- Haskell-Cafe - Haskell.org

Closest analog to popen()
by Dimitry Golubovsky 14 Apr '05

14 Apr '05

Hi, Does there exist any analog of popen in the standard Haskell libraries? I need to execute an external program (not a Haskell funciton) - maybe in a shell, being able to feed something to its stdin, and finally catch the exit code. Does there exist a way to do this without dealing with multithreading, writing my own ffi and other stuff? Something like rawSystem, but acting asynchronously (returning a writeable pipe handle, and a process ID), and later I would use getProcessStatus to catch the exit code. PS I have found http://community.moertel.com/ss/space/Tom's+Haskell+code/POpen.hs so far, but it does not seem to be part of the standard library. -- Dimitry Golubovsky Anywhere on the Web

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predicates in XML toolboxes
by Henning Thielemann 14 Apr '05

14 Apr '05

The XML toolboxes HaXml, HXML and the XML toolbox uses one function type (called filter) for different purposes. The functions of types predicates a -> Bool selectors, transformators a -> a list-valued functions a -> [a] are all implemented with the one type (a -> [a]). In this design predicates don't return False or True, but an empty list or a one-element list containing the input value. Transformators return single element lists. In my opinion this means a significant loss of type safety and quality of documentation. Are there more type safe XML/HTML processor libraries in Haskell?

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Passing Constructors as arguments
by Ralph Hodgson 13 Apr '05

13 Apr '05

I am learning Haskell and have set a small exercise for myself on a frames and slots program. Would appreciate some help on how to pass constructors of data structures as arguments to a function. Thanks, -- Ralph __________ A Frames test > module Frames > where Define frame slots: > type FirstName = String -- first name > type LastName = String -- last name > type Organization = String > type Email = String > type WorkPhone = String > type CellPhone = String > type TelephoneNumber = String Define slots for a contact > data ContactProperty = FN FirstName > | LN LastName > | OR Organization > | EM Email > | WP TelephoneNumber > | MP TelephoneNumber > deriving (Show, Eq) > data Contact = Contact [ContactProperty] > deriving (Show, Eq) > type Contacts = [ Contact] Now I need a way to extract properties from the frame. Start by testing pattern matching without using parameters. Then I need to find out how to pass a constructor as a parameter. > getProperty:: [ContactProperty] -> FirstName > getProperty ((FN fn):_) = fn > getProperty (_:xs) = getProperty xs > getProperty [] = "unknown" > firstName:: Contact -> FirstName > firstName (Contact cpl) = getProperty cpl Define Contacts > c1::Contacts > c1 = > [ ( Contact [(FN "Ralph"),(LN "Hodgson"),(OR "TopQuadrant"),(EM "rhodgson(a)topquadrant.com")]), > ( Contact [(FN "Mills"),(LN "Davis"),(EM "mdavis(a)project10x.com")])] Tests > t1=firstName $ head c1 -- should be "Ralph" > t2=firstName $ last c1 -- should be "Mills" ___________________________

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foldr1 min [(maxBound::Int)%1,1 % 2]
by Bo Herlin 13 Apr '05

13 Apr '05

Hi How come > foldr1 min [(maxBound::Int) % 1,1 % 2] 2147483647 % 1 but > foldr1 min [2147483647 % 1,1 % 2] 1 % 2 Why??? /Bo Herlin

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[Q] A typing problem
by Didier Verna 13 Apr '05

13 Apr '05

Hi ! I'm trying to write a function that combines folding and mapping. This function would take two arguments (a folding function and a mapping function), and would return a function of one argument (a list of 'a''s) returning a 'a'. The idea is to write something like sumsquare = foldmap (+) square Here's what I write: foldmap :: (c -> c -> c) -> (c -> c) -> (c -> c -> c) foldmap f m = foldr1 f (map m) But this gives a typing error: ERROR "src/ssq4.hs":2 - Type error in application *** Expression : foldr1 f (map m) *** Term : map m *** Type : [b] -> [b] *** Does not match : [a] What can I do to get what I want ? Thanks. -- Didier Verna, didier(a)lrde.epita.fr, http://www.lrde.epita.fr/~didier EPITA / LRDE, 14-16 rue Voltaire Tel.+33 (1) 44 08 01 85 94276 Le Kremlin-Bicêtre, France Fax.+33 (1) 53 14 59 22 didier(a)xemacs.org

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Re: [Haskell-cafe] Binary file r/w example
by Bo Herlin 13 Apr '05

13 Apr '05

Yes, I am, and the IOExtension module IS one of my problems. The other problem is that I am interested in this Ptr thing, and I havent seen any examples in using it, and is seems like Ptr is not in any of my Haskell-books or in any of the tutorials that I have found. Henning Thielemann wrote: > > On Wed, 13 Apr 2005, Bo Herlin wrote: > >> Hi, does anyone have a small but complete example on how to read a >> (portion of) a binary file, like r/w the header of a Midi-file? >> >> It seems like readBinaryFile (ans writeBinaryFile) is deprecated and I >> should use hGetBuf (and hPutBuf), but what is this Ptr thing? >> >> How would I implement readBinaryFile using hGetBuf? > > > > Are you aware of Haskore (www.haskell.org/haskore and > cvs.haskell.org/darcs/haskore) which provides a MidiFile reader and > writer? It has no good solution for reading and writing binary files, > but uses a platform dependent IOExtension module. :-( >

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Binary file r/w example
by Bo Herlin 13 Apr '05

13 Apr '05

Hi, does anyone have a small but complete example on how to read a (portion of) a binary file, like r/w the header of a Midi-file? It seems like readBinaryFile (ans writeBinaryFile) is deprecated and I should use hGetBuf (and hPutBuf), but what is this Ptr thing? How would I implement readBinaryFile using hGetBuf? Best/ Bo Herlin

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Installing and running QuickCheck
by Adam Wyner 12 Apr '05

12 Apr '05

Hi, I'd like to use QuickCheck for testing Haskell programs. I'm using Hugs in Windows. I'm a newbie to Haskell. Just running QuickCheck.hs itself, which comes with the Hugs98 libraries, I get an error message and the Monad command line, which indicates that quickcheck didn't load. ERROR "C:\Program Files\Hugs98/libraries\QuickCheck.hs":147 - Undefined variable "chr" Monad> Here is the line in QuickCheck.hs which leads to the error. instance Arbitrary Char where arbitrary = choose (32,255) >>= \n -> return (chr n) coarbitrary n = variant (ord n) This code is from the website: http://www.cs.chalmers.se/~rjmh/QuickCheck/QuickCheck.hs I tested it with the following module, as per the instructions in on QuickCheck's manual page: ---- module TestQuickCheck where import QuickCheck prop_RevRev xs = reverse (reverse xs) == xs where types = xs::[Int] ---- Loading just this, I get the same error: Prelude> :l TestQuickCheck ERROR "C:\Program Files\Hugs98/libraries\QuickCheck.hs":147 - Undefined variable "chr" Monad> I know others report using QuickCheck, so this problem must have been resolved. Also, I guess the quickcheck script is for Linux alone? Any scripts for Hugs in Windows? Cheers, Adam

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Re: [Haskell-cafe] Thompson's Exercise 9.13
by Daniel Fischer 10 Apr '05

10 Apr '05

Am Sonntag, 10. April 2005 17:33 schrieben Sie: > On 2005/04/10, at 18:18, Daniel Fischer wrote: > >> But this returns [] and doesn't work. I can't get "left" to know that > >> it is working with the rightmost element. It just throws away every > >> element when its right hand side is empty. > >> > >> I found a solution that works for Strings, but I am looking for a more > >> general solution. This exercise may yet again be one of those that is > >> difficult to solve with my current knowledge of Haskell, but I'm > >> asking > >> anyway. > > > > I cannot imagine how you could do it for Strings, but not for general > > lists. > > Mind sending me the code? > > The code is here: > http://wiki.pragprog.com/cgi-bin/wiki.cgi/ThompsonNineAnswersJD Look > for 9.13 towards the bottom of the page. > > It uses chr 0 as an end of string marker, and so this solution only > works for Strings. Unfortunately, Strings may contain '\0': *Init> finit "this doesn't\0quite\0work\0!" "this doesn't\NULquite\NULwor" so it doesn't work in all cases. The idea is nice, though: import Data.Maybe finit = catMaybes . foldr f [Nothing] . map Just where f x ys = case ys of [Nothing] -> [] _ -> x:ys will do fine. > > -- > Kaoru Hosokawa > khosokawa(a)gmail.com Cheers, Daniel

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List containing different types but all implementing the same class
by Bo Herlin 09 Apr '05

09 Apr '05

Hi everyone. Being new to Haskell I wonder how I can make a list contain different types but all implementing the same class, like this: data X = X data Y = Y class Z a where f :: a -> Int instance Z X where f x = 0 instance Z Y where f y = 1 test1 :: Z a => [a] test1 = [X,Y] test2 = map f test1 Is it possible to make this work? /Bo Herlin

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