How Can You Adapt a Function to Implement an Interface in Go?

Function Implementing an Interface

In Go, an interface defines a set of methods that a type must implement. A type implements an interface if it provides all the methods declared by the interface.

Example 1: Implementing an Interface Directly

The following code defines the Handler interface and a type that implements it:

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

type Counter int

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

Here, the Counter type has a ServeHTTP method that takes the arguments required by the Handler interface. Therefore, Counter implements the Handler interface.

Example 2: Implementing an Interface with a Function

In this example, a function called notFound is not a valid Handler type because it doesn't have the required method:

func notFound(c *Conn, req *Request) {
    // Not a valid Handler
}

To make notFound implement the Handler interface, it needs to be "finagled" into the following form:

// Define a type that implements ServeHTTP
type HandlerFunc func(*Conn, *Request)

func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) {
    f(c, req) // Function call with arguments
}

// Convert function to implement the interface
var Handle404 = HandlerFunc(notFound)

Here, HandlerFunc is a type that takes two arguments and returns nothing. By wrapping notFound within HandlerFunc and assigning it to a variable, notFound becomes a valid Handler type.

Explanation

The key concept here is that the variable Handle404 can be used as a Handler even though it's actually a function. This is because the HandlerFunc type implements the ServeHTTP method for the function. When you call Handle404.ServeHTTP, it is actually calling the notFound function.

This approach allows functions that do not implement the Handler interface directly to be used with it. It's essentially a way to "adapt" a function to fit the requirements of an interface.

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