Aims to learn the knowledge about concurrency in Go
## Flow control with channels
1. We can imagine that channel is a pipe, `chan<-` means to send a value into the channel. `<-chan` means to receive a value from the channel.
2.`select ... case` can execute sending value into a channel
## Range and Close
1. A sender can close a channel to indicate that no more values will be sent.
2. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression. `v, ok := <-ch`, `ok` is `false` if there are no more values to receive and the channel is closed.
3. The loop for `i:= range c` receives values from the channel repeatedly until it is closed.
4. Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.
5. Channels aren't like files; you don't usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a `range` loop.
## Select
1. The `select` statement lets a goroutine wait on multiple communication operations
2. A `select` blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready
## Default selection
1. The `default` case in a `select` is run if no other case is ready.
2. Use a `default` case to try a send or receive without blocking
Channels are great for communication, but what if we don't need communication?
What if we just want to make sure only one goroutine can access a variable at a time to avoid conflicts? This concept is called `mutual exclusion`, and the conventional name for the data structure that provides it is `mutex`
Go's standard library provides mutual exclusion with `sync.Mutex` and its two methods: `Lock` and `Unlock`
