What does a const value do in an anonymous struct field?

Given the following code:

type NodeType int

const (
	NodeText NodeType = iota
	NodeAction
	// etc..
)

type ListNode struct {
	NodeType
}

What is the explanation of including the NodeType type as an anonymous field in the ListNode struct? Does it serve as some sort of identifier for the struct?

For such an anonymous or embedded field, per The Go Programming Language Specification,

> The unqualified type name acts as the field name.

In your case, you can write the following:

var node1 ListNode
node1.NodeType = NodeText

or

node2 := ListNode{NodeText}

or

node3 := ListNode{
	NodeType: NodeText,
}

All three create the same value.

NodeType isn't a constant. It's a type, and it has been embedded into the ListNode struct.

> Struct types have the ability to contain anonymous or embedded fields.
> This is also called embedding a type. When we embed a type into a
> struct, the name of the type acts as the field name for what is then
> an embedded field.

e.g. You would set the NodeType for a ListNode as per below:

ln := &ListNode{NodeType: NodeText}
fmt.Println(ln)
// Outputs: &{0}

Further reading: http://www.goinggo.net/2014/05/methods-interfaces-and-embedded-types.html (scroll down) and https://golang.org/doc/effective_go.html#embedding

That is not an 'anonymous field' the language feature being used is called 'embedding' and it's sort of Go's way of addressing inheritance. In Go, if StructA embeds StructB then fields on StructB and methods with a receiving type of StructB will become directly accessible on StructA. Same rules for exported vs unexported apply however (ie unexported fields in StructB won't be accessible outside the packages scope like normal). I can't say why you want this value in all your structs, this is just an efficient way of doing it. At the end of the day it's just an int and you're just saving a few lines of code by not giving it an actual name, nothing more really.