BTW, the same idea doesn't work for maps, of course. For maps, you could exploit the fact that they are pointer-shaped in the current implementation and use unsafe to compare those pointers: https://play.golang.org/p/JCeXmoUctT2 You should keep in mind though, that this might break in the future, if the map-implementation ever changes.
On Wed, Nov 11, 2020 at 4:41 PM Axel Wagner <axel.wagner...@googlemail.com> wrote: > 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 >>>>>> >>>>>> -- >>>>>> You received this message because you are subscribed to the Google >>>>>> Groups "golang-nuts" group. >>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>> send an email to golang-nuts...@googlegroups.com. >>>>>> To view this discussion on the web visit >>>>>> https://groups.google.com/d/msgid/golang-nuts/b2cb6b5e-febc-407f-b5b3-d9ca196ce68bn%40googlegroups.com >>>>>> <https://groups.google.com/d/msgid/golang-nuts/b2cb6b5e-febc-407f-b5b3-d9ca196ce68bn%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>>> . >>>>>> >>>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "golang-nuts" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to golang-nuts...@googlegroups.com. >>>> >>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/golang-nuts/250107e9-f688-4205-ae52-728221eb2e4cn%40googlegroups.com >>>> <https://groups.google.com/d/msgid/golang-nuts/250107e9-f688-4205-ae52-728221eb2e4cn%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts+unsubscr...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/7714670d-823a-4486-a89e-a1c636051005n%40googlegroups.com >> <https://groups.google.com/d/msgid/golang-nuts/7714670d-823a-4486-a89e-a1c636051005n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAEkBMfFxHd6xRyAKB7jia5gtT46Ng1G5GeTRYk2TjuF5i3xaPA%40mail.gmail.com.
