Type parameter restrictions

May 2, 2023

We’ll get to a full discussion of type parameters later on, but knowing at least what they look like is useful for the next section. So here’s an example, used in the definition of a generic function:

func min[T ~int|~float64](x, y T) T {

In this example, min[T ~int|~float64] represents the type parameter.

With that in mind…

General interfaces

The type T in a term of the form T or ~T cannot be a type parameter, and the type sets of all non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty). Given a type parameter P:

interface {
	P                // illegal: P is a type parameter
	int | ~P         // illegal: P is a type parameter
	~int | MyInt     // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
	float32 | Float  // overlapping type sets but Float is an interface

This means, for example, that within our above min function, the following would be illegal:

func min[T ~int|~float64](x, y T) T {
	var x interface {
		T // illegal because T is a type parameter of the generic function

We also can’t combine types and underlying types that overlap. At least this restriction doesn’t require any special understanding of generics that we haven’t covered yet.

type MyInt int
interface {
	~int | MyInt // Illegal becasue of overlapping type sets

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

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General interfaces

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