Re: To :D or not to :D

Hi,

On 10/12/2015 03:41 PM, Mark Overmeer wrote:
> 
> Hi all,
> 
> Liz and Tux demonstrate powerful Perl6 code at each monthly meeting of
> our Mongers in Amsterdam.  Looking at their examples, I collected a few
> questions of which I want to discuss the first one in this thread.
> 
> 
> When I look at Merijns (Tux') code, I see a huge number of :D attributes.
>     https://github.com/Tux/CSV/blob/master/lib/Text/CSV.pm
> 
> Close to all scalar positional parameter (51x) carry the :D flag.  I count
> only 3 where the parameter does accept undef and the method is able to
> handle it.  I count another 3 where the :D is missing, but the method is
> not able the handle it.
> 
> The same for examples Liz shows us in our core code: most scalar
> positional parameters have :D.
> 
> Writing a sub which is able to handle undef is usually more work than
> implementing "I expect sane values for all of the parameters".
> 
> 
> Questions:
>   . are they using :D correctly?

Yes, though not everybody uses :D as much as they do. Do you check that
all the parameters that your Perl 5 methods/subs receive are defined? If
not, you wouldn't use :D in Perl 6 either.

>   . the simpelest code does not handle the undef case... but now needs
>     the more complex prototype to be correct.
>   . it feels like the wrong default: usually you have to do something
>     extra for the unusual cases, not the 90%+ usual cases.

FWIW you can now (as of a few days ago) control the default with

use invocant :D;

and

use parameters :D;

which means all those :D annotations can go away, and you have to use :_
explicitly if you want to allow all.

That said, I agree that it's the wrong default. And the design documents
even mandate a default to :D, though at the time it was written, it
wasn't clear how to switch off that default, nor how to avoid having to
write

	method new(MyClassHere:U: *@args) { ... }

in the constructor, which would be quite hostile to newbies. It's still
not clear to me how to avoid that.

And I don't know if we can change it now without creating a huge havoc
in the existing ecosystem.

Another concern is that if "everything" defaults to :D, then classes
(and other type objects) aren't really first class objects anymore,
which is a really neat thing to have.

>   . :D looks really ugly, don't you think?  Try to explain to students
>     to add this smiley everywhere.

It's not uglier than a 'die "Must be defined" unless defined $x'

Cheers,
Moritz