August 2010 - Haskell-Cafe

Twidge Short URL not working...
by arn-arn fernch 13 Aug '10

13 Aug '10

Hello, It's my first time using this mailing list thing so forgive me if I made any mistakes. I was using Twidge (http://wiki.github.com/jgoerzen/twidge/) and it's mentioned there is an available support in the Haskell Cafe. I only have a slight problem with Twidge. It's currently not shortening URLs by default though I can see in my .twidgerc this line "shortenurls: yes". Any things I should consider? Thanks!

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universal quantification is to type instantiations as existential quantification is to what
by Joshua Ball 13 Aug '10

13 Aug '10

Hi, If I have a universally quantified type mapInt :: forall a. (Int -> a) -> [Int] -> [a] I can instantiate that function over a type and get a beta-reduced version of the type mapInt [String] :: (Int -> String) -> [Int] -> [String] (I'm borrowing syntax from Pierce here since I don't think Haskell allows me to explicitly pass in a type to a function.) This makes sense to me. The universal quantifier is basically a lambda, but it works at the type level instead of the value level. My question is... what is the analog to an existential type? mapInt :: exists a. (Int -> a) -> [Int] -> [a] (I don't think this is valid syntax either. I understand that I can rewrite this using foralls and a new type variable, doing something that looks like double negation, but the point of my question is to get an intuition for what exactly the universal quantifier means in the context of function application... if this even makes sense.) In particular: 1. If I can "instantiate" variables in a universal quantifier, what is the corresponding word for variables in an existential quantifier? 2. If type instantiation of a universally quantified variable is beta-reduction, what happens when existentially quantified variables are [insert answer to question 1]? 3. Is there any analogue to existential quantifiers in the value world? "Forall" is to "lambda" as "exists" is to what? Also (separate question), is the following statement true? forall T :: * -> *. (forall a. T a) -> (exists a. T a) If so, what does the implementation look like? (What function inhabits it, if it is interpreted as a type?) Josh "Ua" Ball

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Re: Documentation for OpenGL?
by Eitan Goldshtrom 13 Aug '10

13 Aug '10

Woops, nevermind. I WAS looking in the wrong place. I should've known =P. Sorry for the unnecessary request. -Eitan PS. For anyone interested all of the documentation is in Graphics.Rendering.OpenGL.GL.PerFragment on hackage

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Documentation for OpenGL?
by Eitan Goldshtrom 13 Aug '10

13 Aug '10

Hi everyone. I'm looking for something specific. I'm trying to figure out how to use the depthFunc StateVar and I can't find any official documentation for it. I did a ":t depthFunc" in ghci and got "depthFunc :: StateVar (Maybe ComparisonFunction)" so I looked up ComparisonFunction and couldn't find anything official on that either. Is there any documentation for it? Maybe I wasn't looking in the right places? Thanks in advance. -Eitan

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How do you change the mouse position?
by Eitan Goldshtrom 12 Aug '10

12 Aug '10

In C++, maybe Java, I remember using a Robot to change the location of the mouse on the screen. My intention is to do something like in an FPS game where the mouse is always centered to make sure it doesn't run into the edges of the screen. How can I do that in Haskell? I'm using OpenGL, so if there is a utility that specifically works with OpenGL then I believe that would be the most helpful. -Eitan

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ghc in macports
by Ozgur Akgun 12 Aug '10

12 Aug '10

Dear Cafe, I wonder who is maintaining the ghc package in macports, and what the current stategy of doing things is? http://www.macports.org/ports.php?by=name&substr=ghc (ghc 6.10.4) Personally, I'd like to use the macports version, if the ghc version there was resonably recent (having 2 versions, a stable and an edge could be a good idea?) Thanks, Ozgur

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ADT patch/update
by Sergey Mironov 12 Aug '10

12 Aug '10

Hi Cafe. I am searching for materials on one data type-related problem. Suppose we have version control system storing values of user-defined algebraic data type. 'User' (actually, programmer) wants to store his/her data and later update it by applying patches. By patch I mean value of some (another) type representing one simplest update of stored value. Say, for tree type > data Tree a = Node (Tree a) (Tree a) | Leaf a one may write > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE UndecidableInstances #-} > > class Storable a where > type Patch a :: * > update :: Patch a -> a -> a > > data TreeStep = Left | Right -- Like in zippers ? > type family Location a :: * > type instance Location (Tree a) = [TreeStep] > > instance (Storable a) => Storable (Tree a) where > type Patch (Tree a) = (Location (Tree a), Patch a) > update = ... -- Move to specific location and call node's update Patch here is coded explicitly but I guess there may be some generic way of it to calculate. Whether any similar work has been carried out? Will be glad to read comments! -- Thanks, Sergey

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hackage.haskell.org cannot be accessed?
by Magicloud Magiclouds 12 Aug '10

12 Aug '10

Hi, Today I found out that I cannot access hackage.haskell.org. I have tried vpn/proxy to see if my network has something wrong. No luck. -- 竹密岂妨流水过山高哪阻野云飞

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Workshop on ML 2010 - Call for Participation
by Matthew Fluet 12 Aug '10

12 Aug '10

The 2010 ACM SIGPLAN Workshop on ML http://www.cs.rit.edu/~mtf/ml2010 Baltimore, Maryland, United States Sunday, September 26, 2010 co-located with ICFP 2010 Call for Participation ML is a family of programming languages that includes dialects known as Standard ML, Objective Caml, and F#. The development of these languages has inspired a large amount of computer science research, both practical and theoretical. This workshop aims to provide a forum to encourage discussion and research on ML and related technology (higher-order, typed, or strict languages). The format of the 2010 Workshop on ML will be different than that of recent years, returning to a more informal model: a workshop with presentations selected from submitted abstracts but without published proceedings. We hope that this format will encourage the presentation of more exciting (if unpolished) research and deliver a more lively workshop atmosphere. Invited Speaker ~~~~~~~~~~~~~~~ Luke Hoban (Microsoft) -- Bringing F# to Visual Studio 2010 Program ~~~~~~~ 9:00 Invited Talk Visual F#: Bringing F# to Visual Studio 2010 Luke Hoban (Microsoft Research) 10:00 Break 10:30 Probabilistic programming using first-class stores and first-class continuations Oleg Kiselyov (FNMOC); Chung-chieh Shan (Rutgers University) 10:55 Effective progamming in ML Daan Leijen (Microsoft Research); Ross Tate (University of California, San Diego) 11:20 Discussion 11:30 First-class modules and composable signatures in Objective Caml 3.12 Alain Frisch (LexiFi); Jacques Garrigue (Nagoya University Graduate School of Mathematics) 11:55 First-class modules: hidden power and tantalizing promises Jeremy Yallop (Applicative Ltd); Oleg Kiselyov (FNMOC) 12:20 Discussion 12:30 Lunch break 14:00 Deriving a Typed Implementation for Coroutines in ML Konrad Anton (Universitat Freiburg); Peter Thiemann (Universitat Freiburg) 14:25 The Design Rationale for Multi-MLton Suresh Jagannathan (Purdue University); Armand Navabi (Purdue University); KC Sivaramakrishnan (Purdue University); Lukasz Ziarek (Purdue University) 14:50 Discussion 15:00 Mirage: high-performance ML kernels in the cloud Anil Madhavapeddy (University of Cambridge); Thomas Gazagnaire (INRIA Sophia Antipolis) 15:25 Hosting a Standard ML compiler in a Web Browser: Status Report Martin Elsman 15:50 Discussion 16:00 Break 16:30 A simple and effective method for assigning blame for type errors David MacQueen (University of Chicago) 16:55 The MetaOCaml files: Status report and research proposal Oleg Kiselyov (FNMOC); Chung-chieh Shan (Rutgers University) 17:20 Discussion 17:30 Closing Registration and Local Arrangements ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * https://regmaster3.com/2010conf/ICFP10/register.php * http://www.icfpconference.org/icfp2010/local.html Program Chair ~~~~~~~~~~~~~ Matthew Fluet Rochester Institute of Technology Program Committee ~~~~~~~~~~~~~~~~~ Kathleen Fisher AT&T Labs Research Adam Granicz IntelliFactory Daan Leijen Microsoft Research Johan Nordlander Lulea University of Technology Sungwoo Park Pohang University of Science and Technology Daniel Spoonhower Google

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Is there a pure Haskell gzip/bzip compression module out there?
by C K Kashyap 12 Aug '10

12 Aug '10

Hi All, I looked at Hackage and found a couple of pure Haskell modules for un-compression. Are there any for comression? I found Huffman compression - but what I need is something that I can uncompress with standard tools. -- Regards, Kashyap

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