Question

I want to do two things:

In production code, I want to redefine the open command to enable me to add automagic file logging. I work on data processing applications/flows and as part of that, it’s important for the user to know exactly what files are being processed. If they are using an old version of a file, one way for them to find out is by reading through the list of files being processed.

I could just create a new sub that does this logging and returns a file pointer and use that in place of open in my code.

It would be really nice if I could just redefine open and have pre-existing code benefit from this behavior. Can I do this?

In debug code, I’d like to redefine the printf command to insert comments along with the written output indicating which code generated that line. Again, I have a sub that will optionally do this, but converting my existing code is tedious.

Answer 1

For open: This worked for me.

use 5.010; use strict; use warnings; use subs 'open'; use Symbol qw<geniosym>;  sub open (*$;@) {      say 'Opening $_[-1]';     my ( $symb_arg ) = @_;     my $symb;     if ( defined $symb_arg ) {          no strict;         my $caller = caller();         $symb = \*{$symb_arg};     }     else {          $_[0] = geniosym;     }     given ( scalar @_ ) {          when ( 2 ) { return CORE::open( $symb // $_[0], $_[1] ); }         when ( 3 ) { return CORE::open( $symb // $_[0], $_[1], $_[2] ); }     }     return $symb; }  open PERL4_FH, '<', 'D:\temp\TMP24FB.sql'; open my $lex_fh, '<', 'D:\temp\TMP24FB.sql'; 

For Printf: Did you check out this question? -> How can I hook into Perl’s print?