March 2005 - Haskell-Cafe

Functional Reactive Programming
by Andrew Cheadle 17 Mar '05

17 Mar '05

Hi Please excuse the broadcast! Does anyone know where I can get hold of the FRP software bundle from Yale that pre-dates AFRP and Yampa, i.e with behaviours and clocked events. We're trying to resurrect some software and want to start with a working code base before migrating it to AFRP. Many thanks Andy ********************************************************************* * Andrew Cheadle email: a.cheadle(a)doc.ic.ac.uk * * Department of Computing http://www.doc.ic.ac.uk/~amc4/ * * Imperial College * * University of London * *********************************************************************

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Best practices for modular programming in Haskell
by Benjamin Pierce 17 Mar '05

17 Mar '05

For someone coming to Haskell from an OCaml background, one of the hardest things to get used to is the somewhat more bare bones module system that Haskell provides. Naturally, as I've gotten into the Haskell groove I've discovered (or learned from reading other people's code) nice ways of doing *some* of the things that I was used to doing with OCaml modules, but there are still things that I miss. This leads me to wonder how expert Haskell programmers address these issues. For example, here's my biggest current puzzle: What is the best way to expose the *interface* of a module (what would go in the .sig file in ML) to programmers that need to know how to use it? My impression is that there is no universally accepted "best way" to do this. Some people write their modules in the form: module Foo (<exported names>) where <type declarations for exported names> ------------------------------------------------------------- line of dashes <definitions of exported names (and other stuff)> This works fine as long as what you're exporting is just values, but it's awkward for types, since there is no way to *declare* a type (e.g., giving just its kind) without *defining* it. I suppose one could do something like module Foo (T,x) where type T = T_internal x :: T ------------------------------------------------------------- type T_internal = Int x = 5 but it seems clunky. Other people seem to rely on Haddock to generate interfaces as documentation. This is nicer in many ways (e.g., it solves the above problem because Haddock elides the right-hand side of a "data" or "newtype" declaration if the constructors are not exported by the module), but it means that you are stuck browsing interfaces in a web browser can't just have a look with emacs. This is especially annoying if you want to read the interface in tandem with the implementation (because you are trying to understand how the thing works internally but want to use the interface to get a high-level picture of the functionality it presents to the world). Are there other (perhaps better) ways that I'm not aware of? Thanks, Benjamin

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Re: (Newbie) Dynamic Programming, Memoizing Etc.
by oleg＠pobox.com 17 Mar '05

17 Mar '05

Bryce Bockman wrote: > 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). One observation is that `l' is merely `an iteration counter'. Therefore, the code can be simplified. The other observation is that memoizing all intermediate results may be expensive: the memoization table (a list, for example) will be big and sparsely populated. The cost of indexing in such a list can be significant. Here's a solution that seems to be balancing both costs. One can change the balance by modifying sc_upb. > module Foo where > > import Data.Array > > sc_upb = 100 > sc = listArray (1,sc_upb) $ map sc_calc [1..] > > sc_get i | i <= sc_upb = sc ! i > sc_get i = sc_calc i > > sc_calc :: Int -> (Int,Int) > sc_calc 0 = (0,0) > sc_calc 1 = (1,1) > sc_calc x | odd x = let (r,c) = sc_get (3*x + 1) in (r, c+1) > sc_calc x = let (r,c) = sc_get (x `div` 2) in (r, c+1) > > sCalcListLengthRange' a b = map (snd . sc_calc) [a..b] *Foo> maximum $ sCalcListLengthRange 1 10000 262 (1.57 secs, 82200088 bytes) *Foo> maximum $ sCalcListLengthRange' 1 10000 262 (1.18 secs, 61043084 bytes) there seems to be a benefit.

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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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