- Beginners - Haskell.org

package random requires two versions of time
by Ashish Agarwal 05 Aug '10

05 Aug '10

Running 'cabal configure' on one of our code bases gives: package random-1.0.0.2 requires time-1.1.4 package random-1.0.0.2 requires time 1.2.03 in addition to several other such warnings. This one is the oddest however since it is the same version of random that presumably requires two different versions of time. How can that be? Output of 'cabal info random' gives: Dependencies: base >= 3 && <5, time -any This happens only on a Debian system, in which I installed the Haskell Platform using apt-get.

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How to avoid Gmail attachments? ("An HTML attachment was scrubbed")
by Peter Schmitz 04 Aug '10

04 Aug '10

I see that when I post to the Haskell lists ( http://www.haskell.org/mailman/listinfo) from gmail, the end of my post has: "

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Question regarding the ListT Monad Transformer
by Tim Cowlishaw 04 Aug '10

04 Aug '10

Hey all, I'm trying to get my head around the ListT Monad Transformer, and am having trouble working out how to use it. Essentially, I am trying to transform a type of [IO a] to IO [a] I'd assumed that this might work something like this Transform [IO a] to ListT IO a call runListT to transform ListT IO a to IO [a] ..having I'd assumed that this was the correct situation to use a Monad Transformer, as I'm layering monads around each other. However, while the second half of the transformation above is catered for via runListT - I can't seem to work out how to do the first - namely, to write a function of type [IO a] -> ListT IO a . To be honest, I'm not entirely sure I've entirely got my head around Monad Transformers yet - does what I'm describing make any sense at all, and if so, is there any chance you could point me in the right direction towards implementing (and hopefully understanding) this? Cheers, Tim

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Yezod and Snap deployments to ISP
by MH 04 Aug '10

04 Aug '10

I want to build a website using haskell and deploy it to some ISP. I appreciate if you help me understand the following: 1. Do I need to consider using special ISP to deploy Yezod or Snap? 2. Do I need to consider using special ISP to deploy any other Haskell based web app? 3. To start with I need to display a static content and ability to send email from user submitted form. Can I send email with these Web frameworks (I would appreciate the samples)? 4. Can you recommend me any ISP in USA? Thanks a lot. Malik

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run another program from within haskell program
by prad 04 Aug '10

04 Aug '10

i have downloaded data from postgresql into a file my haskell program. i can of course edit this file with say vim. can i run vim on that file from within haskell and after quiting vim have control returned to my haskell program? the situation is like calling vim to edit my claws-mail emails instead of using the native editor. after editing, i quit the external editor and am returned to the native editor. -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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Re: [Haskell-beginners] Boilerplate Code
by Matt Andrew 04 Aug '10

04 Aug '10

Hi all, Thanks for the many replies. I have been looking at the Template Hakell route (thanks Brent). It looks like it's going to take a bit to figure out how to use it properly (a lot of the tutorials/docs seem a little out of date?), but should do the trick once I do. Thanks again to everyone for their replies. Matt

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Boilerplate Code
by Matt Andrew 04 Aug '10

04 Aug '10

Hi all, I am in the process of writing a Scheme interpreter/compiler in Haskell as my first serious project after learning the basics of Haskell. The goal is to really get a feel for Haskell. I am trying to accomplish this as much as I can on my own, but am referring to Jonathan Tang's 'Write Yourself a Scheme in 48 hours' whenever I get really stuck. I have a question regarding a pattern that I have found within my code for which I cannot seem to find an abstraction. I am implementing some of the primitive Scheme type-checker functions with the following code: numberP :: SchemeVal -> SchemeVal numberP (Number _) = Bool True numberP _ = Bool False boolP :: SchemeVal -> SchemeVal boolP (Bool _) = Bool True boolP _ = Bool False symbolP :: SchemeVal -> SchemeVal symbolP (Atom _) = Bool True symbolP _ = Bool False This is a pattern that I could easily provide an abstraction for with a Lisp macro, but I'm having trouble discovering if/how it's possible to do so elegantly in Haskell. The closest (but obviously incorrect) code to what I'm trying to accomplish would be: typeChecker :: SchemeVal -> SchemeVal -> SchemeVal typeChecker (cons _) (cons2 _) = Bool $ cons == cons2 I understand this code drastically misunderstands how pattern matching works, but (hopefully) it expresses what I'm trying to accomplish. Anyone have any suggestions? I do realise that such an abstraction is barely worth it for the amount of code it will save, but this exercise is about learning the ins and outs of Haskell. Appreciate you taking the time to read this, Matt Andrew

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Re: Announcement: Real World Haskell
by Jeff Greer 03 Aug '10

03 Aug '10

Hi Ben, There is plenty of room at linuxagora.com for new reading groups. I'll add it to the list. If you know enough that would be interested in the Hutton book, it could be done concurrently. We are always open for new ideas. Thanks, Jeff Message: 2 > Date: Sun, 01 Aug 2010 02:54:58 +0900 > From: DekuDekuplex(a)Yahoo.com (Benjamin L. Russell) > Subject: [Haskell-beginners] Re: Announcement: Real World Haskell - > Reading Group > To: beginners(a)haskell.org > Message-ID: <m2ocdnedu5.fsf(a)Yahoo.com> > Content-Type: text/plain; charset=utf-8 > > Jeff Greer <jgreer613(a)gmail.com> writes: > > > Hi Russel,Thanks for the suggestion to start a Haskell Reading Group mail > list. At the current time, I can not fit it into my schedul...Â full time > teaching job, gard school, and family....I hope some find the reading group > useful. > > -- http://teacherwikiweb.com > > My pleasure. Incidentally, another great book I would suggest for a > reading group would be _Programming in Haskell_ [1], by Graham Hutton. > > Does anybody have any plans to start a reading group for this book? > > -- Benjamin L. Russell > > [1] Hutton, Graham. _Programming in Haskell._ Cambridge: Cambridge > University Press, > 2007. Print. <http://www.cs.nott.ac.uk/~gmh/book.html<http://www.cs.nott.ac.uk/%7Egmh/book.html> > >. >

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doc on accessing C struct binary data
by Sean Perry 01 Aug '10

01 Aug '10

I would like to read C structs that have been written to disk as binary data. Is there a reference on doing this somewhere. The IO is not too hard, but how do I mimic the C struct in Haskell and still honor the exact sizes of the various struct members?

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Parsec and Validation
by Vladimir Solmon 31 Jul '10

31 Jul '10

I'm attempting to use Parsec to write a parser for times and dates. I'm aware of Data.Time.Format, but it doesn't offer the flexibility I need for this project. My current code (see below) uses Control.Monad.guard to validate the numeric values for hours and minutes inside the parser. The problem I'm running into is that checking for errors in the parsing code causes Text.Parsec.Combinator.choice (in the time function below) to fail and return an error if it cannot parse input using the first option (tTimeHourMin), even if the input should match the second option (t24hrClock). I have several questions: Why is choice failing in the time function below? Is there a better way to do numeric validation while parsing? Should I only use Parsec to validate that the input is syntactically correct and do the numeric validation elsewhere? What are some good examples of validating input using Parsec? Here is a bit of the code I'm working with: time :: Parser TimeOfDay time = choice [ tTimeHourMin, t24hrClock ] tTimeHourMin :: Parser TimeOfDay tTimeHourMin = do hour <- range (0, 23) oneOf " :,." min <- range (0, 59) return (TimeOfDay hour min 0) t24hrClock :: Parser TimeOfDay t24hrClock = do (h, m) <- splitAt 2 <$> count 4 digit let hour = read h let min = read m guard (hour >= 0 && hour <= 23 && min >= 0 && min <= 59) <?> printf "24hr time represented as hhmm" return (TimeOfDay hour min 0) range :: (Int,Int) -> Parser Int range (lower, upper) = do t <- read <$> many1 digit guard (t >= lower && t <= upper) <?> printf "integer in range [%d,%d]" lower upper return t Thanks for any suggestions, vladimir

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