January 2013 - Beginners

Type Inference
by Johannes Engels 16 Jan '13

16 Jan '13

Hello, playing around with type classes, I came up with the idea to implement Matlab-like array slices by overloading the (!) - Operator. My first try goes > {-# LANGUAGE MultiParamTypeClasses , > FunctionalDependencies , > FlexibleContexts , > FlexibleInstances #-} > import Data.Array.Unboxed hiding ((!)) > import qualified Data.Array.Unboxed as AU > class Sliceable arrtype indtype resulttype | arrtype indtype -> resulttype > where (!) :: arrtype -> indtype -> resulttype > instance (Ix i, IArray UArray e) => Sliceable (UArray i e) i e > where (!) = (AU.!) > instance (Ix i, IArray UArray e) => Sliceable (UArray i e) [i] (UArray Int e) > where (!) arr ilist = listArray (0,end_ind) [(AU.!) arr i | i <- ilist] > where end_ind = length ilist - 1 In principle this seems to work, for instance ghci> let arr = listArray (0,99) [0..99] :: UArray Int Double ghci> arr ! (17::Int) 17.0 ghci> arr ! [13..15 :: Int] array (0,2) [(0,13.0),(1,14.0),(2,15.0)] However, the ugly type annotations (::Int) are necessary, otherwise I get an error message: > arr ! 17 No instance for (Sliceable (UArray Int Double) indtype0 resulttype0) arising from a use of `!' Possible fix: add an instance declaration for Sliceable (UArray Int Double) indtype resulttype0 in the expression: arr ! 17 whereas the type annotation is not necessary for the original Array.Unboxed - (!) : > (AU.!) arr 17 17.0 Could somebody please explain why the type annotation is necessary in my case? Is there a trick to circumvent this? Any hint would be welcome! Best regards Johannes

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kind of path search
by Olivier Scalbert 15 Jan '13

15 Jan '13

Hello, I am a beginner in Haskell and in functional programming. Here is the problem I want to solve with Haskell. I am sure that this fascinating language can do it !!!! Suppose I have N nodes: node1, node, ..., nodeN At each node I have a subset of values taken from [v1, v2, ..., vk]. For exemple: node1 : [v1, v3] node2 : [v2, V5, v6] node3 : [v2, v3] I have also a directed graph. From a value you can go to other values. v1 -> v5 v1 -> v6 v2 -> v1 v2 -> v2 v3 -> v2 v6 -> v2 Now, I want to get all the paths from node1, to nodeN, following the node values and the graph. node1.v1 -> node2.v6 -> node3.v2 is a path. node1.v3 -> node2.v2 -> node3.v2 is also a path. My problem size: N around 1000 and each node has 3 to 4 values taken from [1..10]. Thanks for your help !!! Olivier

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Re: [Haskell-beginners] Natural functions versus existential types
by Darren Grant 14 Jan '13

14 Jan '13

On Wed, Jan 9, 2013 at 1:45 PM, Aleksandar Dimitrov < aleks.dimitrov(a)gmail.com> wrote: > On Sun, Jan 06, 2013 at 11:22:51PM -0800, Darren Grant wrote: > > I've seen some cases where Haskell examples dive into existential types > > instead of using natural higher order functions. For instance, the > > "Existential type" wiki page [1] section 2.2 proposes existentials as the > > solution to a heterogeneous list problem where a ray-tracer must > evaluate a > > trace function for different types of objects. Is there a good reason for > > this, or is it just a case of prior language assumptions leading to > > unnatural code? Why could I not just make the list homogeneous by > supplying > > a list of partially evaluated trace functions instead? > > I have never written a ray-tracer, so I don't know about the exact > requirements, > but depending on the code in question, partial evaluation might not be > possible. > > Parametric types can only be instantiated with *one* specific type. A list > [a] > can only be either [Int] or [Bool], but not [Int,Bool] or so. [1] > > So whenever you'd like to have a collection of things that won't have the > same > type, but have a certain operation defined on them, existential types come > in > handy. A partial application only makes sense when you have a function of > higher > arity, but `hits` in the example on the Wiki has only one argument, namely > the > list. At some point, a *collection* of Renderables needs to pass by the > function > `hit`. This collection probably needs to be stored somewhere, presumably a > list, > that is then fed to `hits`. > > Of course, you might set up your types differently, so that everything > that will > need to pass by the function `hits` would also be of one certain type. You > could, for example, instead of storing the heterogeneous Renderables in a > list, > just reduce the different shapes to a Renderable data type that contains > the > important fields. Then, whenever a geometric object gets created, a > Rendereable > also gets created, then stored in that list. But how *viable* that is, I > don't > know. My gut feeling says that it won't be viable at all. Consider the following: If I have a traceable data type like data Sphere, I can define it and a trace function as follows: data Sphere = Sphere { spherepos :: (Double,Double,Double), sphereradius :: Double } traceSphere :: Sphere -> Ray -> [Hit] Where Ray is self-explanatory and [Hit] is a resulting list of hits where ray intersects sphere. I can then define a list of traceable objects thus, [Ray -> [Hit]] Which can also be thought of as, type Trace = Ray -> [Hit] [Trace] Now I am able to define a homogeneous list [Trace] of partially applied functions like so, [traceSphere $ Sphere (0,0,0) 20, traceSphere $ Sphere (10,20,30) 40, traceBox $ Box (0,0,0) (10,10,10) -- where traceBox is also partially applied ] You see what I mean? This is very natural in Haskell. I'm sure there must be counter-examples, but I haven't the comprehension for these. What do you think? > Sometimes I read an argument that existentials are required due to unknown > constraints at compile time, but do not have an intuition for this > > situation yet. For instance, I read that IO requires existentials. Still > > working on that one. :) > > Where did you read that? The *definition* of the IO type does not use > existentials [2]. But I'm not very familiar with the internals. You're right! I'm not sure what I saw, but I would guess that I conflated some application of IO with the definition. Cheers, Darren

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Game of Life try
by Емануела Моллова 14 Jan '13

14 Jan '13

Hello all! :) I'm new to Haskell and I would like to try to implement a simple version of Game of Life. I can imagine it in C++ for example, but I have a little difficulties in Haskell. All I can think for is a mutable array with all elements - 0 in the beginning and the user to be able to write in the coordinates of the cells, which are alive and this cells to become 1. Then to iterate through the array (but I'm not sure if I can iterate, there is no 'while' here) and to make the changes (but maybe I will need a new array for the changes, because when the first change happens, it will affect the result). Also I was thinking of how to make it visible, so I tried to make all 0-s red and all 1-s green, and after each iteration to clear the screen with ANSI so that it looks a bit like animation, but none of these ideas work... Here is an orientation in my ideas: import Data.Array.IO import System.Console.ANSI main :: IO () main = do arr <- newArray ((1,1), (10,10)) 0 :: IO (IOArray (Int, Int) Int) writeAraay arr (1,1) 1 a <- readArray (1,1) setSGR [SetColor Foreground Dull Red] putStr [a] Could you please suggest me what is a good place to store the information about my cells (mutable, unmutable array, lists, tuples...) and any ideas and tutorials at all would be really appreciated! Thank you very much in advance! :)

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Is http://hackage.haskell.org/packages/hackage.html having a challenging serving up archive.tar?
by KC 11 Jan '13

11 Jan '13

:) -- -- Regards, KC

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A Week in and Clueless
by Philip Cote 11 Jan '13

11 Jan '13

So a week into Haskell, I still seem to be not “getting” it which is kind of weird in my case. I came in with knowledge of a lot of functional ideas from using them in Javascript and Python. Or at least I thought I knew them. 5 chapters into "Learn You a Haskell", I admit it's not really sinking in for me even after typing in and running all the examples. I acknowledge that I don't know jack. Any ideas or exercises that might help me along?

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Cabal problems
by Abram Demski 10 Jan '13

10 Jan '13

Hi all, I'm just trying to install some of the games from the game list (on the wiki) so that I have something to look at. I tried the ones which have cabal packages as I assumed that would be easy. Every one I tried so far has failed to install. Below are two of the quickest failures. Any comments are appreciated. Is there something I'm generally not getting? Or are there just various problems with installing possibly-old packages? abram@whitehead:~$ cabal install shu-thing Resolving dependencies... Configuring Shu-thing-1.1.1... Building Shu-thing-1.1.1... Preprocessing executable 'shu-thing' for Shu-thing-1.1.1... [1 of 1] Compiling Main ( Shu-thing.hs, dist/build/shu-thing/shu- thing-tmp/Main.o ) Shu-thing.hs:4:27: Module `Control.Exception' does not export `Exception(ExitException)' cabal: Error: some packages failed to install: Shu-thing-1.1.1 failed during the building phase. The exception was: ExitFailure 1 === abram@whitehead:~$ cabal install spaceInvaders Resolving dependencies... Configuring HGL-3.2.0.0... Building HGL-3.2.0.0... Preprocessing library HGL-3.2.0.0... [ 1 of 27] Compiling Graphics.HGL.Internals.Flag ( Graphics/HGL/Internals/Flag. hs, dist/build/Graphics/HGL/Internals/Flag.o ) [ 2 of 27] Compiling Graphics.HGL.Internals.Utilities ( Graphics/HGL/Internals/Utilities.hs, dist/build/Graphics/HGL/Internals/Utilities.o ) Graphics/HGL/Internals/Utilities.hs:33:31: Expecting one more argument to `E.Exception' In the type signature for `safeTry': safeTry :: IO a -> IO (Either E.Exception a) cabal: Error: some packages failed to install: HGL-3.2.0.0 failed during the building phase. The exception was: ExitFailure 1 -- Abram Demski http://lo-tho.blogspot.com/

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hsmagic & core dump
by Sergey Bushnyak 10 Jan '13

10 Jan '13

Trying to work with images via hsmagic package, use something like this scaleTest :: FilePath -> IO() scaleTest dst = do initializeMagick image <- readImage dst let image' = scaleImage 450 450 image writeImage dst image' but get core dump, when compiling. Types are ok, and path are valid, can it be problem of os or I'm doing something wrong?

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Approach to resolve dependencies
by Sergey Bushnyak 09 Jan '13

09 Jan '13

Currently switched to ghc-7.6.1 and when I want to install new packages, they are broke others, because of use older versions. Observe this: $ cabal install fay Resolving dependencies... In order, the following would be installed: QuickCheck-2.5.1.1 (reinstall) ansi-terminal-0.5.5.1 (new package) ansi-wl-pprint-0.6.4 (new package) attoparsec-0.10.3.0 (reinstall) changes: containers-0.5.2.1 -> 0.5.0.0 ... and others Things like in last row with containers repeats in a lot of packages, which with --reinstall key broke them. So, I wondering what is the easiest and proper way to install new packages with respect to old dependencies? -- Best regards, Sergey Bushnyak

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"stage restriction" when using Data.Heap
by Martin Drautzburg 09 Jan '13

09 Jan '13

When I :load this into ghci import Data.Heap heap = empty :: MinHeap (Int,String) insert (1,"kjh") heap I get GHC stage restriction: `heap' is used in a top-level splice or annotation, and must be imported, not defined locally In the second argument of `insert', namely `heap' In the expression: insert (1, "kjh") heap But when I type in import Data.Heap let heap = empty :: MinHeap (Int,String) insert (1,"kjh") heap everything works and I get fromList [(1,"kjh")] Why is that so and what can I do to prevent the "stage restriction"? -- Martin

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