Just wanna say : It would be nice to employ a custom type as a valid key
for a map!!! :D
El jueves, 24 de enero de 2019, 0:47:32 (UTC-3), mount...@gmail.com
escribió:
>
> thanks lan.
>
> 在 2019年1月23日星期三 UTC+8下午11:26:57，Ian Lance Taylor写道：
>>
>> On Tue, Jan 22, 2019 at 11:47 PM <mount...@gmail.com> wrote:
>> >
>> > i got :
>> > # command-line-arguments
>> > ./key.go:6:18: invalid operation: F() == F() (func can only be compared
>> to nil)
>>
>> Yes, that is what the language spec says and it is what I said: in Go,
>> function values are not comparable.
>>
>> I wrote that code as an example to help explain why function values
>> are not comparable.
>>
>> Ian
>>
>> > 在 2019年1月23日星期三 UTC+8上午11:59:07，mount...@gmail.com写道：
>> >>
>> >> When i use the func, map slice as the key of map, it isn't work!
>> >> So I lookup source why, i find
>> >>
>> >> // spec: "The comparison operators == and != must be fully defined
>> >> // for operands of the key type; thus the key type must not be
>> a
>> >> // function, map, or slice."
>> >> //
>> >> // Delay this check because it requires fully setup types;
>> >> // it is safe to continue in any case (was issue 6667).
>> >> check.later(func() {
>> >> if !Comparable(typ.key) {
>> >> check.errorf(e.Key.Pos(), "invalid map key type %s",
>> typ.key)
>> >> }
>> >> })
>> >> // Comparable reports whether values of type T are comparable.
>> >> func Comparable(T Type) bool {
>> >> switch t := T.Underlying().(type) {
>> >> case *Basic:
>> >> // assume invalid types to be comparable
>> >> // to avoid follow-up errors
>> >> return t.kind != UntypedNil
>> >> case *Pointer, *Interface, *Chan:
>> >> return true
>> >> case *Struct:
>> >> for _, f := range t.fields {
>> >> if !Comparable(f.typ) {
>> >> return false
>> >> }
>> >> }
>> >> return true
>> >> case *Array:
>> >> return Comparable(t.elem)
>> >> }
>> >> return false
>> >> }
>> >
>> > --
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>
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