Why does Go have typed nil?

It sounds like you’re asking about this error message:

http://play.golang.org/p/h80rmDYCTI

package main

import "fmt"

type A struct {}
type B struct {}

func (a *A) Foo() {
    fmt.Println("A")
}

func (b *B) Foo() {
    fmt.Println("B")
}

func main() {
    n := nil
    n.Foo()
}

This prints:

prog.go:17: use of untyped nil
 [process exited with non-zero status]

In that example, should the program print “A” or “B”?

You have to help the compiler decide. The way you do that is by specifying the type of n.

For example:

http://play.golang.org/p/zMxUFYgxpy

func main() {
    var n *A
    n.Foo()
}

prints “A”.

In other languages, n.Foo() might crash immediately if n is nil or its equivalent. Go’s language designers decided to let you determine what should happen instead. If you access the pointer without checking for nil, you get the same behavior as in other languages.