March 2002 - Haskell-Cafe

Continuation Passing Style
by Tom Bevan 14 Mar '02

14 Mar '02

I noticed that Ralf Hinze posted a CPS monad yesterday. Would someone be kind enough to post a simple example of a function that uses CPS. Thanks Tom

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(no subject)
by 정종길 14 Mar '02

14 Mar '02

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Hiring Haskell programmers
by Mark Carroll 14 Mar '02

14 Mar '02

How easy is it to hire reasonable Haskell programmers? Of course, this may mean, hiring people with the aptitude and interest to quickly learn Haskell. Has anyone any experience of this that they can share? -- Mark

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Date Sorting...
by Michael Ruth 14 Mar '02

14 Mar '02

Hello, I am pretty new to Functional programming, and I am having the following problem. I need to sort a list of dates provided in an input file. One date per line.. day month year is the order. And each is represented by integers seperated by spaces. And once the list is sorted output the results to a file. I having trouble knowing where to start. This is what I was thinking the best line of attack was: --read in the lines one by one into a list --then iterate thru the list while breaking the strings into [Int] list -- of 3 elements [day,month,year] --then quicksort overall list --then print out overall list line by line I understand this is a imperative approach, but this is the way I was taught how to program, well that and OOP. How do I go about writing this in a functional sense ? And is there another way to do this AKA another plan of attack ?. Thanks very much for your time and consideration. Michael _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.

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Re: Debugging Haskell
by Jerry, JiJie 13 Mar '02

13 Mar '02

Thank you very much, it works :) As I think it might be of help to the others, I'm cc'ing this mail to the mailing list where my question was initially posted with the revised code attached. Sincerely, Jerry * Simon Marlow <simonmar(a)microsoft.com> [020313 20:16]: > [ redirected to glasgow-haskell-bugs(a)haskell.org ] > > > Good day everyone, I was fiddling around with this tiny echo > > client/server haskell program from 'The Great Language Shootout' > > site (http://www.bagley.org/~doug/shootout/) and got stuck. > > > > The code (attached) has been reformatted with minimal API tweak > > (mkPortNumber, writeSocket, readSocket) to please my ghc-5.02.2, and > > all what I get is something stuck forever after the first > > iteration: > > > > $ ./echo 3 > > Client wrote: Hello there sailor > > Server recv: Hello there sailor > > Server read: Hello there sailor > > Server wrote: Hello there sailor > > > > After adding all these print statement, I still don't have a clue > > what's jammed there. Hope someone here can shred some light. > > It turns out to be a bug in the network library; we weren't putting the > socket returned from accept into non-blocking mode. It works fine if > you use Handles rather than send/recv because the act of making a Handle > from a file descriptor sets non-blocking mode on the FD, so a workaround > for your program is just to insert a call to socketToHandle on the > socket returned from accept (you don't have to use the Handle, just > calling socketToHandle has the desired effect). > > Thanks for the report. > > > BTW, I'd also like to take this chance to ask how to debug a haskell > > program in general? > > With putStr or IOExts.trace, or using one of the more sophisticated > debugging tools such as the nhc98 tracing system or Andy Gill's Observe > library. You should be able to find links on the www.haskell.org pages > to these projects. > > Cheers, > Simon >

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Question about something in Hudak's book
by Ludovic Kuty 13 Mar '02

13 Mar '02

Hello, I have just begun reading Hudak's The Haskell School of Expression and i am wondering something about the manner he calculates the area of a polygon (made of vertices) on page 27. He wrote: area(Polygon (v1:vs)) = polyArea vs where polyArea :: [Vertex] -> Float polyArea (v2:v3:vs') = triArea v1 v2 v3 + polyArea(v3:vs') polyArea _ = 0 i do not understand why variable v1 is pattern-matched in the definition of area and made "global" to polyArea instead of doing it with polyArea like this: area(Polygon vs) = polyArea vs where polyArea :: [Vertex] -> Float polyArea (v1:v2:v3:vs') = triArea v1 v2 v3 + polyArea(v3:vs') polyArea _ = 0 Is it an idiom or some sort of optimization ? Or just the way he likes it ? TIA Ludovic Kuty

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Re: IO and syntax
by Jyrinx 13 Mar '02

13 Mar '02

> readIL' should be defined differently. Try: > > readIL' ans = > do > a <- ... > b <- ... > if a==0 && b==0 then return ans else readIL' ((a,b):ans) > > (note that I added "return".) > I don't know what the ...s should be, but they definitely should not be > what they are: readDec is some kind of number reader, but it doesn't > deal with actually getting input. Other people here will know more. > One way is to use getStr (or is it getStrLn??), split it at > the space, and then use read to get the numbers. However, I suspect > there is a better way. Good luck. Ah! You were right - I was confused as to the purpose of readDec and friends. I changed it to the following, which works: readIncompatList :: IO [(Player, Player)] readIncompatList = readIL' [] where readIL' ans = do ln <- getLine let [a, b] = map read (words ln) if a == 0 && b == 0 then return ans else readIL' ((a,b):ans) Thanks! Jyrinx jyrinx_list(a)mindspring.com (It occurs to me ... I love Haskell: that "let [a, b] = map read (words ln)" would be so much longer in C ... :-) )

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Hiring and porting
by Mark Carroll 13 Mar '02

13 Mar '02

Thanks, everyone, for your responses! It's all been very helpful. Some things I should mention, then: We're based in central Ohio, but are not currently hiring FPers. Whether we will be in the future depends somewhat on this porting issue. However, if we do decide to hire any Haskell programmers, I shall mention that here. Ideally, yes, we'd look to hire practising programmers who already know Haskell and wish that they could be using it in their job, although I'd fear that there may not be enough such people available to us. The larger application is a simulation engine that involves parsing, symbolic processing, etc. in which correctness in important and that will require ongoing improvement and modification, so I was thinking that use of Haskell would be appropriate. (Use of C++ would make it faster, though, I expect. (-:) However, I do fear that Ashley's correct in suggesting that you'd probably need to rewrite everything to sensibly translate the Haskell to C or Java or whatever, and it is both reasonable and plausible that some larger clients will demand use of a more mainstream language in anything that we deliver to them so that they don't rely on us for maintenance. So, first we have to figure out if we should use Haskell at all, because a likely need for a non-trivial port in the future could easily negate, in time and cost, the initial productivity benefits we might gain from Haskell. -- Mark

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Re: IO and syntax
by Ashley Yakeley 13 Mar '02

13 Mar '02

At 2002-03-12 20:18, Jyrinx wrote: >Of course, this "probable cause" is ridiculous at face value ... anyway, >my code makes as much sense to me as it can at the moment ... how should >I get it to make sense to GHC? (I couldn't find any examples on I can see the bug. It's a very common one when using monads and the 'do' notation -- confusing 'a' with 'IO a'. When in doubt, add type signatures to everything. You'll soon narrow it down. -- Ashley Yakeley, Seattle WA

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IO and syntax
by Jyrinx 12 Mar '02

12 Mar '02

I just got back from a high-school programming competition where the only allowed languages were C[++] and Java (*grmbl*). To see just how cool Haskell is, I'm redoing some of the problems in this more enlightened language :-) Anyway, that explains the rather strict requirements for my program's interface. At one point, the user must enter pairs of numbers like so: 1 4 2 4 3 2 4 1 4 3 0 0 (where two zeros terminate the input). Here is my latest attempt at a function to do this part of the problem (and it's the easy part ...) --- snip --- -- type Player = Int readIncompatList :: IO [(Player, Player)] readIncompatList = readIL' [] where readIL' ans = do a <- readDec b <- readDec if a == 0 && b == 0 then ans else readIL' ((a, b):ans) --- snip --- GHC complains with the following: --- snip --- prob13.hs:37: Couldn't match `String -> t' against `[t1]' Expected type: String -> t Inferred type: [t1] Probable cause: `(:)' is applied to too many arguments in the call ((:) (a, b) ans) In the first argument of `readIL'', namely `((a, b) : ans)' --- snip --- Of course, this "probable cause" is ridiculous at face value ... anyway, my code makes as much sense to me as it can at the moment ... how should I get it to make sense to GHC? (I couldn't find any examples on something as mundane as console I/O; funny how people are so fixated on the real stuff :-) ) Thanks! Jyrinx jyrinx_list(a)mindspring.com

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