Overview of Go Type System

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

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.

Concept: Composite Types

Go supports the following composite types:
Composite types may be denoted as their respective type literals. Following are some literal representation examples of all kinds of composite types.
// 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 1.12), 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 the next section.)

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.

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,
  • a new defined type and its respective source type in type definitions are two distinct types.
  • two types defined in two type definitions are always two distinct types.
  • the new defined type and the source type will share the same underlying type (see below for what are underlying types), and their values can be converted to each other.
  • types can be defined within function bodies.
Some type definition examples:
// The following new defined and source types
// are all basic types.
type (
	MyInt int
	Age   int
	Text  string
)

// The following new defined and source types are
// all composite types.
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}
}

Syntax: Type Alias Declarations

(Type alias declaration is one new kind of type declarations added since Go 1.9.)

As above mentioned, there are only two built-in type aliases in Go, byte (alias of uint8) and rune (alias of int32). They are the only two type aliases before Go 1.9.

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 declaration.
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 names and literals:
  • Age and int
  • table and map[string]int
  • Table and map[Name]Age

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

Note, although aliases table and Table denote the same type, Table is exported so it can be used by other packages but this does not apply to table.

Concept: Defined Types vs. Non-Defined Types

A defined type is a type defined in a type definition.

All basic types are defined. A non-defined type must be a composite type.

In the following example. type alias C and type literal []string both represent the same non-defined types, but type A and type alias B both represent the same defined type.
type A []string
type B = A
type C = []string

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 non-defined 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, unsafe.Pointer or a non-defined type is met, the tracing will be stopped. Take the type declarations 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. A type may have many values, one of them is the zero value of the type. Values of the same type share some common properties.

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 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 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 call the same value size of a type as 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 "field" of the struct type. For example, the following struct type Book has three fields, author, title and pages.
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: Method 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 the 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 container type has a length, either that container type is a formal one or an informal one.

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 1.12), Go doesn't support comparisons (with the == and != operators) between 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 as 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 styles. Please read the following listed articles for details:

Fact: Generics in Go

Up until now (Go 1.12), the generic functionalities in Go are limited to built-in types and functions. Custom generics are still in draft phase now. Please read built-in generics in Go for details.


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