- Haskell-Cafe - Haskell.org

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 22 Jul '18

22 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 20 Jul '18

20 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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What is your favourite Haskell "aha" moment?
by Simon Peyton Jones 20 Jul '18

20 Jul '18

Friends In a few weeks I'm giving a talk to a bunch of genomics folk at the Sanger Institute<https://www.sanger.ac.uk/> about Haskell. They do lots of programming, but they aren't computer scientists. I can tell them plenty about Haskell, but I'm ill-equipped to answer the main question in their minds: why should I even care about Haskell? I'm too much of a biased witness. So I thought I'd ask you for help. War stories perhaps - how using Haskell worked (or didn't) for you. But rather than talk generalities, I'd love to illustrate with copious examples of beautiful code. * Can you identify a few lines of Haskell that best characterise what you think makes Haskell distinctively worth caring about? Something that gave you an "aha" moment, or that feeling of joy when you truly make sense of something for the first time. The challenge is, of course, that this audience will know no Haskell, so muttering about Cartesian Closed Categories isn't going to do it for them. I need examples that I can present in 5 minutes, without needing a long setup. To take a very basic example, consider Quicksort using list comprehensions, compared with its equivalent in C. It's so short, so obviously right, whereas doing the right thing with in-place update in C notoriously prone to fencepost errors etc. But it also makes much less good use of memory, and is likely to run slower. I think I can do that in 5 minutes. Another thing that I think comes over easily is the ability to abstract: generalising sum and product to fold by abstracting out a functional argument; generalising at the type level by polymorphism, including polymorphism over higher-kinded type constructors. Maybe 8 minutes. But you will have more and better ideas, and (crucially) ideas that are more credibly grounded in the day to day reality of writing programs that get work done. Pointers to your favourite blog posts would be another avenue. (I love the Haskell Weekly News.) Finally, I know that some of you use Haskell specifically for genomics work, and maybe some of your insights would be particularly relevant for the Sanger audience. Thank you! Perhaps your responses on this thread (if any) may be helpful to more than just me. Simon

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Monads because of I/O?
by Olaf Klinke 19 Jul '18

19 Jul '18

Paul, monads were not invented because I/O could not be presented in another way in Haskell. Monads are way older than Haskell. It is a concept of category theory which was developed in the 1950s. Actually some concepts of algebra that are even older turn out to be monads. Take Galois theory for example. Once you know the pattern, you find a monad under every stone you turn around. It's one of the luckiest things that people like Moggi and Wadler realized that monads can be applied to structure programs. And don't blame them that monads are not composable - it is simply a mathematical fact. Some monads do compose with any other monad, and those are the monad transformers. If you like Prolog's relational programming model so much, then you should play with those programs that have "no business value". Because Haskell's type inference algorithm, together with multi-parameter type classes and maybe type level natural numbers together give rise to Prolog-like capabilities. All the work is done by the compiler this way. What you say about FSM is certainly true to some extent - they are well understood, can be generated automatically, and there is decent theory to reason about them. That is why this model is used in safety-sensitive environments such as aviation. I once applied for a position in verification in the automotive industry, and the interview partner told me that they struggle mightily with the vast state spaces of the FSMs they are checking. All this speaks in favour of Haskell. The semantics is simple and beautiful, because it is a single-paradigm language. And because of that, clever people can leverage theorem provers to mathematically prove correctness of Haskell code. I don't know of many languages where that is possible. (But then, I'm not an expert on verification.) Olaf

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Defining operators via sections
by Tom Ellis 19 Jul '18

19 Jul '18

I can define operators directly or through their parenthesised name: > let x .+ y = x + y > let (.+) x y = (+) x y > let (.+) x = (+) x > let (.+) = (.+) but I can't define them via sections: > let (x .+) = (x +) <interactive>:11:6: Parse error in pattern: x .+ > let (.+ y) = (+ y) <interactive>:12:6: Parse error in pattern: .+ y It seems natural to write, for example instance Monad Maybe where ... (>>= f) = \case Nothing -> Nothing Just a -> f a Is there any particular reason this is not possible? Thanks, Tom

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cannot upgrade stack
by Roger Mason 19 Jul '18

19 Jul '18

Hello, I'm attempting to upgrade stack on a FreeBSD 11.1 system but it fails: stack upgrade Current Stack version: 1.5.1, available download version: 1.7.1 Newer version detected, downloading Querying for archive location for platform: freebsd-i386-static Querying for archive location for platform: freebsd-i386 Exception occured when trying to perform binary upgrade: Unable to find binary Stack archive for platforms: freebsd-i386-static freebsd-i386 Falling back to source upgrade Selected mirror https://s3.amazonaws.com/hackage.fpcomplete.com/ Downloading timestamp No updates to your package list were found Updating package index Hackage (mirrored at https://s3.amazonaws.com/hackage.fpcomplete.com/) ...Could not parse '/tmp/stack-upgrade2294/stack-1.7.1/stack.yaml': Aeson exception: Error in $['extra-deps'][0]: failed to parse field 'extra-deps': (Invalid package identifier: "rio-0.1.1.0@rev:0","rio-0.1.1.0@rev:0") See http://docs.haskellstack.org/en/stable/yaml_configuration/ If it matters, I'm using ghc-8.0.2_3. Thanks for any help, Roger

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Backpack vs plain old classes and instances
by Clinton Mead 19 Jul '18

19 Jul '18

Hi All I read though Edward Z. Yang's blog post <http://blog.ezyang.com/2016/10/try-backpack-ghc-backpack/> about Backpack but I can't understand what it does above and beyond classes and instances. It seems to me we could just replace a "signature" with a "class" and a "unit" with an "instance" and achieve the same thing. Of course there are issues with orphans instances if we don't own either the class or the associated data, but it would seem to me that "orphan units" are no less problematic than "orphan instances". Edward Kmett's unpacked-containers <https://www.reddit.com/r/haskell/comments/8a5w1n/new_package_unpackedcontai…> gets some speed improvements using Backpack, but is Backpack just optimisation thing, or are there ways you can organise code with Backpack that you couldn't do without just classes and instances? For example, does Backpack solve the issue of packages requiring huge dependency lists to implement instances that most of their users will not use? Thanks, Clinton

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SqlPersistT action in MonadError issue
by Vlatko Basic 18 Jul '18

18 Jul '18

Hello, I'm trying to use SqlPersistT funcs with MonadError, but am failing in writing the runDB correctly. I tried several things, but always getting: • Couldn't match type ‘IOException’ with ‘AppException’ arising from a functional dependency between: constraint ‘MonadError AppException IO’ arising from a use of ‘f1’ instance ‘MonadError IOException IO’ at <no location info> • In the second argument of ‘runPool’, namely ‘f1’ In the second argument of ‘($)’, namely ‘runPool pgConf f1 pgPool’ In a stmt of a 'do' block: liftIO $ runPool pgConf f1 pgPool I understand there already is "instance MonadError IOException IO" and fundep says it can be only one for IO. How to make it compile? Best regards, vlatko Here is the minimal reproducible code: module Test where import Prelude import Control.Exception.Base import Control.Monad.Except import Control.Monad.Trans.Reader import Database.Persist.Postgresql data AppException = ExcText String | ExcIO IOException deriving (Show) type AppM m = ExceptT AppException (ReaderT App m) data App = App { pgConf :: PostgresConf, pgPool :: ConnectionPool} runDB :: (MonadIO m, MonadError AppException m) => AppM m Bool runDB = do App{..} <- lift ask liftIO $ runPool pgConf *f1* pgPool -- error -- liftIO $ runPool pgConf *f2* pgPool -- OK f1 :: (MonadIO m, MonadError AppException m) => SqlPersistT m Bool f1 = throwError $ ExcText "exception" f2 :: MonadIO m => SqlPersistT m Bool f2 = return True

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