不能在返回参数中使用msgs（类型<-chan _）作为类型chan _

Question

Have following code:

func consumeQueue(ch *amqp.Channel, q_Name string) (chan amqp.Delivery) {
    msgs, err := ch.Consume(
        q_Name, // queue
        "",     // consumer
        true,   // auto-ack
        false,  // exclusive
        false,  // no-local
        false,  // no-wait
        nil,    // args
    )
    failOnError(err, "Failed to register a consumer")   
    return msgs
}

It throws following compile time exception:

cannot use msgs (type <-chan amqp.Delivery) as type chan amqp.Delivery in return argument

What is wrong?

Answer 1

It's perfectly straightforward. You're trying to return a directional channel (in this case a read-only channel) as a general channel that can be used for both reading and writing.

There's some basic information about channel directions here

Tl;Dr

Any chan t can be used as a directional channel. This increases overal typesafety and makes code easier to understand/use for others:

func Foo() <-chan struct{} {
}

This tells me Foo returns a channel I can read from. If it were to return just chan struct{}, I'd have to check the docs to see if I'm expected to push something to that channel, or receive from it.

A non-directional channel can be made directional, but it's not allowed to convert back to a non-directional channel for obvious reasons. You can check the spec on channel types for even more details, but the essence of it is this simple sentence:

The optional <- operator specifies the channel direction, send or receive. If no direction is given, the channel is bidirectional. A channel may be constrained only to send or only to receive by conversion or assignment.

solution

In your particular case, the solution is simple: change the return type to match the channel you have:

func consumeQueue(ch *amqp.Channel, q_Name string) (<-chan amqp.Delivery) {

As you've noticed: the compiler shouts at you when you try to convert a directional channel into a bidirectional channel. You'll also get a similar error when you try to convert a read-channel into a write channel (written as chan<- amqp.Delivery). So if you get type error messages from the compiler, just check what types it's mentioning... go's compiler spits out pretty self-explanatory errors IMO