NAME

perlintro -- a brief introduction and overview of Perl 6

AUTHOR

Kirrily "Skud" Robert <skud@cpan.org> (original Author) and Moritz Lenz <moritz@fau2ik3.org> ("translation" to perl 6)

Some fixup by David Koenig <karhu@u.washington.edu>

DESCRIPTION

This document is intended to give you a quick overview of the Perl programming language as of Version 6, along with pointers to further documentation. It is intended as a "bootstrap" guide for those who are new to the language, and provides just enough information for you to be able to read other peoples' Perl and understand roughly what it's doing, or write your own simple scripts.

This introductory document does not aim to be complete. It does not even aim to be entirely accurate. In some cases perfection has been sacrificed in the goal of getting the general idea across. You are strongly advised to follow this introduction with more information from the full Perl manual, the table of contents to which can be found in perltoc.

Throughout this document you'll see references to other parts of the Perl documentation. You can read that documentation using the p6doc command or whatever method you're using to read this document.

What is Perl?

Perl is a general-purpose programming language originally developed for text manipulation and now used for a wide range of tasks including system administration, web development, network programming, GUI development, and more.

The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal). Its major features are that it's easy to use, supports procedural, object-oriented (OO) and a bit of functional programming, and has powerful built-in support for text processing. It can also use the large collection of Perl 5 modules.

Running Perl Programs

Currently, the main interpreter is Pugs, available at http://pugscode.org. This is the most complete interpreter so far, but is kind of slow.

Assuming your code is in a file named "foo.p6", you can invoke your program as:

pugs foo.p6

You can also write one-liners with the -e option:

pugs -e '"Hello World".say'

Basic syntax overview

A Perl script or program consists of one or more statements. These statements are simply written in the script in a straightforward fashion. There is no need to have a "main()" function or anything of that kind.

Perl statements end in a semi-colon:

say "Hello, world";

Comments start with a hash symbol and run to the end of the line. You can also have comments that are in the middle of a line by enclosing the comment with brackets. You can have no space between the bracket and the hash:

# This is a comment say #(this is a comment too) "Hello World";

Whitespace is irrelevant, for the most part. However, there are some places where space is required or not allowed:

say "Hello, world" ; # this works "foo" .say; # this is a syntax error, no space allowed. "foo"\ .say; # this is a "long dot", allowed. backslashed whitespace is ignored.

... except inside quoted strings:

# this would print with a linebreak in the middle say "Hello world";

Double quotes or single quotes may be used around literal strings:

say "Hello, world"; say 'Hello, world';

However, only double quotes "interpolate" variables and special characters such as newlines ("\n"):

print "Hello, $name\n"; # works fine print 'Hello, $name\n'; # prints $name\n literally

(Note that print and say do the same thing, only say appends a newline character at the end.)

Numbers don't need quotes around them:

say 42;

You can use parentheses for functions' arguments or omit them according to your personal taste. They are only required occasionally to clarify issues of precedence.

say("Hello, world"); say "Hello, world";

You can call methods in OO-Style notation:

"Hello, world".say;

If you provide addtional arguments to method calls, you should always use parentheses.

Perl variable types

Perl has three main variable types: scalars, arrays, and hashes.

Scalars

A scalar represents a single value:

my $animal = "camel"; my $answer = 42;

Scalar values can be strings, integers or floating point numbers, and Perl will automatically convert between them as required. There is no need to pre-declare your variable types. But if you feel like, you declare their type explictly:

my Str $animal = "camel"; my Int $answer = 42; my Num $close_to_pi = 3.141;

Scalar values can be used in various ways:

say $animal; say "The animal is $animal"; say "The square of $answer is ", $answer * $answer;

There are some "magic" scalars with names that look like punctuation or line noise. These special variables are used for different purposes, all of them are documented in doc:perlvar. The only one you need to know about for now is $_ which is the "default variable". It's used as the default argument to a number of functions if they start with a period . and it's set implicitly by certain looping constructs.

.print; # prints contents of $_

Arrays

An array represents a list of values:

my @animals = "camel", "llama", "owl"; my @numbers = 23, 42, 69; my @mixed = "camel", 42, 1.23;

Arrays are zero-indexed. Here's how you get at elements in an array:

print @animals[0]; # prints "camel" print @animals[1]; # prints "llama"

You can get the index of the last element of an arry with end:

print @mixed[@mixed.end]; # last element, prints 1.23

You might be tempted to use @array.end + 1 to tell you how many items there are in an array. Don't bother. As it happens, using @array where Perl expects to find a scalar value ("in scalar context") will give you the number of elements in the array:

if @animals < 5 { ... }

If you want to get the number of elements explicitly, you can use elems:

print @animals.elems # prints "3";

To get multiple elements from an you can use a list of indexes in the square brackets:

@animals[0,1]; # gives ("camel", "llama"); @animals[0..2]; # gives ("camel", "llama", "owl"); @animals[1..@animals.end]; # gives all except the first element

This is called an "array slice".

You can do various useful things to lists:

my @sorted = @animals.sort; # or: my @sorted = sort @animals my @backwards = @numbers.reverse;

There are some special arrays too, such as @*ARGS (the command line arguments to your script). These are document in doc:perlvar.

There are also some shortcuts that make life easy. For example, these two lines are the same:

my @list = <foo bar baz; my @list = 'foo', 'bar', 'baz'; >

Hashes

A hash represents a set of key/value pairs:

my %fruit_color = "apple", "red", "banana", "yellow";

You can use whitespace and the "=>" operator to lay them out more nicely:

my %fruit_color = "apple" = "red", "banana" => "yellow", ); >

To get at hash elements:

%fruit_color{"apple"}; # gives "red"

You can get a list of keys and values with the keys and values methods:

my @fruits = %fruit_color.keys; my @colors = %fruit_color.values;

Hashes have no particular internal order, though you can sort the keys and loop through them.

Just like special scalars and arrays, there are also special hashes. The most well known of these is %*ENV which contains environment variables. Read all about it (and other special variables) in doc:perlvar.

Complex data structures

TODO: when do you need captures?

Variable scoping

Throughout the previous section all the examples have used the syntax:

my $var = "value";

my creates lexically scoped variables, that means they are scoped to the block (i.e. a bunch of statements surrounded by curly-braces) in which they are defined.

my $a = "foo"; if $some_condition { my $b = "bar"; say $a; # says "foo" say $b; # says "bar" } say $a; # says "foo" say $b; # error: $b has fallen out of scope

Using no strict; at the top of your Perl scripts allows you to use variables without declaring them, but that is strongly discouraged.

Conditional and looping constructs

Perl has all of the usual conditional and looping constructs.

The conditions can be any Perl expression. See the list of operators in the next section for information on comparison and boolean logic operators, which are commonly used in conditional statements.

if

if blocks test by conditions. Unlike Perl 5, you don't need parentheses. However, you need a space between the condition and the opening bracket:

if condition { ... } elsif other condition { ... } else { ... }

There's also a negated version of it:

unless condition { ... }

However, unless blocks can't have else or elsifs attached.

This is provided as a more readable version of if !condition.

Note that the braces are required in Perl, even if you've only got one line in the block. However, there is a clever way of making your one-line conditional blocks more English like:

# the traditional way if $zippy { print "Yow!"; }

# the Perlish post-condition way
print "Yow!" if $zippy;
print "We have no bananas" unless $bananas;
>

Sometimes the condition is more important than the action. You can say:

$zippy and print "Yow!";

and get the same effect.

while

while condition { ... }

There's also a negated version, for the same reason we have unless:

until condition { ... }

You can also use "while" in a post-condition:

say "LA LA LA" while 1;          # loops forever

If you want the condition to be checked after the loop block, you can use Pascal-like repeat:

repeat {
    ...
} while condition

repeat {
    ...
} until condition

for and loop

For a C-style for-loop use loop:

loop (my $i = 0; $i < $max; $i++){
    ...
}

This kind of loop is rarely needed in Perl since Perl provides the more friendly list scanning for loop.

for @list -> my $i {
    say "This element is $i";
}

# or you can use thte default $_ variable:
for @list {
    say "This element is $_";
}

Builtin operators and functions

Perl comes with a wide selection of builtin functions. Some of the ones we've already seen include say, sort and reverse. A list of them is given at the start of perlfunc and you can easily read about any given function by using "p6doc -f functionname".

Here are a few of the most common used operators:

Arithmetic

cpanm Perl6::Doc

perl -MCPAN -e shell
install Perl6::Doc