How to Convert a Function into an Interface Implementation?

Understanding the Function Implementation of an Interface

Introduction

Interfaces in programming define a set of methods that a type must implement to be considered a valid interface implementation. When a function implements an interface, its method signature must match the interface's method signature.

Interface and Implementation

In this case, we have the following interface:

type Handler interface {
    ServeHTTP(*Conn, *Request)
}

This interface defines a method called ServeHTTP, which takes a Conn and a Request as arguments.

The following implementation of the Counter type satisfies the Handler interface:

type Counter int

func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) {
    fmt.Fprintf(c, "counter = %d\n", ctr);
    ctr++;
}

Converting a Function to Implement the Interface

Sometimes, we may have a function that matches the interface method signature but is not directly implemented as a method. In such cases, the following technique can be used to convert the function into an interface implementation:

// Now we define a type to implement ServeHTTP:
type HandlerFunc func(*Conn, *Request)
func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) {
    f(c, req) // the receiver's a func; call it
}
// Convert function to attach method, implement the interface:
var Handle404 = HandlerFunc(notFound);

In this example, the notFound function is not a method, but it matches the ServeHTTP method's signature. To make it compatible with the Handler interface, we create a HandlerFunc type that wraps the function in a method. Then, we create an instance of HandlerFunc with notFound as its value. This instance, Handle404, now implements the Handler interface and can be used as a valid implementation.

Summary

This technique demonstrates how functions that satisfy an interface's method signature can be converted into interface implementations, even if they are not methods of a custom type. This approach is particularly useful when working with existing functions that need to be adopted to interface-based systems.

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