perl – 采用可选块参数的子例程

不幸的是,这有点痛苦.您需要使用Perl 5.14中引入的关键字API.这意味着您需要在C中实现它(以及它的自定义解析)并使用XS将其链接到Perl.

幸运的是,DOY为Perl关键字API编写了一个很好的包装器,允许您在纯Perl中实现关键字.没有C,没有XS！它被称为Parse::Keyword.

不幸的是,这有很多错误处理封闭的变量.

幸运的是,他们可以使用PadWalker进行解决.

无论如何,这是一个例子：

use v5.14;

BEGIN {
  package My::Print;
  use Exporter::Shiny qw( myprint );
  use Parse::Keyword { myprint => &;_parse_myprint };
  use PadWalker;

  # Here's the actual implementation of the myprint function.
  # When the caller includes a block,this will be the first
  # parameter. When they don't,we'll pass an explicit undef
  # in as the first parameter,to make sure it's nice and
  # unambiguous. This helps us distinguish between these two
  # cases:
  #
  #    myprint { BLOCK } @list_of_coderefs;
  #    myprint @list_of_coderefs;
  #
  sub myprint {
    my $block = shift;
    say for defined($block) ? map($block->($_),@_) : @_;
  }

  # This is a function to handle custom parsing for
  # myprint.
  #
  sub _parse_myprint {

    # There might be whitespace after the myprint
    # keyword,so read and discard that.
    #
    lex_read_space;

    # This variable will be undef if there is no
    # block,but we'll put a coderef in it if there
    # is a block.
    #
    my $block = undef;

    # If the next character is an opening brace...
    #
    if (lex_peek eq '{') {

      # ... then ask Parse::Keyword to parse a block.
      # (This includes parsing the opening and closing
      # braces.) parse_block will return a coderef,# which we will need to fix up (see later).
      #
      $block = _fixup(parse_block);

      # The closing brace may be followed by whitespace.
      #
      lex_read_space;
    }

    # After the optional block,there will be a list
    # of things. Parse that. parse_listexpr returns
    # a coderef,which when called will return the
    # actual list. Again,this needs a fix up.
    #
    my $listexpr = _fixup(parse_listexpr);

    # This is the stuff that we need to return for
    # Parse::Keyword.
    #
    return (

      # All of the above stuff happens at compile-time!
      # The following coderef gets called at run-time,# and gets called in list context. Whatever stuff
      # it returns will then get passed to the real
      # `myprint` function as @_.
      #
      sub { $block,$listexpr->() },# This false value is a signal to Parse::Keyword
      # to say that myprint is an expression,not a
      # full statement. If it was a full statement,then
      # it wouldn't need a semicolon at the end. (Just
      # like you don't need a semicolon after a `foreach`
      # block.)
      #
      !!0,);
  }

  # This is a workaround for a big bug in Parse::Keyword!
  # The coderefs it returns get bound to lexical
  # variables at compile-time. However,we need access
  # to the variables at run-time.
  #
  sub _fixup {

    # This is the coderef generated by Parse::Keyword.
    #
    my $coderef = shift;

    # Find out what variables it closed over. If it didn't
    # close over any variables,then it's fine as it is,# and we don't need to fix it.
    #
    my $closed_over = PadWalker::closed_over($coderef);
    return $coderef unless keys %$closed_over;

    # Otherwise we need to return a new coderef that
    # grabs its caller's lexical variables at run-time,# pumps them into the original coderef,and then
    # calls the original coderef.
    #
    return sub {
      my $caller_pad = PadWalker::peek_my(2);
      my %vars = map +($_ => $caller_pad->{$_}),keys %$closed_over;
      PadWalker::set_closed_over($coderef,%vars);
      goto $coderef;
    };
  }
};

use My::Print qw( myprint );

my $start = "[";
my $end   = "]";

myprint "a","b","c";

myprint { $start . $_ . $end } "a","c";

这会生成以下输出：

a
b
c
[a]
[b]
[c]