How Can I Use Type Parameters in Go Interface Methods?

Using Type Parameters in Interface Methods

In attempting to implement a generic data structure in Go, you encountered an error when defining an iterator interface with a method taking a type parameter. This article addresses the issue and provides a solution.

Interface Definition Error

The initial code defines an interface with a method that takes a function type parameter, which led to the error: "function type cannot have type parameters." Similarly, moving the type parameter to the method resulted in the error: "methods cannot have type parameters."

Solution: Generic Interface Definition

As suggested by the error, Methoden cannot have dedicated type parameters. Instead, the solution is to specify the type parameter on the interface type itself. This enables you to use the type parameter in methods within the interface body.

The corrected code:

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

With the type parameter defined on the interface type, you can now use it in the method as expected:

type MyIterator[T any] struct {
    // implementation of the iterator
}

func (i *MyIterator[T]) ForEachRemaining(action func(T) error) error {
    // implementation of the ForEachRemaining method using T
    return nil
}

Example Usage

The following example demonstrates the usage of the generic iterator interface:

package main

import (
    "fmt"
    "collection"
)

type MyIterator[T any] struct {
    // implementation of the iterator
}

func (i *MyIterator[T]) ForEachRemaining(action func(T) error) error {
    // implementation of the ForEachRemaining method using T
    return nil
}

func main() {
    myIterator := &MyIterator[int]{}
    _ = myIterator.ForEachRemaining(func(num int) error {
        fmt.Println(num)
        return nil
    })
}

This code defines a generic iterator, MyIterator, and uses it to iterate over a slice of integers, printing each value.

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