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Section 4.7.  Simplifying Nested Element References with Arrows

 
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4.7. Simplifying Nested Element References with Arrows

Look at the curly-brace dereferencing again. As in our earlier example, the array reference for Gilligan's provision list is ${$all_with_names[2]}[1]. Now, what if we want to know Gilligan's first provision? We need to dereference this item one more level, so it's yet another layer of braces: ${${$all_with_names[2]}[1]}[0]. That's a really noisy piece of syntax. Can we shorten that? Yes!

Everywhere we write ${DUMMY}[$y], we can write DUMMY->[$y] instead. In other words, we can dereference an array reference, picking out a particular element of that array by simply following the expression defining the array reference with an arrow and a square-bracketed subscript.

For this example, this means we can pick out the array reference for Gilligan with a simple $all_with_names[2]->[1], and Gilligan's first provision with $all_with_names[2]->[1]->[0]. Wow, that's definitely easier on the eyes.

If that weren't already simple enough, there's one more rule: if the arrow ends up between "subscripty kinds of things," such as square brackets, we can also drop the arrow. $all_with_names[2]->[1]->[0] becomes $all_with_names[2][1][0]. Now it's looking even easier on the eyes.

The arrow has to be between non-subscripty things. Why wouldn't it be between subscripty things? Well, imagine a reference to the array @all_with_names:

my $root = \@all_with_names;

Now how do we get to Gilligan's first item?

$root -> [2] -> [1] -> [0]

More simply, using the "drop arrow" rule, we can use:

$root -> [2][1][0]

We cannot drop the first arrow, however, because that would mean an array @root's third element, an entirely unrelated data structure. Let's compare this to the full curly-brace form again:

${${${$root}[2]}[1]}[0]

It looks much better with the arrow. Note, however, that no shortcut gets the entire array from an array reference. If we want all of Gilligan's provisions, we say:

@{$root->[2][1]}

Reading this from the inside out, we can think of it like this:

  • Take $root.

  • Dereference it as an array reference, taking the third element of that array (index number 2).

  • Dereference that as an array reference, taking the second element of that array (index number 1).

  • Dereference that as an array reference, taking the entire array.

The last step doesn't have a shortcut arrow form. Oh well.[*]

[*] It's not that it hasn't been discussed repeatedly by the Perl developers; it's just that nobody has come up with a nice backward-compatible syntax with universal appeal.


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