Question

In Perl, when you have a nested data structure, it is permissible to omit de-referencing arrows to 2d and more level of nesting. In other words, the following two syntaxes are identical:

my $hash_ref = { 1 => [ 11, 12, 13 ], 3 => [31, 32] };

my $elem1 = $hash_ref->{1}->[1]; 
my $elem2 = $hash_ref->{1}[1]; # exactly the same as above

Now, my question is, is there a good reason to choose one style over the other?

It seems to be a popular bone of stylistic contention (Just on SO, I accidentally bumped into this and this in the space of 5 minutes).

So far, almost none of the usual suspects says anything definitive:

• perldoc merely says “you are free to omit the pointer dereferencing arrow”.
• Conway’s “Perl Best Practices” says “whenever possible, dereference with arrows”, but it appears to only apply to the context of dereferencing the main reference, not optional arrows on 2d level of nested data structures.

• “Mastering Perl for Bioinfirmatics” author James Tisdall doesn’t give very solid preference either:

  “The sharp-witted reader may have
  noticed that we seem to be omitting
  arrow operators between array
  subscripts. (After all, these are
  anonymous arrays of anonymous arrays
  of anonymous arrays, etc., so
  shouldn’t they be written
  [$array->[$i]->[$j]->[$k]?) Perl
  allows this; only the arrow operator
  between the variable name and the
  first array subscript is required. It
  make things easier on the eyes and
  helps avoid carpal tunnel syndrome. On
  the other hand, you may prefer to keep
  the dereferencing arrows in place, to
  make it clear you are dealing with
  references. Your choice.”

• UPDATED “Intermediate Perl”, as per its co-author brian d foy, recommends omitting the arrows. See brian’s full answer below.

Personally, I’m on the side of “always put arrows in, since it’s more readable and obvious they’re dealing with a reference”.

UPDATE To be more specific re: readability, in case of a multi-nested expression where subscripts themselves are expressions, the arrows help to “visually tokenize” the expressions by more obviously separating subscripts from one another.

Answer 1

Unless you really enjoy typing or excessively long lines, don’t use the arrows when you don’t need them. Subscripts next to subscripts imply references, so the competent programmer doesn’t need extra clues to figure that out.

I disagree that it’s more readable to have extra arrows. It’s definitely unconventional to have them moving the interesting parts of the term further away from each other.

In Intermediate Perl, where we actually teach references, we tell you to omit the unnecessary arrows.

Also, remember there is no such thing as "readability". There is only what you (and others) have trained your eyes to recognize as patterns. You don’t read things character-by-character then figure out what they mean. You see groups of things that you’ve seen before and recognize them. At the base syntax level that you are talking about, your "readability" is just your ability to recognize patterns. It’s easier to recognize patterns the more you use it, so it’s not surprising that what you do now is more "readable" to you. New styles seem odd at first, but eventually become more recognizable, and thus more "readable".

The example you give in your comments isn’t hard to read because it lacks arrows. It’s still hard to read with arrows:

 $expr1->[$sub1{$x}]{$sub2[$y]-33*$x3}{24456+myFunct($abc)}
 $expr1->[$sub1{$x}]->{$sub2[$y]-33*$x3}->{24456+myFunct($abc)}

I write that sort of code like this, using these sorts of variable names to remind the next coder about the sort of container each level is:

my $index = $sub1{$x};
my $key1  = $sub2[$y]-33*$x3;
my $key2  = 24456+myFunct($abc);

$expr1->[ $index ]{ $key1 }{ $key2 };

To make that even better, hide the details in a subroutine (that’s what they are there for 🙂 so you never have to play with that mess of a data structure directly. This is more readable that any of them:

  my $value = get_value( $index, $key1, $key2 );

  my $value = get_value(
      $sub1{$x},
      $sub2[$y] - 33*$x3,
      24456 + myFunct($abc)
      );