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Maybe not related to coroutines, but that's where ...
# coroutines
o
Maybe not related to coroutines, but that's where I found it: Can anybody explain to me why the following code works? https://pl.kotl.in/O8wu8VKSE In essence the code sets an instance of type 
B
as a value of 
StateFlow<A>
. I would not expect that to be possible. The code not only compiles, it actually runs in the Playground without a run-time exception.
o
because you forced an unsafe cast and you're never type checking, so the generic doesn't come into play (generic type erasure)
if you did 
val a = flow.value
I would expect an exception as kotlin should introduce a cast there
and indeed, that's what happens
o
Well, when the very same implementation is used with a LiveData receiver (casting LiveData to MutableLiveData as opposed to casting StateFlow to MutableStateFlow), compiler doesn't allow posting 
B
to 
LiveData<A>
. But it it has no problem setting 
B
as value of 
StateFlow<A>
w
I suppose it is a bit surprising that 
MutableStateFlow
doesn’t perform any checks when setting value of a wrong type 🤔
o
it shouldn't have to, don't make an unsafe cast!
additionally, it can't -- it has no type information, that's how erasure works
o
@octylFractal If it can't then how is it possible that with LiveData it correctly infers an attempt to set a value of incompatible type, refusing to even compile? I have no problem with the behavior, but I dn't know why it's inconsistent.
And I just realized there's a difference between the two. 
public abstract class LiveData<T>
vs 
public interface StateFlow<out T>
- There's no out-projection in LiveData
o
isn't that just because 
LiveData
is a Java class, not a Kotlin one?
➕ 1
hmm, I see what you mean now
I suspect it is due to that inconsistency with the type parameters -- 
out T
allows such things as:
Copy code
interface Source<out T> {
    fun nextT(): T
}

fun demo(strs: Source<String>) {
    val objects: Source<Any> = strs // This is OK, since T is an out-parameter
    // ...
}
so kotlinc is able to infer 
StateFlow<Any>
from the 
flow
and allow the unsafe set
this is why 
MutableStateFlow
doesn't declare 
out T
, but just 
T
-- since it prevents input from being promoted to 
Any
☝🏻 1
a
That’s why MutableList etc are invariate compared to List in general
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