Implicit satisfaction

Learn that Go interfaces are satisfied implicitly.

Implicit satisfaction

In Go a type satisfies an interface implicitly: there is no implements keyword. If it has the required methods, the type is that interface:

type Ordek struct{}
func (Ordek) Ses() string { return "vak" }

type Robot struct{}
func (Robot) Ses() string { return "bip" }

for _, s := range []Sesli{Ordek{}, Robot{}} {
    fmt.Println(s.Ses())
}

This is called "duck typing", but in Go it is checked at compile time.

Compile-time check

To guarantee that a type satisfies an interface, an empty assignment is written; if a method is missing, compilation fails:

var _ Sesli = Ordek{}      // Ordek must satisfy Sesli
var _ Sesli = (*Robot)(nil) // for pointer-receiver methods

If a method is defined with a pointer receiver, only *T satisfies the interface; a T value doesn't.

Exercise Sound-making types

Write the Sesli interface (Ses() string) and the types Ordek ("vak") and Robot ("bip"). Add var _ Sesli = Ordek{} as a compile-time check and print both types' sounds in order.

Show example solution

package main

import "fmt"

type Sesli interface{ Ses() string }

type Ordek struct{}
type Robot struct{}

func (Ordek) Ses() string { return "vak" }
func (Robot) Ses() string { return "bip" }

var _ Sesli = Ordek{}

func main() {
    for _, s := range []Sesli{Ordek{}, Robot{}} {
        fmt.Println(s.Ses())
    }
}

Test yourself

What is the biggest advantage of implicit satisfaction?
- It runs faster
- No methods need to be written
- Mistakes are impossible
- Another package's type can satisfy your interface without that package changing
Show answer

Another package's type can satisfy your interface without that package changing — The consuming side defines the interface; standard types such as os.File satisfy it without their authors knowing about it.

Satisfaction check

var ____ Sesli = Ordek{}

Show answer

var _ Sesli = Ordek{}

Concept

implements

Not in Go
Implicit satisfaction

Methods are enough
var _ I = T{}

Compile-time check
*T receiver

Only *T satisfies

Next lesson: The error interface

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