- Haskell-Cafe - Haskell.org

A new type of newtype --- Type-level instance diversification and resolution (tl; dr)
by Albert Y. C. Lai 30 May '17

30 May '17

When you store elements in a binary search tree, wouldn't you love to custom-order the comparison order? Wouldn't you love to do so without newtype-wrapping the element type? In fact you may actually opine that your element type should not admit a standard order in the first place (e.g., complex numbers), and whichever order used for the sake of binary search trees should stay just that and not become a general-purpose default. Perhaps more pressing to most people is when you store elements in a HashSet and you want to customize the hash function, again without either newtype-wrapping or fighting over defaults. Here is how to do that. Have a parameterized Ord class like this: {-# LANGUAGE MultiParamTypeClasses, PolyKinds, KindSignatures #-} class MyOrd (resolver :: k) a where mycmp :: p resolver -> a -> a -> Ordering instance MyOrd () Int where mycmp _ x y = compare x y data Rev instance MyOrd Rev Int where mycmp _ x y = compare y x It is also possible to get rid of the proxy parameter and use TypeApplication instead. dmwit from IRC suggests that MyOrd could be tied back to standard Ord by: instance MyOrd () a => Ord a which helps with backward compatibility. So basically use () to resolve to the standard instances (wherever a community-wide standard makes sense) and have existing standard classes tied to the new parameterized classes by resolver ~ (). Now use the parameterized Ord class for binary search trees: {-# LANGUAGE ScopedTypeVariables #-} import Data.Proxy(Proxy(Proxy)) import Data.List(foldl') data BST (resolver :: k) a = Nil | Bin !(BST resolver a) a !(BST resolver a) empty :: BST r a empty = Nil insert :: forall r a. MyOrd r a => a -> BST r a -> BST r a insert a Nil = Bin Nil a Nil insert a t@(Bin left key right) = case mycmp (Proxy :: Proxy r) a key of EQ -> t LT -> Bin (insert a left) key right GT -> Bin left key (insert a right) fromList :: MyOrd r a => [a] -> BST r a fromList xs = foldl' (flip insert) Nil xs toAscList :: BST r a -> [a] toAscList t = run t [] where run Nil = id run (Bin left key right) = run left . (key :) . run right Now we have it. toAscList (fromList [3,1,4,15,9,2,6,5,38] :: BST () Int) evaluates to [1,2,3,4,5,6,9,15,38] toAscList (fromList [3,1,4,15,9,2,6,5,38] :: BST Rev Int) evaluates to [38,15,9,6,5,4,3,2,1] and both terms have the same type [Int]. The second term does not have to have type [Down Int]. Other applications include: Custom-making equivalence relations without calling them Eq; alternative interfaces to groupBy, sortBy, maximumBy, etc.; SML-style functorial programming; and liberating Complex from Ord. I thank jadrian from IRC for proposing "named instance": > so instead of "instance Num n => Monoid (Sum n) where" we'd write e.g. > "instance Sum of Num n => Monoid n where" and use it as "Sum.mempty" which stimulated me to think "oh we can emulate that by multiple parameter type class..."

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Re: [Haskell-cafe] A new type of newtype --- Type-level instance diversification and resolution (tl; dr)
by Oleg 29 May '17

29 May '17

The idea of adding a label (`resolver') is very old. Here is the examples of how far you can take it: http://okmij.org/ftp/Haskell/TypeClass.html#Haskell1 We can represent all of Haskell98 classes plus functional dependencies with only one, predefined type class.

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Call for participation: Trends in Functional Programming, 19-21 june 2017, University of Kent, Canterbury
by Peter Achten 29 May '17

29 May '17

----------------------------- C A L L F O R P A R T I C I P A T I O N ----------------------------- ======== TFP 2017 =========== 18th Symposium on Trends in Functional Programming 19-21 June, 2017 University of Kent, Canterbury https://www.cs.kent.ac.uk/events/tfp17/index.html The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below). Authors of draft papers will be invited to submit revised papers based on the feedback receive at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. TFP 2017 will be the main event of a pair of functional programming events. TFP 2017 will be accompanied by the International Workshop on Trends in Functional Programming in Education (TFPIE), which will take place on 22 June. The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in * Edinburgh (Scotland) in 2003; * Munich (Germany) in 2004; * Tallinn (Estonia) in 2005; * Nottingham (UK) in 2006; * New York (USA) in 2007; * Nijmegen (The Netherlands) in 2008; * Komarno (Slovakia) in 2009; * Oklahoma (USA) in 2010; * Madrid (Spain) in 2011; * St. Andrews (UK) in 2012; * Provo (Utah, USA) in 2013; * Soesterberg (The Netherlands) in 2014; * Inria Sophia-Antipolis (France) in 2015; * and Maryland (USA) in 2016. For further general information about TFP please see the TFP homepage. (http://www.tifp.org/) == SCOPE == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include, but are not limited to: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Debugging and profiling for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2017 program chairs, Scott Owens and Meng Wang. == BEST PAPER AWARDS == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == PAPER SUBMISSIONS == Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which ALL authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. Papers must be submitted at: https://easychair.org/conferences/?conf=tfp17 Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 == INVITED SPEAKERS == Conor McBride University of Strathclyde (UK) Cătălin Hriţcu INRIA Paris (FR) == IMPORTANT DATES == Submission of draft papers: 5 May, 2017 Notification: 12 May, 2017 Registration: 11 June, 2017 TFP Symposium: 19-21 June, 2017 Student papers feedback: 29 June, 2017 Submission for formal review: 2 August, 2017 Notification of acceptance: 3 November, 2017 Camera ready paper: 2 December, 2017 == PROGRAM COMMITTEE == Co-Chairs Meng Wang University of Kent (UK) Scott Owens University of Kent (UK) PC Jeremy Yallop University of Cambridge (UK) Nicolas Wu University of Bristol (UK) Laura Castro University of A Coruña (ES) Gabriel Scherer Northeastern University (US) Edwin Brady University of St Andrews (UK) Janis Voigtländer Radboud University Nijmegen (NL) Peter Achten Radboud University Nijmegen (NL) Tom Schrijvers KU Leuven (BE) Matthew Fluet Rochester Institute of Technology (US) Mauro Jaskelioff CIFASIS/Universidad Nacional de Rosario (AG) Patricia Johann Appalachian State University (US) Bruno Oliveira The University of Hong Kong (HK) Rita Loogen Philipps-Universität Marburg (GE) David Van Horn University of Marylan (US) Soichiro Hidaka Hosei University (JP) Michał Pałka Chalmers University of Technology (SE) Sandrine Blazy University of Rennes 1 - IRISA (FR)

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Ralf Lämmel gives a talk at Haskell User Group - Frankfurt, 21.6.17
by Peter Althainz 29 May '17

29 May '17

Dear All, we are happy to have Ralf Lämmel in Frankfurt, talking about: "An interpreter for every season", targeted at a Haskell-friendly audience. https://www.meetup.com/de-DE/Frankfurt-Haskell-User-Group/events/240346167 regards Peter

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List instance of Alternative: why (++)?
by Theodore Lief Gannon 29 May '17

29 May '17

Fiddling around, I found myself wanting: coalesce :: [a] -> [a] -> [a] -- or -- :: (Foldable t) => t a -> t a -> t a coalesce a b = if null a then b else a I expected this to be (<|>) (it is for Maybe!) but instead I find no canonical implementation of it anywhere, and what seems like a useless instance Alternative []. What's the rationale?

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Yesod Problem - passing values inside a route
by Michael Litchard 28 May '17

28 May '17

Below I have an example from the Yesod <https://www.yesodweb.com/book/forms> book. The form action maps to a route. How do I pass a value to that route. I know it has something to do with ^ but I can't see to make it work right. {-# LANGUAGE MultiParamTypeClasses #-}{-# LANGUAGE OverloadedStrings #-}{-# LANGUAGE QuasiQuotes #-}{-# LANGUAGE TemplateHaskell #-}{-# LANGUAGE TypeFamilies #-}import Control.Applicativeimport Data.Text (Text)import Yesod data App = App mkYesod "App" [parseRoutes|/ HomeR GET/input InputR GET|] instance Yesod App instance RenderMessage App FormMessage where renderMessage _ _ = defaultFormMessage data Person = Person { personName :: Text , personAge :: Int } deriving Show getHomeR :: Handler Html getHomeR = defaultLayout [whamlet| <form action=@{InputR}> <--- problem line -- want to do this <form action=@{InputR ^haskellValue}> <p> My name is <input type=text name=name> and I am <input type=text name=age> years old. <input type=submit value="Introduce myself"> |] getInputR :: Text -> Handler Html getInputR label = do person <- runInputGet $ Person <$> ireq textField label <*> ireq intField "age" defaultLayout [whamlet|<p>#{show person}|] main :: IO () main = warp 3000 App

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stack problems
by Dennis Raddle 28 May '17

28 May '17

Trying to learn stack. I'm following the tutorial on this page: https://docs.haskellstack.org/en/stable/GUIDE/ I built helloworld. Then I modified Lib.hs to include Data.Text.IO, as the tutorial specifies. I typed 'stack build' and got the error that the text package wasn't installed, as the tutorial says I would. I then modified helloworld.cabal to have these lines: build-depends: base , helloworld , text and typed 'stack build' again. I got the exact same output - an error on building Lib.hs, 'Failed to load interface for Data.Text.IO. I checked that I really modified al lthe right files and can't find anything I did wrong.

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ThEdu'17: 2nd Call for Extended Abstracts & Demonstrations
by Achim D. Brucker 28 May '17

28 May '17

(Apologies for duplicates) Call for Extended Abstracts & Demonstrations ThEdu'17 Theorem proving components for Educational software http://www.uc.pt/en/congressos/thedu/thedu17 at CADE 26 International Conference on Automated Deduction 6-11 August 2017 Gothenburg, Sweden http://www.cade-26.info/ ThEdu'17 Scope Computer Theorem Proving is becoming a paradigm as well as a technological base for a new generation of educational software in science, technology, engineering and mathematics. The workshop brings together experts in automated deduction with experts in education in order to further clarify the shape of the new software generation and to discuss existing systems. Topics of interest include: * methods of automated deduction applied to checking students' input; * methods of automated deduction applied to prove post-conditions for particular problem solutions; * combinations of deduction and computation enabling systems to propose next steps; * automated provers specific for dynamic geometry systems; * proof and proving in mathematics education. Important Dates Extended Abstracts: 18 June 2017 Author Notification: 2 July 2017 Final Version: 16 July 2017 Workshop Day: 6 August 2017 Submission Interested researchers are invited to submit extended abstracts and system descriptions. Both kinds of submissions should be approximately 5 pages in length and present original unpublished work not submitted elsewhere. Submission is in PDF format via easychair, https://easychair.org/conferences/?conf=thedu17 formatted according to http://www.easychair.org/publications/easychair.zip The extended abstracts and system descriptions will be made available online. At least one author is expected to presents his/her submission at ThEdu'17. Joint publication in companion with other CADE26 events is under consideration (as a volume in the EPiC Series in Computing). Program Committee Francisco Botana, University of Vigo at Pontevedra, Spain Achim D. Brucker, University of Sheffield, United Kingdom Filip Maric, University of Belgrade, Serbia Walther Neuper, Graz University of Technology, Austria (co-chair) Pavel Pech , University of South Bohemia, Czech Republic Pedro Quaresma, University of Coimbra, Portugal (co-chair) Vanda Santos, CISUC, Portugal Wolfgang Schreiner, Johannes Kepler University, Austria Burkhart Wolff, University Paris-Sud, France -- 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] wiki account request
by Flavio Villanustre 27 May '17

27 May '17

I just created an account for you. You should be receiving a temporary password via email in the next few minutes. Please let me know if you need any further assistance to log in. Best, Flavio On May 27, 2017 02:50, "jzeus" <jzeus(a)protonmail.com> wrote: username "jzeus"

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problems with stack, config.yaml
by Dennis Raddle 27 May '17

27 May '17

A year ago I tried to install stack on my MacBook. Just recently I tried again, and it asked me if I wanted to upgrade rather than install. I did so, but got an error (some message from GHC saying 'the thing that should never happen, happened' or something like that). So I thought, okay let me try uninstalling stack first, then installing it, and going from there. So I did that, and I'm working the stack tutorial, and upon doing "stack setup" I got a parse error in config.yaml. "Error in $.params: failed to parse field 'params', expected Map k a, encountered String" As far as the question of where I got my config.yaml from, I edited something that was there to add some specific information, but I am not sure if I changed anything else. Here is the config.yaml that produces the above error: # This file contains default non-project-specific settings for 'stack', used # in all projects. For more information about stack's configuration, see # http://docs.haskellstack.org/en/stable/yaml_configuration/ # The following parameters are used by "stack new" to automatically fill fields # in the cabal config. We recommend uncommenting them and filling them out if # you intend to use 'stack new'. # See https://docs.haskellstack.org/en/stable/yaml_configuration/#templates templates: params: author-name:Dennis author-email:dennisraddle@gmail.com category:programming copyright:2017 github-username:Dennis

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