raku - Perl 6 : writable multidimensional subscript access with AT-POS

Question

您可以使用 AT-POS 轻松地允许下标访问您自己的类:

class Foo
{
    has @.grid;

    method AT-POS($x) is rw { return-rw @!grid[$x] }
    method Str { '<' ~ @!grid.join(' ') ~ '>' }
    method gist { self.Str }
}

my $foo = Foo.new(:grid(<a b c d e>));
say $foo;
say $foo[2];
$foo[3] = 'z';
say $foo;

输出:

<a b c d e>
c
<a b c z e>

但我需要二维下标访问。我已经想出了如何使这项工作适合阅读，但在写作时它就死了:

class Bar
{
    has @.grid;

    method AT-POS($y, $x) is rw { return-rw @!grid[$y;$x] }
    method Str { '<' ~ @!grid».join(' ').join("\n ") ~ '>' }
    method gist { self.Str }
}

my $bar = Bar.new(:grid(<a b c d e>, <f g h i j>, <k l m n o>));
say $bar;
say $bar[1;2];
$bar[2;3] = 'z';
say $bar;

输出:

<a b c d e
 f g h i j
 k l m n o>
h
Too few positionals passed; expected 3 arguments but got 2
  in method AT-POS at ./p6subscript line 25
  in block <unit> at ./p6subscript line 33

有什么办法可以使这项工作？

Answer 1

不知何故，AT-POS方法没有被调用。 documentation提到了 ASSIGN-POS 的使用相反，所以我们开始:

class Bar
{
    has @.grid is rw;

    method AT-POS($y, $x) is rw { say "AT-POS $y, $x"; return-rw @!grid[$y;$x] }
    method ASSIGN-POS($y, $x, $new) { say "ASSIGN-POS $y, $x"; @!grid[$y;$x] = $new }
    method Str { '<' ~ @!grid».join(' ').join("\n ") ~ '>' }
    method gist { self.Str }
}

my $bar = Bar.new(:grid(<a b c d e>, <f g h i j>, <k l m n o>));
say $bar;
say $bar[1;2];
$bar[2;3] = 'z';
say $bar;

有趣的是，这会产生另一个错误:

Cannot modify an immutable List ((k l m n o))
  in method ASSIGN-POS at semilist-so.p6 line 8
  in block <unit> at semilist-so.p6 line 16

所以问题不是真正的语法，而是你正在使用不可变列表的事实。您应该使用 Array s，它们是可变的，你将能够做到这一点。

class Bar
{
    has @.grid is rw;

    method AT-POS($y, $x) is rw { return-rw @!grid[$y;$x] }
    method ASSIGN-POS($y, $x, $new) { @!grid[$y;$x] = $new }
    method Str { '<' ~ @!grid».join(' ').join("\n ") ~ '>' }
    method gist { self.Str }
}

my $bar = Bar.new(:grid([<a b c d e>], [<f g h i j>], [<k l m n o>]));
say $bar;
say $bar[1;2];
$bar[2;3] = 'z';
say $bar;