Question

I wish to implement the fmt.Stringer interface’s String method. However for a set of types deriving from Node, their String implementations would be a wrapper around a Print interface method they must provide. How can I provide String automatically for all types implementing Node? If I provide the default String on some base class, I lose access to the derived type (and thus the interface method Print).

type Node interface {
    fmt.Stringer
    Print(NodePrinter)
}

type NodeBase struct{}

func (NodeBase) String() string {
    np := NewNodePrinter()
    // somehow call derived type passing the NodePrinter
    return np.Contents()
}

type NodeChild struct {
    NodeBase
    // other stuff
}

func (NodeChild) Print(NodePrinter) {
    // code that prints self to node printer
}

Answer 1

Go explicitly declares that it’s not possible:

When we embed a type, the methods of that type become methods of the outer type, but when they are invoked the receiver of the method is the inner type, not the outer one.

For a solution, I recommend something like this:

func nodeString(n Node) string {
    np := NewNodePrinter()
    // do stuff
    n.Print(np)
    return np.Contents()
}

// Now you can add String method to any Node in one line
func (n NodeChild) String() string { return nodeString(n) }