Jon Pretty

Honest question: why do people still use `Seq`? I thought it was on its way out, and it was a pretty big wart in 2018 that I always banned in my codebases for lack of clarity and how it obfuscates the complexity class of the program...

My guess is, because Java people have been taught to program to the interface, not the implementation. Did you realise the nastiest thing about Seq (as it was in 2.11, at least)? How everybody believes it's immutable but you get the mutable version by default?

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 …]