Comments on: Day 19 – False truth

By: carl

carl — Tue, 21 Dec 2010 05:00:50 +0000

Regardless of the object's type, when you write prefix:, that gets translated into .Bool. You're absolutely right that the .Bool method on the Bool type doesn't do much; it basically looks like this:

method Bool { self }

In other words, a kind of identity operation. As to why there'd be a need to call the .Bool method on something that's already Bool (besides the trivial reason that it's always done): there's not really any good reason to do that... until someone overrides the method, as in this post. An optimizer that could prove that no overriding has taken place would be free to remove the call.

By: skids

skids — Mon, 20 Dec 2010 20:49:43 +0000

The 42 example makes sense.

But, True is a already Bool; why would it call .Bool in order to be converted to a Bool?

By: carl

carl — Sun, 19 Dec 2010 11:14:42 +0000

Dan, what Christopher and Jesper said. The “always return False” role was just an example, and you can overload the .Bool method to return things according to rules as complex as you like.

By: Jesper

Jesper — Sun, 19 Dec 2010 08:33:49 +0000

Maybe it should be noted that this doesn’t take away any functionality.

$r = run($cmd);
say $r == 42; # Still possible to do anything you’d do with a number.
say ?($r + 0); # Will still say true according to the normal
# non-overloaded bool role; it’s not poisonous.

By: Jesper

Jesper — Sun, 19 Dec 2010 08:26:21 +0000

I think run simply returns the process return values in this way:

return ($result == 0 ?? $result but True !! $result but False);

Thus returning a zero which tests true, or a nonzero which tests false.

By: Christopher Osborn

Christopher Osborn — Sun, 19 Dec 2010 07:29:18 +0000

I can’t imagine the example method shown is the one used for the overloaded &run, for the reason you state. However, I think it would be done similarly to this…

return $exit_code but role { method Bool { $_ == 0 } }; # or perhaps !$_

…though I’m sure they use a predefined type.
(Code not guaranteed to be accurate, but I hope you get the idea.)

By: Dan J.

Dan J. — Sun, 19 Dec 2010 03:20:27 +0000

I’m confused on one issue. You say “Normally, for Ints, the .Bool method (and therefore the prefix: operator) returns whether the number is non-zero, but here we make it always return False.”

Does it ALWAYS return False? If so, how does the overloaded &run work? How do you specify the conditions on when it returns True and when it returns False? Specifying when something returns True or False is a very useful feature. Having it ALWAYS return one specific value would be much less so, since there would be no need to every check it’s return value.