- Haskell-Cafe - Haskell.org

Haskell
by J Brown 21 Dec '01

21 Dec '01

hi, I am only a beginner of programming with Haskell, how do I define each of the following: * maxFirst :: (Int,Int) -> (Int,Int) -> (Int,Int) which returns, of its two input tuples, the one whose first component is the highest. * Define a function maximumFirst :: [(Int,Int)] -> (Int,Int) which returns, from its list of input tuples, the tuple whose first component is highest, assuming input list is non-empty. A function is given: maxf :: [Int] -> Int such that if maxf [x1,....,xn] = xi then elem xi [x1,...,xn] == True f xi >= f x1 f xi >= f x2 ...... fxi >= f xn * How do you define the function maxf (above) which returns the element of its non-empty input list for which f is maximal. *and to define a function minf which returns the element from its non-empty input list for which f is minimal. *how do I define the function maxIndexf :: [Int] -> Int such that if maxf xs = i for non-empty xs, then 0 <= i < length xs f (x s !! i) >= f (x s !! 0) f (x s !! i) >= f (x s !! 1) .... f (x s !! i) >= f (last xs) that is it returns the index of the element of its input list for which f is maximal. __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com

2 1

Haskell
by Law 20 Dec '01

20 Dec '01

hi, how do I define this function: tupleWithf :: [Int] -> [(Int, Int)] such that tupleWithf [x1,x2,.....,xn] = [ (f x1, x1), (f x2 x2),...., (f xn, xn) ]

2 1

Re: We need "Documentation"
by Eray Ozkural (exa) 20 Dec '01

20 Dec '01

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Hal, On Thursday 13 December 2001 20:47, Hal Daume III wrote: > I think we should move this off the mailing list. I'm willing to > spear-head such an effort. Anyone who is interested in contributing, > please email me. I'll compile a list of people and we can figure out what > we want to do. I think it's okay to discuss in haskell-cafe. I'd like to contribute too but I have little time nowadays trying to finish writing my msc. thesis. I do have a few suggestions though: 1. Using wiki does not seem to be a bad idea. You may also consider using a word processing system such as LaTeX though. It's easy to convert LaTeX to html and it yields very quality prints as you all know. However, for true collaboration wiki is the king. There is already a nice Haskell wiki appropriate for such a project IIRC. 2. I don't think following the outline of a class based imperative language will be fitting for a high quality Haskell tutorial. I think you should look at OcaML tutorial and the python tutorial, and combine ideas from both as well as your own :) Thanks, - -- Eray Ozkural (exa) <erayo(a)cs.bilkent.edu.tr> Comp. Sci. Dept., Bilkent University, Ankara www: http://www.cs.bilkent.edu.tr/~erayo GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F 7C07 AE16 874D 539C -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE8IMf0fAeuFodNU5wRAunNAJ9eS6w6b5ho45htdzDLxhEFhK5EtACgjVdY dP0ObV/+usABuJgRL7aWV9M= =SLzH -----END PGP SIGNATURE-----

2 1

RE: space efficiency question
by Simon Peyton-Jones 18 Dec '01

18 Dec '01

[redirected to haskell-cafe] No, GHC doesn't test pointer equality first. It's unsound to do so, since it might turn bottom into non-bottom, or remove `seq` calls that kill space leaks. S -----Original Message----- From: Frank Dellaert [mailto:frank@cc.gatech.edu] Sent: 18 December 2001 11:48 To: Simon Peyton-Jones; Haskell(a)haskell.org Subject: Re: space efficiency question Thanks ! Now, a small follow-up question: if I subsequently test 2 Instances whether they were instantiated with the same Attributes value, as in test (Instance a1 _) (Instance a2 _) = (a1==a2) will this be implemented efficiently ? I.e. will it check first whether the pointers happen to be the same, and only then do a full Eq comparison ? Cheers Frank ----- Original Message ----- From: Simon <mailto:simonpj@microsoft.com> Peyton-Jones To: Frank Dellaert <mailto:frank@cc.gatech.edu> ; Haskell(a)haskell.org Sent: Monday, December 17, 2001 10:24 AM Subject: RE: space efficiency question Now: if I have a couple of thousand of these Instances, created with the same Attributes value, as in: a = Attributes ["a","bc"] i0000 = Instance a [1,2] i0001 = Instance a [1,1] ... i7896 = Instance a [2,1] can I be assured that there are not thousands of actual copies of a but that they simply have a pointer to a ? I'm interested in the answer for both ghc (compiled/interpreted) as interpreted Hugs. Yes for GHC: you'll only get one copy of 'a'. Simon

1 0

sending values between actions
by Nuno Silva 17 Dec '01

17 Dec '01

can anyone tell me how to send values from one action to another??? I have main0 = do s <- getLine writeFile "text.txt" s main1 = do .... I'd like to use main0 info on main1 action to build a nXm multisquares box... anyone understood this???? please help... __________________________________________________ Do You Yahoo!? Check out Yahoo! Shopping and Yahoo! Auctions for all of your unique holiday gifts! Buy at http://shopping.yahoo.com or bid at http://auctions.yahoo.com

3 2

Re: Pointers in Haskell??
by Jeff Dalton 17 Dec '01

17 Dec '01

The answers are making this question seem trickier than I'd thought it was, because so far they (both) make it sound like structure-sharing is tied very closely to names / variables. For instance: > In Haskell, you can arrange for a large data structure to be shared by > giving it a name, and then using the name wherever you'd use a pointer in > some other language. The idea here seems to be that you have a name that refers specifically to the struccture you wish to share (as opposed to needing only an expression whose value is that structure), and then there are two possible interpretations: One possible interpretation is: this is *a* way to get sharing, to show that it's possible to have shared structures. The other possible interpretation is: this is *the* (only) way to do it. What I'd expected was that it would suffice to have an expression (and indeed that implementationally it would be much like Lisp or Java so that values were always (except in exceptional cases) secretly "pointers to".) -- Jeff

3 2

Error message
by victoria borisovna rozhkova 17 Dec '01

17 Dec '01

Hello, what does it mean: Variable definition clashes the import. thank you Vika

2 1

modules, classes, instances
by David Feuer 16 Dec '01

16 Dec '01

Well, I've been doing some more stupid thinking, and I've decided that I am not satisfied with the module system in haskell, or the way it deals with namespaces. It seems to me that there are four kinds of things that need to be dealt with: classes, instances, types, values, and possibly some kind of ML-style structures/functors. I see no good reason not to treat a class declaration similarly to the way Haskell treats modules. For example, if I have a class class A x where f1::... f2::... I would like to access the functions by writing A.f1, A.f2, etc. There should, however, be some way to unqualify the names, the way ML allows structures to be opened. Of course, this can be done in Haskell by putting the class declaration inside a module, but doing so can be annoying, and I think this style clearly attaches the method to the class it belongs to. It thereby lets you have a List l=>List.empty::l and also a Bag b=>Bag.empty::b automatically. Then there are types and instance declarations. I have the strange feeling that they go together. So I'm thinking it might be good to have some kind of type/instance module that can export types and instances. I can't understand why it is that instance declarations can't contain nested declarations other than the required ones. I think it would be nice if they could contain other declarations, as modules can. Also, instances could be imported (not just as part of a module, but separately) ML-style structures/functors. For everything else. Finally: I want nested modules!!! They probably couldn't be compiled separately, but they'd provide some namespace control. -- /Times-Bold 40 selectfont/n{moveto}def/m{gsave true charpath clip 72 400 n 300 -4 1{dup 160 300 3 -1 roll 0 360 arc 300 div 1 1 sethsbcolor fill}for grestore 0 -60 rmoveto}def 72 500 n(This message has been)m (brought to you by the)m(letter alpha and the number pi.)m(David Feuer) m(David_Feuer@brown.edu)m showpage

2 1

drawing grids
by Nuno Silva 14 Dec '01

14 Dec '01

hello can anyone tell me how to draw grids?? it's like this: Imagine that I say that I want the grid to be 5X6, it will compute the distbetween the limits and then draw them. if I want a 6X10, it will do the same. thanks __________________________________________________ Do You Yahoo!? Check out Yahoo! Shopping and Yahoo! Auctions for all of your unique holiday gifts! Buy at http://shopping.yahoo.com or bid at http://auctions.yahoo.com

1 0

overloading efficiency
by David Feuer 14 Dec '01

14 Dec '01

I have read that using overloaded functions is expensive because overloading is implemented by passing dictionaries around. These various sources seem to agree that this in necessary to allow separate compilation. I don't understand why this is... Couldn't the functions be specialized at link time? -- /Times-Bold 40 selectfont/n{moveto}def/m{gsave true charpath clip 72 400 n 300 -4 1{dup 160 300 3 -1 roll 0 360 arc 300 div 1 1 sethsbcolor fill}for grestore 0 -60 rmoveto}def 72 500 n(This message has been)m (brought to you by the)m(letter alpha and the number pi.)m(David Feuer) m(David_Feuer@brown.edu)m showpage

2 1