Because nil is a special thing in Go. nil is not a value, it's a predeclared identifier. it represents zero value for pointers, interfaces, maps, slices, chan

Because nil is a special thing in Go. nil is not a value, it's a predeclared identifier. it represents zero value for pointers, interfaces, maps, slices, channels and function types, representing an uninitialized value len(nil) feels like it should work if you think of nil as the same as "value of empty array" but what should be: var p *Struct len(p) ??? There's no "length of zero-valued pointer".