| Message ID | 20220615042424.7606-1-jacob.kroon@gmail.com |
|---|---|
| State | Accepted, archived |
| Commit | 8189f58d0449d16f162b6e8d98c4e5edc6bff875 |
| Headers | show |
| Series | [v4] bitbake-user-manual: Correct description of the ??= operator | expand |
On Wed, 2022-06-15 at 06:24 +0200, Jacob Kroon wrote: > Stating that the assignment is done at the end of parsing is misleading. > The weak default value is the value which a variable will expand to if no value > has been assigned to it using any of the assignment operators. > > Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com> > --- > > Changes in v4: > * Mention that "??=" operator overrides previously defined weak default values > * Add examples when appending/prepending, with and without override-style syntax > > Changes in v3: > * Update commit message > * Change "... via one of ..." to "... via any of ..." > > Changes in v2: > * Add comment to line with immediate variable expansion > > .../bitbake-user-manual-metadata.rst | 55 +++++++++++++------ > 1 file changed, 39 insertions(+), 16 deletions(-) > > diff --git a/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst > index af4b1358..3250c205 100644 > --- a/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst > +++ b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst > @@ -195,22 +195,45 @@ value. However, if ``A`` is not set, the variable is set to "aval". > Setting a weak default value (??=) > ---------------------------------- > > -It is possible to use a "weaker" assignment than in the previous section > -by using the "??=" operator. This assignment behaves identical to "?=" > -except that the assignment is made at the end of the parsing process > -rather than immediately. Consequently, when multiple "??=" assignments > -exist, the last one is used. Also, any "=" or "?=" assignment will > -override the value set with "??=". Here is an example:: > - > - A ??= "somevalue" > - A ??= "someothervalue" > - > -If ``A`` is set before the above statements are > -parsed, the variable retains its value. If ``A`` is not set, the > -variable is set to "someothervalue". > - > -Again, this assignment is a "lazy" or "weak" assignment because it does > -not occur until the end of the parsing process. > +The weak default value of a variable is the value which that variable > +will expand to if no value has been assigned to it via any of the other > +assignment operators. The "??=" operator takes effect immediately, overriding Generally looks good but I'd replace "overriding" with "replacing" since overriding means something very specific to bitbake! Cheers, Richard
On 6/15/22 09:23, Richard Purdie wrote: > On Wed, 2022-06-15 at 06:24 +0200, Jacob Kroon wrote: >> Stating that the assignment is done at the end of parsing is misleading. >> The weak default value is the value which a variable will expand to if no value >> has been assigned to it using any of the assignment operators. >> >> Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com> >> --- >> >> Changes in v4: >> * Mention that "??=" operator overrides previously defined weak default values >> * Add examples when appending/prepending, with and without override-style syntax >> >> Changes in v3: >> * Update commit message >> * Change "... via one of ..." to "... via any of ..." >> >> Changes in v2: >> * Add comment to line with immediate variable expansion >> >> .../bitbake-user-manual-metadata.rst | 55 +++++++++++++------ >> 1 file changed, 39 insertions(+), 16 deletions(-) >> >> diff --git a/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst >> index af4b1358..3250c205 100644 >> --- a/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst >> +++ b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst >> @@ -195,22 +195,45 @@ value. However, if ``A`` is not set, the variable is set to "aval". >> Setting a weak default value (??=) >> ---------------------------------- >> >> -It is possible to use a "weaker" assignment than in the previous section >> -by using the "??=" operator. This assignment behaves identical to "?=" >> -except that the assignment is made at the end of the parsing process >> -rather than immediately. Consequently, when multiple "??=" assignments >> -exist, the last one is used. Also, any "=" or "?=" assignment will >> -override the value set with "??=". Here is an example:: >> - >> - A ??= "somevalue" >> - A ??= "someothervalue" >> - >> -If ``A`` is set before the above statements are >> -parsed, the variable retains its value. If ``A`` is not set, the >> -variable is set to "someothervalue". >> - >> -Again, this assignment is a "lazy" or "weak" assignment because it does >> -not occur until the end of the parsing process. >> +The weak default value of a variable is the value which that variable >> +will expand to if no value has been assigned to it via any of the other >> +assignment operators. The "??=" operator takes effect immediately, overriding > > Generally looks good but I'd replace "overriding" with "replacing" > since overriding means something very specific to bitbake! > Agreed, new version on the way. /Jacob
diff --git a/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst index af4b1358..3250c205 100644 --- a/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst +++ b/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst @@ -195,22 +195,45 @@ value. However, if ``A`` is not set, the variable is set to "aval". Setting a weak default value (??=) ---------------------------------- -It is possible to use a "weaker" assignment than in the previous section -by using the "??=" operator. This assignment behaves identical to "?=" -except that the assignment is made at the end of the parsing process -rather than immediately. Consequently, when multiple "??=" assignments -exist, the last one is used. Also, any "=" or "?=" assignment will -override the value set with "??=". Here is an example:: - - A ??= "somevalue" - A ??= "someothervalue" - -If ``A`` is set before the above statements are -parsed, the variable retains its value. If ``A`` is not set, the -variable is set to "someothervalue". - -Again, this assignment is a "lazy" or "weak" assignment because it does -not occur until the end of the parsing process. +The weak default value of a variable is the value which that variable +will expand to if no value has been assigned to it via any of the other +assignment operators. The "??=" operator takes effect immediately, overriding +any previously defined weak default value. Here is an example:: + + W ??= "x" + A := "${W}" # Immediate variable expansion + W ??= "y" + B := "${W}" # Immediate variable expansion + W ??= "z" + C = "${W}" + W ?= "i" + +After parsing we will have:: + + A = "x" + B = "y" + C = "i" + W = "i" + +Appending and prepending non-override style will not substitute the weak +default value, which means that after parsing:: + + W ??= "x" + W += "y" + +we will have:: + + W = " y" + +On the other hand, override-style appends/prepends/removes are applied after +any active weak default value has been substituted:: + + W ??= "x" + W:append = "y" + +After parsing we will have:: + + W = "xy" Immediate variable expansion (:=) ---------------------------------
Stating that the assignment is done at the end of parsing is misleading. The weak default value is the value which a variable will expand to if no value has been assigned to it using any of the assignment operators. Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com> --- Changes in v4: * Mention that "??=" operator overrides previously defined weak default values * Add examples when appending/prepending, with and without override-style syntax Changes in v3: * Update commit message * Change "... via one of ..." to "... via any of ..." Changes in v2: * Add comment to line with immediate variable expansion .../bitbake-user-manual-metadata.rst | 55 +++++++++++++------ 1 file changed, 39 insertions(+), 16 deletions(-)