anonymous fields in type declaration?

Question

I came across this type declaration:

type Handler func(*Conn)

type Server struct {
  Handshake func(*Config, *http.Request) error
  Handler
}

(this is a simplified version of https://github.com/golang/net/blob/38c17adf51120973d1735785a7c02f8ce8297c5e/websocket/server.go#L55-L66

The second field in the Server structure is anonymous. There is just type and no name.

Here is the grammar for type declarations (https://golang.org/ref/spec#Type_declarations):

TypeDecl     = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec     = identifier Type .

and it clearly requires an identifier name. But yet the section that I referenced that contains the grammar, also mentions anonymous fields.

I do not understand why this syntax is correct and how anonymous fields are used.

dyoo dyoo · Accepted Answer · 2015-01-16T23:04:09

You want to look at the part of the grammar that has to do with structures, not just types. See: Struct types and the use of AnonymousField. Looking just at the production for TypeSpec is focusing attention on the wrong place. Instead, look at FieldDecl; the grammar shows that we have two possibilities: named fields (IdentifierList Type), or anonymous fields (AnonymousField).

Anonymous fields are typically used for embedding. In your example, a Server will act like a Handler because it has embedded that field.

anonymous fields in type declaration?

1 Answers