September 2004 - Haskell-Cafe

RE: [Haskell-cafe] Writing binary files?
by Simon Marlow 15 Sep '04

15 Sep '04

On 15 September 2004 12:32, ross(a)soi.city.ac.uk wrote: > On Mon, Sep 13, 2004 at 12:01:58PM +0100, Glynn Clements wrote: >> My view is that, right now, we have the worst of both worlds, and >> taking a short step backwards (i.e. narrow the Char type and leave >> the rest alone) is a lot simpler (and more feasible) than the long >> journey towards real I18N. > > This being Haskell, I can't imagine a consensus on a step backwards. > In any case, a Char type distinct from bytes and the rest is the most > valuable part of the current situation. The rest is just libraries, > and the solution to that is to create other libraries. (It's true > that the Prelude is harder to work around, but even that can be done, > as with the new exception interface.) Indeed more than one approach > can proceed concurrently, and that's probably what's going to happen: > > The Right Thing proceeds in stages: > 1. new byte-based libraries > 2. conversions sitting on top of these > 3. the ultimate I18N API > > The Quick Fix: alter the existing implementation to use the > encoding determined by the current locale at the borders. I wish I had some more time to work on this, but I implemented a prototype of an "ultimate" i18n API recently. This is derived from the API that Ben Rudiak-Gould proposed last year. It is in two layers: the InputStream/OutputStream classes provide raw byte I/O, and the TextStream class provides a conversion on top of that. The prototype uses iconv for conversions. You can make Streams from all sorts of things: files, sockets, pipes, and even Haskell arrays. InputStream and OutputStreams are just classes, so you can implement your own. IIRC, I managed to get it working with speed comparable to GHC's current IO library. Here's a tarball that works with GHC 6.2.1 on a Unix platform, just --make to build it: http://www.haskell.org/~simonmar/new-io.tar.gz If anyone would like to pick this up and run with it, I'd be delighted. I'm not likely to get back to in the short term, at least. Cheers, Simon

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Re: [Haskell-cafe] Layered I/O
by MR K P SCHUPKE 15 Sep '04

15 Sep '04

My thoughts on I/O, binary and chars can be summerised: 1) Produce a new WordN based IO library. 2) Character strings cannot be separated from their encodings (ie they must be encoded somehow - even if that encoding is ascii). I would approch this using parameterised phantom types, so for example you would have: data Ascii data Chr a = Char Then the encoding becomes explicit in the type system, but of course functions like equality on types cvan still be expressed generically: charEq :: Chr a -> Chr a -> Bool but of course, you must be comparing the same encodings (enforced by type system)! The phantom type could be used in IO, IE to define a new encodeing: data MyEncoding instance ChrToBinary MyEncoding where chrToBinary = ... instance BinaryToChr MyEncoding where binaryToChr = ... Keean.

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complete documentation like the one from Java or C++
by PRISACARIU Cristian 10 Sep '04

10 Sep '04

Hellow, can you tell me where I can find a more explicit documentation for the Haskell library and maybe for some of the extentions. GHC comes with something but is not as I want. I would like them to tell me for a function what it does not only what arguments it takes and what it returnes. And maybe some hints about how it works if not some examples. I would like something that approch the one from Java or .NET or MSDN. thank you for your time and understanding

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(no subject)
by jcp＠cs.yale.edu 10 Sep '04

10 Sep '04

From: John Peterson <peterson-john(a)cs.yale.edu> To: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Functional Reactive Programming Functional Reactive Programming is alive but in need of some new students to push the effort a bit. A lot of us have taken teaching or industrial positions so the old FRP team is a bit depleted. I don't think anyone is working on Yampa directly at the moment. Although it's stable and working well it lacks a critical mass of nice libraries to make it attractive. I'm still plugging on a wxHaskell port to Yampa (the wxFruit stuff). I've made some semantic changes to Yampa so I probably shouldn't say it's real Yampa but pretty close. I should have something to release later this fall. Aside from that, we have a student working in the hybrid modeling area. That's good stuff but not likely to produce software of interest to Joe Haskell. Another student is keeping the robotics side of things alive but it's in the context of a very specialized robotic hardware environment. So there you go! John

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Re: [Haskell] Haskell recruitment?
by Lyle Kopnicky 09 Sep '04

09 Sep '04

I think there are so few such opportunities that it would not overwhelm the list. I should also point out that you can post such openings at http://www.haskell.org/jobs.html by sending an e-mail to peterson-john(a)cs.yale.edu and olaf(a)cs.york.ac.uk. Regards, Lyle Jochen L. Leidner wrote: >Finally, would it be considered spam to post Haskell-realted job postings >in this group? Thanks in advance. > > Regards, > Jochen > > >

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cost of List.// for Ord types?
by David Roundy 08 Sep '04

08 Sep '04

Hello, I was wondering if the list diff operator \\ takes advantage of situations where the list data type is in class Ord, besides being in Eq. If it is only in Eq, then \\ must be O(n^2), while if it is in Ord, a O(nlogn) \\ can be written. Basically, I'm wondering if I should avoid using the standard library \\, and if so, whether there might be some better solution than writing my own (which would involve sorting the lists, and then the obvious picking off of the appropriate values). If there isn't a \\ that takes advantage of Ord, could there be? This being a more fundamental question: can you write a separate version of a function for the case where its argument is within a more specific class? -- David Roundy http://www.abridgegame.org

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Functional Reactive Programming
by Mark Carroll 07 Sep '04

07 Sep '04

I was wondering, how much active development is done on FRP frameworks these days. What direction is it going in, and who are the users? I haven't seen much new on Yampa lately so I wondered how that was doing, or if it was thought largely finished and maybe something else was going on. For instance, maybe more of the activity is in developing wxFruit or suchlike? -- Mark

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Installing HaXML,(using Hugs and WindowsXP)
by Tom Spencer 07 Sep '04

07 Sep '04

Has anyone successfully installed HaXML to be used with winhugs in XP. Using the build.bat included with HaXML it seems that it can only be installed for GHC. Any suggestions? Thanks Tom Spencer PS. Apologies that this is pretty vague

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QuickCheck - Extracting data values from tests
by Jorge Adriano Aires 04 Sep '04

04 Sep '04

Hello all, When using Quickcheck, is there some way to extract generated data values to the IO Monad? I know I can collect and print information about test cases, but that's not enough. Data may be pretty complex, and there may be no parsers for it. If a test suddenly goes wrong, just having it displayed doesn't seem that useful. I'd expect quickCheck to have type: quickCheck :: forall a. (Testable a) => a -> IO [a] Show I could just get the offending data with: please_be_empty <- quickCheck prop_foo Also, even when I'm implementing a generator, I want to see how it is working. Running a verboseCheck on some dummy property helps, but I may want to analyse the data, or some parts of it better - for instance, for many data structures I have alternative "show" functions that take parameters as arguments. Thanks in advance, J.A.

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Partially-applied type synonyms?
by Lyle Kopnicky 03 Sep '04

03 Sep '04

Hi all, I'm trying to write a monad transformer class called MonadPCont, for partial continuations, which fits in with the Control.Monad libraries. I'm having a typing problem. What I have so far looks like this: -------------------- module MonadPCont where import Control.Monad import Control.Monad.Cont import Control.Monad.Trans import Control.Monad.Reader import Control.Monad.Writer import Control.Monad.State import Control.Monad.RWS class (Monad (mc a), Monad (mc r)) => MonadPCont mc a r where shift :: ((forall b. Monad (mc b) => a -> mc b r) -> mc r r) -> mc r a reset :: mc a a -> mc r a instance MonadPCont Cont a r where shift f = Cont (\c -> runCont (f (\x -> Cont (\c' -> c' (c x)))) id) reset m = Cont (\c -> c (runCont m id)) type ContT' m r a = ContT r m a instance Monad m => MonadPCont (ContT' m) a r where shift f = ContT (\c -> runContT (f (\x -> ContT (\c' -> c x >>= c'))) return) reset m = ContT (\c -> runContT m return >>= c) -------------------- The error I get is: MonadPCont.hs:21: Type synonym `ContT'' should have 3 arguments, but has been given 1 In the instance declaration for `MonadPCont (ContT' m) i o' Failed, modules loaded: none. I guess it's not possible to partially apply a synonym for a type constructor. Essentially, I'm trying to do a 'flip', but at the type level. The underlying problem is that ContT is written to take the final result type (call it 'r') as the first parameter, and the underlying monad (call it 'm') as the second parameter, e.g. 'ContT r m a'. This is done so that 'ContT r' can be made an instance of the MonadTrans class. Unfortunately, I need 'PI r -> ContT r m', along with a and r, to be a member of the MonadPCont class (PI is the type binding operator). So I thought I'd define ContT' to take the arguments the other way around. Unfortunately, it can't be partially applied. Any ideas, or is it just not feasible to work this class into the library? Thanks, Lyle Kopnicky

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