In some of my work I have started doing a lot more with higher order and functional Perl programming. A good example is PGObject::Util::DBMethod which provides a way to declaratively map stored procedures in Postgres to object methods. I have linked to the source code on github above because it is a good example of where macros would be very helpful.

Now I will be the first to admit that in these cases, macros are not 100% necessary. The module above can accomplish what it needs to do without them. However the alternative, which means effectively creating a highly generalized anonymous coderef, setting up a custom execution environment for that coderef, and then installing the generalized coderef with the specific execution environment as a method has some significant drawbacks.

Here's the particular section that does the main work: 
sub dbmethod {
    my $name = shift;
    my %defaultargs = @_;
    my ($target) = caller;

    my $coderef = sub {
       my $self = shift @_;
       my %args;
       if ($defaultargs{arg_list}){
           %args = ( args => _process_args($defaultargs{arg_list}, @_)
+ );
       } else {
           %args = @_;
       }
       for my $key (keys %{$defaultargs{args}}){
           $args{args}->{$key} = $defaultargs{args}->{$key} 
                  unless $args{args}->{$key} or $defaultargs{strict_ar
+gs};
           $args{args}->{$key} = $defaultargs{args}->{$key} 
                 if $defaultargs{strict_args};
       }
       for my $key(keys %defaultargs){
           next if grep(/^$key$/, qw(strict_args args returns_objects)
+);
           $args{$key} = $defaultargs{$key} if $defaultargs{$key};
       }
       my @results = $self->call_dbmethod(%args);
       if ($defaultargs{returns_objects}){
           for my $ref(@results){
               $ref = "$target"->new(%$ref);
           }
       }
       if ($defaultargs{merge_back}){
           _merge($self, shift @results);
           return $self;
       }
       return shift @results unless wantarray;
       return @results;
    };
    no strict 'refs';
    *{"${target}::${name}"} = $coderef;
}

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Now that is 40 lines of code and 30 lines of it go into the coderef which is executed when the method is actually run. This doesn't seem too much but it does the work of 5-10 lines of code in an imperative style. In other words, it is 5-6 times as long and intensive as it needs to be.

With macros, it would be quite possible to generate only the code needed for the specific function rather than creating a generalized case which has to handle many non-applicable inputs, and then create a context where it only gets what it needs.

In reply to The Case for Macros in Perl by einhverfr

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