Tour of Go 2.0

Type assertions

A type assertion lets you pull the concrete value out of an interface.

t := i.(T)

This says: “I know i holds a T — give me the T.” If you’re wrong, it panics.

The safer form uses two return values:

t, ok := i.(T)

If i holds a T, you get the value and ok is true. If not, ok is false, t is the zero value, and there’s no panic.

This should feel familiar — it’s the same pattern as reading from a map.

Try it: the last line in main does an unsafe assertion on a float64. Run it and watch it panic.

main.go ▶ Run in Playground 
package main

import "fmt"

func main() {
	var i interface{} = "hello"

	s := i.(string)
	fmt.Println(s)

	s, ok := i.(string)
	fmt.Println(s, ok)

	f, ok := i.(float64)
	fmt.Println(f, ok)

	f = i.(float64) // panic
	fmt.Println(f)
}