interface identity (was Re: Proposed new dependency: github.com/juju/errors (and github.com/juju/errgo))

[John Meinel]

The address of the real value is the same.

John
=:->
On May 28, 2014 6:04 AM, "Andrew Wilkins" <andrew.wilkins at canonical.com>
wrote:

> On Wed, May 28, 2014 at 9:52 AM, Tim Penhey <tim.penhey at canonical.com>wrote:
>
>> On 28/05/14 13:48, Andrew Wilkins wrote:
>> > On Wed, May 28, 2014 at 8:47 AM, Tim Penhey <tim.penhey at canonical.com
>> > <mailto:tim.penhey at canonical.com>> wrote:
>> >
>> >     On 28/05/14 12:43, Nate Finch wrote:
>> >     > This sounds like one of those "if you have to ask this question,
>> >     you're
>> >     > doing something wrong".
>> >     >
>> >     > Can you give an example of where we need this?
>> >
>> >     Sure... let's say we have a stack of errors, for simplicity of the
>> >     argument lets say it is a slice of error interface values.
>> >
>> >      stack []error
>> >
>> >      * an error is pushed on to the stack initially, we now have one
>> error
>> >      * the same error is pushed (or appended - I don't care)
>> >      * we now have the same error twice
>> >      * I push a new error on the stack, so it looks a little like this
>> >        [err1, err1, err2] right?
>> >
>> >     Now iterating through this slice I want to know when the error
>> changes.
>> >
>> >
>> > Can you explain where equality fails?
>>
>> Equality fails when the interface is satisfied by a non-comparable value
>> type, like a struct with a slice in it.
>
>
> I see. In that case, I guess you'd have to use reflect.DeepEquals to
> collapse arbitrary errors.
>
>  > I guess you're thinking you'd like to do something like "x is y" in
>> > Python. There's no such thing as objects in Go, so no universal
>> > definition of identity either.
>>
>> But an interface is effectively two pointers, one to the type and one to
>> the thing that satisfies the interface.  Identity in that case is pretty
>> simple.
>>
>
> If the pointer is the same, which it won't be if you're storing a slice in
> an interface; slices are wider than pointers.
>
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