Use cases for different sbt Key operators

Question

The documentation for sbt seems to be really lacking here, so I\'d like to get a definitive answer on this: what is the difference between \"+=\", \"++=\", \"<+=\", \"<++=

Answer 1

When the operators were deprecated, is there any documentation or porting guide what they mean or how to translate them to a currently used syntax?

You can see example of such translation in recent (Dec. 2016) commits (in scala/scala itself) like:

• fd3610c: avoid deprecated sbt 0.12 operators,
• c6e79dd: Avoid 2 more deprecated sbt 0.12 operators

You will see:

-  incOptions <<= (incOptions in LocalProject("root")),
+  incOptions := (incOptions in LocalProject("root")).value,

or

-    packagedArtifact in (Compile, packageBin) <<= (artifact in (Compile, packageBin), bundle).identityMap,
+    packagedArtifact in (Compile, packageBin) := (((artifact in (Compile, packageBin)).value, bundle.value)),

You can see more about the deprecation of those operators in sbt PR 2711, sbt PR 2716 and sbt/notes/0.13.13/ops_deprecation.md.