I'm pretty new to Scala and try to understand mutable Seq. Since it's in package mutable I expected there is a method that allows us to append element without copying the whole collection.
But there is no += method in the mutable.Seq, but in Buffer is. :+ and +: both copy the collection.
So why is it mutable?
Because mutable and growable isn't the same thing.
(the latter is one specific type of the former: everything, that's growable is mutable, but not everything that's mutable is growable).
mutable.Seq has update, that allows you to change the element at a given index, but it does not grow or shrink.
Buffer is s specialization of Seq, that is both mutable and growable.
As explained in the documentation, mutable.Seq adds an update method to collection.Seq. += on the other hand is defined in Growable.
In Scala standard library, most mutable collections extend the immutable version, which is why they inherit copying :+, +:.
:+, +:." No, they don't. Both mutable and immutable versions (e.g. collection.mutable.Seq and collection.immutable.Seq) extend a common version (in this case, collection.Seq) instead, and that contains :+ and +:. But collection.mutable.Seq doesn't extend collection.immutable.Seq.
collection.Seq have "immutable collection" signature, see scala-lang.org/api/2.12.0/scala/collection/… & co.