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AngloHaskell 2007
by Philippa Cowderoy 21 May '07

21 May '07

I'd like to announce AngloHaskell 2007, or at least the start of the process of organising it! Last year there was a get-together organised around the interviews for the job of GHC maintainer. There were talks, with a largely pragmatic flavour, and there was plenty of socialising. It was fun, and doing it again seems a good idea. In the vein of last year's process, organising an event this year shall take place on the Haskell Wiki page at: http://www.haskell.org/haskellwiki/AngloHaskell and on IRC, in #anglohaskell on irc.freenode.net So far this is almost all the detail available, as neither date nor venue have been set - a summer date seems good though, and by the time we're organised I imagine late July or August looks most likely. Simon Peyton-Jones has offered to ask Microsoft Research for space to give talks in. This worked well last year, but if anyone else wants to offer a venue that's fine! -- flippa(a)flippac.org The task of the academic is not to scale great intellectual mountains, but to flatten them.

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Behavior of groupBy
by Hans van Thiel 21 May '07

21 May '07

Hello All, The standard function groupBy of List.hs doesn't work as I expect in this case: groupBy (\x y -> (last x) == (last y)) ["abc", "bd","cac"] results in: [["abc"],["bd"],["cac"]] where I want: [["abc","cac"], ["bd"]] Am I doing something wrong, or is this a bug? The function is defined (in List.hs in the hugs Data library) as : -- | The 'groupBy' function is the non-overloaded version of 'group'. groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy _ [] = [] groupBy eq (x:xs) = (x:ys) : groupBy eq zs where (ys,zs) = span (eq x) xs Thanks, Hans van Thiel

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how can I select all the 3-element-combination out of a list efficiently
by geniusfat 21 May '07

21 May '07

hi dear haskell lover ;) what I want to do is simply this: select3 :: [a] -> [(a, a, a)] and how can it be done efficiently? thanks in advance! -- View this message in context: http://www.nabble.com/how-can-I-select-all-the-3-element-combination-out-of… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Lazy HTML parsing with HXT, HaXML/polyparse, what else?
by Henning Thielemann 21 May '07

21 May '07

I want to parse and process HTML lazily. I use HXT because the HTML parser is very liberal. However it uses Parsec and is thus strict. HaXML has a so called lazy parser, but it is not what I consider lazy: *Text.XML.HaXml.Html.ParseLazy> Text.XML.HaXml.Pretty.document $ htmlParse "text" $ "<html><head></head><body>"++undefined++"</body></html>" *** Exception: Prelude.undefined *Text.XML.HaXml.Html.ParseLazy> Text.XML.HaXml.Pretty.document $ htmlParse "text" $ "<html><head></head><body>&</body></html>" *** Exception: Expected "</" but found & at file text at line 1 col 26 If it would be lazy, it would return some HTML code before the error. HaXML uses the Polyparse package for parsing which contains a so called lazy parser. However it has return type (Either String a). That is, for the decision whether the parse was successful, the document has to be parsed completely. *Text.ParserCombinators.PolyLazy> runParser (exactly 4 (satisfy Char.isAlpha)) ("abc104"++undefined) ("*** Exception: Parse.satisfy: failed If it would have return type (String, a) it could return both a partial value of type 'a' and the error message as String. It would be even better if it has some handling for incorrect input texts, and returns ([String], a), where [String] is the type of a list of warnings and error messages and 'a' is the type of a total value of parse output. Is there some parser of this type? Unfortunately http://www.haskell.org/haskellwiki/Applications_and_libraries/Compiler_tools does not compare the laziness of the mentioned parsers.

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Ann: Emping 0.2
by Hans van Thiel 21 May '07

21 May '07

Hello All, Version 0.2 of Emping, a utility to derive heuristic rules from a table of nominal data, is available. In addition to a reduced normal form in .csv format, which can be read by Open Office Calc, it now also displays observed implications and equivalences in the reduced rules (if present) and ambiguities in original rules (if present). A bug in the response to a user typing error has been fixed and some code has been simplified, compared to 0.1. An online user guide is at: http://j-van-thiel.speedlinq.nl/emp/empug.html and the Cabal package can be downloaded from there, or from the HackageDB. Thanks, Hans van Thiel

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Unresolved overloading error
by Scott Brown 21 May '07

21 May '07

I have written this code in Haskell which gives an unresolved overloading error. The function bernoulli should give the probability of j successes occuring in n trials, if each trial has a probability of p. > fac 0 = 1 > fac n = n * fac(n - 1) > binom n j = (fac n)/((fac j)*(fac (n - j))) > bernoulli n p j = (binom n j)*(p ^ j) * ((1 - p)^(n - j)) However, bernoulli 6 0.5 3 gives the error: ERROR - Unresolved overloading *** Type : (Fractional a, Integral a) => a *** Expression : bernoulli 6 0.5 3 Why doesn't Haskell infer the types? What kind of type casting or type definition can I use to fix the error? Send instant messages to your online friends http://au.messenger.yahoo.com

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Follow up on 'GroupBy behavior'
by Hans van Thiel 21 May '07

21 May '07

Thanks again for the help, and, to follow up, this now does what I need.. -- partitions a list according to an equivalence relation partition1 :: (a -> a -> Bool) -> [a] -> ([a],[a]) partition1 eq ls = partition ((head ls) `eq`) ls -- partitionBy :: (a -> a -> Bool) -> [a] -> [[a]] partitionBy eq [] = [] partitionBy eq ls = (fst x):(partitionBy eq (snd x)) where x = partition1 eq ls Regards, Hans van Thiel

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A wish for relaxed layout syntax
by Benjamin Franksen 21 May '07

21 May '07

Hi, I often run into the following issue: I want to write a list of lengthy items like this mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] With the current layout rules this is a parse error (at the closing bracket). Normally I avoid this by indenting everything one level more as in mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] but I think this is a little ugly. Same issue comes up with parenthesized do-blocks, I would like to write when (condition met) (do first thing second thing ) So my wish is for a revised layout rule that allows closing brackets (of all sorts: ']', ')', '}') to be on the same indent level as the start of the definition/expression that contains the corresponding opening bracket. Cheers Ben

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SWIG with Haskell ?
by Mark Wassell 21 May '07

21 May '07

Hello, Has anyone got any pointers on using SWIG with Haskell to integrate a C++ library? For the library there other language bindings, so I am in a position to leverage off these. In general what are the particular issues with C++ and Haskell. One obvious one is management of object lifecycle - making sure the object stays around long enough but doesn't out stay its welcome. Is using ForeignPtr sufficient? Cheers Mark

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writer monad help
by iskaldur 20 May '07

20 May '07

I'm trying to learn to use the Writer Monad, but I'm having trouble with the following very simple program: import Control.Monad.Writer foo :: Writer String Int foo = tell "hello" Basically, I was trying to figure out the 'tell' function, so I want foo to return ((), "hello"). But I get this error: Couldn't match `Int' against `()' Expected type: Writer String Int Inferred type: Writer String () In the application `tell "hello"' In the definition of `foo': foo = tell "hello" What am I supposed to do? Write my own version of tell for Writer String Int? How exactly do I do this? -- View this message in context: http://www.nabble.com/writer-monad-help-tf3777133.html#a10680445 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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