Tour of Go 2.0

If with a short statement

Like for, if can start with a short statement that runs before the condition is checked. It’s a handy pattern for computing a value and immediately branching on it.

Variables declared in that short statement are scoped to the if block — they vanish after the closing brace.

Try it — attempt to use v in the last return statement and see the compile error.

main.go ▶ Run in Playground 
package main

import (
	"fmt"
	"math"
)

func pow(x, n, lim float64) float64 {
	if v := math.Pow(x, n); v < lim {
		return v
	}
	return lim
}

func main() {
	fmt.Println(
		pow(3, 2, 10),
		pow(3, 3, 20),
	)
}