- Haskell-Cafe - Haskell.org

Newbie question -- fromInt
by Tim Rowe 28 Apr '05

28 Apr '05

I'm working through "The craft of functional programming" , and I've come to excercise 3.14: "Give a function to return the average of three integers". My attempt at an answer is: averageThree :: Int -> Int -> Int -> Float averageThree a b c = fromInt(a + b + c) / 3 but when I try to load this file I get an error, "Undefined variable fromInt". I get the error on winHugs version Nov 2003 and on Hugs version Nov 2003 (in both cases in Hugs mode). Curiously, when I found a machine with an old (2002) version of Hugs on it the file loaded fine. Has something changed between versions of Hugs, is my code reasonable, and how /do/ I convert an Int to a Float in Hugs version Nov 2003? Thanks in advance, -- Tim Rowe

2 1

RE: [Haskell-cafe] fptools in darcs now available
by Simon Marlow 28 Apr '05

28 Apr '05

On 28 April 2005 14:26, John Goerzen wrote: > To do that though, we should really identify a permanent home for the > canonical fptools darcs repo. I'm not really set up to provide > accounts for those that would need write access, and I don't want to > be the gatekeeper (I suspect nobody else wants that either <g>). > > If cvs.haskell.org is up to the task, that would be an ideal location > IMHO. I haven't yet convinced tailor.py to work with the pserver for > fptools, so if it can access the repo on the local filesystem, that > would be ideal. Plus, one could then cron it to run frequently. cvs.haskell.org is the way to go - we already have all the required accounts set up on that machine, and you get local access to the CVS repo. I've asked the owner of the box if it can be upgraded. Failing that, you'll just have to make darcs more efficient ;-) There's already an old version of darcs installed (0.9.18), and darcs repos in /home/darcs. Cabal is kept there, for example. We need to upgrade that version of darcs, I presume - can you point me at an appropriate RPM? > Now, having said that, I did keep the request in mind. I figure that > this big repo can be split up into smaller ones at any time after the > CVS mirroring is stopped. For each smaller one, the process would be: > > 1. Branch (darcs get) the master repo > > 2. Delete all the files that don't apply to the smaller project > > 3. Rename the smaller project's files as appropriate > > 4. Checkpoint here Won't that leave a lot of useless history in each individual project? When I do 'darcs get' locally to check out a few sub-repositories, won't I get N copies of all the old history? >> - commit mails to the cvs-<blah>@haskell.org lists > > I figure a cron job can do this. Every x minutes, run darcs changes > -s, and send copies of never-before-seen logs to the list. Should be > fairly trivial. I can do this too. (But only after the CVS gateway > is disabled; otherwise, you'd get two copies for every commit) Keep the cvs gateway for the time being; at some point we can seamlessly change to generating commit mails from the darcs repository. BTW, one thing I've always wanted is a hyperlink in the commit mail that takes me straight to the patch. Cheers, Simon

2 1

Ambiguous type signature in class declaration
by Bo Herlin 28 Apr '05

28 Apr '05

Hi I am trying to make a class like this: > class CRank a b where > rank :: a -> b -> Maybe Integer -- Nothing means b is out of range or badly constructed > unrank :: a -> Integer -> Maybe b -- Nothing means rank is out of range > count :: a -> Maybe Integer -- Nothing means infinity with possible instances like > data DPrime = Prime deriving (Show) > > instance CRank DPrime Integer where > rank Prime x = Nothing -- to be implemented: (rank Prime 11) should give (Just 4) > unrank Prime r = Nothing -- to be implemented: (unrank Prime 4) should give (Just 11) > count _ = Nothing -- Nothing means infinity and > data DFibonacci = Fibonacci deriving (Show) > > instance CRank DFibonacci Integer where > rank Fibonacci x = Nothing -- to be implemented > unrank Fibonacci r = Nothing -- to be implemented > count _ = Nothing -- Nothing means infinity but all i get is ERROR "./Cafe.lhs":8 - Ambiguous type signature in class declaration *** ambiguous type : CRank a b => a -> Maybe Integer *** assigned to : count Any suggestions anyone? Thanks in advance /Bo

3 8

REMINDER: Contributions to the HC&A Report (May 2005 edition)
by Andres Loeh 28 Apr '05

28 Apr '05

Dear Haskellers, the deadline for the May 2005 edition of the Haskell Communities and Activities Report is only a few days away -- but this is still enough time to make sure that the report contains a section on *your* project, on the interesting stuff that you've been doing; using or affecting Haskell in some way. Many projects that have been included in former reports have not yet updated their entries. Please have a look at http://haskell.org/communities/topics.html and at the November 2004 edition for reference. * Has your project been listed in previous Reports, but is not yet updated? Please write a short update! * Are you no longer working on a project that was included in the Report? Write up what you are working on instead, and tell me if someone else has picked up the project. * Is some project you have heard about not included in any previous Report? Please let me know ... There is still time to write a completely new entry, on a new compiler, tool, library, company, user group, idea, ... -- as long as there is a connection to the Haskell language, there is a place for it in the Report! Submissions are due by next Tuesday, that is Tuesday, 03 May 2005. Please mail your entries to <hcar at haskell dot org>, in plain text or pseudo-(La)TeX format. More information can be found in the original Call for Contributions at http://www.haskell.org/pipermail/haskell/2005-April/015688.html I look forward to receiving your contributions. Thanks a lot, Andres (current editor) -- Haskell Communities and Activities Report (http://haskell.org/communities) 05/2005 edition submission deadline: 03 May 2005 <hcar(a)haskell.org>

1 0

RE: [Haskell-cafe] Function composition confusion
by Anuj Seth 28 Apr '05

28 Apr '05

Thanks, that clears it up I was thinking that the output of (splitAt blockSize) would get piped to (splitAt blockSize . snd). Did not realize it goes to the whole iterate part. -----Original Message----- From: Henning Thielemann [mailto:lemming@henning-thielemann.de] Sent: Wednesday, April 27, 2005 4:37 PM To: Anuj Seth Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Function composition confusion On Wed, 27 Apr 2005, Anuj Seth wrote: > Excuse the newbie'ness of my question. > Saw an example in Two Dozen Questions by Rexx Page. > > blocks blockSize = > takeWhile ( not . null ) . map fst . > iterate (splitAt blockSize . snd) . splitAt blockSize > > What is the meaning of the . operators outside the parentheses ? The same as inside. :-) (splitAt blockSize) transforms a list to a pair of lists. So imagine a list input at the right of (splitAt blockSize) and a pair output at the left. This pair is piped into the 'iterate' part which transforms a list of pairs into a list of pairs of lists. This in turn is piped to (map fst) and so on. The information contained in this message is proprietary of Amdocs, protected from disclosure, and may be privileged. The information is intended to be conveyed only to the designated recipient(s) of the message. If the reader of this message is not the intended recipient, you are hereby notified that any dissemination, use, distribution or copying of this communication is strictly prohibited and may be unlawful. If you have received this communication in error, please notify us immediately by replying to the message and deleting it from your computer. Thank you.

2 1

Function composition confusion
by Anuj Seth 27 Apr '05

27 Apr '05

Hi, Excuse the newbie'ness of my question. Saw an example in Two Dozen Questions by Rexx Page. blocks blockSize = takeWhile ( not . null ) . map fst . iterate (splitAt blockSize . snd) . splitAt blockSize What is the meaning of the . operators outside the parentheses ? Thsi does not look like function composition to me. Thanks, Anuj. The information contained in this message is proprietary of Amdocs, protected from disclosure, and may be privileged. The information is intended to be conveyed only to the designated recipient(s) of the message. If the reader of this message is not the intended recipient, you are hereby notified that any dissemination, use, distribution or copying of this communication is strictly prohibited and may be unlawful. If you have received this communication in error, please notify us immediately by replying to the message and deleting it from your computer. Thank you.

2 1

Small Project coder needed
by Mike Richards 27 Apr '05

27 Apr '05

Hi everyone, Calling all Haskell coders! I just got back into coding haskell and need to take over from another lecturer at our University.I need to get a solution done out to a problem he set my students last week, and as Im attending a conference on XP for the next couple of days, I doubt I'll have time to get it done between flights. Its not a very large project, and should only take an hour or two of your time to code.I can offer between £30-45 if it meets the specs I set (and functions 100% of course!). Let me know before the weekend, Thanks Mike Richards -- _______________________________________________ Get your free email from http://mail.oxygen.ie Powered by Outblaze

1 0

do statement
by Walt Potter 27 Apr '05

27 Apr '05

Can anyone point me to examples of using the do statement which does not involve IO? I find that the texts I have do not provide enough examples. Thanks, Walt

3 2

RE: [Haskell-cafe] FFI and pointers to pointers
by Simon Marlow 26 Apr '05

26 Apr '05

On 25 April 2005 21:40, John Meacham wrote: > On Mon, Apr 25, 2005 at 12:51:04PM +0100, Simon Marlow wrote: >> The syntax you want is >> >> foreign import ccall "&bar" :: Ptr CInt >> >> But note that GHC 6.4 has a bug whereby this doesn't work as expected >> when compiling via C. Either use the native code generator (-fasm) >> or wait for 6.4.1. > > Any ETA on 6.4.1? I am debating whether to rewrite code to work around > this bug or just wait for the point-release? We don't have any firm plans yet. It makes sense to wait until the flow of bug reports has slowed, and that hasn't happened yet. But I'll take your message as a vote for sooner rather than later. Cheers, Simon

1 0

How to debug GHC
by Monique Louise 26 Apr '05

26 Apr '05

Hi, all, I'm developing a back end for GHC and I have the following problem: my program is throwing an "empty list exception" due to head [] and I need to compile GHC with -prof -auto-all in order to see the stack trace when running it with +RTS -xc -RTS. I changed the makefile but the option +RTS -xc -RTS was not recognized as an available RTS option Does anyone have any idea about how I can do that ? Thanks in advance, -- ________________________________ Monique Louise B.Monteiro Msc Student in Computer Science Center of Informatics Federal University of Pernambuco

1 0