doc: Add '--manifest' example using 'specification->package+output'.

* doc/guix.texi (Invoking guix package): Add --manifest example using
'specification->package+output'.
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Ludovic Courtès 2017-01-17 22:58:37 +01:00
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@ -1685,6 +1685,20 @@ of packages:
(list guile-2.0 "debug")))
@end example
@findex specification->package+output
In this example we have to know which modules define the @code{emacs}
and @code{guile-2.0} variables to provide the right
@code{use-package-modules} line, which can be cumbersome. We can
instead provide regular package specifications and let
@code{specification->package-output} look up the corresponding package
objects, like this:
@example
(packages->manifest
(map (compose list specification->package+output)
'("emacs" "guile@@2.0" "guile@@2.0:debug")))
@end example
@item --roll-back
@cindex rolling back
@cindex undoing transactions