- Haskell-Cafe - Haskell.org

hspec - type issue in error assertion
by Paweł Bałaga 08 Jul '18

08 Jul '18

Hello fellow Haskellers! I'm approaching learning the basics of Haskell by going through https://wiki.haskell.org/99_questions. At the same time I write tests for my code in Hspec. Consider question no. 2: "Find the last but one element of a list". My solution: -- Problem 02 myButLast :: [a] -> a myButLast [] = error "Empty list" myButLast [x] = error "List has only one element" myButLast [x1,x2] = x1 myButLast (x:xs) = myButLast xs and a a test: describe "02 myButLast" $ do it "returns the last but one element of a list" $ do myButLast [] `shouldThrow` anyErrorCall myButLast [1] `shouldThrow` anyErrorCall -- <- this line causes the problem myButLast [1..4] `shouldBe` 3 myButLast ['x','y','z'] `shouldBe` 'y' myButLast "abc" `shouldBe` 'b' Building tests with stack test command causes the below compilation error: • No instance for (Num (IO a0)) arising from the literal ‘1’ > • In the expression: 1 > In the first argument of ‘myButLast’, namely ‘[1]’ > In the first argument of ‘shouldThrow’, namely ‘myButLast [1]’ > | > 27 | myButLast [1] `shouldThrow` anyErrorCall > | > >From what I understand, type of myButLast [1] is different than expected by shouldThrow. What I don't understand is why exactly it behaves so and how to fix this problem. Only that one assertion doesn't compile. The others are fine. Particularly, why does myButLast [] `shouldThrow` anyErrorCall work but with one element it doesn't? Can you please give me a hand?

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functions making their own memos
by Dennis Raddle 08 Jul '18

08 Jul '18

I've been wondering for some time how to handle a common use case in imperative programming, "static" variables that are accessed only inside one function, but not needed elsewhere. I think this probably has something to do with existential quantification, but in any case this is an area I'm ready to learn a lot more about. Any ideas are welcome along with suggested reading. Let's say that I have a function for generating random numbers every time it is called, but I sometimes want it to to re-use the last number depending on the time the last number was geneated: sometimesNew :: Time -> Int -> IO (Time,Int) sometimesNew lastTime lastValue = do currentTime <- <get time for system> if currentTime > lastTime + 10 then do newValue <- <get pseudorandom value> return (currentTime,newValue) else return (lastTime,lastValue) This function depends on the caller to remember its output (last time and last value) and feed that back to it. It would be nice if this function could make some kind of memo inside it itself and no one else needs to know. The case I'm working on right now is doing animation in Purescript on a canvas, in which I can control the motion of an object appearing in the animation by composing functions that control the motion. I might like to write a function that generates an overall circular trajectory, then compose it with a function that generates a slight wobbly motion. The wobbly function needs, basically, to change the motion randomly by interpolating between random positions and changing the random position only occassionally instead of during every animation frame. type MovementFunc = ... So this would generate a large circular movement: circularMovement :: MovementFunc This generates a small wobble. wobble :: MovementFunc Then I can compute the final position once per animation frame by superimposing or composing individual movements: computePosition :: [MovementFunc] -> Time -> IO Position The wobble function might like to have access to a memo of the last randomly chosen position, and how long ago in milliseconds it chose that position. Let's say that every movement function might like to have access to a memo, but the actual data type involved could be different from function to function. I also need to deal with initializing the memo. So the trick is to maintain and initialize these memo types without needing to know the internals of every function. Dennis

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Fwd: [livecode] haskell-art is back
by Alfred Matthews 08 Jul '18

08 Jul '18

Here. ---------- Forwarded message --------- From: Alex McLean <alex(a)slab.org> Date: Sun, Jul 8, 2018, 5:16 PM Subject: [livecode] haskell-art is back To: livecode <livecode(a)we.lurk.org> I finally found some time to recreate the haskell-art list here: https://we.lurk.org/postorius/lists/haskell-art.we.lurk.org/ So vivid and other in-depth haskell discussion can move back there. Cheers ! -- blog: http://slab.org/ _______________________________________________ Livecode mailing list -- livecode(a)we.lurk.org To unsubscribe send an email to livecode-leave(a)we.lurk.org -- Alfred.

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Call for paper: Only one *week* left to prepare your paper for the Workshop in OCL and Textual Modeling (OCL 2018)
by Achim D. Brucker 08 Jul '18

08 Jul '18

(Apologies for duplicates) ********************************************************** ** Only one week left until the submission deadline! *** ********************************************************** CALL FOR PAPERS 18th International Workshop on OCL and Textual Modeling Co-located with MODELS 2018: ACM/IEEE 21th International Conference on Model Driven Engineering Languages and System, October 14, 2018, Copenhagen, Denmark http://oclworkshop.github.io Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but they have weaknesses: for example, detailed visual representations bear the risk of becoming overcrowded faster than textual models and some of the visual features lack the level of precision required to create complete and unambiguous specifications. These weaknesses of graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is to create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. ## Topics of interest Topics of interest include (but are not limited to): - Mappings between textual modeling languages and other languages/formalisms - Mathematical models and/or formal semantics for textual modeling languages - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for: - validation, verification, and testing, - model transformation and code generation, - meta-modeling and DSLs, and - query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - Model-driven security using textual modeling languages - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports: - usage of textual modeling languages and tools in complex domains, - usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks We particularly encourage submissions describing applications and case studies of textual modeling as well as test suites and benchmark collections for evaluating textual modeling tools. ## Venue This workshop will be organized as a part of MODELS 2018 Conference in Copenhagen, Denmark. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. ## Workshop Format The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. ## Submissions Two types of papers will be considered: * Short contributions (between 5 and 7 pages) describing new ideas, innovative tools or position papers. * Full papers (between 10 and 14 pages). in LNCS format. Submissions should be uploaded to [EasyChair](https://easychair.org/conferences/?conf=ocl2018). The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online, as part of the MODELS workshop proceedings, in [CEUR](http://www.ceur-ws.org). ## Important Dates - Submission of papers: July 17, 2018 - Notification: August 17, 2018 - Workshop date: October 14, 2018 -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

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Re: [Haskell-cafe] Bool is not...safe?!
by Olaf Klinke 08 Jul '18

08 Jul '18

Indeed, why use Bool when you can have your own algebraic datatype? Why not data Equality = Equal | NotEqual (==) :: Eq a => a -> a -> Equality when we have essentially the same for 'compare' already? Why not use data Result = Success | Failure as the return type of effectful code? Equality and Result are Booleans with some provenance. But wait! Equality does not come with all the nice functions like (||) and (&&), and Failure still does not tell us what exactly went wrong. Maybe a derivable type class IsomorphicToBool could remedy the former problem. For the latter, Haskell has exception monads. One of the things I love about Haskell is that I do not have to use Int as a return type and remember that negative numbers mean failure of some sort, or worse, the return type is Void but some global variable may now contain the error code of the last function call. In some post in haskell-cafe which I am unable to find right now it was mentioned that the GHC source contains many types that are isomorphic but not equal to Bool. Does anyone know whether the C and C++ folks are now using more informative structs as return types? Cheers, Olaf

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Re: [Haskell-cafe] Bool is not...safe?!
by Olaf Klinke 07 Jul '18

07 Jul '18

>But in Prolog every piece of code is evaluated to TRUTH or to FALSE. Each sentence is a goal which can be achieved and becomes a fact >or can not be – and becomes lie. So, if you inverse semantic in Haskell – program is compiled although with strange behaviour but in >Prolog program become lie. Prolog evaluation model has clean definition in run-time too, not only at compile-time. Execution of >Prolog program is a CHECK: is it truth or lie? I have only very limited Prolog pratice. But what you state about Prolog's semantics is only true for very simple cases, isn't it? As soon as I/O is involved, things get messier. Also, there are actually three outcomes as far as I know: provably true, provably false and "could not be proved" which is different from "false". When I anticipate an exception in an application where the condition prohibits the program from fulfilling its purpose, I simply call error. A common pattern I use is a library function that has return type (Either SomeErr Value), and the application takes it and wraps the result in either (error.show) return, transforming (Either SomeErr Value) into (IO Value). For libraries, one should be more careful, as we don't know what kind of environment the function might be called in - see Joachim Durchholz's post. Olaf

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Re: [Haskell-cafe] Bool is not...safe?!
by Olaf Klinke 07 Jul '18

07 Jul '18

> >> true = return () >> (&&) = liftM2 const >> (||) = mplus >> false = mzero > > I don't think it's by definition yet, but surely by convention, that > this is a restricted form of > > true = pure () > (&&) = (<*) > (||) = (<|>) > false = empty > > All of these functions require only an Applicative or an Alternative, > and except for "true", they're all in the library. True, and after sending I realized I should have used exaclty the definitions you gave. It's just that I don't know whether there exists the concept of "commutative applicative". It is equired for (&&) to be commutative. How could one even state it without the monad bind? The following definitions are from a category theory paper [1] by Anders Kock. import Control.Monad -- tensorial strengths t'' :: (Functor m) => a -> m b -> m (a,b) t'' a mb = fmap (\b -> (a,b)) mb t' :: (Functor m) => m a -> b -> m (a,b) t' ma b = fmap (\a -> (a,b)) ma -- so-called linear extensions in the first and second argument linearext1 :: Functor m => (m c -> c) -> (a -> b -> c) -> m a -> b -> c linearext1 struct f ma b = struct $ (fmap $ uncurry f) $ t' ma b linearext2 :: Functor m => (m c -> c) -> (a -> b -> c) -> a -> m b -> c linearext2 struct f a mb = struct $ (fmap $ uncurry f) $ t'' a mb The first argument to linearext1 and linearext2 is called a "structure map" in the context of [1]. Such structure maps exist when m is a monad. Indeed, we have linearext1 join t'' :: (Monad m) => m a -> m b -> m (a, b) linearext2 join t' :: (Monad m) => m a -> m b -> m (a, b) Now m is called commutative if the above two maps are the same [*]. But join does not exist for applicative functors, it is interdefinable with (>>=). Olaf [1] Anders Kock: "Commutative Monads as a Theory of Distributions". Theory and Applications of Categories, Vol. 26 No. 4 (2012). [*] They are called Fubini maps because a special case is Fubini's theorem about integration by parts.

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ANN: network-2.7.0.2
by evan＠evan-borden.com 07 Jul '18

07 Jul '18

Announcing network-2.7.0.2 * Removing withMVar to avoid the deadlock between "accept" and "close" [#330](https://github.com/haskell/network/pull/330) * "close" does not throw exceptions. A new API: "close'" throws exceptions when necessary. [#337](https://github.com/haskell/network/pull/337) * Fixing the hang of lazy sendAll. [#340](https://github.com/haskell/network/pull/340) * Installing NetDef.h (#334) [#334](https://github.com/haskell/network/pull/334) Thank you to network contributors for this bug fix release. -- Evan Borden

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ANN: network-2.6.3.6
by evan＠evan-borden.com 07 Jul '18

07 Jul '18

Announcing network-2.6.3.6 This is a bug fix release for the 2.6 branch of network. * Removed unnecessary withMVar, which caused locking on close. [#330](https://github.com/haskell/network/pull/330) Thank you to network contributors for finding and diagnosing the bug. -- Evan Borden

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[GSoC 2018] Benchmarking Haskell graph libraries
by Alexandre Moine 06 Jul '18

06 Jul '18

[ Results are here: https://github.com/haskell-perf/graphs/tree/master/results/ ] Hi, I am french 2nd year student in maths and computer sciences and I am running a {Google,Haskell} Summer of Code now. One of my goals is to write a benchmarking suite for graphs libraries in Haskell (Fgl, Alga, Containers and Hash-Graph) and I reached a state where I can produce reasonable numbers. If you are interested, I have wrote a little blog article explaining the results I obtained: https://blog.nyarlathotep.one/2018/07/benchmarking-haskell-graph-libraries/ Feel free to comment or to send feedback on the Github :) -- Alexandre Moine

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