[Rpm-ecosystem] Rich deps syntax finalization

[Neal Gompa]

On Tue, Aug 25, 2015 at 12:01 PM, Florian Festi <ffesti at redhat.com> wrote:

> On 08/25/2015 05:54 PM, Pavel Odvody wrote:
> >> Technically a NOT operator should not be needed. So we are basically
> >> looking for real life examples where it would be really handy or even a
> >> pain if it was missing. What would you do with a NOT operator?
> >>
> >
> > Requires: (PkgA AND (PkgB IF NOT PkgC)
>
> > But I'm really not sure whether it isn't just easier (correct) to handle
> > this through conflicts / virtual provide.
>
> Well, as Requires are (logically) all connected with an AND this is
> equivalent to:
>
> Requires: PkgA
> Requires: (PkgB IF NOT PkgC)
>
> And as IF is equivalent to OR NOT this is equivalent to
>
> Requires: PkgA
> Requires: (PkgB OR PkgC)
>
> That's why I am interested in *real world* examples where we can at
> least argue that writing it in a given way is more clear and the one
> without using NOT.
>
> >> [2] We are talking about Boolean operators here - not if statements.
> >> Those variants are identical to the forward and backward implication
> >> which are identical to (NOT . OR .) and (. OR NOT .)
>
>
> Florian
>
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>
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​​From my point of view, I prefer the expressive syntax (IF, ELSE, THEN,
OR, AND, NOT)​, though I'm not sure if making it case sensitive really
helps anything. Because they have to be grouped in parenthesis anyway, I
don't think the enforced all-caps is really necessary. That said, as a
C/C++, Java, and now Python programmer, I feel quite at home with boolean
logic symbols ( "||", "&&", "!") . I really don't think it would hurt
anything if we supported both kinds.

I consider the NOT evaluation to be useful for intent more than anything
else.

For example: "Requires: (pkgA IF NOT pkgB)" or "Requires: (IF NOT pkgB THEN
pkgA)"
That lines says two things to me:

   - The package prefers pkgB over pkgA
   - Either option works (functionally an OR operation)

This could be handy for declaring priority of dependencies based on what
functionality it provides.

For a more concrete example, suppose I have a package that has a runtime
dependency on a kmod package. However, the kmod package is available in
several forms (akmod, dkms, and finally kmod). I could set up something
like this:

"Requires: (IF NOT (IF NOT module-kmod THEN module-akmod) THEN
module-dkms)"

​If read with the intent given, then that means that if module-kmod isn't
installable, then use module-akmod. If that's not installable, then use
module-dkms.

Or maybe I'm misreading how this would work, but that's how I interpret it.


-- 
真実はいつも一つ！/ Always, there's only one truth!
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