What does " _ = c(ctx, cmd)" mean?

I'm reading the code that go-redis library, and then I want to know how the _ = c(ctx, cmd) works.

// Get Redis `GET key` command. It returns redis.Nil error when key does not exist.
func (c cmdable) Get(ctx context.Context, key string) *StringCmd {
	cmd := NewStringCmd(ctx, "get", key)
	_ = c(ctx, cmd)
	return cmd
}

I found type alias below but I couldn't find this function's body.

type cmdable func(ctx context.Context, cmd Cmder) error

why and how _ = c(ctx, cmd) works without function body?

https://github.com/go-redis/redis/blob/master/commands.go#L780

Thanks!

cmdable is defined as type cmdable func(ctx context.Context, cmd Cmder) error. So because it is an alias of a function it can call itself from a method. This works just like any other function variable:

abc := func(ctx context.Context, cmd Cmder) error {
    fmt.Println("we can do anything here")
    return nil
}

cmd := cmdable(abc)

So the actual function body is provided outside of this struct. They use it in a few places for example, this is the definition of Client:

type Client struct {
	*baseClient
	cmdable
	hooks
	ctx context.Context
}

It embeds cmdable so all exported methods on cmdable will become exported methods on Client as well. During creating of the Client in NewClient the cmdable is set to c.Process:

func NewClient(opt *Options) *Client {
	opt.init()

	c := Client{
		baseClient: newBaseClient(opt, newConnPool(opt)),
		ctx:        context.Background(),
	}
	c.cmdable = c.Process

	return &c
}

So in the case of Client, Client.Process is the body of c. But for Pipeline, Pipeline.Process is set. They do this because you want to queue commands before committing the pipeline.

So this is sort of a combination of inheritance from an abstract struct and a strategy pattern.

The _ = discards the return value which is an error. Using an underscore like this is called a blank identifier. This is not strictly necessary but is likely done to indicate to tools like go vet that this is on purpose and that the author didn't forget to check an error.