Generics in Go

Why Generics?

To write DRY code! Generics allows us to use variable to refer to specific types. This helps us reduce code duplication.

How To Use Generics

Define a variable with the constraint any. The variable does not have to be named T.

func yourFunc[T any](s []T) ([]T) {
	// your code here
}

From this example, we’ve defined T to constraint any and use it to define both the argument and return type to []T. Which means a slice of any type.

Constraints

Constraints are interfaces that allow us to write generics that only operate within the constraints of a given interface type. This means that any interface we define could possible be used as a generic constraint. The any constraint is the same as a empty interface which will match anything.

Create Custom Constraints

The concat function takes a slice of values and concatenates it into a string. This should work with any type that can represent itself as a string.

type stringer interface {
    String() string
}

func concat[T stringer](vals []T) string {
    result := ""
    for _, val := range vals {
        // this is where the .String() method
        // is used. That's why we need a more specific
        // constraint instead of the any constraint
        result += val.String()
    }
    return result
}

Interface Type Lists

We can also create interfaces with just a list of types.

type Number interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | 
	~float32 | ~float64
}

Parametic Constraints

Interface definitions can accept type parameters as well.

type store[P product] interface {
	Sell(P)
}

type product interface {
	Price() float64
	Name() string
}

type guitar struct {
	name string
	manufacturer string
	price float64
}

func (g guitar) Name() string {
	return fmt.Sprintf("%s by %s", b.name, b.manufacturer)
}

func (g guitar) Price() float64 {
	return g.price
}

type guitarStore struct {
	guitarsSold []guitar
}

func (gs *guitarStore) Sell(g guitar) {
	gs.guitarsSold = append(gs.guitarsSold, g)
}