April 2010 - Beginners

Map on Nested Lists
by Ben Derrett 08 Apr '10

08 Apr '10

Hi, I'm trying to write a generalization of the map function that operates on nested lists, e.g. [[a]] or [[[a]]]. (It should map all elements of type a (not list) with f) I was thinking of something along these lines: mapN f *list of type [a] (where a is not a list type)* = map f l mapN f l = (mapN f (head l)):(mapN f (tail l)) Any suggestions about how to go about this? Many thanks, Ben -- Ben Derrett Department of Mathematics MIT Class of 2012 bderrett(a)mit.edu

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Adding module
by legajid＠free.fr 08 Apr '10

08 Apr '10

Hi, i need the System.IO.UTF8 module. But it is not in my installation package (windows xp, ghc 6.10.4); i can't import it, nor display html help about it. Where to find and how to install it ? Thanks, Didier.

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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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