- Haskell-Cafe - Haskell.org

[ANN] lentil - frugal issue tracker
by Francesco Ariis 02 Sep '16

02 Sep '16

I am pleased to announce the release of lentil 1.0 [0]. [0] http://hackage.haskell.org/package/lentil-1.0.0.1 ============================= lentil - frugal issue tracker ============================= lentil is a minimum effort issue tracker; if you litter your code with: -- FIXME: does outPop work with b/w terminals? [ansi] lentil can help you listing, sorting, exporting, tagging slicing and dicing those `todo`s. To see it in action just type: cabal install lentil # or use stack cd ~/your-project-folder/ lentil . Any repo will do, not only Haskell ones! For further information, check the project homepage [1] and the user manual [2]. [1] http://ariis.it/static/articles/lentil/page.html [2] http://ariis.it/static/articles/lentil-manual/page.html Heartfelt "thank you!" to anyone who contributed [3] by providing valuable feedback, suggestions, patches! [3] http://ariis.it/link/repos/lentil/contributors.txt

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StdGen Reliability Across Versions / Systems
by Christopher Howard 01 Sep '16

01 Sep '16

I know that, ff you feed mkStdGen a particular seed, and pull a list of random numbers from that StdGen, that that the list will always be the same if your run your compiled program over and over again on that same system. But which of the following will cause you to get different output?: 1) Changes in GHC version 2) Changes in "random" library version 3) Compiling / Running on a different host computer (but compiling with the same GHC & random versions. -- https://qlfiles.net My PGP public key ID is 0x340EA95A (pgp.mit.edu)

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new edition of "Programming in Haskell"
by Gregory Guthrie 01 Sep '16

01 Sep '16

>> I'm delighted to announce that the new edition of "Programming in >> Haskell" is now available! Further details are provided below, >> and are also available from: http://tinyurl.com/hnfjdgc. This is great, and a welcome update, thank you! +1 for book and update :-) -1 for DRM on eBook :-( -------------------------------------------

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New edition of "Programming in Haskell"
by Graham Hutton 01 Sep '16

01 Sep '16

Dear all, I'm delighted to announce that the new edition of "Programming in Haskell" is now available! Further details are provided below, and are also available from: http://tinyurl.com/hnfjdgc. Best wishes, Graham ================================================================= *** BOOK ANNOUNCEMENT *** Programming in Haskell - 2nd Edition Graham Hutton, University of Nottingham Cambridge University Press, 1st September 2016 320 pages, 120 exercises, ISBN 9781316626221 http://tinyurl.com/hnfjdgc ================================================================= DESCRIPTION: Haskell is a purely functional language that allows programmers to rapidly develop clear, concise, and correct software. The language has grown in popularity in recent years, both in teaching and in industry. This book is based on the author's experience of teaching Haskell for more than twenty years. All concepts are explained from first principles and no programming experience is required, making this book accessible to a broad spectrum of readers. While Part I focuses on basic concepts, Part II introduces the reader to more advanced topics. This new edition has been extensively updated and expanded to include recent and more advanced features of Haskell, new examples and exercises, selected solutions, and freely downloadable lecture slides and example code. The presentation is clean and simple, while also being fully compliant with the latest version of the language, including recent changes concerning applicative, monadic, foldable, and traversable types. ================================================================= CONTENTS: Foreword Preface Part I. Basic Concepts: 1. Introduction 2. First steps 3. Types and classes 4. Defining functions 5. List comprehensions 6. Recursive functions 7. Higher-order functions 8. Declaring types and classes 9. The countdown problem Part II. Going Further: 10. Interactive programming 11. Unbeatable tic-tac-toe 12. Monads and more 13. Monadic parsing 14. Foldables and friends 15. Lazy evaluation 16. Reasoning about programs 17. Calculating compilers Appendix A. Selected solutions Appendix B. Standard prelude Bibliography Index ================================================================= AUTHOR: Graham Hutton is Professor of Computer Science at the University of Nottingham. He has taught Haskell to thousands of students and received numerous best lecturer awards. Hutton has served as an editor of the Journal of Functional Programming, Chair of the Haskell Symposium and the International Conference on Functional Programming, and Vice-Chair of the ACM Special Interest Group on Programming Languages, and he is an ACM Distinguished Scientist. ================================================================= This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Announcing containers 0.5.8.1
by David Feuer 01 Sep '16

01 Sep '16

There's a lot to see in this one. There are plenty of brand-new functions in Data.Map, Data.Set, and Data.Sequence, including a highly-optimized lens-inspired map alteration function and a brand-new API for merging maps efficiently. Several key map, set, and sequence functions have sped up considerably. The full change log is below. I have tried to give appropriate credit to all the non-maintainers who contributed to this release, some of whom made very major contributions indeed. If I missed anyone, please give a shout. General package changes: Remove all attempts to support nhc98 and any versions of GHC before 7.0. Integrate benchmarks with Cabal. (Thanks, Gabriel Gonzalez!) Make Cabal report required extensions properly, and stop using default extensions. Note that we do not report extensions conditionally enabled based on GHC version, as doing so would lead to a maintenance nightmare with no obvious benefits. Use BangPatterns throughout to reduce noise. This extension is now required to compile containers. Improve QuickCheck properties taking arbitrary functions by using Test.QuickCheck.Function.Fun instead of evil Show instances for functions. Expose several internal modules through Cabal (as requested by Edward Kmett). These remain completely unsupported. New exports and instances: Add alterF, restrictKeys, and withoutKeys to Data.Map and Data.IntMap. Add take, drop, splitAt, takeWhileAntitone, dropWhileAntitone, and spanAntitone for Data.Map and Data.Set. Thanks to Cale Gibbard for suggesting these. Add merge, mergeA, and associated merge tactics for Data.Map. Many thanks to Cale Gibbard, Ryan Trinkle, and Dan Doel for inspiring the merge idea and helping refine the interface. Add fromDescList, fromDescListWith, fromDescListWithKey, and fromDistinctDescList to Data.Map. Add fromDescList and fromDistinctDescList to Data.Set. Add Empty, :<|, and :|> pattern synonyms for Data.Sequence, as originally envisioned in the finger tree paper by Paterson and Hinze. Add adjust', (!?), lookup, chunksOf, cycleTaking, insertAt, deleteAt, intersperse, foldMapWithIndex, and traverseWithIndex for Data.Sequence. Derive Generic and Generic1 for Data.Tree.Tree, Data.Sequence.ViewL, and Data.Sequence.ViewR. Add foldTree for Data.Tree. (Thanks, Daniel Wagner!) Semantic changes: Make Data.Sequence.splitAt strict in its arguments. Previously, it returned a lazy pair. Fix completely erroneous definition of length for Data.Sequence.ViewR. Make Data.Map.Strict.traverseWithKey force result values before installing them in the new map. Make Data.Tree.drawTree handle newlines better. (Thanks, recursion-ninja!) Deprecations: All functions in Data.Map proper that have been documented as deprecated since version 0.5 or before now have DEPRECATED pragmas and will actually be removed after another cycle or two. Tree printing functions in Data.Map intended for library debugging are now deprecated. They will continue to be available for the foreseeable future in an internal module. Performance changes: Substantially speed up splitAt, zipWith, take, drop, fromList, partition, foldl', and foldr' for Data.Sequence. Special thanks to Lennart Spitzner for digging into the performance problems with previous versions of fromList and finding a way to make it really fast. Slightly optimize replicateA. Stop traverse from performing many unnecessary fmap operations. Most operations in Data.Sequence advertised as taking logarithmic time (including >< and adjust) now use their full allotted time to avoid potentially building up chains of thunks in the tree. In general, the only remaining operations that avoid doing more than they really need are the particular bulk creation and transformation functions that really benefit from the extra laziness. There are some situations where this change may slow programs down, but I think having more predictable and usually better performance more than makes up for that. Add rewrite rules to fuse fmap with reverse for Data.Sequence. Switch from hedge algorithms to divide-and-conquer algorithms for union, intersection, difference, and merge in both Data.Map and Data.Set. These algorithms are simpler, are known to be asymptotically optimal, and are faster according to our benchmarks. Speed up adjust for Data.Map. Allow map to inline, and define a custom (<$). This considerably improves mapping with a constant function. Remove non-essential laziness throughout the Data.Map.Lazy implementation. Slightly speed up deletion and alteration functions for Data.IntMap.

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Which tab width do you prefer for Haskell code?
by Christopher Done 31 Aug '16

31 Aug '16

Hi everyone, I'm trying to get a feel for the modern Haskell community's opinion on default tab-width for HIndent. Here is a link to the online poll: http://doodle.com/poll/82xf854t9mmuv22h#table Quote: > For the HIndent tool we'd like to choose the standard tab width. > For now, it is 4, the width set by the Johan Tibell style guide. > A survey of 1000 top-downloaded packages on Hackage reveals about 50-50 either way:https://gist.github.com/chrisdone/e3d1796d8f5df717b91db54aae36c440 > HIndent is meant as a "community standard" tool; it should reflect the community's actual practice. If there's a clear preference for 2, we'll switch to that. Otherwise, we remain with 4. Cheers!

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Cache miss performance costs for Haskell programs?
by Rob Stewart 31 Aug '16

31 Aug '16

Hi, Any Haskell profiling and performance tuning blog or tutorial will advise the use of memory space and runtime profiling, using GHC tooling. Far less is said about the impact of increased cache miss rates as program size increases. The paper "Secrets of the Glasgow Haskell Compiler inliner", in the Journal of Functional Programming, July 2002, talks a lot about the benefits of inlining, i.e. it's part of GHCs simplifier as it enables many other optimisations, some that ultimately reduce program size. Not much is said about detrimental effect that bad inlining choices has to runtime. The paper says: "Bloated programs are bad (increased compilation time, lower cache hit rates)" in Section 2.2. I'd really like to see how badly Haskell runtimes are affected as cache hit rates decrease. Is anyone aware of any empirical studies, or papers, or blog posts, that show examples where: For the same Haskell program, increasing inlining causes lower cache hit rates, which slows down runtime due to costly cycles to fetch from main memory more often. Thanks, -- Rob

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Re: [Haskell-cafe] [Haskell] I repeat my Question becaue I not really sure to do it right
by Ivan Lazar Miljenovic 29 Aug '16

29 Aug '16

On 28 August 2016 at 23:15, <CCUTM(a)web.de> wrote: > Hello it is possible that this is the solution. > But I don't know how I can istall it. See the Ubuntu section here: http://www.leksah.org/download.html > > Gesendet: Sonntag, 28. August 2016 um 15:07 Uhr > Von: "Ivan Lazar Miljenovic" <ivan.miljenovic(a)gmail.com> > An: CCUTM(a)web.de > Betreff: Re: Re: [Haskell] I repeat my Question becaue I not really sure to > do it right > Do you have the -dev libraries installed for gtk, glib, etc.? > > Or is there a gtk2hs package available for Ubuntu? > > On Sun, 28 Aug 2016, 11:05 PM <CCUTM(a)web.de> wrote: >> >> I'm seem rubish, >> but exactly that have I do >> And I have at first install cabal. >> But this is comming: >> >> Resolving dependencies... >> Configuring glib-0.13.4.0... >> Building glib-0.13.4.0... >> Failed to install glib-0.13.4.0 >> Build log ( /home/thomas/.cabal/logs/glib-0.13.4.0.log ): >> [1 of 1] Compiling Main ( >> /tmp/cabal-tmp-4585/glib-0.13.4.0/dist/setup/setup.hs, >> /tmp/cabal-tmp-4585/glib-0.13.4.0/dist/setup/Main.o ) >> Linking /tmp/cabal-tmp-4585/glib-0.13.4.0/dist/setup/setup ... >> Configuring glib-0.13.4.0... >> Building glib-0.13.4.0... >> Preprocessing library glib-0.13.4.0... >> /usr/bin/ld: dist/build/System/Glib/StoreValue_hsc_make.o: Die Umlagerung >> von >> dist/build/System/Glib/StoreValue_hsc_make.o: error adding symbols: >> Ungültiger Wert >> collect2: error: ld returned 1 exit status >> linking dist/build/System/Glib/StoreValue_hsc_make.o failed (exit code 1) >> command was: /usr/bin/gcc dist/build/System/Glib/StoreValue_hsc_make.o >> dist/build/System/Glib/StoreValue_hsc_utils.o -o >> dist/build/System/Glib/StoreValue_hsc_make -fno-PIE -fno-stack-protector >> -lgobject-2.0 -lglib-2.0 >> -L/home/thomas/.cabal/lib/x86_64-linux-ghc-7.10.3/utf8-string-1.0.1.1-L8eKHa7Iv9q7FVKUYW6u4b >> -Wl,-R,/home/thomas/.cabal/lib/x86_64-linux-ghc-7.10.3/utf8-string-1.0.1.1-L8eKHa7Iv9q7FVKUYW6u4b >> -L/home/thomas/.cabal/lib/x86_64-linux-ghc-7.10.3/text-1.2.2.1-HmqVQnZSpjaC156ABqPhne >> -Wl,-R,/home/thomas/.cabal/lib/x86_64-linux-ghc-7.10.3/text-1.2.2.1-HmqVQnZSpjaC156ABqPhne >> -L/usr/lib/ghc/binar_3uXFWMoAGBg0xKP9MHKRwi >> -Wl,-R,/usr/lib/ghc/binar_3uXFWMoAGBg0xKP9MHKRwi >> -L/usr/lib/ghc/conta_2C3ZI8RgPO2LBMidXKTvIU >> -Wl,-R,/usr/lib/ghc/conta_2C3ZI8RgPO2LBMidXKTvIU >> -L/usr/lib/ghc/bytes_6VWy06pWzJq9evDvK2d4w6 >> -Wl,-R,/usr/lib/ghc/bytes_6VWy06pWzJq9evDvK2d4w6 >> -L/usr/lib/ghc/deeps_6vMKxt5sPFR0XsbRWvvq59 >> -Wl,-R,/usr/lib/ghc/deeps_6vMKxt5sPFR0XsbRWvvq59 >> -L/usr/lib/ghc/array_67iodizgJQIIxYVTp4emlA >> -Wl,-R,/usr/lib/ghc/array_67iodizgJQIIxYVTp4emlA >> -L/usr/lib/ghc/base_HQfYBxpPvuw8OunzQu6JGM >> -Wl,-R,/usr/lib/ghc/base_HQfYBxpPvuw8OunzQu6JGM >> -L/usr/lib/ghc/integ_2aU3IZNMF9a7mQ0OzsZ0dS >> -Wl,-R,/usr/lib/ghc/integ_2aU3IZNMF9a7mQ0OzsZ0dS -lgmp >> -L/usr/lib/ghc/ghcpr_8TmvWUcS1U1IKHT0levwg3 >> -Wl,-R,/usr/lib/ghc/ghcpr_8TmvWUcS1U1IKHT0levwg3 -L/usr/lib/ghc/rts >> -Wl,-R,/usr/lib/ghc/rts -lm -lrt -ldl -lffi >> cabal: Error: some packages failed to install: >> ghcjs-dom-0.2.4.0 depends on glib-0.13.4.0 which failed to install. >> gio-0.13.3.0 depends on glib-0.13.4.0 which failed to install. >> glib-0.13.4.0 failed during the building phase. The exception was: >> ExitFailure 1 >> gtk3-0.14.5 depends on glib-0.13.4.0 which failed to install. >> gtksourceview3-0.13.3.0 depends on glib-0.13.4.0 which failed to install. >> jsaddle-0.3.0.3 depends on glib-0.13.4.0 which failed to install. >> leksah-0.15.2.0 depends on glib-0.13.4.0 which failed to install. >> leksah-server-0.15.2.0 depends on glib-0.13.4.0 which failed to install. >> ltk-0.15.0.5 depends on glib-0.13.4.0 which failed to install. >> pango-0.13.3.0 depends on glib-0.13.4.0 which failed to install. >> vcsgui-0.1.3.0 depends on glib-0.13.4.0 which failed to install. >> webkitgtk3-0.14.2.0 depends on glib-0.13.4.0 which failed to install. >> webkitgtk3-javascriptcore-0.13.2.0 depends on glib-0.13.4.0 which failed >> to >> install. >> >> Please can you help me!!? >> >> >> Gesendet: Sonntag, 28. August 2016 um 14:50 Uhr >> >> Von: "Ivan Lazar Miljenovic" <ivan.miljenovic(a)gmail.com> >> An: CCUTM(a)web.de >> Cc: "Haskell List" <haskell(a)haskell.org> >> Betreff: Re: [Haskell] I repeat my Question becaue I not really sure to do >> it right >> On 28 August 2016 at 22:40, <CCUTM(a)web.de> wrote: >> > This is Leksah http://leksah.org/ >> > Please excuse my false name for it >> > And the Question is why can I install it. >> >> What error messages do you have when you try? >> >> From here, this is what you need to do: >> https://github.com/leksah/leksah/wiki/download >> >> > cabal install gtk2hs-buildtools >> >> > cabal install leksah >> >> >> >> > >> > excuse me. >> > Mungo1981 >> > >> > Gesendet: Sonntag, 28. August 2016 um 13:49 Uhr >> > Von: "Ivan Lazar Miljenovic" <ivan.miljenovic(a)gmail.com> >> > An: CCUTM(a)web.de >> > Cc: "Haskell List" <haskell(a)haskell.org> >> > Betreff: Re: [Haskell] I repeat my Question becaue I not really sure to >> > do >> > it right >> > On 28 August 2016 at 20:43, <CCUTM(a)web.de> wrote: >> >> Ok Haskell is good. >> >> Ok Laska is great. >> >> And my Ubuntu Studio ist rubish >> >> So I try to install Laska on Ubuntu Studio >> >> But when I do this, i will get a long list of dependecies >> >> which could not reallise >> >> So I not know what should I do >> > >> > Which question? >> > >> > What is Laska that you're having trouble installing it? >> > >> >> >> >> Mungo1981 >> >> >> >> _______________________________________________ >> >> Haskell mailing list >> >> Haskell(a)haskell.org >> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell >> >> >> > >> > >> > >> > -- >> > Ivan Lazar Miljenovic >> > Ivan.Miljenovic(a)gmail.com >> > http://IvanMiljenovic.wordpress.com >> > >> > _______________________________________________ >> > Haskell mailing list >> > Haskell(a)haskell.org >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell >> > >> >> >> >> -- >> Ivan Lazar Miljenovic >> Ivan.Miljenovic(a)gmail.com >> http://IvanMiljenovic.wordpress.com -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Haskell Tutorials to Spread the Word
by Will Tennien Murphy 28 Aug '16

28 Aug '16

Hey, I'm looking for people to help spread the word about Haskell by translating or writing tutorials. I'm a contributor to py-app.com . Py is an app that teaches programming concepts "on the go"—that is, with a mobile focus. Do you... 1. Want to help spread the word about Haskell? 2. Like helping other people learn? 3. Speak another natural language and can translate existing content? 4. Know a tutorial you'd like to make more mobile-friendly? If you meet ANY of these conditions—especially if you are proficient another language—drop me a line at wtmurp(a)gmail.com. I'm very intent on making things as easy as possible for writers so you can write in whatever you find most convenient[1] and I'll figure out how to get it into the app. [1] except PDF

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managing embedded structs
by Henning Thielemann 28 Aug '16

28 Aug '16

I want to write a Haskell interface to a C library that provides two data structures A and B, where B is contained in A. That is, if A is freed, then B is automatically freed, too. There are two ways to obtain a Ptr B. Either construct it separately or as part of A, i.e. b0 :: Ptr B b0 <- createB b1 :: Ptr B b1 <- getBofA =<< createA Now I want to let the garbage collector manage the resources for B in a ForeignPtr. The first case is easy: fb0 :: ForeignPtr B fb0 <- newForeignPtr deleteB =<< createB But how to create an fb1? fa1 :: ForeignPtr A fa1 <- newForeignPtr deleteA =<< createA fb1 :: ForeignPtr B fb1 <- newForeignPtr ??? =<< withForeignPtr fa1 getBofA Obviously, there is nothing I can insert for ???. fb1 must not have its own finalizer, because fa1 already has one. But I have to make sure that fa1 lives at least as long as fb1. How to do that?

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Is anybody interested in taking over maintaining hasktags?
by Marc Weber 28 Aug '16

28 Aug '16

There is not much to be done - patch requests come in once or twice a year. Current task would be uploading a new version to hackage fixing compilation bug. The patch is already on github Marc Weber

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Re: [Haskell-cafe] [Haskell] I repeat my Question becaue I not really sure to do it right
by Francesco Ariis 28 Aug '16

28 Aug '16

haskell(a)haskell.org is low traffic, for announcements only, I am replying to haskell-cafe(a)haskell.org , where this question is better suited (and most people willing to help are). On Sun, Aug 28, 2016 at 10:50:50PM +1000, Ivan Lazar Miljenovic wrote: > On 28 August 2016 at 22:40, <CCUTM(a)web.de> wrote: > > This is Leksah http://leksah.org/ > > Please excuse my false name for it > > And the Question is why can I install it. > > What error messages do you have when you try? > > From here, this is what you need to do: > https://github.com/leksah/leksah/wiki/download > > > cabal install gtk2hs-buildtools > > > cabal install leksah > > > > > > > excuse me. > > Mungo1981 > > > > Gesendet: Sonntag, 28. August 2016 um 13:49 Uhr > > Von: "Ivan Lazar Miljenovic" <ivan.miljenovic(a)gmail.com> > > An: CCUTM(a)web.de > > Cc: "Haskell List" <haskell(a)haskell.org> > > Betreff: Re: [Haskell] I repeat my Question becaue I not really sure to do > > it right > > On 28 August 2016 at 20:43, <CCUTM(a)web.de> wrote: > >> Ok Haskell is good. > >> Ok Laska is great. > >> And my Ubuntu Studio ist rubish > >> So I try to install Laska on Ubuntu Studio > >> But when I do this, i will get a long list of dependecies > >> which could not reallise > >> So I not know what should I do > > > > Which question? > > > > What is Laska that you're having trouble installing it? > > > >> > >> Mungo1981 > >> > >> _______________________________________________ > >> Haskell mailing list > >> Haskell(a)haskell.org > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell > >> > > > > > > > > -- > > Ivan Lazar Miljenovic > > Ivan.Miljenovic(a)gmail.com > > http://IvanMiljenovic.wordpress.com > > > > _______________________________________________ > > Haskell mailing list > > Haskell(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell > > > > > > -- > Ivan Lazar Miljenovic > Ivan.Miljenovic(a)gmail.com > http://IvanMiljenovic.wordpress.com > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell

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Re: [Haskell-cafe] [Haskell-community] haskell.org download page
by Marco Zocca 28 Aug '16

28 Aug '16

-1 to making HP the first method Stack is easier to get started with and makes tinkering easier. In this sense, it's a more natural choice for beginners. Also, the Minimal option is kind of dead for Mac users since GHCForMacOSX is not maintained anymore. (NB: I speak out of personal experience and have no conflicting interests)

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Using MonadRandom with QuickCheck
by Magnus Grindal Bakken 27 Aug '16

27 Aug '16

Is there some easy way to use the Gen type from QuickCheck with the MonadRandom class? Links to the packages in question: https://hackage.haskell.org/package/MonadRandom https://hackage.haskell.org/package/QuickCheck Some background in case there's an easier way to solve the problem: I'm toying around with an API for a simple turn-based board game. The main type in my API is called GameState, and I'm trying to define an Arbitrary instance for it so I can set up invariants with QuickCheck. The Arbitrary instance should produce game states that are the result of playing a random number of random moves. At certain points in the gameplay the game itself needs to invoke some randomness. I've designed the public game API to have basically just two functions: playAction :: ActionData -> GameState -> GameState evaluateGameState :: MonadRandom m => GameState -> m GameState The usage is supposed to be that the client first calls playAction with an ActionData value that depends on the current move, and afterwards calls evaluateGameState to evaluate whatever random events need to happen at that point (e.g. reshuffling a deck of cards). That way my main "action" logic doesn't need to concern itself with any form of randomness. I'm using MonadRandom from Control.Monad.Random because it seems like a much nicer way to generate random numbers than using RandomGen directly. However I can't figure out how to make it play nice with QuickCheck. I can easily set up my Arbitrary instance to call playAction an arbitrary number of times, but I can't call evaluateGameState since it lives in the other monad. It would work if Gen were an instance of MonadRandom, but I can't quite work out how to write that instance since most functions on Gen only work if the output type is an instance of Arbitrary, while MonadRandom needs to work with any types that are part of the Random class. This part works: instance MonadRandom Gen where getRandomR = choose But I don't know how to define the other functions on MonadRandom for Gen. I think I might also be able to use the PropertyM monad transformer: https://hackage.haskell.org/package/QuickCheck-2.9.1/docs/Test-QuickCheck-M…, but monad transformers always confuse me so I haven't been able to figure out how to do that yet either. I've thought about rewriting the API to use the Gen type instead of MonadRandom, but it feels kind of iffy to have the API depend on a testing library. Has anyone tried to use these libraries together before?

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Confused about logBase
by MarLinn 26 Aug '16

26 Aug '16

Hi, I discovered that in base /logBase/, the functions representing arbitrary logarithms, are defined in terms of two applications of /log/. /log/, the functions representing the natural logarithm, just pass responsibility on to some primitives. That is fine, mathematically speaking. But from an implementation standpoint, I wonder why one would do that. The logarithm that should be the fastest and the most precise one to approximate by a cpu should be the one to base two. In fact if one already has a floating point representation, it should be almost ridiculously easy to compute from the exponent. I suppose it's probably a good idea to use primitive functions for the harder cases to get some speed-up. What I don't see is why the choice seems to have been an /xor/ instead of an /and/. I am by absolutely no means an expert on these things, so I am probably missing something here. Is there actually a good reason for this choice? For example will ghc optimize such logarithms anyway? Cheers, MarLinn

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problem installing GHC on Ubuntu 16.10
by Guillaume Hoffmann 26 Aug '16

26 Aug '16

Hi, Anyone knows why I am not able to install GHC 8 nor 7.10 on my Ubuntu 16.10 machine using the packages from GHC's page nor using stack setup? Here are the errors thrown by stack setup: * http://lpaste.net/180513 * http://lpaste.net/180514 Ubuntu 16.10 provides GHC 7.10 as package but I'd like to have GHC 8. thanks Guillaume

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Re: [Haskell-cafe] Data.Set optimisations [was: library for set of heterogeneous values?]
by Anthony Clayden 26 Aug '16

26 Aug '16

> > On Mon, Aug 15, 2016 at 1:15 AM, Anthony Clayden wrote: > > "indirect jumps are particularly expensive on a modern > > processor architecture, because they fox the branch > > prediction hardware, leading to a stall of 10 or more > > cycles depending on the length of the pipeline." > [Introduction. The paper goes on to estimate a penalty > > around 7 to 9 % in general GHC code.] > The cost model here makes sense if ... Thanks Wren. Is that 7 to 9 % figure unduly alarmist? (It is consistent with Milan's benchmarking.) For example would strictifying or inlining have much greater benefits than worrying about constructors? And it only affects processor-intensive applications(?) > ... > Regardless of how GHC does things, those basics of branch > prediction and primop ordering will remain the same. ... > In practice GHC does the naive thing of ordering branches > to be in order of declaration in the data type definition. > This is good in as much as it doesn't matter how users > textually order their case analyses, since GHC will > reorder them. ... > ...the order of data constructors in a data type's definition > is also used for other magic, like deriving Ord. ... Hmm, I'd hardly describe derivng Ord as "magic". It's what the language report says [Chapter 11]. > Really, we'd like to make GHC smarter here about > how it orders [case branchng code]; > but that gets complicated very quickly. I can see that. I gave some possible rules of thumb in an earlier message. (In particular, recursive constructors are more likely.) > ... noone other than me seems to care very much. ... I guess not many know. We kinda expect GHC will be much smarter than we could achieve by hand-crafting the object code. Those who could write assembler code must be dying out ;-) Judging by the 2006 paper, part of the difficulty seems to be that the C code GHC generates is none too idiomatic for what a C optimiser is tuned for. AntC

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Fwd: Executable runs but dynamic linking fails within ghci
by Ryan G 25 Aug '16

25 Aug '16

I'm working with a GUI library and I can get everything to work when I run my executable. When I try to call my gui function within ghci, it crashes with: user error (unknown GLU entry gluOrtho2D). I'm on Arch running ghc version 7.10.3. The GLURaw maintainer could reproduce this and added some insight I can reproduce this both with LTS Haskell (based on GHC 7.10.3) and > Stackage nightly (based on GHC 8.0.1) on Ubuntu 16.04, too. This is somehow > related to GHCi's dynamic linker, which has changed a few times in the > past, and there are still issues with it. Alas, I don't remember the > details, but perhaps you can get some answers on the haskell-cafe mailing > list. All I can say is that things worked even within GHCi at some point in > the past... [image: :wink:] > > The gory details: Given an API entry name like "gluOrtho2D", the GLURaw > package uses dlopen anddlsym to get the address of the function, see the Haskell > part > <https://github.com/haskell-opengl/GLURaw/blob/2fbff384e9637e1c1763c65a5f843…> and > the C part > <https://github.com/haskell-opengl/GLURaw/blob/2fbff384e9637e1c1763c65a5f843…>. > For some reason, dlsymdoesn't find the GLU entries when running under > GHCi, while the compiled version does. > Any help is appreciated

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Stack and Circle CI
by Nicola Gigante 25 Aug '16

25 Aug '16

Hi all, has any of you had any success testing haskell projects on CircleCI with a setup that involved Stack? CircleCI seems to directly support haskell by specifying the version of GHC but I wanted my dependencies and tests to be handled by Stack also on the testing machines, since it is my main development tool. I’m also new to Circle CI in particular, although I used Travis in the past. I’ve tried collecting some example files but it seems to have problems with the first ‘stack build’ phase since I see no output in the online console. My circle.yml file: dependencies: cache_directories: - "~/.stack" pre: - wget https://github.com/commercialhaskell/stack/releases/download/v0.1.2.0/stack… -O /tmp/stack.gz - gunzip /tmp/stack.gz && chmod +x /tmp/stack - sudo mv /tmp/stack /usr/bin/stack override: - stack setup - stack build test: override: - stack test Anybody? Thanks. Greetings, Nicola

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ANN: Bookkeeper
by Julian Arni 25 Aug '16

25 Aug '16

I was playing around with GHC 8 yesterday, and ended up writing another extensible records library: https://hackage.haskell.org/package/bookkeeper And a blog post about it: https://turingjump.com/blog/bookkeeper/ I hope someone finds this useful! -- Julian K. Arni Haskell Consultant, Turing Jump https://turingjump.com

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cat as banana In AOP
by cutsea110 25 Aug '16

25 Aug '16

Hi,all I'm studying `Algebra of Programming' written by Prof. Richard Bird. My question is for section 3.5 "Concatenation and currying > Exponentials". In page 74, he said ``` cat = (| curry ([outr, cons] . (id+id x apply) . (id+assocr) . distl) |) ``` where (| ... |) means banana operator. But I guess this left hand side `cat' is typo of `ccat` or `curry cat`? I think this is reasoned below. ----------------------------- cat . (alpha x id) = [outr, cons] . (id + id x cat) . phi <= page 73 start expr (maybe this is from equation (3.6)) = {- Theorem 3.1 precondition -} cat = apply . ( (| curry (h . G apply . phi |) x id) ----- (A) = {- on the other hand, exponential's universal property : g = curry f == apply . (g x id) = f -} cat = apply . ((curry cat) x id) ----- (B) = {- from (A) and (B) -} (curry cat) = (| curry (h . G apply . phi |) ----------------------------- This result is curry cat = (| curry ( [outr, cons] . (id + id x apply) . (id + assocr) . distl) |) not cat = rhs(above). How about this?

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Rigid type variables and their role in type checking
by Morten Olsen Lysgaard 24 Aug '16

24 Aug '16

I am making a toy compiler for a polymorphic lambda calculus working of Stephen Diehls excellent Write You a Haskell [1]. I would like to add explicit type annotations to the language, eg. type signatures for functions, but yesterday stumbled over a nut I still haven't been able to crack. The problem is with the semantics of programmer specified type annotations, type variables contained in those signatures and how they interact with the type checker. First off, the WYAH type inference algorithm is a text-book implementation of Hindley-Milner algorithm. It is designed to take a program without any explicit type information, and if type checking succeeds, it outputs the types of every part of the AST. This works fine, the issue surfaced when I added explicit type annotations to the AST. Lets say I have the function: foo1 :: a -> a foo1 x = x The inferred type of `foo1` is `foo1 :: t -> t`, when unified with the annotated type, `a -> a`, the substitution [t = a] is found. Everything works out, as it should. Now consider: foo2 :: Int -> Int foo2 x = x The inferred type of `foo2` is `foo2 :: t -> t` when unified with the annotated type, `Int -> Int`, a substitution [t = Int] is found. Everything works out, as it should. Lastly consider: foo3 :: a -> Int foo3 x = x The inferred type of `foo3` is, again, `foo3 :: t -> t`, when unified with the annotated type, `a -> Int`, we find that `t` must have the same type as `a`, and `t` must be of type `Int`, thus `t = a = Int`. Everything works out!? This is of course not intended behavior, the type annotation on foo3 is horribly wrong, and is blatantly lying about the actual type of the function. When I found this my first instinct was to check how the Haskell compiler would check this function. I found that Haskell complains: Couldn't match expected type `a' with actual type `Int' `a' is a rigid type variable bound by the type signature for foo3 :: a -> Int at foo3.hs:1:9 As far as I understand, Haskell has the notion of a rigid type variable, which my compiler is missing, I am also guessing that these rigid type variables have different rules with respect to unification? Where can I read more about these strange things? I have been trying to form the rules for unification from my intuition about Haskell type checking, but still haven't grasped the whole story, eg. how does one decide if the rigid type variable can be "specialized" or not. In `foo2` it is ok to specialize `a` into `Int`, but in `foo3`, this is not ok. I am struggling to see the underlying rules for this, and would gladly appreciate any pointers to enlightenment! Bonus question! Given the function: foo4 :: a -> a foo4 x = -(x :: Int) The inferred type of `foo4` is `foo4 :: Int -> Int`. When unified with the annotated type, `a -> a`, the substitution [a = Int] is found, everything checks out! >From intuition we know that the type annotation for `foo4` is "to general", that is, `generalness(Int -> Int) < generalness(a -> a)`, but how does one actually express that in code? That is, what is the algorithm that lies behind the function `generalness`? [1] http://dev.stephendiehl.com/fun/

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Re: [Haskell-cafe] How to report builds...?
by Christopher Howard 24 Aug '16

24 Aug '16

On 08/24/2016 12:01 AM, Daniel Trstenjak wrote: > > On Tue, Aug 23, 2016 at 09:32:56PM -0800, Christopher Howard wrote: >> Hi list. Hackage cannot build my package, because for some reason it >> will only use base-4.9.0.0, which does not satisfy the dependencies of >> some of my dependencies. So, I was trying to figure out how to report my >> own builds. However, when I use the "cabal report" command, cabal take >> my username and password, and then (so far as I can tell) does nothing. >> I tried increasing the verbosity, as well as upgrading to the latest >> version of cabal, but this did not work. And I can seem to find the >> documentation for this feature. Please advise. > > But the first thing you could try is to call cabal with the '--allow-newer' > option, which allows newer library versions then specified by the > constraints. This will only work if the newer library versions don't > containt breaking changes, so it's just a way to get going for the > moment but no real solution. > > About the reporting, there's no other way then to contact every single > developer of the breaking libraries. > Well, I know the reason it cannot build with base-4.9.0.0: My library is fully compatible with base (>=4.6 && <4.10), but it depends on the Gloss library (>=1.7 && <1.10). I cannot use Gloss 1.10, because Gloss 1.10 has a breaking API change on one of the Gloss functions I use in my library, which I could only get around if I used a lot of conditional code, which I haven't done yet. However, only Gloss 1.10.* is compatible with base-4.9.0.0. http://hackage.haskell.org/package/mars http://hackage.haskell.org/package/mars-0.2.1.0/reports/ Hackage, it is seems, will only try building with base-4.9.0.0, rather than reverting to an earlier version and compiler. The package builds just fine on my system, using an earlier base and compiler. So, I guess the question is, do I have to further restrict the dependencies of my library (to exclude 4.9.0.0) just because one of my dependencies cannot handle that version? -- https://qlfiles.net My PGP public key ID is 0x340EA95A (pgp.mit.edu)

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How to report builds...?
by Christopher Howard 24 Aug '16

24 Aug '16

Hi list. Hackage cannot build my package, because for some reason it will only use base-4.9.0.0, which does not satisfy the dependencies of some of my dependencies. So, I was trying to figure out how to report my own builds. However, when I use the "cabal report" command, cabal take my username and password, and then (so far as I can tell) does nothing. I tried increasing the verbosity, as well as upgrading to the latest version of cabal, but this did not work. And I can seem to find the documentation for this feature. Please advise. -- https://qlfiles.net My PGP public key ID is 0x340EA95A (pgp.mit.edu)

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Proposal: (.:) operator in base.
by Alex Belanger 23 Aug '16

23 Aug '16

Hi, Some of you might be familiar with (.:) = (.) . (.). It has type :: (c -> d) -> (a -> b -> c) -> a -> b -> d It allows the composition of two functions, the first one, accepting one operand, and the second, two operands. This appears to be a very common pattern, referenced a bit everywhere, almost always defined on lambdabot and found in multiple codebases in the wild. I'd like the know the general sentiment about this operator, as well as how its inclusion in base, probably Data.Function, would be perceived before I actually try to make it happen. Cheers, Alex (nitrix).

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Re: [Haskell-cafe] Proposal: (.:) operator in base.
by David Feuer 23 Aug '16

23 Aug '16

And also sometimes wrong. I don't really understand why, but sometimes eta *expansion* is important when working around RULES. On Aug 23, 2016 5:42 AM, "Alan & Kim Zimmerman" <alan.zimm(a)gmail.com> wrote: Suddenly the eta reduce hlint warning becomes much more important. Alan On 23 Aug 2016 11:22 a.m., "Tom Ellis" <tom-lists-haskell-cafe-2013@ jaguarpaw.co.uk> wrote: > *Very* interesting. Here is the key section of the GHC users guide: > > GHC will only inline the function if it is fully applied, where “fully > applied” means applied to as many arguments as appear (syntactically) > on > the LHS of the function definition > > That this is a good heuristic is *extremely* counterintuitive to me. I > would have supposed that being more explicit, for example > > {-# INLINE (.) f g #-} > > to inline on all static applications of at least two arguments would have > been a much clearer way to communicate this message. > > What's the rationale behind the current behaviour? > > Tom > > > On Tue, Aug 23, 2016 at 09:56:25AM +0100, Matthew Pickering wrote: > > I think the point which no-one has articulated yet is that the > > source-level arity of (.) affects whether GHC will decide to inline it. > > > > Only fully saturated applications are inlined > [...] > > > > On Tue, Aug 23, 2016 at 8:33 AM, Tom Ellis > > <tom-lists-haskell-cafe-2013(a)jaguarpaw.co.uk> wrote: > > > On the contrary, they're exactly the same (on GHC 7.6.3): > > > > > > module Foo where > > > > > > comp1 f g x = f (g x) > > > > > > comp2 f g = \x -> f (g x) > > > > > > > > > % ghc -O0 -dsuppress-all -fforce-recomp -no-link -ddump-prep > test.hs > > > [1 of 1] Compiling Foo ( test.hs, test.o ) > > > > > > ==================== CorePrep ==================== > > > Result size of CorePrep = {terms: 24, types: 36, coercions: 0} > > > > > > comp2 > > > comp2 = > > > \ @ t_aeU @ t1_aeV @ t2_aeW f_sfz g_sfy x_sfx -> > > > let { > > > sat_sfI > > > sat_sfI = g_sfy x_sfx } in > > > f_sfz sat_sfI > > > > > > comp1 > > > comp1 = > > > \ @ t_af5 @ t1_af6 @ t2_af7 f_sfG g_sfF x_sfE -> > > > let { > > > sat_sfJ > > > sat_sfJ = g_sfF x_sfE } in > > > f_sfG sat_sfJ > > > > > > (the same holds for -O2, if you compile them separately) > > > > > > On Tue, Aug 23, 2016 at 05:19:38PM +1000, Ben wrote: > > >> At the semantic level of "does my program compute correct results" > they're > > >> identical. At the operational level of "how fast does my program run" > > >> they're different. > > >> > > >> > > >> On August 23, 2016 5:09:19 PM GMT+10:00, Tom Ellis < > tom-lists-haskell-cafe-2013(a)jaguarpaw.co.uk> wrote: > > >> >On Mon, Aug 22, 2016 at 10:23:07PM -0700, wren romano wrote: > > >> >> (.) f g = \x -> f (g x) > > >> >> > > >> >> vs: > > >> >> > > >> >> (.) f g x = f (g x) > > >> >> > > >> >> has ramifications, though it's fairly easy to guess which one of > > >> >those > > >> >> two will be most performant. > > >> > > > >> >Are these not synonyms? What is the meaning of > > >> > > > >> > fargs var = expr > > >> > > > >> >if not > > >> > > > >> > fargs = \var -> expr > > >> > > > >> >? > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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Re: [Haskell-cafe] Proposal: (.:) operator in base.
by Mike Ledger 23 Aug '16

23 Aug '16

(Forgot reply all) ---------- Forwarded message ---------- From: "Mike Ledger" <eleventynine(a)gmail.com> Date: 18 Aug 2016 7:52 AM Subject: Re: [Haskell-cafe] Proposal: (.:) operator in base. To: "Csongor Kiss" <kiss.csongor.kiss(a)gmail.com> Cc: IMO: I've never seen a compelling use of this; I think it decreases clarity, and barely shaves off any characters from equivalent "pointful" expressions. On Thu, Aug 18, 2016 at 7:45 AM, Csongor Kiss <kiss.csongor.kiss(a)gmail.com> wrote: > Of course I meant to write ((->) e) > > On 17 August 2016 at 22:39:36, Csongor Kiss (kiss.csongor.kiss(a)gmail.com) > wrote: > > Can you provide a bit more detail on why you think (fmap . fmap) captures > the concept behind (.) . (.) ? > It certainly does generalise it in one way - and I do get how, but is it > the *right* generalisation? > > Or does it just coincide with (-> e)’s fmap being defined as (.), and in > fact we would prefer to > generalise the composition aspect, in which case Category would be a more > ‘obvious’ place to look? > > On 17 August 2016 at 22:15:56, Tony Morris (tonymorris(a)gmail.com) wrote: > > You'd generalise it to: > > fmap . fmap :: (Functor f, Functor g) => (a -> b) -> f (g a) -> f (g b) > > And then, would you do the same for Traversable, Foldable and Applicative? > > On 18/08/16 03:43, Alex Belanger wrote: > > Hi, > > Some of you might be familiar with (.:) = (.) . (.). > > It has type :: (c -> d) -> (a -> b -> c) -> a -> b -> d > > It allows the composition of two functions, the first one, accepting one > operand, and the second, two operands. > > This appears to be a very common pattern, referenced a bit everywhere, > almost always defined on lambdabot and found in multiple codebases in the > wild. > > I'd like the know the general sentiment about this operator, as well as > how its inclusion in base, probably Data.Function, would be perceived > before I actually try to make it happen. > > Cheers, > Alex (nitrix). > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to:http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. > > > ------------------------------ > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. >

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library for set of heterogeneous values?
by Anthony Clayden 23 Aug '16

23 Aug '16

I have a (let's call it) database of heterogeneous records. They're not Haskell records, but anonymous/extensible type-labelled rows. (Could be tuples, could be HLists, could be Lens-like, could be something fancier.) There's a small number (dozens) of distinct row types, each with a large number (thousands) of rows. The variety of row-types is not predictable in advance. And indeed a row might 'morph' over time with fields added/removed. So the obvious answer of putting the lot into a giant HList (each element of the list being a row) isn't going to scale. I could have a type-indexed HList in which each element is a Set of homogeneous rows. But performance still suffers from scanning along the list to find the right type index. Is there something better? On hackage there's two packages called HSet, neither giving very much help about their suitability: * `hset` (lower case) [AlekseyUymanov] seems isomorphic to a type-indexed HList. ie Must be unique type in each element (could be a Set type, I guess) * `HSet` (upper case) [athanclark] "Faux heterogeneous sets" seems a lot meatier why the "Faux"? built over hashtables in the ST monad. Has anybody used these? Can give guidance on what they can and can't? Bonus questions: Given a filter specifying a restriction on (some) fields of rows, I want to get a heterog subset: * all rows with at least those fields, matching those restrictions. * the restriction might be merely "has field labelled L". GIven a candidate row for insertion, I want first to scan for quasi-duplicates: * any existing row with a subset of the given fields, and the same value at those fields. * any existing row with a superset of the given fields, and the same value at those fields in common. * ignore records with only a partial overlap of fields. One possible data structure: a "vertical store". Give each row a Globally Unique Id. Have a separate set for each possible field, where the set elements are field value (key) to set of GUId -- records with that value. Then I have a different bonus question: * how to retrieve all field values for a given GUId? Thanks AntC

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Munich Haskell Meeting, 2016-08-25 @ 19:30
by Christian Neukirchen 22 Aug '16

22 Aug '16

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, August 25 at Hirschgarten Biergarten 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-aug-2016/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Re: [Haskell-cafe] Using stringize and string concatenation in ghc preprocessing
by Harendra Kumar 22 Aug '16

22 Aug '16

Thanks Mathieu. This works pretty well for gcc ( https://gcc.gnu.org/onlinedocs/cpp/Traditional-macros.html) but sadly it does not work for clang cpp as Brandon too pointed out earlier that clang does not have a traditional mode. -harendra On 22 August 2016 at 02:01, Boespflug, Mathieu <m(a)tweag.io> wrote: > Hi Harendra, > > I ran into this very problem recently. Turns out -traditional knows string > concatenation too. I seem to remember learning this by browsing the GHC > source code, but now I can't find any occurrence of this pattern. But > here's an example of how to do string concatenation with CPP in > -traditional mode: https://github.com/tweag/sparkle/blob/ > a4e481aa5180b6ec93c219f827aefe932b66a953/inline-java/src/ > Foreign/JNI.hs#L274. > > HTH, > > -- > Mathieu Boespflug > Founder at http://tweag.io. > > On 20 August 2016 at 20:33, Brandon Allbery <allbery.b(a)gmail.com> wrote: > >> On Sat, Aug 20, 2016 at 2:27 PM, Harendra Kumar <harendra.kumar(a)gmail.com >> > wrote: >> >>> But "-optP" seems to only append to the flags that GHC already passes >>> and gcc has no "-no-traditional" option to undo the effect of the >>> "-traditional" that GHC has already passed. I think "-optP" should override >>> the flags passed by ghc rather than appending to them. Is there a reason >>> not to do that? >>> >>> Is there any other better way to achieve this? What is the standard way >>> of doing this if any? >>> >> >> Removing -traditional will break much Haskell source. Go look at the >> history of clang with ghc (clang doesn't do -traditional) to see what >> happens. (tl;dr: without -traditional, cpp knows too much about what >> constitutes valid C, and mangles and/or throws errors on valid Haskell that >> doesn't lex the way C does.) >> >> You might want to look at cpphs as an alternative preprocessor. There are >> some ancient K&R-era hacks that could be used if absolutely necessary, but >> cpphs should be a much simpler and cleaner solution. >> >> -- >> brandon s allbery kf8nh sine nomine >> associates >> allbery.b(a)gmail.com >> ballbery(a)sinenomine.net >> unix, openafs, kerberos, infrastructure, xmonad >> http://sinenomine.net >> >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> >> >

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Rewrite without parenthesis
by Carl Petersen 22 Aug '16

22 Aug '16

How do you rewrite head (tail [1,2,3]) without parenthesis. Thanks!

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Re: [Haskell-cafe] Using stringize and string concatenation in ghc preprocessing
by Brandon Allbery 21 Aug '16

21 Aug '16

On Sun, Aug 21, 2016 at 4:31 PM, Boespflug, Mathieu <m(a)tweag.io> wrote: > I ran into this very problem recently. Turns out -traditional knows string > concatenation too. I seem to remember learning this by browsing the GHC > source code, but now I can't find any occurrence of this pattern. But > here's an example of how to do string concatenation with CPP in > -traditional mode: https://github.com/tweag/sparkle/blob/ > a4e481aa5180b6ec93c219f827aefe932b66a953/inline-java/src/ > Foreign/JNI.hs#L274 > > . > That's the hacky K&R way I mentioned earlier. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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How can I generalize my innerJoinOnId function to innerJoin with Vinyl?
by Cody Goodman 21 Aug '16

21 Aug '16

Hello all! I'll get right to it. As a first step I'd like to generalize innerJoinOnId's type signature to something like: innerJoinOnId :: (Id ∈ fields, Id ∈ fields2, Id ∈ fields2) => [Rec Attr fields] -> [Rec Attr fields2] -> [Rec Attr fields3] Or if possible: innerJoinOnId :: (Id ∈ fields) => [Rec Attr fields] -> [Rec Attr fields] -> [Rec Attr fields] As a next step I'd like to create an innerJoin function with type: innerJoinOn :: (a ∈ fields) => [Rec Attr fields] -> [Rec Attr fields] -> [Rec Attr fields] Where a is supplied and that constraint is carried on to the other inputs. Is this possible in Haskell? Here is both a link and the text of my (working, compilable) code thus far. Link: https://github.com/codygman/vinyl-experiments/blob/master/src/Main.hs Source code: {-# LANGUAGE DataKinds, PolyKinds, TypeOperators, TypeFamilies, FlexibleContexts, FlexibleInstances, NoMonomorphismRestriction, GADTs, TypeSynonymInstances, TemplateHaskell, StandaloneDeriving #-} module Main where import Data.Vinyl import Control.Lens hiding (Identity) import Data.Singletons.TH import Data.Maybe import Control.Monad import Data.Vinyl.TypeLevel (RIndex) data Fields = Id | Name | Age | ActivityName deriving Show type Person = ['Id, 'Name, 'Age] type Activity = ['Id, 'ActivityName] type family ElF (f :: Fields) :: * where ElF 'Id = Int ElF 'Name = String ElF 'Age = Int ElF 'ActivityName = String newtype Attr f = Attr { _unAttr :: ElF f } makeLenses ''Attr genSingletons [ ''Fields ] instance Show (Attr 'Id) where show (Attr x) = "id: " ++ show x instance Show (Attr 'Name) where show (Attr x) = "name: " ++ show x instance Show (Attr 'Age) where show (Attr x) = "age: " ++ show x instance Show (Attr 'ActivityName) where show (Attr x) = "activity: " ++ x (=::) :: sing f -> ElF f -> Attr f _ =:: x = Attr x joy :: Rec Attr ['Id, 'Name, 'Age] joy = (SId =:: 1) :& (SName =:: "Joy") :& (SAge =:: 28) :& RNil jon :: Rec Attr ['Id, 'Name, 'Age] jon = (SId =:: 0) :& (SName =:: "Jon") :& (SAge =:: 23) :& RNil karen :: Rec Attr ['Id, 'Name, 'Age] karen = (SId =:: 2) :& (SName =:: "Karen") :& (SAge =:: 15) :& RNil jonFootball :: Rec Attr ['Id, 'ActivityName] jonFootball = (SId =:: 0) :& (SActivityName =:: "football") :& RNil jonDancing :: Rec Attr ['Id, 'ActivityName] jonDancing = (SId =:: 0) :& (SActivityName =:: "dancing") :& RNil joyRacing :: Rec Attr ['Id, 'ActivityName] joyRacing = (SId =:: 1) :& (SActivityName =:: "racing") :& RNil peopleRows :: [Rec Attr ['Id, 'Name, 'Age]] peopleRows = [joy, jon, karen] activitieRows :: [Rec Attr ['Id, 'ActivityName]] activitieRows = [jonFootball, jonDancing, joyRacing] printActvy :: ('ActivityName ∈ fields) => Rec Attr fields -> IO () printActvy r = print (r ^. rlens SActivityName) isInPplIdx :: ('Id ∈ fields) => [Int] -> Rec Attr fields -> Bool isInPplIdx peopleIdx actvyRow = any (== True) . map (== actvyIdInt) $ peopleIdx where actvyIdInt = actvyRow ^. rlens SId . unAttr mkJoinedRow :: (Eq (ElF r1), RElem r1 ['Id, 'Name, 'Age] (RIndex r1 ['Id, 'Name, 'Age]), RElem r1 ['Id, 'ActivityName] (RIndex r1 ['Id, 'ActivityName]), ElF r1 ~ Int) => sing1 r1 -> [Rec Attr ['Id, 'ActivityName]] -> Rec Attr ['Id, 'Name, 'Age] -> [Rec Attr ['Id, 'Name, 'Age, 'ActivityName]] -- mkJoinedRow :: _ -> [Rec Attr ['Id, 'ActivityName]] -> Rec Attr ['Id, 'Name, 'Age] -> [Rec Attr ['Id, 'Name, 'Age, 'ActivityName]] mkJoinedRow field activities person = do let name = person ^. rlens SName . unAttr age = person ^. rlens SAge . unAttr let filteredActivities = filter (\r -> r ^. rlens field . unAttr == person ^. rlens field . unAttr) activities case listToMaybe filteredActivities of Just _ -> do let activityId actvy = actvy ^. rlens field . unAttr activityName actvy = actvy ^. rlens SActivityName . unAttr (\actvy -> (SId =:: activityId actvy) :& (SName =:: name) :& (SAge =:: age) :& (SActivityName =:: activityName actvy) :& RNil) <$> filteredActivities Nothing -> [] innerJoinOnId :: [Rec Attr ['Id, 'Name, 'Age]] -> [Rec Attr ['Id, 'ActivityName]] -> [Rec Attr ['Id, 'Name, 'Age, 'ActivityName]] innerJoinOnId people activities = do let peopleIdx =(\r -> r ^. rlens SId . unAttr) <$> people let filteredActivites = filter (isInPplIdx peopleIdx) activities join $ map (\p -> mkJoinedRow SId filteredActivites p) people main :: IO () main = mapM_ print $ innerJoinOnId peopleRows activitieRows -- example of main running: -- λ> peopleRows -- [{id: 1, name: "Joy", age: 28},{id: 0, name: "Jon", age: 23},{id: 2, name: "Karen", age: 15}] -- λ> activitieRows -- [{id: 0, activity: football},{id: 0, activity: dancing},{id: 1, activity: racing}] -- λ> main -- {id: 1, name: "Joy", age: 28, activity: racing} -- {id: 0, name: "Jon", age: 23, activity: football} -- {id: 0, name: "Jon", age: 23, activity: dancing} -- Code I wish worked: -- λ> mapM_ print $ innerJoinOn SId peopleRows activitieRows -- λ> mapM_ print $ innerJoinOn SName peopleRows activitieRows -- this line would give a compiler error about activitiesRows not containing 'Name

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Build error with llvm-general and lens
by HMPerson1 20 Aug '16

20 Aug '16

If I have a file > module Lib where > import Control.Lens > data Foo = Foo { _x :: Int , _y :: Int } > makeLenses ''Foo and I compile with `stack build`, it works fine. But if I add `llvm-general` to build-depends, and I run `stack build`, I get > [1 of 1] Compiling Lib ( Lib.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/Lib.o ) > <command line>: can't load .so/.DLL for: /usr/lib/libcurses.so (-lncursesw: cannot open shared object file: No such file or directory) > > -- While building package bug-test-0.1.0.0 using: > /home/user/.stack/setup-exe-cache/x86_64-linux/setup-Simple-Cabal-1.22.5.0-ghc-7.10.3 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.22.5.0 build lib:bug-test --ghc-options " -ddump-hi -ddump-to-file" > Process exited with code: ExitFailure 1 I've made a tiny test case here: https://git.io/v6MVv If I symlink `/usr/lub/libcurses.so` to `/usr/lub/libncursesw.so` (as suggested at https://git.io/v6MV8 and https://ghc.haskell.org/trac/ghc/ticket/9237) I get > [1 of 1] Compiling Lib ( Lib.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/Lib.o ) > > -- While building package bug-test-0.1.0.0 using: > /home/user/.stack/setup-exe-cache/x86_64-linux/setup-Simple-Cabal-1.22.5.0-ghc-7.10.3 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.22.5.0 build lib:bug-test --ghc-options " -ddump-hi -ddump-to-file" > Process exited with code: ExitFailure (-11) (There's no "can't load..." line and the exit code is different) As a workaround, I can separate the lens instances into a separate library, but I would like to avoid doing so if possible. How can I fix this?

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Using stringize and string concatenation in ghc preprocessing
by Harendra Kumar 20 Aug '16

20 Aug '16

Hi, To reduce boilerplate code in an FFI implementation file I am trying to use the stringizing and string concatenation features of the C preprocessor. Since ghc passes '-traditional' to the preprocessor which disables these features I thought I can pass my own flags to the preprocessor like this: {-# OPTIONS_GHC -optP -E -optP -undef #-} But "-optP" seems to only append to the flags that GHC already passes and gcc has no "-no-traditional" option to undo the effect of the "-traditional" that GHC has already passed. I think "-optP" should override the flags passed by ghc rather than appending to them. Is there a reason not to do that? Is there any other better way to achieve this? What is the standard way of doing this if any? -harendra

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GHC-related project for undergraduate thesis
by Eugene Akentyev 20 Aug '16

20 Aug '16

Hello, I am an undergraduate student and would like to do something useful for GHC as undergraduate thesis. This list https://ghc.haskell.org/trac/ghc/wiki/ProjectSuggestions is outdated (last modified 6 years ago) and some projects are already done (?). Do you have any ideas/projects? Thanks.

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ordering of structured values
by Paolino 19 Aug '16

19 Aug '16

Hello Cafè, I'd like to share a piece of code I've found useful to nest ordering on EQ. {-# language GADTs #-} import Data.Monoid (mconcat) import Data.Ord (comparing) data Orderings t where Ascending :: Ord a => (t -> a) -> Orderings t Descending :: Ord a => (t -> a) -> Orderings t comparings :: [Orderings t] -> t -> t -> Ordering comparings c x y = mconcat $ map g c where g (Ascending f) = comparing f x y g (Descending f) = comparing f y x I couldn't find a simpler solution, maybe somebody else has one. I've tried using comparings :: [forall a. Ord a => t -> a] -> t -> t -> Ordering which is not compiling for impredicativity and I'd like to understand better. An example on sorting (Ord a, Ord b, Ord c) => [(a,(b,c)] with 'a' in opposite order, then 'c' and 'b' in ascending order goes like sortBy (comparings [Descending fst, Ascending $ snd . snd, Ascending $ fst . snd]) Thanks for help/comments paolino

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[ANN] mysql-haskell 0.2.0.0
by 韩冬(基础平台部) 19 Aug '16

19 Aug '16

Hi all, I'm happy to announce mysql-haskell, a pure haskell mysql driver, is now available on package: https://github.com/winterland1989/mysql-haskell https://github.com/winterland1989/mysql-haskell The github page lists the motivation for creating this library, which are * prepared statment and binary protocol support. * better concurrency. * replication protocol support. All these targets are met. This release also includes quite complete tests and benchmarks, we think it’s stable enough to be used for pre-production environment, please fetch the code and testing, issues and bugs report are welcomed! Regards, Winterland@DIDI GROUP

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New online course "Functional Programming in Haskell" starts 19 Sept
by Wim Vanderbauwhede 18 Aug '16

18 Aug '16

We have created an introductory six-week online course (MOOC) on "Functional Programming in Haskell". The course will start on 19 September and is entirely free. We think it's a great way to get started with Haskell! https://www.futurelearn.com/courses/functional-programming-haskell The course is developed at Glasgow university's School of Computing Science and hosted by FutureLearn Enjoy! Wim Vanderbauwhede & Jeremy Singer @HaskellMOOC

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[ANN] ghc-typelits-knownnat-0.2: Solving GHCs KnownNat constraints
by Christiaan Baaij 17 Aug '16

17 Aug '16

Hi, If you have ever worked with `GHC.TypeLits`, then you have probably encountered an error very similar to: > • Could not deduce (KnownNat (n + 2)) > arising from a use of ‘natVal’ > from the context: KnownNat n i.e. where GHC cannot infer a `KnownNat (n+2)` constraint, even though a `KnownNat n` constraint is already given. Enter http://hackage.haskell.org/package/ghc-typelits-knownnat A type-checker plugin that can automatically derive "complex" KnownNat constraints from simpler ones. To use it, you simply add the {-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-} pragma to the top of your file. If you want to know more, you can also check out the corresponding blog posts: http://qbaylogic.com/blog/2016/08/10/solving-knownnat-constraints-plugin.ht… http://qbaylogic.com/blog/2016/08/17/solving-knownnat-custom-operations.html Regards, Christiaan

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(no subject)
by Corentin Dupont 17 Aug '16

17 Aug '16

Hi guys, is there some library with a Monad (+Applicative, Functor...) instance of the following type: data Todo a b = Todo [a] | Done b Thanks! Corentin

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[ANN] generic-random-0.2.0.0
by Li-yao Xia 16 Aug '16

16 Aug '16

Hello, This library aims to derive random generators for ADTs. The previous and first release was an implementation of Boltzmann generators, in response to this blogpost by Brent Yorgey. https://byorgey.wordpress.com/2016/03/23/boltzmann-sampling-for-generic-arb… It's nice to look at, but it is an opinionated choice about the resulting probability distribution which is not always suitable. In this release, using GHC.Generics, Generic.Random.Generic wraps common boilerplate to define "arbitrary" in particular, hiding from you long chains of the following: Constructor <$> arbitrary <*> arbitrary <*> arbitrary as well as frequency [(n1, C1 <$> arbitrary), (n2, C2 <$> arbitrary), ...] leaving just the weights [n1, n2, ...] for you to specify. data Tree a = Leaf a | Node (Tree a) (Tree a) deriving Generic instance Arbitrary a => Arbitrary (Tree a) where arbitrary = genericArbitraryFrequency [9, 8] -- equivalent to -- > arbitrary = frequency -- > [ (9, Leaf <$> arbitrary) -- > , (8, Node <$> arbitrary <*> arbitrary) -- > ] It comes with a simple opt-in strategy to bound the size of generated data, especially convenient for recursive data types. Indeed, even for a simple binary tree, the naive generator obtained by producing a leaf half of the times, and an inner node the other half, with two recursively generated children, results in a random tree of *infinite* expected size (though the tree is still "almost surely" finite). There is no theoretical reason the library uses two flavors of generics for different tasks. There are trade-offs, but I can't see impassable obstacles to use one in place of the other. If I had to be consistent now I'd choose GHC.Generics, though I was not fluent enough in that flavor at the time when I wrote the Data part. Property-based testing is the main application I have in mind for this library. Nevertheless, I welcome ideas, questions and suggestions from any domain or of general nature. Github: https://github.com/lysxia/generic-random Hackage: http://hackage.haskell.org/package/generic-random Li-yao

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Proposal: Rational read
by amindfv＠gmail.com 16 Aug '16

16 Aug '16

Is there a reason to not allow '.' as a separator for Rational's Read instance, so that both "1%2" and "0.5" can be read? Tom

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[ANN] aeson 1.0.0.0
by Adam Bergmark 16 Aug '16

16 Aug '16

Hi haskellers, I'm happy to announce that aeson v1.0.0.0 is now available on hackage! While 0.11.x was mostly regression fixes 1.x adds a lot of improvements that had been in the pipeline for a long time. We decided to move to 1.x since there are a lot of changes. Fortunately most of them are backwards-compatible so hopefully migration will be simple for you. Still, we recommend reading the changelog (https://github.com/bos/aeson/blob/master/changelog.md#1000) Thanks to all contributors of this release: * Adam Bergmark * Andrew Martin * Artyom * Ewout * Matthías Páll Gissurarson * Oleg Grenrus * Ryan Scott * lyxia Regards, Adam

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Help! Haskell Platform install broke System.Random.TF
by Jeremy Gibbons 16 Aug '16

16 Aug '16

I've just installed GHC 8.0.1, and it seems that my QuickCheck is now broken. I'm presuming that these two facts are connected. I have a trivially small QuickCheck example: > import Test.QuickCheck > prop_PlusAssociative :: Integer -> Integer -> Integer -> Bool > prop_PlusAssociative x y z = (x+y)+z == x+(y+z) This loads fine, but when I try to test the property I get a mysterious error: jg$ ghci quickcheck.lhs GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( quickcheck.lhs, interpreted ) Ok, modules loaded: Main. *Main> quickCheck prop_PlusAssociative can't load .so/.DLL for: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib (dlopen(/Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib, 5): Library not loaded: @rpath/libHSrandom-1.1-1z8Ujelqc6aKgvPnbRUKkP-ghc7.10.2.dylib Referenced from: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib Reason: image not found) I don't think it's anything to do with QuickCheck itself; I get the same error just trying to use the System.Random.TF library directly: jg$ ghci GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help Prelude> import System.Random.TF.Gen Prelude System.Random.TF.Gen> seedTFGen (1,1,1,1) can't load .so/.DLL for: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib (dlopen(/Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib, 5): Library not loaded: @rpath/libHSrandom-1.1-1z8Ujelqc6aKgvPnbRUKkP-ghc7.10.2.dylib Referenced from: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib Reason: image not found) The "7.10.2" is because I deactivated 8.0.1, then installed 7.10.3, then uninstalled both, then uninstalled all GHCs and deleted /Library/Haskell and reinstalled GHC 7.10.2 (my last working version) from what I hoped would be a clean slate. All to no avail. However, I do appear still to have a ghc-7.4.2, overlooked by uninstall-hs; and that does work fine - apparently not using the same System.Random.TF library. Can anyone suggest what I'm doing wrong, and more importantly, how to fix it? Jeremy Jeremy.Gibbons(a)cs.ox.ac.uk Oxford University Department of Computer Science, Wolfson Building, Parks Road, Oxford OX1 3QD, UK. +44 1865 283521 http://www.cs.ox.ac.uk/people/jeremy.gibbons/

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Proposal: Rational read
by Doug McIlroy 16 Aug '16

16 Aug '16

It has been suggested that (read "0.5" :: Rational) need not work because (show (0.5::Rational)) will never yield the string "0.5". In contrast, (read "1" :: Double) does work, even though (show (1::Double)) will never yield the string "1". The inconsistency is stark. Doug

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Haskell Job In Cambridge UK Working On Compilers, Machine Learning And Computer Vision!
by Joe Hermaszewski 16 Aug '16

16 Aug '16

We have a Haskell job opening in Cambridge, UK. Please see https://www.myrtlesoftware.com/vacancies/ Here is a video of the computer vision algorithms running in hardware we have produced using Haskell: https://www.myrtlesoftware.com/case-studies/fpga/ Myrtle is currently working as part of the UK government’s autonomous vehicles program and we are looking to hire more Haskell developers to join our expanding team. You will be working directly on our technology that translates high-level signal processing code into efficient hardware designs. Applicants should have a deep understanding of Haskell, modern compiler technology and should be familiar with git and CI based development. We are targeting GPU and FPGA backends so experience with either of these would be advantageous although we’re happy to consider strong developers without direct experience of these two areas. Similarly, a familiarity with an HDL, such as VHDL or Verilog, would be viewed positively but is not essential. Applicants should send a resume and covering email, stating your suitability for this role, to jobs(a)myrtlesoftware.com Myrtle is a software company based in Cambridge UK with a long history of working at the forefront of computer graphics and compiler technology. We are best known for having helped produce computer generated images for over twenty major Hollywood movies including the Transformers franchise. Myrtle’s clients have included NYSE and NASDAQ listed companies in LA, Vancouver and London. Closing date for applications 5pm, 27th August 2016

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Which is the most "hackable" compiler?
by MarLinn 15 Aug '16

15 Aug '16

So I have some crazy ideas for language extensions/reinterpretations and ways I might try to implement them. But ghc is a complex beast and my crazy ideas have too many interrelationships for a noob like me to try to bold them onto it. After all, I have never worked on ghc - or any compiler for that matter. I did look at the code, but I reckon maybe I should gather some experience first before diving into that one. It would be even worse to offload my ideas onto someone else before I at least tried them and got some feedback. No fun for anyone. But of course I don't want to start from scratch. As fun as that could be, there's no need to reinvent all the wheels just for a different axle. And I hear rumors that there are other Haskell compilers out there, even if most of them live in the shadow/slipstream of their brother. Alas, I don't have any idea what their statuses and philosophies are. So maybe you can help me here: Have you experienced any of the alternative compilers as especially easy for a newcomer to pick up and play around with? If it helps, I would be satisfied with plain Haskell2010 or even Haskell 98, although some GADT and/or TypeFamilies code to butcher would be nice, too. The ideas are mostly about larger scale structures like whole functions. One representative example idea is "Could it help the implementation - and does it even make sense - to view a module as just a weirdly written zero parameter type class?" As I said: crazy ideas. I'm thankful for any thoughts and ideas. MarLinn

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Re: [Haskell-cafe] The relationship between F-algebras and the Free Monad
by Olaf Klinke 15 Aug '16

15 Aug '16

Dear Will, I used to think of the extra parameter "a" in Free f a as the type of variables in the terms. So in short, while Fix f gives you the closed terms of your AST, Free f a gives you open terms with variables of type a. Indeed the equations Fix f = f (Fix f) Free f a = Pure a | Free (f (Free f a)) are related: Let Var a f x = Var a | App (f x) Then Free f a = Fix (Var a f) Hence the free monad is the fixed point of the functor f where you first sneak in free variables of type a. Moreover, given a function from variables to closed terms, you can turn an open term to a closed term (see below). Your exercise: Implement the below using catamorphisms and so on. -- Olaf newtype Fix f = Fix (f (Fix f)) data Free f a = Pure a | Free (f (Free f a)) data Var a f x = Var a | App (f x) -- tofree.tofix = id -- tofix.tofree = id tofree :: Functor f => Free f a -> Fix (Var a f) tofree (Pure a) = Fix (Var a) tofree (Free fx) = Fix (App (fmap tofree fx)) tofix :: Functor f => Fix (Var a f) -> Free f a tofix (Fix (Var a)) = Pure a tofix (Fix (App fx)) = Free (fmap tofix fx) close :: Functor f => (a -> Fix f) -> Free f a -> Fix f close evaluate (Pure a) = evaluate a close evaluate (Free fx) = Fix (fmap (close evaluate) fx)

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Typeclass resolution errors quite puzzling
by Tom Ellis 15 Aug '16

15 Aug '16

Here's a program with an odd error message (GHC 8.0.1): data A a = A a deriving Eq data B = B main :: IO () main = print (A B == A B) test/main.hs:5:15: error: • No instance for (Eq B) arising from a use of ‘==’ • In the first argument of ‘print’, namely ‘(A B == A B)’ In the expression: print (A B == A B) In an equation for ‘main’: main = print (A B == A B) I get an error about Eq B even though it's Eq A that is manifestly required at the call site. This error is odder when A and B are defined far away from the use of '=='. This is even odder: data A a = A a data B = B instance Ord a => Eq (A a) where main :: IO () main = print (A B == A B) test/main.hs:7:15: error: • No instance for (Ord B) arising from a use of ‘==’ • In the first argument of ‘print’, namely ‘(A B == A B)’ In the expression: print (A B == A B) In an equation for ‘main’: main = print (A B == A B) Now not only is the type puzzling (B instead of A) but the *class* is puzzling (Ord instead of Eq). This occurred to me in practice because 'Data.Graph.Inductive.PatriciaTree.Gr' has '(Eq a, Ord b) => Eq (Gr a b)'. It would have been a lot more helpful to see * No instance for (Ord B) * arising from (Eq A) * arising from the use of '==' Does anyone agree with me that GHC should produce the full trace when it fails to resolve instances rather than just the proximal failure? Tom

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