TL, the simplest reason is this: You can live without the "ok idiom" when retrieving a map value but you cannot when type asserting. Think of the consequences for rest of your program, if you forgot to check the status of assertion that is failed (but you didn't know) because the compiler didn't panic! It would be a travesty.
For compiler's perspective, see Andrey's response. On Thu, Sep 29, 2016 at 10:06 AM, Henrik Johansson <dahankz...@gmail.com> wrote: > I am not sure but perhaps as simple as it is a very natural and known > behavior of maps and to make it work syntactically as the type assertion > would make it weird. > > On Thu, Sep 29, 2016, 07:02 T L <tapir....@gmail.com> wrote: > >> >> >> On Thursday, September 29, 2016 at 12:56:57 PM UTC+8, Micky wrote: >>> >>> The reason is directly stated in the Go language spec: >>> >>> "If the type assertion holds, the value of the expression is the value >>> stored in x and its type is T. If the type assertion is false, a run-time >>> panic <https://golang.org/ref/spec#Run_time_panics> occurs." >>> >>> Here "hold" means if it succeeds. >>> >>> >> I know of the syntax in spec. >> I just want to understand what is the deep reason for the syntax >> inconsistency between map index and type assert. >> >> >>> >>> On Thu, Sep 29, 2016 at 9:53 AM, T L <tapi...@gmail.com> wrote: >>> >>>> >>>> >>>> On Thursday, September 29, 2016 at 12:32:48 PM UTC+8, Henrik Johansson >>>> wrote: >>>>> >>>>> This is just how type assertion works. >>>>> If you don't use the dual return it panics if the actual type is >>>>> different from the one you try to assert. >>>>> >>>> >>>> but what is the underlining reason for the inconsistency between map >>>> index and type assert? >>>> >>>> >>>>> >>>>> On Thu, Sep 29, 2016, 05:26 T L <tapi...@gmail.com> wrote: >>>>> >>>>>> package main >>>>>> >>>>>> func main() { >>>>>> var m = map[string]int{} >>>>>> _, _ = m["abc"] // ok >>>>>> _ = m["abc"] // ok >>>>>> >>>>>> var i interface{} = 789 >>>>>> _, _ = i.(bool) // ok >>>>>> _ = i.(bool) // panic: interface conversion: interface is int, >>>>>> not bool >>>>>> } >>>>>> >>>>>> -- >>>>>> 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...@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. >> > -- > 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. > -- 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.
