Quick note about caller


A quick note about caller.

This is mostly a mute post, as the code below should say it all SYNOPSIS-style:

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';

internal('top', 0);
___balk('something', 'else');
__balk('complaint from top');


sub external {
   internal('external', 0);
   __balk('complaint from external');

sub internal ($hint, $condition) {
   my $caller = ___caller();
   say defined($caller) ? "hint<$hint> caller<$caller>"
      : "hint<$hint> undefined caller";

sub validate ($condition) {
   ___balk(___caller(2), 'validation failed') unless $condition;

sub ___caller ($n = 1) {
   my (undef, undef, undef, $subname) = caller($n + 1);
   return defined($subname) ? ($subname =~ s{\A .* ::}{}rmxs) : '(top)';

sub ___balk ($prefix, $msg) { say "$prefix: $msg"        }
sub __balk  (         $msg) { ___balk(___caller(), $msg) }

The gist of it is that I want to figure out the name of the function that is complaining about something, and caller lets me get that name without forcing me to pass it explicitly (this is implemented by __balk, which wraps ___balk).

So ___caller gives us the name of the caller. As it is itself another call to a function, we have to increment the $n value to get hold of what ___caller’s caller actually wants. This is shown, as an example, in validate, which is meant to be called to do validation of stuff on behalf of its caller internal, which in turn is gets its inputs from its caller (either the top level or external).

The output of the proof of concept above is the following:

hint<top> caller<(top)>
(top): validation failed
something: else
(top): complaint from top
hint<external> caller<external>
external: validation failed
external: complaint from external

This is not a perfect mechanism, as it will not “work” inside anonymous functions, which is a drag for dispatch tables. I usually turn to making the package my dispatch table, usually with a specific prefix to the function names (that I eventually strip off); in this case, checking for the proper callback is as easy as calling __PACKAGE__->can($key), so no big deal.


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