Re: tuple indexes

[Philip Herron <philip.herron@embecosm.com>]

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On 23/06/2021 21:15, Mark Wielaard wrote:
> On Wed, Jun 23, 2021 at 12:51:34AM +0200, Mark Wielaard wrote:
>> Finally, the The Rust Reference says "A tuple index is used to refer
>> to the fields of tuples, tuple structs, and tuple variants." I don't
>> understand how this would work for tuple variants. Does anybody have
>> an example of how to refer to a tuple variant so a tuple index can be
>> used on it?
> Tom pointed out on irc that it doesn't seem possible to access enum
> variant types except through a matching expression. The Rust Reference
> also seems to be inconsistent. As mentioned above it mentions you can
> use a tuple index to refer to a field of a enum tuple variant. But it
> also says "A tuple indexing expression accesses fields of tuples and
> tuple structs." So it probably really isn't possible to use a tuple
> index on enum tuple variants.
>
> I did notice the same issue as for unit tuple struct types. The empty
> tuple wasn't accepted in the parser. The attached patch, also at
> https://code.wildebeest.org/git/user/mjw/gccrs/commit/?h=tuple_enum_variant_unit
> fixes this.
>
> It does include a test case, but most of it is commented out because
> actually resolving enum types isn't implemented yet. If you uncomment
> the rest of the testcase you get:
>
> tuple_enum_variants.rs:3:31: error: failed to resolve TypePath: E
>     3 | fn f(e0: E, e1: E, e2: E) -> (E,E,E,())
>       |                               ^
> tuple_enum_variants.rs:3:31: error: unresolved type
> tuple_enum_variants.rs:3:33: error: failed to resolve TypePath: E
>     3 | fn f(e0: E, e1: E, e2: E) -> (E,E,E,())
>       |                                 ^
> tuple_enum_variants.rs:3:33: error: unresolved type
> tuple_enum_variants.rs:3:35: error: failed to resolve TypePath: E
>     3 | fn f(e0: E, e1: E, e2: E) -> (E,E,E,())
>       |                                   ^
> tuple_enum_variants.rs:3:35: error: unresolved type
> tuple_enum_variants.rs:3:10: error: failed to resolve TypePath: E
>     3 | fn f(e0: E, e1: E, e2: E) -> (E,E,E,())
>       |          ^
> tuple_enum_variants.rs:3:10: error: unresolved type
> tuple_enum_variants.rs:3:17: error: failed to resolve TypePath: E
>     3 | fn f(e0: E, e1: E, e2: E) -> (E,E,E,())
>       |                 ^
> tuple_enum_variants.rs:3:17: error: unresolved type
> tuple_enum_variants.rs:3:24: error: failed to resolve TypePath: E
>     3 | fn f(e0: E, e1: E, e2: E) -> (E,E,E,())
>       |                        ^
> tuple_enum_variants.rs:3:24: error: unresolved type
> tuple_enum_variants.rs:13:12: error: unknown root segment in path E::T0 lookup E
>    13 |   let e0 = E::T0();
>       |            ^
> tuple_enum_variants.rs:14:12: error: unknown root segment in path E::T1 lookup E
>    14 |   let e1 = E::T1(0);
>       |            ^
> tuple_enum_variants.rs:15:12: error: unknown root segment in path E::T2 lookup E
>    15 |   let e2 = E::T2(0,1);
>       |            ^
>
> Cheers,
>
> Mark
>
Hi Mark,

Thanks for the patch, its being merged:
https://github.com/Rust-GCC/gccrs/pull/522

I have open issues about enums and unions they will be fixed as part of
my work into traits. They are a type of algebraic data type so in theory
i can reuse a lot of the existing code to implement them.

Thanks

--Phil



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