Jon Pretty

I was speechless when I learned Seq by default in < 2.13 is mutable

I don't know if there is a difference between repl and sbt/scala project. But if you open a scala project in say IntelliJ and browse to definition on a Seq you can see that it points to the mutable version. Not sure but could repl br using a different predef?

If `collection.mutable._` were imported, that would do it... but I doubt that's where the idea came from. Note that mutable.Seq[T] <:< Seq[T] which means that you can pass a mutable sequence to a method which requests an immutable one. Could that be the source of confusion?

It's related - the Seq type that's in scope by default is defined here: https://www.scala-lang.org/api/2.10.3/#scala.package … That's an alias for scala.collection.Seq, which is the supertype of both mutable and immutable. Meaning that you think your method expects an immutable Seq, but gets a mutable one.

Not sure why I linked the 2.10 doc, that's still the case in 2.12: https://www.scala-lang.org/api/2.12.12/scala/index.html#Seq[+A]=Seq[A …]

Well, I did my test in 2.10...

You’re not testing the “right” thing. Write a method that takes a Seq, then pass a mutable.Seq. This compiles but isn’t coherent with what you’d expect, since unqualified Map, say, is immutable.