gather 是一个语句或者能返回一序列值的 block 前缀。该值来自于 gather 块中的动态作用域的 take 调用。
my $a = gather {
take 1;
take 5;
take 42;
}
say join ', ', @$a; # 1, 5, 42gather/take 能根据上下文按需(惰性地)生成值。如果你想强制惰性求值则使用 lazy 子例程或方法。绑定到一个标量或无符号的容器上也会强制惰性求值。
举个例子:
my @vals = lazy gather {
take 1;
say "Produced a value";
take 2;
}
say @vals[0];
say 'between consumption of two values';
say @vals[1];
# OUTPUT:
# 1
# between consumption of two values
# Produced a value
# 2gather/take 是动态作用域的, 所以你可以从 gather 内部所调用的 subs 或方法中调用 take:
sub weird(@elems, :$direction = 'forward') {
my %direction = (
forward => sub { take $_ for @elems },
backward => sub { take $_ for @elems.reverse },
random => sub { take $_ for @elems.pick(*) },
);
return gather %direction{$direction}();
}
say weird(<a b c>, :direction<backward> ); # (c b a)如果值需要在调用者这边可变, 那么使用 take-rw
my @a = 1...3;
sub f(@list){ gather for @list { take-rw $_ } };
for f(@a) { $_++ };
say @a; # OUTPUT«[2 3 4]»gather .. for
gather for
my @geese = ["African", "Roman Tufted", "Toulouse", "Pilgrim", "Steinbacher"] ;
my @all = ["Mallard", "Hook Bill", "African", "Crested", "Pilgrim","Toulouse", "Blue Swedish"];
my @r = gather for @all -> $a {
take $a if $a !(elem) @geese;
}
say @r;