What are the differences between ++= and += in sbt, say with libraryDependencies?

Question

I'm new to Scala and sbt and I can't understand what the differences are between:

libraryDependencies ++= Seq(...)

and

libraryDependencies += ...

Answer 1

See Appending to previous values: += and ++=:

Assignment with :=is the simplest transformation, but keys have other methods as well. If the T in SettingKey[T] is a sequence, i.e. the key’s value type is a sequence, you can append to the sequence rather than replacing it.

• += will append a single element to the sequence.
• ++= will concatenate another sequence.

For example, the key sourceDirectories in Compile has a Seq[File] as its value.

Answer 2

It is += dep or ++= Seq(dep, dep2, dep3): "Of course, you can also use ++= to add a list of (read: multiple) dependencies all at once".

See collections.Seq for the + ("append an item") and ++ ("append a sequence") operators.

Answer 3

One way to strip this down to a simpler problem is to look at the two parts in separation.

First, there's the ...= part. You're probably familiar with this syntax from Java and other languages, but it performs the ... operator on both the left and right operands, and stores it back into the left operand - in this case, it stores it back into libraryDependencies.

Second, there's the choice of either ++ or +. If you take a look at the Seq Scaladoc then you'll find these two operators. The difference here is that:

1. + takes a single element on the left-hand side, and prepends it to a sequence, returning a new sequence as the result.
2. ++ takes a sequence on the left and a sequence on the right and returns a new sequence of both.

In practice, this means that you will get different results from the two.

• List(1, 2, 3) ++ List(4, 5, 6) will return List(1, 2, 3, 4, 5, 6), but...
• List(1, 2, 3) + List(4, 5, 6) will return List(List(1, 2, 3), 4, 5, 6).