Difficulty with Stringer patch example - <nil> node type

[mdmcconnell]

I'm trying to do something very similar to the Stringer tree patch example at https://expr.medv.io/docs/Visitor-and-Patch, however discovered the exmple is broken and I cannot fix. After changing ast.MemberNode.Field to .Name, I can compile the example (see https://go.dev/play/p/wkZtzPYBKF-). however it crashes with

panic: undefined node type ()

goroutine 1 [running]:
github.com/antonmedv/expr/checker.(*visitor).visit(0xc000093d80, {0x0, 0x0?})
/mnt/f/mattm/home/go/pkg/mod/github.com/antonmedv/[email protected]/checker/checker.go:110 +0x7c8

I imagine the ast.MemberNode is missing information, but I don't understand the node/tree model well enough to fix. Can someone identify what is missing?

[mdmcconnell]

Indeed, the ast.MemberNode is missing a Property naming the method to call, so this:

	ast.Patch(node, &ast.CallNode{
		Callee: &ast.MemberNode{
			Node: *node,
			Name: "String",
		},
	})

Becomes this:

	ast.Patch(node, &ast.CallNode{
		Callee: &ast.MemberNode{
			Node:     *node,
			Name:     "String",
			Property: &ast.StringNode{Value: "String"},
		},
	})