Shell Programming and Scripting

I'm a newbie to perl scripting. Can someone please explain the syntax below. Specifically what does -> and => do?

Code:

$tz->site(['http','https'] => $site);

$tz is a reference variable that refers to the object obtained by blessing some package. site is a sub-routine (object method) in that package to which you're sending two arguments (as a list) viz. ['http','https'] and $site.

The single arrow (->) operator is called the dereference operator. In actual sense, a single arrow is used to refer to a value in a data structure (could be a scalar variable, list, hash, object). Observe the below example:

Code:

my $anonyArrRef = [ 1, 2, 3, 4, 5 ];
print "$anonyArrRef->[2]\n";

This will output 3, the 3rd number in the anonymous array whose reference is held in $anonyArrRef variable.

In your example, $tz is a reference variable that points to some object. And that object contains a sub-routine (object method) that is accessed by the thin arrow operator (->) -- $tz->site()A big arrow operator (=>) is just a notation used to represent key-value pairs while defining a hashmap.

Code:

my %hash = ('k1', 'v1', 'k2', 'v2); # One way to define a hash. Not so pleasing to human eye.
my %hash = ('k1' => 'v1', 'k2' => 'v2'); #Another way. Pleasing to human eye.
my %hash = (   'k1' => 'v1',
               'k2' => 'v2',
               'k3' => 'v3',
           ); # A little more creativity.

In your example, you're sending one key-value pair to sub-routine site, where the key is an anonymous array ['http','https'] and value is a scalar variable $site. Actually, the key would be a reference to this anonymous array.

Hope this helps.

Technically, the -> is a binary infix dereference operator. In your case, since the right side of the operator is neither an array/hash subscript nor a subroutine argument list, it implies a method call with site being the method name and $tz could be an object reference or a class/package name.

The => operator is just a fancy comma. You can use it almost anywhere a , is used. Since this fat-arrow operator has the feature of autoquoting any Perl identifier to its immediate left (and also since it is eye-friendly), it is the preferred way to separate keys and values in a hash.

First, I know almost no perl, but that never stops me!

-> is stolen from C, says point to a child member of, in this case a member function or method of a class/object/struct type.

=> is somewhat synonymous with a comma, and has functions by context as a list separator or left operand discard, like in c you can say "x = ( y == 3 ? printf("Equal\n"), 1 : 0 );", which is assign to x the result of test y equal 3 if true printf and then discard the printf return and supply a 1 else supply a 0, which is an ugly way to say if y is three than printf and set x to 1 else set x to 0. The comma discards the return of the printf, and connects two commands that run together like a {}, especially handyin contexts where {} are not legal. However, the comma has special meanings by context, like separating key from value in a hash table. (A hash table is a lookup by some integer derived (hashed) from the key, to pick one of N buckets where there may be multiple but few key value pairs that have to be searched linearly. However, the hash and bucket select can be far faster than traversing a tree, and buckets are cheap, so you can have many, and so few buckets with many pairs. A good hash helps a lot, too.

Here is some more code. Comment on this.

Code:

my $tz = LWP::UserAgent->new; 
my $site = http://website.com 
$tz->site(['http','https'] => $site);

From what I know, there is no sub-routine 'site' in the LWP::UserAgent module, available in CPAN. Are you using a custom built variant of this module?

Actually I copied it wrong. Below is what it should be. Please explain that.

Code:

my $tz = LWP::UserAgent->new; 
my $proxy = http://website.com 
$tz->proxy(['http','https'] => $proxy);