How can functions be used to implement interfaces in Go?

Functions Implementing Interfaces in Go

In Go, interfaces define a set of methods that a type must implement to be considered a member of that interface. This allows for polymorphic behavior, as types implementing the same interface can be used interchangeably in code.

Example 1: Custom Type Implementing Interface

Consider the following interface for an HTTP handler:

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

The following type, Counter, implements this interface by providing a custom implementation of ServeHTTP:

type Counter int

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

Example 2: Converting Function to Handler

The following function, notFound, does not implement the Handler interface directly:

func notFound(c *Conn, req *Request) {
    c.SetHeader("Content-Type", "text/plain;", "charset=utf-8")
    c.WriteHeader(StatusNotFound)
    c.WriteString("404 page not found\n")
}

To make this function compatible with the Handler interface, Go provides a way to convert a function to a type with a specific method. In this case, the HandlerFunc type is defined:

type HandlerFunc func(*Conn, *Request)

A method, ServeHTTP, is added to this type:

func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) {
    f(c, req)
}

The notFound function can now be converted to an instance of HandlerFunc and assigned to a variable:

var Handle404 = HandlerFunc(notFound)

This allows Handle404 to be used as a Handler, even though the notFound function itself does not directly implement the Handler interface.

Explanation

In summary, Go allows functions to be converted to types with specific methods, addressing situations where a function does not directly fulfill an interface's requirements. By creating a type instance out of the function and defining a method on that type, the function can be accommodated by the interface, enabling polymorphic behavior in the program.

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