Today we have something a bit different. Just a sample package.

An example package

Here is a complete Go package that implements a concurrent prime sieve.

package main

import "fmt"

// Send the sequence 2, 3, 4, … to channel 'ch'.
func generate(ch chan<- int) {
	for i := 2; ; i++ {
		ch <- i  // Send 'i' to channel 'ch'.
	}
}

// Copy the values from channel 'src' to channel 'dst',
// removing those divisible by 'prime'.
func filter(src <-chan int, dst chan<- int, prime int) {
	for i := range src {  // Loop over values received from 'src'.
		if i%prime != 0 {
			dst <- i  // Send 'i' to channel 'dst'.
		}
	}
}

// The prime sieve: Daisy-chain filter processes together.
func sieve() {
	ch := make(chan int)  // Create a new channel.
	go generate(ch)       // Start generate() as a subprocess.
	for {
		prime := <-ch
		fmt.Print(prime, "\n")
		ch1 := make(chan int)
		go filter(ch, ch1, prime)
		ch = ch1
	}
}

func main() {
	sieve()
}

This particular package is a complete program, my virtue of being in package main, containing a main() function.

I’m not going to go through the program line by line. Instead, since this is in the section of the spec about packages, let’s just take a closer look at the main components, and how they relate to a Go package, as review.

1. The package clause, tells us we’re in the main package, the entry point for all Go programs.

package main

2. The import declaration(s), define which external packages we’re importing into this file.

import "fmt"

3. Function declarations

// Send the sequence 2, 3, 4, … to channel 'ch'.
func generate(ch chan<- int) {

The final function in the example is main(), which of course is the function executed by the runtime to begin running the program.

This is a very simple package, with many possible elements ommited, such as package variables, constants, init functions, etc.

Quotes from The Go Programming Language Specification Language version go1.23 (June 13, 2024)