Re: [gentoo-dev] GLEP 29 (USE Flag Grouping) with -@GROUP goodness

[Maurice van der Pot]

The problem with -something seems to be two-fold:
1) it's hard to realize the consequence of double negations
2) with -@GROUP1 and @GROUP2 where both groups share a use flag
   it is order dependent whether or not the common use flag will
   be in the end result

On Sun, Oct 24, 2004 at 04:06:22PM +0100, Ciaran McCreesh wrote:
> Groups may be preceded by a -sign to invert their contents (that is, all
> 'enable' use flags become -flags, and all -flags become enable flags). Be
> warned that this feature can cause confusion (see below). Example usage:
>
> ::
>
> 	# We have the following groups defined...
> 	GROUP1 foo bar
> 	GROUP2 -bar baz -fnord
> 	GROUP3 @GROUP1 -@GROUP2 -bar foo
> 	GROUP4 -foo -bar
>
> 	# And the following...
> 	USE="-@GROUP3 @GROUP4 bar"
>
> 	# which resolves to...
> 	USE="-@GROUP1 @GROUP2 bar -foo -foo -bar bar"
> 	USE="-foo -bar bar -baz fnord bar -foo -foo -bar bar"
> 	USE="-baz fnord -foo bar"

... which is kind of hard to see at a glance.

Whether a use flag is specifically turned off for a group or it is
just not present in the group should make no difference for the meaning 
of GROUP. The same holds for -@GROUP. In other words, -use and -@GROUP 
should only be used in the definition of a group to filter flags out.

Assume MYGNOME="@GNOME -gtk", then USE="... -@MYGNOME ..." should
*not* be equivalent to USE="... -@GNOME gtk ...", instead it should
not influence the state of gtk at all.

> Issues with -flags and -@GROUPS
> -------------------------------
>
> Earlier drafts of this GLEP did not allow -use flags or -@GROUPS. However,
> because of feedback along the lines of "we shouldn't disallow features
> just because some users won't understand them" (for example, [3]_), these
> are now allowed but discouraged.
>
> The problems are best illustrated by example. Say we have the following
> groups defined:
>
> ::
>
> 	KDE X kde qt
> 	GNOME X gtk gtk2 gnome
>
> A user who wants a KDE desktop but no GNOME may do the following:
>
> ::
>
> 	USE="@KDE -@GNOME"
>
> However, this will not give the desired effect -- the ``X`` USE flag will
> end up being disabled.

The problem here is that there is no distinction between a group that 
requires use flags and a group that contains use flags. If the syntax was 
something like GNOME="$X gnome gtk", where $ indicated a requirement, then
specifying USE="@KDE -@GNOME" and USE="-@GNOME @KDE" would be equivalent 
while the end result would still contain X.

> 	KDE X kde qt -gtk -gnome
> 	GNOME X gtk gtk2 gnome -kde -qt
>
> 	USE="@KDE @GNOME"
>
> They would end up with:
> 	USE="X gtk gtk2 gnome -kde -qt"

I would specify it like this:

	KDE $X kde qt -gtk -gnome
	GNOME $X gtk gtk2 gnome -kde -qt

	USE="@KDE @GNOME"

That would result in:
	USE="X kde qt gtk gtk2 gnome"

Exactly what the user wants.

Regards,
Maurice.

-- 
Maurice van der Pot

Gentoo Linux Developer   griffon26@g.o     http://www.gentoo.org
Creator of BiteMe!       griffon26@××××××××.com   http://www.kfk4ever.com