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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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haskell question
by IBRAHIM MOSHAYA 17 Feb '06

17 Feb '06

please can you help me with this function: Significant Figures I need to write a function to return a given number (1st argument) to a given number of significant figures (2nd argument). The function should be called sigFig and have a type signature of the form: sigFig :: (???) => a -> Int -> a For instance: *Main> sigFig 123.456 2 120.0 *Main> sigFig 123.456 4 123.5 *Main> sigFig 0.987 2 0.99 *Main> sigFig 0.432 2 0.43 The type variable a will need to be constrained, but you should aim for the most general implementation you can (replacing ??? above with the appropriate constraint(s)). You may make free use of Prelude functions. Referring to [Jon03] you may wish to consider the use of functions / operators including: /, div, mod, ^, ^^, **, round, fromIntegral, realToFrac thanks alot

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Re: [Haskell-cafe] Decidable type systems? (WAS: Associated TypeSynonyms question)
by Robin Green 17 Feb '06

17 Feb '06

Miles Sabin <miles(a)milessabin.com> wrote: > Can someone explain to me why decidability is of any practical > interest at all? procedure which might never terminate and one which might take > 1,000,000 years to terminate? Actually, why push it out to 1,000,000 > years: in the context of a compiler for a practical programming > language, a decision procedure which might take an hour to terminate > might as well be undecidable But are there any decidable type checking algorithms that have been seriously proposed or used which would take far too long to terminate for real code? If not, then decidability is the only thing that matters. -- Robin

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library sort
by Radu Grigore 16 Feb '06

16 Feb '06

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. -- regards, radu http://rgrig.blogspot.com/

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Cyclical Type Synonyms
by Tom Hawkins 16 Feb '06

16 Feb '06

I want to define a type for a function that returns its own type... type F a = a -> (F a,a) But the compiler gives me an error: "Cycle in type synonym declaration: ...". Why is this a restriction? Of course I can create a user defined type, but then I need an extra mechanism to call the embedded function: data F a = F (a -> (F a ,a)) call :: F a -> a -> (F a, a) call (F f) a = f a -Tom

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