I was trying to show that the current behavior is confusing and that
fmt.Print() needing to resort to panic-and-recover is kinda code smell, but
I sorts-of convinced myself that the current behavior is right, or at least
consistent.
In my code, I got bit because I sometimes use v *Type to denote "I may or
may not have a value here" (where Type is a value-type).
This is probably a bad practice on my behalf, because I break the Liskov
substitution principle: there is a value of `*Type` that is not a valid
value of `Type`, and I let this value slip by.
In this case, `v Type` implements Stringer (i.e. valid callee for
`v.String()`, but `v *Type`, in the strictest sense, does not.
The only reason we can write:
func (Type) String() string {...}
v *Type = &Type{...}
_ = v.String()
and have it compile, is syntactic sugar: `v` gets implicitly de-referenced,
and there's an implicit assumption that it's not nil.
And there's a matching syntactic sugar for converting `Type` to a `*Type`.
So, In the code:
func (Type) String() string {...}
v *Type = nil
r interface{} = v
_, ok = r.(Stringer)
What I really want to ask is "Can I, at runtime, call r.String()?", whereas
the question Go answers is "Is any of `r`, `*r`, or `&r` defines
.String()?" - which matches the static semantics of `r.String()`.
So, while I should probably not use *Type as a replacement for
Optional<Type>, I think it might make sense to have some operator that can
determine, at run-time, if a call `r.String()` is valid (including a
nil-check).
-- Aviv
On Saturday, April 11, 2020 at 4:48:28 PM UTC+3 [email protected] wrote:
> I agree with the OP. The usefulness of nil interfaces is pretty limited.
> Show me a useful case that cant easily be implemented with non-nil
> interfaces.
>
> I would argue that allowing nil interfaces causes more subtle latent bugs
> and makes it harder to reason about the correctness of code when reviewing
> it.
>
> It just feels wrong. I realize I’m probably in the minority here but the
> OP is not alone.
>
> On Apr 11, 2020, at 8:20 AM, 'Axel Wagner' via golang-nuts <
> [email protected]> wrote:
>
> On Fri, Apr 10, 2020 at 7:17 PM <[email protected]> wrote:
>
>> I realize I'm reviving an age-old discussion here and apologize for
>> bringing up the undead. I happend to run into this when my application
>> panicked when some interfaces where initialized with nil mock objects
>> instead of being left uninitialized as in production mode.
>>
>
> Let's imagine a world in which `foo == nil` also is true if `foo` is an
> interface-value containing a nil-pointer. Let's say in this world, someone
> sends a message to golang-nuts. They wrote a mock for the same code. And
> since it's just a mock, they just returned static value from its methods
> and didn't need to care if the pointer was nil or not. They are confused,
> because the passed in this mock, but the code just assumed the field was
> uninitialized and never called into their mock. What would you tell them?
> Why is their confusion less valid?
>
> This would be an example where a nil implementing fooer is never caught:
>>
>> type fooer interface {
>> foo()
>> }
>>
>> type other struct{}
>>
>> func (o *other) foo() {} // implement fooer
>>
>> func main() {
>> var f fooer
>>
>> var p *other // nil
>> f = p // it is a fooer so I can assign it
>>
>> if f == nil {
>> // will not get here
>> }
>> }
>>
>>
>> My confusion comes from the point that the nil interface is apparently
>> not "a nil-pointer with the correct method set" while *other is even if nil.
>>
>
> In the code you posted, even a nil *other is a perfectly fine
> implementation of fooer. You can call `(*other)(nil).foo()` without any
> problems.
> So, as you illustrated, calling methods on a nil-pointer can be totally
> fine. A nil-interface, OTOH, doesn't have any methods to call, as it
> doesn't contain a dynamic value. If you write `(*other)(nil).foo()`, it is
> completely clear what code gets called - even if that code *might* panic.
> If you write `fooer(nil).foo()`, what code should be called in your opinion?
>
> I think it's easy to see that a nil-interface and a nil-pointer stored in
> an interface are very different things. Even from first principles, without
> deep knowledge of the language. And if they are obviously different, I
> don't understand why you'd find it confusing that they are not the same in
> this particular manner.
>
> The above is a case where that might happen. In can be worked around but
>> it is unexpected unless the programmer is deeply rooted in the language
>> definition.
>>
>
> I fully agree with that. What I *don't* agree with, is where you attribute
> the problem here. You say, the problem is that the nil-check is
> ill-behaved. I say that - if anything - the original nil-assignment is
> ill-behaved. Having `(fooer)((*other)(nil)) == nil` be true is semantically
> wrong, because by checking against `nil`, you are checking if you have a
> correct implementation - and you might well have a correct implementation,
> even if it's using a nil-pointer.
>
> Note, that the contained pointer being nil isn't the *only* case in which
> calling the method might panic. For example, what about this code?
> https://play.golang.org/p/lNq0qphez7v
> Shouldn't the `nil`-check also catch that? After all, calling the method
> panics, so it's clearly not a valid implementation - even if x itself is
> not nil. Why is a nil-pointer more special than any other value that causes
> a method to panic?
>
> Seems as of today that there is no tooling to support that check. Maybe
>> it's not a widespread issue.
>>
>
> As of today, the language also isn't changed :) Maybe someone who think
> this is important enough to change the language, could also feel it's
> important enough to write this tooling.
>
>
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