December 2008 - Haskell-Cafe

Infinite grid
by Martijn van Steenbergen 06 Jan '09

06 Jan '09

Hello, I would like to construct an infinite two-dimensional grid of nodes, where a node looks like this: > data Node = Node > { north :: Node > , east :: Node > , south :: Node > , west :: Node > } in such a way that for every node n in the grid it doesn't matter how I travel to n, I always end up in the same memory location for that node. I suspect another way of saying that is that > let f = f . north . east . south . west in f origin should run in constant space. I hope this makes the problem clear. :-) How do I do this? Thanks in advance, Martijn.

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Trouble with interact in ghci
by Adrian Neumann 06 Jan '09

06 Jan '09

Hi, I have a strange problem with "interact" on OS X (ghc 6.10.1). It seems to garble stdin. I have some code here http://hpaste.org/13135#a2 , for testing purpose: *Main> main 1 > 1.0 2 > 1.5 3 > 2.0 *Main> setNonBlockingFD: invalid argument (Bad file descriptor) 11:40:45 ~/Desktop> (I hit ctrl-D) *Main> main 1 > 1.0 2 > 1.5 3 > 2.0 ^CInterrupted. *Main> main *** Exception: <stdin>: hGetContents: illegal operation (handle is closed) *Main> Someone on #haskell suspected my editline to be the culprit. Does anyone of you know what I can do about this problem? Greetings, Adrian

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Haskell, successing crossplatform API standart
by Belka 05 Jan '09

05 Jan '09

Hello! (1) Is anybody aware of SOA approach being supported in Haskell? Found HAIFA package (SOAP, WEB services), but it seems to be a RIP project (with it's last updated in 2006) - trying to install it is a total mess (for me, a newby). (2) Please, perhaps experienced developers could suggest any better crossplatform (crosslanguage) API standart supported in Haskell? For me the main priority is orientation on "being supported everywhere in far future"... Thanks in advance -- View this message in context: http://www.nabble.com/Haskell%2C-successing-crossplatform-API-standart-tp20… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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WriterT [w] IO is not lazy in reading [w]
by Paolino 05 Jan '09

05 Jan '09

As someone suggested me, I can read the logs from Writer and WriterT as computation goes by, if the monoid for the Writer is lazy readable. This has been true until I tried to put the IO inside WriterT > {-# LANGUAGE FlexibleContexts #-} > import Control.Monad.Writer > k :: (MonadWriter [Int] m) => m [Int] > k = let f x = tell [x] >> f (x + 1) in f 0 > works :: [Int] > works = snd $ runWriter k > hangs :: IO [Int] > hangs = snd `liftM` runWriterT k > main = take 20 `liftM` hangs >>= print The main hangs both interpreted and compiled on ghc 6.10.1. The issue is not exposing with IO alone as main = print "test" >> main is a working program. Thanks for explanations. paolino

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ANN: bytestring-trie 0.1.0
by wren ng thornton 05 Jan '09

05 Jan '09

-------------------------------------------- -- Announcing: bytestring-trie 0.1.0 (beta) -------------------------------------------- An efficient finite map from (byte)strings to values. The implementation is based on big-endian patricia trees, like Data.IntMap. We first trie on the Word8 elements of a Data.ByteString, sharing string prefixes where possible, and then trie on the big-endian bit representation of those elements. Patricia trees have efficient algorithms for union and other merging operations, but they're also quick for lookups and insertions. -------------------------------------------- -- Future Extensions -------------------------------------------- * I've done spot checking to make sure it works, but haven't constructed an extensive test suite. Does anyone know of a good data set already out there for testing corner cases? I'm sure other dictionary writers have come up with one. * A Word8 is much smaller than the architecture's natural Word size. Therefore it'd be more efficient for the trie on bits to read off a whole Word at a time. This work is in progress, but I need help from people with 64-bit and big-endian machines in order to verify the code works on those architectures. * Using ByteStrings allows for trieing on any packed byte representation of a value, but they're not Strings. Integration with Data.ByteString.Char8, utf8-string, and utf8-light should happen. * The current implementation also only accepts strict ByteStrings. When chopping up strings to share prefixes we share the smaller string. For very long strings when many deletions are expected, this can still hold onto more memory than necessary. Accepting lazy ByteStrings or adding heuristics for when to copy prefixes instead of sharing will fix this. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie Darcs: http://code.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://code.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring-trie/ -- Live well, ~wren

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What are side effects in Haskell?
by Hans van Thiel 03 Jan '09

03 Jan '09

Hello All, I just saw somewhere that one of the purposes of monads is to capture side effects. I understand what a side effect is in C, for example. Say you want to switch the contents of two variables. Then you need a third temporary variable to store an intermediate result. If this is global, then it will be changed by the operation. But what is a side effect in an fp language like Haskell? As a follow up, I understand why an IO action like getLine or getChar is not a function; its results can be different for different calls. But why not have something like getChar c or getLine str? The types of c or str are pretty clear, aren't they? Just something that's been puzzling me for some time now...thanks. Regards, Hans van Thiel

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Re: [Haskell-cafe] Don't make 'show*' functions
by Donn Cave 03 Jan '09

03 Jan '09

Quoth brian <brianchina60221(a)gmail.com>: | On Fri, Dec 26, 2008 at 1:55 PM, Jeff Heard <jefferson.r.heard(a)gmail.com> wrote: |> Off the top of my head, I would say that the traditional meaning of |> Show could be changed to Serial, where serial encompasses both Read |> and Show -- possibly we could find a more efficient read function, |> several have been proposed. Then a separate class could be made for |> HumanReadable (or Loggable) where the point would bet that we write |> something that can be read by humans without conforming to a |> particular grammar that Haskell could read back in. | | +1. 'Show' sounds like it's for what Java 'toString' and Ruby 'to_s', | etc., do, and that's not right. To be fair, this seems to be a natural ambiguity with string representation. Python kind of blundered into it around 2.0, when someone decided to "fix" the __repr__ function for floating point to display the best decimal value, where it had until then used a rounded value. So users were confounded to see for example that 1.0 - 0.1 is 0.90000000000000002, instead of 0.9, but that does make some sense if __repr__ is to be used for string marshalling. Python has two string representation functions, __str__ and __repr__. The documentation of these two functions is a long standing problem that will probably never be resolved. If as proposed above there had also been a third function that is strictly about marshalling, that might have made it a little easier to focus on the difference betweeen str and repr, where unfortunately the documentation has confused the issue with ambiguous notions like friendliness. For me, "str" is about conversion to string data, where a data type contains or represents string data. Conversion implies loss. For example, an HTML or RFC822 header would convert to its "wire" data. But that's at odds with expectations for numeric types, for example - I'd make str(65) = 'A', but of course it's actually '65'. "repr" portrays the value as a Python object - what you would see for example from the interactive interpreter. Since Python objects are commonly "in the IO monad", there are all kinds of cases where the repr value can't fully marshal the object. Donn

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Parsec : Problems with operator precedence
by Erik de Castro Lopo 03 Jan '09

03 Jan '09

Hi all, I'm using Text.ParserCombinators.Parsec.Expr to parse expressions for a Javascript like language. This language has C-like logical operators ('&&' and '||') and bitwise operators ('&' and '|'). Furthermore, the language definition states that bitwise operators have a higher precedence than the logical ones. I therefore have the following (trimmed): import qualified Text.ParserCombinators.Parsec.Expr as E opTable :: [[ E.Operator Char st Expression ]] opTable = [ -- Operators listed from highest precedence to lowest precedence. {- snip, snip -} [ binaryOp "&" BinOpBinAnd E.AssocLeft ], [ binaryOp "^" BinOpBinXor E.AssocLeft ], [ binaryOp "|" BinOpBinOr E.AssocLeft ], [ binaryOp "&&" BinOpLogAnd E.AssocLeft ], [ binaryOp "||" BinOpLogOr E.AssocLeft ] ] binaryOp :: String -> (SourcePos -> a -> a -> a) -> E.Assoc -> E.Operator Char st a binaryOp name con assoc = E.Infix (reservedOp name >> getPosition >>= return . con) assoc but I still get the following parse error: unexpected "|" expecting end of "|" or term on the line: if (name == null || value == null) If I change the above from a logical to a bitwise OR, the parser accepts it quite happily. Any clues as to what I'm doing wrong here? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- BSD: A psychoactive drug, popular in the 80s, probably developed at UC Berkeley or thereabouts. Similar in many ways to the prescription-only medication called "System V", but infinitely more useful. (Or, at least, more fun.) The full chemical name is "Berkeley Standard Distribution".

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Gitit - Encoding
by Arnaud Bailly 03 Jan '09

03 Jan '09

Hello, I have started using Gitit and I am very happy with it and eager to start hacking. I am running into a practical problem: characters encoding. When I edit pages using accented characters (I am french), the accents get mangled when the page come back from server. The raw files are incorrectly encoded. Where Shall I look for fixing this issue ? Thanks ps: the wiki is live at http://www.notre-ecole.org -- Arnaud Bailly, PhD OQube - Software Engineering web> http://www.oqube.com

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hackage, gtk2hs
by Cetin Sert 02 Jan '09

02 Jan '09

Hi, A package I want to upload only builds with the unreleased gtk2hs version from the darcs repository and not the latest released version 0.9.13 or any lesser. But the repository version seems to not have changed, so if I say in my cabal file: gtk >= 0.9.14 it does not even build with the gtk2hs version from the repos, on the other hand if I say: gtk >= 0.9.13 people using 0.9.13 may be surprised to see the package does not build. What should I do o__O? Wait for the next gtk2hs release before releasing my package? Or is there a way to notify _potential users/those interested_ of the need to use the latest gtk2hs they can get from the darcs repos. Best Regards, Cetin Sert

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