Golang packages
From wikinotes
Basics
- Packages are groups of related code.
- Functions from files belonging to same package can be called as if from same file (no prefix)
- Otherwise, exported functions are callable when a package is imported (and use the packagename as a prefix)
- Unless this is a library, the
main
package is your program's entrypoint (ex. cli). It must have exactly onemain()
function.- Exported symbols in the
main
package are not accessible in subpackagesTODO:
what about nested packages? ex. io/ioutil
Imports
import "fmt" import ( "fmt" // builtin pkgs have no module-path prefix "golang.org/x/example/stringutil" // non-builtin pkgs have module-path prefix (incl. local sub-packages) )
Defining Packages
Anatomy
# project heirarchy myproject/ mypackage/ libfoo.go libbar.go main.go go.mod
myproject/go.mod
// go.mod module github.com/you/myproject go 1.18
myproject/main.go
// main.go package main import "github.com/you/myproject/mypackage" func main() { mypackage.PrintHi(); }
myproject/mypackage/libfoo.go
// mypackage/libfoo.go package mypackage import "fmt" func PrintHi() { fmt.Println("hi") }Scope
Only exported symbols are exposed when a package is imported.
- Exported functions/types/variables are upper-cased.
- If a symbol is not exported, it is only accessible internally within the same package. not even in sub-packages.
TODO:
what about nested packages? ex. io/ioutil
Package Init
Each file within a package can define an
init
function.
This function is evaluated the first time the package is imported.// names/name.go import "fmt" package names var name string func init() { name = "foo" fmt.Println("imported the first time") }Internal Packages
Packages defined within the
internal/
directory can export symbols for internal use, but it will not be publicly available outside of the module.mygolib/ go.mod main.go publiclib/ format.go internal/ logger/ logger.go maths/ division.go multiplication.go// main.go package main import ( "example.com/x/mygolib/pugliclib" // exported functions can be called by package consumers "example.com/x/mygolib/internal/logger" // exported functions only available in 'example.com/x/mygolib' "example.com/x/mygolib/internal/maths" )Checkout the mangos project for a real example of internal packages.