Go中的单向通道有什么意义？

Question

I'm learning Go and so far very impressed with it. I've read all the online docs at golang.org and am halfway through Chrisnall's "The Go Programming Language Phrasebook". I get the concept of channels and think that they will be extremely useful. However, I must have missed something important along the way, as I can't see the point to one-way channels.

If I'm interpreting them correctly, a read-only channel can only be received on and a write-only channel can only be transmitted on, so why have a channel that you can send to and never receive on? Can they be cast from one "direction" to the other? If so, again, what's the point if there's no actual constraint? Are they nothing more than a hint to client code of the channel's purpose?

Answer 1

A read-only channel can be made read-only to whomever receives it, while the sender still has a two-way channel on which they can write. For example:

func F() <-chan int {
    // Create a regular, two-way channel.
    c := make(chan int)

    go func() {
        defer close(c)

        // Do stuff
        c <- 123
    }()

    // Returning it, implicitely converts it to read-only,
    // as per the function return value.
    return c
}

Whom ever calls F(), receives a channel on which they can only read. This is mostly useful to catch potential miss-uses of a channel at compile time. Because read/write-only channels are distinct types, the compiler can use its existing type-checking mechanisms to ensure the caller does not try to write stuff into a channel it has no business writing to.