[OE-core] some simple questions about assignment and appending

[Robert P. J. Day]

  i want to make absolutely sure i understand the mechanics of the
different types of assignment so i'm looking at this snippet from
core-image.bbclass:

... snip ...

PACKAGE_GROUP_qt4-pkgs = "task-core-qt-demos"

CORE_IMAGE_BASE_INSTALL = '\
    task-core-boot \
    task-base-extended \
    \
    ${CORE_IMAGE_EXTRA_INSTALL} \
    '

CORE_IMAGE_EXTRA_INSTALL ?= ""

IMAGE_INSTALL ?= "${CORE_IMAGE_BASE_INSTALL}"

... snip ...

  first, if you're assigning a fixed string (as in the first line
shown in the snippet above), i assume there is no functional
difference in using "=" versus ":=", correct?

  next, is there any value in that null line in the assignment to
CORE_IMAGE_BASE_INSTALL?

  next, is it technically necessary to conditionally do this?

CORE_IMAGE_EXTRA_INSTALL ?= ""

if that variable wasn't set, does it really need to be set to the
empty string?

  finally, what are the evaluation mechanics of the "?=" operator?  in
this line,

IMAGE_INSTALL ?= "${CORE_IMAGE_BASE_INSTALL}"

is the right-hand side evaluated first?  i ask since i find the
snippet above a bit confusing in that you need to follow backwards to
deduce what IMAGE_INSTALL is eventually set to.  i would think this
would be easier to read:

CORE_IMAGE_BASE_INSTALL = '\
    task-core-boot \
    task-base-extended

IMAGE_INSTALL = "${CORE_IMAGE_BASE_INSTALL} ${CORE_IMAGE_EXTRA_INSTALL}"

  in this rewriting, i can *immediately* see what values are used to
construct IMAGE_INSTALL.  in the current code, it's nowhere near as
clear, and you have to backtrack to see how CORE_IMAGE_BASE_INSTALL is
assigned.  or would that not be equivalent based on a misunderstanding
of how assignment works?

rday

-- 

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
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