Why does one function return true and the other false?

Question

go

Why does InitPointer() return true but InitEfaceType() returns false, and why doesn't InitType() compile at all?

package main

import (
  "fmt"
)
type S struct{}

func (s S) F() {}

func InitPointer() *S {
  var s *S
  return s
}
func InitEfaceType() interface{} {
  var s S
  return s
}
func InitType() S {
  var s S
  return s
}
func main() {
  fmt.Println(InitPointer() == nil)
  fmt.Println(InitEfaceType() == nil)
  //fmt.Println(InitType() == nil)
}

Answer 1

Because the InitEfaceType function actually returns a structure, not a pointer.
Structure cannot be nil. If you return a pointer to a structure, then you can already check for nil.
Do fmt.Printf("Type: %T\n", InitEfaceType()) and you will see the type "main.S"
To be able to check the interface for nil you can do like this:

func InitEfaceType() interface{} {
  var s *S
  return s
}
...
fmt.Println(InitEfaceType() == (*S)(nil))

complete code

package main
import (
"fmt"
)
type S struct{}
func (s S) F() {}
func InitPointer() *S {
var s *S
return s
}
func InitEfaceType() interface{} {
var s *S
return s
}
func InitType() S {
var s S
return s
}
func main() {
fmt.Println(InitPointer() == nil)
fmt.Println(InitEfaceType() == (*S)(nil))
//fmt.Println(InitType() == nil)
}

Answer 2

why is InitType() not compiling at all?

returns the type of the structure, not a pointer to it.

InitPointer() returns true

Go initializes variables, so it's nil for a pointer.

and InitEfaceType() is false

because interface{} is not nil.