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Why does HXT use Arrows?
by Gregory Crosswhite 25 Dec '09

25 Dec '09

Something I have been wondering for a while is: Why is HXT built on arrows, rather than monads? I understand that arrows are a generalization of monads, but they also add extra complexity that is often unnecessary. Furthermore, as I understand it they only give you additional power when your arrow cannot implement ArrowApply, since otherwise your arrow is equivalent to a monad anyway, and it looks like the result of parsing an XML document gets you an arrow which implements ArrowApply. Is there some circumstance I am missing where the fact that HXT uses arrows rather than monads lets one do something that one couldn't have done with monads? Cheers, Greg

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problem with editline install
by Andrew U. Frank 25 Dec '09

25 Dec '09

i tried to install editline (because i wanted to install djinn, which depends on it): with cabal install and with downloading and runghc Setup.lhs configure i got the same error: checking for completion_matches... no configure: error: editline not found, so this package cannot be built See `config.log' for more details. in the log i did not see anything helpful (at my level of understanding). i could then install with synaptic because a deb package existed for ubuntu). what did i wrong? thanks for helpful comments andrew ps. in this case a wiki would have been helpful. it would have been a less intruding form to communicate than sending an email to the developer. especially in the case i assume i made some error.

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Haskell type system and Barendregt's Lambda Cube classification
by Vladimir Ivanov 25 Dec '09

25 Dec '09

Dear All, Recently, I've been playing with self-application and fixed-point combinators definition in Haskell. It is possible to type them in Haskell using recursive types. I took Y & U combinators: > newtype Rec a = In { out :: Rec a -> a } > u :: Rec a -> a > u x = out x x > y :: (a -> a) -> a > y f = let uf = f . u in uf (In uf) Recursive types are part of System F-omega, which corresponds to lambda omega in Barendregt's Lambda Cube. For all type systems in Lambda Cube it is proven that they are strongly normalizing. But using "y" I can easily construct a term even without a normal form. So, I got a contradition and conclude that type system implementation in Haskell contains something, that is absent from System F-omega. My question is: what's particular feature in Haskell type system, that breaks strong normalization property? Or am I totally wrong classifying it in terms of Lambda Cube? Best regards, Vladimir Ivanov

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Memory-aware Haskell?
by Roman Cheplyaka 25 Dec '09

25 Dec '09

Imagine some system with hard memory bound (e.g. 64M of physical memory, no swap). I don't want some accidental laziness (maybe even not in my code, but in some used package) to crash my program. So, let's think what we can do at runtime. Suppose RTS takes the parameter -- upper limit of consumed memory. When it sees that memory consumption is close to upper bound, it can: 1. force garbage collection 2. apply some heuristics to find and reduce some chunks which will benefit from reduction in terms of size 3. if nothing helps, throw an exeption. It can be caught in IO and memory-aware program can make apropriate decision -- e.g. abort opening a large file and gracefully warn the user. (And there still is a problem of foreign code whose memory consumption we know nothing about...) What do you think? -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

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Haskell Users in Leipzig
by Matthias-Christian Ott 25 Dec '09

25 Dec '09

Hi, I just wanted to know whether there are any Haskell users and enthusiasts in the Leipzig area who are interested in meeting with other users and perhaps interested in creating a user group. Users of other functional programming languages are of course also welcome. Regards, Matthias-Christian

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pointfree-trouble
by slemi 24 Dec '09

24 Dec '09

hello everybody, i'm a newbie this is my first post here.. i have trouble making a function pointfree: data RealFrac a => Matrix a = Matr [[a]] | Scalar a deriving (Show, Eq) unMatr :: RealFrac a => Matrix a -> [[a]] unMatr = (\(Matr a) -> a) reMatr :: RealFrac a => ([[a]] -> [[a]]) -> (Matrix a -> Matrix a) reMatr a = Matr . (flip (.) unMatr) a this works fine, but if i leave the 'a' in the last function's definition like this: reMatr = Matr . (flip (.) unMatr) it gives an error. can anybody tell me why? (i'm using ghci) -- View this message in context: http://old.nabble.com/pointfree-trouble-tp26881661p26881661.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANNOUNCE: repr-0.3.2
by Bas van Dijk 24 Dec '09

24 Dec '09

Hello, Some months ago I uploaded a little package called 'repr' to hackage. I've now updated the package to work with ghc-6.12.1 and its new base library 4.2.0.0. Back then I forgot to make a proper announcement so I will do that now: 'repr' allows you to render overloaded expressions to their textual representation. For example: *Repr> let rd = 1.5 + 2 + (3 + (-4) * (5 - pi / sqrt 6)) :: Repr Double *Repr> show rd "fromRational (3 % 2) + 2 + (3 + negate 4 * (5 - pi / sqrt 6))" See: http://hackage.haskell.org/package/repr-0.3.2 regards, Bas

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trace
by michael rice 24 Dec '09

24 Dec '09

Can someone provide a simple example of tracing a function. Michael

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Improved message for GHC "No instance for" Errors
by Victor Mateus Oliveira 24 Dec '09

24 Dec '09

Hi, I would like to propose a modification in the ghc message for "no instance for" errors. In the message, the ghc tells that there's no instance for the type you are using and it give you a hint that you can fix it adding a new instance for your type. But it don't tell me any instance alternative that already exists. So, if I don't know the correct type, I need to look at the documentation. At least for me, I get this message when I'm using the function in the wrong way or when I forget to specify the some function signature. It's quite rare the I want to add a new instance. Of course that it's not a problem to look at the documentation, but improving it can save a significant time and help the beginers to guess the error and how fix it. In C++ and Java, when I use g++ or javac and I make a wrong overloading, the compiler show me some alternatives. I think it help more than tell me to add a new overload to the function. It's possible to do it in ghc? I'm new haskell programmer and I enjoying a lot this language. []s Victor -- GNU/Linux user #446397 - http://counter.li.org

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Cabal's License type: MIT commented out?
by Tom Tobin 24 Dec '09

24 Dec '09

I noticed on Hackage that packages that are MIT licensed show up as "OtherLicense". I took a peek inside the Cabal code, and noticed that the License type has lines for MIT, but commented out [1]: > -- -- | The MIT license, similar to the BSD3. Very free license. > -- | MIT Why is this? Both the BSD3 and BSD4 get entries in this module. It would be nice to know up-front that a license is indeed liberal (BSD/MIT) rather than having to open up the package and look at the LICENSE file. (I also use the MIT license for my code, and would like to avoid confusing people once I start distributing Haskell libraries.) [1] http://hackage.haskell.org/packages/archive/Cabal/latest/doc/html/src/Distr…

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