Short Variable

A short variable declaration uses the := operator to declare and initialize one or more local variables simultaneously, relying on the compiler to infer their types from the assigned values. It serves as a syntactic shorthand for a standard var declaration with an initializer, omitting the explicit type signature.

Syntax

identifier := expression
identifier1, identifier2 := expression1, expression2

Lexical Scope Restrictions

Short variable declarations are strictly confined to block scope. They can only be utilized within the body of a function. Package-level variables must be declared using the var keyword; attempting to use := at the package level results in a compile-time syntax error (non-declaration statement outside function body).

Type Inference Mechanics

The compiler resolves the type of the newly declared variable at compile time based on the right-hand side (RHS) expression.

• If the RHS is a typed expression, the variable inherits that exact type.
• If the RHS is an untyped constant, the variable assumes the default type for that constant class: int for integers, float64 for floating-point numbers, complex128 for complex numbers, rune for character literals, string for string literals, and bool for boolean constants.

package main

import (
	"fmt"
	"os"
)

func main() {
	// Type inferred as int (default for untyped integer constant)
	count := 10 

	// Type inferred as float64 (default for untyped float constant)
	pi := 3.14  

	// Type inferred as bool (default for untyped boolean constant)
	isActive := true

	// Type explicitly derived from the RHS function return types
	file, err := os.Open("data.txt") 

	// Variables evaluated to satisfy the compiler's unused variable constraint
	fmt.Println(count, pi, isActive, file, err)
}

Redeclaration and Assignment Rules

Unlike standard var declarations, the := operator permits the redeclaration of existing variables within the same lexical block, subject to a strict condition: at least one non-blank variable on the left-hand side (LHS) must be newly declared. The blank identifier (_) does not satisfy the requirement for a new variable.

When this condition is met, the := operator acts as a standard assignment for the previously declared variables and a declaration for the new ones. The redeclared variables must retain their original type.

package main

import "fmt"

func main() {
	// 'a' and 'b' are newly declared
	a, b := 1, 2 

	// Valid: 'c' is new. 'a' is merely reassigned.
	a, c := 3, 4 

	// Invalid: The blank identifier (_) does not count as a new variable.
	// a, _ := 5, 6 // Compile error: no new variables on left side of :=

	// Variables evaluated to satisfy the compiler's unused variable constraint
	fmt.Println(a, b, c)
}

Variable Shadowing

Because := always constitutes a declaration (for at least the new variables on the LHS), using it within a nested lexical block—such as an if, for, or switch statement—will instantiate a new variable. This new variable will shadow any variable with the identical identifier in an outer scope for the duration of the inner block.

package main

import "fmt"

func main() {
	x := 10 // Outer scope 'x'

	if true {
		x := 5 // Declares a NEW 'x' in the inner scope, shadowing the outer 'x'
		x++    // Modifies the inner 'x' (becomes 6)
		
		fmt.Println(x) // Prints 6
	}

	// Outer 'x' remains 10
	fmt.Println(x) // Prints 10
}

Master Go with Deep Grasping Methodology!

Learn More