# Non-Blocking Channel Operations

This Go code demonstrates the use of the `select` statement with `default` cases to handle multiple channel operations. Let's go through it with inline comments:

```go
package main

import "fmt"

func main() {
    // Creating an unbuffered channel 'messages' for string communication
    messages := make(chan string)

    // Creating an unbuffered channel 'signals' for boolean communication
    signals := make(chan bool)

    // The first 'select' statement attempts to receive a message from 'messages'
    select {
    case msg := <-messages:
        fmt.Println("received message", msg)
    default:
        fmt.Println("no message received")
    }

    // Sending the message "hi" to 'messages'
    msg := "hi"
    select {
    case messages <- msg:
        fmt.Println("sent message", msg)
    default:
        fmt.Println("no message sent")
    }

    // The third 'select' statement tries to receive a message from 'messages'
    // or a signal from 'signals' with a 'default' case for handling no activity
    select {
    case msg := <-messages:
        fmt.Println("received message", msg)
    case sig := <-signals:
        fmt.Println("received signal", sig)
    default:
        fmt.Println("no activity")
    }
}
```

Explanation:

1. `package main`: Indicates that this Go file belongs to the main executable package.
2. `import "fmt"`: Imports the "fmt" package for formatting and printing.
3. `func main() { ... }`: The main function, where the execution of the program begins.
4. `messages := make(chan string)`: Creates an unbuffered channel named 'messages' for string communication.
5. `signals := make(chan bool)`: Creates an unbuffered channel named 'signals' for boolean communication.
6. The first `select` statement attempts to receive a message from 'messages'. Since there is no message at this point, the `default` case is executed, and "no message received" is printed.
7. `msg := "hi"`: Defines a string variable 'msg' with the value "hi".
8. The second `select` statement attempts to send the message "hi" to 'messages'. Since the channel is unbuffered and there is no receiver ready to receive the message, the `default` case is executed, and "no message sent" is printed.
9. The third `select` statement tries to receive a message from 'messages' or a signal from 'signals'. However, neither message nor signal is present, so the `default` case is executed, and "no activity" is printed.

In summary, this code demonstrates the use of the `select` statement with `default` cases to handle channel operations. It shows how to check for activity on channels, handle cases where a channel operation cannot proceed immediately, and provide default actions when no activity occurs.