Master Golang: Generics

Starting with version 1.18, Go has added support for generics, also known as type parameters.

 
package main

import "fmt"

As an example of a generic function, MapKeys takes a map of any type and returns a slice of its keys. This function has two type parameters - K and V; K has the comparable constraint, meaning that we can compare values of this type with the == and != operators. This is required for map keys in Go. V has the any constraint, meaning that it’s not restricted in any way (any is an alias for interface{}).

 
func MapKeys[K comparable, V any](m map[K]V) []K {
    r := make([]K, 0, len(m))
    for k := range m {
        r = append(r, k)
    }
    return r
}

As an example of a generic type, List is a singly-linked list with values of any type.

 
type List[T any] struct {
    head, tail *element[T]
}

type element[T any] struct {
    next *element[T]
    val  T
}

We can define methods on generic types just like we do on regular types, but we have to keep the type parameters in place. The type is List[T], not List.

 
func (lst *List[T]) Push(v T) {
    if lst.tail == nil {
        lst.head = &element[T]{val: v}
        lst.tail = lst.head
    } else {
        lst.tail.next = &element[T]{val: v}
        lst.tail = lst.tail.next
    }
}

func (lst *List[T]) GetAll() []T {
    var elems []T
    for e := lst.head; e != nil; e = e.next {
        elems = append(elems, e.val)
    }
    return elems
}

func main() {
    var m = map[int]string{1: "2", 2: "4", 4: "8"}

When invoking generic functions, we can often rely on type inference. Note that we don’t have to specify the types for K and V when calling MapKeys - the compiler infers them automatically.

 
    fmt.Println("keys:", MapKeys(m))

… though we could also specify them explicitly.

 
    _ = MapKeys[int, string](m)

    lst := List[int]{}
    lst.Push(10)
    lst.Push(13)
    lst.Push(23)
    fmt.Println("list:", lst.GetAll())
}

$ go run generics.go
keys: [4 1 2]
list: [10 13 23]

Next example: Errors.

Gratitude to Mark McGranaghan and Eli Bendersky