April 2009 - Beginners

Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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Selecting a GUI toolkit
by Daniel Carrera 02 May '09

02 May '09

Hello, This question is just idle curiosity. I don't actually plan to write a desktop GUI program. With that said, what library would you recommend to someone who wants to write a cross-platform desktop app with Haskell? Consider the following requirements: 1) Cross-platform. Windows, Linux and preferably OS X too. 2) Maintained, stable, mature, documented, etc. 3) Easy to use. Notice that this has much to do with the Haskell bindings. For example, Qt might be a fabulous toolkit, but qtHaskell might be immature (it looks like it started in Dec 2007). wxHaskell looks good. I like the idea behind wxWidgets, but I have never used it (or any GUI toolkit for that matter). Thank you for your input. Cheers, Daniel.

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Simplifying a 'split' function
by Ben Sanders 01 May '09

01 May '09

In looking through a merge sort implementation example, I came across this code: split :: [a] -> [[a]] split [] = [] split (x:xs) = [x] : split xs I would have written the same code as split :: [a] -> [[a]] split = map (: []) Is there any particular difference here (other than explicit recursion)? And is there any other nicer way to write it? Thanks, Ben Sanders

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List Function
by Nathan Holden 01 May '09

01 May '09

I don't know what you'd call it. Is there a function in any of the basic functions that does this something like this: Sending two lists, [1,2,3] and [2,3,4] it would return [(1,4),(1,5),(1,6),(2,4),(2,5),(2,6),(3,4),(3,5),(3,6)]. I managed to code my way into returning a list of lists, which works. But it seemed like a very basic list/matrix function, so I honestly believe that the Haskell designers probably would've put it in.

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Parsec tutorial
by Patrick LeBoutillier 01 May '09

01 May '09

Hi all, I'm looking for a Parsec tutorial that can get me started in writing a simple SQL parser. I can't seem to find a lot using Google. Does anyone have some resources that could be helpful? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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P.S. "Haskell: Craft of Functional Programming"
by Michael P Mossey 30 Apr '09

30 Apr '09

Oh, and I got HCFP at the local Border's. Apparently it's selling well enough to include it in a general bookstore. Look out Stephen King---Simon Thompson is coming!

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putting in a good word for "Haskell: The Craft of Functional Programming"
by Michael P Mossey 30 Apr '09

30 Apr '09

I'm working through "Haskell: the Craft of Functional Programming" (by Simon Thompson) in depth, and I want to put in a good word for it. His exercises are numerous and they seem to lead me from simpler ideas to more complex ideas in a natural way. As part of learning Haskell, I actually started with other books and tutorials and skipped around in them, reading some of the later chapters before the earlier ones. I like that sometimes. But I'm finding that Thompson's book is helpful for getting an intuition for the language, as opposed to just reading about its features, so I'm working it in order. My other books include "Real World Haskell" and "School of Expression." The wonderful thing about RWH is that it's available online (I do own a hardcopy) where people comment on the text and exercises. -Mike

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International Summer School on Advances in Programming Languages
by voigt＠tcs.inf.tu-dresden.de 30 Apr '09

30 Apr '09

International Summer School on Advances in Programming Languages 25th-28th August, 2009 Heriot-Watt University, Edinburgh, Scotland http://www.macs.hw.ac.uk/~greg/ISS-AiPL Overview This four-day residential International Summer School on Advances in Programming Languages has a major theme of Concurrency, Distribution, and Multicore. Intended primarily for postgraduate research students, the School offers lectures and practical sessions on an engaging blend of cutting edge theoretical and practical techniques from international experts. The Summer School is supported by the Scottish Informatics and Computer Science Alliance (http://www.sicsa.ac.uk/) a Scottish Funding Council Research Pool. Participants from SICSA member institutions may attend at no cost. Confirmed Topics/Speakers - Static and dynamic languages, Prof Phillip Wadler, University of Edinburgh - Compiler technology for data-parallel languages, Dr Sven-Bodo Scholz, University of Hertfordshire - New applications of parametricity, Dr Janis Voigtlaender, Technical University of Dresden - Automatic vectorising compilation, Dr Paul Cockshott, University of Glasgow - Foundational aspects of size analysis, Prof Marko van Eekelen/Dr Olha Shakaravska, Radboud University Nijmegen - Context oriented programming, Dr Pascal Costanza, Vrije Universiteit Brussel - Multi-core programming, Dr Phil Trinder, Heriot-Watt University - Multi-core compilation, Dr Alastair Donaldson, Codeplay Software Ltd - Principles and Applications of Refinement Types, Dr Andrew D. Gordon, Microsoft Research, Cambridge - Resource aware programming in Hume, Prof Greg Michaelson, Heriot-Watt University/ Prof Kevin Hammond, University of St Andrews - Haskell concurrency & parallelism, Dr Satnam Singh, Microsoft Research, Cambridge Location The Summer School is based at Heriot-Watt University's Riccarton campus, set in pleasant parkland to the west of Edinburgh, with easy access to the airport, city and central Scotland: http://www.hw.ac.uk/welcome/directions.htm. The Summer School immediately precedes the 2009 International Conference on Functional Programming (http://www.cs.nott.ac.uk/~gmh/icfp09.html) and takes place during the Edinburgh International Festival (http://www.eif.co.uk/) and the associated Edinburgh Festival Fringe (http://www.edfringe.com/) and Edinburgh International Book Festival (http://www.edbookfest.co.uk/) Steering Committee Prof Greg Michaelson, Heriot-Watt University (Convenor) (mailto:G.Michaelson@hw.ac.uk) Prof Kevin Hammond, University of St Andrews Dr Patricia Johann, University of Strathclyde Prof Phillip Wadler, University of Edinburgh

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Need Help
by kaushal Pathak 29 Apr '09

29 Apr '09

Hi There, Here is a very simple haskell program I have written to find whether a number is prime or not, which doesn't give me any compilation error but gives following run-time error : Any help would help me move ahead, I know that the type concepts is what I am lacking, anly pointer to simple/good article/paper would definitely help. Lots of thannx jut even to look at the mail :-) Regards Kaushal *Run Time Error :* kaushal > isPrime1 171 <interactive>:1:0: Ambiguous type variable `t' in the constraints: `Integral t' arising from a use of `isPrime1' at <interactive>:1:0-11 `Floating t' arising from a use of `isPrime1' at <interactive>:1:0-11 `RealFrac t' arising from a use of `isPrime1' at <interactive>:1:0-11 Probable fix: add a type signature that fixes these type variable(s) kaushal > *Program :* isPrime1 x = let canDivide num 0 = 0 canDivide num 1 = 0 canDivide num divisor = if ((mod num divisor) == 0) then 1 else canDivide num (divisor - 1) in if ( x == 1 ) then putStrLn("1 is Neither prime nor composite!!!") else if ((canDivide x first_div) == 1) then putStrLn(show(x) ++ " is not a Prime Number!!!") else putStrLn(show(x) ++ " is a Prime Number!!!") where first_div :: Integral a => a ; first_div = round (sqrt x)

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Working With TVars
by aditya siram 29 Apr '09

29 Apr '09

Hi all, I have a tuple inside a TVar : > type MySTM = TVar (Int,Int) Whenever I want to edit/read 'a' or 'b' I find myself writing : > editFunction mySTM = do (a',b') <- readTVar mySTM dostuff a' ... This is boilerplate stuff, so I decided to write some accessors. So far I have : > getA , getB :: MySTM -> STM Int > getA mySTM = do > (a',b') <- readTVar mySTM > return a' > > getB mySTM = do > (a',b') <- readTVar mySTM > return b' I want to be able to use these accessors like so: > doSomethingWithA mySTM = do > case (getA mySTM) of > 1 -> doStuff > 0 -> doSomethingElse But getA returns an STM Int, so I still have to do a : > doSomethingWithA = do > a' <- (getA mySTM) > case a' of > 1 -> doStuff > 0 -> doSomethingElse This doesn't really save me a lot of boilerplate. What is the best way of writing a function that just returns my values do I can work with them in the STM monad without unpacking them all the time? Thanks , Deech

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