On Tue, Jan 19, 2021 at 8:02 PM burak serdar <bser...@computer.org> wrote:
>
> On Tue, Jan 19, 2021 at 8:38 PM Ian Lance Taylor <i...@golang.org> wrote:
> >
> > On Tue, Jan 19, 2021 at 7:06 PM burak serdar <bser...@computer.org> wrote:
> > >
> > > In the following program, it is valid to pass an interface to function P:
> > >
> > > func P[T fmt.Stringer](x T) {
> > > fmt.Println(x)
> > > }
> > >
> > > func main() {
> > > var v fmt.Stringer
> > > P(v)
> > > }
> > >
> > > However, there is no way for P to check if x is nil. This does not
> > > compile:
> > >
> > > func P[T fmt.Stringer](x T) {
> > > if x!=nil {
> > > fmt.Println(x)
> > > }
> > > }
> > >
> > > Is it possible to write a generic function that can test if its
> > > argument with a constraint is nil?
> >
> > For an interface type the value "nil" is the zero value of the type,
> > so this is the general zero value issue mentioned at
> > https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#the-zero-value
> >
> > You can write
> >
> > func P[T fmt.Stringer](x T) {
> > var zero T
> > if x!=zero {
> > fmt.Println(x)
> > }
> > }
> >
>
> But that breaks the generic function for non-interface types.
>
> type Str string
>
> func (s Str) String() string {return string(s)}
>
> func main() {
> P(Str(""))
> }
>
> Now the passed parameter is the zero value, but not nil.
OK, but what are you really trying to check? Why do you want to check
that an interface is not the zero value while at the same time you
don't care whether a string is the zero value?
Ian
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