Re: xml pull parser

[PA <petite.abeille@...>]

On Mar 21, 2005, at 17:13, Javier Guerra wrote:

> > What would be the proper syntax then?
>
> my mistake, use 'in' instead of '=' (that's a mistake i repeat all the 
> time)

Thanks :)

> > One way or another, I need to read more about this fabled generic for
> > loop.
>
> you won't regret it.  iterators are a great way to traverse anything, 
> specially
> when done with coroutines (i think they're called 'generators' in that 
> case?)

Yes. I see. I remember now why I don't use the generic for loop. It 
requires a function. That function is then invoked for each iteration 
by the loop itself. In my case, anInputStream.read() return the results 
themselves. So to use the generic for loop I would need the rewrite my 
loop like this:

for aType, aContent, aName, someAttributes in anInputStream.read

Note the lack of parenthesis. This is a gratuitous deviation from how I 
invoked functions: always use parenthesizes.

On the other hand, I would gladly use the generic loop if I could write 
it like this:

for aType, aContent, aName, someAttributes in anInputStream.read()

Note the explicit parenthesis. I favor consistency over magic :)

> it's not an optimization, it's more a mindframe thing.  in low level 
> languages
> you mostly choose your algorithms, but in higher level it's important 
> to see
> what the language offers you and follow the path of least resistance.

Agree. That said, Lua itself allow many variation on the same theme. 
Which you may view as either beneficial or detrimental. I think too 
many variation in how to express the exact same thing is a liability 
overall. How many "for" construct does a language needs? One seems to 
be plenty enough.

> .... unless you want a good fight! ;-)

I'm careful avoiding any such fight :)

> yep.  and there are lots of things done best with an object (i think a 
> XML pull
> parser IS one of them)
>
> personally, i do use object-like constructs in Lua all the time.  i 
> like the
> obj:func() form,

I found the colon notation too much of a good thing and I'm not using 
it at all. I favor the plain dot instead. That way, there is no 
additional confusion: dot or colon. The dot it is always :)

>  and since i seldom use any inheritance, i haven't find the need
> for a full OOP scheme in Lua.

Well, even my LU concoction is far from a full blown OO system. It 
simply boil down to a packaging system to bundle named functions in 
tables. All in all, its main benefit is to provide an organizational 
principal. The OO benefit, as I see it, being mostly polyphormism 
instead of hard core inheritance.

> Even in C++, the data-managing parts of my programs are very shallow 
> class
> hierarchies.  the GUI parts do get deep... but mostly because of the 
> framework i
> happen to use.
>
> > But frankly speaking, I haven't seen a great deal of reuse in Lua. At
> > all. Quite a let down, if you ask me. And here I am writing a dumb
> > little function to decode XML strings. Duh.
>
> very true.  i think code reuse is more at a module level than at 
> object or
> function level.

Well, my so called "objects" are just plain table of functions. It's 
more a packaging scheme than anything else.

> and at the copy-paste level, of course!

Oh, my... :P

> > Because I'm not sure what this fabled "Lua way" is. Concrete 
> > suggestion
> > warmly welcome :)
>
> rici's example is a nice one (much better than mine!)
> concrete 'luaisms' used on his decoder:
>
> - wrap a small module on a do...end block, so a single function is 
> exported
> (decode()), but it has access to local vars (ents[] and maxutf8) and 
> functions
> (entity2char())
> - use a lookup table
> - use a standard function (gsub()), and give it another function as a 
> parameter
> - use 'str and gsub(....)' idiom to check parameters before using them

Yes. I pretty much follow such idioms.

For example, I use anonymous functions quite extensively. As an 
illustration, in my HTTP server, a service accepts an anonymous 
function which acts as an authenticator, by returning true or false for 
a given context:

local anAuthenticator = function( aContext )
    if ( aContext.request().headers().get( "authorization" ) ~= nil ) 
then
	    return true
    end

    return false
end

http://dev.alt.textdrive.com/file/LW/Test.lua

Such function is in turn used when the service has to decide to give 
access to a given resource:

if ( ( anAuthenticator == nil ) or ( anAuthenticator( aContext ) == 
true ) ) then
    this[ aMethod ]( aTask, aContext )
else
    local aChallenge = string.format( "Basic realm=\"%s\"", 
aContext.session().realm() )

    aResponse.headers().put( "www-authenticate", aChallenge )
    aResponse.writeStatus( 401 )
end

http://dev.alt.textdrive.com/file/LW/LWService.lua

> > ......
> > local aValue = thisClass.entities().get( anEntity )
> > ........
> > Is that Lua-esque enough? :))
>
> is that line a lookup table?

Yes.

> if so, why not use tables???

I do. Indirectly. I have a wrapper 
module/package/library/object/whatever around Lua tables which provides 
commonly used functionalities:

http://dev.alt.textdrive.com/file/lu/LUMap.lua

Getting to this lookup table involves a couple of steps:

(1) local aBundle = LUBundle.bundleWithName( "LUXMLInputStream" )
(2) local aFile = LUFile.new( aBundle.path(), "LUXMLInputStream.txt" )
(3) cvars.entities = LUMap.new().load( aFile )

(1) locate where the code is
(2) get the relative path to the specific resource
(3) load the table

By itself, the underlying Lua code is not much, but I use such 
functionality all over the place. And I for once don't enjoy copy & 
paste programming much :P So the code is centralized and reused if 
possible :)

http://dev.alt.textdrive.com/file/lu/LUBundle.lua
http://dev.alt.textdrive.com/file/lu/LUFile.lua

Cheers

--
PA, Onnay Equitursay
http://alt.textdrive.com/