August 2015 - Haskell-Cafe

More concise code using phantom types
by Nikita Kartashov 07 Aug '15

07 Aug '15

Hello! Consider the following code: module Units where data Units a = U Double deriving Eq units :: Double -> a -> Units a units value _ = U value data Meters data Yards meters = undefined :: Meters yards = undefined :: Yards instance Show Meters where show _ = "meters" instance Show Yards where show _ = "yards" extractA :: Units a -> a extractA = undefined instance Show a => Show (Units a) where show u@(U value) = show value ++ " " ++ show $ extractA u main = (print $ units 5 yards) >> (print $ units 5 meters) Is it possible to use something instead extractA function here? For example, substitute "extractA u” with “undefined :: a”? GHC disallows it, so is there a way to explain that I only need a token with type a? Also, with highlighting on lpaste: http://lpaste.net/138219. With regards, Nikita Kartashov

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ANN: happstack-server-tls-cryptonite
by Andrey Sverdlichenko 07 Aug '15

07 Aug '15

Hi, I'd like to announce happstack-server-tls-cryptonite, package adding native TLS support to happstack, utilizing hs-tls and cryptonite packages by Vincent Hanquez. You can now run secure web server without using OpenSSL or other foreign libraries. This package is almost drop-in replacement for happstack-server-tls, just change Happstack.Server.SimpleHTTPS to Happstack.Server.SimpleTLS in imports. Regards, Andrey

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Fwd: Haskell Weekly News
by Kim-Ee Yeoh 07 Aug '15

07 Aug '15

*Editorial:*Over a month has elapsed since the last issue. Is the Weekly News turning into the Monthly News? No. I've been working on getting full-length articles into the News. Imagine articles that investigate, analyze, and give the low-down on hoary issues like cabal hell that cannot be telegrammed in a two-sentence paragraph. But my time isn't quantum mechanical. When I do A, I can't at the same time do B. I know that there are fine haskellers who'd gladly write for pay. However, HWN has never had a foundation in place for paid articles. Building that foundation takes time. Now that much of it is in place, let me duly announce that HWN hereby solicits reviews, reports, criticism pieces, and personal essays. - Flexible word count: anywhere between 250 and 750 is fine. - Send me an outline first. - Published articles pay at 68 bitcents, under 200 euros / dollars right now. - You can count on extensive editing for the requisite polish and also to conform to 'house' style. Interested? Email me with a subject line that starts with "[HWN pitch]" followed by the title of your piece. In the body, outline the main points you'll hit and explain how they engage the HWN readership. So on with the News : *Top picks:* - Galois organizes this year's ICFP contest. The 72-hour countdown has just started! After tantalizing us with whisperings of more computational archaelogy (see 2006 ICFP contest <http://www.boundvariable.org/>) centering on honey bees, national food security, and the Hebrew alphabet; Portland, Oregon reveals a classic offline AI game programming challenge <http://icfpcontest.org/spec.html>. According to redditor skatenerd <https://www.reddit.com/r/haskell/comments/3g1n8f/this_years_icfp_contest_st…>, "it's a lot like Tetris, except you know what pieces are coming." - Rick Dzekman <http://rickdzekman.com/thoughts/the-haskell-user-experience/> offers constructive criticism based on his "poor UX (User Experience) of a great language (Haskell)". Among the low-lying fruit he identifies: a web search doesn't always link to the latest version on the Hackage website, something easily fixed. Not so low-lying are Cabal, a modern IDE, and Haskell wiki documentation. In sympathy with the sentiments are HN readers <https://news.ycombinator.com/item?id=10008169> and /r/haskell <https://www.reddit.com/r/haskell/comments/3ftsh0/the_haskell_user_experienc…> . - Christoffer Stjernlöf brings attention to how only in imperative land does one encounter the awfulness of the same boolean expression getting tested twice in a nested if-inside-if. He uses it to make "the case for controlled side effects", the title of his blog post. Discussions on /r/haskell <http://www.reddit.com/r/haskell/comments/3fzmt6/the_case_for_controlled_sid…> and Hacker News <https://news.ycombinator.com/item?id=10017613>. - Alexey Shmalko introduces Haskell IO as "the command pattern" <http://www.alexeyshmalko.com/2015/io-is-your-command-pattern/> to object-oriented programmers. His article resists mentioning monads or do-notation. Appreciated on /r/haskell <http://www.reddit.com/r/haskell/comments/3fzmt6/the_case_for_controlled_sid…> . - The latest issue of the Functional Works monthly newsletter <http://us10.campaign-archive2.com/?u=30f19e2c5b9f15cfb95906716&id=cee1ab0dc6> reminds us about Chakravarty and Keller's "An Introduction to Computing (with Haskell)", a textbook written from multiple years teaching first-year CS. At 150 pages, the book dodges monads -- nary a single mention -- and zooms into I/O actions. It's out-of-print but readily downloadable <https://www.cse.unsw.edu.au/~chak/papers/#sec:books>. - The program for this year's Haskell Symposium <https://www.haskell.org/haskell-symposium/2015/> is out. PC chaired by Ben Lippmeier, it's truly a smorgasbord. If you can't find a preprint, indefatigable redditors <http://www.reddit.com/r/haskell/comments/3dxxj3/haskell_symposium_2015_acce…> might just have got you covered. - Zhenjiang Hu, John Hughes, and Meng Wang publish "How Functional Programming Mattered" <http://nsr.oxfordjournals.org/content/early/2015/07/13/nsr.nwv042>, a survey of FP's impact outside its academic domain in recent years. It is written in the mold of and complements A History of Haskell: Being Lazy with Class (2007) <http://research.microsoft.com/en-us/um/people/simonpj/papers/history-of-has…>. Comprising 17.5 pages of 2-column ACM-proceedings-style 9-point text, it houses the bibliographical treasure of 122 references, itself a contribution to any debate over priority. /r/haskell <https://www.reddit.com/r/haskell/comments/3f5yew/how_functional_programming…> - Victor Maia writes a lambda calculus evaluator in javascript <https://github.com/SrVictorMaia/optlam> that computes 200^200 mod 31 without breaking a sweat. No mean feat, since all numbers are church numerals. How did he pull it off? His javascript implements a fragment of Lamping's optimal algorithm, which is based on interaction nets. (Optimality here is in the sense of Levy-optimality: keeping the number of beta reductions down to an absolute minimum. By itself, this count is generally an inaccurate measure of efficiency.) He wonders on stackoverflow <http://stackoverflow.com/questions/31707614/why-are-%CE%BB-calculus-optimal…> why optimal evaluation has such magical superpowers. The convo at /r/haskell <http://www.reddit.com/r/haskell/comments/3f21wv/why_are_%CE%BBcalculus_opti…> has yet to crack the mystery. - Philippe Desjardins Proulx <http://phdp.github.io/posts/2015-04-05-automated-reasoning.html> codes and compares a basic arithmetic expression evaluator in all 5 languages of F#, Scala, Haskell, C++, and Julia. The evaluator acts on a given tree: Philippe doesn't touch parsing. Go, Erlang, and Elixir versions are in HN comments <https://news.ycombinator.com/item?id=9995896>. - Redditor gilmi starts a discussion on /r/haskell about the various FRP systems <https://www.reddit.com/r/haskell/comments/3fr5ij/frp_systems_discussion/> out there and whether they're ready for real-world use. Ollie Charles glosses classic vs arrowized systems, Doug Beardsley at Soostone testifies that "reflex is the first thing I've used that makes building web UIs enjoyable for me, which is very exciting", and Joseph Abrahamson channels Conal Elliott in staking out the acronym to mean exclusively those systems that admit continuous time. *Quotes of the Week:* - Andrew Cowie <https://twitter.com/afcowie/status/612764291863703552>: The highlight of my day is "cd ~/src/haskell/stack ; git pull ; stack install". What goodies have they landed this time? - Cale Gibbard <http://ircbrowse.net/browse/haskell?id=20873306&timestamp=1434453345#t14344…>: I basically never turn on IncoherentInstances because it basically means "I don't want anything to work, and I'd like to be confused about why." - Redditor fegu <https://www.reddit.com/r/haskell/comments/3fqiff/dont_give_up_on_haskell/ct…>: Haskell is like a mental drug, after a few initial hits you absolutely crave it. You will forego lucrative .Net career moves just to keep it, hence your long-term career will take a hit. But the days will be brighter, more enjoyable and there is just that glimmer of hope that in the end you will be able to practice Haskell full time in a well-paid position. I wish I was joking. - On ageism in the software industry <https://news.ycombinator.com/item?id=9710936#up_9711829>: Maybe I've been lucky, but this doesn't mesh with my experience at all. The oldest guys at my company are the ones teaching the classes about Haskell or Scalaz. They're the ones trying out Elixir. They're the ones building frameworks for everyone else to build on top of. The youngest people are banging out feature #4,501 for the website. *Real World Haskell of the Week:* - Fed up with the laggy Netflix UI? Check out the snappy Haskell-powered instantwatcher.com. H/T erichmond on HN <https://news.ycombinator.com/item?id=10013434#up_10016932>. -- Kim-Ee

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Serialize to Haskell
by Corentin Dupont 07 Aug '15

07 Aug '15

Hello! I'm wondering if it's possible to serialize some data to a format that is valid Haskell. In practice I would read a file like this: module Foo where myData :: MyData myData = MyData {name = "foo", descr = "test" ... } Reading this into a program is easily feasible with Hint, for example. Then the program would modify some data and serialize it back to: module Foo where myData :: MyData myData = MyData {name = "bar", descr = "test2" ... } In practice I think that the format should be a subset of Haskell that is not Turing-complete. A bit like JSON, which is a subset of Javascript but not Turing complete. Is it possible? Thanks, Corentin

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"cabal sdist" oddity
by Sven Panne 07 Aug '15

07 Aug '15

Cabal from HEAD started to issue the warning "Warning: Cannot run preprocessors. Run 'configure' command first." when running "cabal sdist" after a "cabal install", see e.g. https://travis-ci.org/haskell-opengl/OpenGL/jobs/74581138#L264. Previous versions didn't do this, see e.g. https://travis-ci.org/haskell-opengl/OpenGL/jobs/74581137#L264. Somehow the warning doesn't make sense, because the package has obviously been built before, so configuration must have happened, too. Is this a new bug in Cabal? Cheers, S.

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Syntax extension - adding import support to let/where bindings
by Oliver Charles 07 Aug '15

07 Aug '15

Hi all, I'm sure this has come up before, but a quick bit of Googling didn't reveal any prior discussions (if you known any, let me know), so I'm kicking off a new discussion. I find myself wanting to be able to say something like: foo = ... where import Something.Specific The result would be to import the contents of Something.Specific into the scope of foo and its other where bindings, but not import into the rest of the module that foo is defined in. As a motivating example, I'm currently working on building some HTML in Haskell, and the amount of symbols that come into scope is huge, when you have a DSL for both CSS and HTML - the real pain point being that you get symbols that often conflict. Here's an example of something that doesn't type-check currently image = img [ width 50, style [ width (px 50) ] ] width here is both a symbol in the HTML DSL and the CSS DSL, but to get this to type check I need to write something like image = img [ HTML.width 50, style [ CSS.width (CSS.px 50) ] ] That's not particularly bad here, the really pain is repeatedly having to do this for every single symbol I'm using. I'd prefer to be able to write image = let css = let import CSS in [ width (px 50) ] in let import HTML in img [ width 50, style css ] This is a little bit of a contrived rewrite, but hopefully it shows you what I'd like to be able to do. Please don't fixate too much on this example, it's only intended to illustrate the idea. When defining a large amount of inline styles I think this pays off - I import once to bring all the symbols I need into scope, rather than having to qualify every symbol. In reality, it will lead to me writing code like: myDocument = html where html = div [ style containerStyle ] [ div [ style rowStyle ] "Hello" , div [ style rowStyle ] "World!" ] where import HTML containerStyle = [ backgroundColor ..., padding ..., margin ... ] where import CSS rowStyle = [ backgroundColor ..., padding ..., margin ... ] where import CSS At present the suggestion is a syntax error, so I'm hoping that part won't be too controversial. Thoughts? *ocharles*

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Library for package preprocessing
by Carlo Nucera 06 Aug '15

06 Aug '15

Hi all, I'm writing a tool to analyze source code of a package, which uses haskell-src-extra. As copied from hackage, libraries may contain some extra stuff (for example, cpp directives). What is the modern way to preprocess the package (configuring for the environment, running the cpp ecc) to obtain code which is guaranteed to be parsed by haskell-src-extra? Ideally I'd like to leave most of the logic for this part outside of the package. Could you point me towards some libraries doing a similar thing? Best regards

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Where is Haskell Weekly News syndicated?
by Jeremy 06 Aug '15

06 Aug '15

Haskell Weekly News used to be syndicated on http://contemplatecode.blogspot.com/ and http://sequence.complete.org/hwn. sequence only has HWN up to March 2010, while contemplatecode goes up to the beginning of February. Is there any way that I can subscribe to HWN (preferably as RSS/Atom), and not other stuff that doesn't interest me? -- View this message in context: http://haskell.1045720.n5.nabble.com/Where-is-Haskell-Weekly-News-syndicate… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Voleon Capital Management Hiring Senior Software Engineer
by Charles Weitzer 05 Aug '15

05 Aug '15

Voleon Capital Management LP is a startup quantitative hedge fund located in Berkeley, California. We would like to hire a senior software engineer as soon as possible. Voleon's founders previously worked together at one of the most successful quantitative hedge funds in the world. Our CEO has a PhD in Computer Science from Stanford and has been CEO and founder of a successful Internet infrastructure startup. Our Chief Investment Officer has a PhD in Statistics from Berkeley. Voleon's team includes PhD's from leading departments in statistics, computer science, and mathematics. We have made several unpublished advances in the field of machine learning and in other areas as well. Here is our formal job description: ********************************************************** * Senior Software Engineer * Technology-driven investment firm employing cutting-edge statistical machine learning techniques seeks an exceptionally capable software engineer. You will architect and implement new production trading systems, machine learning infrastructure, data integration pipelines, and large-scale storage systems. The firm researches and deploys systematic trading strategies designed to generate attractive returns without being dependent on the performance of the overall market. Join a team of under 30 people that includes a Berkeley statistics professor as well as over ten PhD's from Berkeley, Chicago, CMU, Princeton, Stanford, and UCLA, led by the founder and CEO of a successful Internet infrastructure technology firm. The firm's offices are walking distance from BART and the UC Berkeley campus in downtown Berkeley, California. We have a casual and collegial office environment, weekly catered lunches, and competitive benefits packages. We seek candidates with a proven track record of writing correct, well-designed software, solving hard problems, and delivering complex projects on time. You should preferably have experience designing and implementing fault-tolerant distributed systems. Experience with building large-scale data infrastructure, stream processing systems, or latency-sensitive programs is a bonus. We are growing rapidly. Willingness to take initiative and a gritty determination to productize are essential. Required experience: - developing with C/C++/Python/Go in a Linux environment with a focus on performance, concurrency, and correctness. - experience with a functional programming language (Haskell, OCaml, Erlang) - working in TCP/IP networking, multi-threading, and server development. - working with common Internet protocols (IP, TCP/UDP, SSL/TLS, HTTP, SNMP, etc.). - architecting and designing highly available systems. - architecting and designing large-scale data management infrastructure. - working in large codebases and building modular, manageable code. Preferred experience: - debugging and performance profiling, including the use of tools such as strace, valgrind, gdb, tcpdump, etc. - working with build and test automation tools. - working with well-defined change management processes. - diagnosing RDBMS performance problems, exploiting indexing, using EXPLAIN PLAN, optimizing at the code layer, etc. - working with messaging queues (RabbitMQ, Redis, etc.) as well as distributed caching systems. Interest in financial applications is essential, but experience in finance is not a primary factor in our hiring. Benefits and compensation are highly competitive. ********************************************************** The above job description is just a starting point in terms of possible duties and seniority. We can be very flexible for the right person. If you are interested or if you know of anyone who might be interested, let me know the best way to get in touch and we can discuss details. Thank you, Charles Weitzer Senior Recruiter Voleon Capital Management LP Charles(a)Voleon.com<mailto:Charles@Voleon.com> Office: 510.704.9870 x 7012 Mobile: (510) 558-9182 www.Voleon.com

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Re: [Haskell-cafe] [Haskell-beginners] How would I increment or otherwise change a value in a record with “Simon-ness”
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 05 Aug '15

05 Aug '15

More suitable for the haskell-cafe. Routing. On 6 August 2015 at 02:52, Michael Litchard <michael(a)schmong.org> wrote: > > The below code is from this tutorial http://dev.stephendiehl.com/hask/ > > it illustrates very well how to operate on values from records with > "Simon-ness" (illustrated below). What I am struggling with is how to > modify values inside records with "Simon-ness", say incrementing "age". I > keep thinking it has to do with the way Label is defined with the > constructor Get. Could I add another constructor Put? > > {-# LANGUAGE DataKinds #-}{-# LANGUAGE KindSignatures #-}{-# LANGUAGE MultiParamTypeClasses #-}{-# LANGUAGE FunctionalDependencies #-}{-# LANGUAGE FlexibleInstances #-}{-# LANGUAGE FlexibleContexts #-}{-# LANGUAGE StandaloneDeriving #-}{-# LANGUAGE ExistentialQuantification #-}{-# LANGUAGE ConstraintKinds #-} > > import GHC.TypeLits > newtype Field (n :: Symbol) v = Field { unField :: v } deriving Show > data Person1 = Person1 > { _age :: Field "age" Int > , _name :: Field "name" String > } > data Person2 = Person2 > { _age' :: Field "age" Int > , _name' :: Field "name" String > , _lib' :: Field "lib" String > } > deriving instance Show Person1deriving instance Show Person2 > data Label (l :: Symbol) = Get > class Has a l b | a l -> b where > from :: a -> Label l -> b > instance Has Person1 "age" Int where > from (Person1 a _) _ = unField a > instance Has Person1 "name" String where > from (Person1 _ a) _ = unField a > instance Has Person2 "age" Int where > from (Person2 a _ _) _ = unField a > instance Has Person2 "name" String where > from (Person2 _ a _) _ = unField a > > age :: Has a "age" b => a -> b > age pnt = from pnt (Get :: Label "age") > > name :: Has a "name" b => a -> b > name pnt = from pnt (Get :: Label "name") > -- Parameterized constraint kind for "Simon-ness" of a record.type Simon a = (Has a "name" String, Has a "age" Int) > > spj :: Person1 > spj = Person1 (Field 56) (Field "Simon Peyton Jones") > > smarlow :: Person2 > smarlow = Person2 (Field 38) (Field "Simon Marlow") (Field "rts") > > > catNames :: (Simon a, Simon b) => a -> b -> String > catNames a b = name a ++ name b > > addAges :: (Simon a, Simon b) => a -> b -> Int > addAges a b = age a + age b > > > names :: String > names = name smarlow ++ "," ++ name spj-- "Simon Marlow,Simon Peyton Jones" > > ages :: Int > ages = age spj + age smarlow-- 94 > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > -- Regards Sumit Sahrawat

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