[go-nuts] Why need a special rule for nil map?

append returns a new slice value, map assignment returns nothing. Map
assignment is a statement, not an expression, so there is nothing to return
leaving panic as the only recourse.

Thanks for your reply, but I can't get the point.

I think(I am not really sure) a map is a piece of memory block, and there
is a pointer in the block, this pointer points to a hash table. If the map
is nil, that pointer can only be zero(nil).
I think(I am not really sure) map assignment just change the the hash
table. A map assignment to a nil map will panic because the hash table
doesn't exist, why not just create one, and assign the address of that hash
table to the map's pointer?

Pretty much, a map value is a pointer to the array of hash buckets that
hold the map's data.

var m map[string]int

x := m

m["hello"] = 42

fmt.Println(x["hello"])

What is printed ?

Thank's very much, I got something.
The code you write will panic for nil map. I think you want to write
*make(map[string]int)*, then that code will print 42, so a map is *really a
pointer to a data struct*. And a nil map M1 is a nil pointer, if we do M2
:= M1, M1 M2 will both hold the same pointer nil. *Without the special nil
map rule, M1["string"] = 42 will change M1's pointer,* and M2 still holds
the nil pointer, that will confuse people, so they choose panic.
I think that might be the reason, thanks very much!