自己类型的接口方法返回值

Question

I am trying to make a method which will take structs of a certain type and do operations on them. However, I need to have a method one can call on an instance of the stuct, and it will return objects of that struct's type. I am getting a compile time error because the return type of the type which implements the interface isn't the same as the interface's method return type, but that's because the interface needs to return values of it's own type.

Interface declaration:

type GraphNode interface {
    Children() []GraphNode
    IsGoal() bool
    GetParent() GraphNode
    SetParent(GraphNode) GraphNode
    GetDepth() float64
    Key() interface{}
}

Type which implements that interface:

type Node struct {
    contents []int
    parent   *Node
    lock     *sync.Mutex
}

func (rootNode *Node) Children() []*Node {
...
}

Error Message:

.\astar_test.go:11: cannot use testNode (type *permutation.Node) as type GraphNode in argument to testGraph.GetGoal:
*permutation.Node does not implement GraphNode (wrong type for Children method)
have Children() []*permutation.Node
want Children() []GraphNode

Method to get parent:

func (node *Node) GetParent() *Node {
    return node.parent
}

The above method fails because it returns a pointer to a node, and the interface returns type GraphNode.

Answer 1

*Node doesn't implement the GraphNode interface because the return type of Children() isn't the same as that defined in the interface. Even if *Node implements GraphNode, you can't use []*Node where []GraphNode is expected. You need to declare Children() to return []GraphNode. The elements of a slice of type []GraphNode can be of type *Node.

For GetParent(), just change it to this:

func (node *Node) GetParent() GraphNode {
    return node.parent
}