February 2006 - Haskell-Cafe

Re: [Haskell-cafe] Infinite loop?
by Donn Cave 19 Feb '06

19 Feb '06

Quoth Barbier de Reuille Pierre <pierre.barbier(a)cirad.fr>: | On Fri, 17 Feb 2006 17:26:08 +0000 | Jon Fairbairn <jon.fairbairn(a)cl.cam.ac.uk> wrote: | | > On 2006-02-17 at 09:22PST Jared Updike wrote: | > > Yep. change one line to: | > > | > > entry <- if isdir && name /= "." && name /= ".." | > > | > > and it does in fact work. | > | > Only if no-one has been tricky with symbolic links. | > | | And how can you, in Haskell, detect symbolic links ? I refer you to the documentation in System.Posix.Files, especially getFileStatus :: FilePath -> IO FileStatus, and isSymbolicLink :: FileStatus -> Bool. I don't think I've used this function, but it looks promising. Donn Cave, donn(a)drizzle.com

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classes inheritance seems not to work.
by asker 19 Feb '06

19 Feb '06

Hi,   I'm trying to make three haskell classes and an instance this way:  ------------------------------------------------------------------------------ class (Eq a) => Graph a where vert :: [a] ady :: a -> [a] class (Graph a) => Paths a where gps :: a -> a -> [[a]] class (Paths a) => Minimum a where mgps :: a -> a -> [a] instance Minimum Char where vert = ['a' .. 'f'] -- This is the line of the error message. ady 'a' = ['b', 'c'] ady 'b' = ['d', 'c', 'e'] ady 'd' = ['f'] gps = bepgr -- bepgr is defined and implemented in other code's part mgps = head gps ------------------------------------------------------------------------------   When I try to run this I get: "ERROR: filepath:line -No member vert in class "Minimum"" I don't know why does this happen, am I doing something wrong?   Thank you. -- View this message in context: http://www.nabble.com/classes-inheritance-seems-not-to-work.-t1148006.html#… Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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libreadline dependency
by J. Scott Thayer, M.D. 19 Feb '06

19 Feb '06

Running Suse 9.3 64bit on Acer Ferrarri laptop. Haskell wants libreadline 4 while I have libreadline 5. Is there a definitive fix for this? The answers in archived messages were 'sub-optimal'.

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Re: [Haskell-cafe] Problems instancing a class
by Benjamin Franksen 18 Feb '06

18 Feb '06

On Friday 17 February 2006 21:03, Juan Carlos Arevalo Baeza wrote: > What really got me is that if I don't use a type synonym: > > instance MyClass (HT.HashTable String String) where > htLookup h var = > do result <- HT.lookup h var > case result of > Nothing -> return "" > Just s -> return s > > it still complains: > > Illegal instance declaration for `MyClass (HashTable String String)' > (The instance type must be of form (T a b c) > where T is not a synonym, and a,b,c are distinct type variables) > In the instance declaration for `MyClass (HashTable String String)' > > even though the documentation in GHC states that HashTable is > declared as "data". (Guessing) Maybe because 'String' is not a type variable? > How exactly is type undecidable if this is allowed? I found here: > > http://hackage.haskell.org/trac/haskell-prime/wiki/FlexibleInstances > > an example of a non-terminating instance: > > instance C b b => C (Maybe a) b > > I don't see how/where/when this would be non-terminating. I > mean... if you now define an instance "C Int Int", then you actually > also have "C (Maybe Int) Int", C (Maybe (Maybe Int)) Int" and so on, > I can see that. But how exactly is that a problem? Can you show me a > use of C that cannot work with that instance? What is (or better: might be) non-terminating is the process (algorithm) of /type inference/. This: http://www.haskell.org//pipermail/haskell-prime/2006-February/000609.html message contains a detailed explanation. Ben

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Re: (Newbie Question) How to get the value of an "IO String" Monad
by Peter 17 Feb '06

17 Feb '06

Hello All, Thanks Cale, Udo, Daniel, Lemmih, Ketil, and Marc for your very helpful posts, and Haskell resources. Every post really helped a lot. Thanks for all the help, Peter

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(Newbie Question) How to get the value of an "IO String" Monad?
by Peter 17 Feb '06

17 Feb '06

Hello, For the purpose of familiarizing myself with Haskell, and also because I love Haskell :), I am trying to re-make a script that I made in Python that sends a request to a server and extracts the list of email addresses from a Mailman web-page by using an XML Parser on the page's HTML that has been converted to XHTML by "HTML Tidy". However, I cannot seem to figure out a way to get the state of a Monad; Specifically I cannot get the value of an "IO String" Monad. I have read some tutorials on Monads but I guess I must have missed something. I have read that the >>= operator is the only way to extract the state of an action as a string, and pipe it to a function. So far so good. But, That does not seem to work, because as I understand the >>= operator, it expects the function on the right hand side to return an IO Monad, which completely defeats the purpose here. So, How am I supposed to get the value of an IO Monad, such as "IO String", without returning an IO Monad? If this is of any help, here is the function I am stuck on: recv_headers' :: Socket.Socket -> String -> IO [[String]] recv_headers' sock bulk | received == "" = error "Connection died unexpectedly." | received == "\n" && endswith bulk "\r\n\r" = return [["foo", "bar"]] | otherwise = recv_headers' sock (bulk ++ received) where received = (Socket.recv sock 1) --- End code --- And here is the (expected) error I get from trying to compare "IO String" to "String": MemberBackup.hs:29:18: Couldn't match `IO String' against `[Char]' Expected type: IO String Inferred type: [Char] In the second argument of `(==)', namely `""' In a pattern guard for the definition of `recv_headers'': received == "" Failed, modules loaded: none. --- End error --- Thanks for the help, Peter

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Problems instancing a class
by Juan Carlos Arevalo Baeza 17 Feb '06

17 Feb '06

So, consider this code: import Data.HashTable as HT class MyClass a where htLookup :: a -> String -> IO String type Context = HT.HashTable String String instance MyClass Context where htLookup h var = do result <- HT.lookup h var case result of Nothing -> return "" Just s -> return s This doesn't compile. GHC says: Illegal instance declaration for `MyClass Context' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `MyClass Context' If I use "data" instead of "type", it works: data Context = C (HT.HashTable String String) instance MyClass Context where htLookup (C h) var = do result <- HT.lookup h var case result of Nothing -> return "" Just s -> return s Why? What's going on here? JCAB

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Badly designed Parsec combinators?
by Juan Carlos Arevalo Baeza 17 Feb '06

17 Feb '06

So... I see no reason why someone can't just do it themselves, but... I was playing around with Parsec (as included in GHC 6.4.1), and I found two functions that are... not quite what I believe they should be. optional :: GenParser tok st a -> GenParser tok st () optional p = do{ p; return ()} <|> return () Now, this completely loses the result of the optional parser. Better would be: optional :: GenParser tok st a -> GenParser tok st (Maybe a) optional p = do{ x <- p; return (Just x) } <|> return Nothing Same thing with manyTill: manyTill :: GenParser tok st a -> GenParser tok st end -> GenParser tok st [a] manyTill p end = scan where scan = do{ end; return [] } <|> do{ x <- p; xs <- scan; return (x:xs) } Better: manyTill :: GenParser tok st a -> GenParser tok st end -> GenParser tok st ([a], end) manyTill p end = scan where scan = do{ endr <- end; return ([], endr) } <|> do{ x <- p; (xs, endr) <- scan; return (x:xs, endr) } Is there any reason I can't see why they are the way they are? JCAB

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Re: HaXml: ampersand in attribute value
by Koen.Roelandt＠mineco.fgov.be 17 Feb '06

17 Feb '06

> But speaking of HaXml bugs, I'm pretty sure HaXml doesn't handle > % correctly. It seem to treat % specially everywhere, but I think > it is only special inside DTDs. I have many XML files produced by > other tools that the HaXml parser fails to process because of this. I had a similar problem where the parser choked on % signs in attribute values in the XML file. I solved it by playing around in Lex.hs. I could check for the (ugly) solution... > Yes, it is expected. An ampersand indicates the start of a reference, > e.g. < or  If there is no semicolon to indicate the end of the > reference, then it is a parse error. The XML specification is quite > clear that neither & nor < are valid standalone characters in an > attribute value. Which is exactly the problem. The & is part of a reference, namely ë. Regards, Koen.

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RE: [Haskell-cafe] library sort
by Bayley, Alistair 17 Feb '06

17 Feb '06

> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Radu Grigore > > Is there a sort function in the libraries that come with GHC > (6.4)? My search at > http://www.haskell.org/ghc/docs/latest/html/libraries/index.html > has failed, but I can't believe there is none. Data.List.sort (and sortBy): http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-List.htm l#v%3Asort ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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