Day 15 – Building a (little) Spacecraft with Perl 6

Showing off long ears

In the previous post, we have encountered some kind of special elves’ magic:

enum Fuel <Solid Liquid Gas>;

class SpeedChoice does ASNChoice {
    method ASN-choice { { mph => (1 => Int), kmph => (0 => Int) } }
}

class Rocket does ASNSequence {
    has Str $.name is UTF8String;
    has Str $.message is default-value("Hello World") is UTF8String;
    has Fuel $.fuel;
    has SpeedChoice $.speed is optional;
    has Str @.payload is UTF8String;

    method ASN-order { <$!name $!message $!fuel $!speed @!payload> }
}

my $rocket = Rocket.new(
    name => 'Falcon',
    fuel => Solid,
    speed => SpeedChoice.new((mph => 18000)),
    payload => [ "Car", "GPS" ]);

my $rocket-bytes = ASN::Serializer.serialize($rocket, :mode(Implicit));

#`[ Result:
      Blob.new(
          0x30, 0x1B, # Outermost SEQUENCE
          0x0C, 0x06, 0x46, 0x61, 0x6C, 0x63, 0x6F, 0x6E, # NAME, MESSAGE is missing
          0x0A, 0x01, 0x00, # ENUMERATED
          0x81, 0x02, 0x46, 0x50, # CHOICE
          0x30, 0x0A, # SEQUENCE OF UTF8String
              0x0C, 0x03, 0x43, 0x61, 0x72,  # UTF8String
              0x0C, 0x03, 0x47, 0x50, 0x53); # UTF8String
]

say ASN::Parser.new(:type(Rocket)).parse($rocket-bytes) eqv $rocket; # Certainly true!

However, as an elf I know once quoted, ‘It’s hard to tell the difference between mastered technique and magic’. So the mystery can be resolved? Let’s glance over the way it all works.