Packages: Go-Code organisieren

Was ist ein Package?

// file: math/calc.go
package math

// Exported — accessible from other packages (capital letter)
func Add(a, b int) int { return a + b }

// Unexported — package-private (lowercase)
func helper() int { return 42 }

Package-Deklaration und Importpfad

// Import path: github.com/myapp/util/stringutil
// Package name: stringutil (last segment by convention)
package stringutil

func Reverse(s string) string {
    r := []rune(s)
    for i, j := 0, len(r)-1; i < j; i, j = i+1, j-1 {
        r[i], r[j] = r[j], r[i]
    }
    return string(r)
}

Packages importieren

package main

import (
    "fmt"                         // standard library
    "strings"                     // standard library
    "github.com/myapp/util/math"  // third-party or local
)

func main() {
    fmt.Println(strings.ToUpper("hello"))
    fmt.Println(math.Add(1, 2))
}

Exportierte und nicht exportierte Bezeichner

package server

// Exported: accessible from other packages
type Server struct {
    Addr    string   // exported field
    timeout int      // unexported field
}

// Exported method
func (s *Server) Start() error { return nil }

// Unexported helper
func (s *Server) listen() {}

Package main und func main

package main

import "fmt"

func main() {
    fmt.Println("Hello, Go!")
    // Program starts here and ends here
}

// All other packages are libraries — no main() needed

init()-Funktion

package config

import "fmt"

var DB string

func init() {
    DB = "localhost:5432/mydb"
    fmt.Println("config package initialized")
}

// init runs once when the package is first imported
// Multiple init() functions per file/package are allowed

Blank-Import für Seiteneffekte

package main

import (
    "database/sql"
    _ "github.com/lib/pq"  // registers PostgreSQL driver via init()
)

func main() {
    db, err := sql.Open("postgres", "host=localhost dbname=test")
    _ = db
    _ = err
}

Importe mit Alias

package main

import (
    "fmt"
    gomath "math"              // alias to avoid collision
    mymath "github.com/me/math"
)

func main() {
    fmt.Println(gomath.Sqrt(16))     // 4
    fmt.Println(mymath.Add(1, 2))    // 3
}

Konventionen für Package-Namen

Zirkuläre Importe

// WRONG: A imports B, B imports A — compile error
// package a imports "myapp/b"
// package b imports "myapp/a"

// SOLUTION: extract shared types to package c
// package a imports "myapp/c"
// package b imports "myapp/c"
// No circular dependency

Wissenscheck

Zusammenfassung: Packages