- Haskell-Cafe - Haskell.org

About my commitment to my passions, Haskell and the world
by Jesus Gonzalez 17 Jun '15

17 Jun '15

Hi everybody, I have a very important announcement. I’ve decided to dedicate the next big part of my life to improve the world through Haskell. And you can help! (I’m not asking for money…) I plan to go to live at a “poor” village where my grandparents live. Poor because it is very hard to get a higher education. My grandfather founded some time ago some schools nearby the village, but if people wanted to have a bright future, they had to leave the village. My father moved to the nearest capital city. The story repeated. My father and my mother (together with other people) founded a school at the city where I was born. I left this city to get an even higher education at one of the best universities of my continent. At college, I got the opportunity to study a year as an exchange student at one of the best CS universities in the world. Knowing that probably I would never be able to study at such a good university again, I took the hardest subjects I could. The first semester, I took 2 undergrad courses an 2 grad courses (Program Transformations, Software Design, etc…) This first semester was hard, but at the end I got ok grades, so I thought that maybe I could give a little bit more. For the next semester, I tried to encode the knowledge I acquired as clearly as I could. Very quickly, I got more interested in how to model this knowledge more than the knowledge itself. When I failed with Java, I turned to Haskell and so it begun. I did enjoy my time also. I got to see the inauguration of the Gates Computer Science Building, I found the time to play the piano and learned how to drive the bike without hands very well. When I returned, my parents kindly forced me to finish colleague. When I graduated, I got a scholarship for grad school (that I took, but I left 3 months later) and a job making iOS and Android apps. I couldn’t use Haskell but I did work some months with F#. Now that I’ve seen the world, the good, the bad and the ugly things, Now that I’ve tasted the impoverishing money and repented, Now I’m ready, I flipped the arrows. The hardest thing so far has been to finish my NDA… I’m still on it… It looks like the story will repeat once more. I want to retreat my self and found a next generation school where my grandparents live. I believe that we need to create high performant Haskellers and that we owe to teach children as young and fast as they can happily take it. I would appreciate encouragement, tutorials for all ages and levels of math encoded in Haskell, advice, anything good. I’m already burning bridges! I’ll keep reporting.

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[Hiring] Infrastructure Engineer @ Elastic (makers of Elasticsearch) Location: Anywhere
by Joe Hillenbrand 16 Jun '15

16 Jun '15

Although not strictly a Haskell developer position. Haskel experience/interest is a huge plus. https://www.elastic.co/about/careers/engineering/infrastructure-engineer?id… (Note: the id param is required or else you'll get a 404)

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Type constraint with RankNTypes gradually slows down function?
by Ben Gunton 16 Jun '15

16 Jun '15

When I write a function with the type constraint "embedded" in the function's type declaration, instead of at the beginning, the function takes longer each time it is run. The simplest example I could write is this: http://lpaste.net/134563. "run1" is near instantaneous the first time, but gets marginally slower after each iteration... after thousands of iterations it takes about a millisecond to run (and keeps getting worse). "run2" is always near instantaneous. What is happening in "run1" that makes it slow down? PS: The example is fairly contrived, I know "run1"'s type declaration wouldn't be used in practice. Thanks for any guidance! {-# LANGUAGE RankNTypes #-} {-# LANGUAGE FlexibleContexts #-} module Main where import Control.Monad.State.Strict import qualified Data.Time.Clock as Clock import Control.Exception run1 :: (Int -> (Num Int => State Int Bool)) -> Int -> IO () run1 f state = do t1 <- Clock.getCurrentTime evaluate $ runState (f 1) state t2 <- Clock.getCurrentTime print $ Clock.diffUTCTime t2 t1 run1 f state run2 :: Num s => (Int -> State s Bool) -> s -> IO () run2 f state = do t1 <- Clock.getCurrentTime evaluate $ runState (f 1) state t2 <- Clock.getCurrentTime print $ Clock.diffUTCTime t2 t1 run2 f state main :: IO () main = run1 (const $ return False) 1 --main = run2 (const $ return False) 1

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Space leak with replicateM
by Ken Takusagawa II 16 Jun '15

16 Jun '15

In the following program, the function "test1" results in huge memory usage, but substituting "test2", which does essentially the same thing, does not. GHC 7.10.1, AMD64. Is there a different implementation of replicateM that avoids the space leak? module Main where { import Control.Monad; numbers :: [Int]; numbers=[1..200]; -- has a space leak test1 :: [[Int]]; test1 = replicateM 4 numbers; -- no space leak test2 :: [[Int]]; test2 = do { x1 <- numbers; x2 <- numbers; x3 <- numbers; x4 <- numbers; return [x1,x2,x3,x4]; }; main :: IO(); main = print $ length $ test1; } Thanks, --ken

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Question about constraining functions to particular ADT constructors
by Román González 16 Jun '15

16 Jun '15

Hello there, I've been thinking on different approaches to constraint particular functions to a particular constructor of an ADT in order to reduce representation of invalid states. Say for example I've this types: data Address = Address { ... } newtype Email = Email String data Package = Package { ... } data EmailMsg = EmailMsg { ... } data User = RealUser Address | VirtualUser Email And I would like to implement two functions: deliverPackage :: User -> Package -> IO Bool sendEmail :: User -> EmailMsg -> IO () I would like to constraint both deliverPackage and sendEmail to receive only the semantically correct constructor of User. I know of an approach I could use, that is wrapping each constructor in it's own newtype, and create some smart constructor that way, that approach works, but I find it rather verbose. Is there any other well known approach to deal with this scenarios?

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Link to BayHac presentation and code files.
by David Banas 16 Jun '15

16 Jun '15

Hi all, Great to have seen you all again at BayHac! I couldn’t find any reference to a central repository for presentation/code upload. So, here’s my stuff (“Cracking the Vigenere Cipher in Haskell”): https://github.com/capn-freako/BayHac_2015 Many thanks to Graham Hutton for permission to post his code, along with my own. Cheers, -db

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returning a polymorphic function
by Paolino 15 Jun '15

15 Jun '15

Hello list, I'm trying to accomplish something along this line {-# LANGUAGE ScopedTypeVariables #-} f :: IO (a -> a) f = return id main = do (g :: a -> a) <- f print $ g 1 print $ g "ciao" Is it possible or I have to call f more than once to get different g's Thanks paolino

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PhD student or postdoc position announcement
by Thiemann, Rene 15 Jun '15

15 Jun '15

2-year postdoc or 3-year PhD-student position at the University of Innsbruck ============================================================================ The Computational Logic research group at the University of Innsbruck has one open position funded by the FWF (Austrian science fund) via the START project „Certifying Termination and Complexity Proofs of Programs“. The project aims at increasing the reliability in current complexity and termination provers by independently checking the generated proofs. To this end, several analysis techniques will be formalized in the theorem prover Isabelle/HOL, with a focus on term rewriting and Haskell. For this project, we are looking for an enthusiastic young researcher with a background in computational logic. Knowledge of automated termination analysis, complexity analysis, or theorem proving would be an asset. Candidates with a strong theoretical background in related areas are also encouraged to apply. The PhD-student candidate must have a Master's or equivalent degree. Knowledge of German is not essential. The salary is determined by the FWF-funding scheme (3.546,00 EUR monthly gross salary for postdocs, and 2.024,90 EUR for PhD-students, cf. http://www.fwf.ac.at/en/research-funding/personnel-costs/) Applications (including a CV, a publication list (only for postdocs), and a letter of recommendation) may be emailed to the project leader René Thiemann (rene.thiemann at uibk.ac.at) no later than July 6, 2015. We plan to make decisions on these position until July 10. The preferred starting date is summer or autumn 2015. Informal inquiries are also welcome via email. The city of Innsbruck is superbly located in the beautiful surroundings of the Tyrolean Alps. The combination of the Alpine environment and urban life in this historic town provides a high quality of living. Further information is available from the following links: START Project: http://cl-informatik.uibk.ac.at/research/projects/certifying-termination-an… Institute of Computer Science: http://informatik.uibk.ac.at/ University of Innsbruck: http://www.uibk.ac.at/ City of Innsbruck: http://www.innsbruck.at/

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Data.Hashable instance generation?
by Matti Nykänen 15 Jun '15

15 Jun '15

Hello, A practical question about getting hashing to work. Having used Data.Map before, I wanted to try Data.HashMap.Lazy instead: ---- {-# LANGUAGE DeriveGeneric #-} import Data.HashMap.Lazy as HM import GHC.Generics (Generic) import Data.Hashable data Colour = Red | Green | Blue deriving Generic instance Hashable Colour foo = HM.insert Red ---- The data and its instance definition are directly from the web page <https://hackage.haskell.org/package/hashable-1.2.1.0/docs/Data-Hashable.html>. However, I det the following error: ---- GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( htest.hs, interpreted ) htest.hs:13:8: No instance for (hashable-1.1.2.5:Data.Hashable.Hashable Colour) arising from a use of `insert' Possible fix: add an instance declaration for (hashable-1.1.2.5:Data.Hashable.Hashable Colour) In the expression: insert Red In an equation for `foo': foo = insert Red Failed, modules loaded: none. ---- I do not know what is wrong or how I could fix it. Note that my Haskell had originally the older hashable-1.0.0 package, but I cabal-installed this hashable-1.1.2.5 because surfing the net suggested that it would be much better. Could it be the case that the instance goes somehow in the wrong class? -- Matti Nykänen

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Haskell dev role in Strats at Standard Chartered London
by Don Stewart 15 Jun '15

15 Jun '15

I'm hiring another Haskell dev to join the Strats team in London. Details here: https://donsbot.wordpress.com/2015/06/15/haskell-dev-role-in-strats-at-stan…

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