How about something like this?
Type switches
A *generic type switch* allows a generic function to provide specialized
behaviour based on its type arguments (for example to use a more efficient
implementation for some types).
A type switch refines the type of a type parameter. Cases match actual
types against the generic type parameter in turn. Within the body of a
case, the named type parameter has the actual type. A single case may not
list more than one type.
By default, matching is exact: for a case to match, the type parameter must
be exactly the specified type. By wrapping the specified type in a
type qualifier,
the matching is not exact, but *matches* the kind of type instead. For some
non-interface type X, type(X) matches any type with an underlying type of X.
When X is an interface type, type(X) matches any type that implements the
interface. Type-list interfaces are allowed.
func F(type T, U)() {
switch T {
case int:
// T is exactly the type int.
case error:
// T is exactly error, not some type that happens to satisfy the
error inferface.
case type(string):
// T is any type with an underlying type of string
case type(io.ReadCloser):
// T is any type that satisfies the io.ReadCloser interface.
case type(interface{
type int, int64
}):
// T has an underlying type of either int or int64.
case string:
// Compile error: duplicate case (matched by generic(string)
above).
case type(io.Reader):
// Compile error: duplicate case (matched by
generic(io.ReadCloser) above).
case U:
// Both type parameters are exactly the same type.
}
}
*Discussion*
It might be a good idea to allow a type switch to specify a list of types
rather than a single type, so several generic parameters can be specialized
at the same time. For example:
switch T, U {
case int, string:
}
This is syntactically rather more useful than in a normal type switch,
because there's no way to avoid nesting type switches (in a normal type
switch, it's possible to assign the specialized value to another variable
with wider scope, but that's not possible in general with generic type
switches). However, it may be too confusing syntactically with the normal
comma-list in a type switch statement which means "any of these types". A
possible way of avoiding confusion that might be to require brackets:
switch T, U {
case (int, string):
}
The ability to use a concrete type directly in a type(X) case is just
syntax sugar for type(interface{type X}) and could be omitted. It depends
how commonly used this might be.
On Tue, 7 Jul 2020 at 04:51, Steven Blenkinsop <steven...@gmail.com> wrote:
> On Mon, Jul 6, 2020 at 6:29 PM, roger peppe <rogpe...@gmail.com> wrote:
>
>>
>> I've also been playing around in this area. I've been trying something
>> similar to this approach: https://go2goplay.golang.org/p/sHko_EMhJjA
>> But this isn't ideal - note that we lose type safety when assigning back
>> to the generic hash function, because there's no way to let the compiler
>> know that the actual type of the generic type parameter is known at that
>> point.
>>
>> I also noticed an interesting wrinkle to doing a type switch on a value
>> of the type:
>>
>> We can't do this:
>>
>> case Hasher:
>> hi = func(t Hasher) uintptr {
>> return t
>> }
>>
>> because the actual type probably isn't Hasher - it's the actual type, not
>> the interface type.
>>
>> If a type switch on a type argument was implemented (and I definitely
>> support it) we'd have to think carefully about what semantics would be
>> desired in this case - if one of the types is an interface type, would one
>> get the usual interface-subtype rules for matching or not?
>>
>> For the record, I quite like the idea of using a separate syntax for
>> type-based checking:
>>
>> type switch T {
>> case int:
>> case string:
>> }
>>
>> That way, we could use exact-match rules for that construct without risk
>> of confusion with the normal type switch rules, and perhaps the difference
>> might make it clearer that T would change its type inside the body of the
>> switch.
>>
> If the idea is to be able to refine the constraints on a type parameter,
> then you need to be able to match based on "T satisfies this interface".
> If you want to be able to match based on underlying type, you could match
> against e.g. interface { type int }.
>
> I'm not sure how useful matching on a precise type is in this context.
> Interfaces can't express exact type constraints as currently designed, so
> exact type matching can't ever handle all cases. You'd need a really good
> reason to be treating exactly one type out of the set with the same
> underlying type differently in order for it to make sense.
>
> If you needed to, though, you could plausibly do it by:
>
> type switch *T {
> case interface { type *fmt.Stringer }:
> // T has to be the interface type `fmt.Stringer`
> case interface { type *int }:
> // T has to be the specific concrete type `int`
> }
>
> Whether to support the following is what could potentially cause confusion:
>
> func F(type T interface { type int })(t T) {
> type switch T {
> case int:
> ...
> }
> }
>
> Someone could plausibly believe this covers all possible cases, when it
> doesn't cover type MyInt int, for example. This could either be
> disallowed or picked up by a lint. Disallowing it would force the above
> workaround syntax using pointer types in interface type lists. This is a
> bit kludgey, but then it probably shouldn't be easier to do the thing which
> will more often be wrong than the thing that will more often be right.
>
> The other thing is that interface constraints as currently designed don't
> really lend themselves to refinement in the first place. Sure, if you have
> an interface with a type list, you can match on a subset of that type list,
> but you can't ask whether a type "is in this type list *or* satisfies
> this interface" or "satisfies at least one interface in this list". Easiest
> solution might be to allow comma separated embedded interfaces:
>
> type CustomOrdered(type T) interface {
> Less(T) bool
> }
>
> type AnyOrdered(type T) interface {
> constraints.Ordered, (CustomOrdered(T))
> }
>
> (I should also mention I don't really *like* having to define the above
> interfaces so they have to be applied reflexively to work. The fact that it
> will be done automatically in type parameter lists helps, but it still feels
> wrong to have to do it. I understand that this is to sidestep needing to add
> a second kind of generic type name in interface definitions, though.)
>
>
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