On Wed, Jan 18, 2023 at 5:14 PM 'Christian Stewart' via golang-nuts <
golang-nuts@googlegroups.com> wrote:
> The thing I ran into with this today was, if you want to compare two
> interfaces:
>
> var foo1 Thing
> var foo2 Thing
>
> Ordinarily you can do foo1 == foo2 and it does pointer-wise comparison.
>
No, it does not. Consider the following example. Do all three vars have the
same address?
package main
type Thing struct{}
var foo1 Thing
func main() {
var foo2 Thing
var foo3 Thing
println(foo1 == foo2, &foo1, &foo2)
println(foo2 == foo3, &foo2, &foo3)
}
The output will probably surprise you. See
https://go.dev/ref/spec#Comparison_operators. An empty struct is handled
thusly:
- Struct values are comparable if all their fields are comparable. Two
struct values are equal if their corresponding non-blank
<https://go.dev/ref/spec#Blank_identifier> fields are equal.
Obviously, depending on how "Thing" is defined the results may be
different. Perhaps you are thinking of the case where "Thing" is an
interface.
But in a generic function, as an example:
>
> // IsEmpty checks if the interface is equal to nil.
> func IsEmpty[T Block](blk Block) bool {
> var empty T
> return empty == blk
> }
>
> ... that doesn't work due to the errors mentioned in this thread. And
> I can't make the type constraint comparable either, that doesn't seem
> to work.
>
> On Wed, Jan 18, 2023 at 4:59 PM burak serdar <bser...@computer.org> wrote:
> >
> >
> >
> > On Wed, Jan 18, 2023 at 5:52 PM Andrew Athan <andrew...@gmail.com>
> wrote:
> >>
> >> (Possibly related to issues such as those discussed in
> https://groups.google.com/g/golang-nuts/c/pO2sclKEoQs/m/5JYjveKgCQAJ ?)
> >>
> >> If I do:
> >>
> >> ```
> >> func Foo[V any](v V)bool {
> >> return v==v
> >> }
> >> ```
> >>
> >> golang 1.19 reports:
> >>
> >> ```
> >> invalid operation: v == v (incomparable types in type set)
> >> ```
> >>
> >> There are two issues with this in my mind: (1) It is ambiguous in that
> I cannot be sure, as a new user, whether what's being indicated is an
> incompatibility between the actual types on either side of the equality
> test or between the set of possible types that COULD be on either side of
> the equality test due to V's any type constraing in either the case where
> the left and right side are same or different types (2) In this case, the
> type V appears to be determinable at compile time and yet it is claimed
> this equality test is in some way problematic.
> >>
> >> I'm sure I'm misunderstanding something about golang generics.
> >>
> >> Can someone comment on this?
> >>
> >> Thanks in advance :)
> >
> >
> >
> > The problem here is that a type constraint is different from a type, but
> the same identifier is overloaded in the language so "any" as a type
> constraint means something else from "any" as a type.
> >
> > The error is there, because without that Foo([]string{}) would satisfy
> the constraint but it is still a compile error. Foo[V comparable)(v V) bool
> is the correct declaration.
> >
> >>
> >>
> >> --
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--
Kurtis Rader
Caretaker of the exceptional canines Junior and Hank
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