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Vital visual notations?
by George Russell 14 Nov '03

14 Nov '03

Martin Ewig wrote (snipped) > I think that some visual notations are more readable than text > (but not all). In particular, if you try to teach lambda calculus > or type inference to beginners, visual notations can be extremely > helpful. For some people, perhaps. I don't have a very good mind for visual things and I much prefer symbolic versions of lambda calculus. Perhaps beginners need to be shown it both ways.

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AW: Generic Haskell Diffs?
by Markus.Schnell＠infineon.com 14 Nov '03

14 Nov '03

let (Just mail) = lookup "Generic Haskell Diffs?" mailbox in replace "Are there" "Are they" mail > -----Ursprüngliche Nachricht----- > Von: haskell-bounces(a)haskell.org > [mailto:haskell-bounces@haskell.org] Im Auftrag von > Markus.Schnell(a)infineon.com > Gesendet: Freitag, 14. November 2003 10:49 > An: haskell(a)haskell.org > Betreff: Generic Haskell Diffs? > > > According to the communities report there are different > generic haskell projects (Jeuring/Hinze and PJ/Lämmel) out there. > But I don't understand their relation. > Can you use both at the same time? > Is one building on the other? > Are there adressing different issues? > > A clarifying sentence or two would be heartily welcome. > > Cheers, > Markus > > > -- > Markus Schnell > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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[rpms] updated greencard rpm package for ghc-6.0.1 available
by Jens Petersen 14 Nov '03

14 Nov '03

FYI a new greencard rpm package built with ghc-6.0.1 is now available from : http://haskell.org/~petersen/rpms/greencard/?C=M&O=D Cheers, Jens

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ANNOLUNCE: attribute 0.2
by Abraham Egnor 14 Nov '03

14 Nov '03

I apologize for the frequency, but based on initial feedback I made some interface-breaking changes and thought it wise to release. The major changes are: * use functions instead of a datatype for Property construction. From the user's point of view, this just means the colon goes at the end instead of the beginning, i.e. (=:) instead of (:=). * split non-haskell98 functions (MRef, attrMRef[T]) off into their own module - Data.Attribute.Util. Data.Attribute itself is now pure Haskell 98. * added an MRef instance for lazy STRefs. The new tarball is at "http://abe.egnor.name/attribute-0.2.tar.bz2". Abe

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StateT space leak
by Wojtek Moczydlowski 14 Nov '03

14 Nov '03

Consider the following program: module A where import Control.Monad.State f :: StateT Int IO () f = (sequence_ $ repeat $ return ()) t = runStateT f 0 When t is evaluated under ghci or hugs, the program quickly runs out of heap memory. What's going on here? Is this inherent in StateT monad? If so, then this is very surprising, I would expect f to run in constant space for any sensible monad. Wojtek

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ANNOUNCE: attribute 0.1
by Abraham Egnor 13 Nov '03

13 Nov '03

Attribute is a library for storing and retrieving named values from haskell datatypes in arbitrary monads. Many of the haskell GUI libraries have implemented something similar; in one of my current projects, I discovered that such a thing would be useful. However, I didn't want to tie it to my specific use, the result of which is this library. The README included with the source is hopefully enough documentation to get started, the text of which is included at the end of this email. A tarball is available at "http://abe.egnor.name/attribute-0.1.tar.bz2". Source can also be obtained via arch: tla register-archive http://ofb.net/~abe/archive/2003 tla get abe-tla@ofb.net--2003/attribute--main === README === This is attribute, monadic attributes for haskell datatypes. See COPYRIGHT for copying information. Building: edit the Makefile for the install path make make install (as root) The only dependency is a recent version of ghc (>=6). Use: Abstractly, an attribute represents a value that can be retrieved from or stored into a specific type in a specific monad; an attribute can either be readable, writable, or both, represented by the types Read, Write, and ReadWrite. A note on naming conventions: I've used general words (such as Read, Write, set, get, etc.) for most functions; this does not follow Haskell convention, but does follow the ideas at "http://haskell.org/hawiki/UsingQualifiedNames", which makes far more sense to me. If you can't live without prefixes, qualify the import. Example: "ReadWrite Int String IO" represents a String that can be both extracted from and stored into an Int in the IO monad (although such a property is unlikely to be useful). A more useful attribute might be something like: contents :: Read FilePath String IO which would represent the contents of a file, probably read in via getContents or some such. Attributes can be constructed directly from setter or getter functions: data (Monad m) => Read o d m = Read (o -> m d) data (Monad m) => Write o d m = Write (o -> d -> m ()) data (Monad m) => ReadWrite o d m = ReadWrite (o -> m d) (o -> d -> m ()) A few convenience functions are provided for constructing attributes: attrMRef :: (MRef r m) => (a -> b -> b) -> (b -> a) -> ReadWrite (r b) a m attrMRefT :: String -> ExpQ attrMRef takes a pure mutator and extractor, and creates an attribute that applies those functions to a monadic reference. Instances for MRef are provided for both IORef and STRef. attrMRefT simplifies a common case, where you have a pure datatype defined with named records and you'd like to make attributes for some of the records: data Foo = Foo { fooBar :: Int, fooBaz :: String } bar :: (MRef r m) => ReadWrite (r Foo) Int m bar = $(attrMRefT "Main:fooBar") baz :: (MRef r m) => ReadWrite (r Foo) String m baz = $(attrMRefT "Main:fooBaz") The String passed to attrMRefT is the name of one of the records; the current implementation of template haskell requires that it be prefixed with the name of the module in which it's defined. attributes are bound to values by creating a Property; the constructors for property are ":=", ":~", "::=", and "::~", which are pure set, pure mutate, monadic set, and monadic mutate respectively. To reuse the Foo example from above: test :: IO (Int, String) test = do ref <- newMRef $ Foo { fooBar = 3, fooBaz = "hello" } set ref [ bar := 5, baz :~ (++" world") ] bar' <- get ref bar baz' <- get ref baz return (bar', baz') will return (5, "hello world"). Note that because attributes created with attrMRef or attrMRefT are qualified by monad type, this example could be changed to use the ST monad simply by changing the type signature. Two functions are provided for manipulating attributes: "set" and "get". set :: (Monad m) => o -> [Property o m] -> m () get :: (Monad m, CanRead a) => o -> a o d m -> m d The "CanRead" class constraint simply enforces the readability of the particular attribute; both Read and ReadWrite are instances. There is a similarly used "CanWrite" class: class CanRead a where aGet :: (Monad m) => (a o d m) -> (o -> m d) class CanWrite a where aSet :: (Monad m) => (a o d m) -> (o -> d -> m ()) While you are certainly free to define new instances of the classes, I have yet to find a use case where the simple Read/Write/ReadWrite types do not suffice. See the files in src/test/ for examples. Have fun! Abe Egnor (abe-attribute(a)ofb.net)

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Re: time since the epoch
by Stefan Karrmann 13 Nov '03

13 Nov '03

Hi, a while ago time calculation was subject on this list. Now, I have a time library based on the TAI (international atomic time) time scale. >Peter Thiemann (Thu, Feb 06, 2003 at 12:40:14PM -0800): >> John's code illustrates TimeDiff's deficiencies perfectly: >> >> There is also a more fundamental problem with the TimeDiff data >> type. While seconds, minutes, hours, and days are clearly specified >> amounts of time, the duration of a month or a year may vary depending >> on the reference point where the time difference is applied. > newtype TimeDiff = TimeDiff Rational > deriving (Eq, Ord) >> Hmm, this is underspecified! >> As another poster said, (pointing out http://cr.yp.to/libtai, but it >> is better to look at http://cr.yp.to/time.html, which has a discussion >> on UTC vs TAI vs UNIX time) the official source of time is TAI, so it >> is best to base a time library >> *on the number of TAI seconds since a reference date* >> (which is btw what the libtai is all about). >> For compatibility with UNIX time, "Arthur David Olson's popular time >> library uses an epoch of 1970-01-01 00:00:10 TAI" >> [http://cr.yp.to/proto/utctai.html] >> So this mostly means that you need to set your system clock correctly:-) Sincerly, -- Stefan Karrmann

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ANNOUNCE: Release of Vital, an interactive visual programming environment for Haskell
by Keith Hanna 13 Nov '03

13 Nov '03

Dear Haskellers, I am pleased to announce the release of Vital, an interactive visual programming environment based on Haskell. Vital provides the user with a workspace within which Haskell expressions and declarations can be located and their values displayed textually or graphically. The display is 'live' in the sense that the effects of graphical Copy and Paste operations carried out displayed values are automatically reflected in the Haskell source code. The long-term aim of the Vital project is to make Haskell available in a form that supports the open-ended, incremental style of program development that end users (engineers, scientists, analysts, etc.) often prefer. It may also be useful in teaching Haskell. This release of the Vital system is available from: http://www.cs.kent.ac.uk/projects/vital/ The site includes screenshots and a downloadable implementation with demo programs. If you happen to have Java installed on your machine, you can (in principle!) run the Vital interpreter and these demo programs simply by clicking on this link http://www.cs.kent.ac.uk/projects/vital/download/start.jnlp Enjoy! Keith Hanna

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haskell httpd
by S. Alexander Jacobson 13 Nov '03

13 Nov '03

I've been working on a new project and evaluating the Twisted python framework which relies on asynchIO rather than threading to achieve speed. The resulting idioms feel like they would be much more elegant in Haskell. So my question at this point is: Is there a reasonably efficient Haskell httpd implementation around that uses poll/select? -Alex- ___________________________________________________________________ S. Alexander Jacobson Check out my new blog!!! 1-212-787-1914 voice http://alexjacobson.com

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set representation question
by Hal Daume III 13 Nov '03

13 Nov '03

this is somewhat off topic, but i'll add "does anyone have a haskell implementation of..." to the beginning to make it more on topic :). anyway there are a lot of smart people here... i'm looking for a representation for a set of natural numbers. right now, my representation is sorted array, which works well. *all* i care about is being able to quickly calculate the size of the intersection of two sets. these sets are, in general, very sparse, which means that the intersections tend to be small. for example, i might have two sets reprsented by the arrays: {0,1,10,346,398,1039,3289,3853,9811,89231,50913} {0,3,98,183,398,1038,5319,7642,9811,13893,93123} and all i need to be able to do is respond with "3" very very quickly. using sorted arrays, this takes O(n+m) time (where n and m are the sizes of the arrays). i'm willing to pay some up front cost for creating a data structure that could do this more quickly (i.e., in logarithmic time). is such a thing possible? "does anyone have a haskell implementation of" such a thing? :P thanks! - hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume

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