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

[John Meinel]

I think we need concrete examples which Tim should have in the test suite.

John
=:->
On May 28, 2014 6:50 AM, "Andrew Wilkins" <andrew.wilkins at canonical.com>
wrote:

> On Wed, May 28, 2014 at 10:39 AM, John Meinel <john at arbash-meinel.com>wrote:
>
>> The address of the real value is the same.
>>
> Are you referring to the backing array? That is not what is being
> compared, so that's not a useful property.
>
>> 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.
>>>
>>> --
>>> Juju-dev mailing list
>>> Juju-dev at lists.ubuntu.com
>>> Modify settings or unsubscribe at:
>>> https://lists.ubuntu.com/mailman/listinfo/juju-dev
>>>
>>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.ubuntu.com/archives/juju-dev/attachments/20140528/27cbf329/attachment-0001.html>