Perl 6 Cookbook: Introduction to Strings

AUTHOR

Scott Penrose

Definition

In Perl 6, a string is a sequence of zero or more characters and other simple objects, forming a single unit of data of the type Str. A string is data usually in its reportable or printable form. You can write a Perl string using single-quotes '', double-quotes "", and various quote-like operators. You can create and manipulate strings programmatically, in many ways. In fact, this is what Perl does best.

Description

Perl 6 interprets strings depending on context. A string may consist of zero or more characters, including letters, spaces, numbers, and other characters.

Declaring and assigning Strings

A string can be created explicitly, by declaring a variable using the Str keyword, and assigning a string value to it.

my Str $string = 'This Str is holding a String';

Or a string can be declared implicitly by assigning a string value to a variable, in this case a scalar. It automatically becomes a string variable.

my $scalar = 'Party time';

Displaying Strings

We display strings using print or say:

print ""        ; # output is an empty string
    print "Hello\n" ; # output is Hello followed by a new line
    say   "Hello"   ; # same
    say   'Hello'   ; # same

Both print and say also accept a list of things to display, and will attempt to join all the things into a string:

say "Hello", " World", "!"; # Hello World!

Strings can be appended to one another, using the concatenation operator, ~

Beware when using print and say; concatenation is slower than joining.

Here, three strings are concatenated into a single string. Output is Hello World! followed by a newline.

my Str $str;
    $str = "Hello" ~ " World" ~ "!";
    say $str; # Hello World!
    $str.say; # As above

Introspection

Perl 6 has extensive support for introspection, that is, to see the internals of types and objects during runtime. It is therefore possible to find out if a variable is a string and act upon that information.

We know we get a String when we declare something explicitly as Str:

my Str $string = 'This is $string: a scalar holding a String';
    say $string;
    say '$string is ', $string.^name;                 # Str

We can also easily show that a general variable containing a String, is in fact just that:

my $scalar = 'This is $scalar holding a String';
    say $scalar;
    say '$scalar is ', $scalar.^name;                 # Str

Numbers as strings

A number may be interpreted as a string, depending on the context:

say 1; # 1 is interpreted as a number
    say 2, " is a number interpreted as a string"; # 2 is a number interpreted as a string.
    say 1+2*3, " is a number interpreted as a string"; # 7 is a number interpreted as a string.

Note that Perl 6 ensures that the arithmetic expression before the first comma is evaluated even without enclosing parentheses, and that it is only afterwards that it is interpreted as a string.

Strings as numbers

Conversely, sometimes a string might be interpreted as a number:

print  +""     ; # a num-ified empty string evaluates as 0
    print  "1" + 1 ; # 2

The string, "1" is treated as a number in this context, added to the number 1 by the + operator, which returns the number, 2, as a string for output.

Context sensitivity is the essence of Perl. Keeping this in mind, what would you expect to be the output string, for the following?

my $string = "1" ~ "1" + 10; # 12, 21, or even... "111"?
    say $string;

But, "1+1", surrounded by quotation marks, either '' or "", stringifies the expression, so that it is evaluated as a string.

say "1 + 1"; # literally: 1 + 1

To force the interpretation of a string for any programmatic value it might contain, use the built-in EVAL() call:

say EVAL "1 + 1";    # 2

On the command-line, you may pass a string to the perl 6 interpretor, to have it evaluated as a program expression, by using the -e switch:

./perl6 -e "say 1+1"; # 2
    ./perl6 -e 'say "1+1"'; # 1+1

Assignments of non-strings set the variable to the appropriate type:

my $scalar = 1234;
    say $scalar; # 1234
    say '$scalar is ', $scalar.^name   # $scalar is Int

An object can be stringified, by using the ~ operator immediately prior to the variable's sigil

say '~$scalar is ', (~$scalar).^name; # ~$scalar is Str

Quotes, Interpolation and Quote-like operators

Single-quoted Strings

Strings that are written with single quotes are almost verbatim. However, backslashes are an escape character. This is so that you can write literal single-quotes within a single-quoted string, and also be able to write a backslash at the end of a single-quote-enclosed string:

say 'n\'     ; # Error: perl sees no closing '
    say '\\'     ; # \
    say 'n\''    ; # n'
    say 'n\n'    ; # n\n
    say 'n\\n'   ; # n\n
    say 'n\\\n'  ; # n\\n better spelled as:
    say 'n\\\\n' ; # n\\n

A few other backslashy escapes work in single quotes too

Double-quoted Strings

If you want to interpolate variables and other special characters within a literal string, use double quotes around the value:

my $var1 = 'dog' ;
    say "The quick brown fox jumps over the lazy $var1";

Interpolation

Double-quoted strings interpolate the elements of an array or a hash, closures, functions, backslashed control characters, and other good stuff. Single-quoted strings do not.

# literal whitespace
    my $squot = '    The quick brown fox jumps over the lazy dog.
    	dog.';
    my $dquot = "    The quick brown fox jumps over the lazy
    	dog.";
    say $squot;
    say $dquot;

# Double-quotes interpolate special backslash values,
    # but single-quotes do not
    say 'The quick brown fox\n\tjumps over the lazy dog\n';
    say "The quick brown fox\n\tjumps over the lazy dog\n";

# interpolate array elements:
    my @animal = ("fox", "dog");
    say 'The quick brown @animal[0] jumps over the lazy @animal[1]';
    say "The quick brown @animal[0] jumps over the lazy @animal[1]";

# interpolate hash elements:
    my %animal = (quick => 'fox', lazy => 'dog');
    say 'The quick brown %animal{\'quick\'} jumps over the lazy %animal{\'lazy\'}.';
    say "The quick brown %animal{'quick'} jumps over the lazy %animal{'lazy'}.";

# interpolate methods, closures, and functions:
    say '@animal.elems() {@animal.elems} &elems(@animal)';
    say "@animal.elems() {@animal.elems} &elems(@animal)";

Perl's Quote-like Operators

It's often useful to use something other than single or double quotes when declaring strings. To do so use the q// and qq// quote operators, which provide advanced interpolation control:

# Single quoted strings
    say 'I have to escape my \'single quotes\' in this string';
    say q/This string allows 'single quotes' seamlessly/;

# Double quoted strings
    say "I have to escape my \"double quotes\" in this string";
    say qq/This string allows "double quotes" seamlessly/;

The slashes in q// and qq// can be replaced with most of the delimiters that worked in Perl 5. All of Unicode above Latin-1 is reserved for user-defined quotes.

# Single quoted strings
    say q'Many delimiters are available for quoting';
    say q"Many delimiters are available for quoting";
    say q`Many delimiters are available for quoting`;
    say q[Many delimiters are available for quoting];
    say q<Many delimiters are available for quoting>;
    say q{Many delimiters are available for quoting};
    say q?Many delimiters are available for quoting?;

# But not the colon B<:>
    q:illegal_perl6:; #legal perl 5

# Also a space is needed below, because q() is a function call
    say q (Many delimiters are available for quoting);

Advanced Interpolation Control

Perl 6 allows very fine control over string quoting using the q// quote operator with specialized adverbs. For instance, q:s// signifies that we only want scalars interpolated. These adverbs can also be expressed in a short form, for instance q:s// can be expressed as qs//.

:s          :scalar         Interpolate $ vars
    :a          :array          Interpolate @ vars
    :h          :hash           Interpolate % vars
    :f          :function       Interpolate & calls
    :c          :closure        Interpolate {...} expressions
    :b          :backslash      Interpolate \n, \t, etc. (implies :m)
    :w          :words          Split result on words (no quote protection)
    :ww         :quotewords     Split result on words (with quote protection)
    :x          :exec           Execute as command and return results
    :t          :to             Interpret result as heredoc terminator

# Raw quoting: no escaping at all (unless otherwise adverbed)
    say Q/(no interpolation) even backslash has no special meaning: \\ \/;

# Single quoting:
    say 'Lots of options for single quotes';
    say q/Lots of options for single quotes/;

# Double quoting: interpolates scalars, arrays, hashes, functions,
    # closures, and backslash codes
    say "Plenty of ways to double quote too";
    say qq/Plenty of ways to double quote too/;

# Interpolate scalars only:
    say q:s/The quick brown $var1 jumps over the lazy dog/;
    say qs/The quick brown $var1 jumps over the lazy dog/;

# Interpolate @ vars only:
    say q:a/The quick brown @animal[0] jumps over the lazy @animal[1]/;
    say qa/The quick brown @animal[0] jumps over the lazy @animal[1]/;
    say qa/We have @animal.elems() elements in @animal[]/;

# interpolate % vars only:
    say q:h/The quick brown %animal{'quick'} jumps over the.../;
    say qh/The quick brown %animal{'quick'} jumps over the.../;
    say qh/We have %animal.elems() key in %animal{}/;

# interpolate functions only: both & and () are required
    sub get_animal ($tag) { return %animal{$tag}; }
    say q:f/The quick brown &get_animal('quick') jumps.../;
    say qf/The quick brown &get_animal('quick') jumps.../;

# interpolate closures only:
    say q:c/The quick brown { 'fox'; } jumps.../;
    say qc/The quick brown { 'fox'; } jumps.../;

# interpolate backslash codes only:
    say q:b/The quick brown fox\n\tJumps over the lazy dog/;
    say qb/The quick brown fox\n\tJumps over the lazy dog/;

Adverbs can be strung together to make a specialized quoting environment for your string.

# interpolate only scalars and array variables:
    say q:s:a/The quick brown $fox jumps over the lazy @animal[1]/;

Special adverbs and synonyms

=over 4

:w Split on words

my ($fox,$dog)     = q:w/brown lazy/;
    my @array          = qw/fox dog/;

The <> synonym for q:w has many uses

@animals           = <fox dog monkey>;
    say @animals[0]    ; # fox
    %animal            = <brown fox lazy dog>;
    say %animal<lazy>  ; # dog

:ww Split on Quoted Words

@animals           = q:ww<"brown fox" "lazy dog">;
    # Quoted words and variable interpolation
    @animals           = qq:ww/"brown $fox" "lazy %animal{'lazy'}"/;

The «» synonym for qq:ww has many uses, also spelled <<>>

($fox,$dog)          = «brown lazy»;
    ($fox,$dog)          = <<brown lazy>>; # same
    %animal              = « $fox fox lazy "lazy dog" »;
    say %animal« $dog »  ; # lazy dog
    say %animal<<$dog>>    ; # lazy dog

:x Execute

TODO

=back

:t Defining Multiline Strings (Here Documents)

Multiline strings (here documents) can be defined using the q// and qq// operators with the :to adverb added.

# A double quoted multiline string:
    my $a = qq:to/EOF/;
        This is a multiline here document terminated by EOF on a
        line by itself with any amount of whitespace before or
        after the termination string. Leading whitespace equivalent
        to the indentation of the delimiter will be removed from
        all preceding lines.
        EOF

When defined in this way the whitespace at the start of each line will be removed up to the same amount of indentation used by the closing delimiter, a tab character being equal to 8 normal spaces.

A here document can be as exacting with adverbs as any other quoted string. For instance you can specify that you only want scalars interpolated by adding the :s adverb.

# This multiline string will only interpolate scalars
    my $multiline = q:s:to/EOF/;
        This $scalar will be interpolated, but this @array won't be.
        EOF

These adverbs apply to the body of the heredoc, not to the terminator, because the terminator has to be known at compile time. This means that q:s:to/EO$thing/ doesn't do what you mean.

#!/usr/bin/env raku

use v6;



my Str $string = 'This Str is holding a String';
my $scalar = 'Party time'; # Also a string



print ""        ; # output is an empty string
print "Hello\n" ; # output string is Hello followed by a new line
say   "Hello"   ; # same
say   'Hello'   ; # same



say "Hello", " World", "!"; # Hello World!



{
  my Str $string;
  $string = "Hello" ~ " World" ~ "!";
  say $string; # Hello World!
  $string.say; # As above
}



{
  my Str $string = 'This is $string: a scalar holding a String';
  say $string;
  say '$string is ', $string.^name;                 # Str

  my $scalar = 'This is $scalar holding a String';
  say $scalar;
  say '$scalar is ', $scalar.^name;                 # Str
}



say 1; # 1 is a number
say 2, " is a number interpreted as a string"; # 2 is a number interpreted as a string.
say 1+2*3, " is a number interpreted as a string"; # 7 is a number interpreted as a string.



say   +""     ; # a num-ified empty string evaluates as 0
say  "1" + 1  ; # 2



{
  my $string = "1" ~ "1" + 10; # 12, 21, or even... "111"?
  say $string;
  say "1 + 1"; # literally: 1 + 1
  say EVAL "1 + 1";    # 2
}



$scalar = 1234;
say $scalar; # 1234
say '$scalar is ', $scalar.^name;
say '~$scalar is ', (~$scalar).^name;



# literal whitespace
my $squot = '    The quick brown fox jumps over the lazy dog.
	dog.';
my $dquot = "    The quick brown fox jumps over the lazy
	dog.";
say $squot;
say $dquot;

# Double-quotes interpolate special backslash values,
# but single-quotes do not
say 'The quick brown fox\n\tjumps over the lazy dog\n';
say "The quick brown fox\n\tjumps over the lazy dog\n";

# Variables
my $var1 = 'dog' ;
say 'The quick brown fox jumps over the lazy $var1';
say "The quick brown fox jumps over the lazy $var1";

# interpolate array elements:
my @animal = ("fox", "dog");
say 'The quick brown @animal[0] jumps over the lazy @animal[1]';
say "The quick brown @animal[0] jumps over the lazy @animal[1]";

# interpolate hash elements:
my %animal = (quick => 'fox', lazy => 'dog');
say 'The quick brown %animal{\'quick\'} jumps over the lazy %animal{\'lazy\'}.';
say "The quick brown %animal{'quick'} jumps over the lazy %animal{'lazy'}.";

# interpolate methods, closures, and functions:
say '@animal.elems() {@animal.elems} &elems(@animal)';
say "@animal.elems() {@animal.elems} &elems(@animal)";



# vim: expandtab shiftwidth=4 ft=perl6