in reply to Re^2: Refactoring Perl #7 - Remove Assignments to Parameters
in thread Refactoring Perl #7 - Remove Assignments to Parameters

by_val does not modify its parameter ($_[0]), it modifies a copy of its parameter ($var). Without realizing it, you've used the very refactoring tool you are trying to present to us (my $var = shift;).

Using a temporary variable instead of assigning to (or even reading from) a parameter is standard practice in Perl!

Update: Example: 

sub inc_original {
   $_[0]++;
   return $_[0];
}

sub inc_refactored {
   my ($var) = @_;
   $var++;
   return $var;
}

{  my $i = 3;
   my $j = inc_original($i);
   print("$i + 1 = $j\n");    # 4 + 1 = 4
}
{  my $i = 3;
   my $j = inc_refactored($i);
   print("$i + 1 = $j\n");    # 3 + 1 = 4
}

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Re^4: Refactoring Perl #7 - Remove Assignments to Parameters
by agianni (Hermit) on Aug 23, 2007 at 15:57 UTC

    Excellent! I guess this refactoring pattern maps nicely to TheDamian's Always unpack @_ first best practice (PBP, p. 178).

    Update: Actually, after thinking about it a bit, it doesn't map exactly. Consider: 

    my $array_ref = [ qw( 1 4 9 10 15 23 34 84 100 ) ];
add_n_to_values_and_print( $array_ref, 15 );

sub add_n_to_values_and_print{
    my ( $list, $add ) = @_;

    for my $value ( @$list ){
        $value += $add;
        print $value;
    }
}

    [download]

    This follows Mr. Conway's admonition to unpack @_ but it doesn't accomplish the goal of avoding changes to the reference passed in that Fowler is after. What we really need to do is: 

    my $array_ref = [ qw( 1 4 9 10 15 23 34 84 100 ) ];
add_n_to_values_and_print( $array_ref, 15 );

sub add_n_to_values_and_print{
    my ( $list, $add ) = @_;

    # dereference and work with a local copy of the array
    my @local_list = @$list;
    for my $value ( @local_list ){
        $value += $add;
        print $value;
    }
}

    [download]

    Truthfully, that's not a very good example, as I wouldn't generally write code that actually updates the value I would probably write print $value + $add, but for the sake of an example...

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