On Friday, October 14, 2016 at 1:33:52 AM UTC+8, di...@veryhaha.com wrote: > > > > On Friday, October 14, 2016 at 1:28:01 AM UTC+8, di...@veryhaha.com wrote: >> >> >> >> On Thursday, October 13, 2016 at 11:22:14 PM UTC+8, Chris Manghane wrote: >>> >>> In that example y is a nil interface value of type l. The last line >>> implies that for a type assertion to another interface type, the operation >>> will only be possible if the underlying value implements both interfaces. >>> That is, the value must have an m() method as well as all of io.reader >>> methods or the type assertion will like the above assertion to a string >>> type. >>> >> >> But it looks it is not essential for the underlying value implements >> io.Reader. >> The code always compiles, whether or not the underlying value implements >> io.Reader. >> The difference is: >> * if the underlying value implements io.Reader, then r is a non-nil >> io.Reader value >> * if the underlying value doesn't implement io.Reader, then r is a nil >> io.Reader value >> > > sorry, my mistake, if the underlying value doesn't implement io.Reader, > the last line will panic. >
but it compiles even if the underlying value doesn't implement io.Reader, not like the " s := y.(string) ", which doesn't compile. > > >> >> >>> >>> On Oct 13, 2016 7:48 AM, <di...@veryhaha.com> wrote: >>> >>> >>> >>> On Thursday, October 13, 2016 at 8:52:14 PM UTC+8, Jan Mercl wrote: >>> >>>> On Thu, Oct 13, 2016 at 2:42 PM <di...@veryhaha.com> wrote: >>>> >>>> > I don't understand the comment of the last line. Can someone explain >>>> it for me? >>>> >>>> "r has type io.Reader" means that the type if expr.(T) is T. >>>> >>>> "and y must implement both I and io.Reader" >>>> >>>> y is either nil or it implements I, because that's how it was declared >>>> and nothing not implementing I can be assigned to it. The dynamic type of >>>> y >>>> can implement any number of interfaces, so it can implement both I and >>>> io.Reader. The later is checked at run time when the type assertion >>>> expression y.(io.Reader) is actually evaluated. >>>> >>>> -- >>>> >>>> -j >>>> >>> >>> So "y must implement both I and io.Reader" means short form of "the >>> dynamic type of y must implement both I and io.Reader"? >>> My brain really can't accept this short form. >>> >>> And in that example, it gives people the impression y is nil. I really >>> think it is a bad example. >>> >>> >>> >>> >>> >>> >>> I feel that comment (and that example) is very not professional for >>> these reason: >>> 1. y is a value of type I, >>> >>> >>> -- >>> 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. >>> For more options, visit https://groups.google.com/d/optout. >>> >>> >>> -- 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. For more options, visit https://groups.google.com/d/optout.
