- Beginners - Haskell.org

Re: [Haskell-beginners] Boilerplate Code
by Matt Andrew 06 Aug '10

06 Aug '10

I had a play today and managed to figure out how to write the code I needed. If anyone is curious the resulting code is: {-# LANGUAGE TemplateHaskell #-} module Template where import Language.Haskell.TH typeChecker :: String -> Q Exp typeChecker schemeType = do x <- newName "x" return $ LamE [VarP x] (CaseE (VarE x) [Match (ConP ty [WildP]) (NormalB (AppE (ConE bool) (ConE true))) [],Match WildP (NormalB (AppE (ConE bool) (ConE false))) []]) where ty = mkName schemeType bool = mkName "Bool" true = mkName "True" false = mkName "False" I then import this module into where I am defining the predicates (templates have to be defined in a seperate module), where the code becomes: numberP :: SchemeVal -> SchemeVal numberP = $(typeChecker "Number") boolP :: SchemeVal -> SchemeVal boolP = $(typeChecker "Bool") symbolP :: SchemeVal -> SchemeVal symbolP = $(typeChecker "Symbol") So, as I always knew, not really worth the effort as I've written a 13 line module to save 3 lines of code, but a good learning experience =) Thanks again for the point in the right direction. Cheers, Matt

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getopt
by Gaius Hammond 05 Aug '10

05 Aug '10

Hi all, I have used GetOpt in Python and Ocaml so I am reasonably familiar with it, and I have been studying the technique here: http://www.haskell.org/haskellwiki/High-level_option_handling_with_GetOpt Can anyone explain to me what is actually happening here: opts <- foldl (>>=) (return startOptions) actions let Options { optVerbose = verbose , optInput = input , optOutput = output } = opts input >>= output I understand the fold is evaluating everything in the actions list overlaying them onto the startOptions record, then getting these out of IO and into the opts record - but where are "verbose", "input" and "output" coming from? Are they just aliases? Any insight gratefully received :-) Thanks, G

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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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