I think I'm missing something. How is nil not an inhabitant?
On Thu, Jun 18, 2020 at 6:48 PM Bryan C. Mills <bcmi...@google.com> wrote:
>
> On Thu, Jun 18, 2020 at 4:19 PM 'Axel Wagner' via golang-nuts
> <golang-nuts@googlegroups.com> wrote:
>>
>> Addendum: In Go, every type needs to be inhabited by at least one value -
>> it's zero value. And we already have a type that can take *exactly* one
>> value, namely struct{}.
>
>
> That is true, but struct{} is the unit type — not the bottom element of the
> interface-type lattice.
> struct{} can be added as a field of any struct with no effect on
> comparability or equality, and adds no bits of information (it is a
> multiplicative unit).
> As an interface value, struct{} carries one “bit” of information: its type.
> But struct{} has no methods and is not assignable to any interface type:
> therefore, it is not the bottom type of the interface lattice.
>
> In contrast, the interface of the empty type-list, if such a thing could
> exist at run-time, would presumably have only one value as well: the nil
> interface value.
> The nil interface value is a member of every other interface type, so the
> empty type-list would be assignable to every other interface type (it is the
> bottom of the interface lattice).
> Conceptually, it has all possible methods, but there would be no point in
> calling them: they all result in a nil-panic.
> As a struct field, the empty interface is also a multiplicative unit (it adds
> no bits of information).
> However, it is also an additive unit: the union of the empty type-list and
> any other interface is identical to the other interface (because every other
> interface already admits the nil interface value).
>
>> On Thu, Jun 18, 2020, 22:13 Axel Wagner <axel.wagner...@googlemail.com>
>> wrote:
>>>
>>> These arguments would be more convincing, if Go wouldn't already reject
>>> interfaces impossible to implement: https://play.golang.org/p/dYm8js26qml
>>>
>>> On Thu, Jun 18, 2020, 17:26 Jesper Louis Andersen
>>> <jesper.louis.ander...@gmail.com> wrote:
>>>>
>>>> It is a type which cannot be inhabited by a term. These exist and often
>>>> have uses. As Bryan wrote they also completes the type lattice, so
>>>> rejecting them is often a lot of work for little gain.
>>>>
>>>> If you want examples, look up phantom types, where an uninhabited type is
>>>> used as a tag for ensuring compile time restrictions.
>>>>
>>>> On Wed, Jun 17, 2020, 22:09 jimmy frasche <soapboxcic...@gmail.com> wrote:
>>>>>
>>>>> This isn't a bug per se, but I can file one if requested.
>>>>>
>>>>> https://go2goplay.golang.org/p/AWynhg6ya7h
>>>>>
>>>>> Since embedding interfaces with type lists uses the intersection of
>>>>> the items in the type list, it's possible to create an interface that
>>>>> cannot be satisfied by any type.
>>>>>
>>>>> Currently this does not cause an error until you attempt to
>>>>> instantiate something that uses such an interface as a bound.
>>>>>
>>>>> I think it would be more useful to raise the error when defining the
>>>>> interface that cannot be used as it's likely an error—or at least I
>>>>> can see no valid use for creating an unsatisfiable constraint.
>>>>>
>>>>> --
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>>>>
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>>
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