Perl 6 Basics Tablet

Herbert Breunung — Tue, 17 Apr 2012 00:54:12 -0000

Intro - Chapter: 0:History 1:Design 2:Basics 3:Var 4:Op 5:IO 6:Block 7:Sub 8:OOP 9:Rx 10:Meta
Overview - Appendix: A:Index B:Tables C:Cook D:Delta E:Best of F:Links G:Glossary

1st law of language redesign: Everyone wants the colon for their particular syntax.
2nd law of language redesign: Larry gets the colon for whatever he wants.

Basics doesn't mean here not always easy but fundamental.

Defaults

Please start your Perl 6 program with one of the following lines.

#!/usr/bin/perl6
use v6.0;
v6;

or just start with a keyword like module or class. That marks your code as Perl 6 (in case the interpreter defaults to Perl 5) and makes it possible to mix Perl 5 and 6 in one source file.

To even that little obstacle, you can leave out the usual use strict; and use warnings; in front of every script, because thats now default. Also use utf; is obsolete since any Perl 6 source code is always treated as unicode and any UTF character can be used anywhere in the code. Even the features of the pragmas constant and vars are now part of the core language.

Also the functionality of many useful and famous modules like Moose (object system), Parse::RecDescent++, exception handling, List::[More]Utils, Export, English an advanced pretty printer and much more is already built in. So you get a lot extra for a little v6;.

Statements

Unless you use blocks, a Perl program executes one statement after another in linear progression (from left to right, from up to down). They have to be separated by a semicolon (;), except before and after a closing curly brace, where it is optional.

$coderef = sub { fetch_data(); compute() }

Spaces and Indentation

Perl doesn't care about indentation. And spaces are still in many places without meaning. However these have become fewer.

Comments

Single Line

Like in Perl 5 and many other languages of its league, a "#" tells the compiler to ignore the rest of the line.

my $var = 'good'; # that code is boring

Multi Line

If many lines has to be commented, use #` followed by any pair of braces that surround the comment.

$things = #`( i wonder how many of these
I will need, hm maybe 3, or 4, better 5 ) 5; # same as $things = 5;

POD

Even POD is there to embedd documentation, it can be used just for inserting comments.

=begin comment
...
=end comment

$=

all POD variables

Quoting

Quoting is like regular expression a sublanguage inside the main language with its own syntactical rules. It is parsed by a special grammar as to be found in the special variable $~Quote. The operator with the same name (the generic quoting operator Q) does almost nothing, just provides a mechanism to mark the beginning and end of text sequence.

Delimiter

The examples in this chapter use almost every time slashes for that purpose, but any not alphanumerical character or pair of matching (bracing) character can be used as well.

Q /.../ or Q |...| or Q *...* or Q "..." or Q[...] ...

An extended delimiter mechanism is delivered by heredocs.

Interpolation

Inside of these delimiters, every character will be taken literally. Any additional meaning has to be added by quoting adverbs that have to follow the Q. Most of them have a short and a long name and some of the most useful have an additional syntax that replaces them altogether with the Q operator (like single or double quotes).

:b aka :backslash # control character (implies at least :q)
:s aka :scalar # scalar variable: $name
:a aka :array # array variable: @name[...]
:h aka :hash # hash variable: %name{...}
:c aka :closure # anonymous blocks: {...}
:f aka :function # callable routines: &name(...)

Q :b /\t\n/; # tab and new line character
Q :s /$poem/; # content of $poem
Q :a /@primes[]/; # all number separated by single spaces
Q :a /@primes[0]/; # returns '2', the first prime
Q :a /me@primes.de/; # returns literally the mail adress, you need the square braces to interpolate arrays
Q :h /%dev{}/; # all developer names (values, not keys) separated by single spaces, angle brackets work too
Q :h /%dev[rakudo] %dev/; # just 2 values
Q :h /%dev/; # literally '%dev', you need braces here too
Q :c /There are {2**6} hexagrams in I Ging./; # returns: 'There are 64 hexagrams in I Ging.', inserts the result of the closure
Q :c /Perl 6 Compiler: {%dev.keys}./; # use it too for method calls
Q :h /Perl 6 Compiler: %dev.keys./; # no interpolation
Q :f :a /Here it Tom with the weather: &fetch_report($day)./; # inserts report of that day, even inside Strings the correctness of arguments will be checked!
Q :f :a /fetch_report($day)/; # interpolates just $day
Q :f :a /&fetch_report/; # literal string '&fetch_report', even if the subroutine takes no arguments

Single Quotes

They provide the most basic mechanism in a convenient syntax. All the following are synonyms:

Q :single /.../;
Q :q /.../;
q /.../;
'...'

The backslash (\) liberates here just itself and the single quote from its special meaning. Or to put it simple \\ translates (or interpolates) to \ and \' to '. For anything more you need additional adverbs.

'Welcome in Larry\'s madhouse'
'\'\\'; # string contains: '\
q |\||; # string contains: |

Double Quotes

Double quoting combines all the previous mentioned adverbs for interpolation (also :q - implied by :b), thatswhy all the following are synonymous.

Q :s, :a, :h, :f, :c, :b /.../;
Q :double /.../;
Q :qq /.../;
qq /.../;
"..."

But further adverbs can also be added using q/.../ or qq/.../.

Quote Words

While other quote operators return a single string item, this one can return arrays because he splits the string on any whitespace (\s aka ).

Q :words /.../;
Q :w /.../; # :q implied
qw/.../; # like Perl 5's qw/.../
<...>

Q :quotewords /.../; # qw/.../ with quote protextion
Q :ww /.../; # :qq implied
<<>> # have also a unicode alias (chevron)

The second group of aliases mark a modified version, where single and double quoted strings (inside the quote) are treated as one word. Thats called quote protection.

my @steps = ; # 3 steps to success: ["one", "\"two", "three\""]
my @steps = <>; # now only 2 steps: ["one", "two three"]

Please note also that :quotewords (double pointy braces) implies :double (double quotes), which means all interpolation rules apply here also.

<$pi> eq '$pi'
<<$pi>> eq "$pi" # == '3.14159...'

The same pointy braces (quote operators) are also in used, when writing hash slices.

Heredocs

Are now normal quoted strings, only with a special delimiter, defined by the adverbs to and heredoc. Heredocs can be nested.

Q :to 'EOT';
...
...
EOT

To make templates in which variables and closures are evaluated, take the normal double quote and just add the adverb for the heredoc delimiter or define with other adverbs what exactly you want to have evaluated.

qq:heredoc 'EOT';
EOT

Paths

Pathstrings have their own quote operator. This way you get the warnings early if there is something incompatible with convention.

Q :path /.../;
Q :p /.../;
qp /.../;

Regex

Even being a completely different language then quoting on its own (as to be defined in $~Regex and $~P5Regex), regular expressions can be built using the general quoting operator with the right adverb.

Q :regex /.../ aka rx/.../
Q :subst /.../.../ aka s/.../.../
Q :trans /.../.../ aka tr/.../.../ aka .trans("..." => "...")

Code

The following 3 aliases quote code that will be run immediately (on runtime) and replaced with the result.

Q :exec /.../;
Q :x /.../;
qx /.../;

In Perl 5 qx/.../ aka ... did a system call and not just run eval. To get that behaviour use:

qqx/$cmd @args[]/ # do system call and insert result, alias to that is gone

However there is yet another adverb for quasi quoting, meaning: the quoted code will be parsed and compiled into a abstract syntax tree (AST - internal representation of the compiler) during compile time. Result is the compiled AST. Parsing will be done by using the grammar stored in $~Quasi. This gets important when writing macros.

Q :code /.../;

Number Literals

Unlike strings, numbers don't need quoting. But if there is a non number character in it, there will be an error. Chars of a number definition are: (0-9,.,+,-,e,E,i,_) including the radix prefixes: (0b,0o,0d,0x) and the prefix for version numbering (v). The + and can act also as operator that convert into the numerical context, which still means: take from left to right all digits and stop with the first none number character.

A single underscore is allowed only between any two digits, delimiter helping readability.

3_456_789; # same as 3456789

$int = 2;
$real = 2.2;

Radix Prefixes

0b binary - base 2, digits 0..1
0o ocatal - base 8, digits 0..7
0d decimal - base 10, digits 0..9
0x hexadecimal - base 16, digits 0..9,a..f (case insensitive)

General Radix Form

:10<42> # same as 0d42 or 42

Scientific Notation

$float = 60.2e23 # becomes automatically 6.02e24
$float = 6.02E-23 # capital E works too

Rational Number

To distinguish them from a division operation, you have to groupe them with braces.

(3/7)
(3/7).numerator
(3/7).denominator
(3/7).nude.perl

As always, .perl gives you an almost source like code formatting which results here in 3/7. Adding .nude you get (3/7), the nude source code. There are 2 different immutable value types representing both rational number. FatRat has unlimited precision and Rat has just enough to be evaled into a Real type. When you explicitly type a variable to one o them, the braces become optional.

my Rat $pi_approx = 22/7;
my FatRat $pi_approx = 2222222222/6981317007; # much more precision

Complex Number

have also there own immutable value type.

1+2i
my $c = 5.2+1e42i;
say $c.WHAT; # returns 'Complex', which is the classname of the value object

Version Number

v1.2.3 # okay
v1.2.* # okay, wildcard version
v1.2.3+ # okay, wildcard version
v1.2.3beta # illegal
Version('1.2.3beta') # okay

Formatting

perl

The .perl method returns a string that arranges any set of values in almost the same format, as the would be defined it source code. It's a built in Data::Dumper (pretty printer).

@a.perl # evals to: [1, 2, 3, 4, 5]
%h.perl # evals to: {"akey" => "avalue", "bkey" => "bvalue"}

This works with data of any nesting depth.

pretty

fmt

Small brother of sprinf that works as a method of a variable. If thats a pair or list it formats of course several values in the same way with same syntax as sprinf.

$result = '5.123456789';
say $result.fmt('%.2f'); # "5.12\n"
@nr = 1..5;
say @nr.fmt("+%d."); # "+1.+2.+3.+4.+5.\n"
say @nr.fmt("%d.",','); # "1.,2.,3.,4.,5.\n"
say @nr.fmt("%d %d"); # ERROR
%p6c = sorear => 'niecza', fglock => 'perlito';
say %p6c.fmt("%s!"); # "sorear!\nfglock!\n"
say %p6c.fmt("%s",','); # "sorear,fglock!\n"
say %p6c.fmt("%s:%s"); # "sorear:niecza\nfglock:perlito\n"
say %p6c.fmt("%s:%s",); # "sorear:niecza,fglock:perlito\n"
say %p6c.fmt("%s %s %s"); # ERROR

sprintf

Since Algol68 known function, that inserts into a string any number of special formatted values.

pack

Formats

moved from core language to a module.

Date and Time