Go Generics – How to Use a Type Parameter in an Interface Method

genericsgointerface

I am trying to write a sample program to try and implementing the a data structure using Go generics.

As part of this I want to define a iterator interface. I have the following code:

package collection

type Iterator interface {
    ForEachRemaining(action func[T any](T) error) error
    // other methods
}

It keeps giving me following error

function type cannot have type parameters

Moving the type parameter to the method also doesn't work:

type Iterator interface {
    ForEachRemaining[T any](action func(T) error) error
    // other methods
}

Gives error:

methods cannot have type parameters

Is there any way to define generic interface

Best Answer

As the error suggests, methods cannot have type parameters of their own as per the latest design. However, they can use the generics from the interface or struct they belong to.

What you need is to specify the type parameter on the interface type as follows:

type Iterator[T any] interface {
    // ...
}

and then use the T as any other type parameter in methods within the interface body. For example:

package main

import "fmt"

type Iterator[T any] interface {
    ForEachRemaining(action func(T) error) error
    // other methods
}

func main() {
    fmt.Println("This program compiles")
}

Try it on Go playground.

Related Solutions

Go – Implementing Generic Functions

As of Go 1.18 you can write a generic function Print as below:

package main

import (
    "fmt"
)

// T can be any type
func Print[T any](s []T) {
    for _, v := range s {
        fmt.Print(v)
    }
}

func main() {
    // Passing list of string works
    Print([]string{"Hello, ", "world\n"})

    // You can pass a list of int to the same function as well
    Print([]int{1, 2})
}

Output:

Hello, world
12

Go Interface Type Constraints – Conversion Issues Explained

The language specifications explicitly disallow using interfaces with type elements as anything other than type parameter constraints (the quote is under the paragraph Interface types):

Interfaces that are not basic may only be used as type constraints, or as elements of other interfaces used as constraints. They cannot be the types of values or variables, or components of other, non-interface types.

An interface that embeds comparable or another non-basic interface is also non-basic. Your Number interface contains a union, hence it is non-basic too.

A few examples:

// basic: only methods
type A1 interface {
    GetName() string
}

// basic: only methods and/or embeds basic interface
type B1 interface {
    A1
    SetValue(v int)
}

// non-basic: embeds comparable
type Message interface {
    comparable
    Content() string
}

// non-basic: has a type element (union)
type Number interface {
    int | int64 | float64
}

// non-basic: embeds a non-basic interface
type SpecialNumber interface {
    Number
    IsSpecial() bool
}

In the initialization of the variable a, you are attempting to use Number in a type conversion Number(1), and this is not allowed.

You can only use Number as a type parameter constraint, i.e. to restrict the types allowed for instantiation of a generic type or function. For example:

type Coordinates[T Number] struct {
    x, y T
}

func sum[T Number](a, b T) T {
    return a + b
}

Best Answer

Related Solutions

Go – Implementing Generic Functions

Go Interface Type Constraints – Conversion Issues Explained

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