perl-meta-signatures.md

Meta signatures

Abstract

Imagine every function of instance of its own class with every function argument as class field.

use has word to allocate context variable

	sub foo {
		has $x => isa => <Constraint>, default => 1;
		has $y => isa => <Constraint>, default => 2;

		return $x + $y;
	}

import signature of another fuction

Treat function reference as a role

	sub Parent::foo {
		has $x => default => 1;
		has $y => default => 2;
	}

	sub Child::foo {
		does \& Parent::foo;

		return 2 * $x + $y;
	}

works for anonymous functions as well

	my $foo = sub { ... };
	my $bar = sub { does $foo; ... }

OOP - override shortcut

	override bar {
		return 2 * $x + $y;
	}

OOP - returns constraint

	package Parent {
		sub foo {
			returns Number::Even;
		}
	}

	package Child {
		override foo {
			returns Number::odd;
			# every return will
		}
	}

dependencies - conditionally required

	sub foo {
		has $x => required => 1;
		has $y => required => sub { $x % 2 };
	}

dependencies - address parameter before defined

	sub volume {
		has $x => required => sub { ! exists $area };
		has $y => required => sub { ! exists $area };
		has $z => required => 1;
		has $area => default => sub { $x * $y };

		return $area * $z;
	}

consistency with class declaration

using modified Object::Pad example

class Point {
	has $x = 0;
	has $y = 0;

	method move {
		has $dX;
		has $dY;

		$x += $dX;
		$y += $dY;
	}
}

multiple dispatch

continuation of previous example

{
	method move {
		has $dX =>
			required => sub { exists $dY && ! exists $radius },
			default  => sub { $distance * cos $radius },
			;
		has $dY =>
			required => sub { exists $dX && ! exists $radius },
			default  => sub { $distance * sin $radius },
			;
		has $radius => required => sub { ! exists $dX || ! exists $dY };
		has $distance => required => sub { exists $radius };
		$x += $dX;
		$y += $dY;
	}
}

$obj->move (1, 1);
$obj->move (radius => 0.707, distance => 0.707);
$obj->move (dX => 1, dY => 1);
$obj->move (1, dX => 1);
$obj->move (1, dY => 1);