February 2021 - Haskell-Cafe

Do something about Cabal?
by Ignat Insarov 03 Jan '22

03 Jan '22

# Do something about Cabal? Hello. Cabal is the second most used tool in Haskell after GHC. It has many problems. It may be noticed that there is one and a half developers working on it. This is clearly not enough to address these problems. I propose that this is a good place to invest in. ### Problems I have in mind: * Poor communication, lack of open source development process. The whole Cabal–Stack schism appears to be an outcome of poor communication. One of the leading developers of Cabal is even banned from participation somewhere in Stack circles.[1] Personally, I reported several issues to Cabal and every single time it resulted in sadness. Observe a vicious circle: core developers are overworked ⇒ they are being unfriendly ⇒ there are fewer contributors ⇒ core developers are overworked. I have no hard evidence but it appears that presently, more people that strive to improve the Haskell build experience are outside the Cabal cabal than are inside. * User experience is an afterthought. Cabal's user experience is horrifying. A collection of complaints is being compiled elsewhere.[2] There are also bugs being opened to Cabal because of this, requiring triage and therefore wasting the precious time of the few overworked developers. Stack is much more friendly — this shows by example that the user experience problem is not inherent and may be solved. It is ordinary to receive output like this: ``` % cabal run example-executable Warning: The package list for 'hackage.haskell.org' is 84 days old. Run 'cabal update' to get the latest list of available packages. Resolving dependencies... cabal: Could not resolve dependencies: [__0] trying: example-0.1.0.6 (user goal) [__1] next goal: opaleye (dependency of example) [__1] rejecting: opaleye-0.7.1.0, opaleye-0.7.0.0 (constraint from project config TODO requires ==0.6.7006.1) [__1] rejecting: opaleye-0.6.7006.1 (conflict: example => opaleye^>=0.7) [__1] skipping: opaleye-0.6.7006.0, opaleye-0.6.7005.0, opaleye-0.6.7004.2, opaleye-0.6.7004.1, opaleye-0.6.7004.0, opaleye-0.6.7003.1, opaleye-0.6.7003.0, opaleye-0.6.1.0, opaleye-0.6.0.0, opaleye-0.5.4.0, opaleye-0.5.3.1, opaleye-0.5.3.0, opaleye-0.5.2.2, opaleye-0.5.2.0, opaleye-0.5.1.1, opaleye-0.5.1.0, opaleye-0.5.0.0, opaleye-0.4.2.0, opaleye-0.4.1.0, opaleye-0.4.0.0, opaleye-0.3.1.2, opaleye-0.3.1, opaleye-0.3, opaleye-0.2, opaleye-0.6.7002.0, opaleye-0.6.7001.0, opaleye-0.6.7000.0, opaleye-0.5.2.1, opaleye-0.3.1.1 (has the same characteristics that caused the previous version to fail: excluded by constraint '^>=0.7' from example) [__1] fail (backjumping, conflict set: example, opaleye) After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: opaleye, example ``` There are so many things that are wrong here. Even a sneaky _«to do»_ remark. If you wonder, in this case the solution is to remove and re-generate `cabal.project.freeze`. Even the name of the program — it is actually _«cabal-install»_ — is incomprehensible, it should be re-branded to Cabal, which is how everyone calls it anyway. * Features are not being introduced. There is no reason for two build tools to exist. The killer feature of Stack — snapshots — should be supported by Cabal. Possibly Cabal itself should be refactored and split so that there are separate tools for packaging, version resolution and human interaction — I do not know. But certainly the way things are presently is a waste of developer effort and a source of confusion for everyone. ### My proposition, in particular. * Ask all the people that show compassion to the cause of a great Haskell build tool to unite and work together on a better Cabal. This includes the developers of Stack and everyone that expressed unhappiness with the current state of Cabal. These people should be seen as a blessing, not as an obstacle. * Put in place a clear process for contributing and decision making, so that it does not come down to the privileged opinion of one of the core developers. * Make a model of user experience that Cabal should conform to, and make conformance a priority. Surely there are among us people that know a thing or two about user experience — call for them to step forward. Every issue that stems from misunderstanding, re-assign to the model instead of closing. * Merge the support of Stackage snapshots into Cabal. Ask the core developers of Stack to join the effort. Transition from Stack to Cabal should be one well discoverable command that just works. I realize that this letter is largely an opinion piece. You can also see it as an _«ideal piece»_. Without an ideal, without a vision, we are stuck with the present. I do not insist that my vision is the best. But the present reality is not the best vision either. I propose, foremost, that we work and fight for a better future. [1]: https://github.com/commercialhaskell/stackage/issues/4472 [2]: https://github.com/tomjaguarpaw/tilapia/issues?q=is%3Aissue+is%3Aopen+cabal

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Re: [Haskell-cafe] How to do reversible parsing?
by Johannes Waldmann 10 Jul '21

10 Jul '21

Rendell and Ostermann, Invertible syntax descriptions, Haskell Symposium '10 https://doi.org/10.1145/1863523.1863525 https://hackage.haskell.org/package/invertible-syntax For the general background, see, e.g., Grohne and Voigtländer 2017 Formalizing semantic bidirectionalization ... https://doi.org/10.1016/j.jlamp.2016.04.002 And there's a workshop series on Bidirectional Transformations (from 2012 onwards) - J.

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Why does my module take so long to compile?
by Troels Henriksen 28 Mar '21

28 Mar '21

I have a program with about 220 modules. One of these modules takes _much_ longer to compile than the others (several minutes longer). I don't understand why. It compiled much faster with GHC 8.8.3, but got orders of magnitude slower when I changed to GHC 8.10.2. I don't understand why, since I don't think there is nothing special about the module - e.g. no type-level programming. Is there any good way to "profile" the stack or cabal compilation process to at least get an intuition for what takes so long? Note that the module is in the middle of the dependency tree of my program, so it is not viable to separate it out and compile it manually with GHC. It's slow even when I pass '--fast' to stack which should disable the optimiser, so I guess the type checker may be spending a lot of time on handling the admittedly large amount of typeclass constraints I have on some of my definitions. I'm mostly looking for techniques and tools I can also use next time this problem occurs, not the specific problem here, but if anyone's curious, the module in question is this one: https://github.com/diku-dk/futhark/blob/master/src/Futhark/Pass/ExtractKern… -- \ Troels /\ Henriksen

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Global Package Install
by A. Mc. 22 Mar '21

22 Mar '21

Hello, I am trying to add the package System.Random to be able to run it the same way Data.List or Data.Char, et cetera does in ghci and in doing a ghc command line build. I've tried using $ stack ghci --package random which I have to run every single time I want to load a module that imports anything from System.Random and I've already tried $ cabal install --lib random which only works with cabal, and I've used the command $ stack install random, which says 'cache populated' but if a restart and load a dependent module, still is not working for me. I've also tried to update the .yaml file by putting System.Random into the brackets is the sr directory, but this I'm guessing isn't what you're supposed to do as it doesn't work either. I'm using Cygwin on a Windows 10 operating system. Your assistance in this matter would be greatly appreciated. Thank you for your time.

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Profiling analyzing ghc compilation
by Martin Heuschober 05 Mar '21

05 Mar '21

Hi, Is there a way to profile (time/space) GHC compilation itself, I'd be interested in what compilation steps in compiling my project take long and how much RAM it uses/why it uses so much RAM, so I can identify what to rewrite, remove, and split to get a leaner build. Thanks, Martin PS.: I hope this is the right place to ask this.

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International Conference on Applied Data Science and Intelligence - (ADSI 2021) Prague
by matteo＠confscience.com 03 Mar '21

03 Mar '21

Call for papers ************************************************* International Conference on Applied Data Science and Intelligence - (ADSI 2021) Prague- Czech Republic, October 14-15, 2021 https://confscience.com/adsi/ Submission deadline: April 1, 2021 All papers accepted in ADSI 2021 will be published in Springer CCIS (Communications in Computer and Information Science). CCIS is abstracted/indexed in Scopus, SCImago, EI-Compendex, Mathematical Reviews, DBLP, Google Scholar, and Thomson Reuters Conference Proceedings Citation (Former ISI Proceedings) *************************************************************************** IMPORTANT DATES: - Paper Submission: April 1, 2021 - Acceptance Notification: July 1, 2021 - Final Manuscript Due: September 1, 2021 *************************************************************************** The ADSI 2021 conference will be held in Conjunction with: International Conference on Recent Theories and Applications in Transportation and Mobility (RTATM 2021) International Conference on Informatics Revolution for Smarter Healthcare (IRSH 2021) *************************************************************************** TOPICS: Authors are invited to submit their original papers to address the topics of the conference, including but not limited to: FUNDAMENTALS AND THEORIES - Theoretical Models - Spatial and temporal multi-models - Multi-dimensional data - Data acquisition and pre-processing - Data inference - Data Classification and Taxonomy - Data Metrics - New approaches for collaboration and competition - Self-organization, self-healing, fault-tolerance approaches - Spatial reasoning - Context awareness - Intelligent mobility - New approaches to supervised and unsupervised learning - New approaches for security, privacy, trust, and ethics in data science - Real-time data analytics - Multi-Agent Systems for data science - Distributed data analytics - Data authenticity - New theories and approaches for Deep learning - New approaches for Business Intelligence - Fuzzy logic - Decision trees - Support vector machines - Evolutionary computation - Statistical methods - Collaborative filtering - Data engineering - Content mining - Indexing schemes - Information retrieval - Metadata use and management INTELLIGENT DATA PROCESSING AND ANALYTICS - Multi-level data processing - Data analytics optimization - Smart data mining - Machine Learning - Deep Learning - Bio-Inspired Computing - Secure data analytics - Privacy in data analytics - Trust in Big Data - Business intelligence - Visualization Analytics - Intelligence as a Service (IaaS) - Data Science as a Service (DSaaS) - Natural Language Processing - Signal Processing - Simulation and Modeling - Data-Intensive Computing SYSTEMS AND INFRASTRUCTURES - Data storage infrastructure - Data warehouses - Data Query and Indexing Technologies - Software Defined Infrastructures - Software Defined Networks (SDN) - Distributed data systems - Smart grid computing - Intelligent data management - Big Data computing - Smart data networking - Internet of Things - Cyber Physical Systems - Blockchain - Fog and Edge intelligence - Parallel Computing systems - Open Source systems for data science - Embedded intelligence - Embedded data science - In-Memory computing - Intelligent drones - Internet of Drones - Real-time data acquisition systems APPLICATIONS - Intelligent Hazard management - Intelligent data science in healthcare - Intelligent data science in farming - Intelligent data science in Oil and Gas - Smart logistics - Intelligent data science in transportation - Intelligent data science in surveillance - Xtech (Fintech, Agritech, etc.) - Intelligent drones - Digital transformation - Bioinformatics - Marketing - Social Science - E-learning and E-services *************************************************************************** OUTSTANDING PAPERS: Based on the peer review scores as well as the presentations at the conference, the authors of outstanding papers will be invited to extend their works for a potential publication in journals special issues with high impact factors. *************************************************************************** PAPER SUBMISSION: Papers must be submitted electronically as PDF files via easychair (https://easychair.org/conferences/?conf=adsi2021) All papers will be peer reviewed. Length of Full papers: 12-15 pages long (written in the LNCS/CCIS one-column page format, 400 words per page) Length of Short papers: less than 12 pages For more information, please refer to the conference website: https://confscience.com/adsi/ *************************************************************************** CONTACT For more information, please send an email to info-adsi(a)confscience.com

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DatatypeContexts / alternative
by CASANOVA Juan 01 Mar '21

01 Mar '21

Hello again, Haskell Cafe, Here again with another doubt on how am I supposed to. This one should be fairly easier, I believe. In short, I would like to have a functor type that can only be applied to certain type parameters. Why? Well, something like this: module DatatypeContextsExample where import Data.Map import Data.Bifunctor data Foo t = Foo (Map t t) instance Functor Foo where fmap f (Foo m) = Foo (fromList (fmap (bimap f f) (toList m))) This does not compile, because I am using toList and fromList, which require (Ord t). But I cannot really have Foo be a functor in this way without it. The thing is, every time I am going to use it, t is actually going to be Ord. But how do I tell Haskell to have this constraint? DatatypeContexts seems to be the extension that allows this: data Ord t => Foo t = Foo (Map t t) But this has two issues. First, DatatypeContexts is deprecated. Second, more importantly, it still does not type check! It produces the exact same error, saying that Ord t could not be inferred. It should be inferred from the very fact that I am using the type Foo, but GHC does not seem to want to understand this. I could also try with GADTs: data Foo t where Foo :: Ord t => Map t t -> Foo t But this still gives the same error on the fmap definition, stating it cannot use Foo without Ord. But I am pattern-matching against it, shouldn't that be enough to know that (Ord t) is going to hold? I don't understand it. Is there anything simple I am missing? Any other simple way to achieve this? Any fundamental problem with what I am trying to do? Thanks as usual, Juan. The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. Is e buidheann carthannais a th' ann an Oilthigh Dhùn Èideann, clàraichte an Alba, àireamh clàraidh SC005336.

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Desktop GUI in Haskell
by A. Mc. 28 Feb '21

28 Feb '21

Hello, I'm attempting to use GTK+3 to create a Desktop, but I am having a problem installing all of the dependencies. I keep getting the following error: GUI cabal: version >=0.9.0 of pkg-config is required I am using Cygwin on a Windows 10 operating system. What is needed to fix this? Is this the best way to do a desktop GUI in Haskell? Thanks in advance and thank you for your time.

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[ANN] HGeometry-0.12
by Lemmih 28 Feb '21

28 Feb '21

It is our great pleasure to announce version 0.12 of HGeometry! HGeometry is a swiss army knife for computational geometry. Wondering if a point is inside a polygon? HGeometry will tell you. Looking for the shortest path inside a maze? HGeometry will light your way. Planning a museum heist? HGeometry will show you the camera blindspots in O(n log n) time! Check out our website for more: https://hgeometry.org/ HGeometry is open to contributors at all levels of Haskell experience and we have a list of weekend-sized projects. Ask in the Discord server for guidance and/or pair programming. New in 0.12: - Algorithms: - Visibility polygon in O(n log n). - Earclip triangulation in O(n^2) worst case, O(n) expected case. - Single-source shortest path in O(n). - Planar point locator in O(log n). - Point set diameter in O(n log n). - Convex hull of a polygon in O(n). - Diameter of a convex polygon in O(n). - Check if a point lies inside a convex polygon in O(n). - Bug fixes and improved numerical robustness. - Property testing and several methods for generating random polygons. - See changelog.org for an exhaustive list of changes. Future developments: - Bring Haddock documentation coverage up to 100%. Currently, we're in the low eighties. - A holistic approach to demonstrating correctness: - Proven algorithms. - Unit testing for hand-picked corner cases. - Property testing with randomly generated polygons / geometric data. - Exhaustive property testing with real-world datasets (osm). - Benchmarks to show the relative performance of HGeometry's many algorithms. Contributors: - Frank Staals, @noinia - David Himmelstrup, @Lemmih - Hunter DeMeyer, @1ndy Website: https://hgeometry.org GitHub: https://github.com/nionia/hgeometry Hackage: https://hackage.haskell.org/package/hgeometry Discord: https://discord.gg/HQwbD9jWqg

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What is your favourite graph library?
by Ignat Insarov 28 Feb '21

28 Feb '21

Hello! I really like graphs. People seem to like thinking in terms of dots and arrows, and graph theory provides a formalization for this. (Homogeneous binary) relations, finite state automata, networks and categories are sorts of graphs too. But unfortunately it is not possible to represent a graph as an algebraic type. For example, a list is a fixed point of a maybe tuple, and every usual linear container _(an array, a vector, a sequence…)_ is isomorphous to a list, so we have an easy time prototyping and specifying the behaviour of any such thing. With graphs, we have a hard time! The most popular library for graphs in Haskell is `fgl`. Unfortunately, it is really easy to get weird behaviour with it, because folds tear away the limbs from contexts so that your observation of said contexts depends on the order of folding. I have been poking at it, trying to make graphs resemble a comonad… but of course you can have an empty graph, and a comonad cannot be empty. There is also `alga` — it seemed promising, but now it is deprecated for some reason… The usual `containers` also provide something they call a graph, but it does not offer much in terms of instances and algorithms — a graph is merely a type alias to an array. So, is there any cool library that I missed? Or some promising research? Do you know of any packages that use graphs heavily and in interesting ways? Are you curious to see progress in this area?

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