immutable collections by default (`mutable`/covariant/protocol/nominal)

[KotlinIsland]

Using Sequence when you want an covariant or immutable list is not always optimal.

We propose:

• ListLike/DictLike etc protocols
• A mutable use site modifier that will make mutation methods accessible (append, clear, etc)
  • update the definition of the collections with markers on the mutable methods
• Make the default type of the collections immutable and covariant

such that

Sequence
↑
ListLike
↑
list
↑
mutable[list]

A collection coming from third party code (inc std) would always be a mutable[list].

[Zeckie]

Making collections immutable by default will be a pretty major change. Are you proposing a new language, forked from python?

[KotlinIsland]

Making collections immutable by default will be a pretty major change. Are you proposing a new language, forked from python?

Making collections immutable for static analysis purposes only, similar to making classes final by default, this will have no effect on any runtime behaviour, ie:

[0][0]=0

Would get a static error, but would run and complete without issue.