- Haskell-Cafe - Haskell.org

Looking for pointfree version
by Edsko de Vries 16 Feb '09

16 Feb '09

Hi, Is there a nice way to write down :: Focus -> [Focus] down p = concat [downPar p, downNew p, downTrans p] in point-free style? (In doesn't make much difference what these functions do; if it helps, their types are downPar, downNew, downTrans :: Focus -> [Focus]). Ideally, I would like to write something like down = downPar ... downNew ... downTrans but I'm not sure what should be on the dots. This works: down = concat . flip map [downPar, downNew, downTrans] . flip ($) but is extremely ugly and doesn't really explain what's going on :) (It seems to me I should be able to take advantage of the list monad, somehow). Pointers appreciated! Edsko

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Help needed with apache config for hackage
by Duncan Coutts 16 Feb '09

16 Feb '09

Hi folks, Does anyone have any experience with apache configuration, particularly mime types and handling browser quirks and would like to help us with an issue we have on hackage? http://hackage.haskell.org/trac/hackage/ticket/498 The problem is described in the ticket but basically IE6 gets confused by the Content-Type and Content-Encoding and ends up saving ".tar.gz" files with the wrong name ".tar.tar". We need help working out how to configure apache to use a workaround and with testing that the solution actually works. Thanks! Duncan

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ANNOUNCE: spacepart-0.1.0.0 (was called data-spacepart)
by Corey O'Connor 16 Feb '09

16 Feb '09

0.1.0.0 - Renamed Math.Geometry to Data.SpacePart.AABB - Renamed Data.QuadTree to Data.SpacePart.QuadTree - Added Data.SpacePart.QuadTree.query. Returns all elements that intersect a given boundary. - The inclusive nature of the boundary's min extent should take precedence of the exclusive nature of the max extent. Before this change many of the tests failed when boundaries of 0 area were involved. One case that did not work was constructing a quadtree containing elements of 0 area. This change corrected this. The tests all_elements_inserted_query_prop and element_bounds_query_is_element_prop still fail if any element involved is of 0 area. - Cannot create quadtrees with initial bounds of 0 area. - Removed requirement on elements being an instance of the Intersectable class. The only required instance is of Data.SpacePart.AABB.HasBoundary. - Changed package name to spacepart instead of data-spacepart. The last release of data-spacepart used a data based version number. This version number policy did not work well with the standard package version policy. - Added some QuickCheck based checks. Run with test/run_verify - Cleaned up the module exports. Cheers, Corey O'Connor

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Type families not as useful over functions
by John Ky 16 Feb '09

16 Feb '09

Hi Haskell Cafe, I tried using type families over functions, but when I try it complains that the two lines marked conflict with each other. class Broadcast a where type Return a broadcast :: a -> Return a instance Broadcast [a -> r] where type Return [a -> r] = a -> [r] -- Conflict! broadcast fs a = [] instance Broadcast [a -> b -> r] where type Return [a -> b -> r] = a -> b -> [r] -- Conflict! broadcast fs a b = [] Given that in Haskell, every function of n+1 arguments is also a function of n arguments, this is likely the cause of the conflict. In this case, currying is not my friend. Unfortunately this means I'm stuck with numbered function names: bc0 :: [r] -> [r] bc0 rs = rs bc1 :: [a -> r] -> a -> [r] bc1 [] a = [] bc1 (r:rs) a = (r a):bc1 rs a bc2 rs a b = rs `bc1` a `bc1` b bc3 rs a b c = rs `bc1` a `bc1` b `bc1` c -- etc Cheers, -John

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Haddock Markup
by David Waern 16 Feb '09

16 Feb '09

I got the subject line wrong, so I'm reposting this: ==== Hi everyone, I received this question from Lennart Augustsson (via Simon M) and thought I'd send out an inquiry to the Haskell community in general (Lennart, I hope you don't mind): Lennart writes: > We have some local patches for haddock that extends the <<blah>> > syntax so you can put TeX formulae in the documentation. > It looks like, <<! LaTeX stuff here !>>, but I'd like to extend it so > you can process the string with any command. > > Are you interested in folding this into the main branch? So the question is about extending the Haddock markup language. When modifying the language we should think about the tension between familiarity, presentation features (pictures, math, whatever) and visual portability across different mediums (HTML, ghci, IDE tooltips, etc). And here I should say that Haddock already supports pictures using the << url >> syntax. IMHO, adding <<! LaTeX !>> for TeX math is fine, because: - math in documentation is often useful - if you're going to write math, you need a format, even when the medium is plain text as in ghci. - TeX formulae seem to be relatively widely used and understood. As for running arbitrary commands, I think we are opening up to a lot of unfamiliar syntax. I'd like to hear what everyone thinks about that. There was also a thread about Haddock markup on haskell-cafe@ about a year ago, which originated with the interesting idea of using Markdown (or a Pandoc-extended version of it) instead of the current language: http://www.mail-archive.com/haskell-cafe@haskell.org/msg38054.html I think the original idea there is pretty nice, but let's first focus on the current markup language in order to answer Lennart's question. That thread contains some useful opinions on this matter, also. So, any comments? :) David

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The wheather is ⊥
by Henk-Jan van Tuyl 15 Feb '09

15 Feb '09

The temperature is now NaN°: http://traviswalters.net/?p=58 -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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Need some ideas to map XML Schema types to Haskell types
by Praki Prakash 15 Feb '09

15 Feb '09

Hi, I need to work with a web service that has a few thousand types in its schema. I am trying to automate the generation of data types and serializers from the schema. My first thought was to map each data type to a Haskell record and generate pickle methods thereafter for serialziation/deserialization. However, I am seeing a few challenges in mapping Schema types to Haskell records. First issue is the naming conflicts with record fields. If the number of types were smaller, I could have manually mangled the name with a unique prefix. Using a type class for shared name fields seems like an option but it looks like quite a bit of work. Is there any other technique I can use? Second issue is with capturing the Schema type hierarchy. Is there any way to relate types? If not, what would be a good technique to represent type hierarchies? Thanks, Praki

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Epic failure
by Andrew Coppin 15 Feb '09

15 Feb '09

OK, what did I do wrong here? module Main () where import Data.List top = 10 ^ 8 :: Int main = do let numbers = [1 .. top] print $ foldl' (+) 0 numbers -- Runtime is 20 seconds. #include <stdio.h> int main(void) { int total = 0; int n; for (n = 1, n < 100000000; n++) total += n; printf("%d", n); } // Runtime is 0.8 seconds. module Main () where import Data.List top = 10 ^ 8 :: Int kernel i o = if i < top then o `seq` kernel (i+1) (o+i) else o main = do print $ kernel 1 0 -- Runtime is 0.5 seconds. Clearly these two nearly identical Haskell programs should have exactly the same runtime. Instead, one is 40x slower. Clearly something has gone catastrophically wrong here. The whole point of GHC's extensive optimiser passes are to turn the first example into the second example - but something somewhere hasn't worked. Any suggestions?

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Help with Stack Space Overflow / Memory Issues
by Travis B. Hartwell 15 Feb '09

15 Feb '09

Hello, I'm writing a small program to process Delicious [1] RSS feeds. I like look at the recent feeds to see what others have bookmarked recently. But, there are a lot of duplicates in the recent feeds as an entry is shown for each person who bookmarks an individual URL. I decided to write a small program that would trim out those that I've seen before. I wrote a small program that read a feed (initially just a on-disk copy of an RSS feed) and removed the duplicate items just within that feed. It worked great. Then, I wanted to add persistence, so this would maintain state from one run to the next. I decided to use Data.Binary to serialize the Data.Map I was using and re-load it each time. Unfortunately, making this change caused a "Stack Space Overflow" error and I couldn't track down what was wrong. This was with GHC 6.8.2. I recently upgraded to GHC 6.10.1 and the memory just grows unbounded, until it actually locks up my machine. This happens even when I comment out the code for the serialization / de-serialization of the map, so essentially the only difference from my prior version is the function where the map is initialized returns IO [Item] instead of [Item]. The latest version of my code is up on github [2], and the sample RSS feed I was processing is included in the repo. I'd appreciate some help in how to attack this problem. I've even tried profiling this (back when I was using 6.8.2) and there was nothing enlightening there, at least with my limited Haskell experience. I am unsure of how to get this to work, or if the problem is even my code. Additionally, I am unsure if my serialization code would work anyway. Because Haskell is not pass-by-reference, would the changes to the seenMap propogate back to my deDupWithSerializedMap function where it is serialized? If not, how would I go about doing this? I think part of my problem might be the difference between pure and impure code and how to separate it. Thanks for the help! [1] http://www.delicious.com/ [2] http://github.com/Nafai77/recent-feeds/tree/master --------------- Travis B. Hartwell Software Toolsmith Blog: http://www.travishartwell.net/blog Where to find me: http://www.travishartwell.net/blog/static/where_to_find_me

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Interactive debugging?
by Colin Paul Adams 15 Feb '09

15 Feb '09

Can this be done with Haskell? In particular, I'm using ghc 6.10.1. Can I get symbols for use with gdb, for instance? -- Colin Adams Preston Lancashire

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