January 2008 - Haskell-Cafe

Re: HList error with hFoldr
by oleg＠okmij.org 02 Feb '08

02 Feb '08

Denis Bueno wrote: > For the moment my HLists only contain Ints. ... > -- Why should the following generate an error? > testApplyDistSum = hFoldr ApplyDistSum 0 ((4,4) .*. hNil) Here is the problem: in Haskell, 4 is not an Int. (4::Int) is an Int, but just 4 is a _polymorphic_ constant of the type Num a => a. Because it is polymorphic and so does not have any defined type, the type checker cannot chose an appropriate instance for ApplyDistSum. The error message says this directly: > No instance for (Apply ApplyDistSum ((t, t1), r1) r) Note the appearance of lower-case identifiers t, t1, r1, and r in the error message. They stand for the types that the type checker cannot figure out. When you use hFoldr in a larger expression, the types often become constrained by other components of the expression and so type variables become instantiated. In the simple cases as above, there is no information available to instantiate the type variables, hence the error. You have to give the type-checker hints in the form of type annotations: testApplyDistSum = hFoldr ApplyDistSum (0::Int) ((4::Int,4::Int) .*. hNil) In general, it is possible to avoid the type annotation on the accumulator (in this case, 0). The type checker could figure out the type of the result. But that requires specifying functional dependencies. The type annotations might be easier to start with. Incidentally, it is generally a good idea in Haskell to write Int constants as (1::Int), explicitly specifying their type. Here's a bit elaborated example: {-# OPTIONS -fglasgow-exts #-} module B where import HList class (Num i) => MetricSpace e i where dist :: e -> e -> i instance Num i => MetricSpace Int i where x `dist` y = fromIntegral $ abs (y - x) instance Floating i => MetricSpace Float i where x `dist` y = realToFrac $ abs (y - x) data ApplyDistSum = ApplyDistSum instance (MetricSpace e r) => Apply ApplyDistSum ((e, e), r) r where apply _ (p, v) = v + uncurry dist p testApplyDistSum = hFoldr ApplyDistSum (0::Double) ((4::Int,5::Int) .*. hNil) testApplyDistSum1 = hFoldr ApplyDistSum (0::Double) ( (4::Float,5.1::Float) .*. (4::Int,5::Int) .*. hNil)

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Simple network client
by Timo B. Hübel 01 Feb '08

01 Feb '08

Hello, I am using the very simple interactTCP example from [1] to play around with Haskell network programming but I just can't get a simple client for that example to work (it works like a charm with my telnet client, as described in the article). This is what I am trying to do with the client: main = withSocketsDo $ do hdl <- connectTo "localhost" (PortNumber 1234) hSetBuffering hdl NoBuffering hPutStr hdl "test message" res <- hGetContents hdl putStrLn (show res) The server looks like this: interactTCP :: Int -> (String -> IO String) -> IO () interactTCP port f = withSocketsDo $ do servSock <- listenOn $ PortNumber (fromIntegral port) waitLoop f servSock waitLoop f servSock = do bracket (fmap (\(h,_,_)->h) $ accept servSock) hClose (\h -> do hSetBuffering h NoBuffering hGetContents h >>= f >>= hPutStr h) waitLoop f servSock main = interactTCP 1234 (return . map toUpper) But is seems as some deadlocking occurs. Both programs just hang around doing nothing. By inserting some debug output I was able to make sure that the client successfully connects, but the data interchange just does not start. Because the whole thing works using telnet, I suspect that I am doing something fundamentally wrong in the client ... Any hints are greatly appreciated. Thanks, Timo [1] http://stephan.walter.name/blog/computers/programming/haskell/interacttcp.h…

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anybody can tell me the pronuncation of "haskell"?
by slofslb＠sina.com 01 Feb '08

01 Feb '08

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hxt memory useage
by Matthew Pocock 01 Feb '08

01 Feb '08

Hi, I've been using hxt to process xml files. Now that my files are getting a bit bigger (30m) I'm finding that hxt uses inordinate amounts of memory. I have 8g on my box, and it's running out. As far as I can tell, this memory is getting used up while parsing the text, rather than in any down-stream processing by xpickle. Is this a known issue? Matthew

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derive Pretty?
by Greg Fitzgerald 01 Feb '08

01 Feb '08

Is it possible to automatically derive instances of Pretty<http://haskell.org/ghc/docs/latest/html/libraries/haskell-src/Language-Hask…>? If no, what do most do when it comes to pretty-printing large data types? Thanks, Greg

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Relevance and applicability of category theory
by aaltman＠pdx.edu 31 Jan '08

31 Jan '08

Category theory seems to have an inconsistent relationship to Haskell - both documentation and the language's implementations of categorical concepts. I come from a math background that makes the Haskell's tight coupling to its mathematical foundation very appealing. But, I may have found some inconsistencies. Whether you agree or not I hope we can get some discussion going that will clarify the wikibooks, etc. My issues: 1. Are Haskell monads useful in a truly categorical sense? 2. Is Haskell's functor class misnamed? 3. Haskell arrows and Haskell monads have a misleading relationship 1. Categorical monads are a class of functors, that is, morphisms on Cat. Haskell's monads are at least a bit closer to the categorical idea than Haskell's functors by virtue of having the same domain and codomain: Hask -> Hask. I think applicability of Haskell monads beyond sequencing computation, and the validity of their definition, would be much more clear if someone explained the meaning of adjoint functors to and from Hask. In other words, provide the mathematical characterization to make Haskell monads a precise representation of categorical monads on Hask. 2. Functors are structure preserving maps in the category Cat. The Haskell Functor class represents structure preserving maps in the category Hask, which seem to me more like the categorical notion of arrows or the algebraic notion of homomorphisms. If we talk about functors relating to Haskell, it seems more likely we'd be referring to functors to or from the category Hask than functions (or arrows or whatever) between elements of Hask. Maybe categorical functors are useful as an abstract characterization of programs themselves: a program defines a functor from the problem domain into Hask, or from Hask into output in a problem domain. The name "Arrow" is already taken; what about renaming Functor to Automorphism? 3. I believe the documentation stating that Haskell arrows are a generalization of Haskell monads, but arrows are a categorical thing too and in that context bear a much more distant relationship to monads. Does a Haskell arrow have Hask as domain and codomain? Or is one particular element in Hask its domain and possibly another its codomain? Those are not at all the same thing. My guess is that the concept of Hask is too murky to be any use right now. The documentation may be more clear by avoiding it until we work out a formal definition... or is there one already? - Aaron Altman

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announcing darcs 2.0.0pre3
by David Roundy 31 Jan '08

31 Jan '08

We are happy to announce the third prerelease version of darcs 2! Darcs 2 features numerous improvements, and it seems that we have fixed most of the regressions, so we're looking for help, from users willing to try this release out. Read below, to see how you can benefit from this new release, and how you can help us to make the final darcs 2 release the best ever! The third prerelease features (apart from numerous bug and performance regression fixes) a completely rewritten rollback command and new progress-reporting functionality. If darcs takes more than a couple of seconds for a given operation and provides you with no feedback as to what it's up to, let us know, so we can fine-tune the progress-reporting output! I would especially like to thank Mark Stosberg for his tireless work triaging old bugs in our bug tracker and writing tests to be added to our test suite for those still present! For more details, and instructions on obtaining darcs 2.0.0pre3, see: http://wiki.darcs.net/index.html/DarcsTwo David

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RE: [Haskell] Announce: Yi 0.3
by Bayley, Alistair 31 Jan '08

31 Jan '08

> From: haskell-bounces(a)haskell.org > [mailto:haskell-bounces@haskell.org] On Behalf Of > Jean-Philippe Bernardy > > * homepage: http://haskell.org/haskellwiki/Yi Took a quick look around and saw this: * GTK frontend works in Win32 So Yi works on Windows? hs-plugins was broken for Windows for a while, so I'm wondering if this has been fixed too. Does Yi use hs-plugins, or does it go straight down to ghc? (looking at the yi.cabal file seems to indicate no dependency on hs-plugins) Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: [Haskell-cafe] RE: [Haskell] Announce: Yi 0.3
by Peter Verswyvelen 31 Jan '08

31 Jan '08

Is YI also an IDE for creating Haskell code? I guess it's not, it's "just" a text editor right? I mean, if it talks to GHC directly, it could neatly take over some tricks used in Visual Haskell (on the fly type inference, good completion, etc), only much much easier (without the COM layer, directly in Haskell).

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Re: [Haskell-cafe] Simple network client
by Gary Bickford 31 Jan '08

31 Jan '08

Wasn't there a Linux file system (possibly a FUSE user-space one) that worked on writable CDs? IIRC it worked by marking the previous copy of the file as erased, and writing a new copy. On Wed, 2008-01-30 at 23:05 -0500, haskell-cafe-request(a)haskell.org wrote: > > PS: I would love to see an immutable filesystem that does not allow > writing to files, it only creates new files and garbage collects files > that have no incoming reference anymore... Just like a garbage > collected heap, and a bit like an OLAP databases (as far as I remember > my DB theory...) Besides the performance bottleneck, does something > like that exists? > > Plan 9's venti is somewhat similar to this. though it's really a > storage backend that you implement a filesystem on top of, and the fs > winds up having a write cache, which is mutable in practice. The > interesting thing is that the block's location is the cryptographic > hash of its contents, which leads to all sorts of neat properties (as > well as requiring immutability). > -- "Isn't it funny how the Global Village includes everybody but the villagers?" (http://wiki.laptop.org/go/OLPC_Publications)

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