Perl 6 中的引号结构

作者: 焉知非鱼 | 来源:发表于2016-03-12 00:21 被阅读22次

    The Q Lang


    在 Perl 6 中, 字符串通常使用一些引号结构来表示. 这些引号结构中,最简单的就是 Q, 通过便捷方式 「…」Q 后跟着由任意一对儿分隔符包围着的文本. 大多数时候, 你需要的只是 '…'"…".

    Literal strings: Q


    Q[A literal string]
    「More plainly.」
    Q ^Almost any non-word character can be a delimiter!^
    

    分隔符能够嵌套, 但是在普通的 Q 形式中, 反斜线转义是不允许的. 换种说法就是, Q 字符串尽可能被作为字面量.

    Q<Make sure you <match> opening and closing delimiters>
    Q{This is still a closing brace → \}
    

    这些例子产生:

    A literal string
    More plainly.
    Almost any non-word character can be a delimiter!
    Make sure you <match> opening and closing delimiters
    This is still a closing brace → \
    

    Escaping: q


    'Very plain'
    q[This back\slash stays]
    q[This back\\slash stays] # Identical output
    q{This is not a closing brace → \}, but this is → }
    Q :q $There are no backslashes here, only lots of \$\$\$!$
    '(Just kidding. There\'s no money in that string)'
    'No $interpolation {here}!'
    Q:q#Just a literal "\n" here#
    

    q 形式的引号结构允许使用反斜线转义可能会结束字符串的字符. 反斜线自身也能被转义, 就像上面的第三个例子那样. 通常的形式是 '...'q 后跟着分隔符, 但是它也能作为 Q 上的副词使用, 就像上面的第五个和最后一个例子那样.

    这些例子产生:

    Very plain
    This back\slash stays
    This back\slash stays
    This is not a closing brace → } but this is →
    There are no backslashes here, only lots of $$$!
    (Just kidding. There's no money in that string)
    No $interpolation {here}!
    Just a literal "\n" here
    

    Interpolation: qq


    my $color = 'blue';
    say "My favorite color is $color!" # My favorite color is blue!
    

    qq 形式 -- 通常使用双引号写成 -- 允许变量的插值, 例如字符串中能写入变量, 以使变量的内容能插入到字符串中. 在 qq 引起字符串中, 也能转义变量.

    say "The \$color variable contains the value '$color'";
    # The $color variable contatins the value 'blue'
    

    qq 的另外一种功能是使用花括号在字符串中插值 Perl 6 代码:

    my ($x, $y, $z) = 4, 3.5, 3;
    say "This room is $x m by $y m by $z m."
    say "Therefore its volume should be { $x * $y * $z } m³!"
    

    输出:

    This room is 4 m by 3.5 m by 3 m.
    Therefore its volume should be 42 m³!
    

    默认情况下, 只有带有 '$' 符号的变量才能正常插值. 这时, "documentation@perl6.org" 不会插值 @perl6 变量. 如果呢确实想那么做, 在变量名后面添加一个 []:

    my @neighbors = "Felix", "Danielle", "Lucinda";
    say "@neighbors[] and I try our best to coexist peacefully."
    

    输出:

    Felix Danielle Lucinda and I try our best to coexist peacefully.
    

    通常使用一个方法调用会更合适. 只有在 qq 引号中, 方法调用后面有圆括号, 就能进行插值:

    say "@neighbors.join(', ') and I try our best to coexist peacefully."
    

    输出:

    Felix, Danielle, Lucinda and I try our best to coexist peacefully.
    

    "@example.com" 产生 @example.com.

    Word quoting: qw


    <a b c> eqv ('a', 'b', 'c')
    qw|! @ # $ % ^ & * \| < > | eqv '! @ # $ % ^ & | < >'.words
    Q:w { [ ] \{ \} } eqv ('[', ']', '{', '}')
    

    :w 通常写作 <...>qw, 把字符串分割为"words" (单词). 在这种情景下, 单词被定义为由空格分割的一串非空白字符. q:wqw 继承了 q 的插值和转义语法, 还有单引号字符串分割符, 而 QwQ:w 继承了 Q 的非转义语法.

    my @directions = 'left', 'right,', 'up', 'down';
    

    这样读和写都更容易:

    my @directions = <left right up down>;
    

    Word quoting with interpolation: qqw


    qw 形式的 word quoting 不会进行变量插值:

    my $a = 42; say qw{$a b c};  # $a b c
    

    因此, 如果你想在引号字符串中进行变量插值, 你需要使用 qqw 变体:

    my $a = 42;
    my @list = qqw{$a b c};
    say @list;                # 42 b c
    

    或者同样的:

    my $a = 42;
    my @list = «$a b c»;
    say @list;                # 42 b c
    

    Shell quoting: qx


    把一个字符串作为一个外部程序运行, 在 Perl 6 中反引号不再用于 shell quoting, 并且 qx 不再插值 Perl 变量, 因此:

    my $world = "there";
    say qx{echo "hello $world"}
    

    仅仅打印 hello. 然而, 如果你在调用 perl6 之前声明了一个环境变量, 这在 qx 里是可用的, 例如:

    WORLD="there" perl6
    > say qx{echo "hello $WORLD"}
    

    现在会打印 hello there.

    调用 qx 会返回结果, 所以这个结果能被赋值给一个变量以便后来使用:

    my $output = qx{echo "hello!"};
    say $output;    # hello!
    

    Shell quoting with interpolation: qqx


    带插值的 Shell quoting:

    my $world = "there";
    say qqx{echo "hello $world"};  # hello there
    

    再一次, 外部命令的输出结果可以保存在一个变量中:

    my $word = "cool";
    my $option = "-i";
    my $file = "/usr/share/dict/words";
    my $output = qqx{grep $option $word $file};
    # runs the command: grep -i cool /usr/share/dict/words
    say $output;      # Cooley
    Cooley's
    Coolidge
    Coolidge's
    cool
     ...
    

    Heredocs: :to


    heredocs 是多行字符串字面量的便捷方式, 你能选择自己的分隔符:

    say q:to/END/;
    Here is
    some multi-line
    string
    END
    

    heredoc 的内容从下一行开始.

    my $escaped = my-escaping-function(q:to/TERMINATOR/, language => 'html');
    Here are the contents of the heredoc.
    Potentially multiple lines.
    TERMINATOR
    

    如果终止分隔符缩进了, 同等数量的缩进会从字符串字面量上移除. 因此下面这个 heredoc

    say q:to/END/;
        Here is
        some multi line
            string
        END
    

    输出:

    Here is
    some multi line
        string
    

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