- Beginners - Haskell.org

small question
by Mohamed 19 Oct '10

19 Oct '10

Hello folks, just a quickie here and would appreciate any help... In this expression: instance OBSERVATIONS Drinkability WaterWell Volunteer where observe (Drinkability waterWell) volunteer = volunteer {vReport = if (odorQuale (perceive (Odor waterWell) volunteer) == True && clarityQuale (perceive (Clarity waterWell) volunteer) == True && fullnessQuale (perceive (Fullness waterWell) volunteer) == True) then (if (honesty) then "drinkable" else "undrinkable") else (if (honesty) then "undrinkable" else "drinkable")} I get the following error: Couldn't match expected type `Bool' against inferred type `Volunteer -> Bool' In the expression: honesty In the expression: (if honesty then "drinkable" else "undrinkable") In the `vReport' field of a record It is worth mention that Volunteer id defined as follows: data Volunteer = Volunteer {vid:: Id, vloc:: Id, odorQuale:: Bool, clarityQuale:: Bool, fullnessQuale:: Bool, vQuale:: Bool, vReputation:: Float, honesty:: Bool, vReport:: Report} deriving Show Where Report is defined as: type Report = String I understand that the vReport field is not receiving a String as it should, but I have completely failed in knowing why! I just think the nesting of the if statements is correct but wonder why is it not working. What is the correct code to do what I intend from the abofe nonfunctioning if statements? cheers, m

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Re: [Haskell-beginners] small question
by Mohamed 19 Oct '10

19 Oct '10

Stephan and Daniel, Thank you both very much. It worked! cheers, M ________________________________ From: Daniel Fischer <daniel.is.fischer(a)web.de> To: Mohamed <bishrma(a)yahoo.com> Sent: Tue, October 19, 2010 4:36:37 PM Subject: Re: [Haskell-beginners] small question Hi, On Tuesday 19 October 2010 16:13:05 you wrote: > Hello folks, > > just a quickie here and would appreciate any help... Stephen already told you that it should be if (honesty volunteer) ... , so > > In this expression: > > instance OBSERVATIONS Drinkability WaterWell Volunteer where > observe (Drinkability waterWell) volunteer = volunteer {vReport = > if (odorQuale (perceive (Odor waterWell) volunteer) == True && > clarityQuale (perceive (Clarity waterWell) volunteer) == True && > fullnessQuale (perceive (Fullness waterWell) volunteer) == True) then > (if (honesty) then "drinkable" else "undrinkable") else (if (honesty) > then "undrinkable" else "drinkable")} Don't use 'if condition == True'. For Bool values, (== True) is the identity function, so 'condition == True' is exactly the same as 'condition'. Cheers, Daniel

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Defining custom parser using Parsec
by Jimmy Wylie 19 Oct '10

19 Oct '10

Hi everyone, I'm working on a digital forensics application that will take a file with lines of the following format: "MD5|name|inode|mode_as_string|UID|GID|size|atime|mtime|ctime|crtime" This string represents the metadata associated with a particular file in the filesystem. I created a data type to represent the information that I will need to perform my analysis: data Event = Event { fn :: B.ByteString, mftNum :: B.ByteString, ft :: B.ByteString, fs :: Integer, time :: Integer, at :: AccessType mt :: AccessType ct :: AccessType crt :: AccessType } deriving (Show) data AccessType = ATime | MTime | CTime | CrTime deriving (Show) I would like to create a function that takes the Bytestring representing the file and returns a list of Events: createEvents :: ByteString -> [Event] (For now I'm creating a list, but depending on the type of analysis I decide to do, I may change this data structure) I understand that I can use the Parsec Library to do this. I read RWH, and noticed they have the endBy and sepBy combinators, but my issue with these is that using these funcitons performs too many transformations on the data. endBy will return a list of strings, which then will be used by sepBy which will then return a [[ByteString]] which I will then have to iterate through to create the final [Event]. What I would like to do is define a custom parser, that will go from the ByteString to the [Event] without the overhead of those intermediate steps. This function needs to be as fast as possible, as these files can be rather large, and I will be performing many different tests and analysis on the data. I don't want the parsing to be a bottleneck. I'm under the impression that the Parsec library will allow me to define a custom parser to do this, but I'm having problems understanding the library, and the documentation for it. A gentle shove in the right direction would be greatly appreciated. Thanks for your help, Jimmy

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GHC-generated executables size
by . 19 Oct '10

19 Oct '10

Hi, I was playing around with ghc again, and I was wondering what makes the executables so large and how I could make them smaller (strip works, but is there anything more I can do?) More specifically, I am compiling a program that uses the GTK+ bindings, HDBC, and some things from Prelude. The program simply displays a window, and reads and writes values from/into a data base file. Not much, really. Anyway, the program size is 20MB without stripping, and 10MB after stripping ... Any hints? Thanks and good night for now .. Christian

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Gtk2Hs in Windows (next question :) )
by C Gosch 18 Oct '10

18 Oct '10

Hello, this is a question only for the gtk2hs users on this list. It might be the wrong list, and I could probably figure it out myself given enough time, but I am asking anyway just in case someone has solved the problem already: I am trying to install gtk2hs in Windows 7 with cabal install gtk on the command line (cmd.exe). I managed to make the gtk libraries available, but the build process ends with this error message: setup.exe: gtk-0.11.2: library-dirs: c:/devel/dist/win32/libpng-1.4.3-1/lib doesn't exist or isn't a directory (use --force to override) cabal.exe: Error: some packages failed to install: gtk-0.11.2 failed during the building phase. The exception was: ExitFailure 1 The directory c:/devel/dist/win32/libpng-1.4.3-1/lib does in fact not exist, and I am wondering why it is considered at all .. I think a libpng came together with the gtk package. I have tried to use --extra-lib-dirs, but that leads to the same error message. Any ideas? Thanks and my apologies if this is too gtk-/windows-specific for this list. Christian

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Instance but no instance
by Jeroen van Maanen 18 Oct '10

18 Oct '10

I am trying to upgrade some update facility in my program to allow for IO: class Updater modelType updateType where update :: updateType -> modelType -> Maybe modelType class UpdaterIO modelType updateType where updateIO :: updateType -> modelType -> IO (Maybe modelType) pureUpdateToIO :: (updateType -> modelType -> (Maybe modelType)) -> updateType -> modelType -> IO (Maybe modelType) pureUpdateToIO updateFunction update model = return $ updateFunction update model instance (Updater modelType updateType) => UpdaterIO modelType updateType where updateIO = pureUpdateToIO update My code still works for all my old Updater instances even when I use them in the IO monad: do maybeUpdatedModel <- updateIO newUpdate oldModel So far so good. However, when I try to upgrade one of the Updater instances to an UpdateIO instance I get into a kind of a catch 22 situation where not only maybeUpdatedModel = update newUpdate oldModel results in a "No instance for Updater [...] [...]" compiler error as expected, but also do maybeUpdatedModel <- updateIO newUpdate oldModel unexpectedly results in an "Overlapping instances for UpdaterIO [...] [...]" compiler error. Where one of the overlapping instance declarations is: instance (Updater modelType updateType) => UpdaterIO modelType updateType How is that possible? Cheers, Jeroen

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GHC-generated executables size
by Christian Gosch 17 Oct '10

17 Oct '10

Hi, I was playing around with ghc again, and I was wondering what makes the executables so large and how I could make them smaller (strip works, but is there anything more I can do?) More specifically, I am compiling a program that uses the GTK+ bindings, HDBC, and some things from Prelude. The program simply displays a window, and reads and writes values from/into a data base file. Not much, really. Anyway, the program size is 20MB without stripping, and 10MB after stripping ... Any hints? Thanks and good night for now .. Christian

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installing ghc in home directory
by Alexander Katovsky 17 Oct '10

17 Oct '10

Hello, I want to install GHC in my home directory on Ubuntu. But when I get to configuring the haskell platform, I need to tell GHC about the non-standard location of the GMP library. GHC does not appear to pass LDPATH or LD_LIBRARY_PATH or LDFLAGS to ld. Making an alias 'ghc=ghc -L$HOME/lib' will not work here. Any suggestions? Thanks, Alex.

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A problem with a simple csv file parser
by Andy Elvey 16 Oct '10

16 Oct '10

Hi all - I'm just trying to get the simple csv file parser from "Real World Haskell" working. However, I've got a problem. Here's the code - -- A simple csv parser Module parseCSV where import Text.ParserCombinators.Parsec csvFile = endBy line eol line = sepBy cell (char ',') cell = many (noneOf ",\n") eol = char '\n' parseCSV :: String -> Either ParseError [[String]] parseCSV input = parse csvFile "(unknown)" input ( Btw, I'm running Ubuntu 10.04, GHC 6.12.1 ) When I try to run this (using "runhaskell parseCSV.hs test.csv" ) I get the error - parseCSV.hs:4:16: parse error on input `where' This has me really puzzled - as far as I can see, the first line looks identical to a number of similar programs that I've found (while searching for a solution to this). So, if anyone can let me know what is causing this, that'd be great! Many thanks in advance - - Andy

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Newbie performance question
by Jordan Ellis 16 Oct '10

16 Oct '10

I'm trying (really trying!) to get my head around Haskell, but one (among many) significant stumbling blocks for me so far has been trying to figure out how my code is actually going to perform. Here's a contrived example of the sort of thing that confuses me. Let's say I have a function which produces a list of odd integers from 1 to n: f n = filter (odd) [1..n] ...and I have another function that makes a list of all sums of pairs of odd numbers from 1 to n: g n = [a + b | a <- (f n), b <- (f n)] I assume that in most languages, (f n) would get called twice (once for a, and once for b), but what happens in Haskell? Does GHC recognize that, since f is a pure function, it only has to be evaluated once for a given n? If not, would a 'where clause' make any difference? e.g., g n = [a + b | a <- h, b <- h] where h = f n Thanks.

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