you can "employ" any custom type as key but the ones that doesn't
defines equality operations, see
https://golang.org/ref/spec#Comparison_operators
as Ian already pointed out, you just can't build your custom type on
top of func, map or slice to be a valid key, anything else you can.
On Fri, Jan 25, 2019 at 12:47 PM Victor Giordano <vitucho3...@gmail.com> wrote:
>
> 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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