- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] type (++) = (<>)
by Doug McIlroy 12 Aug '17

12 Aug '17

> What do you think of making (++) the same as (<>) This seems to be a call for returning to the old situation in which (++) was an operator of class MonadPlus. Why was that abolished in Haskell 98? Doug

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Where is the hole?
by Baa 11 Aug '17

11 Aug '17

Hello, Dear List! I have code (this is the fragment only): {-# LANGUAGE CPP #-} ... ... let user' = ... ... else defect $ logger # ("authentication failure (user: " ++ user' ++ ")") #: __LINE__ ... ... and I get compilation error: • Found hole: __LINE__ :: Int Or perhaps ‘__LINE__’ is mis-spelled, or not in scope • In the second argument of ‘(#:)’, namely ‘__LINE__’ In the second argument of ‘($)’, namely ‘logger # ("authentication failure (user: " ++ user' ++ ")") #: __LINE__’ In the expression: defect $ logger # ("authentication failure (user: " ++ user' ++ ")") #: __LINE__ • Relevant bindings include... ... etc ... But if I change variable name to `userx` (from user'), all is compiled succesflully, no any holes. Where is the hole? --- Best regards, Paul

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How to bring existentially quantified type variables into scope
by Wolfgang Jeltsch 11 Aug '17

11 Aug '17

Hi! GHC has some support for referring to existentially quantified type variables. For example, consider the following definition: > data SomeList = forall a . SomeList [a] With the ScopedTypeVariables extension, you can get hold of the concrete type a used in a SomeList value: > f :: SomeList -> SomeList > f (SomeList [x :: a]) = SomeList xs where > > xs :: [a] > xs = [x, x] However, this approach does not work if there are no fields whose type involves the existentially quantified type variable. Consider, for example, the following definition: > data IsListType a where > > IsListType :: IsListType [b] For each type T, we can match a value of type IsListType T against the pattern IsListType. If this succeeds, we know that T is a list type, but we do not have access to the respective element type. Is there a way to determine existentially quantified type variables like the b in the above example? All the best, Wolfgang

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command-line Haskell with 'ghc -e'
by Takenobu Tani 11 Aug '17

11 Aug '17

Hi cafe, I wrote a simple note: command-line Haskell with 'ghc -e' https://github.com/takenobu-hs/commandline-haskell May Haskell with you :) Regards, Takenobu

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FFI and a struct containing a string and allocating memory space
by Brian Sammon 11 Aug '17

11 Aug '17

If I have a struct like struct EventInfo { char * event_name; int year; int month; int date; } and an associated haskell datatype that I want to marshall into it, it seems to me that I can't take advantage of the speed advantages of alloca (and its friends like "with") The only safe way I've thought of to do this is to do this is to have the poke function for EventInfo (the associated haskell type) call malloc (or newCAString or something) and then poking the string into the newly allocated space. I realized after a few successful runs of using withCString in the poke function for EventInfo, that I don't think that this (calling withCString from the poke function) was a safe approach, and I'm thinking it was just dumb luck that this worked at all. Thoughts? Am I on the right track? Am I missing any better approaches? (I'm using Haskell 8.0.1 and a Storable instance for EventInfo, BTW)

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Investigating single-letter type variables
by Jeffrey Brown 11 Aug '17

11 Aug '17

Haskellers tend to use uninformative single-letter type variables. A case in point: Megaparsec> :i ParsecT type role ParsecT nominal nominal representational representational newtype ParsecT e s (m :: * -> *) a ... I've gotten used to the conventions that "a" stands for anything and "m" stands for monad -- but without digging into the code it wasn't initially obvious to me whether "s" stood for stream or state. Single-letter type variables don't seem to always be the standard, though: Megaparsec> :i between between :: Applicative m => m open -> m close -> m a -> m a -- Defined in ‘Control.Applicative.Combinators’ My questions are: (1) Are the brevity gains worth the confusion costs? (2) If I'm looking at a new library for the first time and trying to figure out what a type variable stands for, is there a canonical way to do it? Flailing through documentation at random? Unifying types by hand? -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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DSLDI 2017: Final Call for Talk Proposals (deadline extended)
by Lindsey Kuper 09 Aug '17

09 Aug '17

********************************************************************* FINAL CALL FOR TALK PROPOSALS DSLDI 2017 Fifth Workshop on Domain-Specific Language Design and Implementation October 22, 2017 Vancouver, Canada Co-located with SPLASH http://2017.splashcon.org/track/dsldi-2017 https://twitter.com/wsdsldi ********************************************************************* **Deadline for talk proposals extended to Friday, 11 August 2017** Well-designed and implemented domain-specific languages (DSLs) can achieve both usability and performance benefits over general-purpose programming languages. By raising the level of abstraction and exploiting domain knowledge, DSLs can make programming more accessible, increase programmer productivity, and support domain-specific optimizations. ## Workshop Goal Domain-Specific Language Design and Implementation (DSLDI) is a workshop intended to bring together researchers and practitioners interested in discussing how DSLs should be designed, implemented, supported by tools, and applied in realistic contexts. The focus of the workshop is on all aspects of this process, from soliciting domain knowledge from experts, through the design and implementation of the language, to evaluating whether and how a DSL is successful. More generally, we are interested in continuing to build a community that can drive forward the development of modern DSLs. ## Workshop Format DSLDI is a single-day workshop and will consist of an invited speaker followed by moderated audience discussions structured around a series of short talks. The role of the talks is to facilitate interesting and substantive discussion. Therefore, we welcome and encourage talks that express strong opinions, describe open problems, propose new research directions, and report on early research in progress. Proposed talks should be on topics within DSLDI’s area of interest, which include but are not limited to: * solicitation and representation of domain knowledge * DSL design principles and processes * DSL implementation techniques and language workbenches * domain-specific optimizations * human factors of DSLs * tool support for DSL users * community and educational support for DSL users * applications of DSLs to existing and emerging domains * studies of usability, performance, or other benefits of DSLs * experience reports of DSLs deployed in practice ## Call for Talk Proposals We solicit talk proposals in the form of short abstracts (max. 2 pages). A good talk proposal describes an interesting position, open problem, demonstration, or early achievement. The submissions will be reviewed on relevance and clarity, and used to plan the mostly interactive sessions of the workshop day. Publication of accepted abstracts and slides on the website is voluntary. * Deadline for talk proposals: August 11th, 2017 * Notification: September 11th, 2017 * Workshop: October 22nd, 2017 * Submission website: https://dsldi17.hotcrp.com/ ## Workshop Organization Co-chairs: * Lindsey Kuper (lindsey(a)composition.al) Intel Labs * Eric Walkingshaw (eric.walkingshaw(a)oregonstate.edu) Oregon State University Follow us on Twitter at https://twitter.com/wsdsldi Program committee: * Nada Amin (EPFL/University of Cambridge) * Eric Holk (Google) * Gabriele Keller (Data61, CSIRO (formerly NICTA) and UNSW) * Rebekah Leslie-Hurd (Intel Labs) * Chris Martens (NCSU) * Lee Pike (Galois) * Jonathan Ragan-Kelley (UC Berkeley) * Jesús Sánchez Cuadrado (Autonomous University of Madrid) * Vincent St-Amour (Northwestern University) * Philip Wadler (University of Edinburgh)

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Combining pool and Conduit
by Pieter Laeremans 08 Aug '17

08 Aug '17

Hello, I would like to combine a "Connection Pool" with the conduit library. I have an application were a stream of results from a web api needs to be analysed and stored in a database. Since the analysis is composed of different parts, I combine some parts together ( .|) Some of those parts need a database connection to do their work. Initially I created a Connection and passed it around to each part that needed it, this is sub-optimal. So I wanted to try the "resource-pool" library to address this problem. (https://hackage.haskell.org/package/resource-pool) So instead of a Connection I pass around a (Pool Connection) But I run into problems , and I don't know how to fix them. Here is a simple snippet of code that demonstrates the problem: ------------------------------------------- {-# LANGUAGE RankNTypes #-} module Lib where import Data.Pool import Database.HDBC import Database.HDBC.ODBC import Data.Conduit import Conduit createConnectionPool :: Pool Connection createConnectionPool = undefined processFoo :: Pool Connection -> Consumer String IO () processFoo pool = withResource pool $ \conn -> do mapM_C print ------------------------- I can not get this to compile: src/Lib.hs:15:19: error: • No instance for (MonadBaseControl IO (ConduitM String o IO)) arising from a use of ‘withResource’ • In the expression: withResource pool In the expression: withResource pool $ \ conn -> do { mapM_C print } In an equation for ‘processFoo’: processFoo pool = withResource pool $ \ conn -> do { mapM_C print } But I have no Idea how to tackle this problem. Any suggestions ? Thanks, Pieter -- Pieter Laeremans pieter(a)laeremans.org

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Extended deadline: IFL 2017 (29th Symposium on Implementation and Application of Functional Languages)
by publicityifl＠gmail.com 08 Aug '17

08 Aug '17

<div dir="ltr">Hello, Please, find below the third call for papers for IFL 2017. Note that some of the deadlines have been extended. Details are given below. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL --- IFL 2017 - CALL FOR PAPERS: DEADLINES EXTENDED ============================================== 29th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES ======================================================================= University of Bristol, UK In cooperation with ACM SIGPLAN Wednesday 30 August - Friday 1 September, 2017 <a href="http://iflconference.org/">http://iflconference.org/</a> Scope ----- The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2017 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Peer-review ----------- Following the IFL tradition, IFL 2017 will use a post-symposium review process to produce the formal proceedings. All participants of IFL 2017 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: <a href="http://www.sigplan.org/Resources/Policies/Republication">http://www.sigplan.org/Resources/Policies/Republication</a> The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings are not subject to the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. The formal proceedings will appear in the International Conference Proceedings Series of the ACM Digital Library. Important dates --------------- Note that the original deadlines for submission and registration have been extended. | | | | ----------------------| -----------------------------------------------------------| | Thu 17 August 2017 | Submission deadline draft papers | | Fri 18 August 2017 | Notification of acceptance for presentation | | Mon 21 August 2017 | Early registration deadline | | Mon 21 August 2017 | Submission deadline for pre-symposium proceedings | | Fri 25 August 2017 | Late registration deadline | | Wed 30 August 2017 - Fri 1 September 2017 | IFL Symposium | | Mon 4 December 2017 | Submission deadline for post-symposium proceedings | | Wed 31 January 2018 | Notification of acceptance for post-symposium proceedings | | Mon 12 March 2018 | Camera-ready version for post-symposium proceedings | Submission details ------------------ Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must use the new ACM two columns conference format, which can be found at: <a href="http://www.acm.org/publications/proceedings-template">http://www.acm.org/publications/proceedings-template</a> For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. Authors submit through EasyChair: <a href="https://easychair.org/conferences/?conf=ifl2017">https://easychair.org/conferences/?conf=ifl2017</a> Topics ------ IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2017, please contact the PC chair at <a href="mailto:nicolas.wu@bristol.ac.uk">nicolas.wu(a)bristol.ac.uk</a>. Topics of interest include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications Peter Landin Prize ------------------ The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee ------------------- Chair: Nicolas Wu, University of Bristol, UK - Kenichi Asai, Ochanomizu University, Japan - Sandrine Blazy, University of Rennes 1, France - Carlos Camarao, Universidade Federal de Minas Gerais, Brazil - Stephen Dolan, University of Cambridge, UK - Jurriaan Hage, Utrecht University, Netherlands - Yukiyoshi Kameyama, University of Tsukuba, Japan - Benjamin Lerner, Brown University, USA - Bas Lijnse, Radboud University, Netherlands - Garrett Morris, University of Kansas, USA - Miguel Pagano, Universidad Nacional de C√≥rdoba, Argentina - Tomas Petricek, Alan Turing Institute, UK - Maciej Pir√≥g, University of Wroclaw, Poland - Exequiel Rivas, Universidad Nacional de Rosario, Argentina - Neil Sculthorpe, Nottingham Trent University, UK - Melinda Toth, Eotvos Lorand University, Hungary - Phil Trinder, Glasgow University, UK - Kanae Tsushima, National Institute of Informatics, Japan - Marcos Viera, Universidad de la Republica, Uruguay - Meng Wang, University of Kent, UK Venue ----- The IFL 2017 will be held in association with the Department of Computer Science, University of Bristol, UK. Bristol is located in South West England, and can be easily reached from Bristol Airport. See the website for more information on the venue. </div>  <a style='display: block; margin: 32px 0 40px 0; padding: 10px; font-size: 1em; text-align: center; border: 0; border-top: 1px solid gray; ' href='https://goo.gl/2ksdRv'>powered by GSM. Free mail merge and email marketing software for Gmail.</a>

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Convert Data.Vector.Unboxed.Mutable.MVector to Data.Vector.Unboxed.Vector
by Cody Goodman 06 Aug '17

06 Aug '17

I want to do this so I can use the Data.Vector.Unboxed.Generic functions requiring the Vector constraint, namely the maxIndex function. Implementing maxIndex for Data.Vector.Unboxed.Mutable.MVector would be useful as well, but I'm even more confused at how to do that or where to begin. Here is some stubbed out code demonstrating this. {-# Language ScopedTypeVariables #-} module Main where import Control.Monad.Primitive import qualified Data.Vector.Unboxed import qualified Data.Vector.Generic as VG import qualified Data.Vector.Unboxed.Mutable as VM import qualified Data.Vector.Unboxed as V toImmutable :: VM.MVector RealWorld Int -> V.Vector Int toImmutable = undefined main :: IO () main = do v :: VM.MVector RealWorld Int <- VM.new 1 VM.write v 0 (3 :: Int) x <- VM.read v 0 -- y <- VG.head . toImmutable $ v -- print y print $ VM.length v {- Thanks, Cody -}

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