- Haskell-Cafe - Haskell.org

(no subject)
by Noriyuki Sugita 05 Jan '02

05 Jan '02

1 0

Compiler error using IO
by Tom Bevan 04 Jan '02

04 Jan '02

When trying to right IO I continually run across this sort of compile error which I just don't understand. Any help would be very muich appreciated. Function: while :: IO Bool -> IO () -> IO () while test action = do res <- test if res then do action while test action else return () Error: io.hs: 112 lines, 2463 characters. Reading file "io.hs": Parsing ERROR io.hs:110 - Last generator in do {...} must be an expression Tom

2 1

Intro to Functional Prog. by Bird, Section 2.5
by Shawn P. Garbett 02 Jan '02

02 Jan '02

I've been working on understanding Haskell using hugs98 and playing with some of the examples in Bird's book. In section 2.5, page 46 of the second edition, he gives an example of dependent types: But I get this error I've been unable to solve: ---------------------------------------------------------------------------------------------------------------- --Type checking --ERROR sec2-5.hs:8 - Type error in application --*** Expression : case' (Left' f,Right' g) --*** Term : (Left' f,Right' g) --*** Type : (Either' (d -> e) f,Either' g (h -> i)) --*** Does not match : (a -> b,c -> b) data Either' a b = Left' a | Right' b case' :: (a->g, b->g) -> Either' a b -> g case' (f, g) (Left' x) = f x case' (f, g) (Right' y) = g y plus' :: (a->b , g->d) -> Either' a g -> Either' b d plus' (f,g) = case' (Left' f, Right' g) ----------------------------------------------------------------------------- Please note that the version I've entered is correct by the errata published for this section.

3 2

RE: haskell sockets
by Simon Marlow 02 Jan '02

02 Jan '02

> i want to write/read data to the soccerserver of the robocup. > the sample > code below works, but i want to have a handle for writing to > the server > instead of SocketPrim.sendTo. how can i do that ? SocketPrim.socketToHandle gives you back a Handle which you can use to read/write to/from using Haskell's normal IO operations. Note that you need to call SocketPrim.connect, even for a Datagram socket, to fix the endpoint for the socket. Cheers, Simon

1 0

PHOTOS XXX GRATUITES !!!
by sandy000＠hotmail.com 29 Dec '01

29 Dec '01

1 0

haskell sockets
by Thomas Rabe 22 Dec '01

22 Dec '01

hello, i want to write/read data to the soccerserver of the robocup. the sample code below works, but i want to have a handle for writing to the server instead of SocketPrim.sendTo. how can i do that ? import IO import Socket import SocketPrim local = "127.0.0.1" portnum = 6000 local' = iNADDR_ANY portnum' = aNY_PORT main = withSocketsDo $ do -- create udp socket s1 <- socket AF_INET Datagram 0 ia <- inet_addr local -- bind socket to first free port given by system bindSocket s1 (SockAddrInet ( portnum') local') -- write to soccerserver on port 6000 SocketPrim.sendTo s1 "(init myteam)" (SockAddrInet ( portnum) ia) h' <- socketToHandle s1 ReadWriteMode hSetBuffering h' LineBuffering -- hPutsStr not works: Reason: Transport endpoint is not connected ?! -- hPutStr h' "foo" -- bad style infinity loop, just for testing testloop h' testloop h = do b <- hWaitForInput h 2000 if b == True then do line <- hGetChar h hFlush h putChar line else putStrLn "End of Input" testloop h Greetings, Thomas Rabe. -- mail: raven at drehmoment dot org -- home: http://hal9000.drehmoment.org -- more: http://drehmoment.org

1 0

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