Tour of Go 2.0

Type parameters

Generics let you write one function that works across many types — no duplicating logic, no empty interfaces. The secret is type parameters: extra brackets before a function’s arguments that say “this function works for any type T.”

func Index[T comparable](s []T, x T) int

Here, T must satisfy the built-in constraint comparable — meaning Go can use == and != on it. That’s how the function compares values without knowing their concrete type ahead of time.

Try it: run the example as-is. Index works on both []int and []string with no changes. That’s the whole point — write once, use everywhere.

main.go ▶ Run in Playground 
package main

import "fmt"

// Index returns the index of x in s, or -1 if not found.
func Index[T comparable](s []T, x T) int {
	for i, v := range s {
		// v and x are type T, which has the comparable
		// constraint, so we can use == here.
		if v == x {
			return i
		}
	}
	return -1
}

func main() {
	// Index works on a slice of ints
	si := []int{10, 20, 15, -10}
	fmt.Println(Index(si, 15))

	// Index also works on a slice of strings
	ss := []string{"foo", "bar", "baz"}
	fmt.Println(Index(ss, "hello"))
}