- Beginners - Haskell.org

Help in Haskell
by Andre Paulo Machado 30 Nov '08

30 Nov '08

Hi folks..... Somebody could help me, please? Please, I would like a help in haskell programming, because I have with following question: I need to create a list of strings, like as a graph. See: initiaList = [ ("BSB,"SSA"), ("CNF","SSA"), ("CNF","GIG"), ("CNF","GRU"), ("GIG","CNF"), ("GIG","GRU"), ("GRU","BSB"), ("GRU","GIG"), ("GRU","CNF"), ("SSA","CNF") ] With this list above, I will create a function that returns a path with max number of links: example: maxLink = 4 (result until 4) command: myFunction "BSB" "GRU" 4 result: 1st Option: BSB -> SSA -> CNF -> GRU 2nd Option: BSB -> SSA -> CNF -> GIG -> GRU ---------------------------------------------------------------------------- command: myFunction "BSB" "GRU" 3 result: 1st Option: BSB -> SSA -> CNF -> GRU ---------------------------------------------------------------------------- How do this? I'm waiting from you, thank you so much. Andre

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Recommended GUI library?
by Colin Paul Adams 29 Nov '08

29 Nov '08

Looking at the wiki, it says that none of the high-level libraries are in a really good state for production usage (i.e. they are still all reasearch-level). What would people recommend? I'd really prefer to use a high-level library. -- Colin Adams Preston Lancashire

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Possible to update Haskell syntax
by cm 27 Nov '08

27 Nov '08

I'm very interested in learning Haskell, but one frustration is that lists always require commas. As an example [1,2,3,4] is a valid expression, but [1 2 3 4] is not. In this ideal world, another example would be that [(fun1 3 4) 7] would be a valid expression and be a two-element list equivalent to [fun1 3 4, 7]. If what I am suggesting is syntactically possible and doesn't break more advanced features of the language that I (as a beginner) am unaware of, I'm interested in patching the open source for my own use (as well as others who might want it).

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FFI, Foreign.Marshal.Array, etc.
by Alexey Beshenov 27 Nov '08

27 Nov '08

Hi! I'm playing with the FFI. Could anyone help me with a practical example? I have a function int solve_sys (double **a, int n, double b[]); where a is an array of size n*n and b is an array of size n (it contains the needed result if solve_sys returns 0). How could I wrap it in something like solveSys :: [[Double]] -> [Double] -> [Double] ? -- Setting Orange, Aftermath 38 YOLD 3174 Alexey Beshenov http://beshenov.ru/

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Ensuring consistency in typeclass instances?
by Colin Paul Adams 27 Nov '08

27 Nov '08

Hello, I'm just reading through Real World Haskell - chapter 6 (typeclasses). The definition of Eq is shown, and it mentions that you can define either one or both of the two classes. What would happen if I were two define both == and /= for an instance, in such a way that they were not opposites? If I were doing this in Eiffel, the function definitions would have postconditions to state the relationships, and the first execution would trigger a violation, telling me what was wrong. Is there any similar facility in Haskell? -- Colin Adams Preston Lancashire

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Not quite another Haskell tutorial, but ...
by Janis Voigtlaender 27 Nov '08

27 Nov '08

... I submitted my Habilitation thesis last week. The first few chapters of it try to give an introduction to Haskell with emphasis on types and reasoning principles. That might be an interesting read for some, so I made it accessible at http://wwwtcs.inf.tu-dresden.de/~voigt/habil.pdf. And yes, the November HCAR will also be coming. Rather soon now that this cause for delay is out of the way. Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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GTK + Reactive
by Levi Stephen 25 Nov '08

25 Nov '08

Hi, I'm trying to set up a simple program combining Reactive and GTK. I have the program below so far, but mainQuit is never called. After looking at a couple of adapters I tried adding the commented out line, but this didn't work for me. I'm sure I've probably missed something, but I'm not sure what. Thanks, Levi main = do initGUI clock <- makeClock (windowDestroy, sink) <- makeEvent clock w <- windowNew w `onDestroy` sink () widgetShowAll w -- forkE (tSync clock) $ quitOnDestroy windowDestroy mainGUI quitOnDestroy :: Event () -> Event (IO ()) quitOnDestroy e = fmap (const mainQuit) e tSync :: Clock TimeT-> ITime -> IO () tSync clock t = sleepPast (cGetTime clock) (exact t)

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Trouble with formatting, Real World Haskell example
by Robert Kosara 25 Nov '08

25 Nov '08

I'm having some trouble getting the InteractWith example from Real World Haskell's Chapter 4 to compile. When I use spaces (like below), it says "Interact.hs:10:30: Empty 'do' construct." When I use tabs (4 spaces per tab), I get "Interact.hs:16:13: parse error on input `='." In either case, I don't see the problem. Any help would be greatly appreciated. I'm also confused about the formatting of the example in the book (page 72). The last line ("myFunction = id") seems to be indented between the where and the next line, why is that? Is that simply a layout problem? To me, it seems it should be on the same level as the where. Below is my program: -- Interact.hs, simple filter in Haskell import System.Environment (getArgs) interactWith function inputFile outputFile = do input <- readFile inputFile writeFile outputFile (function input) main = mainWith myFunction where mainWith function = do args <- getArgs case args of [input, output] -> interactWith function input output _ -> putStrLn "Usage: Interact inputFile outputFile" myFunction = id Regards, Robert

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Efficient sieve of erastothenes, for solving project euler problem #10?
by Malcolm Reynolds 24 Nov '08

24 Nov '08

Hello all, I'm attempting to learn Haskell by going through the project euler problems. Number 10, http://projecteuler.net/index.php?section=problems&id=10 , involves summing prime numbers. It's easy in terms of coding something up that works, but I'm having a lot of trouble getting decent performance. I've learned a reasonable amount of ML at uni but Haskell is the first lazy language I've used.. I think the inefficiency is possibly due to the laziness but I'm not positive. I'd love if someone could show me how to do this in Haskell somewhere near as fast as C - at the moment I have a C version which runs in about a tenth of a second ( http://github.com/malcster/project-euler-solutions--c-/tree/master/10.c ). My haskell attempts are http://github.com/malcster/project-euler-solutions/tree/master/10better.hs (using the sieve) and http://github.com/malcster/project-euler-solutions/tree/master/10.hs (using possibly an even worse method, but seems to be a bit faster). If anyone could point out any neat strictness annotations or anything else I could put in, that would be cool. Cheers, Malcolm

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Type polymorphism with size
by Michael Snoyman 19 Nov '08

19 Nov '08

I am trying to write some code to read flat files from a mainframe system. This includes some character fields. This is a fixed width file, so each field will have a consistent length between records, but there are fields of different length within a record. For example, I might have a "name" field length 20 and an eye color field length 5. I am trying to use the binary library to read in this file. I've written a binary type, MFChar2, for reading in a 2-length character field. It is defined as such (you can safely ignore the ebcdicToAscii piece, it is just doing character conversion): data MFChar2 = MFChar2 [Word8] instance Binary MFChar2 where put = undefined get = do ebcdic <- replicateM 2 getWord8 return $ MFChar2 $ map ebcdicToAscii ebcdic What I would like to do is have some kind of generic "MFChar" data type which could take any character length, but I can't figure out how to do it. Any help would be appreciated. Thanks, Michael

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