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Interfaces don't contain interfaces

April 25, 2023

As we learned recently, Go interface elements may contain a type term of a non-interface type. It’s worth re-iterating that these are non-interface types.

In particular, as the spec states:

General interfaces

…

By construction, an interface’s type set never contains an interface type.

That is to say, that the following is invalid:

type interface foo {
  /* some interface elements */
}

type interface bar {
  foo
}

Actually, I lied. That’s not invalid. It’s a case of embedding. But the point is, putting the name of an interface inside of an interface like that does not work as putting the name of a type, such as int or a struct, in an interface.

Quotes from The Go Programming Language Specification Version of December 15, 2022

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Underlying types

We’ve made it through the complete list of types in Go. Now we’re going to dig into some of the fundamentals, with a tour of general “Properties of types and values”. Properties of types and values Underlying types Each type T has an underlying type: If T is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is T itself. Otherwise, T’s underlying type is the underlying type of the type to which T refers in its declaration.

Restrictions on underlying type terms

General interfaces … In a term of the form ~T, the underlying type of T must be itself, and T cannot be an interface. type MyInt int interface { ~[]byte // the underlying type of []byte is itself ~MyInt // illegal: the underlying type of MyInt is not MyInt ~error // illegal: error is an interface } Once again, the spec examples are pretty well explained. TL;DR; the ~ prefix must always be associated with an underlying data type.

Interface examples with type elements

Let’s look at some interface examples from the spec: General interfaces … // An interface representing only the type int. interface { int } // An interface representing all types with underlying type int. interface { ~int } // An interface representing all types with underlying type int that implement the String method. interface { ~int String() string } // An interface representing an empty type set: there is no type that is both an int and a string.