Re: how to set constants from command line?

[Parrot Raiser]

It's not really a constant he wants, but a value that's read-only to
everything but the setting routine. Some sort of object, perhaps?

Re: how to set constants from command line?

[Larry Wall]

On Sun, Dec 15, 2013 at 11:28:02AM -0500, Parrot Raiser wrote:
: It's not really a constant he wants, but a value that's read-only to
: everything but the setting routine. Some sort of object, perhaps?

Any parameter to sub MAIN is readonly by default, since parameters
in general are readonly by default.

Larry

how to set constants from command line?

[Richard Hainsworth]

I would like to set a series of magic numbers from the command line.

I have in a program

constant N-SCENARIOS = 10;

which works great. But I would like to set an option in the command 
line, such as

perl6 program.p6 scenarios=15

and then within program have the option assigned to the constant.

I tried this with a BEGIN block, something like

BEGIN {
  if @*ARGS[0] ~~ / scenarios \= (.d+) / { constant N-SCENARIOS = $0 }
}

say N-SCENARIOS;

But this does not work.

I could have a normal scalar
my $scenarios ;

But that doesn't seem as elegant as creating a constant.

Any neat trick I am missing?

Richard

Re: how to set constants from command line?

[Moritz Lenz]

Hi Richard,

On 12/12/2013 08:56 AM, Richard Hainsworth wrote:

I would like to set a series of magic numbers from the command line.

I have in a program

constant N-SCENARIOS = 10;

which works great. But I would like to set an option in the command
line, such as
perl6 program.p6 scenarios=15


.. then it's not a constant. So don't use a constant. Use a variable 
instead.



I could have a normal scalar
my $scenarios ;

But that doesn't seem as elegant as creating a constant.


Using a constant for something that isn't a constant doesn't strike me 
as elegant at all.


Note that in Perl 6, there is no guarantee that command line arguments 
are known at compile time, so mucking with @*ARGS in BEGIN seems like a 
very bad idea.


Cheers,
Moritz