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RE: Date Sorting Round Three
by Garner, Robin 15 Mar '02

15 Mar '02

The problem you are trying to come to grips with here is one of the watersheds in the learning curve that most people learning Haskell encounter, namely how to use values returned in Monads. I could point out that you have already solved the same problem when you used getLine. The question you need to ask yourself is not, "how do I convert a value of type IO String to a value of type String", but "how do I use the result of a calculation within the IO Monad". Look at where you've already solved the problem, and the answer will appear almost immediately. Robin -----Original Message----- From: Michael Ruth [mailto:meruth@hotmail.com] Sent: Friday, 15 March 2002 11:30 To: haskell-cafe(a)haskell.org Subject: Date Sorting Round Three long set of code here, I think I figured out what you all were telling me. Compose small functions then compose bigger functions from smaller functions and so on... The problem I am having is I have an (IO String) where I need a (String) and it reports it as such: ERROR "C:\Documents and Settings\drunkenmike\My Documents\Programming\Csci4501\H askell\datesort.hs":41 - Type error in application *** Expression : separateIntoLines (getInput x) *** Term : getInput x *** Type : IO String *** Does not match : [Char] I am gonna mark line 41 with a * at the beginning of the line. I have been looking high and low for the answer to IOString -> String.... all I think I need to do (I think anyways) By the way, thanks to everyone for their help. import IO --- -- Control Flow --- datesort :: IO() datesort = do { putStr "Enter a filename: "; theFile <- getLine; sortAndOutput (readInAndGetReady theFile); } -- -- Composite functions -- 1) Read in & Get ready to sort -- 2) sort & output -- -- readInAndGetReady :: String -> [String] *readInAndGetReady x = fixDates (makeSortableDates (linesToDates (separateIntoLines (getInput x)))) sortAndOutput :: [String] -> IO() sortAndOutput x = writeTheFile (getOutput (makePrintableDates (qsort x))) -- -- Make the date printable. Printable means a string consisting of -- dates in the form dd mm yyyy (more or less undoes the make sortable f x) -- makePrintableDates :: [String] -> [String] makePrintableDates x = map makePrintableDate x -- -- Make the date printable. Printable means a string consisting of -- dates in the form dd mm yyyy (more or less undoes the make sortable f x) -- makePrintableDate :: String -> String makePrintableDate x = unwords (reverse (words x)) -- -- Make all the dates sortable. Sortable means a string consisting of -- dates in the form yyyy mm dd -- makeSortableDates :: [[String]] -> [String] makeSortableDates x = map makeSortableDate x -- -- Make the date sortable. Sortable means a string consisting of -- dates in the form yyyy mm dd -- makeSortableDate :: [String] -> String makeSortableDate x = unwords (reverse x) -- -- Get the input from the given file -- getInput :: String -> IO String getInput myfile = readFile myfile -- -- Seperate the input file into an list of Strings -- separateIntoLines :: String -> [String] separateIntoLines str = lines str -- -- Transform all the lines into dates -- linesToDates :: [String] -> [[String]] linesToDates x = map lineToDate x -- -- Transform the lines into dates -- lineToDate :: String -> [String] lineToDate x = words x -- -- Transform the dates into lines -- getOutput :: [String] -> String getOutput x = unlines x -- -- Fix all the dates so that they are in the form 0x if x < 10 -- fixDates:: [String] -> [String] fixDates x = map fixPrefixes x -- -- Fix the dates so that they are in the form 0x if x < 10 -- fixPrefixes :: String -> String fixPrefixes p | p == "1" = "01" | p == "2" = "02" | p == "3" = "03" | p == "4" = "04" | p == "5" = "05" | p == "6" = "06" | p == "7" = "07" | p == "8" = "08" | p == "9" = "09" | otherwise = p -- -- Quicksort the List of strings -- qsort :: [String] -> [String] qsort [] = [] qsort (x:xs) = qsort elts_lt_x ++ [x] ++ qsort elts_greq_x where elts_lt_x = [y | y <- xs, y < x] elts_greq_x = [y | y <- xs, y >= x] -- -- Write the ouput string to a file -- writeTheFile :: String -> IO() writeTheFile x = writeFile "output.txt" x _________________________________________________________________ Chat with friends online, try MSN Messenger: http://messenger.msn.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Date Sorting Round Three
by Michael Ruth 15 Mar '02

15 Mar '02

long set of code here, I think I figured out what you all were telling me. Compose small functions then compose bigger functions from smaller functions and so on... The problem I am having is I have an (IO String) where I need a (String) and it reports it as such: ERROR "C:\Documents and Settings\drunkenmike\My Documents\Programming\Csci4501\H askell\datesort.hs":41 - Type error in application *** Expression : separateIntoLines (getInput x) *** Term : getInput x *** Type : IO String *** Does not match : [Char] I am gonna mark line 41 with a * at the beginning of the line. I have been looking high and low for the answer to IOString -> String.... all I think I need to do (I think anyways) By the way, thanks to everyone for their help. import IO --- -- Control Flow --- datesort :: IO() datesort = do { putStr "Enter a filename: "; theFile <- getLine; sortAndOutput (readInAndGetReady theFile); } -- -- Composite functions -- 1) Read in & Get ready to sort -- 2) sort & output -- -- readInAndGetReady :: String -> [String] *readInAndGetReady x = fixDates (makeSortableDates (linesToDates (separateIntoLines (getInput x)))) sortAndOutput :: [String] -> IO() sortAndOutput x = writeTheFile (getOutput (makePrintableDates (qsort x))) -- -- Make the date printable. Printable means a string consisting of -- dates in the form dd mm yyyy (more or less undoes the make sortable f x) -- makePrintableDates :: [String] -> [String] makePrintableDates x = map makePrintableDate x -- -- Make the date printable. Printable means a string consisting of -- dates in the form dd mm yyyy (more or less undoes the make sortable f x) -- makePrintableDate :: String -> String makePrintableDate x = unwords (reverse (words x)) -- -- Make all the dates sortable. Sortable means a string consisting of -- dates in the form yyyy mm dd -- makeSortableDates :: [[String]] -> [String] makeSortableDates x = map makeSortableDate x -- -- Make the date sortable. Sortable means a string consisting of -- dates in the form yyyy mm dd -- makeSortableDate :: [String] -> String makeSortableDate x = unwords (reverse x) -- -- Get the input from the given file -- getInput :: String -> IO String getInput myfile = readFile myfile -- -- Seperate the input file into an list of Strings -- separateIntoLines :: String -> [String] separateIntoLines str = lines str -- -- Transform all the lines into dates -- linesToDates :: [String] -> [[String]] linesToDates x = map lineToDate x -- -- Transform the lines into dates -- lineToDate :: String -> [String] lineToDate x = words x -- -- Transform the dates into lines -- getOutput :: [String] -> String getOutput x = unlines x -- -- Fix all the dates so that they are in the form 0x if x < 10 -- fixDates:: [String] -> [String] fixDates x = map fixPrefixes x -- -- Fix the dates so that they are in the form 0x if x < 10 -- fixPrefixes :: String -> String fixPrefixes p | p == "1" = "01" | p == "2" = "02" | p == "3" = "03" | p == "4" = "04" | p == "5" = "05" | p == "6" = "06" | p == "7" = "07" | p == "8" = "08" | p == "9" = "09" | otherwise = p -- -- Quicksort the List of strings -- qsort :: [String] -> [String] qsort [] = [] qsort (x:xs) = qsort elts_lt_x ++ [x] ++ qsort elts_greq_x where elts_lt_x = [y | y <- xs, y < x] elts_greq_x = [y | y <- xs, y >= x] -- -- Write the ouput string to a file -- writeTheFile :: String -> IO() writeTheFile x = writeFile "output.txt" x _________________________________________________________________ Chat with friends online, try MSN Messenger: http://messenger.msn.com

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Continuation Passing Style
by Tom Bevan 15 Mar '02

15 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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