On Wednesday, June 24, 2020 at 3:02:43 PM UTC-7, Ian Lance Taylor wrote:
>
> On Wed, Jun 24, 2020 at 2:33 PM i3dmaster <[email protected] <javascript:>>
> wrote:
> >
> > example:
> >
> > type Key interface {
> > type comparable, fmt.Stringer
> > }
> >
> > type Value interface {
> > type interface{}, copyable
> > }
> >
> > type Container(type K Key, V Value) interface {
> > Add(K, V) bool
> > }
> >
> > type HashMap(type K Key, V Value) struct{ m map[K]V }
> >
> > func (h *HashMap(K,V)) Add(k K, v V) bool {
> > h.m[k] =v
> > return true
> > }
> >
> > But of course, copyable isn't defined... So where to find some of these
> predefined constraints? Or if not, how could one define a such constraint?
> >
> > Also how to specify constrains like
> >
> > 1. A type that I can call len() against it?
> > 2. A type that I can range over it?
>
> Your message came through as white text on a black background that is
> hard to read. On this mailing list, please send plain text as plain
> text. Thanks.
>
> Oh god, I am sorry about that. I was using new browser + black background
and didn't realize it even decorated the message.
> The current generics design draft doesn't support the kind of
> constraints you are looking for.
>
Is there or will there be a plan? Using generic types with [], map, etc can
work on some use cases,
but there are likely also use cases where custom container types would more
appreciated to abtract
or confine certain API surfaces. I wish Go authors wouldn't completely
eliminate these possiblities.
>
> I'm not even sure what you mean by "copyable". All Go types can be
> copied.
>
Hm... I thought there are some aren't at least recommened to copy, e.g.
Mutex, Cond, or any types that explicitly documented not to copy.
>
> Typically instead of writing generic type constraints like supports
> len or range people write code that takes a slice of some generic
> type.
>
> Note that you are using type lists in a way that doesn't look right.
> To write a constraint that is a combination of other constraints, you
> should use embedding, as in
>
> type Key interface {
> comparable
> fmt.Stringer
> }
>
Interesting, maybe I wasn't still quite not understanding the draft spec...
What's the difference here?
// Numeric is a constraint that matches any numeric type.
// It would likely be in a constraints package in the standard library.
type Numeric interface {
type int, int8, int16, int32, int64, uint, uint8, uint16, uint32,
uint64, uintptr, float32, float64, complex64, complex128
}
Also, btw, it compiles just fine.
> Ian
>
--
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 [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/golang-nuts/a311297b-865a-417d-a383-3734ee07723co%40googlegroups.com.
