Blog / Go Language Spec

Type declarations

July 18, 2023

We’ve already talked about the different types availabe in Go, but now we’ll see how to declare them… just in case it wasn’t already obvious.

Type declarations

A type declaration binds an identifier, the type name, to a type. Type declarations come in two forms: alias declarations and type definitions.

TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = AliasDecl | TypeDef .

We’ll talk about these two different forms in the coming days. But for now, let me point out that a type alias is a very specific concept in Go, distinct from a standard type declaration. This is something that confuses many newcomers, who often think of any type delcaration as an “alias”.

To spell it out:

type MyInt int

In this example, MyInt is not an alias for int. It is a distinct type. Now, it does have the same underlying type… but it is not an alias. That is to say that int and MyInt are distinct types. They may have distinct method sets. They cannot be converted to one another without, well, conversion.

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

Share this

Related Content

Alias declarations

Alias declarations An alias declaration binds an identifier to the given type. AliasDecl = identifier "=" Type . Within the scope of the identifier, it serves as an alias for the type. type ( nodeList = []*Node // nodeList and []*Node are identical types Polar = polar // Polar and polar denote identical types ) Aliases are a feature you likely won’t use frequently. You may never use it, in fact.

Type identities of other types

Type identity … Two pointer types are identical if they have identical base types. Given: type ( P = *int Q = P R *int ) P, Q, and *int are all identical to each other, but different from *int32 and R. Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is.

Type identities of compound types

So we now know that types are either identical or different. let’s look at the details, with examples. Type identity … In detail: Two array types are identical if they have identical element types and the same array length. So given: type ( X = [10]int Y = X Z [10]int ) Types X, Y, and [10]int are identical to each other, but not to [9]int or [10]int32. Type Z is also different from all the others, due to the fact that it is a named type.