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Go Type System Overview

This article will introduce all kinds of types in Go and the concepts regarding Go type system. Without knowing these fundamental concepts, it is hard to have a thorough understanding of Go.

Concept: Basic Types

Built-in basic types in Go have been also introduced in built-in basic types and basic value literals. For completeness of the current article, these built-in basic types are re-listed here.

Note, byte is a built-in alias of uint8, and rune is a built-in alias of int32. We can also declare custom type aliases (see below).

Except string types, Go 101 article series will not try to explain more on other basic types.

The 17 built-in basic types are predeclared types.

Concept: Composite Types

Go supports the following composite types:

Unnamed composite types may be denoted by their respective type literals. Following are some literal representation examples of all kinds of unnamed composite types (name and unnamed types will be explained below). 
// Assume T is an arbitrary type and Tkey is
// a type supporting comparison (== and !=).

*T         // a pointer type
[5]T       // an array type
[]T        // a slice type
map[Tkey]T // a map type

// a struct type
struct {
	name string
	age  int
}

// a function type
func(int) (bool, string)

// an interface type
interface {
	Method0(string) int
	Method1() (int, bool)
}

// some channel types
chan T
chan<- T
<-chan T

Comparable and incomparable types will be explained below.

Fact: Kinds of Types

Each of the above mentioned basic and composite types corresponds to one kind of types. Besides these kinds, the unsafe pointer types introduced in the unsafe standard package also belong to one kind of types in Go. So, up to now (Go 1.24), Go has 26 kinds of types.

Syntax: Type Definitions

(Type definition, or type definition declaration, initially called type declaration, was the only type declaration way before Go 1.9. Since Go 1.9, type definition has become one of two forms of type declarations. The new form is called type alias declaration, which will be introduced in a section below.)

In Go, we can define new types by using the following form. In the syntax, type is a keyword. 
// Define a solo new type.
type NewTypeName SourceType

// Define multiple new types together.
type (
	NewTypeName1 SourceType1
	NewTypeName2 SourceType2
)

New type names must be identifiers. But please note that, type names declared at package level can't be init.

The second type declaration in the above example includes two type specifications. If a type declaration contains more than one type specification, the type specifications must be enclosed within a pair of ().

Note,

Some type definition examples: 
// The following new defined and source types are all
// basic types. The source types are all predeclared.
type (
	MyInt int
	Age   int
	Text  string
)

// The following new defined and source types are all
// composite types. The source types are all unnamed.
type IntPtr *int
type Book struct{author, title string; pages int}
type Convert func(in0 int, in1 bool)(out0 int, out1 string)
type StringArray [5]string
type StringSlice []string

func f() {
	// The names of the three defined types
	// can be only used within the function.
	type PersonAge map[string]int
	type MessageQueue chan string
	type Reader interface{Read([]byte) int}
}

Please note that, from Go 1.9 to Go 1.17, the Go specification ever thought predeclared built-in types are defined types. But since Go 1.18, the Go specification clarifies that predeclared built-in types are not defined types.

Concept: Custom Generic Types and Instantiated Types

Since Go 1.18, Go has supported custom generic types (and functions). A generic type must be instantiated to be used as value types.

A generic type is a defined type and its instantiated types are named types (named types are explained in the next section).

The other two important concepts in custom generics are constraints and type parameters.

This book doesn't talk about custom generics in detail. Please read the Go Generics 101 book on how to declare and use generic types and functions.

Concept: Named Types vs. Unnamed Types

Before Go 1.9, the terminology named type is defined accurately in Go specification. A named type was defined as a type that is represented by an identifier. Along with the custom type alias feature introduced in Go 1.9 (see the next section), the named type terminology was ever removed from Go specification and it was replaced by the defined type terminology. Since Go 1.18, along with the introduction of custom generics, the named type terminology has been added back to Go specification.

A named type may be

Other value types are called unnamed types. An unnamed type must be a composite type (not vice versa).

Syntax: Type Alias Declarations

Since Go 1.9, we can declare custom type aliases by using the following syntax. The syntax of alias declaration is much like type definition, but please note there is a = in each type alias specification. 
type (
	Name = string
	Age  = int
)

type table = map[string]int
type Table = map[Name]Age

Type alias names must be identifiers. Like type definitions, type aliases can also be declared within function bodies.

A type name (or literal) and its aliases all denote an identical type. By the above declarations, Name is an alias of string, so both denote the same type. The same applies to the other three pairs of type notations (either names or literals):

In fact, the literals map[string]int and map[Name]Age also both denote the same type. So, similarly, aliases table and Table also denote the same type.

Note, although a type alias always has a name, it might denote an unnamed type. For example, the table and Table aliases both denote the same unnamed type map[string]int.

Concept: Underlying Types

In Go, each type has an underlying type. Rules:

Examples: 
// The underlying types of the following ones are both int.
type (
	MyInt int
	Age   MyInt
)

// The following new types have different underlying types.
type (
	IntSlice   []int   // underlying type is []int
	MyIntSlice []MyInt // underlying type is []MyInt
	AgeSlice   []Age   // underlying type is []Age
)

// The underlying types of []Age, Ages, and AgeSlice
// are all the unnamed type []Age.
type Ages AgeSlice

How can an underlying type be traced given a user declared type? The rule is, when a built-in basic type or an unnamed type is met, the tracing should be stopped. Take the type declarations shown above as examples, let's trace their underlying types.

MyInt → int
Age → MyInt → int
IntSlice → []int
MyIntSlice → []MyInt → []int
AgeSlice → []Age → []MyInt → []int
Ages → AgeSlice → []Age → []MyInt → []int

In Go,

The concept of underlying type plays an important role in value conversions, assignments and comparisons in Go.

Concept: Values

An instance of a type is called a 'value' of the type. Values of the same type share some common properties. A type may have many values. One of them is the zero value of the type.

Each type has a zero value, which can be viewed as the default value of the type. The predeclared nil identifier can used to represent zero values of slices, maps, functions, channels, pointers (including type-unsafe pointers) and interfaces. For more information on nil, please read nil in Go.

There are several kinds of value representation forms in code, including literals, named constants, variables and expressions, though the former three can be viewed as special cases of the latter one.

A value can be typed or untyped.

All kinds of basic value literals have been introduced in the article href="l">basic types and basic value literals. There are two more kinds of literals in Go, composite literals and function literals.

Function literals, as the name implies, are used to represent function values. A function declaration is composed of a function literal and an identifier (the function name).

Composite literals are used to represent values of struct types and container types (arrays, slices and maps), Please read structs in Go and containers in Go for more details.

There are no literals to represent values of pointers, channels and interfaces.

Concept: Value Parts

At run time, many values are stored somewhere in memory. In Go, each of such values has a direct part. However, some of them have one or more indirect parts. Each value part occupies a continuous memory segment. The indirect underlying parts of a value are referenced by its direct part through (safe or unsafe) pointers.

The terminology value part is not defined in Go specification. It is just used in Go 101 to make some explanations simpler and help Go programmers understand Go types and values better.

Concept: Value Sizes

When a value is stored in memory, the number of bytes occupied by the direct part of the value is called the size of the value. As all values of the same type have the same value size, we often simply call this the size of the type.

We can use the Sizeof function in the unsafe standard package to get the size of any value.

Go specification doesn't specify value size requirements for non-numeric types. The requirements for value sizes of all kinds of basic numeric types are listed in the article basic types and basic value literals.

Concept: Base Type of a Pointer Type

For a pointer type, assume its underlying type can be denoted as *T in literal, then T is called the base type of the pointer type.

More information on pointer types and values can be found in the article pointers in Go.

Concept: Fields of a Struct Type

A struct type consists a collection of member variable declarations. Each of the member variable declarations is called a "field" of the struct type. For example, the following struct type Book has three fields, author, title and pages. 
type Book struct {
	author string
	title  string
	pages  int
}

More information on struct types and values can be found in the article structs in Go.

Concept: Signature of Function Types

The signature of a function type is composed of the input parameter definition list and the output result definition list of the function.

The function name and body are not parts of a function signature. Parameter and result types are important for a function signature, but parameter and result names are not important.

Please read functions in Go for more details on function types and function values.

Concept: Methods and Method Set of a Type

In Go, some types can have methods. Methods can also be called member functions. The method set of a type is composed of all the methods of the type.

Concept: Dynamic Type and Dynamic Value of an Interface Value

Interface values are values whose types are interface types.

Each interface value can box a non-interface value in it. The value boxed in an interface value is called the dynamic value of the interface value. The type of the dynamic value is called the dynamic type of the interface value. An interface value boxing nothing is a zero interface value. A zero interface value has neither a dynamic value nor a dynamic type.

An interface type can specify zero or several methods, which form the method set of the interface type.

If the method set of a type, which is either an interface type or a non-interface type, is the super set of the method set of an interface type, we say the type implements the interface type.

For more about interface types and values, please read interfaces in Go.

Concept: Concrete Value and Concrete Type of a Value

For a (typed) non-interface value, its concrete value is itself and its concrete type is the type of the value.

A zero interface value has neither concrete type nor concrete value. For a non-zero interface value, its concrete value is its dynamic value and its concrete type is its dynamic type.

Concept: Container Types

Arrays, slices and maps can be viewed as formal container types.

Sometimes, string and channel types can also be viewed as container types informally.

Each value of a formal or informal container type has a length.

More information on formal container types and values can be found in the article containers in Go.

Concept: Key Type of a Map Type

If the underlying type of a map type can be denoted as map[Tkey]T, then Tkey is called the key type of the map type. Tkey must be a comparable type (see below).

Concept: Element Type of a Container Type

The types of the elements stored in values of a container type must be identical. The identical type of the elements is called the element type of the container type.

Concept: Directions of Channel Types

Channel values can be viewed as synchronized first-in-first-out (FIFO) queues. Channel types and values have directions.

More information on channel types and values can be found in the article channels in Go.

Fact: Types Which Support or Don't Support Comparisons

Currently (Go 1.24), Go doesn't support comparisons (with the == and != operators) for values of the following types:

Above listed types are called incomparable types. All other types are called comparable types. Compilers forbid comparing two values of incomparable types.

Note, incomparable types are also called uncomparable types in many articles.

The key type of any map type must be a comparable type.

We can learn more about the detailed rules of comparisons from the article value conversions, assignments and comparisons in Go.

Fact: Object-Oriented Programming in Go

Go is not a full-featured object-oriented programming language, but Go really supports some object-oriented programming elements. Please read the following listed articles for details:

Fact: Generics in Go

Before Go 1.18, the generic functionalities in Go were limited to built-in types and functions. Since Go 1.18, custom generics has already been supported. Please read the generics in Go article for built-in generics and the Go Generics 101 book for custom generics.

(more articles ↡)

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