[Nix-dev] [Want to commit this week] deepOverride replacement when callPackage is used

[Michael Raskin]

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		Hello.
	[Summary: right now it is hard to build a package with recursively
replaced glib; there is a way that works - look at code snippets and
comment on them.
	I see no reasons not to commit these changes in a couple of days after
checking them on Empathy build, unless some problems with the changes
are pointed out]

	Soon after we had a discussion about avoiding deepOverride,
all-packages.nix changed to callPackage style and the proposed solution
stopped working.

	Part one: gtkLibs220 has no __overrides. So gtk depends on default glib
unless you basically copy-paste everything. Proposed solution
(instantiation checked, gives the same unless you use overrides):

- ------------ CODE ------------------
  gtkLibs220 = recurseIntoAttrs (makeOverridable
    (overrides :
      (let
         callPackage = newScope self;
         self = rec {
 [snip]
         } // overrides;
         in self)) {});
- ------------ END CODE --------------

	Part two: callPackage uses default pkgs (with overrides from
config.nix). When you override gtkLibs, gnome subset gets old glib
whatever I do (and I guess overriding gtk would be even worse).
	Proposal:

- ------------ CODE ------------------
move

  callPackage = newScope {};
  newScope = extra: lib.callPackageWith (defaultScope // extra);
  defaultScope = pkgs // pkgs.xorg;

inside pkgsFun result body.
- ------------ END CODE --------------

	Possible alternatives: convert pkgsFun the same way as gtkLibs220.

	My plan: finish building Empathy (which needs that and more) as a
check, commit "conservative part" (some telepathy and gnome libraries)
right after that. If no reasons are given why the proposed composition
changes are bad, commit them too.
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