Re: Adding an "Ignore trailing spaces" text filter

[Angel Ezquerra <angel ezquerra gmail com>]

On Fri, Jan 25, 2013 at 9:02 PM, Kai Willadsen <kai willadsen gmail com> wrote:
> On 22 January 2013 03:45, Angel Ezquerra <angel ezquerra gmail com> wrote:
>> Hi,
>>
>> it would be nice if meld came with a built-in "Ignore trailing spaces"
>> text filter, such as:
>>
>> [ \t\r\f\v]*$
>>
>> This would nicely complement the existing "All whitespace" and
>> "Leading whitespace".
>>
>> I often need to do this because some editors automatically remove
>> trailing spaces which often creates spurious differences.
>
> I don't disagree, but our filter list is really meant as a base for
> people to customise, rather than as a comprehensive list of filters
> you might need. Part of that is because of the limited nature of the
> filtering, as you mention below.

That makes sense. However I find that ignoring trailing white spaces
is very common and is a built-in feature of some diff tools, so it
would be nice if meld had it out of the box.

>> Also, it would be cool if you could specify a "replacement" string
>> with your text filter expressions. Currently the implicit replacement
>> string is an empty string. This is often what you want but it does not
>> let you do things such as creating an "ignore white space changes"
>> text filter such as the ones that are provided with mercurial diff.
>> The way this works is that all white space "blocks" (i.e. contiguous
>> strings of white spaces and tabs) are replaced by a single space
>> before the comparison is run. This means if the amount of whitespaces
>> between words changes the diff tool ill ignore it.
>>
>> Another thing that could be done with this sort of "advanced filter"
>> would be to compare files that have different indent options (e.g. one
>> indents with tabs and the other with spaces), by replacing [\t]{1}
>> with " ".
>
> The existing 'leading whitespace' filter will get you most of that as
> it stands, with the only difference being that there's no way to pick
> out significant whitespace.

Yes but that would not let me do a "ignore white space changes"
filter. That sort of filter would ignore differences such as between:

~~~
    int var1 = 5;
    int variable2 = 7;
~~~

and:

~~~
    int var1 =        5;
    int variable2 = 7;
~~~

Which is common code alignment pattern, particularly in VHDL and Verilog code.


> The problem with replacing text is that it breaks user expectation
> when people discover that text filters *don't* apply to Meld's inline
> highlighting, only to large-scale differences. They *should* apply to
> the inline highlighting, but doing so requires keeping a reverse
> mapping from the transformed text to the visible text so that we can
> correctly apply the highlight, and no one has written that yet.
>
> In the end, we actually need a significantly more sophisticated filter
> mechanism to fix these problems (including the long-standing line
> break limitation) and I just don't have the time. If anyone is keen on
> working on it, then I'm happy to give some pointers, but it's not
> likely to be a quick fix.

I'm not sure I understand the problem, but I think what you said can
be summarized as "It is harder than it looks" :-)

Cheers,

Angel