On Tuesday, June 20, 2017 at 2:19:25 AM UTC-4, Volker Dobler wrote:
>
> On Tuesday, 20 June 2017 06:52:58 UTC+2, Will Hawkins wrote:
>>
>> I know that there is a difference between interface values and dynamic
>> types and dynamic values. Is it possible that the documentation about the
>> %T is misleading? Should it be more specific that it returns the variable's
>> "dynamic type."
>> [...]
>>
> Should the documentation for %T be updated to say that it prints the
>> variable's dynamic type?
>>
>
> Well, no.
> Look at fmt.Printf: It take a format string and a bunch of interface{}
> values to
> be printed. If %T would print the static type it always would print
> interface{}.
> To be useful Printf has too look inside it's arguments and inside is a
> HelloInt.
>
Mr. Dobler,
Thank you for your reply!
I agree with your statement that to be useful, the Printf function must do
what it does. And, of course I want it to be useful :-)
However, I am still concerned about this language from the spec:
"The *static type* (or just *type*) of a variable is the type given in its
declaration, the type provided in the new call or composite literal, or the
type of an element of a structured variable."
Where it says "just *type*" means that anywhere the documentation uses the
word "type" without qualification, it is referring to the static type. In
the case of Printf, then, *yes*, %T should always print "interface{}"
because that is the variable's static type.
I 200% (that's more than 100% :-)) agree with you that is *not* useful.
However, based on the language in the spec, it is how I would expect it to
operate.
I sincerely appreciate your willingness to read through my message and
offer your reply. I am already learning very much!
Will
> The HelloListen basically gets lost while repacking the actual HelloInt
> into the
> empty interface{} of Printf's arguments.
> So all is fine, all is consistent and no documentation is misleading.
>
> It is just that fmt.Printf is not a magical function working outside of
> the type
> system. Any argument you supply is assigned to an empty interface. This
> is how argument passing in Go works. You might try:
> var empty interface{}
> empty = hellolisten
> fmt.Printf("empty: %T\n", empty)
> which makes this repacking into an interface{} explicit and now it is
> obvious
> that it will print main.HelloInt.
>
> V.
>
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