为什么通道的方向变化不兼容？

Question

I like to program providing interfaces as restrictively as possible, both to avoid bad usage as to be explicit and self-documented.

So, I like to provide directional channels when they are supposed to be used unidirectionally by the user, but of course, internally I have a bidirectional channel copy.

Assigment for the following works:

var internal chan int
var external <-chan int
external = internal

But now I want to provide to the user a <-chan chan<- int type (in the return of a function), but the following won't work:

var internal chan chan int
var external <-chan chan<- int
external = internal // this fails

I have two questions:

• Exactly why that doesn't work?
• So, I can declare a variable of <-chan chan<- type, but... can't use such a type in any practical sense? (Because even though there're directional channels, they're AFAIK aways used in orchestration with bidirectional ones, and since assignment is not possible, they can't be used this way)

Answer 1

The reason why this does not work

The specification says this about channel assignability:

A [channel] value x is assignable to a variable of type T ("x is assignable to T") [when] x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a named type.

This reflects exactly what you're experiencing:

• chan (chan int) to <- chan (chan int) works
• chan (chan int) to <- chan (chan<- int) does not

The reason for this is that the element types (the ones after the chan keyword) are not equal.

I can declare it but not use it?

You can use it but not the way you want to. It is not possible to assign the variables the way you do but by correcting the element types you can indeed use it:

var internal chan chan<- int
var external <-chan chan<- int
external = internal

If you only have your chan chan int type you need to copy your values (Examples on play):

var internal chan chan int
var internalCopy chan chan<- int

go func() { for e := range internal { internalCopy <- e } }()

var external <-chan chan<- int
external = internalCopy