为什么 sub 在 raku 的 "return"ed 映射中使用时不能访问动态变量？

Question

当在地图中使用时，似乎 sub 无法访问动态变量，并且该地图是“return”ed。

考虑这段代码：

sub start {
    my $*something = 'foobar';
    
    # WORKS
    say 'first say-something:';
    say-something;
    
    # WORKS
    say 'mapped say-something, no return:';
    my @foo = (^2).map({say-something});
    
    # ERROR: Dynamic variable $*something not found
    say 'mapped say-something, with return:';
    return (^2).map({say-something});
}

sub say-something {
    say $*something;
    1
}

start;

这将输出：

first say-something:
foobar
mapped say-something, no return:
foobar
foobar
mapped say-something, with return:

Dynamic variable $*something not found
  in sub say-something at main.raku line 18
  in block <unit> at main.raku line 14

为什么sub无法访问动态变量？有解决办法吗？

Answer 1

例程 map 是 lazy，因此块运行直到 start 返回。

来自文档：

multi method map(Hash:D \hash)
multi method map(Iterable:D \iterable)
multi method map(|c)
multi method map(\SELF: &block;; :$label, :$item)
multi sub map(&code, +values)
Examples applied to lists are included here for the purpose of illustration.

For a list, it invokes &code for each element and gathers the return values in a sequence and returns it. This happens lazily, i.e. &code is only invoked when the return values are accessed.

所以在你的情况下，地图被评估一次 start returns，因此动态变量已经超出范围。

解决此问题的方法是通过添加 .eager

强制 map 急切地 发生

return (^2).map({say-something}).eager

为什么 sub 在 raku 的 "return"ed 映射中使用时不能访问动态变量？

Why can't a sub access dynamic variables when used in a "return"ed map in raku?

subroutine

rakudo

dynamic-variables

raku