January 2017 - Haskell-Cafe

install of ghc-8.0.1 failing
by Michael Litchard 07 Jan '17

07 Jan '17

I'm trying to install ghc-8.0.1 on ppc64 arch. Below is the truncated config.log, followed by the error that stops make. It was created by The Glorious Glasgow Haskell Compilation System configure 8.0.1, which was generated by GNU Autoconf 2.69. Invocation command line was $ ./configure ## --------- ## ## Platform. ## ## --------- ## hostname = ubuntu-ppc64 uname -m = ppc64 uname -r = 4.4.0-53-powerpc64-smp uname -s = Linux uname -v = #74-Ubuntu SMP Fri Dec 2 16:41:54 UTC 2016 /usr/bin/uname -p = unknown /bin/uname -X = unknown /bin/arch = unknown /usr/bin/arch -k = unknown /usr/convex/getsysinfo = unknown /usr/bin/hostinfo = unknown /bin/machine = unknown /usr/bin/oslevel = unknown /bin/universe = unknown PATH: /home/lambdafan/.cabal/bin PATH: /home/lambdafan/bin PATH: /home/lambdafan/.local/bin PATH: /usr/local/sbin PATH: /usr/local/bin PATH: /usr/sbin PATH: /usr/bin PATH: /sbin PATH: /bin PATH: /usr/games PATH: /usr/local/games PATH: /snap/bin PATH: /home/lambdafan/ghc-8.0.1 <snip> configure: exit 0 Now, the error that make stops at E: Unable to locate package gmake lambdafan@ubuntu-ppc64:~/ghc-8.0.1$ make ===--- building phase 0 make --no-print-directory -f ghc.mk phase=0 phase_0_builds make[1]: Nothing to be done for 'phase_0_builds'. ===--- building phase 1 make --no-print-directory -f ghc.mk phase=1 phase_1_builds utils/genapply/ghc.mk:23: utils/genapply/dist/build/.depend.haskell: No such file or directory utils/genapply/ghc.mk:23: utils/genapply/dist/build/.depend.c_asm: No such file or directory make[1]: *** No rule to make target '/home/kgardas/sfw/ghc-7.10.1/ lib/ghc-7.10.1/base_I5BErHzyOm07EBNpKBEeUv/Prelude.hi', needed by 'utils/deriveConstants/dist/build/Main.o'. Stop. Makefile:129: recipe for target 'all' failed make: *** [all] Error 2 I looked in utils/genapply/dist/build/ to find only this ls -la utils/genapply/dist/build/ total 12 drwxr-xr-x 3 lambdafan lambdafan 4096 Jan 4 13:07 . drwxr-xr-x 3 lambdafan lambdafan 4096 Jan 4 13:07 .. drwxr-xr-x 2 lambdafan lambdafan 4096 Jan 4 13:07 autogen advice on how to proceed?

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Build base with stack
by Robin Palotai 06 Jan '17

06 Jan '17

Hello Cafe, Do you know a way to force a rebuild of base with stack? I don't want to modify it, just want to feed it through a customized ghc pipeline. Using stack is desirable for consistency with how other packages are run through that pipeline. When I download the appropriate base package and put it in a multi project stack directory, I see all the dependent packages are rebuilt, but not base. Thanks! Robin

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Problems with truncate big float values in Haskell
by Henson 05 Jan '17

05 Jan '17

Hi, Firstly I desire one year brilliant for all on group. I have one simple problem but not solved for me for some mistake mine. I have one Rational number (3 + sqrt 5) and I need get the last three digits from integral part of that number. The question is the number is arbitrary and that number always is power of n and n is between 2(Two) and 2000000000(Two billions). I have attempted that on ghci for various values and get same result with different powers: let x = (truncate ((3 + sqrt 5) ^ 2000000)) `mod` 1000 and result is 216. let y = (truncate ((3 + sqrt 5) ^ 20000000)) `mod` 1000 and result is 216. let w = (truncate ((3 + sqrt 5) ^ 200000000)) `mod` 1000 and result is 216. let z = (truncate ((3 + sqrt 5) ^ 2000000000)) `mod` 1000 and result is 216. The result is same for all: 216, and is incorrect because any that expressions have one exclusive value. What I do for complete that task correctly and what I'm mistaken here?! Thank you, Josenildo Silva

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Re: [Haskell-cafe] Problems with truncate big float values in Haskell
by Manuel Gómez 05 Jan '17

05 Jan '17

On Thu, Jan 5, 2017 at 3:51 PM, Henson <jfsihenson(a)gmail.com> wrote: > Ok! I have mistake the correct number. Really the number is Irrational and > the problem is the values are Infinity but How I work with BigFloat precision > for Arbitrary numbers in Haskell?! > > I need get the values for arbitrary precision numbers in haskell about > samples posted not is better implementation for that because range of double > implemented in various languages. > > The correct question here about that is How to resolve that problem?! > > I need get three last digits for arbitrary numbers and after search in > various places not found response for that. > > If exists any material about that but not only for explanations about Floats > and Doubles it's a good start for resolve question. I haven't used it, but this package looks like it might help in your situation: http://hackage.haskell.org/package/cyclotomic

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How to tie knots in XML-style data structures
by Auke Booij 05 Jan '17

05 Jan '17

I am reading an XML file, where some nodes have attributes that refer to other nodes. For example: <blah name="first" type="5"> <yada blah="second"></yada> </blah> <blah name="second" type="3"> </blah> Here, the point is that the inner <yada> refers to a <blah> that is not its parent by means of a "blah" attribute. But there might also be a mistake in the XML file, which I would then want to report: the inner <yada> might refer to a <blah> which does not exist. Using an XML parsing library, this XML file can easily be read into a data type such as the following: data Blah = Blah {blahName :: String, blahType :: Int, blahYadas :: [Yada]} data Yada = Yada {yadaBlah :: String} But for my application, it is useful to have the Yadas refer back to actual Blahs, instead of just their name Strings. So I'd like to parse the XML into the following data structure: data Blah' = Blah' {blah'Name :: String, blah'Type :: Int, blah'Yadas :: [Yada']} data Yada' = Yada' {yada'Blah :: Blah} (the only change is the type of yada'Bla from String to Blah) 1. How can I write a function that takes a Blah, and outputs a Blah', and reports a nice error when there is no <blah> with the name specified in the <yada>? Can it be done at all without using partial functions like fromJust (when using Maybe to "report" errors)? 2. Is there a better way to define the types Blah, Yada, Blah' and Yada' (e.g. with less repetition)? After all, all I'm changing is the type of one field, but I end up having to redefine all my types.

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threadDelay delays less time than expected (Windows)
by Lian Hung Hon 04 Jan '17

04 Jan '17

Dear Haskellers, I am baffled by a strange bug (?) with threadDelay on Windows. It delays for less time than expected (500 seconds vs expected 24 hours), and is consistently reproducible on my system. Can someone have a look (need some patience, ~8 minutes waiting), just to be sure? I glanced around trac but didn't find any related issues. How to reproduce: module Main where import Data.Time import Control.Concurrent import Control.Monad main :: IO () main = do _ <- forkIO thread getLine >>= print -- Just to keep the main thread running thread :: IO () thread = forever $ do now <- getCurrentTime print now threadDelay 86400000000 print "done waiting" Expected result: 2016-12-28 13:07:49.5113098 UTC ... (next timing should be the next day 2016-12-29) Actual result: 2016-12-28 13:07:49.5113098 UTC "done waiting" 2016-12-28 13:16:10.2159485 UTC "done waiting" 2016-12-28 13:24:30.8735845 UTC "done waiting" 2016-12-28 13:32:51.5292203 UTC ... System: Windows 7 Enterprise Service Pack 1, ghc 7.10.3 This seems specific to Windows, it doesn't appear on Debian 8. Help is appreciated! Regards, Hon Rant: It has cost me significant debugging time! Please tell me I made a rookie mistake or misread the docs somewhere!

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Deploying Scotty based web app to Amazon Elastic Bean - One approach
by Guru Devanla 02 Jan '17

02 Jan '17

Hello All, Recently, I have been working on a web app in Haskell using Scotty as the web framework. I had to go through different sources and compile together an approach I took to deploy a dockerized version of web app to Amazon Elastic Beanstalk. There may be other approaches available and I am happy to hear about it. Meanwhile, I documented my experience with a simple Hello World app and documented the steps I had to take to get my app up and running on Elastic Beanstalk. I look forward to suggestions on a better way to do this. https://github.com/gdevanla/helloscotty/blob/master/README.md Hopefully, this documentation is helpful, and does not proposed an anti-pattern! Thanks

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Code review - wrapping servant client to hide required parameters
by Nigel Rantor 02 Jan '17

02 Jan '17

Hello, So, I am still learning and would like someone to cast an eye over some code to see if I am missing something in terms of making it cleaner/more concise. I have begun wrapping the They Work For You API using the Servant Client library. All code is already online for you to grab at; https://github.com/wiggly/twfy-api-client https://hackage.haskell.org/package/twfy-api-client The API is not under my control so I can't change how it works or how it accepts parameters. Every API call requires a Key to be passed as a query string parameter. For a particular session/user this will always be the same, and there is no point in allowing the user to not provide it. Since it is a query parameter Servant forces the type to be within Maybe. For example; type TwfyAPI = "getConstituency" :> QueryParam "key" ApiKey :> QueryParam "name" T.Text :> QueryParam "postcode" T.Text :> Get '[JsonIso8859] Constituency Gives the following function signature; getConstituency :: Maybe ApiKey -- ^ API key -> Maybe T.Text -- ^ Name -> Maybe T.Text -- ^ Post code -> ClientM Constituency To make the API easier to use I have created a Client module that wraps these so that I can hide the passing of the API key for every call, and also allow the user to not have to deal with http client managers or URL parsing if they don't want to. The upshot is that I get a signature that now accepts a data type I have created that contains all the information required to run the client action and return a result, like so (where A.getConstituency is the function above taking a Maybe ApiKey as first argument); getConstituency :: Client -> Maybe T.Text -- ^ Name -> Maybe T.Text -- ^ Post code -> IO (Either ServantError Constituency) getConstituency c = do let key = Just $ clientApiKey c env = clientEnv c in (\name postCode -> (flip runClientM) env (A.getConstituency key name postCode)) So, this wraps up all of the calling of the servant client with an appropriate environment and arguments. I have two questions; 1) What do people think of this style of wrapping the servant client calls? 2) I am now returning a lambda that accepts the remaining arguments to the call, is there any way I can make this cleaner? i.e. is it possible for me to not have to specifically re-write all of the function parameters using something like uncurry or other machinery? I have played with trying to use uncurry but got nowhere fast. Thanks in advance, Nigel

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ANN: hledger 1.1
by Simon Michael 01 Jan '17

01 Jan '17

Two months since last release, I'm pleased to announce hledger 1.1 ! Thank you to contributors Johannes Gerer, Nikolay Orlyuk, and Shubham Lagwankar, along with all others who contributed in other ways. hledger (http://hledger.org) is a dependable, precise, cross-platform program for tracking money, time, or any other commodity, using double-entry accounting and a simple plain text file format. Inspired by the Ledger project, hledger provides command-line, curses and web interfaces, and aims to be a robust, practical tool for daily use. Notable user-visible changes in this release: - hledger-ui --watch live-updates as files or the current date change, useful eg when reconciling or for a time log dashboard - Ledger-style balance assignments are now supported in journal files Full release notes: http://hledger.org/release-notes#hledger-1.1 To install this release: Get stack, eg from http://haskell-lang.org/get-started $ stack install --resolver=nightly hledger [hledger-ui] [hledger-web] [hledger-api] $ ~/.local/bin/hledger or see http://hledger.org/download for more installation options. Our IRC channel is #hledger on Freenode, and you can find out more at http://hledger.org. I hope you enjoy these tools and that they help you achieve your goals. If you like hledger, your support and participation is appreciated! Fund us on Gratipay or Bountysource, give feedback, report bugs, send pull requests, write about it, etc. Happy 2017! Best, -Simon

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How to call python from haskell
by jun zhang 31 Dec '16

31 Dec '16

Dear cafe I had try to use the http://hackage.haskell.org/package/cpython <http://hackage.haskell.org/package/cpython> package. I can call python function once by the package, but when I call function second time it will occur a "Segmentation fault”. Does there any stable package support call python from haskell? Thanks. Julian

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