data.List import qualified Data.Set as Set nearest_k :: Ord a => Int -> [(Set.Set a, v)] -> Set.Set a -> [(Set.Set a, v)] nearest_k k bs b = take k bs' where bs' = sortOn (hamming b) bs hamming :: Ord a => Set.Set a -> (Set.Set a, v) -> Int hamming x (y, _) = hamming_distance x y hamming_distance :: Ord a => Set.Set a -> Set.Set a -> Int hamming_distance xs ys = Set.size (Set.difference xs ys) + Set.size (Set.difference ys xs) subsets :: [a] -> [[a]] subsets [] = [[]] subsets (x:xs) = subsets xs ++ map (x:) (subsets xs) int_lists :: [[Int]] int_lists = subsets [1..20] values :: [(Set.Set Int, Int)] values = map f (zip [1..] int_lists) where f (i, x) = (Set.fromList x, i) test = nearest_k 8 values (Set.fromList [1,2,3]) ---- This works ok for the test above (with sets of ints), but is rather slow in my actual application (in which the sets are large sets of ground atoms of first-order logic). Is there some major optimization I should be doing here? thanks, Richard

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Help evaluating TQueue variants
by David Feuer 25 May '18

25 May '18

I've written three related variations on TQueue (from the stm package). All three are designed to avoid the reader starvation problem described in GHC Trac 9539 [1] while limiting contention. Two are based on Okasaki's scheduled (i.e., real-time) banker's queues, while the third is based on his amortized-time banker's queue. I am hoping one or more people with programs that use TQueue heavily would be willing and able to try out these alternatives and see how they perform in their applications. I'm interested in how they compare to the current TQueue implementation, and also how they compare to each other. The queues are currently in a GitHub repository [2], but if it would be helpful I could put them up on Hackage. Thank you for your time. [1] https://ghc.haskell.org/trac/ghc/ticket/9539 [2] https://github.com/treeowl/stm-queues

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haskell in the cloud
by Dennis Raddle 24 May '18

24 May '18

I have a CPU-intensive Haskell application. I have it working with simple multi-core concurrency. I'm wondering if I can run this on a cloud virtual machine with 128 cores or so, paying by the CPU minute. I'll run it for maybe 15 minutes a day so I'm probably best off paying just for the CPU used. What platform would be recommended? For ease of use? For best price/performance? It's a backtracking optimization algorithm that builds data, one element at a time. It's not hard at all to make it concurrent: at the first 3 or so levels of element choices would be about 100 to 500 combinations. Even if the simplest method of running concurrent Haskell on multiple cores doesn't work, I could just divide these first cases into batches and run them on individual machines. D

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how do cabal internal libraries work?
by Evan Laforge 24 May '18

24 May '18

The example at https://www.haskell.org/cabal/users-guide/developing-packages.html#library doesn't seem to work as I expect. The doc for the library field is out of date, but if you scroll down it mentions "internal libraries". But depending on the internal library doesn't seem to have any effect, in that cabal still complains I didn't mention Foo.Internal, and doesn't see the build-depends from it. Am I misinterpreting how the feature is supposed to work? This is with Cabal 2.2.0.1 and cabal-install 2.2.0.0, ghc 8.4.1: % cat testing.cabal name: foo version: 1.0 license: BSD3 cabal-version: >= 1.23 build-type: Simple library foo-internal exposed-modules: Foo.Internal build-depends: base, text library exposed-modules: Foo.Public build-depends: foo-internal, base % cat Foo/Internal.hs module Foo.Internal where import Data.Text % cat Foo/Public.hs module Foo.Public where import Foo.Internal % cabal build Resolving dependencies... Configuring foo-1.0... Warning: Packages using 'cabal-version: >= 1.10' must specify the 'default-language' field for each component (e.g. Haskell98 or Haskell2010). If a component uses different languages in different modules then list the other ones in the 'other-languages' field. Preprocessing library 'foo-internal' for foo-1.0.. Building library 'foo-internal' for foo-1.0.. [1 of 1] Compiling Foo.Internal ( Foo/Internal.hs, dist/build/foo-internal/Foo/Internal.o ) [1 of 1] Compiling Foo.Internal ( Foo/Internal.hs, dist/build/foo-internal/Foo/Internal.p_o ) Preprocessing library for foo-1.0.. Building library for foo-1.0.. <no location info>: warning: [-Wmissing-home-modules] These modules are needed for compilation but not listed in your .cabal file's other-modules: Foo.Internal [1 of 2] Compiling Foo.Internal ( Foo/Internal.hs, dist/build/Foo/Internal.o ) Foo/Internal.hs:2:1: error: Could not find module ‘Data.Text’ Use -v to see a list of the files searched for. | 2 | import Data.Text | ^^^^^^^^^^^^^^^^

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profunctorial vs vanlaarhoven lenses
by Paolino 24 May '18

24 May '18

Hello, I'm trying to write a lens for a datatype which seems easy in the Twan van Laarhoven encoding but I cannot find it as easy in the profunctorial one data Q5 a b = Q51 a (Identity b) | Q52 [b] lq5Twan :: Applicative f => (b -> f b') -> Q5 a b -> f (Q5 a b') lq5Twan f (Q51 a bs) = Q51 a <$> traverse f bs lq5Twan f (Q52 bs) = Q52 <$> traverse f bs data BT tt tt' b t t' a = BT1 (tt -> b) (t a) | BT2 (tt' -> b) (t' a) deriving (Functor,Foldable,Traversable) runBT (BT1 f x) = f x runBT (BT2 f x) = f x lq5Profunctor :: forall p a b b' . Traversing p => p b b' -> p (Q5 a b) (Q5 a b') lq5Profunctor = dimap pre post . second' . traverse' where pre (Q51 a x) = ((), BT1 (Q51 a) x) pre (Q52 bs) = ((), BT2 Q52 bs) post ((),x) = runBT x The issue here is that I need to project type 'b' into 2 different Traversable (sketched [] and Identity) so the traverse' must handle both together. Is the lq5Twan correct ? Which simpler ways to write the lq5Profunctor we have ? Thanks for help. paolino

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Question of defining Type Class
by Shinichi Higashino 22 May '18

22 May '18

I am confusing since I am trying to define Abstract sense of Matrix; class Mat - is a more abstract concept since 1x1 Matrix is isomorphic to Num - is a more abstract concept since 1xn Matrix is isomorphic to Vector So, class Mat a => Num a where … class Mat a => Vec a where … should be . However, I know that the Initial object of Ring Category is Integer . So, I should start with Z and then I should define class ( Module R on Z ) But I could not find that kind of definition in Haskell. Is it Data.Ring ? I am so confusing now… Please help me if you are drinking some tea just now!

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Tezos is hiring 40 functional programmers
by Ryan Trinkle 22 May '18

22 May '18

Hi everyone, I received this from Tezos - one of Obsidian's clients - and I thought it would be of interest to people here. Tezos is a blockchain and a platform of distributed applications written in OCaml, which brings scalability, formal verification and self-amendment to the blockchain space. Tezos has 40 open positions in the world including 25 in France. Remote and part-time accepted. We are looking for profiles ranging from masters (with possible funding for PhD) to PhD with extensive experience. Expertise in a functional language such as OCaml, Haskell, Clojure or equivalent is required, as well as skills in one of the following topics: algorithms, compilers, design of programming languages, cryptography, p2p, web programming / UI in OCaml, SAT / SMT, linear and integer programming, formal verification, stochastic processes, computer algebra or numerical optimization. Highly competitive salaries. If you are interested, please send your CV (resumé) to careers(a)tezos.com Best, Ryan

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tutorials, Freer or "data a la carte"
by Dennis Raddle 21 May '18

21 May '18

I'm interested in Control.Monad.Freer, but I would need some kind of tutorial aimed at a low-intermediate level. Also I think "data a la carte" might be applicable to me, but again I would need some kind of tutorial. D

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