December 2005 - Haskell-Cafe

HXT : need to retrieve a part of a document by a XPath expression
by Simon Odou 10 Dec '05

10 Dec '05

Hi everyone, I am using the HXT (Haskell XML Toolbox) library to parse XML documents. And I would like to be able to retrieve a part of a document by a XPath expression. I saw the "getXPath" function which nearly does the work but not exactly: if you have the xpath "//grandfather/father/son", calling getXPath on the "grandfather" node gives the "son" subtree; if you have the xpath "father/son", calling getXPath on the "father" node gives the "son" subtree. I would like to get the "son" subtree when I call getXPath-like on the son. Something like: if you have the xpath "father/son" or "//grandfather/father/son", calling getXPath-like on the "son" node gives the "son" subtree. I understand why getXPath is done like that. But is there an easy way to get the previous result? Thanks -- Simon Odou simon.odou(a)gmail.com

1 0

Can't Haskell catch up with Clean's uniqueness typing?
by haskell-cafe.mail.zooloo＠xoxy.net 10 Dec '05

10 Dec '05

Hi all, being occupied with learning both languages, I'm getting curious if Haskell couldn't achieve most of the performance gains resulting from uniqueness typing in Clean by *automatically* determining the reference count of arguments wherever possible and subsequently allowing them to be physically replaced immediately by (the corresponding part of) the function's result. Are there any principal obstacles, or *could* this be done, or *is* this even done already, e. g. in ghc? Regards, zooloo -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.362 / Virus Database: 267.13.12/192 - Release Date: 05.12.2005

18 31

What is a Boxed Array?
by John Velman 09 Dec '05

09 Dec '05

I've tried google and google scholar, wikipedia, and planetMath. Can't find a description. Can someone point me to a freely available reference? Thanks, John Velman

5 5

mapOfmap & composition
by raptor 09 Dec '05

09 Dec '05

hi, Is there a function like "map (map f) list" i.e. map of map ? And second why when I try do do function composition i.e. : newf = map . f But this works : newf x = map (f x) thanx

4 3

Monad applied in Java
by Ben.Yu＠combined.com 09 Dec '05

09 Dec '05

My apology to those who don't care about Java. Please trash this note. I recently published two open source Java projects that are based on the Monad computation model learned from Haskell community. 1. A port for Parsec. This library is called Jparsec. It implements monadic parser combinator in Java. Not nearly as concise and clean because of the limitation of the Java language, but it implements the same idea. And it looks like the only monadic parser combinator lib in Java so far, according to my google homework. ;--) url: http://docs.codehaus.org/display/JPARSEC/home 2. A light-weight Dependency Injection container that's built around the Monad model. If you see <sequence id="mybean"> <bean var="a1" class="A1"/> <bean var="a2" class="A2"/> <method class="X" name="create" args="$a1,$a2"/> </sequence> You'll know what I'm talking about --- do-notation. With a bunch of "bind" calls under the hood, it is equivalent as saying: do a1 = bean "A1" a2 = bean "A2" method "X" "create" [a1,a2] url: http://docs.codehaus.org/display/YAN/home This is the only ioc container that uses Monad model in Java. Just FYI. Any comment is welcome. Ben.

1 0

preffered 'map'
by raptor 09 Dec '05

09 Dec '05

hi, I imported : import Data.Map as Map but now anywhere when I want ot use "map" it complains for name clashes, so I have to specifiy Prelude.map all the time. Is there a way to specify that i mean Prelude not Data 'map' (but not fqn) I use Hugs, 'cause error messages are more understandable. tia.

5 4

how to nicely implement phantom type coersion?
by David Roundy 09 Dec '05

09 Dec '05

Hello all, I've been working on a sort of static consistency-checking framework for darcs patches using GADTs and phantom existential types, and am looking for a bit of advice on whether I might be able to create a class to avoid some tedious coding overhead. The basic idea is that a patch will have type (Patch a b) where "a" and "b" are phantom types. Sequential patches will have identical ending and beginning types. So that a sequential pair, for example, can be written as data Sequential a c where Sequential :: Patch a b -> Patch b c -> Sequential a c The trickiness is that we need to be able to check for equality of two patches, and if they are truly equal, then we know that their ending states are also equal. We do this with a couple of operators: (=\/=) :: Patch a b -> Patch a c -> Maybe (EqContext b c) (=/\=) :: Patch a z -> Patch b z -> Maybe (EqContext a b) data EqContext a b = EqContext { coerce_start :: Patch a z -> Patch b z, coerce_end :: Patch z a -> Patch z b, backwards_coerce_start :: Patch b z -> Patch a z, backwards_coerce_end :: Patch z b -> Patch z a } where we use the EqContext to encapsulate unsafeCoerce so that it can only be used safely. The problem is that it's tedious figuring out whether to use coerce_start or coerce_end, or backwards_coerce_end, etc. Of course, the huge advantage is that you can't use these functions to write buggy code (at least in the sense of breaking the static enforcement of patch ordering). So I'm wondering if there might be some sort of multi-parameter class one could define that would ease the use of an EqContext. For example I can imagine a (somewhat lame) class class EqTypes a b where safe_coerce_end :: Patch x a -> Patch x b safe_coerce_start :: Patch x a -> Patch x b safe_coerce_start = invert $ safe_coerce_end $ invert p (note that invert :: Patch a b -> Patch b a) then we could perhaps write some template haskell to define a couple of instances of EqTypes from a data type EqContext... except that I suspect that this is isn't possible, since the EqContext only exists at runtime, so unless one could define classes dynamically at runtime I'm out of luck. So is there some other approach that I can use for easily coercing phantom types based on runtime checks? Any suggestions? What I'd really love would be to be able to create a function that is polymorphic in kind rather than in type such as (inventing a rather poor syntax off the top of my head): data EqContext a b = EqContext { safe_coerce :: f(a,b) -> f(b,a) } where f(a,b) is a function of two types that returns a type, so the value of f(a,b) might be (Patch a b) or (Patch x b) or something like that. But I'm not sure if this is possible in Haskell, and if it is, then it definitely requires some sort of tricky extension that I'm not familiar with... -- David Roundy http://www.darcs.net

7 7

RE: [Haskell-cafe] STM reference?
by Simon Peyton-Jones 09 Dec '05

09 Dec '05

GHC now has a Wiki http://cvs.haskell.org/trac/ghc/wiki, so I've started a new page to give pointers to material on concurrent programming using GHC. (The link is near the bottom.) Please add to it. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Bulat Ziganshin | Sent: 09 December 2005 07:07 | To: Scherrer, Chad | Cc: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] STM reference? | | Hello Chad, | | Friday, December 09, 2005, 3:09:29 AM, you wrote: | | SC> Can anyone suggest some references to learn how to program using GHC | SC> with threads? I've read a little bit about STM, but it's still pretty | SC> mysterious to me. Is this the best approach to take? I've never used | SC> threads in any language, but monads are fairly comfortable for me. | | STM is just additional instrument, you can completely avoid it (as we | had do until 6.4 release). Below is a list of papers related to | imperative programming in Haskell. first contains pragmatic | introduction in this world, including all concurrency stuffs, second | is about an real high-concurrency application designed by GHC team. | other papers is of more theoretical (and more strict) character | | Tackling the awkward squad: monadic input/output, concurrency, exceptions, and foreign-language | calls in Haskell | [http://research.microsoft.com/Users/simonpj/papers/marktoberdorf/markto berdorf.ps.gz] | Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server, | [http://www.haskell.org/~simonmar/papers/web-server.ps.gz] | [http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz] | [http://www.haskell.org/~simonmar/papers/conc-ffi.pdf] | The Concurrent Haskell Foreign Function Interface | [http://www.haskell.org/ghc/docs/papers/threads.ps.gz] | Asynchronous Exceptions in Haskell [http://www.haskell.org/~simonmar/papers/async.ps.gz] | [http://research.microsoft.com/~simonpj/papers/stm/stm.pdf] | [http://www.haskell.org/ghc/docs/papers/except_ps.gz] | Imperative Functional Programming [http://www.haskell.org/ghc/docs/papers/imperative.ps.gz] | | | | -- | Best regards, | Bulat mailto:bulatz@HotPOP.com | | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

STM reference?
by Scherrer, Chad 09 Dec '05

09 Dec '05

Hi, Can anyone suggest some references to learn how to program using GHC with threads? I've read a little bit about STM, but it's still pretty mysterious to me. Is this the best approach to take? I've never used threads in any language, but monads are fairly comfortable for me. Thanks! Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

2 1

busting up a structured binary file
by Brian McQueen 08 Dec '05

08 Dec '05

Can someone point me to a sample so I can get started? This will be my first Haskkel project, after reading about it for some months now. I need to extract text from a structed binary file. Its a local database for a commercial app of proprietary structure, though the structure has been determined. So I need to move along a number of bytes, take a few chunks of so many more bytes and grab the string and move on to the next record, dumping the contents - ultimately dumping it to XML. It'll need to run on windows so I guess I had better use Hugs. So a sample and any advice would be an excellent start for me, and would be greatly appreciated. Thanks Brian McQueen

2 1