Type Compositions

Go 1 doesn't support user-defined generic types. Go only supports generic for first-level citizen composite types in Go. We can use composite types to create infinite custom types by using all kinds of first-level citizen types in Go.

Following of this article will show type composition examples and explain how to read these composited types.

Type Composition Examples

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,

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

The Current Status Of The Built-in Generic Functionalites In Go

Besides the built-in generic for composite types, there are several built-in functions also support generic. Such as the built-in len function can be used to get the length of values of arrays, slices, maps, strings and channels. Generally, the functions in the unsafe standard package are also viewed as built-in functions.

Go doesn't support generic for custom types and functions, at least for Go 1. This really brings some inconveniences in Go programming sometimes. For example, the types of the arguments and results of most functions in the math standard package are float64. When we want to use these functions on values of other kinds of numeric types, we must first convert the values to float64 values as arguments, and we must convert the float64 results back to the original numeric types, which is not only inconvenient, but also is not efficient.

Luckily, the scenarios requiring custom general types and functions are not many in Go programming. If we do need custom generic, currently, we can use code generating for compile-time generic and use reflections (including type assertion and type switch) for run-time generic.

The future Of Generic In Go

Go language design and development team wouldn't mind supporting generic feature in Go, it is just that they haven't found a generic solution which will keep Go simple and clean yet. So, it is (very) possible that Go 2 will support custom generic.

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