- Haskell-Cafe - Haskell.org

Is it possible to zip sequences any faster?
by David Feuer 28 Dec '14

28 Dec '14

containers-0.5.6 has a new implementation of zipWith in Data.Sequence that, like the old one, offers O(n) zipping, but unlike the old one offers immediate O((log(min{i,n-i}))^2) access to the elements, where i is the requested index. I haven't been able to think of a way to bring that down from polylogarithmic to logarithmic. The difficulty is that t splits one of the sequences repeatedly, so reaching an element the first time is kind of like pushing a snow shovel all the way down the driveway, rather than throwing snow off to either side. Does anyone have an idea for doing it better, or a good argument that it can't be done? Thanks, David Feuer

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How to call hUpdateAtHNat with a value known only at runtime
by Phil Darnowsky 28 Dec '14

28 Dec '14

Hello all, I'm working on something that involves HLists where I'm making use of the hUpdateAtHNat function for array-like semantics, which works great if you know the index of the element you want at compile time: Prelude> :module +Data.HList Prelude Data.HList> let hl = hEnd $ hBuild "foobar" 123 True Loading [a bunch of packages] Prelude Data.HList> hl H["foobar", 123, True] Prelude Data.HList> hUpdateAtHNat hZero 90909 hl H[90909, 123, True] Prelude Data.HList> hUpdateAtHNat (hSucc hZero) 90909 hl H["foobar", 90909, True] Prelude Data.HList> hUpdateAtHNat (hSucc $ hSucc hZero) 90909 hl H["foobar", 123, 90909] So far so good. What's giving me trouble is figuring out how to update at an arbitrary index, where that index is only known at runtime, because hZero and hSucc are of different types: Prelude Data.HList> :t hZero hZero :: Proxy 'HZero Prelude Data.HList> :t hSucc hZero hSucc hZero :: Proxy ('HSucc 'HZero) What I'd ideally like is some kind of function intToHNatProxy that would take an Int and return a Proxy of the appropriate type, but I so far only kinda sorta understand how proxies and lifted types work, so I haven't yet succeeded in constructing one. Any advice would be very welcome. Thanks, Phil

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ANN: Groundhog-inspector, a tool for generating datatypes from database
by Boris Lykah 28 Dec '14

28 Dec '14

Hello everyone, I am pleased to announce groundhog-inspector. It analyzes database schema and creates corresponding datatypes and mapping configuration for Groundhog. It works with PostgreSQL, MySQL, and Sqlite. Composite keys, constraints, references across schemas, and other details of schema are reflected in the output. Groundhog-inspector can be used as a library and provides a standalone tool for simple scenarios. Here is an example of the standalone tool usage. JSON mapping in the output was manually converted to YAML for brevity. $ sqlite3 dbfile "CREATE TABLE mytable (id INTEGER PRIMARY KEY NOT NULL, str VARCHAR NOT NULL, ref INTEGER references mytable)" $ groundhog_inspector sqlite dbfile data Mytable = Mytable {mytableStr :: String, mytableRef :: (Maybe (AutoKey Mytable))} - entity: "Mytable" dbName: "mytable" constructors: - name: "Mytable" fields: - name: "mytableStr" dbName: "str" - name: "mytableRef" dbName: "ref" * [Hackage] (http://hackage.haskell.org/package/groundhog-inspector) * [GitHub] ( https://github.com/lykahb/groundhog/tree/master/groundhog-inspector/examples ) I would be happy to answer the questions. Enjoy! Regards, Boris Lykah

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ghc 7.10.1-rc1 ghc no longer supports single-file style package databases (dist/package.conf.inplace) use 'ghc-pkg init' to create the database with the correct format.
by Konstantine Rybnikov 27 Dec '14

27 Dec '14

Hi! I just did `sudo make install` on ghc 7.10.1-rc1, removed ~/.ghc and did `cabal update`. I'm now trying to upgrade to latest cabal by issuing `cabal install cabal-install`. I get the following: ``` ... on the commandline: Warning: -package-name is deprecated: Use -this-package-key instead ghc: ghc no longer supports single-file style package databases (dist/package.conf.inplace) use 'ghc-pkg init' to create the database with the correct format. Failed to install zlib-0.5.4.2 cabal: Error: some packages failed to install: HTTP-3001.0.4 depends on text-1.2.0.3 which failed to install. cabal-install-0.4.0 depends on zlib-0.5.4.2 which failed to install. mtl-2.2.1 failed during the building phase. The exception was: ExitFailure 1 network-2.6.0.2 failed during the building phase. The exception was: ExitFailure 1 parsec-3.1.7 depends on text-1.2.0.3 which failed to install. text-1.2.0.3 failed during the building phase. The exception was: ExitFailure 1 zlib-0.5.4.2 failed during the building phase. The exception was: ExitFailure 1 ``` Does anyone know a quick-fix? Thanks!

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compile ghc on Nexus 7 Android Tablet
by Cody Goodman 27 Dec '14

27 Dec '14

How would I go about this? I see a lot of stuff about compiling for android/arm, but not too much about putting ghc on an android arm tablet. Has anyone done this? I'm sure it's easier with ubuntu touch, but I'd like to try it using vanilla android. Thanks!

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Tutorials on Machines?
by gonzaw 27 Dec '14

27 Dec '14

Hi. I've been recently getting into the streaming libraries (pipes, conduit, etc), and noticed there was this talk about this "Machines" library which supposedly handles streaming too. However, I'm getting a little confused after reading its source. From what it seems, machines are a completely different beast than pipes or conduits, and I don't seem to find good guides on what exactly they are, other than a generic "a machine is a thingy that takes inputs and maybe produces some output or something". Pipes and conduits I see as constructions that only allow streaming. You put this construction in "between" a source of inputs and a consumer of outputs, and it streams from one to the other, possibly with side-effects, state, termination, etc. The thing is that the tutorials on them, and the examples that get used to show their power, make sense in this context, so you can easily grok them. But it's harder to grok Machines. I see the source and notice how you can model Moore machines, Mealy machines, Stacks, and other constructions, and all of this can be made into machines. But at the same time you can construct machines by yourself with Plans, using similar syntax used by pipes/conduit ("yield", "await"). So it confuses me, since I'm not sure what the connection between concepts like Mealy machines, and concepts like "yield"/"await" (which are also used in other streaming libraries), is. I'd like to know if there is a good tutorial on what all of these are, how they relate to each other, and allow you to easily understand all those constructions (moore/mealy machines, plans, processes, etc) and understand when you can use one or the other. Thanks. -- View this message in context: http://haskell.1045720.n5.nabble.com/Tutorials-on-Machines-tp5762883.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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FRP Tutorial
by Lucas Paul 27 Dec '14

27 Dec '14

I've been trying to get my feet wet with Functional Reactive Programming, and I'd like to play around with a library and some examples, particularly one that's already set up to work with a GUI framework like Gtk2Hs or wx. But it's been nightmarish trying to find something that works. - Grapefruit examples wouldn't compile because they were trying to import things I couldn't find in Hackage. (Codebreaker.hs, for instance) Examples using git-level stuff? - reactive-banana-wx wouldn't build because I have array-0.5 on the system level and cabal couldn't resolve dependencies. - threepenny-gui example Buttons.hs wouldn't compile (16:30: ‘jsStatic’ is not a (visible) constructor field name) Am I just having a run of bad luck, or is FRP really elusive right now for some reason? Should I just be learning some other GUI discipline if I want to write interfaces for my Haskell stuff? Sagely advice would be very much appreciated. - Lucas

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Generic "to tuple" function
by Michael Orlitzky 27 Dec '14

27 Dec '14

I have tons of code like this: from_tuple = uncurry MLBBoxScoreGameBreakdownXml to_tuple (MLBBoxScoreGameBreakdownXml a h) = (a,h) The first function, from_tuple, generalizes easily with `uncurryN` from Data.Tuple.Curry. But I'm wondering, is there any generic function that will take a simple data type and cram it into a tuple? So instead of writing, from_tuple = uncurryN MLBBoxScoreGameBreakdownTeamXml to_tuple (MLBBoxScoreGameBreakdownTeamXml w x y z) = (w,x,y,z) I could just do, from_tuple = uncurryN MLBBoxScoreGameBreakdownTeamXml to_tuple = toTupleGeneric because things get real ugly when there are ~20 fields in the data type.

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Computer Language Benchmarks Game: Mandelbrot
by Henk-Jan van Tuyl 27 Dec '14

27 Dec '14

L.S., The holidays a good time to do something about the Haskell entries for the Computer Language Benchmarks Game. I made the Mandelbrot entry[0] a bit smaller, see the attached file. I removed superfluous imports, type declarations and all bangs (except in the line "data T = T !Int !Int !Int !Double"). Both with and without bangs, there is a memory leak; in both cases, the run time is the same (tested on Windows 8.1, GHC 7.8.3). If no one knows any further improvements, I will submit my version to the Benchmarks Game. By the way, would it be a good idea to create a public (Darcs) repository for the Benchmarks Game entries? Regards, Henk-Jan van Tuyl [0] http://benchmarksgame.alioth.debian.org/u64q/program.php?test=mandelbrot&la… -- 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://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming

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Re: [Haskell-cafe] Handling Postgresql array types
by Riaan 27 Dec '14

27 Dec '14

Thank you very much. I have actually started to have a look at opaleye independently after the reddit discussion and I would like to experiment with it. I assume that I somehow have to declare my own queryRunnerColumn for the array. I am afraid that I did not understand the example so I was not able to generalise it. Date: Fri, 26 Dec 2014 11:02:12 +0000 From: Tom Ellis <tom-lists-haskell-cafe-2013(a)jaguarpaw.co.uk> To: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Handling Postgresql array types Message-ID: <20141226110212.GJ31575@weber> Content-Type: text/plain; charset=us-ascii On Fri, Dec 26, 2014 at 10:52:46AM +0000, Tom Ellis wrote: > On Fri, Dec 26, 2014 at 02:36:07PM +1100, Riaan wrote: > > So went back to HDBC. But now my queries are starting to get fairly long > > and I have been looking at libraries like Persistent with Esqualeto, > > HaskellDB and Groundhog to make my queries a little more composable and > > type safe. > > Don't forget to look at Opaleye too :) It doesn't really have much support > for arrays at the moment, but if you let me know what you want to do I'm > happy to help you. > > > So to my question. Does anyone have experience with one of these libraries > > in dealing with postgresql arrays > > postgresql-simple has > > instance (FromField a, Typeable a) => FromField (PGArray a) > > Does that not do exactly what you want? import Database.PostgreSQL.Simple (query_, ConnectInfo(..), connect, Only) import Database.PostgreSQL.Simple.Types (PGArray) import Data.String (fromString) connectInfo :: ConnectInfo connectInfo = ConnectInfo { connectHost = "localhost" , connectPort = 25433 , connectUser = "tom" , connectPassword = "tom" , connectDatabase = "opaleye_test" } arrayQuery :: String arrayQuery = "select '{{1,2}, {3,4}}' :: integer[][]" main :: IO () main = do conn <- connect connectInfo results <- query_ conn (fromString arrayQuery) :: IO [Only (PGArray (PGArray Int))] print results -- Output -- -- ghci> main -- [Only {fromOnly = PGArray {fromPGArray = [PGArray {fromPGArray = [1,2]},PGArray {fromPGArray = [3,4]}]}}]

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