I've given some thought to it and it may be cool t...

# language-proposals

[Pere Casafont]

I've given some thought to it and it may be cool to add thread safety to this by implicitly creating readonly locks on objects when passed as "const"

[Andreas Sinz]

does that apply to the instance itself, its properties or everything inside the properties?

[Pere Casafont]

It applies only to the instance itself, as by allowing only readonly operations on it we don't need anything else

[Andreas Sinz]

Two follow up questions:
1. How does the compiler know which is a readonly-operation?
2. If you have a const foo: Foo, is it ok to pass it into a fun bar(foo: Foo) or is it necessary to annotate the parameter with const too?

[Pere Casafont]

1. assignments, sets and methods containing any of those affecting their members
2. yes, to be able to pass a const to a function its signature needs that param to be specified const

[Pere Casafont]

oh, as 1. I've mentioned the non-readonly operations 😛

[Andreas Sinz]

What advantage does that have over a read-only interface?

[Pere Casafont]

We wouldn't need to have 2 implementations of each object we want to be writable or read-only

[Pere Casafont]

a normal reference to List would already be treated as MutableList, and a readonly/const one would be what List is now

[Pere Casafont]

given Kotlin's smart nature, in many cases no extra keywords would be needed though

[Pere Casafont]

for example:

val thing = MyThing()
thing.readOnlyOperation()
functionForReadOnly(thing)

the fact that thing is const/readonly can be statically proven, so it wouldn't need any extra keyword in its declaration

[Pere Casafont]

the function call wouldn't complain

[Andreas Sinz]

if you call a mutating method on a const thing, does it throw an exception or is it not allowed (compiler error)?

[Pere Casafont]

compiler error

[Andreas Sinz]

in your List example, how would Variance work with a const List<T>?

[Pere Casafont]

nice question, I still have to think well about generics, but it would be nice if T could be given a mutability modifier so you could have an immutable list of immutable items, an immutable list of mutable items and the same with mutable lists

[Pere Casafont]

All that depends a lot on whether we decide to make the immutability totally recursive or just down to 1 level