Golang：在Go中输入By吗？

Question

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~!

Answer 1

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.