also, of course, you could still use operators with them, while now also
knowing the exact semantics of those operators (e.g. in regards to
overflow), which might also be useful.
On Fri, Aug 21, 2020 at 11:42 PM Axel Wagner <axel.wagner...@googlemail.com>
wrote:
>
>
> On Fri, Aug 21, 2020 at 11:30 PM roger peppe <rogpe...@gmail.com> wrote:
>
>> On Fri, 21 Aug 2020 at 22:10, jimmy frasche <soapboxcic...@gmail.com>
>> wrote:
>>
>>> I'd assume that would fail to compile as you're returning a []T not a
>>> []int
>>>
>>
>> If that's the case, then I'm not sure that such a type switch would be
>> very useful. It would tell you what type the values are, but you can't do
>> anything with them because all the values would still be of the original
>> type.
>>
>
> You can reasonably convert them to their underlying type and *then* use
> them as such.
> That would make it useful while not allowing what you posit.
>
> I had assumed that the intention was that within the arm of the type
>> switch, the switched type would take on the specified type.
>> That would allow (for example) specialising to use underlying machine
>> operations on []T when T is a known type such as byte.
>>
>
> It would, however, prevent you from calling methods on the type or pass it
> to a function taking an interface compatible with the constraint.
> Also, I shudder to even imagine how this could be put into a spec.
>
>
>>
>>
>> On Fri, Aug 21, 2020 at 2:07 PM roger peppe <rogpe...@gmail.com> wrote:
>>> >
>>> >
>>> > On Fri, 21 Aug 2020 at 01:28, Ian Lance Taylor <i...@golang.org>
>>> wrote:
>>> >>
>>> >> After many discussions and reading many comments, we plan to move
>>> >> forward with some changes and clarifications to the generics design
>>> >> draft.
>>> >>
>>> >> 1.
>>> >>
>>> >> We’re going to settle on square brackets for the generics syntax.
>>> >> We’re going to drop the “type” keyword before type parameters, as
>>> >> using square brackets is sufficient to distinguish the type parameter
>>> >> list from the ordinary parameter list. To avoid the ambiguity with
>>> >> array declarations, we will require that all type parameters provide a
>>> >> constraint. This has the advantage of giving type parameter lists the
>>> >> exact same syntax as ordinary parameter lists (other than using square
>>> >> brackets). To simplify the common case of a type parameter that has
>>> >> no constraints, we will introduce a new predeclared identifier “any”
>>> >> as an alias for “interface{}”.
>>> >>
>>> >> The result is declarations that look like this:
>>> >>
>>> >> type Vector[T any] []T
>>> >> func Print[T any](s []T) { … }
>>> >> func Index[T comparable](s []T, e T) { … }
>>> >>
>>> >> We feel that the cost of the new predeclared identifier “any” is
>>> >> outweighed by the simplification achieved by making all parameter
>>> >> lists syntactically the same: as each regular parameter always has a
>>> >> type, each type parameter always has a constraint (its meta-type).
>>> >>
>>> >> Changing “[type T]” to “[T any]” seems about equally readable and
>>> >> saves one character. We’ll be able to streamline a lot of existing
>>> >> code in the standard library and elsewhere by replacing “interface{}”
>>> >> with “any”.
>>> >>
>>> >> 2.
>>> >>
>>> >> We’re going to simplify the rule for type list satisfaction. The type
>>> >> argument will satisfy the constraint if the type argument is identical
>>> >> to any type in the type list, or if the underlying type of the type
>>> >> argument is identical to any type in the type list. What we are
>>> >> removing here is any use of the underlying types of the types in the
>>> >> type list. This tweaked rule means that the type list can decide
>>> >> whether to accept an exact defined type, other than a predeclared
>>> >> type, or whether to accept any type with a matching underlying type.
>>> >>
>>> >> This is a subtle change that we don’t expect to affect any existing
>>> >> experimental code.
>>> >>
>>> >> We think that this definition might work if we permit interface types
>>> >> with type lists to be used outside of type constraints. Such
>>> >> interfaces would effectively act like sum types. That is not part of
>>> >> this design draft, but it’s an obvious thing to consider for the
>>> >> future.
>>> >>
>>> >> Note that a type list can mention type parameters (that is, other type
>>> >> parameters in the same type parameter list). These will be checked by
>>> >> first replacing the type parameter(s) with the corresponding type
>>> >> argument(s), and then using the rule described above.
>>> >>
>>> >> 3.
>>> >>
>>> >> We’re going to clarify that when considering the operations permitted
>>> >> for a value whose type is a type parameter, we will ignore the methods
>>> >> of any types in the type list. The general rule is that the generic
>>> >> function can use any operation permitted by every type in the type
>>> >> list. However, this will only apply to operators and predeclared
>>> >> functions (such as "len" and "cap"). It won’t apply to methods, for
>>> >> the case where the type list includes a list of types that all define
>>> >> some method. Any methods must be listed separately in the interface
>>> >> type, not inherited from the type list.
>>> >>
>>> >> This rule seems generally clear, and avoids some complex reasoning
>>> >> involving type lists that include structs with embedded type
>>> >> parameters.
>>> >>
>>> >> 4.
>>> >>
>>> >> We’re going to permit type switches on type parameters that have type
>>> >> lists, without the “.(type)” syntax. The “(.type)” syntax exists to
>>> >> clarify code like “switch v := x.(type)”. A type switch on a type
>>> >> parameter won’t be able to use the “:=” syntax anyhow, so there is no
>>> >> reason to require “.(type)”. In a type switch on a type parameter
>>> >> with a type list, every case listed must be a type that appears in the
>>> >> type list (“default” is also permitted, of course). A case will be
>>> >> chosen if it is the type matched by the type argument, although as
>>> >> discussed above it may not be the exact type argument: it may be the
>>> >> underlying type of the type argument.
>>> >
>>> >
>>> > Here's one interesting implication of this: it allows us to do type
>>> conversions that were not previously possible.
>>> >
>>> > For example, if we have "type I int", we can use a type switch to
>>> convert some type []I to type []int:
>>> > https://go2goplay.golang.org/p/-860Zlz7-cn
>>> >
>>> > func F[type T intlike](ts []T) []int {
>>> > switch T {
>>> > case int:
>>> > return ts
>>> > }
>>> > return nil
>>> > }
>>> >
>>> > It seems to me that this kind of thing will allow us to perform a
>>> similar conversion (convert some part of the type to its underlying type)
>>> on any type.
>>> >
>>> > In the early days of Go, the spec allowed this kind of conversion as a
>>> normal type conversion. I wonder if it might be reasonable to revert to
>>> those more relaxed semantics. I think they're potentially useful, for
>>> example, when dealing with named types obtained from modules with two
>>> different major versions without incurring copies.
>>> >
>>> > Although in the above-linked issue Robert talks about runtime costs
>>> such as "possibly re-mapping method tables", I don't see that this would
>>> necessarily be the case. Thoughts?
>>> >
>>> > --
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>>> .
>>>
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