- Haskell-Cafe - Haskell.org

Free theorem for `forall z. (A,z) -> (B,z)`?
by Conal Elliott 25 Jul '18

25 Jul '18

Suppose `g :: forall z. (A,z) -> (B,z)`. Is it necessarily the case that `g = first f` for some `f :: A -> B` (where `first f (a,b) = (f a, b)`), perhaps as a free theorem for the type of `g`? Note that `(A,)` and `(B,)` are functors and that `g` is a natural transformation, so `g . fmap h == fmap h . g` for any `h :: u -> v`. Equivalently, `g . second h == second h . g` (where `second h (a,b) = (a, h b)`). Thanks, -- Conal

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Design of an extensible system
by Marc Busqué 25 Jul '18

25 Jul '18

Hi there, I'm currently programming my first non toy project in Haskell. I come from Ruby background, where everything is possible no matter how much entropy the universe gains after you finish your work, so I'm still not sure how to do certain things without this side-effect :) I'll try to be concise. My idea is that the project would consist of several different Haskell packages. The central one, call it Core, would define a certain data type (with info about certain URLs). Other packages would define each one an strategy to do something with that data type (processing the URL info). Each strategy would have its own dependencies, and as a user probably won't need all of the strategies, I would like to keep them in separated packages so she or he has no need to download the unneeded stuff. The problem is that Core should act as the central dashboard, whether through a CLI or even a web frontend. For example, in the CLI case, I would like to be able to install Core package along with package StrategyA and package StrategyB and then do something like the following: ``` bash> core --action=fetch --strategy=A bash> core --action=fetch --strategy=B ``` Two generic ideas come to my mind but I don't know how to translate them to Haskell: - Infer module name from the `--strategy` argument and call it dynamically. However, as far as I know, in Haskell modules are not first class citizens, so no way I can dynamically call them. I have read something to achieve this through Template Haskell or through outdaded [plugins](http://hackage.haskell.org/package/plugins) package, but I'm not sure whether the need is the same and anyway it seems quite unnatural to Haskell (I would not like to go against it, but to learn to do thing in its way). - Define from Core some kind of global list where strategy packages can register a map between CLI argument value and a defined instance of the core data type. But, again, how to do global things in Haskell (and I would prefer not to need external tools like RDBMS, filesystem or whatever)? And, anyway, surely I don't want to do this because concurrency and everything... Any idea? Thanks in advance! Marc Busqué http://waiting-for-dev.github.io/about/

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Expressing disjunctions of constraints
by Olaf Klinke 24 Jul '18

24 Jul '18

Dear cafe, today I wrote a type class with three parameters. class Foo a b c | a b -> c where foo :: a -> b -> c instance Foo A R A where ... instance Foo R A A where ... In addition to the functional dependency I'd like to express that at least one of the parameters a and b is c, that is, Foo is the union of the two classes class Foo1 a b where foo1 :: a -> b -> a class Foo2 a b where foo2 :: a -> b -> b and furthermore the choice between Foo1 and Foo2 determines one parameter to be a fixed type, say R. I understand that logical disjunction poses problems for instance resolution [1] because adding an instance may affect the instance dictionary lookup. This should not happen in my case because in the end this would be a one-parameter type class. The ultimate aim is to use the same symbol for both a function foo :: R -> a -> a and for flip foo. Of course writing flip foo is not that much of a nuisance, I just wonder whether Haskell is expressive enough to do this. Olaf [1] https://stackoverflow.com/questions/10255148/how-can-i-combine-two-type-con…

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animation application
by Dennis Raddle 24 Jul '18

24 Jul '18

I'm working on an application for animating words, images, and shapes. I'm actually doing it in Purescript and it will become part of a math game that runs in the browser, but I'm asking my question on the Haskell list because the issues seem central to a functional/pure paradigm. Let's say that the things being animated are "objects." Objects could include photos, text, shapes, lines, etc. They can have fixed positions, colors, and other attributes, or their attributes can change over time. How do I compute attributes that change over time? In some cases, a function with a single argument, the current time, would suffice. I might want state, though -- perhaps an object's movement is pseudorandom and needs to refer to a seed and past position state. Objects might even need to observe each other's state. For instance, imagine we have two words, W1 and W2, and they move according to some stateful algorithm. Meanwhile we want to draw a line connecting their centers, so we need a line object L that can read the centers of W1 and W2. How do I represent the state of W1, W2, and L? I could define a general Object data type: (here there are no fields mentioned) data Object = W1 | W2 | L I will need state and the ability to customize the drawing and positioning functions, so to flesh out the fields of, say , W1. type St = StateT WholeScene IO data Object = W1 { leftEdge :: Int , rightEdge :: Int , topEdge :: Int , bottomEdge :: Int , computePosition :: W1 -> Time -> St W1 , draw :: W1 -> St () , ... other state ... } Okay, so now I could define W2, which will be different in some ways, perhaps needing different state. But it will look a lot alike. It might have the same edge fields. In fact, all my objects will have similar position, color, etc. attributes. I certainly don't want to reinvent the wheel for everything. I also want objects to be able to read each other's state. Consider a basic positioning task: some object Obj1 keeps its left edge 100 pixels to the right of some other object's right edge. So the Obj1 function for computing position needs access to the other objects. Also consider that all these objects need to be put into a list together, to hold the state of the entire scene. They need to be members of the same class, and have a way to look each other up. One way to do this is have a single Object type with numerous constructors, and define similar functions on every constructor type (say, a function for accessing or setting the left edge). I also thought about having an Object be an existential type, and the fields that are custom to one object will be the existentially defined data. I actually implemented this in Purescript, but it got out of hand, probably due to my own weakness of understanding how existential types work. Any ideas? D

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Why does this result in a runtime loop instead of a type error or typechecker loop?
by Theodore Lief Gannon 24 Jul '18

24 Jul '18

λ. :set -XFlexibleContexts -XFlexibleInstances -XMultiParamTypeClasses -XUndecidableInstances λ. import RIO λ. import Data.Has λ. class HasBool a where boolL :: Lens' a Bool λ. instance {-#OVERLAPPABLE#-} HasBool a => Has Bool a where hasLens = boolL λ. instance {-#OVERLAPPABLE#-} Has Bool a => HasBool a where boolL = hasLens λ. runRIO () $ view boolL ^CInterrupted. The RIO environment () doesn't contain a Bool. I can see how the typechecker might get lost in this, but I can compile an equivalent program; it loops on execution.

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Munich Haskell Meeting, 2018-07-25 @ 19:30
by Leah Neukirchen 23 Jul '18

23 Jul '18

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, July 25 at Max Emanuel Brauerei 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-jul-2018/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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Job: Haskell developer at GetShop.TV (interactive television platform)
by Nickolay Kudasov 23 Jul '18

23 Jul '18

Hello, Cafe! We are seeking a Haskell developer to join our team and help us build an interactive television platform at scale in Russia! Candidates of all ages, nationalities, genders, etc. are welcome :) Full-time position in Moscow, Russia is preferable, but remote/part-time is possible! Please, feel free to send your CV directly to me via e-mail or via a form at http://getshop.tv/ourteam/sr-haskell-developer/ GetShop.TV is an interactive television platform that connects viewers with advertisers and content providers on both live TV and video-on-demand services. Right now the company is expanding its platform, connecting more TV channels and operators in Russia. At the same time we are working on automation and scalability. Our backend in 100% Haskell (Servant) and some of our internal services rely on GHCJS (Miso). We are seeking a senior Haskell developer to help us build a stable, automated, typesafe service at scale. Some of the challenges and tasks include - dealing and optimising for an uneven, spiky, but predictable load - integration with multiple imperfect services and platforms - automating processes that aren't automated yet - improving our automatic reporting system - performing regular code reviews - tutoring several junior developers Some buzzwords that we use are Git Flow, GitHub, Servant (with servant-swagger), Miso, acid-state, PostgreSQL, hasql, postgresql-query, hspec, HUnit, QuickCheck, critetion, Circle CI, Ansible, Docker, Telegram (via telegram-bot-simple). We expect candidates to be familiar with most of those, as well as having an industry experience in functional programming. We would be thrilled if a candidate were also interested in and/or capable of - DevOps with Ansible/Docker/Nix - technical writing (Russian or English) At the company we encourage learning and knowledge sharing, which include internal tech talks. We also support open source contributions and participations in meetups and conferences. We'll gladly accept submissions from candidates of all ages, nationalities, genders, etc. Our focus will be on the candidates located in Moscow, Russia or willing to relocate. However, remote/part time is possible for strong candidates or after a probationary period. Our office in located right in the center of Moscow, a walking distance from Red Square, Arbat, Hermitage Garden, Moscow Zoo. It's also less than a minute from Pushkinskaya, Tverskaya and Chekhovskaya stations. The office is equipped with a kitchen, microwave, coffee machine, offers tea and fresh fruit. Kind regards, Nickolay Kudasov CTO GetShop.TV

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GHC, cabal and nightlies via Chocolatey
by Phyx 23 Jul '18

23 Jul '18

Hi All, For those that use a cabal based workflow on Windows or are interested in doing Windows testing for their packages using AppVeyor I've written up some instructions on how to do this using Chocolatey https://hub.zhox.com/posts/chocolatey-introduction/. The standard packages are pulled directly from Haskell.org As a trial I'm also making my own Windows Nightlies for GHC and Cabal available via my NuGet package stream. Details available on the post. Thanks, Tamar

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Re: [Haskell-cafe] What is your favourite Haskell "aha" moment?
by Johannes Waldmann 22 Jul '18

22 Jul '18

> - Large-scale refactoring with types: > this is a huge selling point for Haskell in general I fully agree with the general idea, but the specific example > data Shape > = Circle Double > | Square Double > | Rectangle Double Double -- ^ added is not convincing (well, to programmers that know about static typing) Think of a Java program interface Shape { .. } class Circle implement Shape { ... } When you add class Rectangle { } , then Shape foo = new Rectangle () would be an error, until you put class Rectangle implements Shape { } then the compiler tells you what methods are missing. I think the extra value of types in Haskell (for everything, including refactoring) is that they tend to express more of the program's properties (w.r.t. statically typed imperative programs). Examples: the distinction between a and IO a, between IO a and STM a. In Data.Set, function "fromList" has an Ord constraint, but "toList" does not. Does "singleton" need the constraint? I used this as an exam question just last week. - J.W.

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fyi 10 benchmarks game programs broken
by Isaac Gouy 21 Jul '18

21 Jul '18

https://benchmarksgame-team.pages.debian.net/benchmarksgame/measurements/gh… Several of those programs fail to compile: Variable not in scope: rdeepseq For example: https://benchmarksgame-team.pages.debian.net/benchmarksgame/program/fannkuc… best wishes, Isaac

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