Flag proposal

[Andrei Alexandrescu]

On 6/10/11 2:12 PM, Michel Fortin wrote:
> On 2011-06-10 14:52:53 -0400, Robert Clipsham
> <robert at octarineparrot.com> said:
>
>> On 10/06/2011 19:06, Andrei Alexandrescu wrote:
>>> Ugly is in the eye of the beholder, but I fail to see how the added
>>> punctuation makes Flag!"param".yes significantly more verbose than param
>>> : true.
>>
>> foo(param: true, otherParam: false);
>> foo(Flag!"param".yes, Flag!"otherParam".no);
>>
>> I don't know about you, but I find the former is far more legible. I'd
>> hate to see my code littered with Flag!"something".
>
> I have to say I totally agree with Robert.
>
> I agree with the need for a way to name parameters, but instantiating a
> pseudo-boolean type from a template for each function parameter is worse
> than the initial problem. Templates are already a difficult concept to
> grasp for most people (because they're so abstract), we really don't
> need to replace every boolean parameter with a template type.

I disagree with the "templates are difficult" mantra. It is a meme from 
C++ much like "macros are evil" are one from C. It's a simple construct, 
you use it.

>>> The problem that named parameters are still optional remains. Or we need
>>> to add one extra language feature to specify required named parameters.
>>
>> void foo(bool param, bool otherParam, bool thisOneIsntRequired = false);
>>
>> Call it with or without named parameters, two are required, one is not.
>>
>> foo(otherParam: true, param: false);
>> foo(true, false);
>> foo(otherParam: true, param: false, thisOneIsntRequired: true);
>
> And just try to think of the signature for the function above if it was
> using Flag!
>
> void foo(Flag!"param" param, Flag!"otherParam" otherParam,
> Flag!"thisOneIsntRequired" thisOneIsntRequired =
> Flag!"thisOneIsntRequired".no);
>
> Do we really want to expose that in the documentation?

Yes, that's the whole point, though I agree the hiccup in the default 
initializer is annoying. The documentation doesn't need the names 
anymore; the user would just say:

void foo(
   Flag!"compressed",
   Flag!"encrypted",
   Flag!"buffered" = Flag!"buffered".no);

Save for the "ehm" default parameter this looks palatable to me.


Andrei