It feels like it’s been a month since we started on interfaces. Today we’re covering the final section on this topic!

Implementing an interface

A type T implements an interface I if

• T is not an interface and is an element of the type set of I; or
• T is an interface and the type set of T is a subset of the type set of I.

A value of type T implements an interface if T implements the interface.

All three of these points should be pretty intuitive, especially if you’ve read the rest of the interface posts, but let’s give an example of each just to be sure.

1. T is not an interface and is an element of the type set of I

   type Float interface {
   	~float32 | ~float64
   }
   
   type myFloat float64 // myFloat implements Float; it is not
                        // an interface, and it is an element
                        // of the type set of Float.
   
   type Fooer interface {
   	Foo()
   }
   
   type myFooer struct {} // myFooer implements Fooer; it is not
                          // an interface, and it is an element
                          // of the type set of Fooer by virtue of
                          // the fact that it implements the Foo()
                          // method
   
   func (f *myFooer) Foo() {}
   

2. T is an interface and the type set of T is a subset of the typset of I

   type FooBarer interface {
   	Foo()
   	Bar()
   }
   
   var fb FooBarer
   var f Fooer = fb // FooBarer implements Fooer, because
                    // it is an interface, and has a type set
                    // that is a subset of FooBarer's type set.
   

3. A value of type T implements an interface if T implements the interface.

   This just extends the type implementation rules to values of that type.

Quotes from The Go Programming Language Specification Version of December 15, 2022