This is a Go grammar question and seems a stupid question but I've been checking the Go language specification to find some official words or definitions to define what type is a xxx type, say, what type is an interface type?
For example, I see words like these:
The method set of an interface type is its interface.
Or
An embedded field must be specified as a type name T or as a pointer to a non-interface type name *T, and T itself may not be a pointer type.
Or
Consider a struct type T with two methods ...
type T struct {
a int
}
....
A type literal like struct {...} is a struct type, what about A in type A struct {...} and B in type B interface{...}? Is A a struct type and B an interface type?
Yes, from the above sample for the struct type T, I can tell that a defined type (by the "type" declaration) whose given type is a struct type or interface type is also a struct or interface type. So A is a struct type and B is an interface type also. But where are the official definitions of this rule?
For defined types I can only find the following relating to type categories:
A type definition creates a new, distinct type with the same underlying type and operations as the given type, and binds an identifier to it.
So my understanding is that the defined type is a new, distinct type with the given type, but they are in the same type category, say, interface types or struct types. Still, there are no such definitions.
TLDR;
The kind of type
Tis interface if its underlying type is an interface type.The kind of type
Tis struct if its underlying type is a struct type.
Spec: Struct types and Spec: Interface types specifies exactly what are the struct and interface types:
StructType = "struct" "{" { FieldDecl ";" } "}" . FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] . EmbeddedField = [ "*" ] TypeName . Tag = string_lit . InterfaceType = "interface" "{" { MethodSpec ";" } "}" . MethodSpec = MethodName Signature | InterfaceTypeName . MethodName = identifier . InterfaceTypeName = TypeName .
So for example these are struct types:
struct { A int }
struct {}
struct { _ int }
and these are interface types:
interface { String() string }
interface {}
We may use a type declaration to create a new type, such as:
type Point struct { X, Y int }
The above type definition creates a new, distinct type with the same underlying type and operations as the given type, and binds an identifier to it. The definition of underlying type is recursive:
Each type
Thas an underlying type: IfTis one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type isTitself. Otherwise,T's underlying type is the underlying type of the type to whichTrefers in its type declaration.
When we talk about arbitrary types being structs or interfaces, we're talking about their kind.
In the light of this, basically your question is equivalent to this:
"When is the kind of an arbitrary type interface or struct?"
The answer to this question is not in the spec, but this is how we could define it:
The kind of a type
Tis interface if its underlying type is an interface type.
Similarly:
The kind of a type
Tis struct if its underlying type is a struct type.
So for example:
type Point struct { X, Y int }
type PP Point
Is the type struct { X, Y int } of kind struct? Yes, because since it's a type literal, its underlying type is itself, and it's by definition a struct type.
Is Point a struct? Since the underlying type of Point is the underlying type of the type to which it refers in its type declaration, which is a type literal (see above), it is of struct type (its kind is struct).
Is PP a struct? Since its underlying type is the underlying type of the type to which it refers in its type declaration (which is Point), whose underlying type is a struct type literal, yes, it is also a struct type.
This kind we're talking about is represented by the reflect.Kind type. There are reflect.Interface and reflect.Struct constants (of type reflect.Kind) to represent the struct and interface kinds. And the reflect.Type type descriptor has a Type.Kind() method to access this kind.
This is how you can check if the type (kind) of some value is a struct for example:
func isStruct(i interface{}) bool {
return reflect.TypeOf(i).Kind() == reflect.Struct
}
Testing it (try it on the Go Playground):
fmt.Println(isStruct(Point{})) // true
fmt.Println(isStruct(PP{})) // true
fmt.Println(isStruct(struct{}{})) // true
fmt.Println(isStruct("text")) // false
Checking for interface type is a little more complicated because passing an interface value to a function that expects interface{} will not pass the interface value as-is but the concrete value "stored" in it, and as an interface{} value. We would have to pass a pointer to interface (which otherwise rarely makes sense in Go), access the element type and check its kind. For details, see this answer: What is the difference between reflect.ValueOf() and Value.Elem() in go?
An interface type is a type introduced by the interface keyword, or a name for such a type as defined by type name interface (plus of course the actual requirements for that interface).
Any type, whether it is an interface type or not, implements an interface type if it has the appropriate set of named methods. That is, a struct type may be sufficient to be used with some interface type. Even a non-struct type may be sufficient:
type foo int
func (receiver_arg foo) method1() { ... }
Type-name foo now implements any interface that requires a method named method1 (provided of course that it implements the rest of any required methods).
... the defined type [via the
typekeyword] is a new, distinct type with the given type, but they are in the same type category ...
Yes, just so. Using reflect, you'll find that they have the same Kind. The word kind isn't in the spec like this but it's quite useful, and the link here enumerates all the fundamental type-kinds in Go.