Type Compositions

Type compositions in Go are designed very intuitive and easy to interpret. It is hardly to get lost in understanding Go composite types, even if for some very complex ones. Following are several type composistion examples, from simpler ones to more complex ones.

Let's view an simple composite type literal.

[3][4]int

When interpreting a composite type, we should look at it from left to right. The [3] on the left in the above type literal indicates that this type is an array type. The whole right part following the [4]int is another array type, which is the element type of the first array type. The first array type can be viewed as a two-dimention array type.

An example on using this two-dimention array type.

package main

import (
	"fmt"
)

func main() {
	matrix := [3][4]int{
		{1, 0, 0, 1},
		{0, 1, 0, 1},
		{0, 0, 1, 1},
	}
	
	matrix[1][1] = 3
	a := matrix[1] // type of a is [4]int
	fmt.Println(a) // [0 3 0 1]
}

Similarly,

• [][]string is a slice type whose element type is another slice type []string.
• **bool is a pointer type whose base type is another pointer type *bool.
• chan chan int is a channel type whose element type is another channel type chan int.
• map[int]map[int]string is a map type whose element type is another map type map[string]int. The key types of the two map types are both int.
• func(int32) func(int32) is a function type whose only return result type is another function type func(int32). The two function types both have only one input parameter with type int32.

Let's view another type.

chan *[16]byte

The chan keyword at the left most indicates this type is a channel type. The whole right part *[16]byte, which is a pointer type, denotes the element type of this channel type. The base type if the pointer type is [16]byte, which is an array type. The element type of the array type is byte.

An example on using this channel type.

package main

import "fmt"
import "time"
import "crypto/rand"

func main() {
	c := make(chan *[16]byte)
	
	go func() {
		// Use double arrays to avoid data racing.
		var dataA, dataB = new([16]byte), new([16]byte)
		for {
			_, err := rand.Read(dataA[:])
			if err != nil {
				close(c)
			} else {
				c <- dataA
				dataA, dataB = dataB, dataA
			}
		}
	}()
	
	for data := range c {
		fmt.Println((*data)[:])
		time.Sleep(time.Second / 2)
	}
}

Similarly, type map[string][]func(int) int is a map type. The key type of this map type is string. The remaining right part []func(int) int denotes the element type of the map type. The [] indicates the element type is a slice type, whose element type is a function type func(int) int.

An example on using the just explained map type.

package main

import "fmt"

func main() {
	plusone := func(x int) int {return x + 1}
	square := func(x int) int {return x * x}
	double := func(x int) int {return x + x}
	
	transforms := map[string][]func(int) int {
		"increase,increase,increase": {plusone, plusone, plusone},
		"square,increase,double": {square, plusone, double},
		"double,square,square": {double, double, square},
	}
	
	for _, n := range []int{2, 3, 5, 7} {
		fmt.Println(">>>", n)
		for name, transfers := range transforms {
			result := n
			for _, xfer := range transfers {
				result = xfer(result)
			}
			fmt.Printf("   %v: %v \n", name, result)
		}
	}
}

Below is a type which looks some complex.

[]map[struct {
	a int
	b struct {
		x string
		y bool
	}
}]interface {
	Build([]byte, struct{a int; b struct {x string; y bool}}) error
	Update(dt float64)
	Destroy()
}

Let's read it from left to right. The starting [] at the left most indicates this type is a slice type. The following map keyword shows the element type of the slice type is a map type. The struct type denoted by the struct literal enclosed in the [] following the map keyword is the key type of the map type. The element type of the map type is an interface type which specifies three methods. The key type, a struct type, has two fields, one field a is of int type, and the other field b is of another struct type struct {x string; y bool}.

To get a better readibility, we often decomposite such a type into multiple type declarations. The type alias T declared in the following code and the just explained type above denote the identical type.

type B = struct {
	x string
	y bool
}

type K = struct {
	a int
	b B
}

type E = interface {
	Build([]byte, K) error
	Update(dt float64)
	Destroy()
}

type T = []map[K]E