https://github.com/ronna-s/go-ood/ [Clone me!]
This workshop is aimed to clarify the OOP features that Go provides. It is named A Path to OOD and not OOP because different language features mean different design concepts.
All exercises can be completed using the go tool, podman or a combination podman and the make tool. If you are planning to use the go tool directly you can skip this step.
If planning to use podman and you don't have the make tool, run:
podman machine init
podman machine start
podman build . -t go-oodIf you have the make tool and podman, run:
make buildThe term Object-Oriented Programming means different things to different people (and we will learn about that), but first, here's what we can all agree on: The central idea behind Object-Oriented is to divide software into "things" or "objects" or "instances" that communicate via "messages" or "methods" or "member functions". Or in short, combining data and functionality. This core idea has not changed in the 4-5+ decades since it was conceptualized. It is meant to allow the developer to build code and separate responsibilities or concerns just like in the real world which is what we are familiar with and how we generally think and solve problems.
To understand the benefits of OO, we are going to help a gopher get out of a maze, without knowing anything about how the maze is implemented (or the gopher).
*This exercise is heavily inspired by the Intro to CS first home assignment that Prof. Jeff Rosenschein gave my Intro to CS class in 2003.
To get a sense of what strong OOP can do for us, solve a maze given a Gopher that can perform 4 actions:
// Gopher is an interface to an object that can move around a maze
type Gopher interface {
Finished() bool // Has the Gopher reached the target cell?
Move() error // The Gopher moves one step in its current direction
TurnLeft() // The Gopher will turn left
TurnRight() // The Gopher will turn right
}Find the function SolveMaze(g Gopher) in cmd/maze/maze.go and implement it.
# go tool
go test github.com/ronna-s/go-ood/cmd/maze
# make + podman (linux, mac)
make test-maze
# podman directly (linux, mac)
podman run -v $(pwd):/root --rm -it go-ood go test github.com/ronna-s/go-ood/cmd/maze
# podman on windows + powershell
podman run -v $(PWD):/root --rm -it go-ood go test github.com/ronna-s/go-ood/cmd/maze
# podman on windows without powershell
podman run -v %cd%:/root --rm -it go-ood go test github.com/ronna-s/go-ood/cmd/mazeThe test checks for very basic navigation. You can also check what your code is doing by running:
# go tool
go run cmd/maze/maze.go > tmp/maze.html
# make + podman
make run-maze > tmp/maze.html
# any other setup with podman
[podman command from before up to go-ood] go run cmd/maze/maze.go > tmp/maze.html Open tmp/maze.html file in your browser to see the results of your code. You can run the app multiple times to see your gopher running through different mazes.
Done? If not, don't worry. You have the entire conference ;)
This section is meant to highlight most of the functionality that supports OO in Go.
The following code defines a new type A with the underlying type bool.
It then defines a new type B with the underlying type A.
We can instantiate the variable a (of type A) with false (a constant) but to convert a to type B, we have to be explicit because they are different types.
package main
import "fmt"
type A bool
type B A
func main() {
var a A = false
var b B = !B(a) //try changing this to: var b B = a
fmt.Println(a, b)
}We can add methods to any type in Go using receivers. This is how we add a method using a value receiver.
package main
import "fmt"
type A int
func (a A) Zero() bool {
return a == 0
}
func main() {
var a A
fmt.Println(a.Zero())
}We can create more complex types using structs.
package main
import "fmt"
type Person struct {
Name string
Hobbies []Hobby
}
type Hobby string
func main() {
p := Person{Name: "Batman", Hobbies: []Hobby{"Vigilantism"}}
fmt.Println("Name:", p.Name, "Hobbies :", p.Hobbies)
}If your method requires changing the value of the receiver (outside the method), we must use a pointer. From A Tour of Go
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func main() {
v := Vertex{3, 4}
v.Scale(10)
fmt.Println(v.Abs())
}This is NOT what is referred to as pass by value and pass by reference. That terminology is used to describe languages with constructors and particularly copy constructors. In Go there is no copy constructor but de facto everything is passed by value and something is always copied. The difference is, that if you use a pointer explicitly (and pointers are explicit in Go), what is copied is the address of the value, so the memory is still accessible.
Teaser: what will the following code do?
package main
import "fmt"
type A struct{}
func (a *A) Foo() string {
return "Hi from foo"
}
func main() {
var a *A //a is nil, the zero value of the pointer
fmt.Println(a)
fmt.Println(a.Foo())
}This code worked because under the hood, the method a.Foo() is just sugar syntax to the function Foo on the Type level that takes the receiver as a first parameter.
//https://go.dev/play/p/zVtRx_mX2rq
package main
import "fmt"
type A struct{}
func (a A) Foo() int {
return 1
}
func main() {
var a A
fmt.Println(a.Foo())
fmt.Println(A.Foo(a)) //exactly the same
}You can define an alias to any type, but what does alias mean?
package main
import "fmt"
type A int
type B = A
func (b B) Foo() int {
return int(b)
}
func main() {
var a A = 5
fmt.Println(a.Foo())
var b B
fmt.Printf("a:%T b:%T\n", a, b)
}One of Go's strongest features is the Interfaces. Go interfaces are implicit and therefore we can plug into them any code that we found on the web anywhere. This also means that a package does not need to provide interfaces to its clients, only for its own dependencies when necessary.
From "A Tour of Go"
package main
import "fmt"
type I interface {
M()
}
type T struct {
S string
}
// This method means type T implements the interface I,
// but we don't need to explicitly declare that it does so.
func (t T) M() {
fmt.Println(t.S)
}
func main() {
var i I = T{"hello"}
i.M()
}When type B embeds type A we say that type B is composed of type A.
Upon embedding, the methods of A become available to B, we call this "promotion".
package main
import "fmt"
type A int //Creates a new type A with an underlying type int
// Foo is now a method of my A
func (a A) Foo() int {
return int(a)
}
type B struct {
// B embeds A so B now has method Foo()
A
}
func (b B) Bar() int {
return int(b.A)
}
type I interface {
Foo() int
}
// to implement J we have to provide implementation for Foo() and Bar()
type J interface {
I
Bar() int
}
func main() {
var j J = B{1}
fmt.Println(j.Foo()) // 1
fmt.Println(j.Bar()) // 1
}We can embed as many types as we want.
The empty interface interface{} (now also comes as the built-in alias any), defines an interface with no methods, and therefore requires no methods.
This is why any type (including primitive types that have no method) can be passed around as any or interface{}.
We can check at run time if an interface implements another interface and convert it.
package main
import "fmt"
type A struct{}
func (a *A) Foo() string {
return "Hi from foo"
}
type I interface {
Foo() string
}
func main() {
var a *A //a is nil
var b interface{} = a
if val, ok := b.(I); ok {
fmt.Println(val.Foo())
} else {
fmt.Println("val doesn't implement I")
}
}Now that we have the basics we can go back and understand the code.
Let's review the code that made this possible and examine the Go features it uses.
Run:
# make tool + podman
make godoc
# using podman
podman run --rm -p 8080:8080 go-ood godoc -http=:8080
# or, install godoc and run
go install golang.org/x/tools/cmd/godoc@v0.1.12
godoc -http=:8080 #assuming $GOBIN is part of your path. For help run `go help install`The repo started with one package in the pkg directory called maze which offers a basic maze generator and nothing else. Go to: http://127.0.0.1:8080/pkg/github.com/ronna-s/go-ood/pkg/maze
The package defines 5 types:
- Cell - an alias type to int
- Coords - a new type defined as a pair of integers (an array of 2 ints)
- Direction - a new type with an underlying type int (enum)
- Maze - a generated 2D maze that is a struct
- Wall - a struct that holds 2 neighboring cells
We see that:
- There are no constructors in Go (since there are no classes), but we can create functions that serve as constructors.
- The godoc tool identified our constructor function New and added it under the Maze type.
- We have structs and they can have fields.
- We can define a new type out of any underlying type.
- Any type can have methods (except for primitives).
- That means that any type satisfies the interface{} - an interface with no methods.
- You can alias to any type.
- If you want to add methods to primitives, just define a new type with the desired primitive underlying type.
- Methods are added to types using Receivers (value or pointer receivers).
- Methods that can change/mutate the value of the type need a pointer receiver (the common practice says not to mix receiver types).
Let's proceed to examine the maze code, navigate around to see the travel package, then the robot package and finally the main package in cmd/maze
That package defines the interface that abstracted away our robot.Robot struct into the Gopher interface. This ability that Go provides is not common.
The common OOP languages approach is that class A must inherit from class B or implement interface I in order to be used as an instance of B or I, but our Robot type has no idea that Gopher type even exists. Gopher is defined in a completely different package that is not imported by robot. Go was written for the 21st century and allows you to plug-in types into your code from anywhere on the internet so long that they have the correct method signatures.
Scripting languages achieve this with duck-typing, but Go is type-safe and we get compile time validation of our code. Implicit interfaces mean that packages don't have to provide interfaces to the user, the user can define their own interface with the smallest subset of functionality that they need.
In fact our robot.Robot has another public method Steps that is not part of the Gopher interface because we don't need to use it.
This makes plugging-in code and defining and mocking dependencies safely a natural thing in Go and makes the code minimal to its usage.
In conclusion: before you write code make sure it's necessary. Be lazy. Be minimal. Be Marie Kondo.
The problem with object-oriented languages is they've got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle. (Joe Armstrong)
Just like in the real world, wherever there are things, there can be a mess. That's why Marie Kondo is rich. Just as you can write insane procedural code, you can write sane OO code. You and your team should define best practices that match your needs. This workshop is meant to give you the tools to make better design choices.
It is important to know that in common OOP languages:
- Objects are instances of a class because only classes can define methods (that's how we support messaging).
- Classes have constructor methods that allow for safe instantiation of objects.
- Classes can inherit methods and fields from other classes as well as override them and sometimes overload them (we will get to that later).
- In case of overriding and overloading methods, the method that will eventually run is decided at runtime. This is called late binding or dynamic binding.
Go doesn't offer classes, which means there are no constructors (or destructors) and no inheritance, etc. There is also no late or late late or late late late binding in Go (but there's something else, we'll get to that). These are technical concepts that have become synonymous with Object-Oriented Programming. Go does have a variety of very strong features for Object-Oriented Programming that enable Gophers to express their code in a manner that follows the OO principals. In the end, the answer to the question is Go an OOP language depends on the answer to the question "is t an object" in this sample code
package main
import "fmt"
type MyThing int //Creates a new type MyThing with an underlying type int
// Foo is now a method of my MyThing, in many languages to have a method you have to have a class or a struct
func (t MyThing) Foo() int {
return int(t)
}
func main() {
var t MyThing = 1
fmt.Println(t.Foo()) // Q: is t an object?
}Whether you think t is an object or not, no gopher is complete without all the tools in the gopher toolbox so let's get (re)acquainted with them.
Go doesn't provide us constructors that ensure that users of our types initialize them correctly, but as we saw, we can provide our own ctor function to make our types easy to use. Developers coming from other language often make types and fields private to ensure that users don't make mistakes. If your type is not straight-forward, the Go common practices are:
- Provide a CTOR function.
- The CTOR should return the type that works with all the methods properly so if the type has methods with pointer receivers it will likely return a pointer.
- Leave things public, comment clearly that the zero value of the type is not ready to use or should not be copied, etc.
- Provide default functionality when a required field is zero value.
When you do it right, the godoc will nest your CTOR function inside your type.
In software, if A inherits from B, it's the code equivalent to saying A is B. On the other hand, we use composition to express that A is made of B. In Go, we don't have inheritance, but we have unlimited composition. Since we don't have inheritance, to express that A is I we use interfaces. To express that A is made of B or composed of B we use embedding.
The difference between inheritance and composition can be seen here.
Most OO languages limit inheritance to allow every class to inherit functionality from exactly one other class. That means that you can't express that an instance of class A is an instance of class B and class C, for example: a truck can't be both a vehicle and also a container of goods. In the case where you need to express this you will end up doing the same as you would do in Go with interfaces, except as we saw the Go implicit interface implementation is far more powerful. In addition, common language that offer inheritance often force you to inherit from a common Object class which is why objects can only be class instances (and can't be just values with methods, like in Go).
Alan Kay is considered to be the person who coined the term Object-Oriented, and he applied his ideas about OO to his language Smalltalk. In his mind, an object should take in messages and check if it can handle it or not at runtime, a concept very close to duck-typing and extreme late binding. He later criticized heavily C++ and Java for going in a very different direction to this.
Using interface type assertion (or conversion), we can check at runtime if a type has a method (or a set of method) of the exact signature we would like to invoke, and call it. The result of this check is cached for performance.
In addition, the receiver (used to define methods) is inspired by Oberon-2 which is an OO version of Oberon.
Yes and no. Although Go has types and methods and allows an object-oriented style of programming, there is no type hierarchy. The concept of “interface” in Go provides a different approach that we believe is easy to use and in some ways more general. There are also ways to embed types in other types to provide something analogous—but not identical—to subclassing. Moreover, methods in Go are more general than in C++ or Java: they can be defined for any sort of data, even built-in types such as plain, “unboxed” integers. They are not restricted to structs (classes).
Also, the lack of a type hierarchy makes “objects” in Go feel much more lightweight than in languages such as C++ or Java.
Go is object-oriented, even though it doesn't have the notion of a class. The type system is more general. Any type—even basic types such as integers and strings—can have methods. This allows inheritance and other object-oriented techniques to apply more broadly than with classes alone. For instance, Go's formatted printing library, package fmt, uses interfaces and methods to provide a way to use a printf-like API to print any value, ranging from basic types to arbitrary user-defined objects, with perfect type safety.
We are going to add 2 types of players to the game P&P - Platforms and Programmers who will attempt to take on a Production environment.
The roles that we will implement are pnpdev.Gopher, pnpdev.Rubyist.
The player roles are going to be composed of the struct pnpdev.Character for common traits like XP and Health.
Gopher and Rubyist will also need to implement their own methods for their individual Skills and AsciiArt.
Run the game with the minion player:
# go tool
go run cmd/pnp/pnp.go
# make + podman
make run-pnp
# any other setup with podman
[podman command from before] go run github.com/ronna-s/go-ood/cmd/pnp.go// Player represents a P&P player
type Player interface {
Alive() bool
Health() int
XP() int
ApplyXPDiff(int) int
ApplyHealthDiff(int) int
Skills() []Skill
Art() string
}We already have a type Minion in package pkg/pnpdev with some implementations for a player.
- We will extract the methods
Alive,ApplyXPDiff,ApplyHealthDiff,HealthandXPto a common typeCharacter. - We will embed
CharacterinsideMinion. - Create new types
GopherandRubyistthat implement thepnp.Playerinterface, use the failing tests to do this purposefully, see how to run the tests below. - Add
NewGopher()andNewRubyist()in cmd/pnp/pnp.go to our list of players. - Run the game.
- We notice that the Gopher and the Rubyist's names are not properly serialized... We will fix that in a moment.
To test our players:
# make + podman
make test-pnp
# go tool
go test github.com/ronna-s/go-ood/pkg/pnpdev
# any other setup with podman
[podman command from before] go test github.com/ronna-s/go-ood/pkg/pnpdevWe are not done with exercise 2.
As we saw our Rubyist and Gopher's name were not displayed properly.
We fix this by adding the String() string method to them:
func (r Rubyist) String() string {
return "Rubyist"
}
func (g Gopher) String() string {
return "Gopher"
}We run the game and see that it works as expected but what actually happened here? - String() is not part of the Player interface?
We can check if a type implements an interface at runtime:
https://go.dev/play/p/6Ia8aGJS7Bc
package main
import "fmt"
type fooer interface {
Foo() string
}
type A struct{}
func (_ A) Foo() string {
return "Hello from A"
}
func main() {
var a interface{} = A{}
var i interface{} = 5
if v, ok := a.(fooer); ok {
fmt.Println(v.Foo())
} else {
panic("should not be called")
}
if v, ok := i.(fooer); ok {
panic("should not be called")
} else {
fmt.Println("v is nil:", v)
}
}Go's print function checked at runtime if our types have the method String() string by checking if it implements an interface with this method and then invoked it.
Russ Cox compared this to duck typing and explained how it works here.
It's particularly interesting that this information about what types implement what interfaces is cached at runtime to maintain performance. Even though we achieved this behavior without actual receivers that take in messages and check if they can handle them, from design perspective we achieved a similar goal.
This feature only makes sense when interfaces are implicit because in languages when the interface is explicit there's no way a type can suddenly implement a private interface that is used in our code.
If you are going to add your own type assertions, remember that the code execution becomes unpredictable, you therefore should comply with a couple of rules:
- The user of your code might not know what interfaces they are expected to implement or might provide them but cause a panic. Provide default behavior and in addition use
deferandrecoverto prevent crashing the app or return errors if the interface allows it. - If your type is expected to implement an interface, to protect against changes add a line to your code that will fail to compile if your type doesn't implement the interface, like so:
// In the global scope directly
var _ interface{ String() string } = NewGopher()
var _ interface{ String() string } = NewRubyist()Whether you choose the common structures with cmd, pkg, etc. you should probably define some guidelines for your team. Here are a few suggestions:
- Support multiple binaries: Your packages structure should allow compiling multiple binaries (have multiple main packages that should be easy to find).
- An inner package is usually expected to extend the functionality of the upper package and import it (not the other way around), for example:
net/httpimage/draw- and the example in this repo
maze/travel
- There are some exceptions to this for instance
image/coloris a dependency forimage, but it's not the rule. In an application it's very easy to have cyclic imports this way. - A package does not provide interfaces except for those it uses for its dependencies.
- Use godoc to see what your package looks like without the code. It helps.
- Keep your packages' hierarchy relatively flat. Just like your functions, imports don't do spaghetti well.
- Try to adhere to open/close principals.
- Your packages should describe tangible things that have clear boundaries - domain, app, utils, aren't those things.
- Package path with
internalcannot be imported. It's for code that you don't want to allow to import, not for your entire application. It's especially useful for anyone using your APIs to be able to import your models for instance.
I like this simple explanation by (Gabriele Tomassetti)[https://tomassetti.me/code-generation/]
The reasons to use code generation are fundamentally four:
- productivity;
- simplification;
- portability;
- consistency
It's about automating a process of writing repetitive error-prone code. Code generation is similar to meta-programming but we compile it and that makes it safer to run. Consider the simple stringer Consider Mockery Both were used to generate code for this workshop.
Also, as a favour to me, please commit your generated code. A codebase is expected to be complete and runnable.
Default variables are very useful Singleton values. Examples: net/http.
// https://go.dev/play/p/8hiAeuJ90uz
package main
import (
"errors"
"fmt"
"net/http"
)
func main() {
http.ErrBodyNotAllowed = errors.New("my error")
fmt.Println(http.ErrBodyNotAllowed)
}Generics are not specifically related to Object-Oriented, but generics do enable more flexible work with types, which benefits OO.
It was a long time consensus that "real gophers" don't need generics, so much so that around the time the generics draft of 2020 was released, many gophers still expressed that they are not likely to use them.
Let's understand first the point that they were trying to make.
Consider this code, made using C++.
We see here generic code (templates) that allows an event to add functions (listeners) to its subscribers.
Let's ignore for a second that this code adds functions, not objects and let's assume it did take in objects with the function Handle(e Event).
We don't need generics in Go to make this work because interfaces are implicit. As we saw already in C++ an object has to be aware of it's implementations, this is why to allow plugging-in of functionality we have to use generics in C++ (and in Java).
In Go this code would look something like this:
package main
import "fmt"
type Listener interface {
Handle(Event)
}
type Event struct {
Lis []Listener
}
func (e *Event) Add(l Listener) {
e.Lis = append(e.Lis, l)
}
func main() {
var l Listener
var e Event
e.Add(l)
fmt.Println(e)
}We didn't need generics at all!
However, there are cases in Go where we have to use generics and until recently we used code generation for. Those cases are when the behavior is derived from the type or leaks to the type's behavior:
For example: The linked list
// https://go.dev/play/p/ZpAqvVFAIDZ
package main
import "fmt"
type Node[T any] struct { // any is builtin for interface{}
Value T
Next *Node[T]
}
func main() {
n1 := Node[int]{1, nil}
n2 := Node[int]{3, &n1}
fmt.Println(n2.Value, n2.Next.Value)
}Example 2 - Addition
package main
import "fmt"
type A int
// Add takes any type with underlying type int
func Add[T ~int](i T, j T) T {
return i + j
}
func main() {
var i, j A
fmt.Println(Add(i, j))
}Of course, you might not be likely to use linked lists in your day to day, but you are likely to use:
- Repositories, databases, data structures that are type specific, etc.
- Event handlers and processors that are specific to a type.
- The concurrent map in the sync package which uses the empty interface.
- The heap
The common thread to these examples is that before generics we had to trade generalizing certain behavior for type safety (or generate code to do so), now we can have both.
Implement a new generic Heap OOP style in pkg/heap (as usual failing tests provided).
The heap is used by TOP OF THE POP! cmd/top to print the top 10 Artists and Songs.
Test:
# go tool
go test github.com/ronna-s/go-ood/pkg/heap
# make + podman
make test-heap
# any other setup with podman
[podman command from before] go test github.com/ronna-s/go-ood/pkg/heapRun our TOP OF THE POP app:
# go tool
go run cmd/top/top.go
# make + podman
make run-heap
# any other setup with podman
[podman command from before] go run cmd/top/top.goWhat we've learned today:
- The value of OOP
- Defining types that fit our needs
- Writing methods
- Value receivers and pointer receivers
- Organizing packages
- Interfaces
- Composition
- Generics
- To generate code otherwise