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"Duck Typing" defined

April 24, 2023

I’ve already introduced the concept of duck typing, but now we have more or less a formal definition:

General interfaces

…

The quantification “the set of all non-interface types” refers not just to all (non-interface) types declared in the program at hand, but all possible types in all possible programs, and hence is infinite. Similarly, given the set of all non-interface types that implement a particular method, the intersection of the method sets of those types will contain exactly that method, even if all types in the program at hand always pair that method with another method.

Of note, an interface matches all possible types in all possible programs. To re-iterate, there is no explicit syntax to indicate that a type “implements” 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.