Why does ZChannel.toStream requires InErr, InElem and InDone to be 'Any'?

[pvgoran]

I have a channel that doesn't accept any input (neither errors nor elements nor the final value): ZChannel[Env, Nothing, Nothing, Nothing, OutErr, OutElem, OutDone]. I want to turn it into a ZStream:

val channel : ZChannel[Env, Nothing, Nothing, Nothing, OutErr, OutElem, OutDone] = ???
val stream : ZStream[Env, OutErr, OutElem] = channel.mapOut(Chunk(_)).toStream

I get an error "Cannot prove that Any <:< Nothing".

Why is it designed this way? A ZStream does not have any input by design, and my channel does not have any input due to its type, so turning the latter into the former should be a natural conversion.

What's worse, I don't see any type-safe way to transform my channel into a channel that can be converted to a stream. I can use contramapInZIO and contramapZIO to get a channel with InElem=Any and InDone=Any, but I InErr seems to be out of reach. So I may need to resort to asInstanceOf which is dangerous.

(Also see #7202.)

[vigoo]

You should represent the fact that the channel is not taking any input by using Any in the contravariant type parameters (Env, InErr, InElem, InDone).

[pvgoran]

Could you elaborate, please? Why is Any more appropriate than Nothing (which is the explicit "no input is possible" type, as far as I understand)?

[vigoo]

You can think of it as the input type is not bound (it can be Anything) because it is not using it at all.

This also means that because of the contravariance, a channel of type InElem=Any is a subtype of any other channel with a specific input element type; you can use a channel that is not consuming any input in place of any other channel consuming input regardless of the input type, as it is never reading anything.

[adamgfraser]

A ZChannel[Env, Nothing, Nothing, Nothing, OutErr, OutElem, OutDone] is a channel that can never be run. It requires inputs that could never possibly exist. A ZChannel[Env, Any, Any, Any, OutErr, OutElem, OutDone] is a channel that can be run with any possible inputs, and thus does not do anything with those inputs because those inputs contain no useful information.

Another way to think about this is that Nothing is a subtype of every other type. So if I had a ZChannel[Env, Nothing, Nothing, Nothing, OutErr, OutElem, OutDone] that would be a subtype of a ZChannel[Env, Foo, Foo, Foo, OutErr, OutElem, OutDone]. If we could run a channel of type of ZChannel[Env, Nothing, Nothing, Nothing, OutErr, OutElem, OutDone] that would imply we could also run a channel of type ZChannel[Env, Foo, Foo, Foo, OutErr, OutElem, OutDone]. But that is not the case because the channel requires an input of type Foo and we have no way to produce that input.

You should represent your channel as a ZChannel[Env, Any, Any, Any, OutErr, OutElem, OutDone] and then it can naturally be converted to a stream by chunking its elements and ignoring its done value.