March 2002 - Haskell-Cafe

Re: Gaussian Integers
by Ashley Yakeley 19 Mar '02

19 Mar '02

(moved to Cafe) At 2002-03-19 03:33, dominic.j.steinitz(a)britishairways.com wrote: >I've already declared Gaussian a as being of class Eq so why does it need >to be told again? No you haven't. You've declared 'Eq a => Eq (Gaussian a)' -- 'Gaussian a' is Eq if 'a' is, not 'Eq (Gaussian a)'. Try this instance Eq a => Num (Gaussian a) where ... ...and actually looking at your code, I think you want this: instance Num a => Num (Gaussian a) where ... -- Ashley Yakeley, Seattle WA

1 0

partial application
by Cagdas Ozgenc 19 Mar '02

19 Mar '02

Greetings. Is there a reason why partial application cannot be applied in arbitrary order? Was it a technical difficulty in the design of Haskell? Or is it just following beta reduction rigorously? Thanks

9 10

Reading and processing data file
by Ramachandran M 18 Mar '02

18 Mar '02

I'm working on an assignment at my uni that requires me to prompt the user for an input and to locate that item in a data file. For example, if the user enters a flight no. of IC009, the program should locate that particular record in the file and output the flight's origin and destination. How should I go about writing this in Haskell? Any pointers would be greatly appreciated. Thanks. _________________________________________________________________ Chat with friends online, try MSN Messenger: http://messenger.msn.com

1 0

Partial application on any argument(s) regardless of argument order (Was: Re: partial application)
by Dimitre Novatchev 18 Mar '02

18 Mar '02

Koen Claessen koen(a)cs.chalmers.se wrote: > Cagdas Ozgenc wondered: > > | Is there a reason why partial application cannot be > | applied in arbitrary order? Was it a technical > | difficulty in the design of Haskell? Or is it just > | following beta reduction rigorously? > > Alastair David Reid answered: > > | Haskell doesn't dictate any particular evaluation > | order. If you look at the name of the Haskell report, > | it calls Haskell a "non-strict" language not a "lazy" > | language. > > Somehow I do not think that Cagdas was talking about > evaluation order at all. I think he referred to the > following: Suppose I have a function f which is defined to > have 3 arguments: Partial application on any subset of arguments regardless of their order has been implemented in the FXSL functional programming library and is described here: http://www.topxml.com/xsl/articles/df/ Cheers, Dimitre Novatchev. __________________________________________________ Do You Yahoo!? Yahoo! Sports - live college hoops coverage http://sports.yahoo.com/

1 0

Re: partial application
by C.Reinke 18 Mar '02

18 Mar '02

> ..I was thinking of the following syntax. I orginally had the > idea for C++, where you can't do partial application at all. ? FC++: Functional Programming in C++ http://www.cc.gatech.edu/~yannis/fc++/ (I wouldn't be defending C++ otherwise, but its set of things you can't do at all must be a lot smaller than in other languages;-) They seem to support the _ notation, btw. Laurent Dami had a lambda-calculus variant that allowed for partial application by name instead of by position (amongst several other interesting features:) http://cui.unige.ch/~dami/publis.html Claus -- Research Opportunity: Refactoring Functional Programs (UKC) Closing date: Friday 22 March 2002 !! http://www.cs.ukc.ac.uk/vacancies_dir/r02-24.html

1 0

Re: partial application
by Peter G. Hancock 18 Mar '02

18 Mar '02

Apropos of section notation, don't forget flip, alias the C combinator. It allows you to express partial applications "to the next argument but one" is in operator section notation. We can express (usually horribly) any affine lambda-abstraction with flip, (.), id and const. Peter Hancock

1 0

Re: Hiring Haskell programmers
by John Hughes 15 Mar '02

15 Mar '02

> Depends where you are, I guess. In Sydney, it would be > easy. We are teaching Haskell to about 1500 first-year > students every year. > Melbourne University is teaching similar numbers (perhaps slightly less; probably more than 1000) of first-year students Haskell every year. -- Nick Nethercote njn25(a)cam.ac.uk Goodness me, we're only teaching 160 here in Gothenburg! I feel inadequate! However, we're teaching 20 or so a year to an advanced level, so I reckon there are around 100 really skillful Haskell programmers in Gothenburg by now. That should be enough for a while... John Hughes

1 0

Re: Continuation Passing Style
by Ashley Yakeley 14 Mar '02

14 Mar '02

At 2002-03-14 16:10, Tom Bevan wrote: >I noticed that Ralf Hinze posted a CPS monad yesterday. He did? Anyway I have one here: <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hscheme/Source/C ontinuationPassing.hs?rev=HEAD&content-type=text/plain> Basically it's this: newtype CPS ex p a = MkCPS {unCPS :: (ex -> p) -> (a -> p) -> p}; 'ex' is exception, so that I can make it an instance of MonadError. >Would someone be kind enough to post a simple example of a function that >uses CPS. Once you've made it an instance of Monad, of course you can just use 'do' notation. But I use CPS here (for my Scheme interpreter) because you can do call-with-current-continuation. -- Ashley Yakeley, Seattle WA

1 0

RE: Date Sorting Round Three
by Garner, Robin 14 Mar '02

14 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

1 0

Date Sorting Round Three
by Michael Ruth 14 Mar '02

14 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

1 0