Blog / Go Language Spec

Circular generic types

July 27, 2023

There’s a lot to cover within type definitions! We’re several days in, and still going strong…

Type definitions

…

In a type definition the given type cannot be a type parameter.

type T[P any] P    // illegal: P is a type parameter

func f[T any]() {
	type L T   // illegal: T is a type parameter declared by the enclosing function
}

I expect this will surprise noone. We’ve already seen that struct, array, and slice types cannot reference themselves. This is just the logical application of the same idea to generic types.

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

Share this

Related Content

Composite keys

This week’s episode of the Cup o’ Go podcast is out! Keep up with the Go community in just 15 minutes per week! Have a listen! Composite literals … The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key. If you’ve written any Go code, you probably already know about keys in maps:

Assignability of composite components

Composite literals … The types of the elements and keys must be [assignable(https://go.dev/ref/spec#Assignability)] to the respective field, element, and key types of type T; there is no additional conversion. This shouldn’t be surprising, but let’s examine the implications just the same. type myStr string s := myStr("my string") var x = map[int]string{ 3: "oink", // Valid, untyped numeric constant 3 is assignable to an int. int(4): "quack", // Valid, int(4) is of type int int64(12): "moo", // Invalid, int64(12) is not converted to int 8: s, // Invalid, type myStr is type string 9: string(s), // Valid; explicit conversion to type string } Quotes from The Go Programming Language Specification Version of August 2, 2023

Composite literals

Earlier this year, near the beginning of this series on the Go spec, we went through literal representations of all of the basic types in Go: ints, strings, etc. If this were an Intro to Go style book, the very next section would have been on composite literals. But it’s not. So we had to wait nearly 6 months to get to that topic… But here we are, at long last!