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Implement ??= operator

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??= is a lazy, conditional assignment.  Whereas a ?= immediately assigns to
the variable if the variable has not yet been set, ??= does not apply the
default assignment until the end of the parse.  As a result, the final ??= for
a given variable is used, as opposed to the first as in ?=.

Note that the initial implementation relies upon finalise() to apply the
defaults, so a "bitbake -e" without specifying a recipe will not show the
defaults as set by ??=.  Moving application of the default into getVar adds
too large a performance hit.  We may want to revisit this later.

(Bitbake rev: 74f50fbca194c9c72bd2a540f4b9de458cb08e2d)

Signed-off-by: Chris Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>
This commit is contained in:
Chris Larson
2010-02-25 09:42:28 -07:00
committed by Richard Purdie
parent ada2a8494a
commit 57044b9a62
3 changed files with 19 additions and 2 deletions
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bitbake/doc/manual/usermanual.xml
+7 -1
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@@ -91,7 +91,13 @@ share common metadata between many packages.</para></listitem>
<section>
<title>Setting a default value (?=)</title>
<para><screen><varname>A</varname> ?= "aval"</screen></para>
<para>If <varname>A</varname> is set before the above is called, it will retain it's previous value. If <varname>A</varname> is unset prior to the above call, <varname>A</varname> will be set to <literal>aval</literal>.</para>
<para>If <varname>A</varname> is set before the above is called, it will retain it's previous value. If <varname>A</varname> is unset prior to the above call, <varname>A</varname> will be set to <literal>aval</literal>. Note that this assignment is immediate, so if there are multiple ?= assignments to a single variable, the first of those will be used.</para>
</section>
<section>
<title>Setting a default value (??=)</title>
<para><screen><varname>A</varname> ??= "somevalue"</screen></para>
<para><screen><varname>A</varname> ??= "someothervalue"</screen></para>
<para>If <varname>A</varname> is set before the above, it will retain that value. If <varname>A</varname> is unset prior to the above, <varname>A</varname> will be set to <literal>someothervalue</literal>. This is a lazy version of ??=, in that the assignment does not occur until the end of the parsing process, so that the last, rather than the first, ??= assignment to a given variable will be used.</para>
</section>
<section>
<title>Immediate variable expansion (:=)</title>