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.htmlThe 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 5It 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 5For backward compatibility perl 5 allows you to refer to $main'foo but $main::foo is recommended.
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