- Beginners - Haskell.org

Functional Data Structure with Cheapest Modification?
by Travis Erdman 07 Apr '10

07 Apr '10

In Real World Haskell, the authors state that "the attraction of a tree to a functional programmer is cheap modification." They go on to explain that in a tree of say 10,000 nodes, approx 9,985 would be reused (on average) when updating a single node (rather than having to copy all 10k nodes, like a Haskell list would have to do). In my application, I have a collection of (only) about 200 objects. So from the surface, not large at all. However, each object in the collection is a large tree that might be up to 20 MB in size. The application pulls objects one at a time from the collection, performs some processing, and delivers a new, updated object that needs to be inserted in the collection for future processing (the original object is now obsolete). I'm guessing a Data.IntMap might involve copying over about 8 objects for each single object update, which while better than copying all 200, is still not perfect. What would be the most efficient Haskelly way to maintain my collection? I'm wondering also if there is some sort of "pointer list" I could use. I think recreating a list of 200 pointers (199 of which are unchanged) might be a pretty efficient solution, if it exists. Sadly, I've tried to understand Mutable Arrays (of a few different flavors), but all the monadic stuff is over my head at the moment, and I couldn't find sufficient documentation/tutorial that I could understand to implement them.

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haskell platform
by Irvin 05 Apr '10

05 Apr '10

Hi I'm having trouble installing Haskell platform using Ubuntu (Karmic Koala). When I run ./configure I get the message configure: error: The Haskell Platform version 2010.1.0.0 requires ghc-6.12.1 It is recommended that you upgrade to ghc-6.12.1 however if you really want to use this release with an unsupported version of GHC then use ./configure --enable-unsupported-ghc-version In synaptic the latest version of ghc is 6.10.xx; I've tried doing ./configure --enable-unsupported-ghc-version, but make fails to set up Cabal. Any suggestions? Should I just try to set up everything piece-meal? Do I need to remove the currently installed version of ghc using synaptic before I try to use the 6.12.1 installer or build it from source. Thanks. Best, Irvin

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recursively retrieving widgets handlers in wxhaskell
by legajid 05 Apr '10

05 Apr '10

Hi, trying wxHaskell GUI interface, i would like to get a list of all widgets in a frame. My frame is composed of 4 widgets and a panel containing 2 widgets. wlf <- get frame children gives a list of 5 widgets (4 + 1 panel) wlp <- get panel children gives a list of 2 widgets Making wlf ++ wlp gives the whole list of widgets in the frame. But, for a more general use, i want to explore recursively the hierarchy from frame, without having to explicitly know wether the frame contains other containers ... Since the <- syntax to get a list of children, i can't figure out how to write the recursive function to get all children of a frame. I can't write (x <- get widget children) ++ f (rest of list of widgets) , as for usual functions using lists. Thanks for helping. Didier.

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(->) instance for ArrowApply and laziness
by aditya siram 03 Apr '10

03 Apr '10

Hi all, Could someone help me understand how the (->) instance for ArrowApply works? It looks like this: > instance ArrowLoop (->) where > loop f b = let (c,d) = f (b,d) in c This models recursion for arrows and I need help understanding how it does that. I have a strong feeling it has to do with the magic value 'd'. I've read the Patterson tutorial but I still don't grok it. Thanks! -deech

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Question about IO, particularly hGetContents
by Ken Overton 02 Apr '10

02 Apr '10

Hello all, I was playing with some Haskell code that read in text files and processed them and often found myself writing empty result-files. I've pared the problem down to the following small example. -- example program import IO main = do rhdl <- openFile "test.in" ReadMode content <- hGetContents rhdl putStrLn content -- if I cmt out this line, content will be empty hClose rhdl putStrLn "Content: " putStrLn content -- if the first 'putStrLn' call was commented, this will print a blank line -- end example For some reason, if I comment out the 'putStrLn content' between hGetContents and hClose, the data from the hGetContents call is not stored. Can somebody verify this behavior and (if so) explain why it's happening? Thanks, -- kov

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Addable function combinators
by Edgar Klerks 31 Mar '10

31 Mar '10

(I am sorry for the spam, but my browser seems to chopoff the reply-to field. I have changed mailclient so I can manually add it. Hopefully the issue is resolved) Hi Brent, I already solved it myself. I wanted to find an systematic way to produce composition operators which composes functions with different arity together. I stated it a bit vague. For example <*||> is (c -> d) -> (a -> b -> c) -> a -> b -> d I can add this types with the composition operator to create a new operator which combines functions with different arity. In this case a function with arity 1 with a function with arity 4: <*||>.<*||> = <*||||> But I couldn't find an systematic way to level the arity of the function on the left side. I wanted to find an operator what did this, like so: <*|> `op` <*|> = <**|> But I found another way already, which can be found in the followup. So I guess this question is closed. But thanks anyway for your response. I really appreciate the way the people on this list react to new haskellers. Your are really helpful, which sometimes on other lists is quitte the oposite. Greets, Edgar

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Addable function combinators
by Edgar Klerks 31 Mar '10

31 Mar '10

Hi Brent, I already solved it myself. I wanted to find an systematic way to produce composition operators which composes functions with different arity together. I stated it a bit vague. For example <*||> is (c -> d) -> (a -> b -> c) -> a -> b -> d I can add this types with the composition operator to create a new operator which combines functions with different arity. In this case a function with arity 1 with a function with arity 4: <*||>.<*||> = <*||||> But I couldn't find an systematic way to level the arity of the function on the left side. I wanted to find an operator what did this, like so: <*|> `op` <*|> = <**|> But I found another way already, which can be found in the followup. So I guess this question is closed. But thanks anyway for your response. I really appreciate the way the people on this list react to new haskellers. Your are really helpful, which sometimes on other lists is quitte the oposite. Greets, Edgar

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hiding documentation for non-exposed modules
by Ashish Agarwal 31 Mar '10

31 Mar '10

I'm using cabal to build and install a new library I'm writing. In the .cabal file I've exposed only a single module. However, the haddock generated documentation includes even the non-exposed modules, which is not what I want since end-users should not see these. Is there a way to tell cabal to build documentation only for exposed modules?

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What should I use (the equivalent of Windows schtasks) to schedule a task to run?
by Hong Yang 30 Mar '10

30 Mar '10

I searched "schedul" on hayoo!, but had no clue. It seems that I can use scheduleEventAt from Control.Monad.Event.Classes. But what is type "e"? It seems like an event. How do I define an event? I have not learned Monad yet. Can some one give me a simple example? Suppose I want to run an external program "pg" which in Haskell's language has the type of String -> IO (). Thanks, Hong

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Addable function combinators
by edgar klerks 30 Mar '10

30 Mar '10

He All, I was toying a bit around and found a function, which combines an function of arity 1 with 2. Then I produced a whole sequence of these function, the number of * stands for the arity of the left function and the number of | stands for arity the right function. so <*|||> :: (b -> c -> d) -> (a -> c) -> a -> b -> d: (<*****|>) = flip (<*|||||>) (<****|>) = flip (<*||||>) (<***|>) = flip (<*|||>) (<**|>) = flip (<*||>) (<*|>) = (.) (<*||>) = (<*|>).(<*|>) (<*|||>) =(<*||>).(<*|>) (<*||||>) = (<*||>).(<*||>) (<*|||||>) = (<*||||>).(<*|>) con = (flip.) .(.) (<**||>) = (<*||>).flip (<**|||>) = (<*|>).(<**||>) (<**||||>) = (<*|>).(<**|||>) (<***||>) = (<*||>).(con flip) (<***|||>) = (<*|>).(<***||>) (<****||>) = (<*||>).(con.con $ flip) I found it interesting, but now I am stuck. is it possible to make the stars addable instead of the | or both? Greets, Edgar Edgar

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