Tue, 09 Dec 2008

Quoting and Parsing


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NAME

"Perl 5 to 6" Lesson 23 - Quoting and Parsing

SYNOPSIS

    my @animals = <dog cat tiger>
    # or
    my @animals = qw/dog cat tiger/;
    # or 
    
    my $interface = q{eth0};
    my $ips = q :s :x /ifconfig $interface/;

    # -----------

    sub if {
        warn "if() calls a sub\n";
    }
    if();

DESCRIPTION

Quoting

Perl 6 has a powerful mechanism of quoting strings, you have exact control over what features you want in your string.

Perl 5 had single quotes, double quotes and qw(...) (single quotes, splitted on whitespaces) as well as the q(..) and qq(...) forms which are basically synonyms for single and double quotes.

Perl 6 in turn defines a quote operator named Q that can take various modifiers. The :b (backslash) modifier allows interpolation of backslash escape sequences like \n, the :s modifier allows interpolation of scalar variables, :c allows the interpolation of closures ("1 + 2 = { 1 + 2 }") and so on, :w splits on words as qw/.../ does.

You can arbitrarily combine those modifiers. For example you might wish a form of qw/../ that interpolates only scalars, but nothing else? No problem:

    my $stuff = "honey";
    my @list = Q :w :s/milk toast $stuff with\tfunny\nescapes/;
    say @list[*-1];                     # prints with\nfunny\nescapes

Here's a list of what modifiers are available, mostly stolen from S02 directly. All of these also have long names, which I omitted here.

    Features:
        :q          Interpolate \\, \q and \'
        :b          Other backslash escape sequences like \n, \t
    Operations:
        :x          Execute as shell command, return result
        :w          Split on whitespaces
        :ww         Split on whitespaces, with quote protection
    Variable interpolation
        :s          Interpolate scalars   ($stuff)
        :a          Interpolate arrays    (@stuff[])
        :h          Interpolate hashes    (%stuff{})
        :f          Interpolate functions (&stuff())
    Other
        :c          Interpolate closures  ({code})
        :qq         Interpolate with :s, :a, :h, :f, :c, :b
        :regex      parse as regex

There are some short forms which make life easier for you:

    q       Q:q
    qq      Q:qq
    m       Q:regex

You can also omit the first colon : if the quoting symbol is a short form, and write it as a singe word:

    symbol      short for
    qw          q:w
    Qw          Q:w
    qx          q:x
    Qc          Q:c
    # and so on.

However there is one form that does not work, and some Perl 5 programmers will miss it: you can't write qw(...) with the round parenthesis in Perl 6. It is interpreted as a call to sub qw.

Parsing

This is where parsing comes into play: Every construct of the form identifier(...) is parsed as sub call. Yes, every.

    if($x<3)

is parsed as a call to sub if. You can disambiguate with whitespaces:

    if ($x < 3) { say '<3' }

Or just omit the parens altogether:

    if $x < 3 { say '<3' }

This implies that Perl 6 has no keywords. Actually there are keywords like use or if, but they are only special if used in a special syntax.

MOTIVATION

Various combinations of the quoting modifiers are already used internally, for example q:w to parse <...>, and :regex for m/.../. It makes sense to expose these also to the user, who gains flexibility, and can very easily write macros that provide a shortcut for the exact quoting semantics he wants.

And when you limit the specialty of keywords, you have far less troubles with backwards compatibility if you want to change what you consider a "keyword".

SEE ALSO

http://perlcabal.org/syn/S02.html#Literals

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