February 2006 - Haskell-Cafe

Unexpected results with simple IO
by Maurício 21 Feb '06

21 Feb '06

Dear Haskell users, I have a problem using IO. The small test program below asks the user to guess from a list of random numbers between 1 and 10. Everything works well excepts for one problem: all the messages ("Guess a number...", "Right..." and "Wrong...") are printed after the program finishes, i.e., I have to use it blind. I'm afraid I misunderstand something important about lazyness or monads... What am I doing wrong? Thanks, Maurício module Main where import Random main = do r_gen <- getStdGen --random generator let r_list = (randomRs (1,10) r_gen) --random list guess_loop (r_list) guess_loop (r:r_others) = do putStrLn "Guess a number between 1 and 10:" n <- readLn if n==r then do putStrLn "Right! :)" return () else do putStrLn "Wrong... :(" guess_loop r_others

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Code completion? (IDE)?
by Marc Weber 21 Feb '06

21 Feb '06

vim7 has introduced "omni-completion"... So I'm interested wether there are any projects which support any kind of completion.? I've read the thread about Eclipse IDE haskell plugin. Would you recommend picking up code from a haskell compiler and adapt it for this purpose? I'm interested but don't plan to implement this in the near future. Marc

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Lazy read
by Neil Mitchell 21 Feb '06

21 Feb '06

Hi, I have a nice big data structure that I serialise to a file using "show", and read back in using "read". This data structure has deriving (Read, Show), which makes it all nice and easy to save and load this data structure, without worrying about parsing code etc. The problem is that this data structure can become quite big, and while show operates lazily, read does not. I understand the reason for this - that it internally calls reads and that on a parse error reads will not return a result - i.e. it has to get to the end to determine whether the parse was valid or not. However, in my particular case, I know the data structure is valid, and if it isn't then I'm quite happy with the parser throwing an error half way through - after having lazily returned half of the data. A short example, which shows up the exact same issue is: head ((read $ show [1..n]) :: [Int]) The time of this operation is dependant on n, because of the strictness of read. What is the best way to modify the code to have read operate lazily? Is there any method that doesn't require writing a custom parser? Is there any standard way for solving a problem like this? Thanks Neil

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RE: [Haskell-cafe] building ghc on win with gcc of cygwin, without cygwin1.dll (-mno-cygwin flag) ?
by Simon Peyton-Jones 20 Feb '06

20 Feb '06

| Googling around I've found that there exists the -mno-cygwin flag which | you can use to not include this lib.. So would it might be possible to | get a ghc build not using cygwin.dll with just cygwin ? I don't know. It sounds plausible. But we only have enough resource to maintain one route for building GHC on Windows. We'd be delighted if other folk were engaged enough to push other routes through and make them work. Another route we don't have, but which in principle is definitely feasible is to compile GHC using Cygwin for Cygwin. The advantage would be that the resulting GHC would have a working Posix library. The disadvantage is that programs it compiles would require Cygwin to run. Anyway, it's up for grabs. Please grab away! We need more Windows-savvy expertise on GHC. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Marc | Weber | Sent: 12 February 2006 04:45 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] building ghc on win with gcc of cygwin,without cygwin1.dll (-mno-cygwin flag) | ? | | Hi.. I've found the nice documentation about building ghc on | haskell.org/ghc -> documentation. | There is one chapter (#1) about C compilers and environments to use: | Either MSYS or cywin and | gcc of MinGW because gcc of cygwin will link to cygwin1.dll by default | which may change and therefore can brake your apps. | Googling around I've found that there exists the -mno-cygwin flag which | you can use to not include this lib.. So would it might be possible to | get a ghc build not using cygwin.dll with just cygwin ? | I haven't read anything about that in those docs. | | Or does this flag simply include cygwin.dll within your apps (statically | linked) ? | | #1 :http://www.haskell.org/ghc/docs/latest/html/building/platforms.html#ghc -cygwin | | Marc | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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HaXml: ampersand in attribute value
by Koen.Roelandt＠mineco.fgov.be 20 Feb '06

20 Feb '06

HaXml seems to choke on finding an ampersand in an attribute value. Is this normal? Is there any workaround? Cheers, Koen.

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Haskell as scripting language?
by Marc Weber 20 Feb '06

20 Feb '06

Is there a way to use haskell as scripting language in a) your own project? b) other projects such as vim (beeing written in C)? At the moment I'm interested, I don't have any real project..

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RE: [Haskell-cafe] Numerical methods in Haskell
by Simon Peyton-Jones 20 Feb '06

20 Feb '06

| Between google searching and looking through the activity | report, I take it that no one has really developed serious | libraries for matrix manipulations, diff eqs, etc. | | Are there any practical reasons for this or is it just a | matter of the haskell community being small and there not | being many people interested in something so specialized? The latter I think, but it's just the sort of thing that a functional language should be good at. Two other difficulties (a) It's hard to compete with existing libraries. The obvious thing is not to compete; instead, just call them. But somehow that doesn't seem to be as motivating. Perhaps some bindings exist though? (b) A concern about efficiency, because numerical computation is typically an area where people really care about how many instructions you take. It's a legitimate concern, but I don't think that it'll turn out to be justified. With unboxed arrays, and/or calling external libraries for the inner loops -- and the potential for aggressive fusion and/or parallelism, there is plenty of upward potential. I also want to work on nested data parallelism (a la NESL, and NEPAL) which fits right in here. I'd love to see a little community of matrix manipulators spinning up. Simon

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Shapes Compiling Error
by Chatzianastassiou Achilleas 19 Feb '06

19 Feb '06

Hi everybody, I have written some code for several shapes, and i get a compiling error: parse error on input `:' though I can't find where the error is... data Shape = Ellipse Radius Radius | Polygon [Vertex] deriving Show type Radius = Float type Vertex = (Float, Float) area :: Shape -> Float area (Rectangle s1 s2) = s1 * s2 area (RtTriangle s1 s1) = s1 *s2 /2 area (Ellipse r1 r2) = pi * r1 * r2 area (Polygon(v1:v2:v3:vs)) = triArea v1 v2 v3 + area(Polygon(v1:v3:vs)) area(Polygon _ ) = 0 area (Polygon(v1:vs')) = polyArea vs where polyArea :: [Vertex] -> Float polyArea(v2:v3:vs') = triArea v1 v2 v3 + polyArea(v3:vs') polyArea _ = 0 triArea :: Vertex -> Vertex -> Vertex -> Float triArea v1 v2 v3 = let a = distBeetween v1 v2 b = distBeetween v2 v3 c = distBeetween v3 v1 s = 0.5 * (a + b + c) in sqrt (s * (s - a) * (s - b) * (s - c)) distBeetween :: Vertex -> Vertex -> Float distBeetween (x1, y1) (x2, y2) = sqrt ((x1 - x2) ^ 2 + (y1 - y2) ^ 2) Any suggestions? Thanks in advance, Skag55 _________________________________________________________________ On the road to retirement? Check out MSN Life Events for advice on how to get there! http://lifeevents.msn.com/category.aspx?cid=Retirement

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GADTs and bar :: Foo t1 -> Foo t2
by Daniel McAllansmith 19 Feb '06

19 Feb '06

Hello, I have a recursive type > data Foo = A | B [Foo] | C [Foo] that I would like to restrict so that a C can only contain Bs, and a B can only contain As. If I use a GADT as follows the bar function, understandably, will not type check. > data AType > data BType > data CType > data Foo a where > A :: Foo AType > B :: [Foo AType] -> Foo BType > C :: [Foo BType] -> Foo CType > --get the successor of the Foo > bar c@(C [] ) = c > bar (C (b:_)) = b > bar b@(B [] ) = b > bar (B (a:_)) = a > bar a@A = a How do I achieve this restriction? Do I need to have some sort of successor class with dependent types? Ta Daniel

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Infinite loop?
by rgo 19 Feb '06

19 Feb '06

Hi all, my program probably goes into infinite loop... But i cannot understand where and why. code: import System.Directory data MyFile = MyDir { dir_name :: String, dir_files :: [MyFile] } | MyFile { file_name :: String } read_dir_entries :: [FilePath] -> IO [MyFile] read_dir_entries [] = return [] read_dir_entries (name:names) = do isdir <- doesDirectoryExist name; entry <- if isdir then read_dir name else return (MyFile {file_name = name}); entries <- read_dir_entries names; return (entry:entries) read_dir :: FilePath -> IO MyFile read_dir name = do content <- getDirectoryContents name; files <- read_dir_entries content; return MyDir { dir_name = name, dir_files = files } instance Show MyFile where show (MyDir {dir_name = name, dir_files = files}) = "\n" ++ name ++ ":\n" ++ show files show (MyFile {file_name = name}) = "\t" ++ name ++ "\n" main = do cwd <- getCurrentDirectory; dir <- read_dir cwd; print dir;

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