May 2007 - Haskell-Cafe

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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HaskellWiki XHTML not well-formed or valid
by Isaac Dupree 20 May '07

20 May '07

I mentioned this a few months ago in a place no-one noticed, it seems... http://www.haskell.org/haskellwiki/Talk:Haskell#Well-formedness_.2F_validity Please fix, or tell me who to contact (and how), or how to fix it myself? :-) Isaac

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Random idea
by Andrew Coppin 20 May '07

20 May '07

Greetings. I was thinking... we already have Lambdabot sitting in an IRC channel. How hard would it be to mangle Lambdabot to the point where it works over HTTP? You know - so you could type some Haskell into a form on a web page, hit [submit], and get the result sent back to you? (Again, assuming it can be computed in a sane amount of time/space, and truncated to some reasonable textual length.) I think it might be kinda neat to have such a thing linked from the Haskell.org homepage. You know, sort of "hey, wanna try Haskell without installing any stuff? Click here!" ...on the other hand, maybe I'm just strange... :-$

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Debunking tail recursion
by Jules Bean 20 May '07

20 May '07

A conversation on #haskell just showed that it's quite hard to explain (at least it is for me) to people attached to tail recursion why that is a red herring in Haskell. I had a poke around the wiki and couldn't see a page which explains it clearly. In fact http://www.haskell.org/haskellwiki/Performance/Accumulating_parameter is actuall slightly misleading since it suggests that the performance gain is from the tail recursion whereas really it's from the change from a function which accumulates an enormous thunk to an function which can do the work as it goes along. If someone has the energy and the clarity to explain this clearly and make a wiki page about it, that would be great. If someone finds an existing wiki page I'm too unperceptive to notice, even better! Jules

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Profiling, measuring time
by Steffen Mazanek 20 May '07

20 May '07

Hello, I have written a function f, that performs a quite complex operation on its argument. Furthermore I have another function genInput that takes a number and constructs an argument for f of this size. What I want now is a list [(n,time)] that gives me for every size of input n the time, f needs to compute this input. How can I do this? I have found the module Time and tried diffClockTimes, but I ever got zero as a delta. Help is greatly appreciated. There is probably a way to do this via ghcs profiling but I hope that there is also a "normal" Haskell way of doing this. Best regards, Steffen -- Dipl.-Inform. Steffen Mazanek Institut für Softwaretechnologie Fakultät Informatik Universität der Bundeswehr München 85577 Neubiberg Tel: +49 (0)89 6004-2505 Fax: +49 (0)89 6004-4447 E-Mail: steffen.mazanek(a)unibw.de

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