How to write a program with multiple packages in Google Go

This blog post explains how to write a Google Go program which contains multiple packages. Let's suppose your software contains multiple packages: a and b, a consisting of two source files: a1.go and a2.go, and b contains only 1 source file importing package a. Here is an example:

$ cat a1.go
package a
type Rectangle struct {
  width int;
  Height int;
}

$ cat a2.go
package a
func (r *Rectangle) GetArea() int { return r.width * r.Height }
func (r *Rectangle) Populate() {
  r.width = 2;
  r.Height = 3;
}

$ cat b.go
package main
import (
  "a";
)
func main() {
  r := new(a.Rectangle);
  r.Populate();
  // Fields (etc.) starting with a small letter are undefined.
  // (private): b.go:9: r.width undefined (type a.Rectangle has no field width)
  // print(r.width, "\n");
  print(r.GetArea(), "\n");
  print(r.Height, "\n");
  print(r, "\n");
  print("done\n")
}

$ cat abc.sh
#! /bin/bash --
set -ex
rm -f *.6 6.out
6g -o a.6 a1.go a2.go
6g -I. b.go
6l b.6  # links a.6 as well

$ ./abc.sh
...
$ ./6.out
6
3
0x7f18309691a8
done

Notes:

• Symbols and structure fields starting with lower case are not visible in other packages.
• You have to specify -I. for the compiler so it will find a.6 in the current directory.
• The linker figures out that it has to load prerequisite a.6 when linking b.6.