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”.
>
This is great. I like the fact that the type parameter list is just a
normally formed parameter list now.
I did quite like the fact that you could see defined types with a grep for
"type", but that's
not always the case anyway with type blocks, so I'll put that feeling aside.
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.
>
This is also a great forward-looking change.
>
> 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.
>
Ditto.
>
> 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. To make that rule very clear,
> type switches will not be permitted for type parameters that do not
> have type lists. It is already possible to switch on a value “x”
> whose type is a type parameter without a type list by writing code
> like “switch (interface{})(x).(type)” (which may now be written as
> “switch any(x).(type)”). That construct is not the simplest, but it
> uses only features already present in the language, and we don’t
> expect it to be widely needed.
>
I'm less keen on this restriction. That latter construct has at least one
significant
pitfall: if the type parameter is an interface type and x is nil, the type
switch won't
choose the correct type. Instead, you'd need to do something like “switch
(interface{})(&x).(type)”
and use the pointer type in the type switch (
https://go2goplay.golang.org/p/IdIuWkMTaBF).
It might not even be obvious that we're switching on an interface type when
we're using
a type parameter in the switch case. For example:
https://go2goplay.golang.org/p/5ridyxPoJOZ.
Another issue with this approach is that you may need to use non-type-safe
(and somewhat
verbose) code to work around it. Here's an example:
https://go2goplay.golang.org/p/A-_mVRxhvl4
This fails because even though we know the type of the value in the type
switch, the
compiler can't make the connection between that and the actual type of K, so
it's not possible to use values of the known type where values of type K
are expected.
The workaround is to do a dynamic type conversion:
https://go2goplay.golang.org/p/PBYDoESWXjW,
but it's not hard to get wrong, resulting in a runtime panic
<https://go2goplay.golang.org/p/WdAaUAPXpEU>.
I suggest that we allow type switches on non-type-list type parameters and
make it clear
that the type matching is exact - there would be no way to use a type
switch to
match against a type parameter's underlying type.
cheers,
rog.
>
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