Type definitions
- Go course
- Beginner
- Lesson 23/90
- 4 min
Learn to define new types with the type keyword.
Type definitions
The type keyword creates a new, named type from an existing one. The new type holds the same data but the compiler treats it as a separate type:
type Celsius float64
type Fahrenheit float64
var c Celsius = 25
var f Fahrenheit = 77
// c + f → compile error: different typesThis catches mixed-up units at compile time.
Adding methods and aliases
You can add methods to your own type; this is the real power of type definitions:
func (c Celsius) Fahrenheit() Fahrenheit {
return Fahrenheit(c*9/5 + 32)
}
fmt.Println(Celsius(100).Fahrenheit()) // 212type Ad = string (with an equals sign) isn't a new type, just an alias; byte and rune are defined this way.
Practice Temperature type
Define type Celsius float64 and type Fahrenheit float64. Compute c*9/5 + 32 with the method func (c Celsius) Fahrenheit() Fahrenheit and print Celsius(25).Fahrenheit().
Starter code
package main
import "fmt"
func main() {
// buraya yaz
}Sample solution
package main
import "fmt"
type Celsius float64
type Fahrenheit float64
func (c Celsius) Fahrenheit() Fahrenheit {
return Fahrenheit(c*9/5 + 32)
}
func main() {
fmt.Println(Celsius(25).Fahrenheit())
}Expected output
77
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 After <code>type Yas int</code>, why doesn't <code>var y Yas = 5; var n int = y</code> compile?
- Yas and int are different types; int(y) is needed Correct answer
- 5 can't be a Yas
- Yas isn't a number
- It compiles
Explanation: There is no implicit assignment between named types; even with the same underlying type, an explicit conversion is written.
2 New type
____ Metre float64
var uzunluk Metre = 3.5type Metre float64
var uzunluk Metre = 3.5Explanation: The type keyword defines a new, named type.
3 Definition and meaning
| Term | Meaning |
|---|---|
type Celsius float64 |
A new type |
type Ad = string |
An alias |
Celsius(25) |
A conversion |
func (c Celsius) |
A method |