Possible to create class aliases dynamically? - raku

Question

Got this class:
class Mass-lb is Mass {
method new(Rat:D() $value = 1.0) {
self.bless(
:abbr('lb'),
:base_value(453.59237),
:$value,
);
}
}
I have created aliases like this:
class Mass-lbs is Mass-lb { }
class Mass-pound is Mass-lb { }
class Mass-pounds is Mass-lb { }
class Mass-pnds is Mass-lb { }
But I'd prefer to do something like this:
my #lb-syn = < lbs pounds pound pnds >;
for #lb-syn {
EVAL 'class ::("Mass-$_") is Mass-lb {}';
}
This throws an error:
Name ::("Mass-$_") is not compile-time known, and can not serve as a package name
PHP has a built-in for creating aliases: https://www.php.net/manual/en/function.class-alias.php
I couldn't find anything similar for raku.

Answer 1

In RakuAST there's a class that you can call to create a new type. But that the RakuAST branch hasn't landed yet.
Until then, your approach using EVAL is valid, you just need to make it a bit simpler:
class Mass-lb { }
BEGIN "constant Mass-$_ = Mass-lb".EVAL
for <lbs pounds pound pnds>;
my $mlb = Mass-lbs.new;
Make sure the aliases are created at BEGIN time.
No need to subclass, you can use a constant for aliasing.
Since constants are our by default, they're visible outside of the EVAL.

Answer 2

Alternatively, you could use raku Physics::Unit and Physics::Measure...
use Physics::Unit;
use Physics::Measure :ALL;
# define a new custom Unit
Unit.new( defn => 'lbm', names => <Mass-lb Mass-lbs Mass-pound Mass-pounds Mass-pnds> );
say GetUnit('Mass-lbs').names; #[Mass-lb Mass-lbs Mass-pound Mass-pounds Mass-pnds]
# use the Unit in a Measure
my $mass = ♎️'42 Mass-pnds';
say $mass; #42Mass-lb
say $mass.^name; #(..Mass) ...class name
# convert to another Unit
my $kgm = $mass.in: 'kg';
say $kgm; #19.05087954kg
# convert back
say $kgm.in: 'Mass-pound'; #42Mass-lb
# raku Rats mean that the back conversion is identical
say $kgm cmp $mass; #Same
# with % or abs Error
my $mass2 = ♎️'42 Mass-pnds ±3%';
say $mass2; #42Mass-lb ±1.26
say $mass2.in: 'kg'; #19.05087954kg ±0.5715263862
More info at github Physics::Unit and Physics::Measure...