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Exploring and Mastering the Full Go Featureset

Variables

Learn about variables in Go.

We'll cover the following...

Unused variables

Go programs with unused variables do not compile:

“The presence of an unused variable may indicate a bug […] Go refuses to compile programs with unused variables or imports, trading short-term convenience for long-term build speed and program clarity.”
https://golang.org/doc/faq

Exceptions to that rule are global variables and function arguments:

Go (1.16.5)
package main
var unusedGlobal int // this is ok
func f1(unusedArg int) {// unused function arguments are also ok
     // error: a declared but not used
     a, b := 1,2
     // b is used here, but a is only assigned to, doesn't count as “used”
     a = b
}

Short variable declaration

Short variable declarations only work inside functions:

Go (1.16.5)
package main
v1 := 1 // error: non-declaration statement outside function body
var v2 = 2 // this is ok
func main() {
    v3 := 3 // this is ok
    fmt.Println(v3)
}

They also don’t work when setting field values:

Go (1.16.5)
package main
type myStruct struct {
    Field int
}
func main() {
    var s myStruct
    // error: non-name s.Field on the left side of :=
    s.Field := 1
    var newVar int
    s.Field, newVar = 1, 2 // this is actually ok
}