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Typeclasses and instances
by Mark Carroll 17 Mar '05

17 Mar '05

If I have, newtype Floating a => Vector a = Vector [a] if I want to make it an instance of Functor (with the obvious meaning), how do I write that? Thanks, Mark -- Haskell vacancies in Columbus, Ohio, USA: see http://www.aetion.com/jobs.html

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(Newbie) Dynamic Programming, Memoizing Etc.
by Bryce Bockman 16 Mar '05

16 Mar '05

How would you guys memoize the following code. simpleCalc :: (Int,Int) -> (Int,Int) simpleCalc (1,l) = (1,l+1) simpleCalc (x,l) | (odd x) = simpleCalc (((3*x) + 1), 1 + l) | otherwise = simpleCalc ((x `div` 2), 1 + l) sCalc x = simpleCalc (x,0) sCalcListRange a b = map sCalc [a..b] sCalcListLengthRange a b = map snd (sCalcListRange a b) The key is I need to calculate maximum (sCalcListLengthRange 1 1000000). This is a common programming contest problem that I'm playing with for fun. My first attempt fails miserably. It begins to thrash much more quickly then simple version above: calcListLengthRange a b = map snd (calcListRange a b) calcListRange a b = map calcList [a .. b] where calcList = ((map calc' [0 ..]) !!) where calc' :: Int -> (Int,Int) calc' i = calcList' (i,0) calcList' :: (Int,Int) -> (Int,Int) calcList' (1,l) = (1,l+1) calcList' (x,l) | (odd x) = ((fst((calcList ((3*x) + 1))), snd(calcLi\ st((3*x) + 1)) + 1 + l)) | otherwise = ((fst((calcList (x `div` 2))), snd(calcLi\ st((x `div` 2))) + 1 + l)) I tried to associate a table entry with a particular solution to the problem, so taht calcList would just be a lookup for those we had finished calculating. Sorry this code is ugly I know, but I'm just learning. Cheers, Bryce

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What is MonadPlus good for?
by Benjamin Pierce 16 Mar '05

16 Mar '05

I have seen lots of examples that show how it's useful to make some type constructor into an instance of Monad. Where can I find examples showing why it's good to take the trouble to show that something is also a MonadPlus? (I know there are many examples of things that *are* MonadPluses; what I want to know is why this is interesting. :-) Thanks, - Benjamin

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RE: [Haskell-cafe] invalid character encoding
by Simon Marlow 16 Mar '05

16 Mar '05

On 16 March 2005 03:54, Ian Lynagh wrote: > On Tue, Mar 15, 2005 at 10:44:28AM +0000, Ross Paterson wrote: >> On Mon, Mar 14, 2005 at 07:38:09PM -0600, John Goerzen wrote: >>> I've got some gzip (and Ian Lynagh's Inflate) code that breaks >>> under the new hugs with: >>> >>> <handle>: IO.getContents: protocol error (invalid character >>> encoding) >>> >>> What is going on, and how can I fix it? >> >> A Haskell 98 Handle is a character stream, and doesn't support binary >> I/O. This would have bitten you sooner or later on systems that do >> CRLF conversion, but Hugs is now much stricter, because character >> streams now use the encoding determined by the current locale (for >> the C locale, that means ASCII only). > > Do you have a list of functions which behave differently in the new > release to how they did in the previous release? > (I'm not interested in changes that will affect only whether something > compiles, not how it behaves given it compiles both before and after). > > Simons, Malcolm, are there any such functions in the new ghc/nhc98? > > Also, are you all agreed that the hugs interpretation of the report is > correct, and thus ghc at least is buggy in this respect? (I'm afraid I > haven't been able to test nhc98 yet). GHC (and nhc98) assumes a locale of ISO8859-1 for I/O. You could consider that to be a bug, I suppose. We don't plan to do anything about it in the context of the current IO library, at least. Cheers, Simon

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re parser question
by Carter Tazio Schonwald 16 Mar '05

16 Mar '05

term :: Parser Int term = do f <- factor do symbol "*" e <- expr return (f * t) <--------------- replace t with e +++ return f I hope that helps, Carter Schonwald

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wxHaskell htmlWindow usage
by David Owen 15 Mar '05

15 Mar '05

Good day all, I am attempting to display html pages using wxHaskell's HtmlWindow functions. I am hoping that it is indeed possible for me to do just that and that I haven't misunderstood what HtmlWindow is used for. My code so far will create a frame, a button and an HtmlWindow and other assorted things. I would like an html file to be displayed in the window when the button is clicked. At the moment, only the path (a string) of the html file that I would like to display is printed in the window when the button is clicked. Here is some of the code i've been trying to get to do this: do -- the main frame f <- frame [text := "Browser", fullRepaintOnResize := False] -- create html window hw <- htmlWindowCreate f 1 (Rect 50 150 50 150) 5 "theWindow" -- 'load page' button go <- button f [text := "Load Page", on command := htmlWindowSetPage hw "c:/example.html"] (I've left out the layout part here) htmlWindowSetPage might be the wrong function to use for this or I might just be using it incorrectly. This may well be more complicated than I'm hoping it is too! Thankyou to whoever has a look at this. bye all David

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RE: [Haskell-cafe] Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

Thanks folks! Writing it in a lispy manner seems to work. I see what Arthur means about the layout - I think I'm still thinking too much in C. :) Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

Curiouser and curiouser... expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t + e) +++ return t solves the undefined variable problem but introduces a new 'Last operator in do {...} must be an expression' error, which then disappears if I explicitly return e expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return e return (t + e) +++ return t to give me the original undefined variable t error at the line expr = do t <- term . It looks in scope to me... :( Thanks, Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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RE: [Haskell-cafe] Parser problem continued
by Nicola Whitehead 15 Mar '05

15 Mar '05

perhaps like this: > expr = do t <- term > (do symbol "+" > e <- expr > return (t+e) > ) > +++ > (return t) > > >although I think you may also want a 'try' before the first alternative. No, that still gives the same undefined variable error. :( Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland ********************************************************************* Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. *********************************************************************

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RE: [Haskell-cafe] wxHaskell htmlWindow usage
by Bayley, Alistair 15 Mar '05

15 Mar '05

> From: David Owen [mailto:d_w_o@hotmail.com] > > htmlWindowSetPage might be the wrong function to use for this > or I might > just be using it incorrectly. This may well be more > complicated than I'm > hoping it is too! I-Am-Not-A-WxHaskell-Expert, but... a quick google finds this page: http://www.wxwidgets.org/manuals/2.4.2/wx499.htm#wxhtml ... where is is explained that wxHtmlWindow.LoadPage is used to load a page from a file, while wxHtmlWindow.SetPage is used to display a literal HTML string in the widget. And in the wxHaskell docs there are two matching functions: - htmlWindowSetPage - htmlWindowLoadPage which probably correspond. So, I guess you want to use htmlWindowLoadPage. Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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