11.1. What do the ->, => and :: symbols mean?

The -> is the "infix dereference operator". In other words it is the means by which one calls a sub with a pass by reference (among other things you can do with ->). As stated above most things in calls to perl/Tk routines are passed by reference. The -> is used in perl just as in C or C++. (Most of the widget primitives are elements of the Tk:: "perl class".) A simple example of dereferencing would be:

    $x = { def => bar };  # $x is a reference to an anon. hash
    print $x->{def},"\n"; # prints ``bar''
Note that in the case of calling perl/Tk subs there may be more than one way to call by reference. Compare
    my($top) = MainWindow->new;
with
    my($top) = new MainWindow;
But in general you will be making extensive use of calls like:
    $top -> Widge-type;
There is a clear and succint discussion of references, dereferences, and even closures in man perlref(1) or see the perl 5 info page at:
    http://www.metronet.com/perlinfo/perl5.html
The use of the => operator is quite common in perl/Tk scripts. Quoting from man perlop(1):
The => digraph is simply a synonym for the comma operator. It's useful for documenting arguments that come in pairs.
You could say that => is used for aesthetic or organizational reasons. Note in the following how hard it is to keep track of whether or not every -option has an argument:
    $query -> Button(-in,\$reply,-side,'left',-padx,2m,-pady,
     2m,-ipadx,2m,-ipady,1m)->pack(-side,'bottom');
As opposed to:
    $query ->Button( -in => \$reply,
                     -side => 'left',
                     -padx => 2m,
                     -pady => 2m,
                     -ipadx => 2m,
                     -ipady => 1m
                    )->pack(-side => 'bottom');
By the way if you wanted the numeric "greater than or equal" you would use >= not =>.

While the :: symbol can be thought of as similar to the period in a C struct, it is much more akin to the :: class scope operator in C++:

    a.b.c;       /* something in C */
    a::b::c();   // function in C++ 
    $a::b::c;    # a scalar in Perl 5
    @a::b::c;    # a list in Perl 5
    %a::b::c;    # an associative array or "hash" in Perl 5
    &a::b::c;    # a function in Perl 5
It is also analogous to the single forward quotation mark in perl 4:
    $main'foo;   # a $foo scalar in perl 4
    $main::foo;  # a $foo scalar in Perl 5
For backward compatibility perl 5 allows you to refer to $main'foo but $main::foo is recommended.

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