Personally, in a classroom context, I'd avoid the term "untyped nil" (or "typed nil", for that matter) for interface values - specifically *because* I find it confusing. That is, I would point out that it is sometimes used that way (so people don't get confused if they hear it), but insist on the parlance from the spec:
- An interface value can be nil, in which case it has no dynamic type/value - An interface value can be non-nil, in which case its dynamic value might or might not be nil (depending on whether the dynamic type allows it) https://golang.org/ref/spec#Variables That is, instead of calling something a typed or untyped nil (neither of which is parlance defined by the language - "untyped nil" is an error message from the compiler, that might be more appropriately worded as "can't infer type of nil-identifier"), you talk about whether an interface value has a dynamic value and whether or not that dynamic value is nil. I will never be able to remember what an untyped nil is supposed to be (that is, which of the cases it to denotes) and I will never understand why someone came up with that, if there is already an authoritatively and unambiguously defined term :) PS: I know that Roger already mentioned that it's technically incorrect, I just wanted to make explicit what I think *is* correct and what I think ought to be taught instead. :) On Fri, Dec 20, 2019 at 9:59 AM roger peppe <rogpe...@gmail.com> wrote: > It's probably also worth saying that this is a bit more fuzzy with nil > than with other constants, because in some sense every nil interface value, > although having a static type (the interface type) is untyped in the sense > that it doesn't have a dynamic type. > > This is a common source of confusion. > > A few different kinds of nil: > > - statically untyped nil, as described by Ian - this is a > compile-time-only thing > - statically typed nil - any nil value at runtime. > - dynamically untyped nil - a value of interface type that isn't > associated with an underlying dynamic type > - dynamically typed nil - a value of interface type with an underlying > dynamic type that's nil. > > Although technically incorrect, I think "untyped nil" is often used to > refer to the third of those things. > > > > On Fri, 20 Dec 2019 at 06:53, Ian Lance Taylor <i...@golang.org> wrote: > >> On Thu, Dec 19, 2019 at 4:42 PM Victor Giordano <vitucho3...@gmail.com> >> wrote: >> > >> > Hello guys, just wanna clarify if the words "Untyped nil" refers to the >> nil value placed literally within a expresion, instead of being placed >> "behind" a variable, a constanst or a function call return value. So >> basically, can i say (in a classroom) that untyped nil means literal nil? >> > Is that correct? >> >> I wouldn't say that they are exactly the same. Go supports untyped >> constants, as described at https://golang.org/blog/constants. The >> predeclared identifier "nil" is similar to an untyped constant, except >> that it doesn't have a default type. If you write "var a *byte = nil" >> then the "nil" acquires the type "*byte" from context. But if you >> write "var a = nil" then there is type in context. Since "nil" >> doesn't have a default type, the case "var a = nil" is a use of an >> untyped nil, which is an error. >> >> So an untyped nil is the use of a literal nil when there is no type >> context. >> >> Ian >> >> -- >> You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts+unsubscr...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/CAOyqgcXv8puXkY%2BC3R_GJG6ZcNo_%2BNJuv5bPhvsn0LGr_0Wpjg%40mail.gmail.com >> . >> > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/CAJhgacjAHmbggMSotha3Yr0_O9ATv1V8GKdB65kkLCKGovwBqA%40mail.gmail.com > <https://groups.google.com/d/msgid/golang-nuts/CAJhgacjAHmbggMSotha3Yr0_O9ATv1V8GKdB65kkLCKGovwBqA%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAEkBMfGg8HZE0vTAM8W7vvKrn-fDo%2BhGT7Y87sFHssvsP2t8Tw%40mail.gmail.com.
