Final thoughs on type parameter declarations

August 3, 2023

Are you as curious about Fuzzing in Go as I am? On Monday’s live stream, I’ll be live coding my way through John Arundel’s 4-part series on Go Fuzzing. I hope you’ll join me!

Two more small points before we move on from type parameter declarations…

Type parameter declarations

Type parameters may also be declared by the receiver specification of a method declaration associated with a generic type.

No surprises here. But we’ll look more closely when we get to the section on method declarations.

Within a type parameter list of a generic type T, a type constraint may not (directly, or indirectly through the type parameter list of another generic type) refer to `T``.

type T1[P T1[P]] …                    // illegal: T1 refers to itself
type T2[P interface{ T2[int] }] …     // illegal: T2 refers to itself
type T3[P interface{ m(T3[int])}] …   // illegal: T3 refers to itself
type T4[P T5[P]] …                    // illegal: T4 refers to T5 and
type T5[P T4[P]] …                    //          T5 refers to T4

type T6[P int] struct{ f *T6[P] }     // ok: reference to T6 is not in type parameter list

No surprises here, either. By now, we’re pretty accustomed to the idea that Go doesn’t like circular references, be it in slice/array definitions, struct definitions, or even imports! So avoid circular references in your type parameter declarations, too!

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

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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:

Type parameters in method definitions

Yesterday we saw that when a method is defined on a generic type, the receiver must include type parameters. Now for all the relevant details: Method declarations … … Syntactically, this type parameter declaration looks like an instantiation of the receiver base type: the type arguments must be identifiers denoting the type parameters being declared, one for each type parameter of the receiver base type. The type parameter names do not need to match their corresponding parameter names in the receiver base type definition, and all non-blank parameter names must be unique in the receiver parameter section and the method signature.

Methods on generic types

Ready to dive back into generics, this time with regard to methods? Great, ‘cause here we go! Method declarations … If the receiver base type is a generic type, the receiver specification must declare corresponding type parameters for the method to use. Okay. Before we continue, let’s make sure we all know exactly what we’re talking about. What does it mean for a receiver base type to be a generic type?