Flag proposal

[Andrei Alexandrescu]

On 6/11/11 9:30 AM, Steven Schveighoffer wrote:
> On Sat, 11 Jun 2011 09:25:19 -0400, Michel Fortin
> <michel.fortin at michelf.com> wrote:
>
>> On 2011-06-11 09:05:52 -0400, "Steven Schveighoffer"
>> <schveiguy at yahoo.com> said:
>>
>>> As far as the negation, I think we need one more layer of type:
>>> struct FlagParam(string pname)
>>> {
>>> Flag!pname value;
>>> alias value this;
>>> this(Flag!pname x) { this.value = x }
>>> FlagParam op???() const { return FlagParam(cast(Flag!pname)!value); }
>>> }
>>> BTW, is there a way to hook "!"? Maybe this won't work... The idea is
>>> to have Yes.abc return FlagParam!"abc"(Flag!"abc".yes).
>>
>> If you need to convert a bool to a Flag!"abc", you can do any of these:
>>
>> func(expression ? Yes.abc : No.abc);
>>
>> or
>>
>> func(cast(Flag!"abc")expression);
>>
>> or, with your proposal:
>>
>> func(FlagParam!"abc"(expression));
>>
>> None of this is very appealing, but I find the first is the most
>> readable.
>
> We need another struct besides Yes and No that allows creation with
> another opDispatch:
>
> Foo.abc(expression);
>
> The best name for Foo is Flag, but that's taken...
>
> But still, is there a way to override the type returned by !flag? I
> think there isn't... That's kind of sucky. Flipping of a flag is a very
> common feature, I would expect to be able to do it easily.

I agree that's a drawback. I couldn't find a reasonable workaround.

Andrei