https://play.golang.org/p/qTBiAdR9djt
`a` is the address of the first element of `l`.
`b` is the address of the first element of `l[0]`, which is of type
`[]interface{}` after the assignment.
Both `l` and `l[0]` refer to the same underlying array, so their first
elements have the same address.
In practice, you probably want to walk the value with a depth-first search
and use `reflect` to extract the addresses and check for slices and the
like and use a `map[reflect.Value]bool` with pointers to keep track which
values you've already seen. The code isn't trivial enough for me to just
write it down at the moment, but once the idea is clear, it should be
doable.
On Wed, Nov 11, 2020 at 2:00 PM arpad ryszka <arpad.rys...@gmail.com> wrote:
> `l[0] = l` would be fine for me, indeed. Though I am not sure I understand
> the suggested solution. Notice that the type of the slice is
> `[]interface{}`. This...
>
> ll := l[0].([]interface{})
> println(&l == &ll)
>
> ...would print `false`.
>
> I think the main challenge is that l and ll, or l[0] for that matter, are
> not the same values. I tried with reflection, too, but couldn't figure a
> way.
>
> PS: I am quite aware how unearthy this problem is, and how it should not
> ever happen in case of actual programs. My use case is that I want to
> pretty print in-memory objects for debug purposes during testing, and one
> of the features would be to point out such cycles as this.
>
>
> On Wednesday, November 11, 2020 at 8:13:55 AM UTC+1
> axel.wa...@googlemail.com wrote:
>
>> The question was about detecting cyclic references. For that, the `l[0] =
>> l` case is enough.
>>
>> On Wed, Nov 11, 2020 at 2:31 AM jake...@gmail.com <jake...@gmail.com>
>> wrote:
>>
>>> FYI, that does detect the simple case of l[0] = l, but not more
>>> complicated circular cases like l[1] = l[1:]
>>>
>>> On Tuesday, November 10, 2020 at 2:38:26 AM UTC-5
>>> axel.wa...@googlemail.com wrote:
>>>
>>>> If the slice is empty, it doesn't reference anything.
>>>> If it is not empty, &x[0] can be used to identify the slice
>>>> (potentially also using len/cap, if it's interesting).
>>>>
>>>> On Tue, Nov 10, 2020 at 4:11 AM arpad ryszka <arpad....@gmail.com>
>>>> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> is there a way to detect the cyclic reference in the following example
>>>>> somehow? Either via reflection or by other means? My understanding is that
>>>>> slices cannot be compared with == (unless to nil), or used as keys in
>>>>> maps.
>>>>> Is there a different way?
>>>>>
>>>>> l := []interface{}{"foo"}
>>>>> l[0] = l
>>>>> fmt.Println(l)
>>>>>
>>>>> or here it is as a playground link:
>>>>> https://play.golang.org/p/T0qZlF8m-vi
>>>>>
>>>>> Cheers,
>>>>> Arpad
>>>>>
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