Feedback thread 'Examples - How to use Filter' wiki page

[Max]

With Adamant, we went a little further and here is the result: a short howto trying to make easier filters understanding and use, and we hope it will help.

wow - this is really a great guide. any chance you could move it to the wiki?

[SlowThinker]

I have read the text very quickly, and so I might miss something, but ...

------------------------------
I think you should explain how 'parentheses are added' to the code when it is parsed, because filters are not always intuitive in this aspect.
examples:

[and]A[/and] [or]B[/or] [and]C[/and] [not]D[/not] ...
is processed like
(((((Universum and A) or B) and C) and_not D) ... )

[SLF]x= y= terrain= radius=xyz [and]...[/and][/SLF]
is processed like
function_radius=xyz([SLF]x= y= terrain= [and]...[/and][/SLF])

-------------------------------

Code: Select all

[filter] 
	race=orc 
	x,y=16,22 
[/filter] 
 
[filter] 
	x,y=16,22 
	race=orc 
[/filter]

These two tags are identical because the internal representation of WML tags in lua doesn't distinguish any order among WML attributes (this is true in general, not only in filters).

[pyrophorus]

Hi !
Thanks to everyone for the feedback !

Don't thank me.
Pyrophorus is the one who deserves the honour.
He was the one who wrote these pages;

Not only... I wrote, but Adamant provided me with examples and questions, read the drafts, and IMO really oriented the work and deserves to sign it with me.

wow - this is really a great guide. any chance you could move it to the wiki?

Well, maybe later. But I'm rather reluctant to do that because to me, wiki syntax sucks. But if anyone wants to do the job, be my guest

I think you should explain how 'parentheses are added' to the code when it is parsed, because filters are not always intuitive in this aspect.
examples:

[and]A[/and] [or]B[/or] [and]C[/and] [not]D[/not] ...

I'm not writing a reference manual here, and your example is exactly what I would strongly discourage readers to do. If you translate it in natural speech; it would become:
(A) or (B) and (C) (not D)
What does this means ? It can be read as:
( A or B) and (C and (not D)) -> [and] A [or]B[/or] [/and] [and] C [not]D[/not] [/and]
or:
A or ( B and C and (not D)) -> A [or] B C [not] D [/not] [/or]
or even otherwise. These "translations" are unambiguous when the original writing is not. Of course, filter execution will be only one of those because there are operators precedence rules. But writing filters like this is IMO the best way to write code that will not work as expected and to spend hours in trial and error process. If it's clear in one's head (and in natural writing) it will be too in the code. If not... well...

These two tags are identical because the internal representation of WML tags in lua doesn't distinguish any order among WML attributes (this is true in general, not only in filters).

The point was already quoted by Sapient. My purpose is more to illustrate the idea of "more restrictive criterion first" than to give a description of how things works internally (I said twice it was not my purpose). Anyway, since programs are deterministic, here too, one could explain exactly in which order criteria are evaluated: "lua doesn't distinguish any order among WML attributes" is of course an approximation of something maybe very complex and in any way far beyond the scope of our paper.
The point is not important, but as I said, it can be useful to use logical operators to enforce an evaluation order in some special cases (when using filter_wml particularly).

Friendly,

[SlowThinker]

These "translations" are unambiguous when the original writing is not. Of course, filter execution will be only one of those because there are operators precedence rules.

There are no operators precedence rules in filters. Multiple operator tags (and similarly multiple Meta-Condition Tags) are processed the way I explained above:

(((((Universum and A) or B) and C) and_not D) ... )

Maybe you are confused by wiki, which is not very clear (see this post: http://forums.wesnoth.org/viewtopic.php ... 23#p531123 )

"lua doesn't distinguish any order among WML attributes" is of course an approximation of something maybe very complex and in any way far beyond the scope of our paper

I believe "Wesnoth cannot distinguish any order among WML attributes" is a principle that would help WML users to understand things.

[pyrophorus]

One again (and the last one), my goal is to help WML users who are not easy with progamming and formal descriptions. How could they read this ?

(((((Universum and A) or B) and C) and_not D) ... )

...

"Wesnoth cannot distinguish any order among WML attributes"[/i] is a principle that would help WML users to understand things.

I'm not saying you're wrong. I'm saying these "explanations" look contradictory and so explain nothing and confuse readers. More of it, my readers are not interested in knowing exactly how WML works: they want to create filters which work as they expect. And if one works as I suggest, grouping conditions and creating blocks with [and] tags, using temporary variables to reduce complexity, they will get what they want.
For myself, I'm not interested in theoretical discussion about WML because this language is an ugly hack. Not because it was badly designed. Because it was intended to follow the KISS principle as far as possible. The result is powerful and yet rather easy to use. But because it's a tradeof between consistency and simplicity, it has black patches and traps. My opinion is it's better to shun them when you don't need to dig into.

Friendly,

[Adamant14]

I second every single word.
Mind, this page is written for guys like me.
WML is the only computer language I ever learned,
and I think I am not the only one who only knows WML.
And this page is made for us.
There is the Wiki, and the Wiki is great, and no one wants to replace the Wiki,
this page is more a extension for the Wiki.
Pyrophorus explains all those things about filter's in a very clear and easy readable way.
In my opinion he made a very good job.
Thank you Pyrophorus.

[pyrophorus]

Hi !
I updated the HowTo, taking in account the observations given here by the readers.
I tried to clear the order problem and to give more clear hints to avoid pitfalls.
Hope it is better now.

Friendly,

[Adamant14]

Examples how to use filters is now also available as a wiki page.

If you have any comments or suggestions, feel free to write it here in this thread.