- Haskell-Cafe - Haskell.org

Can I allow multiple rigid type variables?
by Cody Goodman 14 Jan '17

14 Jan '17

I know my question doesn't quite make sense and you can't have "multiple rigid type variables" but I couldn't think of a better way to characterize it or explain it. So I'll also try to give more detail below along with the code example and error at the end. I have an inner join function using vinyl which does cool stuff like giving a type error if you try to join on a column that doesn't exist. I previously had an implementation that blindly built an index based on the left argument. I then tried to dispatch based on the lengths of the left and right producer arguments. The problem is my return type is: Rec f (as ++ bs) That means I have two different producers, one of as and one of bs. I then do a case statement on the lengths and I want to store the producer to use for a given piece in that variable. The problem is ghc "pins" the type variable as either (Rec f as) or (Rec f bs) when I need to make that choice at runtime. My code and error is both at github here: https://github.com/codygman/frames-diff/blob/inner-join-fix/Frames/Diff.hs#… and below: -- | Performs an inner join and keeps any duplicate column -- Recommend keeping columns in producers disjoint because accessing -- anything but the leftmost duplicate column could prove difficult. -- see: https://github.com/VinylRecords/Vinyl/issues/55#issuecomment-269891633 innerJoin :: (MonadIO m, Ord k) => Producer (Rec f leftRows) IO () -- leftProducer -> Getting k (Rec f leftRows) k -- leftProducer lens -> Producer (Rec f rightRows) IO () -- rightProducer -> Getting k (Rec f rightRows) k -- rightProducer lens -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) innerJoin leftProducer leftLens rightProducer rightLens = do leftProducerLen <- P.liftIO $ P.length leftProducer rightProducerLen <- P.liftIO $ P.length rightProducer let curProducer = case rightProducerLen < leftProducerLen of True -> rightProducer -- False -> leftProducer let curKeymapProducer = case rightProducerLen < leftProducerLen of True -> leftProducer -- False -> rightProducer let curLensLookup = case rightProducerLen < leftProducerLen of True -> rightLens -- False -> leftLens let curLensInsert = case rightProducerLen < leftProducerLen of True -> leftLens -- False -> rightLens let appender km row = case rightProducerLen < leftProducerLen of True -> case M.lookup (view curLensLookup row) km of Just otherRow -> pure $ rappend otherRow row Nothing -> P.mzero -- False -> case M.lookup (view curLensLookup row) km of -- Just otherRow -> pure $ rappend row otherRow -- Nothing -> P.mzero keyMap <- P.liftIO $ P.fold (\m r -> M.insert (view curLensInsert r) r m) M.empty id curKeymapProducer pure $ curProducer >-> P.mapM (\r -> appender keyMap r) -- error if I uncomment my false cases (specifically the False case for curProducer) -- [5 of 5] Compiling Frames.Diff ( Frames/Diff.hs, interpreted ) -- Frames/Diff.hs:125:32: error: -- • Couldn't match type ‘leftRows’ with ‘rightRows’ -- ‘leftRows’ is a rigid type variable bound by -- the type signature for: -- innerJoin :: forall (m :: * -> *) k (f :: * -- -> *) (leftRows :: [*]) (rightRows :: [*]). -- (MonadIO m, Ord k) => -- Producer (Rec f leftRows) IO () -- -> Getting k (Rec f leftRows) k -- -> Producer (Rec f rightRows) IO () -- -> Getting k (Rec f rightRows) k -- -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) -- at Frames/Diff.hs:113:14 -- ‘rightRows’ is a rigid type variable bound by -- the type signature for: -- innerJoin :: forall (m :: * -> *) k (f :: * -- -> *) (leftRows :: [*]) (rightRows :: [*]). -- (MonadIO m, Ord k) => -- Producer (Rec f leftRows) IO () -- -> Getting k (Rec f leftRows) k -- -> Producer (Rec f rightRows) IO () -- -> Getting k (Rec f rightRows) k -- -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) -- at Frames/Diff.hs:113:14 -- Expected type: Producer (Rec f rightRows) IO () -- Actual type: Producer (Rec f leftRows) IO () -- • In the expression: leftProducer -- In a case alternative: False -> leftProducer -- In the expression: -- case rightProducerLen < leftProducerLen of { -- True -> rightProducer -- False -> leftProducer } -- • Relevant bindings include -- curProducer :: Producer (Rec f rightRows) IO () -- (bound at Frames/Diff.hs:123:7) -- rightLens :: Getting k (Rec f rightRows) k -- (bound at Frames/Diff.hs:119:47) -- rightProducer :: Producer (Rec f rightRows) IO () -- (bound at Frames/Diff.hs:119:33) -- leftLens :: Getting k (Rec f leftRows) k -- (bound at Frames/Diff.hs:119:24) -- leftProducer :: Producer (Rec f leftRows) IO () -- (bound at Frames/Diff.hs:119:11) -- innerJoin :: Producer (Rec f leftRows) IO () -- -> Getting k (Rec f leftRows) k -- -> Producer (Rec f rightRows) IO () -- -> Getting k (Rec f rightRows) k -- -> m (P.Proxy P.X () () (Rec f (leftRows ++ rightRows)) IO ()) -- (bound at Frames/Diff.hs:119:1)

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ANN: `dump-core` a prettier GHC core viewer
by Iavor Diatchki 13 Jan '17

13 Jan '17

Hello, Over the holidays I wrote a small GHC plugin to help me do some low-level optimizations of Haskell code. I thought it might be of use to other people too, so please try it out! When enabled, the plugin will save the Core generated by GHC in JSON format, and also render it in HTML for human inspection. The plugin is available on Hackage: http://hackage.haskell.org/package/dump-core The instructions on how to use it are in the README file. You may also read about it at the github page: http://hackage.haskell.org/package/dump-core There are many things that could probably be improved, just let me know. Also, if you are good at design, I could use some help making things look prettier :) Happy hacking, -Iavor

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[ANN] wiringPi-0.1.0.0
by Patrick Pelletier 11 Jan '17

11 Jan '17

https://hackage.haskell.org/package/wiringPi-0.1.0.0 https://github.com/ppelleti/hs-wiringPi This is a Haskell binding to the wiringPi library, which allows you to interface with the GPIO pins on the Raspberry Pi. Unlike some other solutions for using the Raspberry Pi's GPIO pins, wiringPi provides access to more advanced features, such as enabling the internal pull-up or pull-down resistors. To use this library, you must either run as root, or set the WIRINGPI_GPIOMEM environment variable. (Set it to anything; the value does not matter.) However, PWM will not work if WIRINGPI_GPIOMEM is set. This library will only build on the Raspberry Pi. Before building this library, you must install the "wiringPi" C library on your Raspberry Pi, like this: sudo apt-get install wiringpi Tested on a Raspberry Pi Model B, with Raspbian Jessie Lite, using the system-provided Haskell compiler. (GHC 7.6.3.) This Haskell binding is licensed under the 3-clause BSD license, and the examples in the examples directory are in the public domain. However, be aware that the wiringPi C library itself is licensed under the LGPLv3+. --Patrick

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[ANN] powermate-1.0
by Patrick Pelletier 10 Jan '17

10 Jan '17

As the new maintainer of the "powermate" package, I'm pleased to announce the first new release in nearly 9 years! This library is for interfacing the Griffin PowerMate USB (a big silver knob you can turn and click) with Haskell on Linux. You can read events from the PowerMate, and control the brightness, pulse speed, and pulse waveform of the built-in blue LED. https://hackage.haskell.org/package/powermate-1.0 https://github.com/ppelleti/powermate Version 1.0 is a major release with some breaking changes. The complete list of changes since version 0.1 is: * Now works with GHC 7.2.2 - 8.0.1. * Added Eq, Ord, Show, and Read instances for Event and Status. * Added example programs powermate-print and powermate-pulse. * Added documentation. * An open PowerMate is now represented by the opaque type 'PowerMate' instead of 'Handle'. A new function 'closeDevice' exists to close the opaque 'PowerMate' type. * Fixed a bug where the PowerMate would get confused and start dropping button presses if you set the LED. * Removed extraneous printing to stdout. * Removed getUSBName and readEventWithSkip; I didn't see the utility in these functions. * readEvent no longer returns a Maybe. There was no meaning to 'Nothing' and it just had to be ignored. * Removed the MPD module. It wasn't directly related to the PowerMate functionality, and the "libmpd" package seems to be an alternative which is more complete and better maintained. --Patrick

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Call for interest: Haskell in middle school math education
by Chris Smith 08 Jan '17

08 Jan '17

Hello fellow Haskellers! Are you interested in getting Haskell into middle school (ages 11-14) math or other classes? I'd love to hear from you! Many of you are probably already aware that I've spent the last several years teaching Haskell to children and teens via my CodeWorld project ( http://code.world and http://github.com/google/codeworld) Well, 2017 is my year to make a bigger success of the whole effort. So, let's get Haskell into K-12 schools as a part of mathematics education. By incorporating a programming language like Haskell that's faithful to modeling using true functions, variables, and equations, we can make mathematical modeling and algebraic reasoning tangible and creative in a way it's never been before. There's a lot that's already been done, but the task is still daunting. With some help from some coworkers of mine at Google, and in conjunction with the New York Haskell User Group, I've put together a small budget of around $60K in USD, to start this ball rolling a bit faster with some paid help. Some possible goals include: - Produce quality instructional materials to help teachers around the world incorporate Haskell into their classes and after-school activities. - Produce videos and online resources to support students learning on their own. - Build software features make the CodeWorld tools more helpful, easier to use, easier for students share their work more broadly, and more supportive when students need help. - Make what we already have available to more students, and educate more teachers. - Spread the word and look for more support so we can keep up the effort after the initial investment is finished. Everything produced as part of the project will be distributed under either the Apache 2.0 open source license, or the latest Creative Commons attribution-only license, so that it's freely available for any educational institution (or anyone else) to use, modify, redistribute, etc. So here's where you can step in: - Join the codeworld-discuss@ mailing list (https://groups.google.com/ forum/#!forum/codeworld-discuss) - Introduce yourself, and tell us a bit about what vision you have, and what role you might want to play. This could be a spectator, teacher at a school near you, volunteer contributions on the side, or even a leadership role with a salary attached! Cognizant that solo decisions can be poor ones, I hope to follow some initial sharing of ideas with gathering a small handful of like-minded individuals - I know you're out there - to make decisions about the best way to manage the effort, and go from there. Some background reading: - Blog post on the defining decisions of my effort so far <https://cdsmith.wordpress.com/2016/05/22/codeworlds-big-decisions/> - The web site and its source code <https://github.com/google/codeworld> - A huge bag of partially incomplete stuff already produced <https://drive.google.com/drive/u/1/folders/0B-qIu_nqxaMoYTQ4ODZjMjMtNWRiOC0…> - A similarly motivated project using Racket <http://www.bootstrapworld.org/> that gets a lot right I look forward to hearing from you.

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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 06 Jan '17

06 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 06 Jan '17

06 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 06 Jan '17

06 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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