Introduction to Generics

Generics let functions and types work with multiple concrete types while keeping compile-time type checking.

Go added type parameters in Go 1.18. A type parameter such as T stands for a type selected at instantiation, and its constraint determines which operations are permitted. any accepts any type, but does not make every operation valid on T.

Use generics when several types genuinely share the same behavior. For simple operations that already have clear concrete types, a non-generic function is often easier to maintain.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

See the official generics tutorial.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func first[T any](values []T) T { return values[0] }
func main() {
	fmt.Println("First number:", first([]int{8, 9}))
	fmt.Println("First word:", first([]string{"Go", "Rust"}))
}

Output

First number: 8
First word: Go

The compiler infers T=int for the first invocation and T=string for the second. The returned values preserve their specific types instead of becoming any values.

• The example requires nonempty slices: indexing an empty slice would panic.
• any does not permit arithmetic on arbitrary T values.

Write a generic last function and try it with both string and int slices.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Introduction to Generics in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.