Why does Go have typed nil?

Question

Why does Go have typed nil? It throws an explicit interface conformation check for convenience. What's the problem of untyped nil and what did the designers want to solve with typed nil?

Answer 1

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.

Answer 2

This is due to type safety. nil is actually the value of uninitialized variables in Go. The nil values for slices, maps, functions, channels, pointers and interfaces are not the same type, and not comparable. See The language spec for more details.

EDIT: As pointed out by @newacct the correct technical term for this is the "zero value" for the type:

When memory is allocated to store a value, either through a declaration or a call of make or new, and no explicit initialization is provided, the memory is given a default initialization. Each element of such a value is set to the zero value for its type: false for booleans, 0 for integers, 0.0 for floats, "" for strings, and nil for pointers, functions, interfaces, slices, channels, and maps.

Playground example

There is also some information regarding nil interfaces and errors at Why is my nil error value not equal to nil? in the Go FAQ.