Blog / Go Language Spec

Resolving type parameter ambiguities

August 2, 2023

First today, one rather mundane sentence from the spec…

Type parameter declarations

…

Just as each ordinary function parameter has a parameter type, each type parameter has a corresponding (meta-)type which is called its type constraint.

Followed by a bit of WTF…

A parsing ambiguity arises when the type parameter list for a generic type declares a single type parameter P with a constraint C such that the text P C forms a valid expression:

type T[P *C] …
type T[P (C)] …
type T[P *C|Q] …
…

If you’re up for it, I’ll try to break this thing down. If you’re not up for it, no worries. See you tomorrow! 👋😆

Let’s consider the first example provided: type T[P *C] …. How can this be ambiguous?

Well, it looks like an array type declaration, where the array length is derrived from a constant expression, such as in this example:

const P = 3
const C = 4

type T [P * C]int // T is of type [12]int

Other expressions are possible, too, which is what leads to this possible ambiguity.

So, we need a way to solve this WTF scenario, and Go gives us two of them…

In these rare cases, the type parameter list is indistinguishable from an expression and the type declaration is parsed as an array type declaration. To resolve the ambiguity, embed the constraint in an interface or use a trailing comma:

type T[P interface{*C}] …
type T[P *C,] …

So either wrap your constraint in interface{…}, or add a magical comma at the end. Poof! problem solved.

But still. WTF?!

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

Share this

Related Content

Type parameter declarations

Type parameter declarations A type parameter list declares the type parameters of a generic function or type declaration. The type parameter list looks like an ordinary function parameter list except that the type parameter names must all be present and the list is enclosed in square brackets rather than parentheses. TypeParameters = "[" TypeParamList [ "," ] "]" . TypeParamList = TypeParamDecl { "," TypeParamDecl } . TypeParamDecl = IdentifierList TypeConstraint .

Generic methods

Type definitions … A generic type may also have methods associated with it. In this case, the method receivers must declare the same number of type parameters as present in the generic type definition. // The method Len returns the number of elements in the linked list l. func (l *List[T]) Len() int { … } This probably needs further explanation. Or an example. It did for me. Let’s look at a simple generic type:

Definitions of generic types

Type definitions … If the type definition specifies type parameters, the type name denotes a generic type. Generic types must be instantiated when they are used. type List[T any] struct { next *List[T] value T } We haven’t covered instantiation yet, but we will. For now, let’s just summarize: Instantiation is process by which a generic type’s type parameters are substituted with the actual type arguments to be used.