Go generics
last modified April 11, 2024
In this article we show how to work with generics in Golang.
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
With generics, we can write code that can be used with various types. Functions and other types can be written to use any of a set of types. Generics help reduce code duplication.
Go uses parameter types for generics. They are specified inside square brackets
[].
func Println[T any](v T) {
fmt.Println(v)
}
By convention, generic parameter types are specified with uppercase letters,
typically T, K, or V.
Go generics example
In the first example, we define a simple generic function.
package main
import (
"fmt"
)
func Println[T any](v T) {
fmt.Println(v)
}
func main() {
Println[string]("an old falcon")
Println[int](23)
Println[float64](3.34)
Println[bool](true)
Println[[]int]([]int{1, 2, 3, 4, 5})
}
Our custom Println function can accept any type and print it.
Println[string]("an old falcon")
Println[int](23)
Println[float64](3.34)
Println[bool](true)
Println[[]int]([]int{1, 2, 3, 4, 5})
We print a string value, an int64 value, a float64 value, a bool value and a integer slice value.
$ go run main.go an old falcon 23 3.34 true [1 2 3 4 5]
Go generics omit type
In many cases, we can omit the type when calling the generic function. The compiler will infer the type from the function arguments.
package main
import (
"fmt"
)
func Println[T any](v T) {
fmt.Println(v)
}
func main() {
Println("an old falcon")
Println(23)
Println(3.34)
Println(true)
Println([]int{1, 2, 3, 4, 5})
}
In the program we omit the parameter type declarations when calling the
Println function.
$ go run main.go an old falcon 23 3.34 true [1 2 3 4 5]
Go generic union type
We can restrict the generic type parameter to the types in the union.
package main
import (
"fmt"
)
func Println[T string | int](v T) {
fmt.Println(v)
}
func main() {
Println("an old falcon")
Println(23)
// Println(3.34)
// Println(true)
// Println([]int{1, 2, 3, 4, 5})
}
In the program, the Println function can accept either strings or
integers.
$ go run main.go an old falcon 23
Go generics tilde
The ~ tilde token is used in the form ~T to denote the
set of types whose underlying type is T.
package main
import (
"fmt"
)
type mystring string
func Println[T ~string | int](v T) {
fmt.Println(v)
}
func main() {
Println("an old falcon")
Println(23)
Println(mystring("rainy day"))
}
In the program, we have a custom mystring type. With
~string syntax, we tell the compiler to include any type that
approximates to string.
$ go run main.go an old falcon 23 rainy day
Go generic filter function
The filter function processes a collection and produces a new collection containing exactly those elements for which the given predicate returns true.
In the next example, we create a generic version of the filter function.
package main
import (
"fmt"
"strings"
)
func filter[T any](data []T, f func(T) bool) []T {
fltd := make([]T, 0, len(data))
for _, e := range data {
if f(e) {
fltd = append(fltd, e)
}
}
return fltd
}
func main() {
words := []string{"war", "cup", "water", "tree", "storm"}
res := filter(words, func(s string) bool {
return strings.HasPrefix(s, "w")
})
fmt.Println(res)
vals := []int{-1, 0, 2, 5, -9, 3, 4, 7}
res2 := filter(vals, func(e int) bool {
return e > 0
})
fmt.Println(res2)
}
In the program we use a generic filter function to filter strings and integers.
func filter[T any](data []T, f func(T) bool) []T {
fltd := make([]T, 0, len(data))
for _, e := range data {
if f(e) {
fltd = append(fltd, e)
}
}
return fltd
}
The filter function builds a new slice which includes only elements that satisfy
the given condition. The function works on parameter type T with
constraint any. It takes a collection and a predicate function as
parameters. We call the predicate on each element and add it to the
fltd slice if it matches the predicate's condition.
res := filter(words, func(s string) bool {
return strings.HasPrefix(s, "w")
})
Here we filter out all words that start with 'w'.
$ go run main.go [war water] [2 5 3 4 7]
Go generic ForEach function
In the next program, we create a generic ForEach function.
package main
import "fmt"
func ForEach[T any](data []T, f func(e T, i int, data []T)) {
for i, e := range data {
f(e, i, data)
}
}
func main() {
vals := []int{-1, 0, 2, 1, 5, 4}
ForEach(vals, func(e int, i int, data []int) {
fmt.Printf("e at %d: %d\n", i, e)
})
fmt.Println("-------------------------")
words := []string{"sky", "forest", "word", "cup", "coin"}
ForEach(words, func(e string, i int, data []string) {
fmt.Printf("e at %d: %s\n", i, e)
})
}
The generic ForEach function takes a generic slice and a closure function as parameters. The closure is used to perform a task on each of the elements.
func ForEach[T any](data []T, f func(e T, i int, data []T)) {
for i, e := range data {
f(e, i, data)
}
}
In the ForEach function, we use the for loop to
go over the elements of the generic slice and call the closure on each element.
ForEach(vals, func(e int, i int, data []int) {
fmt.Printf("e at %d: %d\n", i, e)
})
When actually calling the ForEach function, we pass a closure with
concrete types. The elements have int type, the index has
int type, and the collection is of int[] type.
$ go run main.go e at 0: -1 e at 1: 0 e at 2: 2 e at 3: 1 e at 4: 5 e at 5: 4 ------------------------- e at 0: sky e at 1: forest e at 2: word e at 3: cup e at 4: coin
Source
Getting started with generics - tutorial
In this article we have covered generics in Golang.
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