This is from Golang.org http://golang.org/pkg/sort/
// By is the type of a "less" function that defines the ordering of its Planet arguments.
type By func(p1, p2 *Planet) bool
I've never seen this structure. How come func comes after type? And what is type here?
I've seen the following structures but
type aaaaaa interface { aaa() string }
type dfdfdf struct { }
Never seen like
type By func(p1, p2 *Planet) bool
How this is possible in Go? type can take other things than interface, struct keywords?
Thanks~!
type By func(p1, p2 *Planet) bool is an example of defining a type from a function value.
We can see that by creating a new By value and printing the type using fmt.Printf. In the example below I stumped out Planet as a string - the type doesn't matter for the purposes of the example.
type.go
package main
import(
"fmt"
)
type Planet string
type By func(p1, p2 *Planet) bool
func main() {
fmt.Printf("The type is '%T'", new(By))
fmt.Println()
}
Output:
mike@tester:~/Go/src/test$ go run type.go
The type is '*main.By'
EDIT: Updated per nemo's comment. The new keyword returns a pointer to the new value. func does not return a function pointer like I had incorrectly thought but instead returns a function value.
You can define a new type in go with any base type including another user defined type.
For instance if you define a new type File
type File struct {}
with some methods
func (f *File) Close() { ... }
func (f *File) Size() { ... }
You could then define a new type called:
type SpecialFile File
and define your own different methods on it.
func (f *SpecialFile) Close() { (*File)(f).Close() }
The important thing to note is the SpecialFile type doesn't have a Size method on it even though it's base type is a File. You have to cast it to a *File in order to call the Size method.
You can do this for types you don't even own if you want that aren't even in the same package.