October 2011 - Beginners

tokenizing a string and parsing the string
by kolli kolli 12 Oct '11

12 Oct '11

Hi, Can anyone help how to tokenize a string and parse it.

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quickCheck generation question
by Joe Van Dyk 11 Oct '11

11 Oct '11

I'm going through the 99 Haskell problems and am trying to write quickCheck properties for each one. -- 3 (find k"th element of a list) element_at xs x = xs !! x prop_3a xs x = (x < length xs && x >= 0) ==> element_at xs (x::Int) == (xs !! x::Int) When I run prop_3a through quickCheck, it gives up, saying that it only passed 1 test. This makes sense, because the random x is going to be larger than the number of elements in the random list in most cases. So what I need to do is write a generator that generates a random list with length larger than a random integer, but I'm not sure how to do that. Thanks! Joe

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help for the lhs2Tex install
by kolli kolli 10 Oct '11

10 Oct '11

hi, when I am trying to install lhs2tex on my machine(windows).I am getting the following error.Can you please me install lhs2tex.... Microsoft Windows [Version 6.1.7601] Copyright (c) 2009 Microsoft Corporation. All rights reserved. C:\Users\namratha>cabal install lhs2Tex Resolving dependencies... [1 of 1] Compiling Main ( C:\Users\namratha\AppData\Local\Temp\lhs2t ex-1.171640\lhs2tex-1.17\Setup.hs, C:\Users\namratha\AppData\Local\Temp\lhs2tex- 1.171640\lhs2tex-1.17\dist\setup\Main.o ) Linking C:\Users\namratha\AppData\Local\Temp\lhs2tex-1.171640\lhs2tex-1.17\dist\ setup\setup.exe ... Configuring lhs2tex-1.17... setup.exe: kpsewhich command not found cabal: Error: some packages failed to install: lhs2tex-1.17 failed during the configure step. The exception was: ExitFailure 1

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Recursive update of Records with IO
by Alia 10 Oct '11

10 Oct '11

Hi folks, (Apologies if there is duplication, I didn't see my post archived so I assumed I had to subscribe to be able to post) In the hope of tapping the collective wisdom, I'd like to share a problem which seems to be worth sharing: how to recursively update a single record instance with a list of IO tainted functions. A simple pure IO-less version of what I am trying to do is probably the best way to explain this: <pure_version> module Test where data Person = Person {name :: String, age :: Int} deriving Show setName :: String -> Person -> Person setName s p = p {name=s} setAge :: Int -> Person -> Person setAge i p = p {age=i} update :: [Person -> Person] -> Person -> Person update [] p = p update [f] p = f p update (f:fs) p = update fs p' where p' = f p p1 = Person {name="sue", age=12} p2 = update [(setName "sam"), (setAge 32)] p1 </pure_version> This works very nicely. Now if the setter functions involve some IO, I believe the type signatures should probably look like this: setName :: String -> Person -> IO Person setAge :: Int -> Person -> IO Person update :: [Person -> IO Person] -> Person -> IO Person and the setter functions should look like this for example: setName :: String -> Person -> IO Person setName s p = do putStrLn "setting name" return p {name=s} setAge :: Int -> Person -> IO Person setAge i p = do putStrLn "setting age" return p {age=i} but I'm stuck on how the update function would look.. Any help would be much appreciated. Best, AK

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What's the difference between "operator" and "function"?
by Costello, Roger L. 10 Oct '11

10 Oct '11

Hi Folks, In a book that I am reading the author distinguishes between "operator" and "function." Example: The "+" is an operator. The "map" is a function. What's the difference between operator and function? /Roger

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Fwd: Real World Haskell: From Portable to System.Win32
by Paulo M. Pocinho 08 Oct '11

08 Oct '11

createFile is probably what I should be looking for. As a side note, I really think this is not a big deal. The portable code is a lot better. I wonder if that exercise was a pun on working with Haskell code just to step back into foul territory. And yes, extra careful on the parameters, or it may end up corrupting other files, launch the missiles and whatnot. One thing I noticed so far using Haskell, usually I have to spend more time thinking before hitting the keyboard. Yet the code is always elegant. Meanwhile, I've been taking a peek inside System.IO.HVFS from MissingH [1]. Looks nice. Thanks for the replies. Cheers! -- [1] http://hackage.haskell.org/packages/archive/MissingH/1.1.1.0/doc/html/Syste… On 8 October 2011 20:34, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: > On Sat, 08 Oct 2011 18:45:43 +0200, Chaddaï Fouché > <chaddai.fouche(a)gmail.com> wrote: > >> On Sat, Oct 8, 2011 at 7:47 AM, Paulo Pocinho <pocinho(a)gmail.com> wrote: >>> >>> getFileTime :: HANDLE -> IO (FILETIME, FILETIME, FILETIME) >>> >>> I suppose that means getFileTime takes a file handle (with type >>> HANDLE) and returns an IO type with CreationTime LastAccessTime >>> LastWriteTime [3]. >>> >>> The thing is I don't know how to get a HANDLE type. I understand I can >>> get a file handle by opening it. I've been trying the following: >>> >>> openFile :: FilePath -> IOMode -> IO Handle >>> fh <- openFile "myfile.hs" ReadMode >>> >> >> Yes, the case is significant and that means that Handle and HANDLE >> aren't the same, apparently HANDLE is specific to the win32 api (which >> I don't know well), a rapid inspection of System.Win32.File seems to >> reveal that the only function to get an HANDLE is createFile which has >> a ton of parameters you'll need to set carefully to avoid overwriting >> your existing file (you should probably define another function with >> most of these already fixed for convenience) but should do the trick. > > You can use the information about createFile at: > http://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).a… > > Regards, > Henk-Jan van Tuyl > > > -- > http://Van.Tuyl.eu/ > http://members.chello.nl/hjgtuyl/tourdemonad.html > -- >

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Real World Haskell: From Portable to System.Win32
by Paulo Pocinho 08 Oct '11

08 Oct '11

Hello Haskellers. I'm having a bit of trouble solving the challenge in chapter 9 of the book Real World Haskell [1]. I understand the portable code and have no difficulty with it. More specifically, I want to know how to get a file's last modified date. Perhaps in understanding this bit I may also extrapolate to similar parts of System.Win32 more easily. A simple example using portable code, and also formats the date: getFileModifiedDate :: FilePath -> IO String getFileModifiedDate f = do m <- getModificationTime f c <- toCalendarTime m return $ formatCalendarTime undefined "%Y-%m-%d %T" c However, the exercise is to "port the code from this chapter to your platform's native API", which in my case is System.Win32. Using the simple example, I know that I can use getFileTime from System.Win32 [2]: getFileTime :: HANDLE -> IO (FILETIME, FILETIME, FILETIME) I suppose that means getFileTime takes a file handle (with type HANDLE) and returns an IO type with CreationTime LastAccessTime LastWriteTime [3]. The thing is I don't know how to get a HANDLE type. I understand I can get a file handle by opening it. I've been trying the following: openFile :: FilePath -> IOMode -> IO Handle fh <- openFile "myfile.hs" ReadMode Right now I cannot use getFileTime on it because this has type Handle: Couldn't match expected type `HANDLE' with actual type `Handle' In the first argument of `getFileTime', namely `fh' In a stmt of an interactive GHCi command: nf <- getFileTime fh I think I got lost at inspecting the type HANDLE = Ptr () If anyone could make a useful example I would appreciate it very much. Perhaps it's just me going around in circles. Regards, P. M. Pocinho -- [1] http://book.realworldhaskell.org/read/io-case-study-a-library-for-searching… [2] http://hackage.haskell.org/packages/archive/Win32/2.2.1.0/doc/html/System-W… [3] http://msdn.microsoft.com/en-us/library/windows/desktop/ms724320%28v=VS.85%…

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sorting
by kolli kolli 08 Oct '11

08 Oct '11

hi, can anyone help me sorting elements of a list ["ala", "zero", "jera"] in alphabetical order

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module not found
by Tyler Hayes 08 Oct '11

08 Oct '11

Hello, Very beginner here. I downloaded the Haskore package with cabal: cabal install haskore but when I import Haskore in my source, I get: Could not find module `Haskore' I've got to be missing something. Thanks, Tyler

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More than one GHC version installed
by Paulo Pocinho 06 Oct '11

06 Oct '11

Hi. I'd like to know if it is possible to have a separate install directory with GHC 7.0.4 and another with GHC 7.2.1 in the same Windows system. I was thinking about having a standard PATH environment "C:\Haskell\ghc\bin". Then have a folder structure like "C:\Haskell\ghc-{7.2.1, 7.0.4}", renaming either one of them into "C:\Haskell\ghc" depending on what I would like to be using. I have taken into consideration to renaming the cabal install directory in the same manner. However, the problem with this approach is that the package database remains somewhere I can't find. Does the default GHC store the package database in the registry or something? I'd appreciate if you could hint me where I could swap the databases for this setup.

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