TheDamian suggests the use of get_foo and set_foo.This is because you have some attributes for which there will be a get_foo but no set_foo so the example was that a foo sub that is both a getter and setter has no way to indicate that you can not actually set foo, because if it dies then you have to wrap all your get/setters in eval { ... } and nobody wants that.

package Foo;
use Readonly my $SPAM_FACTOR = 22; 
use Readonly my $SPAM_FUDGE  = 42;  # seasonally adjusted

# cause-cause-bundles-of-problems constructor
sub new { bless {spam=>4} , shift } 

# you can get or set bar with $that->bar
sub bar {
   my ($this, $newbar) = @_;
   if (@_) $this->{bar} = $newbar;
   
   return $this->{bar}; 
}

# you can't set baz, because it's derrived automatically from other at
+tribute 
sub baz {
   my ($this) = @_;
   # just ignore @_ ...   perhaps warn "you cant change baz"
   return $this->{spam} * $SPAM_FACTOR + $SPAM_FUDGE;
}

# some time later
pacakge main;

my $foo = Foo->new();
$foo->bar( 'moose' ); # neat, bar is moose
$foo->baz( 'shoe' ); # not so much, worst of all, it looks right!

printf "I have a lovely foo, it has %s bar and %s baz " , $foo->bar, $
+foo->baz;
# hey! where's my shoe!?
[download]

if you use the get_bar and a set_bar convention, perl will complain bitterly when you call set_baz when the pacakge Foo does not provide one and you'll know straight away that you can't set_baz after all.

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