#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;
use Sub::Identify;
use Sub::Install;

sub g {
    my $i = shift;
    return 1 if $i == 0;
    return $i * g->($i-1);
}

# Decorator

sub trace_sub {
    my $sub     = shift;
    my $subname = Sub::Identify::sub_name( $sub );
    my $lvl     = 0;
    
    Sub::Install::reinstall_sub({
        code => sub {
            my @args = @_;
            print "  " x $lvl++ ."called $subname(" . join(', ', @args) .")\n" ;
            my $value = $sub->(@args);
            print "  " x --$lvl ."$subname(". join(', ', @args) .") returns $value\n"; 
            return $value;
        },
        as   => $subname,
    });
    
    return;
}

trace_sub(\&g);
print g(4);