- Haskell-Cafe - Haskell.org

is identity the only polymorphic function without typeclasses?
by Cagdas Ozgenc 02 Mar '03

02 Mar '03

Greetings, Is identity function the only meaningful function one can write without constraining the type variable using a typeclass? If not, could you please give a counter-example? Thanks

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Re:
by wr 28 Feb '03

28 Feb '03

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Hugs' graphics lib
by Daniel Jian 27 Feb '03

27 Feb '03

Hi all, I have just installed Hugs to my system. And then I also compiled its drawing library. My question is how can I include that library in my code? I know this is rather silly a question, but I have tried to play around with the environment variable in .bashrc, and I have added:-P$HOME/local/graphics-2.0.4/lib/x11 to the HUGSFLAGS. But Hugs still complains that it cannot include the lib. I am just wondering, can I make an executable file using hugs? How can I do that (if I can)? Many thanks! Blessings, Daniel Jian __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

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Buenas noches !!!
by Olga Lucia Quintero Fonseca 24 Feb '03

24 Feb '03

Buenas noches, quisiera consultarles sobre lo siguiente, tengo estas dos funciones -- Calcula Raiz Wn wn n = cos c :+ (-1*sin c) where c=2*pi/n twiddlefactor r k = concat [(map (^i) [(wn rs)^i | i<-[0..s-1]])| i<-[0..r-1]] where rs = (r^k) ; s = (r^(k-1)) la dificultad que tengo es que twiddlefactor no me acepta enteros compila pero no me acepta los parametros, sin embargo si la defino como --Calcula vector de Twiddle Factor twiddlefactor r s rs= concat [(map (^i) diag) | i<-[0..r-1]] where diag = [(wn rs)^i | i<-[0..s-1]] no tengo problema, lo que sucede es que posteriormente esta definicion me es inconveniente pues el valor de rs es r^k y lo necesito en un programa recursivo, que me esta dando inconvenientes precisamente por que debo estar reenviando el valor r^k. Agradezco la ayuda que puedan darme

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Re: Simple problem from Hudak's SOE
by C.Reinke 24 Feb '03

24 Feb '03

> I checked out the type of "snd" in hugs and basically > figured out what it does, but I was wondering where I could find the > definition of this function. Is it possible to look at it in hugs? If your hugs is configured correctly to use an editor, simply type ":find snd". This underappreciated feature is one reason why you want to set the editor in hugs!-) http://cvs.haskell.org/Hugs/pages/hugsman/started.html hth, Claus

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Debugging haskell
by Sengan.Baring-Gould＠nsc.com 24 Feb '03

24 Feb '03

http://www.catb.org/~esr/writings/taoup/html/ch01s06.html states that "debugging often occupies three-quarters or more of development time". I don't think that is my experience in Haskell... more like 1/4 at most. I was wondering what others felt. Sengan

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Simple problem from Hudak's SOE
by M. Parker 24 Feb '03

24 Feb '03

I'm a real newbie to Haskell, and I'm having trouble with a particular problem dealing with higher-order functions. Exercise 5.9 in Hudak's "School of Expression" asks us to write a function, "makeChange," s.t. it makes change for a given amount using coins in a coin supply (represented by a list of decreasing integers). For example, make Change 99 [5,1] ==> [19,4] This chapter is about higher order functions, so I'm assuming he wants us to compute the result using the higher order functions defined in the chapter (map, foldl, foldr). I devised two solutions: {-Solution 1-} makeChange money coinList = zipWith div (scanl mod money coinList) coinList {-Solution 2-} makeChange' money (coin:coins) = let money' = money `mod` coin numCoins = money `div` coin in (numCoins: makeChange' money' coins) makeChange' 0 _ = [] makeChange' _ [] = [] However, my problem is that neither solution uses the higher-order functions defined in the chapter. So is it possible to solve this problem using map and fold? Furthermore, Hudak makes the case that we should strive to find the higher-order solutions instead of the recursive ones because the latter leads to clearer and more concise solutions. Although solution 1 is more concise, I feel like Solution 2 is clearer to me than Solution 1, but maybe this is just because I'm new to haskell and higher order functions. It just seems like its easier to understand the actual algorithm in solution 2 than in solution 1. Thanks, Matt Parker University of North Texas undergrad http://www.cs.unt.edu

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by Domain Monitor 23 Feb '03

23 Feb '03

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Re: can this be written more easily?
by Dean Herington 23 Feb '03

23 Feb '03

[moved to haskell-cafe] While I largely agree with what Nils said, it does seem that arrays are a good match for your application. It is true, unfortunately, as you're discovering, that mutable arrays are awkward in a pure functional language. I think the most appropriate way to deal with them would depend on the larger perspective of your application's control structure. If the array manipulation is necessarily intermixed with input/output, for example, you might consider an essentially imperative approach using `IOArray`s in the `IO` monad. If the input/ouput and array manipulation tend to alternate, `STArray`s in the `ST` monad might offer a more functional approach. If the amount of array manipulation is small, then the approach you've shown so far might be entirely adequate. Dean "Mike T. Machenry" wrote: > I guess I figured that Arrays were the natural data type for the tickets > since it has a fixed size and the elements all have a specific player > associated with them. I am coming from a Scheme background so I am already > very fluent in list manipulation. I'm not an imperative programer, so that's > not really the problem. I just think that Arrays represent this data much > better. > > -mike > > On Fri, Feb 21, 2003 at 01:00:30PM +0100, Nils Decker wrote: > > "Mike T. Machenry" <dskippy(a)ccs.neu.edu> wrote: > > > Hey Everyone, > > > > > > I am having a hard time making a data structure that I can > > > incrimentally > > > update. Mostly because dealing with arrays is so tough. Does anyone > > > think I'm going about this completely the wrong way? This is what I > > > have. > > IMO there is normally no need to use arrays in haskell. You should > > use lists instead, because it is much more easy to use recursion > > over lists. Once you got the pattern, it feels like the natural > > way in haskell. > > It also helps to define more types than just one large type for everything. > > > > > data GameState = GameState { > > > dTickets :: Array Player (Array Ticket Int), > > > fTickets :: Array FugitiveTicket Int, > > > history :: [Move], > > > dLocations :: Array Player Stop, > > > fLocations :: Set Stop > > > } > > > > > > removeTicket :: GameState -> Detective -> Ticket -> GameState > > > removeTicket s d t = > > > s { tickets = (tickets s) // [(d,[(t,((tickets s)!d!t - 1))])] } > > > > why not use > > > > data Ticket = Ticket Int {-value-} deriving (Eq, Show) > > type Tickets = [Ticket] > > > > removeTicket :: Ticket -> Tickets -> Tickets > > removeTicket _ [] = fail "not there" > > removeTicket x (t:ts) > > | x == t = ts > > | otherwise = t : (removeTicket x ts) > > > > Is there a reason, to have different fields and types for detectives? > > data PlayerNames = MrX | Red | Green | Blue deriving (Eq,Show) > > If MrX needs special treatment ( computed move or information shown > > to the player ) you can patternmatch for MrX. For the usage of tickets > > and the history of moves there should be no difference. > > > > There might even be no reason to have PlayerNames as instance of enum. > > As i understand, you want to use succ(player) to find the next player > > to move. It can be easier to have a function that recurses over a > > list of players to run one round. > > > > > This remove ticket function is just terrible and it's common for me to > > > have to do operations like this. It's been hard to make this a > > > function that I can pattern match on, because which piece of data is > > > manipulated depends on the parmeter d (Detective) > > Just split up the huge record and have tiny functions to deal with > > every specific part of it > > > > Summary in a few words: Use many small functions instead of a few > > big ones. Use lists instead of arrays. If you use arrays, first understand > > why you can not use lists in that specific case. Learn to recurse > > over lists! Learn to use map, foldl and foldr. They save you a lot of > > typing and make most functions dealing with lists short and clear. > > > > You should derive Show for all your types and test every new function > > in hugs or ghci. > > > > Regards > > Nils Decker > > > > PS: I have learned haskell a year ago after using imperative languages > > all of my life. At first it is hard to get used to some concepts, > > but then they are wonderful. > > > > PPS: There is another list called haskell-cafe. It is used for discussion > > of problems while this list is meant for short threads and announcements. > > You might want to subscribe to it. > > > > > > -- > > Freedom of speech is wonderful - right up there with the freedom > > not to listen. > > > > Nils Decker <nils(a)ndecker.de>

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a monadic if or case?
by David Roundy 20 Feb '03

20 Feb '03

Hello all. I was just thinking that there ought to be a better way to write the following code. It seems to be a common case that within a 'do' I bind a variable that I only intend to use once, in an if or case statement. It occurred to me that there ought to be a better way to do this. For example, it seems like the following code: whatisit :: String -> IO String whatisit f = do isdir <- doesDirectoryExist f if isdir then return "dir" else do isfile <- doesFileExist f if isfile then return "file" else return "nothing" could be replaced by something like: whatisit :: String -> IO String whatisit f = do ifM doesDirectoryExist f then return "dir" else ifM doesFileExist f then return "file" else return "nothing" Is there any way I could do something like this? -- David Roundy http://civet.berkeley.edu/droundy/

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