July 2007 - Haskell-Cafe

Re: [Haskell-cafe] Callback functions?
by Dan Piponi 02 Jul '07

02 Jul '07

On 7/2/07, Hugh Perkins <hughperkins(a)gmail.com> wrote: > Anyway, so the question is: how do we write callback functions in > FP/Haskell? Can someone provide a simple, but functional example? What makes a callback different from any other kind of function? In Haskell, as in other functional programming languages, you're completely free to pass functions into other functions and store them in datastructures. Take a look at the 'callbacks' used in the HOpenGL examples. They're just ordinary functions: http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/libraries/GLUT/examples/R… -- Dan

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Re: [Haskell-cafe] Very simple parser
by Gregory Propf 02 Jul '07

02 Jul '07

This was a bit baffling too. It appears that there's an implied argument to runTick. This also works and makes it more explicit. I suppose the compiler just works out that the only place to put the 'n' is after tick. runTick :: Int -> (String,Int) runTick n = runState tick n ----- Original Message ---- From: Arie Peterson <ariep(a)xs4all.nl> To: haskell-cafe(a)haskell.org Sent: Monday, July 2, 2007 4:51:59 PM Subject: Re: [Haskell-cafe] Very simple parser To get you going, start with the example from the documentation, modified slightly: > tick :: State Int String > tick = do > n <- get > put (n+1) > return (show n) If you want to actually run 'tick', use the 'runState' function: > runTick :: Int -> (String,Int) > runTick = runState tick ____________________________________________________________________________________ Sucker-punch spam with award-winning protection. Try the free Yahoo! Mail Beta. http://advision.webevents.yahoo.com/mailbeta/features_spam.html

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Very simple parser
by Gregory Propf 02 Jul '07

02 Jul '07

As a programming exercise I'm trying to use the State monad to create a simple parser. It's for a very simple assembly language for a simple virtual machine. The state is a string of instructions. I want to be able to call something like getNextInstruction to pull out the next instruction and then update the state (string). I know I can do this non-monadically by just passing the string explicitly each time but I'd like to learn more about the State monad. I also know about Parsec and Happy and so forth but this is just an exercise so I want to do it this way. Any ideas? I can't seem to get anything to work. I've tried different things but I suspect I'm just missing something basic. Can someone post a simple prototype for this? Just assume the instructions are integers. ____________________________________________________________________________________ Get the Yahoo! toolbar and be alerted to new email wherever you're surfing. http://new.toolbar.yahoo.com/toolbar/features/mail/index.php

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Callback functions?
by Hugh Perkins 02 Jul '07

02 Jul '07

This sounds something like using a continuation function. In imperative languages, we might have something like: class MyDisplayForm { void ButtonGo_Press() { Processor.Go( new CallbackDelegate( this.SetStatus ) ); } public void SetStatus( string message ) { StatusLbl.Text = message; Application.DoEvents(); } } So: we call Processor.Go, passing in a pointer to the function SetStatus (a "delegate" in C# terminology, an "interface" in Java, a pointer in C, ...). The Processor.Go function is going to update the status bar in MyDisplayForm as it goes along with useful messages "Processing blah.txt...", etc. It does that by calling the passed in "callback" function, with the appropriate status message as a parameter. This is also implicit to the Observer pattern, which may or may not be useful in FP (?) Anyway, so the question is: how do we write callback functions in FP/Haskell? Can someone provide a simple, but functional example?

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XmlSerializer.deserialize?
by Hugh Perkins 02 Jul '07

02 Jul '07

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Re: [Haskell-cafe] N00b question
by Jim Burton 02 Jul '07

02 Jul '07

poop wrote: > > So I'm working my way thorough haskell, doing some programming problems, > and I have this one so far: > Hi, I haven't spotted the problem in your code but there's an alternative solution to Euler Problem 14 on the wiki: http://www.haskell.org/haskellwiki/Euler_problems/11_to_20#Problem_14 -- it may be helpful as a comparison? Regards, > p14next n = if (mod n 2 == 0) then (div n 2) else (3*n + 1) > > p14seqlen n = f' 1 n where > f' accum n > | n == 1 = accum > | otherwise = f' (accum + 1) (p14next n) > > sndmax a b = if snd a > snd b then a else b > > p14 n = fst (f' (0,0) n) where > f' maxTuple n > | n == 0 = maxTuple > | otherwise = f' (sndmax maxTuple (n, p14seqlen n)) (n-1) > > the goal is to be able to run: > p14 999999 > > and not get a stack overflow message. I read what was available on tail > recursion and I think I have it working with tail recursion now, at least > it gives me a stack overflow message really quickly compared to before :) > > I have not been able to find any documentation on seq and ($!) which was > also mentioned as a way to get rid of stack overflow messages so if those > are what is required an explanation of those would be most helpful. > > -- View this message in context: http://www.nabble.com/N00b-question-tf4010616.html#a11395620 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Tree Guidance
by Hans van Thiel 02 Jul '07

02 Jul '07

Hello All, Both CFP and SOE have chapters on trees and there is a standard library Data.Tree. I expected to find all kinds of functions there, as in Data.List, but instead the functions are defined as instances of more general structures. It uses: import Control.Applicative (Applicative(..), (<$>)) import Control.Monad import Data.Monoid (Monoid(..)) import Data.Sequence (Seq, empty, singleton, (<|), (|>), fromList, ViewL(..), ViewR(..), viewl, viewr) import Data.Foldable (Foldable(foldMap), toList) import Data.Traversable (Traversable(traverse)) import Data.Typeable When looking at those it is not clear (to me) what those all mean and I have not been able to find much on the WiKi either. Could someone with more experience provide some guidance on how to learn to use these libraries (and ultimately on the tree type in Data.Tree, which is what I want). There is a Zipper Monad with defined functions, which look promising, but I have not heard of this before, and it appears not to be standard. http://www.haskell.org/haskellwiki/Zipper_monad/TravelBTree http://www.haskell.org/haskellwiki/Zipper_monad Does anybody have any experience with these libraries they could share? Ideally I would want a N-ary tree, where the branches are what's actually the represented data, and so I would like to access it both from the root and the leaves. In an imperative language I would just add an up-pointer in each node, but I have no idea how expensive this would be in Haskell, or if it's necessary at all. Many thanks for your help, Hans van Thiel

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Re: [Haskell-cafe] Re: Parsers are monadic?
by Gregory Propf 02 Jul '07

02 Jul '07

Thanks, that was helpful. I didn't realize that there were pure functional monads. ---------------------------------------------------------- "Monadic" just means a calculation using a mathematical structure called a monad. All impure calculations in Haskell are monadic, but not all monadic calculations are impure. Does this answer your question? --Eric _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ____________________________________________________________________________________ Be a better Globetrotter. Get better travel answers from someone who knows. Yahoo! Answers - Check it out. http://answers.yahoo.com/dir/?link=list&sid=396545469

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TT check
by Andrew Coppin 01 Jul '07

01 Jul '07

Can somebody check that I've implemented this correctly? *****X*X*X*XX***X*X*XXX*X*X*XX***X*X*X*XX**X*X*XX**X*X*X*XX ***X*X*XXX*X*X*XX**XX*X*XX***X*X*XXX*X*XX****X*X*X*XX****X* X*X*XX***X*X*XXX*X*X*XXX*X*XXXX****X*X*XXX****X*X*X*XX***X* X*XXX*X*X*XXX*X*XXXX A parser for this is given by decode (c:cs) = case c of 'X' -> X '*' -> let (e0,cs0) = decode cs; (e1,cs1) = decode cs0 in (e0 `apply` e1, cs1) The letter X stands for the following combinator: X = \x -> xSK K = \xy -> x S = \fgx -> fx(gx) Assuming my Haskell code is correct, the above *should* be a program to compute the sum of the Church numberal 2 and the Church numeral 2. In other words, this is the world's most verbose way to define the act of putting two and two together. ;-) But... I don't know... that's A LOT of expression there! Have a slipped up somewhere? Does anybody have a way to check?

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Name Decoration and Undefined References
by SevenThunders 30 Jun '07

30 Jun '07

I have a very complex project that has to play nice with a lot of C code written by other people. My own code is half Haskell and half C. My build process is rather complex since I generate about 5 different libraries, some of them export Haskell routines. A supreme effort was made to try to make Haskell somewhat invisible, although ultimately it requires a lot of gcc command line work with a bunch of undefines and a links to Haskell static libraries. In the end I'm not sure if it's such a good idea and probably would be better off completing the final link stage just using ghc. In the process of doing all this I attempted to link one of my libraries to some test code written in Haskell and I proceeded to get a few unresolved links whilst running ghc. What happened is that I have a Haskell binary, lets say called foo.o generated from foo.hs that appears in two libraries say called libmyh.a and libmyc.a and that contains a Haskell function called bar. Library libmyh.a ultimately gets built using cabal and libmyc.a gets built using gcc directly. The binary is compiled twice (for complicated reasons) prior to linking to each library. However in the library libmyh.a, the bar symbol is prepended with an ascii string that contains the library name (say libmyh). However the other library shows that the symbol does not have this name decoration since it was linked directly with gcc. It appears however when trying to link against libmyc.a that ghc expects to see the original name decoration for bar. I presume this is due to some side information, perhaps what's contained in foo.hi that informs ghc what names to actually link to. I now ask the question, how can I control this process? Build tools such as CMake and Cabal seem to have their own ideas about what directories to place binaries. It seems like I need to somehow make sure that all files related to the compilation process of a given library are isolated in the same directory or something. Thus it's not enough to specify binaries on the command line of ghc, it's got to have access to some other files; I presume the .hi files. This makes things a bit confusing I'm afraid. -- View this message in context: http://www.nabble.com/Name-Decoration-and-Undefined-References-tf4003458.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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