Variable Declaration / Short Declaration

// Declaration with var (explicit type)
var variableName type
var variableName type = value

// Declaration with var (type inference)
var variableName = value

// Short declaration operator (only allowed inside functions)
variableName := value

// Declaring multiple variables at once
var x, y int = 1, 2
a, b := 10, 20

package main

import "fmt"

// At the package level, use var (:= is not allowed here)
var appName string = "MyApp"
var version = 1.0 // type inference

func main() {
	// Inside a function, both styles are available
	var msg string = "Hello"
	lang := "Go" // short declaration (most commonly used)

	// Declaring multiple variables at once
	var x, y int = 10, 20
	a, b := 100, 200

	fmt.Println(appName, version)
	fmt.Println(msg, lang)
	fmt.Println(x, y)
	fmt.Println(a, b)

	// var block
	var (
		name = "Tanaka"
		age  = 30
	)
	fmt.Println(name, age)
}

Inside functions, the short declaration := is the most concise form and is commonly used in Go code. At the package level (outside any function), only var can be used.

With type inference, Go automatically determines the type from the value on the right-hand side. For example, x := 42 produces an int, and x := 3.14 produces a float64. Use var when you want to specify the type explicitly or initialize a variable to its zero value.

:= must introduce at least one new variable on the left-hand side. Writing only already-declared variables on the left will cause a compile error. Also note that declaring a variable but never using it is itself a compile error in Go.