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Errors in non-monadic code
by jabolopes＠google.com 19 Aug '13

19 Aug '13

Hi, What is the proper way to implement a non-monadic function that checks whether a given value is correct and gives a proper error message otherwise ? What is the recommended option ? * Either String a check val | valid val = Right val | otherwise = Left errorMsg * Maybe String check val | valid val = Nothing | otherwise = Just errorMsg Cheers, Jose

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inv f g = f . g . f
by Christopher Done 19 Aug '13

19 Aug '13

Anyone ever needed this? Me and John Wiegley were discussing a decent name for it, John suggested inv as in involution. E.g. inv reverse (take 10) inv reverse (dropWhile isDigit) trim = inv reverse (dropWhile isSpace) . dropWhile isSpace That seems to be the only use-case I've ever come across. There's also this one: co f g = f g . g which means you can write trim = co (inv reverse) (dropWhile isSpace) but that's optimizing an ever rarer use-case.

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Database.postgreSQL.Simple - ambigious type
by Hartmut Pfarr 18 Aug '13

18 Aug '13

Hello, I've a problem connecting to my postgresql database. Can You help me fix the ambigious type signature? (The example is identical to the first 5-liner-example in the package documentation) http://hackage.haskell.org/packages/archive/postgresql-simple/0.3.5.0/doc/h… Kind regards Hartmut ------------------------------------------------------------------------ {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" return () ------------------------------------------------------------------------ But this leads to error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) Couldn't match expected type `IO a0' with actual type `q0 -> IO [r0]' In the return type of a call of `query' Probable cause: `query' is applied to too few arguments In a stmt of a 'do' block: query conn "select 2 + 2" In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2"; return () } ------------------------------------------------------------------------ OK, I see, that a parameter q is missing. I change the source code to {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" ( ) {- added ( ) here -} return () ------------------------------------------------------------------------ Now, I run into next error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) No instance for (FromRow r0) arising from a use of `query' The type variable `r0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance (FromField a, FromField b) => FromRow (a, b) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c) => FromRow (a, b, c) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c, FromField d) => FromRow (a, b, c, d) -- Defined in `Database.PostgreSQL.Simple.FromRow' ...plus 10 others In a stmt of a 'do' block: query conn "select 2 + 2" () In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () } In an equation for `main': main = do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () }

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MongoDB single insertions super slow
by Kyle Hanson 18 Aug '13

18 Aug '13

I am trying to write a simple benchmark of testing 1000 single inserts in mongodb, but doing single insertions is super slow. It takes nearly 40 seconds to perform these operations with the haskell mongodb driver, but under a second to perform the same number of operations of single inserts in python. Here are the two files I am working with: https://gist.github.com/hansonkd/6263648 I checked the BSON serialization, and writing it to a socket instead of mongodb driver, makes the script run in under a second so the problem seems to be with the mongodb driver. Any insights would be appreciated. -- Kyle Hanson

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ANN: processing-1.2.0.0. Graphic web applications with processing.js.
by Daniel Díaz Casanueva 18 Aug '13

18 Aug '13

Hello readers of Haskell-Cafe. Yesterday I uploaded a new version of the processing library [1]. == What is this library for? == The library is oriented to create graphic applications that run in a web browser, with no need of running a server. It provides the user with a set of functions and combinators that can be used to create an abstract piece code. This code is rendered to the processing language syntax. Once the code is uploaded, using processing.js [2], the output code is runnable from any browser with JavaScript support. To quickly give you a taste of the library, look at this animation, created with a simple recursive function: http://daniel-diaz.github.io/projects/processing/mill.html The code is here: https://github.com/Daniel-Diaz/processing/blob/master/examples/mill.hs == What's new? == After a couple of weeks working hard to have this library properly working, I think it have reached a point of reasonable usability. As a proof, I have written a simplified version of the Pac-Man game [3]. However, although I have implemented some code optimizations, it seems that the game still does not run smoothly in every web browser. I will definitely keep trying to improve the performance of the code generated by the library. I encourage you to try the library if you are interested, but keep in mind that it is still in an early stage, and may contain bugs, lack some important features, have an incomplete documentation or suffer from API changes. == References == [1] Library at Hackage - http://hackage.haskell.org/package/processing-1.2.0.0 [2] Processing.js - http://processingjs.org [3] Pac-Man game - http://daniel-diaz.github.io/projects/processing/pacman.html

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Alternative name for return
by J. Stutterheim 18 Aug '13

18 Aug '13

Dear Cafe, Suppose we now have the opportunity to change the name of the `return` function in Monad, what would be a "better" name for it? (for some definition of better) N.B. I am _not_ proposing that we actually change the name of `return`. I do currently have the opportunity to pick names for common functions in a non-Haskell related project, so I was wondering if there perhaps is a better name for `return`. - Jurriën

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Applicative is like an Arrow
by Thiago Negri 18 Aug '13

18 Aug '13

I just stumbled upon the Applicative term. Arrows are quite difficult for me to understand at the moment. I guess it needs time to digest. But, as I understand so far, Applicative and Arrows looks like the same thing. Please, enlight me.

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continuations and monads
by Christopher Howard 17 Aug '13

17 Aug '13

Q: Are the "continuations" in Scheme related to the "monads" from Haskell? If so, could someone elaborate on that?

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Haddock multiple definitions
by jabolopes＠google.com 16 Aug '13

16 Aug '13

Hi, I am using GHC: 6.12.1 Haddock: 2.6.0 and the following does not work with Haddock (GHC is fine!): -- Main -- | Blah blah blah (x, y, z) = (1, 2, 3) $ haddock ... /tmp/Main.hs:2:0: parse error on input `(' Is this a bug? Or it's just not part of Haddock? This seems like an interesting feature to document several definitions together, for example, error codes: -- | Syscall error codes for blah... -- -- errA when blah -- ... (errA, errB, errC) = ... Cheers, Jose

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Call for participation IFL 2013
by publicityifl＠gmail.com 16 Aug '13

16 Aug '13

CALL FOR PARTICIPATION 25th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2013 RADBOUD UNIVERSITY NIJMEGEN, THE NETHERLANDS ACM In-Cooperation / ACM SIGPLAN AUGUST 28 - 30 2013 "Landgoed Holthurnsche Hof" http://ifl2013.cs.ru.nl [program available - late registration still open] We are proud to announce that the 25th edition of the IFL series returns to its roots at the Radboud University Nijmegen in the Netherlands. The symposium is held from 28th to 30th of August 2013. 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 2013 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. Following the IFL tradition, IFL 2013 will use a post-symposium review process to produce the formal proceedings which will be published in the ACM Digital Library. All participants of IFL 2013 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: http://www.sigplan.org/Resources/Policies/Republication 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 do not count as publication for 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. Invited Speaker --------------- Lennart Augustsson, currently employed by the Standard Chartered Bank, well-known for his work on Haskell, parallel Haskell, Cayenne, and Bluespec, is the invited speaker of IFL 2013. The title and abstract of his talk is: "Implementation and Application of Functional Languages - A personal perspective" It is now over 30 years ago since I implemented my first functional language, and over 15 years ago since I wrote my first commercial application. In this talk I will look back to those bygone days and remind you of things that you might have forgotten or never known. The talk will be absolutely free of anything new. Peter Landin Prize ------------------ The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honoured 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 ------------------- Thomas Arts, Quviq, Gothenburg, Sweden Andrew Butterfield, Trinity College, Dublin, Ireland Edwin Brady, University of St. Andrews, UK Clemens Grelck, University of Amsterdam, Netherlands Adam Granicz, IntelliFactory, Budapest, Hungary Jeremy Gibbons, University of Oxford, UK Fritz Henglein, University of Copenhagen, Denmark Stephan Herhut, Intel Labs, Santa Clara, US Ralf Hinze (co-chair), University of Oxford, UK Zoltán Horváth, Eötvös Loránd University, Budapest, Hungary Zhenjiang Hu, University of Tokyo, Japan Mauro Jaskelioff, Universidad Nacional de Rosario, Argentina Johan Jeuring, University of Utrecht, Netherlands Rita Loogen, University of Marburg, Germany Marco T. Morazán, Seton Hall University, New Jersey, US Dominic Orchard, University of Cambridge, UK Rinus Plasmeijer (chair), Radboud University Nijmegen, Netherlands Tim Sheard, Portland State University, US Sam Tobin-Hochstadt, Northeastern University / Indiana University, US Peter Thiemann, University of Freiburg, Germany Simon Thompson, University of Kent, UK Venue ----- The 25th IFL is organized by the Radboud University Nijmegen, Model Based Software Development Department at the Nijmegen Institute for Computing and Information Sciences. The event is held in the Landgoed Holthurnsche Hof, a rural estate in the woodlands surrounding Nijmegen. It can be reached quickly and easily by public transport. Program ------- Wednesday August 28 ------------------- 8:15 registration 8:50 opening 9:00 Marcos Viera First Class Syntax, Semantics, and Their Composition Doaitse Swierstra 9:25 Olivier Danvy Circularity and Lambda Abstraction Peter Thiemann Ian Zerny 9:50 break 10:20 Loic Denuziere Piglets to the rescue Ernesto Rodriguez Adam Granicz 10:45 Simon Fowler Correct and Secure Web Programming using Dependent Types Edwin Brady and Embedded Domain-Specific Languages 11:10 break 11:40 Jennifer Hackett The Under-Performing Unfold: A new approach to optimising Graham Hutton corecursive programs Mauro Jaskelioff 12:05 Laurence Edward Day Compilation á la Carte Graham Hutton 12:30 Artjoms Sinkarovs Functionally Redundant Declarations for Improved Sven-Bodo Scholz Performance Portability 12:55 lunch 14:00 Xavier Clerc OCaml-Java: Typing Java Accesses from OCaml Programs 14:25 Jonathan Protzenko The implementation of the Mezzo type-checker 14:50 break 15:20 Ralf Lämmel The 101haskell chrestomathy Thomas Schmorleiz Andrei Varanovich 15:45 Chide Groenouwe Instant playful access to serious programming for non-programmers with a visual functional programming language 16:10 break 16:40 Arjan Boeijink Supercompiling Haskell to Hardware Philip Hölzenspies Christiaan Baaij Jan Kuper 17:05 Bas van Gijzel Towards a framework for the implementation and verification Henrik Nilson of translations between argumentation models 17:30 end of talks 18:00 dinner 20:00 25th IFL reunion 23:00 Thursday August 29 ------------------ 9:00 Bas Lijnse Supporting Semi-Structured Work using Higher Order Tasks Jan Martin Jansen 9:25 Viktória Zsók A Prototype of CPS Systems 9:50 break 10:20 Majed Al Saeed A Critical Analysis of Parallel Functional Profilers Patrick Maier Phil Trinder Lilia Georgieva 10:45 Vladimir Janjic Using Erlang Skeletons to Parallelise Realistic Christopher Brown Medium-Scale Parallel Programs Kevin Hammond 11:10 break 11:40 Lennart Augustsson Invited talk: Implementation and Application of Functional Languages A personal perspective 12:40 lunch 13:45 - 23:00 Social event: excursion, symposium dinner, Peter Landin award Friday August 30 ---------------- 9:00 Malak Aljabri The Design and Implementation of GUMSMP: Hans-Wolfgang Loidl a Multilevel Parallel Haskell Implementation Phil Trinder 9:25 Henrique Ferreiro Kindergarten Cop: Laura Castro Dynamic Nursery Resizing for Increasing Parallelism in GHC Vladimir Janjic David Castro Kevin Hammond 9:50 break 10:20 Fangyong Tang User-Defined Shape Constraints in SAC Clemens Grelck 10:45 Leaf Petersen Measuring the Haskell Gap Todd Anderson Hai Liu Neal Glew 11:10 break 11:40 Marco T. Morazán Immediate Dominators in Linear Time 12:05 Merijn Verstraaten On Predicting the Impact of Resource Redistribution Sven-Bodo Scholz in Streaming Applications 12:30 IFL 2014 12:40 lunch 14:00 Melinda Tóth Reduction of regression tests for Erlang based on impact analysis István Bozó Zoltán Horváth 14:25 Macías López A DSL for Web Services Automatic Test Data Generation Henrique Ferreiro Laura M. Castro Thomas Arts 14:50 break 15:20 Kanae Tsushima A Weighted Type Error Slicer Kenichi Asai 15:45 Ben Thorner A Type Inference Debugger for ML in Education Kathryn Gray 16:10 break 16:40 Nicolas Wu Structured Sharing for Dynamic Programming 17:05 Clemens Grelck Towards Persistent and Parallel Asynchronous Adaptive Specialisation Heinz Wiesinger for Data-Parallel Array Processing in SAC 17:30 closing

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