Re: [RFC] A new mechanism to access the active field of unions

[Jose E. Marchesi]

> On Thu, Mar 04, 2021 at 09:01:51PM +0100, Jose E. Marchesi wrote:
>> 
>> >
>> > So, to summarize:
>> >
>> >
>> >   VAL isa TYPE
>> >
>> >     Checks whether VAL is of type TYPE.
>> >     This already works.
>> >
>> >   VAL isa IDENTIFIER
>> >
>> >     Checks whether VAL is an union value _and_ has a field named
>> >     IDENTIFIER.
>> >
>> >   VAL isa STR
>> >
>> >     Checks whether VAL is an union value _and_ has a field named STR.
>> >
>> > If reusing `isa' like that is confusing, we can come with a new syntax
>> > for the second and third usages, like:
>> >
>> >     VAL?.IDENTIFIER
>> >     VAL?."STR"
>> >
>> > or something like that...
>> 
>
> I think the most general form is `VAL keyword STR`. (keyword can be `isa` or
> something better)
>
> My reason:
>
> ```poke
> var field = "field1";
> var isa1 = u isa "field1";
> var isa2 = u isa field;
>
> assert (isa1 && isa2);
> ```
>
> If we choose the `VAL keyword IDENTIFIER`, we will lose the power to save
> the field name in a variable.

Yes, the `VAL keyword STRING' is more general.

Still, I was thinking on supporting _both_, since for most cases the
user will want to specify a constant name, and it is faster and easier
to just write an identifier.

>
>> Gotta say that my favorite option for now is to overload `isa'.
>> 
>
>
> I don't think it's a good idea to give `isa` a completely different
> syntax and semantics.

Yes I can see why.

> In fact you're just *implicitly* adding a new keyword to the language,
> it's just not obvious (An `isa` for types, and another one for unions
> which accepts an identifier on the RHS).

I don't think that the propety of "being a keyword" is accumulative,
i.e. `if' is no more (nor less) keyword than `else', unless you accept
the possibility of removing certain uses from the language at some
point.

In any case, the alternative is to add a new keyword to the language
"explicitly" :(

Unless... we adopt the attribute syntax and support only the STR
version:

  VAL'holds("foo")


> If you do so, "you're acting exactly like C++ committee here" :P
> (I'm sorry for being rude here :D)

What!?! You... I demand satisfaction!  Choose your weapon.