August 2010 - Beginners

Argument order and partial function application
by Tom Murphy 19 Aug '10

19 Aug '10

Hi all, Thanks for this great resource. I've got a basic question about partial function application: Let's say I have a very basic function: volumeFunction length width height = length * width * height Its type signature is: volumeFunction :: Int -> Int -> Int -> Int This implies that I first pass it the length, *then* the width, *then* the height. So while it is very easy for me to write volumeWithLengthTen = volumeFunction 10 , it's not possible to create a volumeWithWidthTen or volumeWithHeightTen. I can see how I might be able to "hack" my way into accessing the "width" argument with infix functions, but there's no way at all that I can see to access the height. This seems arbitrary to me: my volumeFunction doesn't need them in the order they're given. Is there a way around this? Could I somehow combine three smaller functions? I appreciate any insight. Tom

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Restoring interleaved lists?
by Patrick LeBoutillier 19 Aug '10

19 Aug '10

Hi all, I'm parsing a file that represents a type of musical notation (Guitar Pro file format). In the format there are t tracks, each composed of m measures. In the file the measures are all stored sequentially, i.e. from measure 1 to measure t*m, in the following fashion: - measure 1 of track 1 ; - measure 1 of track 2 ; - ... - measure 1 of track t ; - measure 2 of track 1 ; - measure 2 of track 2 ; - ... - measure 2 of track t ; - ... - measure m of track 1 ; - measure m of track 2 ; - ... - measure m of track t ; I need a function that, given t and the list of t*m measures, can spilt the measures by track, returning a list of t lists, each containing m measures. I cannot figure out how to do this, even though it seems to me like it shouldn't be too hard... I can't figure out how to "update" the lists of lists when I want to add a new element. Do anyone have any ideas? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Question about ghc and cabal: finding packages
by C Gosch 19 Aug '10

19 Aug '10

Hi everyone, I have installed ghc 6.12.3 and the Haskell platform; now, when I install packages from hackage using cabal install <package-name>, they get installed in ~/.cabal, which is fair enough. However, when I call a runhaskell Setup.lhs configure that used to work on a different computer with the same software setup, ghc (or is it cabal?) complains that it cannot find the packages I have just installed. Checking again revealed that they are indeed installed, just not in the ghc installation directory, but under ~/.cabal. How can I tell cabal and/or ghc where to look for libraries? Thanks, again, for any help! Christian

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forM vs mapM
by Gaius Hammond 19 Aug '10

19 Aug '10

Hi all, Can anyone give me an example of a situation where you would use forM but *not* mapM or vice versa? As far as I can see one is simply a flip of the other. Does forM make any guarantees about order of execution? Thanks, G ------------------

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2 Questions: mutable data and copying results
by C Gosch 18 Aug '10

18 Aug '10

Hello to all Haskell experts! I have two more beginner questions: I ran into texts mentioning types of "mutable" data. Does that mean the data is actually changed in place? How would I go about implementing such a data type, as I expect much higher performance in some applications? Do I do that with the Foreign module and Ptr types? The second question is this: say I have some functions working with a data type, say a graph. Some of them might want to change the graph and return a new graph as a result. If they change say only one graph node, it would be extremely bad for performance if the whole graph got copied any time such a change takes place. Is the compiler smart enough to make the code only copy the parts that have changed (copying lazily, so to speak)? Is that even possible? Thank you in advance for any hint! Christian

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monad nomad gonad gomad
by prad 17 Aug '10

17 Aug '10

ok so taking MAN's suggestion: "I think It's time for you to get serious with the monads" that's just what i'm going to do! i found the following references: You Could Have Invented Monads! http://blog.sigfpe.com/2006/08/you-could-have-invented-monads-and.html an interesting way to become familiar with the idea Monads for the Working Haskell Programmer http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm seems to have several practical ideas there (though i don't understand them yet) A tour of the Haskell Monad functions http://members.chello.nl/hjgtuyl/tourdemonad.html a nice reference with brief explanations Explaining Haskell IO without Monads http://neilmitchell.blogspot.com/2010/01/haskell-io-without-monads.html possibly a good conceptual aid for background All About Monads http://www.haskell.org/all_about_monads/html/index.html this seems to have everything though it'll take some work to dig in in any case, monads seem to be a rather important concept to getting anything done in haskell. for instance, i have a program which is generating the output it is supposed to (i can print it), but i can't seem to get it into another function and keep getting the error i've seen so many times: Couldn't match expected type `[String]' against inferred type `IO [String]' so it's time to understand them. besides, the stuff looks rather intriguing and certainly appears to take one into computer language design theory. any sources others have found useful would be appreciated as well as suggestions on how to proceed through the above. in friendship, prad -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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hClose: illegal operation (handle is finalized) and/or <<loop>>
by Travis Erdman 17 Aug '10

17 Aug '10

> Perhaps in your {BOOLEANS} you are referring to > 'recurseChildren', which in turn needs the {BOOLEANS}? Uggh! Indirectly, yes, I am. OK, back to the drawing board for me on this one. Thanks!

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hClose: illegal operation (handle is finalized) and/or <<loop>>
by Travis Erdman 17 Aug '10

17 Aug '10

> > putStrLn $ show result > > > > <<loop>> > > That is the *real* error message. > > It means that where you define result, you refer to result > also on the right-hand side in a way that is an endless > loop instead of well-behaved recursion. > > We would have to see more of your code to be able to > say much more. Thank you very much ... now I have isolated the offending "loop" code: I am folding with a function that takes a tree and the input data, and delivers a "processed"/"updated" tree. The function that is used in the fold is "calcTree". calcTree is also recursive ... it updates the root node, then recursively calls itself to update each of its children, which is all working fine. Here are simplified versions of my structure: data Tree a = Node a [Tree a] calcTree (Node oldData oldsubtree) input = Node newData recurseChildren where {-calculate newData from oldData and input-} recurseChildren = if {BOOLEANS} then oldsubtree else map (\old -> calcTree old input) oldsubtree And, in words: if certain conditions become true, there will be no change in any decedent node from the current node (ie all newData = oldData, and tree structure never changes), so rather than waste exponential time traversing all the decedent nodes, I'd like to just re-use the ones I already have, as they are identical. This is "pruning" the tree. If I remove this pruning, the "<<LOOP>>" objection ceases and the code executes as expected ... except it takes lots longer than it should, because there are usually lots of oppt'ys for pruning, and I'd rather not explicitly traverse all of them. How can I make this work? Thanks again for all the A's to my Q's !

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Man/dir/help pages for haskell?
by Kevin Burke 17 Aug '10

17 Aug '10

Hello, I've been learning Haskell by working through the O'Reilly book, but a lot of the coding I do is offline. One thing that's been really helpful in other languages are the terminal instructions and detail for functions. Is there something similar for Haskell? I know I can type :help at the prompt but that only gives you info on the broad commands. Thanks for your help, Kevin

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hClose: illegal operation (handle is finalized) and/or <<loop>>
by Travis Erdman 17 Aug '10

17 Aug '10

I read in some data from a file (via decodeFile), process it, and write the result back to a file. My last line of code is .... encodeFile (filename++".sol") (gametype,gstree,newiterlist,newstamplist,result) and it is all of the sudden now causing this run-time error: mincfr: kclv5.sol: hClose: illegal operation (handle is finalized) It has been working fine for quite a while, but now quite mysteriously gives this error. (But obviously I have been heavily tinkering with the processing part of the program). If I remove the encodeFile line and instead ... putStrLn $ show result I get this run-time error: <<loop>> Help? thanks, Travis

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